The reducibility of rubber refers to the phenomenon of a decrease in the performance of vulcanized rubber during normal vulcanization time or under high temperature (over 160 degrees) vulcanization conditions. (Generally refers to the sulfur sulfide system, as for the peroxide sulfide system, according to the sulfide mechanism, its reducibility should be relatively small, unless degradation occurs at high temperatures.). After the occurrence of vulcanization reduction phenomenon, the tensile strength, constant elongation stress, and dynamic fatigue performance of the vulcanized rubber decrease, and the crosslinking density decreases. From the vulcanization curve, after reaching the maximum torque, the torque gradually decreases with the prolongation of vulcanization time.

The causes of rubber vulcanization reduction can be summarized as follows:

1) The fracture and rearrangement of cross-linked bonds, especially the rearrangement of polysulfide bonds, and the resulting changes in network structure.

2) Rubber macromolecules undergo cracking at high temperatures and long-term vulcanization temperatures. In fact, those who have studied Polymer chemistry, Organic Chemistry and other Polymer chemistry courses can understand the generation of reducibility at the molecular level.

In the final analysis, vulcanization and crosslinking of rubber is also a process of chemical reaction, that is, there is a chemical reaction formula and Chemical equilibrium, as shown in the diagram below; When a certain condition (temperature, time, or both) is favorable for positive equilibrium, the reaction proceeds in the positive direction, as shown in the figure A+B to C+D; When changes in conditions (temperature or time, or both) begin to be unfavorable for positive equilibrium and favorable for negative equilibrium, the reaction will proceed in the opposite direction, as shown in the figure below from C+D to A+B.

Therefore, the optimal vulcanization time is actually the condition (temperature/time) under which the above reaction reaches a positive equilibrium, which is reflected in the vulcanization curve as T100. However, in engineering, T90 is generally chosen as the process vulcanization time for ease of operation (such as considering the influence of product thickness).

How to control and reduce sulfurization reducibility?

1) Selection of rubber type: the reducibility is related to the Degree of unsaturation of rubber. The lower the Degree of unsaturation, the less the reduction phenomenon. For example, EPDM and IIR have better resistance to reduction compared to natural, styrene butadiene, cis-1,4-polybutadiene rubber, etc

2) The impact of vulcanization system: The traditional sulfur vulcanization system (such as natural rubber, S2. 5 parts+CZ0. 6 parts vulcanization system) has the most severe reducibility, while the semi effective sulfur vulcanization system (such as natural rubber, S1. 2 parts+CZ1. 8 parts vulcanization system) has average reducibility, while the effective vulcanization system (such as natural rubber, S0. 3 parts+TMTD1. 0 parts+CZ2.0~3.0 parts vulcanization system, or using sulfur donor DTDM to replace sulfur) has basically no reduction phenomenon.

3) Other additives such as Si-69 in the semi effective vulcanization system of natural rubber can achieve good resistance to reduction; The addition of HVA-2 to the traditional sulfur vulcanization system in natural rubber also has a significant effect.