Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
The increasing significance of polypropylene for packing materials is leading to a more widespread use of this semi-crystalline thermoplastic as film, vacuum-formed packagings and blow moldings. This paper deals both with the extrusion of semi-finished ware (film/sheet) and with the biaxial stretching, thermoforming and stretch blow molding of PP. It begins with the influence of cooling on the crystallization and the structure, taking into account various nucleation possibilities. It was found that regranulated scrap as a nucleation additive with PP homopolymer can lead to a fine, spherulitic structure of high crystallinity, which is of particular significance for its use as thermoformed sheet. The relationship between crystallinity or density and mechanical properties was not clear provided there was an influence of defects (e.g. in the spherulite border in the case of a coarse structure). What proved to be dependent on this was both an increase and a decrease in the modulus of elasticity with rising density.With the biaxial stretching of the sheet of PP homopolymer and copolymer in a stretching frame, there were marked differences in the overall properties of the stretched sheets as far as the mechanical strength data were concerned. Whereas, in the case of the copolymer, the stress at break and modulus of elasticity could only be raised slightly, in the case of the homopolymer, increases in Young's modulus of 100% and more were achieved.With biaxial stretching, only a small influence on the cooling rate was noticeable, and yet the effect of the thermal history from the extrusion was particularly marked during the subsequent thermoforming process. There, a coarse structure led to poorer product properties, it is impossible to compensate this by varying the forming temperature or the mold temperature. The forming temperature nevertheless proved to be the decisive parameter for the quality of the molding; as the forming temperature rises, there is a major increase in the molding accuracy of the radii and in the gloss. The thermal shrinkage drops. We can conclude from this that polypropylene should be formed just below its melt point.With the aid of theoretical observations, the cooling and conditioning phase for a PP stretch-blowing process are explained “out of first heat”, which was realized and examined in a machine concept - 8-stage machine. With cycle times of approximately 15 s, it was possible to manufacture 1 1 bottles with a crystallinity of 40-45%.
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