Adhesive bonding surface preparation

In addition to the effect of the chemistry and the physical properties of the adhesives, the surface preparation of the adherend also has a marked effect. Aluminum surface treatments vary from simple solvent wiping to anodization. Steel treatments vary from abrasive grit blasting to acid etching. In general, it is found that the better the initial surface preparation, the more durable the bond. 

Consideration of all the above energy relationships and derived values can be of great help in selecting adhesives and surface preparation conditions for attaining predictable, strong adhesive bonds. Several examples are presented here. 

The overriding consideration with respect to the adhesion of sealants is that the sealant and surface preparation should be selected to ensure that the adhesion strength exceeds the cohesive strength of the sealant. This can necessitate that extra care be used in the substrate bond surface preparation or that a primer be used. Ensuring that any failure that occurs will be cohesive in the sealant rather than adhesively to the substrate is a primary design consideration. 

Polymers exhibit excellent physical and chemical properties, relatively inexpensive, and easy to process. However, very often they do not possess the surface properties needed for better adhesive bonding. They are hydrophobic in nature and in general exhibit insufficient adhesive bond strength due to relatively low surface energy (Chan et al. 1996). Therefore, in terms of successful application of polymers to form structural parts using adhesive bonding, surface properties like hydrophiKcity and roughness are essentia] (Noeske et al. 2004). Due to these reasons, modification of polymeric surface is the most important factor. The main purpose of surface preparation is to improve the adhesion properties to such an extent that the interfacial failure from polymer to adhesive does not take place. 

Abstract This chapter constitutes one of the very few reviews in the existing literature on shoe bonding, and it gives an updated overview of the upper to sole bonding by means of adhesives. The surface preparation of rubber soles and both the formulations of polyurethane and polychloroprene adhesives are described in more detail. The preparation of adhesive joints and adhesion tests are also revised. Finally, the most recent development and technology in shoe bonding is described. 

Depending on fashion, each year different materials have been and are currently used in the manufacturing of shoes, ranging from rubber soles (vulcanized styrene-butadiene rubber (SBR), thermoplastic rubber, EPDM) to different polymers (leather, polyurethanes, ethylene-vinyl acetate (EVA) copolymers, polyvinyl chloride (PVC), polyethylene, Phylon). To produce adequate adhesive joints, surface preparation of those materials is required (see part B Surface treatments). Surface preparation procedures for these materials must be quickly developed and the validity of these treatments is generally too short. Several procedures have been established to optimize the upper to sole bonding, most of them are based in the use of organic solvents. Due to environmental and health issues, solvents should be removed from the surface preparation procedure and several environmental friendly procedures for the surface preparation of several materials have been proposed. 

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. 

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

The surface preparation must enable and promote the formation of bonds across the adherend/primer-adhesive interface. These bonds may be chemical (covalent, acid-base, van der Waals, hydrogen, etc.), physical (mechanical interlocking), diffusional (not likely with adhesive bonding to metals), or some combination of these (Chapters 7-9). 

Direct bonding. In many high-volume production applications (i.e., the automotive and appliance industries), elaborate surface preparation of steel ad-herends is undesirable or impossible. Thus, there has been widespread interest in bonding directly to steel coil surfaces that contain various protective oils [55,56,113-116], Debski et al. proposed that epoxy adhesives, particularly those curing at high temperatures, could form suitable bonds to oily steel surfaces by two mechanisms (1) thermodynamic displacement of the oil from the steel surface, and (2) absorption of the oil into the bulk adhesives [55,56]. The relative importance of these two mechanisms depends on the polarity of the oil and the surface area/volume ratio of the adhesive (which can be affected by adherend surface roughness). 

ASTM D3933, Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding, Phosphoric Acid Anodization, A.STM, West Conshohocken, PA. El-Mashri, S.M., Jones, R.G. and Forty, A.J., Philo.s. Mag. A, 48, 665 (1983). 

Wegman, R.F., Surface Preparation Techniques for Adhesive Bonding. Noyes Publications, Park Ridge, NJ, 1989. 

Mazza, J.J. and Kuhbander, R.J., Grit blast/silane (GBS) aluminum surface preparation for structural adhesive bonding, WL-TR-94-4111. Materials Laboratory, Air Force Materiel Command, September 1999. 

The final section in this volume deals with applications of adhesion science. The applications described include methods by which durable adhesive bonds can be manufactured by the use of appropriate surface preparation (Davis and Venables) to unique methods for composite repair (Lopata et al.) Adhesive applications find their way into the generation of wood products (Dunky and Pizzi) and also find their way into the construction of commercial and military aircraft (Pate). The chapter by Spotnitz et al. shows that adhesion science finds its way into the life sciences in their discussion of tissue adhesives. 

Several environment-friendly surface preparation for the treatment of mbber soles with radiations have been recently studied. These treatments are clean (no chemicals or reactions by-products are produced) and fast, and furthermore online bonding at shoe factory can be produced, so the future trend in surface modification of substrates in shoe industry will be likely directed to the industrial application of those treatments. Corona discharge, low-pressure RF gas plasma, and ultraviolet (UV) treatments have been successfully used at laboratory scale to improve the adhesion of several sole materials in shoe industry. Recently, surface modification of SBR and TR by UV radiation has been industrially demonstrated in shoe industry... 

When the direct-on process is utilized, surface preparation requirements are more critical to ensure effective enamel adhesion. The acid etch is often deeper and the nickel deposition is always thicker. Typically, the nickel coating is 0.01 to 0.02 g/m2 for direct-on coating as compared to 0.002 to 0.007 g/m2 for two-coat applications. A few porcelain enamelers prefer to omit the nickel deposition step. Although the nickel enhances enamel bonding, product quality requirements may not require nickel deposition. The omission of the nickel step necessitates the utilization of a heavy acid etch to ensure a clean, properly conditioned surface for enamel bonding.3-6... 

Effect of Surface Preparation on the Durability of Structural Adhesive Bonds... 

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