WOOD REPLACEMENT

Keepax, C. (1975). Scanning electron microscopy of wood replaced by iron corrosion products. /. Archaeol. Sci. 2,145-150. 

The manufacture of cellular PVC/wood composites has been studied. The properties achieved, as foam density was reduced, were examined showing suitability for many wood replacement applications (196). 

A major disadvantage of composites of wood with thermoplastics materials is a relatively high specific gravity compared with those of many natural wood products. A PVC-wood composite, for example, has a specific gravity of about 1.3 g/cc. The manufacture of cellular PVC-based wood composites was studied and the properties that were achieved as the foam density was reduced were examined. Overall, even with densities as low as 0.6 g/cc, the physical properties should be adequate for many wood replacement applications. The composites also exhibited the aesthetics of wood and economics that were favourable compared with those of both rigid and cellular PVC. 6 refs. 

Recycled PVC supermarket trays have been used successfully in production-scale trials to make the foamed layer in coextruded cladding. The extruded product had satisfactory foam density, foam structure and colour. Impact properties were better than those of the control made from virgin PVC this is attributed to the high levels of impact modifier used in tray formulations. These trials demonstrated that PVC straight-on trays can be recycled into foamed extrusions for wood replacement products. 8 refs. 

Styrene and butadiene also form copolymers known as high impact polystyrene, or rubber-modified polystyrene, when the content of butadiene is 10%. This type of material has excellent mechanical properties, and it is widely used in practice for the manufacturing of numerous objects, including parts for household appliances, furniture, etc. Rubber-modified polystyrene is commonly used as wood replacement and also for packaging. The synthesis of this material typically is done by dissolving polybutadiene in styrene monomer, followed by free radical polymerization achieved using a peroxide catalyst. This procedure leads to block or graft type copolymers. 

The use of cellular styrene plastics for insulation has been widespread for many years (25). More recently, polystyrene "structural" foams have been used, especially in wood replacement applications. Such use is expected to grow in the future, particularly as wood becomes less available and greater demands are placed on more efficient use of "short" plastic materials. 

The French patented Ugine Kuhlman process uses a die with an internal mandrel, as shown in Figure 10. The outer skin is quickly frozen via a water-cooled sizing die while the mandrel forms an internal void in the extrudate that is filled by the inward foaming action. The process creates a thick-skinned part with a foamed core. The process is very important for structural members such as window frames using PVC as a wood replacement. 

Applications coated fabrics, cornices, curtain rails, doorframes, guide rails for shutters, furniture profiles, roofing products, plastisol foams, shoe soles, skirting boards, sofit boards, wall and ceiling panelhng, wallpaper, window sills, wood-replacement products... 

Foam composite wood replacement material. [Data from Zehner, B. E.,... 

Artificial wood replacer, multi-layer plywood, heavy-duty... 

Seaspray, Sprayable wood replacement, Seawolf Industries, Inc. 

Foamed PVC has been commercially available for at least 20 years, but is now becoming increasingly important because of recent improvements in technology and a broadening of applications. It has becomes one of the fastest growing markets in the vinyl industry, sparking enthusiasm among processors over new opportunities for vinyl foam, particularly in many wood-replacement applications. 

Comparing Table 2.8 to Table 2.3, apart from the food contact restrictions, the applications for recyclates are quite similar to the applications for virgin materials. Mixed plastic waste (MPW) appears in this table. This is a material consisting of various plastic wastes rather than just one single polymer type. The outlets for materials of this type tend to be limited to wood replacement products for reasons that will become apparent later. 

Mixed Plastic Waste (MPW) Wood replacement products such as pallets and fencing... 

Recycled plastic is finding use as a wood replacement product. It does not splinter, rot or warp like conventional wood. It does not require preservation treatments and its lifetime can exceed over 50 years. There are now a number of plastic lumber type products available ranging from those made of entirely of one material, generally HOPE, to mixtures of plastic, wood, rubber and other materials. A number of factors are pushing the use of plastic lumbeg especially in the USA where alternatives to hardwood lumber have appeal on environmental grounds. The market in the USA is estimated to amount to 10 billion [3] and the industry is expanding rapidly. 

In Chapter 1, chemical recycling and energy recovery were introduced as alternatives to mechanical recycling. We have seen how plastics can be reprocessed if they are pure streams, if they can be sorted, or if suitable blends can be obtained. We have also seen potential applications for mixed plastics where the quality of the final product is reduced, but the inherent properties of all plastics such as rot-resistance can be utilised. An example of this is in wood replacement applications. However, material that is highly contaminated and mixed may be difficult or perhaps even impossible to recycle mechanically. Remember that cleaning and sorting can be difficult, costly and laborious. 

Foams can be manufactured on standard extruders the main problem being to control the amount of foaming on leaving the extruder. This is usually achieved with several sizing and cooling rollers. PVC foam for wood replacement is becoming an important market. 

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