The durability of paper prints usually refers to the durability of paper and ink used in printing. This article focuses on the durability of paper used in printing. The durability of paper refers to the degree to which the paper can retain its color and folding resistance after a long time. For printing paper, durability expresses the service life of the paper print or the longest time it can be kept. Knowing, understanding and even predicting the durability of the paper, you can choose the paper accordingly according to the shelf life requirements of the printed matter. Especially for paper used in folding cartons, try to avoid the phenomenon that the inner packaging has not expired and the paper has yellowed. Usually the time taken for the paper to turn yellow to a certain degree or the folding endurance to a certain degree is called the durability of the paper.
There are many factors that affect the durability of paper, including the type of pulp used in papermaking, the content of sizing materials, paints, acids and metal compounds, as well as the temperature, humidity, and light when stored or used.
1 Main factors affecting paper durability
The durability of paper is determined by both internal and external factors. The internal factors include the type and quality of pulp, the content of rubber compounds, paints, acids and metal compounds, and other components in the paper, which are determined by the pulp and paper process. External factors refer to the storage or use conditions of paper, such as temperature, relative humidity, light, and atmospheric pollution.
1) The effect of residual lignin and pulp washing in pulp, the number of bleaching stages and methods on paper durability
The residual lignin content in the pulp, the washing degree of the pulp, the number and method of bleaching stages, etc., all obviously affect the stability of the whiteness of the paper. Lignin is different from the linear molecules of fiber and hemicellulose. It is a three-dimensional network-like macromolecular structure. Its molecular formula is represented by C138H138O35. Lignin is a harmful component in the paper. The residual lignin in the paper will make the paper brittle and reduce the mechanical strength. After being exposed to sunlight, it will show the yellowish brown color of the lignin, affecting the whiteness of the paper. Lignin can be dissolved in acid and alkali solutions and can be decomposed under the action of various oxidants. In the papermaking and pulping process, this property can be used to preserve the cellulose and hemicellulose in the raw materials as much as possible while removing lignin. The main reason why newsprint returns to yellow quickly is that the ground wood pulp used does not remove the lignin in the wood raw materials, and it is generally not bleached or only single-stage hydrogen peroxide bleaching.
2) The effect of the pH value of the paper itself on the durability of the paper
The research results show that the pH of the paper itself is the most important factor affecting the durability of the paper. If the pH of the paper is low, excessive acidity in the paper will accelerate the degradation of cellulose. In acid paper, hydrogen ions promote the "oxygen bridge" of cellulose macromolecules, and promote the hydrolysis and oxidation of cellulose, and these effects are relatively strong. The paper prints will be seriously discolored, the stronger the acidity, the more serious this phenomenon. There may be multiple sources of acidity in paper, but the most important source is alum used in sizing. The hydrolysis of alum leads to the formation of sulfuric acid in the paper. Therefore, the manufacture of archival document paper with better durability requires the neutral sizing method without adding alum. The source of acidity in paper is also pulping and bleaching. The paper made from sulfite pulp has a special effect on plant fibers due to low pH and acidic cooking conditions, so the durability is not as good as the paper made from sulfate pulp; if the residual chlorine content in the pulp is higher after bleaching, the pulp The chloride ion reacts with water to produce hydrochloric acid and hypochlorous acid, which makes the paper's pH lower and causes the paper's durability to decrease.
For paper with a high pH value, strong alkalinity is also not conducive to improving the service life of the paper, because the oxidative degradation rate of cellulose will be greatly accelerated under the condition of higher alkalinity. Therefore, for printing paper, strong acid and strong alkali have a great destructive effect on the durability of the printed matter. [next]
3) The effect of paper fiber type and quality on paper durability
The type and quality of fibers used in papermaking is also an important factor in determining the durability of paper. Fibers derived from cotton, rags, linen, sandalwood, etc., can be used as raw materials for the manufacture of paper with good durability, because the pulp made from these materials has a high cellulose content and a high degree of polymerization of cellulose . Straw fiber is just the opposite, so it is not suitable for making durable paper.
Cellulose is a white, odorless, odorless carbohydrate, insoluble in water and dilute acids, dilute alkalis and general organic solvents, but can be hydrolyzed under the action of concentrated sulfuric acid. Its chemical formula is (C6H10O5) It is a chain polymer compound composed of a large number of glucose groups. The strength of cellulose is determined by the degree of polymerization. The greater the degree of polymerization, the greater the strength. Bast fiber has a higher degree of polymerization, followed by cotton fiber, wood fiber and grass fiber. Therefore, paper made of cotton and hemp fibers has higher mechanical strength than paper made of grass fibers. Highly polymerized cotton and hemp fibers are mostly used to make high-quality paper.
4) The effect of various additives in paper on the durability of paper
The presence of added substances in the paper, such as various rubber compounds, fillers, pigments and other various additives, also affects the chemical stability and durability of the paper to a certain extent.
â‘ Filler. The whiteness of the filler is quite different due to its type and grade. If a filler with a higher whiteness than the paste is used, the more the amount of filler, the higher the whiteness of the paper.
â‘¡Dye. Each dye has a certain characteristic wavelength at which light is absorbed most, which is commonly referred to as the maximum absorption wavelength. When selecting a dye, it should be such that the dye does not affect its reflectivity in the vicinity of the light wavelength at which the whiteness is measured, which can correct the color of the paper without significantly affecting the whiteness of the paper.
â‘¢ Slurry. Pulp whiteness is the most important factor affecting paper whiteness. The whiteness of high-grade paper made from 100% bleached pulp and high-grade coating printing paper using it as base paper is about 80%. In contrast, the whiteness of medium-grade paper and medium-grade coated printing paper equipped with mechanical wood pulp will be lower. The whiteness of such paper depends on the whiteness of the used pulp and the ratio of the pulp.
â‘£ Paint. The whiteness of the coating printing paper is determined by the whiteness of the base paper, the whiteness of the coating and the coating amount. If the whiteness of the base paper is higher than that of the coating, some light passing through the coating will be reflected back by the surface of the base paper and reflected again by the surface of the deep coating, then the whiteness can also be improved. If the whiteness of the base paper is low, the amount of light absorbed by the surface of the base paper increases, and the light reflected by the coating layer decreases, thereby reducing the whiteness of the coated surface. Of course, when the coating layer is completely opaque, the whiteness of the coated paper is not affected by the base paper. However, the thickness and light transmission properties of the coating layer determine that it does not have the ability to completely hide the whiteness of the base paper, so the whiteness of the base paper also affects the whiteness of the coated paper.
If titanium dioxide, a pigment with high whiteness and high opacity, is added to the coating, not only can the whiteness of the coating itself be improved, but also because of its high opacity covering the original color of the base paper, the coating layer The whiteness of the surface is further improved.
To improve the whiteness of paper, the most important thing is to use pulp with a relatively high whiteness, that is, to strengthen the bleaching of the pulp. Secondly, the filler should be selected. When the whiteness of the filler is higher than that of the fiber component, the yellow tone of the pulp can be diluted and the whiteness of the paper can be significantly improved. The whiteness of the filler has a large difference according to its type and grade. The higher the whiteness of the selected filler than the pulp, the greater the amount added, the more obvious the whiteness of the paper will increase.
Regardless of coated paper or uncoated paper, fluorescent whitening agent can be added to the pulp or paint to improve the whiteness of the paper. This is because the fluorescent whitening agent can absorb invisible ultraviolet light and convert the ultraviolet light into blue light at 450 nm, thereby increasing the reflectance of blue light and increasing the whiteness value. However, fluorescent whitening agents have a very weak whitening effect on pastes and coatings with low whiteness. This is because ultraviolet light is absorbed by deep-colored fibers or coatings and cannot be converted into visible light.
In the production of non-coated paper, a small amount of magenta, lake blue or magenta is often added to the paper material after adding glue, alum and filling to achieve the purpose of whitening. Adding a small amount of blue-violet or red-blue dye to the pulp can absorb the yellow or orange-yellow spectrum of 500-600nm, so that the yellowish pulp pulp disappears, even slightly blue or slightly red. The whiteness of the paper that has been treated in this way has not been improved with a whiteness meter, but the human eye looks white. This is because the added dye does not enhance the reflected light of the paper, but absorbs a part of the yellow light to be reflected. Therefore, when observed with the naked eye, the whiteness of the paper has been improved to a certain extent. [next]
2 Methods to improve paper durability
In general, there are several methods to improve the durability of paper:
1) In order to improve light stability, adding paper-making additives (such as calcium carbonate, titanium white, zinc white), ultraviolet protection agent, fluorescent whitening agent, etc. can prevent the whiteness from decreasing.
Plant fibers generally have their own colors, so white paper must be added with whitening agents to improve the whiteness of the paper. The fillers commonly used in paper such as calcium carbonate, titanium white, zinc white, etc. are white pigments, so the addition of fillers Can increase the whiteness of the paper. The degree of increase in whiteness depends in part on the whiteness of the filler and in part on the particle size and hiding power of the pigment. The whiteness of the paper also depends on the whiteness and beating degree of the pulp. If these fillers are added to a high-whiteness slurry, the effect of increasing the filler will be reduced.
â‘¡Change the papermaking raw materials, such as changing softwood and hardwood to non-wood fiber. Fibers obtained from cotton, rags, linen, sandalwood, etc. have high cellulose content in the pulp and a high degree of polymerization of cellulose, which can be used to make paper with good durability.
③ Proper use of paper can also improve the durability of paper. The durability of paper is closely related to the storage conditions, so we must strictly control the temperature, humidity and light of the product packaging box, books, and documentation storage environment. The temperature is preferably 12 ~ 25 ℃, too high will cause a small amount of chemical substances in the paper (such as certain acidic substances) to increase the damage to cellulose, too low will make the paper moisture content freeze The internal structure of the paper is destroyed. The relative humidity should generally be 40% to 55%, which is compatible with the moisture balance conditions of the paper.
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