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Forest engineering

Rapid and economic drying

Researcher from Embrapa develops method for treating wood

In the mid 90’s, when he started to advocate to his colleagues a new method for speeding up the drying of timber, which would be capable of drastically reducing the time and money spent on this vital task for industries in this sector, forestry engineer Osmar José Romeiro de Aguiar, from Embrapa Eastern Amazon, in Belém, Pará, would frequently be told that he was crazy. Based on relaxing the tensions from drying that occur inside the timber (the outer part gets dry and the inner part remains moist) when placed in an oven, the new process promised wonders.

According to its inventor, this method reduces the water content in any kind of sawed wood – regardless of the species, thickness and original degree of humidity – to the desired levels, usually around 10%, in a period between ten and 20 times less than the normal method. And it would do this without causing any splitting or warping in the boards, and without resorting to preliminary drying in the open air. This technique consists of leaving the cut wood in a natural environment for long periods, usually months, for the planks to progressively lose a major part of their liquid content before being forwarded for as shorter stage inside a dryer, an industrial oven, which will conclude the reduction of the substance’s water level.

The excess of the water naturally present in a plank of pine, mahogany, or any other tree is removed because damp wood is unstable and difficult to work. In a state like this, it does not sandpaper well, it does not glue well, and there is no way for it to keep its original size. Besides this, there are other advantages in lowering the level of humidity in battens of oak or pau d’arco: dry wood weighs less (and reduces the cost of freight), is more resistant and rots less, as it stands up better to attacks from fungi and insects.

The questions that go away
The importance of drying timber – the sector produces about 90 million cubic meters of timber in logs and has sales of R$ 2.5 billion a year – led to a confrontation between the old technique and the one proposed by the researcher from Embrapa. “But where is it written that the procedure ought to be like that? If I don’t do the preliminary drying and adopt your method, will the timber not split or burn? And who will pay for the loss?” were the questions he would hear from colleagues and lumber dealers, when he had finished putting forward his ideas.

Today, Aguiar still hears this kind of query, but less often, because he has two trump cards up his sleeve that give him credibility: he defended his method of drying in a thesis for a doctorate at the National School for Rural Engineering, Waters and Forests (Engref), in Nancy, France, and in partnership with the French, the co-authors of the discovery, obtained a patent for the process in Brazil, the European Union, the United States, Canada, Australia, Indonesia and Malaysia.

“Now, they are coming after me to give lectures in companies and universities,” explains the researcher, who, for a question of industrial secrecy, cannot disclose all the details of his intellectual creation. He believes that the interest aroused is due to an estimate of the costs of drying with his method, on average 60% lower than the traditional methods. The savings generated by his invention derives, in good measure, from the advantage of using ovens, which use a great amount of electrical and thermal (steam energy) for a shorter period than they are used in the well-known methods.

In the conventional approach, the time spent to dry planks of oak – a noble wood used for luxury furniture, musical instruments and barrels for aging beverages – varies from a year and a half to two years. First, the wood remains drying in the open air for some 18 months and, next, it stays 40 days in an oven at 45° C. It calls for patience. “Under my method, the oak is put straight into the industrial dryer, at temperatures in excess of 70° C, and dries out in 20 days”, Aguiar avers.

Molecular fluidity
The technical name for the new method is drying based on the rheological properties of the wood, characterized by the molecular fluidity of its natural polymers, when heated to the right temperature. This method exploits the vitreous transition of lignin, a complex organic substance that is deposited on the cell walls of the trunk and gives it hardness and rigidity. If kept at its point of vitreous transition, within a temperature range, usually in excess of 70° C, the wood momentarily loses its rigid nature and acquires viscosity and elasticity. It is this more flexible condition that allows a relaxation of the tensions found in the traditional method, and this is how a quicker and more homogeneous process is made possible.

In this vitreous condition, less rigid and more viscous, the wood suffers more blandly the impact of the tensions from drying that are inherent to the process of reducing its water content; consequently, it shows fewer splits, according to Aguiar. “Think of a heated PVC tube. If you heat it a little, but not too much, it doesn’t change its physical state, but it does become more malleable. It then becomes possible to alter its outline without spoiling it or breaking it. When it cools down, the tube becomes rigid once again and keeps its new shape”, is Aguiar’s comparison.

“Something similar happens to wood that is heated and in a drying process, while it is within the range of vitreous transition. As lignin becomes temporarily viscous and soft, the wood loses water and undergoes molecular rearrangements without any great traumas. When the temperature falls, the wood becomes hard again and maintains its new water content.” If the range of vitreous transition is exceeded, all the wood inside the oven may be lost, which is a risk with the new process, which works with higher temperatures than usual.

Depending on the tree and the original level of humidity of a batch of timber, this molecular rearrangement inside the material can be brutal by the end of the drying process. In visual terms, a perceptible consequence of the drying process – and hence of these internal deformations resulting from the loss of water – is the shrinkage of the wood. When dry, a plank of eucalyptus 1 meter in width can be reduced to a mere 85 centimeters.

Aguiar hopes that the process that he invented really proves to be more effective than the usual one – a lot of people are not yet convinced of this, and, for the moment, only one company from Belém is using it commercially. “When a license for the use of the patent is negotiated with some large company, my method is going to be much better understood, and it will be disseminated,” he believes.