94
4
leading to an inefficient output rate of the resulting polymer. This
calls for a requirement of a thorough viscometric plot analysis for
polymers like nylon and judgement of its processing temperature
prior to its processing. A corollary to this results in the selection of
a temperature profile in the extruder that should be maintained in
such a way to prevent the nylon from flowing like water, ultimately
reducing the ease of processability.
Furthermore, the incorporation of fillers, such as glass fibre, that
tend to break at high shear need to be carefully processed. Glass
fibres are mostly fed into the extruder from a secondary feeder
located somewhere amidst the flow path. The temperature across
the primary feeder to the secondary feeder needs to be maintained
in such a way that the glass fibres are fully wetted by the molten
matrix. While incorporating inorganic particulate fillers from the
primary feeder, it is of utmost importance that the temperature at
the primary feeder junction is higher than the melting or glass transition temperature of the matrix. This way the filler gets a high
enough percentage of the residence time to get impregnated by the
molten polymer matrix.
4.3.2 Rheology of Incorporation of Elastomer
Rheology is also very important when it comes to incorporating
elastomer into the matrix of thermoplastics. To disperse elastomer
particles uniformly into the matrix of a thermoplastic, one has to
implement high shear rates. High shear rates are responsible for
developing an intermediate morphology that has elongated the dispersed elastomer particles throughout the polymer matrix. On
application of a high shear rate for a sufficiently long time, the elongated elastomer particles stretch further in order to develop striations during the mixing stage. At the end of the mixing time, the
striations get stretched to such an extent that they eventually collapse and yield elastomer particles of a fine size. The point to be
noted is that the final dispersion and distribution of elastomer particles in the final polymer matrix is a strong function of shear rate,
temperature, and a third factor, which is the difference in the viscosity of the thermoplastic as well as the elastomer. Elastomers are
known to have much higher viscosities when compared to thermoplastics owing to their molecular weights. The higher the viscosity
of the elastomer to be incorporated, the more difficult it is to get
dispersed in the matrix. Hence a higher shear rate is needed that
eventually calls for a higher power requirement. A thorough knowledge about the viscosity of the polymers at the processing temperature at high shear rates is necessary to ensure that a finely dispersed
morphology is obtained at an optimum power consumption.
4.3.3 Rheology in the Dispersion of Fillers
Polymer composites are supersets comprised of polymers incorporated with fillers of varying sizes. Fillers with sizes in microns
Chapter 4 · Rheology in Processing of Polymeric Composites
4
leading to an inefficient output rate of the resulting polymer. This
calls for a requirement of a thorough viscometric plot analysis for
polymers like nylon and judgement of its processing temperature
prior to its processing. A corollary to this results in the selection of
a temperature profile in the extruder that should be maintained in
such a way to prevent the nylon from flowing like water, ultimately
reducing the ease of processability.
Furthermore, the incorporation of fillers, such as glass fibre, that
tend to break at high shear need to be carefully processed. Glass
fibres are mostly fed into the extruder from a secondary feeder
located somewhere amidst the flow path. The temperature across
the primary feeder to the secondary feeder needs to be maintained
in such a way that the glass fibres are fully wetted by the molten
matrix. While incorporating inorganic particulate fillers from the
primary feeder, it is of utmost importance that the temperature at
the primary feeder junction is higher than the melting or glass transition temperature of the matrix. This way the filler gets a high
enough percentage of the residence time to get impregnated by the
molten polymer matrix.
4.3.2 Rheology of Incorporation of Elastomer
Rheology is also very important when it comes to incorporating
elastomer into the matrix of thermoplastics. To disperse elastomer
particles uniformly into the matrix of a thermoplastic, one has to
implement high shear rates. High shear rates are responsible for
developing an intermediate morphology that has elongated the dispersed elastomer particles throughout the polymer matrix. On
application of a high shear rate for a sufficiently long time, the elongated elastomer particles stretch further in order to develop striations during the mixing stage. At the end of the mixing time, the
striations get stretched to such an extent that they eventually collapse and yield elastomer particles of a fine size. The point to be
noted is that the final dispersion and distribution of elastomer particles in the final polymer matrix is a strong function of shear rate,
temperature, and a third factor, which is the difference in the viscosity of the thermoplastic as well as the elastomer. Elastomers are
known to have much higher viscosities when compared to thermoplastics owing to their molecular weights. The higher the viscosity
of the elastomer to be incorporated, the more difficult it is to get
dispersed in the matrix. Hence a higher shear rate is needed that
eventually calls for a higher power requirement. A thorough knowledge about the viscosity of the polymers at the processing temperature at high shear rates is necessary to ensure that a finely dispersed
morphology is obtained at an optimum power consumption.
4.3.3 Rheology in the Dispersion of Fillers
Polymer composites are supersets comprised of polymers incorporated with fillers of varying sizes. Fillers with sizes in microns
Chapter 4 · Rheology in Processing of Polymeric Composites