ISSN:
0730-6679
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Chemical and physical processes are attracting increasing attention from plastics processors. The reasons for this are basically twofold. On the one hand, more and more processors are recognizing that the quality of their products is highly dependent upon physical processes (cooling, temperature control, blending, etc.) and, on the other hand, new methods of physical and chemical treatment are being developed which are bringing improvements in quality and hence, virtually inevitably, greater profitability in production. This technology has made its impact both on discontinuous processing (reaction injection molding), on cable and pipe extrusion, and on the direct polymerization of tapes and films. The formulations that can be applied here are monomer casting systems. A prerequisite for the industrial utilization of this system is a process analysis of the reaction molding compound processing in a parallel-running twin screw extruder to allow the influence of the process parameters on product quality (e.g., molecular weight) to be clarified and the polymerization to be integrated into other processing sequences, such as panel extrusion. The modification of tried and tested plastics through blending with other polymers, however, can also change the properties of the material in such a way that cost-intensive plastics can be replaced or even new fields of application can be opened up. The attainable properties are just as much a function of the individual components and their percentage content as they are of the blending technique, as investigalions on a corotating twin-screw extruder have shown. The melting zone takes on primary significance, since it is here that both the homogenization and desired or undesired high shearing takes place. The crystallization of semicrystalline plastics also has a considerable influence on product quality. To date this has been influenced by the selective addition of nucleiproducing foreign substances. A new process substitutes electron-irradiated granules for these nucleating agents, such that the drawbacks of the foreign particles (toxicity, notch effect, and optical impairment) are eliminated. The effectiveness of this measure is reflected in better mechanical characteristic values. These both allow a higher product quality and a shortening of cooling time and hence higher productivity. Modijication of material properties through subsequent crosslinking represents the state of the art in rubber and polyethylene extrusion for cables and pipes. Special peroxides and the application of microwave energy make it possible to crosslink extrudates (pipes, profiles) made of HDPE and EPDM. The kinetics of crosslinking are explained, taking the example of crosslinking with peroxides. It is seen that the process technologv has to be adapted to the compound in terms of residence time and temperature. Under these conditions the microwave energy crosslinking plant operates with considerably shorter plant lengths and hence with lower energy losses.
Additional Material:
50 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/adv.1986.060060308
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