Reactive Diluents

The technique of reducing the viscosity of an epoxy of an epoxy adhesive by adding a reactive diluents is a low viscosity liquid that is capable of combining chemically with the resin/hardener system. The reactive groups are usually epoxide and many reactive diluents are glycidyl ethers. In some cases the compounds many themselves form useful adhesive; generally speaking they do not.

Those reactive diluents that after comparative evaluation have been classed as high efficiency diluents” include allyl and butyl glycidyl ether, octylene oxide. Others that are used include 1,4-butanediol diglycidyl ether, phenyl and cresyl glycidyl ethers. Some of the paracid epoxidised cyclo-olefines referred to other article, such as butadiene dioxide, and limonene dioxide are useful reaction diluents.

Reactive diluents, being low molecular weight liquids with a relarively high epoxide content, are in some cases rather unpleasant to handle and are toxic. In this connection, however, it is reported that some of the olefine (and terpene) oxides, for example, limonene dioxide, dodecene oxide, octylene oxide, and a-pinene oxide are of a low order of toxicity.

A few epoxy resin compositions are marketed containing a proportion of reactive diluent, but normally the addition is made by the user, especially in the case of adhesives.

Versamid Complex Polymer

The Versamid are complex polymeric substances having relatively high melocular weights in properties to their amine content. From this it follows that the amount required for curing is large and the proportion not very critical. This is confirmed by Floyd, Peerman and Wirrcoff in reporting on versamid/epoxy resin system, including their reaction mechanism. Niermann and Gunther considered that the best high temperature adhesion is given by stoichiometrc proportion combined with curing at 100 – 150 oC, a fact that should not be taken as inconsistent with the general belief and experience that within sensible limits the ratio of Versamid to epoxy is not at all critical. According to the epoxy value of the resin, the stoichiometric proportion may vary from less than one half the weight of epoxy resin to more than an equal weight.

Versamids, having a long carbon chain between functional groups, increase the flexiblity of an epoxy adhesive and therefore increase peel strength. A remarkable flexible adhesive can be made by substantially increasing the amount of Versamid above that needed to produce satisfactory curing, but in this case chemical resistance is reduced. Although flexibilishing an adhesive strength (depending to some extent on the type of test), the hot strength is reduced the cohesive strength of the glue suffers.

Although the Versamid are capable of curing bisphenol A resins at room temperature the degree of crosslinking is not high and adhesive strength is increased by curing at a higher temperature, for example at 60oC for an hour; alternatively, the strength of glued joint that have been cured initially at room temperature can be increased by post curing. The rate of curing can be accelerated by adding one of the more reactive amines such as triethylenetetramine, or tertiary amines, or some organic acids. N,N-dimethyl-1,3-propylene diamine is a particularly useful accelerator. Phenols, and certain compounds made by the Mannich reaction also accelerate Versamid curing, but one common example, namely tris-(dimethylaminomethyl) phenol, is reported to reduce adhesive strength by 20-30 %.

Miscellaneous Polymers

Copolymers of butadiene and acrylonitrile are not readily compatible with an epoxy resin; nevertheless, a low molecular weight butadiene acrylonitrile copolymer is claimed to give especially good metal and glass to metal adhesion, without using pressure. In another example high peel strength is claimed; and in a similar system it is said to be advantageous to incorporate a phenolic resin; they are largely inreactive. But as reactive flexiblishing compounds, saturated and unsaturated polyesters with terminal carboxyl groups are of interest in adhesive formulations.

Polymethylmethacrylate, in substantial quantity, with a mixture of an epoxy and a thermosetting phenolic resins, is used for bonding PVC to metal. Polyvinyl chloride is not an early material to bond, and therefore it is significant that another epoxy adhesive claiming to give improve adhesion of PVC to metal also incorporates an acrylate polymer. This adhesive is include of vinyl polymer.

Having regard to the individual reactivity of polyisocyanates and epoxy resins, it is not surprising that there has been interest in using them together. A relatively early patent describes a metal adhesive produced by reacting equal parts by weight of a diisocyanate ad a bisphenol epoxy resin. A later patent describes a metal adhesive produced by applying separately and “polymerizing together” an epoxide resin and a polyurethane pre-polymer; a force of 36.2 kg/cm2, which is rather low, is said to be necessary to break the bond, but no details of the test are given. It is not clear if the term “polymerizing together” means polymerizing individually at the same time, or co-polymerizing. In this connection it is interesting to see that a reaction between epoxide and isocyanate groups to form an oxazolidone has been described. And in another patent an adhesive is made by reacting a polyepoxide and a diisocyanate, and subsequently adding an amine, which will of course react with both.

Epoxy resins may be mixed with styrene to produce adhesives of low viscosity without added solvent and, rather surprisingly, a reaction is said to take place. Another adhesive formulation containing styrene, together also with a furfural acetone monomer, has been reported from Russia.

Vinyl Polymers

Certain vinyl polymers such as the polyvinyl acetals, in particular polyvinyl butyral and polyvinyl formal, also polyvinyl esters, are compatible with bisphenol Aglycidyl ethers, and can be added to improve the impact strength and peel strength of an adhesive. Polyvinyl butyral is probably the best in this respect. A 10-20% addition effects a marked improvement by naturally reduces hot strength.

Ease of solubility in the epoxy resin depends to some extent on the grade of the vinyl polymer used; and on its molecular weight. Polyvinyl esters and acetals can be dissolved in melted (solid) epoxy resin as well as liquids, but heating to a temperature of at least 100oC is usually necessary.

A composition having remarkably good adhesion to metal, combined with toughness, consists of a mixture of a plasticised polyvinyl resin particularly polyvinyl chloride or its copolymers which will crosslink by reacting with both resins. This use of polyvinyl chloride is interesting in view of its poor solubility characteristics. Although the composition in intended primarily as a metal coating, it gives excellent strength as a conventional adhesive. A somewhat related type of adhesive, recommended for bonding both synthetic and minearal fibers, consist of a dispersion of plasticised polyvinyl chloride (a plastisol), an epoxy resin and hardeners.