VOCs explained

24 July 2014


In the last issue I defined VOCs (volatile organic compounds) and in this issue I shall give an overview of the VOCs that are emitted from wood products. As I am sure readers will know, wood consists almost entirely of organic components and this fact is behind the excellent environmental credentials of wood products. The volatile components are almost exclusively derived from the extractives.

To some extent, Table 1 gives a false impression because it implies that hardwoods and softwoods are rather similar when, of course, there are many differences between them. It is interesting to note that as the proportion of a component decreases, then the diversity of molecules within that component increases.

For example, the largest component, cellulose, is made up of glucose sugars joined together to form long polymer chains in both hardwoods and softwoods, whereas the extractives consist of many hundreds of different molecules.

Extractives are usually classified by the solvent used to extract them, ie water, alkali, ethanol, ether, etc. No single solvent can extract all the extractives that are present in wood because the polarity of the extractives ranges from polar to non-polar. Therefore, some extractives may be extracted with water (polar) but also with ethanol (slightly less polar). Consequently, extraction of extractives is normally carried out using a sequence of solvents from polar to non-polar.

The groups of compounds found in wood include: terpenes; lignins; stilbenes; flavonoids; fats; waxes; fatty acids; alcohols; and steroids.

Each of these groups can contain tens of different molecules. It is fair to say that not all the compounds present in wood extractives have been identified. The diversity between wood species, in terms of wood durability, colour, paint-ability and so on, is largely due to the extractives which are present. They are formed by trees to protect themselves from attack from bacteria, fungi, insects and, in some cases, animals.

Parenchyma cells create the extractives, which then migrate into other cells to provide a more complete protection.

Softwoods tend to contain a lot of terpenes, which is the basic component of turpentine and the resin that exudes from cut logs (see photo above). The most commonly found terpene is a-pinene.

To non-chemists it may seem strange that the other two compounds are also monoterpenes. What amazes me is that the three similar compounds result in such different smells of pine, citronella and rose oil. Research has shown that freshly made softwood based panels liberate a lot of terpenes in the first few months after production.

In addition, the amount of saturated aldehydes, like hexanal, increases during the first two weeks after manufacture and then falls with time. Overall, the terpenes tend to make up 70-90% of the TVOC observed (total VOC).

VOCs from hardwoods tend to be saturated aldehydes like hexanal (as for softwoods), organic acids like acetic acid (vinegar) and aromatics like 2-pentylfuran (a food flavouring!).

It would seem that VOC emission from hardwoods is generally at least an order of magnitude less than softwoods. This may surprise many readers, as many hardwoods are known to contain a lot of extractives. However, many of these are not volatile and so are not detected during air sampling.

 

Dr Mark Irle
Three monoterpenes. Top: a-pinene – the most common terpene found in softwood extractives. Middle: citronellal – part of the lemon scent of citronella. Bottom: geranial – part of the rose scent in a rose