Published: Jan 1954
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To complement the growing interest in and demand for a more exact definition of the thermal stability for electrical insulation, a new approach to this problem is presented for elastomeric and plastomeric types of insulating materials. Plasticized polyvinyl chloride insulating compounds have been selected as a plastomeric electrical insulation for demonstrating this new scheme for assigning a thermal stability index to a given formulation. Inasmuch as there are several facets of thermal stability, it should be made dear that this procedure is based on measurement of degradation by a mechanical test method to give a quantitative value for embrittlement. Such an index of thermal stability of a plasticized polyvinyl chloride (PVC) compound is based first on a new test method and second on interpretation of test data derived therefrom. High polymer physicists have developed a theory (1) with the aid of spring and dashpot models to explain the unique mechanical behavior of elastomers and plastomers under mechanical stress. By this analogy a logical explanation has been derived to explain why PVC does not follow Hooke's law (2). Flexibility of a metallic material is easily expressed by its modulus of elasticity, since a stress causes an instantaneous and proportionate strain almost to the point of rupture. On the other hand, when PVC is dynamically stressed, the strain does not change proportionately up to the breaking point. This makes it impossible to assign a single value of modulus of elasticity to such plastic materials that has significance over the full elastoplastic range. Likewise, to assign a value for flexibility to PVC is most difficult, due to the nonlinear relationship between the dynamic stress and strain, as in a tension test. For these reasons it is difficult to obtain a responsive index of flexibility of plastic materials. Accordingly, this problem has been investigated to find a reliable and sensitive test for flexibility of plastics. The Durometer hardness test has been found through practice to be totally unsatisfactory. During the course of this study, it was found that the deflection of a PVC compound under a constant stress approaches a constant value. Elongation of PVC in the form of standard dumbbell specimens having a fixed unit loading is plotted against time under constant stress as shown in Fig. 1. Inspection of these curves shows that after a 5-min loading period has elapsed the elongation reaches a nearly constant value. This elongation after 5 min is taken as an index of flexibility and is defined as the static elongation of the material. Unit loading based on the original cross-sectional area may be set at any fixed value to suit the particular compound under test. It is usually desirable to employ a loading corresponding to approximately one third the ultimate tensile strength of the vinyl compound being tested. In the case of most flexible PVC compounds, a unit loading of 1000 psi has been found most useful. Higher unit loadings would be objectionable since excessive cold flow would interfere with the precision in measuring elastic deformation as shown in Fig. 1. Slope of the horizontal section of these elongation versus time curves represents an index of plastic or cold flow, while the vertal segment of this same curve is mostly the result of elastic deformation. The rapid change in slope of the elongation versus time curve shows a fairly well defined separation between these two deformation properties, namely, elastic and plastic elongation. It is not possible to completely isolate these two types of deformation; but from point of view of this test, a reliable and sensitive index of flexibility is obtained. This static elongation is basically a measure of the elastic deformation of flexible materials by a technique that can separately dileneate most of the plastic flow. Inspection of elongation of vinyl compound A in Fig. 1 will show that at 1000 psi there is more plastic flow than at a lower unit loading of 500 psi for the same compound. To clarify the significance of this static elongation test more fully, it should be pointed out that the elongation of compound A at 1000 psi is almost exactly twice the elongation at 500 psi. As demonstrated by compound A, which is typical of many plasticized PVC materials, a generalization may be made that the elongation is directly proportional to the unit loading for any given compound. Different compounds may have different elongations at the same unit loading as can be seen by comparing the curves for compounds A and B at 1000 psi in Fig. 1. Obviously compounds A and B have a greatly different degree of flexibility. Soft or very flexible PVC compounds will have a high elongation after a 5-min stress period, while harder materials will have a lower elongation. Since a 1 C variation in room temperature will cause a 4 per cent absolute change in elongation at 1000 psi, room temperature must be carefully controlled during this test. For the sake of brevity this elongation after 5 min will henceforth be referred to as “static elongation.” It is interesting to note that other work stimulated by the need for a sensitive measure of flexibility of plastics has resulted in a measure of stiffness based on a constant strain (3). Although this constant strain method appears to give data similar to the constant stress technique described here, the strain method requires considerably more elaborate apparatus. For the static elongation test with constant stress only two clamps and a set of weights are needed.
Bartlett, R. C.
Natvar Corp., Woodbridge, N. J.