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.