Which accelerator for which rubber?

Basic rule: unsaturation

Sulfur vulcanization proceeds through the double bonds on the rubber chain. The more double bonds, the easier the cure.

NR, SBR and BR have high unsaturation and cure easily with medium-speed systems. EPDM and IIR (butyl) contain few double bonds and need faster, stronger accelerator systems.

NR, SBR, BR: sulfenamide base

In general-purpose compounds the primary accelerator is mostly a sulfenamide (CBS or TBBS). Processing safety is high and thick parts fill the mold easily.

If more speed is needed, a small amount of thiuram (TMTD, TBzTD) or guanidine (DPG) is added as a secondary accelerator. In silica-filled tire treads, DPG also has the job of balancing the silica surface.

EPDM: multi-accelerator systems

Because of the low unsaturation of EPDM, a single accelerator is often not enough. Thiazole, thiuram and dithiocarbamate are used together; the classic example is the MBT + TMTD + ZDBC blend.

One reason for using many accelerators is to prevent blooming: each one stays below its solubility limit. In products with nitrosamine restrictions, ZBEC, TBzTD and dithiophosphates (ZDTP) are preferred.

NBR: sulfenamide or sulfurless systems

Sulfenamide + thiuram systems are common in NBR. If heat resistance is the priority, switch to EV systems with low sulfur or sulfur donors (TMTD, DTDM, CLD).

The solubility of sulfur in NBR is low; sulfur should be added to the compound early and dispersed well.

IIR (butyl): fast systems or resin

Butyl rubber contains very little unsaturation. Thiurams and dithiocarbamates (TMTD, ZDBC, TDEC, copper dithiocarbamate) are used together.

Where very high heat resistance is required (tire curing bladder), the cure is done with a brominated alkylphenol resin instead of sulfur.

CR (chloroprene) and ECO

Chloroprene is vulcanized not with sulfur but with metal oxides (MgO + ZnO); thioureas are used as accelerators. ETU was the standard for a long time; because of its SVHC status, MTT, DETU and DBTU are the alternatives.

MBTS acts as a scorch retarder in chloroprene and is added in small amounts for processing safety.

Latex

Latex products cure at low temperature and in a short time. For this reason the fastest class, dithiocarbamates, is used: ZDEC, ZDBC, ZDMC. Water-soluble sodium salts (SDBC) are used in the dispersion.

In gloves, balloons and food-contact products, nitrosamine-safe ZBEC stands out.

Summary table

The table is a starting point; the exact system is determined by rheometer trials according to the compound formulation, the process and the product specification.