Glycidyl Ether Adhesives

The number of epoxide groups per molecule and the rigidity of the molecular structure are factors that effect hot strength of an adhesive. In the search, therefore, for higher hot strength, glycidyl ethers of certain polynuclear phenols including phenolic resins, and also those of some polyhydric mononuclear phenols such as resorcinol and hydroquinone, have been examined.

The preparation and properties of some these glycidyl ethers have been described in papers and patent specifications; and important one is the glycidyl polyester of tetraphenylolethanc; a tetraphenol made by reacting ordinary phenol with glyoxal, a di-aldehyde. The relationship between strength and temperature of this resin hardened with two different amines and two different anhydride is shown on the figure.

Attention has been given to the adhesive properties of this resin when cured with diaminodiphenyl sulphone and also when modified by the incorporation of a polyvinyl acetal.

Dearbors et al have compared certain di-, tri-, and tetra-glycidyl ethers cured with anhydrides. Although adhesion was not among the properties examined, much of the information given is important in considering the usefulness of these resin as adhesives.

In a patent describing epoxy resins from certain trisphenols, adhesive strengths on aluminum alloy have been quoted, using 4,4 – diaminodiphenyl sulphone as hardener. These, for the glycidyl ether of 1,2,2-tris (hydroxyphenyl) ethane are reproduced in certain condition. The glued joints in this test were 0.126 mm thick and the glued area 2.5 cm wide by 1.25 cm overlap. The lower strength of the trisphenol glycidyl ether at 21 oC compared with bisphenol A glycidyl ether results from the brittleness of the former adhesive being emphasized by the type of stress imposed in the testing of an overlap joints.

Impact Structural Adhesives

Versatile as the bisphenol A adhesives are, they cannot yet be used satisfactorily to provide what may termed an impact structural adhesive. This type of adhesive requires initially the property of a pressure sensitive adhesive, but should subsequently cure and develop the resistance to creep expected of a structural adhesive. The difficulties are clearly very great but with the variety of rubbery and sticky polymers available, hardly inuperable. One approach has been through the use of other glycidyl ethers, and progress has been reported with the epichlorohydrin derivatives of the bisphenol of cashew nut shell oil, which is mainly 1,8-bis(hydroxyphenyl) pentadecane.

Durability
There is much everyday evidence on which to form the opinion that the durability of epoxy resin is excellent, but the most material, organic and inorganic, they suffer to some extent from exposure to weather, they do not, however, degrade rapidly or badly.

The durability of bisphenol A epoxy adhesives is under investigation in weathering tests being made by the Wright Air Development Center, and with tests still in progress it appears that epoxy phenolic adhesives are performing well. But question of the permeability of thin adhesive layers, as mentioned in the article before is always an important factor. The effect of permeability is such that water may penetrate a thin film of adhesive and cause separation at its interface is dependent in part on the type of metal, and so supports the axiom that the behavior of an adhesive depends on the adherent.