Improvement of selected properties of Wood-Polymer Composites (WPC) – Silane modification of wood particles
Description:... Wood Polymer Composites are a new group of hybrid materials, which combine the advantages of synthetic polymers such as polyolefines and natural polymers such as wood; whereas the synthetic polymer is used as matrix material and the wood as reinforcement material or filler. As matrix material, principally every thermoplastic polymer with a processing temperature below 200°C can be used due to the temperature sensivity of wood. Wood Polymer Composites are processed typically with processing technologies from the plastic industry such as extrusion and injection molding. The present study was conducted to explore the possibility of wood particle modification with different types of silanes. It was the aim to contribute the silanes as compatibilizers or coupling agent and therefore improve the mechanical properties and the resistance against water. Norway spruce (Picea abies) as representative wood species was used in three different particle types. The size distribution for the wood particles ranges from 70-2500 µm. Four commercial available silanes with various functional groups (amino, di-amino, alkyl) were used as modification agents. The concentrations varied between 1.5%, 3.0%, 4.5% and 7.5%. As reference system commonly used maleated acid anhydride based coupling agents were used. The pre-treated wood particles were compounded via extrusion with polypropylene and samples were produced via injection and compression molding. The following properties were tested; tensile, bending, and impact strength, water uptake (cold and boiling water test), descent rate, weathering tests and durability test against basidiomycetes. SEM-EDX investigations proved the presence of silane either in the cell wall structure, or on the wood particle surface. Due to the structure and the functionality of the silanes it was expected that the silane treated wood particles are able to improve the mechanical properties. It was shown that the silanes had no significant effect as compatibilizer or coupling agent. The mechanical properties strongly increase with the usage of coupling agent. Both coupling agents were based on maleic acid anhydride grafted on a polymer backbone, whereas Type I reaches an optimum regarding the mechanical properties at 3%, the coupling agent Type II still improves the mechanical properties up to a ratio of 4.5% with no clear optimum. The silane pretreatment influences the improvement not significantly, compare to the improvement caused by the coupling agent. The used wood particles showed mechanical degradation during the compounding process. The biggest degradation was monitored for the wood particles of Lignocel® Type 9. Due to its fine structure it can be assumed that the Arbocel® C100 wood particles consist of only cell wall fragments of the wooden cell wall, which express a better resistance against mechanical destruction during the process. The Wood Polymer Composites showed a lower decay rate compared to solid wood. The moisture content within the Wood Polymer Composite samples ranges at the low optimum level for fungi attack and were not significantly improved by an accelerated wetting before the test. The core remains relatively dry. The main protection seems to come from the encapsulation by the polymer. The weathered Wood Polymer Composite samples showed a strong color change within a relatively short period of exposure. The color changed independently from the silane type or the silane ratio. Also an increase of cracks and gaps on the sample surface was observed compared to the unexposed sample and the pure polymer reference.
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