With the development of the market, people pay more and more attention to the quality, performance and grade of paper, which promotes the development of coated processing paper and coated board. Coating processing paper by applying a variety of pigments, adhesives or special coatings on the surface of the paper to meet different performance requirements, such as surface strength, appearance color, wear resistance, rust prevention and environmental protection properties. Pigment coated paper is composed of base paper, pigment, adhesive and additives, and the coated base paper usually occupies 70% of the volume. Base paper, as a coating substrate, has an important effect on the post-processing properties of coated paper. Although coating can improve some properties of the base paper, it cannot completely eliminate its defects. 

Therefore, in order to ensure the quality of coated paper, Qingdao JIERUIXIN has made the following requirements for the performance indicators of base paper:

1. Full range distribution

The full width distribution of base paper is a very important indicator for post-processing, especially for paper. The full range distribution here includes the distribution of quantity, thickness and moisture.

1.1Quantitative distribution

There are important indicators to describe quantitative distribution, which are banner quantitative difference and quantitative deviation. The banner quantification difference is the difference between the maximum and minimum quantification values obtained by sampling the entire banner. Quantitative deviation is the difference between the maximum or minimum allowable quantification and the identified quantification. In actual production, the banner quantitative difference should be a fluctuation range, according to the advanced level of paper machine equipment, the fluctuation range is not the same. It is generally believed that the banner weight difference of coated base paper is not more than 3%.

1.2Thickness uniformity

Thickness uniformity is commonly used to describe thickness distribution in research fields. In addition to the quantitative uniformity of thickness, paper moisture is also an important factor. During the calender process, the portion of the web with a higher moisture content is usually thinner. The unevenness of the additional pressure of the calendering machine and the pressing part, the wear of the calendering machine roll surface, the uneven temperature of the roll surface, and the improper distribution of the medium and high will lead to the uneven transverse thickness of the paper. This uniformity of transverse thickness is a very important factor causing the uneven tension of the full width of coated paper, especially on both sides thick, thin in the middle or one side thick, one side thin, it is more harmful.

1.3Overall moisture uniformity

In the general ordinary paper machine, the full moisture is usually high in the middle and low on both sides, more generally, the operating side is lower than the transmission side, the better difference is below, and the tension of the lower water part is tighter. In addition to the uneven press dehydration, the temperature difference between the two ends of the drying cylinder and the middle part of the dryer, the water filtration difficulty of the forming net, the press felt and the poor local air permeability of the dry blanket also have a greater impact on the moisture uniformity of the paper. For paper machines with surface sizing, the uniformity of sizing machine pressure is also important. The non-uniformity of paper weight has little effect on the difference of paper moisture.

2.Strength of base paper

The strength of base paper includes mechanical strength and the strength of base paper bond and surface. The mechanical strength of the base paper is a prerequisite to ensure the smooth operation of the coating process, and the bonding and surface strength is an important indicator to ensure the wet drawing strength, printing pressure and speed during the coating operation and printing, especially when the high-speed coating operation of high concentration shear of the scraper coating is more important. However, tear degree and tensile strength are the main factors that have important influence on the post-processing.

2.1 Mechanical strength of the base paper 

The mechanical strength of the base paper is mainly determined by the shape of the fiber and the interweaving and binding condition of the base paper, which mainly refers to the tearing degree and tensile strength.

(1) Tear degree 

Tear degree refers to the force required to tear a pre-notched sample to a certain length, expressed in millinewtons. The breaking degree of paper is the ability of paper and paper board to resist cracking. It has a certain relationship with the folding resistance of paper. Tear cracking also depends on the length of the fiber and its internal organization direction and the strength of the fiber itself, but also related to the bond strength and interweaving of the paper. Generally, the force of tearing paper is much smaller than the force required to break the paper, and the work done when tearing consists of two parts, that is, the work done by pulling apart the fiber and the work done by breaking the fiber. Tear strength is an important index that affects the smooth process of coating.

(2)nsile strength

The tensile strength of paper is one of the basic properties of paper strength, and the tensile strength measured in the paper industry refers to the breaking load of a certain width, which should strictly be the breaking strength. Tensile strength refers to the maximum tension that a paper or board can withstand. The factors affecting the tensile strength of base paper involve various aspects, including beating, paper making, sizing, filling and moisture control, which will not be discussed in detail here.

2.2 Surface strength 

The surface strength of base paper refers to the combined strength of fiber and fiber, fiber and filler. The surface strength of the base paper is affected by the wet charge, the slurry handling technique and the internal sizing degree of the base paper. In the production of coated base paper, when the value of the wax rod is greater than 14, the internal binding force of the paper is better, and the surface strength of the coated paper produced is greater than the value of the wax rod, the surface strength of the coated paper is different, and the block is dropped during printing.

3. Smoothness and breathability of base paper

3.1 Smoothness 

Smoothness measures the time required for a specific volume of air to pass between the sample surface and a glass surface under certain pressure and vacuum, expressed in seconds. It depends on the base paper's surface condition; uneven paper results in poor smoothness. Smoothness is closely related to ink absorption—smoother paper, often more compact, absorbs ink less effectively, especially when calendered. Thus, for paper requiring high ink absorption, smoothness should not be too high. For coated paper, smoothness also affects paint absorption.

3.2 Air permeability 

measures the volume of air passing through the paper per minute or the time required under specific area and vacuum conditions. It identifies the gaps between paper layers and tests moisture resistance, varying with the paper's density. The air permeability of base paper significantly affects the surface roughness of coated paper.