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Adhesion of coating materials to woodJozef Kdela & Eva Liptkov

Version of record first published: 02 Apr 2012.

To cite this article:Jozef Kdela & Eva Liptkov (2006): Adhesion of coating materials to wood,Journal of Adhesion Science and Technology, 20:8, 875-895

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J. Adhesion Sci. Technol., Vol. 20, No. 8, pp. 875 895 (2006) VSP 2006.Also available online - www.vsppub.com

Adhesion of coating materials to wood

JOZEF KDELA 1, and EVA LIPTKOV 2

1 Technical University in Zvolen, Faculty of Wood Sciences and Technology, Department of WoodScience, Masarykova 24, 960 53 Zvolen, Slovak Republic

2M. R. tefnika 9, 960 01 Zvolen, Slovak Republic

Received in final form 20 March 2006

AbstractThis study analyses the work of adhesion calculated according to the YoungDuprequation and the work of adhesion calculated based on the interactions at the interface between thecoating material in the liquid phase and the wood. It has been shown that the adhesion of coatingmaterials to wood and their ability to spread over the substrate surface are primarily controlled byinteractions between the surface forces at the interface. The values of these interactions confirmedthat the tested materials should spread spontaneously over the wood surface. There are also discussedthe secondary factors that can significantly influence the interactions between the surface forces and,

consequently, the final stability of the examined system.The values of surface free energy for the hardened coating materials were considerably higher than

the original values of surface free energy for the corresponding liquid coatings especially due tothe several times higher polar component. We also observed a considerable increase of the work ofadhesion of the coating materials to the wood due to the hardening process. This increase resultedmainly from interactions between the polar components of surface free energy of the adjacent phases.The increase in adhesion of the coating materials during the hardening process was accompanied bya decrease in the interfacial free energy at the interface with the substrate.

Mechanical testing of the stability of the coating materialwood system showed that the failurein the system always occurred in accordance with the calculated values of the work of adhesion ofthe coating material to the wood and the work of cohesion of the coating material and of the wood

substrate.

Keywords: Coating materials; wetting; contact angle; surface free energy; spreading coefficient;adhesion; cohesion; tensile strength; wood.

1. INTRODUCTION

The final surface quality of wood coated with synthetic or natural paint (coating)

materials is a result of the chemical composition of the paint materials, properties

To whom correspondence should be addressed. E-mail: [email protected]

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876 J Kdela and E. Liptkov

of the substrate and interactions between the two materials. The interaction betweenthe coating and wood materials comprises:

interactions between the coating material and the wood at their interface, secondary influences arising from specific properties of the coating material and

the wood substrate.

Examination of the interactions across the interface between the wood and a coatingmaterial requires focusing on two basic systems:

solid materialliquid,

two solid materials with different physical and chemical properties.

The first case is considered when we study the interface between wood and coating

material during its application. Interactions between surfaces occurring across theinterface in such a case are primarily reflected in the wetting and spreading of thecoating material over the substrate surface. This determines, together with woodsubstrate morphology, the area of contact interface (coatingwood) and the finalsmoothness of the coating layer. Consequently, the study of wood wetting withcoating materials is necessary for understanding adhesion of these materials to thewood substrate.

The solid coatingwood system is formed after the hardening (drying) of thecoating material on the substrate surface. Our hypothesis is that the developmentand stability of this system is primarily controlled by the interactions between theunsaturated active zones at the interface between the solid coating and wood and bythe cohesion within the particular constituents of the system.

The purpose of this study was to explore the interactions between the surfaceforces at the interface of coating and wood material in both liquid and solid phases,and, based on the results obtained, to elucidate the mechanism of adhesion ofcoating materials in both liquid and solid phases to wood.

2. WOODCOATING MATERIAL SYSTEM

2.1. Wood

Wood is a heterogeneous, anisotropic, porous and hydrophillic natural material.Variously shaped and variously arranged anatomical elements of wood constitute

a heterogeneous porous structure (Fig. 1). The pores with radiir 100 nm are,in general, present in the cell walls. The pores with r >100 nm correspond to thelumens of cell elements and the intercellular space. The latter represents a majorpart of wood porosity. At the same time, it is necessary to consider the specificfeatures of individual wood species (shape and size of the pores, their mutuallinking and spatial arrangement). The heterogeneity of wood porous structure isnaturally reflected in a higher variability of the wood surface roughness and unevenpenetration of the liquid coating materials into wood. The result is the formation of

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Adhesion of coating materials to wood 877

Figure 1. Distribution of pores in beech wood as determined using mercury porosimetry.

a wood layer pre-impregnated with the coating material [1, 2]. The thickness of thepre-impregnated layer is dependent on several factors.

The wood porosity related to penetration of a film-forming material into woodhas a twofold effect: it increases the contact area between the wood and the coatingmaterial, and can cause defects in the final coating quality.

Chemical properties of wood depend on its chemical composition (cellulose,hemicelluloses, lignin and extractives). The quality of wood surface with appliedcoating is also influenced by the extractives [38] that can cause:

migration of wood extractives into the coating,

undesired changes in the substrate colour,

inhibition of coating material hardening.Chemical composition of wood is also reflected in its hydrophilic nature. Thesorption capacity of wood is dependent on the density of functional groups con-stituting the adsorption grid. The equilibrium moisture content in the wood alsodepends on the ambient environment (relative air humidity and temperature). Woodmoisture is necessary to consider both at the application stage of film-forming ma-terials as well as after the film hardening because with varying wood moisture con-tent, the dimensions (swelling, shrinking) also change, which causes developmentof stresses in the solid coating. Excessive moisture content in wood can result inpoor adhesion properties of the coating material.

Chemical composition of wood also determines the thermodynamic propertiesof its surface (surface free energy, dispersion and polar components, etc.). Wemeasured the values of surface free energy ranging from 60 to 70 mJ m2 depending

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Table 1.

Surface free energy of selected wood species , with dispersion and polar components d and p

(microtomed surface)

Wood species d p

[mJm2]

Spruce 60.3 27.8 32.5Pine 66.9 25.0 42.0Birch 67.8 24.6 43.2Aspen 68.3 24.4 43.9Beech 68.1 24.5 43.6Walnut sapwood 67.8 24.6 43.2Walnut heartwood 65.0 26.0 39.0

Oak 63.9 26.5 37.3Elm 65.4 25.7 39.7

on the wood species (Table 1) and on mechanical treatment of the wood surface[912].

Surface free energy is of an additive nature and consists of dispersion, i.e. non-polar and a polar component that determine the polar/non-polar character of thewood surface. As seen in Table 1, the polar component of surface free energyis significantly higher than the non-polar component. Several authors [13 16],

however, have reported that the non-polar component is higher. However, this valuecan be considerably influenced by the method of measurement of the equilibriumcontact angle discussed in the following text.

Both physical and mechanical properties of wood influence a wide range ofcharacteristics (density, moisture content, swelling and shrinkage, permeability,strength, elasticity, hardness. . .) and have an indirect impact on the quality ofsurface treatment and on the stability of the woodsolid coating system.

2.2. Coating materials

The basic characteristics of coating materials primarily depend on the chemicalcomposition of the film-forming constituent. In the drying process, both physicaland chemical events occur leading to the formation of thermoplastic or thermoset-ting films with specific physical and chemical properties. Today, the production ofsuch films can be well controlled and their properties adjusted precisely accordingto the requirements.

The values of surface free energy for the selected types of coating materialsin their liquid form and the separate values for dispersion and polar componentsare summarized in Table 2. The surface free energy for all organic solvent-

borne coatings is virtually the same and ranges from 27.01 to 27.61 mJ m

2

. Thedispersion component of surface free energy is highly dominant (the ratio of thedispersion to polar component is about 9 : 1). The surface free energy of the water-based acrylate lacquer is considerably higher than the organic solvent-borne types,

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Table 2.

Basic statistical characteristics of cumulative values of surface free energy for selected coatingmaterials in the liquid phase L and separate values for dispersion and polar components dL and

p

L [17]

Coating material L s dL s pL s Number of

measurements[mJm2] n

Nitrocellulose lacquer 27.61 0.65 24.76 0.41 2.85 0.23 20Acid-hardened lacquer 27.22 0.50 24.51 0.30 2.71 0.18 20Polyurethane lacquer 27.14 0.64 24.39 0.42 2.65 0.22 20Polyester lacquer 27.01 0.59 24.37 0.39 2.64 0.21 20Water-based acrylate lacquer 34.69 1.89 28.54 0.82 6.15 1.08 20

s standard deviation.

and this can be attributed to the higher proportion of polar component. In the caseof water-based acrylate lacquer, the value of surface free energy is dependent on theamount of water contained in the coating.

The values of surface free energy of coating materials are approximately 2.5-timeslower than the values of surface free energy of wood. The values dL are nearlyequivalent to the values of the dispersion component of the surface free energy ofwood; on the other hand, the values of polar component of surface free energy ofcoating materials are several times lower in comparison to the values of the polarcomponent of surface free energy of wood.

We examined the wood wetting with coating materials. In most cases we observedthat the wetting of wood substrate with the coating material was incomplete [17].

3. INTERACTIONS OF WOOD WITH COATING MATERIALS IN THE LIQUID

PHASE

Application of coating materials onto the wood surface entails creation of an

interface between the coating material and the wood.The process of wetting and adhesion at the interface between the coating and thewood material in the liquid phase will be influenced by the chemical constitutionof the wood substance as well as morphology of the wood surface. There exista number of properties of these two contacting phases: geometry of the substratesurface, viscosity of coating materials, wood moisture content, various stress statesin the system, etc. [2, 9, 11, 1821] which must to be considered when studyingcoating-wood systems because they also may cause a considerable distortion of thepattern of interactions among the surface forces, and influence the conditions at theinterface between the wood and the liquid coating material. They can also have anegative impact on the stability of the final solid coatingwood system.

The wetting process was evaluated by using the Youngs concept of energyequilibrium at the interfaces between the solid, liquid and gaseous phases, according

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to the equation

LVcos =SV SL, (1)

whereSVis the interfacialfree energy at the solid materialgas interface,LVis theinterfacial free energy at the liquidgas interface, SLis the interfacial free energyat the solidliquid interface and is the equilibrium contact angle.

The capacity of a liquid to wet the surface of a specific solid material is assessedbased on the experimentally measured contact angle .

As we can see in the literature [22 28], there are a number of ways to determinethe contact angle, because the actual wetting process always occurs in conditionsmore or less different from the ideal equilibrium state.

The penetration of the liquid into the substrate at the interface between the coating

and wood material is, in general, characterized by changing contact angle. Thischange depends on time and, to a considerable extent, also on wood morphologyand geometry of the wood external surface [17, 29].

The problem of changing contact angle has been discussed in detail in [30, 31].In these works we suggest that for the calculation of thermodynamic characteristicsthe most suitable angle is the one corresponding to an ideally smooth wood surfacew.

The wood surface is more or less rough as a result of the wood morphologyand the way of treatment and at the moment of the contact between the drop

and wood surface, the void spaces on the surface under the drop remain partiallyfilled with air. In case when the substrate surface in the contact spot with the liquidconsists of two different components with surface free energy values S1and S2,the equilibrium contact angleis expressed as:

cos =f1S1VS1L

LV+f2

S2VS2L

LV, (2)

where f1 and f2 represent the proportions of the separate components at the siteoccupied by the liquid.

Modifying equation (2) we obtaincos =f1cos 1 +f2cos 2, (3)

where1and 2are values of contact angles corresponding to the separate substratecomponents.

If one of the components is the wood substance and the other is the air, then thevalue ofS2Von the right side of equation (2) is equal to zero, SLis identical to thesurface stress of the liquid LVand equation (3) can be arranged in the followingway:

cos 0 =f1cos w f2, (4)where 0 is the contact angle at zero time, i.e. at the moment the drop justcontacts the substrate, wis the contact angle on ideal smooth surface, in our case

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corresponding to the surface of wood substance with molecular-sized roughness,f1is the proportion of wood substance andf2is the proportion of air-filled unevennessunder the liquid.

The continually spreading drop at the same time infiltrates the substrate, and,consequently, the unevenness on the surface is filled with the liquid, the dropdiameter gets larger, and the height of the drop decreases. Just at the moment whenall the unevenness under the drop is filled with the liquid, there occurs a changein the nature of the interface. At this time the advancing contact angle is reversedto a receding one, the value S2Von the right side of equation (2) is equal to thesurface tension of the liquid LV, and SLis equal to zero. We consider this stateas equilibrated. Consequently, the equilibrium contact angle uat the woodliquidinterface can be expressed as:

cos u =f1cos w +f2. (5)

The meanings of the other symbols in equation (5) are the same as in equation (4).Having reached this point, the liquid starts to recede from the already-wetted

surface, and, as a result of progressive penetration, the liquid gradually penetratesthe substrate. The drop diameter starts decreasing, the height and contact angle keepdecreasing too.

As

f1 +f2 =1, (6)it is possible to calculate the values of contact angle w by substituting theexperimentally obtained values of0and uin equations (4), (5) and (6).

In such a way, we eliminate the influence of wood surface morphology, becausenow the values of the wetting angle w are only dependent on the wood surfacechemical constitution. The same is true for the values of surface free energy andfor other thermodynamic characteristics. The contact angle w is more suitablefor calculation of surface free energy values and also for the values of othercharacteristics describing the thermodynamic conditions at the interface than either

the contact angle0or another contact angle measured at a specified time [31].The conditions at the interface between the wood and liquid coating materials

can also be evaluated through the reversible work of adhesion Wa, according to thefollowing equation:

Wa =SV +LV SL. (7)

Rearranging equations (1) and (7) we obtain the YoungDupr equation

Wa =LV(1+cos ). (8)

Equation (8) enables us to calculate the values of adhesion of a liquid material toa solid substrate, because the values ofLVand on the right side can be obtainedexperimentally.

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Table 3.

Work of adhesion of coating materials to wood Wa, work of cohesion of coating materials Wc andspreading coefficientS

Wood Coating material w Wa Wc Sspecies

[degree] [mJ m2]

Spruce Nitrocellulose lacquer 11.8 54.64 55.22 0.58Acid-hardened lacquer 15.2 53.48 54.44 0.96Polyurethane lacquer 13.1 53.57 54.08 0.51Polyester lacquer 14.4 53.17 54.02 0.85Water-based acrylate lacquer 16.7 67.91 69.38 1.47

Beech Nitrocellulose lacquer 15.4 54.23 55.22 0.99Acid-hardened lacquer 15.7 53.42 54.44 1.02Polyurethane lacquer 15.7 53.27 54.08 0.81Polyester lacquer 14.2 53.19 54.02 0.83Water-based acrylate lacquer 27.7 65.40 69.38 3.98

Oak Nitrocellulose lacquer 14.6 54.32 55.22 0.90Acid-hardened lacquer 19.5 52.88 54.44 1.56Polyurethane lacquer 15.7 53.27 54.08 0.81Polyester lacquer 13.1 53.32 54.02 0.70Water-based acrylate lacquer 20.8 67.11 69.38 2.27

Both the cohesion of a coating material and its adhesion to wood also affect thespreading capacity of the coating material on the wood surface as expressed throughthe spreading coefficientS[32] according to the relation

S=Wa Wc, (9)

whereWcis work of cohesion which is

Wc =2LV. (10)

Equation (9) means that the coating material will spontaneously spread on the

wood surface in the case when S >0, i.e., whenWa > Wc.Substituting the mean values of surface free energy of wood (Table 1) and themean values of contact angle w (Table 3) into equations (7)(9), we obtainedthe values of the work of adhesion of coating materials to wood Wa, the valuesof cohesion of the coating materials Wc and the values of spreading coefficient S(Table 3).

The values of the work of adhesion Wa are lower than the values of the workof cohesion. This fact is also reflected in the negative values of the spreadingcoefficient S. Wetting and spreading of coating materials decrease with increasingviscosity (Fig. 2).

A different method for evaluation of the interface between the wood and thecoating material is derived from the works dealing with studies of intermolecularforces occurring at interfaces [3335]. These authors show that it is possible to

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Figure 2. Influence of dynamic viscosity of nitrocellulose lacquer on values of contact angle wmeasured on spruce, beech and oak wood substrates.

determine the values of interfacial free energy for the solid materialliquid interfaceSL, according to the relation

SL =

d

S

d

L

2

+

p

S

p

L

2

(11)

by knowing the values of the dispersion (non-polar) and polar components of thesurface free energy of the substrate dS and

pS and of the non-polar and polar

components of the surface free energy of the liquid material dL andp

L .The work of adhesionWaand the spreading coefficientScan be expressed as:

Wa =2

dS d

L +2

p

S p

L , (12)

S=2

dS dL +2

p

S p

L 2

dL +p

L

. (13)

The term 2(dL + p

L )on the right side of equation (13) corresponds to the termrepresenting the work of cohesion of the liquid in equation (10).

The work of adhesion of coating materials to wood, the work of cohesion ofcoating materials and spreading coefficient were also calculated from equations (12)and (13) (Table 4). The table also contains the values of non-polar and polarcomponents of the work of adhesionWda andW

pa.

As it follows from the results in Table 4, the adhesion of a coating material inthe liquid phase to wood is a result of interactions between non-polar and polarforces at the interface between the wood and the coating material. Except for thewater-based acrylate lacquer, there were found significant interactions between thenon-polar components of the surface free energy of the neighbouring phases.

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Table 4.

Work of adhesion of coating materials to wood Wa, work of cohesion of coating materials Wc andspreading coefficientS[17]

Wood Coating material Wa Wda Wpa Wc S

species[mJm2]

Spruce Nitrocellulose lacquer 71.74 52.52 19.22 55.22 16.52Acid-hardened lacquer 71.00 52.25 18.75 54.44 16.56Polyurethane lacquer 70.66 52.12 18.54 54.08 16.58Polyester lacquer 70.60 52.10 18.50 54.02 16.59Water-based acrylate lacquer 84.63 56.39 28.24 69.38 15.25

Beech Nitrocellulose lacquer 71.53 49.24 22.29 55.22 16.31

Acid-hardened lacquer 70.72 48.99 21.73 54.44 16.28Polyurethane lacquer 70.36 48.87 21.49 54.08 16.28Polyester lacquer 70.30 48.85 21.45 54.02 16.28Water-based acrylate lacquer 85.60 52.86 17.14 69.38 16.23

Oak Nitrocellulose lacquer 71.89 51.26 20.63 55.22 16.67Acid-hardened lacquer 71.12 51.00 20.12 54.44 16.68Polyurethane lacquer 70.76 50.87 19.89 54.08 16.69Polyester lacquer 70.71 50.85 19.86 54.02 16.69Water-based acrylate lacquer 85.34 55.03 30.31 69.38 15.96

The interactions between the non-polar and polar forces at the interface betweenthe wood and an organic solvent-soluble coating material result in rather high valuesof interfacial free energy at the interface between the wood and the coating materialSL, resulting from high proportion of the polar component (Table 5). In the case ofthe interface between the wood and the water-based acrylate lacquer, the values ofinterfacial free energy are lower than in solvent-soluble lacquers, and the differencebetween non-polar and polar components is also lower.

The work of adhesion of coating materials to wood as calculated accordingto equation (12) is significantly higher than the work of cohesion of coating

materials Wc. Consequently, all the values of spreading coefficient Sare greaterthan zero (Table 4). This means that the coating materials have intrinsic ability tospontaneously spread over the wood surface. This fact has been confirmed also byPecina and Paprzycki [20].

The values of the work of adhesion of coating materials to wood determinedusing equation (12) are evidently higher (Table 4) compared to those obtainedusing equation (8)(Table 3). From the viewpoint of thermodynamic equilibrium,however, equivalence of equations (8) and (12) is to be expected. According toLiptkovet al. [17], these differences stem from the secondary influences due torheological properties of coating materials because in the case of ideal liquids, thevalidity of this equivalence has been confirmed.

Experimental results demonstrate that the values of the work of adhesion andspreading coefficient obtained using equations (8) and (9) and the corresponding

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Table 5.

Values of interfacial free energy between the coating materials and wood, SLand its non-polar andpolar componentsdSLand

pSL[17]

Wood Coating material SL dSL pSL

species[mJm2]

Spruce Nitrocellulose lacquer 16.14 0.10 16.04Acid-hardened lacquer 16.49 0.11 16.38Polyurethane lacquer 16.64 0.11 16.53Polyester lacquer 16.67 0.11 16.56Water-soluble acrylate lacquer 10.33 0.01 10.32

Beech Nitrocellulose lacquer 24.14 0.00 24.14

Acid-hardened lacquer 24.55 0.00 24.55Polyurethane lacquer 24.75 0.01 24.74Polyester lacquer 24.77 0.00 24.77Water-soluble acrylate lacquer 17.14 0.15 16.99

Oak Nitrocellulose lacquer 19.59 0.03 19.56Acid-hardened lacquer 19.97 0.04 19.93Polyurethane lacquer 20.14 0.05 20.09Polyester lacquer 20.17 0.05 20.12Water-soluble acrylate lacquer 13.22 0.04 13.18

values obtained using contact angles measured for woodcoating material in theliquid state do not correspond to the thermodynamic conditions at the interfacebetween a solid and a liquid, as a result of the influence of secondary factors.

The values of the work of adhesionWaand of the spreading coefficientSresultingfrom interactions between surface forces at the interface between the wood and acoating material can only be determined using equations (12) and (13), i.e., fromthe experimentally determined values of the dispersion and polar components of thesurface free energy of the neighbouring phases.

However, a comparison of the effects of surface force interactions at the solid-

liquid interface determined using equations (8) and (9) and those determined usingequations (12) and (13) allows us to evaluate the actual state at the interface.The results obtained in such a manner point out the necessity to eliminate thesecondary influence of rheological properties of coating materials on their wettingand spreading over the wood surface in the process of application. One possibleway to address this issue is to decrease the viscosity of the coating material beforethe application and choose a suitable application technique.

4. INTERACTIONS OF WOOD WITH COATING MATERIALS IN THE SOLID

PHASE

The process of drying a coating material on a substrate surface encompasses severalmechanical and physical events resulting in changes in the chemical composition of

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Figure 3. Average values of surface free energy of wood and of coating materials in solid andliquid phases, together with their dispersion and polar components (white columns: liquid phase,grey columns: solid phase, dark grey columns: wood, 1 nitrocellulose lacquer, 2 acid-hardenedlacquer, 3 polyurethane lacquer, 4 polyester lacquer, 5 water-based acrylate lacquer) [36].

Table 6.

Equilibrium contact angle at solid coating water interface u, surface free energy values of solidcoatingsS2, their dispersion and polar components dS2and

pS2[36]

Coating material u s S2 s dS2 s pS2 s n

[] [mJ m2]

Nitrocellulose lacquer 61.3 7.1 46.57 4.17 31.32 0.59 15.25 4.55 60Acid-hardened lacquer 43.5 11.0 57.37 5.62 28.99 1.69 28.38 7.21 60Polyurethane lacquer 48.3 5.1 54.13 3.06 30.25 0.66 23.88 3.56 60Polyester lacquer 53.9 8.0 50.87 4.62 30.82 1.05 20.05 5.61 60Water-based lacquer 48.1 5.8 54.27 3.34 30.20 1.00 24.07 4.32 60

s standard deviation,n number of measurements.

On the other hand, the values of surface free energy of the solid coatings werelower than the values of surface free energy of wood and were, depending on thewood species and the type of the coating, 6895% of the original surface free energyvalueS1(Fig. 3). The smallest differences are for systems based on spruce wood.The ratio between the dispersion components d

S2/d

S1was in all cases greater than

one; on the other hand, the polar component ratiopS2/p

S1was smaller than one.From the mean values of surface free energy of wood (Table 1) and the mean

values of surface free energy of the solid coatings (Table 6) we calculated, using

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Table 7.

Work of adhesion at the interface of wood with a solid coating Wa, and its dispersion and polarcomponentsWda andW

pa [36]

Wood Coating material Wa Wda Wpa

species[mJm2]

Spruce Nitrocellulose lacquer 103.59 59.07 44.52Acid-hardened lacquer 117.57 56.83 60.74Polyurethane lacquer 113.81 58.04 55.77Polyester lacquer 109.64 58.59 51.05Water-based acrylate lacquer 113.94 58.00 55.94


Acid-hardened lacquer 123.62 53.28 70.34Polyurethane lacquer 119.01 54.42 64.59Polyester lacquer 114.05 54.93 59.12Water-based acrylate lacquer 119.15 54.38 64.77

Oak Nitrocellulose lacquer 105.37 57.65 47.72Acid-hardened lacquer 120.57 55.46 65.11Polyurethane lacquer 116.43 56.65 59.78Polyester lacquer 111.91 57.19 54.73Water-based acrylate lacquer 116.57 56.61 59.96

equation (10), the values of the work of adhesion of coating materials to wood Waafter their hardening or drying ( Table 7). In this table we also present the valuesof interactions between non-polar and polar components Wda and W

pa at the solid

coatingwood interface.The values of Wa for the wood species decreased from beech, through oak to

spruce. The results of the work of adhesion summarised in Table 7 show that theadhesion of coating materials to wood is a result of interactions between non-polarand polar components at the interface between the wood and the coating materials,and the magnitudes of these components are dependent on chemical composition of

the substrate and the solid coating applied.Comparison of the values of the work of adhesion determined at the interfacebetween coating materials in the liquid phase and the wood substrate (Table 4)with the values given in Table 7 revealed that the work of adhesion of thecoating materials to wood increased after drying. This increase primarily followsfrom interactions between polar components of the surface free energy of theneighbouring phases.

The increase in the work of adhesion of coating materials during drying isaccompanied by a drop in interfacial free energy at the interface with the substrate

S1S2 (Table 8). The values of

S1S2 are relatively low, and they have mostly

polar character. The dispersion component dS1S2, resulting from the non-saturateddispersion component of surface free energy of the solid coating is lower by an orderof magnitude and, consequently, can be neglected.

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Table 8.

Interfacial free energy of wood with solid coating S1S2and its dispersion and polar componentsdS1S2andpS1S2[36]

Wood Coating material S1S2 dS1S2 pS1S2

species[mJm2]

Spruce Nitrocellulose lacquer 3.33 0.10 3.23Acid-hardened lacquer 0.15 0.01 0.14Polyurethane lacquer 0.70 0.05 0.65Polyester lacquer 1.57 0.07 1.50Water-based acrylate lacquer 0.68 0.05 0.63


Acid-hardened lacquer 1.81 0.19 1.62Polyurethane lacquer 3.22 0.30 2.92Polyester lacquer 4.87 0.36 4.51Water-based acrylate lacquer 3.17 0.30 2.87

Oak Nitrocellulose lacquer 5.06 0.20 4.86Acid-hardened lacquer 0.66 0.05 0.61Polyurethane lacquer 1.61 0.12 1.49Polyester lacquer 2.83 0.16 2.67Water-based acrylate lacquer 1.51 0.06 1.45

Table 9 shows the values of the work of adhesion of solid coatings to wood Wa,the values of the work of cohesion for wood WC1and the work of cohesion of thesolid coatingsWC2. In all cases shown in Table 9 the following holds:

WC1 > Wa > WC2. (18)

If we suppose that the stability of the solid coatingwood system results onlyfrom the cohesion forces within the principal components and from the adhesion attheir interface, the results obtained indicate that the coatings are the weakest link in

the system. Under mechanical loading of the system, the failure will occur in thecoating. However, it is necessary to emphasize that the term wood cohesion meanscohesion of the ideal compact wood substance, i.e., the wood substance withoutpores; and the cohesion of the coating material means the cohesion of free solidfilms of the corresponding material.

In the case of wood, it is necessary to consider its porous structure, containingcertain types of defects. In such cases, the strength of wood is considerably lowerthan the strength of the corresponding solid wood substance. The higher is the woodporosity, the lower is its density and the lower is also its strength.

The strength of solid coating films applied onto wood can also be lower than thetheoretical value, because the film structure may be changed to some extent [20]and, moreover, there may be defects in the coating films with negative influenceon the film strength [37, 38]. The impact of these factors on the stability of a specific

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Table 9.

Mean values of the work of adhesion of solid coating materials to wood Wa, the work of cohesion ofwoodWC1and the work of cohesion of solid coating WC2[36]

Wood Coating material Wa WC1 WC2species

[mJm2]

Spruce Nitrocellulose lacquer 103.59 120.54 93.14Acid-hardened lacquer 117.57 114.74Polyurethane lacquer 113.81 108.26Polyester lacquer 109.64 101.74Water-based acrylate lacquer 113.94 108.54

Beech Nitrocellulose lacquer 106.94 136.12 93.14Acid-hardened lacquer 123.62 114.74Polyurethane lacquer 119.01 108.26Polyester lacquer 114.05 101.74Water-based acrylate lacquer 119.15 108.54

Oak Nitrocellulose lacquer 105.37 127.74 93.14Acid-hardened lacquer 120.57 114.74Polyurethane lacquer 116.43 108.26Polyester lacquer 111.91 101.74Water-based acrylate lacquer 116.57 108.54

solid coatingwood system can be verified by mechanical loading of the system intension (pull-off test). This method of testing the stability of the solid coatingwoodsystem is frequently used in practice. Such testing is often considered as a test ofthe coating practical adhesion to the substrate.

The results of analysis of the failure sides of coating and wood surface after thecoating detachment, performed using scanning electron microscopy, allow us toconclude that the failure was most frequently in the coating (i.e., cohesive failure,

Fig. 4). Consequently, the study into interactions between intermolecular forces atthe solid coatingwood interface confirmed that the weakest link occurred just inthe coating. If we evaluate such failures only visually, they can be misinterpreted asadhesion failures, particularly in the case of transparent coating materials.

In the case of oak and spruce wood, we observed partial wood failure as evidencedby the presence of wood fibres on the coating film (Fig. 5a). However, it wasnot always a typical cohesive failure over the whole substrate surface, with a fewexceptions when the wood substrate contained early wood (Fig. 5b). In such cases,the work of cohesion of the wood was smaller than the work of cohesion of thecoating material.

However, it is important and necessary to emphasize that not even in a single caseadhesion failure was observed, which is consistent with the results of the work ofadhesion and the work of cohesion of coating materials and wood.

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(a)

(b)

Figure 4. Cohesive failure in the coating, following mechanical loading of the polyester lacquerbeech wood system. (a) backside of the coating pulled apart from wood; (b) wood surface after thecoating was torn away the wood remains covered with a lacquer layer.

In case of ground specimens, the fibres can also be torn away from the coatedsubstrate (Fig. 6). In common practical evaluation, such a case is considered asa cohesive failure in the substrate which is not correct and is in contradictionwith the results of thermodynamics at the interface. The phenomenon, occurringin grinding, is caused by broken wood fibres, pre-impregnated with the coatingmaterial [9, 12]. If a wood specimen with a hardened coating material is subjected totension, the fibres are picked up from the wood; however, this cannot be considereda cohesive failure in the substrate. The conclusion that this is not a cohesive failurein the substrate follows also from the strength values which are significantly lowerthan the tensile strength of the wood in the direction considered.

Mechanical tests have confirmed that the results of the work of adhesion of solidcoatings to wood and their work of cohesion calculated using equations (15), (16)

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(a)

(b)

Figure 5. Backside of the coating film of water-based acrylate lacquer torn away from (a) oak wood

tangential surface, (b) spruce wood tangential surface (with visible wood debris).

and (17) provide us with information on the stability at the interface between thecoating material and wood and on the weakest link in solid coatingwood system.

The condition for an adhesion failure is

WC1 > Wa < WC2. (19)

Such cases were observed when there was high moisture content in the substrateor in the case of contamination of the wood surface before the surface treatmentitself.

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Figure 6. Backside of the detached polyester film covered with fibres torn from tangential beechwood surface treated by grinding.

5. CONCLUSIONS

The analysis of the results discussed in this study allows us to draw the followingconclusions.

Adhesion of coating materials to wood and their ability to spread over the woodsurface primarily result from interactions between the surface forces at the solidliquid interface. The values of these interactions showed that the materials testedshould spread spontaneously over the wood surface.

The secondary influence of the rheological properties of coating materials causesa limited wetting abilitiy on the wood surface. Negative influence of coatingrheological properties on the wood surface wetting with the coating materialsand the spreading of these materials over the surface is possible to overcome byapplying appropriate mechanical force during the wetting process (suitable methodof application).

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Comparison between the effects of interactions among surface forces at the solidliquid interface calculated using equations (8) and (9) and those calculated usingequations (12) and (13) provide an actual pattern of the interface.

The values of surface free energy for the hardened coating materials are consid-erably higher than original surface free energy values for the corresponding liquidcoatings especially due to the higher polar component. At the same time, thechange in the dispersion component is comparatively small. The value of surfacefree energy is also significantly influenced by the chemical composition of the coat-ing material.

It has been shown that the adhesion of coating materials to wood increases duringthe drying process. This increase was primarily controlled by interactions betweenthe polar components of the surface free energies of the neighbouring phases. An

increase in adhesion of coating materials during the drying process is accompaniedby a drop in interfacial free energy at the interface with the substrate.

Mechanical testing of the stability of the coating materialwood system showedthat the failure in the system always occurred in accordance with the calculatedvalues of the work of adhesion of the coating material to wood and the work ofcohesion of the coating material and the wood substrate.

Acknowledgements

We thank Professor Douglas J. Gardner for helpful comments on the manuscript.This work was supported by the Slovak Grant Agency for Science (Grant No.1/0558/03).

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