A REVIEW OF INTERFACIAL ASPECTS IN
WOOD COATINGS
Mari de Meijer
Drywood Coatings
TOPICS:
• Coating penetration into substrate
• Wood surface energy and wetting
• Adhesion
• Wood surface preparation
PENETRATION OF COATINGS
• Techniques for assessment
• Influence of wood anatomy
• Influence coating properties
• Relevance to performance
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
Examples softwood
Examples softwood
Examples softwood
Examples hardwood
Schematic overview of possible penetration
1.
flo w in to op en en d of lon g itud in al
trac h eid
2.
flo w in to ra y trac h eid
3.
flo w in to ra y p aren c h ym a
4.
flo w from ray p aren c h ym a in to
lon g itud in al latew ood trac h eid
5.
flo w from ray trac h eid in to lon g itud in al
trac h eid
Influence coating properties
Model capillary flow,
static situation
L=
2 cos q g L
r r Lg
Length filled capillary (L),
liquid surface tension (g L)
cosine of the contact angle (q ) of
wetting liquid
capillary radius (r)
acceleration of gravity g (9,8 m s-2)
density of the liquid (rL)
Influence coating properties
Model capillary flow,
dynamic situation
L=
g L cos q r t
2h
Length filled capillary (L),
liquid surface tension (g L)
cosine of the contact angle (q ) of
wetting liquid
capillary radius (r)
viscosity paint (h)
time (t)
CAPILLARY UPTAKE
CELL WALL
INCREASING SOLIDS
CONTENT PAINT
WOOD
SELECTIVE
UPTAKE
WATER OR
SOLVENT
FLOW
COATING
VISCOSITY - SOLIDS
VISCOSITY - SOLIDS
WATER
SOLUBLE
LINSEED OIL
WETTING COATING
gcoating < gwood
C A P IL L A R Y
P E N E T R A T IO N
Viscosity can also be
limiting the wetting
LATEW O O D
EARLYW O O D
WETTING COATING
c o n ta c ta n g le (d e g re s s )
140
120
100
a c1 /E W
a c2 /E W
a c3 /E W
h sa /E W
sb a /E W
w b a /E W
a c1 /L W
a c2 /L W
a c3 /L W
h sa /L W
sb a /L W
w b a /L W
80
60
40
20
0
0
50
100
150
200
T im e , s
250
300
350
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
Wood surface energy and wetting
• Critical surface energy
• Polar and disperse components
• Lifshitz-van-der-Waals and (Lewis) acidbase components
• Young’s equation: gs= gsl + gl cos q
• Drop or Wilhelmy plate with various
liquids
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
? !!!
Overview of literature data (mJ m-2)
W ood
S p ecies
Type of
m ea su re m en t
B eech
sessile d ro p
gc
g
3
P
g
g
D
S 1
1 9 .1 8
3 1 .8 8
5 0 .0
4 5 .5 3
2 4 .4 8
6 8 .8
B eech
sessile d ro p
B eech
sessile d ro p
5 0 .6
5 3 .1
6 .9
60
C h erry
W ilh elm y p late
4 8 .1
3 8 .1
1 6 .1 9
5 4 .3
C h erry
W ilh elm y p late
3 5 .1
2 0 .0 9
5 5 .2
D o u glas fir
W ilh elm y p late
1 1 .8
3 6 .2
48
D o u glas fir
sessile d ro p
5 2 .8
1 9 .2
2 8 .8
48
D o u glas fir
sessile d ro p
1 1 .5
3 7 .5
49
M ap le
W ilh elm y p late
4 6 .8
5 6 .0 7
8 .7 7
6 4 .8
M ap le
W ilh elm y p late
4 0 .9 3
2 0 .1 3
6 1 .1
M ap le
W ilh elm y p late
1 6 .4
4 0 .2
5 6 .6
P in e
P in e
4
4
5
AB
g
S 2
2 .8 6
3 .2 9
6 .1 3
4 4 .8
4 5 .5
0 .4 6
3 3 .1 9
7 .8 5
5 3 .3
4 3 .2
0 .7 1
1 3 .2 9
6 .1 5
4 9 .4
sessile d ro p
4 0 .7
1 .7 3
8 .4 1
7 .6 3
4 8 .3
W ilh elm y p late
3 8 .9
0 .0 5
1 7 .3 3
1 .8 6
4 0 .8
3 9 .7
0 .4 6
3 7 .7 4
8 .3 0
4 8 .0
4 9 .4
0 .8 1
1 1 .3 5
6 .0 6
5 5 .5
42
5 4 .3
6 8 .1
3
7 1 .1
R ed o ak
W ilh elm y p late
4 6 .8
4 2 .2
1 0 .4
5 2 .6
R ed o ak
W ilh elm y p late
S p ru ce
W ilh elm y p late
S p ru ce
g
3 8 .7
sessile d ro p
6
-
5 4 .3
8 3 .8
S p ru ce
g
6 .8 4
0 .4
5
+
2 8 .0 0
8 3 .4
P in e
g
0 .4 2
5 0 .9
6
LW
4 7 .5
sessile d ro p
P in e
g
3 5 .0 4
1 6 .8 7
5 1 .9
45
1 6 .5
45
6 1 .5
sessile d ro p
5 1 .8
7 1 .6
2
7 3 .6
sessile d ro p
5 3 .2
4 1 .9
1 3 .9
5 5 .8
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)
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)
Peel tape test
ta p e
c o a tin g
180 °
w ood
w a te r
e n tr y
Peel tape test
2 0 0 ,0
ta p e
peak
1 7 5 ,0
w ith e a rly w o o d
w ood
w a te r
e n tr y
p e e l fo rc e N /m m o r J /m 2
c o a tin g
180 °
c o rre s p o n d in g
1 5 0 ,0
bands
1 2 5 ,0
v a lle y c o rre s p o n d in g
1 0 0 ,0
w ith la te w o o d b a n d s
7 5 ,0
5 0 ,0
w o o d s tru c tu re u n d e r
c o a tin g w h ic h is p e e le d a w a y
2 5 ,0
0 ,0
0 ,0 0
1 0 ,0 0
2 0 ,0 0
3 0 ,0 0
p e e le d d is ta n c e m m
4 0 ,0 0
5 0 ,0 0
Mechanical anchoring
a d h e s io n s tre n g th J /m
2
350
300
e a rly w o o d (h ig h e r p e n e tra tio n )
250
la te w o o d (lo w e r p e n e tra tio n )
200
150
100
50
0
a cr y l i c1
a cr y l i c2
a cr y l i c3
a cr y l i c4
a lkyd -
s o l ve n t
hig h
em u l si on
a lkyd
sol i d
a lkyd
Mechanical anchoring
torn out coating
m ate r ia l
Moisture & adhesion
a d h e s io n
s tr e n g th
J /m
2
473
500
450
400
350
300
232
195
250
200
78
150
76
298
71
100
140
50
116
53
58
e a r ly w o o d
55
0
la te w o o d
Ac 1
Ac 2
Ac 3
Ac 1
liq u id
Ac 2
liq u id
Ac 3
liq u id
vapour
vapour
vapour
• Strong impact on adhesion: dry >> vapour > liquid
• Dry state: too high to measure > 600 J/m2
Moisture & adhesion
Factors influencing the measured adhesion:
WT =
work of
adhesion
g
cw
interfacial work
of adhesion
+ Wp -

work stored
in plastic
deformation
stored strain
energy
• Interfacial work of adhesion: molecular interaction
• Plastic deformation: negligible
• Stored strain energy due to internal stress :
differential hygroscopic expansion coating and wood
Moisture & adhesion
 = c. E.
 
coating
  wood
)
2
1 
c:
coating thickness
E:
:
 coating:
 wood:
coating elasticity
poisson ratio (0.4)
swelling coating
swelling wood
Moisture & adhesion
M a x im um s w e lling 6 5 % RH to liquid w a te r
 = c. E.
 
coating
  wood
)
2
1 
p in e w o o d
7
SBA
2,7
HSA
2,7
W BA
2,6
A c3
c:
coating thickness
E:
:
 coating:
 wood:
coating elasticity
poisson ratio (0.4)
swelling coating
swelling wood
46
78
A c2
11
A c1
0
20
40
60
vo lu m e t r ic s w e llin g
80
Calculated – measured adhesion
Wacw = g c + g w - g cw
W
a
cw
2

g
LW
c
g
LW
w

Wawet = gCL + gWL - gCW

c
g g

w


c
g g

w
)
Calculated – measured adhesion
C o a tin g
W
EW
p
LW
W
a
W
C W dr y
a LW -AB
w et
Ac 1
152
107
0 .0 8 7
0 .0 2 0
Ac 2
142
115
0 .0 9 6
0 .0 1 0
Ac 3
156
110
0 .0 9 4
0 .0 5 0
WBA
238
126
0 .0 9 8
0 .0 1 9
HSA
682
200
0 .0 9 3
0 .0 1 4
SBA
580
255
0 .0 9 3
0 .0 3 0
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
Wood surface preparation
 Sanding: reduction of penetration
 Rough sawing: increase in coating uptake
 Planing: possibility of cell compression
Wood surface preparation
 Sanding: reduction of penetration
 Rough sawing: increase in coating uptake
 Planing: possibility of cell compression
Deformed cells
Source: SHR Timber Research
Cell compression
• Solventborne: expansion during weathering
• Waterborne: expansion during coating
application
Exposed
to water
F iguu r 9
Coated with
solventborne
paint
b ron d e k an t + celd e form atie
op los m id d elh ou d en d e
Source: SHR Timber Research
a r on d e k an t + c eld eform atie
on afg ew erk t
alk yd s p u it v erf
Coated with
waterborne
alkyd paint
c ron d e k an t + celd eform atie
w aterg ed rag en
alk yd d om p el v erf
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
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
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.
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
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.
• Thank you for your long lasting attention!!