Reactive Extrusion Systems

Leon P.B.M. Janssen

• Publisher: CRC Press
• Year: 2004
• Print ISBN: 9780824747817
• eBook ISBN: 9781135544409
• Topic: Physical Sciences
• Subtopic: Chemical & Biochemical Engineering

Citing recently realized applications for extruders as polymerization, modification, and degradation reactors and presenting a telling array of new research results and illustrative experimental cases, Reactive Extrusion Systems sheds light on the complex set of interactions underlying reactions in extruders. The book succeeds as a three-part surve

1
Introduction

I. HISTORY

The possibilities to use an extruder as a polymerization reactor were already recognized in 1950. A patent of Dow Chemical Company (1) described a polymerization unit in which a single-screw extruder was used as the main polymerization device. For the first part of the polymerization process, when low viscosities prevailed, a continuous stirred-tank reactor with a residence time of 90 h was used as a prepolymerizer, after which the material was transferred to the single-screw extruder for the high-viscosity part of the reaction. In this extruder a residence time of 18 h was necessary for the thermal polymerization of styrene. The first polymerizations described in open literature were the polymerization of nylon (2) and several polycondensation reactions (e.g., Ref. 3). In the mid-1970s, the first theoretical considerations concerning reactive extrusions appeared. Meyuhas et al. (4) stated that an extruder is the best plug flow reactor for viscous materials, but that some distribution in molecular weight of the polymer formed can not be prevented. In this study, a prepolymerization is advised to avoid low viscous material to be fed to the extruder. Mack and Herter (5) proposed twin-screw technology for reactive extrusion because of difficulties in scaling up single screw extruders that could be avoided in twin-screw extruders. Residence times of half an hour were possible in self-wiping twin-screw extruders. Mack and Herter also concluded that a combination of a stirred-tank reactor, a single-screw extruder, and a twin-screw extruder was most suitable for the production of polyesters.

In more recent years, the radical polymerization of several methacrylates was studied in a counterrotating twin-screw extruder (6–9). Stuber and Tirrel (6) and Dey and Biesenberger (7) studied the radical polymerization of methylmethacrylate, Ganzeveld and Janssen (8) described the polymerization of n-butylmethacrylate, while Jongbloed et al. (9) also studied a copolymerization of butylmethacrylate and 2-hydroxy-propylmethacrylate. Both Jongbloed and Ganzeveld found maximum conversions of roughly 96% in one step based on gravimetric analyses, while Dey claimed complete conversion after pre-polymerization, as measured by gas chromatography. Similar differences in conversion are sometimes found in literature. A possible explanation for this was given by Van der Goot et al. (10), who pointed out that the method of analysis can have a distinct influence on the final conversion found. They measured the conversion during reactive extrusion of styrene in the same samples both with a gravimetrical method and by using gas chromatography. It was found that the conversions with gas chromatography were up to 3% higher than that when measured by gravimetry. Jongbloed et al. (11) compared a self-wiping corotating twin screw with a counter rotating closely intermeshing twin-screw extruder for the copolymerization of n-butylmethacrylate with 2-hydroxy-propylmethacrylate. Apart from the polymerizations already mentioned, the counterrotating extruder was also used for the polycondensation of urethanes (12) and the anionic polymerization of ε-caprolactam (13). The possibilities for reactive extrusion of urethanes were also recognized by several other authors (14–16).

Other reactions have been described in various types of extruders, like the use of corotating twin-screw extruders for the anionic polymerization of ε-caprolactam (17–19). The anionic polymerization of styrene was investigated by Michaeli et al. (20), who also published work on the copolymerization of styrene with isoprene (21). The radical polymerization of styrene and several copolymerizations with styrene as the main component were described in a patent by Kelley (22). This patent also describes the synthesis of high-impact polystyrene (HIPS). Van der Goot and Janssen (10) investigated the polymerization of styrene and the influence of prepolymerization on the maximum stable throughput. A comprehensive overview of reactive extrusion is given in a monograph, edited by Xanthos (23). In this work Brown gives a listing of over 600 reactive extrusion processes that have appeared in open and patent literature between 1966 and 1983. The most surprising conclusion from this survey is that during that period more than 600 patents on reactive extrusion were granted to 150 companies, but only 57 technical papers were found. Only three of them were from companies holding five or more patents.

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) describes the ionic polymerization of acetals in the same type of machines and Jakopin (26) reports on reactive compounding, where the good mixing action of a co-kneader is an advantage.

Foster and Lindt developed models for devolatilization in connection with reactive extrusion (27, 28). In later work it has been described that in a reflux flask reactor a significant acceleration of a transesterification reaction could be achieved if a boiling inert hydrocarbon solvent was present (29). The same significant enhancement was found in reactive extrusion with simultaneous devolatilization, during a monoesterification reaction between styrene-maleic anhydride copolymer and alcohol (30).

Reactive extrusion is also attractive for grafting or modification reactions, where micromixing is an important factor for obtaining a homogeneous end product. Typical examples are the free-radical grafting of maleic acid, glycidyl methacrylate, or acrylic acid onto polyolefines (31–34). The resulting functionalized polymers can be used for blending with other polymers; also the adhesive properties of these polymers to metals or glass fibers improve (35). Apart from the desired grafting reactions, unwanted side reactions such as cross-linking of polyethylene and chain scission in polypropylene may occur (36).

Little is known about the stability of the reactive extrusion process. Especially if a transition of very low to very high viscosities occurs, as is the case during polymerizations starting from low-molecular-weight monomers, a sharp transition between high conversion and low conversion of the reaction can occur as a result of very small changes in operating conditions. This may be accompanied by severe fluctuations in throughput and conversion. Van der Goot et al. (37, 38) noted that stability can be increased by increasing reaction speed or viscosity buildup, which later could be explained qualitatively by multiplicity in stability of the length over which the extruder is fully filled with material (39).

II. ADVANTAGES AND DISADVANTAGES OF REACTIVE EXTRUSION

Due to its specific properties, the extruder has certain advantages as a reactor for polymerization and modification reactions:

• The extruder is a stable pump for highly viscous media. This guarantees a constant throughput which is vital for its operation as a continuous reactor.
• The process is continuous in contrast to various conventional processes that operate batchwise.
• The mixing can be adjusted to the requirements for optimal reaction conditions by a judicious screw design.
• Devolatilization of the reaction product in the extruder makes it possible to remove and recycle unreacted components.
• No or only a small amount of solvent has to be used in an extruder-polymerization process; therefore, no expensive extra separation steps are needed. Due to the absence of volatile solvents, the process is also more environmentally friendly. This is important as legislation increases strongly in this area.

However, there are also some restrictions to the use of extruders as polymerization reactors as well as to the type of extruder to be used.

• As the extruder is a reactor with a relatively expensive volume, the residence time needed for the reaction should be short. Therefore, the reaction kinetics have to be sufficiently fast to acquire an economically feasible process.
• There is a limitation to the reactions that can be performed in extruders based on the heat of reaction and the viscosity reached. If the reaction enthalpy is very large, the temperature rise in the extruder is too large to control. Moreover, the viscosity of the reaction product has to be sufficiently high to be able to obtain a stable transport of the material and to make the use of an extruder profitable.
• On scaling up the equipment, the surface-to-volume ratio decreases, which limits the heat removal in production machines. Moreover, in production-size machines thermal i...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE
• 1. INTRODUCTION
• 2. EXTRUDERS
• 3. CHEMICAL KINETICS
• 4. RHEOLOGY AND RHEOKINETICS
• 5. MIXING AND REACTIONS
• 6. HEAT BALANCES AND HEAT TRANSFER
• 7. CHAIN-GROWTH HOMOPOLYMERIZATIONS
• 8. COPOLYMERIZATIONS
• 9. STEP-GROWTH POLYMERIZATIONS
• 10. MODIFICATION REACTIONS
• 11. REACTIVE COMPOUNDING
• 12. SCALE-UP
• 13. STABILITY
• 14. ECONOMIC FEASIBILITY