Frictional Properties of Tetrafluoroethylene-Perfluoro(propy1 vinyl ether) Copolymers Robert C. Bowers" and William A. Zisman Laboratory for Chemical Physics, Naval Research Laboratory. Washington. D. C. 20375
The frictional properties of several new experimental random copolymers of tetrafluoroethylene and minor proportions of perfluoro(propy1 vinyl ether) have been studied both as thick samples and as thin films on harder substrates. These copolymers have advantage over polytetrafluoroethylene (PTFE) in that they can be melted and made to flow, so that it should be possible to form water-resistant and melt-adhesive solid-lubricant films without the necessity of roughening the substrate or of using adhesive binders. For bulk samples, the kinetic coefficient of friction ( P k ) of the new copolymers is considerably greater than that of PTFE. However, when applied as a thin film or a hard backing, the kinetic coefficient of friction is only 0.05-0.06.
Introduction The extremely low critical surface tension of wetting, the unique low coefficient of friction, and the outstanding chemical resistance of polytetrafluoroethylene (PTFE) have created a widespread interest in the study of this polymer. Numerous applications have been found for PTFE based on one or more of these properties. One disadvantage of PTFE is the difficulty in molding or forming it into a desired shape since it does not melt or flow in the same manner as conventional thermoplastics. At temperatures above the crystalline melting point (327") it becomes an amorphous gel but still retains considerable mechanical strength. This limitation was overcome by copolymerizing tetrafluoroethylene with a smaller proportion of hexafluoropropylene. The properties of this copolymer (TFEHFP) are similar to PTFE. There was, however, a considerable increase in the coefficient of friction for the bulk polymer (Bowers and Zisman, 1963) and for thin FEP films on hard substrates (Bowers, 1971) as well as a reduction on high-temperature mechanical properties. Recently E. I. du Pont de Nemours & Co. has developed several new random copolymers of tetrafluoroethylene and perfluoro( propyl vinyl ether) (F&=CF-0CFzCFzCF3). Like TFE-HFP resins, these new perfluoroalkoxy copolymers have the advantage over P T F E in that, while maintaining their remarkable chemical and oxidative resistance, they can be melted and made to flow. These new copolymers have recently been shown to have a low critical surface tension of wetting, in the range 16-19 dyn/cm (Reardon and Zisman, 1974). Therefore, in the liquid state they would be expected to have an extremely low surface tension and consequently to spread over a wide variety of solid surfaces. This is a condition for the formation of strong adhesive bonds (Zisman, 1963). Thus, it appears possible to form melt adhesive solid lubricant films without the necessity of roughening the substrate or of using a binder or primer which is required with P T F E films. This paper will discuss the frictional properties of these new copolymers which contain either 3.8, 5.4, or 6.6% of the perfluoro(propy1 vinyl ether) comonomer, (corresponding to mole ratios of 0.015, 0.021, and 0.027).
Apparatus and Procedure The apparatus used to measure friction and wear was a "stick-slip'' type machine which is similar to that developed by Goodzeit, et al. (1956), and which we have previously described (Bowers and Zisman, 1968). With this device the coefficient of friction was determined between an elastically restrained slider and a plane surface which was driven, in these studies, a t a constant velocity of 0.01 cm/sec. Except where otherwise noted, the sliders were fh-in. diameter spheres of 52100 steel which had been degreased by refluxing reagent grade benzene in a Soxhlet extractor. The plane surfaces were the new copolymers, used either as a thin film or as bulk material. These copolymers were made available by Du Pont. The 3.8% copolymer was available in two average molecular weights. Unless otherwise noted, the 3.8% copolymer used was the one with the higher molecular weight. Each first traverse was made with an unused area of the slider over an unused area of the plane polymer. The length of each first traverse was 7 mm. Each subsequent traverse was made with the same area of the slider used for the first traverse over the center 3 mm of the first traverse. Preparation of Thin Films The most useful method of film preparation consisted merely of rubbing a pellet or a small piece of copolymer sheet material over substrates which were heated to a temperature approximately 75" below the melting point of the polymer (-300-310"). This produced a very thin film containing some relatively large pieces which formed predominantly a t the end of each rubbing stroke. These could, therefore, be largely confined to areas near the edges of the substrate and avoided during friction measurements. We had previously demonstrated that a thin film of polyethylene could be transferred to steel by rubbing the polyethylene at room temperature (Bowers, et al., 1953). The low and constant value of the kinetic coefficient of friction ( M ~ )for this film established that the film was continuous and that it was not penetrated by the steel slider. The polymer, transferred by a single traverse Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 2, 1974
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of a polyethylene slider onto a steel platen, was examined by electron diffraction. These patterns established that the transferred polyethylene crystallites were oriented so that the long axes of the polymer molecules were in the plane of the steel surface and were nearly all pointed in the direction of sliding (Bowers, e t al., 1953). Subsequently thin films were prepared on hard surfaces a t elevated temperatures by the rubbing technique, for high-density polyethylene (HDPE), polyvinylidene fluoride, polytetrafluoroethylene, a copolymer of ethylene and tetrafluoroethylene, and a tetrafluoroethylene-hexafluoropropylene copolymer (Bowers, 1971). The transfer of polymers to clean surfaces has recently been studied in greater detail by Pooley and Tabor (1972), who found that, under certain conditions, PTFE transferred as a n extremely thin film. The film was estimated from electron micrographs to be as small as 2.5 n m (25 A) or about 5 times the diameter of the PTFE molecule. Under similar conditions HDPE transferred as discrete streaks less than 10 n m thick. At room temperature after one traverse of a PTFE slider over clean glass, there remained areas of uncoated surface along the track. At 100" the coverage was much greater and the P T F E film more coherent. With other polymers, and with P T F E and HDPE under some conditions, the thin transferred polymer films were covered with large lumps of polymer (Pooley and Tabor, 1972). There is, therefore, ample evidence that a thin coherent film can be prepared by the rubbing technique. Experimental Section a n d Results Bulk Polymers. The kinetic coefficients of friction for the new 3.8, 5.4, and 6.6 wt % copolymers, determined a t a 1-kg load, are shown in Figure 1 as a function of the number of unilateral traverses. These specimens were cut from 31-mil sheets and were cleaned by washing with a n aqueous detergent ("Tide") solution, rinsing copiously with distilled water, drying in a desiccator, and finally refluxing in Freon TF (trichlorotrifluoroethane). They were then annealed by heating a t 260" for 24 hr and cooling slowly to room temperature. The copolymers were annealed in order to ensure that each sample had a similar thermal history. The frictional behavior of the annealed samples did not vary significantly from the corresponding samples which were cleaned but not heat treated. This is in contrast to the critical surface tension of wetting which decreased noticeably after a similar annealing process (Reardon and Zisman, 1974). The principal feature of Figure 1 is the initial and gradual decrease in the average value of before a constant value is reached after approximately 15 traverses. It is also evident that there are only small differences in c(k among the three perfluoroalkoxy copolymers. For comparison the values of Kk for PTFE and for a TFE-HFP resin are included in Figure 1. These were taken from a n earlier publication (Bowers and Zisman, 1963). The constant value of Pk for the perfluoroalkoxy copolymers is much higher than c($ for P T F E and slightly lower than Fk for the TFE-HFP copolymer. Several features which are not evident in Figure l were observed with the three perfluoroalkoxy copolymers. On the first traverse the static coefficient of friction ( p s ) was much greater than ls for any subsequent traverse. On the first traverse ps was usually 0.35-0.40, but after several traverses it was about 0.22. The kinetic coefficient of friction also decreased with sliding distance during the first traverse. For example, with the 6.6 wt ?% copolymer, P k decreased from 0.27 to 0.21 after sliding several millimeters. If a slider used for a 50traverse set was then made to traverse a fresh area of the copolymer, both l~~ and P k were intermediate between the 116
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3 6,6WT. e/'
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0 5 . 4 WT %COMONOMER
C 3.8 WT
%COMONOMER
i 1
5
IO
I
I
I5
20
I
25
30 TRAVERSE NUMBER
I
35
40
45
50
Figure 1. The kinetic coefficient of friction as a function of the
number of traverses for thick samples of the copolymer containing 3.8, 5.4, and 6.6 wt 9 G of the perfluoro(propy1vinyl ether) comonomer. 1st and 50th traverse values. The initial decrease in with repeated traverses (Figure 1) and the preceding observations indicate that both the condition of the slider and the polymer were changing. They suggest that a film of polymer was quickly transferred to the slider and that both this transferred film and the bulk polymer gradually became oriented to a greater degree. Two disks, 1h in. thick, of the 3.8% copolymer were molded at ca. 800 psi and 320". One disk, molded from pellets, had the same molecular weight as the 3.8% sheet material of Figure 1. The second was molded from powder and had a lower molecular weight. The surface of each disk on which friction was to be measured was prepared by abrasion under water on 600-A grit silicon carbide paper. Friction measurements were made at loads of 1 and 10 kg. In contrast to P k for the copolymer sheets, fik for the disks remained constant during each series of multiple traverses; l~~ for the lower molecular weight sample was 0.14-0.15 and for the other disk it was 0.13-0.14. The reason for this difference between the frictional behavior of the sheet material and the molded disks is not known. The cause may be related to the differences in preparation. The disks were formed from the copolymer melt and were cooled very slowly. The highest value of ps occurred on the initial traverse and was 0.23 a t the 1-kg load for each disk. On subsequent traverses pS was usually between 0.16 and 0.18. Thus it appears that a film is transferred to the slider a t the onset of relative motion. Neither this film nor the bulk polymer is significantly altered by repeated sliding. Thin Films. Films of the high and low molecular weight 3.8 wt % copolymer were prepared on soda lime glass substrates by the rubbing technique a t 230". Fifty traverses were made a t a 10-kg load and 10 traverses a t a 1-kg load on each film. Except for the initial traverse, Kk was nearly constant during each traverse. During the first pass P k decreased rapidly with sliding distance from 0.10 or 0.11 to 0.07 or 0.06. On subsequent traverses P k was only 0.05-0.06. These films were next heated to 350" for 2 hr to fuse them to the substrate. After they were slowly cooled to room temperature, several series of multiple traverses were made a t 10 kg. On a few areas the films began to fail after 10-30 traverses as evidenced by a n increase in friction and by substrate damage. There were. however, areas where P k remained low (0.05-0.06) during the 50traverse series. The areas of film failure do not mean that the film was degraded by the high temperature. The original films may not have been continuous. These two speci-
mens were then soaked in distilled water for 3 weeks. Three series of multiple traverses a t 10 kg were made on the lower molecular weight film. Twice the film failed after approximately 25 traverses. On one area gk remained at 0.05-0.06 for the entire 50 traverses. On the higher molecular weight film two determinations at 10 kg were made. The kinetic coefficient of friction was 0.05-0.06 throughout the 50-traverse series on both areas. Some preliminary studies were made on a thin film of a copolymer containing an unknown amount of perfluoro(propy1 vinyl ether). This film was prepared on glass by rubbing the polymer at a temperature of ca. 225”. Fifty traverses were made at a 10-kg load on the original surface, on the same surface after fusing above the copolymer melting point, and on the film after benzene refluxing for 3 hr and Freon TF refluxing for 2 hr. In every case pk remained low throughout the multiple-traverse series. A rubbed film of the same copolymer on an acidcleaned sapphire disk was studied a t loads of 1 and 10 kg with a steel slider and a t 1, 2, 5 , and 10 kg with an acidcleaned sapphire slider. The average kinetic coefficient of friction was the same as pk for this film on the glass substrate regardless of the load or slider. The film did not show any evidence of breaking down even after 50 traverses a t 10 kg with a sapphire slider. Two films of the lower molecular weight 3.8% copolymer were prepared from an aqueous dispersion of finely divided particles. A drop of dispersion was placed on a glass substrate and the water was allowed to evaporate a t room temperature. The sample was then placed in a clean vacuum oven a t 260” for 3 hr to remove the dispersing agent. It was then heated in an air oven to melt and fuse the particles. The second film was prepared by the same procedure using a drop of dispersion which had been diluted by a factor of 100. Although this method did not produce uniform films, they were useful for friction measurements. The first film was approximately l mil thick; the thickness of the second was roughly 1% that of the first. With the thicker film a t a 1-kg load on the initial traverse, p s was 0.18 and pk varied between 0.08 and 0.12. On subsequent traverses p s was 0.13 or 0.14 and the average value of pk varied from 0.08 to 0.12. At 10 kg ps and pk were both about 0.07. With the thinner film, a t a 1-kg load pCswas 0.10 on the initial traverse and 0.07-0.08 on the following traverses. The kinetic coefficient of friction a t 1 kg and both p s and ,uk a t 10 kg were 0.05-0.06, i e., the values obtained with other films of this 3.8% copolymer. The higher friction for the thicker film resulted from the larger area of contact which in turn is a consequence of the larger ratio of the load supported by the film. Increasing the load diminishes this effect. The study of the perfluoroalkoxy copolymers as thin films on soda lime glass indicated that these films are not affected by repeated rinsings in benzene or Freon TF or by prolonged soaking in distilled water. No significant difference was found between the two methods used to prepare the film or between the average molecular weights for the 3.8% copolymers. Discussion The friction between two clean sliding solids results mainly from adhesion a t the many small areas of intimate contact (real area) which are scattered over the geometric area. The strength of each adhesive junction is nearly always comparable to or greater than the shear strength of the materials so that during sliding shearing takes place near the interface but within the weaker solid. The frictional force is, to a first approximation, equal to the product of the shear strength (S) of the softer material and the
sheared area (A), i . e . , the real area of contact. The real area is determined by the mean yield pressure (P,) of the softer solid. Since P , is equal to the ratio of the normal load ( W) to the real area of contact and the coefficient of friction ( p ) is by definition equal to the frictional force (F) divided by the normal load, then p = F/
W = SA/P,A
= S/P,
(1) This is the adhesion theory of friction as originated and developed by Bowden and Tabor (1954). Solids which are very hard (large P,) are also very strong (large S),and conversely weak solids are soft so that the ratio of S to P, does not vary greatly among solids. A method for obtaining a small ratio of S to P is the use of a very thin film of a relatively easily sheared solid made to adhere to a much harder substrate which essentially determines the pressure. The relationship for thin films is more complex than eq 1 would indicate because the shear strength of the film is itself a function of pressure (Bowers, 1971; Bowers and Zisman, 1968). For a series of high polymers S has been reported to vary nearly linearly with pressure (Bowers, 1971) so that S=kP+C
(2)
It follows that for a thin polymer film on a hard backing p
= SIP = k
-+ C / P
(3)
In eq 3, P is equal to P, only if the load and geometry produce a pressure great enough to cause gross plastic deformation of the substrate. In general the film will deform plastically and the substrate elastically so that the pressure on the film will be determined by the elastic properties of the substrate (Bowers and Zisman, 1968). From the experimental values of pk obtained for the 3.8 wt % copolymer as a thin film and as a thick sample the values of k and of C in eq 3 can be estimated and a theoretical curve drawn for pk as a function of pressure. It is evident from eq 3 that as P is increased, p approaches K The kinetic coefficient of friction after repeated traverses for a thin film of the 3.8 wt % copolymer was 0.05-0.06. These values were obtained for pressures, as calculated from the Hertz equation for elastic deformation, ranging from 3500 kg/cm2 (steel on Pyrex a t a 1-kg load) to 18,300 kg/cm* (sapphire on sapphire a t a 10-kg load). Therefore, k is approximately 0.05. The constant C can be determined from pk for the bulk polymer. When friction is measured on bulk polymer, the points of contact deform plastically until the real area is great enough to support the normal load. Thus the pressure is equal to the mean yield pressure of the polymer, which can be determined from hardness measurements. The measured Vickers diamond pyramid hardness for this copolymer was 4.3. This corresponds to a mean yield pressure of 460 kg/cm2 (Tabor, 1951). If p k is taken as being equal to 0.14 (Figure 1) a t a pressure of 460 kg/cm2, the constant C is equal to 41 kg/cm2. A calculated curve of pk as a function of pressure for the 3.8% copolymer is given in Figure 2. The three perfluoroalkoxy copolymers and PTFE may be considered as a series in which P T F E contains 0% of the perfluoro(propy1 vinyl ether). The addition of the 3.8 wt % of this ether causes a large increase in friction of the bulk polymer (Figure 1). A further increase to 6.6 wt % causes only a very minor increase. These findings are consistent with the recent studies of Pooley and Tabor (1972), who have shown for the reverse combination, L e . , polymer sliding on clean glass, that lower friction and polymer transfer occur with polymers which have “smooth molecular profiles.” Such profiles are unique for high density polyethylene and PTFE. Chemically related polymers of Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 2, 1974
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E
2 0.04 0.02
~
I
2
3
4
5
PRESSURE
6
7
8
9
1
(IO3kg/cm21
Figure 2. A t h e o r e t i c a l p l o t of C(k us. pressure for t h e copolymer c o n t a i n i n g 3.8 w t % of t h e perfluoro(propy1 v i n y l ether) comonomer.
HFPE or PTFE whose smooth molecular profile is altered, by the substitution of other atoms for the hydrogen or for the fluorine atoms, by the addition of a side chain, or by cross-linking, have much greater friction and wear. They concluded that HDPE and PTFE sliders acquired a preferred orientation as soon as sliding commenced. The idea that polymer orientation could reduce friction had been suggested earlier (Bowers, et al., 1953; Bowers and Zisman, 1963). I t appears that the exceptionally low coefficient of friction of bulk PTFE is not primarily dependent upon the fluorine atoms as such or upon the low surface energy, but rather upon the molecular structure. The smooth molecular profile permits the formation of a highly crystalline polymer. Crystallinity tends to increase the hardness or mean yield pressure. The same smoothness also permits relatively easy shear, perhaps analogous to lamellar solids like MoS2. Hence S in eq 1 is small and P relatively high, both of which will decrease p . Therefore, the introduction of pendent perfluoroalkoxy groups into polytetrafluoroethylene molecules would be expected to increase the friction of that polymer. Summary a n d Conclusions Recently several new experimental random copolymers of tetrafluoroethylene and perfluoro(propy1 vinyl ether) were made available by E. I. du Pont de Nemours & Co. The advantage of these copolymers over polytetrafluoroethylene is that, while maintaining the remarkable chemical resistance, they can be melted and made to flow. These copolymers have exceptionally low critical surface tensions of wetting, 16-19 dyn/cm, and in the liquid state they would be expected to have extremely low surface ten-
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sions. They should, therefore, wet nearly all solid surfaces. This is a condition for the formation of strong adhesive bonds. Thin films of the polymer could readily be transferred to heated solid surfaces by a rubbing technique. Pellets of the copolymers were rubbed over a surface a t a temperature which was approximately 75" below the polymer melting point. The transferred film was subsequently fused to the surface a t a temperature above the melting point. These films were very durable and did not break down after 50 unilateral traverses of a %-in. diameter slider a t a 10-kg load. This corresponds to a pressure of 8000 kg/cm2 for steel on glass and 18,000 kg/cm2 for sapphire on sapphire. No decrease in film durability was evident even after these films were soaked for 3 weeks or were treated for several hours in refluxing benzene and for several hours in refluxing Freon TF. Thus it is likely that water-resistant and melt-adhesive solid-lubricant films of this highly stable new fluorocarbon copolymer could be fabricated without the necessity of roughening the substrate or of using adhesive binders. For bulk samples, pk of the perfluoro(propy1 vinyl ether) copolymer is less than pk of TFE-HFP but considerably greater than that of PTFE. When applied as a thin film on a harder backing, however, the kinetic coefficient of friction is only 0.05-0.06. From the friction data for bulk copolymers and for thin films, pk as a function of pressure was determined for one of the new copolymers. The friction results are related to the molecular structure of the copolymers. Acknowledgment The authors are grateful to the E. I. du Pont de Nemours & Co. for making available samples of experimental copolymers of tetrafluoroethylene and perfluoro(propy1 vinyl ether). Literature Cited Bowden, F. P., Tabor, D., "The Friction and Lubrication of Solids," Oxford University Press, London, 1954. Bowers, R. C., J. A w l . Phys.. 42, 4961 (1971). Bowers, R. C., Clinton, W. C., Zisman, W. A , . Lubric. €ng.. 9, 204 (1953). Bowers, R . C., Zisman. W. A., Mod. Piast.. 41, 139 (1963). Bowers, R. C.. Zisman, W. A . , J. Appi. Phys., 39,5385 (1968). Goodzeit, C. L., Hunnicutt. R. P., Roach, A . E., Trans. ASME. 78, 1669 (1956). Pooley, C. M., Tabor, D., R o c . Roy. Soc.. Ser. A . 329,251 (1972). Reardon, J. P.. Zisman, W.A . , to be submitted for publication, 1974. Tabor, D., "The Hardness of Metals," Oxford University Press, London, 1951. Zisman, W.A . , Ind. Eng. Chern.. 55, 18 (Oct 1963). Receiuedfor reuiew O c t o b e r 18, 1973 A c c e p t e d D e c e m b e r 11,1973
