Reactive extrusion processing reactions

Thermal quenching of the crosslinking reaction In an actual reactive extrusion process, the degree of crosslinking can be controlled by adjusting the residence time at elevated temperature. 

The first results of degradation of polyamides shown above are based on fast melt modification reactions. The determination of the gross kinetical reaction rate was impossible. Only a very vague estimation could be made during reactive extrusion processes. 

Reactive extrusion is well surveyed by a recent text (Xanthos, 1992), and has been referred to as the deliberate performance of chemical reactions during continuous extrusion of polymers and/or polymerizable monomers (Xanthos, 1992). This usually represents chemical reactions conducted during a continuous extrusion process at relatively short residence times. Examples of types of reactions and example polymer systems used in reactive-extrusion processes are given in Table 2.1. 

In addition to these complications, Moad (1999) notes that, for typical reactive modifications, the amount of modification can be quite small (0.5-2 mol%) and therefore very difficult to characterize. However, Moad (1999) does suggest some techniques such as chemical methods, FT-IR, NMR and DSC that may be useful to aid characterization. Janssen (1998) also notes complications of thermal, hydrodynamic and chemical instabilities that can occur in reactive extrusion that must be addressed by combining knowledge of the chemistry and of the physics (flow behaviour, mixing) of the reactive extrusion process. Xanthos (1992) presents the importance of understanding both the chemistry and the reaction engineering fundamentals of reactive extrusion, in order better to understand and model the process in practice. 

Polybutadiene-PET block copolymers (PBD-b-PET) ccmtaining about 10 wt% PBD blocks have been developed by a reactive extrusion process via a transesterification reaction between PET and hydroxyterminated polybuadienes using a cobalt salt as a transesterification catalyst (Tibbitt et al. 2007). Such a PBD-b-PET copolymer containing preblended cobalt catalyst is sold commercially as Amosorb DEC (ColorMatrix USA) resin, for use as an oxygenscavenger concentrate for blending with PET to make enhanced barrier PET bottles. 

Execution of chemical reactions during extrusion of polymers and/or polymerizable mmiomers. The reactants must be in a physical form suitable for extrusion processing. Reactions have been performed on molten polymers, on Uquefied or dispersed monomers, or on polymers dissolved or suspended in or plasticized by a solvent (also reactive compounding or reactive processing). 

The reactive extrusion process for grafting maleic anhydride monomer (MAH) onto polypropylene backbone involves a balance of simultaneous reaction steps as described in Chapter 3 ... 

The equations above provide a possibility to describe the viscosity changes during the reactive extrusion process in a relatively simple way. Only few parameters have to be adjusted for an accurate description of the viscosity changes on a physical basis. Figure 4.3 shows the general tendency for the viscosity development in the different types of reaction. 

For reactive extrusion of butylmethacrylate the influence of the rotation rate of the screws and the throughput on the product were also investigated by Jongbloed et al. (11) in a self-wiping corotating twin-screw extruder. The inhibited monomer was mixed with a combination of two peroxide initiators with different half-life values. A combination of initiators was used to prevent a deficiency of radicals towards the end of the reactive extrusion process, where due to the reaction, the temperature increased significantly. The total initiator concentration was 0.07mol/l. The mixture of monomer and initiators was fed to the extruder at room temperature. 

Prepolymerization can improve a reactive extrusion process for several reasons. First of all, the heat, due to the polymerization reaction in the extruder decreases, since part of the heat of polymerization is already released in the prepolymerization reactor. This decreased heat is important, because too high temperatures limit the conversion. This limitation in conversion is connected to the occurrence of a ceiling temperature, the... 

From the interaction diagram as given in Fig. 13.3, the gel effect is identified as a possible source of chemical instabilities. Nevertheless, there are two other chemical effects that influence the stability of reactive extrusion The ceiling temperature and the phase separation. The stability of a reactive extrusion process is influenced by these three factors because they have a direct influence on the viscosity and reaction speed and therefore on the hydrodynamic instabilities. 

With the pressurized hydrolysis technology, water under high temperature and high pressure is applied to hydrolyze and break the crosslinks of the paint film, and then a kneading extruder is used to convert the decomposed film into particles and disperse them in the TP resin. Two types of the processing system have been developed. One is the autoclave reaction and screw extrusion processing system (batch system), and the other is the twin screw reactive extrusion processing system (continuous system). 

The performance of single-component polymerizations in a twin screw extruder is dependent on several extruder parameters. Each parameter has its specific influence on the conversion of the reaction and the average molar mass of the polymer formed. To understand the influence of the parameters, a theoretical analysis of the reactive extrusion process was developed in the form of a reactive extrusion interaction diagram. The conclusions which can be drawn from the theoretical and the experimental investigations of a single-component reaction (the polymerization of butylmethacrylate) in a twin screw extruder are as follows ... 

For example, the functionalization of a conventional EP rubber by reaction with maleic anhydride, followed by the melt blending of polyamide 6 caused an interfacial reactive compatibilization leading to a significantly improved dispersibility of the rubber into much smaller particles (Fig. 8.2) even when the entire process was done as a 1-step, sequential reactive extrusion process on a twin screw extruder [11]. 

At the downstream of the extrusion process a suitable reaction catalyst, reactive diluent, e.g., crosslinking monomer, is fed into the molten polymer mix. 

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