Page 70 - Handbook of Surface Improvement and Modification

5

Tack-free Surface

5.1 METHODS AND MECHANISMS OF TACK REDUCTION

Adhesives are materials developed to form bonds with surfaces. Abhesives are films or
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coatings that prevent or greatly reduce adhesion. An abherend is an anti-adhesive surface.
Abhesive materials are also called release agents, non-stick surface coatings, parting
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agents, or antistick agents.
The opposite to adhesion, the requirement for increased abhesion, include the fol-
lowing characteristics: prevention of mechanical interlocking, diffusion, low absorption
and surface reactivity, low surface tension and electrostatic attraction, and weak boundary
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layers on the surface. The increased abhesion can be achieved by complete curing of ther-
mosetts (e.g., epoxies or polyurethanes), elimination from formulation low molecular
additives (e.g., plasticizers in PVC), low polydispersity and increased molecular weight
(small molecular weight polymers and oligomers have a tendency to reside on the surface
and increase tack), development of a technological process which renders a smooth, defect
free, non-porous surface coating with materials which have low reactivity, prevent diffu-
sion, have low surface tension, and are resistant to degradation on exposure to conditions
of material performance (e.g., lacquers or clearcoats), lamination with polymeric layers
which are known to be excellent abherents, use of additives which can migrate to surface
and form a protective layer on the surface, or dusting or coating with inorganic powders
which stick to the surface and change its properties.
Usually, usefulness of all of the above methods should be analyzed and promising
approaches implemented. Our discussion below concentrates on additives because this is a
subject of this book.
Several common materials used in additives have low surface tension. These include
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carnauba wax − 38, polyethylene − 25-36, polytetrafluoroethylene − 18.5. The lower the
surface tension the better the abhesive properties. The next section contains data on the
most common additives which are used to reduce surface tack of materials.