Rubber Vulcanization Additives: How to Build a Reliable Cure System
Every vulcanized rubber product depends on a well-designed cure system. This guide walks through the main rubber vulcanization additives, what each one does and how they work together – useful whether you are troubleshooting an existing compound or building a new one.
The four building blocks of a sulphur cure system
1. Vulcanizing agent
Sulphur forms the cross-links between polymer chains. It can be added as powder, as a sulfur masterbatch (S 80E, S 75SBR, S 75NBR) or as insoluble sulphur (IS 50E–IS 75E) where bloom must be avoided.
2. Activators
Zinc oxide, used with a fatty acid such as stearic acid, activates the accelerator and improves cure efficiency. Predispersed zinc oxide (ZNO 80E, ZNO 85E, ZNO 85NBR) improves dispersion of what is otherwise a fine, dusty powder.
3. Accelerators
Accelerators reduce cure time and temperature and shape the cross-link network. Primary accelerators (CBS, TBBS, MBTS) are commonly boosted with secondary accelerators (TMTD, ZDEC, ZDBC, DPG). See our accelerator masterbatch range.
4. Retarders
Retarders such as CTPI delay the onset of cure, giving more processing safety (scorch time) without greatly slowing the cure itself.
Sulphur donors and efficient cure systems
Conventional cure systems use relatively high sulphur with low accelerator, producing mainly polysulphidic cross-links with good flexibility and tear strength. Semi-efficient (semi-EV) and efficient (EV) systems reduce elemental sulphur and use more accelerator or a sulphur donor – such as DTDM, TMTD, TBzTD, DPTT or DPTH – to form shorter, more heat-stable cross-links. These give better heat ageing and reversion resistance.
Other additives that affect cure
- Antioxidants (MBI, MMBI, ZMMBI, NDBC) protect the vulcanizate from heat and oxidation.
- Blowing agents (azodicarbonamide grades, OBSH, TSH) must decompose in step with the cure for good cell structure in sponge rubber.
- Adhesion promoters (HMT, resorcinol) work alongside the cure to bond rubber to fabric or metal.
Why dispersion of vulcanization additives matters
Curatives are used at low levels, so poor dispersion has a big impact: under-cured areas, surface defects, inconsistent hardness and failed tests. Using predispersed, polymer-bound masterbatches for sulphur, ZnO and accelerators is one of the simplest ways to make a cure system more robust. They are also dust-free, which improves housekeeping and reduces exposure to chemical dust.
Quick troubleshooting
| Symptom | Possible cause | Things to review |
|---|---|---|
| Scorch in processing | Cure system too fast | Primary accelerator choice, retarder, mixing temperature |
| Sulphur bloom | Excess soluble sulphur | Insoluble sulphur, sulphur level, EV/semi-EV system |
| Variable cure between batches | Poor curative dispersion or weighing | Switch to predispersed curatives |
| Poor heat ageing | Highly polysulphidic network | Sulphur donor, semi-EV system, antioxidant |
Need a hand with your compound? Chemipat has made rubber additives in Salford since 1976 and can suggest grades or develop a bespoke blend. Contact us. For the basics, read What is vulcanization?
