The Adhesive

The adhesive can be either a multi-component system or it can be a one-component adhesive. However, the aim with both is to provide the plastic and the elastic properties coupled with good tack and adhesion. They require these for a pressure sensitive adhesive. In the multi-component adhesive, the basis is an elastomer (such as natural or synthetic rubber) or one of the newer elastoplasticts which, although it may show auto-adhesion (the ability to stick to it self), shows no indication of adhesion. At this point, the elastomer has high internal strength and is highly elastic.

To this, is added one or more synthetic resins. Initially, the resin dissolves in the rubber. However, as more and more resin is added, eventually a two-phase system develops. At this point, the elastomer/resin blend becomes a pressure sensitive adhesive. If excess resins were added, eventually the rubber would dissolve in the resin, resulting in a hard, brittle solid.

Liquid tackifiers can be added, usually of the resin or liquid polymer family, which can impart tack to adhesive. However, they can also plasticize the adhesive, resulting in loss of cohesive strength, so must be used with caution.

There are times when the addition of plasticizers or softeners to the adhesive is desirable. such materials could be oils, soft greases (such as lanoline) or waxes (such as paraffin wax or beeswax).

In order to give added internal strength or to extend the polymer blend or add whiteness, a filler or reinforcing agent can be added. Such fillers could be zinc oxide or compounds containing silica.

In order to give heat stability, shear resistance or other characteristics demanding high internal strength of the adhesive, cross-linking can be added. This converts the adhesive into a loosely linked three-dimensional network.

And finally, the hole compound, particularly the elastomer, should be protected against aging so anti oxidants and ultraviolet stabilizers are added. The one-component adhesive achieves most of the aforementioned by correct design of an organic copolymer, usually a polyacrylate-acetate base. The adhesive is cast as a thin film onto a suitable carrier. This can be done either by dissolving the adhesive in a suitable solvent, applying it as a thin coating onto the moving carrier, then drying off the solvent in an oven or by making the adhesive thermoplastic by heating then rolling it into a thin sheet and then transferring it to the carrier by suitable rollers. The latter method is known as calendaring.

 

Note: This material is an excerpt from Shuford Literature.

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