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Paper Moisture Content And The Characteristics Of Its Equilibrium Moisture Changes Nov 16, 2017

Paper moisture content


    Paper moisture content of a great impact on the printing, water is too low, brittle paper, printing easy to produce static electricity; content is too high, it will make the ink difficult to dry. Moisture changes on the various characteristics of the paper also has a great influence, as the water changes, its quantitative, tensile strength, flexibility, folding and so will change, the size of the paper also occurs telescopic, or even curl, Alice Side, wrinkling, lotus leaf and other phenomena.


    All kinds of paper have an optimum moisture content, at this value, the paper is in good condition all kinds of performance, cast coated paper, cast coated white cardboard optimal moisture content of 7% ± 2%, white cardboard 4% -7%, single-coated white cardboard 8% ± 2%. This is the moisture content of the paper when it leaves the factory.


    During printing, the paper acts as a hygroscopic material that absorbs moisture from the air at a rate that depends on the relative humidity and temperature of the air; it also destroys the moisture in the air at a rate that depends on the moisture content of the paper And the environment temperature and humidity. When the moisture absorption rate and the same rate of water loss, paper and air in equilibrium, the same water content in the paper, in which case the moisture contained in the paper called the balance of water.


    The so-called relative humidity, refers to the temperature of the air at a certain temperature and the air content of the air can reach the saturation moisture content ratio. The same kind of paper has different moisture balance under different relative humidity.


    Balance moisture


    In addition, the balance of water changes have the following characteristics:


    ① by the nature of the paper


    Under the same relative humidity, the more hydrophilic the paper, the higher the equilibrium moisture; the other hand, the lower. Papers with no excipients balance the water higher, while adding filler, sizing, coated paper, lower water balance. The same variety, its thick products than the thinner balance of water is higher, this is because thick products rich in water absorption of the high rate of raw paper.


    ② affected by temperature


    The same relative humidity, the temperature changes of about 15 ℃, the paper changes the maximum moisture content of about 0.5%. However, in the process of printing, the water content of the paper should be controlled within ± 0.1%. Otherwise, the accuracy of overprinting will be affected. Therefore, the color printing workshop shall control the temperature change within ± 3 ° C while controlling the relative humidity.


    ③ balanced by the trip - hygroscopic or desiccation effects


    Paper at a certain relative humidity, moisture absorption by the low moisture balance to reach the moisture content, than at the same relative humidity by the high moisture desiccant to reach the equilibrium moisture content is lower, which is the paper's moisture absorption hysteresis effect. To make the paper water content consistent with the original. Must be taken "overkill" approach. That is balanced at a relative humidity of the paper, if the moisture in high humidity moisture to reach the original equilibrium moisture, must be placed in a lower relative humidity environment, and vice versa.


    In addition, the speed of hygroscopic and dehumidification are also different, dehumidification speed is much slower. However, no matter moisture absorption or dehumidification, the starting speed is relatively fast, the closer to balance the slower. This process is also related to paper and air mobility. Under the standard wet temperature, a single cigarette paper processing 35min, you can reach 5.8% of the balance of moisture; all kinds of printing paper you need 2-4h to reach 5% -8% of the balance of water; . This hysteresis between the change in relative humidity and the change in the moisture content of the paper leads to a hysteresis effect between the change in paper deformation and the relative humidity.


    ④ by the direction of the paper


    The so-called direction, refers to the balance of moisture in the paper is not vertical and horizontal, the horizontal expansion rate much larger than the vertical expansion and contraction, therefore, the horizontal expansion and contraction of the paper to be larger than the vertical. Tests show that a single fiber in the horizontal expansion equivalent to about 20 times the longitudinal. However, the arrangement direction of the fibers inside the paper is varied, but the number of fibers arranged along the longitudinal direction is usually more.


    Therefore, the difference between the vertical and horizontal direction of the paper is not as large as that of the single fiber. Taking the relative humidity from 50% to 60% for example, the ratio of longitudinal and transverse telescoping is about 3: 7, that is, about 2.3 times. This ratio varies according to the degree of orientation of the fibers, and the stronger the directivity, the greater the ratio. Copying fibers in a random and irregular arrangement is one of the key issues in reducing lateral expansion and preventing overprinting.


    ⑤ affected by the two sides of the paper


    A very flat piece of paper, the environment in which it is changed, the paper will absorb moisture or moisture in the air. If it is hygroscopic, the reverse side of the elongation of the front than the large, the paper will curl to the front; if it is off, the reverse side of the shortening is larger than the front, the paper will curl to the back. Whatever direction is curled, it is always the longitudinal axis of the paper, which is caused by the different degrees of orientation of the paper on the front and the back. Especially cardboard, in this area even worse.