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ADHESIVES - AN OVERVIEW



Toughened Acrylics

Rating - Structural.

Preferred materials - Most materials especially plastics and metals.

Toughened acrylics are relatively fast curing and offer high strength and toughness, plus have more flexibility than common epoxies. They tolerate minimal surface preparation and bond well to a wide range of materials.

These adhesives were developed as an extension of earlier work on the single-component, anaerobic adhesives. The use of an initiator, in addition to the adhesive, made a higher level of performance possible. The initiator is applied to one surface and the adhesive to the other. When they are brought together, as the joint is closed, the initiator dissolves in the adhesive and the polymerisation reaction starts.

Although two components are required, the early versions of these adhesives did not require mixing prior to application. However, as a consequence, they did not cope well with large gaps. Newer acrylic adhesives, developed to fill gaps, are two-part, pre-mixed systems and some confuse the classifier by combining acrylic reactive monomers with epoxy or polyurethane pre-polymers. The cure can be accelerated by the application of heat.

Anaerobics

Rating - Sealant/Structural

Preferred materials - Metals.

Anaerobic adhesives are often known as 'locking compounds', being used to secure, seal and retain turned, threaded, or similarly close fitting parts. They are also used to bond coaxial assemblies and seal flange-faces.

A member of the acrylic family of adhesives, they are essentially low viscosiy liquids, although they can be formulated into pastes or thixotropic liquids. They are single-part adhesives which cure when air is excluded, hence the name anaerobic.

Solely under the influence of anaerobic conditions the cure rate is quite slow; in the presence of metal the cure rate is much faster and this, in practice, is how they are used - confined between closely fitting metal parts. The close fit excludes air and the metal surface speeds the rate of cure to a commercially useful degree.

Since hardening only takes place in the absence of air, these adhesives have the advantage that material outside the joint does not normally cure and hence can be wiped off after the assembly has reached handling strength. They are unique among adhesives in that they are made with different strength characteristics ranging from relatively weak materials, which allow the easy dismantling of large parts, to very strong materials for permanent fixing. Within each strength-band there will usually be several products of different viscosities, allowing different gaps to be filled at the same level of controlled strength. They can also be toughened (see toughened adhesives section) to provide greatly improved peel and impact values.

Cyanoacrylates

Rating - Structural (small parts).

Preferred materials - Most non-porous materials especially plastics and rubber.

Cyanoacrylate adhesives cure through reaction with moisture held on the surface to be bonded. They need close fitting joints and usually solidify in seconds which has resulted in the universal generic name of 'superglues'.

Curing is started the film of moisture found on most surfaces which is usually slightly alkaline and it is this alkalinity which causes the polymerisation. The rate of cure is normally extremely fast. Because of the cyanoacrylate's need for slightly alkaline conditions, any acidic surfaces, such as wood, will tend to inhibit the cure and in extreme cases prevent curing altogether.

Most grades are low viscosity liquids and therefore they need close fitting joints. Newer developments of gelled variants allow cure in wider gaps and application to vertical surfaces. Cyanoacrylates are suited to small plastic parts and to rubber bonding.

Epoxy (One-Part and Two-Part)

Rating - Structural.

Preferred materials - Most materials especially plastics and metals.

Epoxy adhesives have good strength and chemical resistance, do not produce volatiles during curing, and have low shrinkage. Therefore they form extremely strong and durable bonds with most materials in well-designed joints.

Due to the nature of the chemistry and the curing reaction, great versatility in formulation can be achieved since there are many resins and many different hardeners. Epoxy adhesives can be used to join most materials. They are available in a wide variety of forms, from low-viscosity liquids to solid pastes or films. Development of toughened formulations (see later section) has dramatically increased the demanding uses of these adhesives in many industries.

Throughout all the variations, the mechanism of curing is always the same. This mechanism requires exact quantities of resin and hardener, hence the need for precise mix-ratios and the thorough mixing of resin and hardener in two-part systems. Without these, the polymer will not form correctly and inferior properties may result - typically lower strength and reduced environmental resistance.

Two-part epoxy adhesives start to react under ambient conditions once the two components have been mixed together and are often termed room-temperature (RT) curing adhesives because of this. The reaction mechanism is still affected by temperature and as a rule of thumb the reaction rate approximately doubles for every 10C rise in temperature i.e. an epoxy which takes 1 hour to cure at 20C, will cure in 15 minutes at 40C. Conversely the cure time will double as the temperature drops by 10C. Complete cure times at ambient temperatures for two-part systems range from ~10mins to several days.

Single-part epoxy adhesives are available in liquid, paste or film form. These adhesives require heat to cure. The resin and catalytic hardener are pre-mixed but curing does not occur because the catalyst is in an inactive form at room temperature. It only becomes reactive as the temperature is raised, usually in excess of 100C. The higher the temperature, the faster the reaction becomes and hence shorter curing times of less than ten minutes can be obtained. Cure of the two-part adhesives can also be accelerated by heat.

Hot-melt Adhesives

Rating - Semi/Non-structural.

Preferred materials - Wide ranging.

Hot-melts are based on modern thermoplastics and are used for fast assembly of structures designed to be only lightly loaded. They are applied as a melt - a hot and usually viscous liquid - that cools and develops strength by its consequent solidification.

The formulation of a hot-melt can be quite complex, based on polymer resins such as ethylene vinyl acetate (EVA), polyamide or polyurethane. High tack is needed to 'grab' the substrate, and thermal degradation of the melt must be suppressed. Some products, based on polyurethane technology, are also formulated to react with atmospheric moisture, which improves their heat and moisture-resistance (reactive hot melts).

The strength obtained from this class of adhesive is often modest and they are likely to fail in demanding environments - creep under warm conditions being a particular problem. The new moisture-reacting types are better. Hot-melts find extensive use in fast assembly of structures designed to be only lightly loaded. Their chemical composition means that they can be used to directly bond some plastics with low surface energy, e.g. polypropylene, which with other adhesive types require a surface pre-treatment.

The term hot-melt can also apply to a technology where pressure-sensitive adhesives, elastosols or single-part epoxies can be deposited by this method.

Polyurethanes

Rating - Structural (lower modulus, tough systems).

Preferred materials - Most materials especially plastics and metals.

These adhesives, often abbreviated to PU or PUR, which get their name from the chemical form of the hardened material, are another example of a two-part, chemically curing adhesive.

Polyurethane adhesives are chemically reactive formulations that may be one-part or two-part systems and can be fast curing. They provide strong impact-resistant joints and have better low-temperature strength than most other adhesives. A fast cure usually necessitates applying the adhesives by machine. They are often used with primers.

Single-part formulations are available, which are partially polymerised and stable until cure is initiated by the action of absorbed atmospheric moisture. Their reaction rate is slower because it takes time to absorb the necessary water. Polyurethanes can be supplied as reactive chemicals, solvent solutions, pastes or hot melts.

They provide strong, resilient joints which are impact resistant and have good low-temperature strength compared with many other adhesives. Polyurethanes find major uses in the bonding of glass fibre reinforced plastics (GRP), direct glazing of automobiles and lamination of both insulation panels and flexible packaging materials.

Silicones

Rating - Sealants/Structural (bonding glass).

Preferred materials -glass.

Silicones are not very strong adhesives but are known for their flexibility and high temperature resistance. They are available in single or two-part forms. The latter function like the two-part epoxies, the former like the single-part polyurethanes. When the single-part adhesives cure they liberate either alcohol or acetic acid (vinegar).

They are often used as bath and shower sealants. Their adhesion to surfaces is only fair but, like their flexibility, their durability is excellent. The two-part versions need a hardening agent to be mixed into the resin. Two forms are available - those which liberate acid on curing and those that do not. As might be anticipated, the two-part adhesive systems give a better cure in thick sections than do the single-part types and are often used in more demanding applications.

PVA adhesives and related emulsion systems

Rating - Semi-structural.

Preferred materials - Porous systems including wood and board

This large class of adhesives is based on polymers such as polyvinyl acetate (PVA), acrylic, rubber or polyurethane. PVAs are familiar as the household glues which bond paper, cardboard and wood, these adhesives cure by loss of their fluid carrier, in this case water. The polymer, dispersed in the water, separates out as a sticky mass which gives good initial tack and then dries to a strong film which bonds the substrates together. The rate of cure is controlled by the speed with which water is lost and generally, to obtain sound joints, one of the substrates must be absorbent.

Alternatively, the water is driven off completely before the joint is closed. Two such coated surfaces will bond immediately, in the case of rubber-based contact adhesives. Alternatively, the adhesive can be designed to be reactivated by solvent, heat or to be pressure-sensitive.

OTHER ADHESIVES NOT INCLUDED IN SELECTOR AT PRESENT

Amino or Urea based adhesives

Rating - Structural (material specific).

Preferred materials - Interior wood.

These are usually two-part systems consisting of resin and hardening agents and based on products from the reaction of urea and formaldehyde. Curing is normally achieved under pressure but without heat, although heating can be used to accelerate the cure. As the adhesive cures, water is liberated and this tends to limit their use to porous substrates. Due to their poor environmental resistance they are normally used in the manufacture of interior wooden structures.

Phenolics and Resorcinolic adhesives

Rating - Structural (have shock susceptibility).

Preferred materials - Metals and wood.

Phenolics were the first structural adhesives for metals, and have a long history of successful use for joining metals and wood. Although the starting materials used to make the two adhesive types in this group are different, they are chemically similar and the curing mechanism is the same. Consequently they can be considered together.

During the curing reaction, which must be done at elevated temperatures, water is liberated as the reaction proceeds. This means that either the substrates must be porous or a high pressure must be imposed to prevent the formation of voids. The brittle, basic resins can be modified with other more rubbery polymers (to give vinyl or nitrile phenolics).

These types of adhesive are particularly durable to both elevated temperatures and harsh environments and would see much greater use in engineering industries if they were easier to use and less susceptible to shock.

Polyimides (PIs) and Bismaleimides (BMIs) - High Temperature Adhesives

Rating - Structural.

Preferred materials - Metals and ceramics.

Few adhesives are suitable for prolonged use at temperatures between 200C and 350C. For specialist aerospace and electronic applications, in cases where brittle ceramic adhesives are not appropriate, then more exotic, synthetic polymers must be considered. Polyimide and bismaleimide adhesives are the most established types in this class. They are available as liquids or films, but are relatively expensive and difficult to handle. However, they are superior to most other adhesive types with regard to long-term strength retention at elevated temperatures.

Plastisols and Elastosols

Rating - Non-structural.

Preferred materials - Wide ranging.

Plastisols are modified PVC dispersions, which require heat to harden. The resultant joints are often resilient and tough. These adhesives take the form of viscous, immobile pastes which cure by a combination of solvent loss and chemical reaction. Plastisols consist of a suspension, or partial solution, of polyvinylchloride (PVC) particles in a combination of solvents, plasticisers and reacting monomers. After application, some solvent evaporation may be allowed before the joint is closed. On heating to about 180C, the PVC dissolves in the plasticiser and reacts with any monomers present to form a cross-linked, semi-thermoset material. Once cooled to room temperature, a tough, flexible and durable bond is formed.

In the past plastisol adhesives were used in combination with spot welding, which took place through the film of unreacted adhesive. Welding held the parts together during the heating cycle. However, local burning of the plastisol released hydrochloric acid which can, and often did, initiate corrosion. A more recent variation avoids this problem by replacing the PVC with a crosslinkable butadiene polymer (Elastosol).

Rubber Adhesives

Rating - Non-structural.

Preferred materials - Wide ranging.

Rubber adhesives are based on solutions of latexes and solidify through loss of the solvent medium. They are not suitable for sustained loadings.

Solvent Based Adhesives

Rating - Non-structural.

Preferred materials - Wide ranging.

This is a very large group of adhesives which all consist of a polymeric resin dissolved in an organic solvent. The adhesives are often based on rubber, but other plastics may be employed, such as acrylic, polystyrene or PVC. In use, the solvent is allowed to evaporate from the surfaces to be bonded before the joint is made. A well-known example is rubber-based contact adhesive. For safety and environmental reasons, water-based emulsions of similar polymers, or in some cases hot melts, are replacing these adhesives in many applications.

Water based Adhesives

Rating - Non-structural.

Preferred materials - Porous systems.

Adhesives, which are applied when dissolved in water, include natural products such as starch, dextrin, casein or gelatin, and synthetic polymers including polyvinylpyrollidone (PVP). Casein is obtained from milk and was one of the first adhesives used for bonding wooden aircraft components. Starches and dextrins are still used in high volume, e.g. to bond cigarette papers and corrugated cardboard, respectively. PVP is commonly used in the office and home as the main ingredient in the familiar stick adhesive for paper.

These adhesives provide modest strength compared with most other types, but they form a vital component in rapid manufacturing processes which are devised and controlled to very high engineering standards. They can be used as intentionally temporary joints in specialist applications. An additional benefit in the current age of environmental consciousness is that many of this class of adhesives are obtained from renewable resources, do not inhibit biodegradation or prevent recycling.

Pressure Sensitive Adhesives (PSAs)

Rating - Semi-structural (susceptible to creep).

Preferred materials - Wide ranging.

This large group of adhesives uses many polymers (acrylics, rubbers, polyurethanes), together with plasticisers and tackifying resins to form a permanently tacky (sticky) adhesive. They can be deposited from solvents, water emulsions or hot melts and find uses in stationery, labels, laminated products and as the active ingredient in pressure-sensitive tape adhesives (see later).

The name comes from the fact that moderate pressure alone is sufficient to spread the viscous adhesive layer on to the surface and achieve useful adhesive strength. Although they do not solidify or chemically cure, they are often able to withstand adverse environments. This type of adhesive is generally not suitable for sustained loading.

All forms are based on a backing material, such as cellulose, polyester or PVC, which has been coated with a pressure-sensitive adhesive - a tacky, semi-solid polymer, either acrylic, polyurethane or rubber. Some have a separate release strip, while others do not.

Double-sided tape has been coated on both sides and must have a release strip. The central core can either be a thin plastic film (as above), a foamed plastic layer (to allow for movement or irregular bondlines), or nothing (a transfer tape). Generally, most pressure-sensitive tapes give high tack but fairly low strength. Some versions develop higher strength upon ageing. Newer, higher strength products can be used in more rigorous applications, previously the domain of reactive, curing adhesives.

Radiation-cured Adhesives

Rating - Can be structural.

Preferred materials - Various including glass, plastics and ceramics.

Adhesives in this class become active and cure when exposed to radiation, usually ultra-violet (UV) light. The mechanism depends upon special modifications to the monomer's structure and the inclusion of light-sensitive compounds that start the reaction.

Generally specific types of acrylics, epoxies and silicones can fall into this class. They are widely used for bonding glass and transparent plastics. Alternatively, UV light can be used to rapidly 'tag' the edge of an opaque joint, with the cure completed thermally or by a two-part reactive system.

Toughened Adhesives

Rating - Structural.

Preferred materials - Wide ranging.

Almost all of the reactive, chemically curing, high-strength adhesives suffer from some combination of low peel strength, impact, or environmental resistance. Although the situation can be improved by the use of plasticisers, there is usually a consequent loss of shear strength.

The basic problem is the brittle nature of the polymers, which are normally formed when the high-strength adhesives are created. Once a crack starts in such a polymer, it grows very quickly and disintegration follows. The situation can be eased by using a variety of techniques to make the material more flexible, but this tends to reduce strength and environmental resistance.

During the last 25 years a process known as 'toughening' has been extensively developed using methods evolved earlier by the plastics industry. A toughened adhesive has small, rubber-like particles dispersed throughout the glassy matrix. The profound effect of these particles is to change the mechanical response of the adhesive and make it much more resistant to crack propagation. So far, this technology has only been successfully applied to the acrylic and epoxy-based adhesives.

The epoxy and anaerobic versions of acrylic adhesives were the first types to be toughened by this technique and the resulting adhesives have similar shear strength but greatly improved peel and impact values. The development of toughening has allowed adhesives to be used in more demanding conditions and, as a consequence, in a greater range of industries, particularly where load-bearing joints are required.



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