In this issue I am going to explain the gas analysis method (EN717-2 or ISO12460-3) and in the next (June/July) I will describe the various chamber methods.

There are two approaches to measuring formaldehyde: either extract the formaldehyde in the panel; or measure the concentration of formaldehyde in the air surrounding a specimen. Most methods fall in to the latter category. In some ways this is appropriate because the quality of the air we breathe is important to all of us.

The specimens used in the gas analysis method are relatively small, being only 400mm long and 50mm wide, whereas wood based panels in use tend often to be 2m long and 1m wide. Consequently, the permeability of the panel faces has a great influence on emission rates. The edges of the gas analysis specimens are therefore sealed so that only the formaldehyde emission through the faces is measured.

The gas analyser itself is a small cylinder 555mm long and 96mm in diameter. This gives a chamber volume of just over four litres. The chamber is heated to 60^oC and clean, dry air is pumped through it at 60 litres per hour. When the air leaves the chamber it passes through two impingers (glass bottles that bubble the air through distilled water) in series to capture any formaldehyde in the air.

As mentioned above, the sample edges are sealed and any method of edge sealing is permitted, but most laboratories use aluminium tape. The problem with such tape is that it thermally expands slightly during the test and the panel sample shrinks because it dries out. On occasions, therefore, the sealing tape can buckle, exposing some of the edge. This can lead to variable results from the sample panel.

We minimise this by overlapping the tape, by about 1mm, on to the faces. Although this is not strictly following the standard it does provide more consistent results and does not seem to reduce the amount of formaldehyde observed during a test.

The test lasts for four hours. At the end of each hour the air is automatically diverted to a new pair of impingers. After the test, the formaldehyde concentration in the water is determined.

The European and ISO standards specify the Hantzsch reaction for determining formaldehyde concentration: Acetyl acetone and ammonium acetate are added to the water containing formaldehyde. These react with formaldehyde to produce diacetyldihydrolutidine (DDL). The concentration of DDL is then easily determined by light spectroscopy because DDL absorbs light at 412nm.

A previously-prepared calibration curve is then used to determine the formaldehyde concentration.

The gas analysis method is relatively quick, at four hours, plus another 1½ hours for the analysis of the water. Consequently, the method is used by laboratories such as mine for product development. When conducting screening tests, however, we have found that a two-hour test provides a sufficiently reliable indication, giving a result about 10% higher than if a full four-hour test is used.

Formaldehyde emission is dependent on the moisture content of the samples; EN 636, the European standard for plywood, recommends at least four weeks conditioning in 65% relative humidity at 20^oC prior to testing. This is no good for production control, therefore there is a need to provide moisture content corrections for this method.

The use of 60^oC and dry air in the gas analysis method causes the specimen to dry, so formaldehyde emission falls during the test. The mechanisms behind this phenomenon are not completely understood and we need to understand them if significant improvements are to be made in the future.

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