Deformation properties of polymeric composite materials during thermal aging

Abstract

The irreversible changes with time occurring in the physicomechanical of polymeric composite materials (PCM) on prolonged exposure to elevated temperatures may be due to thermal aging of the binder, reinforcing agent and interphase layer. The polymeric binder, which under the operational conditions to which the PCM are subjected, can be present therein in both a vitreous and highly elastic state, is most strongly subject to the thermal aging. At temperatures below the glass transition temperature Tg, the physical aging is typical of that of a polymer [ 1, 2], while at temperatures above Tg - extrahardening [3], formation of ordered crystalline type structures [4], and thermodegradation processes are characteristic. This applies equally to thermoreactive and thermoplastic polymers. In the present work we shall consider only the thermoreactive polymers. Physical aging - a process of a relaxational transition from a thermodynamically nonequilibrium state into an equilibrium state - was discovered in creep tests of polymers [1], and was later studied by the differential scanning calorimetry (DSC) [5-7] and dilatometry [8] methods. It should be noted that not all the methods have the same sensitivity toward the change in the structure of the polymer, and therefore the results may turn out to be uncomparable [7]. Moreover, some of the methods have an effect on the aging process itself. Thus for example, the DSC method involves heating of the samples, which in certain cases can lead to additional aging. For the same reason, i.e., to avoid additional aging during tests, the tests for short-term creep of the polymeric material of different ages are usually carried out at room temperature [1, 5, 7, 8]. When high-temperature creep has to be studied, the experimental data for a short-term creep of the PM samples of different ages are compared at a higher, but constant temperature. The effect of the influence of the physical aging on the creep of the PM is expressed in the following: the compliance diagrams of PM of different ages obtained under the same testing conditions are combined into a single diagram by simple shifting along a logarithmic time scale. The value of the shift -In aag is dependent on the time tag and temperature of the aging Tag, In accordance with the principle of analogies on a reduced time scale t', the viscoelastic compliance is determined as follows: