Roymech engineering encyclopedia

Adhesives



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Adhesives Notes


Introduction.. (Refer also to notes Adhesive theories )

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 strength of the adhesive termed cohesion.   Adhesives can also provide a sealing function.

There are many advantages in using adhesives compared to other fastening methods as listed below.

  • Fatigue - There are few, if any, stress concentrations associated with adhesive joints
  • Surface Area - For a lap joint the stress is distributed over a relatively large area. This provides a strong joint with lower specific stresses.
  • An adhesive joint can provide a sealing function in addition to a fastening function.
  • For a high production process an adhesive joint can be extremely economical.
  • Compared to welding there are no high temperatures required.
  • Adhesives are available for attaching a wide variety of different materials.
  • Compared to bolted joints and rivetted joints there is less need for machined holes, and additional machined components.
  • The is no weight penalty associated with adhesive joints.
  • The stresses developed in an adhesive joint are almost entirely imposed by the operating conditions.
  • For softer materials adhesives are the only real engineering option.

Theoretically an adhesive joint is the ideal joint for permanently /semi-permanently fastening components together. Limitations of adhesive joints are listed below.

  • Certain adhesives are vulnerable to attack by chemicals /water.
  • For the best joints surface preparation is important.
  • Some adhesives require time to achieve design strength.
  • An adhesive joint is intuitively not considered a sound joint.
  • Many adhesives have a temperature limitation.
  • For mass production, reliable joints jigs and fixture are important.
  • Many adhesives are toxic and require care when being applied.
  • Adhesive joints are often vulnerable to localised high stress due to peeling effect.
Adhesive Joints

When designing adhesives joint it is better the the operating load is transferred as a shear stress as opposed to a tensile stress.  Adhesives are also poor in withstanding peel and cleavage loads.  The most typical design of adhesive joints are 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 as bending moments are reduced and , as a result peel loads are reduced.

a tensile.

The sketches below indicate simple methods of providing improved adhesive joints be reducing the peel load.




Co-Axial joints /Screw Threads

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.


Surface Preparation

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..

  • Degreasing with steam, organic solvents or alkaline cleaners
  • Mechanical methods such as blasting, brushing or grinding with emery-paper.
  • Dipping in boiling water after etching in caustic soda.
  • Yellow or green chromate treatment
  • Anodising in chromic, phosphoric or sulphuric acid.
  • Applying a suitable primer.

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.





Different Types of adhesive

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.

Natural Adhesives..Natural Adhesives

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.

Elastomer Adhesives...Rubber Adhesives

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.


Thermoplastic Adhesives...Thermoplastic 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...Thermoset Adhesives

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.

Toughened Adhesives

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.





Shear and Peel Strengths of Generic Adhesive types

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