Full definition
The chemical substance that creates cross-links between polymer chains during vulcanization, transforming raw rubber from a plastic, thermoplastic-like material into an elastic, thermoset material with defined mechanical properties. Primary curing agents: (1) Sulfur — the most common, for unsaturated rubbers (NR, SBR, NBR, CR); works with accelerators and activators; conventional (2-3 phr sulfur), semi-efficient (1-1.5 phr), and efficient (<0.5 phr + sulfur donor) cure systems. (2) Organic peroxides (DCP, DBPH) — for saturated rubbers (EPDM, silicone, HNBR) and when C-C cross-links are needed for superior heat resistance; no bloom or discoloration. (3) Metal oxides (ZnO for CR, MgO for CSM) — specific to halogenated rubbers. (4) Resins (phenolic resols) — for butyl rubber. (5) Platinum catalysts — for addition-cure silicone (LSR). The choice of curing agent determines the type of cross-link bonds and thus the final properties: sulfur bonds (flexible, good fatigue), peroxide bonds (rigid, better heat resistance).