Scorch is the premature onset of vulcanization (crosslinking) in an uncured rubber compound during processing, before the rubber is intentionally cured.

If scorch starts too early, the compound stiffens, loses flow, and can become unprocessable.

What’s happening chemically

Rubber compounds contain a vulcanization system. Scorch occurs when these components begin forming crosslinks at processing temperatures (mixing, extrusion, calendering) instead of in the mold or press.

Once crosslinks start forming:

• Viscosity rises rapidly

• Flow drops

• The process window collapses

This is irreversible.

Where scorch shows up in practice

Typical processing steps where scorch is a risk:

• Internal mixing (especially final stages)

• Extrusion

• Calendering

• Preheating before molding

• Long residence times in hot equipment

Classic symptoms:

• Sudden viscosity rise

• Poor surface finish

• Die swell changes

• Extruder torque spikes

• Burnt or lumpy compound

You can measure through a Mooney scorch test or Rheometer test

Mooney scorch test

Common notation: MS(1+4) 125 °C

• Measures viscosity increase over time

• Key values:

  • t₅ → time to 5 Mooney unit rise

  • t₃₅ → time to 35 MU rise

• Shorter time → worse scorch safety

Rheometer tests (MDR / RPA)

• Measures torque vs time during heating

• Scorch time often defined as:

  • ts₂ or ts₅ (time to 2 or 5 dN·m torque increase)

• Also gives full cure behavior (rate, plateau torque)

Factors that influence scorch

Formulation

• Accelerator type

  • Fast (e.g., TMTD) → poor scorch safety

  • Delayed-action (e.g., CBS, TBBS) → better safety

• Sulfur level

• Accelerator/sulfur ratio

• Presence of retarders (e.g., CTP/PVI)

• Fillers and surface chemistry

• Plasticizers and resins

Processing

• Temperature

• Shear

• Residence time

• Batch-to-batch variability

Controlling scorch

• Use delayed-action accelerators

• Add scorch retarders

• Optimize mixing sequence

• Lower processing temperatures

• Reduce shear and residence time

• Improve cooling between steps

It’s always a balance:

Too much scorch safety → slow cure, long cycle times
Too little scorch safety → processing disaster