Discussion on the influencing factors of poor adhesion of paper core of gypsum board

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Discussion on the influencing factors of poor adhesion of paper core of gypsum board
1 Impact of face protection paper
1.1 The air permeability of paper is too low. Air permeability actually refers to the air permeability resistance of the paperboard. The air permeability of paper mainly affects the forming speed of gypsum plasterboard. At the same production speed, the greater the air permeability of paper (s/100ml), the wetter the plate from the dryer, and the greater the plate deflection. In order to ensure the production quality, it is necessary to reduce the production speed and output. However, if the air permeability is too low, the moisture in the gypsum board evaporates too fast during the drying process. The moisture in the starch in the gypsum core evaporates through the junction of the paper core and migrates to the paper, which affects the dry bonding performance of the paper and the core, and makes the board brittle.
1.2 The water absorption of paper is too high or too low. The water absorption of paper is related to the sizing degree of face paper. In the process of gypsum hydration, the growing needle like crystals will grow into the face paper, thus forming a good combination with the face paper. If the sizing is too heavy, the paper surface is relatively hard, which is not conducive to crystal penetration; If the size is too small and the water absorption is too large, the water in the gypsum slurry will be largely absorbed and evaporated quickly, which is not conducive to the growth of gypsum crystals, and will also affect the good adhesion of the paper core. The proper air permeability of paper depends on the actual situation of the production line and cannot be generalized. In general, the production line with large water solid ratio should select the face paper with large quantitative and slightly high water absorption. Generally, the production line with water paste ratio of 60-70 uses 220 g/m2 of face paper, and its water absorption is generally 15-20 g/m2. The determination of the quantity and water absorption of paperboard must be combined with the production process and industrial and mining conditions of gypsum board. Such as production speed, drying process, etc.
2. Influencing factors of gypsum and other raw materials
Gypsum is the main raw material for the production of gypsum plasterboard, and the performance of gypsum is an important factor affecting the performance of gypsum plasterboard. The grade and impurity content of gypsum, the setting time of building gypsum and the use of various additives will have a certain impact on the bonding performance of face paper and gypsum core.
2.1 The grade of gypsum is too low
Generally, the grade of natural gypsum is required to be no less than 75%. If the content of calcium sulfate dihydrate in gypsum is too low, too few gypsum crystals will be formed during the forming process of gypsum board, which will affect the bonding between face paper and gypsum core.
2.2 Content of K+, Na+and other impurities in gypsum
Generally, the content of K+, Na+and other impurities is not too high, but it has a great impact on the bonding performance of gypsum plasterboard. Therefore, it is generally required that the content of K+, Na+in gypsum plasterboard is not higher than 0.02%. If the content of K+, Na+and other impurities in gypsum is high, a layer of salt mold will be formed at the junction of paper core during the molding and drying process of paper faced gypsum board. When the board is in a low temperature and high humidity environment, the board is very easy to absorb water, making the paper core detached.
2.3 Effect of additives such as foaming agent
In order to improve the physical properties of building gypsum and gypsum plasterboard, additives such as water reducing agent, foaming agent and setting regulator are often added in the production of gypsum plasterboard. The use of these additives will also have a certain negative impact on the plate. Firstly, some additives contain a certain amount of inorganic salt of Na+, which will increase the accumulation of Na+in gypsum slurry and affect the bonding of paper core to a certain extent; Secondly, if you neglect to control the PH value of additives in the selection and procurement process, the PH value of gypsum slurry will fluctuate due to its influence, which will affect the setting time of slurry and the bonding performance of paper core; In addition, the foam stability of foaming agent has always been an important indicator that affects its performance and use effect, while most of this indicator of domestic foaming agent is in the required critical state. For example, if the foam diameter of foaming agent is too large or the stability is too low, more large bubbles will be formed after it is mixed with the slurry, even more concentrated, which will result in poor local bonding of gypsum plasterboard.
3. Influence of process control factors of gypsum plasterboard
3.1 The gypsum slurry is too thin and contains a large number of large bubbles or moisture, which causes a large number of large voids in the board core or the board is too light after drying, reducing the contact point between gypsum crystals and face protection paper, and affecting the good bonding between the face protection paper and the gypsum core.
3.2 If the setting speed of gypsum slurry is too fast or too slow, it will also have a negative impact on the paper core bonding performance of gypsum board. Gypsum slurry shall be initially set after passing through the slab and finally set on the molding belt. If the setting is too fast, the initial setting will occur before the plate pass, and the crystals extending into the face protection paper will be damaged when passing the plate pass. If the setting is too slow and the final setting is not completed on the forming belt, the unformed plates will be subject to continuous vibration during the subsequent roller conveying, which will affect their bonding and cutting. Even when entering the dryer, the board core and the paper are not firmly bonded. After entering the drying area, when the temperature reaches above 80 ℃, the paste will not continue to set, and the moisture in the board will rapidly evaporate outward, which will cause the paper core to be seriously non stick.
3.3 The drying speed and temperature are not coordinated, which causes the board to contain water or burn too much. With the change of gypsum or building gypsum properties, the molding and other production processes should be adjusted in time to adapt to the material properties. Otherwise, it will cause the over burning or under burning of the board, and when the board is over burnt or under burnt, it will show poor adhesion of the paper core to varying degrees.
4 The amount of modified starch is too little or the mobility of starch is not enough.
Although starch is not the main factor affecting the adhesion between the face covering paper and the gypsum core, the modified starch in the gypsum continuously migrates to the junction of gypsum and paper with the evaporation of water, protecting the gypsum crystal on the interface from being damaged by overheating, and strengthening the adhesion between the paper and the core.
5 The foaming agent is not uniformly added or the foam stability of the foaming agent is poor, which often results in poor local adhesion. Poor adhesion is easy to occur in the paper core at the large gap formed by more foam or small bubbles. The uniformity of foaming agent addition is related to the addition mode and mixing speed; The stability of foam is mainly related to the performance of foaming agent itself.
6 Conclusion
The factors affecting the cohesiveness of paper core of gypsum plasterboard involve many aspects such as raw materials and process control. In most cases, it is the result of a combination of several factors, which is difficult to control accurately. In actual production, only by combining the actual production situation, selecting the raw and auxiliary materials suitable for the production process, monitoring their fluctuations, and timely adjusting the process control indicators to achieve the harmony between the previous and next processes can the problem of poor adhesion of paper core be minimized, And ultimately improve the quality and production efficiency of gypsum board.
Zhou Yanhui, Shijiazhuang Jingjin Mechanical Equipment Installation Co., Ltd