Comparison of injection moulded, natural fibre-reinforced composites with PP and PLA as matrices

Poly(lactic acid) (PLA) and polypropylene (PP) were comparatively investigated as matrices for injection-moulded composites containing small (1-3 wt%) amounts of short sisal fibre. The morphology, thermal and dynamic mechanical properties, as well as degradation characteristics were investigated. The scanning electron microscopy (SEM) micrographs of the composites show more intimate contact and better interaction between the fibres and PLA, compared to PP. This improved interaction was confirmed by the Fourier-transform infrared (FTIR) spectroscopy results which showed the presence of hydrogen bonding interaction between PLA and the fibres. The thermal stability (as determined through thermogravimetric analysis [TGA]) of both polymers increased with increasing fibre content, with a more significant improvement in the case of PP. The differential scanning calorimetry (DSC) results showed a significant influence of the fibres on the cold crystallization and melting behaviour of PLA, even at the low fibre contents of 1-3%. The influence of the fibres on the melting characteristics of the PP was negligible, but it had a significant influence on the nonisothermal crystallization temperature range. Both the storage and loss moduli of the PLA decreased with increasing fibre content below the glass transition of PLA, but the influence on the loss modulus was more significant. The dynamic mechanical analysis (DMA) results clearly show cold crystallization of PLA around 110 degrees C, and the presence of fibres gave rise to higher modulus values between the cold crystallization and melting of the PLA. The presence of fibres also had an influence on the dynamic mechanical properties of PP. This article further describes basic biodegradation observations for the investigated samples.