Volume 6, Number 2, May 1984, pp.2-5

Coatings and Their Effect on Dimensional Stability of Wood

(Dr. Arno P. Schniewind, Professor of Forestry and Dr. Donald G. Arganbright, Director of Forest Products Laboratory University of California-Berkeley, both with the Forest Products Laboratory in Richmond, California, have authored "Coatings and Their Effect on Dimensional Stability of Wood." This article is based on a presentation made by Dr. Arganbright at the 1983 WAAC meeting.)

Coatings on wood fulfill a variety of functions, ranging from the purely aesthetic to protection from the elements in exterior exposures. In this article we propose to examine the role of coatings in maintaining dimensional stability of wooden objects. The discussion will focus on interior exposures as they apply to objects in museums or similar environments. Although the same principles apply to exterior exposures, the latter represent special problems because variations in relative humidity and temperature are more severe in terms of rate and magnitude of change (surface temperatures may become as high as 160° F), and because impingement of liquid water including intrusion of wind driven rain can be expected on a more or less regular basis. Interior exposures, on the other hand, require at most, protection from an occasional, accidental wetting.

Purposes of Coatings as Stabilizing Agents

There are a variety of treatments that can be used to influence the dimensional stability of wood objects. One method takes advantage of the anisotropic nature of wood by cross laminating pieces of wood, such as the veneers in plywood, to introduce mechanical restraint to shrinking and swelling. Since the direction of least shrinkage (the direction parallel to grain), coincides with the direction of highest strength and stiffness, a high degree of stabilization can be achieved in the plane of the laminations. Another class of treatments is bulking treatments which introduce various types of non-volatile agents into the cell wall in place of adsorbed water, leaving wood in a permanently swollen state. Polyethylene glycol as used in the treatment of water-logged wood falls into this category. Alternately, heat or chemical treatments can be used to alter the molecular structure of wood components by deactivating or removing sorption sites and thus reducing hygroscopicity. All treatments and methods described so far limit the total amount of possible shrinking or swelling.

Coatings, on the other hand, have no effect on the total amount of shrinking or swelling, but can have a major effect on the rate of exchange of water vapor between wood and the surrounding atmosphere. There is no known coating which adheres to wood and is also completely impervious to water vapor. Even the most effective coating will permit the eventual equilibration of a coated wood object to the relative humidity and temperature conditions of its surroundings.

The purposes of coatings may be stated as follows: 1) to reduce moisture content gradients within a piece of wood, thereby reducing its tendency to warp, 2) to protect a piece of wood from daily fluctuations in relative humidity and temperature, or from unexpected changes in the atmospheric conditions of its immediate environment, 3) to provide surface protection in the event of an accident, 4) to provide protection against vapors and gases other than moisture, and 5) to balance moisture movement and associated stresses, thus preventing warpage. An example of point five would be the sealing of the back of a panel painting.

Permeability of Films and Coatings

Since coatings achieve the purposes outlined in the previous section by controlling the rate of exchange of (principally) water vapor between the wooden object and the surrounding atmosphere, it is appropriate to examine some of the variables involved. The transfer of vapors and gases takes place by diffusion through the protective film or coating in response to gradients in partial pressures. These pressures reflect differences of concentration on the opposite faces of the film. The permeability of a coating can be measured to demonstrate the ease with which such an exchange of vapor or gas takes place. Table 1 shows that permeability very much depends on the nature of the gas and also on temperature.

Since we are more concerned with water vapor than gases, the permeability of various plastic films to water vapor at 25° C is shown in Table 2. The films show a striking range of permeability values; the lowest and highest of which differ by a factor of more than 1000. The least permeable is Saran, which explains its wide use for wrapping food items. Comparison with the data of Table 1 shows that the permeability of polyethylene film to water vapor is of the same order of magnitude as to carbon dioxide.

The permeability of some commonly used coatings is shown in Table 3. The data in this table cannot be compared with that in Tables 1 and 2 because a different measurement method was used and the units differ. In any case, the range of permeability values for coatings (Table 3) is not as great as that of the plastic films (Table 2.) It is noteworthy, however, that the values for the coatings are much lower than those for wood. Table 3 also indicates that the permeability of pine is much greater in the longitudinal direction than in the radial direction. This characteristic applies to all species of wood.

Coating Effectiveness

The total rate of exchange of water vapor permitted by a coating depends on its specific permeability. This disposition is a function of the coating's chemical nature, any additives and solvents used, and the film thickness. Urethane, for example, can be a very effective moisture barrier, not because its permeability is particularly low (Table 3), but because it can be applied in film thicknesses which are 5 to 10 times greater than those of some other coatings.

Since wood is so much more permeable parallel to the grain as compared with perpendicular to the grain, it is important to take special care when applying coatings to endgrain surfaces. Because these surfaces absorb liquids much more readily, it is difficult to achieve a satisfactory film formation. A recent study (Miller and Boxall, 1984) in England, for example, found that paint performance in exterior exposure was improved by the use of suitable endgrain sealers (solvent thinned wood primers were best) under the paint. The sealer provided a more effective exclusion of moisture.

A simple measure of coating effectiveness in interior exposure is the moisture-excluding effectiveness (MEE) equation. The formula is:

           W* - W**
    mee=----------- (100)
             W**

W* is the weight gain of a coated sample which has been exposed to 97% RH at 25° C for seven days, and W** is the weight gain of an uncoated control sample which has been exposed to the same conditions. MEE values will range from 0% for a totally ineffective coating to 100% for one which is completely impermeable. Table 4 shows some values of MEE which range from less than 10% for penetrating oils and surface treatments with wax to nearly 100% for aluminum foil set between two coats of varnish.

Similar measures can be calculated from the results of Richard (1978) who studied the effectiveness of some commonly used conservation materials in reducing dimensional movement of wood samples under cyclically varying RH conditions. The calculated values are shown in Table 5. Since neither the time nor the RH of exposure were the same, the data of Tables 4 and 5 cannot be compared to each other. However, the values in Table 5 clearly show that there can be substantial differences in the effectiveness of various coating materials commonly used in conservation in regard to controlling moisture exchange.

Internal Stresses and Balance

Wooden objects that are subject to changing environmental conditions, whether coated or not, will be subject to a loss or gain of moisture. In general these changes tend to be confined to the surface layers of the object. In the short term, this condition may lead to substantial stresses in the surface layers, if the bulk of the object does not also change moisture content. The shrinkage and swelling which might otherwise take place is restrained. Internal stresses can thus develop even if there is little or no discernible overall dimensional movement.

If such localized stresses are disposed symmetrically about the object, they balance each other and the object will retain its shape. Any unbalance, however, will lead to changes in shape commonly referred to as warping. Ready examples are found in panel paintings, which will warp under changing RH conditions unless the moisture exchange is equal for both front and back. This information would seem to indicate that panel paintings should be coated on the reverse with the same build up of ground, paint and varnish layers as on the obverse.

While the above suggests that all surfaces of a wood object should have coatings of equivalent permeability, there is an additional consideration which should be taken into account. For any given type of wood surface, the exchange of moisture depends on the sum of two resistances and not just on the permeability of a coating. One layer of resistance is established by the permeability of a coating but the other is determined by the nature of the boundary layer of air. For example, the relatively stagnant layer of air trapped behind a panel painting hanging on the wall might have a much higher resistance than the layer of air at the front which is probably subject to normal convection currents or even streams of air from ventilators or air conditioning outlets. Balance will therefore require equalizing the sum of all resistances to moisture on all the surfaces.

In summary, coatings can be effective in achieving a measure of dimensional stability for wood objects. Their effectiveness depends on their permeability, which can differ widely among various types of coatings, and on the thickness of the film. Localized moisture changes, unless they are balanced within the object, lead to internal stresses which result in warpage. Total balance in exchange of moisture from the surface requires balancing the sum of coating resistance as well as the resistance of the boundary layer of air which surrounds the object.

References

Miller, E.R. and J. Boxall. 1984. "The Effectiveness of End-grain Sealers in Improving Paint Performance on Softwood Joinery." Holz Roh-Werkstoff 42(1):27-34;

Nilsson, E. and C. M. Hansen. 1981. "Evaporation and Vapor Diffusion Resistance in Permeation Measurements by the Cup Method." J.Coatings Techn. 53(680):61-64;

Richard, M. 1978. "Factors Affecting the Dimensional Response of Wood." In: N. S. Brommelle, A. Moncrieff and P. Smith Eds., Conservation of Wood in Painting and the Decorative Arts. International Institute for Conservation of Historic and Artistic Works, London;

Rogers, C. E., J. A. Meyer, V. Stannett, and M. Zwarc. 1962. "Permeability of Plastic Films and Coated Papers to Gases and Vapors." pp.12-27, Tappi Monograph No.23, Techn. Assoc. Pulp Paper Ind., New York.

Rowell, R. M. and R. L. Youngs. 1981. "Dimensional Stabilization of Wood in Use." U.S. Department of Agriculture, Forest Products Lab., Research Note FPL-0243.

Tables

Data from Rogers et al., 1962.

Data from Nilsson and Hansen, 1981.

Data from Rowell and Youngs, 1981.

^*Similar to MEE values but not directly comparable. Data calculated from the results of Richard, 1978.