The Fluorocarbon Polymers

Although many fluorocarbon polymers are commercially available (Appendix 16.H), poly(tetrafluoroethylene) (PTFE) is estimated to command about 90% of the market. This polymer, under DuPont s trade name. Teflon, has been made since the early 1940s. Both PTFE and poly(chlorotrifluoroethylene) were developed to meet wartime demands, especially for the corrosive processes involving separation of radioactive isotopes. Many other fluorinated polymers have been commercialized over the years. 

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This proposed mechanism for the fluorocarbon polymer formation is of course not the only mechanism which is conceivable, but is, we believe, the most probable in... 

Outgasslng of Preformed Organic Polymers. The percent condensable-time curves for several heat vulcanized (HV) preformed organic seals are shown in Figure 3. The curves are generally linear and only one condensable product was evolved. The fluorocarbon polymers usually produced very little if any condensable products as indicated by sample F. The acrylic polymers, such as sample G, produced a very small quantity of unidentified products and gave linear, low slope condensable-time curves. 

In surveying the possibilities for elastomers which retain a degree of real flexibility at cryogenic temperatures, only two presented themselves as candidates, namely, the polyurethanes and the fluorocarbon polymers. For liquid oxygen service, it was indeed fortunate that one of these types of materials was LOX-compatible. If the fluorocarbon polymers were subject to the embrittlement observed with many other elastomers, the problem of a flexible, LOX-compatible material would be considerably more serious. 

The equipment used consisted of two plates mounted on an Instron testing machine, with gasket deflection measured by an extensometer driven by an appropriate linkage. For cryogenic testing, the entire fixture was immersed in liquid nitrogen. A number of modulus values for various materials are shown in Table II. Note the excellent values obtained for the fluorocarbon polymer-112 glass laminate. 

Anszver by Author The degree of fiber bundle wetting can be controlled by the melt viscosity of the fluorocarbon polymer, which is a function of laminating temperature. Laminating pressure has also been demonstrated to have the expected effect on fiber bundle wetting. 

Precaution At temps. > 600 F some polymer fumes may be emitted due to oxidation of the fluorocarbon polymer smoking should not be permitted when working with this material FluoHT [Micro Powders]... 

Adhesion of metals to polymers has been an intensively studied subject over the past decades This is due to the wide application of polymers to electronic packaging and, to a lesser extent, to device inter-connect The increasing demand in density for devices and speed for packaging, in turn, prompts searches for polymers with reduced dielectric constants than that of the widely used polyimide. Some fluorocarbon polymers, notably Teflon, have lower dielectric constants, 2.1, vis-i-vi the values of 3.0-3.5 for polyimides. The fluorocarbon polymers, however, have very weak adhesion to metals. An enhancement in adhesion is thus a primary requirement for the application of such polymers to technologies. A wide range of studies have been made in the past to understand and enhance the adhesion between metals and fluorocarbon polymers In this paper we review some of our earlier work, and present new observations related to the enhanced adhesion between metals and fluorocaiton polymers. We present results address three contributions to enhanced adhesion between metals and fluorocarbon polymers chemical, mechanical, and thermal. 

Structure of Perftuorinated Membranes. It is generally accepted that the fluorocarbon polymers undergo phase separation on a molecular scale when swollen by... 

Differential scanning calorimetric studies have shown that Nafion contains at least three types of water—one tightly bound to the ions, one weakly bound in the secondary hydration shell to the ion or another component of the fluorocarbon polymer (e.g., to the ether oxygens), and one free from strong interactions with ions or polymer chains. Thus, the K" " form of Nafion contains 3.8 water molecules per fixed ionic group in the primary hydration shell, 0.15 molecules as loosely bound water, and 3.8 water molecules as free water per each ionic group [49]. 

The binder properties are useful in other pigmented coatings such as clay-based paints (119). The zinc-lithium silicate coatings can be applied by anodic electrodeposition on steel (120). In another-typc of use, lithium polysilicate provides an intermediate bond between the fluorocarbon polymer coating and metal on antisticking cook ware (121). 

Ohsaka et al. [393] reported on the preparation by electrochemical oxidation and properties of thin films of 1-naphthylamine in acetonitrile solutions. These films had conductivities of 10 to 10" S cm". Aminocoronene, which consists of seven fused aromatic rings and an amino group, has also been polymerized [394]. Eaves et al. [395] polymerized perfluorocyclopentene in dimethyl formamide in the presence of tetrabutlyammonium perchlorate. The fluorocarbon polymers could exhibit enhanced environmental stability in comparison with hydrocarbon polymers. 

The plasma etch rates for the fluorocarbon polymers were determined under two rqiresentative plasma etch processes, an oxide etch and a poly etcL The fluoroaromatic acetals are a significant inqnovement over the fluoroaromatic ester etch rates. The most promisii fiuoroaromatic acetal is the HFIP/BOM copolymer in which plasma etch rates are comparable or superior to those of the ESCAP polymer under typical oxide or poly etch conations. When film removal time is used as the figure of merit in evaluating plasma etch resistance, the HFIP/BOM polymer provides over a two-fold inqrrovement in plasma etch resistance over die ESCAP polymer. 

The fluorocarbon polymers have a number of outstanding properties which make them useful in the electronics and electrical industries ... 

Figure 1. Insert - Chemical Structure of DuPont s Nation. Remainder - Cartoon Depicting Cluster-channel Network in Nation membranes. The dark sections represent regions containing mostly the fluorocarbon polymer backbone. The unshaded sections represent the ionic clusters which contain most of the absorbed water. The cations associated with the sulfonate groups have been omitted for clarity.

Idealized structures representing the fluorocarbon polymer products are shown in Figure 4 which are in agreement with the elemental analysis. The analysis of the fluorinated acrylonitrile indicates that some decomposition of the nitrile groups occurred in our product. The nitrogen-fluorine stretch at 935 cm and the elemental analysis indicate the presence of a large number of successfully fluorinated NF2 groups. 

Certain fluorocarbon processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. Blatz first described the use of fluorocarbon polymer process aids with melt extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1 2, wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers (4). 

Nelson et al. [16] have shown that the etching solution attacks only the surface regions of the fluorocarbon polymer. They examined a cut section of etched PTFE and, from optically microscopy, determined that the depth of the coloured surface region was about 1 /xm. However, they reported that the bulk electrical properties of the treated and untreated polymer were virtually identical and that their electron diffraction patterns were indistinguishable, indicating that no changes had been induced in the bulk polymer s crystallinity. These observations may be attributed to the thinness of the treated surface layer and thus the properties of the layer are not measured when analytical techniques are employed which monitor the bulk properties of the material. Indeed, Benderley [17] has since reported that the treated layer does have a substantially lower surface resistivity than the untreated material. 

As might be expected, the chemical changes induced by etching the fluorocarbon polymer in air cause a large increase in the surface free energy of... 

Like the fluorocarbon polymers discussed above, polyolefins such as low- and high-density polyethylene, polypropylene and poly (4-methyl 1-pentene) cannot usually be joined by adhesives to give reproducible high strengths unless some form of surface pretreatment is first employed. 

Acrylic pol niiers of linear and of cross-linked varieties are important classes of materials, and the fluorocarbon polymers are likewise important for substantially different reasons. The broad resistance of fluorocarbons to physical and chemical attack suggests that acrylics could be enhanced by the introduction into the molecules of substantial amounts of fluorine, provided such an addition did not compromise the characteristic acrylic properties. Fluorocarbons also possess a range of unusual surface chemical properties which could make for greater versatility in the acrylics if imparted thereto. For example, a fluoroacrylic resin in the liquid, precured state is expected to be of low surface tension and excellent wetting capability for difficult-to-wet fillers, such as powdered Teflon, whereas the cured fluoracrylic can be expected to be relatively non-wetting and non-absorptive of most liquid systems, particularly those that are water based. In order to attempt such an enhancement of acrylic properties, the synthesis of a series of fluorine-bearing acrylics of various functionalities was undertaken. 

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