Acidity: Common Cause -- of Panel Gluing Problems

How pH contributes to adhesion problems in glued-up panels. 1998.

by Professor Gene Wengert

Editor's note: The following article is an excerpt from the book "The Wood Doctor,s Rx," by Gene Wengert, retired Professor and Extension Specialist in Wood Processing, Department of Forestry, University of Wisconsin-Madison.

Prepared by Marshall White, Virginia Tech, Blacksburg, VA

The Problem
In recent years, the staff at the Brooks Forest Products Center at Virginia Polytechnic Institute and State University has assisted in troubleshooting delamination problems in area furniture, plywood, and panel plants. One common problem the staff encountered can be summarized in the general inquiry: "We are experiencing an acute delamination problem not known before. We have changed nothing in our processing. We recently changed to a new cross band or face veneer species. How can we improve the bonding?"

The staff's examination of several delaminated samples indicated that the new veneer exhibited little or no wood failure. The problem was that no bonding had occurred. Often the new species of veneer has been an imported variety not previously used in the operation.

In an effort to discover the cause of delamination, 20 wood species, imported and domestic, were tested for wetting (and relative acidity (pH)). Unknown to the technician who performed the tests in our laboratory, two of the samples tested had apparently caused gluing problems in two area plants. The samples wetted to a similar degree with water and adhesive, using the droplet test, indicating that lack of wetting, spreading, or contact was not a problem in any species tested.

Next, the samples were tested for relative acidity or pH. pH is a numerical measure of the acidity of aqueous solutions. The pH scale goes from 0 to 14. Low pH values represent acidic solutions and high values basic, with 7 as neutral. Generally, wood is acidic and as expected all these woods exhibited pH in the acidic range. Two, however, Ilomba and prima-vera, were more nearly neutral than the others. Coincidentally, these two specific samples were the only ones of those tested which had caused gluing problems in board plants.

The pH of Wood
Many chemicals in wood are water soluble. These wood extractives in contact with water or other solvents may form a solution, suspension, or mixture which will have a pH different from the solvent. These extractives in wood differ both in amount and in kind from species to species. Therefor, the pH of different mixtures with wood and solvent varies.

Most wood extractives are weakly acidic in water and, therefore, most domestic woods are in the pH range indicated as weakly acidic. The heartwood of certain domestic species is more acidic than sapwood because the heartwood often contains more of these soluble extractives. Although information on many imported woods is lacking, they are believed to vary greatly in pH.

The pH of wood is a critical factor in the gluing process. Plural component thermosets, especially high-temperature-setting adhesives (e.g., certain urea formaldehyde and phenol formaldehyde resins), are pH sensitive - that is, the reaction rate, rate of polymerization or cure time is greatly affected by changes in pH of anything it contacts, such as wood. Moreover, most urea formaldehydes are acid-curing. Therefore, one-component dry adhesives contain an acid salt such as ammonium chloride to accelerate polymerization during hot pressing.

The combination of the acid salt in the glue and the acid in the wood is described in Figure 43-2.

This is a schematic diagram of a magnified glue line showing the glue and the wood cells, some filled with adhesive. The bonding area between the glue and the wood is called the interphase. It is composed of both wood and glue and resembles a mixture of both.

This critical bonding zone is affected by the chemical composition of both the wood and the adhesive. The acid salt in the glue plus the acidity of the wood surface determine the pH in the interphase. The pH of the glue (pH1 in Figure 43-2), "buffer" one another at the interphase. The interphase acidity (pH3), differs from that of the glue and the wood.

The pH of the heartwood of most species is similar, about 3.5 - 4.5, and the acid catalyst in the adhesive mix is adjusted to give the optimum combined pH environment at the interphase during cure. The press time and temperature are then recommended based on that particular bond environment. If, however, the pH of the wood changes - for example, a new veneer species is used which has in this case a higher or more neutral pH (e.g., 5 - 6.5), the pH environment in the interphase has changed and therefore the adhesive cure rate and bond integrity have changed, too. For example, the reaction rate of an acid-catalyzed urea formaldehyde adhesive is slowed by the now neutral wood surface and if no changes are made in press time and temperature, the glue line will emerge from the press uncured and will likely delaminate.

In any acid-catalyzed adhesion, as the wood substrate becomes more basic, the reaction rate of the adhesive in the bonding area (interphase) slows. The opposite also can be true, although less common due to the nature of most thermosets. An extremely acidic wood below 3.5 can accelerate polymerization and actually pre-cure the glue line prior to pressing, which also results in a poor bond.

Although tropical import woods are a common cause of this problem, it is by no means exclusively imported woods that result in pH-related poor bonds. The sapwood and heartwood of some domestic woods can be unusually acidic or basic. Also, often some surface contaminant has caused the unusually high pH. During production or in transit, veneers may be exposed to contaminants that may alter the acidity of the surface.

The Solution
The pH of certain wood species by no means renders them unsuitable for gluing. Every good quality control lab should have a pH meter, which can be used to diagnose the problem. Correcting the problem then is simple. One method is to extend the press cycle or sometimes increase the press temperature.

This, however, can interfere with production rates and product quality and should be considered a remedial solution only. The best procedure to use is to report your problem and the results of your pH test to the adhesive manufacturer. He can then adjust the pH of the catalyst or resin which, in combination with the wood veneer, will provide the proper curing environment at the press time and temperature compatible with your production schedule and equipment.

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison