HNBR and FEPM compounds are tough to mix for the same reason they perform well: strong, saturated, high-viscosity polymers with high filler loadings. Add a peroxide and the process window narrows.
The two conflicting needs
- Enough shear and temperature to disperse reinforcing filler into a stiff polymer.
- Low enough temperature that the peroxide does not decompose and start crosslinking in the mixer.
The standard solution is to separate them.
A two-stage approach
Stage 1, masterbatch. Polymer, fillers, acid acceptors, antioxidants, plasticiser. No peroxide and no reactive coagent. Mix to a dump temperature suited to filler dispersion. Sheet off, cool and rest.
Stage 2, productive. Remix the masterbatch briefly, add peroxide and coagent at a low stock temperature. Dump low. Many plants use a cooled mixer or a two-roll mill for this step to keep temperatures down.
Peroxide choice and scorch
Peroxides are chosen by their decomposition behaviour. Faster-decomposing types give shorter cure but a narrower mixing window. Choose a peroxide whose half-life at the dump temperature is long compared with the time the stock spends hot. Supplier tables of half-life against temperature are the guide.
Coagents
Liquid or solid coagents (such as TAIC, acrylates and methacrylates) improve cure efficiency. They can also cause premature crosslinking if added hot and some are volatile or irritating. Add them in the productive stage, and weigh carefully, because small deviations in coagent move crosslink density.
Dispersion
Because these compounds are stiff, dispersion is hardest to achieve. Use low fill factors, check the batch band on the mill, and test dispersion. Poor dispersion lowers tear strength and decompression resistance.
Cooling and storage
Sheet off and cool quickly. Store at a controlled temperature, and use within the stock's validated shelf life. Scorch safety falls with age and with warm storage.
Control charts
Track ML, MH, ts2 and t90 for every batch. A fall in ts2 with the same recipe means mixing temperature rose, the peroxide is older, or the rotor is worn.
