Acetylated wood is a type of modified wood that is produced through a chemical modification process and does not contain any toxic substances. [1] It produced from a chemical reaction (named as acetylation), involving acetic anhydride and a modification process to make wood highly resistant to biological attacks by fungi and wood-boring insects and durable to environmental conditions. [2] [3] It is a new wood product in the field of wood science, following decades of research and experimentation.
The chemical modification occurs through the reaction of wood polymers especially the free hydroxyl groups present in lignin and hemicelluloses, without the need of a catalyst, forming bonds between them. The substances used, such as anhydrides, modify the structural components of wood without leaving toxic residues. This process prevents approximately 80-90% of hydroxyl (-OH) groups from forming hydrogen bonds with water molecules, effectively "locking" the cellular walls with the material. The chemical reagents employed are non-toxic, and the potential of recycling and disposal of acetylated wood can be accomplished without any restrictions. [4]
Acetylated wood is characterized by its very light colour and has been shown to possess high durability [5] and strong hydrophobic properties, as various research studies have indicated. [6] [7] [8] This wood is suitable for outdoor wooden structures, [9] as well as exterior flooring and decks. It is primarily produced from pine wood (Radiata pine), although beech is also occasionally used with this technology. Acetylated wood has minimal moisture absorption, significantly enhancing dimensional stability and natural resilience. [10]
The first patent for wood acetylation was filed in 1930 by H. Suida, an Austrian chemist. [11] Further fundamental research was conducted in the United States at the Forest Products Laboratory by the researchers Alfred J. Stamm and Harold Tarkow during the 1940s. [12]
The primary material for acetylated wood is one that contains lignocellulose. Since wood acetylation aims to produce durable wood for outdoor use, wood, which typically contains lignin and cellulose, is usually the material of choice. Acetylation is typically performed using acetic anhydride in specialized reactors made of stainless steel. Given that acetic acid decomposes when it comes into contact with the added acetic anhydride, the moisture content of the raw material significantly influences the consumption of acetic anhydride. Therefore, it is advantageous to use raw materials that are as dry as possible (2-4%). In essence, any type of wood can be acetylated. However, since each type of wood behaves slightly differently during acetylation, the process needs to be adjusted to suit the specific wood type to achieve the desired product properties.
The process begins with the introduction of chemical reactants into wood raw material, and the injection of the acetic anhydride solution. The solution can be introduced under vacuum, excess pressure, or at atmospheric pressure.
The acetylation process of wood involves impregnating the wood with an acetic anhydride solution under pressure or vacuum, heating the solution and the wood, and achieving a temperature of around 120 °C to allow the actual acetylation reaction between the wood and the reagent. The excess acetic anhydride solution that is not absorbed by the wood can be removed either before or after the acetylation reaction. To extract unreacted acetic anhydride and the resulting acetic acid from the wood, a final step involving distillation with water or steam is carried out to ensure that the wood is essentially free from acetic anhydride and acetic acid. This step is mainly to prevent the release of acetic acid odors from the final product into the environment, which is undesirable. The solution resulting from dehydration, vacuum extraction, and post-processing is collected and separated. The entire production process is time-consuming. [13]
The degree of acetylation (i.e., acetyl content) needs to be estimated by various methods to achieve successful results. A minimum acetylation degree of at least 20%-22% is required for high-strength wood. Due to the incorporation of acetyl groups, similar to water molecules, causing swelling of wood, the increase in volume can serve as a parameter for the degree of acetylation. On the other hand, the percentage increase in weight, known as WPG (Weight Percent Gain), can be used after acetylation. This also allows for conclusions to be drawn regarding the quantity of anhydrous oxalic acid that ultimately becomes chemically bound to the wood in the cell walls.
Further parameters that can be used to assess the degree of acetylation include resistance to water leaching, electrical conductivity, or methods such as HPLC analysis or spectrophotometry. [13]
Acetylation permanently alters the chemical composition of wood. This chemical modification positively affects various mechanical and physical properties, as well as the wood's resistance to insects and microorganisms that degrade it.
Depending on the type of wood and the extent of modification, acetylation typically results in wood discoloration and an increase in density and hardness. [14] Depending on the level of post-processing, the final wood product may have a more or less pronounced acetic acid odor, easily detectable during cutting.
During the acetylation process, the hydrophilic hydroxyl groups within the polymer structure of the wood's cell walls chemically react to form highly hydrophobic acetoxy groups. As a result, the hydrophobic nature of wood significantly limits its ability to absorb or release water. This means that acetylated wood exhibits a much lower equilibrium moisture content compared to natural, non-acetylated wood.
The maximum equilibrium moisture content of regular wood is typically around 30% (referred to usually as fiber saturation point of wood), while wood that has undergone acetylation treatment (with a WPG of 20%) has a final equilibrium moisture content of only 10-12%. Furthermore, water absorption rate is greatly reduced during the acetylation process. [15]
Replacing the hydrophilic hydroxyl groups within the polymer structure of wood's cell walls, e.g., cellulose, lignin, hemicelluloses, with hydrophobic acetyl groups also positively impacts its dimensional stability. This refers to the wood's ability to maintain its dimensions in changing climatic conditions. Acetylated wood exhibits approximately 70 to 80% greater dimensional stability compared to untreated natural wood. Consequently, it experiences significantly reduced swelling and shrinkage. [16]
Acetylation enhances the biological (natural) resistance of wood. Its resistance to decay by fungi, insects, and other factors is significantly improved. The reduction of maximum equilibrium moisture content to the range of 10-20% in acetylated wood prevents the minimal moisture required for fungal growth. Additionally, the molecular structures of the cell wall, which are easily degraded by fungi, change with acetylation in such a way that fungal hyphae can no longer penetrate through the porous cell wall and break it down.
Various wood species can be fully protected from brown, white, or soft rot through acetylation and upgraded to the highest resistance class, class 1, such as exceptionally durable tropical species like teak, merbau, azobe, and iroko. Acetylation can also improve wood's resistance to decay by bacteria and termites. [15]
In addition to wood decay caused by fungi, wood exposed to sunlight and rain-hail-snow undergoes photochemical degradation and gradual decomposition. On the wooden surface exposed to sunlight, low molecular weight degradation products from photo-oxidative reactions are slowly and gradually produced, primarily triggered by ultraviolet radiation. These degradation products are washed away by rain over time, resulting in either wood bleaching and a change in surface structure or localized dark spots.
Because acetylated wood has a much lower equilibrium moisture content than untreated wood, the degradation products are washed away more slowly. Additionally, the photooxidative degradation of acetylated wood's polymers occurs more slowly. Acetylation cannot stop the process of photochemical degradation in wood, but it can significantly slow it down. [15] Recent research has shown that acetylated wood is best treated with surface finishes or even paints. [17]
The company named "Accsys Technologies," based in London (formerly "Titan Wood"), was initially the sole industry producing acetylated wood. In 2007, the production of this product on a larger scale began using radiata pine (Pinus radiata) sourced from Australia and New Zealand, under the brand name Accoya at the company's manufacturing facilities in Arnhem, in the Netherlands. [18]
A U.S. company, named Eastman Chemical, also produced acetylated wood for a short period but stopped production for unknown reasons in 2014. [3]
Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.
Engineered wood, also called mass timber, composite wood, human-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of wood, together with adhesives, or other methods of fixation to form composite material. The panels vary in size but can range upwards of 64 by 8 feet and in the case of cross-laminated timber (CLT) can be of any thickness from a few inches to 16 inches (410 mm) or more. These products are engineered to precise design specifications, which are tested to meet national or international standards and provide uniformity and predictability in their structural performance. Engineered wood products are used in a variety of applications, from home construction to commercial buildings to industrial products. The products can be used for joists and beams that replace steel in many building projects. The term mass timber describes a group of building materials that can replace concrete assemblies.
Wood easily degrades without sufficient preservation. Apart from structural wood preservation measures, there are a number of different chemical preservatives and processes that can extend the life of wood, timber, and their associated products, including engineered wood. These generally increase the durability and resistance from being destroyed by insects or fungi.
In biochemistry, cellulose acetate refers to any acetate ester of cellulose, usually cellulose diacetate. It was first prepared in 1865. A bioplastic, cellulose acetate is used as a film base in photography, as a component in some coatings, and as a frame material for eyeglasses; it is also used as a synthetic fiber in the manufacture of cigarette filters and playing cards. In photographic film, cellulose acetate film replaced nitrate film in the 1950s, being far less flammable and cheaper to produce.
Fischer esterification or Fischer–Speier esterification is a special type of esterification by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. The reaction was first described by Emil Fischer and Arthur Speier in 1895. Most carboxylic acids are suitable for the reaction, but the alcohol should generally be primary or secondary. Tertiary alcohols are prone to elimination. Contrary to common misconception found in organic chemistry textbooks, phenols can also be esterified to give good to near quantitative yield of products. Commonly used catalysts for a Fischer esterification include sulfuric acid, p-toluenesulfonic acid, and Lewis acids such as scandium(III) triflate. For more valuable or sensitive substrates other, milder procedures such as Steglich esterification are used. The reaction is often carried out without a solvent or in a non-polar solvent that can facilitate Dean–Stark distillation to remove the water byproduct. Typical reaction times vary from 1–10 hours at temperatures of 60–110 °C.
In chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed acetate esters or simply acetates. Deacetylation is the opposite reaction, the removal of an acetyl group from a chemical compound.
Wood–plastic composites (WPCs) are composite materials made of wood fiber/wood flour and thermoplastic(s) such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), or polylactic acid (PLA).
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, which is formed by its reaction with moisture in the air.
Cellulose triacetate, triacetate, CTA or TAC is a chemical compound produced from cellulose and a source of acetate esters, typically acetic anhydride. Triacetate is commonly used for the creation of fibres and film base. It is chemically similar to cellulose acetate. Its distinguishing characteristic is that in triacetate, at least "92 percent of the hydroxyl groups are acetylated." During the manufacture of triacetate, the cellulose is completely acetylated; whereas in normal cellulose acetate or cellulose diacetate, it is only partially acetylated. Triacetate is significantly more heat resistant than cellulose acetate.
An organic acid anhydride is an acid anhydride that is also an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid, the formula of the anhydride being (RC(O))2O. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. Thus, (CH3CO)2O is called acetic anhydride.Mixed (or unsymmetrical) acid anhydrides, such as acetic formic anhydride (see below), are known, whereby reaction occurs between two different carboxylic acids. Nomenclature of unsymmetrical acid anhydrides list the names of both of the reacted carboxylic acids before the word "anhydride" (for example, the dehydration reaction between benzoic acid and propanoic acid would yield "benzoic propanoic anhydride").
Modified starch, also called starch derivatives, is prepared by physically, enzymatically, or chemically treating native starch to change its properties. Modified starches are used in practically all starch applications, such as in food products as a thickening agent, stabilizer or emulsifier; in pharmaceuticals as a disintegrant; or as binder in coated paper. They are also used in many other applications.
Thebacon, or dihydrocodeinone enol acetate, is a semisynthetic opioid that is similar to hydrocodone and is most commonly synthesised from thebaine. Thebacon was invented in Germany in 1924, four years after the first synthesis of hydrocodone. Thebacon is a derivative of acetyldihydrocodeine, where only the 6–7 double bond is saturated. Thebacon is marketed as its hydrochloride salt under the trade name Acedicon, and as its bitartrate under Diacodin and other trade names. The hydrochloride salt has a free base conversion ratio of 0.846. Other salts used in research and other settings include thebacon's phosphate, hydrobromide, citrate, hydroiodide, and sulfate.
A wood-decay or xylophagous fungus is any species of fungus that digests moist wood, causing it to rot. Some species of wood-decay fungi attack dead wood, such as brown rot, and some, such as Armillaria, are parasitic and colonize living trees. Excessive moisture above the fibre saturation point in wood is required for fungal colonization and proliferation. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. Wood-decay fungi consume wood in various ways; for example, some attack the carbohydrates in wood, and some others decay lignin. The rate of decay of wooden materials in various climates can be estimated by empirical models.
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In analytical chemistry, the hydroxyl value is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize the acetic acid taken up on acetylation of one gram of a chemical substance that contains free hydroxyl groups. The analytical method used to determine hydroxyl value traditionally involves acetylation of the free hydroxyl groups of the substance with acetic anhydride in pyridine solvent. After completion of the reaction, water is added, and the remaining unreacted acetic anhydride is converted to acetic acid and measured by titration with potassium hydroxide.
Thermally modified wood is engineered wood that has been modified by a controlled pyrolysis process of wood being heated to (> 180 °C) in an oxygen free atmosphere. This process changes to the chemical structures of wood's cell wall components lignin, cellulose and hemicellulose which decreases its hygroscopy and thus increases dimensional stability. Low oxygen content prevents the wood from burning at these high temperatures. Several different technologies use different media including nitrogen gas, steam and hot oil. All processes degrade strength and toughness of the treated lumber to some degree.
Wood science is the scientific field which predominantly studies and investigates elements associated with the formation, the physical and chemical composition, and the macro- and microstructure of wood as a bio-based and lignocellulosic material. Wood science additionally delves into the biological, chemical, physical, and mechanical properties and characteristics of wood as a natural material.
Furfurylated wood is the end product of treating wood with furfuryl alcohol. This chemical process is also called furfurylation. Furfurylation is a commercially used wood modification process to enhance the physical, mechanical and biological properties of wood. In this process -which is based on principles of wood science- the cell walls of the wood swell with furfuryl alcohol, which polymerizes within the cell walls. This treatment reduces the water absorption capacity of the wood, thus minimizing its tendency to deform with changes in moisture. The hardness and rot resistance of the modified wood are also improved compared to natural, untreated wood. The furfuryl alcohol used is obtained through the hydrogenation of furfural from biological agriwaste residues, such as husks and bagasse.
Holger Militz is a German wood scientist and professor at the University of Goettingen, who is an elected fellow (FIAWS) of the International Academy of Wood Science.