The development of epoxy resins has been directed towards many different field of application, one important application being adhesive bonding. It follows that much of the information which is available does not relate specially to adhesive use, but at the same time can't be separated from it; this, as will be seen from the follow article, applies particularly to methods of curing.
In the unhardened state the chemical structure of an epoxy resin (less well known also as an ethxyline resin) is characterized by the epoxy group.
There are two broad methods of adding an epoxide group in the preparation of epoxy resins or epoxy compounds leading to resins. In the first method a substance already containing an epoxide group such as epichlorohydrin (or other epihalohydrin) is reacted with a substance having repleacable hydrogen atoms, and in the second, peracitic acid (or
other appropriate oxygen-donating compound) is used to effect epoxidation of an olefinic compound. The product of the first method is a glycidyl compound, which in the most important cases is a glycidyl ether, amine or ester. The glycidyl ether include the bisphenol. A epoxy resins, the type that is usually mean when reffering to epoxy resins in
a general way.
In view of the much larger scale on which the bisphenol. A resins are used, especially in the field of adhesion, they will be deal with separately as Part I of this section. Part II will deal with other glycidyl resin and also with the epoxy resins produced by the second method referred to above. None of the resins discussed in Part II is at present widely used as an adhesive, but some of the polyfunctional glycidyl resins have resistance. Other types have great potentially, and after further development valuable adhesive may be evolved.
Epoxy adhesives comprise a liquid or fusible solid containing epoxide groups, and a curing agent containing functional groups with which the epoxide groups combine to form a cross linked polymer. A type of so-called curing agent, of little importance in adhesives, is that which catalylises homo-polymerization of the epoxy compound.