This study deals with medium density wood fibre (MDF) and Kraft fibre reinforcedpolypropylene (PP) composites produced using extrusion followed by injectionmoulding. Initially, composites were produced with MDF fibre using 10, 20, 30, 40,50 and 60 wt% fibre, and 1, 2, 3 and 4 wt% maleated polypropylene (MAPP) as acoupling agent. A fibre content of 50 wt% with 3 wt% MAPP was found to beoptimum. Alkali treatment of fibre was carried out to improve the interfacialbonding. After treatment, fibre surface charge was found to increase, but single fibretensile strength (TS) and Young's modulus were (YM) decreased. Alkali treatmentreduced composite TS but increased YM.The effects of hemicellulose and residual lignin content were assessed withKraft fibre (subjected to different stages of a standard Kraft pulping process andtherefore consisting of different hemicellulose and residual lignin contents). Fibresurface charge was found to increase with decreasing residual lignin content.Composites containing higher amounts of lignin lead to lower TS and lower thermalstability. Composites were subjected to accelerated weathering for 1000 hours. TSand YM were found to decrease during weathering, and the extent of reduction wasfound to be higher for composites with higher residual lignin. The reduction ofmechanical properties was found to be due to degradation of lignin and PP chainscission as evaluated by increase in PP crystallinity after weathering. As low lignin(bleached) Kraft fibre composites were found to provide superior mechanicalproperties, as well as more stable during accelerated weathering, further studyincluding optimisation of MAPP content, effects of fibre contents, fibre length, fibrebeating, hygrothermal ageing and recycling were carried out with bleached Kraftfibre.MAPP contents of 1, 2, 3, 4, 5, 7 and 10 wt% were used in Kraft fibrereinforced PP composites, and 3-5 wt% was found to be most favourable. Compositefibre content was varied between 30-50 wt%, and 40 wt% found to provide themaximum TS. To investigate the effects of fibre length on composites, fibre fractionsof different length distribution were separated using a pressure screen. TS, YM andimpact strength were found to decrease and failure strain (FS) increased withdecreasing fibre length. To improve the interfacial bonding, the fibre was treated bymechanical beater. Fibre beating increased the TS of composites up to a certainpoint, beyond which TS decreased. Hygothermal ageing of composites was carriedout by immersing specimens in distilled water at 30, 50 and 70 C over an 8-monthperiod. Equilibrium moisture content and diffusion coefficient increased withincreased fibre content in composites as well as with increased immersiontemperature. Composites without coupling agent showed higher water uptake anddiffusion coefficient than that of with coupling agent. After hygrothermal ageing theTS and YM decreased but FS and impact strength were found to increase.An investigation into the effects of recycling was carried out with compositescontaining either 40 wt% or 50 wt% fibre (bleached Kraft) with 4 wt% MAPP, andrecycled up to eight times. For composites with 40 wt% fibre, TS and YM werefound to decrease with increased recycling by up to 25% for TS and 17% for YM(after being recycled 8 times). Although TS was lower for virgin composites with 50wt% fibre than for those with 40 wt% fibre, this initially increased with recycling byup to 14% (after being recycled 2 times), which was considered to be due toimproved fibre dispersion, but then decreased with further recycling, and an overall11% reduction of TS was found after recycling 8 times compared to the virgincomposites. YM was higher for virgin composites with 50 wt% fibre than those withfor 40 wt% fibre, and also initially increased with recycling but decreased uponfurther recycling. Recycling was found to increase thermal stability. The TS ofcomposites made by combining recycled with virgin materials was also assessed.Hygrothermal ageing behaviour of recycled composites was also investigated byimmersing specimens in distilled water at 50 C over a 9 month period. It was foundthat the diffusion coefficient and the equilibrium moisture contents of compositesdecreased with increased number of times the materials were recycled. Afterhygrothermal ageing, TS and YM of composites were found to decrease. However,the extent of reduction was found to decrease with increased recycling.
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