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Acetylated Wood

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Term	Definition
Acetylated Wood	What is acetylated wood? Acetylated wood is wood that has undergone a chemical modification process using acetic anhydride to reduce its susceptibility to decay caused by wood-destroying fungi and insects, thereby significantly extending its service life in outdoor applications. Through this process, the molecular composition of the wood is permanently altered, resulting in improved physical and mechanical properties such as dimensional stability, increased density and hardness, and enhanced weather resistance. Since acetic anhydride and the acetic acid produced during the reaction are non-toxic, acetylated wood can be utilized and disposed of without restrictions. Photo © by depositphotos.com \| kropic 1. Production The acetylation process is based on a chemical reaction between the hydroxyl groups present in the cellulose, lignin, and hemicellulose of the wood and acetic anhydride. The goal is to convert these hydrophilic –OH groups into hydrophobic acetoxy groups, thereby substantially reducing the wood’s ability to absorb water. The raw material used is typically lignocellulosic construction wood, as the aim is to produce wood suitable for outdoor use with enhanced durability. Since the moisture content of the raw material directly influences the consumption of acetic anhydride, it is advantageous to use as dry wood as possible. Acetylation is carried out in special stainless-steel reactors. Initially, the reactor is loaded with wood, and an acetic anhydride solution is injected. This injection can take place under vacuum, overpressure, or at atmospheric pressure to ensure deep penetration of the solution into the wood. If the acetic anhydride solution is not pre-heated (typically between 70 and 150 °C), it is heated during the impregnation process. Under a pressure of approximately 2 to 5 bar, the wood is saturated with the solution. Subsequently, the wood and solution are heated to around 120 °C to initiate the actual reaction, during which the hydroxyl groups of the cell walls react with acetic anhydride to form acetyl groups and release acetic acid. To avoid any undesirable odor emissions of acetic acid during the use of the final product, a post-treatment is performed. In this process, the wood is purified by distillation with water or steam to remove nearly all unreacted substances. The resulting solution is collected, and the acetic acid contained therein can be recovered as acetic anhydride and recycled back into the process. Process and Reactions The acetylation process begins by charging the reactor with raw material and injecting the acetic anhydride solution. The addition of the solution can occur under vacuum, overpressure, or at atmospheric pressure to ensure thorough penetration of the wood. A pressure impregnation is then performed at pressures of about 2 to 5 bar, during which the wood and the reagent are heated—typically to around 120 °C—to drive the reaction. In the reaction, the hydroxyl groups (–OH) of the cell wall polymers, particularly in lignin and hemicellulose, are esterified to form acetyl groups, releasing acetic acid in the process. Excess reagent is removed either before or immediately after the acetylation reaction. A final vacuum extraction is then applied to remove both the unreacted acetic anhydride and the formed acetic acid. Finally, the wood is cleaned by distillation with water or steam, ensuring that the acetylated wood is essentially free from acetic anhydride and acetic acid, which prevents any undesirable odor emissions in later use. Evaluation of the Acetylation Process To assess the success of acetylation—that is, the degree of acetylation—various methods can be used. One measure is the volumetric increase of the wood, as the incorporation of acetyl groups leads to swelling. Additionally, the weight percent gain (WPG) of the wood mass after acetylation can be used to determine the amount of acetic anhydride bound in the wood. Other parameters, such as leach resistance, electrical conductivity, and analytical techniques like HPLC or spectrophotometry, may also be employed to evaluate the acetylation outcome. 2. Properties Acetylation permanently alters the molecular structure of wood, influencing several key properties: Material Compatibility Due to residual acetic acid in the wood, there is a potential for corrosion of metals that come into contact with acetylated wood. It is therefore recommended to use corrosion-resistant fasteners when assembling acetylated wood components. Appearance Depending on the wood species and the intensity of the treatment, acetylation can lead to a slight darkening of the wood. When exposed to UV light, the wood may experience some lightening, while weathering in outdoor conditions can cause discoloration—especially in light-colored woods such as maple or birch. Water Absorption A key advantage of acetylation is the significant reduction in water absorption. The hydrophilic hydroxyl groups of the cell wall polymers are converted into hydrophobic acetoxy groups, substantially lowering the equilibrium moisture content. For example, the equilibrium moisture content of untreated wood, which can be around 25–30%, can be reduced to approximately 10–12% in acetylated wood with a typical WPG of about 20. Additionally, the rate of water uptake is significantly reduced. Mechanical Properties Acetylation affects the mechanical properties of wood in several ways. Dimensional stability is significantly improved, with acetylated wood exhibiting a 70–80% reduction in swelling and shrinkage compared to untreated wood. The process also increases density and hardness, resulting in a 10–30% improvement in compressive strength and enhanced surface hardness. Most strength properties of solid wood change by less than 10%. Durability The chemical modification achieved through acetylation greatly enhances the natural durability of wood, making it more resistant to decay caused by fungi, insects, and other degradative agents. By reducing the maximum equilibrium moisture content to 10–20%, the conditions required for fungal growth are not met. Moreover, the acetylation process modifies the molecular structures in the cell wall that are easily degraded by fungi, thereby preventing the ingress and breakdown by fungal hyphae at certain levels of acetylation. As a result, various wood species can be protected against brown rot, white rot, or soft rot, achieving the highest durability class. Weathering Resistance In addition to biological decay, wood exposed to sunlight and precipitation undergoes a photochemical weathering process. In acetylated wood, the degradation products formed by photooxidative reactions are lighter in color compared to those in untreated wood. The reduced moisture uptake and altered cell wall structure result in a slower leaching of these degradation products, thereby retarding the overall photooxidative deterioration process. Although acetylation does not entirely prevent photochemical weathering, it significantly slows it down. 3. Products Initially, Accsys Technologies (formerly Titan Wood) in London was the sole manufacturer of acetylated wood products. Since 2007, large-scale production under the brand name Accoya has been underway at the Arnhem facility in the Netherlands, producing acetylated wood from Monterey pine (Pinus radiata). Other manufacturers, such as Eastman Chemical, now also offer acetylated wood. The product properties, such as density, swelling and shrinkage values, and mechanical characteristics, vary depending on the wood species and the degree of acetylation. 4. Alternative Modification Processes Besides acetylation, other chemical modification processes exist to improve the durability and stability of wood, such as thermal modification and furfurylation. These methods aim for similar goals—increasing dimensional stability, durability, and weather resistance—but differ in their chemical processes and the resulting material properties. 5. Applications Acetylated wood is used to enhance the performance of domestic wood species like pine or ash for outdoor applications, enabling them to reach the quality standards of tropical hardwoods. Due to its properties, acetylated wood is commonly used as decking, in facades, and in windows and doors, making it a viable alternative to tropical wood. A notable aspect of the process is its cyclical nature: at the end of the treatment, the wood is cleaned by distillation with water or steam, and the resulting solution is collected to recover acetic anhydride from the acetic acid, which is then recycled into the process. Summary Acetylated wood is produced by treating wood with acetic anhydride, which binds acetyl groups to the cell walls and substantially reduces the wood's capacity to absorb water. This results in significant improvements in dimensional stability, increased density and hardness, and enhanced durability, making the wood particularly suitable for outdoor use. The process provides an environmentally friendly alternative to traditional wood preservatives by using non-toxic chemicals and allows for unrestricted utilization and disposal of the modified wood. Future developments aim to further optimize the process, reduce costs, and lower energy consumption, thereby broadening the sustainable applications of this innovative wood modification technique. Synonyms: Acetyl wood, Chemically modified wood (by acetylation), Acetyl-modified wood

Term

Definition

What is acetylated wood?

Acetylated wood is wood that has undergone a chemical modification process using acetic anhydride to reduce its susceptibility to decay caused by wood-destroying fungi and insects, thereby significantly extending its service life in outdoor applications. Through this process, the molecular composition of the wood is permanently altered, resulting in improved physical and mechanical properties such as dimensional stability, increased density and hardness, and enhanced weather resistance. Since acetic anhydride and the acetic acid produced during the reaction are non-toxic, acetylated wood can be utilized and disposed of without restrictions.

Acetylated wood for outdoor use Photo © by depositphotos.com | kropic

1. Production

The acetylation process is based on a chemical reaction between the hydroxyl groups present in the cellulose, lignin, and hemicellulose of the wood and acetic anhydride. The goal is to convert these hydrophilic –OH groups into hydrophobic acetoxy groups, thereby substantially reducing the wood’s ability to absorb water.

The raw material used is typically lignocellulosic construction wood, as the aim is to produce wood suitable for outdoor use with enhanced durability. Since the moisture content of the raw material directly influences the consumption of acetic anhydride, it is advantageous to use as dry wood as possible.

Acetylation is carried out in special stainless-steel reactors. Initially, the reactor is loaded with wood, and an acetic anhydride solution is injected. This injection can take place under vacuum, overpressure, or at atmospheric pressure to ensure deep penetration of the solution into the wood. If the acetic anhydride solution is not pre-heated (typically between 70 and 150 °C), it is heated during the impregnation process. Under a pressure of approximately 2 to 5 bar, the wood is saturated with the solution. Subsequently, the wood and solution are heated to around 120 °C to initiate the actual reaction, during which the hydroxyl groups of the cell walls react with acetic anhydride to form acetyl groups and release acetic acid.

To avoid any undesirable odor emissions of acetic acid during the use of the final product, a post-treatment is performed. In this process, the wood is purified by distillation with water or steam to remove nearly all unreacted substances. The resulting solution is collected, and the acetic acid contained therein can be recovered as acetic anhydride and recycled back into the process.

Process and Reactions

The acetylation process begins by charging the reactor with raw material and injecting the acetic anhydride solution. The addition of the solution can occur under vacuum, overpressure, or at atmospheric pressure to ensure thorough penetration of the wood. A pressure impregnation is then performed at pressures of about 2 to 5 bar, during which the wood and the reagent are heated—typically to around 120 °C—to drive the reaction. In the reaction, the hydroxyl groups (–OH) of the cell wall polymers, particularly in lignin and hemicellulose, are esterified to form acetyl groups, releasing acetic acid in the process.

Excess reagent is removed either before or immediately after the acetylation reaction. A final vacuum extraction is then applied to remove both the unreacted acetic anhydride and the formed acetic acid. Finally, the wood is cleaned by distillation with water or steam, ensuring that the acetylated wood is essentially free from acetic anhydride and acetic acid, which prevents any undesirable odor emissions in later use.

Evaluation of the Acetylation Process

To assess the success of acetylation—that is, the degree of acetylation—various methods can be used. One measure is the volumetric increase of the wood, as the incorporation of acetyl groups leads to swelling. Additionally, the weight percent gain (WPG) of the wood mass after acetylation can be used to determine the amount of acetic anhydride bound in the wood. Other parameters, such as leach resistance, electrical conductivity, and analytical techniques like HPLC or spectrophotometry, may also be employed to evaluate the acetylation outcome.

2. Properties

Acetylation permanently alters the molecular structure of wood, influencing several key properties:

Material Compatibility
Due to residual acetic acid in the wood, there is a potential for corrosion of metals that come into contact with acetylated wood. It is therefore recommended to use corrosion-resistant fasteners when assembling acetylated wood components.

Appearance
Depending on the wood species and the intensity of the treatment, acetylation can lead to a slight darkening of the wood. When exposed to UV light, the wood may experience some lightening, while weathering in outdoor conditions can cause discoloration—especially in light-colored woods such as maple or birch.

Water Absorption
A key advantage of acetylation is the significant reduction in water absorption. The hydrophilic hydroxyl groups of the cell wall polymers are converted into hydrophobic acetoxy groups, substantially lowering the equilibrium moisture content. For example, the equilibrium moisture content of untreated wood, which can be around 25–30%, can be reduced to approximately 10–12% in acetylated wood with a typical WPG of about 20. Additionally, the rate of water uptake is significantly reduced.

Mechanical Properties
Acetylation affects the mechanical properties of wood in several ways. Dimensional stability is significantly improved, with acetylated wood exhibiting a 70–80% reduction in swelling and shrinkage compared to untreated wood. The process also increases density and hardness, resulting in a 10–30% improvement in compressive strength and enhanced surface hardness. Most strength properties of solid wood change by less than 10%.

Durability
The chemical modification achieved through acetylation greatly enhances the natural durability of wood, making it more resistant to decay caused by fungi, insects, and other degradative agents. By reducing the maximum equilibrium moisture content to 10–20%, the conditions required for fungal growth are not met. Moreover, the acetylation process modifies the molecular structures in the cell wall that are easily degraded by fungi, thereby preventing the ingress and breakdown by fungal hyphae at certain levels of acetylation. As a result, various wood species can be protected against brown rot, white rot, or soft rot, achieving the highest durability class.

Weathering Resistance
In addition to biological decay, wood exposed to sunlight and precipitation undergoes a photochemical weathering process. In acetylated wood, the degradation products formed by photooxidative reactions are lighter in color compared to those in untreated wood. The reduced moisture uptake and altered cell wall structure result in a slower leaching of these degradation products, thereby retarding the overall photooxidative deterioration process. Although acetylation does not entirely prevent photochemical weathering, it significantly slows it down.

3. Products

Initially, Accsys Technologies (formerly Titan Wood) in London was the sole manufacturer of acetylated wood products. Since 2007, large-scale production under the brand name Accoya has been underway at the Arnhem facility in the Netherlands, producing acetylated wood from Monterey pine (Pinus radiata). Other manufacturers, such as Eastman Chemical, now also offer acetylated wood. The product properties, such as density, swelling and shrinkage values, and mechanical characteristics, vary depending on the wood species and the degree of acetylation.

4. Alternative Modification Processes

Besides acetylation, other chemical modification processes exist to improve the durability and stability of wood, such as thermal modification and furfurylation. These methods aim for similar goals—increasing dimensional stability, durability, and weather resistance—but differ in their chemical processes and the resulting material properties.

5. Applications

Acetylated wood is used to enhance the performance of domestic wood species like pine or ash for outdoor applications, enabling them to reach the quality standards of tropical hardwoods. Due to its properties, acetylated wood is commonly used as decking, in facades, and in windows and doors, making it a viable alternative to tropical wood.

A notable aspect of the process is its cyclical nature: at the end of the treatment, the wood is cleaned by distillation with water or steam, and the resulting solution is collected to recover acetic anhydride from the acetic acid, which is then recycled into the process.

Summary

Acetylated wood is produced by treating wood with acetic anhydride, which binds acetyl groups to the cell walls and substantially reduces the wood's capacity to absorb water. This results in significant improvements in dimensional stability, increased density and hardness, and enhanced durability, making the wood particularly suitable for outdoor use. The process provides an environmentally friendly alternative to traditional wood preservatives by using non-toxic chemicals and allows for unrestricted utilization and disposal of the modified wood. Future developments aim to further optimize the process, reduce costs, and lower energy consumption, thereby broadening the sustainable applications of this innovative wood modification technique.

Synonyms: Acetyl wood, Chemically modified wood (by acetylation), Acetyl-modified wood