What Is a Rubber Compound and How Is It Made?

Definition

A rubber compound is the intermediate product obtained by mixing a raw polymer (natural rubber NR, or synthetic rubbers such as SBR, EPDM, NBR and CR) homogeneously with fillers, process oils, protective agents and a vulcanisation (cure) system. Raw rubber on its own is soft, tacky and weak; once compounded and cured it becomes an elastic, durable material with a permanent shape. The moulder or part manufacturer takes the compound, shapes it in a mould or on a calender and cures it.

Ingredients of a rubber compound

• Polymer (base rubber): the backbone of the recipe; it sets the basic character such as elasticity, heat and oil resistance. A single polymer or a blend (for example NR/SBR) can be used.
• Reinforcing filler — carbon black: raises strength, abrasion resistance and hardness, and gives black compounds their colour. Its grade (N330, N550, N660…) and loading change the mechanical values directly.
• Mineral fillers: calcium carbonate, kaolin, silica; used to tune cost and hardness, and in some applications for special properties.
• Process oils and plasticisers: soften the mix and improve processing and filler dispersion.
• Protective agents (antioxidants, antiozonants, waxes): delay ageing by heat, oxygen and ozone.
• Activators: zinc oxide and stearic acid; make the cure system work efficiently.
• Cure system: sulphur plus accelerators (or peroxide). It defines how fast and at what temperature the compound cures, and its final properties. It is usually added in the last stage to avoid premature curing (scorch).

How it is made: step by step

1. Recipe and weighing. Each ingredient is weighed per the recipe (in phr, parts per hundred rubber). Weighing accuracy is the first condition for batch-to-batch consistency; at Metekim raw-material weighing, carbon-black dosing and oil feeding are automated.
2. First-stage mixing (masterbatch). Polymer, carbon black, fillers and oil are mixed under high shear in an internal mixer (Banbury type, tangential or intermeshing). The aim is a homogeneous dispersion of filler in the polymer, under temperature control.
3. Second stage (final compound). After the masterbatch has cooled, the cure system is added at low temperature so the compound can wait without curing until it reaches the mould.
4. Sheeting and cooling. The compound is sheeted on an open mill or calender to the required thickness, cooled, and wound with a liner or palletised with an anti-tack agent.
5. Testing and release. A sample is taken from every batch; rheometer, Mooney viscosity and hardness are measured. If the values are within specification the batch is labelled and shipped.

How do we know a batch is good?

Testing every batch means the moulder can work with the same cure time and the same hardness every time. Batches at Metekim pass these tests and ship with a certificate of analysis on request; see the laboratory page.

Masterbatch or final compound?

A compound without its cure system is a masterbatch; with the cure system added and ready for the mould it is a final compound. Companies with their own mixing room may buy masterbatch and add the cure system themselves; moulders without a mixing room buy final compound. We supply both: see rubber compounds and toll compounding.

Frequently asked questions