Mixer crystallisation

Bales of natural rubber especially, and many synthetic rubbers benefit from storage in a hot room prior to being prepared for the mixing department. Natural rubber in particular arrives in factories in a frozen (hard) state due to crystallisation and will require a few days storage in warm conditions so that damage does not occur to the mixer from the hard rubber charge. Crystallisation occurs very rapidly at temperatures as low as -26 °C but the rate between 0 and 10 °C is still appreciable. Crystallisation is a reversible physical change without detrimental effects. 

In acid-catalysed processes, the addition ofp-toluene sulfonic acid was found to cause a reduction in torque in the mixer, darkening, a loss of melt viscosity and enhanced crystalhsabiHty of the PC in the presence of 10-20% PCL. Shuster et al. [131] suggested that the enhanced crystalHnity meant that there was no transesterification and p-toluene sulfonic acid promoted carbonate-ncarbonate and not carbonate-l-ester interchange. Previously Smith et al. [132] had reported that transesterification produced block copolymer and loss of crystallisation and crystalhsabiHty. 

The chapter on mixers is comparatively short - not because mixers are unimportant, but because mixing is such an integral part of other intensive processes (heat exchange, reactions and crystallisation/precipitation as examples), that the topic is addressed continuously through other equipment and application chapters. The point is well made by Wu et al. (2(X)7) who stresses in their recent review that PI needs to be implemented via increased mixing, as well as heat and mass transfer - hence the success in this area of the spinning disc and the oscillatory baffle reactors. 

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