Q&A about printing paper (150-162)

150. What are the chemical properties of paper?
A: The chemical properties of paper depend mainly on the

The type of raw materials used, the method and degree of pulping and bleaching, the type and amount of non-fibrous ingredients added, the processing and processing after papermaking, and the like. For printing paper, its chemical properties are mainly moisture, ash, sizing, acidity, and durability of the paper.

151. How does the water content of paper express?
A: The moisture content of paper is expressed as a percentage of the weight of water in the paper and the weight of the paper. It is often referred to as water.

152. What are the effects of moisture in paper on printing?
A: The amount of water in the paper has a great influence on printing. Paper with low moisture content is brittle and will generate excessive static electricity in the printing process. If the paper has high water content, the printing ink will be difficult to dry. The increase and decrease of moisture in paper has a great influence on various properties of paper. With the change of moisture, the paper's quantification, tensile strength, flexibility, and folding resistance will change, and the paper size will expand and contract. Sometimes curling, curling, wrinkling, and "foliage" can occur.

153. Analyze the effect of water content on paper performance from the papermaking process?
A: It is mainly because of the relationship between fiber and water. Plant fibers are hydrophilic substances that bind to each other through the action of water, but if moisture increases to a certain value, the fibers loosen. Therefore, the binding strength of fibers decreases with the increase of moisture content. For example, the tensile strength of paper reaches the maximum when the moisture content is balanced with a relative humidity of about 30% to 35%, and then decreases significantly. At the same time, too low water content can also affect the properties of the paper. If the water content is too low, the rigidity of the paper will increase and the brittleness will occur; the ability of the fibers to slip and disperse stress will decrease, resulting in a decrease in the flexibility and folding resistance of the paper. .
Therefore, the paper should have a proper moisture content under its optimum performance, and the appropriate moisture content should be determined according to the variety and performance characteristics of the paper.

154. What is the balance of paper moisture?
A: Paper is a hygroscopic material that can absorb moisture from the air. The rate of absorption depends on the moisture content of the air and the humidity of the air. It also transfers moisture to the air and loses moisture. The rate of water loss depends on the moisture content of the paper. And its temperature. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in balance, and the moisture content in the paper remains unchanged. The moisture contained in the paper under this condition is called balanced moisture.

155. How does the moisture absorption of the paper affect the strength of the paper?
A: The strength of paper is determined by the strength of the bond between the fibers and the strength of the individual fibers. The strength, such as tensile strength, surface strength, etc., formed by the bonding between the fibers decreases as the moisture increases. Because of the increase in water content, the binding force between the fibers weakens, and they are easily broken under the action of external forces. However, due to the movement between the fibers before the breakage, the paper will stretch, that is, the tensile strength will decrease and the elongation will increase. On the other hand, an increase in moisture causes the paper to become soft, so the degree of flexion, tear, and the like associated with the softness increase with the increase in moisture.

156. How to control the shape stability and static electricity of paper?
A: To avoid or reduce the occurrence of paper deformation and static electricity, it is necessary to control the relative humidity and water content of the environment. The relative humidity refers to the water content in the air at a certain temperature and the air can reach saturation at this temperature. The ratio of water content. According to studies, the relative humidity of the environment is 40% to 60%, and the corresponding paper moisture is less than 3.5%. When the relative humidity of the workshop is lower than 40%, static electricity is prone to occur. Therefore, appropriate adjustments should be made to prevent static electricity.
157. Why do we need to adjust humidity on paper? how to proceed?
Answer: Since the paper has hygroscopicity, the paper should be subjected to humidity conditioning before printing so that the paper can be adapted to the temperature and humidity conditions of the printing room to be suitable for printing. The humidity conditioning of paper is particularly important in offset printing because the paper must continue to be moistened multiple times by the fountain solution during the offset printing process.
The method of humidity adjustment is that, before printing, the paper is placed in a pre-conditioned environment that is more humid than the printing room, and then placed in an environment with the same humidity in the printing room so that it does not continue to absorb moisture during the printing process. The balance of moisture can be readjusted when passing through the press.

158. There are several ways to measure the moisture content of paper?
A: The direct methods for measuring the water content of paper include drying, distillation, and chemical methods.

159. What is the ash content of paper?
A: After the paper is burned, there will be white inorganic residue, which is called ash. The ash content of paper refers to the ratio of the quality of the residue and the quality of the original sample after the paper and paperboard are burned at the temperature specified in the standard. Expressed as a percentage. The ash of the paper is mainly derived from fillers used for filling and pigments used for coating.

160. What is the relationship between printed ash content and other paper properties?
A: In the non-coated printing paper, the ash mainly comes from the filler. Many properties of the non-coated printing paper are related to the ash content, as shown in Table 1. The ash content of coated paper is not as single as that of non-coated paper because it includes two factors: ash contained in the base paper and ash content in the paint. In general coated paper, the ash content in the coating is much higher than that in the base paper. The ash content is much higher.

161. How is the acidity and alkalinity of paper expressed?
Answer: The acidity and alkalinity of the paper can be expressed by pH value, and the production process is different, and the pH value is also different. The pH of most uncoated papers is between 4.5 and 7.0, which is weakly acidic, while the pH value of coated printing papers is mostly between 7 and 9 and is slightly alkaline. However, whether it is coated or uncoated paper, since the carbon dioxide in the air reacts with the acidic components of the paper and the paper to generate weak acids, the pH value decreases as the storage time increases. Therefore, the paper is mostly weakly acidic.

162. The origin of paper acidity?
A: The acidity and alkalinity of the paper is mainly caused by the alum added to the slurry during internal sizing. At the same time, it is related to the factors such as organic acid and bleaching residue left in the pulp. Due to the effect of basic fillers and pigments, the pigment is passed through the pigment. The coated paper may be less acidic or slightly alkaline.

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