Thiokol

The Thiokol polysulphide compounds that are compatible with epoxy resins are liquid polymers that are potentially rubbers. They can be used by themselves to cure epoxy resins but the rate of cure is slow at room temperature and the products are not very useful as adhesives. The liquid Thiokols really serve as flexibilisers, and are incorporated in an adhesive to which a more conventional curing agent is also added; the most commonly used is Thiokol LP.3. In applications where maximum flexibility is required the amount added may equal or even exceed the amount of epoxy resin. The type of curing agent preferred is an amine rather than an anhydride.

Not all amine are completely compatible, but those are incompatible can be used with a “coupling agent”, which, however, necessitates a three-package adhesive system. Both diethylenetriamine and triethylenetetramine are incompatible amines in the sense that they will separate fro the polysulphide in a few days, but since they would cure the epoxy resin in a much shorter time, this delayed incompatibility does not cause inconvenience. The Thiokol are usually added to liquid glycidyl ethers because, although they are compatible with the solid resins in the melted state, the temperature needed may cause fairly rapid gelation.

Joint strengths obtainable with various Thiokol/epoxy resin combinations have been published and good adhesion to a wide variety of materials including glass is claimed. Peel strength and bend strength, not particularly good in unmodified bisphenol A resins, are shown to be markedly improved, and in consequence a Thiokol-modified adhesive is recommended for gluing skins to honeycomb in metal sandwich construction. The room temperature strength of one Thiokol/amine cured adhesive (cured at 120oC) with a ration of LP 3. to epoxy resin of 1 to 3 is quoted as 4300 psi in shear, 36 lb/in, width in peel strength, and 227 lb. in bend strength.

A large number of high boiling organic liquids, many of which are commonly classified as plasticisers, are compatible with, but substantially inert towards bisphenol A resins. They have a flexibilising effect more or less in proportion to the quantity added and are of use primarily in non-structural applications, to give increased peel strength. Typical examples are the polypropylene glycols, other alcohols such as cyclohexanol, diacetone alcohol and phenyl “Cellosolve.” Among esters are dibutyl phthalate and dioctyl phthalate, as well as many others.

The Ratio of Hardener to Resin

Having concluded the section dealing with hardeners, it is appropriate at this point to say a few words about circulating the stoichiometric amount of hardener to add to the resin.

With commercially available hardeners the recommendations of the resin manufacturer should be observed, but when it is desirable to evaluate a new hardening compound it is necessary to know something about the chemical structure of both resin and hardener in order to ascertain the proportions in which they should be mixed. Whereas the hardener is usually a substance of known chemical composition, the epoxy resin, so far as its epoxide content is concerned, is not. Therefore the epoxide equivalent of epoxide, must be determined experimentally, that viscosity and melting point give an indication of the value. Methods of finding the epoxide equivalent are available from number to react with all the epoxide equivalent are available from a number of source; in general they entail measuring the amount of acid required to react with all the epoxide groups.

Theoretically, the optimum ratio of hardener to resin should satisfy the requirement that one reactive hydrogen atom of the hardener is provided for each epoxide group of the resin. For example, if a diprimary amine (four reactive hydrogens) having a molecular weight of 100 is required to be used as hardener of a resin which has been found to have one gram equivalent of epoxide per 250 g, then the stoichiometric ratio of hardener to resin is 25:250.

The well-known technique of separate application of resin and hardener has not been overlooked, but it would not appear to be at all easy to obtain a satisfactory, much less a stoichiometric ratio without considerable care. The use of a catalytic hardener would, be a more reliable adaptation of this idea.

Hardening Composition of Epoxy Resins

One interesting hardening composition is an aniline formaldehyde resin, also in solution in triaryl phophite. Here it is appropriate to mention the growing of phosphorus compound in the curing of epoxy resins; recently a number of organic phosphates have been describe as curing assistants, for any type of polyepoxide and almost any type of hardener, including anhydride, polyamine, polyamide and polysulphide. Although a curing assistant may accelerate the rate of curing, it is also said to improve the ratio of usable life to curing time, in other words to increase the efficiency of the hardener system.

Styrene maleic anhydride copolymers of high molecular weigh are wellknown plastic; recently, related low molecular weight substances in the form of linier polyanhydrides (or partial esters thereof) have been recommended as curing agents, and these are soluble in heated resin.

Quaternary ammonium salts made by neutralizing quaternary ammonium hydroxide with organic acids have been suggested as hardener, and it is important to note, for both glycidyl epoxies.

Certain tertiary amines which can be prepared by the Mannich reaction, in particular tris phenol, are used in some adhesive applications. Their behaviour is catalytic, they initiate polymerization of epoxide groups and also accelerate curing with crosslinking curing agents. A similar reaction product from phenol, an aliphatic amine and also formaldehyde, has superior properties.