Strength (Low, Variable, Too High)

Paper strength can be regarded as being a result of the strengths or individual fibers, plus the strengths of bonds between those fibers. Usually the bonds are weaker than the fibers themselves. That fact implies that it is worth paying a lot of attention to factors that affect inter-fiber bonding. It is not possible in the context of this guide to do justice to all of the many aspects of paper strength, including all of the different kinds of strength evaluation. Therefore, the reader is encouraged to follow up by reading some of the references given after this section.

DRY STRENGTH TOO LOW

The primary tools by which papermakers can increase the dry-strength properties of paper are selection or purchase of a suitable quality and type of fibers, increased refining, the use of dry-strength additives, and changing the conditions of wet-pressing (if possible, given the equipment).

The proportion of softwood kraft fibers can be increased if one wants to improve dry-strength in general, and tear strength and folding endurance in particular. Virgin kraft pulps generally have a moderate strength advantage over recycled kraft pulps of the same type, especially if freeness is held constant when making the comparison. The difference has been attributed to closing up of pores in the cell walls of the recycled kraft fibers, making them stiffer and less capable of developing bonded area. Thermomechanical pulps (TMP), especially chemithermomechanical pulps (CTMP), are noted for higher tensile strength compared to stone groundwood, since the pulping process is somewhat less destructive of fiber length.

Refining (sometimes called "beating") can be defined as the repeated passage of wood pulp through zones of compression and shearing. Refiners usually consist of pairs of surfaces with raised metal bars that rotate relate to each other. Important variables include the energy input per unit mass of fiber (after subtracting out the energy required in the case of water alone), the rate of rotation, and the total length of bar edges encountered by fibers during one pass. The effects of refining are most often evaluated by freeness tests. This practice is somewhat unfortunate, since the reduction in rate of dewatering of the pulp is an undesired side-effect of refining, not the main goal. It has been shown that fines in the furnish, often produced during the refining process, tend to dominate the observed changes in freeness as pulp is refined. In theory it would be better to evaluate the extent of refining by measuring the strength of test sheets and by measuring the water content remaining in plugs of fiber that have been centrifuged under standard conditions (water retention value). Kraft fibers in particular are known to become more water-swollen during refining, and there is often a high correlation between water retention value and inter-fiber bond strength.

One of the first considerations in improving paper strength ought to be whether the refining conditions are at their optimum. Often there are opportunities to switch to refiner plates with a finer bar pattern, offering a lower energy input per number of bars encountered by a typical fiber (a measure of the intensity of the refining action). A finer pattern often is less energy-efficient in terms of freeness reduction, but there can be a substantially reduced tendency for fiber shortening. Rather, one achieves more of the desired effects of making the fibers more flexible and partially delaminating the outer layers, creating some fibrillation of the surfaces. It is expected that kraft fibers refined at low intensity ought to develop higher strength (especially tensile strength) compared to the same fibers refined to the same freeness at higher intensity. Some rules-of-thumb regarding optimum refining levels are given by Baker [1995].

Before turning to chemical factors, it is worth noting that wet-pressing can have a major impact on paper strength. This factor is sometimes overlooked due to the fact that papermakers generally keep wet-press nips near to their maximum practical pressure, short of crushing the sheet. A sheet that comes into a wet-press nip too wet, relative to the applied pressure may be crushed, meaning that the fiber structure created during formation is disrupted, often resulting in breaks.

There has been some debate as to whether dry-strength chemicals increase the relative bonded area within paper or whether they increase the strength of bonding per unit of bonded area. The answer can depend on some definitions. Usually relative bonded area is defined based on optical tests, comparing the light scattering coefficient of a paper sample with a corresponding sheet that was formed from an organic solvent, such as butanol. The latter sample will have almost zero strength, due to the non-swollen condition of the fibers during the formation process, the inability of hydrogen bonds to form, and the inability of cellulosic macromolecules on the adjacent surfaces to intermingle. But the optical tests cannot sense effects of distances that are less than about a quarter of a wavelength of light, e.g. about 50 nm, which is much larger than the range of a typical chemical bond (except possibly polymeric bridges in the expanded form). That means that the optically bonded area generally is expected to be much higher than the actual area of contact on a molecular level. Experiments have shown that dry-strength chemicals such as cationic starch tend to increase the strength per unit of optically bonded area to a greater extent than they increase the optically bonded area [Howard, Jowsay 1989].

Once the pulp has been refined to an optimum extent (most important variable), moderate improvements in strength can be achieved by adding chemicals (secondary effect). The most popular dry-strength additive for the wet end in the U.S. is cationic starch. Cationic versions of corn, potato, and tapioca starches usually are formed by alkaline treatment of slurries of starch grains with an epoxide chemical that contains a quaternary ammonium group. In the case of wet-end products, there is no intentional degradation of the starch molecular mass, and most wet-end starches need to be cooked before use. Batch conditions often involve about 20 minutes of stirring below the boiling point of water. Alternatively, starch can be solubilized in a jet cooker.

The optimum addition point for cationic starch is usually complicated by local situations, including the availability of inlet taps and the need to use many of those addition points for other additives. A general principle is that adsorption of dry-strength agent onto long fibers is expected to yield a greater positive effect on paper strength than an equal amount added to the fines fraction. That means that there is sometimes an advantage of mixing cationic starch with the thick stock before it is diluted with fines-rich white water at the fan pump.

Strength benefits of cationic starch and similar additives (including cationic guar gum) tend to show diminishing returns with increasing dosage. The practical upper limit of starch addition usually is related to the available surface area of the wetted solids in the furnish, and to some extent also on the amount of negatively charged carboxylate groups of those surfaces. Common practical maximum addition levels of cationic starch, depending on the nature of the furnish and the need for strength, often lie between 1% and 1.5% (20 to 30 lb/ton). Evidence that the adsorption capacity of the fiber surface for cationic starch has been exceeded often comes in the form of increased foaming. This is because starch that is in solution, rather than on fiber surfaces, can act as a stabilizer for foam bubbles.

In cases where the dry-strength effects of cationic starch alone are not sufficient, one has options of (a) using a microparticle additive that may make it possible to increase the amount of starch that can be retained and also promote faster dewatering so that the sheet can enter the wet-press section with less water and become consolidated more effectively, or (b) using synthetic dry-strength additives. Anionic and amphoteric acrylamide polymers have been shown to have a superior dry-strengthening ability in some tests. Anionic acrylamide products and carboxymethyl cellulose (CMC) can be added in sequence with a suitable high-charge cationic polymer in order to achieve efficient retention on the fiber surfaces.

The size press is a very important tool for increasing paper strength, partly because of the fact that the practical addition levels are typically much higher, compared to wet-end addition. Also, it is possible to apply relatively inexpensive starch products. "Unmodified" corn starch, which is probably the major size-press additive used in the U.S., needs to be reduced in molecular mass by treatment with enzymes or oxidizing agent just before use in order to reduce the viscosity. Though the degradation decreases the strength of the resulting starch film, this is a necessary compromise that papermakers make in order to run the equipment effectively.

Sometimes poor performance of size-press starch can be traced to an undesired process of crystal formation, known as retrogradation. Retrogradation is most prominent in the linear component of most starch products, the amylose. Retrogradation is much less of a problem in the case of wet-end starches, since the molecules usually are substituted with cationic groups. Hydroxyethylated starch products for the size press are noted for high strength efficiency, as well as high resistance to retrogradation, and their performance often justifies their higher cost.

See the page related to hold out at the size press for a discussion of how to achieve a balance between internal bonding (if the starch penetrates into the paper) versus surface strength (if the starch is held out effectively).

Conditions needed to maximize tensile strength of paper will not necessarily maximize either the compression strength or stiffness. Such differences can be expected, due to the fact that the latter properties demand less flexibility of the overall product. By contrast, tensile strength can benefit from some ability of the paper to stretch and deform so that the load can be borne more evenly among fibers in the paper.

VARIABLE STRENGTH

The search for root causes of strength variability ought to begin with the fiber furnish, including measurements of freeness and fiber length distribution as a function of time. Other factors to track are filler content, formation uniformity, first-pass retention, and any known changes in wet-end additives.

Sometimes variable performance of starch products as dry-strength additives can be traced to its biological degradation. Early warning signs can include foul odors and unexpected lowering of the pH of the starch slurries.

DRY STRENGTH TOO HIGH

Decreased bonding strength sometimes can be advantageous for such products as facial tissue, especially when it is produced from recycled fibers that may have been refined to various levels, and which may contain some cationic starch. High inter-fiber bonding can adversely affect the soft feel of tissue paper. Decreased bonding also may be an advantage in some paper products that need to be bulky.

Some of the first factors to consider when dealing with excessive dry-strength levels relate to some of the variables mentioned in previous subsections, when it was assumed that increased strength was the main goal. Refining should be decreased, though it may still be necessary to refine the stock enough to redisperse any fiber bundles or flakes of broke or recycled fibers. In some cases the wet-press loading can be decreased. Cationic starch and other dry-strength chemicals should be reduced or taken out entirely.

Debonding agents can be used to decrease inter-fiber bonding. Though a variety of molecular types have been evaluated, debonders usually consist of cationic surfactants. Important variables include the length and number of alkyl tails attached to the positively charged group, which is usually a quaternary ammonium salt. The debonding agents are believed to work by covering the fiber surfaces and reducing the opportunities for hydrogen bond formation between the surfaces. Another consequence of such treatment is that the paper usually has a lower apparent density, especially before it is calendered.

References

Baker, C. F., "Good Practice for Refining the Types of Fiber Found in Modern Paper Furnishes," Tappi J. 78 (2): 147 (1995).

Bhardwaj, N. K., Bajpai, P., and Bajpai, P., "Enhancement of Strength and Drainage of Secondary Fibers," Appita J. 50 (3): 230 (1997).

Carlsson, G., Lindstrom, T., and Soremark, C., "Effect of Cationic Polyacrylamides on Some Dry-Strength Properties of Paper," Svensk Papperstid. 80 (6): 173 (1977).

Conte, J. S., and Bender, G. W., "Softening and Debonding Agents," in K. J. Hipolit, Ed., Chemical Processing Aids in Papermaking: A Practical Guide, TAPPI Press, Atlanta, 1992.

Howard, R. C., and Jowsay, C. J., "Effect of Cationic Starch on the Tensile Strength of Paper," J. Pulp Paper Sci. 15 (6): J225 (1989).

Kure, K.-A., Sabourin, M. J., Dahlqvist, G., and Helle, T., "Adjusting Refining Intensity by Changing Refiner Plate Design and Rotational Speed - Effects on Structural Fiber Properties," J. Pulp Paper Sci. 26 (10): 346 (2000).

Linke, W. F., "Retention and Bonding of Synthetic Dry Strength Resins," Tappi 51 (11): 59A (1968).

Liu, J., and Hsieh, J., "Application of Debonding Agents in Tissue Manufacturing," Proc. TAPPI 2000 Papermakers Conf., 71 (2000).

Page, D. H., "A Theory for the Tensile Strength of Paper," Tappi 52 (4): 674 (1969).

Poffenberger, C., Deac, Y., and Zeman, W., "Novel Hydrophilic Softeners for Tissue and Towel Applications," Proc. TAPPI 2000 Papermakers Conf., 85 (2000).

Retulainen, E., and Nieminen, K., "Fiber Properties as Control Variables in Papermaking. Part 2. Strengthening Interfiber Bonds and Reducing Grammage," Paperi Puu 78 (5): 305 (1996).

Roberts, J. C., Au, C. O., Clay, G. A., and Lough, C., "The Effect of C14-labelled Cationic and Native Starches on Dry Strength and Formation," Tappi J. 69 (10): 88 (1986).

Robinson, J. V., "Fiber Bonding," in Casey, J. P., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. II, Wiley-Interscience, New York, 1980, Ch. 7, p. 915.

Smith, D. C., "Chemical Additives for Improved Compression Strength of Unbleached Board," Proc. TAPPI 1992 Papermakers Conf., 393 (1992).

Strazdins, E., "Chemicals Aids Can Offset Strength Loss in Secondary Fiber Furnish Use," Pulp Paper 58 (3): 73 (1984).

Tanaka, A., "Inter-fiber Bonding Effects of Beating, Starch or Filler," Nordic Pulp Paper Res. J. 16 (4): 306 (2001).

Young, J. H., "Fiber Preparation and Approach Flow," in Casey, J. P., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. II, Wiley-Interscience, New York, 1980, Ch. 6, p. 821.

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.


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