Fluorination of polyethylene

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. 

Fluorination of polyethylene surfaces leads to an increase in the surface energy, some degree of cross-linking and a reduction of the free volume of the polymer. All of these effects impart on the surface of the polymer a barrier that is very impermeable to hydrocarbon solvents. A blow-moulding process, in which a low concentration of fluorine in nitrogen is used as the blow-moulding gas, is used for the production of plastic fuel tanks for the automotive industry (Airopak , Air Products) [51]. Post-treatment of hydrocarbon surfaces with fluorine is an alternative technology and techniques for the surface fluorination of natural and synthetic rubber have been described [52]. 

Interest in the direct fluorination of polymers in Japan was established about the same time the initial work on direct fluorination from Rice University was announced. The Japanese workers Okada and Makuuchi(19) studied the fluorination of polyethylene powder and described the reaction as a fiery intense reaction, but were finally able to control it by dilution with nitrogen or helium and use of evacuated reaction vessels. The focus of their study was the rate of reaction. 

Hayes and Dixon have also contributed toward the development of an industrial process for surface fluorination of polyethylene bottles to improve solvent resistance (32)... 

It is often desirable to replace hydrogens with halogens. Fluorination of polyethylene can be carried out in the dark by simply exposing the polymer, either in sheet or in powder form, to fluorine gas. The reaction is exothermic and it is best to dilute the gas with nitrogen 9 1 to allow a gradual introduction of the fluorine and avoid destruction of the polymer [132, 133]. In this manner, however, only the surface layers are fluorinated and the substitutions occur only a few molecular layers deep. 

H. Gisser (Frankford Arsenal) The direct fluorination of polyethylene film leaves a solid structure equivalent to the original state of low crystallinity unless recrystallization or molecular motion has taken place. These may be time and temperature dependent. Have any friction or other measurements been run on directly fluorinated polyethylene to explore the nature of the surface ... 

Dynamics of the Fluorination Process. We have previously outlined in Fig. 7 the basic mechanism for the fluorination of polyethylene. The activation energy for the abstraction reaction is known to be very small ( 0.2 Kcal/mole for straight chain hydrocarbons ), and this is at least an order of magnitude smaller than the typical activation energies quoted for diffusion in polyethylene and simple f1uoropolymers in general (cf. ref. 18). It is entirely reasonable, therefore, that reaction other than at the very surface of the polymer will be diffusion controlled. 

Although the initial abstraction reaction has a low activation energy,successive replacement of hydrogen by fluorine will be expected to increase the activation energy both from the concomitant strengthening of C-H bonds and adverse polar effects in the transition state. Of some relevance in this connection are the relative rates of fluorination at different sites reported for gas phase studies of 1-f1uorobutane. These are shown in Fig. 15. It is clear that introduction of fluorine deactivates both a and 3 sites. We may use this data as a basis for discussing the fluorination of polyethylene. 

D. T, Clark We have not as yet studied the direct fluorination of polypropylene but it does feature in our future plans now that we have sorted out the complexities of the surface fluorination of polyethylene. I agree that abstraction of hydrogen should be somewhat more facile than for polyethylene leading perhaps to more selective fluorination - the relative rates of abstraction being in the order... 

Surface fluorination of polyethylene accomplished by the contact with fluorine gas was proposed in 1954 (10). Enthalpy balance of this reaction is given by (11)... 

Carbon-fluorine bonds are quite strong (slightly stronger than C—H bonds) and like polyethylene Teflon is a very stable inert material We are all familiar with the most characteristic property of Teflon its nonstick surface This can be understood by com paring Teflon and polyethylene The high electronegativity of fluorine makes C—P bonds less polarizable than C—H bonds causing the dispersion forces m Teflon to be less than those m polyethylene Thus the surface of Teflon is even less sticky than the already slick surface of polyethylene... 

Chemical Hazards. Chemical manufacturers and employees contend with various ha2ards inherent ia productioa of evea commonplace materials. For example, some catalysts used ia the manufacture of polyethylene (see Olefin polymers) ignite when exposed to air or explode if allowed to become too warm the basic ingredient ia fluorocarboa polymers, eg, Tefloa (see Fluorine compounds, organic), can become violently self-reactive if overheated or contaminated with caustic substances (45,46) one of the raw materials for the manufacture of acryflc fibers (see Fibers, acrylic) is the highly toxic hydrogen cyanide (see Cyanides). 

The best known aspect, and the first one to find commercialization in the direct fluorination area, was the fluorination of polymer surfaces. This Lagow-Margrave invention, trademarked Fluorokote, involved many types of polymeric materials in various forms e.g., polyethylene bottles, polypropylene objects, and rubber gloves. Polyethylene bottles are easily given fluorocarbon surfaces (>0.1 mm), and this has been commercialized. Air Products has at least 20 licenses for what is known as their Aeropak process and Union Carbide has a Linde Fluorination process as well. Applications in chemical, pharmaceutical, and cosmetic storage are widespread. 

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. 

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. 

In the previous papers, we reported that the radiation-induced cross-linking of polyethylene was accelerated by acetylene (4) and by the mixtures of acetylene and some fluorine-containing monomers (5). 

Polytetrafluorethylene or PTFE (sometimes called TFE) is the most commonly used, the best known and, perhaps, has the highest performance, with excellent performance/ cost ratios. Its main drawback is the impossibility of processing it by conventional molten-state methods. All the hydrogen atoms of polyethylene are replaced by fluorine, leading to the following formula ... 

Here we would like to emphasize that treatment with XeFj fluorinates the polymer surfaces, with the depth and degree of fluorination depending upon the reaction conditions. We never observed the formation of a perfluorinated surface layer after treatment with XeF,. Results of XPS analysis of XeFj-treated polyethylene film —(—CHj—CHj—) — showed that the maximum degree of fluorination corresponded to the composition —(—CF2—CH2—) —. After that, visible destruction of the polymer surface, i.e., cleavage of C—C bonds in the polymer macromolecule, would start. These results differ from those of the fluorination of polymer powders it was reported by Lagow and Margrave in the early 1970s that polymer powders were fully converted into perfluorinated polymers under the action of a F /N mixture. ... 

Potassium fluoride-poly(hydrogen fluoride) salts are useful fluorine sources for bromo- and iodofluorinations of various alkenes (Method F).49 The best yields arc obtained with KH3F4 and KH4F5. The salts are prepared by mixing the calculated amounts of potassium hydrogen difluoride and anhydrous hydrogen fluoride in a reaction vessel made of polyethylene or Hastel-loy B. The mixture is stirred and heated at 60-70 C to form a homogeneous solution of the salt which solidifies at room temperature. 

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