Much more to learn

The questions arise from a number of observations of standard test methods: 1. The 3rd and 4th hour results of EN ISO 12460-3 (formerly EN 717-2) are invariably lower than the 2nd hour result. Also, the 1st-hour emission is often higher than that of the 2nd hour if the panel is less than 9mm thick.

2. The typical curve obtained during a test using EN 717-1 (or ISO 16000-9), like that in Figure 1, can be turned on its head, ie formaldehyde concentration starts low and increases to a plateau with time, if the panel has a low moisture content prior to the chamber.

3. Very dry specimens of panels do not seem to emit any formaldehyde and the same specimens will release formaldehyde when they are re-humidified.

All the above phenomena are caused by changes to the specimens’ moisture content. For example, in point 1 above, the specimens are heated at 60°C during the test, so they dry out over the four hours of the test; the link between the moisture content of a specimen and its formaldehyde release is well known.

For example, the perforator method (EN ISO 12460-5, formerly EN 120) contains a correction factor for specimens that have moisture contents of less than 3% and greater than 10%. When I speak to most people about this they tend to say that this phenomenon is caused by hydrolysis of the resin and/or other formaldehyde-containing compounds. So, if there is no water present then there is no hydrolysis – and no formaldehyde emission.

To some extent, this is logical as the polymerisation of urea formaldehyde (UF) is a condensation reaction (see Figure 2) and, therefore, the reverse is a hydrolysis reaction; so, no water means no breakdown of the UF polymer.

A closer look, however, shows that although the breakdown of the polymer is by hydrolysis, the separation of formaldehyde from urea does not involve water, or any other compound.

Consequently, one would expect to observe formaldehyde emission even in completely dry conditions, especially if one takes into account the presence of a myriad of other formaldehyde-containing compounds which are present, either in the resin or the wood polymers. These include formals, hemi-formals and methylols. The breakdown of these is purely thermodynamic and does not necessarily require water.

Just one example is shown in Figure 3.

So, although we have standard test methods, and mountains of data on formaldehyde emission, I believe there is still much to be learned by understanding what is actually happening at the molecular level.