Creep behaviour of biopolymers and modified flax fibre composites

Creep-recovery behaviour of natural fibre-biopolymer composite systems, poly(lactic acid)-flax and poly(hydroxybutyrate)-flax, with various additives, was quantitatively resolved into elastic deformation, viscoelastic deformation and viscous flow. Comparative interpretation of creep behaviour between composites with different additives was presented and the factors that influenced creep were identified. In comparison with unmodified composites, the additive that proved to be the most effective in reducing creep was thiodiphenol, which formed hydrogen bonds at the interface of polymer and fibre; while a decrease was observed with tributyl citrate, a plasticiser. The suppression of creep in the composites with thiodiphenol was due to reduction in the viscous flow contribution to total creep and a significant increase in both the elastic and viscoelastic deformation. The reduction in creep for the plasticised composites was mainly attributed to a reduction in the viscous flow contribution and an increase in the viscoelastic deformation. The elastic component did not show any changes from the unmodified composite. With hyperbranched polyester, a toughness modifier, an increase in permanent strain and reduction in the recoverable strain were attributed to the hyperbranched molecules being able to easily flow past each other as they cannot form chain entanglements due to their globular structure.