Polyvinylidene fluoride melt viscosity

Fluoropolymer manufacturers and suppliers have developed time-temperature-shear-rate data for melt viscosity or melt flow rate (index) to provide an assessment of the thermal stability of these polymers. Figures 6.1 and 6.2 show the melt viscosity of a few commercial grades of polyvinylidene fluoride as a function of temperature at a fixed shear rate. The relationships between melt viscosity and shear rate, and shear stress versus shear rate, are presented in Figs. 

S-6.5. Melt flow index/rate and its relationship with melt viscosity of polyvinylidene fluoride are given in Tables 6.2 and 6.3. 

Preparation of oriented fibers from mixtures of polyvinylidene fluoride with different inherent viscosities (molecular weight) has been reported. [" 1 In one example, two types of PVDF polymers with inherent viscosities of 1.49 and 0.95 dl/g were blended together at a weight ratio of 70%/30%. The mixture had an inherent viscosity of 1.33 dl/g. After pelletization, the PVDF blend was melt spun into monofilaments with a diameter of 520 pm. A single-screw extruder (32-mm diameter) was used to spin the filaments at a temperature of 300°C. Birefringence, which is defined as the difference in refractive index in two selected perpen-... 

For example, two such polymers had melt viscosities of 11,000 poise (sheath) and 39,000 poise (core) at 260°C and 100 sec. Sheath and core were extruded, respectively, in a 25 mm diameter extruder at 265°C and 35 mm diameter extruder at 275°C. A 1.5 mm diameter concentric die allowed coating of the core with the lower viscosity polyvinylidene fluoride. The filament exited the die in a 250°C air atmosphere followed by cooling in a water bath at 30°C. The fiber next entered a hot oil bath at 165°C where it was stretched at a draw ratio of 5.4. It was subsequently stretched in a second hot oil bath at a draw ratio of 1.22 at 170°C. The last step consisted of relaxation by 5% in hot air at 80°C. A transparent lustrous fiber with a diameter of 128 pm was obtained in which the sheath comprised of 20% by volume. The filament had a birefringence of 0.0395, a tensile strength of 911 MPa, and a break elongation of 20%. Knot strength was 832 MPa and a break elongation of 15%. 

Polyvinylidene fluoride is an engineering resin that represents the basis of melt processable fluoropolymers and is used primarily in injection molding of fittings and molded components as well as extrusion of profiles such as pipes, tubing and sheets. The current commercial resins are limited in their market applications due to their molecular structure, which is essentially linear. Their viscosity can only be controlled by adjusting the Mw. Therefore, a balance between a low viscosity for ease of processing and a high viscosity for sag resistance cannot be achieved in practice. 

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