Compounding dispersive mixing

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. 

Manas-Zloczower, I., Dispersive mixing of solid additives, in Mixing and Compounding of Polymers—Theory and Practice. (I. Manas-Zloczower and Z. Tadmor, Ed.) Hanser Publishers, Munich, 1994, pp. 55-83. 

Vertical and lateral mixing are not considered here. It is assumed that a given mass flux of a compound is mixed instantaneously into the corresponding volume flux of water. Of course, close to the input, vertical and lateral mixing are the most important mechanisms to reduce the concentration in the river. Also note that processes 3, 4, and 5 are only effective in reducing the concentration of the compound for the case of an episodic input. If a compound is continuously added to the river, the sediments reach an equilibrium with the river water and thus do not act as a sink anymore. Similarly, dispersion does not reduce the concentration of a compound that is permanently added to the river. 

Manas-Zloczower, I. 1994. Dispersive mixing of solid additivesMining and Continuous Compounding of Polymefsadited by Mansa-Zloczower, I. and Tadmor, Z., Munich Carl Hanser Verlag, pp. 55-83. 

The most common example of dispersive mixing of particulate solid agglomerates is the dispersion and mixing of carbon black into a rubber compound. The dispersion of such a system is schematically represented in Fig. 3.22. However, the break up of particulate agglomerates is best explained using an ideal system of two small spherical particles that need to be separated and dispersed during a mixing process. 

Each process has a place in the compounding picture because of how it relates to the three key compounding parameters of dispersive mixing, distributive mixing, and thermal control. There is an overlap in the capabilities of these processes, but there are formulations that are best produced by only a single process. 

KBr disks here the finely ground solid compound is mixed with anhydrous KBr and squeezed under pressure. The KBr becomes fluid and forms a disk containing the solid compound dispersed evenly within it and suitable for obtaining a spectrum. The advantage of the KBr disk technique is the absence of the spectrum from the mulling liquid, but the disadvantages are the equipment required (Fig. 28.4) and the practice required to obtain suitable transparent disks, which are very delicate and rapidly absorb atmospheric moisture. 

Figure 1-2 presents an illustration of several types of mixing screws. These show variations in thread pitch as different profiles or offsets in the diameter of the screws between the threaded lands. Screw profiles vary for the type of compound being mixed. In some cases, very high shear is required to assure proper dispersion of ingredients. For other compounds, aggressive shear will... 

Internal mixers must be ran in a full or nearly full condition, so a batch recipe is calculated to provide an appropriate volume. If not filled, the ingredients will not be properly sheared and heat transfer will be compromised. Typical commercial mixers have a batch size of at least 100 pounds of compound. A mixer of this size will have a drive motor of no less than 75 horsepower. Proper dispersive mixing is a balance between proper shear, sequence of addition of ingredients, and thermal stability. Mixers have extensive monitoring instrumentation that provides continuous feedback about thermal conditions, rotor torque, and rotor speed. Once a mixing process has been developed, a standard protocol is followed for preparation of the compound. 

Mixing requirements for polymer compounding can be divided into two basic disciplines—dispersive and distributive. Dispersive mixing breaks down a particle into smaller units, while distributive mixing homogenizes the spatial relationship of the particles (whether dispersed or not). 

As it seems that the dispersive mixing of the additives could be improved by increasing the melt shear rate, in the video conference the 3D simulation expert proposes a modification of the screw configuration as well as the screw speed. This is accepted by the team leader, a new MOREX simulation is done and the compounding extruder configuration is fixed. 

Experimentally, IR spectroscopy can be accomplished in a number of ways by Nujol mull, KBr disc or the diffuse reflectance technique. In the KBr disc technique, the compound is mixed with KBr and compressed into a disc using a press and die. This can be a disadvantage if the compound undergoes a polymorphic transformation under pressure (see Chan and Doelker, 1985). One way to overcome this problem is to use the diffuse reflectance Fourier transform (DRIFT) technique, whereby a few milligrams of compound is dispersed in approximately 250 mg of KBr and the spectrum obtained by reflection from the surface. 

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