Surface treatment adhesive bonds

Studies have been conducted on the effects of corrosive salt-spray environment on bondlines of different bonded systems. The system variations included clad and bare alloys, surface treatments, adhesive primers, and adhesives. Five specimens were fabricated for each of the bonded systems. The specimens were then placed in a salt-spray environment of 5% NaCl at 35 °C. The change in wedge-test crack length of each specimen was recorded periodically. At the end of 1 month, one specimen was randomly selected from each bonded system and opened for visual inspection of the bondline condition, both in the stressed zone (crack-tip zone) and in the unstressed zone. The same procedure was carried out after 2, 3, 6, and 12 months, when the last specimen was removed from test. The conclusions were as follows... 

Adhesive failure is a rupture of an adhesive bond, such that the separation is at the adhesive—adherend interface. This failure is mainly due to a material mismatch or inadequate surface treatment. Adhesive failure should be avoided. 

Table 9.11 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics Eire to be joined with adhesives. Solvent and heat welding are other methods of fastening plastics that do not require chemical alteration of the surface. Welding procedures win be discussed in another section of this chapter. The effects of plastic suT ce treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some common plastic materials that require special physical or chemical treatments to achieve adequate surfaces for adhesive bonding are listed in the following sections. 

To be effective, the reinforcement must form a strong adhesive bond with the plastics for certain reinforcements special cleaning, sizing, finishing, etc. treatments are used to improve bond. Also used alone or in conjunction with fiber surface treatments are bonding additives in the plastic to promote good adhesion of the fiber to the plastic. 

As with metallic substrates, the effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Listed below are some common plastic materials that require special physical or chemical treatments to achieve adequate surfaces for adhesive bonding. 

TABLE 7.17 Effects of Surface Treatments on Bonding to Polyethylene with Various Types of Adhesives (from Ref. 27)... 

Flame treatment is widely used to prepare polyolefin surfaces for adhesive bonding, particularly in labelling operations. This method is purported to burn-off contaminants and weak boundary layers, and also produces surface oxidation. Although flame treatment can be readily automated on a container labelling line, it is very impractical for most product assembly operations. 

D-2651. Practice for Preparation of Metal Surfaces for Adhesive Bonding. Includes procedures for washing, steaming, solvent cleaning (degreasing), mechanical abrasion and chemical treatment or etching. 

Thelen, E., et al. Treatment of Metal Surfaces for Adhesive Bonding, W.A.D.C. Tech. Rept. 55-87, Part V (Feb. 1958) from The Franklin Institute Laboratories. 

The development of SPM and particularly AFM has not been ignored by the adhesion world. The analysis of failure surfaces by AFM is still in its infancy but it is already clear that AFM may sometimes be more useful than SEM. Thus AFM has been used successfully in the analysis of the failure surfaces of adhesively bonded galvanized steel [149] and in the study of the adsorption of polymers on conducting polymer substrates [150,151]. Some of the author s unpublished studies have shown that the corona treatment of polyolefines gives rise to a characteristic morphology and that failure interfaces from peel joints of polyolefine substrates can yield more useful information from AFM than from SEM investigations [152]. 

Fay and Maddison (1990) measured times to failure at 100% r.h and 42 8°C for joints in steel with a number of surface treatments, and bonded with a toughened epoxide adhesive. Their results showed that failure was hastened by increasing stress and that time to failure can be much increased by selection of an appropriate surface treatment. 

Light anodic treatment and various corrosion-preventive treatments produce good surfaces for adhesive bonding. These treatments have been developed by magnesium alloy producers, such as Dow Chemical Company (www.dow.com). Midland, MI, and others. Details are available from ASM Metals Handbook, Vol. and MIL-M-45202,... 

Metals require surface treatment to remove contamination and prepare their surfaces for adhesion bonding. Both initial bond strength and bond durability are affected by the surface preparation technique. Most metals require unique methods of treatment for formation of optimal bond strength. Most treatment methods for surface preparation use or generate chemicals that may have a variety of adverse health effects and must be used with extreme caution. 

Bonding. Surface treatment, such as chemical etch, corona, or flame treatments, is required for adhesive bonding of Tefzel. Polyester and epoxy compounds are suitable adhesives. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

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