Reactive mixing equipment

Alginate impression materials are chemically reactive mixtures. AH factors that influence reaction rates are, therefore, important in the use of these materials, ie, correct proportioning temperature of the water, powder, and mixing equipment and spatulation rate and duration. 

Van Vliet, E., Turbulent reactive mixing in process equipment , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2003). 

First and foremost, the laminar mixing flow created in the reactive processing equipment, must reduce the striation thickness to a level where the diffusion characteristic time, tD = r2/ )AB, is small compared to the reaction characteristic time. Since the molecular diffusivities of low molecular weight components in polymeric melts (see Section 8.3) are very small and of the order of 10 6 cm2/s, the striation thickness must be reduced to the micron level in order to get a characteristic time t of the order of 1 s. Shear flow can accomplish this in reasonable mixing times because the striation thickness is inversely proportional to the total shear (see Section 7.3)... 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

Reactive Interactions. Likewise, your material supplier may not be much help for reactive interactions on your compatibility chart. For these combinations, the first thing you need to know is how much heat or gas can be generated. In some cases, this can be as simple as using the heat of mixing published in a technical reference book. In others, it may involve use of special equipment to accurately measure the amount of heat and pressure generated during a complex chemical reaction. 

The advantage of the interfacial polymerization process is that it is a low-temperature process requiring ordinary equipment. It also allows one to prepare those polyamides that are unstable in the melt polymerization process. Random or block polymers can be prepared easily depending on the reactivity of the reactants and their mixing (consecutively versus all at once.)... 

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