Bildung orientierter Randschichten in Spritzgu�platten aus Polypropylen

Abstract

Zusammenfassung: An 2 mm dicken unter verschiedenen Bedingungen spritzgegossenen Platten aus zwei Polypropylentypen mit unterschiedlicher Molmassenverteilung wird durch Messung der optischen Doppelbrechung die Dicke der hochorientierten Randschichten bestimmt. Durch solche Messungen an Platten, die mit und ohne Nachdruck gespritzt wurden, lassen sich die w/ihrend der Fiillphase von den wfihrend der Nachdruckphase entstandenen Orientierungsanteilen trennen. M6glichkeiten der theoretischen Berechnung der Randschichtdicke werden diskutiert.Hieraus ergibt sich, dab die hochorientierten Randschichten bereits wghreud des Fiillvorganges entstehen und deren Dicke vermutlich durch die Konkurrenz zwischen scherinduzierter Kristallisation und Relaxation bestimmt wird. Beim Spritzen ohne Nachdruck kann die Schmelze in der Mitre der Platte zwischen den Randschichten relaxieren und stark unterkfihlt werden, bevor sic kristalliert, wie dutch zusfitzliche Messungen nachgewiesen wurde.Abstract: The thickness of highly oriented skin layers is determined on 2-ram-thick plates injected under different processing conditions by birefringence measurements. Two technical polypropylene types of different molar mass distributions are used. Measurements on samples injected with and without pressure hold after filling make it possible to decide between chain orientation caused during injection and chain orientation caused by the pressure hold after filling. Different models for the calculation of the oriented layer thickness are discussed.The results reveal that the highly oriented skin layers are already formed during the filling phase and that thickness is probably determined through the opposing development of stress-induced crystallization and relaxation. Further measurements indicate that at the injection molding without pressure hold after filling, the melt can relax between the skin layers and can he strongly supercooled before it crystallizes.

Crystallization of Polypropylene and Poly(Ethylene Terephthalate)

“…Their observed T xMgh ~ 394K at R Tx = -20°C/min can be compared with the values of 358K and 387K, obtained using Eqs. 39b and 40c, which provide the best fits to the data of Koppelmann [59] and Varga et al [39], respectively, at the same R Tx .…”

“…under cooling. The dependences of T Xthw and T sMgh on R Tx , observed by Koppelmann [59] and Varga, Menczel, and Solti [39] in unrelated experiments, and curves for our best-fit equations to these sets of data are shown in Fig. 18.…”

“…Equations 39 and 40 give the best fits to the T Xjlm , and T xMgh data of Koppelmann [59] and Varga et al [39], respectively. We have used the positive sign for the argument of the exponential function for consistency with Eq.…”

“…Varga et al [39] also provided T z<mix data, so Fig. 18 [59], for the temperature limits T^ and 7^ of nonisothermal crystallization under cooling, as functions of the cooling rate R Tx (in C C/ min) and our best-fit equations to these data, (b) Data of Varga et al [39] for T At>H ., T tmal , and T lhish in nonisothermal crystallization under cooling and our best-fit equations to these data. The two sets of results are plotted using the same axes to facilitate comparison.…”

Modeling of polymer crystallization in plates, pipes, and rods during cooling

“…The temperature near the crystallization front might be increased by the liberation of the latent heat of fusion but may still be well below the melting temperature of the crystals. Polymers are poor heat conductors, hence, except for thin films, the temperature increases during crystallization in bulk 1–7. The temperature increase in a polymer due to crystallization either during cooling3–7 or at isothermal ambient conditions1 has been experimentally measured.…”