| dc.description.abstract | This study deals with medium density wood fibre (MDF) and Kraft fibre reinforced
polypropylene (PP) composites produced using extrusion followed by injection
moulding. 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 a
coupling agent. A fibre content of 50 wt% with 3 wt% MAPP was found to be
optimum. Alkali treatment of fibre was carried out to improve the interfacial
bonding. After treatment, fibre surface charge was found to increase, but single fibre
tensile strength (TS) and Young's modulus were (YM) decreased. Alkali treatment
reduced composite TS but increased YM.
The effects of hemicellulose and residual lignin content were assessed with
Kraft fibre (subjected to different stages of a standard Kraft pulping process and
therefore consisting of different hemicellulose and residual lignin contents). Fibre
surface charge was found to increase with decreasing residual lignin content.
Composites containing higher amounts of lignin lead to lower TS and lower thermal
stability. Composites were subjected to accelerated weathering for 1000 hours. TS
and YM were found to decrease during weathering, and the extent of reduction was
found to be higher for composites with higher residual lignin. The reduction of
mechanical properties was found to be due to degradation of lignin and PP chain
scission as evaluated by increase in PP crystallinity after weathering. As low lignin
(bleached) Kraft fibre composites were found to provide superior mechanical
properties, as well as more stable during accelerated weathering, further study
including optimisation of MAPP content, effects of fibre contents, fibre length, fibre
beating, hygrothermal ageing and recycling were carried out with bleached Kraft
fibre.
MAPP contents of 1, 2, 3, 4, 5, 7 and 10 wt% were used in Kraft fibre
reinforced PP composites, and 3-5 wt% was found to be most favourable. Composite
fibre content was varied between 30-50 wt%, and 40 wt% found to provide the
maximum TS. To investigate the effects of fibre length on composites, fibre fractions
of different length distribution were separated using a pressure screen. TS, YM and
impact strength were found to decrease and failure strain (FS) increased with
decreasing fibre length. To improve the interfacial bonding, the fibre was treated by
mechanical beater. Fibre beating increased the TS of composites up to a certain
point, beyond which TS decreased. Hygothermal ageing of composites was carried
out by immersing specimens in distilled water at 30, 50 and 70 C over an 8-month
period. Equilibrium moisture content and diffusion coefficient increased with
increased fibre content in composites as well as with increased immersion
temperature. Composites without coupling agent showed higher water uptake and
diffusion coefficient than that of with coupling agent. After hygrothermal ageing the
TS and YM decreased but FS and impact strength were found to increase.
An investigation into the effects of recycling was carried out with composites
containing either 40 wt% or 50 wt% fibre (bleached Kraft) with 4 wt% MAPP, and
recycled up to eight times. For composites with 40 wt% fibre, TS and YM were
found 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 50
wt% fibre than for those with 40 wt% fibre, this initially increased with recycling by
up to 14% (after being recycled 2 times), which was considered to be due to
improved fibre dispersion, but then decreased with further recycling, and an overall
11% reduction of TS was found after recycling 8 times compared to the virgin
composites. YM was higher for virgin composites with 50 wt% fibre than those with
for 40 wt% fibre, and also initially increased with recycling but decreased upon
further recycling. Recycling was found to increase thermal stability. The TS of
composites made by combining recycled with virgin materials was also assessed.
Hygrothermal ageing behaviour of recycled composites was also investigated by
immersing specimens in distilled water at 50 C over a 9 month period. It was found
that the diffusion coefficient and the equilibrium moisture contents of composites
decreased with increased number of times the materials were recycled. After
hygrothermal ageing, TS and YM of composites were found to decrease. However,
the extent of reduction was found to decrease with increased recycling. | en_NZ |
