The resistance of a PVC melt to the applied shear forces during
the various
processing conditions that can be found in use, is a critical property
for
the polymer compound during its processing experience. This is particularly
true for injection moulding processes where the high shear gradients seen
can give rise to surface defects which in some cases can be unacceptable and
cause components to be rejected. Again conventional processing aids are
often incorporated to improve the melt properties but also often can
increase the melt viscocity and affect output potential.
Benefits in injection molding
CPCC containing moulding compounds demonstrate excellent processing behaviour
and produce components with high gloss without the need for conventional processing aids (see also surface
finish). GCCs fail to match this performance even when used in combination
with process aid. Table 1 illustrates the reformulation trends possible
to optimise process aid in moulding compounds.
Formulation with
process aid + GCC
Typical alternative
CPCC formulation
PVC
100
100
Stabiliser
2
2
Process aid
2
0
1µ GCC
3
-
CPCC
-
5
The CPCC formulation represents a significant cost saving and features
improved surface finish and lower reject rates.
Benefits in rigid PVC foam
The use of CPCC in rigid PVC foam formulations is seen to allow the development
of a more regular and uniform cell structure (see figure 1). This property
is derived from the beneficial improvement in melt reinforcement that
accompanies the incorporation of these ultrafine calcium carbonates. Melt
elasticity and elongation are improved which allows optimisation in the
use of other additives necessary for successful foam production. There
has been speculation as to whether CPCC can act as a “nucleating”
agent due to its very small particle size. This has yet to be confirmed
although in theory this would be an expected outcome of their use.
