Effect of compaction pressure on water absorption behaviour and mechanical properties of hemp fibre reinforced vinyl ester composites

Natural bast fibres such as hemp, jute and flax have been used as eco-friendly forms of reinforcements in composite materials. The advantages of these natural fibres include low cost, renewable, low density, high specific strength and modulus. In this study, hemp fibre reinforced vinyl ester (VE) composite laminates were fabricated using compression moulding method by applying different compaction pressures ranging from 1.5 to 4.5 MPa and the effect of compaction pressure on water absorption behaviour, flexural and low velocity impact properties were evaluated. The maximum percentage weight gain for 1.5, 2.5, 3.5 and 4.5 MPa of compaction pressures of hemp reinforced vinyl ester composite specimens immersed at room temperature for 504 hrs were recorded at 7.08, 6.75, 5.75 and 6.33%, respectively. The water uptake behaviour for all specimens was linear in the beginning, and then slowly approached to the saturation after a prolonged period of time following Fickian diffusion behaviour. The results show that percentage water absorption decreases with increasing compaction pressure up to 3.5 MPa. Increasing the compaction pressure up to 3.5 MPa increased the flexural strength and and modulus as well as impact energy absorption capability which is related to the reduced void content. However, water replence behaviour, flexural and impact properties were decreased with increased compaction pressure above 3.5 MPa. The experimental results showed that the samples prepared at a compaction pressure of 3.5 MPa exhibited best water repelance and mechanical properties. The failure mechanism of impacted speciemens was analysed by scanning electron microscopy (SEM).