Лако-красочные материалы — производство

Sticking things together is a common enough task, and materials exhibiting adhesive properties have been employed in a sophisticated manner since earliest times. Natural adhesives such as starch, animal glues and plant resins have been used for centuries, and are still used widely today for packaging and for joining wood. Rubber — based adhesives were introduced in the shoe and tyre industries towards the end of the nineteenth century, but the birth of modern structural adhesives is generally dated from the early twentieth century with the introduction of phenol-formaldehyde resins. Mainly as a result of the Second World War, many natural products were not available in the early 1940s and this spurred the further development of synthetic resins. The construction of wooden wartime aircraft was, nevertheless, facilitated by the availability of phenol-, resorcinol — and urea-formaldehyde adhesives, and since then reactive formaldehyde-based adhesives have continued to be used in the manufacture of timber-based building elements such as plywood, chipboard, and laminated timber beams.

Over the past four or five decades the natural adhesives have

been improved, and there has been an intense development of synthetic adhesives to meet more technically demanding applications. These synthetic polymers and ancillary products, which include thermoplastic and thermosetting types, have been developed to possess a balance of properties that enables them to adhere readily to other materials, to have an adequate cohesive strength and appropriate mechanical characteristics when cured, to possess good durability, and to meet various application and manufacturing requirements.

Thermoplastic adhesives may be softened by heating and rehard­ened on cooling, and included in this group are polyvinyl acetates (PVACs). Since the 1950s they have been used extensively as general-purpose adhesives for bonding slightly porous materials, from floor screeds to timber; they are, however, sensitive to wet alkaline service conditions, effectively restricting them to indoor use. Similar adhesives suitable for external situations are based on other polymer dispersions such as styrene butadiene rubbers (SBRs), acrylic polymers, and copolymers of vinyl acetate with other monomers. Cyanocrylates, or ‘superglues’, also belong to this class of thermoplastic adhesives and are very useful for bonding small parts involving plastics, rubber, metal, glass, and even human tissue.

Thermosetting materials are so called because, when cured, the molecular chains are locked permanently together in a large three — dimensional structure; they may, therefore, be regarded as structural resins. Unlike thermoplastics they do not melt or flow when heated, but become more rubbery ar. d lose strength with increasing temperature. Phenolic resins, and their modifications, belong to this group of adhesives and are numbered among the early structural adhesives used extensively within the aerospace industry for bonding metal parts. Epoxides and polyesters also belong to this group of thermosetting adhesives, and they find widespread use in civil engineering applications. Unsaturated polyesters are often used as binders in glass-reinforced plastics, or as mortars in conjunction with stone and cementitious materials. However, high shrinkage on curing, poor resistance to creep and low tolerance of damp conditions significantly restricts their application. Epoxides, on the other hand, are generally tolerant of many surface and environmental conditions, possess relatively high strength, and shrink very little on curing. There are available a range of epoxy materials which cure at ambient or elevated temperatures, whose mechanical and physical characteristics vary widely. Indeed the general term epoxy may include materials which vary from flexible semi-elastic coatings and sealants to epoxy resin based concretes. Epoxy adhesives are available as single — or two-component materials in liquid, paste or filmic form, which may additionally be ‘toughened’.

The increasing use of adhesives in a diversity of demanding situations has given confidence in the successful application of synthetic polymers, and has provided the spur for further fundamental research and the development of improved products. In the future it is possible that acrylates and polyurethanes, and their toughened variants, may challenge the epoxides — particularly as they are perceived to be safer to use and less environmentally harmful. Structural silicone adhesives may also be introduced for certain applications where gap-filling and flexibility are required, but where high strength is relatively unimportant; they also possess the added advantage of very high thermal and environmental stability.