Accelerator Masterbatch vs Powder Accelerators: A Guide for Rubber Compounders
Accelerators are used in small amounts but have an outsized effect on how a rubber compound cures. That makes how you add them just as important as which ones you choose. In this guide we look at why many compounders switch from loose powder accelerators to an accelerator masterbatch, and how to make the change.
What does a rubber accelerator do?
Sulphur on its own vulcanizes rubber very slowly and at high temperature. Accelerators speed the reaction, lower the cure temperature and help build a more efficient cross-link network, improving tensile strength, compression set and ageing. Most compounds use a primary accelerator (a sulfenamide such as CBS or TBBS, or a thiazole such as MBTS) with a smaller dose of secondary accelerator (a thiuram, dithiocarbamate or guanidine) to fine-tune cure speed.
The problem with powder accelerators
- Dispersion: fine powders can agglomerate. Undispersed specks create local over- or under-cure.
- Dust: airborne accelerator dust is a health and housekeeping issue, and can contaminate other compounds in the mill room.
- Weighing accuracy: at dosages below 1 phr, small weighing errors become large percentage errors.
- Flow and storage: powders can cake, bridge in hoppers and carry static.
How an accelerator masterbatch helps
In a masterbatch, the accelerator is already dispersed in a polymer binder at a fixed active content – typically 70–80%. The result is a dust-free form that wets into the rubber quickly. Compounders typically see more consistent rheometer curves, cleaner working areas and simpler weighing. It is the same principle behind all polymer-bound masterbatches.
Which accelerator masterbatch should you choose?
| If you need… | Consider |
|---|---|
| Good scorch safety, delayed action | CBS 80E/80NR, TBBS 80E/80NR |
| A general-purpose thiazole cure | MBTS 80E, MBTS 75E, MBT 80E |
| A fast secondary accelerator | TMTD 80E, TMTM 80E, ZDEC 75E, ZDBC 80E |
| A thiuram alternative where nitrosamines are a concern | TBzTD 70E, ZBEC 80E |
| Activation of a thiazole system | DPG 75E |
The binder should suit your base polymer: E (EPDM), NR, SBR, NBR or CPE.
Converting a formulation
To keep the same amount of active accelerator, divide your powder dosage by the active fraction:
Masterbatch phr = powder phr ÷ (active % ÷ 100)
Example: 1.2 phr of powder CBS becomes 1.2 ÷ 0.8 = 1.5 phr of CBS 80E. The binder adds a small amount of polymer, which is rarely significant but worth noting in tight specifications. Always confirm cure on a rheometer after switching.
Need help?
Chemipat has manufactured predispersed rubber chemicals in Salford since 1976. We can recommend a grade, supply samples or develop a bespoke accelerator blend. Contact our team.
