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Tuesday, December 2, 2008

Adhesive Fillers

The fillers added to epoxy resin adhesives are almost exclusively inorganic. The list is long and includes especially oxides and silicates. Powdered metals are used to a small extent, particularly to increase electrical or thermal conductivity. The percentage of the filler added varies widely from a few per cent to an equal weight or more.

Fillers are added for a variety of reasons: they alter the physical state of the uncured adhesive by making it thicker, in some cases creating partial thixotropy-often a valuable property in glue. The effect on the cured adhesive is often to increase shear strength, an effluent that, as mentioned later, seems to be specific to some substances. Depending on the amount added, the effect of adding a filler to a substantially non-brittle adhesive is to increase brittleness and thereby reduce peel strength. On the other hand, the addition of a filler to brittle adhesive, for example a phenilic/epoxy system is to increase bend strength, and also perhaps to impart a small amount of peel strength.

A filler that has created special interest is arsenic pentoxide because it increase thermal stability, provided the system is given a relatively severe curing cycle. In the use of this filler there is the interesting possibility of it reacting to form part of the cured resin.

An adhesive system based on a novolak epoxy (not a bisphenol a resin), arsenic pentoxide, a silicone phenol condensation polymer and aluminum powder, is useful up to 524 oC; (at 538 oC a thermite reaction is said to destroy the bond). In addition to arsenic pentoxide, arsenic trioxide, vanadium pentoxide, barium oxide and magnesium oxide also improve heat stability. And in vanadium to arsenic pentoxide, antimony trioxide, antimony pentoxide, and might therefore be classified as reactive filler.