Rubber mixing processes

Rubber mixing processes are used to produce an elastomeric compound with ingredients sufficiently incorporated, dispersed, distributed and plasticized that it will be able to be processed and will cure consistently in subsequent secondary processing. 

Mixing mills are open two-roller mills consisting of two parallel horizontal rollers rotating in opposite directions, that pull the ingredients through the gap (or nip) between the rollers. The back roller usually rotates faster than the front roller, and most of the work is done on the slower front roller. Rollers can be heated centrally. 

Mixing extruders are automated-feed internal mixers in which the feed is conveyed from the feed hopper into the mixing section (note that the extrusion section is principally used for conveying, not mixing). The mixing section is similar to that of an internal mixer where material is intensively sheared between the rotor and the wall. 

Quality-control tests and important process variables 

Key process and material variables are viscosity (or viscosity reduction) 

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Schwartz (2001) and Haberstroh and Linhart (2004) note that rubber mixing processes are inherently extremely complex and use artificial neural networks to evaluate rheometric properties and process models for mixing of rubber compounds. 

For development of the rubber mixing process and increasing its efficiency and economics, much research work is needed in the following areas ... 

The paddle ridge width of a batch mixer was varied between 0.2-20 mm and the effects on the rubber mixing process were studied photographically. The distribution of carbon black in the mix was uniform with a width below 11 mm, while a width below 15 mm was required for efficient dispersion. 9 refs. Artieles from this journal can be requested for translation by subseribers to the Rapra produced International Polymer Science and Technology. 

PROGRESS IN THE DEVELOPMENT OF THE CONTINUOUS RUBBER MIXING PROCESS BY TWIN-SCREW EXTRUSION... 

Journal of Polymer Engineering Vol.21, No.2-3, March-June 2001, p.251-6 ANALYSIS OF RUBBER MIXING PROCESS IN THE TRANSFERMIX EXTRUDER Haberstroh E Kremers A Krasche A Institut fuer Kunststoffveraibeitung... 

Results are presented of studies undertaken to evaluate the use of filler dispersion control as a means for optimising rubber mixing processes. By image analysis of a freshly cut uncured rubber surface and classification of the number and size of agglomerates, it was possible to predict the extrusion performance of a compound, and a clear relationship was established between the maximum agglomerate size and the extmsion performance. 40 refs. EUROPEAN COMMUNITY EUROPEAN UNION GERMANY SCANDINAVIA SWEDEN USA WESTERN EUROPE... 

Clarke,. 1. and Freakley, P.K., 1995. Modes of dispersive mixing and filler agglomerate size distributions in rubber compounds, Plast. Rubber Compos. Process. Appl. 24, 261-266. 

Reclaim Rubber. The process of reclaiming mbber by chemical digestion has been in use since the late 1800s. Early processes involved the treatment of the mbber cord mix with acid. Acids attack cotton, rayon, and nylon. The acid treatment was used to remove the reinforcing components. 

In order to "cure" or "vulcanize" an elastomer, ie, cross-link the macromolecular chains (Fig. 2), certain chemical ingredients are mixed or compounded with the mbber, depending on its nature (4,5). The mixing process depends on the type of elastomer a high viscosity type, eg, natural mbber, requires powerhil mixers (such as the Banbury type or mbber mills), while the more Hquid polymers can be handled by ordinary rotary mixers, etc (see Rubber... 

FIGURE 35.11 Typical energy balance of one batch-mixing process on a GK320E mixer (styrene-butadiene rubber/carbon black [SBR/CB] compound). 

FIGURE 35.13 Typical fingerprint of a masterbatch mixing process of a solution-based styrene-butadiene rubber (S-SBR)/Silica/TESPT tread compound on a GK 320E (Harburg Freudenberger) with PES5 rotors. 

Results of a Fingerprint Analysis of a Masterbatch and Remill Mixing Process of a Styrene-Butadiene Rubber-Carbon Black (SBR-CB) Compound on a CK320E Intermeshing Mixer with PES3 Rotors (Harburg Freudenberger)... 

Rubber which has been masticated in an operation entirely separate from the mixing process proper. See Mastication and Mixing. Premix... 

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

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 researches of ZnCFO compatibility with the matrix of isoprene rubber in plasticorder "Brabender" PLE 6511 have shown, that the disperse process of composite is accompanied by lower power consumption and its best compatibility in comparison with zinc oxide (fig. 4). The absence of ZnCFO particles as extraneous impurities in rubber mix also was visually observed, while the zinc oxide particles were well appreciable [7],... 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

That is, the analysis of the received results, has shown an opportunity of equal-mass replacement of the traditional activator - zinc oxide on the new polymer - inorganic composite (5,0 phr) at maintenance of a high activation level of sulfur vulcanization process of rubber mixes on the basis of diene isoprene rubber and improvement of the physical-mechanical properties complex of their vulcanizates. 

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