Rubber compounding means adding to 100 parts of raw rubber about 2-3 parts of sulfur as the cross-linker, 5 parts of zinc oxide and 1-2 parts of stearic acid as activators, 30-50 parts of carbon black as reinforcing filler and a small amount of accelerator, then curing at 140-160 °C. The sulfur sets how many cross-links form, which sets stiffness and elasticity, and the filler sets strength and wear. More sulfur or filler gives a harder, less stretchy rubber, so every change should be tested on a small trial strip.
Part of Polymer Engineering
Rubber compounding is the design of a rubber recipe: the choice and amount of cross-linker, activators, fillers and accelerators that turn raw rubber into a vulcanised product with chosen properties.
Cross-Link Density Sets Stiffness And Stretch
Raw rubber is a mass of loose, tangled chains that flows slowly and goes sticky when warm. Vulcanisation ties chains together at points along their length, as covered in Sulfur Vulcanization. Each link stops chains from sliding past each other permanently, so rubber snaps back after stretching. A few links give a soft, stretchy rubber. Many links give a stiff one, and at 25-35% sulfur the corpus gives hard ebonite, used for electrical insulation. Vulcanization puts soft rubber at 2-3% and the general case at 5%.
So the first compounding decision is a target stiffness, not an amount of sulfur out of the air.
The Basic Recipe
Compounding Recipe gives it in parts per hundred of rubber (phr):
| Ingredient | Parts per 100 | Job |
|---|---|---|
| Sulfur | 2-3 | Cross-linker |
| Zinc oxide | 5 | Activator |
| Stearic acid | 1-2 | Activator |
| Carbon black | 30-50 | Reinforcing filler |
| Accelerator (MBT or TMTD) | 0.5-1 | Speeds the cure, if available |
Worked example, scaling: for a 3.5 kg rubber batch, divide by 100 and multiply. Sulfur at 2.5 parts: 3,500 × 2.5 / 100 = 87.5 g. Zinc oxide at 5 parts: 3,500 × 5 / 100 = 175 g. Carbon black at 40 parts: 3,500 × 40 / 100 = 1,400 g. The small items are tiny in proportion, so weigh them to a gram.
Fillers Do More Than Fill
Carbon black particles bind to the rubber chains and make the compound tougher and more wear-resistant, which is why tyres are black. The same article says it can be made crudely by burning oil, natural gas or resinous wood in a restricted air supply and collecting the soot; see also Carbon Pigments. A cheaper filler such as chalk or clay mostly adds bulk and lowers cost with little reinforcement. Optimising filler means finding the point where strength stops rising and mixing gets difficult, and the right amount is found by trial. Too much makes the compound stiff, hard to process and brittle.
Mixing And Curing Follow From The Recipe
Mixing is done in stages: the two-roll mill is described in Mixing. Cure time depends on temperature: Curing Temperature and Time gives 2-4 hours at 140 °C, 1-2 hours at 150 °C and 30-60 minutes at 160 °C for sulfur-only compounds, and warns not to extrapolate below the table. Salvaged rubber can be blended in: Blending with Fresh Rubber gives 20-50% reclaim, and Reclaimed Rubber puts it at 50-70% of the original strength.
Trial Strips Before Production
Cure a strip of every new recipe and test it: stretch it to several times its length and see whether it snaps back, check it stays firm in boiling water, and note any stickiness. Overcured rubber cannot be repaired.
Fine powders burn, and mills catch hands
Sulfur and carbon black are fine powders that can ignite in a dust cloud, so weigh and add gently and keep flame out. Zinc oxide and accelerators irritate lungs and skin. Open-roll mills pull in hands faster than anyone can react, so they need a reachable stop. See Mixing.
Common Mistakes
- Adding more sulfur to cure faster — it just makes a stiffer rubber. Raise temperature or use accelerator instead.
- Changing two things at once — you cannot tell which did what.
- Skipping the trial strip — a bad batch is not recoverable.
What’s Next
- Learn next: Surface Treatments — a very different way of changing a material’s behaviour
- Combine with: Curing Temperature and Time — the temperature half of the recipe