Mechanical, thermooxidative and biodegradable properties of composites from epoxyurethanes and chemically modified hemp woody core

Natural fibre reinforced polymer composites nowadays are considered to be attractive cheap, safe and eco-friendly materials. The main problem of such composites related to the hydrophilicity of plant fibres may be successfully solved by chemical modification of their surface. However, some characteristics of the materials may be suppressed after this procedure. Therefore, the aim of the research is to find out the impact of chemical modification of filler on thermooxidative stability, tensile and flexural strength, as well as on biodegradability of polymer composites. The novelty of this work is in the examining new materials on the basis of Si-containing epoxyurethanes and chemically treated hemp woody core (HWC). Woody core that is the side product of hemp industry requiring its apropriate utilization was exposed to mercerization with sodium hydroxide solution and to further functionalization with epoxidized soybean oil (ESO) or 3-aminopropyltriethoxysilane (APS). Raw and surface treated HWC was used as reinforcement for two types of organic-inorganic epoxyurethane matrices made from sodium silicate, polyurethane prepolymer based on polyisocyanate and castor oil, and either diglycidyl ether of bisphenol-A (DGEBA) or ESO as epoxy component. Functionalization of HWC led to better mechanical properties of composites. Compared to the corresponding materials including untreated filler, maximum increase in flexural strength (26 %) was observed for the samples with ESO-containing epoxyurethane and silanized HWC, while maximum increase in tensile strength (53 %) was revealed for the ones with DGEBA-containing epoxyurethane and oil treated HWC. Thermooxidative stability was also higher for composites reinforced with functionalized HWC. The specimens with APS-treated HWC performed the best at thermal decomposition. The values of their T50% were up to 68 °C more than those for composites with unmodified filler. At the same time, the samples based on APS- or ESO-treated HWC were the most resistant to biodegradation, which may be concluded from their smallest weight loss during soil burial test.