Liederman, D., Voltz. S. E.. Oleck, S. M . , paper presented at Division of Petroleum Chemistry. American Chemical Society. Dallas, Texas, Apr 1973. Liederman. D., Voltz, S. E., Snyder, P. W.. paper presented at 3rd North American Meetina. Catalysis Society. San Francisco, Calif., Feb 1974. Mooi, J . . Kuebrick.-J. P., Johnson, M.. F. L.. Chloupek, F. J., paper presented at 38th Midyear Meeting, Division of Refining, American Petroleum Institute. Philadelphia, Pa., May 1973.
Shelef, M., Dalla Betta, R. A., Larson, J. A,, Otto, K., Yao, H. C . , paper presented at 74th National Meeting, American Institute of Chemical Engineers, New Orleans, La.. Mar 1973. Voltz, S . E., Morgan, C. R.. Liederman. D.. Jacob, S.. ind. Eng. Chem.. Prod. Res. Develop., 12. 294 (1973).
Receiuedfor review June 11,1974 Accepted August 29, 1974
Solvent Effect on Wetting Properties of Highly Fluorinated Polymers Marianne K. Bernett Laboratory for Chemical Physics, Naval Research Laboratory, Washington, D.C. 20375
Films of polymers with highly fluorinated side chains were prepared from three different solutions. At ambient temperatures no significant difference was noted in the wettability of the various films. After exposure to elevated temperatures. however, films prepared from hexafluoroxylene or from the solvent analyzed as perfluorobutyltetrahydrofuran remained superior to those prepared from CC12FCCIF2; films prepared from the latter solvent occasionally showed very deleterious effects.
Polymers with highly fluorinated side chains exhibit very low surface energies provided these chains are sufficiently adlineated to present a surface constitution of closely packed trifluoromethyl groups (Bernett and Zisman, 1962; Pittman, 1972). By virtue of their unusual low surface energy, thin coatings of such polymers on a multitude of substrates have proved essential for many uses, such as stain and soil repellants on fabrics, abhesive agents, nonstick releasers, or barrier films for prevention of liquid spreading (Bernett and Zisman, 1964a, 1965). More recent applications are coatings on mechanical, electrical, and electronic equipment to provide water, oil, or fuel resistance, or to reduce electrical leakage. The critical surface tension of wetting of 10.6 dyn/cm for a substituted methacrylate with the repeating unit (-CH2-C(CH3)(COOCH2C~Fl~)-),(Bernett and Zisman, 1962) remains the lowest reported value for a bulk material (Shafrin, 1973). Because this polymer can easily be applied from solution as a thin film by casting or painting with a brush directly onto the substrate and allowing the solvent to evaporate. it has proved extremely useful as a barrier coating on delicate mechanisms (Bernett and Zisman, 1965) or miniature ball bearings (FitzSimmons, et al., 1965). The presently used solvent, hexafluoroxylene (HFX), assures the formation of uniform, smooth, and well-adherent films. However, the comparatively high boiling point (116'C) and the stringent purity requirements for HFX elicited a search for alternate solvents. Recently the polymer became available in two alternative solvents: (1) a more volatile and more easily available 250
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 4 , 1974
CC12F-CClFz(Freon TF, bp = 48°C) and (2) a slightly more volatile solvent analyzed a t this laboratory as isomers of perfluorobutyltetrahydrofuran, PBTF (bp = 92102°C). This report compares the fluoropolymer films prepared from one Freon T F and two different PBTF solutions, respectively, with films prepared from HFX solution as to wettability and stability after exposure to elevated temperatures in air or in contact with organic liquids.
Experimental Section Materials and Preparation of Films. The fluoropolymer was presented in five forms: (1) as a solution of 20% solids in HFX (Polymer A); (2) polymer A diluted to 2% with HFX (Pierce Chemical Co.) (polymer A-1); (3) as a solution of 2% solids in Freon T F (polymer A-2); (4) as a solution of 2% solids in PBTF-a (polymer A-3); and (5) as a solution of 2% solids in PBTF-b (polymer A-4). The difference between PBTF-a and PBTF-b is the additional presence of 1-270 lower boiling components in PBTF-a. Films of any of the above solutions were applied to acid-cleaned microscope slides by three different methods: (i) dipping slides into solutions for 30 sec one to three times and withdrawing slowly; (ii) painting by brush one to four times (with drying times of several hours in between applications for methods i and ii); and (iii) casting from a pipet. All films were finally air-dried for 24 hr a t ambient temperatures, then cured a t 50°C under vacuum for 7 hr. Films prepared by brushing or dipping were of the order of several thousand gngstroms in thickness and
Table I. Effect of Exposure to 210°C in Air on Wettability of Films of Fluoropolymer Cast from Different Solvents
Polymer A Polymer A-1 Polymer A - 2 Polymer A-3 Polymer A-4
0 5 11 0 5 11 0 5 11 0 5 11 0 5 11
75
73 73 75 73
73 74 69 66 74 73 73 74 73 73
98 96 98 98 97 98 97 95 91 100 99 99 100 99 98
showed interference colors; those prepared by casting were thicker and were transparent. All were smooth, nonbrittle, and adhered well to the substrate. Contact Angles. All liquids of the three homologous series, n-alkanes, research grade dimethylsiloxanes (from 3 to 12 repeating units), and Dow Corning DC 200 silicone fluids (Dow Corning, 1960), and the additional diagnostic liquid methylene iodide were further purified by percolation through alumina and Florisil just before use. Contact angles (0) of these liquids were measured at 23°C on the films by use of a contact angle goniometer (Fox and Zisman, 1950); the reported values represent the average of a t least three measurements; reproducibility was ~k0.5".
Film Wettability When cos 0 of each member of a homologous series of liquids on a smooth, clean, solid, low-energy surface is plotted against the surface tension ( ~ L v ) for each of those liquids, a straight line results; the intercept at cos 0 = 1 (0 = 0') is referred to as the critical surface tension of wetting (rc)for that particular surface (Fox and Zisman, 1950). yc for films of polymer A had previously been found to be 10.6 dyn/cm (Bernett and Zisman, 1962), a very low value for a bulk solid organic surface. As expected, films of the recently developed polymers A-2, A-3, and A-4 had the almost identical yc of 10.5 dyn/cm, regardless of method of preparation. Stability of Films An earlier study (Bernett and Zisman, 196413) had established that prolonged immersion of polymer A films in oils at ambient temperatures did not affect their wettability or their adhesion to smooth surfaces of metals or glass. This study is concerned with wettability at elevated temperatures and the possible effects caused by the use of different solvents. Exposure to 210°C i n Air. The cast films were exposed to 210°C in a clean oven and were examined after intervals of 5 and 11 hr. Adhesion and transparency of all films remained excellent, although A-3 and A-4 films became somewhat pitted. No adverse effect on the wettability on the polymer A, polymer A-1, polymer A-3, and polymer A-4 films was noted as shown by the minimal decrease in contact angles of hexadecane and methylene iodide before and after exposure to 210°C (Table I). Some deleterious effects on the wettability of the polymer A-2 films were indicated by the decrease in 0 with increase in exposure time. Immersion in Oils a t 100°C. Cast films prepared from polymer A, polymer A-1, polymer A-2, polymer A-3, and
Table 11. Effect of 18-Hr Immersion in Oils at 100°C on Wettability of Films of Fluoropolymer Cast from Different Solvents
Before immersion
DC 200, 50 cSt
Bis (2-ethylhexyl) sebacate
Versilube F-50
Nye instrument oil
NRL MB-BOB
Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer
A A-1 A-2 A-3 A-4 A A-2 A A-2 A-3 A-4 A A-2 A-3 A-4 A A-2 A-3 A-4 A A-1
Polymer A-2
Polymer A-3
75 75 74 74 74 71 59 70 42 69 67 65 58 66 66 70 sPr 69 67 72 61 20 (brush l x ) 59 (brush 4x) ia Spr (brush 1 X ) 48 (brush 4x) 60
98 98 97 100 100 96 a4 92 53 94 94 79 a5 91 93 92 45 92 94 96 a4 50 80 55 48 74 86
iia 116 116 115 115 113 99 110 63
loa 109 114 99 110 loa io8 30
loa
(brush l x ) (brush 4x1 (brush 14 (brush 4 4
107 116 10 5 50 90 32 30 65
(brush l x ) (brush 4x) (brush l x ) (brush 4X)
loa
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polymer A-4 were immersed for 18 hr a t 100°C in one of the following liquids: dimethylsiloxane DC200 (50 cSt), bis(2-ethylhexyl)sebacate,Versilube F-50 (chlorinated polysiloxane from General Electric co.), Nye synthetic instrument oil (formulated to NRL MB-SOB) and NRL MB-2OB (mixture of Hercolube A, diester, and additives) (Blachly, et al., 1970). The specimens were then drained free of liquid, washed with Tide solution, rinsed with distilled water, and dried. Adhesion and transpa:ency again remained excellent during immersion; however, after several days of storage in air a t ambient temperature, A-3 and A-4 films that had been in contact with the bis(2-ethylhexy1)sebacate and the Nye instrument oil turned very slightly hazy. The results in Table I1 show that of the four types of films the polymer A, polymer A-3, and polymer A-4 films were the most compatible with the oils a t the elevated temperatures. The considerably lowered contact angles of the polymer A-2 films indicate much lower resistance to such prolonged contact. Especially affected were the films immersed in Nye instrument oil and NRL MB-BOB, where the resulting contact angles seemed to be a function of film thickness: when the polymer A-2 film was applied by brush one time only, 0 for all three wetting liquids after immersion were very low. When the films were applied four times on the same area (with short drying intervals), 0 on these thicker films increased. Film thickness seemed a controlling factor also for polymer A-1; the results show that, here too, the thicker films were less adversely affected: the cast films as well as the four-layer brushed-on films became only slightly more wettable, whereas the wettability of the brushed-on (one layer) films was greatly increased. For any particular method of preparation, however, the effect of immersion in the heated oils was much more deleterious to the wettability of the polymer A-2 films than to the wettability of the other polymer films. Contact with Versilube F-50 seemed to be more deleterious since it not only increased the wettability of all films but also left them vulnerable to attack by hydrocarbons, as evidenced by outlines of the drop remaining on the films. Conclusions Wettability and adhesion of films prepared from polymers A, A-1, A-3, and A-4 were almost unaltered by exposures to 210°C in air for 11 hr and to 100°C in oils for 18 hr. As anticipated, films of polymer A-2 (Freon TF solution) and A-3 (PBTF solution) had the same yc as films of polymer A (HFX solution) ; however, exposure to elevated temperatures in air, and especially in contact with oils, produced generally bad and occasionally disastrous effects in wettability on polymer A-2 films. A t ambient temperatures, Freon T F may be a suitable
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substitute for HFX in many instances. For certain requirements, however, such as prevention of organic liquid spreading a t elevated temperatures, polymer A in HFX or polymers A-3 and A-4 in PBTF are superior to polymer A-2 in Freon T F provided there are no trace impurities present in either solvent. In fact, polymer A-2 may prove to be quite inadequate in contact with certain types of oils. The results indicate that whenever Freon TF is used as solvent for these fluoropolymers, the performance of the films is adversely affected. Addendum Since the submission of the manuscript an additional phenomenon was observed which was considered worth adding to this report. After 3 months storage, a very faint suspension of extremely fine white particles was noticed near the liquid/air interface in the PBTF-b solution. Some of the particles also had adhered to the vessel a t or above the interface. A similar suspension, but even more faint, was observed in the PBTF-a solution after storage of 8 months. None of the solutions had been inspected during the time of storage; thus.the onset of the particle separation is not known. No suspensions had developed on storage in the pure PBTF-a or PBTF-b solvents. Subsequent heating of the solutions to 65°C for 7 hr or cooling to 5°C for 48 hr did not appreciably alter the appearance of the suspensions. The suspensions are sufficiently faint to pose no real problems if films are applied by dipping or painting; they may prove troublesome, though, if films are prepared by casting or spraying through very fine nozzles. Acknowledgments The polymer solutions in the various solvents were made available by the 3M Company. I wish to thank my associate, J . P. Reardon, for carrying out the analysis of the solvent in polymer A-3 and A-4. Literature Cited Bernett, M. K . , Zisman. W . A , . d . Phys. Chem.. 66. 1207 (1962) Bernett, M. K., Zisman, W . A , , Advan. Chem. Ser., No, 43, 332 (1964a). Bernett, M . K.. Zisman, W . A , , Naval Res. Lab. NRL Report 6039 (Feb 1964b). Bernett, M. K . , Zisman. W. A , , (to U.S. Govt ) , U.S. Patent 3.225.866 ( D e c 2 8 . 1965). Blachly, C. H., FitzSimmons. V . G., Ravner, H.. Navai Res Lab.. NRL Report 7087 ( 6 March 1970). Dow Corning Silicone Notes 3-106b (1960) FitzSimmons. V. G . , Murphy, C. M., Romans. J. B.. Singleterry. c. R . , Naval Res. Lab., NRL Report 6356 (Dec 19651 Fox. H.W.. Zisman. W. A . , d . CoiioidSci. 5 , 514 11950) Pittman, A . G.. "Fluoropolymers." pp 419-449. Wiley-lnterscience. New York, N . Y.. 1972. Shafrin, E. G., in "Polymer Handbook." 2nd ed. Interscience. New York. N. Y . , 1973.
Received for reuieu' June 13, 1974 Accepted July 10, 1974
