Rubber cures faster as it gets hotter, and in the carrier’s table for sulfur-only compounds the cure time halves for every 10 °C: 2-4 hours at 140 °C, 1-2 hours at 150 °C, 30-60 minutes at 160 °C. That pattern cannot be trusted below the tabulated range, because it predicts 8-16 hours at 120 °C while the corpus elsewhere says the cure takes weeks below 120 °C. Thick parts also need extra time for heat to reach the centre, so cure time is set by temperature, thickness and a trial strip, never by the table alone.
Part of Rubber Processing
The temperature-time relationship for vulcanising rubber, and where the corpus’s figures can and cannot be extended.
The Cure Is A Race Between A Reaction That Speeds Up And A Rubber That Degrades
The carrier gives a three-row table of cure time against temperature.
| Temperature | With accelerator | Sulfur only |
|---|---|---|
| 140 °C | 15-30 minutes | 2-4 hours |
| 150 °C | 10-20 minutes | 1-2 hours |
| 160 °C | 5-15 minutes | 30-60 minutes |
The carrier also says undercured rubber is soft and sticky and overcured rubber becomes brittle. The corpus’s chemistry Heat Treatment article describes the other failure: above 170 °C the rubber degrades and turns sticky, and below 120 °C the cure takes weeks. The window is therefore about 120-170 °C, and the tabulated rows sit in the middle of it.
What You Need
- A thermometer or proxy that tells 140 °C from 160 °C. The corpus’s heat-treatment article offers vegetable oil beginning to smoke as a sign of excess heat and gives melting points for tin (232 °C) and lead (327 °C), which bracket nothing in the cure window.
- A timer, started when the centre of the part reaches temperature, not when it goes into the bath.
- Test strips cured alongside, and the tests in Quality Testing.
Method: Turning The Table Into A Cure
Step 1: Read the pattern in the sulfur-only column. 140 to 150 °C halves 2-4 hours to 1-2 hours (2 ÷ 2 = 1, 4 ÷ 2 = 2). 150 to 160 °C halves 1-2 hours to 30-60 minutes. A factor of 2 per 10 °C holds exactly in that column.
Step 2: Read the accelerated column. 140 to 150 °C goes 15-30 to 10-20 minutes, a factor of 15 ÷ 10 = 1.5 on both ends. 150 to 160 °C goes 10-20 to 5-15, a factor of 10 ÷ 5 = 2 at the low end and 20 ÷ 15 ≈ 1.3 at the high end. The accelerated factor therefore ranges from 1.3 to 2 per 10 °C. The corpus states neither factor; these are read off the table.
Step 3: Add heat-penetration time for thick parts. The cure clock only starts when the centre is at temperature. A rough heat-up time for a slab heated from both faces is the half-thickness squared divided by the thermal diffusivity. Taking diffusivity of rubber as about 0.1 mm²/s (recalled, not from the corpus):
- 10 mm slab, half-thickness 5 mm: 5² ÷ 0.1 = 250 seconds, about 4 minutes per time constant, so reaching near the bath temperature takes several of them, on the order of 15-20 minutes.
- 25 mm slab, half-thickness 12.5 mm: 12.5² ÷ 0.1 = 1,563 seconds, about 26 minutes per time constant.
The Heat Treatment article’s guideline of 15-20 minutes for 1-2 mm up to 75-120 minutes for over 10 mm at 140 °C shows the same trend: thicker parts take longer, and an undercured soft core inside a cured shell is the failure.
Step 4: Do not extrapolate below the table.
- Sulfur-only, halving per 10 °C backwards from 140 °C: 130 °C is 4-8 hours, 120 °C is 8-16 hours, 100 °C is 32-64 hours (2 × 2^4 = 32 and 4 × 2^4 = 64).
- Accelerated, with the 1.3-2 factor per 10 °C over four steps (1.3^4 ≈ 2.9 and 2^4 = 16): from 15-30 minutes at 140 °C, the range at 100 °C runs from about 15 × 2.9 = 43 minutes to 30 × 16 = 480 minutes, which is 8 hours.
Compare with the corpus. Vulcanization gives steam curing at 100 °C for 4-8 hours, which fits the top of the accelerated extrapolation. Rubber Products boils a wrapped tube for 2-4 hours. The Heat Treatment article says weeks below 120 °C, which fits neither extrapolation, since 120 °C comes out at 8-16 hours. The corpus does not resolve this. Until it does, treat everything outside 140-160 °C as an untested estimate and cure a trial strip.
Worked Example
Plan a 5 mm gasket at 150 °C with no accelerator. Table: 1-2 hours at temperature. Add heat-up: half-thickness 2.5 mm, 2.5² ÷ 0.1 = 62.5 seconds per time constant, so about 5 minutes to approach bath temperature. Total about 1 hour 5 minutes to 2 hours 5 minutes in the bath. Run a strip of the same thickness to confirm.
Common Mistakes
- Timing from when the mould goes in. The centre lags the bath.
- Raising the temperature to speed up a thick part. Above 170 °C the rubber degrades.
- Extrapolating the table. The corpus’s own pages disagree about low temperatures.
- Using the thumbnail test on a hot sample. Cool it first.
Safety
Hot oil, steam and a sealed pot
A 140-160 °C oil bath burns, and vegetable oil begins to smoke at 200-230 °C according to the Heat Treatment article, so the margin to smoking is only 40-70 °C at 160 °C (200 − 160 = 40 and 230 − 160 = 70). Never leave the bath unattended and keep water out of it. Steam curing in a sealed pot is a pressure vessel: the Vulcanization article reports 120-134 °C from a sealed metal pot, which needs a safety valve and a vessel built to hold the pressure.
What’s Next
- Learn next: Compression Molding — curing under pressure in a mould
- Combine with: Compounding Recipe — the accelerator choice that sets the column to use