Fluoroelastomer compounds are made with the same equipment as any other rubber, but they punish careless mixing.
What the polymer brings
- High Mooney viscosity in many grades, so the gum takes more energy to break down.
- Curative-incorporated gums are a common supply form. The curing agent and accelerator are already in the gum, so only acid acceptors, filler and processing aids are added, and the stock is scorch-sensitive from the start.
- Raw gum and peroxide types need curatives added by the compounder, and the compound has its own scorch window.
- Stickiness. FKM sticks to hot metal and to itself.
A sensible order
- Blend the dry ingredients (metal oxides, calcium hydroxide, filler, wax) off line before adding, so they go in together.
- Band or soften the gum on a mill, or load it into the mixer.
- Add the dry blend in one or two portions.
- Add liquid processing aids and wax late.
- Dump at a controlled low temperature, sheet off and cool quickly.
Temperature
Keep stock temperature moderate, and consult the polymer supplier's guidance. Very hot stock can start cure in a curative-incorporated gum and can attack the metal surfaces. Many fluoroelastomer mixing procedures aim for dump temperatures well below those used for general rubbers.
Metal oxides and hydroxides
Magnesium oxide, calcium hydroxide and similar acid acceptors are fine powders. They need good dispersion, because poor dispersion gives patchy cure and variable set. They also take up moisture over time, so handle and store them dry.
Contamination
Fluoroelastomers are very sensitive to contamination by other rubbers, silicone release agents and oils. Even small amounts affect cure and mould release. Use dedicated equipment where possible, or clean thoroughly with a purge compound, and track which compounds were run before.
Safety
At normal mixing temperatures FKM is stable. Overheating, for example if a batch is left hot in a stuck mixer, can release hydrogen fluoride. Operators need training and ventilation, and procedures for a batch that cannot be dumped.
Checks after mixing
Rheometer curve for cure (ML, MH, ts2, t90), Mooney viscosity, density and a visual check for white specks of undispersed oxide. These tell you whether the batch is ready for moulding.
Sources and further reading
- Public fluoroelastomer datasheets recommend standard water-cooled internal mixers or two-roll mills, blending dry ingredients before adding them to the masticated gum, and banding the gum on the mill before adding the blend.
