98
4
4.4.3 Dependence of Processing on Rheology
The major constituents of polymeric composite are the reinforcement and the polymeric matrix. The role of reinforcement during
processing is minimal as long as its physical orientation is maintained, and it is chemically and thermally inert. Usually, most of the
reinforcements are thermally and chemically inert during the processing time and increase in temperature for polymeric composites.
Of course, the particulate and short fibre reinforcements in the
matrix affect the viscosity of the overall mixture, so the viscosity is
increased. However, the increase in viscosity applies to the matrix
only. Therefore, the major role in the processing of polymeric composites is played by the matrices because its physical state from liquid to solid determines the completion of the processing of the
polymeric composite. The fabrication process is dependent upon
the behaviour of polymers in terms of the response of flow with
time and temperature.
The aim of any fabrication process is to achieve a high production rate with a reduction in processing time. Therefore, the curing
or solidification of the matrix has to take place at the fastest rate
without any defect occurring in the part. It is mandatory that temperature and time—both—need to be optimized to shorten the processing time. The flow of the matrix needs to be assured to percolate
throughout the reinforcement to wet it. The lowest viscosity of the
matrix resin enables easy percolation of it around the fibres to facilitate their wetting. Soon after the complete wettability is achieved,
the solidification of the matrix resin must take place to form the
desired composite.
It is amply evident from this discussion that the flow of the
matrix in processing controls the overall quality of the composite,
including the processing time and cost.
4.4.4 Change of Viscosity with Time
and Temperature
Time and temperature both have a profound effect on the viscosity
of the matrix in composites. Within the elapsed time, the viscosity
of the composite system—cured or solidified—increases due to the
advancement of cross-linking of the polymeric network in thermosetting matrices and also with solidification to the melt in thermoplastic matrices.
However, the effect of temperature is reversed in the case of
thermoplastic matrices. With increasing temperature, the viscosity
of thermoplastic matrices comes down. Also in the case of thermosetting matrices, the viscosity drops initially at moderate temperatures due to the physical separation of the polymeric chains of the
matrix, which helps in the percolation of the matrix around the
fibres in composites. However, by increasing the temperature further, the viscosity starts increasing due to the onset of cross-linking
of the polymeric network structure. The viscosity behaviour in
polymeric matrices is shown in . Fig. 4.7. The temperatures
Chapter 4 · Rheology in Processing of Polymeric Composites
4
4.4.3 Dependence of Processing on Rheology
The major constituents of polymeric composite are the reinforcement and the polymeric matrix. The role of reinforcement during
processing is minimal as long as its physical orientation is maintained, and it is chemically and thermally inert. Usually, most of the
reinforcements are thermally and chemically inert during the processing time and increase in temperature for polymeric composites.
Of course, the particulate and short fibre reinforcements in the
matrix affect the viscosity of the overall mixture, so the viscosity is
increased. However, the increase in viscosity applies to the matrix
only. Therefore, the major role in the processing of polymeric composites is played by the matrices because its physical state from liquid to solid determines the completion of the processing of the
polymeric composite. The fabrication process is dependent upon
the behaviour of polymers in terms of the response of flow with
time and temperature.
The aim of any fabrication process is to achieve a high production rate with a reduction in processing time. Therefore, the curing
or solidification of the matrix has to take place at the fastest rate
without any defect occurring in the part. It is mandatory that temperature and time—both—need to be optimized to shorten the processing time. The flow of the matrix needs to be assured to percolate
throughout the reinforcement to wet it. The lowest viscosity of the
matrix resin enables easy percolation of it around the fibres to facilitate their wetting. Soon after the complete wettability is achieved,
the solidification of the matrix resin must take place to form the
desired composite.
It is amply evident from this discussion that the flow of the
matrix in processing controls the overall quality of the composite,
including the processing time and cost.
4.4.4 Change of Viscosity with Time
and Temperature
Time and temperature both have a profound effect on the viscosity
of the matrix in composites. Within the elapsed time, the viscosity
of the composite system—cured or solidified—increases due to the
advancement of cross-linking of the polymeric network in thermosetting matrices and also with solidification to the melt in thermoplastic matrices.
However, the effect of temperature is reversed in the case of
thermoplastic matrices. With increasing temperature, the viscosity
of thermoplastic matrices comes down. Also in the case of thermosetting matrices, the viscosity drops initially at moderate temperatures due to the physical separation of the polymeric chains of the
matrix, which helps in the percolation of the matrix around the
fibres in composites. However, by increasing the temperature further, the viscosity starts increasing due to the onset of cross-linking
of the polymeric network structure. The viscosity behaviour in
polymeric matrices is shown in . Fig. 4.7. The temperatures
Chapter 4 · Rheology in Processing of Polymeric Composites