A chamber method differs from the gas analysis method described last time in that formaldehyde emission is measured in a controlled atmosphere that is close to typical ambient conditions in a room. Chamber volumes vary considerably, but the one cubic metre chamber is probably the most common.
Samples are placed in the chamber and then clean air is continually pumped into it at a precise rate, known as the air exchange rate. Pumping air into the chamber generates a slight pressure within it and this prevents the possibility of air from the laboratory entering the chamber. The temperature, relative humidity and speed of the air in the chamber are controlled and are kept constant during a test. The panel samples in the chamber will therefore reach equilibrium with these conditions and it is at this point that the formaldehyde emission rate is derived.
The decay curve of a typical test result is shown in. What is clear from this graph is that these tests take quite a long time to complete – typically between two and four weeks. Consequently, these methods are not really suited to product development but rather product verification.
At various points during the test, known volumes of air (between 30 and 120 litres depending on the test method) are bubbled through water (EN and ISO standards) or sodium bisulphite solution (ASTM) to capture the formaldehyde in the air. In the case of EN and ISO standards, the concentration of formaldehyde in the water is subsequently measured using the Hantzsch reaction as described last time, whereas the ASTM methods specify the chromotropic acid method.
All methods result in a solution that is analysed using a spectrometer to measure the absorption of light at a wavelength of either 412nm (EN and ISO) or 580nm (ASTM). A previously-prepared calibration curve is then used to determine the concentration of formaldehyde in the solution and, therefore, in the original sample of air.
Over the course of a week or two, enough data points are collected to provide an indication of the steady-state formaldehyde emission of the wood based panel.
Each of the standards specifies a different method for determining when a steady-state condition is achieved.
All, however, accept a change in formaldehyde emission of less than 5% over a given period as representing a quasi steady-state condition. In addition, all the standards propose that the test is stopped after 28 days, even if the steady-state condition is not reached.
I find it disappointing that there is not one accepted chamber method that everyone can follow across the world, and, given the complexity of tests for formaldehyde, I do not hold out much hope of a single test for other volatile organic compounds – of which there many.
