A REVIEW OF INTERFACIAL ASPECTS IN WOOD COATINGS

Mari de MeijerDrywood 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. 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

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

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

CAPILLARY UPTAKE

CELL WALL

WOOD

COATINGCOATING

SELECTIVE UPTAKEWATER ORSOLVENT

INC

REA

SIN

G S

OLI

DS

C

ON

TEN

T P

AIN

T

FLOW

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)

VISCOSITY - SOLIDS

WATER SOLUBLE LINSEED OIL

WETTING COATING

coating < wood

Viscosity can also be limiting the wetting

EARLYWOOD

LATEWOOD

CAPILLARYPENETRATION

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

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) acid-

base components

• Young’s equation: s= sl + l cos

• 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)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

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

woodwood

coatingcoating

tapetape

180 °

waterwaterentryentry

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

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)

Mechanical anchoring

torn out coatingmateria l

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

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

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

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

Calculated – measured adhesion

wcwcLWw

LWc

acwW 2

Wacw = c + w - cw

Wawet = CL + WL - CW

Calculated – measured adhesion

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

Coated with solventborne paint

Coated with waterborne alkyd paint

Source: SHR Timber Research

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!!