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Muttana Suresh Babu, Gudavalli Srikanth &Soumitra Biswas
Abstract
Due to legal provisions against deforestation besides a shortage of forest timber, the world has been looking at various options for wood substitutes. The current research trends focusing on thermoplastics, utilising natural fibres compounded with a thermoplastic resin (polyethylene, polypropylene, PVC etc.) have yielded excellent wood substitutes. Compared to conventional wood, thermoplastics exhibit improved product features.
The product utilizes natural fibres/fillers & thermoplastic wastes which on intimate compounding produce pellets. These pellets are further extruded into various shapes, forms & profiles.
This new class of materials is being accepted and utilized for wide-ranging applications in building & construction, interiors & internal finishes, automotives, garden, outdoor products, industrial & infrastructure and other niche applications in the developed countries. The paper discusses in detail about the raw materials, chemical modifications necessary, process technology, range of product possibilities, properties of wood-plastic composites and market scenario.
1.0 INTRODUCTION The term 'composite' in the building materials industry is often used in a broad sense to describe woody materials bonded together by adhesives covering the entire gamut of plywood, oriented strand board, wafer board, particle board, fibre board and other types of panelling products.
Thermoplastic composites, which combine wood flour or other natural fibres/fillers with thermoplastic matrices, were introduced to the US market in the early 1990's. Such natural fibre-thermoplastic composites find a wide array of applications in the building & construction industry such as door & window frames, decking material, railings for the parapet wall systems, furniture sections (park benches etc.) and others.
These can also cater to a number of industrial applications. All such products are fabricated by extrusion through properly designed dies. Thermoplastic composite is consistent & uniform in properties due to intimate compounding of resin & natural fibre/filler.
While positioned against wood, they score much better in terms of dimensional stability, no water absorption and hence, no swelling in moist weather, better fungal resistance, reduced thermal linear expansion etc. The workability of the composite is also quite good in terms of its ability to be sawn and nailed or screwed by conventional methods.
2.0 BACKGROUND Historically, mankind has been familiar with several applications of composite materials for housing & building construction. Search for stronger & stiffer fibres and feasibility to utilize widespread inorganic wastes and by-products have provided directions to the production and use of natural fibres such as coir, jute, sisal etc. as quite inexpensive and effective reinforcing fibres and hydraulic binders as alternative to conventional one.
In the recent times, the world has seen a total transformation in materials technology and a vast variety of industrially produced composites have come into existence. These developments have mainly taken place to meet housing & building requirements of ever growing world population, particularly in the developing world.
The use of natural fibre as reinforcing elements for thermoplastics is gaining wide acceptance for various building & construction applications since several decades.
The first attempt was made in 1983, by American Wood Stock (now part of Lear Corporation in Sheboyan, Wisconsin, USA) in developing a composite automotive interior substrates/panels with 50% wood flour compounded in polypropylene resin matrix by using Italian extrusion technology.
Later in 1990s, some of the US companies began experimenting with 50% wood flour compounded with polyethylene resin matrix, thereby producing various thermoplastic composite products. These products were used as deck boards, landscape timbers, picnic tables, industrial flooring etc.
Further in 1993, there has been a dramatic increase in the utilization of wood thermoplastic composites. During that period, the products such as subsils for doors, windows, door profiles etc. entered the market. Continuous progress in materials engineering has helped in achieving the perfection in behaviour of materials. Composition of various constituents of materials in a tailor made form has resulted into improved product properties.
3.0 CURRENT SCENARIO Thermoplastic natural fibre composites enjoy very good market demand in Western countries. In fact, demand for wood composites has grown by 35-40 % over the last five years. A recent study by Principia Partners (Exton, Pa., USA) has an estimated business volume of over US $600 million for 500,000 tons of composite material being consumed in the US market alone.
There exist over 50 companies in US producing & marketing wood composite products. Future projection for the material has been estimated at US $2.0 billion for North America & Western Europe by 2007. In the present scenario, the market for decking, window & door profiles has been continuously growing.
Most of the research has been undertaken by focusing on wood as a reinforcing element in thermoplastic composite; other locally available natural fibres/fillers need to be explored for applications development.
4.0 PROCESS TECHNOLOGY The wood-thermoplastic composite products are fabricated by compounding wood flour (or natural fibres) in a thermoplastic resin along with appropriate additives into pellets and subsequently extruding the pellets into various profiles. The process involves, raw material preparation, compounding, profile extrusion, tooling, cooling and finishing. In general, wood flour is made from sawdust, planar shavings, sanding dust and scraps produced in wood recycling industries. Hardwood, softwood, even MDF waste are being used. Other cellulose sources may be used as well, namely, straw, flax, rice hulls, peanut hulls, coir, jute, bamboo dust, kenaf etc.
The most commonly used thermoplastic resins are polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC). Additives such as coupling agents, UV stabilizers, pigments, fire retardants, lubricants, fungicides and foaming agents are added for increased performance level of the finished product.
Compounding is the first stage of manufacturing thermoplastic composite products. It is a process of feeding and dispersing of fillers and additives in the molten polymer resin - this can be done either in batch (i.e. internal & thermo kinetic mixers) or continuous mixers (i.e. extruders, kneaders). Batch compounding system has few advantages over continuous one, where the processing parameters such as residence time, shear and temperatures, could be controlled very easily for better quality & performance of the end product. However, continuous compounding system provides uniform quality throughout. The compounded material can either be immediately extruded into final form of the end product or made into pellets for future processing.
Many of the industries now prefer to produce a pelletized feedstock-this enables for further processing easily by other forming techniques (i.e. extrusion, injection and compression moulding), for obtaining the desired shape of the product. Extrusion produces continuous products/profiles. However, injection & compression moulding techniques are used for producing complicated shapes/pieces with shorter length.
Extrusion is a continuous manufacturing process of heating, compressing and conveying a premixed blend of raw ingredients. The process encompasses shaping of the material by the extrusion dies, cooling of a product upon its exit from the die and down stream handling including cutting, embossing, sanding and other treatments.
This process saves the cost of machining and fabrication. Shape of the final product obtained would be replication of the die opening. The products/profiles made by this process can be cut into small pieces, moulded, routed, sanded etc., with the same tools, feeds and speeds as with wood.
Two constituents of the product i.e. wood and plastic have great synergy in nature: plastics expand & contract due to temperature changes and wood expands & contracts with humidity changes. Adding wood to plastic significantly decreases thermal linear expansion, and the plastic mitigates moisture movement into the part.
Because of the low degradation temperature of wood, only thermoplastic resins that melt or can be processed at temperatures below 2000 C are commonly used in the fabrication of wood thermoplastic composites.
There are various types of extruders presently available viz. single-screw extruders, counter-rotating twin-screw extruders and custom extruder set-ups. A single-screw system is the least expensive type, but typically uses pre-compounded pellets, an added processing step that adds cost, and overall output is lower.
Twin-screw extruders are preferred for wood products because the mixing action blends the polymer and the wood filler together in a single process, with low screw speed and low-shear mixing, which keep temperatures down. By using low viscosity resins temperatures within the screw could be brought down by reducing the shear forces created during mixing.
Advanced Engineered Wood Composites Center of University of Maine, Orono, USA has developed a system called "Woodtruder", which comprises a counter-rotating type twin extruder along with a separate single-screw extruder. This keeps wood feeding and drying separate from polymer melting until well down the barrel. Strandex Corporation, Madison, Wisconsin, USA has been a major player in the licensing of wood composite technologies. The Strandex process uses a hot die mounted on to the extruder thus eliminating the need for hydraulic pullers, calibrators and cooling jackets.
The extruder profile is cooled by water spray as it exits from the die. As the profile shape remains intact while cooling, cost of machining is avoided. The profiles are run through a brushing tool while still warm to achieve a textured appearance.
However, selection of screw design & process equipment would depend on various factors such as raw materials (polymers, natural fibres, additives), processing parameters, end-use of the product etc. 
4.1 Raw Materials
4.1.1 Wood Flour: Wood flour is a finely ground wood cellulose. When the mesh size is above 20 mesh or below 850 microns, the product generally is considered to be wood flour. Mesh size is the measurement of number of openings in a screen per linear inch.
The collected wood flour from various sources (sawdust, planar shavings, sanding dust and scraps) are hammer milled to form very fine powder, classified by the standard mesh size that it can pass through. Most wood-filled thermoplastic manufacturers specify flour in the 30-80 mesh range. Bulk density of wood flour is relatively higher compared to wood fibres. The moisture content of wood has a significant effect on the processing and final composite product quality. Pre-drying the wood flour to less than 1% moisture content could be very helpful.
Wood flour with less moisture content is less likely to burn during compounding with thermoplastics. The chemical & physical properties of hard wood flour are given in Table 1.0. The particle size for other natural fibres/fillers such as pine needles, maple, oak, bamboo dust, jute and coir vary from 10 - 80 mesh. Table 1.0: Hard Wood Flour- Chemical & Physical Properties
(Source: PJ Murphy Forest products Corporation, Montville)
4.1.2 Thermoplastic Resins: The choice of resin being used in thermoplastic composites depends on many factors. As wood has a tendency towards degradation at higher temperatures, the thermoplastic resins that can be processed below 2000C, should be selected for processing of composites. The most commonly used thermoplastic resins are PE, PP and PVC.
PE is the cheapest one and has excellent toughness and impact strength, but the lowest in service temperature. The resin can be used both in virgin & recycled form. All polyethylene grades (i.e. LDPE, MDPE, HDPE) are used for the manufacture of thermoplastic composites.
These types of resins have tendency to stabilize easily against thermal degradation during the process. At present, many industries are using PE as resin matrix for fabricating exterior building components.
PVC is the strongest resin with the highest service temperature, but is the most brittle, measured by percent elongation. It has high stiffness capability, mechanical strength & weather resistance property. PVC resins are being used in thermoplastic composites for manufacture of window panels and now being used in decking as well. PP has good use temperature characteristics and lowest density compared to PE & PVC. The thermoplastic composites made of wood polypropylene are best suited for automotive applications and consumer products and these products have recently been investigated for use in building profiles. 4.1.3 Additives: Thermoplastic composites contain not only the natural fibres/fillers and thermoplastic resin alone, but also many other additives in smaller quantities in order to bring better performance for the final product.
The additives such as coupling agents, pigments, lubricants, UV stabilizers, fungicides and foaming agents can be used for improved performance during the processing of thermoplastic composites. The main functions of additives in thermoplastic composites are given under:
In general, the wood filler increases the stiffness of thermoplastic but decreases the impact strength of the composite. The addition of coupling agents and compactabilizers helps improve the inherently poor bonding between the hydrophilic wood filler and the hydrophobic polymer and can help recover some of the impact strength.
Adding lubricants in thermoplastic composites, helps in increasing the flow of raw material blend, dissipation of heat generated during the process, reducing the viscosity of the blend at higher shear rate, promoting the dispersion of natural fibre fillers in the resin matrix and would also help in reducing the adhesion between resin & screw of the extruder, friction between resin & process equipment.
Colour pigments, heat & light stabilizers are also added in thermoplastic composites for improving material properties. Most of the research has been focused on maleated polypropylene (MAPP) and maleated polyethylene for its use in thermoplastics as compactibilizers for better results. The additives used in wood-filled thermoplastic composites are summarized as follows (Table 2.0) :
Table 2.0: Functions of Additives used in Thermoplastic Composites
4.2 Chemical Modification
The chemical constituents for most of the natural fibres, such as wood, coir, jute, flax, sisal etc., are cellulose, hemicellulose and lignin. They have certain drawbacks when they are exposed to adverse/ aggressive environmental conditions. They are very much prone to swell/warp & shrink when exposed to moist & hot weather conditions respectively. They also burn, decay and degrade by acids, bases and ultraviolet radiation. In order to develop composites with better mechanical properties and environmental performance, it is necessary to impart hydrophobicity to the fibres by chemical reaction with suitable coupling agents or by coating with appropriate resins. Such surface modification of natural fibre would not only decrease moisture absorption, but would also concomitantly increase wettability of fibres with resin and improve the interfacial bond strength, which are critical factors for obtaining better mechanical properties of composites. 4.2.1. Degradation of Natural Fibres The natural fibres absorb moisture as the cell wall polymers contain hydroxyl and other oxygenated groups that attract moisture through hydrogen bonding. The hemicelluloses are mainly responsible for moisture sorption in the natural fibre, but the other non-crystalline cellulose, lignin also play major role in this. Chemical properties of some natural fibres are shown in Table 3. In general, any natural fibre absorbs moisture to certain level i.e. Fibre Saturation Point (FSP). Absorption above or below the FSP causes swelling & shrinking of the fibre - this leads to dimensional instability in the final composite product made of the natural fibre as reinforcing element. When natural fibres are exposed to outdoors, they undergo photochemical degradation caused by ultraviolet radiation. The degradation takes place primarily in lignin component, which is responsible for the characteristic color changes. As the lignin degrades, the surface becomes richer with cellulose content - this results in rough surface of the composite and also accounts for a significant loss in surface fibres. Table 3.0 : Chemical Properties of some Natural Fibres
(Source: Catalogue of Building Materials & Technology Promotion Council (BMTPC), New Delhi)
4.2.2. Chemical Modification of Natural Fibres Chemical modification can be defined as a chemical reaction between some reactive parts of the constituent of the natural fibre and chemical reagent, with or without a catalyst, to form a covalent bond between the two. As the natural fibres result from the chemistry of cell wall components, modification of chemistry of the cell wall polymers can change the basic properties of a fibre.
The chemicals to be used for chemical modification must be capable of reacting with lignocellulosic hydroxyls under neutral, mildy alkaline or acid conditions at temperatures below 1500C. The chemical system should be simple and capable of swelling the structure to facilitate penetration.
The complete molecule should react quickly with the lignocelluloses components yielding stable chemical bonds and the treated lignocelluloses must still possess the desirable properties of untreated lignocellulosics. The hydrophobic nature of reagent should be selected in this aspect.
Many chemical reaction systems have been investigated for the modification of natural fibres. The chemicals include anhydrides such as phthalic, succinic, malaic, propionic and butyric anhydride, acid chlorides, ketene carboxylic acids, different types of isocyanates, formaldehyde, acetaldehyde, difunctional aldehydes, chloral, phthaldehydic acid, dimenthyl sulphate, alkyl chlorides, beta- propiolactone, acrylonitrile, epoxides such as ethylene, propylene & butylenes oxide and difunctional epoxides. Most of the research has been done on the reaction of acetic anhydride with cell wall polymer hydroxyl groups to give an acetylated fibre. Different types of natural fibres, such as wood, bamboo, bagasse, jute, kenaf etc. have been acetylated using a variety of procedures. Acetylation is one of the chemical modification procedures for natural fibres; there could be other procedures, which can be looked into. 5.0 PRODUCT PROPERTIES Thermoplastic composites exhibit better chemical & mechanical properties when compared to conventional wood. These tailor-made composites have better dimensional stability and fungal resistance when exposed to moist weather and they exhibit low thermal linear expansion at adverse environmental conditions.
These composites have an ability to create more complex shapes both on-line and in subsequent forming operations. These products could be painted, screwed, nailed with same conventional tools used for wood.
The other features such as low cost, aesthetic elegance, environmental acceptability and durability in the long run are considered to be attractive features for opting these composites as alternatives. Various extruded composite profiles are shown in Fig. 3.0.
However, the mechanical properties of end product depend on the factors such as raw material selection, formulation, manufacturing technique & processing parameters. Increasing wood flour content & its particle size would also affect the performance of the composite. The comparison of mechanical properties of wood flour-polypropylene composites with hardwood fibre composites is shown in Table 4.0. Table 4.0: Mechanical Properties of wood-polypropylene composites*
(Source: Forest Products Journal, June 2002,Vol. 52,No: 6, Pages: 10-18)
 
6.0 APPLICATIONS & MARKET
Typical applications of thermoplastic composite products in various sectors are listed in the Table 5.0. Table 5.0 : Applications of Thermoplastic Composites
In developed countries, the demand scenario has recorded substantial growth for thermoplastic products, especially for building & construction applications that have limited structural requirements. Products such as window & door panels, lumbers have already been inducted & widely used in the US market.
Other applications of thermoplastic composites are also expected to growth significantly in the near future. Efforts are underway in European countries for commercial exploitation of this technology.
The future market for thermoplastic composites has been estimated at US $2.0 billion for North America & Western Europe by 2007. Worldwide, around 100 companies have been identified till now, who are involved in manufacturing of thermoplastic composites.
Majority of these are based in the USA and Japan. It is expected that the market would take off imminently, in the other parts of the world and would provide significant opportunities for industries interested to adopt the technology.
 
7.0 CONCLUSION
Utilization of natural fibres/thermoplastic waste as reinforcing element in thermoplastic composites, in a cost effective manner, has become an attractive feature for developing such composites for a wide gamut of applications.
These composites assume importance especially due to its capability of utilizing the natural fibre resources such as wood flour, rice husk, bamboo dust, jute, flax, coir, sisal, plastic waste etc. as reinforcing elements.
The mouldability of plastic allows complex product design. In addition, the process of pelletization & extrusion is environmentally benign as waste biomass and thermoplastics can be used to make these products.
This technology & products have widely been demonstrated & utilized in many developed countries. There exists flexibility for manufacturers/developers for choosing appropriate raw materials, compactibilizers/ coupling agents, process technology, process equipment, process parameters for developing products for specific applications. As India is endowed with abundant natural fibre resources and good expertise in polymers, exploring this technology would go a long way in creating new business opportunities in this area. In view of the crucial need for developing indigenous capability in composite technology, the Advanced Composites Programme was launched by the Technology Information, Forecasting & Assessment Council (TIFAC), Department of Science & Technology (Govt. of India).
The programme has been an attempt to enhance the utilisation & application of composite as an important performance material in various sectors and to improve upon the laboratory-industry linkages towards development & commercialisation.
The programme has been an experiment to bring about a culture of development especially for the technology starved SMEs. Poised with the successes of developing natural fibre based composites such as jute-coir composite boards, jute-thermoplastic composites for shoe components, bamboo composite laminates etc., the Advanced Composites Programme is seriously contemplating indigenous development & productionization of thermoplastic composite technology in partnership with an industry.
Serious development efforts would be necessary to explore the suitability of indigenous biomass viz. bamboo, jute, coir, pine needle etc.
References: 1. John A Youngquist (1995), "Unlikely Partners? The marriage of wood and nonwood materials", Forest Products Journal, Vol. 45, No.10, pages 25-30 2. Sara Black (2003), "Composites do wood one better", Composite Technology Magazine, issue: June 2004, page no: 18-21 3. Craig Clemons (2002), "Wood-Plastic Composites in the United States", Forest Products Journal, June 2002, Vol. 52, No.6, pages 10-18 4. Brent English (1996), "Wood Fibre-Reinforced Plastics in Construction" Proceedings of Conference on the use of Recycled Wood & Paper in Building Applications, Wisconsin, September 1996, page No: 79 - 81 5. Donna A Johnson, Rod Jacobson, W. Dan Maclean, " Wheat Straw as a Reinforcing Filler in Plastic Composites", The Fourth International conference on Woodfiber - Plastic Composite, page nos: 200 - 205 6. A C Karmaker, J A Youngquist (1996), " Injection Molding of Polypropylene Reinforced with Short Jute Fibers", Journal of Applied Polymer Science, Vol.62, page nos: 1147 - 1151 7. Alcides L Leao, Rowell, Nilton Tavares (1998), "Applications of natural fibres in Automotive Industry in Brazil - Thermoforming Process" Science and Technology of Polymers and Advanced Materials, page nos: 755-760 8. A K Rana, A Mandal, B C Mitra, R Jacobson, R Rowell, A N Banerjee (1998), Short Jute Fiber- Reinforced Polypropylene Composites: Effect of Compactibilizer, Journal of Applied Polymer Science, Vol.69, pages 329-338 9. Roger M Rowell (1995) " Chemical modification of Agricultural Fibres for Property Enhanced Composites", Proceedings of a seminar; 1995 May; Copenhagen, Denmark Academy of Technical Sciences, Page Nos: 49-70 10. Chris Edwards (2001), "Thermoplastic composites -Opportunity or Threat", COMPOSITES 2001Convention and Trade Show, Composites Fabricators Association, Tampa, USA, 11. Nicole M Stark (1997), " Effect of Species and Particle Size on Properties of Wood-Flour- Filled Polypropylene Composites", Proceedings of Functional Fillers for Thermoplastics and Thermo sets. 12. Research Report (2003) on 'Wood Plastic Composites Study - Technologies and UK Market Opportunities, The Waste and Resources Action Programme (WRAP), Banbury, UK
For further information, please contact Mr. S. Biswas at advcomp@tifac.org.in
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For further details log on website :
http://www.tifac.org.in/index.php?option=com_content&id=535:thermoplastic-composites-technology-a-business-opportunities&catid=85:publications&Itemid=952
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