An Improved Adhesive System for Textile-Reinforced Rubber Products

Abstract

Abstract Resorcinol Formaldehyde Latex (RFL) adhesive systems have been used for over forty years to bond synthetic fibers to rubber compounds. The original RFL formulation was developed by Charch and Maney. The adhesive was applied primarily to rayon fibers and later extended to nylon and glass fibers. At the same time, the formulation evolved with the introduction of synthetic latexes and preformed resorcinol formaldehyde resin solutions. The RFL treated fibers are used to reinforce rubber products (e.g., tires, conveyor belts, hoses, V-belts). With the commercialization of polyester fibers, it was found that the RFL system did not satisfactorily adhere polyester to rubber compounds. The difficulty in bonding to polyester fibers has been attributed to the lack of reactive sites in the polymer structure, as well as its hydrophobic nature. New adhesive systems were developed which consisted of free isocyanates in solvent solutions. These systems were undesirable due to the associated health and fire risks. As a result, the use of aqueous dispersions of blocked isocyanates was investigated. Blocked isocyanates consist of isocyanate molecules which are chemically reacted with a blocking agent to prevent reactions with other materials at room temperature conditions. At higher temperatures, the blocking materials dissociate to free the reactive isocyanate groups. The blocked isocyanates are generally available in solid form and are chemically stable in water. Common blocking materials include phenols, oximes, and lactams. Shoaf developed a two-step, water based, adhesive system that effectively bonded polyester to rubber. This system consists of a first-step dip application which contains a polyepoxide and blocked aromatic polyisocyanate. In the second step, the treated fiber is overcoated with an RFL mixture. The reaction product of the polyepoxide and polyisocyanate has been identified as a polyurethane structure which physically adheres to polyester. Iyengar attributed this physical interaction to the similarity between the cohesive energy density values for the polyester and polyurethane structures.