Adhesive, selection ambient temperature

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

From the above discussions, it may be concluded that to obtain significant water ingress into a typical adhesive joint at ambient temperature, and to observe any appreciable degree of environmental attack, may take many months, if not years, of exposing the joint to the aqueous environment. Accelerated-ageing tests are therefore a very essential tool in order to accelerate the kinetics of environmental attack, and so obtain durability data in a realistic time-scale for the development and selection of adhesive systems. As noted above, the lack of current knowledge on the details of the kinetics of the mechanisms of environmental attack certainly hinders the development of such tests. 

The addition of a thermoplastic rubber at 10-30 %w produces a truly thermoplastic product with elasticity, resilience, and high adhesive strength. Such mixtures can form the basis for a variety of sealants as discussed in the section on permanent crosslinking. Although the selection of the asphalt and its modifications with aromatic or paraffinic oils is not simple, a balance between resistance to phase separation at 300 F and the formation of a coherent network at ambient temperatures can be obtained by empirical tests. 

Below the MFFT the film is chalky and non-continuous. At temperatures very much higher than the Tg, the film will be soft with poor adhesion, and abrasion resistance. Most emulsion resins intended for ambient temperature applications have a in the range of 0°C-30°C. Although it is possible to include additives specifically to reduce the MFFT, these could possibly leach out of the film, and it is considered a much more desirable practice to control the Tg (and hence the MFFT) of the polymer by selection of the monomer type and composition. For this reason most emulsion polymers are copolymers. In addition to T, other factors dictate the choice of monomer, such as the performance characteristics conferred by a particular monomer. A copolymer is designed to give a balance of properties in the final film, including such parameters as adhesion, flexibility, abrasion resistance, water resistance and ease of film forming (coalescence). 

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