Comprehensive lists of the ISO and ASTM standards for the accelerated ageing of plastics, rubbers, composites and adhesives are given in Appendices 2 and 3 in reference [7] – see also Appendix 1 and Test Methods and Standards Module. This section provides an overview of the standard test methods available for conducting accelerated testing of adhesives in hostile environments.
ISO 175 [8] and ASTM D 543 [9] cover the absorption of chemical liquids into polymers and ISO 62 [10] and ASTM D 570 [11] the absorption of water. In each of these tests standards, samples are weighed and then immersed at constant temperature in the test liquid. After an agreed time period the samples are removed from the medium and surface liquid wiped off using a dry cloth before weighing. Samples can then be returned to the medium for continuing exposure provided that the time out of the medium is minimised. The balance used should have a resolution of 0.1 mg or better, which can be relaxed to 1 mg if water absorption is greater than 1 %. The increase in mass divided by the initial mass, measured at regular time intervals, is plotted against time in order to define the absorption curve. Results can be reported either as the mass gain after a particular period of immersion or as the mass at saturation defined as the weight gain from 3 successive measurements that differ in value by less than 1% of the overall weight gain.
ISO 2578 [12] gives a standard procedure for the thermal stability of polymers using the Arrhenius expression. The standard enables the user to define the highest temperature to which a plastic can be subjected for a chosen length of time before a particular property will have degraded unacceptably. The property of the polymer and the level at which it is considered unacceptable will depend on the particular application. The significance of this standard is that it is the only one to specify the Arrhenius method or any other procedure for making long-term prediction from multi-point polymer ageing data.
ISO 176 [13] and ASTM D 1203 [14] cover the stability of plasticisers in polymers. Both standards include methods for measuring the amount of plasticisers that is lost due to absorption on activated charcoal. These test methods are both essentially simple quality control tests.
The constant tensile deformation test is a relatively new test that is currently being developed as an ISO standard as ISO DIS 22088 Part 5 [15]. The test method involves applying a constant deformation to the specimen and monitoring the stress relaxation that occurs while it is immersed in the chemical environment. The test is repeated using progressively smaller levels of deformation until the stress relaxation curves of consecutive tests superimpose on one another as shown in Figure 7. The applied stress required to produce this level of deformation is defined as the critical stress. The environmental stress corrosion resistance (ESC) of the material is determined by comparing the critical stress obtained in the environment to that obtained in air. Although the test has been developed for thermoplastics, it could be applied to thermosets (including rigid adhesives). The mode of loading does not have to be restricted to tension (i.e. flexure, shear and compression) – see also creep rupture.
Figure 7: Stress relaxation curves obtained using progressively smaller levels of deformation (1> 5) until consecutive curves superimpose on one another (4 and 5) [14].
Note: S0 is the initial stress and S is the stress at time t.
