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Industry news
Changes in paper moisture content can cause paper deformation.
Recently, some friends reported that after die-cutting, the protective film and the release paper will curl together. We analyzed that there may be some relationship with the moisture content of the paper.
1. Changes in water content of paper 1. The internal conditions of paper water absorption are well known. The main component of paper is natural plant fiber. and other components, mainly cellulose. Hemicellulose also has a certain content in most papers, while lignin only has a certain content in newsprint, and the content of lignin in other papers is very small. Cellulose is a polysaccharide carbohydrate composed of many glucose molecules. In the molecular structure formula, each polymerized monomer of the long-chain polymerized molecule on cellulose contains a plurality of hydroxyl groups. Hemicellulose is also a higher carbohydrate and also contains many hydroxyl groups. Therefore, paper is a highly polar hydrophilic substance, which has a strong polar adsorption effect on water. In addition, in terms of physical structure, paper is a net-like interwoven fabric formed by natural fibers and fillers on a paper machine. There are many gaps between fibers and between fibers and fillers. These gaps are equivalent to many capillary pores. In addition, so many fibers also have cell cavities. These cell cavities and other capillary pores must have a capillary adsorption effect of water. This polar adsorption and capillary adsorption of paper determines that paper is a highly absorbent material. It not only absorbs a large amount of water in contact with water, but also absorbs water from humid air. Of course, paper can also be dehydrated in dry air. 2. Direct water absorption When paper is in contact with water, many kinds of paper can absorb water directly and quickly. Regarding the direct absorption of water by the release paper, it is worth noting the absorption capacity and absorption speed. The suppliers of paper used in the die-cutting industry are as follows: Yellow paper: Japan Sanyo National Policy, Nippon Paper, Japan Prince, Taiwan Yongfengyu, Korea Guoyi, Shouguang Chenming, Chibi Chenming, Hunan Yuanjiang, Dafeng Paper, Anhui and other paper mills. White paper: UPM (UPM), Jinguang (APP), Shouguang Chenming, Huang Hailin, Ningxia Meili and other paper mills. Glassin: Europe - Dai Weilin, Aoslong, Duncan, Japan - Lindeco, Jiaxing Minfeng (Avery supplier), Wenzhou Xinfeng, Linhai Xiecheng and other paper mills. Glassine paper itself has a relatively low moisture content. If the production and supply of the label material is directly coated on the surface material, and the glassine release paper is only used in combination, it will generally not cause deformation. This production process generally uses water glue and hot sol labels. The oil glue is generally first coated on the release paper and then transferred to the material, which requires a process of drying the release paper in an environment of 70 degrees Celsius. Since the coating weight of special electronic tapes is generally relatively high (60 grams of dry glue, 120, 150 or 200 grams of wet glue), the running speed of the release paper inside the oven is extremely slow, and the paper is easy to be exposed to such a temperature for a long time. Water is lost, so glassine release paper is generally not used as the release paper for special electronic tapes. It is only used in the composite die-cutting process, because it has excellent flatness (protective film composite), stiffness (no substrate tape), and die-cutting performance. The use of white paper and yellow paper generally needs to be coated with plastic and then coated with silicon. At present, 91-98 grams of single-plastic and single-silicon [1.40-1.70 yuan per square meter], 105-110 grams of double-plastic single-silicon [1.60-1.90 yuan per square meter], 115-125 grams of Double plastic double silicon [2.10-2.60 yuan per square meter], 130-140 grams of single plastic single silicon [2.00-2.50 yuan per square meter], 155-165 grams of double plastic single (double) silicon [2.80-3.30 yuan There is more release paper per square meter. The most widely used release liner in the field of special electronic tapes is the plastic coated release liner. The plastic film melted and cast at high temperature is evenly attached to the surface of the paper to obtain flatness (whether the base paper is flat enough also has a certain influence, and the plastic coating is only to overcome some), and the stiffness is obtained by a relatively high amount of plastic coating. Top-coated to avoid water loss and warping to improve die-cutting performance. Glassine can be very suitable when full cutting is required, and plastic-coated release paper is more suitable when half cutting is required. Because the plastic-coated release paper has a certain elasticity, has a buffer, and has better impact toughness. At present, the base paper used in the electronic die-cutting industry is the base paper produced by some good factories. Of course, it cannot be ruled out that some factories also use paper from other factories in order to reduce costs and obtain profits. At present, the base paper used in the electronic die-cutting industry is basically all wood pulp paper. Of course, there are also factories that use half wood pulp, bamboo pulp and straw pulp paper. Paper using the latter as a raw material for papermaking is in no way comparable to all-wood pulp paper in terms of physical properties. All-wood pulp high-grade release base paper pays special attention to the flatness, stiffness and die-cutting performance of the paper. The price difference between high-quality base paper and inferior base paper will be more than 5,000 yuan per ton. One ton of high-quality paper can buy two tons of low-quality paper. The water absorption capacity of paper refers to the maximum amount of water that a unit weight of paper can absorb. Generally speaking, the looser the tissue structure of the paper, the greater the ability to absorb water. Under the same degree of looseness, the greater the amount of non-plant fibers such as fillers and sizing materials in the paper, the smaller the water absorption capacity. Therefore, general newsprint and letterpress paper have stronger water absorption capacity than double offset paper, and coated paper has weaker water absorption capacity than double offset paper. When the paper absorbs the same amount of water, the paper with high water absorption capacity will deform less, and the paper with low water absorption capacity will deform more. The water absorption speed refers to the speed at which the paper absorbs water. Generally speaking, the paper with high water absorption capacity has a high water absorption speed, but the water absorption speed is not equal to the water absorption capacity. There are papers with a high water absorption speed, but the water absorption capacity is not large. There are various methods to detect the water absorption speed of paper. Some are immersing a paper of a certain thickness in water to measure the time it takes to be completely soaked; some are inserting a strip of paper of a certain length and width into water, and measuring the time when the paper is soaked in water. the height of. 3. Balance water absorption and dehumidification All kinds of paper are not completely free of moisture. Generally, it contains about 5-8% water. The moisture content of paper is not constant without contact with water, but changes with the relative humidity and temperature of the surrounding air. When a paper with a certain water content is placed in the air of a certain temperature and relative humidity, it will absorb water from the air or dehumidify from the air until the paper reaches a constant weight, that is, the water content in the paper is reached. A state of equilibrium in which the water vapor pressure in the air is equal to the water vapor pressure in the air. At this time, the paper no longer absorbs water from the air, nor does it remove water from the air, or the paper absorbs water from the air at the same speed as the paper removes water from the air. The tax content in this state is called the equilibrium moisture content of the paper at this temperature and humidity. 4. The speed of moisture absorption and dehumidification It takes time for paper to absorb and dehumidify. Generally speaking, the dehumidification speed is much slower than the moisture absorption speed. However, whether it is moisture absorption or dehumidification, the speed at the beginning is relatively fast, and the closer it is to equilibrium, the slower it is, and it takes a long time to reach full equilibrium. The speed of moisture absorption and dehydration is affected by the quality of the paper, and loose paper absorbs and dehumidifies faster than dense paper. The speed of moisture absorption and dehumidification is also related to the environmental port, especially the dehumidification process. The fluidity of the air in the environment port has a great influence on the dehumidification rate.
2. Moisture absorption and deformation of paper After paper absorbs water, the fibers swell and stretch, and shrink after drying and dehydration. In addition, the penetration or loss of water between fibers can cause the bonding between fibers to become tight or loose. Therefore, moisture absorption and dehumidification will inevitably cause changes in paper size. This is why tapes and release paper are stored in a relatively constant temperature and humidity environment. For the high-demand slitting workshop, the temperature and humidity of the workshop must be controlled to bring certain benefits to the die-cutting accuracy. Unbalanced moisture absorption or dehumidification of the paper can easily cause the paper edge to be completely wavy (lotus leaf edge) or tight edge, and the paper surface is prone to wrinkles. Serious more "seekers" phenomenon. 3. Adjustment of paper humidity There are generally two ways to debug paper. One is called natural humidity control, and the other is called forced humidity control. The natural humidity control method is to hang the paper. After two days or so, the humidity of the paper tends to be balanced with the indoor humidity. This method is also called "hanging method", but this method actually makes the paper absorb moisture. Some more. Debugging directly in the die-cutting workshop basically belongs to the natural humidity control method. The forced humidity control method is to complete the humidity control in a short time (tens of minutes to several hours) in a high temperature and high humidity environment. This method may not reach the equilibrium water, but has reached or exceeded the equilibrium water under the conditions of the die-cutting shop, so the time taken is greatly shortened. When the paper has been completely wavy (ruffled edge) or tight edge, the above two methods are more cumbersome and time-consuming, and generally small die-cutting factories cannot have such conditions and experience. Just introduce a simple method. Put the paper in a small sealed room and raise the room temperature with a few electric heaters. Once the temperature in the sealed room rises, the relative humidity of the air will be greatly reduced, so that the absorbed water can be released faster, and the wavy bending can be eliminated. There is an even easier way to correct the wave shape, which is to directly irradiate the side of the paper with infrared rays on the paper feed table. For paper with tight edges, steam spray can be used to correct it. A large amount of water vapor is sprayed around the paper, so that the paper edge absorbs moisture and stretches in a short time, so that the tight edge phenomenon can be corrected. Fourth, the water resistance of the paper surface Because the die-cutting industry is generally used in higher-grade paper. Generally, we will choose paper with a relatively high sizing degree. In this way, the surface of the paper has a better water resistance.
