Internal Mixer Cycles

As with open mill mixing, when using synthetic polyisoprene in internal mixers, cycle times are reduced and discharge temperatures may be up to 20 °C lower than those obtained with natural rubber, without sacrificing good dispersion of ingredients. 

Mixing. Ethylene—acrylic elastomers are processed in the same manner as other elastomers. An internal mixer is used for large-scale production and a mbber mill for smaller scales. In either case, it is important to keep the compound as cool as possible and to avoid overmixing. Ethylene—acryflc elastomers require no breakdown period prior to addition of ingredients. Mixing cycles for a one-pass mix are short, typically 2.5—3.5 min. When compounds are mixed on a mbber mill, care should be taken to add the processing aids as soon as possible, after the polymer has been banded on the mill. Normal mill mixing procedures are followed otherwise. 

Internal mixing is widely used with fluorocarbon elastomers. Gumstocks and compounds that are particularly successful fall in the viscosity ranges discussed earlier, and use both incorporated bisphenol-type and peroxide cure systems. A typical internal mix cycle mns 6—8 min with a drop temperature of 90—120°C. The typical formulations in Tables 4 and 7 are readily mixed in an internal mixer. 

The use of stabilisers (antioxidants) may, however, have adverse effects in that they inhibit cross-linking of the rubber. The influence of phenolic antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C has been studied. It was found that alloys containing 1% of certain phenolic antioxidants were gel-deficient in the rubber phase.The gel-deficient blends were blotchy in appearance, and had lower flow rates compared with the normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added later in the mixing cycle after the rubber had reached a moderate degree of cross-linking. 

It is well known that the cycle time of the mixing process in an industrial-sized internal mixer is limited by the increase of the compound temperamre. For example, typical mixing times on a 400 L tangential mixer with four wing rotors is of the order 200 s as can be derived from Figure 35.2. Especially during the last mixing phase the temperature increase of the compound is approximately l°C/s. 

The simultaneous addition of the elastomers and all the compounding ingredients to an internal mixer at the beginning of the mixing cycle. A saving in mixing time is claimed but dispersion may be adversely affected. 

Depending on the complexity of the formulation, size of the internal mixer, and application for which the compound is intended, the mix cycle can be divided into a sequence of stages. For an all-natural-rubber compound containing 50 phr carbon black, 3 phr of aromatic oil, an antioxidant system, and a semi-EV vulcanization system, a typical Banbury mix cycle will be as follows ... 

The sequence of addition in an internal mixer is typically (1) polymers and peptizer (2) plasticizers, most carbon black or silica, and oils (3) balance of fillers and antioxidants and (4) vulcanization system components. Construction of a mix cycle is governed by a set of empirical rules ... 

There are three basic procedures for mixing rubber compounds in an internal mixer namely, the conventional method, the rapid oil addition method, and the upside-down mix method. Many variations of these three methods are also used to suit the special characteristics of individual formulations and equipment. It is, in general, necessary to add particulate fillers early in the mixing cycle, so that good dispersion is achieved as a result of the high shear stress and high viscosity at the lower temperatures then prevailing. Similarly, the oils and plasticizers which reduce viscosity should be added later. Upside-down procedures and variants of it are attempts to implement these ideas in practice. 

Most fluorocarbon elastomer gums contain a cure system, and, in the final step, the cure additives are incorporated in a two-roll mill, in an internal mixer, or in a mixing extruder. The cure system comprises an organic onium cure accelerator, such as triphenylbenzylphosphonium chloride, and a bisphenol cross-linking agent, such as hexafluoroisopropylidenediphenol. These cure systems improve compression-set performance and processing safety and accelerate cure cycles. 

Morikawa, A., Min, K., and White, J. L., Flow visualization of the rubber compounding cycles in an internal mixer based on elastomer blends, Int. Polym. Proc. 4 11 (1989). 

As a tangential rotor type mixer, a Werner and Pfleiderer GK-2 internal mixer was used with a chamber volume of 3150 cm. The mixer was operated at a fill factor of 76%, the optimum with regard to mixing efficiency for such a type mixer. Rotor speeds of 46 and 50 rpm were used, running at a friction factor of 1.1. A ram pressure of 5 bars (maximum oil pressure on the plunger of the ram) and a wall temperature of 40°C held constant throughout the mixing cycle. 

Banbury and other internal mixers are equipped with a ram, actuated by an air cylinder, to feed the material into the mixing chamber during the mixing cycle. 

With a two-mill set-up, the first mill will roll out the batch and quickly reduce the temperature. It is usual to have an operator on this mill to do several cuts to allow the stock to pass through the nip and then send the stock on a conveyor to the second mill. This mill can be fitted with a stock blender and a variable speed drive so that the stock can be continuously fed into the rubber strip cooler at a constant speed. For example with a 270-litre internal mixer on a 3-min mixing cycle, the speed of a 800 mm x 10 mm strip would be 9m/min. This continuous lower speed allows for better cooling and drying and avoids too many splices in the strip between batches. 

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