CONTROLLED SURFACE-FLUORINATION PROCESS

Abstract: Method for fluorinating the surface of a polymeric solid for improving a surface property of such solid while substantially maintaining the bulk physical properties of said solid. This method comprises contacting the polymeric solid with dilute fluoring gas at a temperature not substantially above about room temperature and at a pressure not substantially above about 1 atmosphere for a time adequate to partially fluorinate the solid surface without forming substantially any -CF2-groups.

Description: CONTROLLED SURFACE-FLUORINATION PROCESS Background of the Invention The present invention is directed to the surface modification of solid poly¬ meric substrates and more particularly to controlled surface fluorination thereof. Synthetic polymers and plastics provide the basis for a large part of consumer and industrial products encountered presently and likely will increase in the future. Polymeric solids, for present purposes, comprehends an object constructed from, coated with, or otherwise composed of a polymer in solid form. Examples of such polymeric solids include, for example, synthetic fibers for clothing, polyvinyl chloride (PVC) and like plastic storage bags (eg. blood bags), polycarbonate safety glasses, to name but just a few of such items. Such polymeric solids are chosen for an intended use because of particular bulk properties which they possess. That is, particular polymeric solids possess good tensile strength, desirable oxygen perme¬ ability capabilities, high impact resistance, resistance to corrosion, good optical properties, and a variety of other properties. While the bulk properties of such polymeric solids may be quite good, the outer surface of such polymeric solids often can be degraded or eroded by a variety of conditions encountered in every-day use. That is, some polymeric solids while providing good tensile strength, may become easily soiled and scratched, while other polymeric solids of good optical trans- missivity can become easily soiled and are resistant to cleaning. Other polymeric solids may be subject to the phenomena known as plasticizer migration. It will be seen that the polymeric solids require a surface treatment in order to "materially enhance their durability in every-day use. One such surface treatment proposed in the art comprises subjecting the polymeric solid to a fluorine-containing source in order to surface- tuorinate the polymeric solid. For example, U.S. Patent No. 4,296,151 proposes to treat normally solid polymers of aliphatic mono-1-olefins and elasto eric and resinous polymers of conjugated dienes and vinyl-substituted aromatic compounds with fluorine gas in order to render the surface water-wettable and resistant to hydrocarbons, thus improving the printability of the treated solid. U.S. Patent No. 4,264,750 discloses the use of cold plasma fluorination techniques to completely fluorinate a hydro¬ carbon or amide polymer. Such cold plasma process is conducted with various OMPI sA*. fluorine-containing gases in the substantial absence of oxygen. U.S. Patent No. 4,120,032 proposes an improved barrier coating polymeric film by treating such film with combination of fluorine and bromine. U.S. Patent No. 4,076,916 conducts the fluorination of polymers by contacting the polymer initially with dilute fluorine gas (eg. 6%) followed by the gradual increase in fluorine concentration. U.S. Patents Nos. 3,758,450 and 3,775,489 disclose surface fluorination processes utilizing such fluorination process. U.S. Patent No. 4,020,223 proposes to fluorinate polyolefin and polyacrylonitrile fiber form by treating such fiber form with elemental fluorine in the substantial absence of oxygen. While control of the fluorination reaction is an object within each of these prior art proposals, none of such proposals recognizes any universality in the surface fluorination process for virtually all types of polymeric solids. Moreover, no apparent predictability in the fluorination processes of the prior art is readily apparent. The present invention, however, provides an analysis of the chemical reactions occurring at the surface of the polymeric solids during the fluorination process and provides limitations thereon for achieving improved surface properties of the solid while substantially maintaining the bulk physical properties of the polymeric solid itself. Broad Statement of the Invention The present invention is a method for fluorinating the surface of a polymeric solid for improving a surface property of such solid while substantially maintaining the bulk physical properties of said solid. Such method comprises contacting the polymeric solid with dilute fluorine gas at a temperature not substantially above about room temperature and at a pressure not substantially above about one atmosphere for a time adequate to partially fluorinate the solid surface without forming substantially any -CFΛ- groups. Representative dilute fluorine gas con¬ centrations generally are less than about 4% by volume with contact times generally ranging from about 1-30 minutes. Advantages of the present invention include the ability to improve the surface properties of the solid without deleteriously effecting the bulk physical properties of the solid which make it advantageous for particular uses. Additionally, a method for pre-deter ining the desirable fluorine level to which the surface of the polymeric solid should be fluorinated is disclosed. These and other advantages of the present invention readily will become apparent based upon the disclosure contained herein. Detailed Description of the Invention The ultimate goal achieved by the present invention is the improvement of a surface property of a polymeric solid by surface fluorination techniques wherein the bulk physical properties of the solid are substantially maintained. Surface proper- ties of polymeric solids which may be improved according to the precepts of the present invention include, for example, providing improved dirt resistance, better washability, improved bondability, improved printability, reduced friction and wear characteristics, and a variety of additional surface properties. Improving such surface properties of polymeric solids, though, has been within the grasp of the prior art to a limited extent; however, such art sacrificed important bulk physical properties of the polymeric solid in an attempt to improve the surface properties thereof. It is the achievement of both improved surface properties and retention of bulk physical properties that is accomplished according to the precepts of the present invention. Based upon the outlined goal, several fundamental discoveries and realizations occurred during the course of developing the present invention. Several of these realizations are limiting cases which form a framework within which the present invention is operable. One discovery or realization is that a polyvinyl fluoride (PVF) polymeric solid does not meet the criteria established. Thus, the fluorination reaction should not be conducted to an extent wherein virtually all reactive carbon atoms in the polymeric solid contain a fluorine atom. A second realization or discovery is that polyvinylidene fluoride also does not meet the criteria established for the present invention. That is, the fluorination of carbon atoms in the polymeric solid to contain two fluorine atoms is highly undesirable. It will be demonstrated that -CF„- groups generally renders the surface of the polymeric solid to have poor bondability, poor printability, etc.; and can deleteri¬ ously affect bulk physical properties by reducing optical properties, decreasing flexibility especially at lower temperatures, and like loss of valuable properties. Another discovery correlative to the ~CF„- limitation is that it was dis¬ covered that as low as a 20% content of -CF„- groups of the surface of a polymeric solid causes the polymeric solid to experience the same loss of properties that is experienced when virtually all carbon atoms are difluorinated. This evidence leads to the conclusion that surface fluorination must be carried out under conditions and to an extent such that virtually no -CF„- groups are formed if the retention of bulk physical properties with minimum adverse surface property modification is desired. Thus, the surface fluorination modification or treatment of the present invention is carried to an extent such that only the surface is fluorinated and only partial -CHF- formation is permitted to occur. The prior art, then, that proposes dilute fluorination, the use of cold plasma fluorination techniques, and the like, misses the substance of the present invention entirely. The conditions established during the surface fluorination process are relevant only insofar as the control of -CHF- groups to the substantial exclusion of -CF 2 - groups is concerned. Accordingly, the process of the present invention desirably is operated at room temperature, at a fluorination pressure of not substantially above about one atmosphere, preferably with a fluorine gas of less than about 4% concentration in an inert carrier gas, with fluorination times of 1-30 minutes generally practiced. By operating within the precepts of the present invention, and preferably under the preferred operating conditions noted above, the surface of a polymeric solid can be partially fluorinated to form -CHF- groups to the substantial exclusion of -CF_- group formation to achieve advantageous surface property modification without sacrifice of the bulk physical properties which led to the initial selection of the particular polymeric solid being subjected to the treatment. It must be recognized that the fluorinating agents, especially those that are mild and selective towards -CHF- formation, may be useful in the process of the present invention. A correlary to the invention is that the presence or absence of oxygen, water vapor, chlorine or bromine gas, and like potential reactants are important only insofar as the control of fluorine group formation is concerned. As the Examples will demonstrate, the presence of water vapor does not adversely affect operation of the present invention, as some prior art has taught. Whether oxygen in the the process leads to the formation of carbonyl groups or whether the presence of oxygen retards the rate of fluorination is not of great concern to the process of the present invention so long as its presence is taken into account in controlling the type and proportion of fluorine group formation on the surface of the polymeric solid. Thus, the process of the present invention may be termed as an extra dilute phase fluorination reaetion, though such characterization should not be construed as a limitation of the present invention. Diluent carrier gases for the fluorine gas include, for example, nitrogen, rare or noble gases such as neon, argon, helium, and the like, and like non-reactive diluent gases. High pressure and high temperature conditions are to be avoided in order to ensure virtually no ~CF„- group formation. As noted above, the reaction time advantageously ranges from about 1-30 minutes, though the polymeric solid of choice, proportion of fluorine gas, reaction tempera- ture and reaction pressure are variables which necessarily will impact the fluorina¬ tion reaction time which should be practiced. Polymeric solids which may be subjected to the dilute phase surface fluorina¬ tion process of the present invention include, for example, polymeric solids formed from silicone resins, phenolic resins, polyolefins, polyvinyls, polyesters, poly- acrylates, polyethers, polyamides, polysulfones, cellulosic materials, polycarbonates, polyepoxides, polyaerylonitriles (PAN), and a wide variety of other polymeric solids. The polymeric solids may be in the form of sheets or webs, in fiber form, or in any other convenient shape or form as use of such polymeric solids dictates. Such polymers for forming the polymeric solid optionally may be substituted with a variety of substituents and such substituents generally do not adversely affect the surface modification or treatment of the present invention. Such substituents may include, for example, carboxyl, amine, sulfate, halide, silicone, and the like. It must be cautioned, though, that such substituents may be displaced preferentially by the fluorine during the reaction and this must be taken into account in designing the fluorination process for the particular polymeric solid of choice. It is worth noting also that the fluorination process apparently provides a degree of sterilization to the polymeric solid which property may be advantageously exploited in certain applica¬ tions, eg. fluorination of polymeric solids intended for medical use. It would be desirable to be able to correlate a defined or easily experimentally determined variable during the course of the surface fluorination process to the treatment being effected to the polymeric surface, even though such indicia is not the precise modification variable sought to be improved. So long as a relationship between the indicia and the ultimate property can be determined, the monitoring of such indicia becomes important. As general propositions of results achieved through surface fluorination techniques, the surface fluorination of a polymeric solid results in an increase in density with a corresponding decrease of oxygen permeability. Oxygen permeability is believed to be a convenient indicia correlative to the surface modification being affected according to the process of the present invention. A decrease in the refractive index also is believed to be an important indicia correlative to the surface fluorination process also. It is believed that such oxygen permeability variable can be correlated to other surface properties which may be of interest in achieving by the fluorination techniques evolved by the present invention. Two general techniques have been evolved for correlating the surface fluorination to the oxygen permeability of the modified polymeric solid. In this connection reference is made to Salame, "A CORRELATION BETWEEN THE STRUCTURE AND OXYGEN PERMEABILITY OF HIGH POLYMERS", ACS-PoIymer Preprints, Vol. 8, No. 1, pp. 137-144 (1967). The Permachor relationship developed by Salame provides a direct correlation between the structural units of the modified surface and the oxygen permeability thereof. By utilizing the Permachor relationship and the degree of -CHF- formation, the oxygen permeability can be determined ahead of time for any polymeric solid of interest. The oxygen permeability, then, can be related to other surface properties which are of interest. In order to more fully understand this relationship, the following data relating types of fluorinated groups to oxygen permeability is given. Oxygen Permeability - P x 10 12 Structural cc cm % CH 2 ϋnit cm sec cm* ■ Hg Groups % Halogen Groups CH 2 -CH 3 66-83 100 0 CH 2 -CHF 20 47 43% CHF CH 2 -CHC1 4.80 42 58% CHC1 CH 2 -CF 2 2.40 41 59% CF 2 CF 2 -CFC1 1.80 0 43% CF 2 47% CFCl As the above information demonstrates, surface fluorination dramatically affects the oxygen permeability of polymeric solids. Not seen in the above- tabulated data is the deleterious affect which -CF-- groups have on the bulk physical properties of the fluorinated polymer. Yet another method for correlating the degree of surface fluorination to the oxygen permeability of the fluorinated substrate involves the use of the McGϊnniss predictive equation as set forth in the following publications: ACS-Orgaπic Coatings and Plastics Chemistry, Vol. 39, pp 529-534 (1978) and ACS-Organie Coatings and Applied Polymer Science, Vol. 46, pp 214-223 (1981). The McGinniss predictive relationship, as it relates to oxygen permeability and surface fluorination techniques, states that the oxygen permeability of a surface fluorinated polymeric solid can be determined and predicted based upon the refractive index of the polymer and the density of the polymer. Specifically, the product of the refractive index (n) and the density (d) is directly proportional to the oxygen permeability, which according to work on the present invention is proportional to other surface properties of interest , of fluorinated polymeric solids. In order to more fully O PI understand the McGinniss predictive relationship in the context of the present invention, the following data based thereon is given: Oxygen Permeability P x lO 12 _ m Refractive - — 2 c Index Density Polymer Type cm sec cm-Hg (n) (d) n-d Product Poly vinyl dichloride 0.30 1.6 1.66 2.66 Polydifluoroethylene 0.40 1.35 2.0 2.70 (Teflon) CF 2 -CFC1 1.80 1.43 1.92 2.75 Polyvinylidene fluoride 2.40 1.42 1.74 2.47 Poly vinyl chloride 4.8 1.539 1.385 2.13 Poly vinyl fluoride 20.0 1.37 1.46 2.0 Polyvinyl acetate 36.0 1.467 1.19 1.74 Polystyrene 54.0 1.591 1.13 1.80 Polyethylene 36.0 1.52 1.0 1.52 Polypropylene 137.0 1.49 0.85 ^ 1.27 As the above data demonstrates, the oxygen permeability of the polymeric solids are directly proportional to the product of the refractive index and the density of the polymeric solids. Thus, two different methods for determining the oxygen permeability of a fluorinated polymeric solid are given. Use of the process of the present invention to fluorinate the polymeric solid to such fluorine content, then can be conducted. Such oxygen permeability also can be directly related to other surface properties which may be of interest. Also, the use of the Permachor relationship and the McGinniss predictive relationship provides valuable information as to whether a desired property for a given polymeric solid can be achieved without substantially harming the bulk physical properties of the polymeric solid. If the surface modification requires the formation of -CF„- groups, it now is known that loss of valuable bulk physical properties of the polymeric solid can be expected. A different choice of polymer, then, may be dictated. The following examples show in detail how the present invention can be practiced but should not be construed as limiting. In this application, all units are in the metric system and all percentages and proportions are by weight, unless otherwise expressly indicated. Also, all references cited herein are expressly incorporated herein by reference. IN THE EXAMPLES Fluorination reactions were carried out in a 1 liter or 2 liter reaction cell under very dilute fluorination conditions at room temperature (ca. 21°C) and at a total pressure of 1 atmosphere or less. The inert diluting gas in all experiments was nitrogen and the dilute fluorinatϊng gas contained 15% F 2 and 85% N« (supplied by Matheson Division of Searle Medical Products). Typical dilute fluorination for the 1 liter cell was practiced as follows: (1) the solid polymer sample was placed into the cell and the cell evacuated to about 1.8 mm of HG' (about 25 inches of water) with a water aspirator or to 1-2 mm of HG (about 30 inches of water) with a vacuum pump; (2) the evacuated cell was filled to 50% of its volume with N„ (O„ free); (3) the partially pressurized cell then was filled with the dilute fluorinating gas to 17% of the cell volume, then additional N 2 was charged to establish 1 atmosphere in the cell, and the cell held at room temperature for 1-60 minutes depending upon the degree of surface fluorination desired; and (4) the cell was evacuated and the surface fluorinated sample was removed. The total fluorine content in the cell was about 2.55%. Plasticizer migration evaluation of surface fluorinated polyvinyl chloride (PVC) films were determined from weight loss measurements of the sample films before and after being subjected to vacuum treatment at 65> C for various time intervals noted in the examples. Optical properties were determined by visual inspection. EXAMPLE 1 A mixture-of a clear ethylene propylene dϊene monomer rubber (EPDM, Nordel 1320 rubber, E.I. DuPont De Nemours, Company), 5-20 parts of silica, 2% A-174 coupling agent by weight of the silica (A-174 coupling agent is a silane coupling agent, supplied by Union Carbide Corporation), 10 parts of a multifunctional acrylate (Sartomer 350 is trϊmethylolpropane trimethaerylate), and 3 parts of di- tert-butyl peroxide were cured at 160°C for 30 minutes. The cured EPDM film was subjected to the fluorination process described for 2 minutes (Sample 1). Another sample was subjected to the fluorination process for 30 minutes (Sample 2). Each sample was subjected to ESCA analysis (Electron Spectroscopy for Chemical Analysis) in order to determine the types and concentratoϊns of fluorine groups at the surface of the fluorinated EPDM samples. ESCA analysis of Sample 1 (2 minutes fluorination treatment) showed approximately 20% -CHF- content (binding energy 288 to 280 e.v.) and 80% CH,, CH„, and CH content (binding energy 285-287 e.v.). ESCA examination of Sample 2 showed approximately 20% -CF-- content (binding energy 291.2 e.v.), 20% -CHF- content, and 60% CH 3 , CH 2 , CH content. Each sample and a control (unfluorinated) sample was subjected to optical evaluation in order to determine its percent transmittance of light and its dirt resistance. Dirt resistance, recorded as %-haze on a Gardner Laboratory haze meter, is determined by applying talcum powder to the sample and then dry-wiping the powder from the sample with a tissue paper. The samples also were bent for determining flex crazing which is caused by microcracks or related phenomena such that the bent film becomes visually opaque. The results of these tests are displayed below. TABLE 1 Sample % Transmittance % Haze Flex No. Before Talc After Talc Before Talc After Talc Crazing Control 92-93 92-93 4-6 35-42 None 1 91-93 91-93 4-6 12-20 None 2 91-93 91-93 9-16 16-22 Severe These results demonstrate that the dirt resistance of EPDM samples can be improved significantly by surface fluorination; however, the desirable bulk proper¬ ties of good optical performance even under flex stress conditions are lost when -CF« groups are introduced at the surface. Partial fluorination to form only -CHF- groups, though, yielded improved dirt resistance while maintaining optical per- formance and flexibility of the EPDM rubber. * EXAMPLE 2 Thin-film samples of a plasticizer PVC film (20-40 mils, 3/4" by 2", Shamrock 426 PVC film containing 50% by weight bis(2-ethylhexyl) phthalate plasticizer, Diamond Shamrock Corporation) were placed inside of the fluorination cell and subjected to dilute fluorination as described before. Reaction times varied for each sample. Each condition was repeated three times with placement of the sample in three different locations in the cell (top, middle, and bottom) in order to determine whether any significant gradient existed in the cell. Thereafter, the samples were O PI subjected to plasticizer migration analysis as described above. The results of these studies are displayed in the following table. TABLE 2 Fluorina- Plasticizer Loss at 65°C in VACUO, DAYS Sa ple tion (wt%) No. Time ( in.) 10 14 28 56 Control 1 0.61 1.41 3.03 4.05 5.55 11.07 20.0 2 0.43 1.01 2.03 3.13 4.12 6.91 17.8 Avg. 0.52 1.21 2.53 3.59 4.84 8.99 18.9 A Top 3 0.75 1.32 2.48 3.03 4.40 8.87 16.8 Middle 3 0.66 1.40 2.01 2.98 4.23 6.72 17.7 Bottom 3 0.79 1.09 2.41 3.43 4.66 9.97 18.3 Avg. 3 0.73 1.27 2.30 3.15 4.43 8.52 17.6 B Top " 5 0.64 1.23 2.19 2.79 3.60 Middle 5 0.37 0.81 1.61 2.31 2.79 5.28 15.2 Bottom 5 0.98 1.59 2.69 3.24 4.24 7.73 17.3 Avg. 5 0.66 1.21 2.16 2.78 3.54 6.51 16.3 C 60 Sample Degraded The above-tabulated results demonstrate that plasticizer migration can be suppressed by the dilute fluorination process of the present invention. The rate of fluorination loss for the control samples was 0.33%/day, for sample A the rate of loss was 0.30%/day, and for sample B the rate of loss was and 0.28%/day. Thus, there is an average of 9% improvement in plasticizer loss for sample A and 15% improvement for sample B compared to the control samples. These results also show that no significant fluorination gradient is apparent in the cell. Note that even under the dilute fluorination conditions established in the cell, the sample held in the cell for 60 minutes degraded. ' ■ REAT OMPI EXAMPLE 3 EPDM rubbers were prepared according to recipes described in ELASTOMERS, pp 22-29, Palmerton Publishing Co., Atlanta, Georgia (April, 1979) and pp 28-34 (January, 1982). These recipes included 100 parts Nordel 1440 or 2722 EPDM rubber, 50-100 parts filler, and 2-8 parts dicumyl peroxide. Fluorination of these rubbers was carried out for 2 minutes (Sample 1), 3 minutes (Sample 2), and 60 minutes (Sample 3). Coefficient of friction ( < ) measurements then were made on a control sample (unfluorinated) and the fluorinated samples in accordance with ASTMD-1895-69, part 35 (1980) and wear life conditions were determined under ASTM D-3702 part 25. Further discussions on this subject can be found by Savkoor, ACS-Coatings and Plastics Preprints, Vol. 34, No. 1, pp 220-229 (1974). The following results were obtained. TABLE 3 Sample Coefficient of Friction (μ ) No. Initial Final After 60 min. of Wear Control 1.1 0.71 1 0.46 0.53 2 " 0.59 0.68 3 Sample cracked and lost all flexibility These results show that the surface fluorination process improves friction and wear characteristics provided that -CF„- formation is avoided.

Claims: CLAIMS 1. A method for fluorinatϊng the surface of a polymeric solid for improving a surface property of said solid while substantially maintaining the bulk physical properties of said solid, which comprises: contacting said polymeric solid with dilute fluorine gas at a temperature not substantially above about room temperature and at a pressure not above about 1 atmosphere for a time adequate to partially fluorinate said solid surface without forming substantially any -CF„- groups. 2. The process of claim 1 wherein said dilute fluorine gas comprises not substantially above about 4 percent fluorine gas in a diluent inert gas. 3. The method of claim 2 wherein said inert diluent gas comprises nitrogen. 4. The method of claim 1 wherein said contacting is for a time ranging between about 1 and 30 minutes. 5. The process of claim 1 wherein said polymeric solid is made from a material selected from a silicoiE resin, a phenolic resin, a polyolefin, a poly vinyl, a polyester, a polyacrylate, a polyether, a polyamide, a polysulfone, a cellulosic, a polycarbonate, a polyepoxide, and a polyacrylonϊtrile. 6. The method of claim 5 wherein said polymeric solid comprises poly¬ merized ethylene propylene diene monomer. 7. The process of claim 5 wherein said polymeric solid comprises polyvinyl chloride. 8. The method of claim 1 wherein said surface property comprises the oxygen permeability of said polymeric solid.

Inventors: MCGINNISS VINCENT D, SLIEMERS FRANCIS A, MCGINNISS, VINCENT, D., SLIEMERS, FRANCIS, A.
Applicants: BATTELLE DEVELOPMENT CORP, BATTELLE DEVELOPMENT CORPORATION
Application year: 1983
Main classification: C08G85
IPC codes: C08J7