Adhesives used in wood composites such as plywood and veneer lumber are generally cured at lower pressures. In order to have a sufficient contact area between the adhesive and the surface of the substrate to improve the bonding strength, a method of adding a filler and an extender to the adhesive is usually used to satisfy this requirement. According to relevant information, the maximum addition amount of additives can reach 15%. Changes in the raw material market and advances in forest product processing technology have placed increasing demands on additives. At present, the main rubber used in the wood industry at home and abroad is urea-formaldehyde resin glue, which accounts for more than 80% of the total amount of wood adhesive. At home and abroad, additives are generally added during the adjustment of the glue. First, the glue defects such as the glue, lack of glue and bubbling of the plywood can be overcome; secondly, the cost of the adhesive can be reduced. The United States has done a lot of work in the research and development of additives, and has formed a stable industry in this area. The annual output value of additives is 15 million US dollars.
1 History and current status Wood adhesive additives can be divided into two categories according to their use. One is a filler, mainly including lignocellulosic materials, furfural residues and other organic residues, as well as inorganic materials such as clay; and second, extenders, mainly protein and starchy substances such as wheat flour, corn flour, and Starch, etc., in the actual production, starch is applied more. However, the definition of fillers and extenders is not very strict. According to the American Society for Testing and Materials (ASTM), fillers are non-adhesive substances whose function is to improve the properties of adhesives, thereby improving the bonding properties and bonding quality; The extender is a substance with a certain kind of glue. The addition of this substance to the adhesive can reduce the amount of adhesive. Lignocellulosic materials, such as wood flour and nut shell powder, have been used as fillers for synthetic resin adhesives for wood for about 50 years. In 1950, the filler used in structural plywood in the United States was mainly nut shell powder, followed by Douglas fir bark powder. In 1962, Alnus bark powder was developed as a filler. The furfural residue filler developed at the same time becomes a main filler for structural plywood, especially for southern pine plywood adhesives. Particleboard sanding dust (also known as resin wood powder) has been used as an additive for decades. In addition, pine powder, Douglas fir and some hardwood wood powder are used as fillers. Nutshell powder, wood flour, clay and mixtures thereof are also frequently used fillers in some cases. At present, there are many manufacturers using protein and starch substances as extenders for wood adhesives. China's plywood industry mainly uses flour as a bulking agent; in the United States, structural plywood, laminated timber, hardwood plywood and decorative plywood are also mainly used as bulking agents for flour.
2 Market and price The use of China's additives and market conditions have not been reported in detail, and there are no exact statistics. In the United States, additives are mainly used in plastics, adhesives and sealing materials. The amount used in the plastics industry was 25.87 million tons, the amount in adhesives and sealing materials was 42.30 million tons, and the amount in wood adhesives was 72,000 tons. In 1992, the amount of additives used in phenolic resins in the United States accounted for 90% of the total amount of additives used in wood adhesives. The rest were mainly used for urea-formaldehyde resin glue, melamine-formaldehyde resin glue and resorcinol resin glue. The value of the additives used in multi-billion dollar wood adhesives is about $15 million. As US additives have become specialized in production, environmental changes may have a corresponding impact on the additive market. If floods and storms interrupt traffic, the destruction of raw materials, and the occurrence of pests and diseases, the production of nut shells will have a certain impact on the market. The furfural residue used in the United States is mainly imported from Asia. Due to the increase in the production of Asian furfural residues, the price of imported furfural residues in the United States has decreased by 30%, which has caused a local factory to close due to uncompetitive product prices. . In China, due to the single use of the species, the use of flour is basically used, so the above environmental changes have little impact on the market. The price of the additive depends on the type of raw material, the availability of the raw material, the cost of production, the requirements and quality of the adhesive. In 1993, the price of additives for wood adhesives in the United States was 170-270 US dollars / ton. In China, the price of the main used flour has risen to 2,000 to 2,500 yuan / ton, a large increase, many companies have been overwhelmed.
3 various performance indicators of additives
3 1 The physical and chemical properties of the additive, including color, particle size, water content, pH and ash, determine the coating method of the adhesive. Indoor furniture is often light-colored, if you use the test, you can use dark color to judge the wood breaking rate. The adhesive generally used for roll coating has an additive particle size of about 200 mesh, and the spray or spray coated adhesive has a size of about 325 mesh. The moisture content of the additive is preferably from 7% to 12%. The pH and buffer capacity of the additive are critical to its compatibility with the resin, and the ash content plays an important role in tool wear. However, some of the physical and chemical properties of the additive are interlinked. For example, the solubility and dispersion properties of various additives in water, alcohol and sodium hydroxide solution are different, and these characteristics have an important influence on the bonding process. In addition, the molecular affinity of the filler is more sluggish than the protein and starch extenders. In most applications, additives are an important factor in improving gluing properties and achieving a gluing standard for gluing materials. Certain additives, such as walnut shell powder, contain an oily substance that acts like a blowing agent during mixing with the adhesive, and in general, these bubbles have an adverse effect on the gumming.
3 2 Environmental impact of additives The raw materials of each additive have their own inherent composition and characteristics. In some cases, there is a positive impact on the gelation, but in another case it may have an adverse effect. The furfural residue produced from pure rice hulls, compared with those produced from corn cobs and oat hulls, is not as well dispersed in aqueous sodium hydroxide as the latter because the ash content is greater than 20%. In addition, high ash substances have an important influence on the viscosity of the glue, the glue spraying equipment and the cutting tools. Due to the large difference in pH between various resins, the acidity and alkalinity and buffer capacity of the additives have an important influence on the gelation. In addition, the use of additives should also be noted when it affects the environment. For example, asbestos is a good wood adhesive additive, but it is now banned from the production environment and the safety of the end user.
3 3 Addition ratios of extenders and fillers Both fillers and extenders have the effect of increasing the solids content of the adhesive. The incremental effect of the filler is also important. In general, when the additive effect of the filler is not very obvious, the ratio of the filler to the extender is 1.2. Due to the difference in the microstructure of the wood gluing surface, the pores on the surface range from 0 001 to 300 μm, so the bridging effect of the filler during the gluing process is very important. The wetting ability and diffusion rate of the additive at room temperature and under specific conditions is an important performance factor. In general, the addition of 1% or less of filler to the extender is more conducive to the storage, flow and end use properties of the adhesive. 。。 The mixing ratio of filler and extender is 2.1. In recent years, this ratio has fallen to 1.1, and even on some occasions it has dropped to 1.2. The ratio of additive to resin solids content takes into account the final gluing effect. The use of starch extenders should also pay attention to the damage of bacteria and mold to the rubber layer. Based on this consideration, the American Plywood Association stipulates that the maximum amount of protein starch extenders should not exceed 40% of the amount of solid resin.
3 4 Moisture content of additives and its effect on resin pot life and curing time The main solvent of wood adhesives is water. If the moisture content of additives is too high, it will lead to product quality degradation and heat loss. Additives also have an effect on the activity and pot life of the resin. After some additives are added to the resin, the viscosity of the resin increases to a certain extent and then does not increase, and can be stably maintained for several days; some additives increase the viscosity of the resin and the application period is severely shortened. Depending on the application of the resin, the pot life can be considered as an important factor when selecting additives. Various resins vary in type and use, and their pH ranges from 2 to 12. Therefore, the pH of the additive has an effect on the curing time and product quality. In general, roll coating requires less viscosity stability than spray and spray coating. Under alkaline conditions, the nut shell powder additive is more stable than the furfural residue, which increases the viscosity of the resin. Therefore, an additive suitable for roll coating adhesives is not necessarily suitable for spraying and coating adhesives. In addition, the property that the additive remains suspended in the resin without precipitation is also important. To prevent agglomeration and sedimentation, the resin storage device should be equipped with a slow agitation device.
3 5 Effect of additives on bonding strength and tool wear performance An additive should enhance the molecular affinity between different substances. That is, the strength properties of the adhesive layer are improved by reducing the distance between the adhesive and the substrate. Certain additives can increase the tensile strength and stiffness of the adhesive layer and improve the impact resistance of the adhesive layer. Depending on the physicochemical properties of the additive, it also differs in the tool's wear procedure. The tendency of the tool to mechanically and chemically attack the tool can be judged based on the ash content and type of the additive. The influence of additives on the subsequent processing tools of laminated timber and furniture panels should be especially important.
3 6 Effect of additives on the thixotropic properties of the adhesive The thixotropy of the wood adhesive is related to its viscosity, that is, the viscosity of the adhesive before use is small, and the viscosity immediately becomes large when applied. The type of additive has a large effect on this property of the adhesive. The thixotropy and anti-precipitation properties of the adhesive are particularly important for the bonding of laminated materials. The thixotropy of the resin plays an important role in its fluidity, uniform coating and reduced penetration of the gluing process.
4 Technology development trend of additives At present, the production of veneer lumber (LVL) in North America, Europe and Asia is increasing. The phenolic adhesive used in LVL accounts for 30%~32% of the glue used in structural plywood. LVL requires a stable use of the adhesive, which can be achieved by selecting different types of additives. Compared with ordinary plywood, there are many LVL layers, so the choice of additives should pay attention to the wear of the tool. Recently, people have been more interested in the foaming problem of structural plywood. The addition of animal blood and certain proteinaceous substances to the adhesive forms a foam by stirring to increase the volume of the adhesive and reduce the amount of glue per unit area, thereby achieving the purpose of reducing the amount of the adhesive. In addition, there is a growing tendency to use high water content veneers, which require more additives to meet the solids content of the adhesive. This improvement in production processes necessarily imposes higher quality requirements on phenolic adhesives for structural plywood. From the point of view of consumption, in the past, users have added various additives and extenders at the site of use. Today, in the United States, the prepared adhesives are often supplied directly by suppliers. The user may directly request the use of an additive or an additive having a specific property, such as requiring an ash content of more than 3%. At present, the production of oriented strand board in the United States accounts for 30% of the entire structural composite. From the development trend, this ratio will continue to increase, while the ratio of structural plywood is declining, but the decline may also be offset by the increase in LVL production. In China, in order to save the consumption of adhesives, reduce production costs, and expand the resources of wood adhesive fillers, research and development work on agricultural and forestry wastes such as peanut shells, bark, litchi core and wood flour has been carried out, but At present, practical applications are still rare.
5 Conclusion From a global perspective, the raw materials for woodworking adhesive additives are sufficient. In addition to known raw materials, new sources of raw materials will be developed. For example, the acidic hydrolysis residue of waste newspapers is used as an additive, and there are some places where coconut shell powder and bagasse are used as raw materials for making additives. As a producer of additives, only when the products produced can bring benefits and achieve the desired goals, will they expand production, increase investment, and develop new products. Currently in the United States, the expected price of additives can not be lower than 300 US dollars / ton, otherwise it will have an adverse impact on the production of additives.
1 History and current status Wood adhesive additives can be divided into two categories according to their use. One is a filler, mainly including lignocellulosic materials, furfural residues and other organic residues, as well as inorganic materials such as clay; and second, extenders, mainly protein and starchy substances such as wheat flour, corn flour, and Starch, etc., in the actual production, starch is applied more. However, the definition of fillers and extenders is not very strict. According to the American Society for Testing and Materials (ASTM), fillers are non-adhesive substances whose function is to improve the properties of adhesives, thereby improving the bonding properties and bonding quality; The extender is a substance with a certain kind of glue. The addition of this substance to the adhesive can reduce the amount of adhesive. Lignocellulosic materials, such as wood flour and nut shell powder, have been used as fillers for synthetic resin adhesives for wood for about 50 years. In 1950, the filler used in structural plywood in the United States was mainly nut shell powder, followed by Douglas fir bark powder. In 1962, Alnus bark powder was developed as a filler. The furfural residue filler developed at the same time becomes a main filler for structural plywood, especially for southern pine plywood adhesives. Particleboard sanding dust (also known as resin wood powder) has been used as an additive for decades. In addition, pine powder, Douglas fir and some hardwood wood powder are used as fillers. Nutshell powder, wood flour, clay and mixtures thereof are also frequently used fillers in some cases. At present, there are many manufacturers using protein and starch substances as extenders for wood adhesives. China's plywood industry mainly uses flour as a bulking agent; in the United States, structural plywood, laminated timber, hardwood plywood and decorative plywood are also mainly used as bulking agents for flour.
2 Market and price The use of China's additives and market conditions have not been reported in detail, and there are no exact statistics. In the United States, additives are mainly used in plastics, adhesives and sealing materials. The amount used in the plastics industry was 25.87 million tons, the amount in adhesives and sealing materials was 42.30 million tons, and the amount in wood adhesives was 72,000 tons. In 1992, the amount of additives used in phenolic resins in the United States accounted for 90% of the total amount of additives used in wood adhesives. The rest were mainly used for urea-formaldehyde resin glue, melamine-formaldehyde resin glue and resorcinol resin glue. The value of the additives used in multi-billion dollar wood adhesives is about $15 million. As US additives have become specialized in production, environmental changes may have a corresponding impact on the additive market. If floods and storms interrupt traffic, the destruction of raw materials, and the occurrence of pests and diseases, the production of nut shells will have a certain impact on the market. The furfural residue used in the United States is mainly imported from Asia. Due to the increase in the production of Asian furfural residues, the price of imported furfural residues in the United States has decreased by 30%, which has caused a local factory to close due to uncompetitive product prices. . In China, due to the single use of the species, the use of flour is basically used, so the above environmental changes have little impact on the market. The price of the additive depends on the type of raw material, the availability of the raw material, the cost of production, the requirements and quality of the adhesive. In 1993, the price of additives for wood adhesives in the United States was 170-270 US dollars / ton. In China, the price of the main used flour has risen to 2,000 to 2,500 yuan / ton, a large increase, many companies have been overwhelmed.
3 various performance indicators of additives
3 1 The physical and chemical properties of the additive, including color, particle size, water content, pH and ash, determine the coating method of the adhesive. Indoor furniture is often light-colored, if you use the test, you can use dark color to judge the wood breaking rate. The adhesive generally used for roll coating has an additive particle size of about 200 mesh, and the spray or spray coated adhesive has a size of about 325 mesh. The moisture content of the additive is preferably from 7% to 12%. The pH and buffer capacity of the additive are critical to its compatibility with the resin, and the ash content plays an important role in tool wear. However, some of the physical and chemical properties of the additive are interlinked. For example, the solubility and dispersion properties of various additives in water, alcohol and sodium hydroxide solution are different, and these characteristics have an important influence on the bonding process. In addition, the molecular affinity of the filler is more sluggish than the protein and starch extenders. In most applications, additives are an important factor in improving gluing properties and achieving a gluing standard for gluing materials. Certain additives, such as walnut shell powder, contain an oily substance that acts like a blowing agent during mixing with the adhesive, and in general, these bubbles have an adverse effect on the gumming.
3 2 Environmental impact of additives The raw materials of each additive have their own inherent composition and characteristics. In some cases, there is a positive impact on the gelation, but in another case it may have an adverse effect. The furfural residue produced from pure rice hulls, compared with those produced from corn cobs and oat hulls, is not as well dispersed in aqueous sodium hydroxide as the latter because the ash content is greater than 20%. In addition, high ash substances have an important influence on the viscosity of the glue, the glue spraying equipment and the cutting tools. Due to the large difference in pH between various resins, the acidity and alkalinity and buffer capacity of the additives have an important influence on the gelation. In addition, the use of additives should also be noted when it affects the environment. For example, asbestos is a good wood adhesive additive, but it is now banned from the production environment and the safety of the end user.
3 3 Addition ratios of extenders and fillers Both fillers and extenders have the effect of increasing the solids content of the adhesive. The incremental effect of the filler is also important. In general, when the additive effect of the filler is not very obvious, the ratio of the filler to the extender is 1.2. Due to the difference in the microstructure of the wood gluing surface, the pores on the surface range from 0 001 to 300 μm, so the bridging effect of the filler during the gluing process is very important. The wetting ability and diffusion rate of the additive at room temperature and under specific conditions is an important performance factor. In general, the addition of 1% or less of filler to the extender is more conducive to the storage, flow and end use properties of the adhesive. 。。 The mixing ratio of filler and extender is 2.1. In recent years, this ratio has fallen to 1.1, and even on some occasions it has dropped to 1.2. The ratio of additive to resin solids content takes into account the final gluing effect. The use of starch extenders should also pay attention to the damage of bacteria and mold to the rubber layer. Based on this consideration, the American Plywood Association stipulates that the maximum amount of protein starch extenders should not exceed 40% of the amount of solid resin.
3 4 Moisture content of additives and its effect on resin pot life and curing time The main solvent of wood adhesives is water. If the moisture content of additives is too high, it will lead to product quality degradation and heat loss. Additives also have an effect on the activity and pot life of the resin. After some additives are added to the resin, the viscosity of the resin increases to a certain extent and then does not increase, and can be stably maintained for several days; some additives increase the viscosity of the resin and the application period is severely shortened. Depending on the application of the resin, the pot life can be considered as an important factor when selecting additives. Various resins vary in type and use, and their pH ranges from 2 to 12. Therefore, the pH of the additive has an effect on the curing time and product quality. In general, roll coating requires less viscosity stability than spray and spray coating. Under alkaline conditions, the nut shell powder additive is more stable than the furfural residue, which increases the viscosity of the resin. Therefore, an additive suitable for roll coating adhesives is not necessarily suitable for spraying and coating adhesives. In addition, the property that the additive remains suspended in the resin without precipitation is also important. To prevent agglomeration and sedimentation, the resin storage device should be equipped with a slow agitation device.
3 5 Effect of additives on bonding strength and tool wear performance An additive should enhance the molecular affinity between different substances. That is, the strength properties of the adhesive layer are improved by reducing the distance between the adhesive and the substrate. Certain additives can increase the tensile strength and stiffness of the adhesive layer and improve the impact resistance of the adhesive layer. Depending on the physicochemical properties of the additive, it also differs in the tool's wear procedure. The tendency of the tool to mechanically and chemically attack the tool can be judged based on the ash content and type of the additive. The influence of additives on the subsequent processing tools of laminated timber and furniture panels should be especially important.
3 6 Effect of additives on the thixotropic properties of the adhesive The thixotropy of the wood adhesive is related to its viscosity, that is, the viscosity of the adhesive before use is small, and the viscosity immediately becomes large when applied. The type of additive has a large effect on this property of the adhesive. The thixotropy and anti-precipitation properties of the adhesive are particularly important for the bonding of laminated materials. The thixotropy of the resin plays an important role in its fluidity, uniform coating and reduced penetration of the gluing process.
4 Technology development trend of additives At present, the production of veneer lumber (LVL) in North America, Europe and Asia is increasing. The phenolic adhesive used in LVL accounts for 30%~32% of the glue used in structural plywood. LVL requires a stable use of the adhesive, which can be achieved by selecting different types of additives. Compared with ordinary plywood, there are many LVL layers, so the choice of additives should pay attention to the wear of the tool. Recently, people have been more interested in the foaming problem of structural plywood. The addition of animal blood and certain proteinaceous substances to the adhesive forms a foam by stirring to increase the volume of the adhesive and reduce the amount of glue per unit area, thereby achieving the purpose of reducing the amount of the adhesive. In addition, there is a growing tendency to use high water content veneers, which require more additives to meet the solids content of the adhesive. This improvement in production processes necessarily imposes higher quality requirements on phenolic adhesives for structural plywood. From the point of view of consumption, in the past, users have added various additives and extenders at the site of use. Today, in the United States, the prepared adhesives are often supplied directly by suppliers. The user may directly request the use of an additive or an additive having a specific property, such as requiring an ash content of more than 3%. At present, the production of oriented strand board in the United States accounts for 30% of the entire structural composite. From the development trend, this ratio will continue to increase, while the ratio of structural plywood is declining, but the decline may also be offset by the increase in LVL production. In China, in order to save the consumption of adhesives, reduce production costs, and expand the resources of wood adhesive fillers, research and development work on agricultural and forestry wastes such as peanut shells, bark, litchi core and wood flour has been carried out, but At present, practical applications are still rare.
5 Conclusion From a global perspective, the raw materials for woodworking adhesive additives are sufficient. In addition to known raw materials, new sources of raw materials will be developed. For example, the acidic hydrolysis residue of waste newspapers is used as an additive, and there are some places where coconut shell powder and bagasse are used as raw materials for making additives. As a producer of additives, only when the products produced can bring benefits and achieve the desired goals, will they expand production, increase investment, and develop new products. Currently in the United States, the expected price of additives can not be lower than 300 US dollars / ton, otherwise it will have an adverse impact on the production of additives.
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