To make old vulcanised rubber workable again, grind it to crumb of 1-2 mm, heat the crumb to 180-200 °C for 4-8 hours with a reclaiming agent such as pine tar or rosin oil, then pass the softened crumb through a two-roll mill. The result is soft, workable rubber that can be re-compounded and re-vulcanised, but it carries only 50-70% of the original strength. This is partial devulcanisation: it softens the rubber rather than restoring it.
Part of Rubber Processing
Breaking down the crosslinks in scrap rubber enough to make it workable again.
The Heat Treatment That Cures Rubber Is Also The One That Breaks It Down
The chemistry Heat Treatment article warns that above 170 °C rubber degrades and turns sticky, and calls that a failure of a cure. Reclaiming turns the same degradation into a tool: at 180-200 °C, with a reclaiming agent, the crosslinked network breaks down enough that the crumb softens and flows on a mill. The reclaiming range sits just above the point where that article says degradation begins; the corpus does not say that this is the mechanism, only that the carrier gives 180-200 °C.
The carrier’s three steps:
| Step | What | Figure |
|---|---|---|
| Mechanical | Grind to fine crumb with a toothed mill | 1-2 mm particles |
| Thermal | Heat crumb with a reclaiming agent (pine tar or rosin oil) | 180-200 °C, 4-8 hours |
| Mill | Pass the softened crumb through a two-roll mill | Result: soft, workable rubber |
What You Need
- Scrap rubber sorted and stripped; see Reclaimed Rubber.
- A toothed mill or heavy grater to make crumb.
- Pine tar or rosin oil: see Pine Tar.
- A heated vessel that holds 180-200 °C for hours with a lid and a way to measure the temperature. The chemistry heat-treatment article notes that vegetable oil begins to smoke at 200-230 °C, so a bath is at its limit here; hot sand or a covered oven is safer than oil.
- A two-roll mill with its stop; see Compound Mixing.
Method: Devulcanising A Batch
Step 1: Grind to crumb of 1-2 mm.
Step 2: Mix the crumb with the reclaiming agent. The carrier gives no ratio; the corpus has no figure for the amount of pine tar or rosin oil, so treat the proportion as a trial variable and record each batch.
Step 3: Heat to 180-200 °C for 4-8 hours in a covered vessel, stirring occasionally. Test a sample from the pot for softness.
Step 4: Cool to a safe handling temperature.
Step 5: Mill the softened crumb until it forms a continuous sheet.
Step 6: Test a cured strip by the methods in Quality Testing.
Worked Example
Strength lost. The carrier says reclaimed rubber keeps 50-70% of the strength of virgin rubber, so the share lost is 100 − 70 = 30% to 100 − 50 = 50%. That is why the carrier restricts reclaim to floor mats, vibration mounts and low-pressure gaskets. Time at temperature is 4-8 hours at 180-200 °C, a long fuel and attention commitment for one pot; the corpus gives no fuel figure for it.
Common Mistakes
- Treating it as full devulcanisation. The rubber is softened, not made new.
- Too hot or too long. Burnt crumb is useless.
- Skipping the mill. Crumb that is not milled will not bond.
Safety
Hot tar, hot rubber fumes and a long unattended fire
A pot of rubber crumb and pine tar at 180-200 °C is a fire and fume hazard: hot rubber gives off irritating, sulfur-smelling fumes (recalled, not from the corpus), pine tar and rosin oil are flammable and vegetable oil smokes at 200-230 °C. Work outdoors or under a flue, never leave the pot unattended, keep water out of it, keep a lid and sand to hand and never carry the pot by hand. The mill that follows has a nip: see the stop rules in Compound Mixing.
What’s Next
- Learn next: Blending with Fresh Rubber — using the reclaim
- Combine with: Sulfur Vulcanization — re-curing