Vulcanization is a highly intricate process involving the activation of sulfur rings (S8) and the use of accelerators to facilitate the formation of sulfur intermediates. These intermediates play a crucial role in the crosslinking of sulfur with double bonds in elastomers. Without accelerators, vulcanization of elastomers with sulfur alone would take several hours, rendering it commercially impractical. However, by incorporating accelerators into the sulfur curing system, the optimal curing time can be reduced to as little as 2-5 minutes.

The accelerated sulfur curing method is widely employed in various industrial applications due to its ability to yield improved physical properties. This technique offers a significantly faster crosslinking rate and allows for delayed actions required for processing, shaping, and forming the rubber before the formation of the vulcanized network. Depending on the sulfur content and the ratio of accelerator to sulfur, sulfur vulcanization systems are classified into three categories: conventional, semi-efficient (semi-EV), and efficient (EV).

Conventional Vulcanization 

The term “conventional vulcanization” is applied to natural rubber, polyisoprene, and butadiene-based synthetic rubber compounds when relatively higher doses of sulfur (above 1.5 phr) and lower doses of accelerators (0.4 to 1.2 phr) are utilized. In this system, sulfur combines predominantly with the rubber to form “polysulfidic crosslinks.” The amount of free sulfur and the ratio of free sulfur to accelerator vary depending on the specific type of rubber.

The vulcanizate resulting from conventional vulcanization contains a significant proportion of wasted sulfides and exhibits modifications in the main polymer chain. In the case of SBR and BR vulcanizates, poly and disulfidic crosslinks are predominantly formed. This conventional vulcanization system may exhibit drawbacks such as poor reversion resistance, susceptibility to oxidative heat, and limited long-term flex resistance.

However, the vulcanizates produced through conventional vulcanization display favorable tensile and tear strength, exhibit good fatigue resistance, and demonstrate resilience in low-temperature conditions.

• It contains high proportions of sulphur (2.0-3.5 phr).
• low proportions of accelerator (0.4-1.2 phr)
• accelerator to sulphur ratio is 0.1-0.6
• sulphur to accelerator ratio > 1.0
• (low accelerator to sulphur ratio = high sulphur to accelerator ratio)

Efficient Vulcanization

The term “efficient vulcanization” (EV) is employed for natural rubber, polyisoprene, and butadiene-based synthetic rubber compounds when very low doses of sulfur (below 0.4-0.8 phr) and higher doses of accelerators (2 to 5 phr) are utilized. EV systems may utilize a sulfur donor instead of elemental sulfur or a combination of a low concentration of elemental sulfur (less than 0.5 phr) and a high concentration of accelerators. Sulfur donors capable of donating one or two sulfur atoms are used in these systems.

The EV system optimizes the utilization of sulfur for crosslinking and results in a network with thermally stable monosulfidic and disulfidic crosslinks, minimizing chain modifications. However, the short sulfur crosslinks in this system lead to inferior tensile and tear strength, poor flex-fatigue life, and reduced abrasion resistance. On the other hand, EV cure systems offer excellent heat aging resistance and compression set resistance. These cure systems are commonly employed for rubber products with thick cross-sections and for products subjected to static applications.

• Contains very little sulphur (0.4-0.8 phr)
• High proportion of accelerator (2-5 phr)
• Accelerator to sulphur ratio is 2.5-12
• Efficient use of sulphur
• Mainly monosulphidic bonds (75% mono & 25% disulphidic)

Semi-Efficient Vulcanization

This designation refers to rubber compounds made from natural rubber, polyisoprene, and synthetic rubber derived from butadiene. In Semi-EV cure systems, the concentrations of sulfur and accelerators fall between those used in conventional vulcanizing systems and the EV system. The aim of Semi-EV cure systems is to strike a balance between CV and EV cures. These systems have gained popularity in the field of natural rubber (NR), where finding a compromise between heat aging and fatigue life is frequently required. Below are examples of typical Semi-EV cure systems used for different polymers.

• Sulphur levels are intermediate between conventional system and EV system.
• Used for  a compromise in cost and/or performance.
• Particular application in NR where a compromise between heat ageing and fatigue life is sought after.