Mixing

Why mixing decides everything downstream

A formulation is a list of intentions. Mixing is what turns it into a material, and a compound that is badly mixed cannot be rescued by anything that happens afterwards.

Undispersed filler behaves as a flaw rather than as reinforcement. Unevenly distributed curative gives patchy cure. Excess heat picked up during mixing consumes scorch safety that the extruder needed. None of these show up as an ingredient error, because the ingredients were all correct — the problem is that they were not put together properly.

This is why mixing is the most closely controlled step in most rubber plants, and why the mixing record is usually the first thing examined when something goes wrong later.

The internal mixer

Almost all industrial mixing is done in an internal mixer, the design originating in the early twentieth century and still the industry standard. It is an enclosed chamber containing two counter-rotating rotors, in many designs running at slightly different speeds for the same shear-generating reason as a mill's friction ratio, with a hydraulic ram above that pushes material down into the rotor path.

Material is worked between the rotors and the chamber wall, and between the rotors themselves. That working is what breaks down filler agglomerates and blends everything into a uniform mass.

The mixer is closed, which is what allows the ram to apply pressure and what keeps dust contained. It also means you cannot see what is happening inside, so the process is controlled by instrument readings rather than by observation — temperature, power draw, ram position, and time.

The two-roll mill

The mill is an open pair of heavy counter-rotating rolls with an adjustable gap. Material passes through the gap and forms a band around one roll, and the operator cuts and folds it to work it through repeatedly.

Mills are used for cooling and sheeting a batch dropped from the internal mixer, for adding curatives in a second stage, for warming stock before it goes to an extruder, and for small-scale or laboratory mixing.

The rolls usually turn at slightly different speeds. That difference, called friction ratio, is what generates shear in the nip rather than simply squeezing the material through. The speed difference is the main source of shear in the nip; a mill running both rolls at equal speed still works the material, but far less effectively.

What happens during mixing

Four things occur, and they overlap rather than happening in sequence.

• Incorporation — the ingredients are wetted by polymer and drawn into a single coherent mass. Until this happens, nothing else can.
• Dispersion — filler agglomerates are broken down into their constituent aggregates by shear.
• Distribution — everything is moved around so that composition is uniform throughout the batch.
• Plasticization — the compound is worked down to its final viscosity, partly through polymer breakdown.

Distribution and dispersion are often confused and are genuinely different. A compound can be well distributed and badly dispersed — filler spread evenly through the batch but still in lumps. It can also be well dispersed and badly distributed — an under-swept batch is exactly that case. Only good dispersion gives reinforcement; only good distribution gives consistency.

Order of addition

When ingredients go in matters as much as how much of each there is.

The reasoning behind filler-before-oil is worth holding onto. Dispersion needs shear, shear needs viscosity, and oil lowers viscosity. Add oil too early and the batch becomes slippery, the shear collapses, and the filler never breaks down properly. The compound looks mixed and is not.

Why mixing is done in two passes

Dispersing filler well requires temperature, and first-pass drop temperatures of roughly 140 to 160 °C are typical for many compounds, though the figure is formulation and mixer dependent. Curatives cannot survive that without beginning to react, which is scorch.

So the batch is mixed to achieve dispersion, dropped, cooled on a mill or in a batch-off system, and then mixed again at lower temperature with the curatives added. The second pass is short and gentle, because its only job is to distribute the cure system uniformly.

Some compounds are mixed in a single pass, usually where the cure system is slow enough to tolerate it or the dispersion requirement is modest. Others need three passes. The number is a compromise between the dispersion needed, the scorch safety available, and the cost of each pass.

A typical mixing cycle

A conventional first-pass cycle for a black-filled compound looks something like this.

The sweeps matter more than they look. Material collects in the throat above the ram and in the corners of the chamber, and if it is not returned to the rotor path it never gets mixed. A batch that is short on sweeps can be uniform in the bulk and carry unmixed material at the edges.

Note that the cycle is written in time but usually terminated on temperature or energy. Time is a guide; the drop criterion is what actually controls the batch.