AKD is a reactive sizing agent, which has its own specificity to the process conditions, as the user should understand its mechanism and which processes affect its use.
First, AKD sizing mechanism
AKD is a fiber-reactive synthetic sizing agent. Under medium and alkaline conditions, reactive functional groups can react with hydroxyl groups on cellulose to form covalent bonds and fix on fibers, forming a stable layer on the surface of fibers. The film turns the fiber from hydrophilic to hydrophobic so that the paper sheet obtains water resistance.
The sizing process of the AKD neutral sizing agent includes the following processes:
1 Retention - The wet end of the paper machine needs to add cationic retention aids to increase the retention of the AKD. In production, we can improve the performance of AKD in the system by increasing the retention rate. 2 Distribution—In the drying process, heating causes the AKD compound to spread and expand on the cellulose surface. 3 Orientation and fixation - In the process of paper drying and storage, the esterification reaction of the quaternary ring of AKD with the cellulose hydroxyl occurs, and the sizing and fixation on the fibers complete the sizing.
Second, the factors that affect the AKD neutral sizing effect
The amount of 1AKD: It should be as little as possible to ensure the degree of sizing. The AKD sizing curve shows that: AKD has a minimum amount, below which, the sizing degree is almost zero; but more than the optimal amount, the excess part not only can not increase the degree of sizing, but also cause the paper surface friction coefficient is too small, paper Page slippery.
2 Retention agent system: a reasonable retention system should maintain the first-term retention rate of 75% to 85%. Cationic PAMs are used as retention aids in production. Cationic PAMs require higher molecular weight and lower charge density.
3AKD Emulsion Storage Conditions and Hydrolysis in Slurry Suspensions: AKD emulsions can be stored at room temperature for 2 months, not exceeding 30°C or frozen. Practice has proved that when the storage temperature exceeds 30°C or is frozen, the amount of AKD is more than twice the normal amount. If the dilution with water must be used within 48 hours, AKD will be hydrolyzed and lose its sizing effect. Therefore, the unit consumption of AKD in the summer will increase. This is because the system temperature is high, and some AKDs are hydrolyzed, which does not have the effect of sizing.
4 Aluminum Sulfate Use: Because aluminum sulfate is an inexpensive anionic scavenger (ATC), pay attention to pH control.
5 pH value: The sizing effect of AKD has a great relationship with pH value. When pH<6, AKD can hardly produce sizing effect; with the increase of pH value, the sizing efficiency of AKD will gradually increase, especially pH Between 6.7 and 7.5, the degree of sizing of the sheet increases fastest; but when the pH is greater than 8, the rate of increase of the sizing degree begins to slow down. It can be seen that the pH value of the system has an important influence on the sizing fruit, and the sizing effect is too low. The reason is that when the pH value is too low, the chemical reaction rate of the sizing agent is slow, and the slurry pH value With the increase, the reaction rate is accelerated, but at the same time, the hydrolysis rate of AKD is also accelerated, and the sizing effect is reduced due to the hydrolysis of AKD.
6 Zeta potential: The internal charge of the system must be balanced. The more economical ATC is aluminum sulfate or polyaluminum chloride.
7 Adding sites and adding procedures: In order to reduce the degree of hydrolysis of AKD and avoid hydrolysis and demulsification, AKD is often added after the thick paste is diluted and added before the filler. Because fillers absorb almost 100% of AKD and fibers only absorb about 10% of AKD; aluminum sulfate is added in the mixing bath and in the white water system; cationic PAM is added close to the headbox, so it is not subject to any shear The best.
8 Additives that are not conducive to sizing: Surfactants such as defoamers and blanket cleaners are not good for sizing AKD.
9 AKD curing and drying: Curing refers to the development of AKD sizing of paper sheets over a period of time or during heating. The AKD particles remain on the paper like balls. In the drying section, the AKD will diffuse on the surface of the fiber as a monolayer, and then the AKD molecules will be redirected to make the paper resistant to water. This is an energy-consuming process. For quality control purposes, the dryer section must increase the dryer temperature. This process can be accelerated by artificially curing the paper, or it can be left at room temperature for some time to complete the process. A higher drying temperature can speed up the speeding of the AKD and fiber retention and improve the AKD sizing efficiency and aging rate. Therefore, it is very important that the dryer temperature reach 95°C as soon as possible.
10 The machine paper is too high.
Although there are many influencing factors in AKD sizing, as long as the AKD sizing is controlled in the production process, the performance of AKD can be brought into full play so that the advantages of AKD can be fully reflected.
First, AKD sizing mechanism
AKD is a fiber-reactive synthetic sizing agent. Under medium and alkaline conditions, reactive functional groups can react with hydroxyl groups on cellulose to form covalent bonds and fix on fibers, forming a stable layer on the surface of fibers. The film turns the fiber from hydrophilic to hydrophobic so that the paper sheet obtains water resistance.
The sizing process of the AKD neutral sizing agent includes the following processes:
1 Retention - The wet end of the paper machine needs to add cationic retention aids to increase the retention of the AKD. In production, we can improve the performance of AKD in the system by increasing the retention rate. 2 Distribution—In the drying process, heating causes the AKD compound to spread and expand on the cellulose surface. 3 Orientation and fixation - In the process of paper drying and storage, the esterification reaction of the quaternary ring of AKD with the cellulose hydroxyl occurs, and the sizing and fixation on the fibers complete the sizing.
Second, the factors that affect the AKD neutral sizing effect
The amount of 1AKD: It should be as little as possible to ensure the degree of sizing. The AKD sizing curve shows that: AKD has a minimum amount, below which, the sizing degree is almost zero; but more than the optimal amount, the excess part not only can not increase the degree of sizing, but also cause the paper surface friction coefficient is too small, paper Page slippery.
2 Retention agent system: a reasonable retention system should maintain the first-term retention rate of 75% to 85%. Cationic PAMs are used as retention aids in production. Cationic PAMs require higher molecular weight and lower charge density.
3AKD Emulsion Storage Conditions and Hydrolysis in Slurry Suspensions: AKD emulsions can be stored at room temperature for 2 months, not exceeding 30°C or frozen. Practice has proved that when the storage temperature exceeds 30°C or is frozen, the amount of AKD is more than twice the normal amount. If the dilution with water must be used within 48 hours, AKD will be hydrolyzed and lose its sizing effect. Therefore, the unit consumption of AKD in the summer will increase. This is because the system temperature is high, and some AKDs are hydrolyzed, which does not have the effect of sizing.
4 Aluminum Sulfate Use: Because aluminum sulfate is an inexpensive anionic scavenger (ATC), pay attention to pH control.
5 pH value: The sizing effect of AKD has a great relationship with pH value. When pH<6, AKD can hardly produce sizing effect; with the increase of pH value, the sizing efficiency of AKD will gradually increase, especially pH Between 6.7 and 7.5, the degree of sizing of the sheet increases fastest; but when the pH is greater than 8, the rate of increase of the sizing degree begins to slow down. It can be seen that the pH value of the system has an important influence on the sizing fruit, and the sizing effect is too low. The reason is that when the pH value is too low, the chemical reaction rate of the sizing agent is slow, and the slurry pH value With the increase, the reaction rate is accelerated, but at the same time, the hydrolysis rate of AKD is also accelerated, and the sizing effect is reduced due to the hydrolysis of AKD.
6 Zeta potential: The internal charge of the system must be balanced. The more economical ATC is aluminum sulfate or polyaluminum chloride.
7 Adding sites and adding procedures: In order to reduce the degree of hydrolysis of AKD and avoid hydrolysis and demulsification, AKD is often added after the thick paste is diluted and added before the filler. Because fillers absorb almost 100% of AKD and fibers only absorb about 10% of AKD; aluminum sulfate is added in the mixing bath and in the white water system; cationic PAM is added close to the headbox, so it is not subject to any shear The best.
8 Additives that are not conducive to sizing: Surfactants such as defoamers and blanket cleaners are not good for sizing AKD.
9 AKD curing and drying: Curing refers to the development of AKD sizing of paper sheets over a period of time or during heating. The AKD particles remain on the paper like balls. In the drying section, the AKD will diffuse on the surface of the fiber as a monolayer, and then the AKD molecules will be redirected to make the paper resistant to water. This is an energy-consuming process. For quality control purposes, the dryer section must increase the dryer temperature. This process can be accelerated by artificially curing the paper, or it can be left at room temperature for some time to complete the process. A higher drying temperature can speed up the speeding of the AKD and fiber retention and improve the AKD sizing efficiency and aging rate. Therefore, it is very important that the dryer temperature reach 95°C as soon as possible.
10 The machine paper is too high.
Although there are many influencing factors in AKD sizing, as long as the AKD sizing is controlled in the production process, the performance of AKD can be brought into full play so that the advantages of AKD can be fully reflected.
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