Hot Water Effect to Resistance

The hot water (90^oC) resistance of the same adhesive system has been shown to fall rapidly during the first month but to retain at least one-half of its original strength during six months. In a test of this sort, the adherent is important because of the effect of the water at the adhesive/metal interface, and consequently the surface treatment of the adherent is also important.

Melamine, like dicyandiamide a polymer of cyanamide, can be used as a curing agent rather on the lines of dicyandiamide; it has, however, a higher melting point and is less compatible and not soluble in any convenient solvent. Etherified melamine and urea formaldehyde resins, much used in epoxy coating, are not much interest adhesive compositions.

In N.America, more than in Europe, N,N-diallyl melamine is used as a hardener. Its method of use is similar to that of dicyandiamide: it can be mixed in powdered form with the resin, and it dissolves and reacts on heating.

Metal chelate compounds are of interest as curing agents partly because of the improved heat stability to be expected by including inorganic atoms in the structure of the cured resin. A part from this, chelates may also behave as catalyst at elevated temperatures. Triethanolamine borate (m.p. 236⁰C) is an important tertiary amine chelate that has latent catalytic properties. This compound becomes reactive through dissociation at a temperature much below its melting point, its threshold curing temperature being about 105^oC. Other boron containing compound have also been suggested such as aluminum acetoacetic ester have been patented as latent hardeners. And another type of chelate with latent curing properties is a polyamine metallic salt co-ordination complex such as cadmium or zinc bromide diethylenetriamine; with these hardeners the threshold curing temperature is about 60^oC

Other chelates that have recently been proposed as curing agents are certain boron difluoride complexes.

Miscellaneous Curing Agents

Of the miscellaneous compounds that have been proposed as curing agents for epoxy resins, many are probably never tested in adhesive systems unless they arouse the interest of the more experiment-minded user.

A compound that has established itself as an important hardener for adhesives, especially for metal bonding; is dicyandiamide. This substance melts at about 200^oC, and being non-reacting at room temperature is convenient for use in the form of powder on rod. On heating, dicyandiamide. If finely divided, the dicyandiamide will remain in suspension in a liquid resin provided a substance such as Aerosil (a finely divided silicon doxide) is also added to create thixotropy. Dicyanidimide is difficulty soluble substance, but limited amounts can be dissolved in certain polar organic solvents, in particular methoxymethanol (methyl “Cellosolve”) and added as a solution.

An example of the strength/temperature relationship of a bisphenol A epoxy adhesive cured with discyadiamide is shown in the figure.

The hot water (90^oC) resistance of the same adhesive system has been shown to fall rapidly during the first moth but to retain at least one-half of its original strength during six months. In a test of this sort, the adherent is important because of the effect of the water at the adhesive/metal interface, and consequently the surface treatment of the adherent is also
important.

Strength/temperature relationship of bisphenol A resin cured with discyandiamide

Phenolic Resins as Curing Agents

The properties obtained by reacting bisphenol A glycidyl ether with a phenol-formaldehyde resin were first recognized in applications other than adhesive use. The phenolic resin may be either a novolac or resite, and used with or without some other curing agent or catalyst. (This is quite different to reacting epichlorohydrin with a phenolic novolak, as discussed before. With some phenolic resins stability and at the same time to catalyse curing at elevated temperatures. The mechanism of the epoxy/phenelic resin reaction has been studied by Bruin.

So far as adhesive are concerned, the interest is mainly in mixture of a thermosetting phenolic resin with either a solid or a liquid bisphenol A glycidyl ether.

Some important high temperature adhesives of this type are now being used in aircraft construction. Generally speaking, the phenolic resin component used is in considerable excess of the epoxy, sometimes many times in excess, and therefore the amount is much greater than that required simply as a hardener. Because of this, phenolic/epoxy adhesives are here considered to be primarily phenolic adhesives, and as such they are dealt in the article before.

Boron Trifluoride Hardeners

The homopolymerisation of bisphenol A glycidyl ethers via their epoxide and hydroxyl groups can be activated by alkalies especially inorganic alkaline and alkoxides, and by certain acidic substances such as Lewis acids, of wihic boron trifluoride (BF3) is an important example; but the properties of homopolymers are generally considered to be inferior to the addition polymers formed by crosslinking, and there is practically no published information about them. From this is follows that boron trifuoride used purely as a homopolymerisation catalyst for cross linking reaction with both anhydride and amines, especially those reactions involving the slow curing amines. But it would be difficult to establish the extent of catalyzed metathetical cross linking unconfused with catalyzed homopolymerising action.

Boron trifluoride forms chemical complexes with certain compounds, especially amines to give hardeners that are latent in effect until heated. An important one is the BF3 monoethylamine complex, with requires heating to about 95oC for dissociation to occur. This behavior enables one package adhesives with mild curing cycles to be formulated. The complex also has a latent curing effect in association with cross linking hardeners; used with diaminodiphenyl sulphone for example, an addition of one per cent enables the normal curing time to be reduced to about one sixth. However, the remarks at the end of the previous paragraph also apply here.

The nature of the amine affects the reactivity of BF3 amine complexes towards epoxide groups; for example, reactivity decreases in the order ethylamine (primary amine), diethylamine (secondary amine), triethylamine (tertiary amine).

Valuable information the use of boron compounds including BF3 complexes has been given in a paper by Lee and Neville. The use of other boron compounds is mentioned in the next articles.