Permeable adherend

Particular regard should be paid to the highly alkaline nature of concrete and any possible adverse effect on bond strength, especially in the long-term. For new construction, the amount of free moisture associated with pouring wet concrete on to the adhesive-coated steel should be noted. It is also likely that a joint configuration employing permeable adherends such as concrete will be inherently less durable than metal/metal joints. 

Disadvantages. Acetic acid given off by some silicones, need of permeable adherend. 

Solvent-based adhesives are adhesives with polymers dissolved or pasted in organic solvents. The solvents or solvent mixtures are only processing aids and have to be removed, either partly or completely, from the applied liquid adhesive layer through evaporation or penetration prior to the fixing of the adherends. The first case is necessary for solvent-impermeable materials (metals, glass, thermosetting plastics), the second case concerns porous and solvent-permeable materials (paper, cardboard, wood, leather). This process can be accelerated by heat supply. Solvents are mainly esters, ketones, if applicable, portions of different alcohols. The total solvent portion ranges between 75-85%. 

In the case of permeable, respectively, porous materials (papers, cardboards, wood, etc.) the fixing of the adherends is already possible when the minimum... 

One-sided bonding (wet bonding) In this procedure, the adhesive is applied to one adherend only. This is recommendable for bonding of solvent-permeable, respectively, absorbent materials (leather, textiles, wood products). In this case, complete solvent evaporation is not required. 

As already mentioned, moisture is the bane of most adhesive bonds. It is nearly impossible to keep water from a bond exposed to the outside enviromnent [14]. Water can readily diffuse through the adhesive or the adherend, if it is permeable, as a composite might be. Moistine can also wick or travel along the interface and it can migrate via capillary action through cracks and crazes in the adhesive. Once moisture is present, it can attack the bond by [14]... 

Composite materials, like adhesives, are permeable to water. Moisture can therefore diffuse to the adherend/adhesive interface, which is a problem more in initial joint fabrication when a heat-cured adhesive is employed than when the joint is in service. Thus the moisture content of the composite at the time of joining should be low (say 1%) or else moisture may be drawn to the interface by... 

Moisture resistance. The adhesive should be formulated to minimise moisture transport through the adhesive itself. The equilibrium water content (A/ ) should not exceed 3% by weight after immersion in distilled water at 20 °C. The permeability, obtained from the product of the coefficient of diffusion (D) and My, should not exceed 5 x 10 " m-/s at 20 °C (see Fig. A.l). A film of adhesive, approximately 1 mm thick cast in polytetrafluoroethylene-lined moulds and weighing at least 3 g, is suggested for this test(3). This requirement is to enhance the potential for a durable adhesive/adherend interface, even if moisture uptake is not deleterious to the adhesive itself. 

When a solvent-based system is applied to a porous or permeable substrate, the solvent may pass out of the bond throngh the adherends. When the adherends are relatively impermeable, a common practice is to coat both mating surfaces and allow them to become snbstantially dry before bringing them into contact while the adhesive stUl retains some degree of Tack. This technique is familiar to many through its application to the repair of punctures in bicycle tyres or the lamination of decorative ( melamine ) laminates to various substrates. 

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