Bonding, adhesive cleanliness

Bonding rubber to metal is a complex and multifaceted combination of metallurgy, surface science, adhesion science, rubber chemistry, and process engineering, with a multitude of interactions. In all aspects of bonding, scrupulous cleanliness, adherence to process controls and meticulous attention to detail are essential if good adhesion is to be attained on a consistent production basis. 

The priming coat provides the bond between the metal and subsequent coats. It gives electrochemical control of corrosion. Adhesion is dependent largely on the nature of the binder and the cleanliness of the metal surface. 

Metallic adherends were cut from. 032" (.08 cm) thick sheets into lx4-inch (2.54 cm x 10.16 cm) specimens. Cleanliness of the surface, is required in order to facilitate good adhesion. Steel, as bonded within the automotive industry, often experiences a variety of surface contaminants which are not removed prior to bonding. 

The Bonding Process. Before the actual assembly operation, the cleanliness of the shop and tools should be verified. The shop atmosphere should be controlled as closely as possible. Temperature in the range of 18 to 32°C and relative humidity from 20 to 65 percent are best for almost all bonding operations. All parts should be fitted together without adhesive or sealant to indicate possible production problems due to fit. The suitability of fit is established by either visual inspection or direct measurement with gauge or shim. It is desirable that the extremes in mechanical tolerances also be noted and that test specimens be made with the worst possible fit to ensure that the bonding process will always provide reliable joints. 

Cleanliness—the presence of contaminants such as grease, dust, dirt can reduce adhesive-adherent contact, hence weaken the bond. 

A variety of two component adhesives and sealants are currently available for applications requiring the ultimate in bonding and sealing efficiency and performance. Included among these conventional systems are epoxies, urethanes, polyesters, silicones, polysulfides and some modified phenolics and acrylics. Although their performance can be and usually is quite satisfactory, most are limited by pot life and fixturing requirements - and the need for thorough substrate treatment and cleanliness. 

Testing is important in all aspects of materials science and engineering, but it is especially so in adhesives. Such tests evaluate not only the inherent strength of the adhesive, but also the bonding technique, surface cleanliness, effectiveness of surface treatments, etchings of surfaces, application and coverage of the adhesive, and the curing cycle. 

The basic requirements for a good bond to plastic substrates are surface cleanliness, wetting of the surface by the adhesive, solidification of the adhesive, and proper selection of adhesive and joint design. These requirements are briefly defined here and then more thoroughly discussed in following sections. 

Nondestructive testing (NDT) is far more economical, and every assembly can be tested if desired. However, there is no single nondestructive test or technique that will provide the user with a quantitative estimate of bond strength. There are several ultrasonic test methods that provide qualitative values. However, a trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area. Table 7.16 lists the characteristics of faulty joints that can be detected visually. The most difficult defects to be found by any method are those related to improper curing and surface treatments. Therefore, great care and control must be given to surface-preparation procedures and shop cleanliness. 

Good adhesion of primers to metal substrates can best be achieved by ensuring cleanliness of the metal surface and by selection of generic coating primer types based on binders with chemical groups that can bond strongly to surfaces. 

As with adhesive bonding, surface treatment emd cleanliness is of primary importance when decorating plastic parts. Prior to decoration, the svirface of the plastic part must be cleaned of mold release, internal plastics lubricants, and plasticizers. Plastic parts can also become electrostatically charged and attract dust. This could disrupt the even flow of a coating or interfere with adhesion. Solvent or destaticizers may be used to clean and eliminate static from plastic parts prior to decorating. Cleaning of the plastic part requires an understanding of the plastic material to be cleaned and the effect of the solvents and processes on that plastic. 

A prime requirement is good adhesion between fibre and matrix, a condition that will be dependent on features such as chemical bonding and surface cleanliness, as well as mechanical factors. The ratio of surface area to fibre volume should be as high as possible. Considering cylinders of length / and radius r ... 

Cleanliness. To achieve an effective adhesive bond, one must start with a clean surface. Foreign materials such as dirt, oil, moisture, and weak oxide layers must be removed from the substrate surface, or the adhesive will bond to these weak boundary layers rather than the actual substrate. There are various surface preparations that remove or strengthen the weak boundary layer. These treatments generally involve physical or chemical processes or a combination of both. Surface-preparation methods for specific substrates wiU be discussed in a later section. 

The surface-preparation issue is just as serious for fibrous composite structures (see [2]) as for ones made from metal, and is addressed in other chapters of these volumes. Cleanliness alone is not sufficient the adhesive must want to adhere to the substrate, so the surface must be activated appropriately. No glue will bond to a totally inert surface (see [3]). It suffices to say, here, that all the mechanics-based design procedures are rendered worthless if the glue does not stay stuck ... 

As with all adhesives, substrate cleanliness is the key to strong, durable bonds. It is essential that all mold releases, oils, residual acids, etc., be removed initially. This can best be accomplished by wiping the surface with a cloth wetted with acetone or naphtha. If the surface itself is acidic, a wash in a mildly alkaline solution or treatment with a cleaning-activator solution is recommended. For metal bonding, the surface should be solvent cleaned, abraded with sandpaper, followed again by a solvent wash. 

If surfaces are brought into contact they adhere. The coefficient of adhesion is the ratio of the contacting force to the strength of the bond and may be used as a measure of cleanliness. The coefficient of adhesion can be used to monitor the recontamination rate (Figure 13.1). 

Before adhesive bonding, it is essential to thoroughly clean the adherends. Unclean adherends will be unreceptive to optimal adhesion regardless of the quality of materials used, or the stringent control of the application process. Proper surface preparation is extremely important in assuring strong and lasting bonds. For many adherends, surface preparation requirements go far beyond simple cleanliness. 

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