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Deformation Behavior of Poly(viny1idene fluoride)+ D. R. Salnl' and A. V. Shenoy Polymer Science and Engineerlng Qroup, Chemical Engineering Division, National Chemical Laboratory, Pune 4 1 1 008, India
The deformation behavior of poly(viny1idene fluoride) has been analyzed in both the melt state and solid state in order to develop unified grade and temperature invariant curves through the use of simple and appropriate normalizing factors. The advantage of such master curves lies in their ability to provide quick estimates of shear viscosity and normal stress difference as well as long-term mechanical behavior with resonable accuracy useful for design purposes.
Introduction Poly(viny1idene fluoride) (PVDF) has an outstanding combination of properties such as piezoelectricity, toughness, abrasion resistance, low smoke generation, and low flame spread, because of which it has a number of applications. In order to form various products, PVDF has to be processed by conventional polymer processing techniques. During processing, the polymer melt undergoes deformation over a wide range of temperatures and shear rates. Therefore, a knowledge of the rheological behavior of the melt at various shear rates and temperatures is essential for an assessment of the material processiblity as well as process design and optimization. This information is normally made available by the manufacturer of this material in the form of apparent viscosity vs. shear rate curves at a fured temperature or the apparent viscosity vs. temperature at a fixed shear rate. However, the data are limited to a few grades and a fairly narrow range of temperature and shear rate. Thus, if a different grade is to be processed or the same grade at a different temperature, the rheological characteristics of this material would have to be determined on sophisticated instruments like the Weissenberg Rheogoniometer, Instron Capillary Rheometer, Rheometrics Mechanical Spectrometer, etc., all of which are very expensive and require trained operators for data generation. In such circumstances, what is desirable is a quick and easy method for the determination of this data for process design and development work. In the present article, it is shown how the rheological behavior of PVDF (even those available from different manufacturers) can be coalesced on a single temperature-invarient master curve. The idea of generating master curves can be extended even to the product properties, especially those wherein data generation is cumbersome and time-consuming. For example, in assessing long-term mechanical behavior such as creep or stress relaxation of the material, the general practice is to actually carry out stress-strain tests at regular intervals for prolonged periods of days, months, and even years. This results in sets of curves like stress vs. time of stress for each grade of material at different temperatures. Proper normalization can convert such curves into master curves, as shown in the present article. The advantage of such master curves is that they would then eliminate prolonged data generation and act as excellent qualitycontrol tools for process design and development. Shear Viscosity Master Curve Viscosity vs. shear rate data are available for several commercial PVDF materials. As an example, Figure 1 NCL Communication No. 3771. 0196-4321f8611225-0277$01.50/0
shows the viscosity vs. shear rate curves for one grade of DYFLOR 2000 PVDF at six different temperatures between 190 and 290 "C. Figures 2 and 3 show the viscosity vs. shear rate curves at single fixed temperatures of 250 and 220 "C, respectively, for more than six different types of DYFLOR 2000, which includes the high, medium, low, and very low viscosity grades. This entire set of 19 curves was made available to us by Dr. P. Gebauer (Dynamit Nobel AG, D-5210 Troisdorf, FRG). Similar information on viscosity vs. shear rate curves for various grades of KYNAR, the trade name of PVDF manufactured by Pennwalt Corp., at two different temperatures is given in Figures 4 and 5. These data, consisting of 11curves, were made available to us by Luigi Puglia (Pennwalt Corp, Philadelphia, PA 19102). All 30 curves appearing in Figures 1-5 can be coalesced into one single curve through the use of the melt flow index (MFI) as a normalizing parameter, as has been shown by Shenoy et al. (1982, 1983). Thus the 7 vs. curves in Figures 1-5 were replotted as 7 X MFI vs. T/MFI to form the coalesced curve shown in Figure 6. MFI is defined as the weight of polymer in grams extruded in 10 min through a capillary under the influence of a dead weight applied through a piston. The geometric parameters of the capillary and the piston, and also the weight, are specified as per ASTM D1238. As the melt flow apparatus is truly an extrusion rheometer, the value of the shear stress and the apparent shear rate corresponding to a MFI value can be calculated by using the conventional expressions RNF 7=(1) 2Rp2L~
*
* = - 4Q
(2) rRN3 where piston radius RP = 0.4737 cm, nozzle radius RN = 0.105 cm, nozzle length LN = 0.8 cm, force F = test load L (kg) X 9.807 X lo5 dyn, and flow rate Q = MFI/6OOp cm3/s. Note that the shear stress and shear rate values obtained from eq 1 and 2 are average values. Substitution of the above values in eq 1 and 2 gives = (9.13 x 1 0 4 ) ~ (3) 9 = 1.83MFI/p (4) As the MFI value is generated at a fixed temperature and a fixed load, a single point on the shear stress vs. shear rate curve at that specific temperature can be obtained by using eq 3 and 4. This fact can be used to determine the value of MFI from a known shear stress vs. shear rate curve or a viscosity vs. shear rate curve for a specific polymeric system when the MFI is not reported. For PVDF, the test load has been chosen to be 12.5 kg, thus giving a corre0 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
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