Pultrusion of flax/poly(lactic acid) commingled yarns and nonwoven fabrics
The article reports on the successful processing of flax-reinforced poly(lactic acid) (PLA) profiles by thermoplastic pultrusion into sandwich' structures consisting of commingled yarns as outer layers and layers of nonwoven material in the core section. The results showed that a good quality matrix impregnation was obtained. The profiles produced from only commingled yarns had higher impact strengths due to yarns in the middle of the profiles; the matrix of which did not melt during pultrusion. The flexural properties of the composites improved with increasing nonwoven layers. Good PLA impregnation improved the flexural properties. The tensile strength of the sandwich' structures was much lower than that of commingled yarn-based composites, this being attributed to the poor distribution of flax fibres and poor interfacial adhesion leading to poor stress transfer from matrix to fibres. Crystallinity decreased as the number of nonwoven layers increased presumably due to blockage of nucleating sites essential for crystal growth. The storage modulus of composites reinforced with commingled yarns was greater than that of the composites reinforced with sandwich' material, confirming effective transfer of stress from matrix to the commingled flax yarns and indicating that increasing nonwoven layers decreased adhesion between reinforcement and the matrix. The effects of a wide range of pultrusion process parameters on the mechanical properties of resulting profiles were investigated. Higher die temperatures (290 degrees C) and lower pulling speeds (0.5mmin(-1)) improved the mechanical properties due to better melting and lower viscosity of the matrix, enabling increased penetration within the reinforcement. Flexural and tensile properties of microwave-heated samples were very low due to fibre degradation. Scanning electron microscopic images showed that fibre impregnation was good in sandwich' systems, but fibre distribution remained a challenge.