Loo-Teck Ng

Effects of coupling agent on the physical properties of wood-polymer composites

Abstract Wood-polymer composites based on two Australian commercial timbers, radiata pine (pinus radiata) a soft wood and fast growing plantation timber, and blackbutt (eucalyptus piluliaris), a hard wood which is also a plantation timber, were prepared using the vinyl monomer, methyl methacrylate (MMA). It has been established that polymethyl methacrylate does not form bonds with the hydroxyl groups of the cellulose fibres but simply bulks the void spaces within the wood structure. Coupling agents have been known to increase adhesion of wood fibre to polymer matrix. In this study wood samples were pretreated with the coupling agent, vinyltriacetoxysilane, before impregnation with MMA in order to evaluate the effects of the silane coupling agent on the physical properties of wood-polymer composites. These composites were prepared using a catalyst-accelerator method and the polymerisation process was initiated at room temperature. This method involves the impregnation of wood samples using MMA containing benzoyl peroxide (1%), lauroyl peroxide (0.5%) and the accelerator: N,N-dimethylaniline (0.5%). In contrast to the conventional catalyst-heat method, this method requires no heating in the initiation process and thus results in virtually no loss of monomer. In general, there were improvements in dimensional stabilities (positive effects in both anti-shrink and anti-absorption efficiencies), compressive strength and hardness of silane treated wood-polymer composites. The morphology of the composites was examined using scanning electron microscopy and improved adhesion was evident for composites treated with the coupling agent. Since WPC find greatest use in medium to high unit cost products such as flooring and secondary construction items, improvements in both physical and mechanical properties of WPC would have important implications for the wood industry.

Grafting of methyl methacrylate to cellulose and polypropylene with UV and ionising radiation in the presence of additives including CT complexes

Detailed studies of the grafting of polar methyl methacrylate (MMA) to two representative backbone polymers, cellulose and polypropylene (PPE) in the presence of additives, using ionising radiation and UV as initiating sources, are reported. The results are compared with analogous grafting work with non polar styrene previously studied. The additives chosen for examination were predominantly components used in radiation curing formulations since grafting and curing are known to be mechanistically related. The additives were mineral acid, photoinitiators, vinyl ethers, oligomers, polyfunctional monomers including multifunctional acrylates (MFAs) and methacrylates (MFMAs). For the first time charge transfer (CT) monomer complexes have been used as additives in the current work. The CT complexes themselves, being monomers, have also been directly radiation grafted to cellulose. Mechanisms for the above grafting processes are proposed. The significance of this grafting work in analogous radiation curing is discussed. The grafting of the CT complexes, themselves, is shown to lead to new copolymers with potential industrial applications.

Natural Polymer-Based Sulfite Biosensor

As a natural product, chitosan has the inherent properties of being biodegradable, biocompatible and nontoxic. These properties render it an ideal matrix in enzyme immobilization for clinical analysis. In this work, the feasibility of electrochemical biosensor fabrication using chitosan has been demonstrated. As a test case, the enzyme, sulfite oxidase (SOD), was covalently immobilized onto the matrix of chitosan-poly(hydroxyethyl methacrylate) (chitosan-HEMA), a natural/synthetic polymer hybrid material obtainable via a UV curing process. An electron transfer mediator, p-benzoquinone was coupled onto the polymer network for the activation of chitosan-HEMA copolymer after the photo-induced polymerization reaction. The biological activity of the immobilized SOD and the electroactivity of the coupled p-benzoquinone were examined.

The effect of additives on the enhancement of methyl methacrylate grafting to cellulose in the presence of UV and ionising radiation

The role of various additives in the grafting of methyl methacrylate (MMA) to cellulose using UV and ionising radiation has been investigated. MMA grafting yields are hampered by competing homopolymerisation. The styrene comonomer technique was utilised to overcome this problem. The role of acid in these reactions has been studied as well as additives like, an inclusion compound (urea), thermal initiators and photoinitiators. Methanol was used as the swelling agent in all the experiments. Molecular weight studies with homopolymers indicate that both chemical and physical processes are involved in the mechanism of the reaction. The physical process involves a partitioning phenomenon whereas the chemical process concerns additional radical reactions in the radiation initiation step. This grafting mechanism is shown to be applicable to the simple radiation polymerisation of monomers in solution and also analogous UV fast curing systems.

UV-cured natural polymer-based membrane for biosensor application

Chitosan, a natural product, is inherently biodegradable, biocompatible, and nontoxic. These properties make chitosan ideal for inclusion in matrices designed for use in enzyme immobilization for clinical analysis. This study demonstrates the feasibility of using chitosan in electrochemical biosensor fabrication. The enzyme sulfite oxidase (SOX) was covalently immobilized onto the matrix of chitosan–poly(hydroxyethyl methacrylate) (chitosan–pHEMA), a natural/synthetic polymer hybrid obtainable via UV curing. p-Benzoquinone, which served as an electron transfer mediator, was coupled onto the polymer network for activation of the chitosan–pHEMA copolymer, after completion of the photo-induced polymerization reaction. The biological activity of the immobilized SOX and the electroactivity of the coupled p-benzoquinone were investigated.

Additive effects common to radiation grafting and wood plastic composite formation

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.

Significance of grafting in curing processes initiated by UV, excimer and ionising radiation sources

Abstract The grafting of a typical methacrylate monomer (MMA) to polypropylene (PPE) and cellulose initiated by UV and ionising radiation is reported. The effect of additives which constitute components in radiation curing on the grafting process is examined. Additives studied include photoinitiators (PIs), multifunctional acrylates and methacrylates and acrylate oligomers. Synergistic effects when these additives are combined in the same solution are reported. The photografting studies have been extended to include grafting with charge transfer (CT) complexes involving donor/acceptor (DA) monomers to PPE, cellulose and wool. The importance of this work in conventional and PI free curing is discussed, particularly the significance of concurrent grafting during curing.

Role of additives in wood–polymer composites. Relationship to analogous radiation grafting and curing processes

Abstract Wood polymer composites (WPC) were prepared by impregnating an Australian softwood, Pinus radiata with methyl methacrylate which subsequently underwent in situ polymerisation utilising either γ radiation or the catalyst–accelerator method. Novel additives including thermal initiator, crosslinking agents, an inclusion compound and oxygen scavenger were incorporated to improve the polymer loading and properties of the resulting WPC. Polymer loadings of WPC obtained utilising the accelerator–catalyst method corresponded well with those obtained using γ radiation with 20 kGy radiation dose. The mechanistic significance of the current work in analogous radiation grafting and curing processes is discussed.

Effect of monomer structure on radiation grafting of charge transfer complexes to synthetic and naturally occurring polymers

Abstract Effect of monomer structure in photografting charge transfer (CT) complexes to typical substrates like wool, cellulose and polypropylene is reported. The importance of photoinitiators in these processes is examined. Maleic anhydride (MA) with triethylene glycol divinyl ether (DVE-3) is used as reference CT complex in this work. The additional monomers studied include the esters of MA as acceptors and vinyl acetate as donor. The role of solvent in these reactions is discussed, particularly the effect of aromatics in photografting to naturally occurring trunk polymers like wool and cellulose. The effect of the double bond molar ratio of the DA components in grafting is examined. The ultraviolet (UV) conditions for gel formation during photografting, hence the importance of homopolymer yields in these processes is reported. A plausible mechanism to explain the results from this photografting work is proposed. The significance of these photografting studies in the related field of curing, especially in UV and ionising radiation systems, is discussed.

Kinetics study of the photopolymerisation of donor/acceptor pairs using the differential photocalorimetric technique and the relation of the kinetics data to hydrogels formation

Abstract The differential photocalorimetric technique was used to study the rates of photopolymerisation initiated by donor/acceptor pairs with N -vinylpyrrolidinone (NVP) as the donor, and the acceptors were a series of N -hydroxy alkylmaleimides including N -hydroxymethyl maleimide (HMMI), N -(2-hydroxy) ethylmaleimide (HEMI), N -(3-hydroxy) propylmaleimide (HPrMI) and N -(5-hydroxy) pentylmaleimide (HPMI). HPMI/NVP system displays the highest rate of polymerisation followed by HMMI/NVP and then HPrMI/NVP, with HEMI/NVP giving the lowest rate. These donor/acceptor pair systems were subsequently used to initiate the polymerisation of NVP under the influence of UV radiation. With the exception of the HEMI/NVP system, the other three systems produced polymers with characteristics pertaining to that of hydrogel. A previously proposed mechanism (Proceedings of the RadTech Asia ’01 Conference, Kunming, China, 2001, pp. 182–201) was used to explain such phenomenon.