Processing and properties of silane crosslinked wood-polyethylene composites

Sammanfattning: Utilizing wood as filler and reinforcement in thermoplastic polymer-matrix composites, has gained interest during the last decades, mainly as profile extruded exterior building products such as deck boards, railings, and door- and window frame components. This thesis deals with silane-crosslinking of wood-polyethylene composites through reactiveextrusion. The reactive-extrusion aims to graft silanes to the polyethylene in the moltenphase using peroxide as initiator, and a subsequent curing in high humidity and elevated temperature results in crosslinking of the wood-composite. The goal of using this method is to increase the materials strength, toughness and resistance to creep. Long-term mechanical performance is an issue for wood composites of commonly used thermoplastic matrices and improvements of properties may make these composites applicable as primary load-bearing members in structures. The extrusion process, whereonly grafting of silanes should take place, has earlier shown to be difficult for a wood composite, leading to unintentional crosslinking (scorch) in the melt and thereby poor processability of the composite. The objective for this study was to investigate how to control this reactive-extrusion process for wood-polyethylene composites, by studying process-structure relations, and furthermore to understand the relationship between structure and properties of the composites. The process was investigated by use of different material compositions (papers I-III), curing modes (papers I-III) and extrudersettings (paper IV). The structure-properties relations were studied by use of various test and analysis methods (papers I-V). It was found that the process can be controlled such that scorch was suppressed, and the efficiency of curing was increased. Significant differences on the results were shown for extruder-variables like barrel heat, screw speed and screw configurations. The peroxideload in the grafting step also showed a strong effect. Consequently, a composite not scorched and effectively cured could be obtained, and it was concluded that the complexity of this process, in comparison to the silane-crosslinking of a neat polyethylene, is due to the hydrophilic wood present in the process. All crosslinked composites in this study have improved in strength, toughness and creep resistance, compared to a non-crosslinked counterpart. Not scorched composites showed highest strength, toughness and resistance to creep, despite a lower degree of crosslinking in the matrix after curing, compared to highly scorched samples. To investigate the mechanisms leading to properties improvement, dynamic-mechanical analysis and adhesion tests were employed. It showed that direct chemical bonding between wood and matrix polymer is plausible, but do not describe the mechanism for improvements. A model of interphase for silane-crosslinked wood-polyethylene composites was presented, where it was suggested that the wood particles become surface-treated in the extrusion process, leading to an interphase of more efficient stress transfer. Nothing proves that pure chemical bonding between the wood and polyethylene answers to the strength improvements of the silane-crosslinked composites. It was suggested that future work in this field, in terms of processing, should address the use of ethylene-vinyl-silane copolymers (EVS) or blends of polyethylene and EVS, to obviate the silane-grafting step in the process. However, since the bonds of this crosslinking (-Si-O-Si-) are reversible hydrolysable, the long-term stability should be evaluated. For only stabilizing the matrix polymer, crosslinking by use of peroxides only, may be a better option.