Continuous mixing machines

Prepolymer is prepared as already described and cooled to a temperature between 70 and lOO C. This is then used as one stream in the continuous mixing machine. The other stream consists of water, emulsifying agent (e.g. sulphonated castor oil), and a foam stabilizer (silicone oil). Different hardnesses can be achieved by varying the NDI content together with the water content. Density control is obtained by the metering of precise quantities of reaction mixture into the mould which is then closed to enable the material to expand to fill the mould without overspill. After removal from the mould the product requires an oven cure of 24h at 110 C. 

The operation of continuous mixing machines is boring. The first hour or so is interesting, but after that the premix just goes in and the compound just flows out. The excitement of feeding a batch mixer every three minutes and pushing buttons and watching temperatures, times, etc., is just not there and an operator s mind soon wanders away from the task. 

In the 1950s pressure was extended by the tire companies on Parrel-Birmingham to develop a continuous mixing machine to replace the Banbury Mixer. This resulted in the invention of the Parrel Continuous Mixer, which was designed in a series patents filed from the early 1960s [199 to 202], shown in Fig. 9.32. 

There have been considerable efforts to carry out chemical reactions in continuous mixing machines. Much of this activity has involved grafting monomers onto polymer backbones such as maleic anhydride and similar monomers onto polyolefin backbones. There have also been efforts to polymerize monomers in these machines. We will discuss each of these efforts in turn. 

In the era of development of powdered rubbers, machines were developed which would carry out the function of continuously mixing rubbers and filler powders to produce a compound and give the product as an extrudate. These machines functioned adequately for their purpose, but other major problems associated with the concept of powdered rubbers meant that their uptake by industry was extremely limited. 

Fiqure 4.2 (a) Continuous-flow machine with 45-ps dead time, (b) Exploded view of the mixing chamber. Turbulent flow of the two liquids over the platinum sphere gives improved mixing. [Courtesy of M. C. Ramachrandra Shastry and H. Roder.]... 

The first steps in the manufacture of Bregott are pasteurization of the cream, followed by cooling and temperature treatment. The cultures are the same as those used in buttermaking. Measured quantities of cream and soybean oil are mixed in the churn or the oil is continuously injected before churning in a continuous butter machine. The byproduct is sour buttermilk. 

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