A Novel Class of Rubber to Steel Tire Cord Adhesion Promoters

A series of eight novel rubber-brass adhesion promoter systems are introduced and discussed. They can be divided in four categories, viz., homogeneous inorganic polyborosilicates of cobalt or nickel; inorganic cobalt or nickel ions adsorbed on an inert substrate, e.g., silica or alumina; metal-organic cobalt or nickel salts deposited on a similar substrate; and inorganic ion exchangers loaded with cobalt or nickel. The organic systems have the major advantage that they are much more active than if they were used without support. Because of this effect, they can replace the currently used metal-organic cobalt salts but at reduced levels. The inorganic systems all show an improvement of initial and of aged adhesion which is comparable to that of currently used commercial systems. However, they can be used at much higher levels than metal-organic salts since for the first time, promoters are offered which show virtually no negative effect on physical compound properties. Such promoters are insoluble in rubber and act solely as corrosion inhibitors. At high levels, therefore, the performance of the inorganic promoters is far superior to that of the metal-organic systems. A consequence of such promoters being corrosion inhibitors is that nickel salts generally perform as well as the cobalt-containing materials. Another advantage is that such promoters can also reduce the rate of iron corrosion. This was demonstrated for the polyborosilicates. It is expected that the rate of rust formation in the tire will be reduced because of this propensity and also because the inorganic systems have a long-range action due to a slow-release effect. In the development of these unusual promoter systems, extensive use has been made of liquid low molecular weight model systems. A good correlation is reported between crosslinking of NR and of squalene if the levels of sulfur, DCBS accelerator, or ZnO are varied. If levels of cobalt or antioxidant are varied, squalene does not react as NR. For such studies, dodecene-1 is shown to be a better model molecule. This system further has the advantage that it is amenable to gas chromatography analysis which allows the crosslink distribution to be determined quantitatively.