VISCOSITY CHARACTERIZATION OF POLY(TETRAFLUOROETHYLENE) BY CENTRIFUGE BALL VISCOSIMETRY

Low shear rate viscosity measurements of poly(tetrafluoroethylene) (PTFE, also known as Teflon, a registered trademark of Du Pont) melts have been performed at different temperatures by using the centrifuge ball viscometer (CBV). An average measured viscosity value of 5.2 x 10(9) Pa . s was obtained for PTFE-T60 at 380 degrees C. The viscosity eta (Pa . s) was computed by using an empirical relation a(c)(g) = (4.29 +/- 0.68) x 10(2)(eta V infinity)(0.856+/-0.004) (or = 79(eta V infinity)(0.86)) with a(c) (a(c) > 1.1 x 10(-2) gravity) and V infinity being respectively the acceleration and the terminal velocity of a stainless steel ball of 1.58 (or 4.76) +/- 0.01 mm diameter moving in the fluid (e.g., polymer melts) contained in a horizontal cylindrical glass tube having an inside diameter of 4.26 (or 9.99) +/- 0.01 mm. Temperature-dependent viscosities of PTFE melts were obtained, and the activation energy E(a) covered a narrow range of 17-18 kcal/mol. The zero shear viscosity, eta(0) (Pa . s), and the molecular weight, M(W), of three PTFE melts obey the relationship eta(0) = (4.94 +/- 0.07) x 10(-14)M(w)(alpha=3.2+/-0.1) at 380 degrees C. Based on this relationship, the molecular weight (MW) of PTFE-T60 is estimated to be 1.6 x 10(7). If alpha = 3.4, MW = 1.4 x 10(7) for PTFE-T60.