Environmental Conditioning and Testing
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Temperature
Moisture (Water)
Moisture Conditioning
Fickian Diffusion
Moisture Control
Coefficient of Moisture Expansion
Accelerated Conditioning
Other Degradation Agents
Thermal Stability Test Methods
References
In order to determine the effectiveness of different adhesive systems, processing variables and surface treatments, it may be necessary to expose adhesively bonded joints to various environmental and loading conditions that simulate actual service conditions. The resistance of the bonded structure to degradation agents often becomes apparent within a short period. In some circumstances only a few hours of exposure may lead to catastrophic failure or seriously compromise the structural integrity of the joint.
The two predominant factors in climatic exposure are humidity and temperature. The severity of these two factors will depend on geographical location, and need to be taken into account in assessing allowable strengths. A list of possible service environments are listed below:
Static heat ageing or sub-zero exposure
Humidity (inc. hot/wet) exposures
Complete immersion in water at ambient and elevated temperatures
Freeze/thaw and dry/wet cyclic conditions
Continuous or intermittent saltwater immersion or spray
Combined load (i.e. stress) and environmental exposures
Solvents (inc. paint strippers)
Acid and alkali solutions
Diesel and engine oils.
Engineering structures are exposed to various combinations of these environments in service, often resulting in complex synergistic degradation of the material. It is recommended that when comparing the effects of surface treatments, adhesive systems and processing variables on the durability of joints in hostile environments, that all specimens are prepared and conditioned together to account for any variability in the conditioning environment. Also, control specimens are recommended to check improvements in joint performance.
Important factors when selecting a test method are:
Availability of a standard test method;
Ability of the test method to provide consistent and reliable engineering or quality assurance data for a range of materials and service conditions within a relatively short timescale; and
Ability of the test method to provide an improved understanding of the influence of environmental degradation mechanisms and design parameters.
High Temperatures: Prolonged, or even short term, exposure to elevated temperatures will result in a reduction in the short-term properties and possibly irreversible chemical and physical changes within an adhesive. It is recommended when characterising mechanical properties (i.e. stiffness and strength) at elevated temperatures that testing be conducted in a temperature-controlled chamber. It is important that the load cell is thermally isolated from the chamber and that any sensors within the test environment are able to operate at the test temperature. It may be necessary to use thermal compensation to ensure accurate sensor measurements. It is common practice for elevated temperature to allow a soak period of 10 minutes at the test temperature prior to testing. The purpose of “heat-soaking” is to eliminate distortion due to non-uniform temperature distributions. Care needs to be taken when testing moisture pre-conditioned specimens to prevent drying of the specimen during the test (see Moisture Control).
Sub-Zero Temperatures: The basic principles of elevated temperature testing also apply to low temperatures. Additional time, however, may be required to reach the test temperature. Mechanical refrigeration and liquid nitrogen based cooling systems are commercially available with the former tending to be restricted to a minimum operating temperature of approximately -50°C. Liquid nitrogen is capable of changing temperature many times faster than a mechanical refrigerated cooling system and can achieve temperatures around –150°C or better. Even though a mechanically refrigerated system may be capable of achieving a temperature of -50°C, it will take considerably longer to reach those temperatures. All moving parts should be coated in molybdenum grease to prevent stiction. It is recommended that dry nitrogen gas be circulated through the test chamber to prevent moisture condensation and ice formation on the bonded specimen and test apparatus. It is recommended that the test apparatus to be constructed from stainless steel to avoid corrosion products forming on the apparatus surface.
Moisture degradation is probably the major cause of in-service failure in bonded structures. The ubiquitous nature of water combined with the ability to penetrate into the adhesive structure poses considerable problems. With good design it is possible to significantly reduce the rate of moisture diffusing to the adhesive/adherend interfaces. This problem is further exacerbated at elevated temperatures and mechanical stress. Hot and humid environments can often cause rapid loss of strength in metal-epoxy joints within a short duration (i.e. 2 years) with catastrophic consequences. Failure invariably occurs at the adhesive/adherend interface.
The rapid loss of strength for short immersion times in water (shown in Figure 1) is typical for adhesive joints with poor interfacial bonding. Figure 1 shows that there are two competing mechanisms: (i) interfacial degradation (associated with continuous reduction in joint strength); and (ii) plasticisation (softening) of the adhesive leading to a reduction in stress concentrations at the ends of the bond (associated with an “apparent” increase in joint strength with exposure time as observed at elevated immersion temperatures).
Figure 1: Strength retention of moisture conditioned mild steel/epoxy single-lap joints
The major cause of strength loss in adhesively bonded metal joints is associated with interfacial degradation through water-substrate interaction rather than weakening of the adhesive. Water can degrade the strength of adhesive joints through hydration of metal oxide layers. Corrosion products, such as rust, at the interface are considered a post-failure phenomenon. The presence of water in epoxy adhesives results in plasticisation (essentially softening) of the polymeric material. At temperatures below the glass transition temperature
Tg
, polymer property reduction is reversible upon dehydration, whereas above
Tg
, resin properties are permanently degraded. Surface treatments, such as phosphoric acid anodisation and organosilane primer coatings will bestow joints with improved moisture resistance. Figure 2 shows the change in
Tg
with moisture content for an epoxy adhesive as measured using dynamic mechanical thermal analysis (DMTA).
Figure 2: Glass-transition temperature of an epoxy as a function of moisture content
Although it may be possible to relate strength and stiffness reduction of bulk adhesives (Figure 3) with changes in
Tg
, it is generally difficult (if not impossible) to relate joint strength to
Tg
due to interfacial effects, which will often predominate.
Figure 3: Stress-strain curves for different water immersion periods
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