Metal bonding, rubber factors

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. 

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. 

A comparison of these predicted values of E with the measured values plotted in the bar-chart of Fig. 3.5 shows that, for metals and ceramics, the values of E we calculate are about right the bond-stretching idea explains the stiffness of these solids. We can be happy that we can explain the moduli of these classes of solid. But a paradox remains there exists a whole range of polymers and rubbers which have moduli which are lower - by up to a factor of 100- than the lowest we have calculated. Why is this What determines the moduli of these floppy polymers if it is not the springs between the atoms We shall explain this under our next heading. 

There was previously a separate ISO standard for adhesion in shear but this was withdrawn in favour of extending the standard for shear modulus to allow the test to be continued to the failure point, i.e. the two methods have been combined. The composite method is contained in ISO 182715 and uses the same quadruple element test piece as did the separate adhesion standard. The double sandwich construction is intended to provide a very stiff test piece which will remain in alignment under high stresses. The present standard quadruple test piece uses rubber elements 4 1 mm thick and 20 5 mm long and these tolerances are much less tight than previously. The measured adhesion strength in shear is less affected by the test piece shape factor then tension tests8 and the wider tolerances should be perfectly satisfactory. The test piece is strained at a rate of 50 mm/min, in line with the speed for most other adhesion to metal tests, and the result expressed as the maximum force divided by the total bonded area of one of the double sandwiches. The British equivalent BS 903 Part A 1416 is identical. 

The shape factor is an important consideration in the response to any applied load. Shape factor is defined as the ratio of the area of one loaded surface to the total of the unloaded surface that is free to bulge. The ability of the part to move when placed under load is important. If the surfaces are bonded to metal plates, the compressive stress to the compressive strain relationship is quite different. Figure 8.1 illustrates the load bearing of a series of polyurethanes compared to SBR and neoprene rubber compounds. 

The most common situation, both in practice and in experiment, is for the rubber to be bonded to metal plates or held between surfaces that effectively eliminate slip. In this situation the effect of shape factor means that the thinner the rubber the stiffer it appears, and this property is much exploited in the design of rubber mounts and bearings. 

The hardness of the componnd will also become a significant factor the harder the compound the lower will be its ability to deform and follow the contonrs of the metal surface this will affect the level of FSq. If however the increase in hardness of the rubber compound is produced by the addition of sulphur then there will be an increase in the chemically active units on the nitrile rubber surface and thus bond strength because of copper/sulphur bonds being formed gives an increase in FS. 

Although natural adhesives (animal glue, casein, starch, and rosin) are still used for many applications, a host of new adhesive materials based on synthetic polymers have been developed these include polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and rubber materials. Adhesives may be used to join a large variety of materials—metals, ceramics, polymers, composites, skin, and so on—and the choice of which adhesive to use will depend on such factors as (1) the materials to be bonded and their porosities (2) the required adhesive properties (i.e., whether the bond is to be temporary or permanent) (3) maximum/minimum exposme temperatmes and (4) processing conditions. 

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