Low temperature testing

The effect of degradation agents on low temperature behaviour must be relevant in many applications but is virtually never specifically measured. There are low temperature tests for flexible materials (ISO 458 [36] and ISO 974 [37]), but generally DMTA or impact methods might be more appropriate. 

ASTM E647-95a Standard Test Method for Measurement of Fatigue Crack Growth Rates /999 Annual Book of ASTM Standard Volume 3.01 Metals-Mechanical Testing Elevated and Low-Temperature Tests Metallography, American Society for Testing and Materials, 1999. 

High and Low Temperature Tests for Small Arms Ammunition. The purpose of these tests is to determine the effect of high (as high as +165°F) or low temperature (as low as -70°F) storage upon the ballistic performance of ammunition, or components thereof... 

The disadvantage of carrying out a stability test at low temperature is of course the very low rate of the reaction, and hence the very long time required to complete the experiment. Low temperature tests are applied in special cases only, and methods employing elevated temperatures are in general use. 

It is unfortunate that test methods for soft plastics and for rubbers, although very similar, are not identical, for example differences in tensile stress strain, tear and hardness methods. If they were aligned, much of debate about which method to use would be eliminated. For some properties, there is a distinct difference in approach. For example, glass transition temperature is frequently determined for plastics whilst various low temperature tests have been specifically developed for rubbers. 

ASTM also has a standard, D8329, covering conditioning for low temperature testing for which there is no international equivalent. This... 

The effect of mechanical conditioning on low temperature testing is well illustrated by results obtained using the RAPRA dynamic low temperature tester31 where the test piece is slowly cycled in tension and the stress monitored. Figure 5.2 shows results for an SBR compound which illustrates the increasing effect of mechanical conditioning as the temperature is lowered. 

In principle, the shear modulus could be measured using test pieces strained in torsion and in engineering practice components, such as torsion discs and bushes, do operate in this mode. However, it is not common practice to test rubber in this manner except as a low temperature test (see Chapter 15) when a strip test piece is twisted by means of a torsion wire. The instrument traditionally used is not really accurate enough for precise measurement of modulus at room temperature but it would seem reasonable to suppose that an accurate instrument could be devised. 

The term thermal properties is open to more than one interpretation. Specific heat, thermal conductivity and diffusivity clearly come under this heading but the term can be taken to also include heat ageing, low temperature tests and fire resistance. However, these are more properly dealt with, as in this volume, under Effect of Temperature. Thermal analysis is a group of techniques in which a property of a sample is monitored against temperature, or time at a temperature, and, therefore, is also generally concerned with measuring the effect of temperature. Nevertheless, for convenience, a brief overview of thermal analysis is given here. 

Measuring Tg by mechanical methods is usually done with the dynamic methods, as discussed in Chapter 9, but estimates could be made from the standardised low temperature tests discussed in the next section. Indeed, these tests have been the most widely used to study the low temperature behaviour of rubbers although the transition temperature is not specifically derived. Occasionally, electrical methods have also been used. 

One approach using deformation in tension is worthy of note. When the deformation at low temperatures is applied repeatedly the apparent modulus becomes lower until an equilibrium level is reached. Eagles and Fletcher20 described a dynamic low temperature test in which the test piece is continuously cycled in tension and the force monitored so that both initial and equilibrium moduli can be calculated. Furthermore, tests could be made at different applied strains. This method could undoubtedly provide more comprehensive precise data but, despite claims of better reproducibility, it was not adopted as a standard method, principally because it involved rather... 

As the previous sections have shown, there are a large number of low temperature tests in existence. Even when ad hoc bending tests are disregarded, together with the use of the normal range of physical tests, such as tensile modulus and resilience, and the automation of a mechanical test by thermal analysis, there remain several types of specially developed low temperature tests. The various tests do not all have equal relevance to a given product. A test, or tests, should wherever possible, be chosen to provide the information most relevant to the particular application, but for many quality control purposes a test is used simply as a general indication of low temperature behaviour. Whatever the relative merits of the different methods in any situation, the question of correlation between the methods is frequently asked. 

In principle, any of the low temperature tests can be used to study crystallisation effects by conditioning the test pieces at the low temperature for much longer times than is usual. In fact, most of the standard methods include a clause to the effect that the method can be used in this way. In the temperature retraction test, it is suggested that the greater degrees of applied elongation are used when the effects of crystallisation are to be considered, because crystallisation is more rapid in the strained state. 

It would appear that the common standard low temperature tests are not thought totally suitable for measuring effects of crystallisation because a hardness tests has been standardised for this purpose, even although hardness tests are not so commonly used for measuring the immediate effect of low temperature. The international method ISO 338733 and the British method BS 903 Part A6334 are the same and are applicable to unvulcanised as well as vulcanised rubber. This is probably one reason why the hardness test has been introduced because the other methods would not be satisfactory... 

STBs are conducted under three conditions, each requiring three separate test runs. The first condition is a low-temperature test for the purpose of demonstrating that the DRE of two POHCs meets or exceeds 99.9999 percent when they are processed at lower temperatures. 

The second condition is a high-temperature test (HTT) using the same POHCs designated for the low-temperature test, plus ethylene glycol and 21 metal oxides to determine metal emissions. The third condition is an HTT using the pollution abatement system filtration system (PFS). Both HTT conditions use the same surrogates and additives, but additional metal oxides are fed in this third condition test to determine the metal removal efficiency of the PFS. 

Liquid carbon dioxide has multiple applications as a rapid, controllable refrigerant. It is used in one case as an expendable refrigerant for low-temperature testing of aviation, missiles, and electronic components. Carbon dioxide is also used in controlling chemical reactions and for stimulation of oil and gas wells. It is used extensively in food chilling and freezing applications, both in processing and in transportation. 

First, the role of rubber modification in high rate impact is to suppress spallation by inducing the material to yield in the presence of dynamic tensile stresses arising from impact. Second, this yield-spall transition occurs at different strain rates for different rubber contents and may be predictable using quasistatic, low temperature tests of this type. These tests can also provide information concerning the basic nature of the yield process in these materials through the activation parameters which are obtained. Third, the Bauwens-Crowet equation seems to be a good model for the rate and temperature sensitive behavior of the American Cyanamid materials and is therefore a likely candidate for a yield criterion to use in the analytical code work on these materials which we hope to perform as a continuation of this work. 

Stanley HE, Teixeira J. Interpretation of the unusual behaviour of H20 and D20 at low temperatures test of a percolation model. J. Chem. Phys. 1980 73 3404-3422. 

Stanley, H.E. and Teixeira, J. Interpretation of the unusual behavior of H2O and D2O at low temperatures test of a percolation model, /. Chem. Phys., 73, 3404,1980. Truskett, T.M., Debenedetti, P.G., Sastry, S., and Torquato, S. A single-bond approach to orientation-dependent interactions and its implications for liquid water,... 

As has already been stated, the verified possibility of extending the reduced variables principles to ABS resins makes it possible to treat these typical heterophase systems as blends of amorphous homophase polymers and plasticizers. One possible explanation is that over the experimental y range it is not possible to separate the contributions of the two different phases, and the materials will behave as homophase polymer. In fact, long-time molten polymer rheology experiments measure viscoelastic processes over the entire molecule, and, as a consequence, molecular compatibility is evaluated (13). On the other hand, high frequency and/or low temperature tests involve the main chain as well as the side chains of the polymer system the segmental miscibility of the polymer-polymer system is then evaluated. It is important in experimental measurements of polymer compatibility to evaluate the actual size of the volume subject to the test. 

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