The question that confuses more rubber compounders than sulfur itself is which accelerator to use. Accelerators determine cure speed, the scorch-safe processing window, and final crosslink density — yet their names (CBS, TBBS, ZDEC, TRA...) look like alphabet soup. In practice the whole family reduces to a handful of mechanisms ranked by speed, and every choice is a trade-off between scorch safety (how long the compound stays processable at mixing and extrusion temperature) and cure speed (how fast it crosslinks at mold temperature). This guide maps the main classes to their jobs.
Read an accelerator by its family, not its brand
All organic accelerators work by activating sulfur and forming reactive polysulfide complexes with zinc oxide and stearic acid. What differs is the induction behaviour: slow, medium or ultra-fast. The practical ranking from slowest to fastest is roughly: guanidines < thiazoles < sulfenamides < thiurams / dithiocarbamates. A slower family is not "weaker" — it is safer in processing and often gives a flatter, more reversion-resistant cure.
Thiazoles — MBT and ZMBT: the general-purpose workhorses
Accelerator MBT (M) (CAS 149-30-4) is the oldest synthetic accelerator and still the reference mid-speed thiazole. It works across NR, SBR, NBR and reclaim compounds, both as a primary accelerator (typically 0.5–2 phr with 2–2.5 phr sulfur) and as a secondary booster with sulfenamides to lift crosslink density and modulus. Its zinc salt, Accelerator ZMBT (CAS 155-04-4), is slower and less scorch-sensitive, does not bloom to the surface, and is the classic choice for latex compounds where zinc solubility matters. If you keep only one general-purpose accelerator on the shelf, it is usually MBT.
Sulfenamides — CBS, TBBS and DCBS: the industrial standard
Sulfenamides are delayed-action semi-ultra accelerators: during the induction period the compound stays safely processable, then crosslinks rapidly once the amine splits off at cure temperature. This scorch safety is why sulfenamides dominate tyre and industrial rubber goods. All three in this group are N-substituted derivatives of MBT, and the substituent is the whole difference.
- Accelerator CBS (CZ) (CAS 95-33-0) — the cyclohexyl type, the value benchmark and default sulfenamide worldwide. Balanced induction and cure rate; typical use 0.5–2.5 phr.
- Accelerator TBBS (NS) (CAS 95-31-8) — the tert-butyl type. Slightly faster than CBS and, more importantly, it decomposes to a primary amine (tert-butylamine), avoiding the secondary-amine nitrosamine concerns that regulators (EN 71-12, REACH Annex XVII) attach to morpholine- and cyclohexyl-based accelerators. TBBS is the preferred sulfenamide for food-contact and children's-product rubber.
- Accelerator DCBS (DZ) (CAS 4979-32-2) — the dicyclohexyl type with the longest induction period of the sulfenamide family. Choose it for thick articles, high processing temperatures and compounds that must survive long flow paths before curing. Typical use 0.5–2 phr.
Dithiocarbamates — ZDEC, ZDBC, PZ and TDEC: ultra-accelerators
The dithiocarbamate zinc salts are the fastest practical accelerators, curing well at low temperature or in seconds at mold temperature. They are used as primaries in latex and self-curing systems and as secondaries (0.3–1 phr) to speed up sulfenamide or thiazole systems in EPDM and high-temperature molding.
- Accelerator ZDEC (EZ) (CAS 14324-55-1) — diethyl zinc dithiocarbamate, the general-purpose workhorse of the family: latex dipping, foam, and fast NR/SBR molding.
- Accelerator BZ (ZDBC) (CAS 136-23-2) — dibutyl type; low melting point, widely used to accelerate EPDM cable and sponge compounds.
- Accelerator PZ (ZDMC) (CAS 137-30-4) — dimethyl type, the fastest and most scorch-sensitive; classic for room-temperature self-curing and food-contact latex film.
- Accelerator TDEC (CAS 20941-65-5) — the tellurium salt. Exceptionally fast and heat-resistant, used where a scorchy but very rapid cure is required, such as heat-resistant EPDM and cable insulation.
Thiurams — TRA (DPTT): sulfur donors for heat-resistant cures
Accelerator TRA (DPTT) (CAS 120-54-7) is a tetrasulfide thiuram that acts as an ultra-accelerator and — uniquely — as a sulfur donor. In efficient (EV) and semi-EV systems it partially or fully replaces elemental sulfur, producing short, thermally stable monosulfidic crosslinks. The payoff is markedly better heat and reversion resistance, at the cost of a tighter scorch window. Typical use 0.5–2 phr, or 1–3 phr where it serves as the main sulfur source.
Guanidines — DPG: the booster nobody uses alone
Accelerator DPG (CAS 102-06-7) is a medium-speed guanidine that is almost never used as the sole accelerator. Its classic job is as a secondary accelerator with thiazoles or sulfenamides (0.1–1 phr): it activates the primary, raises crosslink density, modulus and hardness, and is a staple of NR truck-tyre and heavy-duty compounds as well as EPDM. It also finds use where a mild, scorch-tolerant boost is needed without moving to a faster family.
Building a cure system — quick reference
| Application | Typical system (phr) | Why |
|---|---|---|
| NR tyre tread / industrial goods | CBS 1.2–1.5 + S 2.25 + DPG 0.2 | Sulfenamide scorch safety with DPG-boosted modulus |
| Molded SBR / NR general goods | TBBS 1.0–1.5 + S 2.0 | Fast, nitrosamine-friendly, good reversion balance |
| Thick articles / high processing heat | DCBS 1.2–1.8 + S 2.25 | Longest induction, best scorch protection |
| Latex dipping & foam | ZDEC 0.5–1 + ZMBT 0.5–1 (low S) | Ultra-fast at low temperature, film-safe zinc salts |
| EPDM fast molding / cable | ZDBC 1–2 or TDEC 0.5–1.5 + thiuram | Fast cure with heat resistance |
| Heat-resistant (EV / semi-EV) | TRA 1–2 as sulfur donor, low/no elemental S | Monosulfidic links, minimal reversion |
Specs to check when sourcing accelerators
- Assay / initial melting point — each grade has a tight melting range (e.g. MBT ~180 °C, CBS ~98–102 °C, TBBS ~104–108 °C); a depressed or wide range means contamination or dilution.
- Ash content — typically ≤ 0.3–0.5%; high ash signals filler extension or metallic impurities that upset cure kinetics.
- Heat loss / moisture — ≤ 0.5%; moisture makes powders lump and can shift scorch behaviour.
- Particle size (for powder grades) — finer, well-dispersed grades reduce dusting and improve distribution; oil-treated or granulated forms are available for cleaner handling.
- Color — light, consistent color matters in white and pastel compounds; darkening indicates oxidation during storage.
Shanghai Better Chemical supplies the full accelerator range above — MBT (M), ZMBT, CBS (CZ), TBBS (NS), DCBS (DZ), ZDEC (EZ), ZDBC (BZ), PZ (ZDMC), TDEC, TRA (DPTT) and DPG — China origin, rubber grade, with TDS/COA/MSDS on request. Tell us your polymer, curing temperature and scorch requirement, and we will recommend the accelerator package and current pricing.