Fluorination surface

In 1954 the surface fluorination of polyethylene sheets by using a soHd CO2 cooled heat sink was patented (44). Later patents covered the fluorination of PVC (45) and polyethylene bottles (46). Studies of surface fluorination of polymer films have been reported (47). The fluorination of polyethylene powder was described (48) as a fiery intense reaction, which was finally controlled by dilution with an inert gas at reduced pressures. Direct fluorination of polymers was achieved in 1970 (8,49). More recently, surface fluorinations of poly(vinyl fluoride), polycarbonates, polystyrene, and poly(methyl methacrylate), and the surface fluorination of containers have been described (50,51). Partially fluorinated poly(ethylene terephthalate) and polyamides such as nylon have excellent soil release properties as well as high wettabiUty (52,53). The most advanced direct fluorination technology in the area of single-compound synthesis and synthesis of high performance fluids is currently practiced by 3M Co. of St. Paul, Minnesota, and by Exfluor Research Corp. of Austin, Texas. 

Surface Fluorination of Polymers. Fluorocarbon-coated objects have many practical appHcations because the chemically adherent surface provides increased thermal stabiHty, resistance to oxidation and corrosive chemicals and solvents, decreased coefficient of friction and thus decreased wear, and decreased permeabiHty to gas flow. Unusual surface effects can be obtained by fluorinating the polymer surfaces only partially (74). 

We are going to discuss the syntheses of fluoropolymers, poly(carbon monofluoride), perfluoropolyethers, perfluorinated nitrogen-containing ladder polymers, and surface fluorination of polymers by direct fluorination. 

Direct fluorination of polymer or polymer membrane surfaces creates a thin layer of partially fluorinated material on the polymer surface. This procedure dramatically changes the permeation rate of gas molecules through polymers. Several publications in collaboration with Professor D. R. Paul62-66 have investigated the gas permeabilities of surface fluorination of low-density polyethylene, polysulfone, poly(4-methyl-1 -pentene), and poly(phenylene oxide) membranes. 

Surface Fluorination of Polymers Using Xenon Difluoride... 

Surface engineering by means of fluorination is an effective way to change surface properties, and is used for both polymer surfaces and inorganic substrates. Polymer surface fluorination has been around a long time. The first patent we know of dates back to 1938,1 but it was only in the 1970s that the introduction of several major industrial applications led to a rapid acceleration in development. 

Surface fluorination is in popular use because most ofthe desirable properties offluoropolymers are largely the result of surface phenomena. Surface fluorination enables us to modify the surface properties of a polymer while retaining others often useful bulk properties (e.g., mechanical strength, elasticity, and ease of processability. 

Surface-fluorinated polymers have a number of desirable inherent properties, including ... 

However, the fact that industrial applications of polymer surface fluorination employ a fluorine/nitrogen mixture as the fluorinating agent complicates matters because fluorine gas is toxic, may explode when brought into contact with organic substances, and causes severe bums on human tissue. Moreover, the use of fluorine requires highly qualified personnel and special safety systems. 

It is appropriate at this point to recall that XeF2 was originally considered to be exotic and was used only as laboratory chemical in very small quantities. While direct polymer surface fluorination with a gaseous F2/N2 mixture was already in use for several significant commercial applications. 

However, one should not forget that apart from the complexity of the synthesis fluoropolymers are very expensive. For example, the price of fluoro-rubber is more than 30-fold that of an ordinary rubber such as butadiene-styrene (SBR) or ethylene-propylene (EPDM). Cost was one of the factors that gave impetus to research polymer surface fluorination, with the object of imparting the properties of fluoropolymers to the surfaces of less expensive polymers without changing their bulk properties. 

It is believed that polymer surface fluorination proceeds via a free radical mechanism, where fluorine abstracts hydrogen atoms from the hydrocarbon, and fluorine atoms are substituted.11 Of course, the precise conditions depend on the nature of the polymer in question and the surface properties required. 

Aeropak process—simultaneous blow-molding and inner-surface fluorination of polymeric containers—is the most well-known example of an in-situ treatment process. 

Surface fluorination changes the polymer surface drastically, the most commercially significant use of polymer surface direct fluorination is the creation of barriers against hydrocarbon permeation. The effectiveness of such barriers is enormous, with reductions in permeation rates of two orders of magnitude. Applications that exploit the enhanced barrier properties of surface-fluorinated polymers include (1) Polymer containers, e.g., gas tanks in cars and trucks, which are produced mostly from high-density polyethylene, where surface fluorination is used to decrease the permeation of fuel to the atmosphere and perfume bottles. (2) Polymeric membranes, to improve selectivity commercial production of surface-fluorinated membranes has already started.13... 

In closing of this general introduction it may be said that the development of the applied chemistry of fluorine, which started with fluorination of substances has led to surface fluorination of materials, which is now an effective tool in changing of surface properties. 

POLYMER SURFACE FLUORINATION WITH XENON DIFLUORIDE... 

In the first stage in order to test the process of various types of polymer films were surface-fluorinated. From 1990 to 1994 it was shown that XeF2 could be used effectively for surface fluorination of a variety of plastics. Polyethylene film and plates,18 aromatic polysulfone,19 polyvinyltrimethylsilane,20 and polycarbonate,21 among other polymeric materials, were fluorinated successfully. 

It is very important to know how surface fluorination affects the stability of tribological characteristics. In the case of Russian-made rubbers we used a standard friction machine (SMT), where abrading was done with a stainless steel instead of a conventional brass grid so that the test would be more rigorous. The results obtained are shown in Table 15.3. 

To validate our idea that treating rubber products with gaseous XeF, causes them to become surface-fluorinated, i.e., that chemically bonded fluorine atoms are incorporated in the surface layer, we did ESCA analyses ofthe samples treated in various modes. The results are shown in Table 15.4. 

Although we made no attempt to elucidate the mechanism of friction decreases in rubbers after surface fluorination, it seems to us that apart from the substitution of H atoms to F in the polymer macromolecule, which forms a fluoropolymer on the surface, there is another phenomenon that makes a significant contribution to the friction decreases, i.e., fluorination of carbon black, which is used in rubber recipes for reinforcement. It appears that when the carbon black in the surface of the rubber is fluorinated it produces a lubricating effect, followed by blooming on the surface of the treated rubber while it is under a friction load. So, in our opinion, two effects contribute to friction decrease of carbon-filled rubbers fluorination of the rubber macromolecules and fluorination of the carbon black rubbers that do not contain carbon black show a much smaller decrease in friction after XeF2 treatment. 

The simple calculations presented below will show that surface fluorination of rubbers with XeF2 is economically advantageous as compared with fluoro-rubbers. The amount ofXeF2 needed for batch treatment of 5000 O-rings made from EPDM rubber (diam. 10.2 x 1.5 mm) is approximately 20 g. That means that the cost ofthe XeF2 needed for one batch treatment, is 20 x 1.5 = 30 for 5000 O-rings or about 0.0006 each. It should be kept in mind that, as mentioned above, the price for fluororubber is thirty times that of EPDM. 

Although we have no specific data on the economics of rubber surface fluorination with the traditional F2/N2 mixture, in our opinion the following facts make the XeF2 technique very attractive for rubber manufacturers ... 

Xenon difluoride has proved to be an effective fluorinating agent for surface fluorination of polymers. 

The technique of surface fluorination with XeF2 is much less sophisticated, expensive, and hazardous than the conventional F2/N2 treatment. 

The XeF2 polymer surface fluorination technique can be easily applied at manufacturing companies that have no experience in dealing with fluorine. 

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