A basic rubber compound is built in parts by weight per 100 parts of raw rubber: 2-3 parts sulfur as the crosslinker, 5 parts zinc oxide and 1-2 parts stearic acid as activators, 30-50 parts carbon black as reinforcing filler, and 0.5-1 part accelerator such as MBT or TMTD if you can get it. For a 1 kg batch of rubber that is only 20-30 g of sulfur and 5-10 g of accelerator, so the small ingredients must be weighed to about a gram or the batch is a guess.
Part of Rubber Processing
The ingredients of a vulcanisable rubber compound, the quantity of each, and what each does.
A Compound Is Rubber Plus A Short List Of Ingredients In Fixed Ratios
The carrier gives the basic compounding recipe by weight:
| Ingredient | Parts per 100 rubber | Purpose | Per 1 kg of rubber |
|---|---|---|---|
| Raw rubber | 100 | Base polymer | 1,000 g |
| Sulfur | 2-3 | Crosslinking agent | 20-30 g |
| Zinc oxide | 5 | Activator | 50 g |
| Stearic acid | 1-2 | Activator | 10-20 g |
| Carbon black | 30-50 | Reinforcing filler | 300-500 g |
| Accelerator | 0.5-1 | Speeds vulcanisation | 5-10 g |
The last column is simple scaling of the carrier’s parts: parts per 100 rubber times 10 gives grams per kilogram. The corpus does not state a total mass for the compound and this article does not add one.
What You Need
- A balance that reads to 1 g or better for the small items.
- Raw rubber: see Acid Coagulation or salvage via Reclaimed Rubber.
- Sulfur, finely ground, and zinc oxide and stearic acid if you have them.
- Carbon black, which the carrier says can be made crudely by burning oil, natural gas or resinous wood in a restricted air supply and collecting the soot.
- Accelerator (MBT or TMTD), if available. Without it the carrier says curing takes much longer, so raise the temperature and time.
- A mill for mixing; see Compound Mixing.
Method: Building A Batch
Step 1: Weigh every ingredient before opening the rubber.
Step 2: Check the sulfur choice against the table in Sulfur Vulcanization. The chemistry Heat Treatment article gives 5-8% sulfur by weight for gaskets and seals, which is 50-80 g per kilogram, two to three times the carrier’s 20-30 g. The two describe different recipes and are not reconciled in the corpus.
Step 3: Mix in stages on the mill. The carrier says to add sulfur, zinc oxide and fillers in stages, cutting and re-rolling until even. In general rubber practice sulfur and accelerator go in last and with the mill kept cool, so the compound does not start to cure on the mill (recalled, not from the corpus).
Step 4: Cure a test strip first, and check it; see Quality Testing.
Worked Example
Weighing error on the smallest item. A 1 kg batch needs 5-10 g of accelerator. If the balance only resolves 5 g, the error is up to 5 ÷ 5 = 100% of the low end; with 1 g resolution it is 1 ÷ 5 = 20% of 5 g and 1 ÷ 10 = 10% of 10 g. A larger batch spreads the same error over more material: for 10 kg of rubber the accelerator is 50-100 g and a 1 g error is 2% to 1%.
For the sulfur: 1 kg of rubber with the carrier’s 2-3 parts takes 20-30 g; with the Heat Treatment article’s 5-8% it takes 50-80 g. Ebonite at 25-35% takes 250-350 g.
Common Mistakes
- Mixing one article’s sulfur with another’s cure time. The recipes differ.
- Weighing small items by volume or by eye.
- Dumping everything in at once. The carrier says to add in stages, cutting and re-rolling.
- No accelerator and no change in cure. The carrier says without it curing takes much longer.
Safety
Dusts: carbon black, sulfur and accelerator
Carbon black and sulfur are fine powders that float in the air, and fine sulfur and carbon dust can burn or explode if a spark meets a cloud (recalled general fact, not from the corpus). Weigh and add them gently, away from flame, and never sweep dry. Mix in ventilated space with a respirator or cloth over the face, and wash hands before eating. Accelerator powders are chemicals that irritate skin and lungs; handle them as you would any unknown powder.
The mill nip
Adding powders to a running two-roll mill puts the hands beside the nip. See the stop-and-reach rules in Compound Mixing and never add material with the hand over the rollers.
What’s Next
- Learn next: Curing Temperature and Time — how long to cure the compound
- Combine with: Compound Mixing — incorporating it evenly