Opportunities in Wet-End Chemistry
Publications

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Abstracts of Publications by Group Members
NOTE: Full article PDF files can be obtained from NC State University's "Repository" by using this link.

Song, J., Yamaguchi, T., Silva, D. J., Hubbe, M. A., and Rojas, O. J. (2010). “Effect of charge asymmetry on adsorption and phase separation of polyampholytes on silica and cellulose surfaces,” J. Phys. Chem. B 114(2), 719-727.

The relation between the properties of polyampholytes in aqueous solution and their adsorption behaviors on
silica and cellulose surfaces was investigated. Four polyampholytes carrying different charge densities but
with the same nominal ratio of positive to negative segments and two structurally similar polyelectrolytes (a
polyacid and a polybase) were investigated by using quartz crystal microgravimetry using silica-coated and
cellulose-coated quartz resonators. Time-resolved mass and rigidity (or viscoelasticity) of the adsorbed layer
was determined from the shifts in frequency (Δf) and energy dissipation (ΔD) of the respective resonator.
Therefore, elucidation of the dynamics and extent of adsorption, as well as the conformational changes of the
adsorbed macromolecules, were possible. The charge properties of the solid surface played a crucial role in
the adsorption of the studied polyampholytes, which was explained by the capability of the surface to polarize
the polyampholyte at the interface. Under the same experimental conditions, the polyampholytes had a higher
nominal charge density phase-separated near the interface, producing a soft, dissipative, and loosely bound
layer. In the case of cellulose substrates, where adsorption was limited, electrostatic and polarization effects
were concluded to be less significant.

Chen, H., Park, A., Heitmann, J. A., and Hubbe, M. A. (2009). "Importance of cellulosic fines relative to the dewatering rates of fiber suspensions," Ind. Eng. Chem. Res. 48(20), 9106-9112.

When cellulosic fines are present in significant amounts they can have a dominant influence on dewatering. Pulp suspensions drain rapidly if the fines have been removed.  In this study, the dependency of gravity dewatering rates on the level and properties of cellulosic fine matter was quantified.  Bleached hardwood kraft pulp was used as a source of primary fines (collected before refining) and secondary fines (collected after refining of fines-free fiber suspensions).  Fractions of fine matter also were obtained from chemithermomechanical (CTMP) pulp.  Size distributions of these fines were characterized using a laser diffraction method.  Results were explainable by a mechanism in which unattached fines are able to move relative to adjacent fibers during the dewatering and consolidation of a mat of fibers.  Due to such movement, fines end up in locations where they plug drainage channels in the mat.  The contribution of the fines to dewatering increased in inverse proportion to particle size and with increasing surface area, as calculated from the light scattering analysis.

Silva, D. J., Rojas, O. J., Hubbe, M. A., Park, S. W., Yamaguchi, T., and Song, J. (2009). “Polyampholytes: Their use in papermaking and their solution and adsorption behaviors,” O Papel 70(9), 40-50.

A high molecular weight random polyampholyte having both positive and negative charge in the same chain was used in this study as a paper strength additive. Changing the balance between the positive and negative groups in response to different environmental solution conditions was found to affect paper strength properties.   In this research the polyampholyte solution and the adsorption behaviors were studied at a molecular level over a large pH range (4.3 up to 8.5) by using up to date techniques, e.g., quartz crystal microbalance with energy dissipation (QCM-D), atomic force microscopy (AFM) and dynamic light scattering (DLS). The results showed that the polymer structure, morphology and the net charge density of the aggregates at solid-liquid interface affected the adsorbed amount and the viscoelastic layer properties.  A higher amount of adsorbed polymer mass and more viscoelastic layers were found for pH close to the isoelectric point (pHIEP 7.3). In this condition the best paper strength properties were found. Also, we found that this polyampholyte worked very well for recycled fiber application.

Silva, D. J., Rojas, O. J., Park, S. W., and Hubbe, M. A. (2009). “Evaluation of adsorbed polyampholyte layers by using quartz crystal microbalance,” 10th Intl. Symp. Process Systems Engineering – PSE2009, de Brito Alves, R. M., do Nascimento, C. A. O, and Chalbaud Biscaia, E. C., Jr., (eds.).

Viscoelastic properties of layers of polyampholytes adsorbed on charged surfaces were studied by quartz microgravimetry. By applying the Voigt viscoelastic model the effective mass and thickness of layers after adsorption from solution at different salt concentrations were calculated. The obtained results were compared with the Sauerbrey
equation, which applies to the case of thin, rigid adsorbed layers. The estimates of mass and thickness from the Voigt model were typically larger, and were more strongly affected by variations in the ionic strength of adsorbing solution. Since the Voigt model uses multiple frequencies and dissipation overtones, it was found that the calculated
adsorbed layer mass was closer to the actual values, while the Sauerbrey approach resulted in underestimation. This observation was explained by the fact that adsorbed layers of polyampholytes were soft and highly dissipative. It was noted that the observed changes in dissipation of the adsorbed polyampholyte layers were comparatively large, which suggests a large amount of coupled water.

Hubbe, M. A., Nanko, H., and McNeal, M. R. (2009). “Retention aid polymer interactions with cellulosic surfaces and suspensions: A Review,” BioRes. 4(2), 850-906.

Retention aids can be defined as very-high-mass, water-soluble polymers that are added to cellulosic fiber slurries before the formation of paper in order to improve the efficiency with which fine particles, including cellulosic fines, are retained in the paper product.  Optimization of retention aid performance can be a key to achieving efficient and environmentally responsible papermaking objectives. This article reviews various published theories related to retention aid use.  Findings related to three main classes of retention aid polymers are considered: cationic acrylamide copolymers (cPAM), anionic acrylamide copolymers (aPAM), and polyethylene oxide (PEO).  While many aspects of the interactions of each of these classes of retention aid products can be understood based on colloid chemistry principles, further research is needed in order to more fully bridge the gap between theory and practice.

Hubbe, M. A., and King, K. (2009). Cost-Saving Strategies in Papermaking Chemistry, TAPPI Press, Atlanta, 237 pp.

The goal of this book is to introduce a variety of strategies for cost savings during the manufacture of paper.  Our focus will be on strategies that involve chemical additives.  Chemicals cost money.  It is possible to achieve savings by prudent and well optimized use of additives.  The target audience for this handbook includes two groups:  1) those who will read the book on their own, and 2) those who are fortunate enough to participate in a course.  As members of the papermaking community we are proud to uphold a TAPPI tradition of providing technical books.  Books are a time-proven medium for dissemination of helpful information, allowing the reader to study the material at a self-selected rate, while providing the opportunity to skip directly to the subject matter of most interest and importance.  Readers of this handbook should include engineers, scientists, paper mill staff, chemical company technical support representatives, students, and people from other disciplines who are interested in promoting the economic success of papermaking operations. 

Lee, S. Y., and Hubbe, M. A. (2009). “Morphologies of synthetic mineral microparticles for papermaking as a function of synthetic conditions,” Colloids Surf. A 339(1-3), 118-125.

Morphological characteristics were determined for a system of Synthetic Mineral Microparticles (SMM), which have been developed to promote drainage of water and retention of fine particles during papermaking. Prior research, as well as our own preliminary research showed that the SMM system can have advantages in both of drainage and retention, compared with montmorillonite (bentonite), which is one of the most popular materials presently used in this kind of application. A partially gelled form of a silica-type microparticle additive is known to perform better than the corresponding sol form, in terms of fine-particle retention during papermaking. For this reason it was of interest to investigate the morphological behavior of SMM as a function of the conditions of synthesis. BET nitrogen adsorption was used to measure the surface area of SMM. The distribution of SMM particle size was investigated in the aqueous state, using a light-scattering technique. The coagulation behavior and morphology of SMM were analyzed using scanning electron microscopy (SEM). It was found that the structural characteristics of SMM particles could be explained in terms of the effects of ionic charges on colloidal stability of primary particles during formation of the SMM.

Hubbe, M. A., Chen, H., and Heitmann, J. A. (2009). "Permeability reduction phenomena in packed beds, fiber mats, and wet webs of paper exposed to flow of liquids and suspensions: A review," BioRes. 4(1), 405-451.

Filter media, including those prepared from cellulosic materials, often suffer from permeability loss during continued use. To help understand such issues, one can take advantage of an extensive body of related research in such fields as industrial filtration, water purification, enhanced oil recovery, chromatography, paper manufacture, and the leaching of pollutants from impoundments. Though the mechanisms that appear to govern permeability-loss phenomena depend a lot on the details of various applications, the published research has revealed a number of common features. In particular, flow through a porous bed or fiber mat can be markedly reduced by deposition of particles or colloidal matter in positions that either block or partially restrict fluid flow.  Progress has been achieved in the development of mechanistic models, as well as the use of such models in numerical simulations to explain various experimental findings.  Further research of this type needs to be applied to cellulosic materials, which tend to be much more elongated in comparison to the bed materials and suspended matter considered most often by most researchers active in research related to permeability loss.

Lee, S. Y., and Hubbe, M. A. (2008). “Application of synthetic mineral microparticles with various metal species,” J. Korea TAPPI 40(5), 1-10.

Hubbe, M. A., and Rojas, O. J. (2008). “Colloidal stability and aggregation of lignocellulosic materials in aqueous suspension: A review,” BioResources 3(4), 1419-1491.

Aqueous dispersions of lignocellulosic materials are used in such fields as papermaking, pharmaceuticals, and preparation of cellulose-based composites. The present review article considers published literature dealing with the ability of cellulosic particle dispersions (fiber, fines, nanorods, etc.) to either remain well dispersed or to agglomerate in response to changes in the composition of the supporting electrolyte solution. In many respects, the colloidal stability and coagulation of lignocellulosics can be understood in terms of well-known concepts, including effects due to osmotic pressure arising from overlapping electrostatic double layers at the charged surfaces.  Details of the morphology and surface properties of lignocellulosic materials give rise to a variety of colloidal behaviors that make them unique.  Adjustments in aqueous conditions, including the pH, salt ions (type and valence), polymers (charged or uncharged), and surfactants can be used to control the dispersion stability of cellulose, lignin, or wood-extractive materials to serve a variety of applications.

Hubbe, M. A. (2008). “Accurate charge-related measurements of samples from the wet end: Testing at low electrical conductivity,” Paper Technol. 49(6), 21-26.

Modest changes in testing procedures can improve the trustworthiness of charge-related measurements at the paper machine wet-end.  This article makes the case for implementing such changes broadly within our industry.  Substantial advantages can be achieved by reducing the salt content of samples of wet-end stock, white water, and similar samples before carrying out tests related to charge.  Improved monitoring and control of charge-related quantities in the wet end has the potential to reduce chemical costs and make the paper machine run more uniformly and efficiently. This paper provides modifications to streaming current (SC) titration procedures and fiber pad streaming potential (SP) procedures.  The SC method is most often used for cationic demand titration endpoint determination, both in the lab and when using online monitoring equipment.  The fiber-pad SP method is most often used for estimating the zeta potential of fiber surfaces, especially for product development and for troubleshooting.

Lee, S. Y., and Hubbe, M. A. (2008). “Polyelectrolyte titrations of synthetic mineral microparticle suspensions to evaluate charge characteristics,” Colloids and Surfaces A 311(3), 175-182.

Colloidal charge properties were determined for a system of Synthetic Mineral Microparticles (SMM), which have been developed to promote drainage of water and retention of fine particles during papermaking. Prior research, as well as our own preliminary research showed that the SMM system can have advantages in both of drainage and retention, compared with montmorillonite (bentonite), which is one of the most popular materials presently used in this kind of application. Streaming current titrations employing highly charged polyelectrolytes and were used to evaluate the charge properties of SMM suspensions and to understand the interactions among SMM particles, fibers, fiber fines, and cationic polyacrylamide (cPAM). Polyelectrolyte titrations were carried out under different conditions of pH to predict the charge properties of SMM under conditions that reflect paper manufacturing practices. It was found that pH variation, caused by the change of Al/Si ratio and partial neutralization of aluminum’s acidity, profoundly affected the charge properties of SMM, due to the variation of Al-ions and the influence ionizable groups on the Si-containing particle surface.

Hubbe, M. A., Heitmann, J. A., and Cole, C. A. (2008).  “Water release from fractionated stock suspensions.  2.  Effects of consistency, flocculants, shear, and order of mixing,” TAPPI J. 7(8), 14-19.

The rate of gravity drainage from a papermaking stock suspension was found to be highly dependent on the initial consistency, as well as on the presence of cellulosic fines.  The results of testing were fitted to a model based on different linear contributions to drainage resistance due to the fibers and due to each type of fines.  Deviations from the model at relatively high consistency were tentatively attributed to flocculation phenomena.  By selectively treating either the fines, the fibers, or the combined furnish with cationic polyelectrolytes it was possible to achieve substantially higher rates of dewatering.  Results were consistent with several mechanisms, which may possibly act in parallel during ordinary papermaking.  Attachment of cellulosic fines to fiber surfaces can prevent those fines from migrating to choke points within a wet web.  Agglomeration of fines to each other can reduce their effective surface area.  Flocculation of the fibers can make the fiber mat less uniform, thus providing larger channels for water to flow from the mat.

Cole, C. A., Hubbe, M. A., and Heitmann, J. A. (2008). “Water release from fractionated stock suspensions.  1.  Effects of the amounts and types of fiber fines,” TAPPI J. 7(7), 28-32.

Tests with a gravity-based freeness device demonstrated a highly non-linear effect of cellulosic fines on resistance to dewatering.  Fines at low to moderate levels had little effect on gravity dewatering, but fines slowed drainage considerably as their level increased beyond a threshold.  Fines created by refining (secondary fines) slowed drainage to a much greater extent than those originally present in bleached hardwood kraft pulp (primary fines).  The results were consistent with a mechanism in which unattached fines can migrate within a wet web to choke points at which they tend to block the flow of water. 

Hamzeh, Y., Ekhtera, M. H., Hubbe, M. A., Izadyar, S., and Pourtahmasi, K. (2008), “Effects of process variables on poly-aluminum chloride (PAC)-rosin sizing performance under neutral papermaking conditions,” Industrial and Engineering Chemistry Research 47, 4302-4307.

The effects of process variables, including poly-aluminum chloride (PAC) and rosin levels and equilibration
time on PAC-rosin sizing performance, were investigated for three alternative sizing processes, include
conventional, reverse sizing, and premixing under neutral papermaking conditions. The individual and
interaction effects of sizing variables were determined. It was found that, in contrast to conventional sizing,
the sizing efficiency increased with increasing equilibration time when reverse and premixing processes were
used for the sizing treatment. In both processes, increasing equilibration time can lead to potential savings in
PAC or rosin. Finally, the optimum conditions for more cost-effective sizing were determined for the three
sizing processes.

Bastidas, J. C., Pawlak, J. J., Venditti, R. A., Heitmann, J. A., Hubbe, M. A., and Kadla, J. F. (2008). “A colloidal probe microscopy study of cellulose/gypsum interactions,” Materials Characterization 59(2), 144-150.

Drywall, which is made primarily of a calcium sulfate dihydrate (gypsum) core with paper on both sides, is one of the most widely used construction materials. Because board failure often occurs at the gypsum core/paper interface, it has become important to know the exact nature of the gypsum/cellulose bond. This study provides data about the nature of this interaction by means of AFM and Colloidal Probe Microscopy. These methods made it possible to distinguish among the different crystal faces and their respective interactions with cellulose. Measured in air, the adhesive forces between the AFM tip and the different faces varied according to f(010) < f(120) < f(111) at 50 % relative humidity. The differences in adhesive force with the different gypsum crystals can be attributed to the differences in surface chemistry. The information obtained will help guide improvements in the crystal production process to obtain better bonding between the crystal and the paper.
  
Hubbe, M. A. (2008). “Minimizing the environmental impact of the papermaking process,” Proceedings of the 2nd IPEC Conference, Tianjin, China, Book A, 37-40.

From an environmental perspective, papermakers have a positive story to tell. The products that we manufacture mainly come from renewable resources. Paper can be recycled. It’s also biodegradable. It also can be incinerated to recover its energy value. However, the process of making paper also can raise some environmental concerns. Decades ago it was common to be able to see environmental impacts of papermaking additives just by looking at a waterway downstream of a paper mill. The color of river water often was a good clue as to what color of paper was being manufactured. Such impacts have been greatly reduced, not only due to implementation of wastewater treatment operations, but also to advances in papermaking chemistry – making it possible to achieve high levels of retention efficiency. In the future we can expect there to be increasing emphasis on the overall environmental impact of each chemical additive that is used.  Issues that will be considered will include the chemical’s toxicity, biodegradability, and its tendency (or lack thereof) to become retained on solid surfaces. Certain papermaking additives also have the potential to reduce the need for fibers – or to enable the papermaker to meet specifications at higher filler levels or lower basis weights. Further opportunities lie in the area of automation, making it possible to use chemical additives more efficiently. Looking toward the future, perhaps energy use will emerge as the dominant issue. Evaporation of water is the most energy-demanding part of the papermaking process. Though it is very difficult to demonstrate conclusively, some of the same chemicals that can promote faster dewatering in the forming section also appear to make it easier to squeeze water out of paper in the press section, reducing the amount of evaporation required.

Wu, N., Hubbe, M. A., Rojas, O. J., and Yamaguchi, T. (2008). “Penetration of high-charge cationic polymers into silica gel particles and cellulosic fibers,” Proceedings of the 2nd IPEC Conference, Tianjin, China, Book B, 626-649.

The gradual penetration of positively charged polymeric additives below the outer surfaces of cellulosic fibers can play an important role in the selection of addition points and optimization of chemical dosages on a paper machine. The present work was undertaken to help understand the role of high and low-molecular-mass fractions of a cationic polyelectrolyte, in determining the charge behavior of the outer surfaces of fibers. Commercial polyelectrolyte samples typically contain a broad range of molecular masses. A dialysis procedure was used to selectively remove the low-mass fraction of poly-diallyldimethylammonium chloride (poly-DADMAC) from aqueous solutions of the polymer. Only cationic polymer samples having a very-low-mass component exhibited penetration into the interior spaces of silica gel, which has a well-defined, narrow pore size distribution. The presence of low-mass components of the polymer also affected the electrical potential associated with the outer surfaces of solids treated with high-mass poly-DADMAC. Preliminary observations showed related behavior in the case of cellulosic fibers. The wide range of time required for measured streaming potential values to decrease to zero, depending on the amount of poly-DADMAC added at time zero, was consistent with a gradual diffusion of the macromolecules below the outer surfaces of the fibers.

Hubbe, M. A., Rojas, O. J., Lucian, L. A., and Sain, M. (2008). “Cellulosic nanocomposites, A review,” BioRes. 3(3), 929-980.

Because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties, a great deal of attention has been paid recently to cellulosic nanofibrillar structures as components in nanocomposites. A first major challenge has been to find efficient ways to liberate cellulosic fibrils from different source materials, including wood, agricultural residues, or bacterial cellulose. A second major challenge has involved the lack of compatibility of cellulosic surfaces with a variety of plastic materials. The water-swellable nature of cellulose, especially in its non-crystalline regions, also can be a concern in various composite materials. This review of recent work shows that considerable progress has been achieved in addressing these issues and that there is potential to use cellulosic nano-components in a wide range of high-tech applications.

Hubbe, M. A., Pawlak, J. J., and Koukoulas, A. A. (2008). "Paper's appearance: A review," BioRes. 3(2), 627-665.

This review article highlights progress in understanding the optical properties of paper.  Paper’s appearance can be defined in terms of its opacity, brightness, color, fluorescent properties, gloss, and various quantities related to its uniformity.  The phenomena that give rise to paper’s optical properties, especially its ability to scatter and absorb visible light, are highly dependent on paper’s structure and its chemical composition.  In an effort to engineer low-cost products having relative high opacity and brightness, it is necessary to optimize the material selection and processing conditions.  The dimensions of solid materials and void structures within the paper are key factors for optimizing the optical properties.  In addition, additives including bleaching agents, mineral particles, dyes, and fluorescent whitening agents can impact paper’s optical properties  Paper’s appearance depends, in subtle ways, on the processes of its manufacture.

Hubbe, M. A., Venditti, R. A., and Rojas, O. J. (2007). "What happens to cellulosic fibers during papermaking and recycling? A review," BioRes. 2(4), 739-788.

Both reversible and irreversible changes take place as cellulosic fibers are manufactured into paper products one or more times.  This review considers both physical and chemical changes.  It is proposed that by understanding these changes one can make better use of cellulosic fibers at various stages of their life cycles, achieving a broad range of paper performance characteristics.  Some of the changes that occur as a result of recycling are inherent to the fibers themselves.  Other changes may result from the presence of various contaminants associated with the fibers as a result of manufacturing processes and uses.  The former category includes an expected loss of swelling ability and decreased wet-flexibility, especially after kraft fibers are dried.  The latter category includes effects of inks, de-inking agents, stickies, and additives used during previous cycles of papermaking.

Hubbe, M. A., Rojas, O. J., Lucia, L. A., and Jung, T. M. (2007). “Consequences of the Nanoporosity of Cellulosic Fibers on their Streaming Potential and their Interactions with Cationic Polyelectrolytes,” Cellulose 14(6), 655-671.

Electrokinetic tests, based on the streaming potential method, were used to elucidate interactions between cationic polyelectrolytes and cellulosic fibers and to reveal aspects of fibers’ nanoporosity.  The fibrillated and nanoporous nature of bleached kraft fibers gave rise to time-dependent changes in streaming potential, following treatment of the wetted fibers with poly-diallyldimethylammonium chloride.  Electrokinetic test results were consistent with an expected longer time required for higher-mass polyelectrolytes to diffuse into pore spaces, compared to lower-mass polyelectrolytes.  Further evidence of the relative inability of polyelectrolyte molecules to diffuse in to the pores of cellulose was obtained by switching back and forth between high and low ionic strength conditions during repeated measurement of streaming potential, after the fibers had been treated with a moderate amount of cationic polymer.  By changing the concentration of sodium sulfate it was possible to switch the sign of streaming potential repeatedly from positive to negative and back again.  Such results imply that a continuous path for liquid flow exists either in a fibrillar layer or within the cell walls.  The same concepts also helped to explain the dosages of high-charge cationic polymer needed to achieve maximum dewatering rates, as well as the results of retention experiments using positively and negatively charged microcrystalline cellulose particles.

Hubbe, M. A. and Panczyk, M. (2007). “Dewatering of refined, bleached hardwood kraft pulp by gravity, vacuum, and centrifugation with applied pressure.  Part 1.  Physical and ionic effects,” O Papel 68(10), 74-87.

Three types of dewatering tests were performed with refined, bleached hardwood kraft suspensions.  A modified water retention value (MWRV) test was used, with pressure applied to damp plugs of fibers during their centrifugation.  Refining levels and drying conditions had dramatic and consistent effects on dewatering rates by gravity (freeness tests), with the application of vacuum, and also the MWRV tests.  Effects of drying were roughly equivalent to a reversal of refining effects.  Though changes in the pH and salt concentration of the aqueous solution had large effects on dewatering by gravity or vacuum, these chemical conditions did not affect MWRV results to a significant extent.

Hubbe, M. A. and Panczyk, M. (2007). “Dewatering of refined, bleached hardwood kraft pulp by gravity, vacuum, and centrifugation with applied pressure.  Part 2.  Effects of wet-end additives,” O Papel 68(10), 88-100.

Several wet-end chemical additives significantly affected dewatering rates according to gravity drainage tests (freeness), the application of vacuum, and a modified water retention value (MWRV) test.  Relatively large increases in dewatering rates, including reductions in MWRV, were obtained by addition of high-charge synthetic cationic polymers to the pulp suspensions.  Results were consistent with charge neutralization and polyelectrolyte complexation within the fibrillated layers of the fiber surfaces.  These mechanisms appear to have caused less water to be held in spaces between the fibers after they had been centrifuged in the presence of applied pressure.  Successive treatment with a cationic acrylamide copolymer (cPAM), followed by colloidal silica, a so-called microparticle program, resulted in very pronounced acceleration of gravity- and vacuum-assisted dewatering.  Though the microparticle system yielded reduced MWRV results under certain conditions, combinations of cPAM and colloidal silica at high levels increased the amount of water retained in compressed fiber pads after centrifugation.

Hubbe, M. A., and Heitmann, J. A. (2007). "Review of factors affecting the release of water from cellulosic fibers during paper manufacture," BioRes. 2(3), 500-533.

The ease with which water becomes released from cellulosic fiber material during the manufacturing of paper can affect both the production rate and the consumption of energy during the manufacturing process.  Important theoretical contributions to dewatering phenomena have been based on flow through packed beds of uniformly distributed fibers.  Such descriptions are able to explain why resistance to dewatering increases as a function of the wet surface area of fibers.  More recent studies have demonstrated a critical role of fine matter.  If the fines are unattached to fibers, then they tend to move freely through the fiber mat and plug channels in the paper web during the dewatering process.  Dewatering also is affected by the deformability of cellulosic fibers, and by whether the fibers easily slide past each other, thereby forming a dense mat.  By emphasizing the role of fine matter, colloidal forces, and conformability of cellulosic materials, one can gain a more realistic understanding of strategies that papermakers use to enhance initial drainage and vacuum-induced dewatering.

Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, Y., Song, J., Sulic, N., and Sezaki, T., "Charge and the dry-strength performance of polyampholytes. Part 2. Colloidal effects," Colloids and Surfaces. A, Physicochemical and Engineering Aspects 301, 23-32 (2007).

Polyampholytes, which are macromolecules that contain both positive and negative ionizable groups, can provide superior strength improvements for paper manufacture, compared to the addition of simple polyelectrolytes.  Colloidal effects, which were measured in solution and in fiber suspensions, were consistent with observed bonding effects.  The same colloidal effects were found to correlate with the effects of pH and of the density of the ionizable groups on the polyampholytes.  Tests were carried out with a series of polyampholytes having a constant ratio of cationic to anionic monomeric groups and molecular mass.  Their charge density varied in the ratio 1:2:4:8.  The greatest strength gains were obtained at intermediate charge density and under conditions of pH favoring instability of the aqueous polymer mixtures.  Colloidal phenomena were elucidated by turbidimetric tests, sediment volumes of treated fiber suspensions, flocculation tendencies of treated fiber suspensions, and zeta potentials of probe particles.

Wang, Y., Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, X., and Sezaki, T. "Charge and the dry-strength performance of polyampholytes. Part 3. Streaming potential analysis," Colloids and Surfaces. A, Physicochemical and Engineering Aspects 301, 33-40 (2007).

Results reported in Part 1 of this series showed that paper strength improvements could be optimized by varying pH and the overall content of ionic groups in random terpolymers containing a fixed molar ratio of acidic and basic monomeric groups.  Further treatment of kraft fiber slurries with polyaluminum chloride (PAC), after polyampholyte addition, yielded significant strength benefits.  The present paper shows how these results can be explained in terms of the streaming potential of glass fibers, which were used as a model substrate.  The data suggest that aluminum ions interact both with the anionic carboxyl groups of the polyampholytes and with anionic silanol groups at fiber surfaces.  The streaming potential of the treated surfaces could be changed by varying the pH, the overall density of charged groups of the polyampholytes, the ratio of cationic to anionic groups on the polymer, and by post-treatment with poly-aluminum chloride.

Hubbe, M. A., Rojas, O. J., Lee, S. Y., Park, S., and Wang, Y. "Distinctive electrokinetic behavior of nanoporous silica particles treated with cationic polyelectrolyte," Colloids and surfaces. A, Physicochemical and Engineering Aspects 292(2-3), 271-278 (2007).

In this study we show, for the first time, that the streaming potential of aqueous suspensions of nanoporous silica gel, after treatment with the cationic polyelectrolyte poly-diallyldimethylammonium chloride (poly-DADMAC), can depend very strongly on the concentration of background electrolyte.  An increase in the electrical conductivity from 60 to 1000 µS/cm resulted in an approximately 1000-fold increase in the amount of poly-DADMAC that was required to reach an endpoint of zero streaming potential.  Results were explained by two contributions to the overall electrokinetic behavior – one due to the outer surfaces and another due to the interior surfaces of nanopore spaces that were inaccessible to the polyelectrolytes.  Experiments with cyclical changes in salt content revealed a high degree of reversibility; such observations help to rule out explanations based on salt-induced desorption or enhancement of pore penetration.  Supplementary tests with non-porous glass fibers showed no evidence of the distinctive electrokinetic behavior observed in the case of nanoporous particles.  Effects of polymer molecular mass and pH, evaluated under similar experimental conditions, agreed with well-established trends. 

Hubbe, M. A. "Paper's resistance to wetting: A review of internal sizing chemicals and their effects," BioResources 2(1), 106-145 (2007).

This review considers research related to internal sizing agents.  Such chemicals, when added as emulsions or in micellar form to slurries of cellulosic fibers before paper is made, can make the product resist water and other fluids.  Significant progress has been achieved to elucidate the modes of action of alkylketene dimer (AKD), alkenylsuccinic anhydride (ASA), rosin products, and other sizing chemicals.  Recent findings generally support a traditional view that efficient hydrophobation requires that the sizing chemicals are efficiently retained on fiber surfaces during the papermaking process, that they become well distributed on a molecular scale, and that they need to be chemically anchored.  A variety of studies have quantified ways in which internal sizing treatments tend to be inefficient, compared to what is theoretically possible.  The inefficient nature of chemical and physical processes associated with internal sizing, as well as competing reactions and some interfering or contributing factors, help to explain apparent inconsis-tencies between the results of some recent studies.

Hubbe, M. A., Rojas, O. J., Sulic, N., and Sezaki, T. "Unique behavior of polyampholytes as dry-strength agents," Appita Journal 60(2), 106-111 (2007).

Polyampholytes yielded superior dry strength increases following their addition to slurries of papermaking fibres.  The bi-ionic polymers achieved greater tensile strength, compared to similar polymers having ionic groups of only positive or negative charge.  Dry-strength efficiency increased with increasing charge density of the polyampholyte.  Strength results were consistent with turbidity data, showing that the polyampholytes generally became less soluble at intermediate values of pH.  In contrast to simple polyelectrolytes, the adsorbed amphoteric macromolecules imbibed significant amounts of water of hydration.  Though high levels of polyampholytes added to the furnish tended to reduce the rate of gravity dewatering, such effects tended to be lower than the drainage inhibition caused by single-charge polyelectrolytes.  The effects of polyampholytes were achieved without the adverse effects often associated with refining, e.g. decreased dewatering rates, fibre shortening, or changes in the conformability of the fibres.

Wang, Y., Hubbe, M. A., Sezaki, T., Wang, X. W., Rojas, O. J., & Argyropoulos, D. S. "Aspects of retention and formation - The role of polyampholyte charge density on its interactions with cellulose," Nordic Pulp & Paper Research Journal 21(5), 638-645 (2006).

Polyampholytes offer considerable promise as dry-strength additives, but the molecular mechanism involved in their adsorption needs to be better understood.  Amphoteric terpolymers of acrylamide, itaconic acid, and N-[3-(NN’,N’-dimethylamine)propyl]acrylamide (DMAPAA) with a constant ratio of basic to acidic groups (5:4) were prepared by random polymerization.  The basic groups ranged from 2.5 to 20 mole percent in the terpolymers.  Analysis by 1H and 13C nuclear magnetic resonance revealed near-quantitative agreement with the make up stoichiometry.
Streaming potential tests showed significant effects of polyampholyte adsorption, depending on the charge density of the polyampholyte, its level of addition, the pH, and the background electrolyte.  Polyampholytes having higher density of ionizable groups yielded more positive streaming potential at low pH values and more negative streaming potentials at high pH values, compared to polyampholytes of lower charge density.  At the extremes of pH, e.g. pH=3 and pH=11, the effects of a polyampholyte on streaming potential were similar to those of single-charged polyelectrolytes having a matched degree of substitution of charged monomeric groups.  Except for the sample having the lowest density of charges, all of the polyampholyte samples showed a broad maximum in adsorbed amount vs. pH within the range of about pH=5 to pH=9, which is intermediate between the pKa values of the respective charged groups.

Hubbe, M. A. "Bonding between cellulosic fibers in the absence and presence of dry-strength agents - A review," BioResources 1(2), 281-318 (2006).

Various water-loving polyelectrolytes, including cationic starch products, are used by papermakers to promote inter-fiber bonding and increase paper’s dry-strength.  Thus, papermakers can meet customer require-ments with a lower net cost of materials, more recycled fibers, or higher mineral content.  In the absence of polymeric additives, key mechanisms governing bond development between cellulosic fibers include capillary action, three-dimensional mixing of macromolecules on facing surfaces, conformability of the materials, and hydrogen bonding.  Dry-strength additives need to adsorb efficiently onto fibers, have a water-loving character, and have a sufficiently high molecular mass.  Though it is possible to achieve significant strength gains by optimal usage of individual polymeric agents, greater strength gains can be achieved by sequential addition of oppositely charged polyelectrolytes.  Superior strength can be achieved by in-situ formation of polyelectrolyte complexes, followed by deposition of those complexes onto fiber surfaces.  Polyampholytes also hold promise as efficient dry-strength additives.  Opportunities for further increases in performance of dry-strength agents may involve fiber surface modification, self-assembled layers, and optimization of the dry film characteristics of dry-strength polymers or systems of polymers.

Song, J., Wang, Y., Hubbe, M. A., Rojas, O. J., Sulic, N., & Sezaki, T. "Charge and the dry-strength performance of polyampholytes. Part 1, Handsheet properties and polymer solution viscosity," Journal of Pulp and Paper Science, 32(3), 156-162 (2006).

Paper strength was increased by adding random terpolymers having a fixed ratio of basic to acidic monomeric groups to bleached kraft fiber slurries over a wide range of pH.  Subsequent treatment of the fiber slurries with polyaluminum chloride (PAC) further increased tensile breaking length.  By contrast, PAC tended to reduce the dry strength contribution of a cationic polyelectrolyte having the same mass and cationic monomer content as one of the polyampholytes .  The reason for adding the PAC last to the mixture was to evaluate possible effects of fresh, highly cationic aluminum species with the polymers as well as the fiber surfaces.  It is proposed that such effects are due to the influence of pH and ionic aluminum species on the molecular conformations, as well as the electrokinetic behavior of solids exposed to the polyampholytes.  Results of solution viscosity tests indicated more expanded polyampholyte conformations resulting from PAC addition, especially in those cases where dry strength advantages of PAC addition were observed. 

Heermann, M. L., Welter, S. R., and Hubbe, M. A., "Effects of High Treatment Levels in a Dry-Strength Additive Program Based on Deposition of Polyelectrolyte Complexes: How Much Glue is Too Much?" Tappi J. 5 (6): 9-14 (2006).

Large increases in paper's dry strength were achieved in this study by depositing high levels of polyelectrolyte complexes onto fibers. A previous study, utilizing glass microfibers, showed an almost linear increase in strength with increasing polymer amounts up to 10%, on a dry basis. Such results prompted questions related to the practical upper limits of addition of dry-strength chemicals. Substantial gains in tensile breaking length were achieved in the present work by sequential addition of balanced amounts of positively and negatively charged polymers to bleached kraft fibers. Strength increased strongly with increasing dosages up to 40% net polymer addition. These results contrasted with past work showing that further addition of single-polymer dry-strength additives often becomes ineffective above about 0.2 to 2% addition, in different cases. Efficient retention of the polymers onto the fibers, even up to a level of equal amounts of polymer and fiber, was evident from the basis weight of handsheets prepared from a constant amount of fiber. Practical upper limits of polymer addition for the dual-polymer system were evident from an increasing stickiness of undried paper handsheets, with increasing polymer dosage. Other problems associated with the highest polymer dosages considered included polymer deposition onto the forming fabric, reduced dewatering rates, and reduced opacity of paper produced with 40% or more dry mass of polyelectrolyte added, based on fiber mass. Important factors affecting the results included the ratio of the two kinds of polymers and the manner in which the polyelectrolyte solutions were added.

Sezaki, T., Hubbe, M. A., Heitmann, J. A., and Argyropoulos, D. S. (2006). "Colloidal effects of acrylamide polyampholytes. Part 2, Adsorption onto cellulosic fibers," Colloids and surfaces. A, Physicochemical and engineering aspects, 289(1-3), 89-95.

Colloidal titrations of commercial acrylamide-based terpolymers having both weak-acidic and weak-basic groups were carried out at pH 3 and 11, using a streaming current technique.  At these pH values it was found that the polyampholytes could be considered as simple polyelectrolytes, though it was necessary to use a modified titration procedure.  The titration endpoint defined by zero streaming current (SC) output deviated from a 1:1 stoichiometry, depending on the salt concentration.  The endpoint also depended at the speed of titration, consistent with a relatively slow rate of forming equilibrated poly-ion complexes between polyampholytes and titrants.  The adsorption of the amphoteric polyacrylamide copolymers onto bleached hardwood fibers was maximized near to its isoelectric pH, such that the net charge of the polymer was relatively low, but opposite to that of the substrate.  Neutral and negatively charged polyampholytes also adsorbed in significant amounts onto the negatively charged fibers, though the amounts were lower than when the net charges were opposite.  Addition of salt generally increased adsorption up to a conductivity value of 1000 mS/cm.  Adsorption increased slightly with increasing time, until reaching a plateau.  The rate of fluid agitation during adsorption did not affect adsorption significantly under the conditions employed. 

Sezaki, T., Hubbe, M. A., Hietmann, J. A., Argyropoulos, D. S., and Wang, X., "Colloidal Effects of Acrylamide Polyampholytes. Part 1. Electrokinetic Behavior," Colloids Surf. A, 281 (1-3), 74-81 (2006).

The colloidal and electrokinetic behavior of three amphoteric acrylamide-based water-soluble terpolymers of high molecular mass was elucidated in terms of their structure and composition, using potentiometric and colloidal titrations, as well as microelectrophoresis, viscometry, and turbidity measurements. Independent variables included polymer composition, pH, and the concentration of salt ions. The electrokinetic properties, titratable charge, and isoelectric pH values of the samples were compared to their monomeric composition, as confirmed by NMR and FTIR analysis. The electrophoretic mobilities of the polyampholytes changed relatively rapidly with pH in the neighborhood of the isoelectric pH values, consistent with an enrichment of excess charges toward the outer parts of the macromolecules. Interactions of the polyampholytes with highly-charged titrants appeared to be less pH-dependent, in the neighborhood of the isoelectric condition, relative to a linear prediction based on the numbers of acidic and basic macromolecular groups. Specific viscosity measurements, in the vicinity of the isoelectric point, were found to increase with increasing salt concentration, which is a typical anti-polyelectrolyte behavior. In a similar manner, salt addition suppressed the development of a turbidity maximum at the isoelectric point.

Hubbe, M. A., Rojas, O. J., and Venditti, R. A., "Control of Tacky Deposits on Paper Machines - A Review," Nordic Pulp Paper Res. J. 21 (2), 154-171 (2006).

Wood-derived pitch and tacky materials of synthetic origin in recovered fiber streams often cause serious deposit problems on papermaking equipment. Ideally such materials would be completely removed in processes such as screening, cleaning, washing, or flotation de-inking. In practice, tacky materials that remain in the fiber furnish can build up within paper machine headboxes, forming fabrics, press sections, and dryer sections, reducing production efficiency. Product quality is likely to suffer, especially if deposited material ends up in the sheet. This review considers a variety of chemical additives that papermakers have used to combat deposit problems. The premise of this article is that knowledge of the chemistry and colloidal behavior of existing deposit-control agents can guide us in the selection, usage practices, and further development of strategies for the control of tacky deposits, especially in the case of pitch, adhesive-based stickies, and wax-like deposits.

Hubbe, M. A., Tripattharanan, T., Heitmann, J. A., and Venditti, R. A. "The Positive Pulse Jar (PPJ): A flexible device for retention studies," Paperi ja Puu = Paper and Timber, 88(1), 39-45 (2006).

A bench-scale apparatus, developed for studies of fine-particle retention, compares effects of contrasting flow conditions and retention chemical strategies, using simulated headbox stock.  The apparatus includes an agitated jar, a forming screen, and a continuous phase of aqueous solution below the screen.  A gear pump provides a constant time-averaged rate of dewatering, and a bellows pump produces a sinusoidal component of flow normal to the screen.  Pulsations of known velocity and displacement distance are used to represent effects due to hydrofoils.  As another alternative, the suspension may be subjected to a uniform time-averaged shear stress during dewatering.  Conditions of flow during dewatering affected not only the efficiency of fine particle retention, but also the distribution of fiber fines and fillers in the thickness direction of the fiber mat.  Retention aid use reduced filtrate turbidity, indicating increased retention efficiency.  Retention also was affected by hydrodynamic shear before dewatering; effects of such shear application were apparent even after subsequent exposure to flow pulsations during dewatering.  The design of the apparatus provides flexibility in evaluating mechanistic questions under quantified conditions of pulsating flow and/or steady shear application to the suspension before or during dewatering.

Lee, S. Y., Hubbe, M. A., & Saka, H. "Prospects for diodiesel as a byproduct of wood pulping - A review," BioResources, 1(1), 150-171 (2006).

Effective utilization of byproducts can affect the profitability of kraft pulping to produce cellulosic fibers from wood.  This review considers opportunities to use tall oil components, obtained from kraft pulping, as a source of raw material for biodiesel fuel, or as a source of additives for petrodiesel.  Considerable progress has been achieved with respect to converting vegetable oils to diesel fuel, and some of what has been learned appears to have potential application for processing of wood-derived fatty acids and related compounds.  Alkaline-catalyzed transesterification strategies, while seemingly well adapted for relatively pure vegetable oil source materials, may not be the best fit for the processing of tall oil fractions.  The promising strategies to consider include acid-catalyzed esterification, enzymatic processes, hydrogenation, and the use of supercritical methanol.

Hubbe, M. A. "Sensing the electrokinetic potential of cellulosic fiber surfaces,"BioResources, 1(1), 93-125 (2006).

The charged nature of a cellulosic fiber surface is expected to play major roles in such phenomena as fiber dispersion, flocculation, adhesion, and adsorption of polyelectrolytes.  This review focuses on the evaluation of such charges by means of electrokinetic measurements, with emphasis on the fiber-pad streaming potential technique.  Results of recent experiments suggest that a continuous network or networks of pores below the outer surface of a kraft fiber can significantly contribute to observed streaming potential data.  At present it is not clear whether the main subsurface contributions to the observed electrokinetic effects come from fibrillar layers on the fiber surfaces or from systems of nanopores within the cell walls of fibers.  Based on the literature it is possible to suggest two conceptual models to account for the fact that the streaming potential of polymer-treated fibers can change in sign, dependent on the concentration of salt.  Additional research is needed to clarify various theoretical and practical points.  There may be opportunities to make more effective use of streaming potential tests in the future by carrying out such tests at reduced salt levels.

Wang, Y., Hubbe, M. A., Sezaki, T., Wang, X., Rojas, O. J., and Argyropoulos, D. S. "The role of polyampholyte charge density on its interactions with cellulose," Nordic Pulp & Paper Research Journal, 21(5), 158-165 (2006).

Polyampholytes offer considerable promise as dry-strength additives, but the molecular mechanism involved in their adsorption needs to be better understood.  Amphoteric terpolymers of acrylamide, itaconic acid, and N-[3-(NN’,N’-dimethylamine)propyl]acrylamide (DMAPAA) with a constant ratio of basic to acidic groups (5:4) were prepared by random polymerization.  The basic groups ranged from 2.5 to 20 mole percent in the terpolymers.  Analysis by 1H and 13C nuclear magnetic resonance revealed near-quantitative agreement with the make up stoichiometry.
Streaming potential tests showed significant effects of polyampholyte adsorption, depending on the charge density of the polyampholyte, its level of addition, the pH, and the background electrolyte.  Polyampholytes having higher density of ionizable groups yielded more positive streaming potential at low pH values and more negative streaming potentials at high pH values, compared to polyampholytes of lower charge density.  At the extremes of pH, e.g. pH=3 and pH=11, the effects of a polyampholyte on streaming potential were similar to those of single-charged polyelectrolytes having a matched degree of substitution of charged monomeric groups.  Except for the sample having the lowest density of charges, all of the polyampholyte samples showed a broad maximum in adsorbed amount vs. pH within the range of about pH=5 to pH=9, which is intermediate between the pKa values of the respective charged groups.

Hubbe, M. A., Emerging Technologies in Flocculation, Pira International, Leatherhead, UK, 2005.

The tendency of fibers used in paper manufacture to become entangled, often leading to a flocculated structure and appearance of paper products, has a huge but often overlooked economic impact. Recent work suggests that if it were practical to form perfectly uniform paper, the tensile strength might be as much as twice that of conventionally formed paper. That implies that there is untapped potential to achieve equivalent product performance with less material. Even if advances in technology can reach only part way to the ideal goal of "perfectly formed paper," the potential savings are of the order of magnitude of tens of billions of Euros per year. In addition to materials savings, paper's uniformity is known to have a major impact on such processes as coating, printing, and the high-speed handling of paper in automated equipment. What can today's paper technologists do to minimize fiber flocculation and to reap benefits of more uniform paper and paperboard? This report focuses on a variety of concepts for managing flocculation. Some of these concepts are reasonably well established, and others have not yet been convincingly demonstrated. Mechanical and hydrodynamic principles underlie many of the most promising strategies. Others strategies focus on the selection and use of various wet-end chemical additives. Certain chemicals can be used in combination with hydrodynamic shear to produce more uniform paper, often at a higher speed of the paper machine. Today's paper technologists also have something else working in their favor; various methods have become available for both online and laboratory evaluation of flocculation.The following topic areas are covered in this report:
- The definition and importance of flocculation
- Measuring the consequences of flocculation
- Advances in mechanical and hydrodynamic aspects of flocculation
- Advances in chemical aspects of flocculation
- Advances in flocculation measurements
- Emerging strategies for management of flocculation

Hubbe, M. A., "Dry-Strength Development by Polyelectrolyte Complex Deposition onto Non-Bonding Glass Fibers," J. Pulp Paper Sci. 31 (4): 159-166 (2005).

Sheets formed from glass microfibers had almost zero tensile strength in the absence of polymer treatment, but paper-like dry-strength was achieved if fiber suspensions were treated with combinations of a cationic and an anionic polyelectrolyte before sheet formation. Strength increases generally were maximized when the ratio between the cationic poly-(diallyldimethylammonium chloride) and carboxymethylcellulose was within about 2:3 and 3:2 stoichiometry of charged groups on the macromolecules. Sequential addition of the polyelectrolytes to fiber slurry generally yielded higher strength than pre-mixing the polyelectrolytes before their addition. Significant effects were observed, depending on the ratio of the two polyelectrolytes, optional cationic pretreatment of the fibers, salt concentrations, and the elapsed time between mixing of polyelectrolytes and their addition to a fiber slurry. Time-dependent electrostatic interactions appear to control the deposition of polyelectrolyte complexes, and it was found that the details of chemical addition strategies can have major effects on the results.

Hubbe, M. A., "Mechanistic Aspects of Microparticle Systems," Tappi J. 4 (11): 23-28 (2005).

Uses of specific microparticle programs and applications will be considered in the following chapters, but there seem to be some common features with respect to "how these programs work." This section will consider mechanistic aspects of microparticle programs in general, weighing evidence for and against various concepts. In principle, an understanding of the mechanisms may make it easier to optimize and control additives flows in a microparticle system. Also there may be clues in the mechanisms that can lead to future developments.

Bastidas, J. C., Pawlak, J. J., Venditti, R. A., Heitmann, J. A., Hubbe, M. A., and Dadla, J. F., "Gypsum-Cellulose Interactions - A Colloidal Probe Microscopy Study," Proc. 13th Internat. Symp. Wood Fiber Pulping Chem., 59th Appita Annual Conf., Vol. 2, New Zealand Forest Council, 2005.

Drywall, which is made primarily of a calcium sulfate dihydrate (gypsum) core with paper on both sides, is one of the most widely used construction materials. Because board failure often occurs at the gypsum core/paper interface, it has become important to know the exact nature of the gypsum/cellulose bond and how crystal morphology affects it. This study provides data about the nature of this interaction by means of AFM and Colloidal Probe Microscopy. These methods made it possible to distinguish among the different crystal faces and their respective interactions with cellulose. Measured in air, the adhesive forces between the AFM tip and the different faces varied according to f(010) < f(120) < f(111)  at 50 % relative humidity. The differences in adhesive force with the different gypsum crystals face can be attributed to the differences in surface chemistry. The information obtained in this study will help guide improvements in the gypsum wallboard production process to obtain better bonding between the crystal and the paper.

Lofton, M. C., Moore, S. M., Hubbe, M. A., and Lee, S. Y., "Polyelectrolyte Complex Deposition as a Mechanism of Paper Dry-Strength Development," Tappi J. 4 (9): 3-7 (2005).

Dual-polymer treatments involving a high-charge cationic polymer followed by anionic carboxymethylcellulose (CMC) increased the strength of handsheets formed from the fiber fraction of recycled xerographic copy paper. The amount of the first additive, poly-diallyldimethylammonium chloride (poly-DADMAC), was varied, whereas the amount of CMC was held constant. Results contrasted with earlier work, in which maximum strength was obtained when the amount of poly-DADMAC was just sufficient to saturate the adsorption capacity of unbleached kraft fibers. Rather, in the case of recycled copy paper, significantly higher tensile strength was obtained when the poly-DADMAC addition exceeded the saturation level by a factor of ten. Tests were performed to evaluate a hypothesis that the strength increase was due to polyelectrolyte complex (PEC) formation in the bulk phase, followed by deposition of PECs onto the fibers. Pre-formed complexes were retained efficiently by the fibers, especially if their surfaces had been pretreated with a saturation level of poly-DADMAC. Surprisingly, such pretreatment increased the retention efficiency of all of the PEC mixtures tested, regardless of which sign of charge was in excess. The results suggested that PEC deposition yielded an additional increase of about 13% in dry strength, beyond what could be achieved by treatments not involving complexation.

Hubbe, M. A., "Paper," in Kirk-Othmer Encyclopedia of Chemical Technology, Online Edition, Wiley, New York, 2005.

Paper can be defined as sheet material comprised of small, discrete fibers that are bonded together. The process of paper formation involves dewatering an aqueous suspension, followed by drying. In most cases the fibers are cellulosic, and hydrogen bonds are an important contribution to the strength of the product. The term paperboard is used in cases where the product is thicker, heavier, and less flexible than conventional paper. In addition to fibers, modern paper often contains substantial amounts of minerals, such as calcium carbonate or kaolin clay. Other components, usually at lesser quantities, can include starch, modified starch products, synthetic polyelectrolytes, and a variety of other additives. Though there is a tradition of hand-made papermaking, going back almost two millennia, most modern paper is manufactured on large, highly automated machines. The fiber slurry is formed and dewatered on a continuous fabric screen or sandwiched between a pair of such screens. Though most of the water used in this process is recycled multiple times, the paper industry remains a major user of fresh water. Substantial gains have been achieved to minimize the loss of fibers and other solid components from the process. Likewise, progress has been achieved in reducing the amount of energy expended in the drying of paper, primarily through increases in the proportion of water that is removed by pressing, rather than evaporation. After reviewing paper's distinguishing characteristics and history, the present chapter describes paper's chemical composition, physical properties, preparation of the fibers, the use of chemical additives, formation, pressing, and drying of the sheet, coating and converting of paper, issues related to the environment and manufacturing efficiency, and a summary of common grades of paper and paperboard

Hubbe, M. A., "Microparticle Programs for Drainage and Retention," in Microparticles and Nano- Microparticles for Papermaking, J. Rodriguez, Ed., Chapter 1, TAPPI Press, (2005).

McNeal, M. R., Nanko, H., and Hubbe, M. A., "Imaging of Macromolecular Events Occurring during the Manufacture of Paper," Proc. XIII Fundamental Research Symposium, Cambridge, 2005.

A novel transmission electron microscopy (TEM) technique, developed to observe the nano-scale interactions of polymeric additives and cellulosic fibrils under idealized laboratory conditions, was applied for the first time in a comprehensive study of the colloidal interactions within a mill producing light-weight coated publication paper. The technique allows the observation of incremental changes in the nano-scale appearance of the papermaking slurry as successive additives are introduced to the system. Such changes include the coagulation of colloidal and dissolved substances present in thermomechanical (TMP) pulp after the addition of a low molecular weight, high charge density polymer, and the subsequent flocculation of the coagulated matter, hydrophobic materials, and fines following the introduction of talc, aluminum sulfate, a high mass cationic polyelectrolyte, and silica nanoparticles. The new results demonstrate that the TEM technique can be applied even in systems as complex as commercial papermaking, leading to a more accurate understanding of what happens on a macromolecular level.

Rojas, O. J., Byrd, M. V., Hubbe, M. A., and Claesson, P. M., "On the Origins of Adhesion in Papermaking Systems," Proc. XIII Fundamental Research Symposium, Cambridge, 2005.

Polyelectrolytes are commonly used as additives to control colloidal stability and adhesive properties of surfaces. This investigation is related to the latter case which is relevant to several papermaking processes such as pretreatment of filler particles with cationic polymers (e.g., polyethyleneimine) to increase deposition on pulp fibers or the development of dry strength. The dry strength of paper is often increased by addition of cationic starch or acrylamides to the fiber furnish, which is subsequently dried. The cationic polymer adsorbs to the negatively charged fibers and mediates an increased fiber-fiber bond. It has been reported that the dry strength of the paper increases with decreasing charge density of the polymer, presumably due to increased polymer-polymer interpenetration and due to increased viscoelastic losses that occur during the rupture of the paper sheet under strain.

Hubbe, M. A., "Why Do Different Charge Demand Tests Give Different Endpoints?" Proc. Pira 2005 Sci. Tech. Advan. Wet End Chem., Lisbon, Portugal, Pira International, Leatherhead, Surrey, UK, 2005.

Have you ever encountered situations where two people, both measuring the same samples taken from a paper machine system, have reported sharply different results for cationic demand tests? Such situations actually are quite common. A lot of needless energy can be spent trying to figure out which of the two sets of data is "correct." Fortunately, there are some understandable reasons to expect variations in the details of charge titration tests to yield significantly different results. By keeping these issues in mind, it is possible to get beyond questions of the correctness of different procedures. The end goal should be to improve the reproducibility of the results of charge demand titrations and to interpret the results with greater confidence

Hubbe, M. A., and Zhang, M., "Recovered Kraft Fibers and Wet-End Dry-Strength Polymers," Proc. TAPPI Practical Papermakers Conf., TAPPI Press, Atlanta, 2005.

The drying and recycling of paper profoundly affects the bulk and surface properties of kraft fibers. Recovered kraft fibers tend to be less porous on a sub-microscopic scale, less flexible, and less able to swell with water, compared to never-dried, refined fibers. Recovered fibers also tend to be less able to form inter-fiber bonds. This review considers how changes associated with the drying and recycling of kraft fibers affect their interactions with dry-strength polymers such as cationic starch, copolymers of acrylamide, and high-charge cationic polymers. A key finding of recent research is that the listed changes in fiber properties occur independently of whether or not dry-strength polymers are present. There are two main ways that strength-enhancing additives can compensate for losses in bonding ability. First, dry-strength polymers present in the original paper can contribute to bonding when the same material is recycled. Second, additional wet-end polymers applied during production of the recycled paper can help compensate for deficiencies of bonding ability. Recently a dual-polymer dry-strength program was adjusted to match the ability of recovered kraft fibers to retain cationic polymers. This approach makes it possible to tailor a treatment system to the type of furnish that is being used.

Hubbe, M. A., and Rojas, O. J., "The Paradox of Papermaking," Chem. Eng. Education 39 (2 ): 146-155 (2005).

Students and educators in chemical engineering, are you aware of the paper industry and its impact in our society? With retirements and with changing technology there is a continual need for new technical and scientific skills to face the challenging goals of our times. The purpose of this article is to introduce some intriguing aspects of papermaking technology. The paradoxical nature of the papermaking process is sure to capture your interest and imagination.

Welf, E. S., Venditti, R. A., Hubbe, M. A., and Pawlak, J., "The Effects of Heating Without Water Removal and Drying on the Swelling as Measured by Water Retention Value and Degradation as Measured by Intrinsic Viscosity of Cellulose Papermaking Fibers," Prog. Paper Recycling 14 (3): 1-9 (2005).

The effects of heating without water removal and drying of bleached kraft fibers were separately investigated. Water swellability as measured using a water retention value method (WRV) and cellulose degree of polymerization as measured using a viscosity method were used to gauge the effects of such treatments on fibers. The drying of fibers at temperatures above 100°C resulted in significant decreases in WRV, as expected. However, heating fibers without water removal at the same temperatures resulted in a decrease in WRV much less than caused by drying. Drying at high temperatures reduced the cellulose viscosity only slightly, whereas heat treatment without water removal at high temperatures resulted in much greater losses in cellulose viscosity. The results of this study indicate that the time-temperature-humidity history of a fiber during papermaking and paper recycling can produce fibers with very different papermaking qualities.

Hubbe, M. A., "Acidic and Alkaline Sizings for Printing, Writing, and Drawing Papers," Book and Paper Annual 23: 139-151 (2005); also in Proc. Contemp. Machine-made Paper, Amer. Inst. Conservation Historic Artistic Works, Oct. 20-23, 2004, Dalton, MA

This review of paper sizing systems describes a recent, quiet revolution with respect to the chemicals used during the manufacture of paper. Before this revolution the primary means of imparting water-resistance to mass-produced paper involved rosin and alum, the latter of which is highly acidic. During the 1980s, and continuing up to today, there has been a dramatic shift to new sizing chemicals that employ an alkaline buffering system. As a side benefit of this change, most printing, writing, and drawing papers now made in the US tend to be brighter and more resistant to embrittlement during storage. Perhaps surprisingly, however, the roots of the revolution may have had little to do with paper's permanence.

Hubbe, M. A., Moore, S. M., and Lee, S. Y., "Effects of Charge Ratios and Cationic Polymer Nature on Polyelectrolyte Complex Deposition onto Cellulose," Indus. Eng. Chem. Res. 44 (9): 3068-3074 (2005).

Sequential addition of poly-diallyldimethylammonium chloride, which is highly cationic, followed by anionic carboxymethylcellulose, has been found to promote inter-fiber bonding during the manufacture of paper, with potential benefits to the recycling of fibers. The present results help to confirm a hypothesis that observed strength gains, in cases where the amount of the first additive exceeded the adsorption capacity of the fibers, were due to formation of polyelectrolyte complexes in the solution phase, followed by their deposition onto fiber surfaces. Complex formation and retention of complexes on fiber surfaces occurred efficiently over a wide range of polymer charge ratios, cationic polymer attributes, and other conditions, regardless of whether or not the fibers had been pre-treated to reverse their net charge.

Hubbe, M. A., Emerging Technologies in Wet End Chemistry, Pira International, Leatherhead, UK, 2005.

Do today's papermakers have what it takes to remain competitive in the years ahead? The answer may depend on how well they make use of emerging technologies, applying recent advances in various branches of science. At the wet end of a paper machine, papermakers already use various chemical additives to enhance product end-use performance and to enhance the efficiency of the manufacturing process. This report looks at emerging strategies that can allow future papermakers to leverage their investments in papermaking equipment and to face new challenges. Such challenges include new end-use applications for paper, a need to reduce costs while maintaining quality, efforts to increase production rates, increased paper recycling, reduced fresh-water use, and increasing competition. Tomorrow's papermakers will face a daunting task of competing not only with each other, often across international boundaries, but also with plastic, electronic media, and a variety of other technology platforms that presently we can only imagine. While this report cannot possibly anticipate all the challenges that papermakers will face in the future, emphasis is placed on the following critical areas of emerging technology:
- Nanotechnology in the wet end
- New wet-end starch technologies
- Process control developments for wet end additives
- Changes in the chemical environment of the wet end
- Efforts to enhance the performance of wet end additives
In each of these key areas it is possible to discern some promising strategies that can be used by papermaking technologists in the coming years. The present report aims to help with decisions to apply new technologies judiciously and effectively to meet both longer-term and shorter-term goals.

Hubbe, M. A., and Wang, F., "Charge-Related Measurements - A Reappraisal. Part 2: Fiber-Pad Streaming Potential," Paper Technol. 45 (9): 27-34 (2004).

Continuing a theme introduced in Part 1, the present article addresses concerns raised in 1995 about the use and interpretation of charge-related measurements. The "fiber-pad streaming potential" method is becoming increasingly popular at paper mills and in the research labs of companies involved with papermaking technology. One drawback of the method, as it is commonly practiced, involves a lack of calibration to more fundamental quantities, such as zeta potential. Nevertheless, progress has been achieved with respect to the precision of the method, especially at increased levels of salinity. Also, one can avoid many theoretical difficulties by using the method to determine endpoints of charge titrations. With these considerations in mind, fiber-pad streaming potential tests can reveal important information about the charge-related properties of fiber suspensions.

Hubbe, M. A., and Chen, J., "Charge-Related Measurements - A Reappraisal. Part 1: Streaming Current," Paper Technol. 45 (8): 17-23 (2004).

A 1995 article in this magazine raised concerns about the use and interpretation of two kinds of measurements that are being carried out in paper mills to evaluate the electrical charges at surfaces in fiber slurries. This article relates to the streaming current method, which is widely used for endpoint detection when testing the charge demand of whitewater or filtrate samples from fiber stock. Although there is still some truth in the statement that the "streaming current detector has no established theoretical basis," subsequent work has helped to define ranges of experimental conditions within which the test gives reliable results. Also, some specific sources of interference have become better understood.

Hubbe, M. A., Chen, J., and Heitmann, J. A., "Measurement and Impact of Charge: A Practical Guide," Solutions! 87 (11): 47-49 (2004).

The electrical charges on the surfaces of fibers and other materials in a papermaking furnish have profound, but subtle effects on both the process and the product. Because "charges" are invisible, they are sometimes overlooked as a source of operational problems and variability. However, the balance of surface charges within a paper machine system can directly affect the performance of retention aid chemicals. Low or variable retention of fine materials during paper formation can lead to other problems. Charge also can impact such things as dewatering rates, sizing efficiency, and deposit control.

About three years ago at NC State University we began a detailed study of one of the most widely used methods for charge determination - the streaming current titration method. One of us (Chen) earned a PhD degree in the process. Details of our research results have appeared or will appear elsewhere (see, for instance Colloids and Surfaces 223: 215, 2003). During this work, and also while putting together and checking the literature review section of the thesis, we have had occasion to think about practical implications of charge measurements. Though the opinions expressed below are our own, we need to acknowledge the substantial research progress by others that helped lead us to the following general conclusions.

Chen, J., Heitmann, J. A., Chang, H.-M., Hubbe, M. A., and Venditti, R. A., "The Effect of Paper Additives on Toner Agglomeration during the Recycling Process," Prog. Paper Recyling 13 (14): 16-23 (2004).

One of the principal contaminants in recovered office paper is toner inks. These toner inks can be removed by agglomeration, followed by conventional screening and cleaning techniques. However, it has been found that, for some types of toner, various paper chemicals can interfere with the agglomeration process. In this research two types of toner were investigated. One toner agglomerated well under most conditions, while the other performed less well and was more susceptible to adverse effects by a variety of paper chemical additives. The surface properties of the two toners were examined, and a model system was used to determine how various copy paper additives influenced agglomeration behavior. It is hypothesized that cationic polymers adsorb onto negatively charged toners and reduce their hydrophobicity, which interferes with the agglomeration process. Market pulp, cured toners, and 1-octadecanol were used as a basic model system. The effect of additives such as calcium carbonate, cationic and anionic starches, polyamine, poly-DADMAC, and polyacrylamide were investigated. It was found that the toner that agglomerated well was uncharged, while the more difficult to agglomerate toner was negatively charged. For the anionic toner, cationic hydrophylic polymers had very adverse effects on agglomeration, and it is believed that this is because these polymers are adsorbed onto the toner. It was found that the cationic polymers can be adsorbed onto the agglomerating agent, causing dispersion and failure of the agglomeration process. Several chemicals which would reduce this type of problem were investigated. They helped to confirm the mechanism and also offer possibilities of treatment of the problem in commercial systems.

Hubbe, M. A., and Gill, R. A., "Filler Particle Shape vs. Paper Properties - A Review," Proc. Spring Tech. Conf., TAPPI Press, Atlanta, 2004.

Contrasting shapes and sizes of mineral filler particles provide today's papermaker with many options to affect paper properties. One can choose between plate-like clay products, irregular-shaped products of grinding, and a diverse assortment of filler shapes that can be achieved by mineral precipitation methods. This review considers the connection between filler morphology and such attributes as apparent density, opacity, strength, and demand for sizing chemicals. Although no one type of filler product will suit every application, published information can help the papermaker deal with a series of compromises. Plate-like particles can be effective for paper products having a high apparent density. More rounded, solid-form particles tend to minimize the demand for sizing chemicals and generally allow more rapid dewatering. Particles with internal voids general offer high light scattering ability, contributing to opacity. Though there is often an inverse relationship between light scattering and strength, it is possible to design fillers that achieve a more favorable balance between these two attributes.

Rojas, O. J., and Hubbe, M. A., "The Dispersion Science of Papermaking," J. Dispersion Sci. Technol., J. Dispersion Sci. Technol. 25 (6): 713-732 (2004).

Paper is formed from a slurry of fibers and much smaller particles that are often called "fines." Ahead of the paper forming process the slurry is subjected to a series of steps, including treatment with polyionic species and passage through unit operations that impose shear forces on the fluid mixture. These steps alternately disperse the solids apart or re-gather them back together. The overall process is optimized to achieve a highly uniform product, while at the same time achieving high efficiency of retaining fines in the sheet and allowing water to drain relatively quickly from the wet paper as it is being formed. As we approach the 1900-year anniversary of the first detailed account of the papermaking process, it is the goal of this review to explore the scientific principles that underlie the art of papermaking, emphasizing the state of dispersion of the fibrous slurries during various procedural phases of the manufacturing process. Some concepts that arise out of the experience of papermakers have potential applications in other fields.

Chen, J., Hubbe, M. A., Heitmann, J. A., Argyropoulos, D. S., and Rojas, O. J., "Dependency of Polyelectrolyte Complex Stoichiometry on the Order of Addition. 2. Aluminum Chloride and Poly-vinylsulfate," Colloids Surf. A 246 (1-3): 71-79 (2004).

In the first part of this series it was shown that the stoichiometry of complexation between oppositely charged polyelectrolytes became increasingly dependent on the order of addition as the concentrations of monovalent and divalent ions were increased. This study considers the effect of aluminum ions on titrations between solutions of a strong poly-acid and a strong poly-base. In addition, the titratable charge of aluminum ion itself was also investigated. It was found that aluminum ions can interfere with the results of charge titrations, and stoichiometric relationships fail to explain the observed results. The word "interfere" implies an unpredictable effect on titration results. Several factors affect this interference. However, by controlling solution pH and degree of charge neutralization of aluminum ions, the titratable charge of the system can be estimated by streaming current titration. The results are consistent with the presence of aluminum polynuclear species within the ranges of aluminum and base addition where the highest titratable charge was reached.

Hubbe, M. A., "Pulp and Paper Chemical Additives," Encyclopedia of Forest Sciences, Chapter (MS 135), Elsevier Science and Technology. (2003).

Chemical additives can be critical to the economic viability of paper machines. Relative to fibers (see Chapters 2 through 6) other materials are present in paper and paperboard at much lower levels. However, additives allow papermakers to differentiate their products to meet the differing needs of customers. They also use additives to make their operations more efficient. This chapter considers the most widely used papermaking additives, focusing on their composition, their modes of preparation and use, and their impacts on either product attributes or process efficiency. The fact that relatively small amounts of additives can make huge differences makes this a fascinating field of work and study.

Hubbe, M. A., Tripattharanan, T., Heitmann, J. A., and Venditti, R. A., "The 'Positive Pulse Jar' (PPJ): A Flexible Device for Retention Studies," Paperi ja Puu 88 (1): 39-45 (2006).

A bench-scale apparatus for studies of fine-particle retention was demonstrated, comparing effects of contrasting flow conditions, with or without retention chemical addition, to a simulated headbox stock. The apparatus consisted of an agitated jar with a forming screen on the bottom and a continuous phase of aqueous solution below the screen. A positive displacement gear pump produced a constant time-averaged rate of dewatering, and a bellows pump was optionally used to produce an additional sinusoidal component of flow normal to the forming screen. Pulsations of known velocity and displacement distance were used to represent effects due to hydrofoils. As another alternative, the suspension was subjected to a uniform time-averaged shear stress during the dewatering. It was shown that the conditions of flow during dewatering affected not only the efficiency of fine particle retention, but also the distribution of fiber fines and fillers in the thickness direction of the fiber mat. Under any given set of flow conditions, retention aid use reduced the filtrate turbidity, indicating increased retention efficiency. Retention also was affected by exposure of the stock to hydrodynamic shear before dewatering; effects of such shear application were apparent even after subsequent exposure to flow pulsations during dewatering. The design of the apparatus provides flexibility in evaluating a variety of mechanistic questions under quantified conditions of pulsating flow and/or steady shear application to the suspension before or during dewatering.

Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Effect of Idealized Flow Conditions on Retention Aid Performance. 1. Cationic Acrylamide Copolymer," Appita J. 57 (5): 404-410 (2004).

The performance of a cationic acrylamide copolymer retention aid was evaluated with respect to five contrasting flow conditions applied either before or during the formation of a fibre mat. Different levels of pre-shearing of a simulated fine-paper headbox stock were applied after chemical addition, but before constant-rate dewatering. Dewatering conditions included simple filtration, flow pulsations of known amplitudes and frequencies of normal to the forming screen, and continuous stirring during dewatering either with an impeller or at a uniform time-averaged shear rate. Under all conditions of flow the retention aid reduced the turbidity of the filtrate, consistent with improved retention of fine materials. However, the effectiveness of the polymer was irreversibly decreased by high shear before dewatering. The irreversible loss was observed even when the slurry subsequently was subjected to vigorous flow pulsations during dewatering. Results were consistent with a polymer-bridging mode of retention aid action.

Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Effect of Idealized Flow Conditions on Retention Aid Performance. 2. Polymer Bridging, Charged Patches, and Charge Neutralization" Appita J. 57 (6): 448-454 (2004).

Retention aids can affect papermaking process efficiency and product quality. The efficiency of these polyelectrolyte treatments may be affected by conditions of flow before and during sheet formation. Four contrasting retention aid systems were compared. Hydrodynamic shear before dewatering decreased the retention efficiency of very-high-mass acrylamide copolymers, consistent with irreversible breakdown of polymeric bridges. Such shear had little effect in the case of a moderately high-mass ethyleneimine copolymer. The relative effectiveness and responses to flow conditions of different polymeric treatments were consistent with concepts of charge neutralization, charged patches, and two types of polymeric bridges. Flow velocity pulsations normal to the plane of the forming screen lowered the retention efficiency for all of the retention aid systems, though not as much as the application of uniform time-averaged shear stress to the suspension during dewatering.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Refining to Overcome Effects of Drying Unbleached Kraft Fibers in the Presence or Absence of Sugar," Progress in Paper Recycling 13 (2): (2): 5-12 (2004).

Previous work has shown that drying of chemical pulps results in lower inter-fiber bonding when the fibers are subsequently redispersed and formed into recycled handsheets. It was also found that the strength loss could be decreased if the initial drying was carried out in the presence of relatively concentrated sugar solutions. The present research was undertaken to determine whether or not the effects of drying and or sugar treatment still remain significant after the fibers are subsequently refined. Unbleached kraft fibers were optionally subjected to oven drying in the presence or absence of 10% dextrose solution and then subjected to various levels of refining in a PFI mill. Although significant differences in fiber flexibility were still apparent after 6000 revolutions of PFI refining, there was no residual effect of either drying history or the presence or absence of sugar on the strength of recycled paper subsequently formed from those fibers. While it is understood that application of additional refining energy to recycled fibers can be expected to produce additional fine material, reducing the freeness of the stock, the present results suggest that such refining can play a critical role in overcoming the adverse effects on strength when kraft fibers are dried.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Effects of Sugar Addition Before Drying on the Wet-Flexibility of Redispersed Kraft Fibers," J. Pulp Paper Sci. 30 (1): in press (2004).

Paper made from recycled, chemically pulped fibers typically has lower strength than paper made from virgin fibers. This was confirmed with respect to tensile and compression strength for an unbleached softwood kraft pulp. It was also confirmed that the addition of sucrose, at high concentration, to virgin pulp before drying could improve the recycled paper strength, compared to a control with no sugar added. The use of glucose was found to be slightly more effective than sucrose. Fibers treated with the sugars were found to have higher flexibility and water retention values than untreated fibers that had been subjected to the same drying conditions.

Chen, J., Heitmann, J. A., and Hubbe, M. A., "Dependency of Polyelectrolyte Complex Stoichiometry on the Order of Addition. 1. Effect of Salt Concentration during Streaming Current Titrations with Strong Poly-acid and Poly-base," Colloids Surf. 223 (1-3): 215-230 (2003).

Titrations were carried out between solutions of a strong poly-acid (polyvinylsulfate, potassium salt) and a strong poly-base (poly-diallyldimethylammonium chloride) over a range of salt concentrations. Streaming current analysis of the titration endpoints appeared to show increasing deviations from 1:1 stoichiometry of complexation with increased salt. The results depended on the direction of the titration, such that a stoichiometric excess of the titrant (second additive) was required to achieve a streaming current reading of zero. These symmetrical results, depending on the order of addition, were obtained despite the fact that the plastic surfaces of the streaming current device had a slight negative charge and differing adsorption tendencies for the two kinds of polymer. A qualitative model of molecular events, based on non-equilibrium entrapment of non-complexed polymer segments was found to be inconsistent with results of tests carried out over a range of initial polymer concentration. Results were better described by a qualitative model involving formation of polyelectrolyte complexes (PECs) in solution, in which near-stoichiometric core complexes are stabilized by an excess of the second additive on their surface. Implications of the latter model were compared with the results of turbidimetric tests, aqueous contact angles on polymer-treated plastic surfaces, and microelectrophoresis of PECs. Results of this study have consequences for interpretation of polyelectrolyte titrations, as well as for industrial operations that involve the mixing of oppositely charged polyelectrolytes.

Hubbe, M. A., Jackson, T. L., and Zhang, M., "Fiber Surface Saturation as a Strategy to Optimize Dual-Polymer Dry Strength Treatment," Tappi J. 2 (11): 7-12 (2003).

The compression strength of unbleached kraft handsheets was maximized when the first component of a dual-polymer treatment was added at a level corresponding to saturation of the fiber surface. The saturation level of poly-diallyldimethylammonium chloride (poly-DADMAC), determined by streaming current analysis, also coincided with a maximum in water retention value (WRV) and a minimum in the light scattering coefficient of the paper. Idealized descriptions of the polymer interactions are proposed to explain the observations.

Hubbe, M. A., "Microparticle Programs for Drainage and Retention," in Micro and Nanoparticles in Papermaking, J. Rodriguez, Ed., Chapter 1, TAPPI Press, accepted (2004).

When papermaking technologists use the word "microparticles," usually they mean certain chemical additive programs that can promote the release of water and help retain fine particles during formation of paper. The prefix "micro" is actually somewhat of a misnomer; some of the most commonly used particles, for which this technology is named, have primary diameters in the nanometer range, 1-5 nm. Microparticles, as well as certain analogous papermaking additives that we will call "micropolymers," are useful only when used in sequence with certain oppositely charged high-mass polymers, hence the term "microparticle programs" as used above. This present book serves as a witness to explosive growth. Before 1980 the subject of microparticle technology, as we know it today, did not exist. Papermakers were generally unaware of potential uses of such additives as colloidal silica and bentonite, except for their occasional uses in water treatment or for control of paper's frictional properties. Now there are at least 550 paper machines that have used these two types of very finely divided minerals - in sequence with cationic starch or cationic acrylamide copolymers - to promote drainage and retention. About 300 have been reported to use colloidal silica, and about 250 have been reported to use bentonite. In addition, there are various paper machines that have used, or are currently using, related retention and drainage programs with highly cross-linked anionic polymers [12] or lignin byproducts playing the role of microparticle. The purpose of this introductory chapter is to review the already-extensive literature related to microparticle programs. This includes descriptions of the materials, how they work together mechanistically, and how the papermaker can take advantage of them to increase paper production rates or product quality.

Hubbe, M. A., Venditti, R. A., Barbour, R. L., and Zhang, M., "Changes to Unbleached Kraft Fibers Due to Drying and Recycling," Progress in Paper Recycling, 12 (3): 11-20 (2003) .

Drying of unbleached kraft pulp in the laboratory revealed two main stages in its response to increasing temperature of drying. The first stage was characterized by significant decreases in water retention value, capacity to adsorb a cationic polymer, dry strength, and apparent density of handsheets formed after re-slurrying the pulp with no additional treatment. These changes, which were independent of the drying temperature, were attributed to the action of capillary forces in the closure of micro-pores in the cell wall during the initial drying. The second stage was characterized by further significant decreases in all of the same parameters when drying temperatures became as high as 150 to 175 oC. In addition, high-temperature drying also resulted in a loss of molecular mass of the cellulose, as revealed by viscosity tests. Surprisingly, neither cellulose molecular mass nor water retention was affected to a significant extent by the value of pH prior to drying, within a range of 3 to 8. The results suggest that whereas some irreversible changes in fiber properties are unavoidable during conventional papermaking practices, further losses in the bonding ability of unbleached kraft fibers can be caused by over-drying.

Hubbe, M. A., "Selecting Lab Tests to Predict Effectiveness of Retention and Drainage Aid Programs," Paper Technol. 44 (9): 20-34 (2003); originally published in Proc. 4th Pira Internat. Conf. Sci. Tech. Advan. Fillers & Pigments for Papermakers, Barcelona, Spain, May 20-21, 2003.

This paper compares laboratory test procedures that predict the performance of chemicals used to enhance retention or dewatering during the manufacture of paper. Key points of difference among the various laboratory methods include the presence or absence of fiber mat formation during the test, the optional application of vacuum, the presence or absence of pressure or velocity pulsations during dewatering, and the use of automation in some test procedures. A well-chosen laboratory test can provide useful information without incurring the high cost and risks associated with having to do full-scale evaluations of many different retention and drainage programs and dosage levels. However, it is important to understand the compromises inherent in different lab-scale tests to guard against premature rejection of specific chemical program options.

Wang, F., and Hubbe, M. A., "Charge Properties of Fibers in the Paper Mill Environment. Part 1. Effect of Electrical Conductivity," J. Pulp Paper Sci. 28 (10): 347-353 (2002).

The electrical conductivity of water used in papermaking processes tends to increase due to water conservation efforts. This study concerns the effect of changes in conductivity on the amounts of highly charged cationic polymers required to neutralize the surfaces of fibers, as measured with a fiber-pad streaming potential method. Streaming potential measurements were carried out at relatively high applied pressure to obtain more precise data under conditions of increased electrical conductivity. The amount of cationic polymer (poly-diallyldimethylammonium chloride) required for neutralization of bleached hardwood kraft fiber surfaces, determined by this test, increased moderately as the electrical conductivity was increased from 0.5 to 10 mS/cm. The amount of cationic polymer required also increased with increasing contact time and with decreasing molecular mass of the polymer. Results are consistent with the porous nature of kraft fibers and the effects of salt on the effective size of macromolecules in solution.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Can Recycled Kraft Fibers Benefit from Chemical Addition Before They Are First Dried?," APPITA J. 55 (2): 135 (2002)

Over 50 chemical treatments of never-dried pulp were compared relative to the strength of paper made subsequently after the fibers had been dried and recycled once. Treatments having the greatest beneficial effect on the compression strength of recycled unbleached kraft paper tended to be polyelectrolytes of relatively high molecular mass. Many of the most effective treatments, either alone or by sequential addition, included both cationic and anionic functional groups on the polymers. Results were consistent with the persistent nature of charged complexes formed by polyelectrolytes at fiber surfaces, and the contribution of such complexes to inter-fiber bonding, even after drying and recycling.

Wang, F., and Hubbe, M. A., "Charge Properties of Fibers in the Paper Mill Environment. 1. Effect of Electrical Conductivity," J. Pulp Paper Science, in print, Nov. 2002.

The electrical conductivity of water used in papermaking tends to increase over time due to water conservation efforts. This study concerns the effect of changes in conductivity on the apparent surface charge of fibers and its measurement with a fiber-pad streaming potential method. Poly-diallyldimethylammonium chloride was added to vary the observed streaming potential from its initial value to zero. The charge of bleached hardwood kraft fibers, determined by this test, increased moderately as the electrical conductivity was increased from 0.5 to 10 mS/cm. The amount of cationic polymer required to reach neutrality increased with increasing contact time and with decreasing molecular mass of the polymer. Results are consistent with the porous nature of kraft fibers and the effects of salt on the effective size of macromolecules in solution.

Wang, F., and Hubbe, M. A., "Development and Evaluation of an Automated Streaming Potential Measurement Device," Colloids and Surfaces A 194: 221 (2001).

The streaming potential and other colloidal properties of aqueous suspensions of bleached kraft fibers were evaluated by a new laboratory instrument, the Streaming Potential Jar (SPJ). This device provides precise streaming potential data under moderately high electrical conductivity levels of 0.5 to 10 mS/cm. Features of the SPJ include automated operation, rapid acquisition and processing of data, continuous stirring, and applied pressures up to 276 kPa. The SPJ also provides data related to drainage rates and the turbidity of the filtrate. Test results showed a high degree of linearity of the streaming potential signals with applied pressure and very little dependence of the results on the solids levels of the fiber slurries. These results, which are consistent with the Helmholtz-Smoluchowski equation, tend to justify the level of applied pressure used in this work. Changes in streaming potential with increasing pH were consistent with expected dissociation of surface-bound carboxyl groups on the fibers. The absolute magnitudes of the streaming potential values of bleached kraft pulps were strongly affected by increasing concentrations of Na2SO4. However, a high repeatability of measurements was obtained throughout the range of conductivities considered; relative standard deviations of streaming potentials were consistently below 3%. Titrations with poly-(diallyldimethylammonium chloride) yielded curves that had shapes similar to those of parallel tests by micro-electrophoresis; however, the amount of titrant needed to reach the endpoints was about three times higher in the case of the streaming potential tests. The disagreement between the endpoints determined by the two types of test is attributed to a diffusion process of the titrant into the porous fibers.

Chen, J., Hubbe, M. A., and Heitmann, J. A., "Measurement of Colloidal Charge in the Paper Mill by Streaming Current," Proc. TAPPI Papermakers Conf., 2001.

The colloidal charge of process water within a paper mill can profoundly affect process efficiency and product quality. With increased pressure for productivity and reduced costs the need for accurate and reliable charge measurements has become more urgent. But the paper machine environment presents challenges that may limit the accuracy of such tests. Recently the streaming current method has become the most widely used means of charge analysis. This report presents new data, helping to define the range of sample types and electrical conductivity where it is possible to achieve accurate and reliable results from streaming current titrations. Within the limits of these ranges the results are consistent with a model of polyelectrolyte adsorption onto the plastic probe surfaces of the test instrument. Outside of these limits the results may be unreliable.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Effect of Chemical Pretreatments of Never-Dried Pulp on the Strength of Recycled Linerboard," Proc. TAPPI Papermakers Conf., 2001.

According to the American Forest and Paper Association (AF&PA) the recovery rate of corrugated boxes used in the US now exceeds 75%. In principle the recycling of boxes saves fiber resources and requires less total energy. However, further progress in old corrugated container (OCC) recycling faces a potential barrier. It is known that recycled kraft fibers have a reduced bonding ability. The approach taken in this study was to pre-treat the never-dried fibers before the first cycle of papermaking. New data have been obtained with never-dried, refined, unbleached kraft pulp. Simple drying, low-shear disintegration, and forming without further refining yielded a loss in compression strength in the range 19 to 26%, depending on the pulp batch. Pretreatment with various chemical agents was able to compensate for some of the strength loss. Two general classes of treatment agent were identified that were able to favorably affect the strength of recycled sheets. Certain low-molecular weight materials such as sucrose appeared to interfere with the mechanism of pore closure during the initial drying. In contrast, certain high-mass polyelectrolytes such as guar gum products, cationic starch, and polyelectrolyte complexes appeared to affect the adhesiveness of the fiber exteriors of the repulped fibers.

Hubbe, M. A., and Wang, F., "Where to Add Retention Aid: Issues of Time and Shear," Solutions! 2002 (3): 61 (2002); TAPPI J. 1 (3): (2002); and Proc. TAPPI Papermakers Conf., 2001; also reprinted in O Papel.

Papermakers continually wrestle with the question of whether to add high-mass acrylamide copolymer retention aids before or after pressure screens in the approach flow to a paper machine forming section. Early addition of a retention aid provides more opportunity for breakage of chemically induced fiber flocs, possibly leading to more uniform formation. Early addition also provides more chance for chemicals to become degraded or lost in the porosity of fiber cell walls. Later addition tends to maximize chemical efficiency in terms of first-pass retention. A new method of fiber floc evaluation was applied in the case of headbox-consistency fiber slurries to help understand what happens to chemically induced fiber flocs when they are exposed to increased time and shear. The extent of flocculation was determined by the force required to move a pair of 6-mm probes through a slurry. Supplementary tests of the streaming potential of fibers were used to help explain the separate effects of time and hydrodynamic shear on the state of flocculation.

Hubbe, M. A., "Reversibility of Polymer-Induced Fiber Flocculation by Shear. 2. Multicomponent Chemical Treatments" Nordic Pulp and Paper Research Journal 16 (4): 369, (2001).

Optical and viscometric tests showed essentially complete dispersal of fiber flocs when polymer-treated chemithermomechanical (CTMP) fiber slurries were exposed to intense shear in a blender. Two types of chemical systems showed evidence that flocs formed again after the shear application were stronger or larger than those in an untreated slurry. Treatments that included poly-diallyldimethylammonium chloride (DADMAC) showed a charge-dependent maximum in the optical test of flocculation that approximately corresponded to charge neutralization. By contrast, treatments involving cationic polyacrylamide (cPAM) following by nano-size anionic materials (microparticles) yielded a net increase in viscometer output after intense shear. In all cases the polyelectrolyte mechanisms holding calcium carbonate filler particles to the fibers or to each other appeared sufficient to withstand intense shear.

Hubbe, M. A., "Method for Determining Electrokinetic Properties of Papermaking Furnish," U. S. Patent 6,176,974, Jan. 3, 2001

A method and apparatus for determining electrokinetic properties of a papermaking furnish includes mixing a sample of furnish in a container with a known amount of a charged additive and then measuring the streaming potential of the resultant ionically modified furnish sample. The amount of charged additive added to the container is increased or decreased as repeated streaming potential measurements are made until a desired streaming potential of the modified furnish sample is obtained. The results, which are expressed as the amount of titrant required to achieve the desired streaming potential in a given volume of furnish, may be used by papermakers to adjust process variables and achieve optimum, stable paper quality and machine runnability.

Wang, F., Tanaka, H., Kitaoka, T., and Hubbe, M. A., "Distribution Characteristics of Rosin Size and their Effect on the Internal Sizing of Paper," Nordic Pulp and Paper Research Journal 15 (5): 416-421 (2000); presentation at International Paper Coating Chemistry Symposium, June 6-8, 2000

It was attempted to systematically elucidate three aspects of acidic and neutral rosin sizing. These were (a) the relationship between sizing efficiency and the retention behavior of rosin sizes at the wet end, (b) the effect of pulp beating and fiber fines on the size distribution and sizing properties of paper, and (c) the distribution characteristics of rosin size on pulp fiber surfaces in internal paper sizing. Pyrolysis-GC and the oxine extraction method were used to determine the retained amounts of rosin size and aluminum in the paper. Scanning electron microscopy (SEM) and SEM-EDXA (energy dispersing X-ray analyzer) were employed to evaluate the distribution of rosin size on the pulp fiber surfaces. Under neutral to alkaline conditions neutral rosin and acid rosin sizes yielded distinctly differing sizing effects. The results depended to a great degree on the chemical stability of rosin particles and the retention efficiency of each type of sizing agent under the wet-end conditions of papermaking. Pulp beating and fiber fines influenced the size distribution and sizing properties of paper. Both pulp beating and the existence of fiber fines were considered as important contributing factors leading to the observed uneven rosin size distribution. Furthermore, the rosin size was unevenly distributed on the fiber surfaces not only for freeze-dried paper, but also for cured paper, and its distribution was similar and correlated to that of aluminum. A continuous rosin size film could not be formed even after drying by heating. It is proposed that an uneven aluminum distribution on fiber surfaces can be a root cause of non-uniform sizing with rosin.

Hubbe, M. A., "Reversibility of Polymer-Induced Fiber Flocculation by Shear. 1. Experimental Methods" Nordic Pulp and Paper Research Journal, in press; presentation at International Paper Coating Chemistry Symposium, June 6-8, 2000

Papermakers desire two seemingly incompatible outcomes. On the one hand, strong agglomeration of fibers and fines can help one to achieve rapid drainage and satisfactory fine-particle retention. On the other hand, papermakers also want uniform distribution of fibers in the sheet. Procedures have been developed in our lab to evaluate effect of different retention and drainage chemical programs under stressed conditions of salt content, fines content, or high levels of charged colloidal matter. In the work described here the same tests were used to compare the reversibility of agglomerative effects of some common classes of retention and drainage programs. Optical and viscometric tests showed increased flocculation following treatment with increased amounts of cationic polyacrylamide. Application of intense hydrodynamic shear caused essentially complete reversal of flocculation. By contrast, treatment with a highly charge density cationic polymer yielded a maximum in flocculation, according to the optical test, at a treatment level corresponding to the point of charge neutralization. Divergent results were obtained when comparing fine-particle retention tests to drainage tests. In general, retention results were consistent with a model in which polymer bridges, i.e. "hard flocs," between fibers may be irreversibly broken by shear. Meanwhile, bonds formed between fibers and fine particles appeared to remain intact. In contrast, drainage results appeared to be more highly dependent on the electrokinetic properties of the furnish, i.e. factors related to "soft floc" formation.

Hubbe, M. A., "Selecting and Interpreting Colloidal Charge Measurements," Proc. Scientific and Technical Advances in Wet End Chemistry," PIRA, Barcelona, June 19-20, 2000.

Achieving an optimum balance between negatively and positively charged materials in a fiber slurry can be critical to the profitable operation of a paper machine. There is no consensus, however, regarding what tests to use and how to interpret the results. Papermakers are free to choose among several competing methods, including micro-electrophoresis, colloidal titrations with a color endpoint, streaming current titrations, and fiber-pad streaming potential methods. Each method has its strengths and weaknesses. It is critical to be aware of the potential weaknesses and interferences with each method before selecting it for a given papermaking application such as process control surveys of paper machine operations. An understanding of potential errors also can improve a user's ability to draw reliable conclusions.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A., "Loss of bonding Strength Dye to Drying and Repulping of Kraft Fibers: Effects of Chemical Additives," Proc. International Symposium on Environmentally Friendly and Emerging Technologies for a Sustainable Pulp and Paper Industry, Taipei, Taiwan, April 25-27, 2000.

Chemical pretreatment of never-dried kraft fibers was found to improve the strength of recycled paperboard. Lab tests with guar gum added to unbleached softwood kraft pulp (linerboard furnish) showed that the additive increased the tensile strength and resistance to compression failure. Strength values fell by 25-35% when the same fibers were reslurried and then formed into second generations of paper. However, the strength of the paper made from pre-treated fibers remained significantly higher than the recycled, untreated control. The results suggest a strategy whereby an initial additive or additives to never-dried fibers yield strength benefits that persist over at least one generation of recycled paper. Expected benefits include reduced overall costs for strength-enhancing chemicals, reduced basis weight requirements to achieve product strength requirements, and an ability to use higher levels of waste fibers.

Tests were carried out to shed light on the mechanism of strength loss and the effects of chemicals relative to strength loss when paper is dried and then recycled. The chemical effects on strength appeared to be governed by at least two significant mechanisms. For instance, it was found that pretreatment of the never-dried pulp with a very high level of sucrose yielded an improvement of the tensile strength of recycled paper, compared to untreated fibers that were dried and recycled. This observation is consistent with the known ability of sucrose to penetrate into the fine pores in the walls of kraft fibers. The sucrose-pretreated fibers retained higher levels of water retention value (WRV), indicating a greater degree of swelling when the once-dried fibers were placed back into water. These results with sucrose are consistent with a mechanism in which loss of bonding ability is related to irreversible closure of pores in the fiber cell wall. Contrasting results were obtained when the never-dried fibers were treated with underivatized guar gums. The guar increased the relative strength of both the primary and secondary sheets, but there was no effect on the water-holding ability of the fibers. It is proposed that the guar's mechanistic role is related to its effects on relative bonded area or shear strength of bonds per unit of bonded area.

Olsen, W. L., Zhu, H., and Hubbe, M. A., "Method of Improving Pulp Freeness using Cellulase and Pectinase Enzymes," U. S. Patent 6,066,233, May 23, 2000.

A method for enhancing the freeness of pulp made from secondary fiber is provided by adding an enzymatic mixture comprised of cellulase and pectinase enzymes to the pulp and treating under conditions to cause a reaction to produce an enzymatically treated pulp. The freeness of the enzymatically treated pulp is increased from the initial freeness of the secondary fiber pulp without a loss in brightness.

Chen, J., Hubbe, M. A., Heitmann, J. A., and Chang, H.-M., "The Effect of Paper Additives on Agglomeration During the Recycling Process," Proc. International Symposium on Environmentally Friendly and Emerging Technologies for a Sustainable Pulp and Paper Industry, Taipei, Taiwan, April 25-27, 2000.

A major contaminant in mixed office waste paper is toner inks. One method for removal of these contaminants is chemically aided agglomeration followed by conventional screening and cleaning techniques. Most agglomeration agents are oil-like hydrophobic or amphipathic materials. Because of their hydrophobicity they wet the surfaces of toners and other hydrophobic materials in aqueous suspensions. Thus they have the ability to form bridges and aid toner agglomeration. One very effective agglomerating chemical was found to be 1-octadecanol. Unfortunately, for some types of toner, it has been found that various paper chemicals can interfere with the agglomeration process. This study was focused on the effect of various paper additives on agglomeration of different toners.

Hubbe, M. A., U.S. Patent 5,936,151, Aug. 10, 1999, "Method and Apparatus for Measuring an Electrical Property of Papermaking Furnish."

An apparatus for determining an electrical characteristic [the endpoint of a streaming potential titration] of a papermaking furnish includes a sample chamber having first and second sections for containing a sample of the papermaking furnish. A fiber-collecting screen having a mesh sufficient to inhibit the passage of suspended fibers separates the first and second sections of the sample chamber. Furnish is urged back and forth through the screen in a plurality of cycles so that as the furnish flows in one direction through the screen, a fiber pad is formed adjacent to the screen and as furnish is move through the screen in a second direction, the fiber pad is expelled from the screen back and redispersed back into the furnish sample. A stirrer or mixer assists in removing the fiber pad from the screen and in redispersing fibers comprising the fiber pad back into the furnish sample. A furnish is urged through the screen, electrodes positioned on opposite sides of the screen produce signal outputs corresponding to an electrical characteristic of the furnish [streaming potential]. A voltmeter receives the electrode outputs and measures voltage across the screen. In one embodiment, furnish is moved through the screen by means of a variable volume chamber in fluid connection with the screen and furnish. In an alternate embodiment, fluid is urged through the screen by reciprocating the screen within the furnish sample. Colloidal charge of the furnish may be determined by moving furnish through the screen in repeated cycles as the furnish sample is titrated with a highly charged additive.

Hubbe, M. A., Wagle, D. G., and Ruckel, E. R., U.S. Patent 5,958,180, Sept. 28, 1999, "Method for Increasing the Strength of a Paper of Paperboard Product."

Processes for increasing the [bonding] strength of cellulosic fibers are carried out by contacting relatively dry cellulosic fibers with an agent in particulate or vapor form comprising a carboxylic acid cyclic anhydride at an elevated temperature for a time sufficient to significantly increase the bonding strength of the fibers. The treated fibers bond more readily to one another and they also hold wet and dry strength aids more strongly. Furthermore, the treatment does not significantly affect the internal chemical structure of the fibers so that paper made from the fibers exhibits overall improved dimensional stability.

Hubbe, M. A., "Difficult Furnishes," Proc. TAPPI '99, 1353-1367; literature review and a proposal to broaden the view of what types of paper furnish components need to be considered in relationship to runnability and product quality.

Changes in paper furnish are making it harder to achieve targets of retention, drainage, formation, and strength. This review takes a closer look at components that tend to make furnish difficult. Prime examples are increasing filler levels, dissolved and colloidal anionic materials, high levels of fines, high conductivity, contaminants, and fibers lacking in ability to form bonds. The review also considers chemical additives and strategies which papermakers use to cope with each furnish deficiency and maintain retention, drainage, formation, and strength at desired levels. The chemical additives and strategies can work by affecting the structure, surface area, state of agglomeration, surface properties, or relative bonded area of the components of a paper sheet.

Hubbe, M. A., "Paper: Wetting and Penetration of Liquids into," Encyclopedia of Materials Technologies, Elsevier; Chapter solicited by subject editor.

End-use applications of paper require a wide range of properties with respect to interactions with liquids. At one extreme are tissue papers and toweling. They are designed to imbibe water quickly. At the other extreme are cup-stock and greaseproof papers that are designed to inhibit passage of liquids. It is remarkable the degree to which variations in the processes and chemical additives used in paper can achieve such diverse products. On a microscopic scale paper is porous, directional, and rough. Papermakers and specialists have used two approaches to define and measure wettability and penetration by liquids. On the one hand tests have been carried out involving smooth, flat surfaces or filaments composed of pure materials such as cellulose films or other ingredients that compose paper. On the other hand, the paper industry has developed a wide range of practical, empirical tests that predict aspects of product performance.

Bennett, M. B. O., Crouse, J. W., Hubbe, M. A., and Paukstra, P. M., "Engineers and Papermakers Bond for Better Papermaking at TAPPI's First Superconference," Tappi J. 81 (3): 239 (1998).

Hubbe, M. A., "Surface Analysis of Paper [Book Review]," Tappi J. 80 (5): 239 (1997).

Read this book and you'll never look at paper in quite the same way. This comprehensive text awakens the reader to substantial progress in surface analysis. It also helps make sense of what can otherwise become a bewildering array of complex and expensive analytical tools and methods.

Hubbe, M. A., "Retention Aids," Proc. TAPPI Short Course on Dyes, Fillers, and Pigments, Chicago, April 1995.

The purpose of this course module is to introduce the subject of retention aids. Students will learn about the basic functions, safe use, and chanical nature of the additives most commonly used as retention aids. Widely known theories of zeta potentail and retention aid mechanisms will be summarized. The course module also describes a popular method for evaluating retention aid effectiveness.

Hubbe, M. A. "Colloidal Aspects of Aluminum Chemistry in Papermaking," presented at the ACS Colloids Symposium in Toronto, June 1993.

The trivalent charge and remarkably small size of the aluminum III cation yield unique characteristics. Papermakers have exploited these unique properties of aluminum salts for many years for such applications as pitch control, drainage, retention, and as a mordant for hydrophobic sizing with rosin. In addition to the traditional use of aluminum sulfate ("papermakers' alum"), papermakers now have the options of using partially hydrolyzed polymeric forms of aluminum salts. The choice of additive and dosage level is further complicated by recent trends toward manufacturing at higher pH. The goal of this paper is to provide fresh insights into the mechanism underlying the use of alum and related products by considering their colloidal behavior.

Hubbe, M. A., "Characterization of Liquid Absorption into Porous Media," with M. B. Lyne, presented at TRI/Princeton 62nd Ann. Conf., April 30, 1992.

Dynamic liquid absorption measurements have achieved widespread use within the paper industry for process control and quality assurance. A test developed by J. A. Bristow has enabled papermakers to predict the performance of their paper when used in printing processes where the paper must absorb ink on a millisecond time scale. The results are sensitive to the sample's porosity, surface roughness, and the chemical nature of its surface; these parameters can be separated by analysis of the data. The surface chemistry in some cases has been shown to change rapidly during the first few seconds of contact with a liquid, which results in accelerated adsorption after an initial delay. Since the surface chemistry and porosity of paper are sensitive to adsorption of moisture, it is essential that all tests be carried out under controlled conditions of humidity. The test method is also applicable to other porous sheet materials.

Lisnyanski, K., and Hubbe, M. A. "System for On-Line Measurement of Color, Opacity, and Reflectance of a Translucent Moving Web," U.S. Patent 5,047,652, Sept. 10, 1991.

Systems for the on-line optical measurement of properties of a translucent moving web, such as paper or plastic, as it is continually produced, colored or otherwise converted. Measured properties include color, reflectance, and opacity. A backing roll has a cylindrical or approximately cylindrical surface which comprises at least one optical standard. The roll is positioned such that a circumferential portion of the roll surface contacts the back web surface where the web characteristic is to be measured, with the web curving around the circumferential portion. An optical sensing device is positioned so as to view the front web surface backed by the optical standard or standards. In several embodiments, two sets of reflectance data are collected, one with a "white" optical standard backing and the other with a "black" optical standard backing. The backing roll surface and the optical sensing device can be arranged such that the sensing device either alternately of simultaneously views portions of the front web surface backed by each of the optical standards. In other embodiments, the backing roll has a uniform optical standard surface.

Hubbe, M. A., "How Do Retention Aids Work," Proc. TAPPI Papermakers Conf., 389-398 (1988).

This paper presents a literature review of mechanisms that have been proposed to explain the action of polymeric retention aids used in paper manufacture. The available experimental evidence can be used to support a variety of hypotheses. For instance, do retention aids work by binding the fine particles to the fibers, or by flocculating the filler into aggregates that can be filtered by the fiber mat? Does the mechanism depend on charge neutralization, or on polymeric bridges? The effects of complicating factors such as time, shear, and detrimental substances are discussed briefly.

Hubbe, M. A., "Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 3. Forces of Adhesion," Colloids Surf. 25: 311-324 (1987).

Colloidal hydrous titania, alumina, and chromium hydroxide spheres were detached from cellulose and glass surfaces by hydrodynamic shear. Independent variables included pH, ionic strength, and the addition of strongly adsorbing ions. The shear stress required for detachment was consistent with a model based on dispersion and electrostatic forces of adhesion. The goodness of the fit of the theory to the data depended on the boundary conditions assumed for the electrostatic forces. In some cases the assumption of constant surface potential was more consistent with the data. In other cases the constant charge assumption yielded a better fit.

Hubbe, M. A., "Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 4. Effects of Polyelectrolytes," Colloids Surf. 25: 325-339 (1987).

Pretreatment of cellulose and glass surfaces with cationic polyelectrolytes greatly increased the forces needed to detach titanium hydrous oxide spheres. The force of adhesion was as much as 30 times greater than the highest values obtained in the absence of polymers. The hydrodynamic shear stress required for detachment increased with pretreatment level, molecular mass, and decreasing cationic charge of the polymer. The results are consistent with the presence of polymeric bridging between the solids.

Hubbe, M. A., "Retention and Hydrodynamic Shear," Tappi J. 69 (8): 116-117 (Aug. 1986).

Recent articles describe the levels of hydrodynamic shear stress in paper machines. High intensities of shear tend to pull particles of filler pigment and fiber fines from the surfaces of fibers. My purpose here is to compare the levels of shear stress relative to the ability of small particles to remain attached. The results imply that suitable synthetic retention aid systems are more than adequate for the shear stresses in the forming section of the machine. Also, smaller particles are more easily detached than larger particles, under specified conditions of treatment and shear stress.

Hubbe, M. A., "Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 1. Experimental System," Colloids and Surfaces 16: 227-248 (1985).

This article describes an improved technique for studying the detachment of very small particles from solids walls exposed to turbulent shear flow. The technique is useful as an assay of the strength of adhesion between solids immersed in solution. It is also useful in determining the mechanism by which detachment takes place. The experiment is designed so that the particles rest on a window in the outer annular wall of a system of coaxial cylinders. This arrangement permits more rapid counting of particles remaining on precisely the same area throughout the experiment. Results are presented for the detachment of uniform colloidal hydrous oxide spheres from cellulose and glass substrates. Independently controlled variables included the applied shear stress, the size of the particles, the composition of the aqueous solution, and the time of shearing.

Hubbe, M. A., "Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids Exposed to Flow. 2. Mechanism of Release," Colloids Surf. 16: 249-270 (1985).

Experiments show how small particles are detached from a flat window exposed to turbulent shear flow. The onset of detachment is governed by the component of hydrodynamic forces which pulls the particles in a downstream sense. An adhesive torque opposes the applied hydrodynamic torque. In the rate-determining step a released particle rolls from its initial site of attachment. Resistance to rolling is proportional to the product of the net adhesive force and a characteristic length of the region of contact. The kinetics of release indicate that the process is governed by random events. Continued shearing at the same average shear stress results in continued entrainment of particles. The data are consistent with an idealized model of fluctuations in the local hydrodynamic force within the viscous sublayer of turbulent shear flow. Brownian motion does not properly account for the effect of shear stress on the rate of detachment.

Hubbe, M. A., "Theory of Detachment of Colloidal Particles from Flat Surfaces Exposed to Flow," Colloids and Surfaces 12: 151-178 (1984).

Theoretical models are presented for the detachment of colloidal particles from solid surfaces exposed to shear flow. The models are most relevant to cases of hard, spherical particles that are small enough to display Brownian motion. It is concluded that the component of hydrodynamic force acting parallel to a sheared wall is usually much larger than the lifting forces. Thus, in most cases, one can expect the downstream component of force to govern the critical or rate-determining step in the process of entrainment. Alternative limiting modes of incipient motion, e.g. rolling, sliding, and lifting, can be distinguished, based on the dependency of the shear stress required for detachment on the size of particles. Rate laws for detachment and the dependency of rates on the applied shear stress permit one to discriminate between processes limited by viscous flow, Brownian motion, and fluctuations in hydrodynamic forces. Finally, it is proposed that separate geometric models of sphere-wall interaction can be employed in computing long- and short-range forces.

Hubbe, M. A., "Bonding of Filler to Cellulose: Effects of Cationic Retention Aids," Proc. TAPPI 1984 Papermakers Conf., 23.

Experiments reveal that pretreatment of cellulose film with cationic polyelectrolytes greatly increases the strength of bonding between the film and model filler particles. Levels of hydrodynamic shear required for detachment of spherical colloidal oxides were affected by the dosage, density of charge, and molecular mass of various cationic agents to which the cellulose had been exposed. The most tenacious bonding was generally achieved at high dosage of cationic agents having low to moderate density of charge. Very high molecular mass was not needed in order to form strong attachments. Both branched and linear polymers were found to be effective. The results suggest strategies for improving the retention of filler on paper machines.

Hubbe, M. A., "Adhesion and Detachment of Biological Cells in Vitro," Prog. Surface Sci. 11 (2): 65-137 (1981).

Adhesion between biological cells and various surfaces is explained in terms of various models, including coagulation at primary or secondary minima of free energy, macromolecular bridges or matrices, and specialized structures at the surfaces of some cells. These models are used to predict the magnitudes of forces necessary to detach a cell in the limiting cases of peeling and simultaneous separation over finite areas of contact. Diverse experimental assays of cellular adhesiveness are reviewed and the forces applied to individual cells are estimated. A very wide range of forces applied to cells in different assays suggests that different mechanisms of bonding are dominant for different types of cells and surfaces under various conditions of growth and chemical environment. The peeling mode of separation is most consistent with the magnitudes of applied force used experimentally in the detachment of cells.

Hubbe, M. A., "A Polarization Resistance Corrosivity Test with a Correction for Resistivity," British Corrosion J. 15 (4): 193-197 (1980).

A polarization resistance test that employs three identical sample probes and compensates for the interference of solution resistivity is described. The corrosion test vessel and probes are treated as a conductivity cell for which the cell constant is found using a standard a-c bridge technique. The constant is used to calculate the contribution of ohmic resistance (IR) to the polarization resistance measurements. Data from two- and three-day exposures of mild steel and brass to salt and acid aqueous solutions yield an excellent linear correlation between corrosion rates predicted by the polarization test and measurements of sample weight loss. The procedure offers a potential increase in precision over some other polarization resistance tests, and the apparatus is fairly simple and can be made portable. The procedure is recommended for non-passive alloys and for industrial process water where the corrosion rate is expected to be in the range 0.01-1.0 amperes per square meter and the solution resistivity is below 10,000 ohm-cm.

Hubbe, M. A., "A Modified Reporting Procedure for Polyelectrolyte Titrations," Tappi 62 (8): 120-121 (1979).

Titration of fibrous slurries with polyelectrolytes to determine the surface charge of the slurry solids has been offered by Halabisky as a means of paper machine wet-end control. Further experience has shown that certain modifications in reporting of the titration results lead to the improved reliability and utility of the original procedure.

Hubbe, M. A., and D. F. Bowers, "Survey of White Water Corrosivity in 30 North European Paper Mills," Paper Trade J. 1978: 53-56 (Nov. 1, 1978).

Reuse of paper machine white water has become an almost universal practice within the industry. Energy costs and environmental restraints on effluent discharge have made some form of this practice mandatory. While the closure of white water systems is becoming a worldwide activity, we describe trends in a recent survey of 30 North European papermaking operations.


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