CHARACTERIZATION OF RUBBERWOOD-POLYMER COMPOSITES

Abstract

Wood-polymer composites based on a local tropical hardwood, Rubberwood (Hevea braziliensis), were prepared by the impregnation of liquid monomers into wood cellular structure followed by in situ polymerization induced by gamma radiation from a cobalt-60 source. The monomers used included methylmethacrylate (MMA), 3: 1 methylmethacrylate/vinylidene chloride (MVDC), 3: 1 methylmethacrylate/bis(2-chloroethyl)vinyl phosphonate (MBP), methylmethacrylate with 5% dioxane (MD), acrylonitrile (AN), and 60:40 styrene/acrylonitrile (STAN). It was found from preliminary studies that ethanol-soluble extractives of rubberwood (RB) inhibited in vitro polymerization of MMA. The polymer loadings of composites prepared from extractive-free wood marginally exceeded those from ordinary wood. Gel permeation chromatography showed that the extractives generally lowered the average molecular weight and raised the polydispersity index of PMMA formed in vitro and in viva in the wood matrix. The physical properties relating to the strength, thermal and dimensional behaviour of the composites were investigated by a variety of methods. The static bending strength of the composite RB-PMMA, expressed in terms of the ultimate tensile strength and modulus of elasticity, was improved relative to untreated RB. Statistical analysis revealed a significant linear relationship between the increase in bending strength and polymer loading. In dynamic mechanical testings, RB-PMMA was found to have a higher dynamic storage modulus than RB, with the percent increase in modulus also depending significantly on polymer loading. A thermal scan of tan delta values between 120° and 200°C showed that the damping pattern of RB was affected to different extents by different polymeric systems. PSTAN caused the greatest change in the damping pattern followed by PMD and PMMA. This observed trend was attributed to the relative extent of monomer penetration and interaction with the wood cell-wall components. The thermal behaviour of RB and five of its composites was studied by oxygen index measurement, differential thermal analysis (DTA) and thermogravimetry (TG). The DTA and TG curves of composites generally deviated from those of RB. The impregnated polymer either reduced the initial temperature of decomposition, increased the peak temperatures of exothermic reactions, or increased the char yield. Only PMBP caused all these desirable effects. Comparison with physical blends of RB and the corresponding polymer provided some evidence of chemical interaction of wood and polymer in the composites RB-PMGP and RB-PST AN. From dimensional stability studies, it was found that RB impregnated with PSTAN showed higher anti-shrink and anti-absorption efficiencies over a longer period of time than RB-PMMA. The dimensional stabilizing effect observed in RB-PSTAN was due to the ability of the styrene-acrylonitrile mixture to swell the wood prior to exposure to moisture. Finally, the distribution of PMMA in RB was studied by permeability experiments, mercury porosimetry and scanning electron microscopy. There was no sign of chemical interaction between PMMA and the cell walls in the composite.