An adhesive is a substance that fastens or bonds materials using surface attachment. The strength of an adhesive joint depends on the strength of the attachment of the adhesive to the material surface (substrate), termed adhesion, and the strength of the adhesive itself, termed cohesion. Adhesives can also provide a sealing function.
In engineering applications, the performance of an adhesive joint depends not only on the adhesive selected, but also on the materials being joined, joint geometry, surface preparation and the loads and environmental conditions experienced in service.
Theoretically an adhesive joint is the ideal joint for permanently /semi-permanently fastening components together.
When designing an adhesive joint it is preferable for the operating load to be transferred as a shear stress rather than a tensile stress. Adhesives are also poor at withstanding peel and cleavage loads.
Typical adhesive joint designs include the lap joint and the axial fit joint. Normal adhesive joints are shown below. The double lap joint is preferred to a single lap joint because bending moments are reduced and, as a result, peel loads are reduced.
The sketches below indicate simple methods of providing improved adhesive joints be reducing the peel load.
Co-axial joints are generally associated with fitted bearings shafts, splines and press fitted
components either permanent or semi-permanent. The anaerobic adhesive is ideally suited for these
applications. They can be formulated such to withstand high shear loads for permanent assemblies or lower
shear loads for assemblies that need to be dismantled.
For joints that with permanent press fits, the use the the anaerobic adhesive provides
many advantages including the elimination of the need for high precision interference fits
and the risk of fretting causing progressive stress concentrations. Anaerobic
adhesives will function very satisfactorily with normal diametrical clearances of 0,05mm.
They can also be used to supplement the performance of interference
fits.
When used with screw threads anaerobic adhesives provide a very low cost convenient alternative
to the use of mechanical methods of locking screw threads (locknuts, lockwashers, wiring etc.).
When used with taper threads used for sealing fluid pressures these adhesives can be used to
seal the threads against the vast majority of fluids used in vessels and piping systems.
When used on bearings and similar systems it is normally recommended that the length of engagement
should be approximately 2 x the diameter. However shorter engagement ratios have been used with no
loss of performance.
It is necessary to ensure that the substrate surfaces of the components being
attached are suitably prepared. For all adhesive joints the minimum surface
preparation is to clean the surfaces such that all loose substrate and grease is removed.
The surfaces to be joined should be flat, some adhesives are more capable of filling
spaces, but the flatter the surface the stronger the joint.
The important surface preparation routines are shown below..
When using cyanoacrylate adhesives it is necessary to ensure that chlorinated solvents are not used for surface penetration.
These solvents have acid stabilisers which prevent curing of the particular solvent.
Materials which are often difficult to bond and which often require enhanced surface protection include
Aluminium (+alloys), Copper ( +alloys), Magnesium (+alloys), Nylon, Polyacetal, Polyethylene, Polypropylene,
PTFE, PVC and Zinc. Glass also generally requires special preparation. It is often possible
to greatly improve the strength of a bonded joint with difficult surfaces using an adhesive primer.
In all cases the adhesive manufacturers literature should be consulted and followed to achieve the optimum
bonding results.
Note: Selecting adhesives from the linked information below.
For each adhesive tabled a short list
of materials bonded (substrates) is provided. If two different materials are to be bonded simply select the adhesive
listing both materials.
Generally set by solvent evaporation. They are generally of low strength and are
susceptible to moisture and mould. Their use is restricted to the joining of low strength
materials. Natural adhesives include animal glues, fish glues, vegetable glues and casein.
Based on natural and synthetic rubbers set by solvent evaporation or heat curing. They have relatively low shear strength and suffer from creep and are therefore used for unstressed joints. They are useful for flexible bonds with plastics and rubbers. Contact adhesives use rubber in a solvent and will join many materials. Elastomer adhesives include natural rubbers, polychloroprenes (neoprene), acrylonitride butadiene (nitrile), butyl rubber adhesives, styrene butadiene rubber adhesives, polyurethane adhesives, polysulphide rubber adhesives and silicone rubber adhesives.
In general, thermoplastic adhesives have low/medium shear strength and may suffer from
creep at high loading. They have good resistance to oils but poor resistance to
water. Thermoplastic adhesives include polyvinyl acetate (PVA), polyvinyl alcohol (PVA),
polyacrylates, polyester acrylics, acrylic solvent cement, cyanoacrylates (superglue), silicone
resins, polyamides and acrylic acid diesters.
Thermoset adhesives set as a result of the build up of molecular chains to produce a rigid crosslinked
structure. They include epoxy resins, which are some of the most widely used
adhesives. There are many different thermoset adhesives available including phenolic
formaldehyde (PF) resins, phenolic neoprene, resorcinol formaldehydes (RF), polyesters,
polyimides and epoxy resins.
During the last 25 years a process known as 'toughening' has been developed.
A toughened adhesive has small, rubber-like particles dispersed throughout the
glassy matrix. The profound effect of these particles is to change
the mechanical characteristics of the adhesive making it much more resistant to
crack propagation. This technology has only been successfully
applied to the acrylic and epoxy-based adhesives.
Important Note: The values below are random values obtained from general reference sources
and suppliers catalogues. They are often rounded values from imperial. They are only of
use to indicate the relative strengths of different adhesives. Links are provided below
with much better quality information. Always consult manufacturers literature
when selecting and designing with adhesives
| Adhesive Type | Shear Strength | Peel Strength | Operating Temp. | |||
| MPa (N/mm2) | N/mm | Min oC | ||||
| Min | Max | Min | Max | Min | Max | Rubber | 0,35 | 3,5 | 1,8 | 7 | -20 | 150 |
| PVA (white glue) | 1,4 | 6,9 | 0,88 | 1,8 | ||
| Cyanoacrilate | 6,9 | 13,8 | 0,88 | 3,5 | 80 | |
| Anaerobic | 6,9 | 13,8 | 0,88 | 1,8 | 200 | |
| Polyurethene | 6,9 | 17,2 | 1,8 | 8,8 | -200 | 150 |
| Rubber modified acrylic | 13,8 | 24,1 | 1,8 | 8,8 | -40 | 90 |
| Epoxy | 10,3 | 27,6 | 0,35 | 1,8 | 200 | |
| Polyimide | 13,8 | 27,6 | 0,18 | 0,88 | 350 | |
| Rubber modified epoxy | 20,7 | 41,4 | 4,4 | 14,0 | 180 | |
Adhesion is the attachment of the adhesive to the surface of the material being bonded, known as the substrate. Cohesion is the internal strength of the adhesive itself. Both are important to the strength of an adhesive joint.
Adhesive joints are generally better suited to carrying loads in shear. Joint designs should minimise peel and cleavage loading because these can produce high local stresses at the edges of the bonded area.
Oil, grease, dirt, loose material and unsuitable surface films can prevent effective bonding between the adhesive and substrate. Suitable cleaning and surface preparation can therefore have a major influence on joint performance.
Anaerobic adhesives are commonly used with close-fitting metallic components, including shafts, bearings, press fits and screw threads. Different formulations can be used for retaining, locking and sealing applications.
Engineering adhesives include natural, elastomer, thermoplastic, thermoset and toughened adhesive systems. Selection depends on the materials being joined, required strength, flexibility, temperature and service environment.
The tabulated values are intended only to indicate typical relative properties of different adhesive types. Actual properties vary with adhesive formulation, substrate, joint design, surface preparation and operating conditions. Current manufacturer data should be used for engineering design.