Cooling early stage

Too fast a cooling rate with thick sections leads at an early stage to the formation of a solid shell with a soft centre. On further cooling the polymer tends either to shrink away from the centre towards the solid shell, resulting in the production of voids, or alternatively the shell tends to collapse with distortion of the product. 

Various clinical manifestations may be present in a patient in shock. For example, in the early stages of shock the extremities may be warm because compensatory mechanisms are initiated and the blood flow to the skin and extremities is maintained. If the condition is untreated, the skin and extremities become cool and clammy because of the failure of the compensatory mechanisms and the progression of shock. Thus, more advanced shock may be referred to as... 

A mixture of 250 g. (1.46 moles) of />-nitrobenzyl chloride, 225 g. (2.74 moles) of fused sodium acetate, and 375 g. (6.25 moles) of glacial acetic acid is refluxed for 8 to 10 hours in a 2-1. flask heated by an oil bath, the temperature of which is maintained at 160-170° (Note 1). After this time the bath is allowed to cool to about 125°, and the acetic acid is removed by distillation under reduced pressure. Care must be taken not to reduce the pressure too rapidly in the early stages of the distillation. As the distillation slows down, the pressure is further reduced until it reaches 50 mm. or lower, and the temperature is slowly raised to 160°. From 2.5 to 3 hours is required for the complete removal of the acetic acid. About 500 ml. of water is added, and the hard cake is broken up with a stirring rod (Note 2). 

In an early stage of warning for a runaway reaction, emergency cooling should be applied in an attempt to control the situation. This often is not sufficient to be a fully preventive measure, particularly if the runaway initiation was not identified quickly. Quenching of the reaction is the usual next step to take. The reaction can be quenched by using one or more of the following procedures ... 

It is most important that the temperature of the reaction mixture be not allowed to rise prematurely, since if it reaches 50° during the early stages the reaction velocity increases so rapidly that the contents are apt to boil out of the flask. For the same reason it appears necessary that a certain ratio between reacting mass and cooled surface be not exceeded. Runs of twice the above size have been carried out in a 5-I. flask without mishap, but temperature control was difficult and these conditions are extremely hazardous. A run of 4 kg. of ethylene chlorohydrin in a 12-I. flask was attempted, but rapidly went out of control. 

The reaction of the excess bromine with diethylamine is exothermic consequently it may be necessary to moderate the reaction by cooling with an ice-water bath during the early stages of the addition. 

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