Mixers, internal

Ghoreishy, M. H. R. and Nassehi, V., 1997. Modelling the transient flow of rubber compounds in the dispersive section of an internal mixer with slip-stick boundary conditions. Adv. Poly. Tech. 16, 45-68. 

Figure 5.1 Initial configuration in a partially filled single blade internal mixer...

Nassehi, V. and Ghoreishy, M. H. R., 1997. Simulation of free surface flow in partially filled internal mixers. Int. Poly. Process. XII, 346-353. 

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

SBR mbber compounds are prepared in two stages the nonproductive and the productive phases. In the nonproductive phase the compound ingredients are mixed, usually in internal mixers (Banbury). The mixing time is usually short and the compound temperature is in the 160—195°C range. 

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. 

The polymei latex is then coagulated by addition of salt oi acid, a combination of both, oi by a fiee2e—thaw process. The cmmb is washed, dewatered, and dried. Since most fluorocarbon elastomer gums are sold with incorporated cure systems, the final step in the process involves incorporation of the curatives. This can be done on a two-roU mill, in an internal mixer, or in a mixing extmder. 

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. 

Blends with PVC. Nitrile mbber may be blended with poly(vinyl chloride) (PVC) by the polymer producer by two different techniques (1) blending of NBR latex with PVC latex followed by co-coagulation and drying, or (2) physically mixing the soHd NBR and PVC powder in mixing equipment such as an internal mixer. NBR—PVC polymer blends are well known for the good ozone resistance that is imparted by the PVC. 

More specific recipes appear in Table 3. The ingredients are added to the elastomers on standard two-roU mills or in internal mixers. Finished compounds are readily extmded, calendered, or molded in standard equipment. Vulcanisation of extmdates is accompHshed in Hve steam autoclaves, Hquid salt baths, fluidized beds, and microwave equipment. 

Polyethylene can be compounded on any of the standard types of mixing equipment used for visco-elastic materials. For laboratory purposes a two-roll mill is suitable operating temperatures varying from about 90°C to about 140°C according to the type of polymer. On the industrial scale, compounding is undertaken either in internal mixers, or more particularly, extrusion compounders. 

Blending of polymer, plasticiser and filler may be carried out using two-roll mills or internal mixers as commonly used in the rubber industry. Alternatively, since the raw polymer is supplied as a free-flowing powder a dry blending process similar to that now widely used with PVC (see Chapter 12) is also used. 

In most of these routes, premixing is carried out in a trough mixer at room temperature to give a damp powdery mass or mush . This may then be fluxed on a two-roll mill, in an internal mixer, or in a continuous compounder such as the Werner and Pfleiderer Plastificator. For many operations the compounded mass... 

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. 

Dry processes which obviate solvent difficulties are now preferred and are similar to those employed with the major thermoplastics. They include the use of two-roll mills, internal mixers, extruders and extrusion compounders. The use of dry blend techniques similar to that used more recently with PVC have also been used. ... 

For the manufacture of medium-shock-resisting grades the preblend of resin, filler and other ingredients does not readily form a hide on the mill rolls. In this case the composition is preblended in an internal mixer before passing on to the mills. 

As an alternative to the wet process described above, moulding compositions may be made by mixing a powdered resin or a methylol derivative with other ingredients on a two-roll mill or in an internal mixer. The condensation reaction proceeds during this process and when deemed sufficiently advanced, the composition is sheeted off and disintegrated to the desired particle size. This dry process is not known to be used in any current commercial operation. 

Before fabrication it is necessary to compound the gum with fillers, vulcruiising agent and other special additives on a two-roll mill or in an internal mixer. 

Ebonite compositions may be prepared without difficulty either in an internal mixer or on a two-roll mill. In addition to the rubber and sulphur, fillers are invariably present in commercial mixes. These materials have the important function of diluting the rubber phase. Because of this the exotherm will be... 

Table 2 Melt Stabilizing Efficiency of Antioxidants in PP (processed in an internal mixer at 190°C). Melt flow index (MFI) Measured at 230 C and 2.16 Kg...

Table 3 UV-Embrittlement Times (EMT) of PP and LDPE Films Containing Different Concentrations of Antioxidants (processed in an internal mixer at 190°C and 150°C, respectively, and exposed to UV light in an accelerated sunlamp-blacklamp UV aging cabinet)...

Figure 8 Internal mixer rotor designs (a) tangential rotor, (b) intermeshing rotor.

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

Elastomer-plastic blends without vulcanization were prepared either in a two roll mill or Banbury mixer. Depending on the nature of plastic and rubber the mixing temperature was changed. Usually the plastic was fed into the two roll mill or an internal mixer after preheating the mixer to a temperature above the melting temperature of the plastic phase. The plastic phase was then added and the required melt viscosity was attained by applying a mechanical shear. The rubber phase was then added and the mixture was then melt mixed for an additional 1 to 3 min when other rubber additives, such as filler, activator, and lubricants or softeners, were added. Mixing was then carried out with controlled shear rate... 

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