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The wet strength of paper refers to the strength of a paper sheet in water or in a humid environment. Under normal circumstances, the strength of the paper is only 5%-10% of its strength when it is wetted without additives. After adding the wet enhancer, the paper can maintain 20%-40% of the original dry strength. . Wet-strength agents are functional additives, most of which are synthetic resins that are resistant to hydrolysis. Adding them to pulp before paper is formed not only improves wet strength, but also creates favorable conditions for the operation of high-speed paper machines.
First, the type and development of wet strength agent
Very long ago, people used sulphuric acid to treat the base paper to make it sheep skin, coat it on the paper surface with waterproof paint, plastic film or metal foil, or sizing paper with gelatin or animal glue, and then make glue paper. Contact with bauxite, formaldehyde or glyoxal at high temperatures forms a protective layer. These methods are quite expensive and the production efficiency is also low, and this method itself can only play the role of water resistance and does not substantially increase the wet strength of the paper.
In 1935, people first discovered that urea-formaldehyde resin (abbreviated as UF resin) was used as a surface sizing agent. After heat treatment, the paper could have a certain wet strength. UF resin that was directly added to the slurry was then developed. However, because it has a negative charge, it is not easily combined with a load-bearing fiber. The retention rate is low and the wet strength effect is poor. In 1942, Melamine formaldehyde resin (MF resin) was developed. , It can be directly added into the pulp, and the paper can obtain good wet strength after heating and drying. In 1946, the cationic modified UF resin was successfully tested. The resin retention rate and wet strength paper efficiency are similar to those of the MF, but the price is low, which creates the conditions for mass production of wet strength paper. The use of urea-formaldehyde resin and melamine resin as wet strength agents not only simplifies the processing but also achieves wet strengths up to 50%. While improving the wet strength, it can also make certain dry strength indicators of the paper (such as crack length, bursting degree and folding resistance, etc.) and sizing degree correspondingly improve.
At the same time, people have also developed other compounds that can make paper and board have a certain wet strength. Such as phenolic resin, polyethyleneimine (PEI), synthetic latex (styrene butadiene latex, nitrile latex, etc.), dialdehyde starch (referred to as DAS), but the phenolic resin color is very deep, and the need for high temperature (150 °C) Mature, synthetic latexes are expensive and need to be made into latex. Polyethylene rubbers are generally only suitable for the production of unstretched wet strength papers; dialdehyde starches do not change the paper absorbency and wetness. The strength is independent of the drying temperature, but it only gives the paper a temporary wet strength. Therefore, these wet strength agents are rarely used except for certain special purposes.
In 1960, we developed a mid-maturing wet strength polyamide polyamine epichlorohydrin resin (PPE or PAE resin). PPE resins have the advantage that many MF resins and UF resins cannot be designed. It is not only a highly efficient moisturizer that matures under moderate conditions, but also does not lose the softness and absorbency of the paper while improving the wet strength. It is especially suitable for medical hygiene paper. And in the production process, the foam is small, the whiteness of the paper is reduced to yellow, and there is good re-slurry performance.
Researchers have developed wet strength agents such as PPA, CPAM, and PA on the basis of PPE resin. Polyamide polyamine resin (PAA) is a cationic water-soluble polymer, which is mainly used for cardboard packaging, paper packaging, liquid packaging materials such as milk and fruit juice, etc. The effect is better than UF resin and MF resin. Cationic polyacrylamide (CPAM) is an enhancer with a dry and wet reinforcing effect, and can make the paper have a higher whiteness. It is easy to disperse into a slurry after recovery of the broke. The curing time is short and the price is moderate. It is suitable for pH. The range is 4.5-7.5. The main drawback is that the wet strength of the reinforced paper is lost by about 40% after it is wetted, and therefore it is not suitable for papers that require long-lasting wet strength. The American paper mill uses it to produce handkerchiefs, facial tissues and other fine papers. Paper species. Polyamine (PA) wet strength agents are among the lowest cost among the three water soluble polymers. This kind of product has good storage stability, and has both dry and enhanced properties. It has stronger wet endurance than CPAM but not as good as PPE resin. The United States uses it to produce toilet paper and corrugated paper.
It can be seen that the wet strength agent has been developed so far and is mainly composed of two major types of products, namely, acidic curing resins represented by MF resins and UF resins and alkaline mature resins represented by PPE resins. Alkaline curing resin is more advantageous than acidic curing resin and will become the development trend of paper industry agent.
Second, the synthesis method of the wet enhancer
(a) Synthetic method of PPE resin
The polyamide polyamine epichlorohydrin resin is abbreviated as PPE, also known as polyamide polyamine epichlorohydrin resin or polyamide-epichlorohydrin resin. PPE is a water-soluble, cationic, thermosetting resin, non-formaldehyde polymer, non-toxic and tasteless, can be used in the pH range of 4-12, with good wet strength. It is characterized by low dosage, suitable for neutral papermaking, and easy handling of broke paper. It can also be used as a flocculant to treat factory wastewater.
1. Quality index of PPE resin: Appearance: light amber particle liquid, solid content: 12.5%±0.3%, specific gravity: 1.03kg/l, viscosity: 40-60cps, freezing point: -1°C, pH4.6-4.9 .
2, PPE resin chemical synthesis method is mainly divided into two steps, the first step to the first synthesis of polyamide, the second step to polyamide as raw material to generate PPE resin. Divinyltriamine and triethylenetetramine can also be used in the synthesis of polyamides.
3. Raw materials: Diethylenetriamine 103g (triethylene tetraamine 146g)
Oxalic acid 146g
Epichlorohydrin 80 g (with triethylenetetraamine, epichlorohydrin 76.4g)
Water amount
10% hydrochloric acid (or sulfuric acid) amount
10% sodium hydroxide amount
4, utensils: open enamel bucket (or three-necked flask), stirrer, glass rod, alcohol lamp (or electric heater), balance (table), measuring cylinder, beaker, viscometer.
5, process conditions: normal temperature and pressure.
6, synthesis method:
first step. Diethylenetriamine 103g (or triethylene tetraamine 146) was taken and placed in an open enamel bucket (or three-necked flask). The stirrer was started and 31 g of water was added. Then slowly add 146g of oxalic acid, at which time the temperature rises to 115-120C. After stirring evenly, start heating and raise the temperature to 130C. At this time, it is necessary to pay attention to the insulation for about half an hour to prevent the foam from overflowing. After the foam disappeared, it continued to heat up to 190-200°C and hydrated for about 1.5-2 hours. The heating was stopped. When the temperature dropped below 160°C, water was slowly added to make the total amount 400 g. At this point, the solid content of the resin is about 50%, and the resin is ready for use after being stirred evenly. The yield of the resin is about 85%.
The second step. To the polyamide liquid prepared in the first step, 600 g of water was added thereto, stirred and 80 g of epichlorohydrin was added dropwise, and the addition was completed in about 3 to 11 minutes. When the heating is started and the temperature is raised to 75C, the heating is stopped, and the stirring is continued. When the viscosity is measured to 25-30 centipoise (solid content 19%), 10% hydrochloric acid (or sulfuric acid) is added immediately to adjust the pH to 4-5, and it can be stored for use.
third step. When used, it is first adjusted to pH 6-7 with 10% sodium hydroxide solution, diluted with appropriate amount of water, and added to the slurry tank after stirring.
7. Note:
(1) The reaction of diethylenetriamine (or triethylenetetramine) with acetic acid is carried out at 160°C-210°C under normal pressure for 0.5-2 hours with a molar ratio of amine to acid of 0.8:1-1.4:1. Preferably, gelation occurs at 0.9:1 to 1.2:1, less than 0.8:1, and greater than 1.4:1 yields a low molecular weight polyamide.
(2) When the polyamide reacts with epichlorohydrin, the temperature is about 45°C to 70°C, and the viscosity is greater than 0.85 centipoise. The reaction is preferably carried out in an aqueous solution to moderate the reaction, usually without adjusting the pH, but the pH during the reaction The value will be reduced. If necessary, the addition of an acid generated in the alkali neutralization unit will favor the conversion of the chlorohydrin group to the epoxy group. When the desired viscosity has been reached, it is diluted with water to make the content of the resin solution about 10%, and cooled to 25°C, and then pH is adjusted to pH 5-6 to make it stable, preferably up to pH=5. The use of 1.0-1.5 moles of epichlorohydrin per secondary amine group of the polyamide gives satisfactory results.
(3) When using PPE resin, the pH should be adjusted to 7-8 with 10% sodium hydroxide, so that it can be activated. PPE resin can be directly added to the slurry. The range of addition is 0.1%-5% (for dry fiber ).
(4) Avoid contact with concentrated acid when using PPE resin.
8. Storage method: PPE resin should be stored in a cool, ventilated and dry place. Try to avoid freezing and high temperature (>32°C). The storage temperature should be higher than 4.4°C. If the product is melted and melted, it should be mixed evenly and used immediately; in the environment below 32°C, it can be stored for 3 months, and the wet strength effect will be reduced for more than 3 months. The general storage period is 6 months.
(B) Synthesis of Cationic UF Resins
Urea-formaldehyde resin is referred to as urea-formaldehyde resin, which is formed by the polymerization of urea and formaldehyde. UF resins are classified into anionic and cationic types. The anionic UF resin is not easily combined with negatively charged fibers, the retention rate is low, and the wet strength effect is poor. The cationic UF resin is a modified cationic UF resin obtained by modifying an anionic UF resin. The wet strength obtained by the modified cationic UF resin can reach 50% or more of the dry strength, and the wet strength can be improved without affecting the other properties of the paper. At the same time, it can also make the paper crack length, burst resistance, folding degree and sizing degree increase correspondingly.
UF resin is a colorless (or grass yellow) clear, uniform, syrupy liquid that mixes with water in any ratio without precipitation. With ionic properties, cationic and anionic. When added to the pulp, the resin is absorbed by the fibers and retained on the fibers. Cationic UF resins are commonly used in the industry.
1. Synthetic method of ethylenediamine modified urea formaldehyde resin
(1) Characteristics: Ethylenediamine modified urea resin is a cationic resin. Because it is cationic, it has a stronger effect of humidification than unmodified urea formaldehyde resin. In addition, it also has good water solubility, is not easy to gel, is easy to manufacture and has low cost. It is made of urea and formaldehyde first, with the addition of ethylenediamine and other auxiliary chemicals.
(2) Raw Material: Urea (95%-100% Crystallized) 1mol
Formaldehyde (51% aqueous solution) 2.5mol
Ethylenediamine (98-100%) 0.1mol
Hydrochloric acid (38%) 0.1mol
Sodium hydroxide (25%) is suitable for controlling the pH of the resin solution
Formic acid (90%) is suitable for controlling the pH of the resin solution
(3) Equipment: measuring cylinder, balance, pH test paper, burette, heater (50°C-100°C), container with thermometer, stirrer and reflux condenser.
(4) Synthesis steps: A. Ethylene diamine salt was formed by reacting 9.5 ml of acid with 6.1 ml of ethylenediamine, and then 147 ml of formaldehyde solution was placed in a vessel equipped with a thermometer, a stirrer, and a reflux condenser.
B. The pH was controlled at 7.0-8.0, followed by 60 g of urea. The pH of the reaction solution was still controlled at 7.0 or above.
C. The temperature was then raised. When the temperature gradually increased to 95°C, the pH was adjusted to 4.2 with formic acid. Due to the basicity of ethylenediamine, the pH of the reaction solution was increased to 6.2 and then reduced to 4.0 with formic acid. After about 20 minutes, the pH of the reaction solution was stable at 5.2-5.6, and the resin reaction was maintained. Performed at 95°C.
D. When the viscosity of the resin liquid reaches about 100 cp, the resin liquid is naturally cooled to 65-75[deg.] C. and the resin reaction is continued.
E. When the desired viscosity of the resin solution (about 0.15 Pa.s) is reached, the solution is rapidly cooled to room temperature, and the resin solution is adjusted to 7.2-7.6 with a 25% NaOH solution. The solid content of the resin liquid obtained at this time is about 51% to 53%, and is diluted with water to a concentration of about 45%.
(5) Storage method: The resin solution with a concentration of 45% is stored below 20°C.
2. Synthetic method of polyamine modified urea formaldehyde resin
(1) Characteristics: The polyamine modified urea-formaldehyde resin is also a cationic resin. Because it has two active groups, it has a strong effect of humidification. In addition, it has a wide range of pH values ​​and good Stability and other advantages.
First, the type and development of wet strength agent
Very long ago, people used sulphuric acid to treat the base paper to make it sheep skin, coat it on the paper surface with waterproof paint, plastic film or metal foil, or sizing paper with gelatin or animal glue, and then make glue paper. Contact with bauxite, formaldehyde or glyoxal at high temperatures forms a protective layer. These methods are quite expensive and the production efficiency is also low, and this method itself can only play the role of water resistance and does not substantially increase the wet strength of the paper.
In 1935, people first discovered that urea-formaldehyde resin (abbreviated as UF resin) was used as a surface sizing agent. After heat treatment, the paper could have a certain wet strength. UF resin that was directly added to the slurry was then developed. However, because it has a negative charge, it is not easily combined with a load-bearing fiber. The retention rate is low and the wet strength effect is poor. In 1942, Melamine formaldehyde resin (MF resin) was developed. , It can be directly added into the pulp, and the paper can obtain good wet strength after heating and drying. In 1946, the cationic modified UF resin was successfully tested. The resin retention rate and wet strength paper efficiency are similar to those of the MF, but the price is low, which creates the conditions for mass production of wet strength paper. The use of urea-formaldehyde resin and melamine resin as wet strength agents not only simplifies the processing but also achieves wet strengths up to 50%. While improving the wet strength, it can also make certain dry strength indicators of the paper (such as crack length, bursting degree and folding resistance, etc.) and sizing degree correspondingly improve.
At the same time, people have also developed other compounds that can make paper and board have a certain wet strength. Such as phenolic resin, polyethyleneimine (PEI), synthetic latex (styrene butadiene latex, nitrile latex, etc.), dialdehyde starch (referred to as DAS), but the phenolic resin color is very deep, and the need for high temperature (150 °C) Mature, synthetic latexes are expensive and need to be made into latex. Polyethylene rubbers are generally only suitable for the production of unstretched wet strength papers; dialdehyde starches do not change the paper absorbency and wetness. The strength is independent of the drying temperature, but it only gives the paper a temporary wet strength. Therefore, these wet strength agents are rarely used except for certain special purposes.
In 1960, we developed a mid-maturing wet strength polyamide polyamine epichlorohydrin resin (PPE or PAE resin). PPE resins have the advantage that many MF resins and UF resins cannot be designed. It is not only a highly efficient moisturizer that matures under moderate conditions, but also does not lose the softness and absorbency of the paper while improving the wet strength. It is especially suitable for medical hygiene paper. And in the production process, the foam is small, the whiteness of the paper is reduced to yellow, and there is good re-slurry performance.
Researchers have developed wet strength agents such as PPA, CPAM, and PA on the basis of PPE resin. Polyamide polyamine resin (PAA) is a cationic water-soluble polymer, which is mainly used for cardboard packaging, paper packaging, liquid packaging materials such as milk and fruit juice, etc. The effect is better than UF resin and MF resin. Cationic polyacrylamide (CPAM) is an enhancer with a dry and wet reinforcing effect, and can make the paper have a higher whiteness. It is easy to disperse into a slurry after recovery of the broke. The curing time is short and the price is moderate. It is suitable for pH. The range is 4.5-7.5. The main drawback is that the wet strength of the reinforced paper is lost by about 40% after it is wetted, and therefore it is not suitable for papers that require long-lasting wet strength. The American paper mill uses it to produce handkerchiefs, facial tissues and other fine papers. Paper species. Polyamine (PA) wet strength agents are among the lowest cost among the three water soluble polymers. This kind of product has good storage stability, and has both dry and enhanced properties. It has stronger wet endurance than CPAM but not as good as PPE resin. The United States uses it to produce toilet paper and corrugated paper.
It can be seen that the wet strength agent has been developed so far and is mainly composed of two major types of products, namely, acidic curing resins represented by MF resins and UF resins and alkaline mature resins represented by PPE resins. Alkaline curing resin is more advantageous than acidic curing resin and will become the development trend of paper industry agent.
Second, the synthesis method of the wet enhancer
(a) Synthetic method of PPE resin
The polyamide polyamine epichlorohydrin resin is abbreviated as PPE, also known as polyamide polyamine epichlorohydrin resin or polyamide-epichlorohydrin resin. PPE is a water-soluble, cationic, thermosetting resin, non-formaldehyde polymer, non-toxic and tasteless, can be used in the pH range of 4-12, with good wet strength. It is characterized by low dosage, suitable for neutral papermaking, and easy handling of broke paper. It can also be used as a flocculant to treat factory wastewater.
1. Quality index of PPE resin: Appearance: light amber particle liquid, solid content: 12.5%±0.3%, specific gravity: 1.03kg/l, viscosity: 40-60cps, freezing point: -1°C, pH4.6-4.9 .
2, PPE resin chemical synthesis method is mainly divided into two steps, the first step to the first synthesis of polyamide, the second step to polyamide as raw material to generate PPE resin. Divinyltriamine and triethylenetetramine can also be used in the synthesis of polyamides.
3. Raw materials: Diethylenetriamine 103g (triethylene tetraamine 146g)
Oxalic acid 146g
Epichlorohydrin 80 g (with triethylenetetraamine, epichlorohydrin 76.4g)
Water amount
10% hydrochloric acid (or sulfuric acid) amount
10% sodium hydroxide amount
4, utensils: open enamel bucket (or three-necked flask), stirrer, glass rod, alcohol lamp (or electric heater), balance (table), measuring cylinder, beaker, viscometer.
5, process conditions: normal temperature and pressure.
6, synthesis method:
first step. Diethylenetriamine 103g (or triethylene tetraamine 146) was taken and placed in an open enamel bucket (or three-necked flask). The stirrer was started and 31 g of water was added. Then slowly add 146g of oxalic acid, at which time the temperature rises to 115-120C. After stirring evenly, start heating and raise the temperature to 130C. At this time, it is necessary to pay attention to the insulation for about half an hour to prevent the foam from overflowing. After the foam disappeared, it continued to heat up to 190-200°C and hydrated for about 1.5-2 hours. The heating was stopped. When the temperature dropped below 160°C, water was slowly added to make the total amount 400 g. At this point, the solid content of the resin is about 50%, and the resin is ready for use after being stirred evenly. The yield of the resin is about 85%.
The second step. To the polyamide liquid prepared in the first step, 600 g of water was added thereto, stirred and 80 g of epichlorohydrin was added dropwise, and the addition was completed in about 3 to 11 minutes. When the heating is started and the temperature is raised to 75C, the heating is stopped, and the stirring is continued. When the viscosity is measured to 25-30 centipoise (solid content 19%), 10% hydrochloric acid (or sulfuric acid) is added immediately to adjust the pH to 4-5, and it can be stored for use.
third step. When used, it is first adjusted to pH 6-7 with 10% sodium hydroxide solution, diluted with appropriate amount of water, and added to the slurry tank after stirring.
7. Note:
(1) The reaction of diethylenetriamine (or triethylenetetramine) with acetic acid is carried out at 160°C-210°C under normal pressure for 0.5-2 hours with a molar ratio of amine to acid of 0.8:1-1.4:1. Preferably, gelation occurs at 0.9:1 to 1.2:1, less than 0.8:1, and greater than 1.4:1 yields a low molecular weight polyamide.
(2) When the polyamide reacts with epichlorohydrin, the temperature is about 45°C to 70°C, and the viscosity is greater than 0.85 centipoise. The reaction is preferably carried out in an aqueous solution to moderate the reaction, usually without adjusting the pH, but the pH during the reaction The value will be reduced. If necessary, the addition of an acid generated in the alkali neutralization unit will favor the conversion of the chlorohydrin group to the epoxy group. When the desired viscosity has been reached, it is diluted with water to make the content of the resin solution about 10%, and cooled to 25°C, and then pH is adjusted to pH 5-6 to make it stable, preferably up to pH=5. The use of 1.0-1.5 moles of epichlorohydrin per secondary amine group of the polyamide gives satisfactory results.
(3) When using PPE resin, the pH should be adjusted to 7-8 with 10% sodium hydroxide, so that it can be activated. PPE resin can be directly added to the slurry. The range of addition is 0.1%-5% (for dry fiber ).
(4) Avoid contact with concentrated acid when using PPE resin.
8. Storage method: PPE resin should be stored in a cool, ventilated and dry place. Try to avoid freezing and high temperature (>32°C). The storage temperature should be higher than 4.4°C. If the product is melted and melted, it should be mixed evenly and used immediately; in the environment below 32°C, it can be stored for 3 months, and the wet strength effect will be reduced for more than 3 months. The general storage period is 6 months.
(B) Synthesis of Cationic UF Resins
Urea-formaldehyde resin is referred to as urea-formaldehyde resin, which is formed by the polymerization of urea and formaldehyde. UF resins are classified into anionic and cationic types. The anionic UF resin is not easily combined with negatively charged fibers, the retention rate is low, and the wet strength effect is poor. The cationic UF resin is a modified cationic UF resin obtained by modifying an anionic UF resin. The wet strength obtained by the modified cationic UF resin can reach 50% or more of the dry strength, and the wet strength can be improved without affecting the other properties of the paper. At the same time, it can also make the paper crack length, burst resistance, folding degree and sizing degree increase correspondingly.
UF resin is a colorless (or grass yellow) clear, uniform, syrupy liquid that mixes with water in any ratio without precipitation. With ionic properties, cationic and anionic. When added to the pulp, the resin is absorbed by the fibers and retained on the fibers. Cationic UF resins are commonly used in the industry.
1. Synthetic method of ethylenediamine modified urea formaldehyde resin
(1) Characteristics: Ethylenediamine modified urea resin is a cationic resin. Because it is cationic, it has a stronger effect of humidification than unmodified urea formaldehyde resin. In addition, it also has good water solubility, is not easy to gel, is easy to manufacture and has low cost. It is made of urea and formaldehyde first, with the addition of ethylenediamine and other auxiliary chemicals.
(2) Raw Material: Urea (95%-100% Crystallized) 1mol
Formaldehyde (51% aqueous solution) 2.5mol
Ethylenediamine (98-100%) 0.1mol
Hydrochloric acid (38%) 0.1mol
Sodium hydroxide (25%) is suitable for controlling the pH of the resin solution
Formic acid (90%) is suitable for controlling the pH of the resin solution
(3) Equipment: measuring cylinder, balance, pH test paper, burette, heater (50°C-100°C), container with thermometer, stirrer and reflux condenser.
(4) Synthesis steps: A. Ethylene diamine salt was formed by reacting 9.5 ml of acid with 6.1 ml of ethylenediamine, and then 147 ml of formaldehyde solution was placed in a vessel equipped with a thermometer, a stirrer, and a reflux condenser.
B. The pH was controlled at 7.0-8.0, followed by 60 g of urea. The pH of the reaction solution was still controlled at 7.0 or above.
C. The temperature was then raised. When the temperature gradually increased to 95°C, the pH was adjusted to 4.2 with formic acid. Due to the basicity of ethylenediamine, the pH of the reaction solution was increased to 6.2 and then reduced to 4.0 with formic acid. After about 20 minutes, the pH of the reaction solution was stable at 5.2-5.6, and the resin reaction was maintained. Performed at 95°C.
D. When the viscosity of the resin liquid reaches about 100 cp, the resin liquid is naturally cooled to 65-75[deg.] C. and the resin reaction is continued.
E. When the desired viscosity of the resin solution (about 0.15 Pa.s) is reached, the solution is rapidly cooled to room temperature, and the resin solution is adjusted to 7.2-7.6 with a 25% NaOH solution. The solid content of the resin liquid obtained at this time is about 51% to 53%, and is diluted with water to a concentration of about 45%.
(5) Storage method: The resin solution with a concentration of 45% is stored below 20°C.
2. Synthetic method of polyamine modified urea formaldehyde resin
(1) Characteristics: The polyamine modified urea-formaldehyde resin is also a cationic resin. Because it has two active groups, it has a strong effect of humidification. In addition, it has a wide range of pH values ​​and good Stability and other advantages.
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