19
Langmuir 1996,11, 19-23
Direct Measurement of the Surface Energy of Corona-Treated Polyethylene Using the Surface Forces Apparatus V. Mangipudi and M. Tirrell" Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Alphonsus V. Pocius Adhesive Technologies Center, 3M, St. Paul, Minnesota 55144 Received July 18, 1994. In Final Form: November 7, 1994@ In this study we report the direct mechanical measurement of enhancement of surface energy of polyethylene films due to corona treatment. Surface energy was measured using the surface forces apparatus (SFA). Unlike practical adhesion tests, the SFA measurements are free from dissipative effects. The results of direct surface energy measurements using the SFA were compared to the contact angle measurements. The extent of modification was controlled by varying the corona energy density. The surface composition is measured using X-ray photoelectron spectroscopy (XPS)and static secondary ion mass spectroscopy (SSIMS). The surface energy of polyethylene increases from 33 to 55 mJ/m2 due to corona treatment. The surface energy estimated from contact angle measurements varied depending on the probe liquids used and the model used to analyze the data. 1. Introduction Many polymer films present low energy surfaces, characterized by their relatively weak adhesion to other materials. Numerous methods have been developed to modify polymer surfaces chemical1y.ls2 Among these methods, corona discharge treatment, simply called CDT, is widely used in industry. Theories proposed for the increased adhesion of corona-treated polymer surfaces include electret f ~ r m a t i o n , ~ the , ~ elimination of weak boundary layer^,^ increased surface roughness due to pitting,6 and introduction of polar groups due to oxidation and other chemical changes in the surface regi~n.~-'OIt is now widely accepted that CDT results in a n increase in surface energy by introduction of polar groups on the surface, thus improving their adhesion and wetting properties. The composition of specific functional groups on the corona-treated surfaces is usually determined using derivatization reactions with group-specific reagents containing XPS tags.7 However, the increase in surface energy has not been measured in a direct manner. The increased adhesion has been measured using practical methods like peel test or lap shear tesL6J1 The increase in surface energy has been monitored indirectly by contact angle measurements.12J3 However, neither of these
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, December 15, 1994. (l)Liston, E. M.; Martinu, L.; Wertheimer, M. R. J. Adhes. Sci. Technol. 1993,7,1091. (2)Wu, S.Polymer Interface and Adhesion; Marcel Dekker: New York, 1982;Chapter 9. (3)Kim. C. Y.:Evans., J.:. Goring. D. A. I. J . A.. m l . Polrm. Sci. 1971, 15,1365. (4)Stradal, M.;Goring, D. A. I. Can. J. Chem. Eng. 1975,53,427. (5)Schonhorn, H.;Ryan, F. W. J.Appl. Polym. Sci. 1974,18, 235. (6)Kim, C. Y.; Goring, D. A. I. J . Appl. Polym. Sci. 1971,15,1357. (7)Gerenser, L.J.;Elman, J. F.; Mason, M. G.;Pochan, J. M. Polymer 1985,26,1162. (8)Owens, D. K. J. Appl. Polym. Sci. 1975,19,265. (9)Briggs, D.; Kendall, C. R. Polymer 1979,20,1053. (10)Briggs, D.; Kendall, C. R. Int. J. Adhes. Adhes. 1982,2,13. (11)Briggs, D.; Rance, D. G.;Kendall, C. R.; Blythe, A. R. Polymer. 1980,895.-(12)Carley, J. F.;Kitze, P. T. Polym. Eng. Sci. 1978,18, 326. (13)Carley, J. F.; Kitze, P. T. Polym. Eng. Sci. 1980,20,330. @
-I
methods measures the true surface energy. There are several uncertainties associated with the contact angle measurements, and the surface energy estimated from these measurements may not be the same as the true surface energy.14J5The adhesive bond strength measured in a practical test includes energy required to break the interfacial bond as well as energy dissipated in the deformation of the substrates and may be several orders of magnitude greater than the true surface energy.16J7 Our earlier work has demonstrated that the surface forces apparatus (SFA) provides a means to measure the thermodynamic or intrinsic work of adhesion between polymer surfaces in a direct mechanical manner.15J8 This is possible because of the small contact area between the surfaces and slow rate of crack propagation in the SFA. We reported the measurement of the surface energies of PET and PE, and the interfacial energy between PET and PE. In this paper, we report the measurement of surface energy of corona treated PE using the SFA. The surface composition is determined using XPS and SSIMS, and the surface energy is correlated to the surface composition. The results of the SFA measurements are compared to the contact angle measurements. 2. Experimental T e c h n i q u e s 2.1. Surface Forces Apparatus. The SFA, originally combines a developed by Israelachvili and co-workers,1Q*20 mechanical means to measure forces with an optical means to determine the distance between the surfaces. The SFA has been primarily used to measure forces acting between surfaces as a (14)Dann, J. R. J. Colloid Interface Sci. 1970,32,302. (15) Mangipudi, V.; Tirrell, M.; Pocius, A. V. J.Adhes. Sci. Technol., in press. Also see Mangipudi, V.; Tirrell, M.; Pocius, A. V. Submitted to Macromolecules. (16)Gent, A. N.; Schultz, J. J . Adhes. 1972,3,281. (17)Andrews, E. G.; Kinloch, A. J. Proc.R.Soc.London,A1973,332, 385. (18)Merrill, W.W.; Pocius,A. V.; Thakkar, B. V.; Tirrell,M. Langmuir 1991,7,1975. (19)Israelachvili, J. N.; Tabor, D. Proc. R.SOC.LondonA 1972,331, 19. (20)Israelachvili, J. N.; Adams, G . E. J. Chem. Soc., Fraday Trans. 1 1978,74, 975.
0743-7463/95/2411-0019$09.00/00 1995 American Chemical Society
20 Langmuir, Vol.11,No. 1, 1995
Letters
Table 1. Summary of Methods for Estimating Surface Energy from the Contact Angle Data method estimated surface energy remarks Zisman’s plot cos 6 varies linearly with ylv; predicts critical surface tension; Lim linearity does not hold universally; A s m a n = YC = ~ l v yc depends on probe liquids based on Bertheolot relation for Good- Girifalcousing ysl = ySv + ylV - 24 (revy d o , 6 attractive constants; valid only in Young‘s equation leads to Fowkes’equation when the solid-liquid interactions (1+ COS e) = $ (3) 0.5 , are dominantly dispersive 2
ref 23 24,25
’
Yl”
yLFis obtained froma plot
cos0 versus ~ I , - O . ~ . $ is the Good-Girifalco interaction parameter.$ 1as dispersive interactions dominate a series expansion of GGF equation;
-
Wu’s equation of state
geometric mean approximation harmonic mean approximation
+
(1
COS
el2 ylv
, 4 y& is the maximum of the plot of ye,+versus ylv dispersive and polar components of solid surface energy are found by solving ylv (1+ COS e) = 2(y: y;)o.6 2($ ypv)o,6 dispersive and polar components of solid surface energy are found by solving ylV (1 cos 0 ) = YC,@
= d2 Y s - ne =
+
+
d d YeYlv
accounts for solid-liquid interactions, though qualitatively
26
an extension of GGF equation; ysspredicted is significantly higher than the critical surface tension similar to geometric mean approximation
27 28
+
function of their separationa21It is only duringthe last few years that SFA has been employed to measure the surface energies of polymer films. The details of the measurements are described elsewhere.16 For use in the SFA, the polymer films need to be thin (-2-6 pm thickness) and smooth. 2.2. Surface Analysis. The composition of corona-treated PE surfaces was determined using X-ray photoelectron spectroscopy (XPS)and static secondary ion mass spectroscopy (SSIMS). X-ray photoelectron spectra were acquired using the Perkin-Elmer Model PHI 5400 spectrometer fitted with a nonmonochromatized Mg Ka X-ray source. The resolution of the spectrometer was 1eV and the pass energy was set at 35.75 eV. The source was operated at 150Wto avoid radiation damage to the sample. All the spectra were referenced t o the C 1s peak for neutral carbon, which was assigned a value of 284.6 eV. The atomic compositions of C and 0 were determined from the XPS spectra. The relative concentrations of different functionalities were estimated fromthe C 1sspectrum. SSIMS(static secondary ion mass spectroscopy)was done on a Model 6000 Perkin-Elmer Physical Electronics Division quadrapole spectrometer. The mass range was 0-1024 amu with a resolution of 0.2 amu. Xe+ ions were used at a beam voltage of 3.5 kV and a beam current of 1.5 nA. Neutralization by an electron gun was necessary in all the measurements. 2.3. Contact Angle Measurements. Contact angles of various liquids on the corona-treated PE film samples were measured using a Rame-Hart goniometer. The “recently advanced”and recedingcontact angles of several liquids, both polar and dispersive liquids, were measured. The liquids used were water (72.2), formamide (58.2), glycerol (63.0),ethylene glycol (45.81, 1,3-propanediol (46.11, 1,2,6-trihydroxyhexane (51.11, methylene iodide (50.81, tricresyl phosphate (40.9),l-bromonaphthalene (44.6), and dimethyl sulfoxide (44.2). The numbers in the parentheses represent the surface tensions measured using the Fisher Surface Tensiomat (Model 21). In contact angle measurements, a small drop of each liquid was placed on the surface using a clean syringe and was allowed to equilibrate for about 60 s. In the case of glycerol, owing to its high viscosity, the equilibration time was longer than 3 min. The surface energy of corona-treated PE was estimated using several different schemes listed in Table 1. 2.4. Preparation of PE Films. The polyethylene (PE)films were coextruded with PET to create a multilayer containing PET (21)Patel, S. S.; Tirrell, M. Annu. Reu. Phys. Chem. 1989,30,387.
layers of 3-4pm thickness and PE layers of 2-4pm. PE surface is corona treated to varying degrees using two different kinds of corona units. The PE/PET bilayer was mounted on a ring, silvered, and then coated with Kraton G1652 adhesive on the back side of PET. The bilayer composite was mounted on the quartz lens, exposing the corona-treated PE surface. 2.5. Corona Treatment. Corona treatment of PE films was carried out in air at room temperature using two types of equipment, both manufactured by Sherman Treaters, Ltd. Type 1is a laboratory unit having k e d power electrodesand a movable ground plate. The ground plate was traversed under the fmed electrode at various speeds. The spacing between the fmed electrode and the film was set at 0.1 mm. Type 2 is a 12-in. industrial unit having rotating powered and ground electrodes. The film is fed into the unit by a web system capable of operating at various speeds. The PE film is taped to the conveyer PET web for passage through the corona unit. Both type 1 and type 2 units were powered by ENI Systems, Inc., power supplies which were capable of measuring dissipated power. Energy dissipated per unit area of film was calculated by determining the power dissipated by the power supply and normalizing the power for web speed and treatment area. The treatment area for type 1 was approximated as the electrode area. For type 2, a stationary web was exposed to corona until a treated area was visible.
3. Theory
3.1. Surface Forces, Surface Energy, and Deformation of Solids and JKR Theory. When two smooth solid bodies are brought into contact in the absence of any applied load, they deform due to the action of surface forces between them. Further, due to the action of these attractiveforces, a finite tensile load is required to separate the surfaces from contact. This tensile load is called the pull-off force, P,. This pull-off force is related to the thermodynamic work of adhesion, W, and the radius of curvature of the surfaces according to the theoretical relations developed by Johnson, Kendall, and Roberts (JKRtheory).22 The pull-off force is measured in the SFA. The basis for the JKR theory is mechanical equilibrium (22) Johnson,K. L.; Kendall, K.; Roberts, A. D. Proc. R.SOC.London,
A 1971,324,301.
Langmuir, Vol. 11, No.1, 1995 21
Letters
Table 2. Surface Analysis of Corona-Treated Polyethylene, XPS Data total 0 1s concentration, atomic %
corona energy density, kJ/m2
percentage composition of carbaneous functional groups +C-H (284.6P
bC-0-
(286.1)
>C=O(287.6)
HO-(!!=O
(289.0)
Type 1 Treatment 28 54 128
10.0 14.3 14.2
86.2 79.6 81.1
12.7 17.8
82.4 71.2
9.4 12.4 11.3
2.8 4.9 4.8
1.6 3.1 2.8
4.3 5.0
2.2 4.6
Type 2 Treatment 2.5 17 a
11.1 13.2
Numbers in parentheses represent the peak positions corresponding to the groups. The width at half-maximum varied from 1.3 to
1.5 eV. Type 1 treatment was done on a laboratory scale unit, and type 2 was done on an industrial set up.
between the energy released due to adhesion and the elastic energy stored due to the deformation of the contact zone. According to the JKR theory the pull-off force required to separate the surfaces from contact is given by
3nWR P, = 2
20
/-=
+Type 1 -+ - T y p e 2
(1)
where thermodynamic work of adhesion; W = 2ys; y, is solid surface energy. The validity ofusing the JKR theory for determining the surface energy of polymer films from the pull-off force measurements was discussed in a n earlier paper.15
3.2. Surface Energy and Contact Angle Analysis. The interfacial energetics between a solid and a liquid determine wetting characteristics. The contact angle (e) of a nonwettingliquid on a solid surface is related to solidvapor, solid-liquid, and liquid-vapor interfacial energies (ysV,ysl, ylv, respectively) via Young's equation, given by Ylv
cos 8 = Ysv
- Ysl
(2)
ylv is the liquid surface tension. ysv is different from the solid surface energy, y,. The exact relation between ya and ysv is not known clearly. In general, when the absorption of vapor is negligible, y, and ysv are taken a s equal. As ysl is not known a priori, Young's equation cannot be used for determining yay,and hence approximate models are used. There are several methods to estimate the surface free energy of a solid from contact angle measurement^.^^-^^ A summary of these models is presented in Table 1, and the details may be obtained from the original papers. In this study, we estimated the solid surface energy of corona-treated PE using different schemes and compared the results to the values of y s obtained from direct force measurements using the SFA. The estimated surface energy is referred to as y:, where "x" denotes the method used for the analysis of contact angle data.
4. Results and Discussion 4.1. Surface Analysis. The surface composition of untreated PE and corona-treated PE was examined using XPS and SSIMS. SIMS data suggested the presence of a variety of types of oxygen functionalities on the surface. In corona-treated samples the amount of oxygen incorporated, obtained from the XPS spectra, varied from 10 (23) Zisman, W. A. Contact Angles, Wettability,and Adhesion; Gould, R. A., Ed.;ACSSymp. Ser.43;AmericanChemical Society: Washington, DC, 1964; p 1. (24) Girifalco, L. A.; Good, R. J. J . Phys. Chem. 1967, 61, 904. (25) Good, R. J.; Girifalco, R. J. J . Phys. Chem. 1960, 64, 561. (26) Wu, S. J . Colloid Interface Sci. 1979, 71, 605. (27) Wu, S.J . Polym. Sci., Part C 1971, 34, 19. (28)Wu, S. J.Adhes. 1973, 5 , 39.
0 0
3 lo' 6 10' 9 10' 1.2 10' Corona Energy Density, Jlm*
Figure 1. Surfaceoxygen concentration(atom %)versus corona energy density (J/m2). 0 denotes type 1on a laboratory unit; W denotes type 2, industrial unit. Type 2 treatment is more effectivethan type 1 treatment in terms of oxygen incorporation. to 18 atom % depending the energy density level. 0 1s composition on the surface of untreated PE was negligibly small (less than 0.4%) indicating t h a t PE surface was clean. It may be noted that corona treatment did not introduce any significant amounts of nitrogen (in all cases N 1s composition was less than 0.2%). These results, shown in Figure 1, also indicate that the amount of oxygen incorporated at a given energy density also depends on the effectiveness of the treatment unit. I t may be noted that type 2 treatment (done on the industrial set up) is more effective than the type 1 treatment (done on a laboratory scale unit). The XPS spectrum of untreated PE shows only one peak in the C 1s region, while the spectra of those that were corona treated showed new peaks a t higher binding energy. The relative composition of the individual carbon-oxygen functionalities was obtained by a simple curve fitting of the peaks in the C 1s region, as shown in Table 2. The peak positions and relative compositions of -C-0, C=O, and O=C-0 functionalities are in agreement with those reported by Gerenser and co-workers.' 4.2. Surface Energy Measurements. The pull-off force (P,) between corona-treated PE surfaces and the radius of curvature ( R )were measured using the SFA. As mentioned in section 2.1, to get accurate measurements of P, and R , it is necessary that the surfaces be smooth. In the case of polymer films, especially after corona treatment, the surfaces were not uniformly smooth. However, it was possible to locate smooth regions on the surface as the size ofthe contact spot was relatively small ( ~ 2 pm 5 contact radius). The value of a pull-off force measured in repetitive experiments a t a given spot was
22 Langmuir, Vol. 11,No. 1, 1995
Letters
L
60
i
60
SFA and Zisman's
(a)
-
L
%
3.+-
t
50;
40
-
0
60
-
50
30
-
: 2
'
'
5
'
"
'
'
I
10
15
SFA Nonpolar liquids Polar liquids
-mv
4 10'
0 "
1
R
-
40-
-
30
i1
30
0
1
3 0 l .
4
L
8 10'
v
1.2 10'
'
20
(b) SFA and GGFs
0 1s Concentration, atom%
Figure 2. Variation of surface epergy of corona-treated PE with surface oxygen concentration. The surface energy is obtained from direct pull-off force measurementsusing the SFA. 0 represents type 1treatment; represents type 2 treatment. constant, within about 3%. From the values of Paand R , the surface energy ( y s ) was determined using the JKR theory (eq 1).Figure 2 shows the measured surface energy plotted as a function of surface oxygen concentration. The surface energy of untreated PE is about 33 mJ/m2, and the surface energy increases as the oxygen concentration on the surface increases. At the highest level of treatment, the surface energy increased to 56 mJ/m2. With increase in the corona treatment level, the density of polar functionalities on the surface increases, resulting in a n enhancement in surface energy. This is clearly demonstrated by the numbers listed in Table 1 and Figure 1. 4.3. Contact Angle Measurements. Advancing and receding contact angles were measured using polar as well as dispersive liquids. There is finite hysteresis in contact angles. The surface energy was estimated using the models listed in Table 1. The values of yz obtained from the advancing and receding angles were not very different. For untreated PE yz and y s were about the same.15 However, in the case of corona-treated PE, y: was found to be less than ys. These values are shown in Figure 3. It may be noted that value of y i estimated by different methods is different. Wu's equation of state, which accounts for solid-liquid interactions, predicts higher values than Zisman's method. It may also be noted t h a t surface energy estimated from contact angle measurements also depends on the chemical constitution (polarity) of the probe liquids. As shown in Figure 3, yz does not change appreciably even as the degree of treatment increases. However, ys increases as the 0 1s concentration increases as anticipated. This illustrates t h a t direct force measurements can be more sensitive to changes in composition on the surface than contact angle measurements. I t may be pointed out t h a t the difference in y s and y: of corona-treated PE is similar to the difference found in the case of poly(ethy1ene terephthalate), which also has polar groups on the surface.15 It has been a practice to resolve surface energy of solids into polar and dispersive components; i.e. ysv = y: y:. These components are determined from the contact angle data of a pair of probe liquids, for which yf and y;' are known. It is desirable to use one polar liquid and one dispersive liquid to get reasonable approximations to the surface energy. We measured the contact angles of water, formamide, methylene iodide, and tricresyl phosphate. The values of yP and y! for these liquids are obtained from l i t e r a t ~ r e . ~y:J ~and ~ ~y:~ ~are ~ ~deter-
+
1
SFA Nonpolar liquids Polar liquids
-m-
30
v E
T
.
.
'
,
,
8 10'
4 IO'
0
1.2 lo'
(c) SFA and Wu's -60
60
P
**
40
tt d
v
Nonpolar liquids Polar liquids
v
30
0
3 IO'
6 10'
9 IO'
1: 40
SFA
-m-
i
30
1.2 10'
Corona Energy Density, J/m2 Figure 3. Comparison of surface energy of corona-treated PE measured using the SFA and that estimated from the contact angle data. represents SFAmeasurements (ys);0 represents ye estimated from nonpolar liquids; v represents ye estimated from polar liquids. ye was determined using (a) Zisman's method, (b)Good-Girifalco-Fowkes' equation; and (c) Wu's equation of state. mined using the geometric or harmonic mean approximations, which are extensions of Good-Girifalco-Fowkes equations due to Wu.27v28The surface energy calculated from these approximate methods is denoted by yea to distinguish these values from ys or ye. ysa determined from geometric and harmonic mean approximations are plotted in Figure 4. The value of ysa was found to be dependent on the pair of probe liquids chosen. For example, the value of ysa obtained from water and methylene iodide contact angles was about 10% higher than the value obtained from formamide and methylene iodide data. This may be due to the difference in solidliquid interactions. This resulted in considerable scatter in ysavalues. The average values of yaaobtained from the data of several pairs of liquids are plotted in Figure 4. It may also be noted that the predictions of harmonic mean
Langmuir, Vol. 11, No. 1, 1995 23
Letters
y e and is in the same range as ys obtained from the SFA measurements. However, due to the scatter in the values of ysait is not clear if the predictions of geometric and harmonic mean approximations are actually close to the true surface energy.
(a) SFA and Geometric mean approximation
6. Conclusions
30 0
3 10'
6 10'
9 10'
1.2 l@
(b) SFA and Harmonic mean approximation
30
le 0
+SFA 0
3 IO'
6 10'
Harmonicmean
9 lo'
I
30
1.2 IO'
Corona Energy Density, J/m2 Figure 4. Comparison of surface energy of corona-treated PE measured using the SFA and that calculated from the contact angle data using geometric mean and harmonic mean approximations t o interfacial energy. (H)represents SFA measurements ( y s ) ; ( 0 )represents ysacalculated using geometric mean approximation in (a)and harmonic mean approximation in (b). The scatter in data is due to difference in ysaobtained for different pairs of liquids. approximation are higher than those for the geometric mean method. In general, ysais significantly higher than
We have presented the direct measurements of surface energy (ys)of corona-treatedpolyethylene using the surface forces apparatus (SFA). As anticipated, ys increases as the extent of surface modification increases. The enhancement in yscan be correlated to the increase in oxygen concentration on the surface. As polar groups are introduced on the surface, the surface energy of PE can increase from about 33 to 55 mJ/m2. SFA can be used to measure directly the true surface energy of polymer films. The SFA results are compared to the surface energy estimated from contact angle measurements. Unlike the SFA measurements, contact angle measurements are not sensitive to small changes in surface composition. Due to the dependence of estimated surface energy values on the probe liquids, the contact angle measurements may not be used to obtain reliable quantitative information. It has been demonstrated that the method of analysis of contact angle data can have significant influence on the values of estimated surface energy. The predictions of less rigorous models like that of Zisman's are considerably lower than the true surface energy, while the geometric and harmonic predict values close to ys. The simplicity and ease of contact angle measurements can be well exploited to predict the surface energy of solids only by correctly choosing both the probe liquids and the method to interpret the data.
Acknowledgment. We thank Mark Strobe1of 3M Co. for helping with type 2 treatment and acknowledge the financial support provided by the 3M Co., St. Paul, MN. LA940566Z
