Basic knowledge of carton paper (3)

Waterproof performance

The waterproof performance of cardboard and corrugating medium is an indispensable property of corrugated boxes. Boxboard waterproof performance is based on the unit of water absorption weight per unit time called the water absorption, generally expressed in g/m2/2mins, the test method is called COBSI (COBB SIZING TEST), box board The water absorption of the positive and negative sides of the paper is equally important. The waterproof property of the corrugated medium is called the degree of sizing or water resistance. The test method is to place a sample of corrugated medium paper in a glass containing a thiocyanide solution. In the dish, a few drops of ferric chloride solution were dropped on the surface of the sample. When the two solutions came into contact, they turned red, and the degree of sizing was expressed as the penetration time of the two solutions (seconds, SEC).

Wet strength (WET STRENGTH) refers to the strength of the original paper is called wet strength after the base paper is immersed in water for 24 hours or placed in high temperature (above 40° C.) high humidity (95% RH) air for a considerable time. The general test base paper has a wet burst strength, a wet tensile strength, a wet ring pressure strength, and the like. This performance is caused by the paper mill adding a wet strength agent to the paste during the papermaking operation, and the fiber is produced after the heat treatment. Waterproof layer to prevent the penetration of moisture, that is to produce a strong wet effect. If a corrugated carton needs to be stored in a humid environment for a long time or it is to be refrigerated, it is necessary to use wet processed paper.

Effect of Relative Humidity on the Physical Strength of Base Paper

The physical strength of the base paper is tested in accordance with the national standard GB10739 Specification for pulp, paper and paperboard. 1 Before the preparation of the sample, the moisture content of the sample should be dried to approximately half the water content in the standard condition (drying temperature must not exceed 60°C). 2 Hang the sample in air at 20±2°C and 65±2%RH so that the surface is fully in contact with the conditioned air. 3 Cardboard or paper with good water resistance needs to be prepared for 24 hours or more, until the full weight has not changed significantly. Physical tests should also be held at the same temperature and humidity. The United States belongs to the continental climate. Therefore, the tests are carried out under the standard conditions of 23±2°C and 50±2% RH. Therefore, the test conditions are different, and the test values ​​obtained cannot compare the advantages and disadvantages.

The base paper used in corrugated cartons is composed of vegetable fibers. The moisture content of plant fibers varies with the ambient air conditions. When the ambient air humidity is high, the base paper will become wet. When the humidity is low, the base paper will dry out. In the air of temperature and humidity, the moisture of the base paper will reach an equilibrium state; when two sheets of paper with large water content are modulated in the same temperature and humidity state, the lower the moisture content, the higher the moisture content; The equilibrium state is reached, but there is still a slight difference in the moisture content between the two. Therefore, before the sample is prepared, the moisture content of the raw paper sample should be dried to about half the moisture content in the standard condition, and the same moisture can be obtained by reabsorbing the moisture. Content, changes in moisture absorption and release from the base paper are shown in Figure 1.

The balance of various types of paper moisture differs depending on the type of pulp. Under the condition of humidity of 60% RH, when the moisture reaches the equilibrium point, the coated paper is about 6%, newsprint is about 10%, and the corrugated paper base paper is about 9.5%. .

The physical strength of the base paper is greatly influenced by the moisture content, and its variation is shown in Figure 2. At a humidity of 65% RH, the strength is 100.

(to be continued)