Лако-красочные материалы — производство

Optimisation of surface pretreatment is the key to maximising joint durability. The adhesive influences the surface oxide layer and the surface oxide layer influences the boundary layer polymer matrix; the whole must therefore be viewed as a unique system for every adherend-adhesive combination. The interplay of chemical bonding

and microstructural and macrostructural behaviour greatly compli­cates the study of adhesion. The chemistry and the mechanics of adhesion are not independent, so that the intrinsic forces of adhesion cannot be equated with measurable adhesion.

The role of surface energies and the mechanisms of adhesion are essential to an understanding of favourable conditions for wetting, and of appropriate surface treatments. Adhesives can be encouraged to wet most adherends if their surfaces are suitably prepared to make them ‘receptive’. Unfortunately, high energy surfaces are readily wetted by atmospheric moisture and airborne contaminants, both prior to bonding and also after the adhesive has cured if these agencies can migrate to the interface. Thus lower energy hydrophobic substrate surfaces are preferred, such as primed surfaces.

In the practice of adhesive bonding for applications in construction, surface pretreatment is likely to be the most difficult process to control. The choice of treatments must be tempered by the scale of operations, the nature of the adherends, the required durability, the adhesive to be used, and the cost. The performance of joints constructed with cold-cure epoxies is likely to be critically dependent upon surface preparation, as exemplified by the experience of the Scottish Irvine Development Corporation. In 1978 they elected to use vertical externally-bonded steel plate reinforcement to strengthen the abutment walls of three pedestrian underpasses. A year later, the plates were reported to be falling off, accompanied by extensive interfacial corrosion; the steel surfaces had been abraded by hand, and the concrete surfaces chemically etched.

Some adhesives, notably the acrylics, the heat-cured epoxies and the plastisols are able to withstand a certain amount of substrate contamination. At room temperature the solubility of oil in all epoxy resins is low, but at higher temperatures appropriate formulation can make these materials into reasonable solvents. Plastisol adhesives, which have been used widely in automobile construction on unprepared steel surfaces, differ because the plasticising oils they contain become very powerful solvents as the curing temperature (say 180 °С) is approached. During curing these oils and the contamination they pick up are incorporated in the hardening adhesive mass. However, the use of apparently tolerant adhesives is not an alternative to good surface preparation, because the pretreatment undoubtedly plays a significant role in subsequent joint durability.

Generally, the adherend surfaces should be clean, dry, and free from oils, grease and all loose and unsound material. Adhesives join materials by attaching to their surface layers, so that it is important to think in terms of bonding, at molecular scale, to oxide layers (which are generally inherently unstable!). Bonds to metal alloys and thermoplastics are problematical but have, in fact, been the subject of intensive research because they are commonplace adherends. Currently, little is known about optimising concrete surface treatments. Indeed for many practical adhesive-substrate interfaces there remain unresolved debates concerning the detailed mechanisms of adhesion and of environmental failure. Despite the extra processes involved, hydration-resistant surface treatments and coatings, coupling agents, or primers would seem to be very worthwhile both in reducing variability in joint performance and in securing environmentally stable interfaces. In recent years, the development of new hydrophobic adhesive formulations and sacri­ficial pretreatment technology have become of considerable interest for bonding under ‘difficult’ conditions.

Experimental assessments of the effects of surface pretreatment are of limited value using mechanical tests unless environmental exposure is included. It is very sound policy to collect and examine information on joints loaded and exposed to natural weathering conditions rather than depend solely on laboratory experiments. It is clear that water is the substance which causes most problems in attaining environmental stability of bonded joints; interfacial failure generally indicates that a better surface pretreatment would impart improved joint performance.