Domestic wood, but durable

Wood properties of spruce and ash are generally desirable - ash has a mechanical strength that is similar to that of oak. Fir stands out with its fine and even structure and has no pitch pockets. But there is a problem: Unfortunately, these two woods are very susceptible to decay. In the standard test they lose about 30% of their mass and according to EN 350-2 wind up in category 5 – "not durable".
Bad luck? Well, not if you run a thermal modification kiln designed by WTT, Brande/DK. The Upper Austrian wood flooring specialist Tilo, Lohnsburg/AT, is member of the WTT-group. Since November 2010, a WTT-thermal pressure installation has been operated there. Ash, fir and pine timber is refined and converted into terrace decking. Thermally treated oak, ash, and larch the flooring specialist uses in the interior. For facades, thermo ash and pine prove very useful. All products are characterised by their dark color. Before the thermally modified timber (TMT) can be displayed under the open sky, they are subjected to a treatment inside the WTT kiln in which the elements are unleashed.

180° C at a pressure of 10 bar

From the outside it looks harmless, but this is deceptive. Only the front side of the WTT chamber is visible as Norbert Reisecker, the manager of Tilo's outdoor product range, leads his visitor to the place where TMT is made. While he speaks about durability classes, the currently most popular thermally treated wood species ("clearly ash") and about graying, pressure inside the chamber is more than 10 bar. Temperature goes up to 180 ° C. This modifies the wood structure from scratch.
What happens chemically is the following: Due to the low oxygen environment, the wood undergoes a partial pyrolysis. Hydroxy-groups which connect hemicellulose with lignin disintegrate. This is also the place where usually water molecules dock which makes wood swell. Without these hydroxy-groups, no more water is absorbed. This, in turn, means that the rate of swelling and shrinking (which causes cracks in the wood) is reduced by 50%. Another advantage: Just as water, wood-degrading fungi find their contact surface significantly reduced.

Further processing is not a problem

TMT leaves the chamber with a residual moisture content of 6%. This way, ash and fir easily make durability class 1 with a calculated product life of over 25 years in the open air. Such information of course highly depends on local conditions. Thermally modified pine reaches durability class 2 (15 to 25 years outdoor product life).
TMT can be milled and planed just as untreated wood. Norbert Reisecker demonstrates this by the example of Tilo's facade profile timber which not only has a diamond-shaped cross-section but also a groove on the front side which both gives the facade a slender appearance. Tilo's thermally treated wood products are used primarily in high-end public building projects and there mainly in terraces. Annually, 50,000 m² of these products leave the production halls in Lohnsburg. Given an average size of 20 m2 per terrace, this means that every year about 2,500 new customers take joy in using natural and lasting flooring made of Tilo-thermo wood.

Once a day, the chamber is filled

The WTT chamber at Tilo runs at full capacity. Every day it processes a full batch of wood. After the passage through the kiln is completed, the door opens and the "newly tanned" goods are moved out automatically. "The chamber at Tilo is equipped with fully automated PLC. Loading and unloading by way of a rail system are also automated", explains Thomas Forstmann. The man from north Germany is in charge of sales for the Danish equipment in the German speaking countries. To Tilo, Forstmann has sold a chamber with a filling capacity of 8 to 10 m³ of sawn timber, according to WTT. All this timber is accommodated in a 12.5 m long double-shell chamber with 1.8 m outer diameter. The interior measures 1.1 by 1.1 m and is 12 m long. About the exact batch time and capacity Tilo is secretive. According WTT, a run takes between 24 to 30 hours – depending on the thickness wood. This is enough to catapult domestic wood from a low to the top durability class.

Energy – a guide to decision making

WTT offers their chambers with lengths in three meter steps between 6 and 18 m which makes the line at Tilo a medium-sized installation. The model has a connected load of 400 kW thermally and 30 kW electrically. According to Reisecker, the comparatively low energy consumption and perfectly functioning reference plants were crucial for their decision in favour of a WTT system. The installation process posed no problems. The Danish manufacturer supplied the boiler and a container with pre-mounted control and heating. The PLC had how-to-routines programmed – just requiring the user to enter dimension, type of wood and desired treatment. The rest is executed automatically.
WTT is permanently connected to the system via remote maintenance line allowing them to check its status at all times. This works perfectly, says Reisecker who is full of admiration. If there is a fault, the employee responsible will be notified by SMS.

Coloring is important

"Thermally treated timber lines such as this one are increasingly used for coloring", says Forstmann. Eight hours at temperatures ranging from 120 to 140 °C suffice to give pale wood a rich tan. At these temperatures even oak is turning darker. And also rustic spruce which often give quaint pubs their special atmosphere can be produced this way. "It has established itself as the second track of usage for our TMT chambers", says the WTT salesman.
But TMT lines are only one part of WTT's extensive product range. It also includes vacuum and energy vacuum dryers, smoking equipment, impregnating lines and even hot-oil systems. In German-speaking countries particularly the thermal treatment equipment is booming. A plant of the same size as Tilo's is currently being commissioned at the premises of Trapa flooring in St. Veit an der Glan/AT.

Preparation step for resin impregnation

A speciality of Tilo is the Natwood treatment in which wood cavities are filled with pine resin. This "Natwood" process is performed after the thermal treatment and enhances durability and dimensional stability of thermally treated wood even further.

WTT Wood Treatment Technology - Facts

Founded: 1977
CEO: Erik Skovmand
Location: Brande/DK
Product range: vacuum drying systems (optionally with smoking-equipment), energy-saving vacuum dryer, thermal wood chambers, pressure-impregnation chambers, creosote impregnation chambers, fusing lines, double vacuum systems, hot-oil systems for linseed oil impregnation
Representative in Austria and Germany: Thomas Forstmann
Installations: 15 wood for TMT-production