Equipment Reactive extruders

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... 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

Example 3. Hydrolysis in an Extruder.69 PET reactive extrusion experiments were carried out on a 25-mm Berstorff ZE25 corotating twin-screw extruder with a barrel length-to-screw ratio of 28 1. The extruder consisted of six barrel sections equipped for heating, cooling, and controlling the temperature of each section of the extruder. Initially reaction extrusion of PET and water was performed with cold water at room temperature injected into the extruder. Typical operating conditions were reaction temperatures of 230-265° C, extruder speeds... 

Reactive processing is limited to polymerization or chemical reactions of polymers in conventional singlescrew or twin-screw extruders, excluding processes in oscillatory kneaders, Banbury-type continuous mixers, or Diskpack equipment. Emphasis is placed on continuous processes that have been implemented commercially or that can serve as models for commercial purposes. 

Polyamide-6 (PA-6) and polypropylene (PP) are both semi-crystalline polymers and the combination of an engineering plastic (PA) and the best commodity product (PP) could lead to new blends with Interesting Intermediate properties. We tested systems containing 50 wt% of each product and the ones obtained by addition of 3% of the reactive PP-g-AM resulting from previous continuous grafting in the extruder. The blends were prepared by simple mixing in the ZSK 30 twin-screw extruder and the samples for mechanical testing were molded by injection in a BILLION equipment. 

It is expected that many novel and useful polymer blends will be obtained using supercritical CO2 by applying both the reactive as well as the non-reactive route, and an extruder may well be a good choice of equipment for reactive blending in the continuous production of various polymer blends. 

Given that small working mass is a more economical means of reactive extrusion, the distinction between different types of extrusion equipment branches off into a whole host of processing attributes. Because of increasing trends in the late 1990s to replace single extruders by twin-screw extruders, our attention is now directed at twin-screw methodology to modify and reinforce polypropylene resins into a wide spectrum of composite materials. 

A special complication occurs when scaling up a reactive extrusion process. Where in small-scale extruders the surface-to-volume ratio is quite large, this ratio diminishes proportionally to the screw diameter. As a consequence, the relative amount of heat transferred in large production machines can be much lower than in similar laboratory size equipment. Moreover, in large-scale equipment the heat that is released in the middle of the channel has to be transported over a much larger distance than in small extruders. This leads to increasing radial temperature differences in larger extruders. Therefore, an analysis of the heat transfer characteristics of the extruder and its dependence on scale is essential for the reactive extrusion process. 

A second consideration in scaling down a reactive extrusion process concerns the axial temperature profile. Because of the different volume to surface ratio between large-scale and small-scale equipment the cooling capacity of the latter will be far better than in production size machines. To obtain similar axial temperature profiles, the temperature settings of the barrel wall in the small-size extruder should be different from those in the large-size machine. The most convenient situation is, of course, if a reliable mathematical model of the reactive extrusion process is available. An alternative is the use of an approximating heat balance over the extruder ... 

For process development, it is often important to scale the prospected process down to lab experiments. A dimensionless number can be defined that indicates whether large thermal inhomogeneities may be expected. Especially in reactive extrusion, neglecting the importance of this number by using too small extruders will inevitably lead to erroneous results and problems in the large-scale equipment. 

Vented extruders are significantly different from non-vented extruders In design and in functional capabilities. A vented extruder is equipped with one or more openings (vent ports) in the extruder barrel, through which volatiles can escape. Thus, the vented extruder can extract volatiles from the polymer in a continuous fashion. This devolatilization adds a functional capability not present in non-vented extruders. Instead of the extraction of volatiles, one can use the vent port to add certain components to the polymer, such as additives, fillers, reactive components, etc. This clearly adds to the versatility of vented extruders, with the additional benefit that the extruder can be operated as a conventional non-vented extruder by simply plugging the vent port and, possibly, changing the screw geometry. 

According to Jakisch et al. [79], FTIR spectroscopy is the preferred method for in-line investigation polymer melts and polymer melt reac-tions/kinetics, allowing quantitative determination of all components. FTIR analysis of compound melts enables additive level stability and effectiveness to be observed over multiple extrusion passes. The use of the ATR principle is suitable for in-line analysis of polymer melts in the extruder. The exit of the extruder was equipped with an on-line IR transmission process control system consisting of a 150 /um thick ZnSe melt flow cell. Characteristics of such systems have been described [71,74]. Another process spectrometer with an in situ ZnSe-ATR dipper probe was mounted at different positions in the extruder. For in-line ATR the residence time plays no role. Only the first 5 /xm (corresponding to the penetration depth of the IR radiation) are examined. Minor components are thus detected with difficulty. Jakisch et al. [79] monitored the conversion of styrene-maleic anhydride copolymers (SMA) with fatty amines into styrene-maleimide copolymer (SMI) during reactive extrusion by means of FTIR. In principle, both mid-IR and near-IR spectroscopy with ATR, transmission and diffuse reflectance probes are suitable for quantitative on- and in-line process analysis of multicomponent polymer... 

Most of the large eompanies active in polymer technology, polymer compounding, and formulation, are moving from classical reaction vessels to processing equipment with respect to polymer chemistry. In this context, extruders are progressively considered as effective continuous reaction vessels. Polymer literature counts about 660 citations on reactive processing over a period of 17 years [11]. Of these, about 90% of the references encountered were patents. 

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