Bonding of Rubber

Rubben-to-Rubber Bonding. In the bonding of unvulcanized rubber mixtures to one another, there is generally no need to use adhesives when the mixtures to be combined with one another are identical or similar in composition. However, adhesives have to be used for combining different types with one another. Various bonding agents based on halogenated polymers are available commercially for this purpose and also may be used for bonding vulcanized to unvulcanized articles.

Rubbei^to-Fabric Bonding. Rubber-to-fabric bonding is of considerable commercial significance in the tire industry and industrial rubber goods such as conveyor belts and rubberized fabrics. In the tire industry, the so-called resorcinol-formaldehyde (RF) latices are still used predominantly for bonding rayon, whereas mainly resorcinol — formaldehyde — vinylpyridine latices are used for bonding nylon and adhesively finished polyester yarns, which, unless specially pretreated, adhere very poorly to rubber.

In industrial rubber goods, which exhibit considerably greater variations in rubber composition, solutions of unvulcanized rubber mixtures are used. They are applied to the fabric as a first coat with the addition of polyisocyanates to improve adhesion. Directly adhering rubber mixtures containing resorcinol-formaldehyde donors and an acidic silica filler are also used for the production of fabric-reinforced rubber materials.

Rubber-to-Metal Bonding. In the Ebonit or hard rubber process [76], the bonding agent is a reaction product of natural or synthetic rubber with 30 — 40 % sulfur, which is thermoplastic in character, adheres firmly to metal, and provides for good adhesion to soft rubber. This process is still in use today in the lining of vessels and, to a certain extent, in the manufacture of large-diameter rolls.

Подпись: AdhesivesThe brass process makes use of the fact that electrodeposited brass of certain composition and crystal structure is capable of directly bonding rubber mixtures, providing their composition is compatible with the adhesion process [77]. Whereas today the brass process is of only minor significance for the production of rubber — metal composites, it is widely used in tire manufacture for bonding the bead wires and for bonding the carcass mixture of steel cord.

Certain polyisocyanate adhesives allow bonding of rubber to metal with far less technical effort than by the brass process. However, polyisocyanates are extremely sensitive to atmospheric moisture and react with compounds containing OH and NH2 groups, such as antiagers. This reaction leads to a weakening of adhesion.

Nowadays, rubber-metal composites are produced with bonding agents in which the polymer base is usually a halogenated polymer, for example, a mixture of chlo­rinated and brominated polybutadiene. After the film of bonding agents applied to the metal component has dried, the metal component is bonded to the rubber mixture during the vulcanization process.

Mechanical and/or chemical pretreatment of the metal is essential for corrosion — resistant bonding (for the effect on bonding of the blasting medium used for blasting, see [78]).

The fillers in the rubber have no significant effect on bonding. However, soft rubber mixtures (30-40 Shore A) are more difficult to bond than hard rubber mixtures because of their high plasticizer content. The degree of vulcanization is another deter­mining factor as far as the quality of bonding is concerned [79]. Bonding errors generally occur if the mixture is scorched before it comes into contact with the bonding agent surface or if the mixture is undervulcanized or severely overheated [80].

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