Biobased epoxy networks reinforced with kenaf fibers: thermal and dynamic-mechanical properties

Recent concerns about the impact in nature of materials at the end of their lifetime, an increasing interest has been devoted to the research and development of environmentally friendly products to replace the petrochemical derived ones. Interesting perspectives as raw materials are offered by vegetable oils which, however, must be chemically modified. Among functional, groups special interest has been paid to the epoxy ring to prepare epoxidized triglycerides that can be used as pre-polymers of new thermosetting matrices with performances comparable to those derived from petrochemical sources. The aim of this work was to study novel biocomposites based on a resin from commercial epoxidized linseed oil (ELO), cured with tetrahydrophtalic anhydride (TPHA) (ELO/TPHA molar ratio (R) equal to 0.8) in presence of 1-methyl imidazole as catalyst (1 wt%). For composite systems, the inclusion of relatively low loadings (3%, 5% and 7% by weight) of kenaf fibres was considered. All materials, cured in a vacuum oven at 150 °C for 15 hours and post-cured at 170 °C for 1 hour, were characterized by thermal and dynamic-mechanical tests. About thermal measurements, non-isothermal calorimetric tests always revealed the achievement of a complete crosslinking while thermogravimetric analysis showed an incipient thermal degradation, approximately at 330 °C, apparently not influenced by additions of kenaf fibres, at least on the range of contents examined by now. Regarding dynamic-mechanical tests, neat bio epoxy resin and composites with kenaf fibres did not show a clear glassy plateau but their storage modulus decreased as temperature increased with trends apparently not affected by the inclusion of natural fibres. About the rubbery region, analogously a continuous diminishing modulus was always observed, probably for the formation of highly heterogeneous networks containing regions with different crosslink density, with values increasing with the fibre content.