In:
Rubber Chemistry and Technology, Rubber Division, ACS, Vol. 21, No. 1 ( 1948-03-01), p. 94-111
Kurzfassung:
The low temperature stiffening of elastomers frequently limits their usefulness. A new laboratory test for measuring their stiffness at low temperatures is described. Strips of the stocks to be tested are mounted around a cylindrical rack in a vertical, cylindrical insulated chamber. The temperature in the chamber is controlled by cooling the base externally with dry ice and by a moderate regulated flow of precooled air through dry ice in the bottom of the chamber. This system gives stable temperatures which are easily controlled. The chamber can be rotated to attach the samples in succession, by means of projecting top grips, to a suitably mounted torsion wire. The stiffness is measured by the angle of twist of the sample when the torsion head is rotated 180°. The relative modulus for any temperature is calculated as the ratio of the modulus at this temperature to that at 25° C. Plots of angle of twist against temperature show a rather sharp break at the low temperature end of the curve. This determines a somewhat subjective “freezing point”. Curves are given to illustrate the wide variety of low temperature stiffening characteristics for elastomers. In unplasticized stocks the chemical composition of the monomers is the dominating factor for these properties for various synthetic rubbers. The stiffness of elastomers which are capable of crystallization on stretching, such as Hevea, Neoprene, and Butyl rubber, depends not only on temperature but also on time of exposure. To study these effects, the foregoing apparatus was used in a cold room. A rather long induction period occurs, during which the stiffness is essentially constant. It then increases and eventually reaches a larger constant value. Several months may be required to complete these changes. x-Ray examination of Hevea and Butyl proved that the increased stiffness on long exposure is due to crystallization. No change was observed in the stiffness of GR-S in the period of 2.5 months at − 30° C. Reduction in the speed of retraction is a critical measure of the deterioration of high elasticity at low temperatures. It gives a wide differeniation at moderately low temperatures between Butyl rubber and Hevea or GR-S, whereas a slow modulus test does not.
Materialart:
Online-Ressource
ISSN:
1943-4804
,
0035-9475
Sprache:
Englisch
Verlag:
Rubber Division, ACS
Publikationsdatum:
1948
