Surface tension of fluoropolymers

Surface Tension of Fluoropolymers I. London Dispersion Term RICHARD

K. S. C H A N

Polymer Research, Air Products and Chemicals, Inc., Trexlertown, Pennsylvania 18087

Received May 10, 1969; accepted October 1, 1969 Surface tension is frequently expressed as the sum of a polar and a nonpolar term. In this paper an empirical approach is proposed for approximating the nonpolar term "~d of the surface tension of fluoropolymers. The experimental data were obtained from contact angle measurements employing a series of linear alkanes. These data are plotted by two different methods to evaluate ~/~d. The critical surface tension ~ obtained from nonpolar contact angle liquids should reasonably approximate the ~'sd of the fluoropolymer surface. This work is based on classical molecular interactions, many concepts of which were established in earlier reports by Fowkes, Good, and Zisman. INTRODUCTION Classical intermolecular forces as t h e y apply to surface tension of materials were reviewed b y Good (1). Fowkes (2) a t t e m p t e d to approximate the London dispersion interaction t e r m for nonfluorinated polymers b y measuring contact angles of wetting. He proposed t h a t the slope of the plot, cos 01 vs, ~T°-5, would approximate the value of %~, where Oz and -y~ are contact angle and surface tension values, respectively. Fowkes recognized t h a t his approach could be invalid for fluoropolymers. Our work supports this point, and d a t a in Fig. 1 show t h a t six fluorochemicals of different surface tension give approximately parallel slopes in a Fowkes plot.

Symbols (reference I gives the dimensions of the notations listed below): ~/ = surface tension; r , = film pressure of solid surface in air; AF = free energy of cohesion (with single subscript) or adhesion (with double subscripts) # = dipole moment; I = ionization potential; a = polarizability; = a t e r m related to ionization potential where U = aI ~ 21 (1); p -- density; 8 = contact angle; Z0 = equilibrium molecular separation at the surface/interface; n -- n u m b e r of molecules or surface-active units per unit volume in the surface/ interface; A -- interaction constants from the potential function of molecules in the surface/interface; M -- molecular weight of repeat unit in polymer. T h e following equations [1] through [4] are

THEORETICAL CONSIDERATIONS This p a p e r discusses the surface tension of fluorinated polymers and presents two plotting methods for approximating the London dispersion term. Let subscripts: s = solid; l = liquid; d = London dispersion term; p = polar interaction term. Journal of Colloid and ~nterface Science, Vol. 32, No. 3, March 1970

492

SURFACE TENSION OF FLUOROPOLYMERS

~

TEFLON

I.C x/ .8

x/"

/ COSO,

×~x

/~

493

PFCMA .//PPFCMA

x

/ j J /WrSMA + / / . . . / ,', . ~ / ~ /- / / ~ / ,d/ / , /, ./PFOA

.6

.4 -

~-/o~

~

_~--'S"

.2

.ZO

.18

.22

i

24

.26

FIG. 1. F o w k e s ' p l o t of f l u o r i n a t e d m o n o m e r s a n d p o l y m e r s .

taken from reference 3:

AF. -= - 2 ~ . ~- --2(3'~d -4- ~o~);

[1]

3~ n~~

[2]

~d =

m z0~ (U~ ~ ) ;

because the geometrical mean of two numbers approaches their arithmetic mean very rapidly as their values approach each other. Let us assume

= ½ (Zo,. + zO,l) -

also,

[3]

For the case of alkane contact angle on fluoropolymer surface, the polar term, A~lv, in Eq. [3] is negligible. F r o m reference (1) Eq. [31 thus becomes ~z(i ~- cos 0l) ~'~ (9ual a,~ =\ 20 /

" \~/

\Nq-

= ½ ( z 0 , . + Zo,,) - Z o , . .

[4]

-UJ

the following equations [5] to [9] are either defined or cited for the purpose of deriving Eq. [10]. Since U = aI, where a ---~2 and I ~ 10 ev (1, 5) for both alkanes and fluoropolymers, one can assume

½(Ul + u~) = ~/ulu~

[5]

[7]

e is the error if one uses the arithmetic mean to represent the geometric mean of Z0,~ and Zo, z, and fl stands for the deviation of Zo,~l from the arithmetic mean of Z0,~ ~nd Zo,z. Thus, Eqs. [6] and [7] give Zo,ol =

(n. nl~ ( 2UI U. '~

[6]

and

zXF~l = 3'l(1 ÷ cos 01) + ~r~ ,.~ 3~ns nz

~/Zo.Zo,,

~ -

~ +

~Zo,.Zo, l.

[8]

When there is no permanent dipole, as in the case of liquid alkanes (1), 2

AFz = -- 2 7 z -

9~r nl UI az 2. 20Z~,z

[9]1

One obtains Eq. [10] b y combining Eqs. [2] 1 T h e f a c t o r 9~/20 in E q s . [4] a n d [9] was t a k e n

from reference 3; it becomes 3~/64 in reference 2. Neither of the two values will affect the results in this paper as shown later in Table I and Table V. Journal of Colloid and Interface Science, Vol. 32, No. 3, March 1970

494

CHAN

t h r o u g h [9]: ~'A (1 + cos 0z) 2 + 0.5w~(1 + cos ~ ) 4

[10]

~-- Q % d ,

A similar a p p r o x i m a t i o n for Z0.z is expected to be less satisfactory; t h u s Zo,~ will be obt a i n e d t h r o u g h surface tension measurements, with the aid of Eq. [9] and t h e following relation:

where

Zo,z = (n~) -1/3. 4(e -- f~) 1-1

= E2(,/Z~o,~

,/Zo,!'~

\'V Zo,~+ 'V go,# 4~

-- ~il~,. Zo,z

[11]

7 3_J-1"

F o r fluoropolymers, ~ is v e r y small; thus, ~ (1 + cos ~)2 ~ 678d 4

[121

T a b l e I lists the Q values of Teflon when fl = 0. T h e value of Q approaches 1 when the series descends to its lower m e m b e r s where e a n d ~ diminish rapidly because Z0,l approaches the value of Z0.8 quickly a t the same time. (See [6] a n d [7].) Therefore, it is reasonable to plot ~/z(1 + cos 0z)2/4 against the Q value of alkanes to obtain the value of %~ at Q = 1. This t y p e of plot is illustrated in the section of Results. Z i s m a n claimed (4) t h a t

= n o m i n a l n o n p o l a r surface tension.

Zo,. = E6"°2 ~×- 102aP*l-l/a _] .

(~'z)0~=o --~ % .

% =

E q u a t i o n [12] states t h a t , when the measurable q u a n t i t y , ~'z (1 -[- cos20)2/4, is plotted against Q, the value of ~/z (1 + cos20)2/4 for Q = 1 should give a good a p p r o x i m a t i o n of %d (the L o n d o n dispersion t e r m in the surface tension of a solid, see Eqs. [1] and [2]). T o calculate Q of Eq. [11] one has to evaluate Zo.,, Zo.z, a n d ~. A n a p p r o x i m a t i o n of Zo,, in the surface is p r o v i d e d b y Z0,~ in bulk, namely, [131

[14]

[15]

W h e n going f r o m higher m e m b e r s in the alkane series to lower m e m b e r s , liquid surface tension and contact angles decrease. Therefore, for lower m e m b e r alkanes, e and 3 are almost negligible, Q --~ 1, and, f r o m Eq. [12], E~'l(1 -t- cos 0l)~ 1 4

Joz=o

=N %d-

[16]

Since the left-hand side of Eq. [14] is exactly % , it can also be written for the case of alkane contact angle on fluoropolymer sur-

TABLE I Q VALVES FOR TEFLON When a = 2, p~ = 2, 5 = 0, and M8 = 50 (gm/mole) Alkane

~'l

C5 C6 C7 C8 C10 C12 C14 C16

15.75 18.2 19.3 21.7 23.7 25.1 26.4 27.4

nla (cm-a X 10-~1) 4.813 d 4. 492 4.115 3.948 3. 498 3.152 2. 886 2. 663

11.24 b 10.49 9,61 9.22 8.17 7.36 6.74 6.22

Zo,1 b (¢m X

0.5923 ~ 0. 6061 O.6240 O.6327 0. 6588 0. 6820 0. 7024 0.7215

1O') 0.446" 0. 457 O.470 0.477 0. 496 0. 514 0. 529 0. 544

Calculated from Eq. [9] with Values of I and a given by reference (5). b Calculated by Eq. [14]. c Calculated from Eq. [11] and a value of Z0.~ = 0.346 X 10-~ (em). d Results calculated from Eq. [9] with factor 9w/20 (3). Results calculated from Eq. [9] with factor 3~/64 (1). Journal of Colloid and Interface Science, VoL 32, No. 3, March 1970

Q" (p~= 2 gm/ml) 1.1454 d 1.1584 1.1752 1.1835 1. 2092 1. 2325 1. 2533 1.2731

0.969" 0. 963 0.955 0.951 0. 939 0. 928 O. 917 O.907

495

SURFACE TENSION OF FLUOROPOLYMERS

of the three. Abbreviations and nominal structures of these and other fluorinated [3'l(1 + COS = N Wd [17] monomers and polymers are shown in Table % 6 .J0z=0 ~ " II, where the polymers contain other isoEquation [17] is not an exact expression. A meric monomers besides the major monomer more exact representation of 7,d should be as indicated. Liquid straight-chain alkanes obtained at Q = 1. In the absence of knowl- were purified by passage through a silicaedge about /~ and Z0,,, it is proposed that alumina column, and then distilled. Some Eq. [17] serves as a good approximation. contact angle data were collected using Namely, in the plot of 0.25 7l (1 + cos 0~)2 alkanes purified by vapor phase chromatogvs. w , the value of 0.25 w (1 + cos 0l) 2 raphy. It was found by gas chromatography at the intercept with the diagonal where that the first method of purification gave 0z = 0, should reasonably approximate we. 95 %, and the second gave 99.99+ % purity. Contact angles obtained from alkanes puriEXPERIMENTAL fied by these two methods agreed within exMaterials and Equipment. Three fluoro- perimental error (1 to 2 degrees). polymers covering a wide range of surface All contact angle measurements were obtension were selected to demonstrate princi- tained with an optical goniometer made by ples. Teflon has the highest surface tension Rame'-hart, Inc., Mountain Lakes, New

faces,

Ol)2]

TABLE II ABBREVIATION AND STRUCTURE OF FLUOROMONOMERS AND T H E I R I:)OLYMERS

Polymer Major, monomer Major monomer structure

PPFOA PPFCMA PFOA PFCMA 1,1 dihydro-pentadecyl fluoro- A fluorinated cyclic octyl acrylate monomer

20.0 -

-

PPFTBMA PFTBMA A fluorinated cyclic monomer

C ) ~ O~C)_,._C)~C)._._," TEFLON

~,5.o o

® ------~o o_o PPFCMA

N.... (1)ft'} O (,..)

+

,oo

_c),.,.~PP FOA @"-'C)-....G..__ ~

q

5"..0

i

II

1.2

1.3

FIG. 2. Nominal nonpolar surface tension vs. O at/~ = 0, p~ = 2 (gm/ml), M8 = 50 (gm/mo]e) using the factor 9 7r/20 in Eq. [9]. Journal of Colloid and Interface Science, Vol. 32, No. 3, March 1970

496

CHAN

Jersey. A platinum syringe with screw head adjustment was used to deliver the drop of alkane to the polymer surface. Procedures. Contact angle measurements:

at least two drops of liquid were advanced to constant angle readings on the left and the right edges of the drop. This procedure was found to duplicate Zisman's data with very

20.0 TEFLON

--

__&.

~

__~--&~A-

A

E

..~

15.0

PPFCMA ..._._~ x ~ y, ~-----~'~"~x

=

~ ''~

q~ 0 +

~._ i,¢

I0.0

-j O

O/O

j O

ET'~"

IO

PPFOA ~

5,0.85

I

I

.90

__

.95

I.O

FIO. 3. Nominal nonpolar surface tension against Q at fl = 0, p~ = 2 ( g m / m l ) , M~ = 50 (gin/mole) using the factor 3~/64 in E q . [9].

22

20 -

TEFLON X •

X

X

X

X--

18E ¢

_

'~

PPFCMA

0

_.+

12

8 --

6 I0

I 12

I 14

16

I 18

I 20

I 22

] 24

YI (dynes/cm)

FIG. 4. Nominal nonpolar surface tension vs. ~/=. Journal of Colloid and Interface ~cience, Vol. 32, :No. 3, M a r c h 1970

0"~0~®

-

I 26

28

SURFACE TENSION OF FLUOROPOLYMERS 1.0

497

\

.8 -

\

.6

x PPFOA

Cos~

"~.~...~

Z) TEFLON

.4

&

"~ X ~

PPFCMA

x ~

.Z

i

i

i

12

r

16

i

i

20 ~ (dynes/cm)

FIG. 5. Zisman plot of PPFOA,

PPFCIV[A,

i

h

i

28

24

and Teflon.

TABLE III

TABLE IV

CONTACT ANGLE DATA IN DEGREES OF STRAIGHTCHAIN ALKANES ON TEFLON, PPFCMA, 2~ND P P F O A

SUMMARY OF "Ysd VALUES OBTAINED BY

DIFFERENT Polymer

Alkanes

PLOTTING

~'sd from Figs. 2 & 3

Polymer . Ca . C~ .

.C8

TeflO1]

--[.22

PPFCMA PPFOA

30 41

26 47

. C1o.

35 51

. C~2 .

C~4

C~8

41 58

43 60

47 62

high reproducibility (4-1°); e.g., the Teflon data given in Figs. 2, 3, 4, and 5. Surface preparation: with the exception of Teflon, which is a molded solid surface, all other polymer or nonpolymer surfaces were prepared b y solution adsorption similar to the method described by Stromberg (8). A mildly etched (by E D T A ) glass slide was immediately dried and used as the absorptive substrate. Contact angle data indicated that the effects of adsorption time and solution concentration are similar to those found b y Stromberg. After immersion in the polymer solution, polymer surfaces were heated for about 1 or 2 hours at 100°C. The fluorinated monomer surfaces were either air dried or vacuum dried at 50°C or lower. I t was found t h a t 2 hours of immersion in a 1% solution was sufficient to ensure equilibrium adsorption and maximum contact angle readings.

PPFOA PPFCMA Teflon

10.4 16.2 19.7

I~ETHODS

T* from

straight-line 7c from curve extrapolation extrapolation of Fig. 4 of Fig, 4

~8 ~15 19

~12 ~16.2 19.5

The contact angle data were reproducible ~--+1 °) not only within the same plate but also for different plates. The adsorption method gave the best surfaces among all methods tried including the use of films produced with a Bird drawdown bar. RESULTS Typical raw data are given in Table I I I for future references. Figure 1 is a Fowkes' plot of six different fluorinated monomers and polymers. The data imply that within the surface tension range of the alkane series, the slope of this plot cannot be used to approximate %~ because all five curves have about the same slope. Figures 2 and 3 show the plot of ~/z(1 + cos 0~)2/4 vs. Q, the first proposed plotting method for the estimation of %~. The two sets of Q values calculated from two different factors in Eq. [9] (see Table IV) evidently give approximately the same ~'oa. Journal of Colloid and Interface Science, Vol. 32, No. 3, March 1970

498

CHAN

Strictly speaking, the Q values are different for different fluoropolymers because the values of Z0,~ are different (see Eq. [11]). Since no reliable data of Z0,~ (or p~ and Ms in the surface region) are available, the same set of Q values for different fluoropolymers are used as an approximation. Figure 4 presents a plot of ~l (1 + cos 0l)2/4 vs. 7l, which is our second approach to evaluate %a. Figure 5 is a Zisman plot using the same data. We have often experienced questionable extrapolation of the Zisman plot for the determination of ~c of P P F O A or other fluoropolymers whose % is low. The results from Figs. 3, 4, and 5 are shown in Table IV. The differences in %d obtained from Figs. 3, 4, and 5 are obvious. In spite of the fact t h a t both Figs. 3 and 4 gave the same results in these few cases, one should carefully use both of the methods with the recognition t h a t the first one is handicapped b y crude values of Q and the second by the restriction of the diagonal where Q ~ 1.

ory. I t was found that both methods of plotting gave about the same %d. Future work will lead to the approximation of the polar term, %p, of the surface tension of fluoropolymers. ACKNOWLEDGMENTS The author wished to acknowledge Drs. D. G. Holland, J. H. Polevy, M. Langsam, all of Air Produces and Chemicals, Inc., for supplying some of the fluoropolymers, and Mr. C. H. Worman for performing the contact angle experiments. The constructive suggestions by Drs. F. M. Fowkes, G. J. Mantell, and R. F. Weimer and critical proofreading by Dr. R. C. Moyer are highly appreciated. REFERENCES 1. GOOD, R. J., Chapter, "Intermolecular and

2. 3.

SUMMARY The above may be summarized as follows: 1) Based on classical molecular interactions, two different types of plotting methods are proposed to empirically approximate the London dispersion term, 7~d, of the surface tension of fluoropolymers. The weakness of each method was described. Whereas Fowkes' plot does not work for fluoropolymers, our proposed methods of plotting would give better accuracy of %~ because of more reliable extrapolation t h a n the Zisman plot. 2) Contact angle data on three fluoropolymers were used to demonstrate the the-

Journal of Colloid and Interface Science, Vol. 32, No.

3, March 1970

4.

5.

6. 7.

8.

Interatomie Forces," in R. L. Patrick, ed., "Treatise on Adhesion and Adhesives." Marcel Dekker, New York, 1967. Fowx~s, F. M., "Chemistry & Physics of Interfaces," 1st Paper of the Symposium on Interfaces, ACS Meeting, June 1964. FOWKES, F. M., J. Colloid and Interface Sci. 28, No. 3/4, 493 (1968). SEAFRIN, E. G., A.ND ZISMAN, W. A., NRL Report 5985, U.S. Naval Research Lab., Washington, D.C., 1963; JARVlS, N. L., AND ZISMXN, W. A., NRL Report 6324, U.S. Naval Research Lab., Washington, D.C., 1965. REED, T. M., IV, ] . Phys. Chem. 59,428 (1955). BUNN, C. W., Trans. Faraday Soc. 35, 482 (1939). DREISBACH, R. R., "Physical Properties of Chemical Compounds," Advan. Chem. Ser. 29 (1961). STROMBERG, R. R., in R. L. Patrick, ed., "Treatise on Adhesion and Adhesives," Vol. 1, p. 69, Marcel Dekker, New York, 1967.