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Plastics often contain fillers or reinforcements
to provide specific properties or reduce costs. Majority of these
fillers or reinforcements are incompatible with polymer matrix.
This incompatiblity results into processing as well as performance
problems. The problems become more intense when the fillers/reinforcements
are used at higher levels. The fillers/reinforcing materials are
required to be modified on surface in order to overcome the problem
of incompatibility.
Surface modification is an important step for manufacture of good
quality fillers/reinforcing materials. Basically two types of surface
modifications are normally used.
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The first modification provides physical bond between filler
and polymer. Normally waxes and fatty acids are employed in this
type of physical bond. Calcium Carbonate is the largest mineral
filler used particularly in PVC, the largest thermoplastics used
for manufacture of filled product. Physical surface modification
is less costly. The physically treated filler often command very
small additional price compared to the untreated filler. However,
this type of physical surface modification is not suitable for clays,
wollastonites, mica or Aluminium Trihydrates.
It also is not suitable for glass reinforcements. These fillers/reinforcements
need chemical bonding at molecular level. |
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Such a chemical surface modification is achieved by the use of
"Coupling Agents" - the special chemicals containing Silanes,
Titanates or Zirconates. All these chemicals are essentially organometallic
in nature and contain alkoxy groups (X) that react with fillers.
There is also an organo functional group (Y) that allows bonding
to the polymermatrix by chemical reaction . The Y component contains
different functional groups to provide bonding to different polymermatrix. |
The Silanes have been in use for a very long time and are the largest
coupling agents. They have been found to perform very well in polar
polymers like PVC etc. as well as for thermosets and elastomers.
They have limited compatibilty with Polyolefins although sometimes
they are used to improve dispersion.
Titanates are more effective in Polyolefins. Titanates, however
tend to discolour polymer that contain Phenols. Zirconates the most
recent coupling agents overcome these limitations of Silanes/Titanates.
However the use of Zirconates is still restricted on account of
high cost.
The chemical bonding can also be achieved by Functionalised Polymers.
These have been developed recently and are less expensive than coupling
agents but are more expensive compared to physical bonding. The
functionalised polymers with acid and anhydride modified PP, PE
or EVA are now increasingly being used in Polyolefins, particularly
for glass or mica reinforcements.
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