A story of bonding: industrial adhesives and their applications

The push to create lighter and lower cost products is forcing designers to consider new materials. These are often difficult to exploit effectively if traditional assembly techniques are used so an increasing number of manufacturers have rekindled an interest in the use of adhesives.

Recently, the number of new plastics, ceramics and application-specific alloys has risen sharply as suppliers have tried to provide designers with the materials they require to meet technical performance criteria at minimal cost. At the same time, more highly developed forms of the established materials are also being offered. A typical example is thin, pre-painted sheet steel, which is often better assembled by bonding.

However, the demanding conditions faced both by vehicle components and vehicles themselves highlight the principal problems which beset adhesives and which must be faced to some degree whatever the application. The difficult areas constitute a formidable barrier to introducing adhesives into any field of production.

This barrier is exacerbated by the practical problems encountered when a new technology is introduced into an established industry. In such circumstances the greatest difficulty is inexperience of some of the engineers charged with investigating and possibly implementing that technology.

The 12 adhesive types

There are some twelve main family groups of adhesives likely to be of value to the engineer. The main family groups are: anaerobic; cyanoacrylate (both being acrylic-based); epoxy; hot melt; phenolic; plastisol; polyurethane; solvent borne rubber; tape; and a large group of further variations on acrylic chemistry.

• Acrylic

  This rapidly developing group of materials, based on a variety of acrylic monomers, has viscosities which are readily modified giving thin liquids, syrupy resins and thixotropic semisolids. Some are truly single component adhesives, such as those cured by UV radiation, while others require some form of hardener. The hardener may be used as a surface primer or mixed directly into the adhesive.

  Extremely versatile, these adhesives will bond almost any substrate well, with the exception of a few rubber-based materials and the difficult thermoplastics such as polyethylene, though even these materials will respond well if properly prepared. Most acrylics are very robust and will cope with demanding environments.

  They possess extremely high strength, impact and peel resistance and therefore are very durable.

• Cyanoacrylate

  These adhesives are a special acrylic variant. They harden - in thin films - because of the catalytic effect of surface moisture. Generally, hardening takes place in a few seconds and for this reason they are frequently used in the assembly of small plastic parts. They bond most types of rubber extremely well, but caution is needed when it is intended to use them in conjunction with unpainted metal components in warm moist conditions.

• Epoxy

  Primarily based on epoxy resins and a variety of hardeners, the epoxy adhesives can be found in many different types of composition. Often they are formulated to meet the specific requirements of individual applications. Very strong and versatile, they are most frequently used on larger components because their viscous nature normally makes them unsuitable for more intricate work. Single component epoxy adhesives are available and they do not require mixing and are cured by heat.

• Polyurethane (PU)

  Like epoxies, polyurethane adhesives can be individually formulated to meet a wide variety of requirements and almost invariably involve two components. Although they have greater susceptibility to moisture attack than the other structural adhesives, polyurethanes can be considered to be good, durable, load-bearing materials provided environmental conditions are not too severe.

Mechanical properties of various adhesive groups

A typical example of use in a low modulus material is the mounting of vehicle windscreens, composite body panels and tailgates. Applications such as these are not very demanding for the adhesives used. Relative to the weight of the components involved, not only is the area actually bonded large, but of even greater significance the length of the bonded edge is very large too.

Stress distribution with regard to an edge for the successful distribution of a load depends on whether the edge in question represents a length or a width. The significance is its relationship to the induced or applied stress. If the edge represents a width then the unit load is proportionately reduced by an increase in the dimension.

If it is a length then the unit load will be proportionally reduced by a dimensional change only if either the adhesive is extremely ductile or the lengths involved are very short. This follows from the disproportionate distribution of stress in joints.

At the other extremity of the modulus spectrum are the heat cured, single part epoxies. These adhesives are amongst the most durable and robust known. Their stiffness and high strength make them suitable candidates for transmission of substantial loads.

When materials with different co-efficients of expansion are to be bonded, care is needed to ensure that unacceptable stress is not induced by their thermal cure. It is also possible that in some designs a lower modulus material, such as a cold or warm cured two-part epoxy, would be better.

It is clear that the lower modulus material is the more suitable for its elastic limit as it is not exceeded by the joint-edge stresses. By contrast, the stiffer material is not so successful.

Co-axial joints are ideal candidates for transmission of substantial forces, particularly because they do not induce peel/cleavage forces in either the adhesive or the composite surfaces being bonded.

However, the most convenient joint is usually some form of lap joint in which cleavage forces are readily introduced unless care is exercised during the conception stage to ensure that they are either suppressed by a design feature or by some mechanical reinforcement.

An additional counter to the damage that peel and cleavage can induce is the use of a low modulus adhesive. Such materials ensure that stress concentrations are reduced to a minimum. The two part epoxies with a shear modulus of about 0.5 GPa were originally empirically formulated to achieve this, while at the same time being sufficiently strong in themselves to withstand substantial loads over prolonged periods.

Except in the more highly stressed joints, considerations such as these may be considered to be somewhat academic. As composites, and other novel materials, are pushed into ever more demanding applications and used in direct competition with conventional materials these issues are becoming increasingly more significant.

Material compatibility with the main adhesive types

• Ceramic

  Ceramic materials will usually bond well unless conditions are warm and moist when a coupling agent should be used.

• Glass

  Glass will usually bond well if a coupling agent is used. This may be incorporated into the adhesive or used separately.

• Metal

  Both heat-cured epoxies and some toughened acrylic types will bond oil contaminated metal surfaces very successfully. However, safety-critical loaded joints must be properly prepared to guarantee a long durable performance. PU-based adhesives must not be used directly on bare metal or oil-contaminated surfaces.

• Aluminium

  Bonds well with epoxies and also with pseudo single-part acrylics. Maximum durability requires either silane, phosphate or a chrome passivation treatment.

• Steel

  Maximum durability requires either silane or chrome passivation treatment.

• Zinc

  Zinc is a weak, readily oxidised metal that behaves poorly unless properly prepared and loaded. Chrome passivation gives bonded joints of excellent durability though if these are to be loaded, low modulus adhesives should be used.

• Thermoplastic

  These have a tendency to stress crack with some single part acrylics. Two part-mixed acrylics and polyurethanes are better although epoxies are generally too stiff for this type of plastic.

• Polyamide (Nylon)

  Bonds poorly unless bonded hot or pre-treated by shot blasting or corona discharge. (Plasma treatment may always be considered as an alternative to corona discharge, but although it is said to be more effective, it requires use of a vacuum chamber.) During hot bonding, the release of absorbed moisture can become a problem.

• Polycarbonate

  These commonly used materials have a strong tendency for unalloyed material to stress crack. Therefore surface preparation is needed such as shot blasting or corona discharge.

• Polyphenylene oxide

  Bonds poorly unless bonded hot or pre-treated by shot blasting or by corona discharge. When hot bonding, absorbed moisture may be released as well as the potential for stress cracking to occur.

• Polypropylene

  Surface preparation needed; shot blast, corona discharge or a chlorinated primer.

• Polyurethane

  Some of these composites may bond well with acrylic adhesives. Epoxies are unsuitable for this polymer, with polyurethane giving the best results.

• PVC

  This polymer bonds very well without surface preparation with acrylic adhesives.

• Thermoset epoxy

  Usually used for only the most demanding circumstances and consequently bonded with either two part or single part epoxies. While the acrylics and PUs can bond with them well the lower moduli of these adhesives generally make them unsuitable for the type of work normally involved.

• Polyester

  Polyesters will bond well with all types of adhesive but single part epoxides are not normally used because of the higher curing temperatures required.

It can be seen that the lowest cost adhesives may not actually provide the most economical solution, which is technically acceptable. Each application requires careful consideration of the costs incurred when extra painting processes may have to be invoked or heating equipment is required to cut production cycle times to an acceptable figure.

It is well established that the thermodynamics of thermoset composites, epoxy and polyester based, favour the bonding process, and were it not for the need to remove the mould release agent and accidental contaminants there would be no requirements to prepare their surfaces prior to bonding.

By contrast, with thermoset composites, the thermodynamics of thermoplastic materials are not generally so favourable and bonding may prove difficult unless the plastic concerned is slightly soluble in the adhesive under consideration. For example, ABS and PVC are slightly soluble in acrylic-based adhesives and for this reason they bond extremely well, though care is needed with the ABS as it tends to stress crack.

Specific steps always have to be taken to prepare thermoplastic composites. This may be brought about either by incorporating special chemical groups in the polymer which improve adhesion or by treating the surface in some way. The latter may range from shot blasting through corona discharge to use of chlorinated polyolefin primers introduced for bonding polypropylene-based composites.

These various techniques appear to work well and good long-term bonds may be anticipated. However, the user should refer to the material supplier for specific advice for there are so many variants on each of the many basic plastic types.