From the instant an adhesive is applied to wood until the adhesive cures, several important steps must take place in order for a good glue bond to occur. In the majority of wood gluing processes, adhesive is applied to the surface of one of the pieces to be glued, and then a second piece of wood is pressed into the adhesive. It is important that the adhesive properly flows and transfers to achieve a glue line which uniformly covers both wood surfaces. Also, the adhesive must penetrate into the wood surface, filling the small voids caused by pores, checks, and other anatomical features. One step further than penetration is wetting. While penetration allows the adhesive to contact the wood deep in the crevices on the wood surface, wetting involves the attraction of the liquid adhesive to the wood surface via molecular forces. Finally, the adhesive must solidify. If any of these actions do not occur, the glue bond will be faulty. Such is the case when gluing wood which is surface inactivated. The definition, causes, and prevention of surface inactivation in wood will be discussed below.
What is Surface Inactivation?
Inactivated wood surfaces occur when the wood will not allow wetting to occur. An attempt to glue surface inactivated wood results in weak glue bonds. Inspection of the broken glue bond shows a glue line with the imprint of the opposite surface (often appearing as a wood grain pattern) and an occasional loose fiber imbedded in the glue. The appearance of the opposite surface indicates that penetration took place (the adhesive filled the voids on the wood surface), however, since the wood was surface inactivated, the adhesive was not attracted to the wood molecularly and a weak bond occurred at the adhesive/wood interface. When gluing thin dense veneers with large vessels or pores, it is possible to experience glue bleed-through (generally a sign of over-penetration) but still have poor bonding due to surface inactivated wood. In such a case, the adhesive simply flows to the outer surface of the veneer through the large pores while failing to wet the surface next to the adhesive.
What Causes Surface Inactivation?
To better understand how to prevent problems associated with surface inactivated wood, a simplified explanation of the wetting phenomenon will be presented. The components which make up wood are held together by strong molecular forces. When wood is machined, essentially what happens at the molecular level is the breaking of the bonds between wood components. Where molecules once joined are now open bonding sites which possess strong attractive forces (open bonding sites are unstable and molecules desire stability). The higher the number of available bonding sites, the greater the total attraction. It is this attraction which gives freshly machined wood its wettability.
Water, gases, microscopic dust and dirt particles, extractives in the wood, and of course, adhesives are all likely candidates for open bonding sites. The longer freshly machined wood is exposed to the atmosphere, the more of these bonding sites will be taken by gases and pollutants, and less will be available for the adhesive. This is why wood loses its wettability over time and the wood surface becomes inactivated.
When the wood is heated, the chance of an inactivated surface is increased. Heat increases the movement of extractives which exist in the wood, increasing the chance that they will move to the wood’s surface and attach to open sites. In addition, severe heat can actually alter the chemistry of wood components, destroying available bonding sites. Such is the case with veneer that has been dried too harshly, a condition often erroneously referred to as casehardening in the veneer industry. Casehardening is actually a condition related to the drying of lumber where stresses are created due to unequal shrinkage of the outer and inner areas of the board. Conditioning of the lumber corrects casehardening.
Preventing Surface Inactivation
To prevent surface inactivation in wood two measures must be taken. First, the wood must be properly dried with care taken not to over dry or over heat the wood. Second, the wood should be glued shortly after machining (including slicing or peeling of veneer). These rules become much more critical when working with naturally hard-to-glue woods such as birch and maple.
Wood may easily be tested for surface inactivation simply by placing a drop of water (using an eye dropper) gently on the surface of the veneer. If the drop sits high on the surface and retains a round shape, surface inactivation is suspect. If the drop sits flat on the surface and begins to take on an oblong shape parallel to the grain, the wood surface is not inactivated. Another test is to time how long it takes for the drop of water to absorb into the wood. If the wood surface is not inactivated, the drop will absorb in several minutes or less. In severely surface inactivated wood, the water will actually evaporate before it can be absorbed.
If surface inactivation is detected, several actions may be taken to increase the chance of good glue bonding. For solid wood, the easiest way to re-activate the wood surface is to sand the surface to be glued, creating a fresh surface (An interesting experiment is to lightly sand a small area on a piece of inactivated wood and place a drop water on both the sanded and the unsanded area for comparison). Also, additives or adjustments may be incorporated into the glue mix to increase the wettability and penetration of the adhesive.
Assistant Professor and Extension Specialist Wood & Paper Science