Rubber to metal bonding: processing

Rubber to metal bonding processing effects J A LINDSAY Importance of good mould design... 

The forming of the required shape and the vulcanisation of the rubber in a rubber to metal bonding process or the production of any rubber component, does not complete the crosslinking and full attainment of the ultimate network stmctnre. Cure times for products are calculated using curemeter information to usually 90 or 95% cure. The 5 to 10% additional cure comes from retained heat in the product after demoulding has taken place. 

The application of instrumentation to the automatic control of a sequence of operations, e.g., injection moulding processes. Once the mould has been loaded with inserts (assuming a rubber to metal bonded part) into the press, the operation of a push-button starts the controller which closes the press, injects the rubber, controls the cure cycle, recharges the injection unit, opens the press, operates the ejectors and presents the mould for cleaning and loading of inserts. 

Rubber to metal bonding is a generic phrase covering a number of interdependent processes. The rubber bonded units that result from the process are used for the isolation of noise and vibration in automotive, engineering and chemical industry applications, such as in rubber lining of process equipment. 

Several years ago the subject was regarded as a "black art". Now the technology allows the production of a uniform, high quality product that is free from failure. Much of the science behind the technology remains cloaked in uncertainty, but it is sure to become an important part of future developments. Today, environmental concerns have an impact on all aspects of commerce. Rubber-to-metal bonding is no exception, so the process is changing to reflect these concerns. 

Further detail may be found in articles on Pre-treatment of metals prior to painting, Pre-treatment of copper, Thermal spray processes and Rubber to metal bonding - pre-treatments. 

Many widely used Tests of adhesion can be applied to the rubber to metal bonds Blister test. Fracture-mechanics test specimens. Non-destructive testing of adhesively-bonded structures. Peel tests. Shear tests. Tensile tests and Wedge test. This particular article is concerned with those aspects that are of practical concern in the rubber-processing industry. 

There are several drawbacks to the use of self-bonding compounds which are worthy of mention. To make a self-bonding compound, one must modify the properties of the bulk rubber compound in order to affect the chemistry of the reactions which are required to take place only at the rubber to metal interface. Processing is adversely affected since the compounds want to bond to the mould and metals alike, and the environmental resistance of the rubber to metal bond is generally poor compared to that which is obtained with the use of a conventional rubber to metal adhesive system. 

The level of environmental resistance required and equipment available may determine whether to evaluate primer/cover coat systems versus one coat adhesive systems. And last, but not least, cost of both the adhesive and the application process is an important factor to consider in any industrial application, rubber to metal bonding being no exception. 

It is possible to clean and phosphate in a single operation. This process of acid phosphate cleaning combines the degreasing with a transformation of the metal surface by removal of oxide and the phosphating of the metal. The process can be used for metals such as steel, aluminium and zinc. This process produces an excellent surface for the adhesion of lacquers but it is not suitable for rubber-to-metal bonding. Tests have shown that the corrosion resistance of the components are not satisfactory. 

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. 

Consideration is given to the different steps involved in rubber-to-metal bonding, including surface preparation of metal substrates, the application of primers and adhesives, and moulding, vulcanisation, curing and posttreatment processes. Factors which can lead to weak adhesion and bond failure are discussed, and approaches to the identification and correction of such problems are outlined. 

Beyond the particular process of cure in this case, the important problem which should not be neglected is the bonding of the rubber to the metal. This problem of adhesion of rubbers to metals is very weU covered in Reference [27] where a full description of the various tests is given. Another case of interest appears with the cure of a rubber when the cure is bound to various fabrics in order to get proofed materials such as hose. 

Rubber is bonded to metal during processing to form a variety of products, and in most cases a very strong bond is ncces.sary for the product to perform satisfactorily. It is usually desirable to measure bond strength by testing the actual product, but this is not always possible or convenient and. particularly for evaluating bonding systems, there is a need for... 

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