The mechanisms of rubber mixing

• The mixing of rubber is a composite operation, involving a number of different mechanisms and stages. These can be separated into four basic processes:
1. Viscosity reduction
2. Incorporation
3. Distributive mixing
4. Dispersive mixing
• As the viscosity and elasticity of a rubber are reduced the rubber can be caused to flow around additives, incorporating and enclosing them in a matrix of rubber. Incorporation and distributive mixing proceed simultaneously, the latter commencing as soon as incorporated additives are available for distribution.
• Distributive mixing occurs concurrently with dispersive mixing which serves the purpose of separating the fragments of agglomerates once they have been fractured.
• In the absence of a mastication stage, mixing time is generally dictated by distributive mixing when large particle size diluent of semi-reinforcing fillers are used; and by dispersive mixing when reinforcing fillers, particularly carbon blacks, are used.
• The smaller the volume fraction of a minor additive, the more mixing is needed to ensure a uniform distribution of that additive. It is more difficult to mix a small amount into a large amount then it is to achieve an acceptable 50-50 mixture.

Material input sequence

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General-Rubber-Goods Compounds

• General-rubber-goods compounds normally have diluent or semi-reinforcing fillers which present few problems for dispersive mixing, resulting in distributive mixing being the rate-determining stage.
• Unless a natural rubber requiring mastication is being used, or the compound includes a high proportion of oil, it is preferable to add the ingredients with the rubber at the start of the mixing cycle, to commence their distributive mixing as early as possible.
• Provided that mixer cooling is effective and a suitable rotor speed is chosen, the curatives and accelerators can usually be added without danger of scorch.

Compounds with Large Amount of Oil

• The main problem is to prevent the oil from lubricating the surfaces of the rotors and chamber wall, causing gross slippage and preventing effective mixing.
• The problem is often overcome by adding the oil and bulk filler together, prior to the rubber, in an upside-down mixing method and allowing time for the oil to be absorbed onto to filler surface and into the interstices between the particles in agglomerates. The amount of free oil is then sufficiently reduced to enable effective mixing to start immediately after rubber is added.

Tire and Conveyor-Belt Type Compounds

• For tire and conveyor-belt type compounds containing substantial quantities of reinforcing fillers (carbon black, silica), dispersive mixing is usually the rate-determining stage.
• The material input sequence should be chosen to maximise the forces acting on filler agglomerates.
• Maximisation of forces is best achieved by withholding oils, waxes and fatty acids from early stages of the mixing cycle and charging the mixer with only the rubber and bulk filler, in addition to any other particulate additives appropriate to the mixing temperatures.
• Rubbers and bulk fillers should all be charged into to the mixer at the same time, unless mastication of a natural rubber is required. When adequate dispersive mixing has been achieved or its efficiency has been much reduced by a temperature rise in the rubber, the oil and other viscosity-reducing ingredients can be added.
• The mixing cycle can be terminated when the oils, waxes, and fatty acids in addition to any other ingredients withheld to minimise their residence time at an elevated temperature, are adequately distributed.
• For compounds containing large quantities of reinforcing fillers and requiring substantial dispersive mixing, three-stage mixing sequences are commonly used, with the following sequence: masterbatch, remill, final mix.
• In the masterbatch stage the rubber and the reinforcing filler are mixed at a high rotor speed (40-60rpm, depending on mixer size) together with any particulate additives which are not temperature sensitive.
• A substantial temperature rise occurs during masterbatch stage, and the batch is dumped when the viscosity reduction is judged to preclude further effective mixing, using time, temperature or energy dump criteria. The final batch temperature is often in the region of 140-160^oC. Following discharge from the internal mixer.
• In the remill stage further dispersive mixing and viscosity reduction take place, which can now continue due to the initial low temperature and high viscosity of the batch. Dispersive mixing is substantially completed during this stage, before the temperature rise again renders it ineffective.
• After remill the compound is ready for the addition of curatives in the final mix stage. Due to the temperature limitations now imposed, this stage is carried out at a low rotor speed, to give a final batch temperature in the region of 90-105^oC.

Weighing of compounds in internal mixers

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• The weighing of the small materials can be carried out automatically but this is usually a very expensive option.
• More common is to have a computer controlled manual weight station on rails which can be moved to the particular small powder demanded by the display screen attached to the scales. The computer also unlocks the appropriate raw material bin to prevent errors in this area. Bar marking of chemicals prior to issue to the mill room can also be used to ensure the correct material is put in the correct bin, again by having a lid which is locked until released by the correct barcode.
• Weighing of polymers usually takes place on the weight/feedbelt located at the mixer feedhopper. Control of this operation is again by computer but with the manual intervention of loading the scale.