Processing stages

Once the fiber has been spun, the remaining processing stages are basically similar, irrespective of the initial spinning route, although handling procedures naturally vary depending whether the process is continuous or not. 

Source Reprinted from Daumit GP, Ko YS, A unique approach to carbon fiber precursor development, High Tech— The Way into the Nineties, ed. Brunsch K, Golden HD, Herkert CM, Proc of the f Int SAMPE Conf, Munich, Elsevier Science Publishers BV, Amsterdam, June 10-12, 201-213, 1986. 

It is necessary to remove all the solvent from the fiber, usually carried out by counter current washing with hot water in conjunction with fiber stretching. 

A finish is usually applied as an aqueous emulsion to act as a lubricant and antistat and penetrates into the interior of the fiber if applied before the collapsed stage. Typical finishes are sorbitan esters of long chain fatty acids, polyoxyethylene derivatives and silicones. 

The relaxation process significantly alters the stress/strain properties of the fiber and can be incorporated in the drying as a continuous stage, or can be carried out by a batch process where the fiber is subjected to a hot wet environment, normally under a slight pressure, in an autoclave, to elevate the boiling point. As the temperature approaches 150°C, water plasticizes the molecules and the internal cracks and fissures fuse together. Silver sulfide staining (Chapter 17) can be used to examine for cracks and fissures. 

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Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Overlapping hatches i.e., more than one hatch at different processing stages resides in the plant at any given time. 

The flow diagram for the viscose process is given in Figure 2. The sequence of reactions necessary to convert cellulose into its xanthate and dissolve it in soda used to be performed batchwise. Fully continuous processes, or mixtures of batch and continuous process stages, are more appropriate for high volume regular viscose staple production. 

Secondary and tertiary processing normally infers material removal processes, e.g. turning, grinding, honing. Adequate machining allowances must be provided for at the primary/secondary processing stage when a secondary/tertiary process is used respectively. 

The process design linear program model is best written with flexibility in mind, such as extra matrix rows to provide flexibility in recycling, adding outside streams intermediate in the process, and determining component incremental values at each processing stage. This subject is discussed more fully later in this chapter. 

Gaseous monomers may also be trapped within the processing equipment and accidents have occurred as a consequence of the resulting pressure buildup. In the case of the polyacetals and poly(vinyl chloride) it is reported that at elevated temperatures these materials form a more or less explosive combination so that it is important to separate these materials rigorously at the processing stage. 

Thermal activation is characterized by two processing stages thermal... 

In storage, held up in process stages, in the working atmosphere, as wastes, etc. 

Fig. 7-18 Modulus vs. temperature dependence going through different processing stages.

Reduction in flammability is achieved by the incorporation of flame retardants into the polymer. Two possible approaches to this are available either the use of additives blended into the polymer at processing stage (additive type) or the use of alternative monomers which confer reduced flammability on the final product (reactive type). A number of elements have been found to assist with conferring flame retardancy on polymers, the main ones being bromine, chlorine, nitrogen, and phosphorus. 

Investigations of new sensors and instruments for on-line characterization of polymer batches at all process stages. 

The batch process control system we ve purchased provides only a starting point for our process research lab we must also identify and test a comprehensive set of controlled devices and real-time process instruments for chemists and engineers to use as building blocks for real feedback control systems. The technologies we are evaluating for characterization of polymer batches at all process stages include ... 

Food additives (like color additives) can be added at various processing stages, but the additions are commonly made at the latest production stage before final marketing to ensure optimal functionality of the additive in the product as sold. Residues of additives used at earlier production stages may be present still in the final formulation. However, if they do not have further functions at that stage, they are considered process aids (to help preparation but without technological effects in the finished product) and are not indicated on labels. Some additives also decompose over time so that the levels present at the end of shelf life may be quite low. Several techniques and information sources can be used to assess contamination levels of chemicals ... 

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