Bonding, adhesive wetting

NR adhesives can be divided in two types wet bonding and dry bonding. Wet bonding adhesives are applied on substrates in a fluid state, the bond being formed by drying. The dry bonding NR adhesives are pressure-sensitive adhesives because the bond is created under pressure. 

Martin E. R. Shanahan Adhesion, Wetting, and Bonding, National Centre for Scientific... 

One of the most important requirements of a structural adhesive bond is durability that is, the ability to retain a significant portion of its load bearing capability for long periods of time under the wide variety of environmental conditions which are likely to be encountered during service life.CjJ Unfortunately, the poor durability of metal/adhesive bonds in wet, hostile environments has proven to be the major obstacle to widespread development and practical usage within many industries. 

Moisture acts as a debonding agent through one of or a combination of the following mechanisms 1) attack of the metallic surface to form a weak, hydrated oxide interface, 2) moisture assisted chemical bond breakdown, or 3) attack of the adhesive. (2 ) A primary drawback to good durability of metal/adhesive bonds in wet environments is the ever present substrate surface oxide. Under normal circumstances, the oxide layer can be altered, but not entirely removed. Since both metal oxides and water are relatively polar, water will preferentially adsorb onto the oxide surface, and so create a weak boundary layer at the adhesive/metal interface. For the purposes of this work, the detrimental effects of moisture upon the adhesive itself will be neglected. The nitrile rubber modified adhesive used here contains few hydrolyzable ester linkages and therefore will be considered to remain essentially stable. 

For comparison, a conventional resol resin adhesive without lignin was prepared (24), and its gluability was examined. This resin was found to require a hot-pressing rate of at least 1.5 min per 1 mm plywood thickness before a satisfactory wet-bond adhesion strength was achieved at the low hot-pressing temperature of 120°C. This indicates that replacing a part of the phenol with lignin does not imply a mere extender addition, but that a positive role is achieved which enhances the reactivity of the adhesive. 

Figure 1. Comparison of wet-bond adhesion strengths of lignin-resol resin adhesives phenolated with and without acid catalyst. Legend phenola-tion with acid catalyst at 80°C for 60 min Q phenolation without catalyst at 200°C for 60 min. Note Numerical values in parentheses are percentages of wood failure hot-press temperature 120°C.

Figure 2. Relationship between hot-press time and wet-bond adhesion strength for lignin-resol resin adhesives with and without alkylresorcinol. Legend Q without alkylresorcinol 0 with 10 parts alkylresorcinol. Note Numerical values in parentheses are percentages of wood failure phenola-tion 200°C, 60 min, without catalyst hot-press 120°C, 6 min.

Stabilizers and anti-oxidants—to help prolong the service life of the adhesive Wetting agents—to improve stability in storage prior to application and, sometimes, to improve the ability of an adhesive to bond more intractable surfaces... 

The discussion centers on how the epoxy adhesive properties depend on the processes that occur during each stage. Special attention is given to the three requisites for good adhesion wetting, solidification, and avoidance of internal stresses. These are fundamental and well-known factors that affect the quality of the adhesive bond. The objective here is not to... 

Adhesives wet, flow, and set to a solid during bond formation. The transformation from liquid adhesive to solid bond can be achieved in a number of ways. Where the adhesive is a polymer, the initial starting material is a liquid monomer or prepolymer that, under the conditions of bonding with heat, pressure, and/or catalyst, polymerizes to the solid polymer in the glue line. It is also usual to apply solutions of preformed polymers in suitable solvents to the faces of adherends, and allow bond formation to take place with evaporation of solvent. Alternatively, polymers that can be melted or softened to flow at elevated temperatures can be applied as hot melt adhesives to form the bond on cooling. With porous adherends like wood, penetration of the pores by liquid or molten adhesives is an important factor in bond formation. 

At some point, they must act as a liquid to flow over and wet the surface of the part they are bonding. The liquid form allows the adhesive to fill gaps, cavities, and spaces at the joint or bond interface. Wetting allows the adhesive to make very close contact with the surface—so close that the molecules of the adhesive and surface material interact, causing a strong bond. 

Primers are applied by bmsh, roller or spray gun, and dried, usually in an extracted, air-circulating oven at 60 to 80 °C. In some applications, the primed substrates are then stored in a clean, dry area until needed in other applications, the primer is cured, by oven-stoving, at the requisite temperature. These components would then be similarly stored. In the first case, the primer co-cures with the adhesive, and in the second, the adhesives wets and bonds to the already cured primer coating. Irrespective of which route is followed, the primer protects the integrity of the pre-treated surface prior to bonding. 

Figure W.2 An illustration of good and poor wetting by an adhesives spreading over a surface. (Ref Schneberger, G.L., Chemical Aspects of Adhesives Bonding, Adhesives Age, March 1970)...

The main reasons for preparing the wood surface before bonding are (1) to produce a close fit between the adherends and a bond-line with uniform thickness (2) to produce a freshly cut or planed surface, free from machine marks and other irregularities, extractives and contaminants and (3) to produce a mechanically sound surface, without crushing or burnishing it, which would inhibit adhesive wetting and penetration. 

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