Impact of ionic liquids on absorption behaviour of natural fibers/biopolyethylene biocomposites

For many years, there has been a growing interest in technologies enabling the replacement of conventional polymer composites with new materials made from renewable raw materials. It is important to assess the behaviour of biocomposites in various environments, including humid conditions. Recently, ionic liquids have been studied as potential modificators of polymers properties, especially flame retardants. In previous study the impact of ionic liquids on thermal and mechanical properties of biocomposites was assessed. In this study the influence of ionic liquids on moisture absorption properties of biocomposites at different relative humidities (RH) was assessed. The biocomposites were built from polyethylene from renewable resources reinforced with flax or hemp fibers. The effect of the addition of 0.5, 1.0, 2.5 and 5 wt.% phosphonium ionic liquids on the moisture absorption properties of biopolyethylene biocomposite reinforced with natural fibers were tested. Mixtures of biopolyethylene, natural fibers and ionic liquid were calendered at 180 °C and then were compounded by injection moulding. The prepared samples were then characterized for their moisture uptake at 30%, 50% and 100% RH. Moisture absorption by biocomposites depended on the structure of the ionic liquid and the type of fiber. The saturation of moisture of about 0.054% was found for samples modified with tributylethylphosphonium diethyl phosphate and reinforced with flax and hemp fibers at RH 100%. The environmental resistance of the materials was found to be improved after the addition of trihexyltetradecylphosphonium bis (2,4,4-trimethylpentyl) phosphinate. Biocomposites with hemp fibers showed slightly less absorption than with flax fibers. It was also observed that ionic liquids: (bis (2,4,4-trimethylpentyl) phosphinate trihexyltetradecylphosphonate) and (bis (2-ethylhexyl) trihexyltetradecylphosphonium phosphate) protect PE biocomposites with plant fibers against mold in high humidity conditions (RH 100%).