Adhesive bonding quality control

Furthermore, the application of existing European or national test and performance standards for epoxy bonded products are much too penalising, since they merely impose severe conditions that are not verified in service, or are inadequate because they were developed originally for other adhesives, namely for phenolic and aminoplastic adhesives used in very thin bondlines [7, 52-58]. Moreover, current standard proposals developed for gap-filling adhesives focus only on the initial bond quality control [59-63]. The lack of standards in this field impedes the objective evaluation of the reliability of a bonded-in rod connection, causing engineers to avoid this type of approach altogether. 

Adhesive remains unchanged until it is cured No multiroll coating needed No complex tension controls needed Adhesive bond established almost instantly Real-time quality control... 

The adhesive bond is established almost instantly. Real-time quality control. 

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should be used only when justified. The most likely occasions for a primer to be used are when (1) the adhesive or sealant cannot be applied immediately after surface preparation, (2) the substrate surface is weak or porous, or (3) the adhesive-adherend interface requires additional protection from service environments such as moisture. 

Both the formulating of adhesives and bonding with adhesives are complex, multiple-part processes, complete with interacting and sometimes unexpected parameters that may contribute to success or failure of the final product. Thus, it is important that the quality control process consider the entire operation from receipt of materials to final product testing. 

A generalized flowchart for the quality control process in formulating epoxy adhesives is shown in Fig. 19.1. A flowchart for controlling the quality of the adhesive bonding process is shown in Fig. 19.2. It must be realized that in both cases, the decisions made in one phase of the process may affect the subsequent phases. Therefore, all the individual phases must be carefully coordinated and controlled. 

FIGURE 19.2 Flowchart of a quality control system for adhesive bonding. 

As with the quality control plan for the formulator described above, acceptance tests on adhesives should be directed toward assurance that incoming materials are identical from lot to lot. However, this plan should also extend to the receipt and control of incoming adherends and other materials used in the bonding process. 

There are several ways that actual joints can be tested. For quality control purposes, a proof test is commonly used, as described above. This test imposes a stress on the specimen but limits it to a point well below where any destruction of the joint can occur. This type of test only looks for serious flaws in the bonding processes such as interface contamination, air entrapment in the joint, or undercured adhesive. The prototype joints can also be tested to destruction by using similar test methods described above for standardized testing. This, however, is generally not done at great frequency because of the cost. Generally, most actual joint tests are performed to ascertain the cause of failure (i.e., forensic analysis of the failed joint) or for nondestructive determination of the adequacy of the bond. 

Since the adhesives only crosslink at specific wavelengths, the parts to be joined can be premounted and fixed with a high degree of positional accuracy under normal workplace conditions. Quality control can also be realized (proof of adhesive application, positional correction) without affecting the adhesion of the bond. 

These standard methods are simple procedures intended for quality control and the comparison of bonding systems, but they could be readily used as the basis for wider investigations such as the effect of peeling angle, test speed, etc., or adhesion to other rigid substrates. 

EIS and other electrochemical methods appear to be useful for the study of the performance of metal pretreatments (cleaning processes, anodization, phosphating, chro-mating, etc.) prior to adhesive bonding. A quick comparison of methods can be achieved, and because the method is fast and straightforward, it can be used as a quality control method. On adhesively bonded system EIS could be performed in more fundamental studies that would provide information on the nature and locus of degradation processes, when immersed in aggressive solutions. 

Proper intervention planning and design, workmanship and quality control are essential since the non-reversibility of adhesion does not allow inspection and correction after cure. Compared to currently used mechanical timber joints, bonding requires more skilled and well-trained operators, and better organisation and record keeping for future traceability of possible problems and defective joints. 

The importance of quality assurance in any industry is widely acknowledged. More stringent customer expectations with regard to quality, together with the realisation that continued improvements in quality are often necessary for a company to sustain good economic performance, mean that some level of quality assurance is mandatory for all those associated with adhesive bonding operations. The use of technical specifications and quality control procedures go some way towards fulfilling the requirements of a quality system, and these measures are outlined here. 

The measurement of the strength of adhesion between adherend and adhesive requires a measurement of the intermolecular forces of attraction this is not currently possible. This aspect of quality control is therefore essentially reduced to assessing the adherend surface characteristics prior to bonding, although some post-bonding simple mechanical tests are appropriate. 

The requirements to form a good adhesive bond, processes for bonding, analytic techniques, and quality control procedmes have been discussed in this book. 

Contaminated adherends will not form durable adhesive bonds regardless of the quality of materials used or the control of process. Many adherends require surface preparation beyond simple cleaning. 

---