Значение и использование

3.1 Low temperature testing of rubber can yield repeatable results only if the preconditioning of the samples is consistent. Properties such as brittleness and modulus are greatly affected by variations in time/temperature exposures. This practice is intended to provide uniform conditioning for the various low temperature tests conducted on rubbers.

Область применения

1.1 This practice covers the characteristic mechanical behavior of rubbers at low temperatures, and outlines the conditioning procedure necessary for testing at these temperatures.

1.2 One of the first stages in establishing a satisfactory technique for low temperature testing is the specification of the time and temperature of exposure of the test specimen. It has been demonstrated that any one or more of the following distinct changes, which are detailed in Table 1, may take place on lowering the test temperature:

TABLE 1 Differentiation Between Crystallization and Glass Transition

Property

Crystallization

Glass Transition

Physical effects
(1, 2, 4, 6, 7)^A

Becomes stiff (hard) but not necessarily brittle

Becomes stiff and brittle

Temperature-volume relation
(1, 2, 3, 4, 5, 8)

Significant decrease in volume

No change in volume, but definite change in coefficient of thermal expansion

Latent heat effect (4, 5, 8)

Heat evolved on crystallization

Usually no heat effect, but definite change in specific heat

Rate (2, 4, 6, 7, 8)

Minutes, hours, days, or even months may be required. In general, as temperature is lowered, rate increases to a maximum and then decreases with increase in deformation. Rate also varies with composition, state of cure, and nuclei remaining from previous crystallizations, or from compounding materials such as carbon black.

Usually rapid; takes place within a definite narrow temperature range regardless of thermal history of specimen. May be limited rate effect (2)

Temperature of occurrence
(4, 5, 7, 8

Optimum temperature is specific to the polymer involved.

Very wide limits, depending on composition

Effect on molecular structure
(1, 2, 5, 6, 8)

Orientation of molecular segments; random if unstrained, approaching parrallelism under strain

Change in type of motion of segments of molecule

Materials exhibiting
properties (5, 7, 8)

Unstretched polymers including natural rubber (low sulfur vulcanizates), chloroprene, Thiokol A polysulfide rubber, butadiene copolymers with high butadiene content, most silicones, some polyurethanes. Butyl rubbers crystallize when strained. Straining increases rate of crystallization of all of the above materials.

All

^A The numbers in parentheses refer to the following references:
(1) Juve, A. E., Whitby, G. S., Davis, C. C., and Dunbrook, R. F., Synthetic Rubber , John Wiley& Sons, New York, NY, 1954, pp. 471–484.
(2) Boyer, R. F., and Spencer, R. S., Advances in Colloid Sciences, Vol II, edited by H. Mark and G. S. Whitby, Interscience Publishers, Inc., New York, NY, 1946, pp. 1–55.
(3) Boyer, R. F., and Spencer, R. S., High Polymer Physics, A Symposium , edited by Howard A. Robinson, Chemical Publishing Co., Inc., Brooklyn, NY, 1948, pp. 170–184.
(4) Wood, L. A., and Bekkedahl, Norman, High Polymer Physics, loc cit, 1948, pp. 258–293.
(5) Schmidt, A. X., and Marlies, C. A., Principles of High Polymer Theory and Practice, McGraw-Hill Book Co., New York, NY, 1948, pp. 175–193.
(6) Treloar, L. R. G., The Physics of Rubber Elasticity, Oxford University Press, London, 1949, pp. 152–191.
(7) Liska, J. W., “Low Temperature Properties of Elastomers,” Symposium on Effects of Low Temperature on the Properties of Materials, STP 78, ASTM, 1946, pp. 27–45.
(8) Turner, Alfrey, Jr., “Mechanical Behavior of High Polymers,” Vol VI of High Polymer Series, Interscience Publishers, Inc., New York, NY, 1948, pp. 80–83 and 340–374.

1.2.1 Simple temperature effects,

1.2.2 Glass transitions, and

1.2.3 First order transitions (crystallization), and solubility and other effects associated with plasticizers.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.