WETTING BEHAVIORS OF PHENOL- AND UREA-FORMALDEHYDERESINS AS COMPATIBILIZERS 1
Sangyeob LeePost-Doctoral Research Associate
Department Forest Products and Center for Advanced Vehicular System (CAVS)Mississippi State University
PO Box 9820Mississippi State, MS 39762-9820
Todd F. ShupetProfessor
Louisiana Forest Products Development CenterSchool of Renewable Natural Resources
Louisiana State University Agricultural Center227 Renewable Natural Resources Bldg.
Baton Rouge, LA 70803
Leslie H. GroomProject Leader
and
Chung Y. HsePrincipal Wood Scientist
USDA Forest Service, Southern Research Station2500 Shreveport HWY.
Pineville, LA 71360
(Received July 2006)
ABSTRACT
Understanding wetting behavior and surface coverage of resins on a wood surface is important to obtainsatisfactory adhesion and optimize adhesive application for wood composite manufacturing. Sessile andmicro-droplets of urea- and phenol-formaldehyde (UF and PF) resins were generated on wood surfaces toobserve wetting behaviors using three directional image generation system and atomic force microscopy(AFM). The generated micro-droplet sizes varied in diameter from 1-100 J.Lmand showed differingwetting behavior based on droplet size and surface conditions. Rougher wood surfaces prevented micro-droplet spreading and resulted in higher contact angles. Contact angles along the fiber direction of theearlywood significantly differed from the angles collected across the fiber direction. Sessile dropletmodels and dimensionless droplet shape factors (DSF) were used to develop the parameters governing thedroplet shape changes from a spherical droplet to an enclosing hemispherical droplet for early- andlatewood surfaces. Droplet dispersing areas with earlywood showed III % with UF and 42% for PF fasterchanges along the fiber surface as compared to across the fiber surface.
Lee et al.-THERMOSETTING RESINS AS COMPATIDILIZERS 483
INTRODUCTION
Previous wetting studies involving contact-angle measurement on wood surface wettabilityhave shown an old and well-defined techniquewith model liquid called probes (Gardner et al.1991; Walinder 2000; Walinder and Strom2001). Wetting studies with wood fiber plasticcomposites (WPC) have evaluated qualitativesurface chemistry and treatment effects on thesurface determined by contact-angle analysis toreveal chemical bonding evidence between fi-bers (Felix and Gatenholm 1991; Hedenberg andGatenholm 1995; Geoghegan and Krausch2003). Untreated wood materials were highlyhydrophilic, but this was substantially reducedby chemical and physical surface modification(Matuana et al. 1998a; Gauthier et al. 1999;Rabinovich 2002). Improved contact-angle mea-surements of the modified surfaces were posi-tively correlated to shear strength properties atthe interface. Single lap-shear-joint tests andpeel tests (Kolosick et al. 1993; Chen et al. 1995;Matuana et al. 1998b; Oksman and Lindberg1998) were used to investigate surface inter-actions at the wood-polymer interface. Assum-ing that the failure of samples occurred at theinterface and that the stress distribution was uni-form, peel and lap shear strengths can be con-sidered as a qualitative measure of interfacialadhesion. In practice, contact-angle measure-ments can be readily obtained on flat wood sur-faces but not on fibers. However, wetting studiesusing thermosets such as urea-formaldehyde(UF) and phenol-formaldehyde (PF) resins haverarely been reported due to the incompatibilityof thermosets in contact with a thermoplasticmatrix.
Some factors influencing surface wetting ofwood by liquid monomers and polymers wereevaluated to study their effect on flexural andshear strength properties of wood-based prod-ucts as well as surface energetic properties at theinterface (Hse 1972a, 1972c; Shupe et al. 1998).The previous research was based on contactangle at the liquid-solid interface. Limited stud-ies have been made to study wetting of solidwood by contact-angle measurement of sessile
or micro-droplets of thermoset resins where theeffect of wood structure was considered. Thedispersing area of thermoset droplets was influ-enced by wood structure, grain directions, glue-line thickness, viscosity, molecular weight dis-tribution, surface roughness, and surface chem-istry (Hse 1968, 1971, 1972a, 1972b; Gardner etal. 1996; Liu et al. 1998; Hse and Kuo 1988;Richter et al. 1994). Mathematical expressionson the droplet volume were based on the re-sponse of equivalent height and average contactangle of droplets with an assumption of the sym-metric droplet dispersing on the wood surface(Chatterjee 2002a, 2002b). However, surfaceroughness at the local sites is heterogeneous dueto the cellular structure of wood and differentgrain angles of the wood specimens.
Cazabat (1992) has shown that the wettingbehaviors between macro- and micro-dropletson the wood surface were different. To deter-mine the optimum level of thermoset and ther-moset wetting characteristics for the wood-basedcomposites, more wetting studies were per-formed (Casilla et al. 1981; Chibowski andPerea-Carpio 2002; Sharma and Rao 2002;Paunov 2003). Therefore, this study examinedthe differences between the behavior of micro-sized adhesive droplets on the earlywood andlatewood of loblolly pine (Pinus taeda L.). Thisstudy also addressed droplet behavior such ascontact angle of thermosets on the surface ofmicrotome sections, heterogeneous wetting, andinterfacial strength properties between thermo-sets applied wood surface and isotactic polypro-pylene (iPP) film. The droplet versus scanningmethod for the contact-angle measurement on awood surface was also examined.
MATERIALS AND EXPERIMENTAL
Materials
Earlywood and latewood from the sapwood ofloblolly pine were selected from areas aroundPineville, LA. Wood samples were microtomedwith each tangential section of 1.4 x 1.4 x 0.06em". The wet microtomed samples were placedbetween glass plates and dried for 48 hours at
484 WOOD AND FIBER SCIENCE, JULY 2007, V. 39(3)
80°C. To nururruze sample warping, a slightload was applied. Isotactic polypropylene (iPP)films (Plastic Suppliers, Inc. Columbus, OR)were used to make lap-shear joints. Two liquidthermosets-urea-formaldehyde (UF; DyneaInc., Chembondw YTT-063-02, 60% solids con-tent) and phenol-formaldehyde (PF: Dynea13B41O, 0.1 Pa'S, Sp. Gr.: 1.202 gcm ", 56%solids content) resin were used as probe drop-lets.
Three directional images
Three image-capture systems (SPOT RT cam-era, and two of SOAR VL-7EX; Scalar, Inc.,Los Gatos, CA) and an image-analysis system(Image-Pro'" Plus, V.S.2) were used to generatecontinual micro-images from sides (along andacross the grain direction) and top for 5 minutes.Figure 1 shows the experimental setup tomeasure wetting characteristics of UF and PFfrom the sides and top as a function of time.Contact-angle measurement used a sessile drop-let method with microscopic magnification at
Side View
~
ri/
L'10 Second
• I
SOx. Two video capture systems were set upfrom the top and the side of the sample droplet.An auto-pipette generated 2-f.d micro-droplets toobserve droplet behavior on the different woodsurfaces. Single images were generated from thevideo files every 5 seconds for 5 minutes. Dur-ing this stage, droplet size and volume changeswere also recorded as a function of time.
Atomic force microscope (AFM) scanning
The technical feasibility of using AFM as amicro-manipulator to measure micro-contactangle measurement and its difference with ses-sile droplets using resins as a probe material wasdetermined. Figure 2 shows the 3-D plot of thesurface and section analysis of the scanned sur-face. The measurement of the topography of asample using AFM involves a micro-fabricatedcantilever with a very small tip being scannedabove the surface of the sample (Wang et al.2001). The scanning method used a NanoscopeIlIa atomic force microscope (AFM; Digital In-
Top View
~
1 Second
10 Second J
FIG. 1. Experimental procedures to collect phenol-formaldehyde resin wetting characteristics from the sides and top ofthe instrumental setup as a function of time.
Lee et at.-THERMOSETTING RESINS AS COMPATIBILIZERS 485
(c)
FIG. 2. Atomic force microscopy (AFM) 3D imagesscanned at the edge of a micro-droplet and section analysis:(a) 3-D surface plot, (b) Section selection, and (c) Sectionanalysis.
struments) mounted on a pneumatic isolationtable with an acoustic hood over the dried drop-let surface. All AFM micrographs were taken ata resolution of 512 x 512 pixels in tappingmode''>'. Micro-droplets were generated usingan air-automated spray, and the size range wasfrom 1 to 100 urn. The micro-droplets weredried for 24 hours before scanning the woodsurface. For the contact-angle measurements,two image-analysis systems were applied. Theanalysis systems were section analysis from theAFM software and Image-Pros Plus.
(A) (B)
Wetting characteristics
The geometry of droplets quickly changedinto a hemisphere shape in 5 seconds. In another5 seconds, an exact hemisphere reflected a re-duced volume (V) of the droplets (Fig. 3). Thus,experimental and mathematical efforts devel-oped several types of simplified hemisphericmodels to estimate a precise droplet volume onwood surfaces. The following equation fromZwillinger (2003) described the volume, surfacearea, and angle of response for a hemisphere anda simplified model for the exact hemisphere:
1 2Va=31Th (3R-h) (1)
where R was the hemisphere radius, determinedby substituting R for droplet height (h). How-ever, the droplets remained as an enclosinghemisphere shape rather than an exact hemi-sphere. Thus, the following equation was re-quired to generate an exact volume for an en-closing hemisphere with contact angle (8) anddroplet height (h) determined from the droplets.
(2)
Accuracy in the droplet volume was obtainedusing balance of gravity and capillary forces,and a well-applied "drop volume" method forestimating interfacial tension (Lando and Oakley1967; Wilkinson and Aronson 1973; Holcomband Zollweg 1990; Chatterjee 2002a, 2002b). A
(C) (D)
FIG. 3. An enclosing hemispherical dimension of the micro-droplets from an exact circle of an adhesive droplet as afunction of time on the wood surface. (A) Exact sphere, (B) Hemisphere, (C) Exact hemisphere, and (D) Enclosinghemisphere.
486 WOOD AND FIBER SCIENCE, JULY 2007, V. 39(3)
drop of resin on the wood surface changed withgravity forces, and a relatively small amount ofdroplets penetrated into the wood surface. Thesurface tension equation was extracted by usingcapillary force response for droplet retention onthe solid surface and each volume condition.The extracted critical surface tension equations(Eqs. 3 and 4) were expressed as:
6.pgR(2 - 3Cose + Cos2e)"Iva = 6Sine (3)
and
6.pgH
"I Vb = l2rSine
where
= density difference between the drop a:t1pcontinuous phase
g = gravity acceleration"I = interfacial tension between the two phasesr = wetted radiuse = three-phase contact angleH = total height
Shape analysis
A dimensionless shape factor (DSF: Eqs. 5and 6) was also generated from the capillaryforces (Cp = [(6.pgVa,b) - (21H}'rSine)/A]) andwet area (A) of the resin droplets (Kwok et al.1997; Chatterjee 2002a, 2002b). The DSF fromeach volume condition was calculated and ex-pressed as:
6.pgR2 3Sin2eDSFVa---------- (5)
2a 2 - 3Cose + cos2e
and
6.pgR2 6Sin2eDSFvb=--=------ (6)
2a 2 - 3Cose + cos2e
Single lap-shear test
Shear strength properties of the single-laplaminates were tested in tension using an Instron
4465 mechanical testing at a crosshead speed of0.13 cm·min.-1 according to ASTM D5573-94(ASTM 1994). Figure 4 shows a schematic drawfor the shear test. The figure also shows inter-faces between resin-applied wood surface andthermoplastic films. Seventy-two PP film lami-nated joints were prepared with a nominal UFand PF sprayed area of 1.1 x 1.0 em". On eachside of the wood strips, a UF or a PF resin wassprayed, and six sheets of PP film were placed inthe middle of the strips to address the interfacialshear development at the resin and polymer in-terface. The single lap-shear specimens werepressed at 0.69 MPa for two minutes at 204°C.At least 18 specimens were tested for each set ofsamples. Analysis of variance was used to de-termine the potential significance of the maineffects for this study with each resin and woodtype. Multiple comparisons were employed todetermine significant differences between thedifferent species using SAS software 9.0e (SAS2004).
Fracture surface
Shear failure was observed using scanningelectron microscopy (SEM; S-3600N) on thefracture surfaces of UF- and PF-loaded micro-tomed wood surfaces and PP film joints, and theinvestigated interfacial adhesion characteristicsat the resin sprayed wood surface and the PPinterface. An ion sputter apparatus (TechnicsHummer V) was used to coat samples with anapproximately 15-nm thin gold layer. Images of75x and 4,000x were generated at 10 kV.
RESULTS AND DISCUSSION
Shape transformation
Figure 3 shows droplets shape changes froman exact sphere of an adhesive droplet to anenclosing hemispherical dimension as a functionof time on the wood surface. The droplet shapeswere transformed from an exact sphere with aninitial contact angle of 1620 to a hemisphereover time (showing little wetting). The shapetransformation occurred quickly, in 5 seconds,
Lee et al.-THERMOSEITING RESINS AS COMPATIBILIZERS 487
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII'~~~~~~~(~~~)-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I§§§§ a §§§§§§§§§§§§§§§§§§§§§§§§§§(b)
(el)
(b)
(A)
(el) 111'I!i!iI. P(b)
(c)
(B)
FIG. 4. Sample schemes for single lap shear test. (A) = Shear sample fabrication, (B) = Shear test, (a) = plates, (b) =wood strips, (c) = Resin, and (d) = Polypropylene film.
from an exact sphere to an exact hemisphereshape. From this result, the critical surface ten-sion and volume were calculated using Eqs. 0),(2), (3), and (4). The surface free energy anddroplet volume were obtained by using the re-sponse of equivalent height of droplet changes(Eq. 1) and the response of the average angle(Eq. 2). Where the droplets were extended by thegravity factor and surface condition of the woodsurface, both methods were judged to be reliableto address substrate characteristics interfacedwith the two resins. Using the two methods, anincreased precision to predict volume changeswas obtained. Additionally, droplet dispersingareas covered by resin types were also differen-tiated on the wood types and fiber directions.Resin droplets dispersed much faster along thefiber direction than across the fiber directionwith earlywood and resulted in increased dropletdispersing areas. The areas with earlywood
showed III % (UF) and 42% (PF) faster changesalong the fiber surface as compared to acrossthe fiber surface (Top view after 10 seconds inFig. I).
Urea-formaldehyde and phenol-formaldehyde wetting
Wetting characteristics of UF and PF dropletson the microtomed wood surface of loblollypine earlywood and latewood are presented inTable 1. Earlywood with UF droplets dispersedrelatively quickly in the longitudinal directionon the surface with higher capillary pressure andthe long fiber length. Weight changes were alsohigher than other combinations. The cell cavi-ties of earlywood are much larger than those oflatewood, which has both smaller cavities andthicker cell walls (Fig. 8c). Thus, the surfaceroughness influenced the dispersing factor and
488 WOOD AND FIBER SCIENCE, JULy 2007, V. 39(3)
TABLE 1. Wetting characteristics of micro urea-formaldehyde and phenol-formaldehyde droplets on the microtome sectionof loblolly pine earlywood and latewood.
Sp. gr. Capillary Weight DispersingContact angle Critical tension
resulted in more resin penetration into the cellwalls. The dispersing ratios (R/I/R 1.) indicatedthat adhesives dropped on the earlywood sur-faces provided more extended resin coveragethan those applied on the latewood surface. Ingeneral, the critical surface tension was used toevaluate adhesive bonding characteristics at theinterface (Scheikl and Dunky 1998; Walinder2000; Khan et al. 2004). Surface tension (r/l)values were close to published values (Hse1971) while r1. was not. There was less penetra-tion into the inner cell structure with latewoodregardless of resin type. This result probably in-fluenced the shear performance at the wood-PPinterface. Volume changes of UF and PF drop-lets were shown as a function of time on thewood surface (Fig. 5). After 15 seconds, thedroplet volumes were fairly constant. The vol-ume changes with UF droplets on both woodtypes showed a similar trend as PF except PF onearlywood surfaces. UF resin gels very quickly
0.0090
0.0080
0.0070'"'E~ 0.0060"§ 0.0050~~ 0.0040
§- 0.0030Q
0.0020
-+- UF- Loblollypine earlywood
--- UF·Loblolly pine latewood.•.•....PF-Loblolly pine earlywood
-- PF-Loblolly pine latewood
0.0010
0.0000o 50 100 150 200
Time (Sec.)
FIG. 5. Volume changes of urea-formaldehyde and phe-nol-formaldehyde droplets as a function of time on the sur-face of microtomed loblolly pine wood (Arrow indicates 15seconds).
and has very high molecular weight (MW) dis-tribution, while PF resin has lower MW distri-bution, respectively.
Single lap-shear strength
Shear strength properties from single lapjoints of the UF and PF sprayed wood surfacesand PP film interface are shown in Fig. 6. Singlelap-shear strength increased 53% (from 1.1 MPato 1.7 MPa) when the latewood was sprayedwith UF resin. It should be noted that 88% woodfailure was observed with UF resin sprayed ontwo wood types, while 25% wood failure wasnoted with PF resin. Single lap-shear tests dem-onstrated that the UF loading at the wood and PPfilm interface improved the interfacial interac-tion. This result is largely attributable to the sur-face tension of wood with UF droplets due to theinherent properties of this resin. The earlywood,which had higher dispersing ratios with bothresin types, showed poor shear performances
~ 2.0~ 1.8•..E- 1.6.: 1.4e. 1.2= 1.0b'" 08" 0.6
= 0.4{:. 0.2
0.0UF
1:311%
250 513%
PF UF PFEarlywood Latewood
\Vood and Resin Types
FiG. 6. Shear strength properties from single lap jointlaminated assemblies of urea-formaldehyde and phenol-formaldehyde sprayed wood surfaces and polypropylenefilm interface (The error bars represent one standard devia-tion).
Lee et al.-THERMOSETTING RESINS AS COMPATIBILIZERS 489
due to over-penetration by capillary action.Thus, the thermoset resin remaining on the woodsurface was beneficial in increasing interfacialinteraction at the wood-PP interface with UFresin.
Dimensionless shape factors
Figure 7 shows dimensionless droplet shapefactors (DSF) with a critical boundary betweenhigh and low retaining regions as a function ofcontact angle. Loblolly pine had relativelyhigher retentions with UF resin than PF and in-fluenced shape transformation of the droplets.The higher retention indicates increased resinpenetration into the wood and in general, con-firms the fact that high resin retention at theinterface is important to obtain effective adhe-sion. Also shear strengths at the wood-PP inter-face increased and showed a lower thermoplastic
1000.00
100.00~'"e.
~ UF _ Earlywood
~ UF _L.1Icwood
-0- PF_E<!r1ywood
~ PF _L1tcwood
-\V;llcrIOil
e
~ 10.00o;-
ii5~.!E
(5
1.00
0.10
0.01o 30 60 90 120
Contact Angle (0)150
FIG.7. Dimensionless shape factor (DSF) changes as afunction of contact angle changes of urea-formaldehyde andphenol-formaldehyde droplets on earlywood and latewoodsurfaces.
failure than when thermoplastics were used forsurface modification. Therefore, the capillaryforces and surface tension played an importantrole in influencing the interfacial strength prop-erties.
Contact angle vs. surface scan
Models were developed for contact-anglemeasurements using two different systems. Themodels differentiated droplet behaviors on twodifferent wood surfaces. Contact angles fromscanned micro-droplets and predicted valueswith the best-fit models are presented in Table 2.Models were performed with ex = 0.05. The R2
values obtained from the models were an excel-lent fit to the data collected from the micro-droplet scanning method. Micro-dropletssprayed on the wood surfaces showed that wet-ting on a small scale is strongly affected byminimal physical surface heterogeneities morethan the relatively larger scale of sessile dropletsand they resulted in higher contact angles. Thus,the AFM technique to scan micro-droplets canbe beneficial to understand micro scale-dropletbehavior of wood adhesives on the surface ofwood materials. Experimental contact angles onthe wood surfaces were measured using AFMand found to validate prediction models.
Shear failure
180SEM micrographs showed the fracture surface
from shear load in Fig. 8. The low magnificationimage shows many fibers exposed from thewood surface. The fibers were produced by PPstretching during the shear failure. PP filmsheets were melted under heat and flowed into
TA8LE 2. Model-generated contact angle measurements versus scanned angle of urea-formaldehyde and phenol-formaldehyde droplets on the tangential surface of microtomed loblolly pine.
Expected ScannedResin Wood type Sessile droplet model R2 angle (0) angle (0)
FIG. 8. Scanning electron microscopy (SEM) micrographs generated with (a) low and (b) high magnification on thefracture surface of wood strip and polypropylene interface, and (c) anatomical structures of typical southern pine from Koch(1972).
radial resin canals exposed on the tangential sec-tion and the tracheid, parenchyma cells, trans-verse resin canals, and epithelial cells. The frac-ture behavior of joints can also be affected bymany other variables such as including the fiberand matrix nature, the fiber-matrix interaction,resin distribution, cell structure, etc. Thus, thesurface fibrillation of the PP matrix may add theinterfacial shear strength of single-lap shearjoints.
CONCLUSIONS
Contact angles of resin droplets, heteroge-neous wetting, and interfacial strength propertiesat the interfaces were evaluated on early- andlatewood of loblolly pine. The results of a sessiledroplet versus scanning method for the contact-angle measurement indicated that micro-dropletbehaviors were affected by the cellular structureof the wood surfaces. Two mathematical formu-las were developed to represent droplet shape
--Lee et al.-THERMOSETTING RESINS AS COMPATIBILIZERS 491
changes from a sphere to a hemisphere and fi-nally an enclosed hemisphere. The formulas alsoprovided a more accurate means of determiningthe sessile droplet volume. With regards to thedispersing behaviors of UP and PF resins, thedroplets dispersed more easily along the fiberdirection. Critical numbers obtained from theforce balance equations show a high retentionregion. Wood structure played an important rolein the interfacial shear strength properties.
REFERENCES
AMERICANSOCIETYFOR TESTINGANDMATERIALS(ASTM).1994. Standard practice for classifying failure modes infiber-reinforced-plastic joints. D 5573-94. American So-ciety for Testing and Materials, West Conshohocken, PA.
CASILLA,R. C., S. CHOW,ANDP. R. STEINER.1981. An im-mersion technique for studying wood wettability. WoodSci. Techno!. 15:31-43.
CAZABAT,A. M. 1992. Wetting from macroscopic to micro-scopic scale. Adv. Colloid Int. Sci. 42:65-67.
CHATTERJEEJ. 2002a. Shape analysis based critical Eotvosnumbers for buoyancy induced partial detachment of oildrops from hydrophilic surfaces. Adv. Colloid Int. Sci.99:163-179.
---. 2002b. Critical Eotvos numbers for buoyancy-induced oil drop detachment based on shape analysis.Adv. Colloid Int. Sci. 98:265-283.
CHEN, M., J. MEISTER,D. W. GUNNELLSANDD. J. GARDNER.1995. A process for coupling wood to thermoplastic us-ing graft copolymers. Adv. Polym. Tech. 14(2):97-109.
CHLBOWSKJ,E., ANDR. PEREA-CARPIO.2002. Problems of con-tact angle and solid surface free energy determination.Adv. Colloid Int. Sci. 98:245-264.
CHOW, S. 1974. Morphologic accessibility of wood adhe-sives. J. App!. Polym. Sci. 18:2785-2796.
FELIX, J. M., ANDP. GATENHOLM.1991. The nature of adhe-sion in composites of modified cellulose fibers and poly-propylene. J. App!. Polym. Sci. 42:609-620.
GAUTHIER,R., H. GAUTHIER,ANDC. JOLY. 1999. Compatibi-lization between lignocellulosic fibers and a polyolefinmatrix. Pages 153-162 in R. M. Rowell, ed. Proc. 5th
Inter!' Conf. Woodfiber-Plastic Composites.GEOGHEGAN,M., ANDG. KRAUSCH.2003. Wetting at polymer
surfaces and interfaces. Prog. Polym. Sci. 28:261-302.GARDNER,D. J., N. C. GENERALLA,D. W. GUNNELLS,AND
M. P. WOLCOTT. 1991. Dynamic wettability of wood.Langmuir 7:2498-2502.
---, M. P. WOLCOTT, L. WILSON, Y. HUANG, ANDM. CARPENTER.1996. Our understanding of wood surfacechemistry in 1995. Pages 29-36 in A. W. Christiansenand A. H. Conner, eds. Wood Adhesives. Forest ProductsSociety. Madison, WI.
HEDENBERG,P., ANDP. GATENHOLM.1995. Conversion ofplastic/cellulose waste into composites. I. Model of in-terphase. 1. App!. Polym. Sci. 56:641-651.
HOLCOMB,C. D., AND1. A. ZOLLWEG.1990. Improve calcu-lation techniques for interfacial tensiometers. J. ColloidInt. Sci. 134:41-50.
HSE, C. Y. 1968. Gluability of southern pine earlywood andlatewood. Forest Prod. J. 18(12):32-36.
---.1971. Properties of phenolic adhesives as related tobond quality in southern pine plywood. Forest Prod. J.21(1):44-52.
---. I972a. Surface tension of phenol-formaldehydewood adhesives. Holzforschung 26:82-85.
---. 1972b. Method for computing a roughness factorfor veneer surfaces. Wood Sci. 4(4):230-233.
---. I972c. Wettability of southern pine veneer by phe-nol formaldehyde wood adhesives. Forest Prod. 1. 22(1):51-56.
---, ANDM. L. Kuo. 1988. Influence of extractives onwood gluing and finishing: A review. Forest Prod. J.38(1):52-56.
KHAN,S., Y. H. CHUI, M. H. SCHNEIDER,ANDA. O. BARRY.2004. Wettability of treated flakes of selected specieswith commercial adhesive resins. J. Instit. Wood Sci.16(5):258-265.
KOLOSIK,P. C., G. E. MYERS, ANDJ. A. KOUTSKY.1993.Bonding mechanisms between polypropylene and wood:coupling agent and crystallinity effects. Pages 15-19 inM. P. Wolcott, ed. Wood Fiber/Polymer Composites:Fundamental concepts, process, and material options.
KWOK,D. Y., T. GIETZELT,K GRUNDKE,H. J. JACOBASCH,ANDA. W. NEUMANN.1997. Contact angle measurementand contact angle interpretation. 1. Contact angle mea-surement by axisymmetric drop shape analysis and a go-niometer sessile drop technique. Langmuir 13:2880-2894.
LANDO,J. L., ANDH. T. OAKLEY.1967. Tabulated correctionfactors for the drop-weight-volume determination of sur-face and interfacial tensions. J. Colloid Int. Sci. 25:526-530.
Lru, F. P., T. G. RIALS,AND1. SIMONSEN.1998. Relationshipof wood surface energy to surface composition. Lang-muir 14:536-541.
MATUANA, L. M., R. T. WOODHAMS,C. B. PARK, ANDJ. J. BALATINECZ.1998a. Effect of surface properties onthe adhesion between PVC and wood veneer laminates.Polym. Eng. Sci. 38(5):765-773.
---, ---, ---, AND---. 1998b. Influence ofinterfacial interactions on the properties of PVC/cellulosic fiber composites. Proc. 56th Ann. Tech. Conf.ANTEC. Part 3 (of 3):3313-3318.
OKSMAN,K, ANDH. LINDBERG.1998. Influence of thermo-plastic elastomers on adhesion in polyethylene-woodflour composites. J. App!. Polym. Sci. 68(11): 1845-1855.
PAUNOVV. N. 2003. Novel method for determining thethree-phase contact angle of colloid particles adsorbed at
492 WOOD AND FIBER SCIENCE, JULY 2007, V. 39(3)
air-water and oil-water interfaces. Langmuir 19:7970-7976.
RABINOVICH,Y. 1., J. J. ADLER, M. S. ESAYANUR,A. ATA,R. K. SINGH,ANDB. M. MOUDGIL.2002. Capillary forcesbetween surfaces with nanoscale roughness. Adv. ColloidInt. Sci. 96:213-230.
RICHTER, K., W. C. FEIST, AND M. T. KNAEBE. 1994. Theeffect of surface roughness on the performance of fin-ishes. Research and work reports, EMPA wood dept.Swiss Federal Lab. for Materials Testing and Research,83 pp.
SAS INSTITUTEINC. 2004. What's New in SAS® 9.0, 9.1,9.1.2, and 9.1.3. SAS Institute Inc., Cary, NC, USA.301 pp.
SCHEIKL, M., AND M. DUNKY. 1998. Measurement of dy-namic and static contact angles on wood for the determi-nation of its surface tension and the penetration of liquidsinto the wood surface. Holzforschung 52(1):89-94.
SHARMA,P. K., ANDK. H. RAO. 2002. Analysis of differentapproaches for evaluation of surface energy of microbialcells by contact angle goniometry. Adv. Colloid Int. Sci.98:341-463.
SHUPE, T. F., C. Y. HSE, E. T. CHOONG,ANDL. H. GROOM.1998. Effect of wood grain and veneer side on loblollypine veneer wettability. Forest Prod. J. 48(6):95-97.
WALINDER,M. 2000. Wetting phenomena on wood: factorsinfluencing measurements of wood wettability. DoctoralThesis. Dept. Manufacturing System, KTH-Royal Insti-tute of Technology, Stockholm, Sweden. 62 pp.
WALINDER,M. E. P., ANDG. STROM.2001. Measurement ofwood wettability by the Wilhelmy method. Part 2. De-termination of apparent contact angles. Holzforschung55:33-41.
WANG, R., M. TAKEDA,K. MUKAl, ANDM. Kino. 2001. ACnon contact mode atomic force microscopy of pure waterwetting on natural mica and SUS304 steel. J. Japan Inst.Metals. 65(12): 1057 -1065.
WILKINSON,M. c., ANDM. P. ARONSON.1973. Applicabilityof the drop-weight technique to the determination of thesurface tension of liquid metals. J. Chem. Soc. FaradayTrans. 69:474-480.
ZWILLINGERD. 2003. CRC: Standard Mathematical Tablesand Formulae. 31st edit., Chapman & Hall/CRC Press,Inc., Boca Raton, FL. 910 pp.
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