MBT in Rubber Compounding: Applications, Dosage and Co-accelerators

MBT (2-mercaptobenzothiazole, CAS 149-30-4) is the fast, low-cost thiazole accelerator – but it is not the right choice for every compound. Its short scorch time makes it excellent for thin, fast-curing articles and continuous processes, and awkward in thick or complex parts. This guide covers where MBT fits, how much to use, and how to manage its scorch behaviour with co-accelerators and retarders.

1. What MBT does in a compound

MBT is a semi-ultra thiazole accelerator. On its own it cures quickly with a short scorch time. That makes it ideal when the part is thin, when cure time must be short, or when the compound is cured continuously – and less suitable for thick sections that need flow time before the onset of cure.

Property	MBT behaviour
Cure speed	Fast
Scorch time	Short – the main limitation
Cost	Lowest of the thiazole family
Typical role	Primary or co-accelerator; also a feedstock for MBTS, CBS, TBBS

2. Where MBT is used

Application	Why MBT suits it
Footwear compounds	Fast cure for thin soles and sheets; economical
Hoses and technical rubber goods	Fast cure in continuous and semi-continuous lines
Belting and sheeting	Short cure cycles; co-accelerated for balance
Latex goods	Works in aqueous systems with a co-accelerator
Reclaim and low-cost compounds	Cheapest thiazole; effective with a co-accelerator
Continuous vulcanisation (LCM, salt bath)	Fast onset suits short residence times

For thick, slow-curing parts – and for most tyre components – the delayed-action sulfenamides (CBS, TBBS) are preferred because they give the compound time to be shaped before cure starts.

3. Dosage

Role	Typical dosage (starting point)
Primary accelerator	About 0.8 – 1.5 phr
Co-accelerator	About 0.3 – 0.8 phr
Sulfur (conventional system)	About 2.0 – 3.0 phr
Activators	Zinc oxide about 3 – 5 phr with stearic acid about 1 – 2 phr

These are starting points for compound development. The final balance of sulfur, MBT, co-accelerator and retarder is set on the cure rheometer and by scorch testing.

4. Co-accelerators that go with MBT

Partner	Effect
Guanidine (DPG, DOTG)	Raises activity and crosslink density; classic general-purpose pairing
Thiuram (TMTD)	Speeds the cure further; also a sulfur donor in EV systems
MBTS	Blends MBT’s speed with better scorch safety
Sulfenamide (CBS, TBBS)	Adds delayed action where processing safety is needed

5. Managing MBT’s scorch

MBT’s weakness is scorch safety. Three practical levers:

Partial substitution. Replace part of the MBT with MBTS (slower, safer) while keeping the cure rate.
Retarders. Add a prevulcanisation inhibitor (for example CTP/PVI, cyclohexylthiophthalimide) at about 0.1-0.3 phr to extend scorch time.
Process control. Add the accelerator late in the mix, keep the dump temperature down (typically below about 120 deg C for the accelerator stage), and cool the compound quickly.

6. Two example systems (starting points)

Component (phr)	NR footwear sole	SBR general-purpose hose
Rubber	100	100
Zinc oxide	4	4
Stearic acid	1.5	1.5
Sulfur	2.5	2.0
MBT	1.0	0.8
DPG	0.4	–
MBTS	–	0.5

Illustrative only – the filler, oil, antioxidant and processing system are omitted, and the accelerators must be balanced on the actual recipe and cure conditions.

7. Mixing and handling

Add accelerators late. Add MBT after the filler and oil stages, at a controlled temperature.
Dust. MBT is a fine powder and a skin sensitizer – use granules where possible, with local exhaust ventilation.
Storage. Sealed, cool and dry, away from oxidising agents; use within the shelf life.

Frequently Asked Questions

How much MBT should I use?

About 0.8-1.5 phr as a primary accelerator, or 0.3-0.8 phr as a co-accelerator, with sulfur and activators per the formulation. Set the final balance by cure rheometry.

Why does my MBT compound scorch?

MBT has a short scorch time. Replace part of it with MBTS, add a retarder (CTP/PVI), and control the mixing temperature and addition order.

Can MBT be used in tyres?

Mostly not in the main tyre components – those use delayed-action sulfenamides (CBS, TBBS). MBT and MBTS are used in some technical rubber goods and continuous-cure processes.

What co-accelerator works with MBT?

Guanidines (DPG, DOTG) and thiurams (TMTD) are common partners; MBTS is used to improve scorch safety.