Crosslinking agents are rubber blend additives that facilitate the formation of chemical links between molecular chains to form a three-dimensional network of connected molecules during the vulcanization process.
a. Sulfur
Sulfur, whether soluble or insoluble, is the most common crosslinking agent for rubber, and can be used to crosslink all rubber types with double bonds. Sulfur produces bonds that create vulcanizates with high flexibility and excellent mechanical properties. The thermal stability of sulfur bonds are more sensitive and the vulcanizates’ heat aging resistance is thus affected negatively.
b. Peroxides
For many technical applications in which mechanical properties are not a primary concern, peroxides are often used for crosslinking. Peroxides create thermostable carbon-carbon bonds that create cross links between the polymer chains. The bonds created, in comparison to ones created using sulfur, have enhanced heat stability and lower compression set at high temperatures. In addition, using peroxide as a crosslinking agent does not result in discoloration of, or from, the material and it is free of nitrosamine-creating chemicals. There are many different peroxides, each of which has its own activation temperature. Some are suitable for use in applications in the foodstuff industry.
c. Other crosslinking agents
This group comprises special crosslinking agents for use with e.g. FKM, ACM, AEM, and ECO.
It includes diamine derivatives and bisphenols for the crosslinking of FKM and ACM/AEM.
Reactive halogenated/non-halogenated octylphenol-formaldehyde resins, which include methylol groups, can be used for crosslinking when vulcanizates with low compression set and high heat resistance are desired. The major application area for reactive phenolic resins is the manufacturing of curing bladders for use by compression molding of car tires.
