Mechanical studies Fluoropolymers

Levyt has described a process for orienting fluoropolymer films using a combination of two rolls (Fig. 6.29) rotated at different speeds while the film was heated to a temperature above its second transition temperature, typically between 145°C and 175°C. Roll B rotates at a faster speed than roll A, resulting in the stretch, where the ratio of the speed of roll B to roll A is equal to the stretch ratio (SR). The stretched film was cooled below the second transition temperature of the pol5mier while it was held under tension. Levy reported that orientation in longitudinal direction (LD) imparted improvement in the mechanical property of the film in the transverse direction (TD). Table 6.13 shows the composition of ETFE and ECTFE resins in this study. Tables 6.14-6.16 present the properties of these resins as a function of stretch ratio. A stretch ratio of 0 was indicative of as-cast, unstretched film. Note that in the absence of any stretch in the transverse direction, with... 

To determine the cause of GDL hydrophobicity loss and associated degradation mechanism(s), changes in the exposed carbon phases (graphite fibers and carbon particles), the fluoropolymer coating (PTFE, EEP, etc.) overlaying the carbon surfaces, and the bonding interface between the sintered fluoropolymer particles and carbon surfaces must be studied. All three of these aspects play at least some role in GDL and MPL hydrophobicity losses, and delineation of the relative importance of each surface or interface is a central theme of this work. The relative... 

It is noticeable that the above mechanism is based on studies of fatty acid amides while other slip agents are also used for various purposes related with reduction of coefficient of fnction. Fluoropolymer additives, for example, are used to improve film extmsion in which they act in a similar manner during process as amides act in the final film. Fluoropolymer additive is also not compatible with polymer matrix. During extrusion it migrates to the surface of metal and forms film which has pronounced effect on production parameters. It reduces melt fraction, viscosity, shear rate, and gate pressure. This makes production faster and eneigy use lower. Some of these additives are developed in such a manner that they migrate only in a molten state but are immobilized within material after material solidifies. This makes them essentially absent from the film surface which in some post process operation is an important requirement. ... 

As the X-ray photoelectron spectroscopy technique is very surface sensitive, the fluoropolymers were carefully transferred - through the air - from the sputtering vessel to the ESCA spectrometer, and analyzed without further chemical or mechanical treatment. All the studied samples were stable in air (see below) and in the spectrometer during the analysis, i.e. no degradation could be detected. 

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