Wood adhesives as a term can encompass many products for a variety of applications, but essentially these products are polymers designed for the joining of timber elements through application to the timber surfaces and binding them through either a chemical reaction or physical interlocking.
Historically the choice of adhesives for woodworking has been animal based glues, especially hide glues which were manufactured from the rendering of animal skins. Although their use has now been largely superseded they are still used in specialist applications such as the manufacture of musical instruments and the repair of antique furniture.
As already mentioned animal glues have been superseded by synthetic glues and these come in many forms for different applications. Some of the most common include urea-formaldehyde, phenol-formaldehyde, resorcinol formaldehyde, polyurethane, epoxy, cyanacrylate, PVA and many others. With many different base resins available and each manufacture developing their own take and brand names the selection of the right glue for a job may be challenging.
Whilst the listing of all adhesive brands and their suitability for different applications is beyond the remit of this page, when selecting an adhesive the user should take note of the base resin and its properties, things to consider include;
Does the bond have to perform in moist environments? Some resin bonds break down in moist conditions and therefore become unstable.
Will the glue line be under load or tension? Some glues creep under load (i.e. they stretch) some of the creep is non permanent and if the load is taken away the joint will return to its former state, where as some creep will leave a permanent change in the glue-line.
Will the adhesive be used in a closed environment? Whilst most adhesives today are inert once they are fully cured, the curing process may lead to the release of chemicals from the bond line and therefore care should be taken
Will the wooden article be used in a closed environment? Some resins will continue to release volatile organic compounds (VOC’s) after they have cured, these may or may not be harmful to humans (depending on what they are and in what concentration they are released). Another chemical that can be released from resins over a long term is formaldehyde and this is discussed further below.
The correct selection of adhesive can lead to a bond line within a wooden construction that is as strong as the timber that surrounds it and has lead to many products that can improve on the properties of solid timber.
Wood Adhesives and Formaldehyde
Many of the wood adhesives (especially those used in industrial applications) contain formaldehyde, a known carcinogen. Formaldehyde is naturally occurring, you are producing it now as you breath, it is produced as tomatoes ripen. However, in high concentrations it has been linked with the formation of cancer.
Because of the health risks some products manufactured using adhesives are regulated through European Standards and these set out permitted limits of formaldehyde release. These limits have reduced throughout recent history and will continue to do so, however it should be noted that these are limits for the release of formaldehyde from a product, not the amount of formaldehyde within the product, once the formaldehyde is locked away in the product it is safe.