Monday, December 29, 2008

Bisphenol A Epoxy Adhesive Properties

Properties and Applications of Bisphenol A Epoxy Adhesives

These are only a few materials that bisphenol A epoxy resins will not bond satisfactorily, the chief ones are polyethylene, polypropylene, polystyrene, polyfluorocarbons, and silicone plastics and rubbers. Even in the case of these exceptions, a satisfactory measure of adhesion can be obtained by specially treating the surface, which usually means that one is eventually sticking to a rather different substance. But the properties of the adhesion in the glued joint, as well as the adhesion to a particular adherend depend on the whole adhesive composition, that is to say, on the type of resin, the curing agent, fillers and modifiers. Equally, the properties of the adhesive become completely meaningful only when related to a particular adherend in a particular use; it is however impossible to assemble all the relevant data.

Wood is by far the world’s most commonly glued material, but there is little information on gluing it with epoxy resin. This is not surprising because there is not a strong case for using it to glue wood, other resin adhesive generally do the work better at lower cost. However, no doubt some applications have been found for which epoxy adhesive are especially appropriate.

As one would expert with the use of a glue that is water insoluble when applied, the moisture content of the wood is more critical than when using, say, a urea formaldehyde adhesive. The water resistance of wood joints glued with bisphenol A resins is a matter in which variable experience is reported. Clarke and Nearn examined a number of parameters that effect adhesion and were of the opinion that it was not possible to obtain water resistant bonds, at least not on maple with the adhesive system they used. Obviously the adhesive system is important, the type of hardener being more important than supposed. The type of timber certainly has an effect, especially on hot-water resistance; an example of the difference between birch and beech is shown below, in the strength of hot pressed plywood joints using diethyl aminopropylamine as hardener.

Birch: Strength; 506 lb (all wood failures), Wet Strength; 519 lb (all wood failures)
Beech: Strength; 370 lb (all wood failures), Wet Strength; 0 lb (all wood failures)

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