Wood is a porous material, large pores such as cell cavities and pits, small, such as microfibril gaps, and wood contains many hydrophilic groups, so that water can easily penetrate into the interior of the wood, and with some in the wood These components are combined in the form of chemical bonds or hydrogen bonds, causing the swelling of the filaments, thereby changing the size of the wood, that is, the dryness of the wood. More importantly, the wood is an anisotropic material, and the various aspects of the wood vary with the moisture content of the wood and unevenly expand, which is prone to warpage, deformation, cracking and other defects, which greatly restricts the application range of wood. Therefore, improving the dimensional stability of wood is of great significance for the efficient use of wood.
The expansion and contraction of wood is ultimately due to the infiltration of water into the microfibrils, which increases the gap between the filaments, which in turn affects the change of wood size. Under certain conditions, the increase of microfibril gap is limited. Stable size can be achieved if, under conditions of full swell, a substance is introduced to these gaps, limiting it to return to its original state when the water is lowered. However, only the water in the wood can cause the change of the size of the wood. This part of the water and the wood are combined in the form of chemical bonds and hydrogen bonds. If the hydrophilic group (mainly an OH) in the wood is removed or reduced, thereby reducing The water absorption of wood can also increase the stability of wood size.
According to the treatment method and effect, Stamm AJ in the United States roughly divides the methods of wood dimensional stability treatment into five categories: (1) Mechanical suppression by cross-lamination, such as veneer production in plywood production. It is to use this principle. (2) Internal or external coating of waterproof coatings, mainly including immersion treatment of organic waterproofing agents such as paint coating and paraffin. (3) Reduce the hygroscopicity of wood, including the extraction of polar substances in wood or the treatment of wood with resin impregnation. (4) Chemical cross-linking of wood cell components, the most common chemical treatment is applied to this principle. (5) The cell wall is preliminarily filled with a chemical, and the resin is impregnated, and an insoluble inorganic salt is immersed in the wood, and an acid, an alcohol, or the like is immersed in the wood to carry out an esterification reaction.
In practical applications, it is often possible to use several methods or one method at the same time. As technology advances, there are newer ways to stabilize the size of wood, such as metallizing or ceramizing wood, which not only increases the dimensional stability of the wood, but also adds many other excellent properties. In this paper, we will study and apply wood size stabilization treatment.
1 Add a water repellent to the wood to add a certain amount of water repellent (water repellent), such as paraffin, dry oil, beeswax, silicone oil or linseed oil. The treatment method is simple in operation, inexpensive, and the waterproof rate can reach 75% to 90%, and the anti-expansion rate (A3 mutual) is 70% to 85%. It is mainly used for the production of particle board or fiberboard. The water repellent is sprayed on the surface of the scrap or fiber in the form of emulsion, and the amount is 1.0% to 2.5%. The surface activity of the treated sheet is decreased, and the gluing property and the mechanical properties of the product are reduced.
2 Paint treatment Painting solid wood or board is an effective way to prevent moisture. Cover the surface of the wood with paint to block the passage of moisture to the interior of the wood. The synthetic resin paints such as nitrocellulose lacquer and aminoalkyd resin are usually used to repeatedly paint the wood, which not only achieves the purpose of preventing moisture, but also serves to beautify the wood products. However, this method is non-permanent, and once the surface of the sheet is damaged, the waterproof effect will decrease. A better effect can be achieved by covering the surface of the paint with a paraffin or olefin resin film. 3 tree è¹ diffusion treatment of low molecular weight phenolic resin (PF), adenine resin (UF), melamine resin (MF), polyacetate (PC) and other resins into the interior of the wood, heating to make it polycondensed inside the wood Insoluble matter, the environment is inserted in the gaps of the filaments, the pits and the cell cavity, so that the gap of the filaments is fully expanded, and at the same time, the moisture is prevented from entering the wood to achieve the purpose of stable size. The amount of retained A3 inter-retaining resin (wood L) increased after treatment. Moreover, the cured resin is deposited inside the cell wall and can strengthen the cells. Therefore, the mechanical strength of the wood treated by this method is increased a lot (up to 50%), but the toughness is decreased. For example, wood is impregnated with 20% PF, and A3 is 70% each when the corpse is 46% to 49%. The wood was impregnated with a low molecular weight MF (molar ratio of 1; 2.5-3). After treatment, A3 reached 47%, and the anti-hygroscopic capacity (A4 500 million) reached 36%, and the color of the material did not change significantly. The wood is impregnated with PC resin. After treatment, the wood A3 is more than 80%, and the equilibrium moisture content is very low (only 3.1%). Moreover, the internal stress of the treated wood is small, the mechanical properties are increased, the material structure is uniform, and the surface is smooth. The wood was impregnated with PVAc (5%) ten MF (50%) by vacuum pressing. The wood was not only treated with stable dimensional and small color difference, but also had certain toughness.
4 acid and alcohol diffusion treatment, immersing a mixture of low molecular organic acid and alcohol into wood, and under certain conditions, causing esterification reaction inside the wood to form polymer insoluble matter filled in the microfibril gap of wood, so that wood Inflating, while the organic acid can partially react with an OH in the wood to improve the dimensional stability of the wood and reduce the water absorption of the wood. For example, a mixture of maleic acid (M) and glycerol (G) (M/G = 3:1) is immersed in wood, and after treatment under certain conditions, the body of the wood is L=23% to 30%, A3 It is about 40% of each other and it can also improve its moisture resistance. However, this method treats wood with a pungent odor, and is also liable to cause slight hydrolysis of wood, and the mechanical properties are also reduced, so it is rarely used.
5 Polyethylene two-day (PEG) diffusion treatment impregnates the wood with a low molecular weight (M=1000-4000) PEG aqueous solution to fully lubricate the wood. After drying, the PEG remains in the wax wall and the cell wall is in an expanded state. Increased wood dimensional stability. If the wood is treated with PEG for 7d, the A3 is 60%, and the treated wood not only has improved wear resistance and toughness, but also has anti-corrosion and flame retardant effects.
6 Polar substances Extract a considerable part of the water in the wood is combined with polar groups (-OH) or polar substances in the wood. The extraction of polar substances from the wood can reduce the water absorption of the wood and thus increase the dimensional stability. Sexuality, but the method easily hydrolyzes the hemicellulose in the wood, causing its mechanical properties to decrease, and also causing discoloration (darkening) of the alkali of the wood. If 1.0% NaHCO: solution is used to extract Xing'an larch, the A3 of the heartwood can reach 44.6% and 62.8%, respectively (the content of the extract in the heartwood and sapwood is different).
7 Heat treatment can make a OH bond in the wood combine to remove water molecules to form a new chemical bond, which reduces the polar group-OH in the wood, reduces the water absorption rate of the wood, improves the dimensional stability, and produces after drying. The hygroscopic hysteresis is also the reason. However, under high heat, the wood will cause a decrease in strength due to partial pyrolysis.
8 Ethylation treatment acetylation is one of the most popular topics in academic research. The principle is to replace the radical (-OH) in wood with acetyl (CH:CO-), thereby reducing the hydrophilic groups in wood. It plays a role in reducing the water absorption rate of wood. Moreover, the introduction of the acetyl group can produce an inflating effect of the insoluble physical microfibril gap of the esterification reaction, thereby achieving the purpose of stabilizing the wood size. The usual method is to use B (such as acetic anhydride, acetyl chloride, sulfuric acid, etc.) under the action of a catalyst (such as ruthenium, magnesium perchlorate, etc.) under certain conditions to treat the wood for several hours and then dry, so that the body can be Up to 25%, 43 mutual reach 70%, while M-set each other also significantly increased. The wood treated by the method has good dimensional stability, enhanced corrosion resistance, low ductility of the fiber product, uniform surface, uniform density, no toxicity, and the strength of the wood itself does not decrease, and has now entered the application stage abroad.
9 Isocyanate treatment of isocyanate groups easily reacts with the warp groups in the wood to reduce the hydrophilic groups (mainly one OH) in the wood to achieve stable size. The principle is: WOOD-OH ten R-N=C=O-WOOD-O-CO-NH-R Commonly used agents are: toluene isocyanate (TDl), 4,4-diphenylmethane diisocyanate (MDl), etc. . It can be treated by liquid phase method or gas phase method, and the retained string decreases as the water content increases. After treatment, the wood has good dimensional stability and long-lasting strength, but the toughness has decreased. For example, when the corpse is 30%, 4â–²g reaches 70%. However, the raw materials required for this treatment method are high in price and highly toxic, and must have strict operation techniques, equipment and working conditions, so their application is also limited.
10 Formaldehyde treatment of formaldehyde Under the catalysis of strong acid or inorganic salt, it can cross-link with an OH in wood to form a methylene compound, thereby reducing the chance of combining wood with moisture and achieving the purpose of stable size. The wood is treated by this method, the formaldehyde retention is small, the dimensional stability is good, and the strength of the wood is much larger than that of the untreated material under the condition of high water content. However, due to the strong acid catalysis, the wood will be partially hydrolyzed under the action of acid and water, the mechanical strength will decrease, and the formaldehyde is toxic. If it is improperly treated, it will pollute the environment, and there are certain limitations in practical application. If the wood is pretreated with tannin, sucrose or polyethylene glycol, it is catalyzed by HCL. After 2h of formaldehyde treatment at '=95°C, the formaldehyde retention string is 2% to 4%, 43 mutual 60. % to 70%.
11 Metallization and ceramization Inject wood cells with low-melting metal or ceramic materials to limit the expansion and contraction of wood, thereby enhancing the dimensional stability of wood and greatly increasing the strength of wood. In fact, what is obtained after this method is a composite material with many excellent properties, which is one of the hotspots of current research. If the alloy (in which the lock is 50%, the lead is 31.2%, the tin is 18.8%, the melting point is 97 ° C), the wood is infused for 20-60 minutes at 130-150 ° C, which can greatly improve the dimensional stability of the wood.
In short, there are many methods for dimensional stability of wood. The specific application should be combined with the requirements of wood use, technical cost, raw material source and cost, treatment process, environmental requirements, and material properties. At present, the application of wood stability treatment is mainly used for making special occasions such as molds, baths and bearings.
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