Advantages of reactive extrusion

SAN can be modified, for example by grafting. For example, maleic anhydride (MA) can be grafted onto SAN. This process occurs preferably in a twin-screw extruder by reactive extrusion. An advantage of reactive extrusion is the absence of solvent as the reaction... 

If heat limitations cause smaller scale extruders to be chosen, the investment costs and operating costs increase and the economic advantage of reactive extrusion disappears. In the example above a rather extreme case with eight extruders with screw diameters of 100 mm was used. It can be concluded that this leads to a price that is approximately 2.5% higher than in the classical batch solution polymerization. 

The calculations are based on a relatively low production capacity of 5000t/year. For highly exothermic reactions large capacities can only be achieved by parallel scale-up. For a continuously operated solution polymerization, however, scale-up can be achieved by an increase of size. This leads to the conclusion that above a certain production volume the economic advantage of reactive extrusion will decrease with increasing scale. 

Most of the research on reactive extrusion is performed in twin screw extruders, although a single-screw co-kneader has definite advantages if micromixing plays an important role. Franz (24) gave an overview of applications of reactive extrusion with a Busskneader in a paper discussing the polycondensation reaction of silanoles to produce silicon oils. Aeppli (25)... 

Reactive extrusion has emerged from a scientific curiosity to an industrial process. Various types of extruders can be used, all with their specific advantages and disadvantages. Further development suffers from lack of kinetic and rheological data at high conversions and from uncertainties about heat transfer and reactor stability. Nonlinear effects in the process can give rise to instabilities that are of thermal, hydrodynamical or chemical origin. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

Many solution polymerizations are batch processes. Contrary to this, reactive extrusion is a continuous process. An important economic advantage of a continuous process is its relative simplicity, resulting in less operator units necessary to run the plant. Moreover, continuous processes require no large storage vessels as they are necessary for a batch process. 

Another trend is to develop processes to synthesize PLA and at the same time take advantage of the fabrication method. An example of this is the reactive extrusion process which is an attractive approach to prepare high molecular weight PLA. Jacobsen et al. studied the polymerization of lactide in a twin-screw extruder and obtained PLLA of molecular weight 100,000 within 7 min... 

In an A/E polymer blend, the reactive additive can react with both polymers as a coupling agent. The advantages of this approach are the availability of many cheap reactive products and also the capability to diffuse in the melt as they possess a low molecular weight. The latter property makes the reaction kinetics suitable for extrusion. On the other hand, the disadvantage of a specific reactivity of the additives can lead to side reactions as the undesired polymer branching. 

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