Practical Parameters for Mixing

Abstract (1) Compounds mixed under high-shear conditions in a laboratory Brabender Plastograph or in larger Banbury mixers can be quantitatively compared on the basis of work input per unit volume. This quantity, unit work Wu, is proposed as a useful parameter for characterizing the effects of polymer, filler, and other ingredient on mixing performance. (2) This work supports the principle that, independent of the size and speed of a mixer, there is a unique relationship between unit work and such in-process properties of the compound as Mooney viscosity, die swell, etc. It has been shown that this relationship extends to other high-shear mixers, such as the Shaw Intermix and a cam-head type Brabender Plasticorder. (3) Practical unit work ranges for passenger tire-tread mixing found typical of factory operations are: first stage, 300–800 MJ/m3; second stage, 150–400 MJ/m3; final mix, 2000–4500 MJ/m3. Mixing to these unit-work levels in the laboratory achieved mixing quality that closely duplicated large-scale high-shear mixing. (4) Process profiles (the changes of certain in-process properties such as Mooney viscosity and die swell as functions of unit work) yield additional behavior indices that are characteristic of each individual rubber—filler mixture: The viscosity-work index (VWI), for instance, reflects the rate of change in the compound viscosity with increasing unit work, while the position of the maximum in the die swell vs. unit work curve is a good indicator of the minimum unit work required to obtain low levels of undispersed carbon black. (5) A second scaling parameter, based on an expanded concept of the total shear-strain performance for Banbury-type mixers, is proposed. It offers an alternative approach to scaled mixing when the actual measurement of unit work is not convenient. The total shear-strain parameter, as derived, is a dimensionless number Γ defined as Γ = [(shear rate) (time) (volumetric throughput ratio) (land-length ratio)]. Its use implies ideal viscous flow during mixing plus an accurate knowledge of mixer rotor and chamber dimensions.