Confocal microscope--a valuable tool for examining wood-coating interface.

The suitability of confocal laser scanning microscopy (CLSM) in examining a wood-coating interface was evaluated using a clear coating system. A comparison of the images of the wood-clear coating interface obtained using CLSM and light microscopy (LM) showed a marked superiority of CLSM in revealing the details of the physical nature of the interaction between the clear coating and the wood cell walls in the surface layer. The most distinct advantage of CLSM was in its ability to clearly resolve penetration of the coating into very fine cracks in cell walls, details not obtainable with LM. The information presented here demonstrates that CLSM has the potential to greatly enhance our understanding of the physical aspects of an interaction between the wood and coating at the interface.

Keywords: Confocal microscopy, photodegradation, appearance, durability, weatherability, polyurethane, wood

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Wood exposed outdoors deteriorates from a variety of physical and biological factors and has to be protected for longer service life. The extent to which surface coatings protect wood depends primarily upon the chemical and physical characteristics of the wood and coatings as well as the strength of the coating adhesion to wood. (1,2) Paints are opaque to sunlight and offer long lasting protection to wood in outdoor applications by preventing sunlight from reaching the wood surface. The life of transparent or semitransparent coatings, such as varnishes and stains, is considerably shorter when used in outdoor situations. Photodegradation of wood cell wall components, particularly in the region of the wood-coating interface, appears to be one of the main contributory factors. Recent microscopic observations of the interface region between the wood and a clear coating provided evidence that weathering-induced degradation of middle lamella and consequent separation of the cells in the surface layers was the primary cause of failure of this coating in an outdoor exposure trial. (3) In another microscopic study conducted to assess the performance of a semitransparent stain film which had been applied to a veneer surface, loss in film adhesion appeared to be caused by photodegradation of lignin in the walls of the outermost cell layer. (4) These observations are consistent with the view that the preweathering of a wood surface has an undesirable effect on coating adhesion. (5)

Clearly, weather-induced changes in the wood cell wall composition/structure in the interface region greatly influence the coating adhesion to wood. Therefore, this region needs to be closely examined to obtain a better understanding of the interaction between wood and applied coatings. However, much of the earlier microscopy work had been devoted to examining the depth of coating penetration into wood (6-9) because of its perceived importance in coating stability. Confocal laser scanning microscopy (CLSM) is one of the novel tools we are employing in our work that is aimed at enhancing the durability of clear coatings in outdoor applications to obtain detailed information about the nature of the mechanical anchorage of these coatings to variously prepared wood surfaces. In this article, we illustrate the use of CLSM in obtaining images of a wood-coating interface high enough in resolution to reveal coating penetration into very fine cracks and openings in the walls of the outermost layer of the wood cells.

EXPERIMENTAL

Panel Preparation

Kiln-dried radiata pine (Pinus radiata) panels measuring 380 X 120 X 20 mm (longitudinal X tangential X radial) were machine-surfaced and sealed with an isocyanate coating prior to applying the polyurethane coating. Three coats of a polyurethane-based clear varnish were spray-applied with light sanding between coats.

Light Microscopy

Hand-and sliding-microtome cut sections of the wood-polymer composite in the region of the wood-coating interface were stained sequentially with 0.1% aqueous toluidine blue and 0.1% Sudan IV (in 80% ethyl alcohol) to enhance the contrast of the cell walls and the coating, respectively. The sections were then mounted in glycerol on a glass slide and examined with a Zeiss Photo-microscope II after placing a cover glass over the sections.

Confocal Laser Scanning Microscopy

The stained sections examined with light microscopy (LM) were marked so that the same sections could be viewed with CLSM. Using confocal fluorescence, the contrast between the coating and the wood tissues in the stained sections was superior to that in the unstained sections, enabling the coating to be clearly differentiated from the underlying wood. The sections were imaged with a Leica TCS/NT CLSM. Confocal images were acquired using an argon/krypton laser with excitation wavelengths of 488, 568, and 647, and a 16 X multi-immersion lens with a numerical aperture of 0.5. Selected regions were zoomed to about three times. Images were collected at 600 and 665 nm.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Images of a wood-coating interface obtained using LM and CLSM are shown in Figures 1 and 2, respectively. Whereas the presence of coating material (stained red) within a ray and in the lumina of underlying tracheids can be clearly resolved in Figure 1, it is difficult to say whether the coating has also penetrated the cell walls, which appear light blue. In comparison, in the CLSM image of this region (Figure 2), it is apparent that the coating has also penetrated the cell walls. Crim-son-colored coating material is present within the cell wall cracks, which are likely to have formed during surface preparation of the wood panel. The presence of the crimson-colored material within the cell wall spaces consisting of a range of dimensions, including very fine cracks, resulting from the mechanical damage can be clearly differentiated from the purple-colored cell wall (Figure 2). In some cases, the outermost cell walls in contact with the coating layer appeared light crimson, an indication that these walls were delaminated, enabling the coating to penetrate into interfibrillar gaps (not illustrated).

Coating adhesion to wood is influenced by several factors, including mechanical anchorage of the coating by wood cells. In this regard, the depth of coating penetration, coating distribution within tissues, and the physical interaction between the coating and the outermost cell layer are all important. While the extent of the coating penetration into wood has been extensively studied by microscopy for different coating and wood types, (6-9) detailed information about the physical nature of a coating's interaction with the cell lumina and cell walls directly exposed to the coating layer is lacking. The information provided here shows the value of CLSM in revealing the intricate nature of penetration pathways of the coating into the cell wall cracks and crevices in the outermost cell layer, which may have formed during planing and subsequent sanding of wood panels. More importantly, it is the ability of CLSM to resolve the very fine cracks that is of great value, emphasizing that the wood-coating interaction at the very interface may play a very crucial role in coating adhesion and needs to be examined more closely. CLSM is an ideal tool for this kind of work because of its capabilities in: (1) high resolution imaging free of artifacts, and (2) optical sectioning, which makes it possible to develop a composite of a series of sections. Light microscopy and scanning electron microscopy (SEM) are the two most widely used microscopy techniques in coatings research, and each has certain drawbacks that can be overcome by the use of CLSM. The resolution of LM is inferior to that of CLSM. Furthermore, using LM makes it difficult to bring both the wood and coating into the same focus, particularly at high magnifications, primarily because of hardness differences between the wood and coating. While the resolution of SEM is much greater than that achievable by CLSM, cell wall distortions due to beam damage pose severe limitations to observing a wood-coating interface. Additionally, coatings are often poorly differentiated from the underlying wood in the conventional mode of SEM viewing.

CONCLUSIONS

In our coatings work, CLSM is proving to be a valuable tool in examining a wood-coating interface at high resolution, which is enabling us to understand the intricate physical interaction between the coating and the wood surface. Undoubtedly, coating penetration into fine cracks in the surface layer of wood cell walls enhances the coating anchorage afforded by the penetration of a coating into larger voids, such as cell lumina. CLSM will form an important part of the performance evaluation system in our future work aimed at enhancing the durability of clear coatings in outdoor applications.

References

(1) Williams, R.S. and Feist, W.C., "Effect of Preweathering, Surface Roughness and Wood Species on the Performance of Paints and Stains," JOURNAL OF COATINGS TECHNOLOGY, 66, No. 728, 109 (1994).

(2) Feist, W.C., "Wood Properties and Finish Durability," JOURNAL OF COATINGS TECHNOLOGY, 74, No. 926, 71 (2002).

(3) Singh, A.P. and Dawson, B.S.W., "The Mechanism of Failure of Clear Coated Wooden Boards as Revealed by Microscopy," IAWA J., 24, No. 1, 1-11 (2003).

(4) Singh, A.P., Dunningham, E.A., and Plackett, D.V., "Assessing the Performance of a Commercial Wood Stain by Transmission Electron Microscopy," Holzforschung, 49, 255-258 (1995).

(5) Williams, R.S. and Feist, W.C., "Duration of Wood Preweathering: Effect on the Service Life of Subsequently Applied Paint," JOURNAL OF COATINGS TECHNOLOGY, 73, No. 930, 65 (2001).

(6) de Meijer, M., Thurich, K., and Militz, H., "Comparative Study on Penetration Characteristics of Modern Wood Coatings," Wood Sci. Technol., 32, 347-365 (1998).

(7) Nussbaum, R.M., Sutcliffe, E.J., and Hellgren, A.-C., "Microautoradiographic Studies of the Penetration of Alkyd, Alkyd Emulsion and Linseed Oil Coatings into Wood," JOURNAL OF COATINGS TECHNOLOGY, 70, No. 878, 49 (1998).

(8) Rijckaert, V., Stevens, M., Van Acker, J., de Meijer, M., and Militz, H., "Quantitative Assessment of the Penetration of Water-Borne and Solvent-Borne Wood Coatings in Scots Pine Sapwood," Holz Roh Werkst., 59, 278-287 (2001).

(9) Van den Bulcke, J., Rijckaert, V., Van Acker, J., and Stevens, M., "Quantitative Measurement of the Penetration of Water-Borne Coatings in Wood with Confocal Laser Microscopy and Image Analysis," Holz Roh Werkst., 61, 304-310 (2003).

Adya P. Singh and Bernard S.W. Dawson--New Zealand Forest Research Institute Ltd.*

* Private Bag 3020, Rotorua, New Zealand. Email: [e-mail]; [e-mail].