Wood-polymer composites problems

Fifteen years ago, when wood-plastic composites were first introduced many people predicted that this process would solve the problem of wood dimensional stability and great claims were made for its future use. Now that the physical properties of wood-polymer composites are better understood, specific commercial products are being produced which take advantage of the desirable aesthetic appearance, the high compression strength, increased hardness and abrasion resistance and improved dimensional stability. Future use of wood-polymer composites will depend upon the imagination of the producer and the market place. 

Reconstruction of structural units and buildings is a comprehensive problem, as they usually consist of different materials, such as concrete, metal, wood, gypsum, ceramics, and so on. These materials are subject to various environmental factors that result in their breakdown. These factors and, therefore, the nature of failure differ depending on their location. In this connection, let us consider the possibilities of using polymer composites for restoration of ground-based, underground, and underwater structural units and buildings. 

The problem of reinforcing PVC with fibres differs from that of polyolefins. This is because PVC needs another additive such as plasticiser, which should be added. The improvement in dispersion and adhesion between the wood particles and the polymer matrix showed that the mechanical strengths can be greatly enhanced, while the rheology or processing of thermoplastics/wood flour composites is seldom studied [34,35]. Recently, the effect of low levels of plasticiser on the rheology and mechanical properties of PVC/newsprint fibre composites has been reported [36]. 

These materials are from renewable sources and are often made from plant materials that can be grown year after year and should come from agricultural nonfood crops. Biodegradable polymers are broken down into CO 2 and water by microorganisms. Although biopolymers may be able to help solve the disposal problems of current plastic packaging, it is not clear if they can really deliver on this promise and whether there is possible competition with the food chain. Cellulose is the most common biopolymer and organic compound on Earth. Other examples are starch, PHB (polyhydroxybutyrate), natural fibers, silk, and wood plastic composites (WPC). 

As already indicated above, what one may consider a surface depends on the property under consideration. Adhesion is very much an outer atomic layer issue, unless one is dealing with materials like fibreboard in which the polymer resin may also be involved in mechanical anchoring onto the wood particles. Gloss and other optical properties are related to the penetration depth of optical radiation. The latter depends on the optical properties of the material, but in general involves more than a few micrometer thickness and therewith much more than the outer atomic layers only. It is thus the penetration depth of the probing technique that needs to be suitably selected with respect to the surface problem under investigation. Examples selected for various depths (< 10 nm, 10 s of nm, 100 nm, micrometer scale) have been presented in Chapter 10 of the book by Garton on Infrared Spectroscopy of Polymer Blends, Composites and Surfaces... 

Because of the extreme structural and chemical complexity of wood, none of the previous gravimetric analyses is completely accurate. Both losses of the desired isolate and contamination are constant problems (3, 4), Therefore, changes in chemical composition attending the environmental decomposition of wood can lead to artifacts in gravimetric analyses of the major constituents. For example, degradation of a structural polymer typically increases its solubility in acid and base (25), producing erroneously low values... 

Particle board from furniture manufacturing waste presents a unique disposal problem because of the urea-formaldehyde (UF) resin used in binding the wood particles and the polymer-laminated coatings. Many thousands of tons of wood wastes are produced in each furniture plant annually in the form of cutoff pieces and sawdust. The composite materials may form toxic and corrosive gases during combustion, and wastes must be landfilled or burned in an incinerator that safely consumes the resin and plastic materials as well as the wood. Landfilling this waste currently costs millions of dollars per year for a good-sized furniture plant. Thus an environmentally and economically feasible... 

Cotton and cellulose fibers like sisal have good high temperature properties (equal to or even above nylon). Some cotton fibers are as long as 4 in. Water removal is a problem in the use of natural fibers, but benefits including renewable resource may offset the difficulties. Polymer-coated cotton fabric is widely used in many different applications. Some excellent work on recycled wood and natural fiber composites for use in construction is being done at the USDA Forest Products Laboratory in Madison, WI. 

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