Fillers for Adhesive 2

The additional of fine size silicon dioxide, one of the cheapest fillers, is shown to produce a remarkable increase in strength, as much as 50% increase being obtained in some cases with the surprisingly low optimum addition of less than 5%.

A special form of silicon dioxide having an enormous surface area and known as “Aerosil” has the property of imparting thixothropy. “Bentone 34” (dimethyldioxtadecyl ammonium bentonite) has the same property. Both of these fillers, in small proportions, produce an adhesive that spreads like cold cream and is sufficiently thioxothropic to resit flow down a vertical surface. When added in larger proportions and in admixture with normal fillers they can impart a putty consistency.

A reduction in the brittleness of an adhesive is usually a desirable change to effect; one aspect of brittleness of an adhesive in impart strength, although brittleness of an adhesive is usually judged by peel and bend strength. Asbestos fillers are known to increase the impart strength of plastic polymers in general, and therefore reduce brittleness. In connection increases the Izod impact strength from 5.2 ft.lb/sq.in.

There are certain anomalies in the results of the evaluation of filler reported from different sources; this is really not surprising and largely arises through investigators studying different parameters. A comprehensive report on the effect of fillers, chiefly inorganic, concludes that in general the strength of cold cured glued joints (but not hor-cured ones) is increased with aluminum adherends but not with steel. The same report also claims a striking increase in cold peel strength with a cold cured adhesive on aluminum, using porcelain flour as filler. From another source the addition of antimony oxide to a hot-cured adhesive is claimed to give an important increase in room temperature tensile shear strength especially with steel, but also with light alloys.

Clearly, there is very little scientific knowledge on which to base the effect of the addition of filler on adhesion, and empirical tests must necessary be made.