Generally speaking, the standard moisture content of paper just off the paper machine is 7% ± 2%, but the moisture content of paper will change during storage and printing processing, which is also a major cause of paper curling, 90% The above paper curls are all due to this.
During the papermaking process of the paper machine, the direction of the pulp fibers is basically the same as that of the net (the longitudinal direction of the paper). The moisture of paper is mainly stored between paper fibers. Therefore, the increase and loss of moisture will lead to the increase or shrinkage of the distance between the paper fibers, which is mainly manifested in the change of the transverse dimension of the paper. When the moisture content of the paper changes uniformly, it will only cause changes in the lateral dimension of the paper, which is a problem that needs to be avoided in printing registration, but it will not cause curling of the paper. Curl is caused by uneven changes in the moisture content of the paper.
There are roughly two situations for the uneven change of paper moisture content:
(1) The edge of the paper and the center of the paper absorb or lose moisture unevenly. For example, stacked sheets or rolls of paper, if there is no isolation package such as stretch film, the paper edge is easy to absorb moisture and cause the "lotus leaf edge" phenomenon, or the paper edge loses moisture and the "tight edge" phenomenon occurs.
(2) The two sides of the paper absorb or lose moisture unevenly. For example, the printing ink on the printing surface of the paper blocks the capillary pores on the surface of the paper, and the non-printing surface is generally not treated, so there is a big difference in the water content of the two sides of the paper. If the paper is exposed to a temperature and humidity environment that is not suitable for its moisture content for a long time, it will inevitably lead to the lateral curl of the paper: when the paper absorbs moisture from the external environment, the lateral expansion between the fibers of the non-printed paper is greater than that of the printed surface, and the paper will Curl to the printing surface; when moisture is lost, the transverse shrinkage between paper fibers on the non-printing surface is greater than that of the printing surface, and the paper will curl to the non-printing surface. Another example is that the packaging of aluminized transfer paper accounts for a large proportion of commodity packaging. Since the printing surface of this type of packaging is made of superimposed aluminum layer and adhesive layer, which hinders the loss of moisture on the printing surface, paper curling is easy to occur during use: if it is used in an air-conditioned environment with low humidity, It is easy to curl to the non-printing surface; when used in an environment with high humidity, it is easy to curl to the printing surface. Therefore, the lateral curl of the paper is the inevitable result of the uneven change of the moisture content in the paper. This provides a good way to identify the cause of lateral curl in the paper.
The paper is produced in different regions and used in different seasons, or in an air-conditioned constant temperature and humidity environment and a non-air-conditioned environment with different temperature and humidity, and paper curling will inevitably occur.
In the wet lamination production process commonly used in composite paper and transfer paper (coating of water-based composite adhesive on the composite surface, coating of waterproof adhesive on the back and drying), if the water content of glue coating, lamination speed and drying links are controlled Improper use will result in a large difference in moisture gain and loss between the two sides of the composite paper and transfer paper, and curls in uncertain directions will inevitably occur.
There may be other causes of curl in paper produced from composite materials. For example, when paper and plastic are combined, the plastic film will be stretched and deformed due to the high temperature of the compound or the excessive tension of the plastic film.
