Surface preparation anaerobic adhesives

Sample preparation of anaerobic adhesives for metal content is an important step, be it by destructive or non-destructive methods. Inactive metal salts are added directly to anaerobic formulations as fillers or for thixotropic reasons. Generally, active transition metals are not added directly to anaerobic adhesives but are prepared as activators in aerosol solvents to be applied to inactive surfaces as part B of an adhesive formulation. In the majority of cases trace metal analysis of anaerobic adhesives is only required for batches with problematic stability and is best done using destructive methods. 

Storage-stable anaerobic formulations can be prepared with no hydroperoxide if the methacrylate resin is aerated in the presence of an amide and a tertiary amine [42]. Anaerobic adhesives have been formulated with alkyl hydroxyethyl peroxides such as t-butyl-2-hydroxyethyl peroxide (XLVII) [43]. An adhesive formulated with t-butylperoxymaleic acid has improved surface adhesion (XLVIII) [44]. 

Recently, the curing characteristics of anaerobic sealants and adhesives (Fig. 13) were reported by Lees (60) and Stamper (64). The present systems can be cured rapidly in most metallic substrates, yet possess shelflives in excess of one year. The factors affecting curing are the cure system, surface preparation, substrate, temperature, and bondline thickness. 

In the mid-seventies, spurred by developments from the DuPont Corporation, a new kind of two-component acrylic adhesive system was introduced to the joining industry. The DuPont technology has been widely licensed and is now referred to by numerous terms such as "second-generation acrylics", "reactive adhesives", "modified acrylics", "toughened acrylics", etc. These adhesives seemed to answer the need for improvements over some of the major disadvantages found in anaerobic structural adhesives, namely those of cost, the need for scrupulous surface preparation, and the ability to bond plastic surfaces. 

Anaerobic adhesives of appropriate viscosity are often used to augment and standardise the performance of shrink and interference-fitted components, where they can reduce hoop stress in the outer component and reduce its tendency to crack or display fretting corrosion. These benefits stem from the ability of anaerobics to fill the voids that are present in even the best-prepared surfaces. This spreads loads evenly over the whole of the faying surfaces instead of concentrating them on the engaged peaks, which represent only a small proportion of the theoretical contact area. 

Quite contrary to popular belief - a belief founded on the use of traditional adhesives - reliable joints can be obtained from unprepared surfaces. Of course, there is no denying that the better the preparation the better the overall performance. But, providing contamination is not gross, perfectly adequate levels of performance can normally be obtained from Anaerobic Cyanoacrylate Plastisol Toughened acrylic and Toughened, heat-cured, epoxide-based adhesives. 

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