THE world of adhesives is undergoing a sea change. From glues, gums and pastes, it has given way lo hi-tech chemical sealants and adhesives that arc fixing vital electronic, automotive and aerospace components.

Both adhesives and sealants are essentially substances that adhere to one or more surfaces. though sealants must remain flexible and also prevent the passage of liquid or gas between surfaces. This holding together of materials by surface attachment is very ancient, but advances in the past few years arc leading to hi-tech adherents and sealants of use in the automotive, aerospace and electronic Industries.

Originally, adhesive bonding was achieved through naturally occurring materials such as coal and pine tars, animal protein, blood albumin, glue, mucilage and paste. The oldest type ol adhesives are those of natural origin, such as starch, asphalt, gelatinised collagen, vegetable proteins, plants saps and resins, rubber and shellac.

Modified natural products such as portland cement, dextrins, nitrated and acetylated celluloses or starches, and fatty amides came next. In the mid-1930s, advances in organic chemistry led to Ihe development of phenolic resins, urethanes, polyvinyl esters and epoxy resins.

Now new hit-tech chemical adhesives are finding a wide range of application* In wood bonding, packaging, tapes and labels, industrial assembly, and biomedical and dental equipment. They are also rendering services in transportation and electronic industries, according to a report in the Journal "Chemical and Engineering News". Although these new synthetic adhesives resemble modified natural products, they perform belter.

For an adhesive to have practical application. 11 must be ut some point be a liquid during the formation of bond in order to wet, spread and form a continuous film.

Once the adhesive has been brought into intimate, continuous contact with the substrate. the adhesive must pass through a transition Into a tough, non flowing, non-liquid, load-bearing Interlayer in Ihe Ixmdcd assembly.

Other necessary features include a measurable shelf-life reasonable cure time and temperatures. relatively non-toxlc and non-polluting constituents and an ability to tolerate ihe bonding of imperfect, mating surfaces.

The use of adhesives In the aulo motive, aerospace and electronic markets has been growing in recent times, having almost doubled in the past eight years.

Across Ihe globe, the automotive industry is replacing Its metal components wilh engineering plastics, and more plastic parts means fewer nuts, bolts, screws or welds, and more adhesive bonds. Chemical or adhesive bonds extend across ' continuous scams instead of concentrating stress at bond points as metal easterners do.

In new models of cars the rear deck, hood, and roof sections arc all bonded assemblies of fiat exterior sheet to plastic reinforced section beams. Almost 5 per cent of all exterior panels arc plastics today, and experts estimate the figure lo shoot lo 50 per cent by 1995. Besides, bonded composite leaf springs, thermoplastic head lamps and plastic lift gates are in use now.

In aerospace, chemical bonds provide the best way lo bond metal and plastic components is Ihc construction of military and commercial aircraft. Chemical bonds also offer

■ light weight alternative lo metal fasteners, besides nak Ing possible the construction of high technology radar transparent aircraft like stealth bombers.

In the field of electronics, new conductive adhesives are increasingly substituting solder attachment of electronic components, while new "snap cure" adhesives ihal cure almost Instantaneously on exposure lo heat are providing' quick attachments of electronic components on higher assembly lines. Chemical bunds also make possible circuits that utilise materials with different coefficients of expansion.

The world wide food packaging industry Is also accelerating the process of replacing metal and glass rigid food containers wilh flexible multilayer laminated plastic containers. Some examples of these include the flexible adhesive-bonded fruit juice puck ages, the flexible, vacuum-scaled plastic containers for fresh-ground coffee, and multiple applications of laminated lid stock for rigid plastic containers.

Research arid development in the major areas of aulomo lives, aerospace and electronics is aiming at eliminating or substantially reducing volatile organic component, is solvent systems to reduce ‘their contribution to atmospheric pollution. This Is especially important In auto and truck assembly operations which use large volumes of adhesives.

New adhesives being used by leading US manufacturers In aulomolives cure without giving off volatile organic compounds, and form gas and water-proof seals. These adhesives form a lough flexible bond with a 50 per cent elongation rate.

Another system designed in the US Includes machinery

and a proprietary »olvcntk-»» ferromagnetic chemical material which delivers a structural, leak proof and pressure-light seal via an induction process The system is essentially used to form ftltefs.

New acrylic pressure-sensitive adhesive coatings allow automakers to fix dash components. emblems, pin stripes and other decorative trim lo the car. Acrylic adhesives art i clear, and ultra violet light-resistant, besides being resistant to gasoline, airborne chemical contaminants and heat. These adhesives make possible new backlit dashboard graphics, and body mouldings fixed lo the side of a car wilh these adhesives replace rivets in drilled holes, thus cllminallng rallies.

Some firms abroad are also making doublc-sixcd acrylic foam tapes nol only to fix body side molding, but also for In-slrunicnl. trim door panel, weather strip attachment. In addition to saving assembly time, the tapes also offer a durable assembly between Ute molding and the metal or plastic substrate lo which it is fixed. These foam adhesives include modifier! epoxtes, phenolics and polylmidcs.

Itolylmidcs arc finding use in acibspacc engines as light weight replacements for titanium faslerncrs, in engine , cores, duels and pipes.

Olliers in the market arc paste adhesives used for hood, deck and door assemblies, and anlifiutter adhesives based on neoprene which eliminates ihc need lo drill holes to fasten Ihc assemblies.

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The use of structural adhesives Is growing rapidly In automotive assemblies, particularly In the ease of robotic installations. Experts are also designing silicone adhesives for bumper and headlamp assemblies, and their research and development efforts are concentrated on blending silicones with organics to form hybrids that have Ihc strength of an organic and the flexibility of silicone.

Because silicones can withstand thermal cycling well, they urc Ideal for automotive underhood adhesives and are in wide use .in ignition modules where a ceramic thick film hybrid component is mounted lo an aluminium back plate with a silicone adhesive.

Silicones' widespread use In electronic (assemblies will continue to rely on their low modules, elastic properties and low loxicily lo workers.

I’olyurctliane adhesives arc another category of adhesives that are emerging In the forefront of adhesive research, along wilh hol-mcll adhesives and electronic adhesives.

Electronic adhesives offer several overriding advantages : ll icy are pure, and the volatile organic compounds are lowered or eliminated. * • •,

The drive is one lo come oul with adhesives with Improved purity, lower volatile organic and choloride contents lo reduce corrosion potential and worker exposure, and in-< <>rporate safer chemical such as diamines us Ute Industry gears itself lo meet hi-tech requirements. — INI Feature.