What Do ML, MH, ts2, and t90 Represent on the Cure Curve?

Four Key Parameters

Minimum torque (ML) is generally associated with the flowability and viscosity of the unvulcanized compound, but it is not equivalent to Mooney viscosity; maximum torque (MH) is generally associated with the stiffness of the cured compound and the degree of crosslinking.

Scorch time (e.g., ts2) is an indicator of processing safety before the compound begins to cure significantly; t90 is the reference time corresponding to approximately 90% of the torque rise.

The relative degree of cure can be approximated as α(t)≈[M(t)−ML]÷[MH−ML]. This is an engineering characterization based on rheometric torque and is not an absolute measurement of true chemical crosslink density.

Three Cure Curve Patterns

Plateau cure: after reaching a high level, the torque remains relatively stable, providing a wide processing window conducive to time tolerance in mass production.

Reversion: torque declines after peaking, indicating breakage or rearrangement of the crosslink network. This warrants particular attention for natural rubber under high-temperature or prolonged curing.

Marching cure: torque continues to rise slowly, making it difficult to define a distinct plateau; evaluation must rely on specified time points and product performance.

Limitations

A good curing plateau does not imply that longer curing is always better; the optimal cure time must ultimately be determined by product performance and actual curing conditions. For thick articles, internal temperature rise lags due to heat conduction, so the t90 of thin laboratory test specimens cannot be directly equated to the overall cure time of large articles.