776-00301 TYRIN IM bro - The Dow Chemical Company

PVC Modification
Polyvinyl-chloride (PVC) is an outstanding
material for the production of rigid extruded
goods, such as pipes or profiles. However
due to its high glass transition temperature,
it has shortcomings in impact strength. It has
therefore to be modified to provide impact
strength at ambient and low temperature.
Impact modifiers provide PVC compounds a
consistently ductile behaviour over a broad
temperature range. They are flexible materials
which are partially compatible with PVC.
They form a discrete phase in the PVC matrix,
which absorbs and dissipates shock energy.
During suspension polymerization of PVC grain
particles with a typical diameter of 65-150
microns are formed. (Figure 1). These consist
of smaller particles ranging from 0.2 to 1.5
microns in size: the primary particles. When
the PVC powder is being processed into a
fabricated product (e.g. by extrusion, calendering)
the grain structure is broken down
mainly by shear and the primary particles are
molten together at their surfaces.
The dominant factor in controlling the
degree of fusion is the melt temperature
and the additive package, consisting of
stablizers, lubricants, fillers, and impact
modifiers can affect the range of processing
conditions required for optimum impact
strength. This process is called fusion or
gelation. The achieved impact strength is
controlled by the degree of fusion of the
PVC matrix. An under fused or over fused
melt lowers the impact strength of the
final product.
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Essentially two classes of impact modifiers
exist, which are physically blended with s-PVC
during extrusion on twin screw extruders.
Acrylic based modifiers are often used,
which are core-shell particles of typically a
butyl-acrylate flexible core with a rigid shell
of poly-methyl-methacrylate. The shell
enables a good flowability of the product
and a good compatibility to PVC to enable a
good dispersion.
TYRIN Chlorinated Polyethylene (CPE) is a
thermoplastic material and its morphology
will develop during the melt processing of
the PVC dry-blend. TYRIN CPE and other low
melting ingredients will coat the PVC particles.
Figure 1: Primary particles structure of s-PVC
65-150 Micron
During subsequent extrusion a phase inversion
occurs and creates discrete CPE particles in a
continuous matrix of PVC.
TYRIN CPE offers an excellent combination of
compatibility, processibility, weatherability,
and impact efficiency in many rigid PVC formulations,
like window profiles, pipes, rigid
and foamed sheet, etc. The use of TYRIN CPE
makes it possible to use higher filler levels
because being thermoplastic in nature, it
flows and encapsulates inorganic filler particles.
This coating provides a mechanism to
control the particle adhesion to the PVC
matrix and thus improves the contribution to
impact resistance, whereas the filler aids
TYRIN dispersion.
Primary
Particles
Skin
Grain
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