When selecting an accelerator system for the production of a specific rubber product, it is essential to consider the following factors:

1. Anticipated compound shelf life
2. Accelerator’s solubility in rubber (high solubility to prevent bloom and enhance dispersibility)
3. Different processing stages that the rubber will undergo
4. Sufficient processing safety for scorch-free operations
5. Faster cure rate for cost-effective production
6. No reversion during oven cure
7. Vulcanization method and available maximum vulcanization temperature
8. Desired cure cycle considering the vulcanization method, temperature, and required vulcanizate properties
9. The accelerator system should demonstrate effectiveness across a broad spectrum of cure temperatures and exhibit compatibility with different types of polymers.
10. No adverse effects on other properties like bonding, aging, adhesion, etc., and non-rubber components in the product
11. No known health hazards associated with the accelerator and its decomposition products during usage
12. No adverse effects during end-use, especially for rubber articles intended for food contact or surgical use
13. Stability of the accelerator as a chemical, avoiding issues with decomposed sulfenamide accelerators
14. Easy handling and dust suppression in its physical form

Choosing an effective acceleration system poses a challenging task in compounding. The selected system must have good stability in raw stock storage and a delay period before vulcanization begins, allowing efficient mixing and processing. It should be compatible with the chosen curing method. For instance, a thick rubber-covered roll may require several hours of curing at temperatures between 200-300°F, while the rubber coating of an insulated electric wire may be cured in seconds at 388°F using saturated steam at 200 psi pressure for continuous vulcanization.

Once vulcanization initiates, it should proceed at an appropriate rate until reaching the desired level of cure. Furthermore, the accelerator must result in a vulcanizate that exhibits good adhesion to fibers or metal in the case of composite articles, appropriate crosslinks for aging or flexing, and no bloom.

Primary & Secondary Accelerators

In rubber compounding, primary accelerators and secondary accelerators are two types of accelerators used to speed up the vulcanization process. Here’s a comparison between them:

Primary Accelerators:

• Function: Primary accelerators are responsible for initiating and promoting the initial stages of vulcanization. They provide fast initial curing rates.
• Usage: Primary accelerators are typically used in higher concentrations compared to secondary accelerators.
• Effect: They contribute to the development of desired physical properties such as tensile strength, modulus, and resilience in the vulcanizate.
• Examples: Primary accelerators include chemicals like mercaptobenzothiazole (MBT), N-cyclohexyl-2-benzothiazolesulfenamide (CBS), and Tetramethylthiuram disulfide (TMTD).

Secondary Accelerators:

• Function: Secondary accelerators work in conjunction with primary accelerators to further enhance the rate of vulcanization and improve the physical properties of the rubber compound.
• Usage: Secondary accelerators are used in lower concentrations compared to primary accelerators.
• Effect: They aid in optimizing the vulcanization process, improve the overall cure rate, and provide additional property enhancements such as heat resistance, aging resistance, and compression set resistance.
• Examples: Secondary accelerators include chemicals like dithiocarbamates (ZDC, ZDBC, ZDEC), thiurams, and guanidines (DPG).