Internal mixer rotors

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

FIGURE 18 Internal mixer rotors from the designs of Sato et al [SI] of Bridgestone and Kobe Steel. 

TABLE III Flow Characteristics of Double- and Four-Flight Internal Mixer Rotors from the Patent Literature... 

There have also been new designs of internal mixer rotors, notably by MiDauer [53] of Werner Pfleiderer (Figure 1.8a) and Johnson et td. [54] of Francis Shaw (Figure 1.8b). Passoni [55] of Pomini has described a completely new design of intermeshing rotor internal mixer in which the rotor inter-axial distances may be... 

Figure 1.8 Post-Cooke intermeshing internal mixer rotors (a) Millauer[53] (b) Johnson etal. [54].

It was found by Min and White [44 to 47] that rotors with a two-wing Banbury rotor induce a circulatory motion in the mixing chamber. These researchers [43] also observed the difference in behavior of various elastomers in an internal mixer. Natural rubber (NR) formed a tight band around internal mixer rotors. However, a narrow molecular weight distribution butyl lithium polymerized polybutadiene (BR) a similarly polymerized butadiene styrene copolymer and an emulsion polymerized butadiene styrene copolymer exhibited tearing to various extents depending upon temperature. 

It became clear in succeeding years that the intermeshing internal mixer rotors did a better job of controlling the rubber temperature than F. H. Banbury s designs because of their higher metal surface areas. One was less likely to chemically damage expensive specialty elastomers with high temperature. 

The compounds were mixed in three steps The first two steps were done in an internal mixer with a mixing chamber volume of 390 mL. The mixing procedures employed in the first two steps are indicated in Table 29.2. The starting temperamre was 50°C and the cooling water was kept at a constant temperature of 50°C. The rotor speed was 100 rpm and the fill factor 66%. After every mixing step the compound was sheeted out on a 100-mL two-roll mill. The third mixing step was done on the same two-roll mill. The accelerators and sulfur were added during this step. 

Silica compounds are generally processed in conventional internal mixers, preferably with intermeshing rotors. These mixers are designed and optimized for carbon black-fiUed compounds in which mixing is based only on physical processes. When a silica-silane reinforcing system is used, additionally a chemical reaction, the sUanization, occurs. One of the main influencing factors of the silanization reaction is the concentration of ethanol in the compound as well as in the mixer [25,26]. As the silanization finally reaches an equilibrium, low concentrations of ethanol in the compound are expected to enhance the reaction rate. 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

Basically two rotor types are applied in the internal mixer The tangential rotor type and the intermeshing rotor type. The latter one interferes with the adjacent rotor and turns at the same rotor speed compulsory. In the drop door or through the side plates a thermocouple is mounted to record the temperature of the mbber. 

An analysis of the flow behavior which is more or less confirmed by numerical simulations is presented by Leblanc. " The total flow in an internal mixer (tangential type) can be divided into a laminar, circulating flow which is pushed by the rotor tip, an axial directed flow, as an effect of the helix angle between axis and rotor wing and a leakage flow between rotor tip and mixing chamber waU (refer to Figure 35.6), or... 

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. 

FI CU RE 35.12 Typical fingerprint of a masterbatch mixing process on an intermeshing internal mixer (GK 320E (Harburg Freudenberger) with PES5 rotors styrene-butadiene rubber/carbon black [SBR/CB] tread compound). 

In general a rotating part of a machine in the mbber industry the term refers particularly to the contoured rolls of an internal mixer and to the mushroom-shaped rotor of the Mooney Viscometer. 

Variable Intermeshing Clearance (VIC) Rotor Internal Mixer... 

Large amounts of energy are consumed during the mixing process and this gives rise to large temperature rises in the rubber batch. Internal mixers therefore have cooling channels in the chamber walls, rotors, and sometimes the ram head, to dissipate this heat. The inlet water temperature is controlled in most modem machines, but in many companies this is still not the case. 

The mastication equipment most commonly employed is standard rubber instrumentation such as roll mills, internal mixers, extruders or laboratory devices modeled on them (e.g., a single-rotor internal masticator described by Wilson and Watson (43), die model improved by Kargin and coworkers (11) and the Brabender plastograph). 

Fig. 20. Internal mixer design showing (a) cross-sectional view of constructional features and (b) typical rotor geometries [129]...

Fig. 22. Rotor designs for (a) single-stage and (b) two-stage continuous internal mixers [147]...

Rubbers and PTFE powder were first premixed in an internal mixer for 5 min at a temperature of 100°C and at a rotor speed of 50 rpm. Figure 10 shows two different crosslinking routes, i.e., thermally or using electron irradiation. In the case of EPDM, crosslinking was performed thermally and also with electron irradiation. 

The infinite parallel plates construct may sound theoretical and impractical, but it is not. The flow in screw extruder channels, between the rotor and the wall of an internal mixer or between the rolls of calenders and roll-mills, to mention a few, can be considered to first approximation as taking place locally between parallel plates in relative motion. 

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