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 40
years of wood coatings research. Research work has covered various types of coatings corresponding
to the state-of-the-art in wood coating technology. It started with studying solventborne alkyds and
drying oils1, followed by a reviewed interest with the introduction of waterborne coatings2. Especially
the introduction of waterborne alkyds have been studied because these types of binders seems to have
a 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
for exterior applications. For interior (furniture or parquet) coatings hardly any work has been
published although 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 most
popular 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 penetrated
with the paint are filled with and additional dye. More recently also confocal laser microscopy has
been 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 a
fluorescent dye might change the properties of the binder or paint to some extent. Since fluorescence
microscopy is limited to magnifications of about 200 x for higher magnifications SEM is the most
widely used techniques although a good contrast between coating and wood is not very easily to
achieve. 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 measuring
the depth in penetration in axial direction or by image analysis. Some typical examples of fluoresence
and SEM images of wood with coating are given below.

Drywood Coatings P.O.Box 3954 7500 DZ Enschede
info@drywood.nl
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
coating
extractive
wood
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 of
a liquid coating in situ. Also the possible penetration of certain components of the coating into the cell
wall n not be assessed. In this respect some more advanced tools might be used in future. Possibilities
might be: magnetic resonance imaging (MRI) microscopy with sufficient resolution to study flow
patterns at a cellular level; environmental scanning microscopy (ESEM) and TOF-SIMS imaging to
study changes in chemical composition during coating penetration. Another approach the study the
dynamical uptake of coating material into the wood is to measure the decrease in volume of coating
droplets 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 be
distinguished as is schematically shown in fig 3. Firstly the outer longitudinal tracheids are filled
directly by coating flowing from the open ends on the surface. This predominantly occurs in the
earlywood. The angle between length axis of the tracheid and the surface has a strong influence on the
importance 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 ray’s proceeds is strongly dependent on
the wood species. In pine the major part of the coating flows through the parenchyma cells, transport
from cell to cell must therefore be possible. In spruce the coating almost solely penetrates the ray
tracheids. A third way of penetration is from ray cells to adjacent longitudinal tracheids in the
latewood. The extent of transport from rays to tracheids is strongly dependent on permeability of the
cross-field pits and almost totally limited to pine sapwood. The importance of the three penetration
mechanisms mentioned above implicates that penetration of the coating can strongly be influenced by
the 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 the
number of rays ending in radial and tangential surfaces. The origin of the wood might influence
penetration because of differences in early- and latewood portions, conditions of the pits, number of
rays and length of longitudinal tracheids. Drying conditions of the wood might also have some
influence on coating penetration because of its impact on pit aspiration.
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
1.
flow into open end of longitudinal
tracheid
2.
flow into ray tracheid
3.
flow into ray parenchyma
4.
flow from ray parenchyma into
longitudinal latewood tracheid
5.
Fig. 3
flow from ray tracheid into
longitudinal tracheid
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 the
first cells of rays and very occasionally axial parenchym and sklerenchym. In more permeable
hardwood 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 tyloses
are present. Extractives appeared to have none or only a very minor influence on the penetration in all
tree wood species studied. Surface preparation can have some influence on coating penetration
because 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 the
depth 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, waterbased
or solvent free. Unpigmented alkyd resins with organic solvent (mostly white spirit) also show a deep
penetration. 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 the
formulations, especially at higher loadings in opaque paints, the penetration of all types of paints is
strongly reduced but the rank-order remains the same. It should be noted that the pigments itself are
still small enough to flow through the pores, only in cross-fields some clogging might occur.
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
To understand these differences the underlying mechanism of the capillary flow process should be
considered. The following two cases should be considered:
L=
2 cos   L
r Lg
(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. Equation
1 predicts a deeper penetration in smaller capillaries. For the very deep penetrating oil based and
unpigmented 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. This
behaviour is predicted by the following equation (known as the Washburn equation):
L=
 L cos  r t
2
(2)
With the time (t) and liquid viscosity (Please note that equation 1 describes an equilibrium
situation 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 the
contact angle between liquid and capillary wall, diameter of the capillary and the reciprocal liquid
viscosity. It should also be noted that according to equation 2, lowering the surface tension if wetting
is complete (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 the
wood 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 the
capillary. The above mentioned process is visualised schematically in fig.4.The selective removal of
solvent or water during the penetration process will increase the polymer fraction in the liquid and
hence the viscosity of the solution6.
WOODEN
INCREASING
FRACTION POLYMER
CELL WALL
SELECTIVE
REMOVAL OF
WATER OR
SOLVENT
CAPILLARY
FLOW
OF BINDER
Fig. 4 Schematic overview of the transport processes during penetration of a liquid containing
polymeric material into a wood cell capillary6
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
The increase in viscosity with increasing solids content is strongly dependent on the physical nature of
the polymer. Dispersions show an almost infinite increase in viscosity at solids contents between 4060 % depending on the nature and the particle size distribution of the dispersion. Emulsions will
remain lower in viscosity until phase transition from an oil in water to a water in oil emulsion takes
place that corresponds with a very sharp increase in viscosity. True solutions of polymers in either
solvent or water retain a low viscosity even at high solids content. In some cases the viscosity might
even drop with increasing solids content. A comparison of various types of binders is given in figures
5a and 5b.
Fig. 5a Relative viscosity at a shear rate of 0.01 s-1 of an acrylic
dispersion, 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 water
soluble modified linseed oil.
(Source: Worlée Technical datasheet)
If it comes to capillary penetration of a coating into wood the most important factor seems the
viscosity increase at higher solids content. The rheological behaviour of coatings at increasing solids
content or during drying is not very well understood in general and only limited work has been
published about it7. This is not only important for substrate penetration but also for properties like
flow, levelling and open time which are still issues that require improvement in waterborne decorative
coatings.
1.4
Relevance of penetration to performance
Apart from the mechanism of penetration of a coating into wood, its usefulness to the overall
performance 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 these
products need to penetrate in the wood and hence a penetrating coating is required. It is for this
reason 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 cracking
and flaking of the topcoat8. This might be explained by reducing stresses between coating and
wood 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 coating
penetration.
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
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 and
adhesion 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 wood
has 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 Lifshitzvan-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 the
ability 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 angle
measurements of sessile drops or dynamic contact angle measurements (Wilhelmy plate). It should
also be emphasised that all these methods are based on Young’s 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 Young’s 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 is
given in table 1. The critical surface tension of most wood species lies within a relatively narrow
range of 40 to 55 mJ m-2, although the wood species vary in chemical composition and the different
researchers used various sets of test liquids. The total surface free energy based on polar and
dispersive 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 the
components seem to be consistently dominant. Even for one specific wood species, the values are
highly 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 Waals
approach, the total surface free energy is much lower, generally below or similar to the critical surface
tension. The surface free energy is primarily composed of the Lifshitz-van der Waals component, but
most 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 different
observations might be explained by the complex nature of the wood surface with respect to contact
angle measurements. Firstly, wood is porous which causes a continuous decrease in contact angle with
sessile drop measurements due to capillary penetration into the wood structure. Secondly the wood
structure causes surface roughness. As a consequence liquid spreading is more pronounced
perpendicular then parallel to the orientation of the wood cells and the roughness of the surface would
affect the measured contact angle data. Differences in spreading between the smoother latewood area’s
and the more rough and porous earlywood areas were also observed by various authors.
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
Table 1
Overview of literature data for the surface free energy of wood (mJ m-2).
Wood
Species
Ash
Type of
measurement
Wilhelmy plate
Ash
Wilhelmy plate
Aspen
Wilhelmy plate
Beech
sessile drop
c
P
42.9 85.16
S 1
2.68
87.8
60.15 13.87
74.0
13.2
3
D
41.8
55
19.18 31.88
50.0
45.53 24.48
68.8
Beech
sessile drop
Beech
sessile drop
50.6
53.1
6.9
60
Cherry
Wilhelmy plate
48.1
38.1 16.19
54.3
Cherry
Wilhelmy plate
35.1 20.09
55.2
Douglas fir
Wilhelmy plate
11.8
36.2
48
Douglas fir
sessile drop
19.2
28.8
48
Douglas fir
sessile drop
11.5
37.5
49
Maple
Wilhelmy plate
46.8 56.07
8.77
64.8
Maple
Wilhelmy plate
40.93 20.13
61.1
Maple
Wilhelmy plate
52.8
42
16.4
40.2
56.6
LW +
-
AB
S 2
42.6
0.00 67.35
0.60
43.2
45.0
0.02 12.64
0.91
45.9
47.5
0.42 28.00
6.84
54.3
38.7
2.86
3.29
6.13
44.8
45.5
0.46 33.19
7.85
53.3
43.2
0.71 13.29
6.15
49.4
Pine
4
sessile drop
40.7
1.73
8.41
7.63
48.3
Pine
4
Wilhelmy plate
38.9
0.05 17.33
1.86
40.8
Pine
5
sessile drop
50.9
83.4
0.4
83.8
6
Pine
Poplar
sessile drop
54.3
68.1
3
71.1
sessile drop
53.1
28.5
25.2
53.7
Red Maple
sessile drop
72.7
3.9
76.6
45.5
0.02 57.01
2.14
47.7
Red oak
Wilhelmy plate
42.2
10.4
52.6
39.7
0.46 37.74
8.30
48.0
Red oak
Wilhelmy plate
35.04 16.87
51.9
Redwood
sessile drop
57
31.5
22.7
54.2
Spruce
Wilhelmy plate
45
16.5
45
61.5
49.4
0.81 11.35
6.06
55.5
37.9
34.0
0.09 58.93
0.39 22.80
4.63
5.98
42.6
40.0
Spruce
46.8
5
sessile drop
51.8
71.6
2
73.6
6
sessile drop
53.2
41.9
13.9
55.8
Wilhelmy plate
Wilhelmy plate
10.8 86.14
31.4 41.65
1.28
5.29
87.4
46.9
Spruce
Walnut
White oak
Ref.
[d]
[d]
[g]7
[f]
[f]
[a]
[d]
[d]
[g]7
[c]
[b]
[d]
[d]
[g]7
[e]
[e]
[a]
[a]
[a]
[h]
[d]
[d]
[b]
[g]7
[a]
[a]
[d]
[d]
S = P + D
S = LW + AB
3
adjusted to ideal surface
4
measured parallel to the grain of the wood
5
earlywood area’s
6
latewood area’s
7
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] Liptáková, E., Kúdela, 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
1
2
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
Another complicating factor is the chemical heterogeneity of the wood surface. Apart from its major
constituent’s cellulose (40-50%), hemicellulose (15-25%) and lignin (20-35%) wood can also contain
5-15 % 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 m2 +
,  = 1.62 mJ m-2, -= 17.2 mJ m-2 and for arabinogalactan (hemicellulose) LW= 37.6 mJ m-2, AB=
12.6 mJ m-2, += 0.75 mJ m-2, -= 53.1 mJ m-2. For extracted lignin it is reported that AB= 10-13 mJ m2
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
depending whether its on the outside, inside or cross-section of the wooden cell wall. Furthermore
water adsorbed 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. Liquids
used 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 the
spreading liquid.
2.3
Wetting by coatings
The wetting of a wood surface of by a coating can also be measured directly by measuring contactangle 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 a
surface energy between 40 and 50 mJ m-2 and most coatings have a surface energy between 30 and 40
mJ m-2 this is generally no a problem. Apart from the surface energy, the spreading of a coating
droplet might also be restricted by the viscosity. The shape and contact angle of the spreading contact
angle 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). In
general there is a good correlation between contact angle and degree of penetration of the coating into
the wood.
CAPILLARY
PENETRATION
contactangle (degress)
140
ac1/EW
ac3/EW
sba/EW
ac1/LW
ac3/LW
sba/LW
120
100
ac2/EW
hsa/EW
wba/EW
ac2/LW
hsa/LW
wba/LW
80
60
40
20
LATEWOOD
0
EARLYWOOD
0
50
100
150
200
250
300
Time, s
Fig. 6 Microscopic image (SEM) showing influence of Fig. 7 Contact angles of coatings on early- (EW) and
Wood structure on the spreading of the coating
latewood (LW) areas of tangential surface of
droplet. Insert at higher magnification shows
pine sapwood. (Source: M. de Meijer,
substrate penetration in the spreading liquid front.
Adhesion aspects of polymeric coatings 2003)
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
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 is
reserved 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 test
with a dolly glued on the coating (ISO 4624)14, shear measurements in torque mode15 and the semiquantitative x-cut or cross hedge (ISO 2409 or ASTM D3359). The first two methods are often
difficult 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 stressdistribution under the dolly and the method of cutting the coating around the dolly. The third method
suffers from reduced reproducibility due to manual influences during the application and removal of
the tape and the visual assessment of the removed coating area. Adhesion measurements by peeling of
coatings 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. Normally
the 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 before
application of the coating. This can be explained by the fact the unprotected earlywood degraded faster
during weathering which lead to a weaker bond strength. A very clear example of the importance of
penetration / mechanical anchoring is given in fig. 8a and 8b. In a peel test the work increases at
penetrated earlywood and decreases on latewood.
Also microscopic analysis of the fractured surfaces after a peel or dolly pull-off adhesion show the
importance 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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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
200
350
peak
corresponding
175
peel force, N/mm
adhesion strength J/m 2
to earlywood
bands
150
125
valley corresponding
to latewood bands
100
75
50
300
earlywood (higher penetration)
250
latewood (lower penetration)
200
150
100
wood structure under
coating which is peeled
25
50
0
0
10
20
30
40
50
0
acrylic1
peeled distance, mm
acrylic2
acrylic3
alkyd-emulsion
high solid alkyd
solvent alkyd
coating type
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.
to r n o ut c o ati ng
m ate r ia l
coating
Fig 9a SEM image of a pigmented alkyd emulsion
Paint peeled from wood in a wet adhesion test
3.4
Fig. 9b SEM image of wood with part of a high solid
alkyd paint that has failed cohesively
Influence of moisture
It is well known from both practical experiences at scientific research that adhesion of coatings on
wood is much weaker under moist conditions and on wood with a high moisture content (this will be
further referred to as wet adhesion). The difference between wet and dry adhesion is most pronounced
with paints based on acrylic dispersions, but also waterborne alkyd paints have a lower wet adhesion
than 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 wet
conditions.
An important factor is the uptake of moisture in the coating, the swelling of the coating as a
consequence of this and the following build-up of hygroscopic stress. The relations between stress and
adhesion and the level of hygroscopic stress are given by the following equations:
=c . E .

coating
  wood
1 

2
(4)
Wp = WaCW + Wd -  


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 change
in environmental conditions. The measured peel work of adhesion (Wp) is a function of: interfacial
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COST E18 – Final Seminar A review of interfacial aspects in wood coatings
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. In
some studies16 this has been clearly demonstrated experimentally. It should also be noted that in such
cases a high film thickness and a high elastic modulus of the coating will reduce that adhesion. The
difference in adhesion after exposure to vapour and liquid water, as is shown in figure 10 can also be
explained 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.
473
500
pine sapw ood
7
SBA
2.7
HSA
2.7
WBA
2.6
450
400
adhesion
strength
J/m2
350
300
232
195
250
46
Ac3
200
78
150
76
71
298
Ac2
78
100
140
50
53
58
earlywood
latewood
0
Ac1
liquid
Ac2
liquid
Ac3
liquid
Ac1
116
55
Ac1
vapour
Ac2
vapour
0
Ac3
vapour
Fig. 10 Differences in peel adhesion strength of 3 acrylic paints
on pine sapwood exposed to liquid water and vapour (98 % RH)
11
20
40
60
80
100
volumetric swelling %
Fig. 11 Volumetric swelling of different
solvent (SBA, HSA) and waterborne
(WBA, Ac1-3) coatings after immersion in
water.
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 intermediate
molecular layer between adhesive and adherent. This molecular layer consists of low molecular weight
impurities of various origins, including water. This theory has never been verified for wood, but it is
known 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 or
coalescing 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) – surface
interface since a random coil of a polymer is repelled, entropically from an impenetrable surface. The
depletion 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-van
der Waals-acid-base parameters:
Wacw =  c + w - cw
(6)

a
Wcw
 2  cLW  LW
  c  w   c  w
w

(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:
Wawet = CL + WL - CW
Page 11 of 16
(8)
COST E18 – Final Seminar A review of interfacial aspects in wood coatings
Table 2 shows results for various types of coatings with the interfacial work of adhesion between
coating and wood calculated according to this equation. The measured differences in interfacial energy
did 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
Wp
Coating
EW
LW
WaCW
Wa LW-ABwet
dry
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: earlywood
LW: latewood
CW: coating-wood
LW-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 seems
to 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 dispersive
components. The improvements in adhesion with such techniques are limited which seems logical
keeping 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 to
viscosity 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 similar
poor adhesion) it might work by the virtue of formation of hydrogen bounds or acid-base
interactions.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 are
claiming formulations based on this principle21.
Page 12 of 16
COST E18 – Final Seminar A review of interfacial aspects in wood coatings
4.
WOOD SURFACE PREPARATION
The wood surface preparation prior to application of a coating has usually received little attention but
might have an important impact on the performance of a coating. The various types of surface
preparation studied are: planing, sanding and rough sawn surfaces. Sanding reduces or even complete
prevents 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 performance
because of that22.
Following several damage complaints about early cracking of solventborne paints on softwood studies
has been done on the influence of planing conditions on durability of wood coatings. It was shown that
sharp planing knifes are essential to prevent compression of wood cells during planing23. An example
of compressed cells is shown in fig. 12. If the compressed cells are coated with a solventborne paint
the cells remain initially compressed but expand during weathering. Because of the extreme expansion
taking place than, most coatings will crack. With waterborne paints the cells will expand during
application 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 shown
in fig. 13.
Deformed cell layer
Fig. 12 Compressed spruce wood due to poor planing
Exposed
to water
Coated with
solventborne
paint
Coated with
waterborne
alkyd paint
Fig. 13
Response
of compressed
to water andc ronde
paints
kant
Figuur
9
b ronde kantwood
+ celdeformatie
a ronde kant + celdeformatie
onafgewerkt
oplosmiddelhoudende
alkyd spuit verf
+ celdeformatie
watergedragen
alkyd dompel verf
Source fig. 12 & 13: SHR report 1.157 Wood Machining and cell deformation (in dutch), 2002.
Page 13 of 16
COST E18 – Final Seminar A review of interfacial aspects in wood coatings
5.
CONCLUSION AND DIRECTIONS FOR FUTURE RESEARCH
The current state of the art in the field of penetration, adhesion and surface chemistry of wood coatings
has 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 of
coatings 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 play
a 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 – acid
base components has been made for many wood species but are of hardly any use in understanding
the adhesion of coatings. In general the surface energy of wood is equal or higher than 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 the
adhesion of a coating, especially under wet conditions. A very deep penetration will not directly
contribute to adhesion but might reduce the differences in dimensional change between coating
and wood and reduce stress in the coating.
4. The adhesion of a coating to wood is particularly critical under wet conditions. Waterborne
coatings (both acrylic and alkyd based) have a lower wet adhesion than solventborne ones. One
reason 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 are
strongly 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 this
field will also contribute in understanding differences between waterborne and solventborne
coatings 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 the
level of solventborne ones. These differences seem to come from differences in penetration and
coating swelling alone. Existing surface energy concepts can not explain observed differences.
Improved knowledge in this field is required to understand why adhesion is sometimes
insufficient.
Page 14 of 16
COST E18 – Final Seminar A review of interfacial aspects in wood coatings
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