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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 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


                                                Page 1 of 16
                 COST E18 – Final Seminar A review of interfacial aspects in wood coatings




                                                                         coating




                                                                      extractive

                                                                                       wood




Fig. 1 SEM-image of pigmented coating        Fig. 2 Fluoresence microscopic image of coating on meranti
       On softwood                           (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.




                                               Page 2 of 16
                  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.      flow from ray tracheid into
                                                                     longitudinal 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 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.




                                                 Page 3 of 16
                 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:

                      2 cos   L
                 L=                                                                          (1)
                         r Lg

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 cos  r t
                 L=                                                                          (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
                            CELL WALL
                                                                         FRACTION POLYMER
                                                                            INCREASING




                         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




                                               Page 4 of 16
                   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 40-
60 % 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   Fig. 5b Viscosity – solids content of a water
dispersion, alkyd emulsion and a solventborne alkyd binder as a        soluble modified linseed oil.
function of binder content. (Source: M. de Meijer,                     (Source: Worlée Technical datasheet)
      Adhesion aspects of polymeric coatings 2003)

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 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 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.




                                                Page 6 of 16
                      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           Type of
    Species        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]
                                  3
    Beech          sessile drop                 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
           4
    Pine           sessile drop                                             40.7   1.73    8.41    7.63    48.3     [e]
           4
    Pine           Wilhelmy plate                                           38.9   0.05 17.33      1.86    40.8     [e]
           5
    Pine           sessile drop        50.9      83.4        0.4    83.8                                            [a]
           6
    Pine           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
               5
    Spruce         sessile drop        51.8      71.6         2     73.6                                            [a]
               6
    Spruce         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]



1
  S = P + D
2
  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




                                                        Page 7 of 16
                 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 m-
  ,  = 1.62 mJ m-2, -= 17.2 mJ m-2 and for arabinogalactan (hemicellulose) LW= 37.6 mJ m-2, AB=
2 +

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 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
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 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 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                                 140
                                        PENETRATION                                                                     ac1/EW     ac2/EW
                                                                                                                        ac3/EW     hsa/EW
                                                                                  120
                                                         contactangle (degress)




                                                                                                                        sba/EW     wba/EW
                                                                                                                        ac1/LW     ac2/LW
                                                                                                                        ac3/LW     hsa/LW
                                                                                  100
                                                                                                                        sba/LW     wba/LW

                                                                                   80


                                                                                   60


                                                                                   40


                                                                                   20

                 LATEWOOD                                                           0
                                    EARLYWOOD                                           0   50   100   150        200     250    300        350
                                                                                                             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)




                                                 Page 8 of 16
                  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 semi-
quantitative 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 stress-
distribution 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.




                                                Page 9 of 16
                                                      COST E18 – Final Seminar A review of interfacial aspects in wood coatings


                   200                                                                                                                350
                                     peak
                   175                corresponding
                                                                                                                                      300
                                    to earlywood                                                                                                 earlywood (higher penetration)
                                     bands
                   150
                                                                                                                                      250




                                                                                                             adhesion strength J/m2
                                                                                                                                                 latewood (lower penetration)
                   125
peel force, N/mm




                                                                                                                                      200
                                                                               valley corresponding
                   100
                                                                               to latewood bands
                                                                                                                                      150
                   75

                   50                                                                                                                 100
                                                                     wood structure under
                                                                    coating which is peeled
                   25                                                                                                                  50


                    0
                                                                                                                                        0
                         0                10                20      30              40                50                                    acrylic1     acrylic2      acrylic3   alkyd-emulsion   high solid alkyd   solvent alkyd

                                                        peeled distance, mm                                                                                                coating type



Fig. 8a Adhesion as a function of peeled distance on pine                                                  Fig. 8b Peel adhesion strength of various coatings on
        sapwood with early- and latewood.                                                                          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                                          Fig. 9b SEM image of wood with part of a high solid
                          Paint peeled from wood in a wet adhesion test                                            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 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:

                                                           wood   2

                    =c. E.                                                                                                           Wp = WaCW + Wd -                                                        
                                                  coating
                                                                              (4)
                                                       1 

                   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



                                                                                                 Page 10 of 16
                            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
             450

             400                                                                                                         SBA        2.7
  adhesion
  strength   350                                                                                                         HSA        2.7
    J/m2     300                                                              232

                                                                                            195
                                                                                                                        WBA         2.6
             250

             200                                                                                                         Ac3                        46
                                                 78
             150             76        71
                                                        298
                                                                                                                         Ac2                                   78
             100
                                                                       140                                               Ac1             11
              50                                                                      116
                      53          58        55                                                      earlywood
               0                                                                                  latewood                      0         20   40        60   80     100
                    Ac1       Ac2        Ac3           Ac1            Ac2            Ac3
                   liquid    liquid     liquid        vapour         vapour         vapour
                                                                                                                                     volumetric swelling %


Fig. 10 Differences in peel adhesion strength of 3 acrylic paints                                               Fig. 11 Volumetric swelling of different
on pine sapwood exposed to liquid water and vapour (98 % RH)                                                    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)

                                        Wcw  2  c  LW   c     c  
                                         a         LW
                                                       w     
                                                                  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                                                                                                 (8)




                                                                                    Page 11 of 16
                  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
                                              p
                  Coating                 W                      a                        a LW-AB
                                                             W       CW dry           W          wet
                                    EW            LW
                       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




                                                     Coated with             Coated with
                                Exposed              solventborne            waterborne
                                to water             paint                   alkyd paint


                                                                    c paints
                Fig. 13 Response of compressed wood to water and ronde kant
                      Figuur 9         b ronde kant + celdeformatie              + celdeformatie
               a ronde kant + celdeformatie     oplosmiddelhoudende        watergedragen
                       onafgewerkt                 alkyd spuit verf      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



6.         REFERENCES
1
 Loon J. van (1966) The interaction between paint and substrate. Journal of the Oil and Color Chemists
Association (49): 844-867; Schneider M.H. (1980) Microscopic distribution of linseed oil after application to
wood surface. Journal of Coatings Technology 52 (665): 64-67; Schneider M.H., Cote W.A. (1967) Studies of
wood and coating interactions using fluorescence microscopy and pyrolysis gas-liquid chromatography. Journal
of Paint Technology 39 (511): 465-471; Schneider M.H., Sharp A.R. (1982) A model for the uptake of linseed
oil by wood. Journal of Coatings Technology 54 (693): 91-96.
2
  G. Rødsrud and E.J. Sutcliff, Alkyd emulsions-properties and application. Results from comparative
investigations of penetration and aging of alkyds, alkyd emulsions and acrylic disperions, Surf. Coat. Int. 77 (1)
(1994), 7-16; Nussbaum R.M. (1994) Penetration of water-borne alkyd emulsions and solvent-borne alkyds into
wood. Holz als Roh- und Werkstoff 52: 389-393; Smulski S., Côté W.A. (1984) Penetration of wood by a water-
borne alkyd resin. Wood Science and Technol. 18: 59-75; R.M. Nussbaum, E.J. Sutcliffe and A.C. Hellgren,
Microautoradiographic studies of the penetration of alkyd, alkyd emulsion and linseed oil coatings into wood , J.
of Coatings Technol. 70 (878) (1998) 49-57.
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  M. de Meijer, K. Thurich and H. Militz, Comparative study on penetration characteristics of modern wood
coatings, Wood Sci. Technol. 32 (1998), 347-365; V. Rijckaert, M. Stevens, J. Van Acker Effect of some
formulation parameters on the penetration and adhesion of water-borne primers into wood. Holz als Roh- und
Werkstoff 59 (2001) 344 – 350; V. Rijckaert, M. Stevens, J. Van Acker,. M. de Meijer and H. Militz
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wood and coating properties Holz als Roh- und Werkstoff 59 (2001) 35 - 45
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difficulties of water, XXIIth Fatipec Conference Budapest Vol 2, 1994, 157-167; R. Hoffman, Factors affecting
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8
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    The Int. Res. Group on Wood Preserv., Stockholm, Doc. No. IRG/WP 98-40121 (1998)
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Hague (2002).
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and dispersion force contributions to the total surface free energy of wood, Wood Science and Technology, 12,
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                                                   Page 15 of 16
                    COST E18 – Final Seminar A review of interfacial aspects in wood coatings



12
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means of wetting method, Holzforschung, 52, 1998, 521-529; Gardner, D.J.; Application of the Lifshitz-van der
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Langmuir 16, 9352-9359 (2000); M. Wålinder, Wetting Phenomena on Wood, Doctoral thesis, Royal Inst.
Technol., Stockholm (2000) ; Van Oss, C.J.; Chaudhury, M.K.; Good, R.J.; Interfacial Lifshitz –van der Waals
and polar interactions in macroscopic systems, Chemical Review, 88, 1988, 927-941
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   M.L. Jansen, Performance testing of exterior wood primers, JOCCA 5 (1986) 117-128; A. Underhaug,
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  S. L. Bardage and J. Bjurman, Adhesion of waterborne paints to wood, J. of Coatings Tech. 70 (878) (1998)
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Adhesion Sci. Technol. 10 (8) (1996) 745-759; M. de Meijer and H. Militz, Wet adhesion of low-voc coatings
on wood a quantitative analysis, Progress in Organic Coatings, 200, 223-240.
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(1964) 1091-1098; P. Ahola, Adhesion between paint and wood substrate, JOCCA 74 (5), (1991) 173-176;
R.S.Williams and W.C. Feist, Durability of paint or solid-color stain applied to preweathered wood, Forest Prod.
J. 43 (1) (1993) 8-14. P.D. Thay and P.D. Evans, The adhesion of an acrylic primer to weathered radiata pine
surfaces, Wood and Fiber Science 30 (2) (1998) 198-204.
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R.M. Nussbaum, The critical time limit to avoid natural surface inactivation of spruce surfaces (Picea Abies)
intended for painting and gluing, Holz als Roh- und Werkstoff 53 (1995) 384; S.M. Kambanis and G. Chip,
Polymer and paint properties affecting wet adhesion, J. of Coatings Technol. 53 (682) (1981) 57-64; J. Ekstedt
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Novel wet adhesion monomers for use in latex paints, Progress in organic coatings 34, 1998, 214-219.
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  Gnatowski, M., Method for protecting wood surfaces and a wood product produced thereby, Patent WO
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     B.F. Tjeerdsma, W. Cobben, Wood machining and cell derformation (in dutch), SHR report 1.157, 2002.




                                                 Page 16 of 16

				
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