Prepress - Paper conditioning

Six, paper humidity treatment

(I) Water content of paper

The main raw material for paper is natural plant fibers. The cell wall of the plant fiber contains cellulose, hemicellulose, and other components of the latest printing technology, including the cellulose and cellulose. Cellulose is a polysaccharide carbohydrate that is formed by the polymerization of many glucose molecules and has the general formula. From the molecular structure, it can be seen that the long-chain polymeric molecules of cellulose contain a plurality of hydroxyl groups in each glucose molecule. Hemicellulose is also a high-grade carbohydrate, close to cellulose. According to the results of hydrolysis, its composition is pentose and hexose, and also contains many hydroxyl groups. Therefore, paper is a highly polar hydrophilic material and has a strong polar adsorption effect on water.

Paper, on the other hand, is an interwoven network of natural fibers and fillers made on a papermaking machine. There are many gaps between the fibers and between the fibers and the filler. This corresponds to a large number of capillary pores. Capillary adsorption.

These polar adsorptivity and capillary adsorption determine that the paper is a strong absorbent material. Not only does it absorb water when in contact with water, but it also has the ability to absorb moisture from moist air. It can also dehydrate the dry air.

The so-called water content of paper refers to the ratio of the mass of water contained in the paper to the total mass of the paper, expressed as a percentage. Paper does not exist completely without water and usually contains left and right moisture, but it varies with the ambient air temperature and relative humidity. After absorbing moisture from the air at a certain temperature and a certain relative humidity until the paper reaches a constant mass, ie, the equilibrium state of the water vapor pressure in the air and the water vapor pressure of the moisture in the paper, the paper no longer draws in water from the air and completes the process. During the water absorption process, the water content in the paper is the equilibrium moisture content at that temperature.

First, the balance of paper moisture depends on the type and purity of the raw material pulp. Hemicellulose has a greater moisture absorption capacity than cellulose. Therefore, at a certain temperature and certain humidity, the raw material pulp has different moisture content. The figure below shows the moisture content of papers made from various pulps at relative humidity. The dashed line in the figure shows the average papermaking moisture content of various pulps. It is a pulp-breaking pulp with a high purity, a low water content as a pulp and a chemical pulp as a chemical pulp, and a chemical pulp and a crushed wood pulp as a banana pulp as a crushed wood pulp.

Second, the equilibrium moisture of the paper is related to the ambient temperature, which decreases with increasing temperature and approximates a straight line relationship. The figure below shows the change in moisture content of offset paper when the relative humidity of the ambient air is kept at 0 and the temperature changes from 12 on the surface to 32 on the surface.

As can be seen from the figure, at a certain relative humidity, the change in water content in the paper is about 45 for every 42 change in temperature. However, during the overprinting process, it is hoped that the moisture content of the paper will not change more than 45, otherwise the overprinting accuracy will be affected. Therefore, the change of the shop's temperature must be controlled, and general changes should not exceed 432. —6— 258 Toppan Printing Technology Volume Diagram Humidity and Moisture Contents of Moisture Content of Various Papers at Relative Humidity

(b) Paper conditioning

All kinds of paper are shipped from paper mills. During transportation and long-term storage, due to the frequent changes in the surrounding climate and the difference in the average temperature between regions, it is impossible for the moisture content of the paper to adapt to the printing shop, and the water content is often not uniform. of. In order to keep the moisture content of the paper uniform on the entire paper surface, and adapt it to the temperature and humidity of the printing shop, at the same time, in order to reduce the sensitivity of the paper to the environmental temperature and humidity, and to improve the stability of the paper size, printing is generally performed. Before, we need to conduct humidity conditioning. The so-called humidity conditioning treatment is to hang the paper in the paper-hanging space before printing, and after a period of time, the paper reaches the equilibrium moisture content under the temperature and humidity conditions between the paper-hanging papers. After hanging, the water content of the paper is even and the dimensional stability can be improved.

The multi-color offset printing test was conducted on papers that had not been hung by air or by different hanging methods, and the state of change in the water content of the paper during the printing process after various hanging methods was obtained.

The moisture content of the test paper was measured before hanging without hanging. If the direct (non-conditioned paper) was used for multiple offset printing, the change in water content of the test paper was as shown in the figure below. Due to the relatively high relative humidity of the workshop, the moisture absorption during the storage of the unprinted paper in the workshop is about the same. After the printing starts, due to the effect of the offset wetting liquid, the moisture content of each printed paper increases one time, and accordingly, each ink is printed. Paper sizes have changed, increasing the difficulty of registration.