The cold resistance of rubber, that is, the ability to maintain elasticity and function properly at specified low temperatures. At low temperature, due to the rapid slowing down of the relaxation process of vulcanized rubber, the hardness, modulus and intramolecular friction increase, and the elasticity decreases significantly, resulting in a decrease in the working ability of rubber products, especially under dynamic conditions. The cold resistance of vulcanizates mainly depends on two basic properties of polymers: glass transition and crystallization. Both cause the rubber to lose its elasticity at low temperatures.

For non-crystalline (amorphous) rubber, as the temperature decreases, the mobility of the rubber molecular segments weakens. After reaching the glass transition temperature (Tg), the molecular segments are frozen and cannot perform internal rotational motion, and the rubber hardens and changes. It is brittle, in a glass-like state, and has lost the high elasticity unique to rubber. Therefore, the cold resistance of amorphous rubber can be characterized by the glass transition temperature (Tg). In fact, even within a certain range above the glass transition temperature, the rubber undergoes a glass transition process that causes the rubber to lose its elastomeric character. The upper limit of this range is called the brittle temperature (Tb), that is, the vulcanizate is only useful above the brittle temperature. Therefore, the brittleness temperature is often used as an indicator of the cold resistance of rubber products in industry, but the brittleness temperature cannot reflect the cold resistance of crystalline rubber.

Because crystalline rubbers generally lose their elasticity at low temperatures much higher than the glass transition temperature, the minimum service temperature limit for these rubbers may sometimes even be 70 to 80°C higher than the glass transition temperature. The crystallization process of rubber is different from vitrification. The crystallization process requires a certain time. When other conditions are the same, the speed and degree of elasticity loss are related to the continuous temperature and time. For example, at the temperature with the maximum crystallization rate, polybutadiene rubber starts to lose elasticity after only 10-15 minutes, while natural rubber starts to lose elasticity after 120-180 minutes. The reduction in the ability of crystalline rubber to work at low temperatures can range from a few hours to several months. Therefore, the evaluation of the cold resistance of crystalline rubber cannot be based only on the short-term test of the sample at low temperature, and the development of the crystallization process during storage and use must be considered.

For example, the tensile cold resistance coefficient of methyl phenyl vinyl silicone rubber (MPVQ) is 1.0 after being placed at -75 ℃ for 5 minutes, but after 30 to 120 minutes, it is reduced to zero. The final result of crystalline rubber crystallization is the same as that of vitrification, the hardness, elastic modulus, and rigidity increase, the contact force during elasticity and deformation decreases, and the volume decreases. For example, at -50°C, the elastic modulus of crystalline polybutadiene rubber is 19 to 29 times higher than that of the same amorphous rubber. The hardness of crystalline vulcanizates can be as high as 90-100 (Shore A). Deformation accelerates the crystallization process and increases the temperature at which the elasticity decreases.