Carbon black
Carbon blacks are identified by an N-number from ASTM D1765. The first digit after N shows particle size, with lower numbers meaning smaller particles and higher reinforcement. Particle size and structure set how the black behaves.
| Grade | Class | Typical use |
|---|---|---|
| N110, N220, N234 | Very high reinforcement | Rarely used in seals; some HNBR for high strength |
| N330, N339 | High reinforcement (HAF) | HNBR where strength and tear matter |
| N550 | FEF, medium reinforcement | Common all-round HNBR filler |
| N660, N683 | GPF and APF | HNBR with moderate hardness |
| N762, N774 | SRF, low structure | Seals where low set and low heat build-up matter |
| N990 | MT, very low reinforcement | Standard in FKM and FEPM; low set and good flow |
Rules of thumb: more black raises hardness, modulus and viscosity; finer black raises tear and tensile strength but also raises compression set and heat build-up. In fluoro-rubbers, which already have good strength, the aim is usually hardness with low set, so MT black dominates.
Non-black fillers
- Silica. Reinforces HNBR for tear and wear and allows light-coloured compounds. It adsorbs cure chemicals and moisture, so peroxide cure efficiency and storage stability need attention. A coupling agent (silane) often helps.
- Calcium metasilicate (wollastonite). A needle-shaped mineral used in light-coloured FKM and FEPM; it adds modulus and tear without large set penalty.
- Barium sulfate. A dense, inert mineral. It resists acids and raises density, with little reinforcement.
- Calcined clay and talc. Used in HNBR for electrical properties, extrusion or barrier behaviour.
- Calcium carbonate. A cheap extender. It reacts with acids, so it is not for acidic or sour fluids.
Acid acceptors
Fluoro-rubbers release small amounts of hydrofluoric acid during cure and in service at high temperature. An acid acceptor neutralises it and protects both the polymer and the tooling. In bisphenol-cured FKM, acid acceptors are also part of the cure chemistry.
- Magnesium oxide (high activity) and calcium hydroxide are the standard pair for bisphenol-cured FKM. A typical starting point is a few phr of magnesium oxide with about twice as much calcium hydroxide. Calcium hydroxide worsens resistance to water and steam.
- Zinc oxide is a common choice in peroxide-cured FKM, and also an activator in sulfur-cured HNBR.
- Hydrotalcite is a synthetic alternative that can be used with peroxide systems.
- Calcium oxide is used as a moisture scavenger to reduce porosity.
Practical notes
- Check the compatibility of the filler with the fluid. Acid-soluble fillers and certain metal oxides can be leached in acids or water.
- Dry fillers before use if moisture-sensitive systems are used.
- Two compounds with the same polymer and the same hardness but different fillers will not behave the same in compression set, tear, swell or low-temperature flexibility.
Sources and further reading
- ASTM D1765 for carbon black classification.