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A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS
Mari de MeijerDrywood Coatings
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TOPICS:
• Coating penetration into substrate
• Wood surface energy and wetting
• Adhesion
• Wood surface preparation
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PENETRATION OF COATINGS
• Techniques for assessment
• Influence of wood anatomy
• Influence coating properties
• Relevance to performance
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Techniques for assessment
Static:
• Light and fluorescence microscopy,dyeing the
coating or the subtrate
• Confocal laser
• SEM (+EDAX)
Dynamic:
• Rate of uptake (volume / droplets)
• No dynamic microscopic techniques
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Examples softwood
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Examples softwood
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Examples softwood
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Examples hardwood
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Schematic overview of possible penetration
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 longitudinaltracheid
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Length filled capillary (L),
liquid surface tension (L
cosine of the contact angle (of wetting liquid
capillary radius (r)
acceleration of gravity g (9,8 m s-2)
density of the liquid (L)
gr
cos 2 = L
L
L
Influence coating properties
Model capillary flow, static situation
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Length filled capillary (L),
liquid surface tension (L
cosine of the contact angle (of wetting liquid
capillary radius (r)
viscosity paint (time (t)
Influence coating properties
Model capillary flow, dynamic situation
2
r t cos = L L
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CAPILLARY UPTAKE
CELL WALL
WOOD
COATINGCOATING
SELECTIVE UPTAKEWATER ORSOLVENT
INC
REA
SIN
G S
OLI
DS
C
ON
TEN
T P
AIN
T
FLOW
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VISCOSITY - SOLIDS
0
3
6
9
12
15
0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65
mass fraction binder
acrylicdispersion
alkydemulsion
solventbornealkydR
ela
tive
vis
cosi
ty
log
(/
o)
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VISCOSITY - SOLIDS
WATER SOLUBLE LINSEED OIL
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WETTING COATING
coating < wood
Viscosity can also be limiting the wetting
EARLYWOOD
LATEWOOD
CAPILLARYPENETRATION
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WETTING COATING
0
20
40
60
80
100
120
140
0 50 100 150 200 250 300 350
Time, s
con
tact
ang
le (
deg
ress
)
ac1/EW ac2/EW
ac3/EW hsa/EW
sba/EW wba/EW
ac1/LW ac2/LW
ac3/LW hsa/LW
sba/LW wba/LW
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Relevance to performance
• Carrier of functional additives like biocides • Improvement of adhesion by providing
mechanical anchoring
• Improving the exterior durability • Esthetical aspects like clarity of grains
(‘anfeuerung’) and pore wetting
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Wood surface energy and wetting
• Critical surface energy
• Polar and disperse components• Lifshitz-van-der-Waals and (Lewis) acid-
base components
• Young’s equation: s= sl + l cos
• Drop or Wilhelmy plate with various liquids
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Wood surface energy and wetting
• Theory assumes:
thermodynamic equilibrium and a chemically homogeneous solid surface, flat and not influenced by chemical interaction or adsorption of the liquid to the surface
? !!!
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Overview of literature data (mJ m-2)WoodSpecies
Type ofmeasurement
c
P D S 1 LW + - AB S 2
Beech sessile drop 19.18 31.88 50.0
Beech sessile drop3 45.53 24.48 68.8
Beech sessile drop 50.6 53.1 6.9 60
Cherry Wilhelmy plate 48.1 38.1 16.19 54.3 47.5 0.42 28.00 6.84 54.3
Cherry Wilhelmy plate 35.1 20.09 55.2
Douglas fir Wilhelmy plate 11.8 36.2 48 38.7 2.86 3.29 6.13 44.8
Douglas fir sessile drop 52.8 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 45.5 0.46 33.19 7.85 53.3
Maple Wilhelmy plate 40.93 20.13 61.1
Maple Wilhelmy plate 42 16.4 40.2 56.6 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
Pine 6 sessile drop 54.3 68.1 3 71.1
Red oak Wilhelmy plate 46.8 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
Spruce Wilhelmy plate 45 16.5 45 61.5 49.4 0.81 11.35 6.06 55.5
Spruce 5 sessile drop 51.8 71.6 2 73.6
Spruce 6 sessile drop 53.2 41.9 13.9 55.8
1 S = P + D
2 S = LW + AB
3 adjusted to ideal surface4 measured parallel to the grain of the wood5 earlywood area’s6 latewood area’s7 data calculated from contact angles reported [a] Scheikl, M., Dunky, M. Holzforschung, 1998, 52, 89-94; [b] Nguyen, T., Johns, W.E. Wood Science andTechnology, 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
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Adhesion / adherence Impact of the measurement technique
Reduction adhesion by energy stored in the coating
because of internal stress
Work expended in deformation during peeling or
torsion of the coating
Impact of mechanical anchoring
Influence of moisture in coating or wood
Molecular forces between coating and wood that
determine the interfacial adhesion (true adhesion)
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Adhesion analysis
X cut of cross-hedge test
dolly pull-off
dolly torques test
peeling in testing machine
atomical level (AFM etc, not on wood)
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Peel tape test
woodwood
coatingcoating
tapetape
180 °
waterwaterentryentry
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Peel tape test
woodwood
coatingcoating
tapetape
180 °
waterwaterentryentry
0,0
25,0
50,0
75,0
100,0
125,0
150,0
175,0
200,0
0,00 10,00 20,00 30,00 40,00 50,00
peeled distance mm
pee
l fo
rce
N/m
m o
r J/
m2
wood structure undercoating which is peeled away
valley corresponding with latewood bands
peak corresponding with earlywood bands
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Mechanical anchoring
0
50
100
150
200
250
300
350
acrylic1 acrylic2 acrylic3 acrylic4 alkyd-emulsion
solventalkyd
highsolidalkyd
ad
he
sio
n s
tre
ng
th J
/m2
earlywood (higher penetration)
latewood (lower penetration)
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Mechanical anchoring
torn out coatingmateria l
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Moisture & adhesion
Ac1liquid
Ac2liquid
Ac3liquid
Ac1vapour
Ac2vapour
Ac3vapour
latewood
earlywood
76 7178
473
232
195
53 58 55
298
140116
0
50
100
150
200
250
300
350
400
450
500
adhesion strength
J/m2
• Strong impact on adhesion: dry >> vapour > liquid• Dry state: too high to measure > 600 J/m2
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Moisture & adhesion
Factors influencing the measured adhesion:
WWTT = = cw cw + W+ Wp p --
• Interfacial work of adhesion: molecular interaction
• Plastic deformation: negligible
• Stored strain energy due to internal stress :
differential hygroscopic expansion coating and wood
work of adhesion interfacial work
of adhesionwork stored in plastic deformation
stored strain energy
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Moisture & adhesion
cc:: coating thicknesscoating thickness
E:E: coating elasticitycoating elasticity
:: poisson ratio poisson ratio (0.4)(0.4)
coatingcoating: : swelling coatingswelling coating
woodwood: : swelling woodswelling wood
= c . E .
coating wood
2
1
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Moisture & adhesion
cc:: coating thicknesscoating thickness
E:E: coating elasticitycoating elasticity
:: poisson ratio poisson ratio (0.4)(0.4)
coatingcoating: : swelling coatingswelling coating
woodwood: : swelling woodswelling wood
= c . E .
coating wood
2
1
Maximum swelling 65 % RH to liquid water
11
46
2,6
2,7
2,7
7
78
0 20 40 60 80
Ac1
Ac2
Ac3
WBA
HSA
SBA
pinewood
volumetric swelling
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Calculated – measured adhesion
wcwcLWw
LWc
acwW 2
Wacw = c + w - cw
Wawet = CL + WL - CW
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Calculated – measured adhesion
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Adhesion promoting technologies
Pretreatment of the wood by flame-ionisation or
plasma- treatment
Incorporation of adhesion promoting monomers in
acrylic dispersions
Reducing the wateruptake and / or swelling of the
coating by crosslinking of the polymer or reducing
the hydrophilicity
Chemical crosslinking between coating and wood
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Wood surface preparation
Sanding: reduction of penetration
Rough sawing: increase in coating uptake
Planing: possibility of cell compression
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Wood surface preparation
Sanding: reduction of penetration
Rough sawing: increase in coating uptake
Planing: possibility of cell compression
Deformed cells
Source: SHR Timber Research
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Cell compression
• Solventborne: expansion during weathering• Waterborne: expansion during coating
application
Exposed to water
Coated with solventborne paint
Coated with waterborne alkyd paint
Source: SHR Timber Research
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CONCLUSIONS
A combination of the anatomical wood structure and
flow of the coating determines coating penetration
Differences in penetration 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
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CONCLUSIONS
• 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 not usefull 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
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CONCLUSIONS• 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
• 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.
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CONCLUSIONS• 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
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GAPS IN KNOWLEDGE • The rheology of coatings at increasing solid content or
during drying is hardly known but is essential to understand differences in penetrating capacity.
• Impact of a penetrating primer on the weathering performance. Seems to be positive, but why?
• Reduction of coating adhesion under wet conditions. Improved knowledge in this field is required to understand
why adhesion is sometimes insufficient.
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• Thank you for your long lasting attention!!