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TRY imagining everyday life without glue-shoes would fall apart, books disintegrate and furniture collapse. More surprisingly, solar panels would drop off satellites and planes would fall from the sky.

Adhesion is only one branch of zygology, the science of joining things together-other branches include welding, riveting and all forms of mechanical joining. Chemical welding, another name for the science of adhesion, is a many-faceted subject requiring an understanding of interatomic forces, chemical composition and physical properties of materials, and the forces to which a final structure will be subjected.

Glues have been used for thousands of years, and up to the early 20th century the only ones of major importance were of animal and vegetable origin. Many natural-based glues are still widely used for bonding porous materials such as paper.

The builders of First World War aeroplanes chose casein glues to construct their wooden frames. Casein is a major protein present in milk. It can be precipitated by acids or by rennet and can be made into a glue by dissolving the precipitate in an alkali and heating it to about 50 掳C. It is an excellent wood glue and has long been used in making furniture. Unfortunately, as the early aviators soon found, casein glues absorb moisture, which encourages the growth of fungal mould, thus weakening it. This is a disadvantage shared by many 鈥渘atural鈥� adhesives.

Since the 1930s the number of synthetic glues has greatly increased. Their main advantage is their resistance to moisture and mould, and their ability to adhere in extreme temperatures and when structures are continually being flexed or bent.

Towards the end of the 19th century, phenol resins made an appearance, and by the early 20th century phenol-formaldehyde resins came on the scene. Phenol-formaldehyde resins became the first synthetic resins to be used as glues. They were widely used in manufacturing plywood. Later, the needs of a growing aircraft industry for materials suitable for metal bonding led to the production of phenolic resins with added synthetic rubber to produce adhesives with 鈥渉igh shear and peel strengths鈥�-that is to say, they do not fracture easily or peel away from surfaces.

Brand new synthetics

Diverse applications

THE 1950s saw the introduction of epoxy resin-based adhesives that were at least as strong as traditional glues. Because they are solid, they tend to be safer to handle and easier to apply than solvent systems. There is no wastage through evaporation. These factors often make them more cost-effective than liquid adhesives.

Today, the number of applications for adhesives is growing fast. They range from industrial processes requiring large quantities of adhesives, to the assembly of components needing only small amounts. Paper, packaging, footwear and woodworking remain the major applications. However, usage has increased significantly in other areas, for example in construction work to fix window frames into walls, in vehicle manufacture to join plastic components such as dashboards to a metal chassis, in electrical assemblies to fix components to circuit boards, and in space to cement solar panels to satellites.

And the tradition of using adhesives in the aviation industry continues apace. Design engineers chose polysulphide adhesives to fix sections of the wings of the European Airbus. Adhesives are ideal for assembling complex components from pieces rather than machining them from blocks of metal and, as a result, cut costs by 30 per cent.

In the past decade, many new synthetic resins have appeared on the scene, such as the cyanoacrylates, and other ingredients of glues have been improved, such as more effective antioxidants (these prevent chemical breakdown by oxygen and sunlight). Such developments have made possible stronger, more durable and versatile adhesives for gluing surfaces which previously were difficult, if not impossible, to bond. For example, 鈥渢hermosetting鈥� plastics and composites have become commonplace in the construction of space exploration probes, bonding panels of lightweight metal and plastics. A thermosetting material hardens permanently, whereas a thermoplastic material softens when heated.

The new adhesives have been developed with more efficient ways of evenly applying and spreading them over large surfaces. As a result, glues are now widely used for joining metal to metal and other materials in load-bearing structures, such as bridges, and for a wide variety of other purposes.

Most adhesives are applied as liquids to join two solid surfaces. The mechanisms by which adhesion occurs are complicated and several theories have been proposed to account for the strong molecular bonds that are formed. The chances are several mechanisms come into play at once (Figure 2).FIG-mg20899502.JPG

The simplest is that of a mechanical hook and eye, whereby the adhesive flows into the minute openings and crevices on the surfaces of the components to be adhered and solidifies. This gives a fixed joint that cannot be fractured without either physically breaking the glue, or the surface of one of the adhering components.

Intermolecular forces

Some serious bonding

WHEN dissimilar substrates or surfaces are bonded, a process of adsorption is involved. The molecules of the adhesives are brought into close contact with those of the substrate, and intermolecular forces then retain the adhesive on the substrate surface. While the adhesive is still liquid, adsorption of the glue is a dynamic process in which desorption-the displacement of other atoms or molecules-and the orientation of the adsorbed molecules all play a part. The intermolecular forces reach an equilibrium which is fixed once the solvent evaporates and the adhesive sets.

Another theory suggests that molecules from the adhesive actually diffuse into the two substrates that are to be joined. This process of absorption means that bonds form not just on the surfaces of the substrates, but deeper inside them.

Because adhesion depends on the formation of chemical bonds, the materials must be chemically compatible. This is why some materials are easier to join than others-for example, it may be difficult to join a plastic to a piece of glass or ceramic, since the latter are inert (unreactive) silica-based substrates. Superglues, such as the cyanoacrylates, may do the trick. Plastics such as Perspex (acrylate) that have double unsaturated bonds are easier to stick than, say, polythene, which has no double unsaturated bonds and is therefore more chemically stable.

When it comes to classifying adhesives there is no adequate single system. The adhesives industry generally uses a classification based on 鈥渆nd use鈥�, such as metal-to-metal adhesives, wood adhesives, general purpose adhesives, paper and packaging adhesives, and so on. The trouble is that a particular adhesive may have several end uses.

Adhesives are also classified according to their physical form, chemical composition, method of application, various processing factors (for example, their setting action) and their suitability for particular working requirements or environments (for instance to withstand the constant flexing of a shoe when we walk or run).

As regards chemical composition, adhesives can be classified into three major groups: natural resins, inorganic materials and synthetic resins. All resin-based adhesives are polymers-that is, they consist of large molecules made up of chains of repeated units (monomers). The natural resins are based on starches, dextrin (a breakdown product of starch), proteins and natural rubbers. They include processed animal hides and bones, and casein. They can be as simple as flour mixed with water (a starch-based adhesive).

The so-called inorganic adhesives are essentially based on silicone. Silicones are polymers with a backbone of alternating silicon and oxygen atoms, with various organic groups attached, usually methyl or phenyl groups. Even though silicones have organic components they are not strictly organic compounds because these are attached to the silicon atoms in the polymer chain, rather than forming a carbon backbone. Some of the best sealants are based on silicones (see below).

There are several other broad chemical classes of adhesives, each with its own specific advantages for particular applications (see Box 2): thermosets, thermoplastics and synthetic elastomers.

The thermosets form bonds which are essentially resistant to heat, catalysts or combinations of these. Unlike thermoplastics, thermosets tend not to deform, or creep, under constant stress. They are often used in heavy load-bearing structures, and in places where they may be subject to severe environmental conditions, such as extremes of temperature, cold, radiation, humidity and adverse chemical atmospheres.

Natural glues can also be thermosetting. They are mostly gelatin-like substances extracted from animal tissues, from fish and from dairy products. Starch and many cellulose-based adhesives are of vegetable origin. Another important natural inorganic adhesive is sodium silicate, or water glass, a viscous colloidal solution of sodium silicates in water. It is used to make silica gel and as a size (a thin gelatinous covering used as a sealer on porous surfaces such as paper) and preservative (for example to preserve eggs by coating their shells).

In contrast, thermoplastic adhesives are easily melted, soluble, soften when heated and are subject to creep under stress. Unlike the thermosetting resins, they do not change chemically when they stick substrates together. As a result of these properties, the thermoplastic adhesives are usually confined to low-load assemblies. Those most widely used are vinyl copolymers, saturated polyesters, polyacrylates and polysulphides.

The elastomeric adhesives have rubber-like properties, in particular an ability to regain their shape after deformation. They are tough and weather resistant and include adhesives based on natural rubber (polyisoprene) and synthetic rubbers (such as styrene-butadiene, polychloroprene and nitrile rubber).

Adhesives can also be classified according to their physical state and how they are applied. Groups include the pressure-sensitive adhesives, hot-melt adhesives, chemical-setting adhesives and solvent-release adhesives.

Finally, glues can be divided into structural and non-structural, though this is somewhat arbitrary as there is no formal definition of 鈥渟tructural鈥� in terms of bond strength. A structural adhesive is normally defined as one which is applied where joints or load-bearing assemblies are subject to large stress loads. Non-structural adhesives cannot support heavy loads and are mostly used to locate the components of an assembly or provide temporary adhesion.

For an adhesive to be designated structural it should at least have a tensile bond strength exceeding 1000 newtons per square centimetre at room temperature. Joint strength (a measure of shear strength) is tested using standard procedures, such as those laid down by the American Society of Testing Materials and by similar organisations in Europe, such as Britain鈥檚 Production Engineering Research Association in Melton Mowbray, Leicestershire.

Tenacious sealants

Modifying additives

SEALANTS are related to adhesives and are used to fill gaps or joints between two surfaces. They are often used to seal, waterproof or weatherproof materials. They must stick tenaciously to surfaces over a wide range of temperatures, and withstand joint movements, infiltration by air, water and dust. Many building materials (such as glass, concrete, wood, masonry and steel) are used with sealants.

The highest quality and most effective sealants are elastomeric in nature, such as those based on polysulphides, silicones, urethanes and fluoropolymers.

The most popular destination for adhesives is in packaging for a multitude of applications including corrugated board, bags, containers, labels, envelopes and a variety of food packages. Altogether, packaging accounts for some 40 per cent of all adhesive use. The building and construction industries use a further 20 per cent or so of the total, in flooring, kitchen and counter tops and dry walling. Various non-rigid bonding uses account for another 15 per cent, including footwear, flocking for mattresses and upholstery, book binding and various medical applications.

Other important uses are in rigid bonding for footwear and in the automobile industry, such as for joining the rubber trim to the doors and windows of vehicles.

The key component of an adhesive ( Figure 1) is the polymeric resin, also known as the binder, which confers strength and cohesiveness. There are as many as 15 different classes of adhesives based on the specific chemical structure of the binder resin that holds them together. The particular use to which the final product is to be put determines the choice of resin. The choice may depend on compatibility with the two surfaces to be joined. Emulsion polymers are the most widely employed resins, notably the styrene-butadiene and polyvinyl acetate copolymers. Epoxies, polyurethanes, phenolics and chloroprene-based materials are also in widespread use.FIG-mg20899501.JPG

Various materials are added to modify an adhesive鈥檚 processing and performance properties. These include hardeners, tackifiers to increase stickiness and improve plasticity, stabilisers, adhesion promoters to form a strong bond, and, as we have already seen, antioxidants. The formulation may also include reinforcing agents or fillers, such as silica, calcium carbonate or carbon black, to improve strength and performance, and to reduce material costs. Additives can also be used to modify the adhesive鈥檚 thermal expansion, its electrical and thermal conductivity, or its shrinkage and heat resistance.

The final ingredient is the solvent, and the basic resin type will determine which one is used. For rubber-based adhesives, aliphatic solvents are normally used, while aromatic and polar solvents (notably esters and ketones) are employed for epoxy, urethane and chloroprene adhesives. Environmental and health regulations impose severe restrictions on what solvents can be used in adhesives because of their potentially toxic effects. For example, 1,1,1-trichloroethane -once employed as a replacement for aromatic and polar solvents because of its low volatility-is now banned in polychloroprene contact adhesives (often used to join aluminium to steel), because of toxic effects on the nervous system.

Despite the development of stronger and stronger adhesives, the general public and some parts of industry remain distrustful. They find it difficult to accept that the chemical binding power of an adhesive can be many times greater than a purely mechanical joining system. It is not widely appreciated, for example, that aeroplanes are largely dependent on adhesives and sealants to hold them together.

Adhesives have several advantages over mechanical fixing methods. Aligning the surfaces to be joined is simpler, and the completed join tends to be neater. Fewer steps are required in the assembly process and adhesives tend to be cheaper than most mechanical fixing methods.

Adhesives can bond dissimilar materials, and mechanical loads can be distributed over a wider area (holes for bolts or rivets, on the other hand, concentrate structural stresses). And adhesives avoid the problems of heat and vibrations that arise during welding and riveting. Finally, the weight of the assembly is reduced overall.

However, the advantages may be offset by some disadvantages. For example, when using glue, assembly time is critical to avoid premature setting. Disassembly and/or repositioning is usually impossible. And cleaning equipment and personnel can be problematic. There may also be a danger of premature deterioration of the adhesive, and most adhesives (except for special types) do not perform well at continuous high temperatures.

Riveting changes

Fashions and trends

TRADITIONAL mechanical fixing devices (such as rivets and bolts) will increasingly be replaced by adhesives. There will also be a decline in the use of many natural-based adhesives in favour of synthetic materials, where the latter can be shown, for example, to have greater bond strength and improved weather resistance.

There has been a trend towards replacing solvent-based products with water-borne systems, which are less damaging to users鈥� health. However, the demand for solvent-based systems is likely to remain steady for at least the next few years.

Solvent-free polyurethanes and epoxy thermosets are due to make an appearance, as well as adhesives based on thermoplastic elastomers and styrenic block copolymers. The molecules of a block copolymer are made up of comparatively long sections of specific polymers, interspersed with segments of different composition, for example to make a styrene-butadiene-styrene chain, or a styrene-isoprene-styrene chain. Block copolymers have potential for hot melt adhesives and pressure-sensitive adhesives. These are activated by heat and pressure respectively.

Customised adhesives-glues that are designed for a specific application-are becoming increasingly popular. Demand has grown so fast in recent years, especially in electronics, that even the large manufacturers have shown an interest in producing them.

Adhesives that can be recycled are increasingly being used (especially for packaging). Such packaging may require special types of adhesive.

For their sheer versatility, the adhesives industry considers hot-melt pressure-sensitive adhesive tapes to be the best bet for the future. They are clean, simple to use and very effective (the glue on Post-it notes, which stick when you press them onto a surface and can be reused, is an example of a pressure-sensitive adhesive). Another sure winner, it suggests, will be adhesives and sealants that can be set, or 鈥渃ured鈥� by ultraviolet radiation or beams of electrons. This will be cleaner, cheaper and more effective than current methods.

There is much still to be learnt about adhesion, and the demand from industry for better materials will ensure that new types will continue to be developed. The future holds many opportunities for scientists and engineers working in this relatively young field of science.

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1: Health and safety

SOME adhesives and the substances used to make them carry severe health and safety warnings. Resins and their precursors, such as the isocyanate component of polyurethanes and the amine-curing agents used in epoxy systems, are particularly hazardous. Some resins cause dizziness and breathing problems, so face masks and protective clothing must be worn when handling them.

Many of the volatile organic compounds (VOCs) used as solvents in adhesives create their own health problems. Aromatic types, esters, ketones and alcohols all cause skin inflammation and also affect the central nervous system. Some solvents cause circulatory disturbances and even renal failure, while others, such as benzene, are known to be carcinogenic.

VOCs also produce harmful 鈥渟econdary pollutants鈥�. Most notable among these is ozone, which at ground level can make breathing difficult, damage trees and corrode a wide range of materials from metals to rubber and fabrics. Increasingly, adhesives manufacturers are using waterborne or solid alternatives to VOCs.

If used carelessly, adhesives pose formidable fire hazards-especially the inflammable solvent-based ones. The dangers of misusing 鈥渟uperglues鈥� are well known, for example they can fix skin to metal.

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2: Chemical classes

Acrylic emulsions are made by polymerising acrylic monomers. Uses: flooring, ceramic tiling, pressure-sensitive glues, labels, binding, packaging and cars.

Polyurethanes (solvent-based) are available in one and two-component systems and as a contact adhesive or as a single-sided coating for heat activation. Uses: flexible laminate packaging, woodworking industries, building, footwear and flooring.

Epoxies are based on resins containing the epoxy or oxirane group, which is produced by the reaction of bisphenol-A and epichlorohydrin. They are available as one-component systems that are heat-cured and as two-component systems that are cured at ambient temperatures using amines or amides. Several grades are available including film, mineral-filled (for example filled with silica) and toughened rubber (rubber reinforced with mineral fillers or fibres). Uses: cars, electronics, aerospace, building and DIY..

Formaldehyde-based adhesives are formed by linking formaldehyde (methanal) molecules with other molecules such as phenol, urea or resorcinol. Urea formaldehyde (above) is probably the most important. Uses: woodworking (furniture, doors, laminates and panels).

Hotmelt adhesives are a blend of elastomeric resin-usually ethylene vinyl acetate, though polyester, polyamide and polyurethane are also used-plus waxes and tackifiers (to increase stickiness and improve plasticity). Thermoplastics are applied in molten form: as heat is lost the glue binds to the substrate. Uses: packaging, bookbinding, woodworking, labels and non-woven disposables, such as nappies.

Cyanoacrylates are esters of alpha-cyanoacrylic acid activated by the presence of small amounts of negatively charged particles. Uses: plastic processing, cars, electrical, mechanical engineering, jewellery and DIY.

Polychloroprene is an elastomeric urethane prepared by the polymerisation of 2-chlorobutadiene, and is applied as a contact adhesive. Uses: woodworking, building, cars and leather..

Polyvinyl acetate latex is an emulsion made by polymerising vinyl latex monomer and co-monomer. Uses: packaging, woodworking, flooring and wall tiling, bookbinding and DIY.

Silicone polymers consist of a backbone chain of alternating silicon and oxygen atoms, with methyl or phenyl side groups.

Styrene-butadienes are available as styrene-butadiene-styrene and as styrene-butadiene-rubber. They are mainly used in pressure-sensitive glues. Uses: packaging, upholstery and building.