Melting of PP/PA6 Polymer Blends in Single Screw Extruders – An Experimental Study S. M. Cunhaa, A. Gaspar-Cunhab, J. A. Covasc IPC – Institute for Polymers and Composites, Department of Polymer Engineering, University of Minho, campus de Azurém, 4800-058 Guimarães, Portugal a
[email protected], b
[email protected], c
[email protected]
Keywords: Melting mechanism, Polymer blends, Single-screw extrusion.
Abstract. A prototype modular single screw extruder fitted with a screw extracting device is used to monitor melting of an immiscible polymer blend (PP/PA6, with different weight ratios) in this popular type of processing equipment. As anticipated, the observed phenomena are much more complex than those involved in extruding PP or PA6, when the well known Maddock/Tadmor mechanism is valid. Consequently a hybrid melting mechanism, involving Maddock/Tadmor and Dispersive melting sequences, is proposed. Introduction Polymer blending is a well established route followed by polymer manufacturers and compounders to prepare materials with improved properties for advanced applications. These materials are then processed using conventional equipment and converted into a variety of products. During material preparation - usually carried out in a twin screw extruder – the melting stage causes major changes in blend morphology [1-3], hence in properties. Melting temperatures, relative viscosity and surface tension should determine which melts first, what is the expected degree of dispersion, whether coalescence occurs and how fine and stable the final morphology will be. Consequently, one wonders if the final blend morphology is sufficiently stable to withstand the second processing experience – now typically in a single screw extruder – or if new morphological changes will take place. After Maddock’s pioneering and seminal work [4], several scientists tried to understand the physical phenomena involved in polymer melting, Tadmor and co-workers [5] being the first to develop a mathematical model able to describe his experimental observations. In this model, the solid pellets are compacted and form a continuous bed, which is separated from the barrel wall by a thin melt film. The relative velocity between the solids and the barrel pushes this film into a melt pool, which is deposited near to the pushing flight. In turn, the melt pool pushes the solid bed against the trailing flight, exposing more solids for melting at the solids-melt film interface. Improvements to this model have been progressively proposed [6-12], but it remains as the description in which major technological developments of various decades were based. In contrast with the Maddock/Tadmor mechanism, melting of individual solid pellets suspended in the melt has also been reported [13-15]. This was modelled by Huang and Peng and denoted as Dispersive Model [16], while Rauwendaal compared analytically the efficiency of the two melting models [17]. The above models and experimental observations considered the processing of simple polymer systems (typically, a homopolymer, homogeneous at the microscopic scale) in single screw extruders. The study of melting of polymer blends is much less common and seems to be limited to twin screw extrusion [18-20]. Therefore, given the industrial relevance of single-screw extrusion as a polymer processing technique, it is important to understand the influence of the melting stage on the morphology induced by the thermo-mechanical environment inside the extruder, this being the aim of a research programme being undertaken at the University of Minho. This paper concerns the
study of the melting of an immiscible PA6/PP polymer blend. No compatibilization was performed, as it is interesting to maximize any eventual changes in the morphological parameters during processing. Materials and experimental procedure Polypropylene (Repsol ISPLEN® PP 030 G1E, viscosity of 256.8 Pa.s for shear rate of 256 s -1 at 240ºC) and Polyamide 6 (DSM Akulon® F130, viscosity of 472 Pa.s at the same conditions) were mixed in various proportions (80/20; 50/50; 20/80 %w/w). The viscosity ratio of the two polymers within the typical extrusion shear rate range is approximately 1.8, thus favoring drop break-up and coalescence, as demonstrated by previous studies [21]. The polymers were pre-mixed in a drum mixer for 10 minutes. PA6 was dried at 70º C during 12h before processing. The blends were processed in a prototype modular single screw extruder, with a screw diameter of 30 mm and L/D = 30, fitted with sampling devices along the barrel (see [22] for details), pressure transducers at the same locations and with a mechanical system for fast screw extraction, as illustrated in Figure 1. Mechanical system for screw extraction Sampling devices 4.8D
4.8D
4.8D
5.8D
5D D1 = 20 mm
9.5 mm
D = 30 mm
4.8D
Heater band
125 mm L1= 10D
L2 = 10D L = 900 mm
L3 = 10D D3 = 26 mm
Pressure transducers
Fig.1 Extruder layout The experiments were performed using the same temperature profile (220/ 230/ 240 / 240/ 240/ 240ºC, from hopper to die), but varying other parameters. Upon reaching operating steady state and after registering the values of various process parameters, the screw rotation was interrupted, the die quickly removed and the screw extracted from the barrel by means of the available mechanical contraption. The whole operation took approximately 1 minute. The polymer helix in the screw channel was further cooled with compressed air and removed for subsequent analysis. Crosssections were obtained at regular down-channel intervals, observed, photographed and analyzed using a commercial software (Image-Pro Plus 4.5). Solid and melted regions were identified and their morphology monitored, providing an overall picture of the melting sequence along the screw. Results and discussion Figure 2 shows representative cross-sections obtained from one of the helixes removed from the extracted screw. Their width is the same (constant pitch screw), but the height varies downstream (see also Figure 1, where the length of the feed, compression and metering zones is identified). The solid pellets and the molten material are easily distinguishable.
Fig. 2 Cross-sections at various screw turns for PA6/PP (50/50) blend. The evolution of melting can be followed in different ways. In Figure 3, it is represented in terms of: i) relative solids area (solids area to total cross-section area), ii) conventional version of relative solids width (i.e., solids to channel width) and iii) number of individual solid particles/pellets suspended in the melt. Data is shown for three different compositions. As expected, the two first variables decrease along the channel. The lack of smoothness of the curves is associated with inherent processing instabilities and also due to the comparatively large size of the pellets in relation to the channel. Melting initiates quite early in the screw and is completed in the compression zone. In practice, it was difficult to adopt lower temperatures in order to delay its onset, due to the difficult processability of the PA6 grade. Identification of the presence of individual particles questions the exclusive development of a Maddock/Tadmor melting mechanism and points to the development of a dispersive mechanism. Since the melting temperature of PP is 165-175ºC and that of PA6 is around 220ºC, and the thermal diffusivity of both materials is also similar (see [23]), it is not surprising that melting initiates earlier and that melting length decreases with increasing PP content. In case of the blend rich in PA6, the evolution of X/W and As/At is similar. Melting is essentially governed by the behavior of PA6 (although this is not represented in the figure for the sake of clarity). A melt pool rich in PP is initially formed, then phase inversion takes place. Conversely, the evolution of As/At for the 20/80 PA6/PP blend seems to indicate that PP melts almost entirely before PA6. When processing pure PP or PA, no solid particles suspended in the melt can be observed. For blends that are rich either in PP or PA, a number of solid particles (most probably PA) remain suspended in the melt (most probably PP). In the first case, the Maddock/Tadmor mechanism seems to be adopted by PP, followed by dispersive melting by PA. In the second case, the Maddock/Tadmor route seems to be predominant. The 50/50 blend exhibits an intermediate behavior, where the simultaneous development of the Maddock/Tadmor and Dispersive models should be equated.
Fig. 3 Evolution of melting along the screw, characterized by several parameters. Experimental data such that discussed above indicate that immiscible polymer blends may follow an hybrid melting mechanism, which encompasses Maddock / Tadmor and Dispersive sequences, that may in fact coexist in some stages. This is shown in Figure 4 in a diagrammatic way. The extent of each stage depends essentially on the relative amount of the blend components and on the respective melting temperatures. Melt film
Melt pool Solid bed
1
2
4 5 Solid particles dispersed in melt pool
3
6 Solid particles surronded by molten polymer
7
Fig. 4 Hybrid melting mechanism
8
-Tadmor mechanism until 4 - 5 Transition mechanism - Dispersive model starting from 6
Conclusions Immiscible polymer blends follow a more complex mechanism than homopolymers, where the application of Tadmor´s melting model generally yields good quality predictions. Parameters such as individual melting temperatures, viscosity ratio and relative percentage of the components determine the sequence of events in terms of the onset of melting, melting mechanism and melting rate. Phase inversion may also occur. Further studies are obviously necessary to determine whether these events have consequences in morphological terms and also whether the compatibilized version of the same blends shows a more conventional behavior. Acknowledgements The authors are grateful to Portuguese Fundação para a Ciência e Tecnologia which support this work under grate SFRH/BD/19997/2004 and DSM, the Netherlands for the PA6 material.
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