A STUDY OF WOOD-PLASTIC COMBINATIONS BASED ON SOME TROPICAL WOODS

Abstract

The objectives of this investigation were to extend an earlier work to prepare wood-polymer combinations (WPC) from a wider variety of tropical hardwoods and to evaluate their modified properties against those of their untreated woods, Selected mainly on their popularity and commercial availability, the hardwoods included the heavy, medium and light types. The monomer systems employed were those used in the earlier work. WPC were prepared by polymerising the monomers "in situ" in oven-dried woods by gamma-radiation. The heavy hardwoods in general gave lower polymer loadings than the medium and light hardwoods. The composite specimens, together with untreated woods, were tested for hardness, compression parallel to the grain, dimensional stability, fire resistance, thermal conductivity and dielectric constant. All the tests were performed at moisture levels and temperatures of the testing laboratories. All the composites increased in hardness, some by as much as five times over the untreated woods depending on the polymer impregnant. This increase was, however, not uniform in all directions. Side-hardness increase was in general greater than end-hardness increase. A greater compressive strength parallel to the grain was also exhibited by the composites, from 26 to 149% increase over the untreated woods. In general, Kempas-polymer composites and W-PSTAN produced the largest increases and W-PAN the least. The included polymers were conceived as filling the internal voids of the woods, strengthening the cell-walls and giving lateral support, thereby resisting compression deformation and failure. In general, the polymer-treated woods showed distinct improvement in dimensional stability over the untreated woods with ASE ranging from 10 to 45% (highest from W-PSTAN and lowest from W-PAN). After one day of soaking in water, the composites attained about 30% or less of their maximum swellage compared to the 50% or more for the untreated woods, showing effective retardation in the initial water-uptake, This was attributed to the bulking action of the polymers filling the cell lumina with probably some interaction with the cell-walls. The polymers PMMA and PMD were found to enhance flammability of the woods, but PVDC, PAN and PSTAN imparted fire resistance to the woods, in decreasing order, The flame-retarding chlorine present in PVDC was responsible for the high fire-resistance of W-PVDC. Ramin- and Keruingpolymer composites showed the highest flammable tendencies obtained.

A reduction in thermal conductivity was exhibited by all the composites, with W-PAN showing the greatest reduction and W-PSTAN, in general, showing the least. An internal lagging effect arising from air pockets created in the polymer impregnants was suggested to be the reason. In the case of W-PAN and Kempas-polymer composites, their reduced flammability could be correlated with their lowered thermal conductivity. The composites of the woods KM, S, C, DRM and LRM gave a lower dielectric constant than the woods. It was suggested that graftcopolymerisation might have taken place to "tie down" some of the molecules thereby reducing the number of polarizable elements. Increasing moisture content was found to increase the dielectric constants of both treated and untreated woods.