Continuous Processes, Rapid Methods

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

This system involves the use of some form of heating by air or steam in a chamber in a manner such that the vulcanization occurs immediately after the rubber is formed in an extruder or calender. This is a suitable process for extruded profiles and calendered sheets and conveyor belts. Liquid curing method (LCM) is also a continuous process which involves the use of suitable hot liquid baths in which extruded profiles can be passed through and vulcanized continuously. Items can be cured rapidly at temperatures from 200°C to 300°C however the compounds must be suitably designed to prevent porosity as this is a common problem with any extrudate. Suitable materials for curing medium includes bismuth-tin alloys, an eutectic mixture of potassium nitrate and... 

The mato feature of a contiguous process is that the final expl product does not remain in the app for a prolonged period of rime and the quantity accumulated at any period of time is much smaller than that in the batch process. The large quantity of expl material accumulated in a batch process, especially at the end of exothermic nitration,presents a serious safety hazard which is more or less absent in current continuous processes The continuous method of nitration is particularly useful when the rate of nitration is rapid (such as in case of manufg NG, NGc, or PETN) and an overheating and expln might take place with the batch process. Nitration of cellulose, on the other hand, does not proceed very rapidly (unless mixed acid contains phosphoric or glacial... 

Reduction of the pH of alkaline solutions to bring them just Inside the boundary of the Instability region results in very rapid and quite reproducible changes in the concentration of small silicate species (IJJ. but the form of the colloidal species Is highly dependent on the method of preparation. Here also, then, is the suggestion that polymerisation and colloid formation may not be part of the same continuous process. 

The technique of thermogravimetric analysis (TGA), which is used for thermomechanical analysis, is one of the most widely used methods due to its rapidity of operation and a lesser need for sample manipulation compared to other conventional analyses. TGA is defined as a continuous process that involves the measurement of mass variation as a function of temperature or of time, at a constant temperature and/or heating rate. The construction of a thermogravimetric balance is shown schematically in Figure 13.4. 

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