The use of structural adhesives can only increase as more and more manufacturers/fabricators become aware of their considerable advantages over conventional joining techniques. This increased use of adhesives, especially in safety-critical areas, makes it necessary to evaluate any possible decrease in performance that may occur when glued joints are subjected to both in service stresses and environmental conditions.

Adhesion is the bond strength measured between the coating and the substrate.   When bonding two items together numerous chemical, physical  and mechanical forces come into play, which may have an effect on each other.  To validate a design these need to be tested before a product is released for service.  There are a large number of different adhesive products, substrates and applications available and all require different adhesive tests depending on the application.

Email: Ken@jcs-tech.co.uk  or phone 01934-644866

Typical test measurements include:

• 180 degree peel tests
• 90 degree peel tests
• Lap shear tests for evaluating the shear strength of adhesives for bonding two surfaces together
• Coating cohesion - adhesion or bond strength of a coating to a substrate
• Climbing drum Peel

For each of these tests variation in humidity and temperature can have a significant effect. To compare two adhesives then the preconditioning and test conditions need to be the same. However for evaluation of an adhesive in service  other conditions may be appropriate such as chemical environment as well temperature etc.

Physical tests can cover a wide range of properties, such as

• Tackiness
• Stringiness
• Flow
• Gel time
• Bond line determination(thickness)
• Filler content
• Particle size analysis , particularly important and cure rate
• Mix ratio
• volume to weight ratio

Clearly there are many more but the above will give a feel for the sort of test than can be conducted

It is possible to determination the chemical nature of an adhesive. Using a variety of chemical techniques it is possible to ensure consistency between batches of adhesive supplied and also identify the chemical make up of an adhesive.

Infra Red Spectrometry

As most adhesives are Organic in nature ( i.e. contain carbon compounds) the first line of investigation is almost always Infra Red(IR) spectrometry. The IR spectrometer looks at bonds between the atoms within the adhesive. Each type of bond behaves in a different way when exposed to Infra red radiation. Using this information it is possible to produce a trace that is like a chemical fingerprint of the adhesive. Comparing traces can quickly identify any changes in formulation.

Chromatography

Chromatography is principally a separation technique, using a range of tubes or pipes containing a variety of chemicals. The constituents of the adhesive under test are held in the tubes for different lengths of time depending on the chemical make up and as they exit the tube they can be analysed by a suitable detector and identified.

 Typically there are two ways of defining failure in bonded joints

Cohesive Failure

This tends to be the best form of failure as this is failure in the adhesive. A good way to check is to look at both sides of the failed item and a complete coating of adhesive on both surfaces is a good indication of cohesive failure

Adhesive Failure

Generally this is a failure to one of the substrates. All the adhesive is on one surface. This tends to point towards problems with surface preparation of the sample. Almost always results in a lower peel of shear value

At JCST we use the following method (for more details of this method see sample preparation in the production section)

1) Ensure area is clean and all materials are available

2) Degrease substrates

3) Vacuum blast or physically abrade surface

4) Degrease substrates after vacuum blast

5) Prime surface if required

5) Bonding using wires to maintain glue line thickness if required

6) Cure adhesive

7) Cut to size if required

8) Final inspection