Langmuir 1991, 7, 2498-2502
2498
Dynamic Wettability of Wood Douglas J. Gardner,* Necitas C. Generalla, David W. Gunnells, and Michael P. Wolcott West Virginia University, Division of Forestry, Morgantown, West Virginia 26506 Received June 25,1990. In Final Form: December 5,1990
Dynamic contact angle (DCA)analysis,utilizing the Wilhelmy plate method of contact anglemeasurement, was used to study the dynamic wettability of red oak (Quercus rubra) and yellow-poplar (Liriodendron tulipifera). The effects of wood elemental and chemical composition, surface roughness, absorption and capillary flow, sample aging, and machine testing speed were examined. DCA analysis appears to be a useful method for studying the dynamic wetting behavior of wood, and when combined with X-ray photoelectron spectroscopy (XPS) and wood chemical composition data, it provides information regarding how wood processing and environmental conditioning effects surface energetics and chemistry. Introduction
Knowledge of solid surface chemistry is crucial to understanding the interaction of liquids with solid substrates, such as adhesives and coatings. A better fundamental understanding of the wood surface is important because it is adhesive bonded in 70% of all applications1. Wettability measurements are a useful method for determining the thermodynamic behavior of wood surface^.^^^ The physical processing of wood (e.g. drying4 and aging5) affect wettability; in particular, wood extractives in some species have a large When wettability studies are coupled with surface analysis for chemical composition using X-ray photoelectron spectroscopy (XPS), these data should provide information regarding both the energetics and chemistry of wood surfaces. XPS is a powerful tool in surface science, capable of determining the elemental carbon and oxygen ratio on wood surfaces as well as the carbon’s oxidation state. Wettability and XPS techniques have been applied widely to physically and chemically homogeneous materials, however, problems exist in their direct application to heterogeneous materials like wood. Wood can be viewed as a heterogeneous composite material of cellulose,hemicellulose, lignin, and extractives. These polymeric compounds are arranged in a cellular structure (Figure 1) resulting in surface roughness on a microscopicscale. These attributes are not unique to wood because material heterogeneity is being used in advanced material design. Polymeric, metallic, and ceramic cellular materials (foams) are widely used in structural capacities that require adhesive bonding. These foams are porous and can be produced with anisotropic cellular structure similar to wood. In addition, composites and copolymers have chemically heterogeneous surfaces that can be physically anisotropic from fiber orientation. The analysis of these solid wood surfaces must consider similar effects that are mandated in the study of wood. Precise measurement of wettability to determine surface thermodynamic characteristics is inherently difficult with (1) Hemingway, R. W.; Conner, A. H. In Adhesives from renewable resources; ACS Symposium Series 385; Hemingway, R. W., Conner, A.
H., Branham, S. J., Eds.; American Chemical Society: Washington, DC, 1989; p 487. (2) Gray, V. R. For. Prod. J . 1962, 12,452. (3) Herczeg, A. For. Prod. J. 1965,15,499. (4) Troughton, G. E.; Chow, S.-Z. Wood Sei. 1971, 3, 129. (5) Nguyen, T.; Johns, W. E. Wood Sei. Technol. 1979,13,29. ( 6 ) Jordan, D. L.; Wellons, J. D. Wood Sei. 1977, 10, p. 22. (7) Casilla, R. C.; Chow, S.; Steiner, P. R.; Warren, S. R. Wood Sci. Technol. 1984,18, 87.
0743-7463/91/2407-2498$02.50/0
Figure 1. Scanning electron micrograph of a red oak surface. Reprinted from Core, H. A.; Cote, W. A.; Day, A. C. Wood Structure and Identification; Syracuse University Press: Syracuse, NY, 1976;p 22. Copyright 1976Syracuse University Press.
wood because of its chemical heterogeneity, surface roughness, and hygroscopic nature. Historically, wood wettability has been measured by static contact angle measurement^^,^,^ and by observing the absorption of a liquid drop into the wood over time.8 Recently, dynamic contact angle (DCA) analysis of wood using the Wilhelmy plate method has been demonstrated.9 A thin plate (wood veneer) is brought into contact with and immersed in a (8) Hemingway, R. W. TAPPI 1969,52,2149.
0 1991 American Chemical Society
Dynamic Wettability of Wood
Langmuir, Vol. 7, No. 11,1991 2499
liquid. The force increase caused by the weight of the liquid in the formed meniscus is related to the liquid surface tension and the contact angle. For the Wilhelmy plate method, the work of adhesion ( WA) can be defined as
w, = pyLVcos e - v p g
(1)
where P is the sample perimeter at interface, V is the volume of liquid in meniscus, p is the liquid density, g is the force due to gravity, y ~ isv the liquid surface tension, and 8 is the contact angle. By use of a liquid of known surface tension, eq 1can be used to calculate the contact angle. The Wilhelmy plate method of DCA analysis addresses the problem of chemical heterogeneity by measuring the interaction of liquid and substrate over a large area in contrast to a static droplet. For absorptive materials, the DCA is used in dynamic mode to measure both the advancing and receding contact angles. Comparisons of these values give an indication of the absorption of the probe liquid by the solid substrate.
Objectives The purpose of this paper is to describe the wettability of wood using DCA analysis. Variables to be examined include wood species, speed of testing, capillary effects, surface roughness, and age of surface. The bulk and surface chemical composition will also be described using elemental and XPS analysis.
Experimental Section Dynamic contact angle measurements were made with a Cahn Instruments DCA 322 on 25 X 50 X 0.75 mm samples of yellowpoplar (Liriodendron tulipifera) and red oak (Quercus rubra) wood veneers. The veneers were conditioned from a green state to 7 9% equilibriummoisture content at ambient temperature (2+ 25 "C).A glass slide served as a control. Distilled and deionized water was used for contact angle measurements unless otherwise indicated. Chemical characterization of the woods included elementalanalysiswith a Perkin-Elmer240C elementalanalyzer, ash content,'o chemical composition analysis including holocellulose,ll a-cellulose,12and Klason lignin, and extractive deterXPS surface analysis was used to define surface mination~.~~ elemental composition. Critical surface tension measurements were obtained by using an acetic acidlwater series. Aged specimenswere used as obtained from the conditioning chamber for DCA measurements, while fresh surfaces were prepared by surfacing (sanding) conditioned specimens immediately before DCA measurements. Capillarity effects on contact angle measurements were determined by measuring the difference in contact angle values between end-sealed (poly(viny1acetate)) veneers and neat veneers. Roughness effects on contact angle measurements were determined by running the DCA experiment with wood veneers both parallel and perpendicular to the grain. The roughness effect was further characterized by coating both a glass slide and wood veneer with poly(viny1 acetate) and obtaining contact angle values. The effect of machine testing speed on contact angle values for wood was measured from 20 (9) Kalnins,M. A,; Katzenberger,C. In Woodandcellulosics: industrial utilization,biotechnology, structure andproperties; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Ellis Horwood, Ltd.: West Sussex, England, 1987; p 409. (10) American Societyfor Testingand Materials. Standardtest method for ash in the analysis of coal and coke from coal; ASTM Philadelphia, PA, 1982; Designation D3174-82. (11) Browning, B. L. Methods of Wood Chemistry, Vol. I& WileyInterscience: New York, 1967. (12) Technical Association of the Pulp and Paper Induetry. TAPPI Standards and Suggested Methods; TAPPI: New York, 1968. (13) Moore, W. E.; Johnson,D. B.Procedures for the Chemical Analysis of Wood and Wood Products; FPL, U S . Forest Service: Madison, WI, 1985.
Table I. Elemental Composition of Yellow-Poplar and Red
Oak Samples. carbon, hydrogen,
samples yellow-poplar sapwood yellow-poplar heartwood red oak sapwood red oak heartwood 0
oxygen,
ash,
%
%
%
%
48.9 49.1 48.7 49.8
5.6 5.1
45.1 44.9 45.2 44.3
0.12
5.6 5.1
0.10 0.34
0.15
Based on moisture-free wood.
to 264 pm/s. The resulting testing speed/contact angle data were plotted and evaluated by using linear and segmented regression analysis.14
Results and Discussion Elemental and Chemical Composition. The elemental analysis and chemical composition of yellow-poplar and red oak wood samples are shown in Tables I and 11. The carbon percentages were higher in the heartwood samples and appear to correlate with the greater extractive content of the heartwood. Lignin content ranged from 18.5% to 21.3% with little difference between the sapwood and heartwood of yellow-poplar. The lignin content of the red oak heartwood was 5 % higher than in the sapwood. a-Cellulose contents ranged from 47 % to 53% , which compared closely with literature values.ls On the basis of summative analysis, high values of holocellulose contents were obtained even after correcting for residual lignin. However, this result agrees with the findings of other workers for fine-grind wood meal.I6 Thus, hemicellulose contents were also high because these were calculated as the difference between holocellulose and a-cellulose contents. Yellow-poplar has a higher hemicellulose content than red oak. According to Browning," high holocellulose values could result from residual acid-soluble lignin and from the possible retention of organic solvents by fine materials which also impede filtrations. As expected, the total extractive content of red oak was higher than that for yellow-poplar and there were more extractives in the heartwood than in the sapwood. XPS Analysis. The XPS analyses of the red oak and yellow-poplar veneers are shown in Table 111. The percent atom composition of the wood surface shows a greater carbon percentage than is found in the bulk wood (Table 11). This seemingly high carbon percentage on the wood surfaces is supported by other XPS studies on ~ o o d . ~ ~ - ~ ~ One possible reason for the high carbon percentages on the wood surfaces could be attributed to the migration of extractives to the surface during dryingS8More detailed work is ongoing in our research group to substantiate the extractives effect on surface carbon percentage. Aging of the wood surface appears to be associated with a large increase in the surface carbon percentages which suggests possible surface modifications by environmental effects such as light or reaction with the atmosphere. The effect of ultraviolet radiation (UV) on wood surfaces was examined by Hon18 using XPS, but UV radiation was shown to oxidize the wood surface. Studies on the interaction of wood surfaces with the atmosphere during (14) SASInstitute, Inc. SAS Language Guide forPersonal Computers, Version 6 Edition; SAS Institute, Inc.: Cary, NC, 1985. (15) Pettersen, R. C. In The Chemistry of Solid Wood;Rowell, Roger, Ed.; American Chemical Society: Washington, DC, 1984; p 57. (16) Erickson, H. D. TAPPI 1962,45,710. (17) Young, R. A.; R a " o n , R. A.; Kelley, S. S.; Gillespie; Wood Sci. 1982, 14, 110. (18) Hon, D. N. S. J. Appl. Polym. Sci. 1984, 29, 2777. (19) Brewis, D. M.; Comyn, J.; Phanapoulos, C. J. Adhes. 1987,21,
303.
2500 Langmuir, Vol. 7, No. 11, 1991
Cardner et al.
Table 11. Chemical Composition of Yellow-Poplar and Red Oak Wood Samples. lignin, holo-cellulose, a-cellulose, hemi-cellulose alcohol-benzene hot water samples % % % (9%) solubles, 7% solubles, % 34.1 1.3 1.1 yellow-poplar sapwood 18.5 81.7 47.6 33.5 2.7 0.3 yellow-poplarheartwood 19.2 81.7 48.2 27.7 2.7 2.5 red oak sapwood 16.3 80.4 52.7 27.2 3.8 2.5 red oak heartwood 21.3 74.1 46.9 0
extractives, %
2.4 3.0 5.2 6.3
Based on moisture-freewood.
Table 111. XPS Analysis of Yellow-Poplar and Red Oak Surfaces % atom composition o/c sample carbon (C) oxygen (0) ratio yellow-poplarsapwood (aged) 80.0 20.0 0.25 yellow-poplarheartwood (aged) 83.0 17.0 0.20 yellow-poplarsapwood (fresh) 70.6 29.4 0.42 red oak heartwood (aged) 82.7 17.3 0.21 red oak heartwood (fresh) 73.8 26.2 0.36
"1
-7 w \
-200
Advandng
450 Receding
-300
-
e 0'
h
C
8
7
B
B
lo 11 li 13 14 15
DEP~H(mm)
Figure 4. grain.
I
I 0!3
1I
1
3
4
5
6
7
6
4
10
1'1
12
15
DEPTH (mm)
DCA analysis of a glass slide.
Figure 2.
400 350I
Receding 8-13'
I
150 1 100-% 'Wl/I
b
50
I
wmr
0on 1
Figure 3.
a B
4
6
6 7 8 DEPTH (mm)
9
10 11 12 13 14 15
DCA analysis of a red oak veneer parallel to the grain.
extended drying at elevated temperatures have shown that oxidation 0ccurs.~*8In this study, the wood was allowed to dry at ambient conditions resulting in a reduced surface, rather than an oxidized surface, as indicated by the XPS results. More detailed examination of the wood aging and drying effects is also being performed by our group. Dynamic Contact Angle Analysis. The DCA analysis of a glass slide and of a red oak veneer parallel to the grain is shown in Figures 2 and 3. The glass slide is an example of a smooth, chemically homogeneous surface which absorbs no water and exhibits little contact angle hysteresis. The wood veneer is a rough, chemically heterogeneous surface which absorbs water and exhibits substantial contact angle hysteresis. Surface Roughness. Surface roughness features as small as 0.05 pm can alter the line of contact between the
DCA analysis of a red oak veneer perpendicularto the
liquid drop and the solidem Localized contact angles are observed on the microscopiclevel although the macroscopic angle is not greatly affected. Surfaces with a roughness of greater than 0.5 pm are reported to cause significant hysteresis for macroscopically measured contact angles.21 This roughness effect is readily apparent when viewing a DCA scan of red oak veneer perpendicular to the grain direction (Figure 4). No advancing contact angle was calculated due to the inconsistent data. The hysteresis was caused by surface roughness from the red oak vessel elements (Figure 1) and also the large amount of water absorption. A relatively smooth receding contact angle was calculated because the veneer was wetted. Although wood surfaces are considered 'bough" from the thermodynamic viewpoint, a %mooth" surface is apparent from the DCA analysis performed on veneers parallel to the grain. This direction produces satisfactory results for determining contact angle measurements as evidenced by the red oak veneer scan shown in Figure 3. To determine if the DCA analysis of wood parallel to the grain approximates a thermodynamically smooth surface, both red oak veneers and glass slides were coated with a thin layer of poly(viny1 acetate) which was then polymerized. The contact angle analyses of these coated samples differed by less than a degree. On the basis of these results, we have limited DCA analysis of wood veneer surfaces parallel to the grain direction. Absorption and Capillarity. Another contact angle hysteresis effect to consider when dealing with wood veneers results from diffusion of the probe liquid into the cell wall polymers and capillary uptake into the wood cell lumens, i.e., pores. The effects of absorption and capillary effects on the measured contact angle are evidenced in Figure 5 which shows a plot of unsealed and end-sealed wood veneers at various DCA testing speeds from 20 to 264 pmls. A t low speeds, water wicks past the water/air/wood interface; thus the wood is partially wetted as the interface moves over it. Because the wood is partially wetted, the (20) Oliver, J. F.; Mason,S.G. J. Colloid Interface Sci. 1977,60,480. (21)Johnson, R. E.: Dettre, R. H. Adu. Chem. 1964,43,85.
Langmuir, Vol. 7, No.11, 1991 2501
Dynamic Wettability of Wood 100
T
.II
8
d
0
-~
20-
'Ob
2b io
eb sb
lbo
1L 140 1c 143 260
. U n m d d
&I
aio2h2hsbo
Figure 5. Effect of DCA testing speed on the contact angle of unsealed and end-sealed red oak veneers. 120 -
2 0 - 0
loa
-
0
do eb eb lb
lao
l~ 1~ l ~ SPEED (mlcmns/sec)
FmohRmgnr.
z t 2io2BozBosbo a ~
Figure 7. Effect of aged and fresh red oak veneer surfaces on advancing contact angle over various DCA testing speeds. 12 ,
.
1
a
8
0.4 -
0.2 -
I
i
io $& do
l& lb 140 160 143
zta bo 2 4 0 * 2 8 0 &
SPEED (mlmW8ec)
OdI
47.9
12 4i I I sb k Ek SB SB do & ei eB eB 7b SURFACE TENSION (dym/an)
*i
4t6h
Figure 6. Effect of DCA testing speed on the advancing and receding contact angles of red oak veneers.
Figure 8. Zisman plot for red oak heartwood.
measured advancing contact angle is lower at slower speeds. The contact angle data for the unsealed veneers increased in a linear manner as testing speed increased while the end-sealed veneers contact angle data exhibited a segmented response to increased test speed. Up to 162 pm/ s, the end-sealed veneer contact angle data increased in a linear manner followed by constant contact angles for testing speeds faster than 162 pm/s. End-sealing reduces capillary uptake of the probe liquid, reducing premature wetting of the wood surface. At faster speeds, the effect of capillarity is reduced because there is less time for the absorption of liquid. Therefore, the difference between measured contact angles for unsealed and end-sealed veneers is reduced at the faster speeds. End-sealing of wood veneers or polymeric foams is recommended as a standard practice for DCA analysis. Test Speed. The effect of DCA testing speed on the advancing and receding contact angles of red oak veneers is shown in Figure 6. Both the advancing and receding contact angle data exhibit a segmented response to increased testing speeds. Up to 162 pm/s, the advancing contact angle data increased in a linear manner followed by constant contact angles for testing speeds faster than 162 pm/s. The receding contact angles increased up to 78.4 pm/s followed by constant contact angles at faster speeds. Because wood absorbs liquid during testing, faster DCA speeds should produce more accurate contact angle data than slow speeds. However, slower test speeds have been recommended for DCA analysis of glass22and human hair.23 It should also be noted that both glass and human hair contact angle data appear to exhibit a segmented
response to increased testing speeds which is similar to the wood veneer data. Over the DCA testing speeds examined, 194 pm/s has proven to be the most precise speed for wood veneers, and it has been chosen as our standard measuring speed. Surface Aging. The effect of aged (unsanded) and fresh (sanded) red oak veneer surfaces on advancing contact angle over various DCA testing speeds is shown in Figure 7. The aged veneer exhibited a segmented response to increased testing speed as previously described for Figure 6 while the fresh veneer surface contact angle data increased in a linear manner. The fresh wood surface is more easily wetted by water than the aged surface. This observation is well supported in the literature519and correlates quite well with the XPS results on aged and fresh surfaces. The higher carbon percentage of the aged surface results in greater hydrophobicity than the lower carbon percentage of the fresh surface. Critical Surface Tension. A frequent objective for performing contact angle analysis on solids is to determine their critical surface tensions by constructing Zisman plots.24 Critical surface tension values are one of many useful physical measurements for evaluating the adhesive bonding characteristics of materials. A typical Zisman plot for red oak heartwood obtained by using an acetic acid/water series is shown in Figure 8. The critical surface tension values for red oak and yellow-poplar veneers obtained by using this series are shown in Figure 9. The presence of wood extractives (Table 11) lowers the critical surface tension of the heartwood for both red oak and
(22) Elliot, C. E.P.; Riddiford, A. C. J. Colloid Interface Sci. 1967,23,
389.
(23) Kamath, Y. K.;Dansizer, C. J.; Weigmann, H.-D.J. Appl. Polym. Sci. 1984, 29,1011. (24) Zisman, W. A. Adv. Chem. 1964,43, 1.
Gardner et al.
2502 Langmuir, Vol. 7, No. 11,1991 SPECIES ROH
i_]ROS 54.4
YPH YPS
47.9
46.1
46.7
SPECIES
Figure 9. Critical surface tension values for red oak and yellowpoplar veneers (key: roh, red oak heartwood; ros, red oak sapwood; yph, yellow-poplarheartwood; yps, yellow-poplarsapwood).
yellow-poplar. The nonpolar nature of heartwood ext r a c t i v e produces ~~~ a more hydrophobic surface, and this result is also supported by the XPS results (Table 111) where the heartwoods of both species have higher surface carbon percentages. (25) Rowe, J. W.; Conner, A. H. Extractiues in eastern hardwoods-a reuiew; USDA Forest Service General Technical Report FPL-18; U.S. Forest Products Laboratory: Madison, WI, 1979; p 8.
Conclusions The results of this study indicate: 1. Dynamic contact angle analysis appears to be a useful method for studying the dynamic wetting behavior of wood. DCA analysis overcomes some of the problems of static angle measurements on wood such as absorption and surface chemical heterogeneity. 2. DCA analysis combined with XPS analysis and wood chemical composition data provide useful information regarding wood surface energetics and chemistry. These techniques should provide useful means for studying wood surface modification by both physical and chemical means. Acknowledgment. We thank Dr. Craig Sass,Research Laboratory, Eastman Chemical Division, Eastman Kodak Co., Kingsport, TN, and Dr. Steven S. Kelley, Bend Research Co., Bend, OR, for performing the XPS analyses. Published with the approval of the Director of the West Virginia Agriculture and Forestry Experiment Station as Scientific Article No. 2253. This research was supported in part by the Michigan State University/USDA:CSRS Eastern Hardwood Research Special Grant Program Nos. 90-34158-4989 and 90-34158-4990, and in part by West Virginia Agriculture and Forestry Experimental Station, McIntire Stennis Grant 955. Registry No. HzO, 7732-18-5.
