The main components of wood are cellulose, hemicellulose and lignin. Cellulose and hemicellulose are hydrophilic structures that contain hydroxyl groups. Hydroxyl groups interact with water molecules to form hydrogen bonds. Consequently, wood is capable of absorbing as much as 100% of its weight in water producing swelling. Evaporation leads to shrinkage. Because this natural water absorption/evaporation cycle occurs non-uniformly, this cycle creates internal stresses within the wood. These stresses cause the wood to check, split and warp.
Research activities to improve the dimensional stability of wood have spanned many decades. Various approaches have been attempted to reduce the affinity of wood for water, such as heat treatment, chemical and enzymatic modification of hydroxyl groups on cellulose or hemicellulose, or provision of a barrier coating (either external or internal).
Currently, three commercial processes are available to impart dimensional stability to wood—thermal treatment, acetylation and furfurylation. Thermal treatment will improve the dimensional stability of wood, however, it also will cause significant loss of mechanical strength. In softwoods, acetylation generally confers an anti-swelling efficiency (ASE) of about 75% with an associated weight increase of about 26% to 28%. Acetylation requires impregnation of acetic anhydride into wood prior to initiation of acetylation. The acetylation process will generate by-product acetic acid in the treated wood. As a result, post-treatment removal of acetic acid is required; however, residual acetic acid will remain in wood. The generation of acetic acid used during wood treatment requires stainless steel, corrosion-resistant treating equipment. In addition, residual acid in the treated wood product requires the use of stainless steel and corrosion-resistant metal fastener hardware. Acetylated wood is also extremely prone to mold growth when exposed to a moist environment. As a result, acetylated wood requires a surface protective coating or colorants if used in an outdoor condition. Furfurylation generally provides treated wood with an ASE of about 60% and a weight gain of about 30%. Furfurylation processes and furfulated wood release undesirable volatile organic compounds (VOC) during the curing process. In addition, furfurylation also results in increased brittleness of the wood. The characteristics, expense and complexity of these processes for enhancing the dimensional stability of wood limit the commercial usefulness of these processes.
Research on improved treatments for enhancing the dimensional stability of wood has included cell-wall bulking treatments. The deposition of bulking agents in wood can be achieved by impregnating non-reactive bulking agents into the wood or by impregnating monomers into the wood followed by polymerization of the monomers within the wood. The bulking agents can be water soluble or insoluble, reactive or non-reactive with wood components. The bulking agents known to those skilled in the art include polyethylene glycol (PEG), phenol, resorcinol, melamine and urea-formaldehydes, phenol furfural, furfuryl-analine and furfuryl alcohol and various vinyl resins such as polystyrene, polymethyl methacrylate, polyacrylonitrile, polyvinyl chloride with the help of wood swelling agents. With the exception of PEG, most bulking agents penetrate into wood but remain in the cell lumen. As a result, these bulking treatments may temporarily retard water absorption by wood, but do not provide long-term stabilization, because the hydroxyl groups in the wood cell wall remain unmodified and are consequently still available to absorb water molecules.
Polyether polyols are generally prepared by reacting epoxides, such as ethylene oxide, propylene oxide or tetrahydrofuran with initiator in the presence of catalyst(s). Common polyether diols are polyethylene glycol (PEG), polypropylene glycol (PPG), and poly(tetramethylene) glycol (also named polytetrahydrofuran or PTMEG). PEGs are generally water soluble, while PPG and PTMEG are water insoluble and can be pressure impregnated into wood.
Water-soluble polyethylene glycol (PEG) polymers have been widely used for treating wood to maintain and improve the dimensional stability of wood and wood products by preventing shrinkage, drying and cracking, and reducing warping. In addition, pre-soaking green wood in a PEG solution can allow kiln drying of green wood at high temperatures without causing cracking, splitting or warping. PEG (even high-molecular weight PEG), however, is highly water soluble and renders it unsuitable for long-term outdoor use, because water and moisture will facilitate the leaching of PEG out of the wood.
The present inventors have discovered that water insoluble polyether polyols, such as polypropylene glycol (PPG) polymer or copolymer, or poly (tetramethylene) glycol (also named polytetrahydrofuran) polymer or copolymer, once impregnated into a wood product, provides excellent wood dimensional stabilization and is less prone to water leaching from the treated wood product than PEG.