ALTERNATIVE USES FOR WASTE-PAPER IN WOOD-BASED COMPOSITE PRODUCTS
The word "waste" projects a vision of a material with no value or useful purpose. However, technology is evolving that holds promise for using waste or recycled paper and, in some cases, even plastics to make an array of high-performance composite products that are in themselves potentially recyclable.
When fibers, resins, and other materials are used as raw materials for products such as paper, they require extensive cleaning and refinement. When recovered fibers, resins, and other materials are used for the manufacture of composites, these materials do not require extensive preparation. This greatly reduces the potential cost of manufacturing.
A case-in-point is the making of composites from recycled paper. In the United States, nearly 80 million tons of 6,000 different paper and paperboard products are produced and over 70 million tons are discarded each year. Few of the paper products found in the municipal solid-waste (MSW) stream are produced solely from fiber and water. Each product consists of a fiber matrix to which some mineral or chemical compound is added to enhance the utility of the product. Thus, many forms of wastepaper contain contaminants (extraneous materials). Whether they are adhesives, inks, dyes, metal foils, plastics, or ordinary household wastes, these contaminants may need to be separated from the wastepaper before the fiber can be recycled into another useful paper product. This is not the case with wood-based composites. In many uses, wood-based composites of varying types are opaque, colored, painted, or overlaid. Consequently, recovered fibers, resins, or other materials used for composites do not require extensive cleaning and refinement. Thus, composites provide an unusually favorable option for the recycling of several highly visible and troublesome classes of MSW-paper of various types, waste wood, plastic bottles, gypsum, and biobased fibers.
A comprehensive waste management program must rely on the aggregate impact of several courses of action: waste reduction, recycling, waste-to-energy schemes, and landfill. The greatest impact likely to result from further research is in the area of recycling. Increased use of recycled biobased resources will allow the markets for fiber composites to grow without increasing the use of virgin timber. Therefore, forest products industries will benefit form such research because less expensive raw materials will be available for producing high quality composites. As part of this effort, the Forest Products Laboratory of the USDA Forest Service has formed a multidisciplinary team of government, university, and industry specialists to prepare a detailed problem analysis to focus research on composites from recycled materials.
The purpose of this report is to describe the potential for producing selected composites from wastepaper and wastepaper fiber. The use of materials that coexist with wastepaper is also considered, including a variety of plastics and other materials that "contaminate" the wastepaper resource. We first discuss the availability of waste materials from the MSW stream and the desirability of developing ways to recycle these materials into useful, high-performing value-added composites. We then briefly describe methods for making selected composites and discuss product properties and attributes. Finally, we outline research and development needs for maximizing the benefits of using recovered waste materials for composite products.
COMPOSITES FROM MSW
Many problems are associated with the use of waste materials, including collection, analysis, separation, clean- up, uniformity, form, and costs. Assuming that these problems can be overcome on a cost-effective basis, some of the resultant reclaimed materials should be useful ingredients for a range of valuable composites, from low-cost, high-volume materials to high-cost, low-volume materials for a wide range of end uses.
Recycling not only extends the life of landfills by removing materials from the MSW stream but also makes available large volumes of valuable raw materials for use by industry in place of virgin resources. Industrial use of such materials reduces both costs for raw materials and the energy required for manufacturing products. The main requirement is that the recycled ingredients meet the quality and quantity criteria of the consuming production operation.
COMPOSITES FROM WASTEPAPER, WOOD FIBER, PLASTICS, AND INORGANIC MATERIALS
Wastepaper, wood, plastics, gypsum, and other materials can be reclaimed from industrial and MSW streams and used for several kinds of composite products: wood fiber-plastic composites, dry-formed wood fiber-based composites, inorganic bonded wood composites, and composites that combine wood and paper fibers with other lignocellulosics like agricultural fibers.
The following sections are not included to be a comprehensive review of recent research on wood. The effects of some important composition and processing variables in the composite processes, including preliminary indications of the effects of recycled ingredients are describes here.
Melt-Blended Thermoformable Composites
A typical composition for a melt-blended composite is about 50 weight percent wood flour or paper fiber with a powdered or palletized thermoplastic such as polypropylene or polyethylene. In the melt-blending process, the paper fiber or wood flour is blended with the melted thermoplastic matrix by shearing or kneading. Currently, the primary commercial process employs twin screw extruders for melting and mixing; the mixture is extruded as sheets that are subsequently shaped by thermoforming into the final product. Limits on the melt viscosity of the mixture restrict the amount of fiber or flour as well as the length of the fibers that can be used. Fiber length is also limited by fiber breakage as a result of the high shear forces during melt mixing.
The 1980s brought a resurgence of research into various aspects of melt-blended composites made from wood flour or paper fiber in virgin thermoplastic matrices.
Currently, the primary application of melt-blended thermoformed composites is for interior door panels and trunk liners in automobiles. Additional large-volume, low-to-moderate cost applications are expected in areas such as packaging (trays, cartons), interior building panels, and door skins.
Nonwoven Mat Composites
In contrast to melt-blended composites, nonwoven mat technology involves room temperature air mixing of lignocellulosic fibers and fiber bundles with other fibrous materials. The resultant mixture passes through a needling step that produces a low-density mat in which the fibers are mechanically entangled. The mat is then shaped and densified by a thermoforming step. With this technology, the amount of lignocellulosics fiber can be greater than 90 weight percent. In addition, the lignocellulosic fiber can be precoated with a thermosetting resin; for example, phenol-formaldehyde. After thermoforming, the product possesses good temperature resistance. Because longer fibers are required, this product can achieve better mechanical properties than that obtained with the melt-blending process. However, high wood fiber contents lead to increased moisture sensitivity.
Brooks developed an interesting method of recycling waste cellulosic materials for the production of medium-density fiberboard and paper. After being shredded, sorted from other waste materials like plastic and metal, and steamed, the cellulosic fibers and fiber bundles are abraided under heat and pressure to break down any hydrogen bonds and to soften any lignin and other resins. The resultant cellulose fibers are then mixed with resin, formed into a mat, and consolidated under pressure to form flat fiberboard and paper products.
Researchers at FPL use nonwoven techniques to undertake a wide range of research with recycled and waste materials. These materials include wood, agricultural fibers, plastics, and paper. The recycled paper materials, which do not have the ink removed, are reduced to a suitable form using several reduction methods. The paper can be hammermilled, shredded, or reduced to fiber by refining. The reduced paper component can be readied for the non-woven process in a variety of ways. It can be coated with thermal setting resin or blended with other lignocellulosic fibers, synthetic fibers, and thermoplastic granules, or prepared by any combination of these methods. Plastic coated paper is an especially interesting material for composites because the plastic coating can serve as a binder.
Nonwoven mat technology can utilize a wide range of wastepaper reduced in form by different methods. The process also allows a variety of binders to be utilized. The resultant mats can be pressed into flat panels or molded into a multitude of shapes.
Low-Density Fiber Mat Composites
Low-density fiber mats can be defined as composites made by the non woven mat process, but not post-formed by heat and pressure. These mats have a bulk density of 48 to 240 kg/m3 (3 to 15 lb/ft3).
One interesting application for low-density fiber mats is for mulch around newly planted seedlings. The mats provide the benefits of natural mulch; in addition, controlled-release fertilizers, repellents, insecticides, and herbicides can be added to the mats as needed. Research results on the combination of mulch and pesticides in agronomic crops have been promising. The addition of such chemicals could be based on silvicultural prescriptions to ensure seedling survival and early development on planting sites where severe nutritional deficiencies, animal damage, insect attack, and weed problems are anticipated. The Forest Service is conducting preliminary research on using paper fiber and other lignocellulosic fiber mats to improve the survival of loblolly pine seedlings in southern Louisiana.
Low-density fiber mats can also be used to replace dirt or sod for grass seeding around new homesites or along highway embankments. The grass seed can be incorporated in a wood, paper, or jute fiber mat. Fiber mats promote seed germination and good moisture retention. Low-density fiber mats can be used for air filters or other types of filters. The density of the mats can be varied, depending on the material being filtered and the volume of material that passes through the mat per unit of time. The FPL is conducting preliminary work on developing wood and paper fiber mats for filters.
All of these applications for low-density fiber mats provide excellent and attractive outlets for recycled wood and paper fiber.
Paper Fiber-Based Composites
For the purpose of this paper, paper fiber-based composites are defined as composite materials consisting mostly of recycled paper fiber. They can be divided into two main categories: wet-laid composites and air-laid composites.
Wet-laid composites. Commercial paper fiber-based composites are made in a process not dissimilar to paper making. Water is used to distribute and randomize recycled paper fibers into thick mats. The fibers are interfelted in a reconstitution process and are characterized by hydrogen bonds produced by the interfelting. The composites are frequently classified as fibrous-felted board products. At certain densities under controlled conditions of hot pressing, rebonding of lignin affects a further bond in the resultant panel product. Binding agents and other materials may be added during manufacture to increase strength or resistance to fire, moisture, or decay. These materials include rosin, alum, asphalt, paraffin, synthetic and natural resins, preservative and fire-resistant chemicals and drying oils. Wax sizing is commonly added to improve water resistance.
There is a great opportunity to produce fiber-based composites of varying densities from recycled paper fibers. One family of products, called Homasote, was first produced in 1916 and is made from old newspapers and other ground-wood paper. Other fiberboard-type products now on the market also use all or partly recycled wood fiber as the raw material base stock. Uses for these types of products include insulating acoustical board; carpet board; wall, ceiling, and floor acoustical insulation panels; nail baseboard; and floor and roof insulation boards. We anticipate that many other uses for paper fiber-based products will be developed as collection, separation, and clean-up processes are further refined and developed.
Air-laid composites. Air-laid composites differ from wet-laid composites in that air is used to distribute and randomize the fibers rather than water. As such, little or no hydrogen bonding takes place and other means are necessary to bind the fibers. Fiber bonding is usually accomplished by using a thermal setting resin. One research project recently completed in this area at FPL was the investigation of air-laid, dry-process hardboards made using old newspapers.
Dry-process hardboards are generally made from softwood fiber and a thermal setting resin. It was thought that the hardboards represent a favorable option for recycling old newspaper fibers. The objective of the research was to determine the effects of various wood fibers to old newspaper ratios (100:0, 50:50, and 0:100) on the mechanical and physical properties of hardboards made from these fibers. Resin was applied at the 3% and 7% levels. Boards were tested for static bending, tensile strength, dimensional stability, and water resistance. As expected, increasing the resin level generally improved all measured properties. Increasing the amount of old newspapers caused a corresponding deterioration in both mechanical and physical properties. Even with the reduction of properties caused by the addition of old newspapers, hardboards made from a 50:50 ratio containing 7% resin equalled or exceeded all American National Standard Institute-American Hardboard Association standards. Based on this research, it is apparent that at least part of the normal hardboard furnish could be supplemented with old newspapers.
Another research project at FPL was prompted by a recent landmark conference on paper recycling (Focus 95+) in Atlanta, Georgia, in which three types of paper were identified as the hardest to reuse for paper products; unsorted mixed office paper, colored paper (the type used for newspaper ads), and magazine paper. The research study explored the properties of fiberboards made from these papers. The papers were hammermilled into small fragments and a thermal setting resin was applied. The resulting blend was pressed into fiberboard panels for testing. The results showed no significant differences between boards made at equivalent levels of adhesive. There is not enough data at this time to draw any conclusions as to the effect of the clay on properties of boards produced in the study.
Wet-Formed Wastepaper-Polyethylene Composites
Ongoing research at FPL is exploring the feasibility of using a wet-laying method for producing paper pulp-polyethylene composites. The advantages of the wet-forming process when compared to either the melt-blending method or the dry-forming process are (a) wet-forming maintains the length of the individual wood fibers (melt-blending shortens the fibers) and (b) wet-forming should result in good interfiber hydrogen bonding, a phenomenon that occurs primarily in paper and paperboard manufacture and to a very small extent in dry processes.
In the wet-forming process, mats of various blends of paper pulp and granulated polyethylene are formed on a screen. The mats are typically about 25-mm (1-in.) thick and have a good dispersion. Several additives, including a debonding agent and a nonionic polymeric surfactant, are being tested for their ability to compatabilize the cellulose-polyethylene system. These additives have been noted to have an immediate effect as surfactants, resulting in reduced foaming and increased dispersion during the mixing process. Research in this area is continuing. Various ratios of paper pulp to polyethylene are being examined, as are various amounts of additives.
Structural Paper Products
Researchers at FPL have developed a structural fiber concept called FPL Spaceboard. The production of Spaceboard is generally undertaken in a wet-forming process not dissimilar to paper making. To make the three-dimensional structural board, wood or paper fibers are suspended in a wet slurry, water is removed via vacuum, and the fibers are press dried against rubber molds with wafflelike configurations to produce two symetrical halves. An adhesive is then used to bond the two halves, creating numerous small cells in the center of the board.
Using this technique, Spaceboard can be made as a laminate or a sandwich, thin enough for strong lightweight corrugated containers or thick enough for wall sections. The result is a fiber composite structural material that is strong in every direction.
Laboratory tests show that Spaceboard is between 30% and 200% stronger than conventional corrugated fiberboard. Its strength is due to the special configuration of the core and the superior strength imparted by the press-dry method that molds the core and facing together.
With further refinement, Spaceboard can provide the wet strength and dimensional stability necessary to build highly engineered structures as well as significantly improved fiberboard containers. Its superior performance parameters are: improved strength and weight characteristics of press-dried fiber facings; unequaled versatility in mechanical design variables (sheet size, cell size, sandwich thickness, and core density); and adaptability to wide range of raw materials.
Spaceboard is a concept in structural fiber construction and, as such, its full potential for application and its economic payoff are just being determined. Spaceboard is currently licensed for packaging and structural applications. It is an excellent outlet for recycled paper fiber, and like other composites described in this report, the paper fiber requires little or no cleaning and deinking.
Inorganic Bonded Wood Composites
Wood particles or paper fibers held together with an inorganic matrix, such as Portland cement or gypsum, form a composite that can be used in a variety of structural and industrial applications. These composites have a unique advantage over some conventional building materials because they combine the characteristics of both the wood fiber and mineral matrix. Some of these composites are water resistant and can withstand the rigors of outdoor applications, and almost all are either fireproof or highly fire-resistant and are very resistant to attack by decay fungi.
A unique feature of inorganic bonded composites is that they are adaptable to either end of the cost and technology spectra for building materials. In the Philippines, cement board is primarily fabricated using manual labor and is used in low-cost housing. In Japan, the fabrication of cement board is automated, and the board is used in very expensive modular housing.
These types of composites, which provide another major recycling opportunity to utilize waste wood, wastepaper, and wastepaper fiber, are made by blending proportionate amounts of the wastes with inorganic materials. The most apparent and widely used example is cement. Portland cement, when combined with water, immediately begins to react in a process called hydration to eventually solidify into a solid stone like mass. The second most-used inorganic binder is gypsum. The gypsum can be mined from natural supplies or derived from flue gas. Gypsum is commonly used in the manufacture of one type of drywall.
Cement board. In general, cement board is a 50/50 mixture of cement and wood or other lignocellulosic fiber. The material is suprisingly lightweight,is easy to nail and saw, and has excellent insulation properties. In addition, cement board is very resistant to moisture, rot, and insect and vermin attack.
Cement board should be of high interest to individuals in the pulping industry. One primary solid-waste problem for that industry is sludge. Generally, sludge is either landfilled, used as a one-time soil conditioner, or burned. Burning may not be the best option because of the high moisture content and low fuel value of sludge. Scientists at FPL believe that sludge can be used in cement board and that its high moisture content will be advantageous.
Sludge could be used in two ways. The first would be to make a conventional cement-bonded composite and substitute a portion of the lignocellulosic content with sludge. A second strategy involves the production of a three-layer board. The center layer would be a low-density composite with quite large wood elements. The outer layers, or faces, would be a high-density mixture of sludge and cement that would be smooth and amenable to paint or other finishes.
Some advantages of using sludge in cement board are (a) it would provide an outlet for mill sludge, (b) it could eliminate landfilling or burning of sludge, and (c) it would extend the forest resource if the sludge-cement board were combined with recycled wood products.
While interest in this technology is growing in Europe, the Philippines, and Japan, the United States is just starting to utilize the excellent properties of cement board. Several companies have started to manufacture roof shingles, shakes, and slates made from cement and paper fiber. Some of the composite slates have 60-year warranties.
Gypsum bonded composites. Another inorganic material that can be used to make composites is gypsum. Gypsum can either be derived by mining from natural sources or can be obtained from flue gas. Flue gas gypsum, now being produced in very large quantities because of Clean Air Act regulations, is the result of introducing lime into the combustion process to reduce sulphur dioxide emissions. Flue gas gypsum can be used in lieu of mined gypsum.
Gypsum bonded wood and paper fiber panels are used as replacements for gypsum wallboard and are reported to have strong nail-and screw-holding properties, high moisture and fire resistance, and improved impact, mold, and mildew resistance. Other reported advantages include improved anti-sag properties (for ceiling boards), better sound insulation, and easy installation (joints do not require taping).
The combination of wood and paper fibers with inorganic binders provides a unique opportunity to utilize recycled, waste, and low-grade wood fiber. Research has clearly indicated that inorganic bonded wood composites can meet structural and industrial needs.
Recycling is a critical element is the long-term management of renewable resources. A successful approach to recycling requires full cooperation between the government and the private sector. Government cannot logically mandate the increased use of recyclable materials without the involvement of industry-the industrial sector has the technical knowledge and equipment to separate and process solid waste and to make useful, economically viable products from waste materials. Industry provides the market for recycled resources, and it must be a full partner in all aspects of the process.
We believe that using recovered wood and paper fiber for bio-based composites, like those discussed in this report, presents tremendous opportunities for growth, for progress, and for further industry competitiveness in a world that is rapidly consuming many nonrenewable resources at an ever-increasing rate.