In rubber compounding, accelerators are chemicals added to:

• Speed up vulcanization

• Lower cure temperature and time

• Control scorch safety

• Tailor final properties (strength, heat resistance, compression set)

Accelerators are the control system of sulfur vulcanization: they decide when curing starts, how fast it happens, and what the rubber becomes. Without accelerators, sulfur curing would be slow, inefficient, and impractical.

What accelerators actually do

Sulfur alone reacts very slowly with rubber chains. Accelerators:

• React with sulfur to form active sulfurating species

• Help sulfur attach to polymer chains more efficiently

• Control crosslink density and structure (mono-, di-, polysulfidic links)

This directly affects:

• Modulus

• Tensile strength

• Heat aging

• Fatigue

• Compression set

Main classes of rubber accelerators

1. Thiazoles

Examples: MBT, MBTS
Traits:

• Medium cure speed

• Moderate scorch safety

• Good processing balance

Used in: NR, SBR, BR
Often combined with: secondary accelerators

2. Sulfenamides (most common today)

Examples: CBS, TBBS, OBS
Traits:

• Delayed action (excellent scorch safety)

• Fast cure once activated

• Broad processing window

Used in: Tires, hoses, belts, molded goods

This is the workhorse accelerator family.

3. Thiurams

Examples: TMTD, TETD
Traits:

• Very fast cure

• Poor scorch safety

• Can act as sulfur donors

Used in: Low-sulfur or sulfur-free systems, thin articles

4. Dithiocarbamates

Examples: ZDEC, ZDBC
Traits:

• Ultra-fast accelerators

• Very low scorch safety

• Low cure temperature

Used in: Latex, low-temperature cures, short cycle times

5. Guanidines

Example: DPG
Traits:

• Slow on their own

• Excellent secondary accelerators

• Improve dispersion and state of cure

Often paired with: thiazoles or sulfenamides

Primary vs secondary accelerators

Most practical compounds use accelerator systems, not single chemicals.

• Primary accelerator
  Controls main cure rate and scorch safety (e.g., CBS)

• Secondary accelerator
  Boosts cure speed or efficiency (e.g., DPG, TMTD)

This lets formulators fine-tune:

• Scorch time

• Cure rate

• Crosslink structure

Interaction with other compounding ingredients

Accelerators don’t work alone:

• Activators (ZnO, stearic acid) are essential

• Fillers can adsorb accelerators (silica especially)

• Retarders (e.g., CTP/PVI) improve scorch safety

• Oils and resins can affect cure kinetics

This is why cure behavior often changes after a formulation tweak.

But there are challenges:

• Faster cure → less scorch safety

• Better heat resistance → lower fatigue

• Short cycle time → narrower processing window

There’s no “best” accelerator—only the best fit for the job.