Melt rheology

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. 

FIGURE 3.4 X-ray diffraction diagrams of isotactic polypropylene crystalline forms. (From Turner-Jones, A. Aizlewood, Z.M. Beckett, D.R. Makromol. Chem., 1964, 75, 134-135. With permission.) 

For an elementary, unifying review of entrance-and-exit effects in viscoelastic fluids, see Hagler [87], For a thorough analysis of viscoelastic fluids, see Bird et al. [88]. 

A systematic study of the basic rheological properties for a wide variety of polypropylene melts has been made by Minoshima et al. [89]. These authors measured shear viscosities at low shear rates in a Rheomatrics mechanical spectrophotometer and at high rates in an Instron capillary rheometer. The principal normal stress difference, Ni, was measured in the mechanical spectrophotometer with a cone and plate device. The elongational viscosity, of special importance to fiber formation, was measured in an apparatus built by Ide and White [90], 

The method consists of heating rods of polypropylene on the surface of a hot silicone oil bath and then stretching them. All the experiments were conducted at 180°C. A description of the melts that were studied is given in Table 3.7. 

Dynamic mechanical properties are measured to evaluate melt rheology of thermoplastics with and without additives which may modify rheological characteristics of these compositions. Dynamic oscillatory shear rheometers are used for these purposes. Two geometries of test fixtures are used including parallel plates and cone and plate. Instrument use for these measurements must be capable of measuring forces (stress or strain) and frequency. Temperature must be controlled in a broad range and various modes of temperature sweeps should be available. Sample geometry is not specified but it should be suitable for measurement in particular experimental setup. 

If temperature sweep is used, the gap setting must be corrected for theraial expansion of material. Typical parameters measured include storage and loss moduli, tan8, complex modulus, complex viscosity, and dynamic viscosity. 

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Suspension- and emulsion-polymerized PVDF exhibit dissimilar behavior in solutions. The suspension resin type is readily soluble in many solvents even in good solvents, solutions of the emulsion resin type contain fractions of microgel, which contain more head-to-head chain defects than the soluble fraction of the resin (116). Concentrated solutions (15 wt %) and melt rheology of various PVDF types also display different behavior (132). The Mark-Houwink relation (rj = KM°-) for PVDF in A/-methylpyrrohdinone (NMP) containing 0.1 molar LiBr at 85°C, for the suspension (115) and emulsion... 

J. M. Dealy and K. Wissbmn, Melt Rheology and Its Role in Plastics Processing, Van Nostiand Reinhold, New Yoik, 1990, pp. 424—440. 

F. N. Cogswek, Polymer Melt Rheology, Halsted Press, a division of John Wiley Sons, Inc., New York, 1981, p. 40. 

COGSWELL, p. N., Polymer Melt Rheology, Geo. Godwin, London (1981)... 

While studying melt rheology in the MPT, it was observed that the m-EPDM-ZnO and m-... 

Dealy, J. M., et al., Melt Rheology Its Role in Plastics Processing, Kluwer, 1990. 

Liquid crystalline polymers (LCPs), 49 melt rheology of, 52 Liquid crystalline (LC) state, 48-49 Liquid crystalline thermotropic polyesters, 20... 

Cogswell FN (1981) Polymer melt rheology a guide for industrial practice. Woodhead, Cambridge... 

Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

Understanding the melt rheology of rubber nanocomposites is crucial from the processing perspective. Bandyopadhyay et al. [37] have studied the melt flow behavior of rubber-silica hybrid nanocomposites in a capillary rheometer. 

Winter, H.H. Evolution of rheology during chemical gelation. Prog. Colloid Polym. Scl,15,104—110,1987. Hempenius, M.A. et al. Melt rheology of arborescent graft polystyrenes. Macromolecules, 31, 2299, 1998. 

Experimental data on the solution properties and melt rheology of highly branched structures are scarcely found in the literature. This might be because of the structural nonuniformity of hyperbranched polymers, which makes it difficult to obtain reliable data. Because of the purely statistical nature of the poly-... 

M. J. McCarthy, R. L. Powell 2001, (Polymer melt rheology by magnetic resonance imaging), Macromolecules 34, 5520. 

PTT exhibits melt rheological behavior similar to that of PET. At low shear rates the melt is nearly Newtonian. It shear-thins when the shear rate is >1000s 1 (Figure 11.10) [68], At the melt processing temperatures of PET, 290°C, and of PTT, 260°C, both polymers have similar viscosities of about 200Pas. However, PTT has a lower non-Newtonian index than PET at high shear rates. The flow behavior can be modeled by the Bueche equation, as follows ... 

Broyer, E. and Macosko, C.W., Comparison of Cone and Plate. Bicone and Parallel Plates Geometries for Melt Rheological Measurements, SPEANTEC Tech. Papers, 21, 343 (1975)... 

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