Reading a cure curve: ML, MH, ts2 and t90 in plain language

A moving-die or rotorless rheometer takes a small sample of uncured compound, heats it to the cure temperature, and oscillates the die at a small angle while measuring torque against time. The resulting curve is a picture of the cure, and it is the quickest way to see whether a batch behaves like the last one. The standard methods are ASTM D5289 and ISO 6502-3.

The curve in four steps

1. Start. The torque starts at a level that depends on how stiff the uncured rubber is.
2. Minimum torque (ML). The compound softens as it heats, so torque dips to a minimum. ML reflects the viscosity of the heated compound. A rise in ML between batches suggests higher polymer viscosity, more filler or poor mixing.
3. Cure. Crosslinks form and torque rises. The slope is the cure rate.
4. Plateau or maximum torque (MH). Torque levels off as cure completes. MH is a rough measure of crosslink density. A fall in MH suggests under-cure or a shortage of curative. A rise suggests extra curative or filler.

The time points

• ts2 (scorch time). Time for torque to rise 2 units above ML. It tells you how long the compound can sit at temperature before it starts to cure, which matters for injection moulding and mould filling. A short ts2 means less processing time.
• t50 and t90. Times to 50% and 90% of the full torque rise. t90 is often used to set the minimum press cure time for the part, adjusted for part thickness and mould heat transfer.
• Cure rate index. Sometimes quoted as 100 divided by t90 minus ts2.

Two shapes to know

• Reversion. Torque falls after the maximum. It can mean the network is being damaged at that temperature and time. This is common in some sulfur systems and is a warning in others.
• Marching modulus. Torque keeps rising slowly and never reaches a clean plateau. It suggests continuing cure, so the cure time should be set from a fixed criterion.

Using the numbers

• Release limits. Set minimum and maximum limits for ML, MH, ts2 and t90 based on measured batch data for that compound. The Pascalor batch passport shows a lot's values against these limits.
• Compare like with like. Test temperature, arc, frequency and test duration change the results. Always state them.
• Trends matter more than single values. A small drift in MH over six batches can signal an ingredient lot change before any physical test fails.
• Rheometer data does not replace physical tests. It indicates cure state, but compression set and tensile properties still need to be measured.

Sources and further reading

• ASTM D5289 and ISO 6502-3 for rotorless curemeters; ASTM D1646 for Mooney viscosity and scorch.