92
4
4.3 Rheology in Material Selection
One of the renowned rheological instruments, the melt flow rate
(MFR) analyser is commonly used in plastic processing industries
to have a prior idea about the viscosity of the melt. For example, if
we have two thermoplastic materials (say polypropylene) with
varying PDI (say 3 and 10) but with similar MFR value, we ought to
have different eases of processability for the two polymers at a particular condition that eventually can be predicted by rheology. On
comparing the viscometric plots of the two polymers, the zero shear
viscosity values for the two polymers shall be very close. However,
the critical shear rate beyond which the shear-thinning regime
begins and the power law index, n, will vary significantly. The polymer with a higher PDI will have a critical shear rate much lower in
comparison to the polymer having a lower PDI. Also, if a polymer
has a narrower PDI, it will show a steeper viscometric plot in comparison to the polymer having a broader PDI.
To answer the question on how to process polymer composites
efficiently, the understanding of rheology for the matrix phase is of
utmost importance. This is mainly because the secondary phase
and the fillers are going to get incorporated into the matrix.
Commonly used matrices include polyethylene, polystyrene, etc.
Polymers like polyethylene are known to have branching inherently
in their polymer chain if no catalyst is used during synthesis.
Whether the polyethylene will have branching or not will also govern the processability of the polymer composites. The presence of
branching will increase the free volume of the macromolecules. In
addition, it will contribute to the broadening of the molecular
weight distribution thereby lowering die swell. The fact is that intermolecular vibration decreases in the case of branched polymers in
comparison to linear polymers; therefore, the lowering of shear viscosity is observed in linear polymers for the same molecular weight.
The lowering of melt viscosity leads to the ease of processability.
This requirement of shear rate is lower than that for linear polymers
of same molecular weight, and eventually it lowers the power consumption for a similar extent while mixing. The drop in shear viscosity of the polymer is in attendance only up to the point where the
long chain branching does not contribute to the entanglement density of the polymer. However, if the lengths of the long chain branching increase beyond a critical limit, it may contribute in the
entanglement density of the matrix. Hence, beyond a critical point
in the lengths of long chain branching, the melt viscosity of the
polymer shall start to show an increase.
Of the previously mentioned four flow patterns, resin percolation is the most important in achieving good bond between the
fibres and the matrix. Darcy’s one-dimensional law of flow through
porous media for one-dimensional penetration is given as
X
kPt
=
2
1 2
µ
/
(4.33)
Chapter 4 · Rheology in Processing of Polymeric Composites
4
4.3 Rheology in Material Selection
One of the renowned rheological instruments, the melt flow rate
(MFR) analyser is commonly used in plastic processing industries
to have a prior idea about the viscosity of the melt. For example, if
we have two thermoplastic materials (say polypropylene) with
varying PDI (say 3 and 10) but with similar MFR value, we ought to
have different eases of processability for the two polymers at a particular condition that eventually can be predicted by rheology. On
comparing the viscometric plots of the two polymers, the zero shear
viscosity values for the two polymers shall be very close. However,
the critical shear rate beyond which the shear-thinning regime
begins and the power law index, n, will vary significantly. The polymer with a higher PDI will have a critical shear rate much lower in
comparison to the polymer having a lower PDI. Also, if a polymer
has a narrower PDI, it will show a steeper viscometric plot in comparison to the polymer having a broader PDI.
To answer the question on how to process polymer composites
efficiently, the understanding of rheology for the matrix phase is of
utmost importance. This is mainly because the secondary phase
and the fillers are going to get incorporated into the matrix.
Commonly used matrices include polyethylene, polystyrene, etc.
Polymers like polyethylene are known to have branching inherently
in their polymer chain if no catalyst is used during synthesis.
Whether the polyethylene will have branching or not will also govern the processability of the polymer composites. The presence of
branching will increase the free volume of the macromolecules. In
addition, it will contribute to the broadening of the molecular
weight distribution thereby lowering die swell. The fact is that intermolecular vibration decreases in the case of branched polymers in
comparison to linear polymers; therefore, the lowering of shear viscosity is observed in linear polymers for the same molecular weight.
The lowering of melt viscosity leads to the ease of processability.
This requirement of shear rate is lower than that for linear polymers
of same molecular weight, and eventually it lowers the power consumption for a similar extent while mixing. The drop in shear viscosity of the polymer is in attendance only up to the point where the
long chain branching does not contribute to the entanglement density of the polymer. However, if the lengths of the long chain branching increase beyond a critical limit, it may contribute in the
entanglement density of the matrix. Hence, beyond a critical point
in the lengths of long chain branching, the melt viscosity of the
polymer shall start to show an increase.
Of the previously mentioned four flow patterns, resin percolation is the most important in achieving good bond between the
fibres and the matrix. Darcy’s one-dimensional law of flow through
porous media for one-dimensional penetration is given as
X
kPt
=
2
1 2
µ
/
(4.33)
Chapter 4 · Rheology in Processing of Polymeric Composites