During the vulcanization process, rubber tensile strength, elongation at break, elasticity and other properties change with the extension of vulcanization time. Usually, the curve of a certain property of rubber with the vulcanization time is used to characterize the vulcanization history and the change law of rubber properties. This is the vulcanization history (Figure), as shown in Figure 1-3. The first half of the figure is the Mooney scorch curve, and the second half is the tensile strength curve. The two parts of the curve constitute a complete vulcanization history. The vulcanization process of rubber can be divided into four stages: scorching stage, hot vulcanization stage, flat vulcanization stage and over-vulcanization stage.
- The burning stage (ab stage) refers to the stage before the rubber is officially vulcanized, from the stage when the rubber is put into the mold to the light vulcanization, including the operation scorch time and the remaining scorch time. Operation scorch time A1 refers to the scorch time consumed during rubber processing due to the effect of heat accumulation. It depends on the degree of processing, such as the number of re-smelting times, the degree of heat refining, and the process conditions of calendering and extruding. The remaining scorch time A2 refers to the time for the rubber to maintain fluidity when heated in the mold. There is no obvious boundary between the operation scorch time and the remaining scorch time. The more processing times of the rubber compound and the longer the operation time, the rubber filling time will be reduced. Therefore, the general rubber compound should be repeated as much as possible. Mechanical processing.
- The thermal vulcanization stage (bc stage) is the stage where the rubber material undergoes crosslinking reaction. The tensile strength of the rubber rises sharply. The slope of the bc curve represents the speed of the vulcanization reaction. The greater the slope, the faster the vulcanization reaction rate. The higher the production efficiency. The hot vulcanization time depends on the vulcanization temperature and formula of the compound.
- The flat vulcanization stage (the cd stage) is the stage where the rearrangement and thermal cracking of cross-linked bonds occur. Due to the dynamic balance of cross-linking and thermal cracking reactions, the tensile strength curve of the rubber compound has a flat zone at this stage, and the vulcanizate maintains the best performance. Therefore, this stage becomes the basis for determining the positive vulcanization time range of the rubber compound in the process. The length of the vulcanization flat time depends on the rubber formula, such as the types and dosage of raw rubber, vulcanizing agents, accelerators and antioxidants.
- The over-vulcanization stage (the part after d) is the stage where the thermal cracking reaction of cross-linking bonds and chain segments occurs. Due to the thermal cracking reaction of cross-linking bonds and chain segments, the tensile properties of the rubber compound are significantly reduced.