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
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)

Electrically conducting adhesives

Incorporating electrically conducting particles in an epoxy adhesive represents a special use of filler. The idea is not new; other conducting cements-thermoplastic as well as thermosetting-are marketed, but more recently there has been an increasing interest in epoxy conducting adhesives.

The function of electrically conducting adhesives is really the same as that of metal solder, to bond and provide electrical contact. But because epoxy adhesives are more versatile, they are able to bond a wide range of metals, and also dissimilar materials, including porcelain and most plastics; one example is the joining together of two conductors, while at the same time bonding them to an isolator.

The common conducting fillers are silver and copper, including silver-plated copper and in some special cases, gold. The amount of metal filler added depends on particle size and shape. At least a part of the filler is usually in the form of flake, and in this form it is generally considered that a smaller amount is needed; but even so, the quantity is large if good particle-to-particle contact and low resistance is to be obtained. Additions of two or three times the weight of resin are not uncommon. Volume resistivity of from 0.01 to 0.001 ohm/cm and shear strength in bonding steel-to-steel of 3,200 p.s.i. have been quoted.

Normal hot or cold curing agents may be used although it is evident that some must be superiors to others from an electrical standpoint. At least one epoxy conducting solder is supplied as a two-tube package.

The whole subject of incorporating metal-fillers in plastics has been dealt with in a book by DELMONTE.

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.

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.