Paper sizing this 6 points clear, but also afraid of sizing effect is not good?

In order to improve the surface strength of paper and paperboard and prevent the diffusion and penetration of water liquid, we often need to perform sizing treatment on paper and paperboard. AKD sizing agent is a mature neutral sizing agent widely used in the paper industry and has been widely recognized by the industry. However, there are still some problems in the actual use of AKD. How should we deal with these problems?

Papermaking sizing generally has two methods: one is surface sizing, that is, after the paper web is formed and dried, the sizing agent can be applied to the surface of the paper through the sizing press, coating machine or calender; Another method is the sizing in the pulp, the sizing agent is added to the papermaking stock, and the combination with the paper web is achieved during the paper sheet forming process. As a result of both processes, the wetting properties of the paper to water can be reduced, and surface sizing can also reduce the porosity of the paper web.

AKD neutral sizing agent has been widely used as a mature sizing agent. As an ideal product for a new generation of medium/alkaline sizing, AKD can significantly improve paper strength, improve printing performance, overcome the shortcomings of rosin sizing difficulties in summer, and reduce the corrosion of equipment. However, in the process of using AKD, there are still some difficult problems to solve:

1. Storage and use of AKD.

There is a phenomenon of slow hydrolysis of AKD raw materials, and the longer the storage time, the higher the degree of hydrolysis. The hydrolysate appears as a waxy floater or milky white viscous substance, which has no effect on the sizing process, but can cause side effects when the hydrolysate is excessive.

2. Paper water absorption is not stable.

In the production process, it was found that the water absorption of the produced paper fluctuated irregularly, and the water absorption of the paper before the roll was significantly different from that after the roll, and the water absorption of the sample after the roll was lower than that before the roll, showing irregular attenuation, with an average attenuation of 3%-5%. This problem may be caused by the following reasons:

(1) The influence of the amount of addition. In actual production, a small amount of AKD will enter the circulatory system with the dehydration of the mesh part, resulting in the gradual accumulation of AKD in the system. After a period of time, the actual concentration of AKD in the system is much greater than the design concentration, resulting in an increase in the amount of AKD retained in the paper.

(2) The influence of temperature. In practice, it is found that the higher the temperature of the roll, the greater the difference in water absorption between the lower roll and the lower roll. The water absorption of the same paper will also be different when the local temperature is different. The higher the temperature, the lower the water absorption. This phenomenon indicates that the higher the temperature is, the more favorable the curing of AKD, the faster the curing speed of AKD, and therefore the greater the change in water absorption of paper.

(3) Influence of entry point. The change of AKD addition point also has a certain effect on water absorption. When the paper reaches the same water absorption index, the amount of AKD added when the AKD adding point is far away from the front box is significantly less than that when the paper is near the front box. This shows that the longer AKD stays in the flow system, the longer the interaction time with fiber and cationic additives, the better the sizing effect on paper.

3. Translucent dots appear on paper.

In the production process, it was found that a large number of waxy translucent points appeared on the paper from time to time, and were distributed irregularly on the paper. The translucent point is caused by the hydrolysis of AKD in the process of use for various reasons, the raw material is reduced to a waxy form, and in the process of paper manufacturing, it is heated by the drying department and melted into oil on the paper. This problem may be caused by the following reasons:

(1) There are hydrolyzed products in raw materials. Influenced by the storage environment and the properties of AKD itself, AKD raw materials will appear a small amount of hydrolysis products after long-term placement, which is specifically manifested in the milky waxy flake substance floating on the surface of the raw material liquid.

(2) Hydrolysis products produced by internal hydrolysis of the system. AKD partially circulates into the system during use, resulting in hydrolysis reaction when AKD circulates in the system for a long time, and the hydrolyzed products will enter the paper with the system, and then produce translucent points.

(3) The effect of system charge potential. Due to the addition of special additives in the process of producing special products, the system charge potential changes, and the paper translucent points increase significantly.

(4) The effect of stuffing. In the production process, it was found that the translucent points produced by the fillers with larger particle size were much less than those produced by the fillers with smaller particle size. The reason for this phenomenon is that the loss rate of the fillers with smaller particle size is relatively large, so the amount of AKD combined with the fillers into the system will increase correspondingly, and the hydrolysis products generated will also increase correspondingly.

(5) The impact of damaged paper. A certain proportion of damaged paper will be added in the production process, and the damaged paper also contains a certain amount of AKD. Therefore, during the damaged paper treatment process, AKD will produce a small amount of hydrolysis products under the action of heating and shearing, which will be added to the system along with the damaged paper.

In view of the above three problems, what should we do?

Countermeasures:

1. Maintain production environment and slow down AKD hydrolysis. For AKD raw materials, attention should be paid to the storage environment, especially the storage temperature and time. In the process of AKD storage, it is necessary to reduce the storage temperature of AKD raw materials (below 25℃) and reduce the storage time to reduce the generation of AKD hydrolyzed products.

2. Stabilize the amount of AKD added and control the actual concentration of AKD in the system. Through production practice investigation, AKD accumulation in the system can not be changed for the time being. At present, the measures can be taken to ensure that the temperature and humidity of the production environment are stable as far as possible to ensure the nature of AKD raw materials; On the premise of ensuring the water absorption index, the amount of AKD added is adjusted according to the actual situation and production experience to achieve real-time regulation and minimize the accumulation of AKD in the system.

3. Control the production operation to reduce the hydrolysis of AKD. In the production process, the use and addition sequence of fillers should be controlled to reduce the effect of charge potential on the hydrolysis of AKD. Regular cleaning of the system to reduce the wall hanging of AKD and filler compounds; In the process of treating damaged paper, pay attention to the control of processing time and temperature to minimize the shear effect on AKD.