The prime objective of this investigation was to demonstrate that Ohio University's Fixed Boundary Extrusion Orientation-Crystallization (FBEOC) process can produce highly oriented fluoropolymer ribbon extrudates, and to determine the effect of the process variables (screw speed, pressure drop, melt residence time) on the extrudate's physical, thermal, and optical properties.
Teflon ®FEP-100, a melt processable fluoropolymer, was extruded through a specially designed converging-diverging die that gave a balanced biaxial deformation of 12:1, and produced 1 1/2" × 1/32" single ribbons. A Brabender 3/4" single screw extruder (Model 200) served as a pressure source, and an eighteen inch length of medium pressure pipe was used to condition the fluoropolymer melt prior to extrusion. Immersing the tip of the die in a water bath caused crystallization of the oriented polymer melt in the fixed boundary, constant cross-sectional area, land of the die, thereby locking in the orientation that was achieved in the die's shaping section. Coating of the die with a Teflon ®dispersion was required in order to reduce the frictional drag between the crystalline polymer and the land of the die.
Continuous extrusion of oriented fluoropolymer ribbons is possible at line speeds up to 24.5 in/min (70.8 in 3/hr) for a freshly coated die, at which point the Teflon ®coating shears off, and the process produces an unacceptable extrudate. The pressure developed at a given screw speed is a strong function of the condition of the die's Teflon ®coating, as higher pressures are required to overcome the increased frictional drag caused by a worn or absent coating. The extrusion rate appears to be independent of the condition of the Teflon ®coating.
The extrudate's tensile strength, modulus, and melting point increase with increasing screw (or line) speed and pressure drop, and increases at a faster rate at the highest operating pressures (4000-7000 PSI). This lends further credence to the existence of a liquid crystalline form in the conditioned polymer melt. The maximum values obtained were 3110 PSI for tensile strength, 47900 PSI for modulus, and 550°K (277°C) for melting point.
The percent crystallinity of the extrudates did not vary with screw speed and pressure drop as expected. However, the method used to determine percent crystallinity (based on endotherm areas obtained from differential scanning calorimetry) may not be an acceptable method for the fluoropolymer studied.
Testing of the extrudates for transparency to light in the ultraviolet, visible, and near-infrared wavelengths (800-220 nm, the wavelengths of light most useful for solar energy applications) shows that the polymer's optical properties are dependent on both the processing conditions (screw speed and pressure drop) and the die coating. The surface roughness caused by a worn or absent coating precludes any increase in transparency that would be realized at the highest pressure drops. The maximum transmittance obtained was 88.2% of the total incident light, which is 4.0% greater than that of the conventionally processed film (30 mil thickness).