The sulfenamide class accelerators include CBS, TBBS, MBS, DCBS etc. and are most popular in the tire
industry due to their delayed action as well as faster cure rate offered by them during vulcanization of
rubber compounds containing furnace blacks.
The sulfenamide accelerators are the reaction products of 2- Mercaptobenzothiazole with basic amines such as Cyclohexylamine / Tert-Butylamine / Morpholine / Dicyclohexyl amine etc.
These accelerators provide wide range of crosslink densities depending on type and dosage of accelerator
used and exhibit flat & reversion resistant cure. Progressive increase in dosage of sulfenamide accelerator
shows improvement in scorch delay, cure rate and state of cure.
Sulfenamide accelerators can be boosted by using DPG, DOTG, TMTM, TMTD type basic accelerators for
further increasing the cure rate (but at the expense of scorch safety).
Sulfenamide accelerators can be effectively retarded using small proportion of Pilgard PVI (CTP) without
affecting their rate of cure.
Vulcanizates of sulfenamide accelerators have a typical 'aminic' odour and exhibit higher stress-strain
properties along with better resilience and flex-fatigue resistance as compared to Thiazoles.
Sulfenamide accelerators decompose rapidly in the presence of steam and hence are preferred for
manufacture of open steam cured rubber products requiring faster onset of cure for better shape retention.
However, this advantage is lost in the case of hot air cured products.
Sulfenamide accelerators are usually incorporated in the rubber compound at the end of mixing cycle when
temperature is above melting point of the accelerator to ensure proper dispersion. Generation of excess heat is avoided to prevent decomposition of sulfenamide accelerator. In case sulfenamide accelerator is to be added at a later stage; addition in the form of sulfenamide rubber master batch is recommended.
Activity of sulfenamide accelerators can be summarised as follows:
1. Scorch safety Longer (CBS < TBBS < MOR < DCBS}
2. Cure Rate Faster (DCBS < MOR < CBS < TBBS)
3. Crosslink density Higher at equal dosage (DCBS < MOR < CBS < TBBS)
Sulfenamide accelerators have limited storage stability and the rate of degradation is greatly influenced by
storage conditions such as humidity & heat and hence, is used on a strictly first in-first out basis.
Sulfenamide accelerators should be stored in a cool & dry atmosphere (Below 30°C & 60% RH) and away from acids /acidic substances / fumes to prevent rapid degradation.