When the rubber is vulcanized, the curve in which its properties change with the vulcanization time is called the vulcanization curve. From the perspective of the process of vulcanization time affecting the modulus of the rubber compound, the entire vulcanization time can be divided into four stages: initial vulcanization stage, under-sulfur stage, normal vulcanization stage and overvulcanization stage.
1. Vulcanization initial stage (also known as scorch period or vulcanization induction period)
Start of vulcanization means the point at which the compound begins to harden during vulcanization and is no longer capable of thermoplastic flow. The initial stage of sulfur is the vulcanization time before this point. At this stage, crosslinking has not yet started and the compound has good fluidity in the mold. The starting speed of rubber vulcanization directly affects the scorch and operation safety of the rubber. The length of this stage depends on the type of compounding agent used, especially the accelerator. The scorch of the rubber compound with super-speed accelerator is relatively short. At this time, the rubber compound is more likely to scorch and the operation safety is poor. When using a slow-acting accelerator (such as sulfenamide) or using it together with a little thiuram accelerator, a longer scorch period and good operational safety can be obtained. However, different vulcanization methods and products have different requirements for the length of scorch time. However, in most cases, a longer scorch time is still desired to ensure the safety of the operation.
2. Under-sulfur stage (also known as pre-sulfur stage)
The stage between the start of vulcanization and normal vulcanization is called the undersulfur stage. At this stage, due to the low degree of crosslinking, most of the properties that rubber products should have are not yet obvious. Especially in the early stage of this stage, the crosslinking degree of the rubber is very low, its performance changes little, and the product has no practical significance. But in the later stage of this stage, when the product is slightly undersulfurized, although the tensile strength, elasticity, elongation, etc. of the product have not yet reached the expected level. However, its tear resistance, abrasion resistance and dynamic crack resistance are superior to those of normal vulcanized rubber. Therefore, if the latter properties are emphasized, the product can be slightly undersulfurized.
3. Normal vulcanization stage
In most cases, the product must reach an appropriate degree of cross-linking during vulcanization, and the stage of reaching an appropriate degree of cross-linking is called the positive vulcanization stage, that is, the normal sulfur stage. At this stage, the physical and mechanical properties of the vulcanized rubber do not reach the highest value at the same time, but reach or approach the optimal value respectively, and its comprehensive performance is the best. The temperature and time taken at this stage are called positive vulcanization temperature and positive vulcanization time.
The positive vulcanization time depends on the required performance of the product and the thickness of the product section. For example, for products that require good tear resistance, the vulcanization time with the highest or close to the highest tear strength should be considered as the positive vulcanization time. For products requiring high wear resistance, the vulcanization time with small wear can be considered as the positive vulcanization time. For thick products, it is necessary to take "post-vulcanization" into consideration when selecting the positive sulfur time. The so-called "post-vulcanization" means that after the sulfur of the product is taken out, due to the poor thermal conductivity of the rubber and the long heat transfer time, the cooling of the product due to heat dissipation is slow, so it can continue to be vulcanized, and it is called "post-vulcanization". ". The tensile strength and hardness caused by "post-vulcanization" further increase, the elasticity and other mechanical properties are reduced, and the service life of the product is thus impaired. Therefore, the thicker the product, the more "post-vulcanization" should be taken into account. In general, the positive vulcanization time can be determined according to the time slightly before the highest value of the tensile strength or the vulcanization time of the highest value of the strong elongation product (the product of tensile strength and elongation).
4. Over vulcanization stage
After the positive sulfur stage, continuing to vulcanize will enter the oversulfur stage. The early stage of this stage is part of the vulcanization flat stage. During the flat period, the physical and mechanical properties of the vulcanized rubber remained basically stable. After the flat period, natural rubber and butyl rubber become soft due to the phenomenon of vulcanization reversion due to more chain scission than cross-linking; synthetic rubber becomes hard and elongated due to the continued dominance of cross-linking and the increase of cyclized structure. The rate is also reduced, and the rubber properties are damaged. The length of the vulcanization flat period not only indicates the thermal stability of the rubber compound, but also has a direct impact on the safe operation of the vulcanization process and the vulcanization quality of thick products.
