Additive effects common to radiation grafting and wood plastic composite formation

Abstract

A range of additives has been developed for enhancing grafting yields in a variety of systems initiated by ionizing radiation. Cellulose has been adopted as the predominant naturally occurring model backbone polymer in these studies because of its structural relationship to wood which is the reference substrate for the work reported in the related second part of this paper concerning composites. Some experiments have been performed with the other major naturally occurring polymer, wool. For comparison purposes with synthetic materials, some studies have also been performed with polypropylene as trunk polymer. Styrene has been used as a predominant monomer in grafting with some experiments utilizing the acrylates like methyl methacrylate. The role of solvent in grafting has been evaluated. UV has been used as initiator to replace ionizing radiation for certain experiments. The additives used were mineral acids, lithium salts, multifunctional acrylates and their methacrylate analogues, urea, oligomer acrylates, silane, fluorinated alkyl esters and thermal free radical initiators. A mechanism to explain the additive effect in terms of reagent partitioning has been proposed. The most efficient of the additives in grafting have been applied to the radiation synthesis of wood-polymer composites using two different types of wood, namely simul from Bangladesh and Pinus radiata from Australia with two different monomers with and without solvent, namely butyl methacrylate and methyl methacrylate. The effect of the additives on the physical properties, such as polymer loading and tensile strength, of the wood composites were determined. The partition concept developed for grafting has been used to explain the reactivity of the additives in wood plastic formation. The polymerization of monomers in wood plastic systems is shown to be related to simple homopolymerization of monomers in solution, a reaction which is also shown to be capable of interpretation in terms of partition phenomena.