De Meijer Paper

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COST E18 Final Seminar A review of interfacial aspects in wood coatings

A review of interfacial aspects in wood coatings:wetting, surface energy, substrate penetration and adhesion.

Mari de Meijer

SUMMARY

This paper gives a review of the state of the art of interfacial aspects in wood coatings

research. It firstly covers the topics of penetration of the coatings into the wood pores from

both an anatomical and a rheological point of view. Secondly results and methods for the

determination of surface energies of wood are briefly reviewed. Thirdly the existing

knowledge on adhesion of coatings on wood is described, including the aspects of wet

adhesion. Finally the major gaps in knowledge are identified.

1. PENETRATION OF COATINGS INTO WOOD

The penetration of primers in the wood substrate has been subject of several studies during the last 40years of wood coatings research. Research work has covered various types of coatings correspondingto the state-of-the-art in wood coating technology. It started with studying solventborne alkyds anddrying oils1, followed by a reviewed interest with the introduction of waterborne coatings2. Especiallythe introduction of waterborne alkyds have been studied because these types of binders seems to havea better penetrating capacity then acrylic dispersions. Also high-solid alkyds and water soluble linseed

oils have been studied more recently3. Most of the work has been done on coating systems intended forexterior applications. For interior (furniture or parquet) coatings hardly any work has been publishedalthough penetration might be relevant for esthetical reasons.

1.1 Existing techniques for assessment

Most of the studies on penetration characteristics are based on microscopic studies. One of the mostpopular techniques is fluorescence microscopy with a fluorescent dye mixed or grafted to the binder or

the paint. Alternatively the wood itself can be stained with a fluorochrome or the pores not penetratedwith the paint are filled with and additional dye. More recently also confocal laser microscopy hasbeen used4. Another method for detection is the use of autoradiography in combination with a 14C

labelled binder. This has the advantage that the binder is chemically hardly changed. The grafting of afluorescent dye might change the properties of the binder or paint to some extent. Since fluorescencemicroscopy is limited to magnifications of about 200 x for higher magnifications SEM is the mostwidely used techniques although a good contrast between coating and wood is not very easily toachieve. For the detection of pigments EDAX analysis can be employed. The majority of the work

done in this field is of qualitative nature but some studies also give quantitative data bast on measuringthe depth in penetration in axial direction or by image analysis. Some typical examples of fluoresenceand SEM images of wood with coating are given below.

Drywood Coatings P.O.Box 3954 7500 DZ Enschede

[email protected]


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Fig. 1 SEM-image of pigmented coating

On softwood

Fig. 2 Fluoresence microscopic image of coating on meranti

(black and white print)

The above described analytical tools are quite sufficient to describe the penetration pattern of a dried

coating into the pore structure of wood they lack the possibility to study the penetration mechanism ofa liquid coating in situ. Also the possible penetration of certain components of the coating into the cellwall n not be assessed. In this respect some more advanced tools might be used in future. Possibilitiesmight be: magnetic resonance imaging (MRI) microscopy with sufficient resolution to study flowpatterns at a cellular level; environmental scanning microscopy (ESEM) and TOF-SIMS imaging tostudy changes in chemical composition during coating penetration. Another approach the study thedynamical uptake of coating material into the wood is to measure the decrease in volume of coatingdroplets deposited on wood surfaces5.

1.2 Influence of anatomical structure

Softwood

If paint is able to flow into the wood cells, three different ways of penetration in softwood can bedistinguished as is schematically shown in fig 3. Firstly the outer longitudinal tracheids are filleddirectly by coating flowing from the open ends on the surface. This predominantly occurs in theearlywood. The angle between length axis of the tracheid and the surface has a strong influence on theimportance of this mechanism. A second way of penetration is through the rays, starting also at the

open cut ends of the ray cells. In which way transport in the rays proceeds is strongly dependent on

the wood species. In pine the major part of the coating flows through the parenchyma cells, transportfrom cell to cell must therefore be possible. In spruce the coating almost solely penetrates the raytracheids. A third way of penetration is from ray cells to adjacent longitudinal tracheids in thelatewood. The extent of transport from rays to tracheids is strongly dependent on permeability of thecross-field pits and almost totally limited to pine sapwood. The importance of the three penetrationmechanisms mentioned above implicates that penetration of the coating can strongly be influenced bythe way in which boards are sawn out of a log. This because of the impact on differences in flat and

standing growth rings, orientation of grain to the surface, width of early and latewood bands and thenumber of rays ending in radial and tangential surfaces. The origin of the wood might influencepenetration because of differences in early- and latewood portions, conditions of the pits, number ofrays and length of longitudinal tracheids. Drying conditions of the wood might also have someinfluence on coating penetration because of its impact on pit aspiration.

coating

extractive

wood


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1. flow into open end of longitudinal

tracheid

2. flow into ray tracheid

3. flow into ray parenchyma

4. flow from ray parenchyma intolongitudinal latewood tracheid

5. flow from ray tracheid intolongitudinal tracheid

Fig. 3 Schematic overview of possible coating penetration patterns in softwood(source: M. de Meijer, K. Thurich and H. Militz, Wood Sci. Technol. 32, 1998)

Hardwood

Penetration in hardwoods, like e.g. dark red meranti is mainly restricted to the filling of vessels and thefirst cells of rays and very occasionally axial parenchym and sklerenchym. In more permeablehardwood species (e.g. beech) vessels will be filled deeper and penetration in axial parenchym and

sklerenchym is more pronounced. The filling of a vessel by the coating is strongly reduced if tylosesare present. Extractives appeared to have none or only a very minor influence on the penetration in alltree wood species studied. Surface preparation can have some influence on coating penetrationbecause sanding reduces the number of open cell capillaries in which paint can flow.

1.3 Influence of coating formulation, rheology and surface energy

The various studies on penetration show fairly consistent results with respect to differences in thedepth of penetration. Unpigmented oil based paints show the deepest penetration, especially through

rays and adjacent tracheids. This is observed for both formulations that are solventbased, waterbasedor solvent free. Unpigmented alkyd resins with organic solvent (mostly white spirit) also show a deeppenetration. Emulsions of alkyd resins do penetrate the other cell layers but clearly to a lesser extent.The penetration of waterborne acrylic dispersions is very limited. When pigments are added to theformulations, especially at higher loadings in opaque paints, the penetration of all types of paints isstrongly reduced but the rank-order remains the same. It should be noted that the pigments itself arestill small enough to flow through the pores, only in cross-fields some clogging might occur.


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To understand these differences the underlying mechanism of the capillary flow process should beconsidered. The following two cases should be considered:

gr

cos2=L

L

L

(1)

With height of liquid in a In this approach the maximum height of capillary rise is determined by the

capillary pressure balanced by the weight of the liquid, and neglecting the effect of viscosity. Equation1 predicts a deeper penetration in smaller capillaries. For the very deep penetrating oil based andunpigmented alkyd paints this is the case with the deepest penetration in the smaller latewood cells.However for most other products the deepest penetration is found in the wider earlywood cells. Thisbehaviour is predicted by the following equation (known as the Washburn equation):

2

r tcos=L

L(2)

With the time (t) and liquid viscosity (). Please note that equation 1 describes an equilibriumsituation whereas equation 2 is a non-equilibrium, time dependent model. Equation 2 states that the

depth of capillary penetration is proportional to the square root of: liquid surface tension, cosine of thecontact angle between liquid and capillary wall, diameter of the capillary and the reciprocal liquidviscosity. It should also be noted that according to equation 2, lowering the surface tension if wetting

is complete ( > 0)will reduce the penetration rate.

The actual limiting factor for most penetration processes following the Washburn equation is the

increase in viscosity during the capillary penetration process. The micro-pores in the cell wall of thewood capillaries, with a size of 0.1- 1 nm, will only allow the lower molecular weight materials like

water and solvent to enter the cell wall. The larger polymeric molecules will remain inside thecapillary. The above mentioned process is visualised schematically in fig.4.The selective removal ofsolvent or water during the penetration process will increase the polymer fraction in the liquid andhence the viscosity of the solution6.

WOODEN

CELL WALL

CAPILLARY

FLOW

OF BINDER

SELECTIVE

REMOVAL OFWATER OR

SOLVENTIN

CR

EASING

FRACTIO

N

POLYMER

Fig. 4 Schematic overview of the transport processes during penetration of a liquid containingpolymeric material into a wood cell capillary6


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The increase in viscosity with increasing solids content is strongly dependent on the physical nature ofthe polymer. Dispersions show an almost infinite increase in viscosity at solids contents between 40-60 % depending on the nature and the particle size distribution of the dispersion. Emulsions willremain lower in viscosity until phase transition from an oil in water to a water in oil emulsion takesplace that corresponds with a very sharp increase in viscosity. True solutions of polymers in eithersolvent or water retain a low viscosity even at high solids content. In some cases the viscosity mighteven drop with increasing solids content. A comparison of various types of binders is given in figures

5a and 5b.

0

3

6

9

12

15

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65

mass fraction binder

acrylic

dispersion

alkyd

emulsion

solventborne

alkydRelativeviscosity

log(

/o)

Fig. 5aRelative viscosity at a shear rate of 0.01 s-1 of an acrylicdispersion, alkyd emulsion and a solventborne alkyd binder as a

function of binder content. (Source: M. de Meijer,

Adhesion aspects of polymeric coatings 2003)

Fig. 5b Viscosity solids content of a watersoluble modified linseed oil.

(Source: Worle Technical datasheet)

If it comes to capillary penetration of a coating into wood the most important factor seems theviscosity increase at higher solids content. The rheological behaviour of coatings at increasing solidscontent or during drying is not very well understood in general and only limited work has beenpublished about it7. This is not only important for substrate penetration but also for properties likeflow, levelling and open time which are still issues that require improvement in waterborne decorativecoatings.

1.4 Relevance of penetration to performance

Apart from the mechanism of penetration of a coating into wood, its usefulness to the overallperformance should be taken into account. The following relevant aspects are here discussed in brief:

Carrier of functional additives like biocides against blue-stain or decay fungi. To be effective theseproducts need to penetrate in the wood and hence a penetrating coating is required. It is for thisreason that blue-stain primers are often based on low viscosity, deep penetrating oils.

Improvement of adhesion by providing mechanical anchoring, this is discussed in section 3. Improving the exterior durability by applying an impregnating primer. Apart from the blue-stain

and adhesion issues some studies have demonstrated that an impregnating primer reduces crackingand flaking of the topcoat8. This might be explained by reducing stresses between coating andwood due to the presence of an intermediate layer9.

Although this aspect has never been described in literature, esthetical aspects like clarity of grains

(anfeuerung) and pore wetting might also be improved by a certain degree of coatingpenetration.


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2. SURFACE ENERGY DETERMINATIONS ON WOOD

2.1 Existing theories and models for surface energy

Knowledge on the surface energy and surface chemistry of wood might help to understand wetting andadhesion phenomena of coatings on wood. In this section the surface energy of wood is discussed.

Specific adhesion issues are discussed in the next section. Measurement of surface energies for woodhas received ongoing attention in recent decades, following the general theoretical developments in

this field. The earliest research was based on measuring critical surface tensions (c )10, later followed

by measurements of polar (P) and disperse (D

) or non-polar energy components of the surface

energy according to either the geometric or harmonic mean methods11. More recently, the Lifshitz-

van-der-Waals (LW) and (Lewis) acid-base components (AB) were used to measure the surface free

energy12. Here the total surface free energy is the sum of the Lifshitz-van der Waals and the combined

acid (+) and base (-) components. In the definition provided by Lewis, the acidity of a surface is

determined by the possibility to accept electrons or donate protons. The basicity is controlled by theability to donate electrons and accept protons. The acid-base interaction does include hydrogen

bonding.

The methods used to determine surface energies of wood are generally based on static contact anglemeasurements of sessile drops or dynamic contact angle measurements (Wilhelmy plate). It should

also be emphasised that all these methods are based on Youngs equation:

s= sl + l cos (3)

where is the surface tension (in mN m -1 or mJ m-2) of the solid (s), the solid-liquid (sl) and the liquid

(l) interface respectively. In principle Youngs equation assumes that the entire system is at

thermodynamic equilibrium and that the solid surface is chemically homogeneous, flat and not

influenced by chemical interaction or adsorption of the liquid to the surface.

2.2 Review of results obtained on wood

An overview of the literature on surface energy data obtained for various wood species and methods isgiven in table 1. The critical surface tension of most wood species lies within a relatively narrowrange of 40 to 55 mJ m-2, although the wood species vary in chemical composition and the differentresearchers used various sets of test liquids. The total surface free energy based on polar anddispersive components shows a larger variation and is generally higher than the critical surface

tensions. The magnitude of the polar and dispersive components is highly variable. None of thecomponents seem to be consistently dominant. Even for one specific wood species, the values arehighly variable. For example, the polar surface energy of beech ranges between 19.6 and 53.1 mJ m-2

and the dispersive component ranges between 6.9 and 32.1 mJ m-2. With the Lifshitz-van der Waalsapproach, the total surface free energy is much lower, generally below or similar to the critical surfacetension. The surface free energy is primarily composed of the Lifshitz-van der Waals component, butmost wood species also show a significant base parameter with only a very low acidic parameter.

Apart from differences in calculation methods, a large part of the variation between differentobservations might be explained by the complex nature of the wood surface with respect to contactangle measurements. Firstly, wood is porous which causes a continuous decrease in contact angle withsessile drop measurements due to capillary penetration into the wood structure. Secondly the woodstructure causes surface roughness. As a consequence liquid spreading is more pronouncedperpendicular then parallel to the orientation of the wood cells and the roughness of the surface wouldaffect the measured contact angle data. Differences in spreading between the smoother latewood areas

and the more rough and porous earlywood areas were also observed by various authors.


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Table 1 Overview of literature data for the surface free energy of wood (mJ m-2).Wood

Species

Type of

measurement c P D S 1 LW + - AB S 2 Ref.

Ash Wilhelmy plate 42.9 85.16 2.68 87.8 42.6 0.00 67.35 0.60 43.2 [d]

Ash Wilhelmy plate 60.15 13.87 74.0 [d]

Aspen Wilhelmy plate 13.2 41.8 55 45.0 0.02 12.64 0.91 45.9 [g]7

Beech sessile drop 19.18 31.88 50.0 [f]

Beech sessile drop3 45.53 24.48 68.8 [f]

Beech sessile drop 50.6 53.1 6.9 60 [a]

Cherry Wilhelmy plate 48.1 38.1 16.19 54.3 47.5 0.42 28.00 6.84 54.3 [d]

Cherry Wilhelmy plate 35.1 20.09 55.2 [d]

Douglas fir Wilhelmy plate 11.8 36.2 48 38.7 2.86 3.29 6.13 44.8 [g]7

Douglas fir sessile drop 52.8 19.2 28.8 48 [c]

Douglas fir sessile drop 11.5 37.5 49 [b]

Maple Wilhelmy plate 46.8 56.07 8.77 64.8 45.5 0.46 33.19 7.85 53.3 [d]

Maple Wilhelmy plate 40.93 20.13 61.1 [d]

Maple Wilhelmy plate 42 16.4 40.2 56.6 43.2 0.71 13.29 6.15 49.4 [g]7

Pine4 sessile drop 40.7 1.73 8.41 7.63 48.3 [e]

Pine4 Wilhelmy plate 38.9 0.05 17.33 1.86 40.8 [e]

Pine5 sessile drop 50.9 83.4 0.4 83.8 [a]

Pine6 sessile drop 54.3 68.1 3 71.1 [a]

Poplar sessile drop 53.1 28.5 25.2 53.7 [a]

Red Maple sessile drop 72.7 3.9 76.6 45.5 0.02 57.01 2.14 47.7 [h]

Red oak Wilhelmy plate 46.8 42.2 10.4 52.6 39.7 0.46 37.74 8.30 48.0 [d]Red oak Wilhelmy plate 35.04 16.87 51.9 [d]

Redwood sessile drop 57 31.5 22.7 54.2 [b]

Spruce Wilhelmy plate 45 16.5 45 61.5 49.4 0.81 11.35 6.06 55.5 [g]7

Spruce5 sessile drop 51.8 71.6 2 73.6 [a]

Spruce6 sessile drop 53.2 41.9 13.9 55.8 [a]

Walnut Wilhelmy plate 10.8 86.14 1.28 87.4 37.9 0.09 58.93 4.63 42.6 [d]

White oak Wilhelmy plate 31.4 41.65 5.29 46.9 34.0 0.39 22.80 5.98 40.0 [d]

1S = P + D

2S = LW + AB

3 adjusted to ideal surface4 measured parallel to the grain of the wood5 earlywood areas66 latewood areas7 data calculated from contact angles reported[a] Scheikl, M., Dunky, M. Holzforschung, 1998, 52, 89-94; [b] Nguyen, T., Johns, W.E. Wood Science and

Technology, 1979, 13, 29-40; [c] Nguyen, T., Johns, W.E Wood Science and Technology, 1978, 12, 63-74; [d]Gardner, D.J. Wood and Fiber Science, 1996, 28 (4), 422-428; [e] Shen, Q., Nylund, J., Rosenholm J.B.

Holzforschung, 1998, 52, 521-529; [f] Liptkov, E., Kdela, J. Holzforschung, 1994, 48, 139-144; [g]Mantanis, G.I., Young, R.A. Wood science and Technology, 1997, 31, 339-353 [h]Maldas, D.C., Kamdem, D.P.

Wood and Fiber Science, 1998, 30 (4), 368-373


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Another complicating factor is the chemical heterogeneity of the wood surface. Apart from its majorconstituents cellulose (40-50%), hemicellulose (15-25%)and lignin (20-35%) wood can also contain5-15 % of material consisting of a wide range of terpenoid, fatty acid or polyphenolic substances.These so-called extractives can have a strong negative impact on the wettability of wood surfaces13.

Available data on isolated wood components show that for cellulose LW= 44 mJ m-2, AB= 17.2 mJ m-2,

+

= 1.62 mJ m

-2

,

-

= 17.2 mJ m

-2

and for arabinogalactan (hemicellulose)

LW

= 37.6 mJ m

-2

,

AB

= 12.6mJ m-2, += 0.75 mJ m-2, -= 53.1 mJ m-2. For extracted lignin it is reported that AB= 10-13 mJ m-2 and

D= 45-50 mJ m-2. Because the cell wall components are not distributed evenly within the cell walls, a

spreading liquid will encounter differences in the chemical composition of the surface dependingwhether its on the outside, inside or cross-section of the wooden cell wall. Furthermore wateradsorbed onto the cell wall will always be present in significant amounts; the exact amount will,however, differ depending on the wood species and the relative humidity of the environment. Liquidsused for the contact angle measurements will also be adsorbed onto the wooden surface and might

even diffuse into it. This means that a thin layer of liquid vapour will be present in front of thespreading liquid.

2.3 Wetting by coatings

The wetting of a wood surface of by a coating can also be measured directly by measuring contact-

angle of a sessile drop of coating on a wooden surface.In order to wet the surface the surface energy

of the coating should be lower than that of the wood (coating < wood).Since most wood surface have asurface energy between 40 and 50 mJ m -2 and most coatings have a surface energy between 30 and 40mJ m-2 this is generally no a problem. Apart from the surface energy, the spreading of a coatingdroplet might also be restricted by the viscosity. The shape and contact angle of the spreading contactangle is influenced by capillary penetration under or at the front of the droplet (see fig. 6). The contact

angle of a coating decrease rapidly initially reaching an equilibrium after approximately (see fig. 7). Ingeneral there is a good correlation between contact angle and degree of penetration of the coating into

the wood.

EARLYWOOD

LATEWOOD

CAPILLARY

PENETRATION

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Time, s

contactangle(degress)

ac1/EW ac2/EW

ac3/EW hsa/EW

sba/EW wba/EW

ac1/LW ac2/LW

ac3/LW hsa/LW

sba/LW wba/LW

Fig. 6 Microscopic image (SEM) showing influence of

Wood structure on the spreading of the coatingdroplet. Insert at higher magnification shows

substrate penetration in the spreading liquid front.

Fig. 7 Contact angles of coatings on early- (EW) and

latewood (LW) areas of tangential surface ofpine sapwood. (Source: M. de Meijer,

Adhesion aspects of polymeric coatings 2003)


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3. ADHESION OF COATINGS TO WOOD

3.1 Practical versus theoretical adhesion (adherence / adhesion)

Understanding, measuring and predicting the adhesion of coatings on wood is rather complex due to

the fact that various mechanisms are involved. Most important topics to take into consideration are:

Impact of the measurement technique itself

Reduction of the measured adhesion by energy stored in the coating because of internal stress.

Work expended in deformation during peeling or torsion of the coating during measurement.

Impact of mechanical anchoring an adhesion.

Influence of moisture in coating or wood.

Molecular forces between coating and wood that determine the interfacial adhesion.

Sometimes the overall or practical adhesion is referred to as adherence, whereas the term adhesion isreserved for the interfacial forces between the two materials.

3.2 Analytical tools

Frequently used techniques to measure the adhesion of a coating on wood are: the axial pull-off testwith a dolly glued on the coating (ISO 4624)14, shear measurements in torque mode15 and the semi-

quantitative x-cut or cross hedge (ISO 2409 or ASTM D3359). The first two methods are oftendifficult to interpret because cohesional failure can occur in the coating, glue or wood. These are often

combined within one fractured surface. Furthermore, the measured force is influenced by the stress-

distribution under the dolly and the method of cutting the coating around the dolly. The third methodsuffers from reduced reproducibility due to manual influences during the application and removal ofthe tape and the visual assessment of the removed coating area. Adhesion measurements by peeling ofcoatings with pressure-sensitive tape in a tensile testing machine are successfully applied on metals,glass and wood16.

3.3 Influence of mechanical anchoring

Some workers have suggested that there is no relation between adhesion and penetration. However,there are many studies on both adhesion of glues and coatings, in which differences in adhesion

between early- and latewood areas correspond to varying degrees of substrate penetration17. Normallythe adhesion strength is higher in the earlywood, which corresponds with its deeper penetration.

Adhesion is only higher in the less penetrated latewood cells, if the wood is preweathered beforeapplication of the coating. This can be explained by the fact the unprotected earlywood degraded fasterduring weathering which lead to a weaker bond strength. A very clear example of the importance ofpenetration / mechanical anchoring is given in fig. 8a and 8b. In a peel test the work increases atpenetrated earlywood and decreases on latewood.

Also microscopic analysis of the fractured surfaces after a peel or dolly pull-off adhesion show theimportance of mechanical anchoring. Two examples of these are given in fig. 9a and 9b showing that

both the penetrated part of the coating can break cohesively or can be pulled out of the wood.


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0

25

50

75

100

125

150

175

200

0 10 20 30 40 50

peeleddistance, mm

peelforce

,

N/mm

wood structure under

coatingwhich is peeled

valley corresponding

to latewood bands

peak

corresponding

to earlywood

bands

0

50

100

150

200

250

300

350

acr yl ic 1 acr yl ic 2 acr yl ic 3 a lky d-emu ls ion h igh s ol id a lk yd s olvent a lky d

coating type

adhesion

strengthJ/m2

earlywood (higher penetration)

latewood (lower penetration)

Fig. 8a Adhesion as a function of peeled distance on pine

sapwood with early- and latewood.

Fig. 8b Peel adhesion strength of various coatings on

pine sapwood after exposure to liquid water.

torn out coatingmaterial

Fig 9a SEM image of a pigmented alkyd emulsion

Paint peeled from wood in a wet adhesion test

Fig. 9b SEM image of wood with part of a high solid

alkyd paint that has failed cohesively

3.4 Influence of moisture

It is well known from both practical experiences at scientific research that adhesion of coatings onwood is much weaker under moist conditions and on wood with a high moisture content (this will befurther referred to as wet adhesion). The difference between wet and dry adhesion is most pronouncedwith paints based on acrylic dispersions, but also waterborne alkyd paints have a lower wet adhesionthan solventborne alkyds (see fig. 8b). Although the reasons for the weaker wet adhesion is not fully

understood some factors can be identified as responsible for lowering the adhesion under wetconditions.

An important factor is the uptake of moisture in the coating, the swelling of the coating as aconsequence of this and the following build-up of hygroscopic stress. The relations between stress andadhesion and the level of hygroscopic stress are given by the following equations:

( )

1c . E .

2

woodcoating= (4) W

p = WaCW + Wd - (5)

With the elastic energy () due to stored hygroscopic strain, thickness (c) ,elasticity (E) and Poisson

ratio () of the coating and hygroscopic expansion (swelling) of coating or wood for a given changein environmental conditions. The measured peel work of adhesion (Wp) is a function of: interfacial

coating


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work of adhesion (WaCW), work expanded in plastic deformation during peeling (Wd) and elastic

energy stored in the coating because of strain (). This means that is the swelling of the coating is

much higher than that of wood, the stress will be high and might exceed the interfacial adhesion. Insome studies16 this has been clearly demonstrated experimentally. It should also be noted that in suchcases a high film thickness and a high elastic modulus of the coating will reduce that adhesion. Thedifference in adhesion after exposure to vapour and liquid water, as is shown in figure 10 can also beexplained by the differences in swelling between exposure to water (which can be very high, see fig.

11) and water vapour were swelling is much lower.

Ac1

liquidAc2

liquidAc3

liquidAc1

vapourAc2

vapourAc3

vapour

latewood

earlywood

76 71

78

473

232

195

53 58 55

298

140116

0

50

100

150

200

250

300

350

400

450

500

adhesion

strength

J/m2

11

46

2.6

2.7

2.7

7

78

0 20 40 60 80 100

Ac1

Ac2

Ac3

WBA

HSA

SBA

pine sapwood

volumetric swelling %

Fig. 10 Differences in peel adhesion strength of 3 acrylic paintson pine sapwood exposed to liquid water and vapour (98 % RH)

Fig. 11 Volumetric swelling of differentsolvent (SBA, HSA) and waterborne(WBA, Ac1-3) coatings after immersion inwater.

In addition to the adhesion reduced by internal stress there might be other factor leading to a lower wet

adhesion. The weak boundary layer theory explains the loss of adhesion as a failure in an intermediatemolecular layer between adhesive and adherent. This molecular layer consists of low molecular weightimpurities of various origins, including water. This theory has never been verified for wood, but it isknown that low molecular weight extractives can easily migrate to the surface and might reduce

adhesion. Also lower molecular weight fractions in the coating (e.g. surfactants, thickeners orcoalescing agents) can influence wet adhesion because they might cause a weak boundary layer18.Another reason for a decrease in adhesion can come from depletion at the polymer (coating) surfaceinterface since a random coil of a polymer is repelled, entropically from an impenetrable surface. Thedepletion effect has to be overcome by adsorption of the polymer to the surface19.

3.5 Calculated measured adhesion

From the surface free energies of both coating (c)and wood (w), the work of adhesion (Wacw)between the two phases can be calculated according to the following equation [73] using Lifshitz-vander Waals-acid-base parameters:

Wacw = c + w - cw (6)

++++=wcwc

LW

w

LW

c

a

cw2W (7)

The interfacial work of adhesion after exposure to water (Wawet) can be obtained from the interfacial

energies between coating-water (CL), wood - water (WL) and coating wood (CW) as follows:

W

a

wet = CL + WL - CW (8)


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Table 2 shows results for various types of coatings with the interfacial work of adhesion betweencoating and wood calculated according to this equation. The measured differences in interfacial energydid not reflect the adhesion differences between the coatings.

Table 2 Measured work of peel adhesion (WP) and calculated interfacial work of adhesion

(Wa

) under wet and dry conditions, all expressed in J m-2

Coating Wp

EW LWWaCW dry W

a LW-ABwet

Ac1 152 107 0.087 0.020

Ac2 142 115 0.096 0.010

Ac3 156 110 0.094 0.050

WBA 238 126 0.098 0.019

HSA 682 200 0.093 0.014

SBA 580 255 0.093 0.030

EW: earlywoodLW: latewood

CW: coating-woodLW-AB: Lifshitz-van der Waal acid-base

3.6 Adhesion promoting technologies

Several attempts can be made to improve the adhesion of coatings on wood but most important seemsto improve the adhesion under wet conditions. The following approaches are described below:

Pretreatment of the wood by flame-ionisation or plasma- treatment. These techniques are aiming

to increase the surface energy of the wood and to change the ratio between polar and dispersivecomponents. The improvements in adhesion with such techniques are limited which seems logicalkeeping in mind that substrate wetting is generally not the limiting factor in getting good adhesion.And even if the wetting of the wood by the coating is incomplete this is more likely to be due toviscosity effects.

Incorporation of adhesion promoting monomers in acrylic dispersions. The monomers used are

usually based on (meth)acrylates, maleates, alkyl or vinyl ester compounds which carry amino,acetoacetate, cyanoacetae, urea, thiourea or cyclic urea groups. The working principle of these

monomers on wood is not described in literature but for adhesion on old alkyd paints (with similarpoor adhesion) it might work by the virtue of formation of hydrogen bounds or acid-baseinteractions.20

Reducing the wateruptake and / or swelling of the coating by crosslinking of the polymer or

reducing the hydrophilicity.

Chemical crosslinking between coating and wood. In principle various types of reactive groups in

two component coatings like isocyanate or expoxides could also react with the hydroxylgroups of

the wood. So far no commercial products based on this principle are available but some areclaiming formulations based on this principle21.


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4. WOOD SURFACE PREPARATION

The wood surface preparation prior to application of a coating has usually received little attention butmight have an important impact on the performance of a coating. The various types of surfacepreparation studied are: planing, sanding and rough sawn surfaces. Sanding reduces or even completeprevents the penetration of the coating due to cell deformation and clogging of capillaries with dust.

Rough sawn surface generally show a higher uptake of paint material and an improved performancebecause of that22.

Following several damage complaints about early cracking of solventborne paints on softwood studieshas been done on the influence of planing conditions on durability of wood coatings. It was shown thatsharp planing knifes are essential to prevent compression of wood cells during planing 23. An exampleof compressed cells is shown in fig. 12. If the compressed cells are coated with a solventborne paintthe cells remain initially compressed but expand during weathering. Because of the extreme expansiontaking place than, most coatings will crack. With waterborne paints the cells will expand duringapplication of the paint. This will lead to grain raising and an uneven surface but cracking during

service will be prevented. A comparison of grain raising with water- and solventborne paints is shownin fig. 13.

Deformed cell layer

Fig. 12 Compressed spruce wood due to poor planing

b ronde kant + celdeformatie

oplosmiddelhoudende

alkyd spuit verf

Figuur 9a ronde kant + celdeformatie

onafgewerkt

c ronde kant + celdeformatie

watergedragen

alkyd dompel verf

Fig. 13 Response of compressed wood to water and paints

Source fig. 12 & 13: SHR report 1.157 Wood Machining and cell deformation (in dutch), 2002.

Exposedto water

Coated with

solventborne

paint

Coated with

waterborne

alkyd paint


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5. CONCLUSION AND DIRECTIONS FOR FUTURE RESEARCH

The current state of the art in the field of penetration, adhesion and surface chemistry of wood coatingshas been described in the previous sections. The following main conclusions can be made:

1. A combination of the anatomical wood structure and the ability of the coating to flow into the

wood capillaries govern the degree of coating penetration. Differences in penetration capacity ofcoatings are mainly determined by the increase in viscosity with solid content due to selective

uptake of water or solvent in the cell wall. Wetting and surface tension of the coating seem to playa minor role and insufficient wetting is often due to a limitation by viscosity.

2. Surface energy determinations in terms of polar dispersive parts or lifshitz vander waals acidbase components has been made for many wood species but are of hardly any use inunderstanding the adhesion of coatings. In general the surface energy of wood is equal or higherthan the surface energy of a liquid coating which means that wetting is not a limiting factor.

3. Penetration of coatings into the outer pores of wood certainly contributes to improving theadhesion of a coating, especially under wet conditions. A very deep penetration will not directlycontribute to adhesion but might reduce the differences in dimensional change between coatingand wood and reduce stress in the coating.

4. The adhesion of a coating to wood is particularly critical under wet conditions. Waterbornecoatings (both acrylic and alkyd based) have a lower wet adhesion than solventborne ones. Onereason might be the higher swelling by moisture but other unknown factors seem to play a role

too.

5. The surface preparation can have a major impact on the coating performance if wood cells arestrongly compressed during planing. The subsequent expansion of the cells can lead to high grain

raising or premature cracking of the coating.

From the current state of the art the following gaps in knowledge can be identified:

The rheology of coatings at increasing solid content or during drying is hardly known but is

essential to understand differences in penetrating capacity. However, a better knowledge in thisfield will also contribute in understanding differences between waterborne and solventbornecoatings with respect to flow, levelling and open time.

Impact of a penetrating primer on the weathering performance. Some work suggests a clear

improvement but the importance and reasons behind it are not known.

Reduction of coating adhesion under wet conditions. Although major improvements have been

made by adjusting binder properties, the wet adhesion of waterborne coatings is still not at thelevel of solventborne ones. These differences seem to come from differences in penetration andcoating swelling alone. Existing surface energy concepts can not explain observed differences.Improved knowledge in this field is required to understand why adhesion is sometimesinsufficient.


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