Polypropylene melt fracture

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Polypropylene melts are viscoelastic fluids. As such, the melts exhibit non-Newtonian viscosity, normal stresses in shear flow, excessive entrance-and-exit pressure drop, die swell, secondary entrance flows, melt fracture, and draw resonance. (Newtonian fluids also exhibit draw resonance.) Polypropylene melts are more viscoelastic than melts of nylon and polyester. 

Many authors have studied the melt fracture of polypropylene. Bartos [97] found for a variety of polypropylene resins that the critical wall shear stress for melt fracture ranged from... 

X10 to 16 X lO dyn/cm. Melts of many other plastics exhibit melt fracture in the same range of shear stress. Vinogradov et al. [98] studied melt fracture in polypropylenes with MFR ranging from 1.17 to 30.5. He also identified three instabilities. His critical shear... 

Melt fracture has been a very perplexing but fascinating problem ever since it was discovered. Another problem that seems to have the same degree of perplexity and fascination is draw resonance. Both are instabilities in polymer flows. (Draw resonance may also occur in Newtonian fluids.) Draw resonance is a periodic variation in the diameter of a spinning thread line above a critical drawdown ratio. Polypropylene and high-density polyethylene are both particularly susceptible to draw resonance. Petrie and Denn have presented a comprehensive review of the numerous theoretical and experimental studies of draw resonance conducted prior to 1976 [99]. 

Die swell is always present in polypropylene spinning because its melt is viscoelastic. The viscoelasticity also gives rise to melt fracture if temperatures are too low or if orifice shear rates are too high. Melt fracture must be avoided. 

Nadella et al. [79] studied polypropylenes with a broad range of melt flows. Table 3.25 lists the properties of the melts they studied. These melts were spun from a screw extruder at a temperature of 230°C. In the case of H-0060, filaments were also spun at 200 and 260°C. With the exception of H-0042, the polymers were spun through a single capillary at 2.1 g/min. The H-0042 could not be extruded at that rate because of melt fracture this material was extruded at 0.5 g/min. The filaments were quenched in stagnant air at 25°C and were taken up at speeds of 50,100,200,400, and 550 m/min. X-ray and birefringence measurements were taken along the descending spin-line. 

Ariffin, A., Ariff, Z. M, Jikan, S. S. (2011). Evaluation on extrudate swell and melt fracture of polypropylene/kaolin composites at high shear stress. J. Reinf. Plus. Compos., 30, 609-619... 

Figure 2.11 Difference between the phenomenon of matte and melt fracture (on distorted extrudates of different polymers) (1) rigid polyvinyl chloride, (2) polyethylene, (3) polypropylene. (4 5) polypropylene viewed from two angles, (6) polymethy-methacrylate, (7) polytetrafluoroethylene. (Reprinted from Ref. 66 with kind permission from Society of Plastics Engineers. Inc., Connecticut, USA.)...

The polysulfone was Udel P-3900, the Polypropylene was a melt fracture grade 9003 (ME =0.8g/10inin) and the two compatibilizers we examined were Fusabond P 51 ID (maleic-anhydride grafted Polypropylene) and Fusabond A 160D (maleic-anhydride grafted EMA). The two binary blends examined contained 20%PP/80% Polysulfone. The ternary blends contained 5% by weight compatibilizer with the ratio of PP and Polysulfone remaining the same. Preparation of Films... 

The mass fraction crystallinity of molded PHB samples is typically around 60%. As shown in Table 3, PHB resembles isotactic polypropylene (iPP) with respect to melting temperature (175-180°C), Young s modulus (3.5-4 GPa) and the tensile strength (40 MPa). In addition, the crystallinity of iPP is approximately 65% [18]. Accordingly, the fracture behavior of PHB may be anticipated to be tough at room temperature. Molded PHB samples do indeed show ductile behavior, but over a period of several days at ambient conditions, they slowly become more brittle [82, 85, 86]. Consequently, the elongation to break of the ultimate PHB (3-8%) is markedly lower than that of iPP (400%). 

SEM photomicrograph of a melt-blended uncompatibilized 90 wt% polypropylene/10 wt% polycyclo-hexylmethacrylate blend fracture surface obtained at room temperature. (From C. Harrats, T. S. Omonov, G. Groeninckx, and P. Moldenaers, Polymer 45,8115,2004. With permission.)... 

Fig. 10.8. Extrema separation vs. temperature of fractured atactic polypropylene labeled with nitroxide radical O, before melt , after melt (Ref )...

Kolbeck and Uhlmann have considered the effect of high stress extrusion on the properties of several polymers - polypropylene, poly(vinylidene fluoride) and polyethylene. There was no substantial heating on deformation up to twenty-five times. Polypropylene and poly(vinylidene fluoride) could not be continuously extruded below the melting point and brittle fracture and necking was observed, while the tensile properties depend on draw ratio with yielding. Above 373 K annealing occurs but the product has a lower elongation to break. The properties of oriented nylon-6, > polyfethylene terephthalate)," - polypropylene, and polyethylene " have been widely studied. 

Polyethylenes are tough, ductile, and easily moulded, but their moduli, yield stresses, and melting points are relatively low. Polypropylenes are stifler, have higher yield stresses and melting points, and also mould well, but in comparison with polyethylenes are more prone to fracture, especially at low temperatures. This is basically a consequence of their higher yield stresses. Rubber toughening substantially improves low-temperature fracture resist-... 

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