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Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste Venice, Italy; 8-11 November 2010 2010 by CISA, Environmental Sanitary Engineering Centre, Italy

THE USE OF POST CONSUMER WOOD WASTE FOR THE PRODUCTION OF WOOD PLASTIC COMPOSITES: A REVIEW

S. MEHMOOD*, A. KHALIQ** AND S.A. RANJHA°

* Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark ** Department of Metallurgical and Materials Engineering, University of Engineering and Technology Lahore, Pakistan °Department of Mechanical Engineering, University of Engineering and Technology Lahore, Pakistan

SUMMARY: The idea of using natural cellulose fibres as reinforcement in composite materials is not a new or recent one. It has been a part of man’s technology since the first ancient builder used straw to reinforce mud bricks. Over the past a few years, renewed interest in using the natural cellulose fibres to reinforce the thermoplastics is due to increasing cost of plastics, and also because of the sustainable aspects of using renewable and biodegradable materials. Such fibres come from natural sources: trees and crops. Cutting of trees at higher rate and increased use of wood in furniture, paper and many other industries are the factors due to which it is difficult to use trees as a source of natural fibres for reinforcement of plastic materials. The aim of this study is to find some alternative sources of natural fibers from post consumer wood waste and how they will be processed. Post consumer wood wastes such as: primary and secondary wood wastes, municipal solid waste, paper waste, paper sludge and agricultural crop residues are very attractive for producing fibers. They can be processed into a usable form by mechanical milling and chemical treatments. Some of these waste materials are already being utilized for some applications so there is a lot of competition among various applications of these waste materials.

1. INTRODUCTION

The idea of using natural cellulose fibres as reinforcement in composite materials is not a new or recent one. It has been a part of man’s technology since the first ancient builder used straw to reinforce mud bricks. Over the past a few years, renewed interest in using the natural cellulose fibres to reinforce the thermoplastics is due to increasing cost of plastics, and also because of the environmental aspects of using renewable and biodegradable materials.

Wood wastes produced during manufacturing of wood base products such as paper, cardboard, crates, pallets, furniture and the by-products of textile industry could serve as reinforcement in matrix of plastic (Hughes, 2004). This depends on suitability and availability of these wastes. The aim of this study is to analyse various important categories of post consumer wood waste (see Figure 1) across the Europe in order to find appropriate source of wood or any other cellulosic based material to reinforce thermoplastics matrix.

Venice 2010, Third International Symposium on Energy from Biomass and Waste

Figure 1. Various categories of wooden wastes.

2. METHODOLOGY

2.1 Primary and secondary wood wastes

There are three main processing stages for wood i.e. harvesting, primary and secondary processing. 50-60% of the total material produced at the harvesting stage can end up as waste in the form of bark, branches, leaves, rejected trees, roots, stumps, thinnings, and top wood (REMADE, 2002). This type of wood waste is too much contaminated with dirt and soil to be used for wood plastic composites.

Primary and secondary wood wastes i.e. post industrial wood wastes from saw mills in the form of saw dust and from wooden product manufacturers in the form of wood chips, cuttings, shavings and off-cuts are one of the best for the production of wood plastic composites.

At the sawmill stage 70 - 80% of the input log is wasted and up to 60% of the saw timber ends up in waste when saw timber is converted into useful products such as furniture and wooden pallets (REMADE, 2002). These wood processing wastes are relatively uncontaminated and have a high degree of traceability (WRAP, 2003). The Table 1 summarizes the types of residue produced by wood processing operations. A lot of end users for this type of wastes already exist such as: wood product panel producers, paper industry, bioenergy, garden and other outdoor landscaping applications, composting, mulch, liquid absorbent, animal bedding and cat litter (REMADE, 2002). As a result there could be a lot of competition for this material. It would be advisable for potential manufacturers of wood plastic composites considering sawdust and wood chips as a feedstock to negotiate a contract with a supplier to ensure consistency of supply to agree appropriate prices (WRAP, 2003).

2.2 Municipal solid waste (MSW)

The main source of post consumer wood waste is municipal solid waste. The sources of wood in MSW include furniture, miscellaneous durable goods (e.g., cabinets for electronic equipment), wood packaging (crates, pallets), windows and materials for construction and some other miscellaneous products (UEA, 2005). These post consumer wood wastes have been successfully used in North America for the production of wood fibre or flour for wood plastic composites (Chelsea centre, 2000). This is a homogeneous source of wood but requires extensive cleaning and preparation due to the associated contaminations (Chelsea centre, 2000).


Table 1 - Common sources and types of wood processing wastes (REMADE, 2002).

In the EU, annual waste wood from MSW accounts for about 35 million tonnes (mainly from construction and windows but also from packaging, furniture …) (UEA, 2005). Only10% of waste wood from MSW is recycled into panel, boards and low value products such as mulch and animal litter (UEA, 2005). The large majority of wood ends either in incineration plants with or without energy recovery or in landfills due to presence of preservative chemicals which is the most commonly reported barriers to the use of reclaimed and/or recycled wood and wood products (WRAP, 2003).

Dr Pascal Kamdem at Michigan State University (2005) has reported the use of contaminated waste wood for wood plastic composite products. However, health concerns during processing and environmental durability need to be addressed. By using sealed equipment for chipping (air and particles) these concerns can be solved. However it is short term solution because end of service life of the composites is a concern and needs attention. If such concerns are properly addressed then composite material can be recycled again and again with no significant loss of properties (English, 1994) and it does not require tree falling for need of wood (UEA, 2005).

However the situation for 10% recyclable wood (homogeneous and clean) is quite different and is used to make panel, boards and some lower value products such as mulch and animal litter (UEA, 2005). Products such as mulch and animal litter have lower position in the product hierarchy than the original waste wood. Going down the hierarchy is easy, but fresh perspectives are needed when moving up the product hierarchy (English, 1994). One approach is to combine wood with thermoplastics to make a wood composite, which has novel, beneficial properties.

2.3 Paper

Post consumer waste paper for recycling can be collected from different sources. Usually the sources are divided into household waste and industrial waste. The amount of recycled paper from industry is about 52%, from offices about 10% and from households 38% (Peltola, 2004). The biggest source of recycled paper is industry. Recycled paper from industry consists of cardboard boxes, corrugated papers, unbleached papers, scrap paper, and unsold magazines and newsprints whereas recycled paper from household consists mainly of newsprint, envelopes, copy paper, magazines, cardboard and corrugated board etc (Peltola, 2004). Some important sources of post consumer waste paper along with their end users are shown in the Table 2.


Table 2 - New life of paper and cardboard (Paperinkerays, 2005).

Repulping and processing decreases fibre length and, therefore, some virgin pulp is always required to maintain the strength and quality of the finished product (DTI, 2002). A recovered fibre can be recycled only about five or six times due to progressive deterioration of its length. Concern was also expressed that long term use of recycled fibres may be unsustainable, as the overall quality of fibre stock would progressively deteriorate, and eventually a high input of virgin fibres would be necessary to restore fibre quality and strength (REMADE, 2000). As a result around 85% of waste paper comprises re-usable fibre, and 15% is waste and this means that 6.705 Mt of unusable material from waste paper arises in CEPI (Confederation of European Paper Industries) paper mills annually and in UK this amount will be 0.65 Mt, which incurs a disposal cost (REMADE, 2000).

Another problem is that of waste paper merchants exporting to the continent and Asia when prices are favorable, leading to domestic waste paper shortages (Remade, 2000). Export of pulp from CEPI countries to other regions of the world rose to 1706 000 tonnes in 2004, compared to 927 000 tonnes in 2000 (CEPI, 2004a). The legislation put by EPCI countries on the export of such waste paper to other regions can assure availability of such paper waste within the countries and this will be a consistent source of feed stock for wood plastic composites (WPCs) along with those paper fibres which are otherwise need to be disposed or used in low quality products due to deterioration of fibre length.

However remarkable increase in water contact angle of Kraft pulp fibre is reported by increased recycling of paper which makes the recycled fibre clearly much less hydrophilic than virgin fibre (Okayama and Takayuki, 2002). Due to decreased hydrophilic nature of fibres, dispersion and coupling agents required for incorporation of fibres into polymer matrix will not be required at all or adding them in polymers will have little effect on properties of the composites. Though due to decreased fibre length, the paper fibres will not be able to provide true reinforcement in wood plastic composites but they will be able to act as a filler.

Scientists at University of Wisconsin have reported the technology that can use post consumer waste paper fibres for wood plastic composites (USDA, 1996). Although most of the research has been focused on old newspapers, many other grades of paper, like mixed office waste and


bulk mailing could also be used. From the paper perspective, the fibres do not require the cleaning methods needed for paper-to-paper recycling. No sludge is produced, no waste water needs to be treated, and no need for deinking. So it can be concluded that use of paper fibres for reinforcement of polymers is more sustainable. However the availability of the used papers source which otherwise are being utilized by paper industry for the purpose of plastic reinforcement depends on demand of wood plastic composite industry.

2.4 Paper sludge

Paper and pulp industries either using virgin wood or white recycled papers as a raw material produce a byproduct known as paper sludge. The sludge is composed of fibre (typically about 50-60 % on a dry weight basis) and the remained being minerals and ash (WRAP, 2004). However paper mills using recycled paper as a raw material produce more sludge than those using virgin wood (Mathur et al, 1999). Across Europe share of recovered paper (see Figure 2) in paper manufacturing has increased from 35.3% in 1992 to 41.8 % in 2002 (CEPI, 2003). This means that quantity of sludge produced by paper mills will increase further in future. In total, UK mills generate around 1 million tonnes of sludge per annum (Aylesford, 2005).

The current practices of sludge disposal such as land spreading, land filling and incineration are becoming increasingly unfavorable due to ecological or economic considerations (University of Toronto, 2005). As paper sludge is rich in cellulose fibres so it has been considered as reinforcing filler for thermoplastics by many researchers. Yang and Kim (2004) are the ones who used paper sludge to reinforce four different kinds of thermoplastics such as PP, HIPP, LDPE and HDPE by using Epolene G-3003TM as coupling agent. Tensile and flexural properties are reported to be improved by increasing the paper sludge content of the composite. However both notched and unnotched izod impact strengths are lowered by increasing paper sludge content of the composite. They have reported difference in densities of 40%-paper sludge-filled PP composites and 40%-glass fibre-filled PP composites was only 0.15 g/cm3. This small difference is probably due to inseparable minerals and ashes attached to cellulose fibres present in paper sludge which increased the density of the composite.

Figure 2. Share of raw materials in paper manufacturing-1992 and 2002 (CEPI, 2003).


As it is clear that the problem with sludge is its composition, it is made up of 50% fibre and 50% fillers – both can be recycled individually but the task becomes more challenging when they are combined (Aylesford, 2005). To increase the potential of recycling of paper sludge into useful applications, WRAP has bought KDS sludge processing plant developed by First American Scientific Corporation (FASC) and sited at Aylesford newsprint in Kent (Aylesford, 2005). The demonstration plant is the first of its kind in Europe (Aylesford, 2005). According to engineers of FASC, KDS MicronexTM (Figure 3) can process sludge containing up to 80 % moisture and can reduce it to less than 5 % moisture, as a result significant cost savings can be achieved by eliminating need for separate drying equipment. The resulting sludge fluff can then be split into its fibre and filler constituents using screening equipment. Possible end uses for fibre include insulation, composite building products lower grade paper applications and fuel briquettes, while the filler material has potential for use in a range of products, particularly for the construction sector.

2.5 Agricultural crop residues

Agricultural crop residues such as corn stover, wheat, rice, barley straw, sorghum stalks, coconut and rice husks, grass, sugarcane bagasse, and pineapple and banana leaves are produced in billions of tonnes around the world and represent an abundant, inexpensive, and readily available sources of lignocelluloseic biomass (Reddy and Yang, 2005). Some of the important cereal and seed crops produced by European Union countries with their contribution to the world production is shown in the Table 3.

A common question is: How much crop residue is produced from a particular crop. Generally as crop yield increases due to moisture and fertility conditions, the amount of straw increases relative to grain (Alberta agriculture, food and rural development, 1999). The ratio of straw to grain production has been estimated by a number of investigators. Colorado State University estimates of crop residues results in figures of 1.3 tonnes of wheat straw per tonne of grain, 1.0 tonne of barley per tonne of grain and 1.2 tonnes of oat straw per tonne of grains (Colorado State University, 1999). Using this estimate straw residue for major crops grown in European Union in 2004 is shown in the Table 4.

Figure 3. KDS being commissioned (First American Scientific Corporation (FASC); 2005).


Table 3 - Production of selected cereals and seed crops by European Union in 2004. (FAO, 2005).

Table 4 - Typical amounts of harvestable straws for selected cereals in European Union in 2004. (Colorado State University, 1999).

Among these enormous amounts of agricultural residues, only a minor quantity of residues is reserved as animal feed or household fuel and a major portion is burned in the field creating environmental pollution (Sain and Panthapulakkal, 2005).

The potential of any natural fibre depends on its chemical structure, especially, cellulose content and mechanical properties of the fibre (Sain et al, 2005). Most agricultural residues have comparatively attractive cellulosic properties similar to wood. For example, wheat straw 38%, rice straw 36%, and bagasse 48% (Reddy and Yang, 2005). As comparison, softwoods have approximately 42% of cellulose on average, and hardwoods have 45% (Vlosky et al, 2004). However the quality of the fibres obtained will determine their end use, and not the cellulosic content (Reddy and Yang, 2005). The fibres obtained from crop residues such as rice straw, wheat straw and corn husk straw and husks are relatively fine as compared to soft and hard wood fibres (Reddy and Yang, 2005). A major difference between straw and wood fibres is the high content of ash which contains silica. Reddy and Yang (2005) has reported that silica blunts the cutting machinery, reduces digestibility of straw, interfaces with pulping process by forming scales on the surface of the reactors and makes combustion more difficult.

Degradability of fibres with temperature above 200oC and moisture absorption due to hydrophilic nature of fibres are some limitations which are common to all natural fibres irrespective of from which source they are obtained. These are reported for the fibres obtained from these crop residues as well. Potential availability and economics of using agro based residues for industrial products such as manufacturing composites for automobile and furniture applications far outweigh their limitations (Reddy and Yang, 2005; Sain and Panthapulakkal, 2005). Due to all these reasons agro based residues were attracted by the scientists and used for manufacturing wood plastic composites and considerable success is reported for wheat straw (Sain et al, 2005), corn stalk (University of Toronto, 2005a) and bagasse (Kokta and Raj, 1991)


and the results for rice husk (Juglison et al, 2004) and corn cob (University of Toronto, 2005a) are not positive.

An increasing population and constraints on growing the crops for industrial purposes makes these waste fibres the most alternative to natural fibres (Reddy and Yang, 2005; Stockmann and Bowyer, 2001) and thus meeting the growing demands of natural fibres for wood plastic composite industry in Europe. At the same time this will add to the creation of rural agricultural based economy.

2.6 Urban forest residues

Urban forest residues include stumps and tree trimmings from metropolitan areas. They have some problems such as higher moisture content, bark and dirt contamination. As such, they are not considered appropriate for wood-plastic composites and are more effectively used as mulch, boiler fuel, or compost augmentation (Chelsea centre, 2000).

3. PREPARATION OF WOOD WASTE FOR REINFORCMENT

A surprising amount of technology and good manufacturing practice is needed to make a viable WPCs product and this good practice starts with material preparation (Chelsea Centre, 2000). The method for the preparation of wood waste varies with the type of waste. Following are some important preparation methods for wood wastes mentioned in Figure 1.

3.1 Preparation of post industrial and MSW wood waste

It can be processed in two ways: processing to produce wood flour and processing to produce wood fibres.

3.1.1 Production of wood flour from post industrial and MSW wood waste

Generally speaking reinforcement can be classified as either wood flour or fibres. Much of wood flour is made commercially by grinding post industrial materials, such as planer shavings, chips, and sawdust into a fine, flour like consistency, typically by hammer milling.

The wood in MSW which includes furniture, miscellaneous durable goods (e.g. cabinets for electronic equipment), wood packaging (crates, pallets), windows and materials for construction and some other miscellaneous products (UEA, 2005) can also be ground like post industrial materials by hammer milling to produce wood flour. But the products like furniture, crates, pallets, doors and window frames contain contaminants such as metallic nails, glue, paint or laminates.

Maximum allowable contamination levels vary from contaminant to contaminant. The clean Washington Centre, USA (1997), has developed a specification for waste wood for use in WPCs production which is given in Table 5. According to the specification there is no acceptable limit for metallic nails so these must be removed from the wood flour. One way of doing this is using the magnets but if the metal is other than iron then the flour can be classified by using classifiers into metallic and wooden part.


Table 5: Maximum Allowable Contamination Levels * (Clean Washington Centre, 1997) *Acceptable contaminant levels are highly variable, and are often driven by appearance issues as well as technical or processing factors. **Varies depending on the colour and market preferences for the finished material.

3.1.2 Production of wood fibres from post industrial and MSW wood waste

In case of wood fibres, it is generally necessary to break down the solid wood, if this has not already been done, into fibrous form. Individual wood fibres are generally of the order of a few millimeters in length and only practicable way of obtaining these fibres is through refining or pulping (Hughes, 2004). Pulp for getting wood fibres can be made by chemical or mechanical processes. Chemical pulping generally refers to the Kraft or Sulphate process, whereby wood in chip form is treated with chemicals at high pressure and temperature and thus individual fibres are separated (WRAP, 2003). High temperature and dynamic stresses are used in order to separate the individual fibres in mechanical pulping (WRAP, 2003).

The properties of cellulosic fibres obtained from two processes are different from each other. Mechanically made fibres which are ground consist of many different sized particles: long fibres, medium-sized fibres and fines. But they retain their lignin where as chemically made fibres lose their lignin during chemical treatment. Lignin is not only the material that is dissolved in the chemical process, since the part of the hemicellulose also dissolves. This means that the yield is reduced and is usually around 50% (Peltola, 2004). For 1 tonne of pulp, two tonnes of wood is needed (Peltola, 2004). As a result of chemical process, fairly pure, long, whole and flexible cellulose fibres are obtained. It has been observed that without the use of expensive coupling agents, the long and pure chemically made cellulose fibres will work mainly as a filler and not give the true reinforcement to WPCs (Peltola, 2004). On the other hand, when mechanically made fibres are used, it has been observed that coupling agents have little or no effect on the properties of composite (Peltola, 2004). This is believed to be due to the presence of jelly like intercellular substance called lignin in mechanically made fibres (Peltola, 2004).

3.2 Preparation of fibre from paper

Many forms of paper such as old newspapers, office paper, plastic coated papers, mixed waste papers, and old corrugated containers contain contaminants such as adhesives, inks, dyes, metal foils, and some inorganic fillers etc. (USDA, 1993). These contaminants must be separated from the waste paper before the fibre can be recycled into paper products. This is not the case when


recycling into wood-based composites such as WPCs (USDA, 1993). So recycled pulp fibre which is achieved by mechanical agitation in water (WRAP, 2003) to separate the fibres and contaminants is not recommended. The reason is that water and energy is used in the process and also we have to handle the sludge produced as a result of the process. Instead of using recycled pulp fibres, mechanically milled fibres produced by hammer milling can be used for WPCs (USDA, 1996).

In many uses, WPCs are opaque, coloured, painted, or overlaid. Consequently, waste paper for recycling into WPCs does not require extensive cleaning and refinement. Thus, composites provide an unusually favorable option for recycling mixed and contaminated paper and other materials (USDA, 1993). Although newspapers appear to be the paper fibre of choice for many melt blended composites, other options such a mixed office paper and corrugated containers may also be the option for composite production (USDA, 1993). High filler and low fibre contents of the magazine paper make them unsuitable because mechanical properties of the composite suffer (USDA, 1993). Some producers of plastic lumber are adding waste paper to their products to increase stiffness and reduce creep (USDA, 1993). Additional large volume, low to- moderate cost applications are in areas such as packaging (trays, cartons, and pallets), interior building panels, and door skins (USDA, 1993).

3.3 Fibre extraction from paper sludges

Sludges from paper making and paper recycling (de-inking sludge) are generated in large quantities throughout the Europe. Primary/Secondary pulp sludge with 45% to 55% moisture, 20% or more solids (Kaolin clay and fibres) makes these materials difficult to handle and process (First American Scientific Corporation, 2005). The contaminated fibres are not suitable for manufacturing WPCs. These must be removed for making them usable for WPCs. The KDS Micronex™ can dewater sludges containing up to 70% moisture, can reduce it to less than 5% moisture and enable the separation of paper fibre from other paper components (i.e. kaolin clay). The paper fibre can be recovered for fuel or recycled as fibre and kaolin clay can be recycled in the papermaking process or other industrial applications (First American Scientific Corporation, 2005). As a result, significant cost savings can be achieved by eliminating the need for separate drying equipment. The patented KDS technology removes water from biomass using only 1/4 of the energy required to dry biomass by conventional methods (Alternative green energy systems Inc, 2005). Fibres obtained by processing newspaper mill sludge with KDS MicronexTM technology has been successfully utilized for making tissue papers by scientists at University of Toronto (2005) which shows that they could probably be utilized for WPCs.

3.4 Fibre Extraction from agricultural by products

Unlike the wood fibres, agro fibres undergo several transformation processes before they are ready for use in composite manufacturing. Natural cellulose fibres are extracted from agricultural by products using bacteria and fungi, mechanical and chemical methods (Reddy and Yang, 2005). However, the basic principle of every process is similar i.e., to attack the noncellulosic substances (Lignin and hemicellulose) and separate cellulosic fibres from them. In biological process; bacteria and fungi perform this function where as in chemical process; acids, alkalies and enzymes are used to separate the fibres. In mechanical processes, high temperature, stresses and shear produced as a result of mechanical forces lead to micro fibril individualization. In principle, fibres may be produced by either chemical or mechanical or biological process or by a combined method.

Sain and Panthapulakkal (2005) characterized the fibres obtained by fungal retting of wheat straws under controlled atmosphere in order to explore the possibilities of using the fibres as


reinforcing materials for thermoplastics. Fibres obtained after retting are reported to have homogeneous and uniform structure with better mechanical properties. However partial removal of lignin and hemicellulose from fibres is reported as a result of fungal retting. The presence of lignin and other extractives on the fibres surface may also improve the compatibility with thermoplastic matrix (Sain and Panthapulakkal, 2004). Although the fibres obtained are of better quality and could probably used as reinforcing materials, it requires relatively longer duration due to natural species involved in the process of retting.

Sain et al (2005) characterized the wheat straw fibres to evaluate their potential as a reinforcing material for polypropylene. Chemical and mechanical pulping was used to generate the fibres and processed fibres were characterized with special emphasis on chemical composition, surface morphology, and physical, and mechanical properties. The results of characterization showed that fibres prepared by chemical process were longer, free from surface irregularities and showed better mechanical properties. It is believed that this is due to removal of lignin and hemicellulose from cellulosic fibres. But fibres prepared by mechanical process were shorter, having larger diameter and lower strength. The lower strength and larger diameter of mechanically processed wheat straw fibres is explained by the high lignin content present in mechanically processed fibres. The reduced length of mechanically processed fibres can be attributed to fibre breakage due to the high shear developed during processing. However, polypropylene composites prepared with chemically processed wheat straw fibres showed comparatively lower strength properties compared to mechanically processed fibres. It is believed that low tensile strength of the composite is due to poor dispersion of longer chemical fibres. But high strength of composite having mechanically processed fibres is due to the ease of dispersion small fibres and lignin content of fibres which helped in binding with polypropylene matrix. Scientists at University of Toronto (2005a) showed that Agricultural residues can be used as an alternative to lignocellulosic materials for manufacturing composites for automobile and furniture applications (Figure 4). Wheat straw and corn stalk exhibited better properties compared to corn cob.

Figure 4. (a) Processed forms of agro residues; (b) Composite processing; and (c) proposed

automotive applications (University of Toronto, 2005a).


4. CONCLUSION

There are a large number of sources of post consumer wood waste which could probably be utilized for WPCs. These include: primary and secondary wood wastes, municipal solid wood, waste paper, paper sludge and agricultural crops residues. Wood obtained from these sources can be used in either fibrous or flour form depending on the application of product and processing route adopted. As the cleanliness of the feedstock is a stringent requirement for manufacturing of WPCs so these wastes must be cleaned before using in order to remove metallic and non metallic contaminants.

4.1 Primary and secondary wood waste

Primary and secondary wood waste: sawdust and wood chips are less contaminated and have higher degree of traceability but there is a lot of competition among paper and board manufacturers for this waste. Its availability for WPCs seems difficult and uncertain. This type of wood waste is ground in hammer mills to get required mesh size of wood flour for use in WPCs. Pulp for getting wood fibres can be made by chemical or mechanical processes.

4.2 Municipal solid wood

Two types of waste wood are obtained from MSW: Contaminated and Uncontaminated. The major portion of the waste wood obtained from MSW is often contaminated with environmentally restricted chemicals such (CCA) so recycling of this wood seems difficult due to environment concerns. Reprocessing of this wood into WPCs does not emit any harmful gases if processing equipment is sealed. However end of service life of the composites is a concern and needs attention. If it is resolved then there is no problem utilizing this wood for WPCs. The small portion of wood from MSW is clean and uncontaminated. There is a lot of competition for clean wood among various applications such mulch, animal litter, panel and boards. However instead of using this wood for low grade applications it is recommended to use for WPC. This will increase the market of recycled wood. This type of wood waste can also be processed just like the same way as primary and secondary wood waste is prepared to be used for WPCs.

4.3 Paper

The utilization of recovered paper for making newspaper and other packaging is very common and infrastructure for it is quite developed so availability of this paper for WPCs is uncertain. But availability of paper which otherwise is exported to other countries is very important for Europe and can function as filler for WPCs. Also the paper which otherwise becomes useless for paper industry due to decreased fibre length after repeated recyclability can be of much importance for WPCs. The use of such paper is not sustainable for paper industry due to increased water requirements for recycling and production of sludge as result of recycling which requires to be managed safely and properly. Traditional process of fibre extraction from paper such as mechanical agitation of paper in water is not sustainable due to increased energy and water requirements of the process. However use of K-mixer for production and dispersion of paper fibres in plastic matrix is sustainable due to less energy equipments of the process and no water is required at all for the process.

4.4 Paper Sludge

Due to increased recyclability of paper the quantity of produced sludge is also increasing. At the


moment a sustainable waste management practice is required for this waste. It consists of paper fibres and some inorganic minerals. The utilization of these fibres for WPCs is only possible if fibres are separated from inorganic minerals which can be done by treating the sludge in sludge processing plant.

4.5 Agricultural Crops Residues

Agricultural crops residues such as wheat and rice straws along with sugarcane bagasse must be utilized for WPCs in order to avoid environmental damage because most of the time these residues are burnt on the field just to get rid of them. The fibres from this biomass can be easily extracted by biological, chemical and mechanical processes which are also processes of fibre preparation for paper industry. Utilization of these residues for WPCs will increase the rural agricultural economy of farmers. Enough quantity of post consumer plastic and wood waste is available which can be utilized by wood plastic composite industry to meet its growing demand. It just requires the attention of authorities and experts who are managing the waste stream.

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