108
4
meric composites. The effect of rheology on the final properties of
polymeric composites is given in . Table 4.5.
Apart from these effects of rheology, some important properties
of composites that are influenced by rheology are explained next.
4.6.1 Product Shrinkage
As a matter of fact, all matrices shrink after curing or moulding;
however, the degree of shrinkage may vary. Shrinkage also can be
a processing aid because it helps in getting the mouldings attached
on to the core of the mould. This makes it easy to remove because
the peripheral ends of the moulding on the core of the mould provide a location to push the moulding out of the mould. In a reverse
situation, expansion of the matrix will lead to the moulding
becoming attached to the cavity of the mould. This would make it
difficult to remove from the mould because the peripheral ends of
the moulding would be inside the cavity [10]. However, minimum
shrinkage is permissible for good product quality and properties.
High shrinkages occur due to a high flow rate of the matrix during
moulding.
. Table 4.5 Rheological properties vs. final properties of polymeric composites
Serial No.
Rheological behaviour of
matrix
Resultant effect
Final property of polymeric
composites
1.
High viscosity
Restricted flow, poor wetting
of fibres, resin-rich or starved
areas
Poor overall properties
2.
Low viscosity
Smooth flow, good wetting of
fibres
Good overall properties
(provided care is taken that resin
does not flush out of the mould)
3.
Quick solidification after
wetting of fibre
Good wetting of fibres
High amorphous content in
semi-crystalline polymers
Reduction in T g
4.
High elongational
viscosity
Matrix retracts after release of
pressure
High loss of modulus
5.
Sedimentation of filler or
additives
Phase separation and
segregation
Nonuniform properties
6.
Collection of matrix in
patches on the fabric
Lack of compatibility
between matrix and
reinforcement
Delamination
Poor overall properties
7.
Low viscosity due to
solvent
Residual solvent and porosity
Low modulus
Defective morphology and
shrinkages
Poor overall properties
8.
Viscosity build-up due to
condensation and
reaction of matrix
Porosity in the composite and
resin-starved areas
Defective morphology
Poor overall properties
Chapter 4 · Rheology in Processing of Polymeric Composites
4
meric composites. The effect of rheology on the final properties of
polymeric composites is given in . Table 4.5.
Apart from these effects of rheology, some important properties
of composites that are influenced by rheology are explained next.
4.6.1 Product Shrinkage
As a matter of fact, all matrices shrink after curing or moulding;
however, the degree of shrinkage may vary. Shrinkage also can be
a processing aid because it helps in getting the mouldings attached
on to the core of the mould. This makes it easy to remove because
the peripheral ends of the moulding on the core of the mould provide a location to push the moulding out of the mould. In a reverse
situation, expansion of the matrix will lead to the moulding
becoming attached to the cavity of the mould. This would make it
difficult to remove from the mould because the peripheral ends of
the moulding would be inside the cavity [10]. However, minimum
shrinkage is permissible for good product quality and properties.
High shrinkages occur due to a high flow rate of the matrix during
moulding.
. Table 4.5 Rheological properties vs. final properties of polymeric composites
Serial No.
Rheological behaviour of
matrix
Resultant effect
Final property of polymeric
composites
1.
High viscosity
Restricted flow, poor wetting
of fibres, resin-rich or starved
areas
Poor overall properties
2.
Low viscosity
Smooth flow, good wetting of
fibres
Good overall properties
(provided care is taken that resin
does not flush out of the mould)
3.
Quick solidification after
wetting of fibre
Good wetting of fibres
High amorphous content in
semi-crystalline polymers
Reduction in T g
4.
High elongational
viscosity
Matrix retracts after release of
pressure
High loss of modulus
5.
Sedimentation of filler or
additives
Phase separation and
segregation
Nonuniform properties
6.
Collection of matrix in
patches on the fabric
Lack of compatibility
between matrix and
reinforcement
Delamination
Poor overall properties
7.
Low viscosity due to
solvent
Residual solvent and porosity
Low modulus
Defective morphology and
shrinkages
Poor overall properties
8.
Viscosity build-up due to
condensation and
reaction of matrix
Porosity in the composite and
resin-starved areas
Defective morphology
Poor overall properties
Chapter 4 · Rheology in Processing of Polymeric Composites