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Adhesives for engineering use may be classified in various ways:

• Chemical group – the main ones being epoxy, phenolic, urethane, anaerobic, acrylic, cyanoacrylate, silicone and polysulphide
• Functional type – structural, hot melt, pressure sensitive etc
• Physical form before curing – 1 or 2 part, liquid, film, paste, solid
• Physical form after curing – rigid or flexible
• Curing process – cross-linking, polymerisation,
• Curing method – heat, UV light, electron beam, moisture etc
• Cured state -Thermoplastic or thermoset

The rigidity of the cured adhesive has a significant effect on the stress distribution in a joint. The tensile and shear properties of a rigid structural epoxy are compared to a flexible polyurethane in the Figure below from which the following can be noted.

• The structural epoxy is glassy in nature whilst the polyurethane is rubbery in nature.
• For the structural epoxy, the initial modulus is high and then yielding occurs prior to failure. The strength is higher in tension than in shear, but the strain to failure which is the reason for joint failure to be susceptible to peel stresses.
• Toughening of the the structural epoxy reduces the yield stresses and increases the strains to failure, reducing the susceptibility to failure from peel stresses.
• The modulus values for the polyurethane is 100 to 1000 times lower than the low strain modulus values of the structural epoxy.
• For the polyurethane a high strain to failure is obtained in both tension and shear so that joint failure is insensitive to peel stresses.

Details of three chemical types (epoxies, acrylics, and polyurethanes) of adhesive are used to illustrate the range of properties available, these are.

Typical properties are shown below. Upper service temperatures (UST) are the values where 50% of 23°C strength is retained. For thermoset adhesives like epoxies and acrylics this is governed by the glass transition temperatures of the adhesive.

Epoxy Group

Structural adhesives, high modulus, slow cure unless heated, durable

۱ part – Shear modulus 2.0-3.5 GPa Elastic limit 40 MPa UST 120°C Bond fill <2mm

۲ part – Shear modulus 0.2-1.0 GPa Elastic limit 25 MPa UST 50°C Bond fill <2mm

Single component epoxies are pre-catalysed and require the application of heat for polymerisation to occur, forming a hard thermoset polymer that will not re-melt on further heating. High cohesive strength is typical and the addition of a rubber phase will toughen the adhesive and improve its fracture and fatigue resistance, by local crack tip blunting. Certain heat curing epoxies also have the benefit of scavenging oil films, particularly useful where thin sheet adherends are coated with anti-corrosion or processing oils. The rheological form can be varied from a low viscosity liquid to a solid paste.

Two part (2k) systems are available and often cure at room temperature, making them suitable for applications where heat curing is impractical – e.g. repair shops. Different mechanical properties of the cured adhesive can be obtained by the careful selection of resin-hardener type and mix ratio. Although the durability of 2k epoxies will not be as good as their heat cured equivalents, modern developments do make them suitable for structural applications.

Epoxies are compatible with most materials, however, bonding to thermoplastics and rubbers can be difficult. Low surface energy polymers (polyolefins and fluoropolymers) make bonding difficult, and require special surface pre-treatments.

Typical applications include major structural components for the aerospace, automotive, and construction industries.

Acrylic Group

Semi-structural adhesives, fairly flexible, fast curing, durable

۱/۲ part Shear modulus 0.1-0.4 GPa Elastic limit 5 MPa UST 80-150°C Bond fill <0.5mm

۳ main types of acrylic adhesives exist, these are:

Anaerobics – cure in absence of air and on contact with iron or copper. These have the highest operating temperatures (up to 150°C) and are typically used for tread locking and sealing applications.

Cyanoacrylates – fast room temperature curing on contact with alkaline agents (present in air vapour) to create strong bonds between metals, plastics, and rubber. Not suitable for high load bearing applications. Operating temperatures of up to 80°C can be achieved, with significant retention of strength.

Toughened – often 2-part mixes with variable cure characteristics based on mix ratios. Most tolerant adhesive type to variation in surface preparation. Bonds most materials well, in particular metals but does not work well with rubbers or low friction polymers. Operating temperatures up to 120°C can be achieved.

Polyurethane Group

Semi-structural, flexible, controllable cure rate, limited durability, pastes

۱/۲ part – Shear modulus 0.05-0.2 GPa Elastic limit 5 MPa UST 100°C Bond fill <40mm

Semi-structural, rapid room temperature curing, adhesives that are flexible and can be considered for both bonding and sealing applications. PU adhesive can be used where large gaps between adherends exist, with the addition of foaming agents within the mix. They can be used with a wide range of adherends including metals, plastics, rubbers, and glass.

The one major restraint with the PU adhesives is their susceptibility to moisture, both during and after curing. The painting of metal surfaces prior to application can help. Protection from high loads in hot humid environments should be considered.

Recent developments in epoxy-polyurethane hybrid adhesives have produced structural adhesives that make use of the PU component to improve impact resistance. These adhesives are being developed primarily for the automotive industry.

All data is provided by Lees WA, Adhesives, Section B15 Kempe’s Engineers Year-Book, 2000.