Water and Humidity
Water and water vapour - humidity - are two of the most damaging environmental factors for adhesive. The possible mechanisms by which water can degrade adhesive joints include:
- hydrolysis or crazing of the adhesive
- degradation or change of the interface resulting in loss of adhesion
- corrosion of the substrate.
One of the most detrimental and insidious effects of water is to change the mechanism by which an adhesive joint can fail. In freshly prepared joints, adhesion should be sufficient for failure to only occur cohesively in either the adhesive or the substrate material. Over time moisture is absorbed by the adhesive resulting in a change in the mechanical properties. For example for the toughened epoxy adhesive shown in Figure 1, the yield strength is reduced but the strain to failure increased by the absorption of moisture and the Tg is reduced. Such a process may take several years to completely affect a joint at ambient temperatures. Joint strength and creep resistance may be reduced but not dramatically. However if there is a loss of adhesion, more dramatic reductions in strength may occur. A programme of accelerated durability testing may therefore be required to ensure that this does not occur during the desired service life.
Figure 1 Effect of moisture uptake (%) on the stress-strain behaviour of an epoxy adhesive
The fatigue life of lap joints is progressively reduced after pre-exposure to moisture, as shown in Figure 2. This is associated with increasing creep due to reductions in Tg of the adhesive as moisture is increasingly absorbed.
Figure 2 Effect of pre-exposure to water on the fatigue life of lap joints
The effects of water on adhesive joints can be very varied, as indicated by the following examples. The significant effect of water on a joint’s lifetime was shown by results from hot-wet ageing of lightly stressed joints, which lost 100% of their strength after three years. With some adhesives, distilled water can be a more aggressive environment than salt water. It has also been found that total immersion of the joint is not as aggressive as wet-dry cycling, such as may occur in-service. The difference is attributed to the availability of oxygen at any crack tip that forms during the wet-dry cycling. It has been noted that water in the natural environment is often a more severe test of a joint’s durability than high humidity conditions in the laboratory.
It has been suggested that the following three factors are important in explaining the effect of water content on adhesive joints:
- the nature of the adhesive-substrate interface
- the solubility of water in the adhesive
- the water activity of the environment
The effects of temperature can be considered in relation to the glass transition temperature (Tg) of an adhesive. For rigid adhesives the normal operational temperatures are below Tg whereas for flexible adhesives they are above Tg. The effects of temperature on strength, stiffness, fracture toughness and elongation/strain are considered.
Stiffness and strength The variation in stiffness and strength of a single lap joint with 40 mm wide, 0.8 mm thick mild steel substrates, 15 mm overlap length with 2 mm adhesive layer thickness is shown in Figure 3 for a rubber based flexible adhesive and toughened epoxy based rigid adhesive. Rapid changes in the stiffness and strength of the joint are associated with the Tg of the adhesives. These variations indicate a Tg of -10C for the flexible adhesive and 60C for the rigid adhesive.
- For rigid adhesives the Tg is an indication of the upper operational temperature.
- For flexible adhesives the Tg is an indication of the lowest temperature for which the flexibility is maintained.
Figure 3 Effect of test temperature on joint stiffness and strength.
Characterising and understanding the toughness properties of polymeric materials and adhesives is complex. Specific details of the various adhesive types and their toughness values can vary significantly, but some generalisations are possible. At higher temperatures, adhesive materials tend to become tougher, due to increases in their fracture energies. At low temperatures the opposite occurs and the fracture energy of adhesives is reduced and they become more brittle.
There are several mechanisms by which toughness is achieved in adhesives. These mechanisms vary, depending on the adhesive type. Different mechanisms influence the extent to which temperature affects the toughness.
In rubbery adhesives there can be large changes in toughness, with high toughness values above the glass transition temperature (Tg) due to the many viscoelastic and frictional processes that can occur. These processes are inhibited at lower temperatures, below the Tg, and the toughness is greatly reduced.
In more rigid adhesives, such as epoxies and phenolics, there are four possible mechanisms of fracture:
- Shear yielding - which involves localised plastic deformation at the crack tip, mainly at a constant volume. Initially the fracture is brittle in nature, which gradually changes to more ductile fracture as the amount of shear deformation increases.
- Crazing - which also involves localised plastic deformation, but which is accompanied by cavitation of the material and hence an increase in volume. The craze consists of a number of microvoids that nucleate ahead of a crack tip under a tensile stress. Crazes are often a precursor to brittle fracture.
- Multiple deformations - these occur in rubber-toughened adhesives and exploit cavitation in the rubber particles and at the rubber/adhesive interface, along with extensive shear yielding over a wide area.
- Crack pinning - uses rigid particles in the adhesive material to impede the progress of a crack front through the material.
The effect of temperature on these adhesives is not as marked as with rubbery adhesives, but the general trend is still for reduced toughness at lower temperatures.
As the temperature is increased, the tendency is for adhesives to have higher elongation or strain to failure. This is due to the increased mobility of the polymer chains in the adhesive material as temperature increases, which allows the material to extend more easily. The opposite occurs at lower temperatures, which restricts the movement of polymer chains in the adhesive and leads to reduced elongation to failure.
Liquids other than water also have the potential to degrade joint performance. With acids, bases and organic liquids this is most likely to be due to direct interaction with the adhesive. An example was a study on sealants, where the durability of aluminium joints was primarily controlled by the amount of absorbed organic liquid. The chemical and physical reactivity of the adhesive towards the particular liquid provides a useful guide to the joint's durability.
With salt and other corrosive chemicals the main problem is likely to be with direct attack and degradation of the substrate material, rather than with any effect on the adhesive. If the substrate is corroded this creates the possibility that the failure mechanism of the joint will change, from a cohesive failure to an interfacial, or near interfacial failure mode. Appropriate tests should therefore be carried out to assess this possibility for corrosive environments.
Standard tests that determine durability
ASTM D3762-03 Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test)
ASTM D2918-99 Standard Test Method for Durability Assessment of Adhesive Joints Stressed in Peel (T-peel)
EN 29142:1993/ISO9142:1990 Adhesives. Guide to the selection of standard laboratory ageing conditions fo testing bonded joints.
ISO 10354:1992 Adhesives. Characterisation of durability of structural adhesive bonded assemblies. Wedge rupture test.
ISO 14615:1997 Durability - Humidity and temperature exposure under load.
See also - creep and fatigue test methods