What does an accelerator do?
Sulphur alone gives a vulcanisation lasting hours. Accelerators, together with zinc oxide and stearic acid, form active sulphurating complexes that cut this to minutes and set the density and type of crosslinks (mono-, di-, polysulphidic). Scorch time (t2) indicates processing safety, optimum cure time (t90) production speed.
Sulphenamides: CBS and DCBS
Delayed action + fast cure = sulphenamide. Vulkacit CZ (CBS) is the standard primary accelerator for tyres and technical goods; Vulkacit DZ (DCBS) starts later and gives the longest scorch safety, indispensable in steel-cord skim compounds. Neither forms nitrosamines.
Thiazoles: MBTS, MBT, ZMBT
Vulkacit DM (MBTS) is the medium-speed, economical and flexible general-purpose accelerator; Vulkacit Merkapto (MBT) is faster but riskier on scorch; Vulkacit ZM (ZMBT) serves latex and EPDM systems. Thiazoles are usually combined with sulphenamides or with thiurams/guanidines.
Thiurams: TMTD
Rhenocure TMTD is an ultra-fast secondary accelerator (0.1–0.5 phr) and a sulphur donor (1–3 phr). It is the basis of efficient vulcanisation (EV) systems in EPDM profiles and heat-resistant parts; it shortens scorch and can form nitrosamines.
Practical selection table
Tyre/technical general: CBS 1–1.5 phr + S 1.5–2.5 phr. Steel-cord skim: DCBS 0.7–1 phr. Thick section/injection: CBS + Vulkalent G 0.2–0.3 phr. EPDM profile: MBTS 1.5 + TMTD 0.8 + (ZDBC). Latex: ZMBT + ZDEC. Heat resistance (EV): TMTD 2–3 phr, sulphur 0–0.5 phr.
What to do about scorch?
First check mixing temperature and the point at which the accelerator is added; then add Vulkalent G (CTP); if that is not enough switch to a more delayed accelerator (CBS→DCBS). Reducing thiuram dosage is often the most effective step.