The most intensive and versatile researches have been conducted by the State Research Center VTT in Finland. The significant contribution has been made by Institute of Nature protection Technologies. On the basis of the technique developed by VTT the technology of heat treatment of Thermowood wood has been created. Wood heats up to temperature not less than 180 degrees Celsius and at the same time the ferry is protected with excessive pressure in the camera. As a result there is high - eco - friendly wood which has passed heat treatment. Its color is more dark, unlike normal wood it reacts to changes of level of humidity more steadily, and its heat - insulating characteristics at the same time are significantly improved. Being carried out at rather high temperatures, processing also increases the tree resilience to rotting. On the other hand, with change of color at the same time also durability at bend decreases.
Technological process of heat treatment of wood in press and vacuum drying tunnel.
The technology of heat treatment of wood on the scale of commercial production has been developed in LLC MB Impuls, and differs from the Thermowood technology in the fact that process of heat treatment of wood happens in press vacuum drying tunnel, at lower temperature and exhaustion, without use of steam. The fulfilled technology has allowed at lower danger of pyrolysis and safety of installation to receive the qualitative modified quality sawn timber, flavourless hot tear. The transfer of moisture and physical and chemical processes happening in wood in vacuum differ from the processes happening in the convective camera with excessive pressure and processing by high - temperature steam at decomposition of acids and humanization of wood. All this has allowed to lower enerozatrata on 1 cube of wood.
Technology of heat treatment of wood it is possible to separate on three the main the period:
- Period 1. Temperature increase and drying at low temperature in vacuum. By means of vacuum and heat temperature in the camera climbs the program approximately to 105gr. s according to the normal program of drying. At high temperature, moisture content decreases almost to zero.
- Period 2. Heat treatment. After drying temperature in the camera increases to 150.C - 170.C. On reaching necessary level, without oxygen access, temperature remains invariable 4 - 8 hours depending on final assignment of product.
- Period 3. Cooling and adjustment of moisture conditions. At final stage temperature decreases by means of system of ventilation; when temperature reaches to 50 - 60.C. As a result of heat treatment the beat changes.
Heating of wood changes number of chemical and physical properties. Change of properties is caused, mainly, by hemicellulose thermodestruction. Desirable changes begin to appear already at temperature of 140.C and do not stop in process of gradual increase in temperature. Decrease in levels of swelling results, the durability of wood increases, resistance to fungi and grizuna improves some extractives of disintegration of acids leave, wood becomes more dark on all thickness, the level of equilibrium moisture decreases, the level of acidity "pH" decreases, thermoinsulating and electric insulation properties increase. However durability and wood hardness can also change when obtaining color on category D.
Standard classification of heat treatment of wood
As their properties are accurately different, soft and strong breeds of tree have separate classification. There are two classes of heat treatment. It is inexpedient to have more than two classes as properties of wood at first, in process of increase in temperature, change slowly. As soon as temperature of heat treatment exceeds 160oC in vacuum, properties begin to change very quickly. Application more, than two classes can create risk of mixing of properties of different classes. Temperature 160oC is sufficient as the maximum value is not so high that impact of heat treatment on structural properties of wood was essential. In forming of tset not only temperature but also humanization of the decaying acids is used. At standard class of heat treatment of wood swelling and compression of wood because of moisture, change of color, and also durability are emphasized as key properties. As the corresponding material for delivery to industrial clients passes heat treatment according to the arrangement between the buyer and the manufacturer, the level of processing can be optimized carefully depending on final scope. In this case material will be subjected to heat treatment, without being categorized according to the standard scheme of classification of processing.
Wood moisture level
Concerning the end result of heat treatment the initial level of moisture content has special value. Processing can subject both the freshly - cut, and dried - up wood. Anyway in the first phase of processing wood is dried up to absolutely dry state. Drying is the longest stage of technology of heat treatment. Freshly - cut wood contains water in two forms: not attached water in gleams of cages and attached water in walls of cages and chemistry. When drying the part of water in gleams of cages passes on capillaries in the direction of fibers because of difference of surface tension and pressure. If time between gleams of cages allows free movement of water, water can do way to several meters. Otherwise drying at the capillary level reaches only several cages from end faces and surface of board. The majority of water is output by means of diffusion through walls of cages in the form of steam and in vacuum along fibers from board end face.
It is required mainly for drying of tree and takes away 80% of the used thermal energy. The general need for energy is only 20% higher, than at normal drying. Need for power supply same, as at normal wood drying.
Nature protection actions
As no chemicals but only water and heat, technology process of heat treatment of wood belongs to environmentally friendly are required. As from wood extractives are allocated, they should be overworked - for example, to burn - in order to avoid unpleasant smell. The longer self - hydrolysis time, the more appears condensation reactions. Products of condensation include β - ketone groups and the interfaced groups of carbonic acid. From all components of wood the lignin is the best of all than the others is capable to maintain heat. The mass of lignin begins to decrease only when temperature exceeds 160.C when β - aril - efirny communications begin to collapse. At high temperatures the maintenance of metoksil of lignin decreases, and some of not condensed elements will be transformed to elements diphenyl of metane type. Respectively, condensation diphenyl of metane type is the most typical reaction at temperatures in the range from 150 to 160.C. This reaction significantly influences properties of lignin at heat treatment, such as color, reactivity and solubility.
Wood contains insignificant quantity few molecular components. Less than 5% of wood are the share of extractives. This group includes, for example, terpenes, fats, wax phenols. Extractives are diverse in different breeds of tree, and the number of compound structures is very high. Extractives are not structural components of wood, the majority of compound structures easily evaporate at heat treatment.
Eko toxicity of waste water from heat treatment of fir - tree has been checked for "STBA" (the project of the European Union - improvement of short - lived breeds of tree by the corresponding pyrolitic heat treatment, 1998) . tests were carried out on the waste water received after testing of "EN 84". These tests are urged to estimate binding of cells of wood. Small samples were affected by water, and water has been tested according to "NF - EN ISO 506341" on (small fresh - water crustaceans), and tests of microtoxicity have been carried out on the shining bacteria. Tests have shown that waste water does not contain the toxic substances harmful to fresh - water and is harmless to bacteria. The wood subjected to heat treatment has been tested as bone substitute (VTT and Surgical clinic at hospital of Turku university). Preliminary tests have shown good results: the birch subjected to heat treatment has the properties similar to properties of bone. The wood subjected to heat treatment is pure, any toxic substances in it it is not revealed.
Density is defined at measurement and weight and the sizes of sample. The wood subjected to heat treatment in press and vacuum drying tunnel has big density, than the raw wood. It happens mainly because of reduction of volume with the same mass of sample when processing as wood loses the weight of the contained moisture.
General information at storage.
The wood subjected to heat treatment on technology has to be stored in the dry place. As no particular temperature condition for storage is provided. The cooled warehouse are also suitable for storage of such wood. Products should be covered or stored in the covered warehouse carefully. Packagings are stored in horizontal position, at the same time the quantity of support has to be sufficient not to allow deformation of the lower boards, at the same time contact with the earth has to be completely excluded. Before application or the further work providing pasting and or processing of surface, material needs to give enough time for "hardening" at the corresponding temperature according to recommendations of manufacturers. At raising of packagings with the processed wood the crane, by means of bifurcate loader or other similar device the gripping mechanism needs to be adjusted on the maximum distance not to damage material. Packaging is opened only just before use of products.