91
4
Substances like water, which are composed of single unit, are
essentially Newtonian as there are no entanglements present for
the applied shear rate to make them disentangled. Such materials
have constant viscosity across the entire window of shear rates. A
drop in viscosity with an increase in shear rate, which is a symptom of disentangling of the entanglements, is called shear-thinning
behaviour. As far as the melt rheology of polymer is concerned,
the polymer shall start to show a shear-thinning behaviour after a
critical shear rate. The resistance to flow (i.e. melt viscosity) of the
polymer will drop eventually on the application of higher shear
rates (analogous to applied force). This can have severe implications on the final product. The sole aim of processing thermoplastics is to make a moulded product. Owing to its shear-thinning
nature, the viscosity of the polymer is much lower than the supposed viscosity—had the polymer been Newtonian. Eventually,
lower viscosity leads to higher flow and, ultimately, higher output
rate or mouldability.
Therefore, there is always a prior need to having the knowledge
of the output rate of the melt from any polymer processing device.
Since the output has a direct correlation to the velocity of the polymer melt when flowing and the velocity of the flowing polymer is
directly dependent on the viscosity of the polymer at the processing
conditions, the importance of rheology in governing the polymer
processing methods is immense. Rheological analysis of thermoplastics can give a wide comparative knowledge about the processability of a particular polymer.
In order to develop a high-quality composite structure, there
should be good bond between the fibres and the matrix. This
requires proper wetting of the fibres by the matrix. Therefore, the
matrix resin must flow. In the processing of composites, following
four types of flows occur.
1. Resin percolation through and along the fibre
2. Transverse flow through the fibres (squeezing flow consisting
of resin shear within the ply in the transverse direction)
3. Intraply shear along the fibre direction
4. Interply slip (cooperative flow among the layers with the layers
in either the same or different orientations)
In polymeric composites, the apparent viscosity (η∗) increases
with an increase in fibre loading and decreases with an increase in
temperature and shearing. This is due to the shear-thinning
behaviour of the matrix. A greater amount of fibre loading may
cause enhanced friction at the interface of the polymer and its
fibres, which may hinder the chain mobility, gradually increasing
viscosity. The incorporation of fibres in polymer systems generally
increases the viscosity. The more nonpolymeric medium in the
matrix, the more the hindrance for the movement of polymeric
chains. The orientation of fibres strongly depends on the amount
of fibre loading. At optimal or lower fibre loading, the flow of the
polymer is more feasible and enables the fibres to align in the
direction of flow. This in turn causes anisotropy in short fibre
composites.
4.2 · Rheology Aspects of a Polymeric Matrix
4
Substances like water, which are composed of single unit, are
essentially Newtonian as there are no entanglements present for
the applied shear rate to make them disentangled. Such materials
have constant viscosity across the entire window of shear rates. A
drop in viscosity with an increase in shear rate, which is a symptom of disentangling of the entanglements, is called shear-thinning
behaviour. As far as the melt rheology of polymer is concerned,
the polymer shall start to show a shear-thinning behaviour after a
critical shear rate. The resistance to flow (i.e. melt viscosity) of the
polymer will drop eventually on the application of higher shear
rates (analogous to applied force). This can have severe implications on the final product. The sole aim of processing thermoplastics is to make a moulded product. Owing to its shear-thinning
nature, the viscosity of the polymer is much lower than the supposed viscosity—had the polymer been Newtonian. Eventually,
lower viscosity leads to higher flow and, ultimately, higher output
rate or mouldability.
Therefore, there is always a prior need to having the knowledge
of the output rate of the melt from any polymer processing device.
Since the output has a direct correlation to the velocity of the polymer melt when flowing and the velocity of the flowing polymer is
directly dependent on the viscosity of the polymer at the processing
conditions, the importance of rheology in governing the polymer
processing methods is immense. Rheological analysis of thermoplastics can give a wide comparative knowledge about the processability of a particular polymer.
In order to develop a high-quality composite structure, there
should be good bond between the fibres and the matrix. This
requires proper wetting of the fibres by the matrix. Therefore, the
matrix resin must flow. In the processing of composites, following
four types of flows occur.
1. Resin percolation through and along the fibre
2. Transverse flow through the fibres (squeezing flow consisting
of resin shear within the ply in the transverse direction)
3. Intraply shear along the fibre direction
4. Interply slip (cooperative flow among the layers with the layers
in either the same or different orientations)
In polymeric composites, the apparent viscosity (η∗) increases
with an increase in fibre loading and decreases with an increase in
temperature and shearing. This is due to the shear-thinning
behaviour of the matrix. A greater amount of fibre loading may
cause enhanced friction at the interface of the polymer and its
fibres, which may hinder the chain mobility, gradually increasing
viscosity. The incorporation of fibres in polymer systems generally
increases the viscosity. The more nonpolymeric medium in the
matrix, the more the hindrance for the movement of polymeric
chains. The orientation of fibres strongly depends on the amount
of fibre loading. At optimal or lower fibre loading, the flow of the
polymer is more feasible and enables the fibres to align in the
direction of flow. This in turn causes anisotropy in short fibre
composites.
4.2 · Rheology Aspects of a Polymeric Matrix