Adhesion and surface pretreatment

The successful performance of many every-day products, and many common materials and construction techniques, is dependent upon adequate adhesion between two or more constituents. Most engineers, however, have only the haziest of ideas about the whole concept of adhesion. For to know how to prepare substrate surfaces for bonding does not necessarily require a knowledge of why adhesive materials should stick to them. It is the intention of this chapter to connect theory with practice, to enable the reader to appreciate why before discussing aspects of surface pretreatment pertinent to applications of adhesives in construction. 

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This test method is preferably used for the comparative evaluation of adhesives and surface pretreatment methods, since it enables the indication of the differences in the adhesive and cohesive behavior of the adhesive layers with high sensitivity. Pressure-sensitive adhesives (adhesive tapes, adhesive labels) are also tested according to this principle. 

Adhesion promoters. Sometimes referred to as coupling agents, these additives have the ability to enhance resin adhesion to surfaces such as glass or metals(8). The most popular type are silanes which can either be mixed with the adhesive itself or applied to the substrate as a primer. They will be further considered in Chapter 3 under adhesion and surface pretreatment. 

Adhesion and surface pretreatment Table 3.3. Pretreatment requirements... 

Replacement of missing teeth using bridges typically employs bonding agents and surface pretreatment procedures (Roulet and Vanherle 2004). Tooth-realigning techniques employed within the dental specialty known as orthodontics employ brackets and wires to force the teeth into more satisfactory positions. The brackets maybe held in place by adhesive agents applied to the surface of the teeth. Aesthetic procedures, such as the placement of ceramic veneers to improve the appearance of the incisors, also typically employ adhesives and surface pretreatments (Mount and Hume 2005). 

Mixed fracture Failure of a bonded joint by proportionate forms of adhesion and cohesion fracture, usually caused by improper adhesive processing and surface pretreatment. 

The working characteristics of the adhesive relevant to the application conditions must be determined. For instance viscosity is often temperature, shear-rate and time-dependent, and this will influence the choice of dispensing equipment, the method of application, the usable life and the open time. The viscosity should therefore be regulated bearing in mind the adherend rugosity and surface pretreatment, the method and location of application, and the cure temperature and duration of application. A thixotropic material may be required for application to vertical or soffit surfaces. Generally, relatively thick bondlines are encountered so that the adhesive should be able to cure in thick and/or uneven layers. It should also be remembered that for about every 8 C change in... 

Figure 10.5 shows the data obtained on an epoxy-nitrile film adhesive on 5052-H34 aluminum alloy after immersion in hot water for 50, 100, 300, 500, and 1000 hours. This test is very useful because it permits a large number of adhesive-bonded specimens with different adhesives, adher-ends, and surface pretreatments to be tested at the same time with a relatively small investment in man hours and equipment. Figure 10.6 shows a comparison of the stressed-durability data and unstressed hot-water-soak data on the same epoxy-nitrile film adhesive, using 2024-T3 aluminum alloy. Note the parallelism of the plots. The curve in the lower left was obtained when lap-shear specimens were subjected to various levels of stress and then exposed to an environment of 60°C and 95% RH until failure. The failure time is plotted as a log function. The curve in the upper right portion is a plot of the data when the same types of lap-shear specimens were subjected to 60°C water for specified periods of time and then tested for their residual strength. In the first case, failure time was recorded. In the latter case, residual strength was determined. The same type of data is obtained with both curves. ... 

Effect of Variation in Adhesive, Curing Conditions, and Surface Pretreatment on Bond Durability"... 

Thus, in many applications, which may involve some of the most critical uses of adhesives technology, the bonded joints are exposed to an environment which also happens to be one of the most potentially damaging. Indeed, the aspect of the durability of adhesive joints to aqueous environments is undoubtedly one of the most important challenges that the adhesives community faces. In particular, there are two main challenges to develop (a) adhesive systems (i.e. combinations of an adhesive/primer/surface pretreatment/substrate type, all of which may interact to affect the joint durability) which possess excellent long-term durability, and which are environmentally friendly and cost effective and (b) test methods and models to accurately rank and predict the service-life from short-term experiments, and thereby convince the potential user that an adequate durability will be realised. Clearly, these two aims are strongly inter-linked. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

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