Heat-Sensitized Gelation
Safety. Ammonium nitrate sensitizer is added just before use. Avoid entraining air. Heat plus a closed mould means burn and scald risk. No copper or brass in latex contact.
Executive summary
Heat-sensitized compounds gel when a heated mould triggers a sensitizer package. Industrial examples include ammonium nitrate, zinc-ammine, and polyvinyl methyl ether. Use this for closed-cavity hollow shells.
Slush moulding: fill the mould, dwell, pour out excess. Thickness follows compound stability and geometry. Rotational moulding: meter compound, multi-axis rotation while heating. More uniform walls.
A handbook Kaysam cycle cites about 4 minutes at 82 °C to gel, then cool, leach, supported dry, and warm-air cure. Dry completely before vulcanizing. Moisture causes blisters.
Read this after Former dip, mould cast, and flat spread routes you to a closed metal mould. Coagulant on a former is a different trigger: Coagulant dipping.
Heat-gel vs coagulant-gel
| Mechanism | Trigger | Typical hardware |
|---|---|---|
| Coagulant | Coacervant on a former | Dip tank, leach |
| Heat-sensitized | Mould temperature plus sensitizer in the compound | Closed metal mould, rotation |
Plaster moulds gel by water absorption plus calcium diffusion, not a heat sensitizer. Keep plaster cards and Kaysam rotation cards apart.
Kaysam industrial cycle
Fill a closed mould. Biaxial rotation. Heat to gel (about 4 minutes at 82 °C is often cited). Cool the mould. Demold the wet gel. Leach in running water. Dry supported at elevated temperature. Cure in warm air.
Sensitizer (20% ammonium nitrate solution) is added just before use.
Slush on metal
Heat-sensitized natural rubber for slush moulding in metal moulds preheated 90–100 °C. Gel 2–5 minutes depending on deposit. Handbook formulations use zinc-ammine (ammonium acetate) or PVME sensitizer packages. Industrial examples, not craft recipes.
Rotational casting
Metered compound. Rotation while heating. Magnesium moulds are preferred over aluminum for alkaline corrosion resistance in balloon-class processes.
Decision cues
text
Closed hollow cavity + metal mould? → heat-sensitized slush or rotational
Need wall uniformity? → prefer rotational (if equipment exists)
Plaster one-off? → absorption gel branch (forming article), not metal slush
Have supported dry + leach? → if no, do not scale gel time aloneDeep dives (optional)
Deep dive: Kaysam full timing
Handbook cycle: biaxial rotation; about 4 minutes at 82 °C to gel; cool 2–5 minutes at 15 °C; leach 15–27 °C; dry 82 °C at least 3 hours, supported; cure 104 °C for 20–60 minutes (thickness-dependent). Sensitizer mixed just before use.
Deep dive: Slush formulation classes
Industrial NR slush formulations use zinc-ammine (ammonium acetate) or PVME sensitizer packages in metal moulds preheated 90–100 °C. Nonionic ethoxylates stabilize zinc oxide / ammonia systems at ambient in the handbook’s section.