1292
~IARIAN K.SBERXETT E ASD IT. A. Z r s ~ a s
Vol. 64
WETTING PROPERTIES OF TETRAFLUOROETHYLESE AXD HESAFLUOROPROPYLENE COPOLYMERS' BY MARIANNE IC. BERNETT AND 'CV. A. ZISMAN U. S.hTavaLResearch Laboratory, Washingtori 25, D.C. Recezced March 65, 1060
It has been shown that the determining factors in the wetting of lorr-energy solid surfaces are the nature of the atoms in the surface and their physical packing. Of all solid organic polymers polytetrafluoroethylene is known to have the lowest surface energy and thus the lowest critical surface tension of wetting (re). In this study the wettability of a series of copolymers of tetrafluoroethylene with hexafluoropropylene by a variety of organic and inorganic liquids was examined; the results show that these solid plastics have critical surface tensions which are even lower than that of solid polytetrafluoroethylene. As anticipated, the progressive increase in the proportion of perfluoromethyl side chains in the polymer introduces a higher concentration of exposed -CFs groups in the surface which in turn progressively reduces yc. By extrapolation y c of a polyhexafluoropropylene has been calculated to give the value of 15 dyiies/cm.
Introduction
A series of investigations2-* on the effect of constitution on the spreading of liquids on solids has formulated the factors determining the wetting of low-energy solid smooth surfaces or of high-energy surfaces modified by adsorbed films. It was demonstrated that wettability is a function of the nature and physical packing of the atoms in the surface, and that it is essentially independent of the nature and arrangement of the underlying atoms and molecules. Polytetrafluoroethylene (poly TFE) has the lowest surface energy of all polymeric solids studied to date. Thus, its critical surface tension of wetting (yc) of 18.5 dynes/cm.2 is characteristic of a surface consisting essentially of close-packed perfluoromethylene -CF2- groups. The surface of lowest energy encountered to date is that of smooth glass or platinum coated with an oriented close-packed monolayer of perfluorododecanoic acid'; therefore, its value of yc, 6-8 dynes/cm., characterizes the resulting properties of a surface of close-packed perfluoromethyl -CFI groups. Perfluoroalkanoic acids of lower molecular weight, when adsorbed on high-energy surfaces, do not pack quite as closely due to the lesser contribution of the intermolecular cohesive forces ; a close-packed monolayer of perfluorobutyric acid, for instance, has a yc of 9.2 dynes/cm. This investigation reports on the wettability of the solid copolymers prepared from tetrafluoroethylene and hexafluoropropylene. It was undertaken to observe the effect on wetting of the progressive replacement of -CF,- by -CFBgroups in the surface of the bulk polymer. Materials and Experimental Procedures The polymers used were molded experimental samples of fluorocarbon resins in the form of sheets 1/16 inch thick which were furnished through the cooperation of the Polychemicals Department of the du Pont Co. The materials (1) Presented before the Division of Colloid Chemistrv. American Chemical Society, a t the 137th National Meeting in Cleveland, Ohio, Ami1 11-14, 1960. (2) H. W. Fox and W. A. Zisman, J . Colloid Sci., 5 , 514 (1950). (3) H. W. Fox and W. A. Zisman, ibid.. 7, 109 (19.52). (4) H. W. Fox and W. .4. Zisnian, ibid., 7,428 (1952). ( 5 ) M. K. Bernett and W. A. Zisman, T I I I JOURNAL, ~ 63, 1241 (1959). (6) M. K. Bernett and W.A. Zisman. ibid., 63, 1911 (1959). (7) E. F. Hare, E. G . Shafrin and W. A . Zisman, ibid., 58, 236 (1954). (8) E. G. Shairin and W.A. Zismsn, %bid., 64,519 (1960).
are copolymers of tetrafluoroethylene (TFE) and heuafluoropropylene (HFP) containing HFP in the following concentrations: 6, 8, 11.5, 14, 16 and 23 mole 70. This general group of polymers is known as the F E P (fluorinated ethylene propylene) resins. The specimen containing 2370 I-IFP is a low molecular weight wax, which represents the highest concentration of hexafluoropropylene that could then be furnished.9 The surfaces of these specimens were smooth and specular, and selected areas were sufficiently flat to be used for the wettability studies without further preparation, other than cleaning with concentrated aqueous solutions of the commerrial detergent Tide.5 All liquids used were highly purified compounds described elsewhere2s4and each was percolated just before use through columns of appropriate adsorbents t o remove polar impurities. The advancing contact angle (e) of each of these liquids was measured on a number of glossy areas of each specimen of the copolymer using the drop buildup method10 and an improved goniometer telescope.2 Variation of 0 was never more than =t2' fr2m the mean; frequently the maximum deviation was + I . All data were observed at 25 i 1" and 50 f 27, R. H.
Experimental Results and Discussion The wetting curves obtained for each solid copolymer by plotting cos e of each liquid n-alkane against the liquid surface tension ~ L are V shown in Fig. 1. For purposes of comparison, the wetting behavior of clean smooth polytetrafluoroethylene2 also is given. It can be seen that the data for each solid surface conform to the now well-established rectilinear relationship between cos 6 and y ~ v . As anticipated, the values of yo obtained by the intercepts of the curves with the line cos 6 = 1 become smaller as perfluoromethylene groups in each surface are replaced by perfluoromethyl groups. An exception is provided by the copolymer containing 6% H F P which exhibits a value of yo higher than that of the homopolymer. Contact angles of all compounds are consistently lower on this surface than on pure poly T F E ; this anomalous behavior in the family of copolymers may be an indication of either decomposition or contamination of the surface, giving rise to a higher-energy surface than poly TFE. Although the individual increments in yo from resin to resin are rather small, the regularity of the results with all but one member of the series of copolymers makes it evident that the value of yc of the 23yGHFP-TFE copolymer is nearly 1 dyne/ em. lower than that of poly TFE. If the respective yo for each surface is plotted against the per cent. substitution of hexafluoropropylene a straight line results, as shown in Fig. 2. (9) C. A Sperati, private communiration. (10) E. G. Shafrrn and W. A. Zlsman, J. Collvrd A C L , I , I b b (1952).
Sept., 1960
j $ T ~ O~F TETRAFLUOROETHYLEXE ~ ~ ~ G AND HEX4FLUOROPROPTLENE COPOLYMERS
By extrapolation an estimated value of yo of 15.6 dynes/cm. is obtained for a hypothetical lOOyo H F P polymer. Values of yo for each copolymer can be calculated from the surface constitution by assuming that the critical surface tensions, like the surface free energies, are additive quantities and may thus be computed to a first approximation. For this purpose the previously established values of 18.5 dynes/cm. for close-packed -CF2- groups2 and of 8 dynes/cm. for close-packed -CFs groups’ were used. Assuming that the copolymerization results in a regular repetitive molecular arrangement of the monomers along the principal chain of each molecule and that this same arrangement occurs in the free surface of the solid, an 8% HFP-TFE copolymer would contain one perfluoromethyl group per 24 perfluoromethylene groups. Applying the above values of yo to weight the additive contribution of each -CF,- and -CF3 group, a theoretical value of 18 1 dynes/cm. is obtained for that particular copolymer surface. Employing these same additive assumptions, calculated values for the surfaces of all the copolymers available as well as for the lOOyo HFP polymer were obtained, and the results are shown in the lower rectilinear plot of Fig. 2. This results in a theoretical value of 13.3 dynes/cm. for the hypothetical 100% H F P polymer. A linear 10070 H F P polymer never has been reported. However, Stuart Briegleb molecular ball models of these copolymers have been constructed and have indicated that chains may consist of no more than three hexafluoropropylene monomers on account of accumulated steric hindrances resulting from the large diameter of the perfluoromethyl group. Whereas a ball model of the 50% copolymer could be made readily, the 75y0HFPTFE copolymer was difficult to assemble because it was nearly rigid structure. In Fig. 3 are shown graphs of cos 8 PS. Y L V for the wetting of a variety of organic and inorganic liquids on the copolymer containing HFP. The intercept with IW 0 = 1 results in a critical surface tcnsion of 1’7.5 dynes/cm., in good agreement with the value of 17.8 dynesicm. obtained for the ?r-alI\:uic liquids (Fig. 1). The concavity of the lorver portion of the curi7.e in Fig. 3 illustrates thc incwase in wetting ?rising from weak hydrogen-bonding of such hydrogc.n-donating liquids as w:tter, glyc*oland formamide with fluorine-containing surfaces.ll Contact angles of these same misccllaiieous liquids on the other HFE-TFE copolymers resulted in similar cos 0 us Y L V curves for each surface with slight progressive displacements to the right as would be predicted from consideration of the H F P mole yo substitution. If the contact angles exhibited by any oiie liquid on the various copolymer surfaces are plotted against the H F P content, a linear relationship is obtained a.3 shown in Fig. 4. Thus the contact angle of hesadecaiie becomes larger for each progressive perfluoromethyl side chain substitution; i e . , it is smallest for 0% substitution (poly TFE) a i d increases in small increments to its largest value (11) k 11. Clliaon. 11. I! l‘ux and W.A . Zismen, THISJOURNAL,67, G22 (1953).
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whcii the IIFI’ content reaches its highest percentage. In Fig. 4 are plotted the wettability result.; obtained with some of these liquids. In almost every instance straight lines were obtained (with somewhat different slopes) demonstrating clearly the decreasing wettability with increase in H F P content. It will be noted that the contact angles of mater, in contrast to those of all other liquids, do not increase with increasing HFP substitution, but remain essentially of equal magnitude throughout. The same results with water were obtained by Allan and Roberts1* in their recent study of the hydrophobic behavior of some perfluorocarbon polymer films. They also used the hydrophobic contact angle as a measure of the roughness of films prepared in various ways from “Teflon” TFE resins and the new du Pont FEP resin. The latter (12) A. G. J. AUan and R. Roberts, J. Polvmer Sa.,39, 1 (1969).
high surface tension, a weak tendency to form hydrogen-bonds with fluorinated surfaces, high permanent dipole moment, and small dimensions ; it thus does not exemplify the wetting behavior of most liquids. Contact angles for the n-alkanes on a 100% HFP polymer were estimated by extrapolation of 0 LIS. 0% H F P curves, such as in Fig. 4. By plotting the cosines of these values against the yL\r of the liquids, the wetting curve for a hypothetical 100% H F P polymer can be constructed, along with the attendant value of yc. Such a curve is plotted as a dash-dot line on Fig. 1. The critical surface teiision in this case is 15.4 dynes/cm., in very good agreement n-ith the value of 15.6 dynes,/cm. obtained previously by other means. Ellison and Zisman in their investigation of halogenated organic solid surfaces,14found that a plot of cos e of various liquids versus the atom per I120 I I I I I cent. fluorine substitution from 0 to 100 in the -0 51 IO 20 30 40 50 60 70 80 solid surface will produce curves of several distinct SURFACE TENSION (DYNES I C M AT 25'C). types; these types me determined by the hydrogenFig. :J.-\lettabilit)by miscellaneous liquids on a HFPbonding ability of the respective liquid to the TFE copolymer containing 23 mole yo of HFP. underlying surface. Strongly hydrogen-bonding liquids, such as water, glycerol or formamide, present a large curvature, whereas non-polar I WATER I71 91 I liquids such as the n-alkanes form straight-line The present study shows rectilinear I graphs. relationships between cos 0 and H F P content for FORMAMIDE 1 5 8 2 ) all liquids employed. However, the maximum rontent is only 23cT,, and the largest change in 0 is METHYLENE IODIDE (5081 only 9 O . These limited changes in the magnitudes I of the variables produce straight-line plots; how-Ln. 80 c'ver, extrapolation to obtain the respective cosines of the contact angles for a 100% H F P polymer may 5 70.1w only a fair approximation in view of the results n -HEXACHLOROPROPYLENE(3811 ( f the previous study by Ellison and Zismnn.14 s 0 60 2 DICYCLOHEXYL 1330) The \ d u e of about 15.5 dynesjcm for the critiI I HEXADECANE (276) r a l u-face tension of wetting for a hypothetical TETRADECANE 126 7 ) 501 OO?/& polyhesafluoropropylene polymer m s obDODECANE 1 2 5 4 1 taiiied by tn-o methods based on experimental 40DECANE I23 9) result?. The value of 1'3.3 dynes#'cm. was derived I 0 additive relations of the atom groups believed ( I = r,,,DYNES/CM AT25'C 1 hy 301 t o occupy the surface. The latter value is predi2001-- wtctl on a number of assumptions, such :LS repeti4 8 12 16 20 24 28 32 t 11-e molecular arraiigement , linear chain formation MOLE PERCENT HEXAFLUORCPROPYLENE. atid regular arraiigement in the free wrface of the Fig. 1.--LAwt of HFI' roiitrJnt on t h \vetting of HFI'-TFI. wlid; because of' the steric hindraiic*eof the bulky copolymcv 111 rniscellancori. liquids perfluoromethyl groups and of its increased inresin had beeti ideiitified earlier ti,\ l'cfloii 1OOX I d fluence at high molal proportions of the HFI' They found large differenre? in t hc contact angle of monomer, oiie or more of these assumptions may water vith changes in the surfaw roughne55, but 110 iiot be d i d . It i:, believed, therefore, that the appreciable difference between smooth surfacw of value of 15.3 dynea,/crn. for yc ~ ( i u l dbe niore TFE or of FEI' resins. The rehults obtained with iiearly cwrrect for a 100% polyhexaAiioropropyrleiie water in the present study corroborate aiid extend polymer. The total shift in yc from a surface comthe latter conclusioii. However, it also shorn that prised ( i f -('E'? - groiips to one cwmpriscd of a mixthe wetting of the copolymers by liquids of lower ture of 41F3mid -('F2- group5 is at I w t only 3 or surface tension than mater does vary appreciably 1 dyiies em. It therefore can be concluded that with differences in the chemical constitution of the 110 copolymer can be produced from the family of copolymers. The physico-chemical properties of FEP copolymers whose solid smooth surface is as mater are, in many respects, unique. It has a non-tvetting as that of a close-packed monoIayer of perfluoromethyl groups. (13) It. 9. Malloiih and 15'. B. Thompson, BI'E Journal, 14, 43 I
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