Wood-plastic composites compatibility

Thus, the role of a coupling agent, by improving compatibility and adhesion in wood plastic composites, become very significant. The following mechanisms in modified WPC specimens with coupling agents can be observed ... 

Wood plastics composites have been successfully and rapidly developed in North America to produce a market of 690,000 tonnes, and they are now becoming more popular in Europe and Asia. Compatibilisers improve the water resistance and rot resistance and transform the mechanical properties, particularly the strength. They also improve the heat distortion temperature. This is because of the poor compatibility between polar cellulosic fibres and nonpolar polyolefins. In practice many manufacturers do not yet use compatibilisers because of their high cost. 

A widely used matrix materiai for wood plastic composites (WPC) is polypropylene (PP). However, the hydrophilic nature of wood lowers the compatibility with the hydrophobic polymer, leading to WPCs with poor dispersions. Therefore, substantial research has been carried out on the surface modification of wood with coupling agents to improve strength properties. Maleated polypropylene has proved to be one of the most suitable materials for improving the interfacial adhesion between wood particles or fibers and polypropylene and hence the mechanical properties of the composite [9]. 

The above results shown in Figs. 12 and 13 can be discussed in connection with the application of the thermoplasticized wood. The thermoplasticized wood can be used as material for molding, and as one way of utilization, can be used as blend composites with synthetic polymers. If this blending is made by grafting as shown above, two benefits can at least be pointed out (a) the thermoplasticity of wood materials is enhanced. (Better results can be obtained with esterified wood.) (b) the compatibility of the plasticized wood with synthetic polymers increases by the grafting. These factors are considered to be advantageous for preparing molded composites with excellent final properties. 

The first synthetic plastics were the phenol-formaldehyde resins introduced by Baekeland in 1907 [1], Melamine and urea also react with formaldehyde to form intermediate methylol compounds which condense to cross-linked polymers much like phenol-formaldehyde resins. Paper, cotton fabric, wood flour or other forms of cellulose have long been used to reinforce these methylol-functional polymers. Methylol groups react with hydroxyl groups of cellulose to form stable ether linkages to bond filler to polymers. Cellulose is so compatible with these resins that no one thought of an interface between them, and the term reinforced composites was not even used to describe these reinforced systems. 

Although the expansion coefficient of epoxy is two-to ten times that of concrete, use of fillers in the epoxy helps to adjust it to the level of concrete [46]. For bonding surfaces (such as steel and concrete, but not composite surfaces), an adhesive-compatible primer coat is usually needed. However, recently an adhesive has been developed which does not need any activators or primers that can be used to bond composites, metals glass, ceramics, plastics and wood successfully [19, 47]. 

On the other hand, Boeglin and co-workers [44], have reported that despite the lack of chemical compatibility between polyolefins and wood (unmodified), adequate mechanical adhesion between the materials can occur, leading to good mechanical properties of the composites. Evidence of the mechanical interlocking of the components in particleboards bonded with recycled polyethylene has been obtained by means of scanning electron microscopy (SEM) micrographs [44]. The authors state that SEM showed wood cells filled with the plastic. Furthermore, particleboards bonded with recycled polyolefins have been proved to have mechanical properties comparable to those of commercial particleboards [45,46]. 

Since wood flour is hydrophilic and plastic used in WPG is hydrophobic, the compatibility of these two is low. Low compatibility of components has a bad influence on the mechanical properties or durability of the composite. Generally, for WPGs, a plastic material in which the base plastic is modifled with maleic acid is used [33-38]. The maleic acid-modifled plastic is prepared by adding maleic acid in a side chain within the main chain of the base plastic (the model of maleic acid modified PP is shown in Figure 5.32). Since the main chain of a maleic acid-modified plastic is the same as that of the base plastic, compatibility is high. On the other hand, the maleic acid-modified portion of the side chain acts on the hydroxyl group on the surface of wood flour and modifies the wood flour surface (Figure 5.32). Thus compatibility between the wood flour and the base plastic is improved [39, 40]. 

In this research, thermal insulation properties of the wood sawdust/PC composites were evaluated by thermal conductivity analysis. Table 1 shows the thermal conductivity of sawdust/PC composites in various treatments. Thermal conductivity is defined as the quantity of heat transmitted through a unit thickness in a direction normal to the surface of that unit area, due to a unit temperature gradient under steady state conditions [6]. The addition of wood can improve thermal insulation of neat PC because thermal conductivity (k) of wood materials (k 0.08 W/m K [9] was normally lower than plastics. Moreover, the lower thermal conductivity was associated with discontinuous phases which were a result from poor compatibility between the wood sawdust and PC matrix [7]. 

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