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Thursday, April 12, 2012

The Stability of Neoprene/Phenolic Adhesives

The substantially sol form of neoprene are not easily soluble in normal phenolic resin, neither resitol nor novolak. They are, however, more readily compatible with the oil soluble phenolic resins based on tertiary butyl phenol, and with terpene modified phenolic resins. The latter resins also contribute tackiness, which may not always be desirable. The problem of compatibility with normal phenolic resins connot be overcome very easily by use of mutual solvent, but the oil soluble phenolic resin have the advantage of being soluble in toluene, and also of reacting with the metal oxides generally added to cure neoprene adhesives. A disadvantage of the oil soluble phenolic resins is their lower heat resistance compared with the best examples of onlinary phenolic resins. The importance of the reactions of the resin component has been discused, who also gives information on the effect of other parameters.

The stability of neoprene/phenolic adhesives depends on the potency of the curing agents and accelerators that are added but, although solutions can be made more stable than films because of the effect of dilution, neither form to particularly stable. It is often more convenient to use film, even though reactivation by means of the solution form, or a solvent, may be necessary to obtain good wetting.

Formulation vary widely as their ratio of phenolic resin to neoprene. A typical formulation contains per 100 parts neoprene, 30-50 parts phenolic resin, 4-8 parts of either or both of magnesium oxide and zinc oxide, perhaps a small amount of sulfur, an accelerator such as thiocarbanillide, an antioxidant, and sometimes up to 50% fillers, which commonly include a proportion of carbon black.

Curing accelerators such as thiourea and diaminodiphenylmethane which can with the phenolic resin are undesible. The previously mentioned reaction between the metal oxides and the phenolic resin does not, however, appear to have moderable concequencies, in fact advantage may be taken of the reaction by allowing the resin and oxide to interact initially in order to reduce sedimentation of the oxide in a solution form of the adhesive.