This teaching support material complements that in lecture C10: Machining, bonding and repair.

Adhesives come in a variety of types, usually distinguished by the chemistry used [1, 2]:

• anaerobics
• acrylic-based adhesives which normally set in the presence of metal and in the absence of air (specifically atmospheric oxygen).
• normally used as thin layers for locking or sealing compounds and have rapid cure time (complementary to the cyanoacrylates).
• cyanoacrylates
• acrylic-based adhesives which require moisture as a vital catalyst to initiate almost instantaneous curing (complementary to the anaerobics).
• normally used as thin layers.
• epoxies
• usually a two-part system of epoxy-resin and a hardener, although premixed single part epoxy (ESP) adhesives are available.
• high strength and good adhesion to many materials and may be used for thicker joints than the above types.
• phenolics
• phenol-formaldehyde resin systems were one of the earliest synthetic adhesives and still offer good performance in severe environments.
• specialised equipment and complex procedures are usually required.
• polyurethanes
• polyurethane chemistry (normally isocyanate and alcohol) can be difficult to handle as the isocyanates have rigorous health and safety requirements.
• good for load-bearing applications in dry conditions, but susceptible to attack by moisture.

The above is not an exhaustive list.  For example other variants include plastisols (based on PVC dispersions) and rubber solutions (where solvent evaporation effect bonding).  In particular, toughened adhesives may be any of the above families of adhesive with the incorporation of low molecular weight rubbers (either chemically incorporated in the polymer backbone or as physical particles).

Adhesives can bond most materials in common engineering use and are especially useful where the substrates are different materials.  For optimum bonding, avoid:

• materials with weak or loose surface layers
• materials troubled by water migration, solvent attack and/or stress cracking.

The advantages of adhesives (compared to welding, brazing, soldering or mechanical fasteners) are:

• joints can be made without blemish, distortion or protrusions
• the net weight of the joint is minimised
• stresses at the joint are more uniformly distributed
• the resulting structure is normally stiffer than for discretely welded/fastened joints
• fatigue life is increased
• joints of complex geometry are relatively easy to make
• capital costs are often reduced
• labour costs are often considerably reduced
• the process can be de-skilled or completely automated

Good joint design is essential for highly-stressed applications.  Bonded joints are best loaded in compression and give acceptable performance in shear.  Tension, especially peel (where at least one component is flexible) and cleavage (where rigid components are involved) should be avoided [3].  See pages 15-16 of reference [2] or page 50 of reference [3] for illustrations of acceptable and unacceptable bad designs for adhesively bonded joints (or the PowerPoint presentation for this lecture: slides 12-15).

Surface preparation is crucial to the achievement of a good bond and for composites normally includes a degrease-abrade-degrease-dry sequence.  The creation of a good bond depends critically on the adhesive wetting the substrate surface.  The wetting of a solid by a liquid depends on the relationship between the interfacial tensions for the three phases: solid/liquid (SL), liquid/vapour (LV) and solid/vapour (SV). The contact angle for a liquid on a given smooth surface is described by Young's equation [4]. A contact angle of <90° will result in wetting (the fluid is hydrophilic when the liquid is water).   A contact angle >90° will not result in wetting (the fluid is hydrophobic when the liquid is water).  Wenzel [5] recognized that the quality of the surface affects the surface wetting properties and modified Young's equation to include roughness as an additional factor.  The Young (left) and Wenzel (right) equations are:

where θ_γ is Young’s contact angle,  θ_W is Wenzel’s contact angle, γ_SL, γ_LV and γ_SV are the interfacial energies per unit area for the solid-liquid (SL), liquid-vapor (LV), solid-vapor (SV) interfaces respectively and r is the the roughness factor (the ratio of the actual area of a rough surface to the geometric projected area).  Marmur [6] has presented a useful summary of the issues arising during the measurement and interpretation of contact angles.

The use of shot-blasting to abrade the surface is inappropriate: it tends to remove too much substrate.  Plastic bead blasting (or similar blast media) permits greater control of material removal.  For adhesively bonded composite components, co-curing is often adopted: the substrate and the adhesive joint are cured simultaneously.

Failure of an adhesive joint is called "debonding".   In this context, debonding differs from the composite failure mode where the fibre/matrix interface breaks. debonding of an adhesive joint is separation of the bulk adhesive from the substrate.

The National Physical Laboratory has an on-line Adhesive Design Toolkit.

The Engineering Sciences Data Unit (ESDU) report 92041 [7], together with ESDUpac A9241, "presents a Fortran computer program for the analysis of the distribution of the stresses and strains in the adhesive of a bonded single lap joint under an applied load, together with the nominal stresses in the adherends".  "Section 1 describes the theory and application of two, alternative, analyses of a single-step lap joint that are included in the program described in Section 2". "The first analysis, the ‘simple shear stress’ analysis, may be used to analyse the shear stress distribution in the adhesive of a bonded lap joint loaded in either tension or shear.  This analysis may be applied to either single or double lap joints, provided that the latter are symmetric, and can allow for the inelastic shear stress-strain properties of the adhesive.  The second analysis, the ‘flexible’ analysis, is applicable to single lap joints loaded in tension and takes account of bending of the joint in deriving the peel stresses resulting from that bending.  This analysis also takes account of thermal stresses arising as a result of differences between joint working temperature and curing temperature but it does not allow for the inelastic shear stress-strain properties of the adhesive".

The Araldite/Huntsman Advanced Materials website offers the following documents:

• Glossary
• User Guide to Adhesives (205 KB PDF)
• Surface Preparation and Pretreatments (192 KB PDF).

The Loctite website offers the following documents:

• Adhesive Sourcebook - LT3355 (full document = 4.8 MB)
• Equipment Sourcebook - LT3669
• Design Guide for Bonding Plastics - LT2197 (9 MB PDF file)
• Pages 1-17: Table of Contents - How to Use ... -  Description of Adhesives - Plastic Properties and Adhesive Performance (acrylic)
• Pages 18-31: Plastic Properties and Adhesive Performance (ABS-Ionomer)
• Pages 32-45: Plastic Properties and Adhesive Performance (liquid crystal polymer-polyetheretherketone)
• Pages 46-53: Plastic Properties and Adhesive Performance (polyetherimide-polyethyleneterephthalate)
• Pages 54-63: Plastic Properties and Adhesive Performance (polyimide-polypropylene)
• Pages 64-96: Plastic Properties and Adhesive Performance (polystyrene-vinylester) - Surface Treatments - Adhesive Joint Design - Processor Rules - Test Methodology - Glossary - Index - Acknowledgements - Disclaimer
• Design Guide for Bonding Metals (LT-3371a vol4)
• Design Guide for Bonding Rubber and Thermoplastic Elastomers (LT-2662)
• Light Cure Product Selector Guide (LT-2731f)
• Light Cure Technology Brochure (LT-2730a)

The Permabond Engineering Adhesives website offers pages on:

• Glossary of adhesive terms
• Acrylics
• Anaerobics
• Cyanoacrylates
• Epoxies:
  • two-part, and
  • single-part
• Ultra-Violet (UV) light curable
• Application Tips:
• Surface Preparation
• Temperature Effects
• Shelf Life and Storage
• Suitable Surfaces

This link lists some major suppliers of adhesives.

References

1. RD Adams (editor), Adhesive Bonding - science, technology and applications, Woodhead Publishing, Cambridge, 2005.  ISBN 1-85573-741-8.  CRC Press, Boca Raton - USA, 2005.  ISBN 0 8493 2584 6.  PU CSH Library.
2. Permabond Adhesives Limited, The Engineers Guide to Adhesives, no date (circa 1990).  MooDLE.
3. WA Lees, Adhesives and the Engineer, Mechanical Engineering Publications, 1989.
4. T Young, An Essay on the Cohesion of Fluids, Philosophical Transactions of the Royal Society of London, 1805, 95, 65-87. (Free access to 3244 KB PDF file)
5. RN Wenzel, Surface roughness and contact angle, Journal of Physical and Colloid Chemistry, 1949, 53, 1466-7.
6. Abraham Marmur, Soft contact: measurement and interpretation of contact angles, Soft Matter, 2006, 2, 12-17.
7. Stress analysis of single lap bonded joints (adhesive and adherends), ESDU 92041, ESDU International plc, London, December 1992 with Amendment A September 2001.

Videos (no longer available in PU CSH Library):

• Tomorrows Technology Today, IMechE Materials Group.
• The taming of the glue, Loctite UK, 1990.

Further reading

• RD Adams, Adhesive bonding: science, technology and applications, Woodhead, Cambridge, 2005. ISBN 1-85573-741-8.  PU CSH Library.
• WA Lees, Adhesives in engineering design, Design Council, London, 1984.  ISBN 0-85072-150-4.
• AJ Kinloch, Adhesion and adhesives: science and technology, Chapman & Hall, London, 1987. ISBN 0-412-27440-x. PU CSH Library.
• JR Panek and JP Cook, Construction sealants and adhesives, Wiley, New York, c1984. ISBN 0-471-09360-2.
• AV Pocius, Adhesion and adhesives technology: an introduction, Hanser/Gardner, 1997. ISBN 1-569-90212-7.  PU CSH Library.
• Guide to the Structural Use of Adhesives, Institution of Structural Engineers, London, 1999.7nbsp; ISBN 1-87426-643-3.
• Mark Geoghegan and Georg Krausch, Wetting at polymer surfaces and interfaces, Progress in Polymer Science, 2003, 28(2), 261-302.