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Advanced Coating Materials

High Performance Starch based Material for Reduced Volatile
Organic Compound Emissions

Abdus Salam, Joel Pawlak and Richard Venditti

Introduction:

This research has the long term goal of developing a novel starch based paper coating that functions as an effective replacement for polyethylene and wax coatings used in the paper industry. This material has been shown to have similar performance properties and visual characteristics to polyethylene and wax with additional benefit of adding significant strength to the paper sheet. This material can be applied directly to the paper sheet without requiring additional handling and transportation of the paper web. This has the potential to save a great deal of energy and significantly reduce CO₂ emission and reduce volatile organic compound (VOC). This material is definitively a better environmental choice when compared to coatings derived from synthetic systems as it does not contain any VOCs.

Currently, there are two primary means to impart water barrier properties to paper. These are coating with polyethylene and coating with paraffin (petroleum) based wax. These materials suffer from serious environmental issues. First, the addition of these materials to paper makes the paper very difficult and inefficient to recycle. Currently, there is no industrially successful process to recycle paper coated with polyethylene and wax. Paraffin wax coated paper is particularly difficult to recycle and even a small amount of contamination in the paper recycling operation results in board quality issues in terms of appearance as well as performance (ex. loss of friction coefficient). The result is that these materials must be landfilled or incinerated. Landfilling can result in a cost of $100 per ton. (Harper/Love (2010)) Neither landfilling nor incineration are good environmental options.

According to the US EPA, the maximum VOC emission for a polyethylene extrusion process would equate to 0.011 % of the polyethylene extruded. For the paper industry, as of 1996, there was 866,000 tons of PE extruded in Western Europe and the United State alone. (Savolainen (1998)) Assuming that this number has grown at a rate of 1 %, current extrusion in these areas would amount to 995,400 tons. This would amount to ~200,000 pounds of VOC emitted in the US and Western Europe from PE extrusion process. One estimate of VOC emissions from wax coating operations place it at 1 % to 3 %. (New Hampshire Department of Environmental Services (2002)) Estimating the usage of wax to be half that of polyethylene extrusion, the total VOC emission would be between 9,954,000 lbs and 29,862,000 lbs annually. Thus, replacing even a portion of these coatings with an alternative VOC free material will have a significant positive impact on the environment. This proposal describes a novel starch based coating that may replace a portion of the paper and paperboard currently coated by polyethylene and waxes.

Starch based coatings have long been used in the paper industry. Starch is typically applied to the surface of the paper sheet during paper manufacture. The objective of traditional starch coatings is to improve the stiffness of the sheet, bind the surface fibers tightly to prevent picking and linting, and to partly seal the surface pores. This traditional starch surface coating is not useful to impart optical properties (gloss, or whiteness) or for providing a barrier. The traditional starch surface may form a layer near the surfaces of the paper sheet, but does not form a continuous film.

The high-performance starch based material in this research has the benefits of traditional starch coating, with the additional benefits of forming a continuous surface film that provides a barrier to the penetration of gases and liquids. It is a replacement for plastics and waxes. This coating is smooth, glossy and transparent as well. The high-performance starch based coating described in this proposal has the ability to significantly reduce VOC in the paper industry and improve the overall environmental performance based on improved recovery and recycling of papers and bio-degradability of the coating. The following section presents the preliminary development work that has been conducted in our laboratory.

Preliminary work:

The starch is reacted with a crosslinker (x) and then a natural polymer (B). All reactions are conducted in water. The material for application is a 4% solids material.

Coating of filter paper

The filter paper was immersed into starch-x-B solution and then passed through a laboratory sizing press (Atlas Laboratory Wringer, Stockport, England), cf. Figure 1c. The filter papers were then dried in an oven at 50°C for overnight.

Measurement of tensile strength

The coated paper was equilibrated in a conditioning room with an atmosphere of 23˚C and 50 % relative humidity for 48 hr prior to testing. The sample size was 30 x 10 mm with a thickness of ~0.2 mm. The tensile strength of the coated paper sheets was measured using an Instron 4411 (Canton, MA.) tensile testing machine in the 23˚C and 50 % relative humidity atmosphere. The crosshead speed was 2 mm/second.

Dynamic contact angle

Dynamic contact angle measurements were performed with a Phoenix 300 Contact Angle Analyzer (Seo Co., Ltd., Korea) on the coated papers. Deionized water was used as the probe fluid. The time interval for image acquisition was 0.5 seconds and data was collected for about 30 seconds.

Results summary:

The following tables contain measurements of critical properties needed for the evaluation of the films. Three different materials were coated onto filter paper and an uncoated sheet was tested as a control. The three materials were an unmodified starch, starch-x, and the final coating material starch-x-B. Table 1 lists the thickness, basis weight and density of the coated paper sheets and the uncoated filter paper. The basis weight illustrates that the starch alone picks up a relatively small amount of coating. A 3.3 % increase in the basis weight is observed for the starch. The Starch-x picks up significantly more coating, increasing the weight by 8.8 %. It is readily apparent that the starch-x-B sample picks up much more coating. The basis weight of this sample is increased by 49 % upon coating. The caliper remained relatively constant, and when combined with increasing basis weight, results in increased density of the paper sheet.

Table 2 displays the physical properties of the coated paper sheets. Note that all measures of strength show that the starch-x-B is significantly stronger than the three control samples. The tensile index is a measurement of strength commonly used in the paper industry to characterize the strength to weight ratio of paper. It is determined by dividing the load at failure by width of the sample and then by the basis weight of the sample. Thus, the tensile index of starch-x-B indicates that the paper sheet is three times stronger than the starch-x coated paper sheet on an equal weight basis. This is a significant improvement in the strength of the paper sheet. In addition to increased strength, the sheet also becomes more extensible. The strain at break increases to about 7 % with starch-x-B coating compared to about 2 % for the other samples. This is a direct indication that the sheet becomes tougher, absorbing more energy during fracture.

The contact angle shown in Table 2 indicates that the starch-x-B coating provides reasonable water resistance. After 30 seconds, the contact angle is relatively unchanged. In fact, after 40 minutes, the contact angle for the starch-x-B is still unchanged and remains stable at 56 degrees (not shown on table).

The surface properties of the starch-x-B coating are also remarkable. They provide an extremely glossy surface and enhanced smoothness to the paper sheet. Overall, the starch-x-B coating provides a smooth, glossy, flexible, water resistant coating for paper, which has an appearance similar to plastic or wax as well as similar performance. This material may ultimately have a number of other uses as well. These uses include wood finishes and coating, coating and paint binder, alternatives to UV cured coatings, polymer additives and as films and fibers. There is a potential of making micro or nano sized particles with this material.

Table 1: Dimensional Properties of Sample Papers

Sample	Thickness (mm)	Basis Weight (g/m²)	Density (g/cm³)
Starch-x-B	223	137.28	0.667
Starch-x	205	100.53	0.448
Starch	197	95.49	0.427
Filter Paper	218	92.39	0.415

Table 2: Physical Properties of Sample Papers

Sample	Tensile				Whiteness Index (CIE)	Dynamic Contact Angle (Degree)		Gloss	Roughness (0.5Mpa)
Sample	Rupture Modulus (MPa)	Tensile Index (N*m/g)	Modulus (MPa)	Strain at Break (%)	Whiteness Index (CIE)	Initial (0 sec)	Final (30 sec)	Gloss	Roughness (0.5Mpa)
Starch-x-B	38.6	62.7	2353.97	7.12	45.8	58	56	82.5	6.4
Starch-x	9.0	18.3	--------	2.05	79.3	50	11	5.0	13.0
Starch	8.0	16.5	--------	2.03	80.2	51	5	5.2	13.3
Filter Paper	5.2	12.1	--------	2.60	79.1	13	5	5.4	12.3

(a) Starch-x (b) Starch-x-B

Figure 1: (a) Starch-x which is an intermediate product in the process created in a semi-dry state. (b) Starch-x-B solution ready for coating on paper. (c) Laboratory size press used for coating paper sheets. (d) Paper coated with starch-x-b. Note the plastic film like appearance and flexibility of the film.

Starch Microcellular Foams

Khaled El-tahlway, Richard Venditti and Joel Pawlak

The main application that may result from this research is the use SMCF coated paper to improve the properties and recyclability of writing paper by replacing traditional paper with inorganic fillers. It is proposed that paper fibers coated with SMCF can be produced with the same or better printing properties as uncoated paper fibers filled with inorganic pigments such as clay or calcium carbonate.

To understand the societal impact of using SMCF coated paper instead of paper filled with inorganic pigments, it should be known that paper is by far the largest fraction of material in municipal waste, about 40% by weight [Tchobanoglous, 1993]. Of the 100 million tons of paper consumed in the US per year approximately 50 million tons of paper are landfilled, the other 50 million tons of paper are recycled [AFPA, 2000]. Paper itself is considered biodegradable but observations have been made that paper can remain intact in a landfill for many decades. Certainly the inorganic pigments in paper (as high as 40% by weight of the paper) does not decompose. By replacing these inorganic pigments by starch, the paper will be significantly more biodegradable.

An equally important aspect is that the use of SMCF coated paper instead of paper filled with inorganic fillers should decrease the amount of industrial solid waste (sludge) that is a by-product of the de-inking process. With present paper technology, in order to produce acceptably clean, bright recycled fibers a wet sludge (50% moisture) consisting of mainly the inorganic fillers, but also the contaminants and fibrous debris is generated. The amount of the solids in the initial wastepaper that is removed in the process is about 30% [McKinney, 1995]. Thus, from the 15 million tons of wastepaper de-inked for printing and writing applications about 10 million tons of wet sludge is produced.

If paper containing inorganic fillers could be replaced by the use of SMCF coated paper then there is a potential to reduce this sludge flow by as much as 90% (i.e. the elimination of 9 million tons of sludge per year). Further, by making writing paper with SMCF coatings instead of inorganic filler the ease of recycling is expected to be enhanced, since coatings serve as a barrier between inks and fibers during printing and this allows for an easier separation of inks and fibers during recycling. This, combined with less sludge generation, should make paper recycling operations more efficient and profitable and thus encourage more paper recycling and decrease the flow of wastepaper to municipal land fills.

To summarize, paper is a very important product for society and the revolutionary concept of using SMCF coated papers should decrease the total amount of sludge landfilled and improve recycling operations. This better utilization of a renewable resource should have a broad, important impact on both society and the environment.

The research is focussed on the improved utilization of natural, renewable resources. To this end, the PI has been involved in developing new materials with new properties from natural resources. This has included applying concepts in physical chemistry and polymer science to topics such as the melt spinning of cellulose and lignin derivatives and the efficient recycling of wood-based fibers for paper products. The research has been equally balanced between both the initial properties of new materials and their recyclability. As an example, one focus of the PI’s research is to develop a better understanding of the mechanisms of strength loss in recycled fibers so that paper products can be designed in the future not for one single use but for multiple effective recycles. In the PI’s opinion, the most powerful strategy for improving the use of natural based resources is the combined product design for both first-use and recycling and the simultaneous complementary development of effective recycling technology. The proposed research herein is just that, the development of a new material (SMCF coated paper) and the simultaneous evaluation of its recyclability (performance in a modern printing and writing paper recycling process).

Starch

Starch is one of the most abundant biopolymers, utilized by plants as the major storage material for carbohydrate [Shannon, 1984; Glenn, 1996]. Starch is a high molecular weight mixture of two glucose-based polymers, amylose and amylopectin. Amylose is a linear polymer with a molecular weight of several hundred thousands whereas amylopectin is a branched polymer with a molecular weight of several million. For wheat and corn starches, the percentage of amylose and amylopectin is typically about 78 and 22%. However, special species have been cultivated to provide amylose contents from 0.8-80%. Starches are hygroscopic and water is a well-known plasticizer for starch.

Starch-based foams have been prepared for many decades [Shogren, 2001; Shogren, 1998]. Through a combination of heat, water and shear the starch-water mixture behaves thermoplastically and can be extruded into shapes. The water can act as a blowing agent, expanding upon a decrease in pressure as the starch-water mixture leaves the barrel of the extruder to form the starch foam. The viscoelastic properties of the starch are important in the foam making process [Shogren, 1998]. The starch initially must have a certain elastic strength during expansion to accommodate the expanding steam and then have enough compressive strength to support the foam structure as the water evaporates. Thus, the temperature and moisture profile for extrusion must be chosen carefully to maintain the appropriate starch properties to form suitable foam.

This method has been used to produce cereals and more recently starch-based packing material [Glenn, 2001a; Nobes, 2001; Shogren, 2001]. These types of foams have pore diameters on the order of 1 mm [Glenn, 1996] and as such are considered to be a macrocellular foam. The blending of starch with other polymers such as poly(vinyl alcohol), poly(vinyl alcohol)/ethylene copolymer, poly(lactic acid) and poly(hydroxyester-ether) has been shown in some cases to improve the mechanical properties of starch based foams. Improvements of the foam properties have been shown to be related to a high concentration of the polymers on the surface and with the polymers high energy dissipation characteristic [Shogren, 2001]. Interestingly, for high amylose content starch, the addition of these polymers did not significantly enhance the properties. Another approach has been to reinforce the starch with either inorganic fillers such as calcium carbonate [Glenn, 2001a] or with cellulose based fibers [Glenn, 2001b]. It was shown that the calcium carbonate did not improve the physical properties but the cellulose fibers did. The effect of these additives on the optical properties was not reported.

In our laboratories we have explored the optical properties of several starch based macrocellular foam products with different densities, adjusted by pressing the starch sheets after extrusion. For sheets of a starch-poly(vinyl alcohol) commercial foam the scattering coefficient was determined to be approximately 10 m²/kg, independent of the thickness of the sheets. This is significantly lower than inorganic fillers utilized in paints and paper applications, 10-60 m²/kg. This result indicates that macrocellular foam simply does not have a large enough value of specific surface area necessary to act as an opacifying agent for applications such as paper or paint. This indicates that the use of microcellular foam is dictated for the needed opacifying power in coatings for applications in which only thin structures are to be produced (see later).

Starch Microcellular Foams (SMCF)

When water and heat are applied to starch granules the granules swell. The amylose material in particular extends from the starch granules and forms a gel in the water phase. If the water is removed in a normal drying process, large capillary forces would act on the starch and collapse the foam structure. In order to preserve this structure, two methods have mainly been used, freeze drying [Coudeville, 1981; Charoenrein, 1989] and solvent exchange [Glenn 1995; Glenn 1996; Glenn 2002]. Solvent exchange involves the replacement of the water with another solvent (or sometimes a succession of solvents) of lower surface tension. Final drying from the solvent with low surface tension, e.g., liquid or critical CO₂ or ethanol, avoids the large compressive forces caused by the drying of high surface tension water and better preserves the SMCF pore structure.

It is apparent from the literature that the lower the surface tension of the liquid the more likely it is to form a low density SMCF. This suggests an area of research investigating the effect surfactants on the drying process of the SMCF and the subsequent foam structure. Could the addition of surfactant sufficiently decrease the compressive forces during drying to maintain a usable foam structure?

The micro-structure of the bulk region of SMCF sheets formed from wheat starch dried from ethanol consisted of a network of strands of amylose forming irregular pores generally less than 2 micrometers [Glenn, 2002]. The surface of the sheets was observed to be not as porous as the bulk. This is very interesting for coating applications in which a smooth non-porous surface may be advantageous.

The specific surface area of SMCF sheets prepared from critical point CO₂ were found to be 116, 50 and 145 m²/gram for unmodified wheat starch, unmodified corn starch and high amylose corn starch [Glenn, 1995]. SMCF sheets produced from solvent exchange with ethanol and air drying were comparable in physical properties and in pore size as the foams compared to critical point CO₂. This specific surface area compares favorably with the specific surface areas of common inorganic fillers used to opacify coatings: kaolin clay (5-8 m²/g) , calcinated clay(16-17), calcium carbonate (2-6), silicates (40-130), and titanium dioxide (8-25) [Eklund, 1991]. Thus, because increased quantities of optical interfaces improves the reflectivity and scattering of light [Eklund, 1991], these high specific surface area SMCF are promising opacifying agents for coating applications.

The dimensions of SMCF sheets and the average pore size could be manipulated by pressing the sheets, pressures from 1.4 to 69 MPa [Glenn, 2002]. Transparent thin regions of the sheets were observed at the highest pressure, indicating that the air-starch interfaces had been locally eliminated decreasing the scattering of light. Other than this qualitative observation, reports of the effect of pressing on the effect of the interaction of the SMCF with light are not readily available.

The application of controlled pressure of SMCF for coating applications is an important research topic. Pressing may be a convenient way to change the size and shape of SMCF pores. Paper typically has a thickness or caliper of about 100 microns. Coatings for paper need to be about 20 microns in thickness. It may be physically convenient to form thicker SMCF and then press the SMCF to a thinner layer. In fact, during paper manufacturing, the paper web travels through several press nips to reduce thickness and thickness variations. This proposal will include investigating the effect of pressing on pore structure and particularly the optical properties of the resulting SMCF.

To summarize, it is presently known that macroscopic foams are not suitable as opacifying agents due to their large pore size. However, SMCF materials have much smaller pore size and correspondingly potentially higher specific surface area. The production of SMCF coatings is possible using reasonable techniques in the laboratory for research purposes. It is also known that the SMCF materials are opaque and that pressing can alter the opacity of the SMCF materials. What is not known is how the pore structure quantitatively relates to the opacity and other optical properties.

Foam applications to paper

Several studies have investigated the application of chemicals to the surface of paper using air in liquid foams applied to paper at very high speeds but then immediately broken, usually by passing the paper/foam through a two roll nip [Anderson, 1986; Eklund, 1986; Riddell, 1978; Trauter, 1986]. There have also been studies of fiber composites and starch foam for bulky, packaging applications [Glenn, 2001b; Gotoh, 2001] but none of these were involved in making thin sheets of opaque paper for printing and writing applications. To the best of the PI's knowledge there were no references available pertaining to paper coated with a thin solid film of foam used as a surface opacifying agent.

Microfibrillar Cellulose

Advances in the Processing of Microfibrillated Celluloses for Bioplastics and Composite Reinforcement Applications

Kelley L. Spence, Richard A. Venditti, Orlando J. Rojas, Youssef Habibi, and Joel J. Pawlak

The world consumption of petroleum-based plastic is approximately 100 million tons annually; these plastics can be recycled, but are likely to be disposed of in landfills, where it will take over 100 years for the material to decompose. With the growing consumption of plastic, the waning area available to potentially be used as landfills, and the fact that petroleum-based plastics are derived from a non-renewable source, the development of a sustainable alternative is imperative to the future of the planet and its inhabitants, Figure 1.

Microfibrillated Celluloses (MFCs), also known as nanofibrillated celluloses, are fibrils with diameters in the range of 10-100 nanometers which are liberated from larger plant-based cellulose fibers. MFCs were first developed in 1983 by Turbak et al. using purified wood pulp and a high pressure homogenizer, utilized to disintegrate the cellulose fibers into sub-structural microfibrils, Figure 2. These individual microfibrils are theoretically stronger than steel and have garnered much attention for the use in composites, coatings, and films because of high specific surface areas, renewability, and unique mechanical properties. MFC films produced at a basis weight of 30 g/m² have tensile strengths of approximately 80-90 MPa; in comparison, Nylon 6,6 has a tensile strength of around 75 MPa and low and high density polyethylene have tensile strengths of approximately 8-31 MPa and 22-31 MPa, respectively. These materials could potentially be used to produce bioplastics, as well as for the utilization as composite reinforcements to reduce the use of petroleum-based plastics.

Contrary to many of the recent MFC studies which focus on highly purified cellulose, this study will investigate the production of MFCs from wood pulp fibers containing aromatic lignin, a native compound found in wood. The removal of this lignin is an expensive part of the MFC manufacturing process; eliminating this step could make the production of these materials more economically feasible, as well as the expectation that the lignin will impart different properties on the material. For example, it was expected that the lignin will affect water interaction properties of these materials as well as mechanical properties.

Pulp samples were used as received from pulp mills in the southeastern United States. These samples included bleached hardwood, bleached softwood, unbleached hardwood, two unbleached softwoods, and thermo-mechanical pulp. Pulps were subjected to a refining pretreatment in order to shorten the fibers and reduce equipment plugging. The pretreated pulps were then processed with a homogenizer, a microfluidizer, or a micro-grinder. Films were generated using a casting-evaporation technique and were testing using TAPPI standard methods. Film characterization included thickness, roughness, optical properties such as opacity, brightness, and scattering coefficient, mechanical properties, and water interaction properties such as water vapor transmission rate and water adsorption. The MFC slurries were evaluated using water retention value, hard-to-remove water content, and the Congo red specific surface area method.

It was found, after homogenization, that the presence of lignin significantly increased film toughness, tensile index, and elastic modulus, opposite to trends observed for the physical properties of paper, Table 1. It is hypothesized that this may be due to the lignin interfering with the hydrogen bonding between microfibrils, making them easier to separate during processing with the homogenizer. This can be observed by the large increase in specific surface area values of the unbleached samples, Table 2.

Unexpectedly, the lignin-containing films had higher water vapor transmission rates than the bleached samples, even though the initial contact angle of these materials was higher, Figure 3. This can be explained by the internal hydrophobic pore structures likely formed in the materials containing lignin; this structure allows for the water vapor to rapidly pass through the film.

Further, three different types of mechanical processing (homogenization, microfluidization, and micro-grinding) were used to generate the MFCs to determine effects on physical properties and the energy efficiency of the processing. Results show that processing resulted in films with high tensile strengths, but the microfluidizer resulted in significantly tougher films than both micro-grinding and homogenization and required less energy to obtain these properties, offering great promise for producing MFC materials with lower energy input, Figure 4. Analysis of the processing methods further allow for process optimization with regards to energy consumption. Unexpectedly, increasing processing pressure and number of passes with the microfluidizer did not improve mechanical properties, Figure 5, suggesting that the optimum processing conditions may be less than 10,000 psi and 5 passes. The refining pretreatment was found to reduce the optimum number of homogenizer passes to approximately 8 from the standard 20 passes, Figure 6, and processing with the grinder was found to eliminate the need for the pretreatment, Figure 7.

These results show that it is possible to produce MFCs containing lignin with properties similar to MFCs made from purified cellulose. The ability to produce MFCs containing lignin could potentially provide new markets for MFC such as composite reinforcements in hydrophobic matrices without surface modification while production with the microfluidizer and the micro-grinder could provide a more economically feasible production method as compared to the homogenizer.

Future research will investigate the mechanism of which the lignin effects the production of the MFCs as well as the mechanical and water interaction properties of the films. The fundamental understanding of lignin with respect to MFC production may allow for cheaper production of these materials as well as new uses.

Figures and Tables