Full definition
Vulcanization accelerators are chemical compounds that are incorporated into rubber formulations to facilitate and enhance the vulcanization process, which transforms raw rubber into a more durable and elastic material. Typically added at concentrations ranging from 0.5 to 2.5 parts per hundred rubber (phr), these accelerators significantly reduce the time required for sulfur vulcanization, enabling cures that traditionally took hours to be completed in just minutes. This acceleration allows for increased production efficiency and lower energy costs in industrial applications.
The major families of vulcanization accelerators include thiazoles, sulfenamides, thiurams, and dithiocarbamates, each with unique characteristics and applications. Thiazoles, such as MBT and MBTS, are considered general-purpose accelerators that provide moderate speed of vulcanization. On the other hand, sulfenamides like CBS and TBBS offer delayed action with excellent scorch safety, making them the most widely used class in industrial rubber applications. Thiurams, including TMTD and TETD, are known for their ultra-fast curing properties and are often used as secondary accelerators or sulfur donors, particularly in efficient or semi-efficient cure systems. Dithiocarbamates, such as ZDEC and ZDBC, are recognized for their low-temperature curing capabilities, making them ideal for latex products.
The selection of a vulcanization accelerator is crucial as it directly impacts the scorch time, which refers to the safe processing time before the rubber begins to cure, the overall cure rate, and the final properties of the vulcanizate. To optimize both processing and performance, it is common practice to employ binary and ternary combinations of accelerators. These combinations allow manufacturers to tailor the curing characteristics to meet specific application requirements, ensuring the final rubber product achieves desired mechanical properties, thermal stability, and resistance to environmental factors. The use of accelerators is always paired with an activator system that typically includes zinc oxide and stearic acid, along with sulfur, to achieve the desired vulcanization effects as classified per ASTM D4818.