| dc.description.abstract |
The study shows that standard plastics like polypropylene and high density
polyethylene can be reinforced by adding nylon short fibres. Compared to the conventional
glass reinforced thermoplastics this novel class of reinforced thermoplastics has the major
advantage of recyclability. Hence such composites represent a new spectrum of recyclable
polymer composites. The fibre length and fibre diameter used for reinforcement are critical
parameters While there is a critical fibre length below which no effective reinforcement
takes place, the reinforcement improves when the fibre diameter decreases due to
increased surface area.While the fibres alone give moderate reinforcement, chemical modification of the
matrix can further improve the strength and modulus of the composites. Maleic anhydride
grafting in presence of styrene was found to be the most efficient chemical modification.
While the fibre addition enhances the viscosity of the melt at lower shear rates, the
enhancement at higher shear rate is only marginal. This shows that processing of the
composite can be done in a similar way to that of the matrix polymer in high shear
operations such as injection moulding. Another significant observation is the decrease in
melt viscosity of the composite upon grafting. Thus chemical modification of matrix makes
processing of the composite easier in addition to improving the mechanical load bearing
capacity.For the development of a useful short fibre composite, selection of proper
materials, optimum design with regard to the particular product and choosing proper
processing parameters are most essential. Since there is a co-influence of many
parameters, analytical solutions are difficult. Hence for selecting proper processing
parameters 'rnold flow' software was utilized. The orientation of the fibres, mechanical
properties, temperature profile, shrinkage, fill time etc. were determined using the software.Another interesting feature of the nylon fibre/PP and nylon fibre/HDPE composites
is their thermal behaviour. Both nylon and PP degrade at the same temperature in single
steps and hence the thermal degradation behaviour of the composites is also being
predictable. It is observed that the thermal behaviour of the matrix or reinforcement does not
affect each other. Almost similar behaviour is observed in the case of nylon fibre/HDPE
composites. Another equally significant factor is the nucleating effect of nylon fibre when the
composite melt cools down. In the presence of the fibre the onset of crystallization occurs at
slightly higher temperature.When the matrix is modified by grafting, the onset of crystallization occurs at still
higher temperature. Hence it may be calculated that one reason for the improvement in
mechanical behaviour of the composite is the difference in crystallization behaviour of the
matrix in presence of the fibre.As mentioned earlier, a major advantage of these composites is their recyclability.
Two basic approaches may be employed for recycling namely, low temperature recycling
and high temperature recycling. In the low temperature recycling, the recycling is done at a
temperature above the melting point of the matrix, but below that of the fibres while in the
high temperature route. the recycling is done at a temperature above the melting points of
both matrix and fibre. The former is particularly interesting in that the recycled material has
equal or even better mechanical properties compared to the initial product. This is possible
because the orientation of the fibre can improve with successive recycling. Hence such
recycled composites can be used for the same applications for which the original composite
was developed. In high temperature recycling, the composite is converted into a blend and
hence the properties will be inferior to that of the original composite, but will be higher than
that of the matrix material alone. |
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