Acetylated wood
Acetylated wood is wood that has been treated with acetic anhydride in a process of chemical wood modification in order to make colonization by wood-destroying fungi or insects more difficult and thus to extend its useful life outdoors. Acetic acid is not toxic. Recycling and disposal of the wood are possible without restrictions.
Acetylated wood hardly absorbs any moisture, which improves dimensional stability and durability .
Manufacturing
The starting material for acetylated wood is lignocellulose-containing material. Since wood acetylation aims to produce durable wood for outdoor use, lumber is usually used. The acetylation is usually done with acetic anhydride in special stainless steel reactors. Since acetic acid is split off when the water contained in the wood comes into contact with the added acetic anhydride , the moisture content of the raw material in particular determines the consumption of acetic anhydride. Accordingly, it is advantageous to use raw material that is as dry as possible. In principle, any type of wood can be acetylated. However, since every type of wood behaves slightly differently during acetylation, the acetylation must be adapted to the respective wood type in order to achieve the desired product properties. For example, oak and spruce are considered heavy, while alder , poplar and birch are easily impregnable types of wood.
Procedure and reactions involved
The process begins by charging the reactor with raw material and injecting the acetic anhydride solution. The solution can be added under vacuum, under excess pressure or at atmospheric pressure . In order to soak the wood as deeply as possible with the solution, it is impregnated at pressures of 2 to 5 bar or under vacuum. If the acetic anhydride solution has not already been injected warm (70–150 ° C), it will be heated up with the wood at the latest now, during the impregnation process. At temperatures of approx. 120 ° C, the wood and the reagent are brought to the actual reaction. The hydroxyl groups (-OH) of the wood cell wall are esterified to form acetic acid esters and acetic acid is released. The esterification takes place mainly on the hydroxyl groups of the cell wall polymers lignin and hemicellulose . A drainage, i.e. H. excess acetic anhydride solution not absorbed by the wood can be withdrawn either before the acetylation reaction or afterwards at the latest. In order to extract the acetic anhydride that has been absorbed by the wood but not reacted, as well as the acetic acid formed from the wood, an ultimate vacuum is created. Finally, the wood is cleaned by distilling off with water or steam so that it is essentially free of acetic anhydride and acetic acid. The main purpose of this is to avoid subsequent, undesired odor evaporation of the acetic acid from the finished product into the environment in the state of use. The solution resulting from dehydration, vacuum extraction and aftertreatment is collected and separated. Acetic anhydride can be recovered from the acetic acid contained therein and fed back into the process.
Assessment of the acetylation success
To achieve success, i.e. H. Various methods can be used to assess the degree of acetylation . Because the storage of the acetyl groups , similar to water molecules, causes the wood to swell , the increase in volume can serve as a parameter for the degree of acetylation. On the other hand, the weight gain WPG (weight percent gain) of the wood mass after acetylation has taken place can be used. In addition, conclusions can be drawn about the amount of acetic anhydride bound in the wood. Washout resistance , electrical conductivity or methods such as HPLC analysis or spectrophotometry can be used as additional parameters to assess the success of acetylation.
properties
With the acetylation , the molecular composition of the wood is permanently changed. The chemical modification influences various mechanical, physical properties as well as the durability of the wood against wood-degrading insects and microorganisms (see below). Depending on the type of wood and the intensity of the impregnation, acetylation usually leads to a dark discoloration and an increase in density and hardness. Depending on the complexity of the after-treatment, the finished product can smell more or less strongly, or not at all, of acetic acid.
Water absorption
During acetylation , the hydrophilic hydroxyl groups of the cell wall polymers react chemically to form hydrophobic acetoxy groups . The hygroscopicity of the wood, i.e. H. the property of giving up or releasing water is thereby severely restricted. As a result, acetylated wood has a lower maximum equilibrium moisture content compared to untreated wood . The maximum equilibrium wood moisture content of untreated wood can be reduced from approx. 25-30% by acetylation with a WPG from 20 to approx. 10 to 12%. At the same time, the rate of water absorption is reduced.
Stamina
The replacement of the hydrophilic hydroxyl groups of the cell wall polymers by hydrophobic acetyl groups also has a positive effect on the stamina , i.e. H. the dimensional stability of the wood in changing ambient climates. Acetylated wood shows a 70 to 80% reduction in swelling and shrinkage compared to untreated wood .
durability
By acetylation natural durability of the wood is, d. H. its resistance to degradation by fungi, insects, etc. significantly improved. Lowering the maximum equilibrium moisture content to 10–20% in the case of acetylated wood prevents the minimum wood moisture required for fungal growth. Furthermore, the molecular structures of the cell wall bandage, which are easily broken down by fungi, are changed by the acetylation in such a way that the fungal hyphae can no longer penetrate the cell wall bandage and break it down above a certain degree of acetylation. Various types of wood can be completely protected against brown , white or soft rot by acetylation and can achieve a durability class of 1. The resistance of the wood to the degradation of tunnel bacteria and termites can also be improved by acetylation.
Weather resistance
In addition to weathering processes of biological origin, such as B. by wood-degrading fungi, wood exposed to sunlight and precipitation is subject to a photochemical weathering process. On the wooden surface facing the sunlight, u. a. Dark-colored, low-molecular-weight degradation products from photo-oxidative reactions, which are mainly triggered by UV radiation . These decomposition products are washed out by the rain over time, which results in bleaching of the wood and a changed surface structure. Because acetylated wood has a lower equilibrium moisture content than untreated wood, the degradation products are washed out more slowly. In addition, the photooxidative decomposition of the acetylated cell wall polymers takes place more slowly. The acetylation therefore can not stop, but some put it off the photochemical decomposition process in the wood.
literature
- Holger Militz: Overview report - Acetylated wood - (Scientific and technological basics, material and economic possibilities and limits, current status of implementation). SGD Süd-Forstliche Versuchsanstalt Rheinland-Pfalz, 2011. online at: FAWF.Wald-RLP.de accessed on January 6, 2014, PDF; 915 KB
- Ulf Lohmann: Wood Lexicon. 4th edition. Nikol Verlagsgesellschaft, Hamburg 2010, ISBN 978-3-86820-086-7 .
- Callum AS Hill: Wood Modification: Chemical, Thermal and Other Processes. Wiley 2006, ISBN 0-470-02172-1 .
- Fuchs, W. (1928). To the knowledge of the genuine lignin, I .: The acetylation of the spruce wood. Reports of the German Chemical Society, 61 (5), 948–951.
Individual evidence
- ↑ a b c d e f g h i j Holger Militz: Overview report - Acetylated wood - (Scientific and technological basics, material and economic possibilities and limits, current status of implementation). SGD Süd-Forstliche Versuchsanstalt Rheinland-Pfalz, 2011, accessed in August 2020.
- ↑ Ulf Lohmann: Wood Lexicon. 4th edition. Nikol Verlagsgesellschaft, Hamburg 2010, ISBN 978-3-86820-086-7 .