1 6 Schwartz & Kemp, PJiyskal Properties of Rubber. 



obtained. This shows a marked decrease in length of the 

 specimen up to a temperature of about 45 °C , after which 

 the rubber began to expand, reaching its original length 

 at a temperature of 58'C. Above the latter temperature 

 the expansion became more and more rapid as the 

 temperature was raised. 



Now rubber is more extensible at lower than at higher 

 temperatures, and consequently the increase in length is 

 less when the temperature is raised. The deformation 

 after the stress is removed is also less. Thus when the 

 specimen ' B ' was heated, the rubber at first attempted 

 to regain its original shape, and a contraction resulted. 

 After a time, however, the natural expansion begins, as is 

 indicated by the upward turn of the curve in Fig. 7 

 at 45 X. 



Experiments were also conducted on spread rubber 

 tape which had been allowed to rest in an unstressed 

 condition for about three years. The specimens were cut 

 to a uniform width of )^' , the average thickness being 22 

 mils. During this period of rest, the rubber had become 

 opaque and hard, a condition which can be destroyed by 

 warming gently, the rubber apparently returning to its 

 original plastic and translucent condition. 



On the first application of heat to a specimen of this 

 rested rubber tape, an expansion was noted, as shown in 

 Fig. 8, the expansion curve bending upwards, indicating a 

 gradual increase in the linear co-efficient of expansion. 

 This continued until a temperature of about 33°C. was 

 attained. At this juncture a sudden increase in the rate 

 of expansion took place, the curve again approximating 

 to a straight line, but with a greatly increased slope. On 

 cooling again the specimen rapidly contracted for a time, 

 after which its length became approximately constant, as 



