Adhesion environmental considerations

Solvent-borne adhesives represent the majority of the volume in the packaging market, with both one- and two-component systems being used. Waterborne polyurethane adhesives are a much smaller segment that has been driven by environmental considerations. Growth has slowed in recent years because of generally inferior performance compared to solvent-based adhesives and because most of the major converters have already made capital investments in solvent recovery systems. 

Tetrahydrofuran (THF) is produced by decarbonylation of furfural to furan followed by catalytic hydrogenation. THF is applied as a solvent for resins and plastics, film castings and adhesives. THF also acts as a solvent in different fine organic syntheses on a commercial scale and as a chemical intermediate. Environmental demands increase the THF application cost because THF users must install solvent recovery systems. A high price and environmental considerations will limit the future growth of THF market. 

Because of toxicity, environmental considerations and inflammability problems, solvent-based adhesives are being replaced by other types of adhesives. 

In practice, there is a large range of materials that can be classified as solvent-based adhesives the solvent can be either water or any of a number of organic liquids, the choice of the latter depending on what is to be dissolved, availability, cost and health and environmental considerations. 

Thermoplastic linear polyurethanes which are usually chain-terminated so that no unreacted free NCO groups remain available. Environmental considerations direct growing attention to these newer non-polluting urethane adhesive forms, e.g. powders, films, aqueous dispersions and 100% solids reactive systems. Some systems do possess blocked diisocyanates which are activated on heating to produce chemically reactive solid systems. 

In this special volume on polymer particles, recent trends and developments in the synthesis of nano- to micron-sized polymer particles by radical polymerization of vinyl monomers in environmentally friendly heterogeneous aqueous and supercritical carbon dioxide fluid media are reviewed by prominent worldwide researchers. Polymer particles are prepared extensively as synthetic emulsions and latexes, which are applied as binders in the industrial fields of paint, paper and inks, and films such as adhesives and coating materials. Considerable attention has recently been directed towards aqueous dispersed systems due to the increased awareness of environmental issues. Moreover, such polymer particles have already been applied to more advanced fields such as bio-, information, and electronic technologies. In addition to the obvious commercial importance of these techniques, it is of fundamental scientific interest to completely elucidate the mechanistic details of macromolecule synthesis in the microreactors that the polymer particles in these heterogeneous systems constitute. 

Over the last 20 years, environmental concerns have become an important consideration in adhesive formulation and use. Firstly, in the plant the adhesive may require particular handling or the use of protective equipment. Secondly, volatile emissions arising from adhesive reactions both in the hot press and subsequently when the panels are in service are subject to tight regulatory control. In particular the reduction formaldehyde emissions from wood-based panels has been a major objective in adhesive development over the last few years, both because formaldehyde-based adhesives are the major type used and because these have been implicated in environmental and health concerns. 

A wide range of adhesive and other binder systems is used in the manufacture of composite panels, dominated by formaldehyde-based systems. Growth in the composite panels industry has depended on adhesives with the required properties being available in sufficient quantities at a suitable eost. The industry requires considerable economies of seale for synthetie adhesive production. The materials are derived from oil (phenol) or natural gas (urea and formaldehyde). Recently in the face of environmental constraints and the inereasing eost of oil and natural gas there is renewed interest in adhesive systems derived from renewable resourees. 

A distinctive feature of dispersion adhesives, in contrast to solvent-based adhesives described in Section 5.2, is that water is used as an incombustible solvent . This bears an advantage regarding possible processing risks and environmentally relevant regulations. Therefore, adhesive and raw material manufacturers made considerable research and development efforts to replace solvent-based adhesives by dispersion adhesives. 

An effective solvent for plastics objects may be required as a component of an adhesion or repair treatment where the edges to be joined require etching or softening. By contrast, a poorly effective solvent for plastic may be sought as a component of a cleaning treatment where the soil is dissolved while the original plastic surface remains undamaged. In addition, selection of an effective and appropriate solvent may also depend on its evaporation rate, environmental or health and safety considerations. 

Traditional pressure-sensitive adhesives were solutions of rubber and resin in solvent, and these dominated the market until well after World War II. From that time, as an increasing array of elastomers became available, as the price of solvents soared, and as environmental opposition to the use of solvents increased, water-based and hot-melt types made substantial inroads into the solvent-based market. This trend is likely to continue, although solvent-borne adhesives will probably always retain niches in areas where drying speed or ability to key into specific surfaces will outweigh environmental, handling, or price considerations. 

---