Using Adhesives on Wood – Best Practices (adapted from http://www.solutionsforwood.ca/_docs/reports/TP09-03UsingAdhesivesonWood.pdf)

Adhesives are widely used to manufacture a wide variety of wood products and proper gluing practices are essential to ensure the best performance. This page provides a summary of wood adhesive types and applications, surface bonding mechanisms, and processing factors and safety. In addition, frequent gluing defects and solutions for preventing them are discussed.
Wood Adhesives – Types and ApplicationsThere are two main types of adhesives used in wood products manufacturing: thermo-set and thermo-plastic. Thermo-set adhesives need an elevated temperature to start curing and once set will not return to their original state. Thermo-plastic adhesives could return to their original state if heated, mixed with the original solvent or moisturized. Each adhesive has its own suggested gluing parameters. For example, epoxy adhesives work well close to freezing, while polyvinyl acetate (PVA) is adversely affected by low temperature. For most of the water-based adhesives, the ideal gluing temperature is close to room temperature (15 to 200C).
Preparing the Surface for Bonding
Adhesives bond to wood surfaces in different ways with mechanical interlocking being the primary and most frequent bonding mechanism. In this process, the adhesive penetrates into the porous surface of the wood fibre and interlocks with the surface wood fibres as it cures. The second method of bonding is through the physical attraction between molecules (e.g., like 2 magnets). The third method of bonding is through a chemical bond where a new product is formed by the chemical combination of wood and adhesive.
To ensure a good bond, a thin glue line on a surface devoid of planer skips, torn, crushed, chipped or loose fibres is recommended. The surface to be glued has to be free of foreign material such as oil, grease, loose fibres, etc. In addition, all wood surfaces to be bonded should be glued within a maximum time of 24 hours after dressing to reducethe risk of surface oxidation and to minimize the effect of any moisture changes that may have occurred after dressing.
Sawn SurfacesSawn surfaces provide a good bonding medium for most adhesives, however, dull saws tend to crush and close the surface wood fibres. It is important to keep saws sharp. Sawn surfaces give a rougher surface than knife cut surfaces but are satisfactory for gluing structural and non-structural joints.
Sanded SurfacesSanded surfaces only give an acceptable bonding surface because the sanding operation tends to crush and close the surface wood fibres thus preventing the adhesive from mechanically interlocking with the top fibre surface.


Knife Cut SurfacesThe knife cut surface given by planers and jointers are the best surfaces for bonding wood. A knife cut surface leaves a smooth, open grain surface thus enhancing the adhesive penetration into the surface fibres of the wood to be bonded. However, if the knives are dull or the feed speed is incorrect, the resulting crushed or burnished surface can impact the
quality of the glue joint.

Practical Bonding Tests
A simple test to evaluate the bonding properties of a surface to be bonded is to wipe a wet rag over a portion of the surface to be bonded, wait one minute, and then remove the excess water. Now compare this wetted surface to an adjacent dry surface. If the wetted surface is much rougher than the dry portion, this means the surface has been altered by the machining operation and the bond could be weaker compared to a sound surface. Another test for surface gluability is the water drop test. Put a drop of water on the surface to be glued and another drop on a freshly sanded portion of the same surface. The water drop should start to be absorbed in the wood within a minute and be totally absorbed in less than 60 minutes. If not, the surface is likely to be resistant to adhesive wetting and the integrity of the joint may be compromised.

Pressure Application
The purpose of applying pressure when gluing is to bring the glued surfaces into close contact with one another. This helps the adhesive penetrate into the surfaces, wets the surfaces to be glued, and improves the spread uniformity of the adhesive over the entire glued surface. A pressure of 175 to 250 PSI is generally recommended for hardwood species and 125 to 150 PSI for softwood species. The higher the density of the wood to be bonded, the higher the pressure should be. Too much pressure will create a starved joint as the adhesive will be pushed into the wood and away from the glue joint. Too low a pressure will not ensure the close contact needed between the two pieces to be bonded.

Press Time
Press time is governed by the cure method of the adhesive. All water soluble adhesives need time for the water to diffuse into the adjacent wood material and for the adhesive to catalyze and create a sound bond between the two pieces.
Moisture Content
Most adhesives will not form a satisfactory bond if the moisture content of the wood is over 20% because adhesives cannot penetrate saturated wood and the high moisture content prevents the adhesive from interlocking with the surface fibres. In general, bond quality is reduced when the moisture content of wood is over 15% or under 5%.

Glue Spread
The glue spread rate should be just enough to completely cover both surfaces. Usually 6 to 8 mils in thickness is a sufficient glue spread rate to cover both surfaces when using waterborne adhesives.