Single screw extruder, improvement

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

GriinschloB, E., A New Style Single Screw Extruder with Improved Plastification and Output Power, 7/it. Polym. Process., 17, 291 (2002)... 

Single screw extruders develop more frictional heat and rely more on back pressure from the die to improve homogeneity. Screw design involves higher compression ratios, e.g., 2 1, which means the volume occupied between two fights at the start of the screw is twice that occupied between two consecutive flights at the end of the screw. 

Single-screw extruders have been widely used for blend preparation however, they do not offer sufficiently high stress levels to improve mixing thus, special designs of screws have been developed such as those with mixing heads or barrier zones that increase residence time and enhance blend mixing. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

Although it is one of the oldest types of machinery for plastics processing, the single-screw extruder has not yet reached the end of its development—as can be seen in improvements over the past few years, based on the continuing industrial demand for improvements. Output, a major focus years ago, remains paramount but qualities such as wear and versatility are of equal importance. Improvements in all equipment are ongoing, with additional new developments occurring in injection molding machines (Chapter 2). 

These dependencies are shown in Fig. 9.2. Evidently, simple shear is inefficient for generation of a large inteifacial area (for smaller deformatiOTis, R < 10, the inefficiency is not as dramatic). The inefficiency can be reduced if a large deformation is obtained not at once, but in a series of smaller deformation steps. The best formula is stretch-twist-fold, the second best is stretch-and-fold, and the third is stretch-and-randomize. Thus, to improve the mixing efficiency of the processing units that operate in the shear mode (like in a single-screw extruder), the flow should be interrupted and the flow lines randomized. For uniform shear strains of randomly oriented elements, the interfacial area increases linearly with strain, and when the process is repeated, the total increase of the interfacial area becomes proportional to the square of strain (y ), etc. ... 

When single-screw extruders are starve fed (Fig. 5.12d), plastic particles do not immediately fill the screw channel. As a result, the first few channels of the feed zone lack the pressure required to compact the polymer particles. Particle conveyance in the imfilled channels is not as steady as transport with filled channels. Consequently, metered feeding is seldom used with single-screw extruders. Such feeding can be used to reduce the motor load, limit temperature rises, add several components through the same hopper, improve mixing in singlescrew extruders, control flow into vented barrel extruders, and feed low-bulk-density materials. 

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