Long-term moisture cycling performance and unexpected strengthening of non-dry flax fibre composites

Abstract

The results of using dried and non-dry fibre (conditioned at 50% and 80% RH) on properties of both flax/polyester and flax/epoxy composites during long-term moisture cycling were determined. The composites' properties are compared to the unaged composites and reference materials such as neat resins, glass/epoxy composite and beechwood. In general, the area damage of composites produced with non-dry fibre is lower than those made of dried fibre for both composites throughout the test. The lower area damage of composites produced with non-dry fibre supports the flexural test results wherein both composites produced with non-dry fibre showed better moisture durability than composites produced with dried fibre. The flexural test results showed that all composites' properties seem to recover or even significantly increase compared to unaged composites until the end of the cycling test. On average, the longitudinal mean modulus and strength of flax/polyester increased by 8% and 5%, respectively, while for flax/epoxy, these increases were 19% and 17%, respectively, after 20 cycles. The improvement in flexural properties of composites during the test is partially related to the stiffening of the resins, but is likely also due to some stiffening and strengthening of the flax fibres, which was unexpected. This was confirmed by impregnated fibre bundle tests on cycled flax fibres. The effect is commensurate to an hornification effect, also resulting in lower water sorption.