Reactive extruders

Reactive extruders and extrusion dies of different designs can be easily included in standard technological scheme of polymer production plants, such as those for polycaproamide synthesis, as shown in Fig. 4.39. In this case, a reactive material premixed in a tank 1 is fed into a static device 2 for prepolymerization, where part of the polymerization process takes place. Then the reactive mixture enters the extruder-reactor 3. The necessary temperature distribution is maintained along the extruder. Transfer of the reactive mass proceeds by a system of two coaxial screws mounted in series in a common barrel. Controlling the relative rotation speed of both screws provides the necessary residence time for the reactive mass in the extrader, so that the material reaching the outlet section of the die is a finished polymer. 

Chen, Y.H., Liu, Y., Wang, Q., Yin, H., Nico, A., and Kierkels, R. 2003. Performance of intumescent flame retardant master batch synthesized through twin-screw reactively extruding technology Effect of component ratio. Polym. Deg. Stab. 81 215-224. 

For example, Nylon 6,6 can be reacted with PET in a reactive extruder to produce a block copolymer. However, as the concentration of A and B groups is relatively low, long residence times are required. Thus, it is more economical to form the block copolymer by other methods. ... 

A reactive extruder may be considered to be a horizontal reactor with one or two internal screws for conveying reactant polymer or monomer in the form of a solid or slurry, melt, or liquid. The most common reactants are polymer or prepolymer melts and gaseous, liquid, or molten low molecular weight compounds. 

These reactions are all commercially significant and the reactions may in some cases take place in a reactive extruder. The mechanism of the reaction depends on whether the reaction is anionically or cationically initiated, and the effects of reaction conditions are those as discussed before. 

These antioxidants include the hindered phenols and are considered to be most effective when the chain-carrying (propagation) radical is an -oxy radical such as alkyl peroxy, R02. Thus, in reactive processing, they would be expected to be of value in suppressing the oxidation reactions which can occur in the earlier zones of a reactive extruder. The chemistry of these systems has been studied in detail (Al-Malaika, 1989, Scott, 1993b), and it has been found in the case of hindered phenols that the effectiveness of these stabilizers is dependent on the chemistry of the oxidation product rather than the simple donor reaction of the phenol hydrogen atom to the propagating radical. 

Figure 3.44. A fibre-optic probe with air cooling and a sapphire window suitable for use in a reactive extruder or other rugged environment. After Hansen and Khettry (1994).

Most attention has been paid to applications of absorption and reflection spectroscopy in the NIR region for industrial process control (Dallin, 1997, Kammona et al, 1999, Fernando and Degamber, 2006). This arises firom the robust nature of the optical components and the availability of commercial systems that are based on gratings as well as interferometers. Transmission and reflectance probes that are designed to interface with extruders, and in particular reactive extruders, are commercially available, and their performance has been assessed quantitatively (Hansen and BQiettry, 1994). The interface for a rugged NIR probe to monitor the molten flow of an extruder is shown in Figure 3.44. 

Figure 6.28. A reactive extruder, showing the metal-at-die interface housing the real-time sensor probes for NIR, Raman and ultrasound (US) sensing of conversion and component concentration (Fischer et ah, 2006). Photograph provided by professor D. Fischer, Leibniz Institute of Pol)mier Research, Dresden.

There are several publications dealing with the comparison of efficiency of compounding, mixing, or reactive extruding in different types of extmders. Most of these studies suffer from the same aspect — the evaluated machines were not operated at a comparable level of performance efficiency. A summary of the reported observations is given in Table 9.14. 

In the second method, fibers, polymer, and maleic anhydride with the addition of peroxide initiator in one step processing are reactively extruded and then proceed with molding or injection to obtain a final composite. 

If the extruder is electrically heated, fluctuations of the wall temperature may also have an effect on the stable operation of reactive extruders. In general, many extruders are equipped with large heating elements to shorten the start-up time. Because the heat has to penetrate to the thermocouple in the wall, a certain time elapses before this thermocouple reacts and the heaters are switched off. With a large heating capacity, this implies that a considerable amount of heat can be released into the barrel wall before the... 

---