Aromatic Hardener

Another aromatic diamine hardener of which the glycidylamine has created interest is 4,4-diaminoxliphenyl suphone. The strength/temperature relationship of this epoxies cured with diaminodiphenyl sulphone, and also with two anhydrides.

Diaminodiphenyl sulphone has also been reacted with a limited amount of epichlorohydrin to give compounds that contain both amino hydrogen and epoxide groups. This is specially interesting because the compounds, whilst stable at room temperature, are self curing at elevated temperatures; they also have heat deflection temperature up to 300oC. One would expect such products to form a useful basis for one package adhesives.

It is only natural to be found to expect that mixture of glycidylamine and glycidyl resin ponding interest is the epoxydation of both the amino and hydroxyl groups in amino phenol. The hardeners used for glycidylamine adhesive can be either anhydrides or amines, but since these epoxy adhesives have their greatest value as high hot strength adhesives, there would not appear to be much ease for using aliphatic amines. A patent refering to glycidyl tertiary amines mentions that hardening can be effected by isocyanates and aldhehydes.

It is interesting, but not surprising, to see that a glycidyl tertiary amine can be used to catalyst the anhydride-curing of a glycidyl ether adhesive. On the whole, there is little detailed information on the adhesive properties of the glycidyl amine resins, but interest in the epoxides is increasing.

Glycidylamine Resin

As well as reacting with hydroxyl groups to form glycidyl ethers, epichlorohydrin also react with a primary or secondary amine to form a glycidylamine. Epoxy resin of this class contain tertiary nitrogen atoms in place of the ether oxygen atoms. The amine analogue of bisphenol A is shown here as an example.

Resin of high functionally can be made from the large number of aromatic, alicyclic, and aliphatic polyamines that are available. The patent position concerning these epoxydes would appear to be somewhat confused. In this case four reactive hydrogen atoms are available, and the preparation of a tetra-epoxide becomes theoritically possible.

Although in practice the reaction result in rather fewer than four epoxide group per molecule, experience has shown glycidylamine resins to be of considerable interest in the formulation of high hot strength adhesives. One such resin is made from the d primary amine, 4,4 diaminodiphenylmethane (“methylene dianiline”), an aromatic diamine mentioned in the last article as a hardener for bishphenol A adhesives. The strength/temperature relationship of this resin cured with methylene dinaline, and also with a authetic anhydride mixture. The strength of a specially formulated adhesive based on the same glycidyl compound and containing fillers and other adhesives, and cured with methylene dianiline. The ageing characteristics of this formulated glycidylamine adhesive cured with methylene dianiline are good.

Glycidyl Ether Properties

Glycidyl Ethers of resorcinol have been shown to have excellent hot strength, even when cured at room temperature they are capable of giving the strength required by US specification.

Wynstra et al. in discussing the effect of the structure of the resin on elevated temperature performance (measured as the flexural strength of a laminate), point out the importance of the overall functionally of the resin/hardener system. But the various characteristics of the molecules, such as the distance between functional groups and the ratio of functionally to molecular weight, are clearly other factors that have an influence on hot-strength.

Since the phenolic hydroxyl group is not involved in the reaction between phenol and formaldehyde, phenolic resin can be converted into glycidyl polyester and formaldehyde, phenolic resin can be converted into glycidyl polyesther by reaction with epichlorohydrin. Phenolic resins of the novolak type are of most interest, and the polyepoxides of ortho-cresol formaldehyde novolak are stated to be of significant importance as adhesives for use in the 316o temperature range.

The properties of an epoxy novolak adhesive cured with pyromellitic dianhydride, and also of an epoxy novolak modified silicone/phenolic adhesive, are described in an informative paper by Janis. Adhesive applications of epoxy resins from low stage phenolic novolaks have been studied by George and Rao; the resins were cured with an aromatic diamine at an elevated temperature and tested in wood, aluminum and wood to aluminum joints.