At present, the wood processing industry still mainly uses traditional formaldehyde series adhesives, which cannot meet the requirements for the bonding of raw material systems in the new situation. With the strengthening of environmental protection requirements and the improvement of people’s environmental awareness, the development and use of environmentally friendly synthetic resin adhesives for efficient wood processing has become a common concern. Isocyanate adhesive does not contain formaldehyde harmful substances and its molecular design is flexible. Starting from the chemical structure and raw material combination, it can achieve different performance of isocyanate resin and is widely used in many fields.
Isocyanate adhesive is a kind of adhesive that contains isocyanate group (-NCO) and a small amount of urethane group (-NHCOO) in the molecular chain and has high polarity and activity. In 1848 Wurtz first used isocyanate and potassium cyanate to synthesize isocyanate. Famous chemists such as Hofmann and Curtius in the 19th century studied their properties. In 1869 Gentier initially identified the structure of the isocyanate. In 1940, the researchers of the German law company found that isocyanates had special adhesive properties. And during the Second World War, 4,4-diphenylmethane diisocyanate (MDI) was applied to the chariot glue of the tank. After the Second World War, Bayer developed the DesmodurR series of polyisocyanates and the Desmocoll series of hydroxyl-terminated polyester polyols, which are still widely used today.
Isocyanate adhesives were developed in the 1950s and have developed rapidly since the 1980s, and have thus far become a wide variety and widely used industry. In 1951 Deppe first applied isocyanate adhesives to the preparation of particleboard. In 1973, Ellingson Lumber Corporation of the United States produced a double-sided MDI particleboard for outdoor use. Wilson J. B and Fujita Wenichiro conducted in-depth studies on the bonding strength, wet strength, and viscoelastic properties of isocyanate-based adhesives. As the advantages of isocyanate adhesives are gradually discovered, their use in wood is becoming more and more widespread. China has developed isocyanate resin adhesives for particleboard, emulsifiable isocyanate resin adhesives for artificial boards, and isocyanate resin adhesives for wood bonding. Other domestic scientific research workers have also done a lot of work on the application of isocyanate adhesives in wood. Junhua et al. of Beihua University processed the phenolic products of corn starch into emulsions under certain conditions of pH and non-toxicity. The harmed synthetic rubber latex is copolymerized into the main agent of API glue. After blocking the isocyanate group of the polyisocyanate compound, it is used as a curing agent for API glue to make a two-component API glue with no aldehyde and water resistance. The three-layer composite solid wood flooring, machine-joined wood board, plywood and glulam and other glued products pressed by the glue completely meet the requirements of the relevant standards. Northeastern Forestry University Ai Jun et al. 1311 studied the strong chemical bonding of isocyanate adhesives for wood-based panels using fluorescence microscopy and Dsc analysis, especially for the bonding of crop straws (wheatgrass, rice straw) in line with China's wooden A Grade superior standard particle board. Tang Chaofa et al. studied low-cost aqueous polymer isocyanate adhesives, and blocked the isocyanates used in cross-linking agents with low-temperature sodium hydroxide method to achieve a -NCO closure rate of more than 50%. At the same time, adding a certain amount of DBP results showed that the The cost API adhesive can meet the production requirements of plywood and blockboard, and the properties of plywood and blockboard produced meet the requirements of the national standard. Xu Xinwu and Others studied the effect of modified isocyanate on the performance of straw particle board. When the density exceeds 0.75g/cm3, the bending performance of the straw particleboard meets the requirements of M3 wood particleboard in the US ASTM A208.1 standard. Researchers are currently studying new types of thermoplastic polyurethane elastomer resins, development of dry composite polyurethane adhesives, reactive flame-retardant polyurethane modified phenolic adhesives, and water-based polyurethane-modified acrylic adhesives.
At present, the application of isocyanate adhesives in the wood industry mainly takes the following forms: waterborne polymer isocyanate adhesives (API); isocyanate prepolymer adhesives; isocyanate blended composite adhesives; the most common are isocyanate and urea formaldehyde resins, tannins, etc. Blends and polyisocyanate monomers are used directly as adhesives, of which the aqueous polymer isocyanate adhesive (API) and isocyanate prepolymer adhesives are the most widely used.
(l) Waterborne polymer isocyanate adhesive (API) Waterborne polymer isocyanate adhesive (API) is a water soluble polymer (usually vinyl acetate emulsion: PVAc), an emulsion (usually styrene-butadiene latex). : SBR, polyacrylic phenol emulsion, ethyl acetate-ethylene copolymer emulsion (EVA, etc.), filler (usually calcium carbonate powder: CaCO3) as the main component of the main agent, and polyfunctional isocyanate compounds (usually P-MDI) It is composed of a cross-linking agent that is the main component. The three-dimensional cross-linking produced by the combination of the two greatly improves the water resistance of the glued joints, so that it can be used as a highly water-resistant wood adhesive. The application development of API adhesives in China is relatively late. It started in the 1990s and is currently used by manufacturers to produce plastic puzzles. The high price is a major factor in limiting the API application. In addition, the API has a short application period, and the inconvenience of the process also brings a lot of trouble to the production. However, as a new star of water-based polyurethane adhesives, API adhesives have attracted people's attention due to their excellent adhesive properties and environmentally friendly properties, and are actively engaged in its research.
(2), isocyanate prepolymer adhesive, prepolymer adhesive structure can be designed according to different occasions, to obtain different properties of the adhesive. In the field of wood processing, the type of isocyanate prepolymer adhesive used is a moisture curing isocyanate adhesive. Moisture curable polyurethane adhesives contain reactive -NCO groups that react with trace amounts of moisture in the air when exposed to air. When bonding, it can react with water on the surface of the substrate and reactive groups such as surface hydroxyl groups to generate urea bond structure. Therefore, the adhesive layer composition of the moisture-curable polyurethane adhesive after curing is a polyurethane-polyurea structure. The moisture-curable polyurethane adhesive is an ideal adhesive for the manufacture of man-made thin wood. Adhesive bonding of the wet material not only facilitates the smooth execution of the cutting process, but also improves the quality of the cut. It can also increase the output rate of the artificial thin wood. The use of moisture curing type isocyanate adhesive can not only fasten raw materials, small diameter wood, tending thinning materials and other small materials directly glued into any large-format sheet, and direct glue can also save a lot of energy consumption. It not only achieves good use of small materials, superior use of inferior materials, but also saves energy and reduces production costs.
Polymeric MDI (PMDI) 4,4-phenylmethane diisocyanate (MDI) is the most suitable diisocyanate in the particleboard manufacturing industry. Polymeric MDI adhesives contain more than 60% of high molecular weight diisocyanate isomers with a degree of polymerization close to six. The reaction of isocyanate groups with hydroxyl groups in the wood component generates urethane covalent bonds between the wood and the glue layer. Simultaneously with the moisture in the wood to generate polyurea, these reactions are the main reaction of MDI used as a wood adhesive. The formation of polyurea is beneficial to the bonding of wood, because, firstly, the molecular weight and its distribution of the adhesive molecules are increased, resulting in better interstitial properties; second, the amide group and its further reaction with isocyanate The urethane groups will accelerate the curing reaction. At the same time, the wood and the isocyanate react together to form an autocatalytic system.
Compared with the commonly used adhesives, MDI adhesives have the advantages of short pressurization time, water resistance and aging resistance of the adhesive layer. The disadvantages are that the use of MDI adhesives is prone to sticking and toxicity during the production of particleboard, and requires metering and pumping systems. Cannot be polluted by water.
The commonly used synthetic adhesives for artificial boards are aldehyde adhesives, namely urea-formaldehyde resin (UF), phenolic resin (PF), melamine-formaldehyde resin (MF) and other adhesives. The glued products of these aldehyde adhesives have formaldehyde escaping during the production and use, pollute the air and cause harm to human health, and are now subject to environmental regulations. In addition, when using aldehyde adhesives in the production of wood-based panels, the moisture content of raw materials must be between 2% and 3%, and the equilibrium moisture content of wood is generally above 10%. Therefore, the raw materials must be dried. The formaldehyde-free isocyanate-based adhesive is different from the ordinary formaldehyde adhesive. As described above, it can react with the moisture in the wood and the hydroxyl groups of the wood to form a tough, water-resistant chemical adhesive. Therefore, in addition to free formaldehyde release, there is sometimes no need to dry the raw materials in production.
The use of isocyanate vinegar adhesive also expands the raw material source of wood-based panels. A large number of crop straws, chaffs, etc., can be made of man-made panels using isocyanate-based cool adhesives, which is an efficient and environmentally friendly adhesive.
Isocyanate adhesive is a kind of adhesive that contains isocyanate group (-NCO) and a small amount of urethane group (-NHCOO) in the molecular chain and has high polarity and activity. In 1848 Wurtz first used isocyanate and potassium cyanate to synthesize isocyanate. Famous chemists such as Hofmann and Curtius in the 19th century studied their properties. In 1869 Gentier initially identified the structure of the isocyanate. In 1940, the researchers of the German law company found that isocyanates had special adhesive properties. And during the Second World War, 4,4-diphenylmethane diisocyanate (MDI) was applied to the chariot glue of the tank. After the Second World War, Bayer developed the DesmodurR series of polyisocyanates and the Desmocoll series of hydroxyl-terminated polyester polyols, which are still widely used today.
Isocyanate adhesives were developed in the 1950s and have developed rapidly since the 1980s, and have thus far become a wide variety and widely used industry. In 1951 Deppe first applied isocyanate adhesives to the preparation of particleboard. In 1973, Ellingson Lumber Corporation of the United States produced a double-sided MDI particleboard for outdoor use. Wilson J. B and Fujita Wenichiro conducted in-depth studies on the bonding strength, wet strength, and viscoelastic properties of isocyanate-based adhesives. As the advantages of isocyanate adhesives are gradually discovered, their use in wood is becoming more and more widespread. China has developed isocyanate resin adhesives for particleboard, emulsifiable isocyanate resin adhesives for artificial boards, and isocyanate resin adhesives for wood bonding. Other domestic scientific research workers have also done a lot of work on the application of isocyanate adhesives in wood. Junhua et al. of Beihua University processed the phenolic products of corn starch into emulsions under certain conditions of pH and non-toxicity. The harmed synthetic rubber latex is copolymerized into the main agent of API glue. After blocking the isocyanate group of the polyisocyanate compound, it is used as a curing agent for API glue to make a two-component API glue with no aldehyde and water resistance. The three-layer composite solid wood flooring, machine-joined wood board, plywood and glulam and other glued products pressed by the glue completely meet the requirements of the relevant standards. Northeastern Forestry University Ai Jun et al. 1311 studied the strong chemical bonding of isocyanate adhesives for wood-based panels using fluorescence microscopy and Dsc analysis, especially for the bonding of crop straws (wheatgrass, rice straw) in line with China's wooden A Grade superior standard particle board. Tang Chaofa et al. studied low-cost aqueous polymer isocyanate adhesives, and blocked the isocyanates used in cross-linking agents with low-temperature sodium hydroxide method to achieve a -NCO closure rate of more than 50%. At the same time, adding a certain amount of DBP results showed that the The cost API adhesive can meet the production requirements of plywood and blockboard, and the properties of plywood and blockboard produced meet the requirements of the national standard. Xu Xinwu and Others studied the effect of modified isocyanate on the performance of straw particle board. When the density exceeds 0.75g/cm3, the bending performance of the straw particleboard meets the requirements of M3 wood particleboard in the US ASTM A208.1 standard. Researchers are currently studying new types of thermoplastic polyurethane elastomer resins, development of dry composite polyurethane adhesives, reactive flame-retardant polyurethane modified phenolic adhesives, and water-based polyurethane-modified acrylic adhesives.
At present, the application of isocyanate adhesives in the wood industry mainly takes the following forms: waterborne polymer isocyanate adhesives (API); isocyanate prepolymer adhesives; isocyanate blended composite adhesives; the most common are isocyanate and urea formaldehyde resins, tannins, etc. Blends and polyisocyanate monomers are used directly as adhesives, of which the aqueous polymer isocyanate adhesive (API) and isocyanate prepolymer adhesives are the most widely used.
(l) Waterborne polymer isocyanate adhesive (API) Waterborne polymer isocyanate adhesive (API) is a water soluble polymer (usually vinyl acetate emulsion: PVAc), an emulsion (usually styrene-butadiene latex). : SBR, polyacrylic phenol emulsion, ethyl acetate-ethylene copolymer emulsion (EVA, etc.), filler (usually calcium carbonate powder: CaCO3) as the main component of the main agent, and polyfunctional isocyanate compounds (usually P-MDI) It is composed of a cross-linking agent that is the main component. The three-dimensional cross-linking produced by the combination of the two greatly improves the water resistance of the glued joints, so that it can be used as a highly water-resistant wood adhesive. The application development of API adhesives in China is relatively late. It started in the 1990s and is currently used by manufacturers to produce plastic puzzles. The high price is a major factor in limiting the API application. In addition, the API has a short application period, and the inconvenience of the process also brings a lot of trouble to the production. However, as a new star of water-based polyurethane adhesives, API adhesives have attracted people's attention due to their excellent adhesive properties and environmentally friendly properties, and are actively engaged in its research.
(2), isocyanate prepolymer adhesive, prepolymer adhesive structure can be designed according to different occasions, to obtain different properties of the adhesive. In the field of wood processing, the type of isocyanate prepolymer adhesive used is a moisture curing isocyanate adhesive. Moisture curable polyurethane adhesives contain reactive -NCO groups that react with trace amounts of moisture in the air when exposed to air. When bonding, it can react with water on the surface of the substrate and reactive groups such as surface hydroxyl groups to generate urea bond structure. Therefore, the adhesive layer composition of the moisture-curable polyurethane adhesive after curing is a polyurethane-polyurea structure. The moisture-curable polyurethane adhesive is an ideal adhesive for the manufacture of man-made thin wood. Adhesive bonding of the wet material not only facilitates the smooth execution of the cutting process, but also improves the quality of the cut. It can also increase the output rate of the artificial thin wood. The use of moisture curing type isocyanate adhesive can not only fasten raw materials, small diameter wood, tending thinning materials and other small materials directly glued into any large-format sheet, and direct glue can also save a lot of energy consumption. It not only achieves good use of small materials, superior use of inferior materials, but also saves energy and reduces production costs.
Polymeric MDI (PMDI) 4,4-phenylmethane diisocyanate (MDI) is the most suitable diisocyanate in the particleboard manufacturing industry. Polymeric MDI adhesives contain more than 60% of high molecular weight diisocyanate isomers with a degree of polymerization close to six. The reaction of isocyanate groups with hydroxyl groups in the wood component generates urethane covalent bonds between the wood and the glue layer. Simultaneously with the moisture in the wood to generate polyurea, these reactions are the main reaction of MDI used as a wood adhesive. The formation of polyurea is beneficial to the bonding of wood, because, firstly, the molecular weight and its distribution of the adhesive molecules are increased, resulting in better interstitial properties; second, the amide group and its further reaction with isocyanate The urethane groups will accelerate the curing reaction. At the same time, the wood and the isocyanate react together to form an autocatalytic system.
Compared with the commonly used adhesives, MDI adhesives have the advantages of short pressurization time, water resistance and aging resistance of the adhesive layer. The disadvantages are that the use of MDI adhesives is prone to sticking and toxicity during the production of particleboard, and requires metering and pumping systems. Cannot be polluted by water.
The commonly used synthetic adhesives for artificial boards are aldehyde adhesives, namely urea-formaldehyde resin (UF), phenolic resin (PF), melamine-formaldehyde resin (MF) and other adhesives. The glued products of these aldehyde adhesives have formaldehyde escaping during the production and use, pollute the air and cause harm to human health, and are now subject to environmental regulations. In addition, when using aldehyde adhesives in the production of wood-based panels, the moisture content of raw materials must be between 2% and 3%, and the equilibrium moisture content of wood is generally above 10%. Therefore, the raw materials must be dried. The formaldehyde-free isocyanate-based adhesive is different from the ordinary formaldehyde adhesive. As described above, it can react with the moisture in the wood and the hydroxyl groups of the wood to form a tough, water-resistant chemical adhesive. Therefore, in addition to free formaldehyde release, there is sometimes no need to dry the raw materials in production.
The use of isocyanate vinegar adhesive also expands the raw material source of wood-based panels. A large number of crop straws, chaffs, etc., can be made of man-made panels using isocyanate-based cool adhesives, which is an efficient and environmentally friendly adhesive.
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