Adhesives in wood construction

Wood adhesives play a key role in modern industrial wood construction. Adhesives help save wood, and they can be used to build light but strong structures and to moderate the expansion and contraction that comes with the inherent moisture retention of wood. Modern industrial wood adhesives have been tailored to meet the needs of the wood industry and are constantly evolving. We asked an industry leader to tell us a little about adhesives and how they affect the properties of wood.

Adhesives are used under controlled conditions in the production of structural wood products. These products include finger-jointed sawn timber, glulam, laminated logs, CLT (cross laminated timber), plywood, and LVL (laminated veneer lumber).

Common adhesive types in use

There are a few chemically different adhesives used in bonding structural wood. The most common adhesive types are phenol-based phenol-formaldehyde (PF), phenol-resorcinol-formaldehyde (PRF), resorcinol-formaldehyde
adhesive (RF), amino resin-based (melamine-urea-formaldehyde adhesive (MUF)), moisture-curing polyurethane adhesive (PU or PUR) and emulsion polymer isocyanate adhesive (EPI). In each case, the requirements of the end product, the service class of the item (1,2 or 3) and the production line type influence the selection.

Phenol-based adhesives are mainly used in structural plywood and LVL production, meaning they are used for gluing veneer. Phenol adhesives have two or three components, cure at high temperatures and create a durable but visible dark adhesive seam. 

MUF adhesives, or two-component melamine-urea-formaldehyde adhesives, are used in the production of many structural wood products, particularly finger-jointed and glulam production. MUF cures at high temperatures and creates a colourless
adhesive joint.

One-component PUR adhesives, otherwise known as polyurethane adhesives, are used in the production of finger joints, glulam, laminated logs, and CLT. Polyurethane adhesive is also used to bond layers of LVL sheets (face-bonding). Polyurethane wood adhesives cure when exposed to moisture at room temperature and create a colourless adhesive joint. Polyurethane adhesives are prepared using a polyol and isocyanate reaction, which creates urethane bonds. Similar ingredients are also used elsewhere in everyday environments, including furniture upholstery foams and sports shoes.

Emulsion polymer isocyanate adhesives are made from dispersion adhesives and isocyanate cures. The adhesive hardeners by drying at room temperature. EPI adhesives are more commonly used outside of Europe in the production of small-dimension finger jointing, glulam and laminated logs.

Adhesive use and emissions

Adhesives based on the curing reaction of formaldehyde (phenol-based/amino resins) and isocyanate (polyurethane/epi) are used in structural bonding because they provide sufficient structural strength and durability when cured. These adhesives are used in industrial environments, and care must be taken to ensure they are used in accordance with their material safety data sheet.

In the curing process, the formaldehyde or isocyanate reacts and creates an adhesive seam with new chemical bonds between the adhesive polymers and between the wood and the glue. For example, moisture-cured polyurethane adhesive cures mainly due to the effect of wood moisture between the wood. Once cured, modern adhesives do not contain solvents and the emissions are almost non-existent. Several adhesives even meet the requirements for emission class M1 as exposed films.

Long-term adhesive bonding and testing

For adhesives, the suitability for structural bonding is indicated by either European standard EN 301 (phenol-based and amino resin adhesives), EN 15425 (polyurethane adhesives) or EN 16254 (EPI adhesives), where EN 301 and EN 15425 type I adhesives are suitable for all Use Categories (1, 2 and 3), and Type II adhesives and EN 16254 Type I adhesives are for Use Categories 1 and 2. To qualify for use, an adhesive must pass the tests required by the applicable standard.

The tests for the different standards (about ten different tests depending on the adhesive) look at temperature and humidity stress resistance under short or long-term loads. The tests are by nature either intense short-term tests (for example, boiling water resistance or a delamination test) or long term tests under constant load and in varying conditions lasting 3, 6 and 12 months. According to the tests, wood fibres usually break down before the adhesive seam, which means that structural protection and consideration of the correct service class play a more key role than adhesive seams for long-term durability.

The most recent standards have introduced a marking to indicate the test result. These markings should be found on the adhesive label or product brochure. For example, a one-component moisture-cured polyurethane adhesive suitable for all use categories will have the general marking EN 15425 1 70 GP 0.3. The marking indicates the standard according to which the adhesive has been tested (EN 15425), the type of adhesive (type 1), the general test temperature (70°C), the intended use (GP = general purpose) and the maximum allowed thickness of the adhesive bond, in this case 0.3 mm. Other application options for all adhesives are FJ (finger-jointing) or SP (special purpose).

Adhesives and wood moisture behaviour

The purpose of adhesive tests is to show that the products are suitable for the main purpose of structural wood adhesive, meaning making an adhesive joint that is stronger than the wood and that is particularly resistant to moisture and temperature fluctuations throughout the designed lifetime of the structure, which may be over 50, 75, or even 100 years. Because the user experience with current adhesives is shorter than their typical designed lifetime, the resulting adhesive bond lines need to be subjected to rigorous, long-term behavioural tests. In the quality control of glulam, laminated logs, and CLT for example, the general delamination test aims to predict the lifetime behaviour and has been developed on the basis of real multi-year environment tests. In the delamination test, the glued piece of wood is impregnated with water in a vacuum/overpressure cycle over the fibre saturation point and quickly dried to near original weight at high temperature.

The moisture retention of wood causes strain on the bond line, which the adhesive should resist for decades. The adhesive forms part of the structure of the bonded wood product and thus affect its moisture resistance. Research shows that wood adhesives can reduce the moisture of wood products compared to solid wood of the same size. Typically, the moisture properties of prefabricated wood products affects the surface layer of the wood product the most and the moisture changes are already less at the depth of the adhesive bond line.

The effect of the adhesive bond line on the transfer of moisture in the wood product depends on the adhesive used, the thickness of the adhesive bond line and the difference in humidity on the different sides of the adhesive bond. When inspected under a microscope, the wood surface being glued is not entirely even. Modern adhesives result in an average bond line thickness of about 0.1 mm, created at the microscopic level between the uneven wood surfaces. The resulting bond line thus contains thicker and thinner areas as well as gaps caused by gas or drying. Adhesive does not form a dense impenetrable film. Instead, the moisture transfers within the wood through the glue joint.

The adhesive bond line moisture content is low when the wood moisture is normal but increases with all adhesive types as the wood approaches its fibre point. Moisture in the adhesive reduces the mechanical strength of the adhesive somewhat compared with a dry joint.

Studies of water, using isotope tracking methods, show that moisture passes through adhesive bond lines and, consequently, glued wood products settle into equilibrium with the surrounding environmental moisture after a sufficient time has
passed. Further, the absorption and evaporation of moisture occurs in different directions in different wood products due to the structure of the wood. For example, each layer in CLT is at an angle of 90 degrees to the previous, which improves moisture stability when compared to solid wood.