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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
Fluoro Epoxies: Surface Properties and Applications Donald L. Hunston,' James R. Grifflth, and Robert C. Bowers Naval Research Laboratory, Chemistry Division, Washington, D.C. 20375
In an effort to develop new, low surface-energy polymers which are mechanically strong and easily fabricated, several highly fluorinated epoxy molecules have been synthesized. Two of these materials, which contained 4 7 % or 53% fluorine by weight, were evaluated both in bulk and in thin films cast from dilute solutions. The wear resistance, high friction, and low surface energy of the unmodified polymers suggest their application where a durable, non-slippery, easily cleaned surface is desired. The films showed little damage when exposed to water, oil, or organic solvents-an important resslt in light of the water degradation problem in composites. When additives are mixed with the epoxy resin, curing can trap these additives in the epoxy matrix and thereby modify the material's surface properties. The addition of powdered Teflon can dramatically lower the coefficient of friction while the presence of certain perfluoro materials can further reduce the surface energy. This flexibility greatly expands the potential of these materials for practical applications.
Introduction Solids which contain a high percentage of fluorine atoms have low surface energies that impart many unique and useful properties. Nearly three decades ago Zisman observed the difficulty in wetting polytetrafluoroethylene (Teflon) as indicated by the high contact angles for a series of liquids on this polymer (Fox and Zisman, 1948). He subsequently determined the critical surface tension of wetting ( y c )for Teflon (Fox and Zisman, 1950) and later demonstrated that the fluorine atoms were responsible for the low surface energy (Fox and Zisman, 1952). One use of a low surface energy material is that of a barrier film to confine a lubricant to an area where it is needed, or prevent a lubricant from creeping or migrating onto components where it would have a deleterious effect. A film which1 has proven to be effective with roller bearings for this application is a polymer of 1H,lH-pentadecafluorooctyl methacrylate (FitzSimmons et al., 1965);this material will be designated here as the Barrier Film. I t has a y c of 10.6 dyn cm-l, which is the lowest value reported for any solid (Bernett and Zisman, 1962). Unfortunately, this polymer has poor wear resistance and therefore cannot be used in exposed areas. Other fluorine-containing compounds such as Teflon are widely used as anti-stick and low-friction surfaces. Here too, however, most of the materials either lack durability and/or cannot easily be processed. Teflon, for example, cannot be melted, nor is there any known solvent. I t would be advantageous, therefore, to obtain a low surface energy material which has resistance to wear and can easily be formed into useful shapes. Such materials would have many important applications. Recently, several highly fluorinated epoxy resins have been developed (Griffith et al., 1972; O'Rear and Griffith, 1973). These epoxies are liquids a t room temperature but can be cured as thin solid films or in molds to form bulk solids of various shapes. As a result it is of interest to examine the friction, wear, and critical surface tension of these materials as well as their resistance to oils and solvents. Materials The molecular structures of the two fluorinated epoxy resins are
This paper not subject to U.S. Copyright.
where RF is C3F7 for the material designated C3 and C7F15 for the material designated C7. For comparison, a nonfluorinated epoxy resin (diglycidyle ether of bisphenol A) was also evaluated. /O\
/O\ O-CH,-CH-CH,
CH,-CH-CH,-O cH3
Although the molecular structure of this compound is somewhat different from that of the fluorinated epoxies, it can be obtained in high purity and serves to represent nonfluorinated epoxies as a general class of materials. T o form films and bulk samples of these compounds, each was mixed with a stoichiometric amount of the curing agent bis-1,4-aminomethylcyclohexane. The mixture was allowed to set in a closed container for several hours until the curing agent dissolved. The resulting fluid was then used to prepare the film and bulk samples. For the bulk specimens the liquid was poured into hollow aluminum planchets and precured under nitrogen at room temperature for several hours. The planchets were then placed in a vacuum oven and the temperature increased to 90 "C a t a rate of 20 "C/h. The samples were kept a t this temperature for several hours to complete the curing process. The resultant specimens were small circular disks about 2.5 cm in diameter and 0.4 cm in thickness. Tests were performed using both sides of the disks; the surface formed against the aluminum and the free surface. Film specimens of the fluoro epoxies were prepared by dissolving the sample-curing agent mixture in hexafluoroxylene (HFX).A small amount of the solution was placed on the surface of a 440C steel plate, the surface having been freshly polished under distilled water on 600A grit silicon carbide paper. The plate was placed in a confined space and the solvent was slowly evaporated. The film was then cured in a vacuum oven following the same procedure that was used for the bulk samples. The film thickness was calculated using the solution concentration, volume of solution used, and the surface area covered (the density of the epoxy is approximately 1.6 g ~ m - ~By ) . varying concentration, a series of films with different thicknesses were made. For all the test samples, the percentages of fluorine by weight in the cured polymers were 53.4%for the C7 compound, 47.4% for the C3 compound, and 0% for the regular epoxy. In addition to the pure epoxy resins, two mixtures were tested as films. The first was prepared by adding a small quantity of the Barrier Film material (ca. 5% by weight of the cured film) to a mixture of C3 epoxy and curing agent dissolved
Published 1978 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
in HFX. T o successfully prepare a film of this type, two things are necessary: a common solvent and sufficient compatibility between the additive and the epoxy-curing agent system so that separation does not occur during the cure. The high fluorine content of the epoxy makes it possible t o meet these requirements. A film of this mixture was prepared in the usual manner. The second mixture that was examined in this work contained powdered Teflon (supplied by DuPont). A small amount of the powder was dispersed in the H F X solution of C3 resin and curing agent. Since the fluoro epoxy has a low yc, it wets Teflon. This makes it possible to suspend the power in solution and, if the powder settles out, to resuspend it. Although some settling of the powder could be observed, it was possible to prepare reasonably uniform films if done quickly. Had it been necessary, however, the settling rate could be substantially decreased by reducing the particle size or increasing the solution viscosity. One additional bulk specimen was prepared, this time using a different curing agent system: (2-hydroxyhexafluoro-2propy1)pyromelliticanhydride and a trace of benzyl dimethylamine. Although the additional fluorine content obtained by using this agent was not expected to significantly affect the surface properties of the cured polymer, some improvement in water resistance might be obtained. The polymer-curing agent system was first blended in an aluminum planchet placed on a 150 “C hot plate and then cured a t 200 “C for 3 h. The resulting disk, which was 2.5 cm in diameter and weighed 2.6843 g when dry, was used for the water absorption studies.
Experimental Section Friction measurements for the bulk specimens and thin films were performed with a “stick-slip” apparatus using a spherical 52100 steel slider. Unilateral traverses were made over the same track at a velocity of 0.01 cm s-l. Film durability was determined by making reciprocal traverses at 0.1 cm sec-l. The criteria for film failure was a rapid increase in the kinetic coefficient of friction ( p ~ ) . The critical surface tension for the bulk and film samples was measured using standard contact angle techniques (Zisman, 1964). T o evaluate their resistance to oils, films of the C3 epoxy and the mixture of C3 epoxy plus Barrier Film were prepared as described previously except that acid cleaned glass slides were used as the substrate. After curing, two slides were placed in Nye Instrument Oil (a diester base oil), two in Versilube F50 (a chlorinated phenyl silicone), and two were retained for reference. The oil samples were then placed in an oven a t 100 “C. After 18 h, the slides were withdrawn, the oil was removed with a detergent, and contact angles for hexadecane were measured. Comparisons with the contact angles on the reference films were used to judge film deterioration. The resistance to solvent was tested in a similar way. Two slides of each sample were placed in HFX, and after 1h they were removed, rinsed several times with fresh HFX, and dried. Repeated rinsing with fresh H F X was used to prevent evaporation of the solvent from redepositing the dissolved materials. The contact angles were again used to test for film deterioration. The resistance to water absorption was measured by monitoring the weight gain for a disk that was immersed in distilled water. Results and Discussion Bulk Samples. The first experiments were performed on the solid disks which had been detergent cleaned. Contact angles, 0, were measured for six straight chain hydrocarbons, methylene iodide, and water. A plot of cosine (0) vs. surface tension of the liquids was then made for each disk and extrapolated to 8 = 0 to determine yc. Although the tests indi-
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cated that the surfaces were not entirely uniform, approximate values of yc could be obtained. For the C3 and C7 disks these values were very similar, falling between 20 and 23 dyn cm-1. The regular epoxy, however, had a yc greater than 27.6 dyn cm-l as indicated by the fact that hexadecane wet the surface. Moreover, the surfaces formed against aluminum tended to have a slightly higher surface energy than the opposite sides. Friction measurements were made on both the fluorinated and unfluorinated epoxies. The surface of the fluorinated epoxies formed against aluminum was reasonably smooth and required only detergent cleaning before use. The other surface was elevated around the circumference. To obtain a level test area the surface was abraded under distilled water on 600A grit silicon carbide paper. Friction was determined as a function of the number of repeat traverses over the same track and there was considerable scatter in the data for each sample. Nevertheless, some consistent trends were evident. The values for the first traverses were between 0.4 and 0.55 with no significant differences among the three epoxies, although the abraded surfaces were usually characterized by lower friction than surfaces formed against aluminum. With repeated traverses p~ always increased dramatically before attaining a constant value between 0.6 and 0.7; a value comparable to p~ obtained for the sliding of one clean metal surface on another. In an effort to investigate the high WKvalues and the initial increase in friction with the number of transverses, several additional tests were performed. First, the C3 disk was submerged in HFX for 1 hour and then rinsed with fresh HFX to remove unreacted monomer and other contaminants from the surface. In subsequent friction tests, however, the behavior was unchanged except that the differences between initial and final values of WK may have been slightly reduced. Consequently, surface contaminants cannot explain the unusual behavior. Two other possible causes were also investigated: first, the generation of heat in the polymer, and second, the buildup of materials on the slider. T o examine these uncertainties, an experiment was performed with a 10-min pause after traverse 50 and a pause for rotation of the slider after traverse 70. The resulting plot of p~ vs. number of traverses was a continuous curve with no discontinuity a t traverse 51 or 71. Thus, the observed behavior is real and not an artifact of the measurement techniques. A similar increase in WK with repeated unilateral traverses has been observed previously for steel sliding on polyvinylidene fluoride and to a lesser extent for steel on an ethylenetetrafluoroethylene copolymer (Bowers, 1971). The increase was attributed to work hardening of the polymers and this provides a possible explanation here too. Of greater interest, however, is the magnitude of p ~At. one time the uniquely low friction of Teflon was attributed to its weak adhesion and low surface energy. Recent studies (Pooley and Tabor, 1972; Bowers and Zisman, 1974), however, have indicated that its low value of p~ is a consequence of its linear structure and “smooth molecular profile”. In general, branching, crosslinking, or even substitution of atoms along a linear chain increases p~ of a polymer. Table I (Bowers and Murphy, 1967) compares both the static ( p s ) and kinetic coefficient of friction for a series of high polymers. Friction was determined under conditions comparable to those for the epoxy experiments described here. The data for p~ represent the “steady-state’’ values usually reached after several unilateral traverses with a steel slider over the same track. It may be concluded that the high friction of the epoxies (0.6 to 0.7) results from the cross-linked nature of these materials; the presence of fluorine has little effect. This combination of a low surface energy and a high coefficient of friction suggests that fluoro epoxies may have a
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
Table I. Frictional Properties of High Polymers" Polymer Polytetrafluoroethylene
0.10
0.05
Tetrafluoroethylene-hexafluoropropylene
0.25
0.18
PS
copolymer Polytrifluorochloroethylene 0.45 Poly(viny1 chloride) 0.45 Poly(viny1idene chloride) 0.68 Poly(viny1idene fluoride) 0.33 Polyethylene (high density) 0.18 Polyethylene (low density) 0.27 Poly(methy1methacrylate) 0.54 Poly(ethy1ene terephthalate) 0.29 Poly(hexamethy1ene adipamide) 0.37 Polycarbonate 0.60 Acetal resin 0.14 From Bowers and Murphy (1967). Intermittent ("stick-slip").
'27
PK
I"'"''''
0.33b 0.40b 0.45b 0.25 0.10
0.26 0.48 0.28 0.34 0.53 0.13 motion
number of interesting applications in situations where an easily cleaned, water or oil repellent material with a nonslippery surface is desired, e.g., for flooring materials. Film Samples. Since there was so little difference in behavior among the three epoxies tested as bulk samples, only the C3 epoxy was tested as a film. In all the friction experiments performed with this material, the variation of I.LKwith number of traverses showed the same pattern. During the first 5 or 10 traverses, p~ decreased about 10%. At this point it leveled off and remained relatively constant for a time (Figure 1).This minimum value (MM) was taken to be characteristic of the film. Eventually, the film began to fail and I.LKincreased gradually until a value >0.5 was reached (steel on steel is about 0.55). Failure of the film was defined as the point at which p~ reached a value half way between I.LMand 0.5. The number of traverses made before the failure point is taken as a rough measure of the relative resistance to failure. With the C3 film I.LM was a function of load, decreasing continuously from 0.24 to 0.19 over the range 1to 10 kg (Figure 1).This is typical of thin films on substrates which are much harder than the film material and is a consequence of an increase in the actual contact pressure resulting from a nonlinear increase in the real area of contact with increasing load (Bowers, 1971; Bowers and Zisman, 1968). Similarly, a t a constant load, a decrease in p~ was observed with a decrease in film thickness (Table 11).This again supports the idea of an increase in contact pressure, in this case resulting from a decrease in real contact area at constant load. Film durability was also determined at a 1 kg load, and Table I1 lists the number of traverses to failure for four different film thicknesses. When the film is relatively thick (20 Hm) the area of contact is so great that the film adheres to the slider and is peeled from the substrate in one large piece. With the films of lesser thickness, the slider seemed to slowly wear away the film so film life decreases with decreasing thickness. Thus, there is an optimum value for thickness, and under the conditions used here it corresponds to ca. 2 pm. Washing the film with HFX had no observable effect on the friction or durability properties. Although the wear resistance of the C3 film was not exceptionally high, it was significantly greater than that of the Barrier Film. The critical surface tension of the C3 film was measured, following the same procedure used with the bulk samples, with eight diagnostic liquids. The value of yc was 18.7 dyn cm-l. After the film was washed with HFX, yc was 19.2. Thus, while the C3 fluorinated epoxy resin had a superior durability, its critical surface tension was far inferior to that of Barrier Film (10.6 dyn cm-l). Mixtures. In view of the above results it was decided to study fluoro epoxy films containing additives. First, a small
LL W
0 V
.2
i! c
-
NUMBER
OF T R A V E R S E S
Figure 1. The kinetic coefficient of friction as a function of traverse number and load on the 2 - ~ m CBfilm. Table 11. Friction and Wear of Films Thickness, pm 20 10
2 0.4
PM
No. of traverses to failure
0.35 0.27 0.24 0.23
1-2 50 75 15
amount of Teflon powder was added to the C3 epoxy in an effort to lower the coefficient of friction. Second, a small amount (ca. 5%) of the Barrier Film was mixed with the C3 epoxy in an attempt to lower the critical surface tension. Films of both mixtures were tested for friction and wear. The addition of the Teflon powder increased the film durability slightly and dramatically reduced friction; WK was comparable to that of Teflon. The value of yc showed very little change, increasing slightly to ca. 20 dyn cm-l. This combination of durability and a low coefficient of friction suggests many interesting applications for this material. Not only does it offer potential as a solid film lubricant, it suggests the possibility of fabricating plastic parts which are "self lubricating". The second mixture that was tested combined the C3 epoxy with the Barrier Film. The presence of this additive was found to reduce yc to the very low value characteristic of the Barrier Film itself without altering the coefficient of friction or durability of the pristine C3 film. This low yc suggested that this film would be resistant to attack by oils and solvents and so tests of these properties were performed with the C3 epoxy and the mixture of Cs epoxy plus Barrier Film. Two different oils were used. Table I11 compares the results with data previously obtained for the Barrier Film alone. All three films show an excellent resistance to attack by oils. In regard to solvent attack, it is known that the films are highly resistant to most liquids and thus only the most aggressive solvent, HFX, was examined here. Initially, both the C3 epoxy and the Barrier Film were soluble in HFX, but the tests indicated that when cured, the C3 film was no longer soluble, at least for short exposures. The Barrier Film, however, remained susceptible to attack. When the C3 epoxy-Barrier Film combination was exposed to HFX, some but not all of the Barrier Film was removed from the surface; the critical surface tension increased from 10 to 14 dyn cm-' (Figure 2). Apparently some of the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
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Table 111. Contact Angles for Hexadecane 0 after immersion in 0 Nye Thick- before instru- Versiness, immer- ment lube wm sion oil F-50 HFX
Film material Barrier Film C3 epoxy Cs epoxy plus Barrier Film Bernett (1973).
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