Adhesive joints - how strong, how long?

TWI Bulletin, March/April 1993

Mehdi Tavakoli, after gaining a BSc in chemistry, studied for an MSc in polymer science and technology at Aston University. He then completed a PhD in the same field at Aston and graduated in 1981. He continued research into polymer degradation and stabilisation. Prior to joining TWI in 1989 as a principal polymer scientist he worked at Bath University on polymer composite manufacturing processes including RTM, fibre/resin compatibility, stress rupture behaviour and damage mechanisms in phenolic resin composites. Since joining TWI, Mehdi has been working mainly on novel surface preparation techniques, including the application of power beams for adhesive bonding, and mechanisms of degradation in polymer welds and adhesively bonded joints.

Structural integrity and reliability of bonded structures are primarily dependent on durability of joints.

Mehdi Tavakoli reviews factors affecting environmental durablity and mechanisms of degradation of adhesively bonded joints.

The most important feature of an adhesive is its ability to retain its bonding capability for long periods under a wide variety of environmental conditions. The permanence of an adhesive joint depends upon many factors other than the adhesive itself or the method of application. These include the means of surface preparation, extremes of temperature encountered in service, design of the joint, loading history and exposure to water and other disruptive or corrosive media.

Despite early reluctance of many design engineers to use adhesives for critical joining applications, it is now being recognised that certain types of thermosetting adhesives are able to provide useful bond strengths over a wide temperature range. Structural adhesives such as epoxies can bond virtually any high strength material to another and can survive many years of outdoor weathering.

Although a number of thermoplastic-based polymers may be used as structural adhesives, the requirement that an adhesive joint be creep resistant generally demands that the adhesive be crosslinked during the process of forming and curing of the bonded joint.

Causes of degradation

Effects of water

Water is one of the commonest environmental agents which can affect the durability of adhesively bonded joints. Most adhesively bonded structures are exposed to water or water vapour. If the relative humidity is high, deterioration in mechanical properties can occur. Water may affect the adhesive or the interface and, in the case of permeable adherends, substrate properties could also be modified. Some of the processes which can take place are reversible, and recovery in strength occurs on drying, but there are also irreversible reactions leading to deterioration of properties.

Moisture enters an adhesively bonded joint commonly through the adhesive or at the interface, although polymer based adherends can also absorb moisture. Diffusion of moisture into adhesives can take place through an exposed edge. The rate of transport of moisture through the adhesive to the interface is given by the permeability which, in turn, is the product of the diffusion coefficient and the solubility.

In general, rapid diffusion may occur with an adhesive in which the diffusion coefficient is high or in which water is relatively soluble, or both. For instance, silicone based adhesives, which are often considered as moisture resistant because of low solubility of water in polysiloxanes, are in fact very permeable to moisture because of their high diffusion coefficient.

Apart from altering the physical properties of the adhesive, the uptake of water and its transport to the interface with the adherend can, in certain cases, lead to displacement of adhesive by moisture and therefore failure of the joint. Displacement normally occurs from the edges of the joint working inwards as diffusion proceeds. The conditions leading to displacement depend upon the nature of interactions between adhesive and adherend. If the interaction is purely physical, i.e. mechanical interlocking, then the relative energies of absorption of water into the substrate and adhesive, compared with the energy of absorption of adhesive into the substrate, determine the equilibrium. A film of water is generated at the interface, particularly when adhesives absorb more than a few percent of water and the substrate has a high surface energy, as is the case with metal oxides.

If the bond between adhesive and adherend involves chemisorption via intermolecular forces rather than physical absorptions, then displacement can only occur after hydrolytic destruction of the chemical bond uniting adhesive and the substrate.

When displacement of a structural adhesive from a primer develops, it is usually a reversible phenomenon and drying normally restores tensile strength. This reversibility, particularly over many cycles, leads to a gradual and permanent loss of strength but the period over which this occurs is usually many years. However, displacement of an adhesive from a metal substrate is generally not reversible and tends to be followed by corrosion of the metal.

If the nature of the adhesive, the surface treatment of the adherends and the resultant interaction between adhesive and adherend are such that water does not displace the adhesive from the adherend, any effect of water or high humidity on joint strength is the result of absorbed water plasticising the adhesive. This reduces the glass transition temperature (Tg) of the adhesive and affects its modulus. Adhesives with Tg>100°C are expected to be less prone to loss of strength in wet conditions.

Where the adhesive used is prone to water attack or displacement, it is customary to seal the edges of the joint, or to coat the bonded area with a polymer having low moisture permeability such as butyl rubber or polysulphide. Use of coupling agents as a primary coat, particularly in conjunction with a sealing agent, is an effective method of reducing the influence of water on joint properties.

Hydrolysis is a particular type of water-related degradation, which can affect either the bulk of the adhesive or the adhesive interface. It requires the presence of water and generally also requires the presence of a strong acid or, more frequently, a strong base. Hydrolytic attack on an ester linkage in the presence of water at high pH is an important example of this mode of attack. As a result of this, main chain scission may occur in the polymer based adhesives. This can lead to loss of cohesive strength within the adhesive and failure of the joint.

Intrusion of water to the interface can disrupt chemical and physical interfacial interactions between the adhesive and the adherend. For instance, water can break up hydrogen bonding between the adhesive and the adherend leading to loss of interfacial interactions and interfacial failure.

The effect of water on bonded structures must be understood in order to predict the long-term durability of any component which experiences water exposure during operation. The adverse effect of water can be significantly accelerated if stress is being imposed simultaneously on the bonded structure. If this is the case in service, both factors must be evaluated at the same time.

Effects of temperature

Both adhesives and joints may be subjected to elevated temperatures during processing or service. Heat curing adhesives are expected to resist thermal and thermal-oxidative degradation during curing. But thermal and residual stresses may be generated in the adhesive during curing, partly due to crosslinking or because of differences between the coefficients of thermal expansion of adherend and adhesive.

Adhesives and the corresponding joints may also be exposed to sub-zero or elevated temperatures during service. At very low temperatures, thermal stress can become significant leading sometimes to failure of the joint, while prolonged exposure of joints to elevated temperatures during service causes degradation of the adhesive and polymer based adherends. This leads to loss in mechanical properties and ultimately joint failure. Oxidation of the surfaces of metallic adherends may weaken the joint when exposed to high temperatures in the presence of air.

Temperature is a common factor in many service environments, so its influence on bond durability should be considered. In evaluating the service life of bonded components, varying temperatures can have both chemical and physical effects on organic based polymers which constitute the bulk of commercial adhesives.

The resistance to degradation of a range of adhesives, based on the heat resistance and thermal ageing properties, is shown in Fig.1.

Fig. 1. Comparison of high temperature structural adhesives:
a) Variation of shear strength with temperature
b) Change in shear strength with time. The polyaromatics (PBI and PI) have the highest shear strength at temperatures above 200°C and best retention of strength on ageing

Stress

Adhesively bonded joints may be exposed to static, dynamic, intermittent or continuous stresses. Such stresses may interact with other environmental factors, such as elevated temperature or moisture, to cause a drastic reduction injoint durability.

Effects of stress on durability can be assessed by determining the time to failure at a given load, or by imposing a load (a fraction of breaking load) for a given time and determining the residual joint strength. As well asexternal load, stresses generated internally by adhesive shrinkage (originated during curing) or by adhesive swelling (through water absorption) can affect joint properties and durability. Moreover, stress may increase the rate ofdiffusion of moisture and environmental fluids into the joint.

Application of external stresses can affect the long-term durability of adhesively bonded joints. Static fatigue, or creep rupture, is the phenomenon of fracture which takes place at some time after the application of a constantload. The applied stress is normally lower than that required to cause fracture under monotonic loading.

Under fluctuating loads, joints fail at stress levels much lower than they can withstand under monotonic loading. Furthermore, for a given stress amplitude, joints fail in a shorter time than in static fatigue. Dynamic or staticfatigue in a hostile environment can significantly reduce the long-term durability of adhesively bonded joints. However, adhesives are generally considered to possess good dynamic fatigue properties in comparison with other methods ofjoining, such as spot welding or riveting.

Radiation

Adhesively bonded joints may be exposed to radiation during service outdoors or in specialist applications. Photo-oxidation is the principal reaction in photo-degradation of polymeric adhesives or adherends during outdoorweathering. Although adhesives differ widely in their resistance to photo-oxidation, almost every adhesive or polymeric adherend eventually deteriorates on continued exposure to solar radiation.

Adequate resistance to, or protection against, photo-oxidation is the most important factor limiting the application of adhesives in joints which must be exposed out of doors. Adhesives used in some fibre optic applications may alsobe exposed to radiation. In this case, resistance of the adhesive to photo-degradation is the major factor in determining reliability of the bonded fibre optic assembly.

Adhesives may be used in extraterrestrial applications. The high-energy radiation encountered in space, however, imposes a serious limitation on these applications. Degradation of adhesives by high energy radiation also limits useof these materials in joints exposed to sources of X-rays, γ-rays or to various types of chemical accelerators. For example, adhesives used in joining components of a medical device or medical implant may beexposed to γ-radiation during sterilisation.

Salt

Adhesively bonded joints used in aircraft, trains, cars and offshore structures may be exposed to salt. Again this may significantly affect long-term durability. Joints involving a metal adherend may suffer loss of strength anddurability directly from corrosion of the metallic adherends when exposed to seawater or salt spray. Water and oxygen, as well as a liquid electrolyte, are required to establish an electrochemical cell at the adhesive/substrateinterface.

Deterioration in joint strength generally occurs much faster in salt water than in fresh water. It is likely that salt water attacks through the bondline instead of through diffusion alone, as in fresh water.

Pretreatments

It is now recognised that proper and adequate surface preparation of adherends before bonding is one of the most important factors for achieving good long term durability and reliability in bonded components. Mechanical treatmentssuch as abrasion and grit blasting, and chemical treatments such as solvent degreasing are often adequate when long term durability is not critical. However, to provide durable joints for metallic adherends, it is essential to use anetching or anodising pretreatment.

Fig. 2. Effect of surface pretreatment on the performance of aluminium-alloy epoxy joints subjected to accelerated ageing in water at 50°C

Aluminium and its alloys which are bonded are particularly sensitive to environmental attack upon outdoor exposure, unless some chemical pretreatment has been used. The most common surface pretreatments for aluminium alloys, usedfor example in aerospace industries, include chromic acid etch, chromic acid anodising and phosphoric acid anodising. Effects of various surface pretreatments on durability of aluminium alloy/toughened epoxy joints are shown in Fig.2; clearly solvent degreasing and grit blasting are less effective compared with anodising and etching treatments. Phosphoric acid anodising is a particularly effective surface treatment for long-term durability in aqueous environments.

Type of adhesive

Chemical composition of an adhesive, including the type of polymer used, fillers, solvents or other carriers, crosslinking agents or other additives, can have a profound effect on joint durability. If environmental agents attack theadhesive, particularly chemically, this can lead to deterioration in properties of the joint. In this regard, adhesives containing hydrolysable linkages such as polyester based polyurethanes or cyanoacrylates are prone tohydrolysis.

Epoxies, phenolic and polyimide based adhesives usually provide better high temperature stability compared with acrylic based systems. Generally, higher curing temperatures lead to higher durability of joints in elevated temperatureservice environments.

All polymer based adhesives attract water to some degree and the mechanical properties of the joints are affected as a result of this interaction. Adhesives containing polar functional groups ( e.g. polyamides or polyurethanes) or polar additives, ( e.g. nitrile rubber as a toughening agent) absorb more water and are affected more by polar fluids than non-polar adhesives ( e.g. natural rubber based pressure sensitive adhesives).

Types of adherends

Durabilities and mechanisms of degradation and failure in adhesively bonded joints are also dependent on the type of substrate used. In general, joints produced using inorganic substrates such as metals, glasses or ceramics are moresusceptible to deterioration than polymeric substrates. For inorganic substrates, environmental attack usually occurs on the adhesive itself or at the interface. The modes of failure are normally cohesive (within the adhesive layer) orinterfacial.

Joints produced from polymeric adherends tend to provide better interfacial strength, and failure may occur cohesively in the adhesive or adherend. For adherends based on fibre reinforced polymers failure may occur between the fibreand the polymer matrix rather than the adhesive/adherend interface when the corresponding joints are subject to environmental attack.

For metallic adherends, water may cause corrosion of the metal itself or react with the oxide layer. Metallic corrosion may occur particularly in joints exposed to corrosive environments such as salt water, or when two differentmetals are in contact with adhesives.

In conclusion

Long term durability of adhesively bonded structures is a principal concern in many applications including aerospace, offshore, transport and electronics. Fundamental knowledge regarding chemical processes which take place at theadhesive or adhesive/adherend interface, and experimental results indicating how chemistry affects mechanical properties, are critical for design, formulation and selection of materials.

Accelerated ageing tests on adhesively bonded joints, based on conditions encountered during service, can provide an indication of durability in real service environments. The validity of accelerated tests depends on whetherextrapolation is possible to service conditions.

Design of accelerated ageing tests should always involve an understanding of the physics and chemistry of the processes occurring at the joint. The nearer to actual conditions an accelerated test can approach, the more reliable itis as a predictor of performance of the joint during service. Assessment of effects of different ageing environments on durability of a number of adhesively bonded polymeric joints, and evaluation of mechanisms of environmental attackare the basis of one of the core research projects currently being carried out at TWI.

References