Ribbon torque

For the impeller ribbon viscometer technique, the power number of an impeller is inversely proportional to the impeller Reynolds number (Eq. 1). As the impeller rotational speed increases, the flow will gradually change from laminar to turbulent, passing through a transition region. Parameter c can be obtained from the calibration fluids. If the same value for c is assumed to apply to a non-Newtonian fluid, then Eq. 4 can be used to calculate the apparent viscosity of that fluid. The range of the impeller method is determined by the minimum and maximum torques that can be measured (5). 

Two Brookfield viscometers were used to collect the data necessary for rheologic property studies of wet grains a Brookfield RVDV III viscometer with a cone-and-plate spindle and a Brookfield HBDV III viscometer with a double helical ribbon impeller attachment. The Brookfield RVDV III had a full-scale torque of 7187 dyn-cm, and the HBDV III had a full-scale torque of 57,496 dyn-cm. Each viscometer had a maximum rotational speed of 250 rpm. Both viscometers had accuracy limits of 5% full-scale torque. 

A helical ribbon impeller, 45 in (1.14 m) diameter, is operated in a 47-in (1.19-m) diameter reactor estimate the fluid viscosity from torque readings. The impeller is a single-turn helix with a 1 1 pitch, so the height of the impeller is the same as the diameter, 45 in (1.14 m). It is a double-flight helix, each blade of which is 4.5 in (0.114 m) wide. 

A torque rheometer is used to determine fusion characteristics of PVC compounds. Components of formulation are first mixed in a beaker in an intensive mixer or ribbon blender. The rheometer is equipped with roller head and the premixed sample is added to a chamber having preselected temperature (typical temperatures used are 140 for flexible compounds, 180 for semirigid compounds, and 197°C for rigid compounds). The amount of the sample added is selected such that it occupies 65% of mixer bowl. Mixing is continued until torque peak is reached. Mixing time and maximum torque are two important results of testing. 

Paddles instead of ribbons decrease sticking problems and the torque required. Paddle blenders as batch granulators can handle wetter paste. These paddle blenders are occasionally used as continuous granulators and have both lower torque and more applications than a continuous ribbon blender. The movement of the paste helps remove material from the paddles. Formulation of a nontacky paste is aided by using a substance such as microcrystalline cellulose that helps absorb the excess moisture from the mix, yet is plastic and nonsticky. 

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