Monday, January 30, 2012

Nitrile Rubber and Phenolic Resin

With nitrile rubbers, the phenolic resin component is more critical and compatibility more of a problem than with the polyvinyl acetal. Since resins having a high pH value tend to less compatible, ammonia and hexamethylene tetramine are preferred to sodium hydroxide as curing catalysts.

Although a wide range of nitrile rubbers, having different but related molecular system is a copolymer of butadiene and acrylonitrile, and is refered to simply as nitrile rubber or more particularly as high nitrile rubber. In spite of this name the monomer ratio is such that butadiene is in excess of acrilonitrile and comprises 60 - 70% of the weight of the copolymer.

The properties of the copolymer can be varied widely simply by varying the ratio of one monomer to the other, and further modification is possible by copolymerising with certain other monomers such as vinyl esters. Monomer ratio together with the variation made possible by using different amounts of phenolic resin, enable adhesives to be formulated that vary from rigid to flexible. Unfortunately, little quantitative information is available on the effect of these parameters on adhesive strength.

The percentage of phenolic resin can be varied between wide limits, but is usually within one half to three times the weight of nitrile rubber, depending on the emphasis given to properties such as peel strength and hot strength.

The solvent generally used to dissolve the prefered grades of nitrile rubber is methyl ethyl ketone, which is a solvent also for the phenolic resin. Small amounts of other solvent also be added, toluene for example. Mixing the two components by milling may be desirable, depending on such things as the molecular weight of the rubber, its age and the monomer ratio. It has frequently been emphasized that the behavior, both in the blending operation and subsequently as an adhesive, depends to quite a large extent on the manufacturing history of the nitrile rubber.

It is sometimes preferred to market, not one solution containing both rubber and resin, but two separate solutions, especially if powerful catalyst are added and at the same time maximum shelf life is required. In this case it is possible to incorporate a catalyst for one component in the solution of the other. The preferred catalyst for the nitrile rubber is magnesium oxide, zinc oxide is also used. Sulfur and an accelerator such as benzothiazyl disulphide are often added together with anti oxidants, tackifiers and fillers including carbon black.