Experiments in Finland on Properties of Wood-Polymer Combinations

Large-scale tests of the physical properties of wood-polymer combinations during 4 years of research in Finland (1965-1969) indicate that birch WPC containing 40-50 percent polyester-styrene copolymerate hardened by gamma radiation differed little from material hardened by chemical catalysis with 0.5 to 1 percent organic peroxide and cured at 60?C. and later at higher temperatures. WPC is produced by filling cells of dry wood with liquid monomer and then polymerizing it by some means. Monomers used in these tests were primarily mixtures of unsaturated polyesters (PE) and styrene (S) and styrene and acrylonitrile (AN). Methyl methacrylate (MMA) was used only for comparisons. The cost of curing with irradiation is high, so heat catalytic processes were investigated. The experiments included over 6,000 tests comparing. WPC and untreated wood. It was found that in WPC samples, flaws in the wood were accentuated and the deviation of the data on strength properties was different from that obtained for untreated wood. WPC samples made with a high monomer and a hard polymer were very brittle. The best methods of production must balance hardness and cleavage properties. Weathering tests have shown that WPC material is not weather proof. Though it absorbs water slowly and does not swell as much as untreated wood, the surface dries rapidly and shrinks while the interior retains its dimensions, and stress and cracking result. Wood fibers, containing no polymer, are unprotected; they become loose, bleach in the sun, and the surface becomes fuzzy and gray. Tests of parquet flooring made of WPC combinations have yielded good results in durability tests, though the product obtained is not evenly colored and is usually more yellow than the original. Mixtures of unsaturated polyesters and styrene produce WPC of quality and cost competitive with MMA-based composites, while allowing more versatility in modifications of the properties of the product.

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