Advances in Probing Wood-Coating Interface by Microscopy: A Review

doi:10.4236/jsemat.2013.31A007

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Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 49-54

http://dx.doi.org/10.4236/jsemat.2013.31A007 Published Online February 2013 (http://www.scirp.org/journal/jsemat) 49

Advances in Probing Wood-Coating Interface by

Microscopy: A Review

Adya P. Singh1*, Byung-Dae Park1, Arif Nuryawan1, Menda Kazayawoko2

1Department of Wood Science and Technology, Kyungpook National University, Daegu, South Korea; 2Departement de Gestion des

Ressources Naturelles, Faculte des Sciences Agronomiques, Universite de Kinshasa, Kinshasa, Democratic Republic of the Congo.

Email: *[email protected]

Received December 18th, 2012; revised January 20th, 2013; accepted January 28th, 2013

ABSTRACT

Surface coatings provide protection to wood products against weathering and other deteriorating factors, such as mois-

ture uptake and microbial invasion. The effectiveness of coatings depends on many factors, including how well the ap-

plied coatings adhere to the wood surface. Coating adhesion to wood involves both chemical and physical interactions

between the coating and wood tissues in contact, and the particular focus of this mini-review will be on the advances

being made in understanding the physical aspects of the interaction by probing wood-coating interface using novel and

high resolution imaging techniques, including confocal laser scanning microscopy (CLSM), SEM-backscattered elec-

tron imaging and correlative microscopy employing light, confocal and scanning electron microscopy.

Keywords: Wood-Coating Interface; Coating Adhesion; Correlative Microscopy; Backscattered Electron Imaging;

Confocal Laser Scanning Microscopy

1. Introduction

Wood products exposed in service under outdoor condi-

tions can deteriorate from the damaging effects of solar

radiation, which can cause weathering of cell walls [1-

10], and cell wall degradation by microorganisms [11,12].

Application of suitable coatings to the product surface

can significantly extend the service life by preventing

solar radiation from reaching the wood surface and or-

ganisms and moisture to come in direct contact with the

underlying wood tissues. Among the factors that influ-

ence coating performance coating adhesion to wood is of

major importance [13], which primarily depends on coat-

ing penetration into wood [14] and mechanical anchorage

through entanglement of coatings within penetrated sur-

face and subsurface layers of wood tissues [15,16]. Ex-

amining wood-coating interface is therefore important to

understand the nature of wood-coating interaction, in-

formation that can be helpful in evaluating adhesion and

performance of applied coatings and ultimately in opti-

mizing formulations and processes to achieve high dura-

bility of wood-coating composites.

Microscopy has been the most widely applied tool to

examine and image wood-coating interface. Whereas the

majority microscopic studies have aimed to record in-

formation on depth to which the applied coatings can

penetrate into wood products made from different wood

types as well as those variously surfaced [14,17-24],

specialized microscopic techniques have also been de-

veloped and employed that can resolve wood-coating

interface more clearly, providing improved understand-

ing of wood-coating interaction at cellular and sub-

cellular levels [15,16,25]. In this mini-review the focus

will be on these technical advances in probing wood-

coating interface, often involving a combination of dif-

ferent types of microscopes that differ in their imaging

capabilities and can provide complementary information.

The information presented is in a broader context to in-

clude places of physical contact of coatings with the sur-

faces of wood products to which they are applied as well

as all contact interfaces between penetrated coatings and

cell walls around cell lumens and lining cell wall cracks

within tissue layers underneath the surface.

2. Novel Microscopic Approaches for

Imaging Wood-Coating Interface

The importance of coating adhesion for coating durabil-

ity and performance has been recognized for a long time.

Among the factors that influence coating adhesion to

wood surface, coating penetration into and entanglement

within surface and subsurface tissues are important, as

improvements in water repellency result in greater di-

mensional stability and less cracking of wood in outdoor

exposures, aspects that have been emphasized in several

*Corresponding a uthor.

Advances in Probing Wood-Coating Interface by Microscopy: A Review

studies [14,23,24,26]. The importance of capillary pene-

tration, i.e. coating penetration into larger pores within

wood tissues such as lumens of vessels, tracheids and

rays, can perhaps be best appreciated from the visual

appearance of microscopic images illustrated in a study

that combined light microscopy and transmission elec-

tron microscopy to understand the mechanism of failure

of a clear coating applied to radiata pine (Pinus radiata)

wood panels [9]. In this outdoor exposure study the clear

coating had failed due mainly to wood failure, involving

cell separation in the middle lamella region, resulting

from preferential degradation of lignin by UV radiation

[4]. Coating adhesion with underlying wood tissues had

not been compromised within the period of exposure,

judging by the presence of outer layers of intact wood

tissues attached to separated segments of the coating.

Even in regions showing partial film detachment, the

coating film was continuous with the coating material

present in rays, resisting coating detachment in contact

regions. Further advances in understanding wood-coating

interactions have come from the development and use of

novel microscopic approaches individually and in com-

bination, which is the main focus of this review. The

review is based on others’ work as well as author’s own

investigations.

3. Imaging of Wood-Coating Interface by

Confocal Laser Scanning Microscopy

(CLSM)

CLSM is proving as an important information tool in

many areas of biological research because of its capabili-

ties in combining non-destructive optical sectioning through

relatively thick samples and 3-D reconstruction based on

images produced from sequential optical sections. This

eliminates the need for using chemical fixation and poly-

mer embedding for serial sectioning, which can alter

cellular structures and is a time-consuming and tedious

way of obtaining information from a large volume of

tissues. CLSM also produces sharper images than achi-

evable by other optical microscopes, because with the

help of an aperture out-of-focus fluorescence is largely

eliminated, which results in an increase in contrast, im-

age clarity and detection sensitivity. Additionally, be-

cause of its capabilities for live cell imaging, in cell bio-

logical, physiological and molecular studies CLSM is

proving to be a highly desirable tool for ob taining spatial

and temporal information at high resolution, making it

possible to understand the intricacies of such vital proc-

esses as molecular and organelle dynamics, intra- and

intercellular communication and signal perception and

processing.

In wood coatings area, a study [15] comparing CLSM

and light microscopy (LM) to image the interface be-

tween P. radiata wood and a clear coating showed

CLSM to be markedly superior to LM in obtaining in-

formation on the physical nature of interaction between

the coating and the surface and outermost subsurface

tissue layers (Figure 1). Whereas the contact interface

appeared ‘fuzzy’ in LM images, CLSM clearly resolved

the interface, enabling coating penetration into fine

cracks, which apparently formed during the surfacing of

wood with planer knives, to be clearly visualized. As the

presence of small size cracks can greatly increase the cell

wall surface area for contact with applied coatings, and

thus enhancing coating adhesion through mechanical

interlocking, it is important to develop and use micro-

Figure 1. (a) LM view of wood-coating interface. The pres-

ence of coating material (red color) in cell lumens can be

resolved but not its penetration into cracks within cell walls

(light blue color). Bar = 20 μm. The micrograph is repro-

duced from JCT Research, Vol. 1, No. 3, 2004; (b) CLSM

view of wood-coating interface. The presence of coating

material (crimson color) within cell wall (purple color)

cracks (arrowheads) can be clearly resolved. Bar = 20 μm.

The micrograph is reproduced from JCT Research, Vol. 1,

No. 3, 2004.

Advances in Probing Wood-Coating Interface by Microscopy: A Review 51

scopic techniques that not only provide information on

coating penetration into wood microcapillaries, such as

lumens of vessels, tracheids and rays [23] but can also

reveal penetration into cell wall cracks formed during the

surfacing of wood products. Although scanning electron

microscopy (SEM) and transmission electron microscopy

(TEM) offer greater resolution compared to CLSM, im-

aging of wood-coating interface using such tools requires

special preparation techniques to visualize coatings [25]

and involve rather labor-intensive and time-consuming

processing of samples [5]. CLSM can provide high reso-

lution images within a relatively short time, particularly

when images are captured in one plane from a single

optical slice, and thus has the potential for wide applica-

tions in wood coatings research.

4. Correlative Microscopy in Wood Coatings

Research

While LM [23], CLSM [15] and SEM [14,24] have pro-

vided valuable understanding of coating penetration into

wood, which can be related to coating adhesion, more

novel technical approaches, such as correlative micros-

copy [16] and X-ray imaging and analysis [27] are pro-

viding more complete information on wood-coating in-

teraction. Here an example is presented where compare-

son of LM, CLSM and SEM was made for their capabili-

ties to resolve greatly distorted surface tissues in a P.

radiata panel that had been band-sawn to produce a

highly rough surface texture in order to understand the

pattern of distribution of a stain coating within these tis-

sues. In this study [16] P. radiata plywood had been

saw-textured to prepare a rough surface which was sub-

sequently coated with a film-forming acrylic stain coat-

ing. For correlative microscopy, sections were trans-

versely cut across wood-coating interface using a sliding

microtome, and stained with toluidine blue, a stain

widely used to contrast lignified cell walls [28]. Th e sec-

tions were initially examined by LM, which was useful

in screening the sections to capture a large number of

images of the highly irregular surface with greatly dis-

torted tissues to compare earlywood and latewood tissues

for micromorphological differences and in the extent of

distortions. However, LM proved inadequate in clearly

resolving individual cells and differentiating cell walls

from cell lumens particularly in the highly distorted tis-

sues. Using LM alone it was therefore not possible to

clearly trace penetration pathways of the applied coating

and the pattern of its distribution within distorted surface

tissues. Same sections were then examined sequentially

by LM, CLSM and SEM, imaging the same tissue region,

to compare imaging capabilities of the three different

microscope systems in resolving distorted tissues with a

view to obtain more complete information on coating

penetration and distribution (Figure 2).

Figure 2. LM (a), CLSM (b) and SEM (c) micrographs of

the same section through the coated face of a saw-textured

plywood. The distorted surface tissues (arrow) appear fuzzy

in the LM image (a) but the same tissue mass is clearly re-

solved in CLSM (b) and SEM (c) images. In (b) the pres-

ence of coating material in cell lumens (arrowhead) and

within a crack across the cell walls (arrow) is resolvable.

The presence of the coating material in cell lumens is also

resolvable in (c) but not as clearly as in (b). Bar in (a) = 200

μm; bar in (b) = 100 μm; bar in (c) = 200 μm. All micro-

graphs in this figure are reproduced from JCT Research,

Vol. 3, No. 3, 2006.

Advances in Probing Wood-Coating Interface by Microscopy: A Review

The surface tissue region examined correlatively and

sequentially by LM, CLSM and SEM was extremely

distorted resulting from twisting of tissues relative to the

plane of the underlying base tissues. The depth into the

tissues from which information can be extracted using

LM is very limited compared to CLSM or SEM. Thus,

whereas the twisted tissue region appeared ‘fuzzy’ under

LM, the features of individual cells within this tissue

mass were clearly resolved by both CLSM and SEM.

CLSM proved most suitable, as the brilliant color con-

trast differentiation achieved between the coating and

cell walls enabled the intricate pattern of coatin g pene tra-

tion and distribution within the distorted tissue mass to

be clearly visualized. In addition to coating penetration

of cell lumens, the presence of coating within the small

size cell wall cracks formed during band-sawing of the

panel surface was confirmed, which helps explain the

observed excellent outdoor performance of a similar tex-

ture-coating system [29].

The example of the work employing correlative mi-

croscopy presented here demonstrates the value of ob-

taining information at cell and cell wall levels. In addi-

tion to coating penetration into cell lumens, coating

penetration into smaller size gaps and spaces (such as

cell wall cracks) within the surface and subsurface tis-

sues is important for effective coating anchorage and

thus coating adhesion to wood [30]. This is particularly

relevant to highly textured wood surfaces where rather

fragile surface tissues have to be stabilized through en-

hanced coating absorption and effective coating penetra-

tion.

5. SEM-Backscattered Electron Imaging of

Wood-Coating Interface

The majority studies employing SEM and associated

techniques have primarily aimed to assess the depth of

coating penetration [14,20,22-24]. SEM is a high reso-

lution imaging tool wih the capability of pro viding cellu-

lar and subcellular level information. However, one ma-

jor disadvantage with SEM in coatings research is that

coatings and wood tissues are not well differentiated be-

cause of a similar grayish appearance.

Here we examine a study where SEM-backscattered

technique was employed to greatly enhance the contrast

of a coating material against the cell walls of penetrated

wood tissues, enabling the pathways of coating penetra-

tion and the pattern of coating distribution to be clearly

resolved (Figure 3). This study used a wood panel-

coating system similar to that described in the section 4

of this paper. Saw-textured radiata pine plywood panels

were coated with a oil modified stain. Sections cut trans-

versely through the wood-coating interface were treated

with osmium tetroxide, a heavy metal stain that reacted

with the oil borne coating but not with wood cell walls.

Figure 3. (a), (b) FE-SEM micrographs of the same section

taken in secondary electron (a) and backscattered electron

(b) imaging modes. In (a) the coating material is poorly

differentiated, from wood cell walls because of a similar

contrast, but in (b) the bright appearing coating material is

clearly resolved. Bars = 100 μm; (c), (d) High magnification

FE-SEM micrographs of the same tissue region in a section

taken in secondary electron (c) and backscattered electron

(d) imaging modes. In (c) the coating material is poorly

differentiated from cell walls, but in (d) the presence of

bright appearing coating material within cell lumens and

fine cell wall cracks (arrows) is clearly resolved. Bars = 10

μm. All micrographs in this figure are reproduced from

JCT Research, Vol. 4, No. 2, 2007.

Advances in Probing Wood-Coating Interface by Microscopy: A Review 53

The osmium treated sections were first imaged with SEI

(secondary electron imaging) mode of a FE-SEM (field

emission scanning electron microscope), and the same

sections were then viewed under BEI (backscattered

electron imaging) mode. Comparison of the two imaging

modes using the same sections showed a distinct advan-

tage of the BEI imaging over the SEI imaging. Whereas

in the SEI mode the coating was poorly differentiated

from wood tissues, in the BEI mode the coating material

appeared bright and thus was readily differentiated from

wood tissues, which enabled the intricate pathway of

coating penetration and distribution within the highly

distorted saw-textured surface tissues to be readily ex-

amined, including penetration of th e coating material into

very small size cell wall cracks present within these tis-

sues. Osmium tetroxide staining of the coating material

was a critical step in the process.

Backscattered electron imaging provides information on

the presence and location of high atomic number sub-

stances within a specimen, which appear bright under

this imaging mode due to high yields of backscattered

electron signals, and this was the basis for coupling the

heavy metal osmium tetroxide with the coating . Osmium

tetroxide has been used previously as a stain or tracer in

other systems involving backscattered electron imaging

[31,32], but in the work described above it was the first

time backscattered electron imaging was used to under-

take high resolution imaging work on the penetration and

distribution of a coating into wood. Similar approach can

also work for examining other coating systems with

wood tissues, provided suitable high atomic number sub-

stances can be employed that can specifically react with

the coatings of interest, and without affinity for wood

cell walls.

6. Conclusion

While light microscopy has been useful in determining

the depth of coating penetration into wood, it is not suit-

able for examining wood-coating interface because of its

inability to clearly resolve particularly the interface be-

tween the outermost surface layer of wood tissues and

the applied coating. The novel and more specific mi-

croscopy approaches employed in more recent years are

proving valuable in wood-coating interaction studies,

particularly involving rough-texture wood surfaces con-

taining hig hl y di st ort ed tissues.

7. Future Prospects

The technical advances discussed in imaging wood-

coating interface are providing greater understanding of

the physical aspects of wood-coating interaction. Com-

bined use of microscopy techniques with complementary

capabilities can provide more complete information, and

in future wood-coating studies th e scope of this approach

should expand .

The majority microscopic studies undertaken have

mainly focused on examining depth of coating penetra-

tion. While understanding of this is impor tant in the con-

text of evaluation of coating adhesion, higher resolution

imaging should be combined to understand particularly

the effect of surface preparation on wood tissues at finer

levels. As documented by CLSM [15], even relatively

smooth surfacing with planer knives can generate cracks

of varying dimensions within cell walls, which collec-

tively can significantly increase cell wall surface areas,

thus enhancing the anchorage of penetrated coating ma-

terial. While LM can still serve as the basic imaging tool

for rapid assessment of coating penetration and distribu-

tion, the scope in future studies should widen to include

the assessment using also tools that can provide 3-D im-

aging capability and high resolution, such as CLSM,

FE-SEM and TEM, which in addition to revealing the

finer aspects of wood-coating interaction can provide

useful information for assessing coating performance,

such as demonstrated in a study employing TEM [5].

8. Acknowledgements

This work was supported by the Korean Ministry of

Education, Science and Technology and the Korean

Federation of Science and Techno logy Societies.

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