Acetylation of wood: journey from analytical technique to commercial reality.

WOOD WAS DESIGNED, after millions of years of evolution, to perform in a wet environment, and nature is programmed to recycle it, in a timely way, back to the basic building blocks of carbon dioxide and water through biological, thermal, aqueous, photochemical, chemical, and mechanical degradations. We learned to use wood, accepting that it changes dimensions with changing moisture content (MC), decomposes by a wide variety of macro- and micro-organisms, burns, and is degraded by acids, bases, and ultraviolet energy. The chemistry of wood has been gradually revealed based on observations over thousands of years.

[ILLUSTRATION OMITTED]

In ancient Africa, natives hardened wood spears by placing a sharpened straight wood stick in the bottom of glowing coals and then pounded the burned end with a rock, repeating this process many times until the end was sharp and hard. It would be many hundreds of years before we understood that pyrolysis of hemicelluloses produced furan resins, which when combined with carbon and compressed, results in an extremely hard composite.

The ancient Egyptians pounded dry wooden wedges into indentations chiseled in granite and water was poured onto the wood to split the giant obelisks from the side of a quarry long before we studied and understood the tremendously large swelling pressure that is exerted when wood swells. The Bible records a message to Noah to build an ark using a wood known to resist decay long before we understood how microorganisms recognized wood as a food source. Finally, the Vikings burned the outside of their ships to make them water resistant and flame retardant without knowing anything about hydrophobicity or the insulating properties of char.

Long before we had formal wood science, we had a long history of observations on the properties and performance of wood. It was well known that wood swelled and shrunk with changing MC, that wood decayed, that wood burned, and that wood was degraded by the sun. People took advantage of the positive properties and also learned to design around wood's weaknesses. It was also well known that wood was widely available, renewable, and sustainable long before we invented these words to describe these phenomena.

As we started to understand the properties and performance of wood, we discovered that the properties of wood are, for the most part, a result of the chemistry of its cell wall components. The polysaccharides (cellulose and hemicelluloses), lignin extractives, inorganics, and the matrix they are in are mainly responsible for the properties and performance of wood. Connecting studies on the chemistry of wood with observations on performance, it became clear that it was mostly the cell wall chemistry and properties that were responsible for the observed performance.

Combining all of the art and science of wood recorded from ancient times to the present, we have discovered that if you change the chemistry of wood, you change its properties and, therefore, you change its performance. From this foundation, the science of chemical modification of wood was born. We first learn by observation, progress by experimentation, and finally advance through understanding.

Chemical modification of wood can be defined as a process of bonding a reactive simple chemical to a reactive part of a cell wall polymer, with or without catalyst, to form a covalent bond between the two. This excludes chemical impregnations (dipping or soaking non-bonding chemicals in carrier solvents), polymer inclusions, coatings, and heat treatments.

Chemical modification of wood has historically been used to 1) isolate various cell wall components; 2) study differences in properties as a result of changing the chemistry; and 3) improve the performance properties of wood.

Many chemical reaction systems have been published for the modification of wood and the systems have been reviewed in the literature. The chemicals include anhydrides (such as acetic, butyric, phthalic, succinic, malaic, propionic, and butyric anhydride), acid chlorides, ketene carboxylic acids, isocyanates, formaldehyde, acetaldehyde, difunctional aldehydes, chloral, phthaldehydic acid, dimethyl sulfate, alkyl chlorides, [beta]-propiolactone, acrylonitrile, and epoxides (such as ethylene, propylene, and butylene oxides, and difunctional epoxides).

None of the studies of the chemical modification of wood and other lignocellulosic resources have gone past the research stage, except acetylation. During the past few years, many laboratories around the world have concentrated their efforts in the acetylation of biomass using liquid acetic anhydride. The successful journey of acetylation from the research stage to the commercial stage is the subject of this article.

History and Process of Acetylation

The acetylation of wood was first performed in Germany by Fuchs (1928), using acetic anhydride and sulfuric acid as a catalyst. Fuchs found an acetyl weight gain of over 40 percent, which meant that he decrystalized the cellulose in the process. He used the reaction to isolate lignin from pine wood. In the same year, Horn (1928) acetylated beech wood to remove hemicelluloses in a similar lignin isolation procedure. Suida and Titsch (1928) acetylated powdered beech and pine using pyridine or dimethylaniline as a catalyst to yield an acetyl weight gain of 30 to 35 percent after 15 to 35 days at 100[degrees]C. Tarkow (1945) first demonstrated that acetylated balsa was resistant to decay. Tarkow (1946) first described the use of wood acetylation to stabilize wood from swelling in water. Since the 1940s, many laboratories around the world have looked at acetylation of many different types of woods and agricultural resources.

The reaction of acetic anhydride with wood results in esterification of the accessible hydroxyl groups in the cell wall, with the formation of by-product acetic acid The by-product acid must be removed from the product as the human nose is quite sensitive to the odor of acetic acid. While this is easily done in the case of wood particles and …