Prebond treatment

Allan, A. J. G. Wettability and friction of polytetrafluoroethylene film effect of prebonding treatments- J. Polymer Sci. 24, 461—466 (1957). 

The sodium dichromate-sulfuric acid process has been found by some to be superior to other ferric chloride methods for the prebond treatment of copper. This dichromate-sulfuric acid method is also defined by ASTM D 2651. Nitric acid and ferric chloride etching processes have also been found to be useful for copper, brass, and bronze substrates in certain applications. 

Surface preparation (surface treatment)—A physical or chemical preparation, or both, of an adherend to render it suitable for adhesive joining. The term prebond treatment is sometimes used, but is deprecated. 

The literature shows that it is possible to improve the properfies of the adhesive joint by surface treatment of the fibres to increase interfacial strength (Luo and Van Ooij, 2002 Holme, 1999). Oxidation prebond treatments are currently the most widely used. Plasma treatments offer some advantages and are becoming more popular. They have the potential to increase the surface functional groups and thereby increase the number of chemical bonds between the treated fibre and the adhesives. Surface modification of textile fibre is stdl an area under active study and more techniques may emerge in the near future. 

The principles involved in producing bonds of optimum strength and durability are the same, irrespective of the nature of the adherends. The surface must be free of contamination, must be receptive to the adhesive (or primer, if used), and the adhesive/adherend interface formed must be stable to environmental exposure. It is the last mentioned factor that is of most concern in a discussion of the durability of adhesive bonds, particularly for metallic adherends. The prebond treatment of the metal surface is of paramount importance in determining joint performance. With wood and plastic adherends, long-term performance is usually determined by other factors, provided that good initial joints are formed. 

Surface preparation of adherends prior to bonding is one of the most important factors in the adhesive-bonding process. Initial bond strength and joint permanence are greatly dependent on the quality of surface that is in contact with the adhesive. Prebond treatments are intended to remove weak boundary layers and provide easily wettable surfaces. As a general rule, all adherends must be treated in some manner prior to bonding. 

Various substrate surface treatments suggested for use with a common epoxy-substrate joint and service environment combinations are discussed in this chapter. Surface preparation processes for a range of specific substrates and detailed process specifications are provided in App. F. The reader is also directed to several excellent texts that provide prebond surface treatment recipes and discuss the basics of surface preparation, the importance of contamination or weak boundary layers, and specific processes for adhesive systems other than epoxy.1,2,3... 

Titanium is widely used in aerospace applications that require high strength-to-weight ratios at elevated temperatures. As a result, a number of different prebonding surface preparation processes have been developed for titanium. These generally follow the same sequence as for steel and other major industrial metal substrates degrease, acid-etch or alkaline-clean, rinse and dry, chemical surface treatment, rinse and dry, and finally prime or bond. Mechanical abrasion is generally not recommended for titanium surfaces. 

To obtain a usable adhesive bond with polyolefins, the surface must be treated. A number of surface preparation methods, including flame, chemical, plasma, and primer treatments, are in use. Figure 16.4 illustrates the epoxy adhesive strength improvements that can be made by using various prebond surface treatments to change the critical surface tension of polyethylene. 

Stressed-bond joint durability is markedly affected by the adherend prebond surface treatment and the adhesive/primer system in contact with it. This is evidenced by the poor performance of FM 123-L/BR 123 (non-CIAP) adhesive/primer system on FPL-etched and chromic acid-anodized 2024-T3 aluminum alloy, clad and bare, and the superior performance of the same systems when BR 127 (corrosion-inhibiting adhesive primer (CIAP)) is substituted for BR 123 (non-CIAP). 

For wetting to occur, the substrate surface has to be chemically or physically altered by some mechanism to raise its surface energy. This is why there are so many prebond surface treatments for plastic substrates. 

Table 7.10 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics are to be joined with adhesives. Specific surface treatments for certain plastics and their effect on surface property characteristics are discussed in Sec. 7.6. Details regarding the surface treatment process parameters may also be found in ASTM D-2093 and various texts on adhesive bonding of plastics. An excellent source of information regarding prebond surface treatments is the suppher of the plastic resin that is being joined. 

From Tables 9.8 and 9.9, it can be forecast that epoxy adhesives will wet dean aluminum or copper surfaces. However, epoxy resin will not wet a substrate having a critical surface tension significantly less than 47 dyn/cm. Epoxies will not, for example, wet either a metal surface contaminated with silicone oil or a clean polyethylene substrate. For wetting to occur, the substrate surface has to be chemically or physically altered by some mechanism to raise its surface energy. This is why there are so many prebond surface treatments for plastic substrates. 

High-pressure water blast has been used for prebond surface treatments to eliminate hazardous materials. The combination of high-pressure water abrasion with subsequent application of an adhesive promoter/primer has been found to provide high strength and durable aluminum bonds. 

To make an economical and practical joint, the surface preparation methods must also meet several other requirements. They must be safe to handle and should not be flammable or toxic. They should be inexpensive and provide fast processing times. The prebond processes should be easy to monitor and control in a production situation. In addition, the process should not in itself leave a weak boundary layer. If chemical solutions are used, they should rinse off easily and not continue to react with the surface past the time when the bond is made. The surface preparation process should allow for practical working time between preparation and application of the adhesive or sealant. Finally, the surface provided by the treatment should not change once the assembled joint is made and placed into service. 

For metals and plastics in particular, prebonding chemical treatment of adherend surfaces is often necessary. The success of high performance metal bonding is so heavily dependent on proper surface preparation that several ASTM standards have been developed which describe procedures for surface treatment and for analysis of the chemicals involved ... 

In the above studies, the influence of various deliberately introduced chemical contaminants on the prebond condition of the adherends has been studied. Also of significance is the influence of prebond handing and the ability of some surface treatments to accommodate physical damage as well as contamination this aspect has been investigated by McNamara etal. (1979). 

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