The relationship between the properties of coated paper and printability
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In the color reproduction system, the quality of printed matter is affected by many factors, and the performance of most printed matter depends on the relationship between the printed ink and the printed material, and the printed matter-paper is divided into coated paper and Two types of non-coated paper.
The coated paper is coated with a coating color on the base paper to give the paper good optical properties and printing properties. Its main purposes are: printing magazines, books and other publishing papers and printing papers such as trademarks, packaging, catalogues, etc., the ratio of the two is about 1:3.
The coated paper can be roughly classified into coated paper, coated paper, and lightweight coated paper, in addition to special coated paper such as polished coated paper. There are also several differences in the classification of such coated papers in countries around the world. The base paper is generally made of Daolin paper, and also coated paper containing ground wood pulp or deinked pulp base paper. Except for special purposes, it is mostly used for advertising posters and magazine pages, so even under the same copying conditions, it is presented. The color will also vary greatly. Therefore, in order to improve the printing quality, it is necessary to first grasp the influence of the paper on the color reproduction, and then make appropriate adjustments on the photographic color separation or printing machine to make the printing quality meet the demand.
First, base paper and paint
1. Base paper
The manufacture of coated paper is simply to apply a coating on the base paper, and after drying, the surface of the coated paper is smoothed by a calender. Therefore, the base paper and the coating are important factors for the quality of the coated paper.
Taking a coating layer having a coating amount of 20 g/m 2 as an example, assuming that the dry specific gravity of the coating is 1.5, the thickness of the coating layer is only about 0.003 cm. Under such a thin thickness, the quality of the base paper does occupy an important position. .
If the speed of coating is slow, since the raw paper after water absorption reaches the drying department, the strength cannot be lost, so it is necessary to apply a proper amount of sizing and maintaining the strength of the wet paper. Conversely, when the coating speed is fast, the paper must have a certain strength to resist the mechanical force brought by the high-speed coating, and uniform water absorption is also very important.
In addition, the base paper for coating must also have the following properties:
(1) The dimensional stability is good and will not change much due to moisture changes.
(2) Strong adsorption to paints.
(3) The base paper must not be bent, otherwise it will cause paint streaks and cause the paper to break.
(4) There must be no wrinkles, holes, etc.
(5) When coating and processing, the paper roll must be wound up to avoid breakage.
The properties of the base paper will affect the coating: homogeneity, smoothness, surface absorbency, surface strength, surface condition, chemical suitability, two-sidedness and optical properties of the paper. Therefore, the quality of a piece of paper lies in the base paper. Whether the situation is good or not.
2. Coating color
Coating preparation is the most basic technology for the production of coated paper, and has a critical impact on the use of the product. Therefore, the coating must meet the operability and printability of the coating operation, and be designed and manufactured according to these conditions; that is, the design of the coating and What kind of coated paper is produced. In terms of quantity, it is still dominated by ordinary coated paper, but at the technical level, it is desirable to have the technology of coated paper. Different products should use their corresponding coatings, coating methods and coaters. Although the various categories must meet the required quality indicators, the most important thing is to get the customer's affirmation.
There are many types of coatings for paper coating, and water-based coatings are more commonly used. The composition of water base coatings can be broadly classified into four categories: pigments, adhesives, additive additives, and water. The combination of these four components is closely related to the quality of the coated paper. Use Table 1 below to list the effect of the composition of the coating on quality.
(1) pigment
Pigments include white clay (white clay), calcium carbonate, sardine white, aluminum hydroxide, titanium dioxide, and plastic pigments.
1. White soil
White clay is classified into kaolin, pyrophyllite, and cericite. Kaolin is further subdivided into: kaolin, kaolin, etc. These clays have been commercialized. However, in the clay for coated paper, kaolinite is basically called kaolin, which is used for coating. Synonymous with white clay.
The kaolin particles are hexagonal and have a particle size of 0.3 to 3 μm. The aspect ratio and particle size of these particles directly affect the quality of coated paper-based pigments. Currently, coated paper pigments are mostly made of kaolin because it can impart a high white gloss to the coated paper, and the required amount of adhesive is suitable. And the dispersion is better, so that the coating can improve the solid content.
2. Calcium carbonate
It is widely used in coated paper and its use is second only to white clay. Especially in Taiwan, which is rich in calcium carbonate, in recent years, it has even gradually replaced the more expensive white clay, and developed many varieties according to the use and production methods of coated paper. Therefore, how to use calcium carbonate moderately can be said to be coated paper coating. Focus. Since the calcium carbonate pigment has high whiteness and porosity, the ink absorbability is very good, so that the printability can be improved, and even the coated paper having no light treatment is very practical.
3. Sardine White
Sardine white is a white pigment synthesized from lime lime and barium sulfate, which has been used as a pigment for coated paper. It has good whiteness, gloss, ink absorption, and a smooth surface. Corresponding to these, there are also advantages such as high viscosity of the coating and high expansion, but also disadvantages such as poor coating workability and large amount of adhesive required. In the past, Shadingbai used it as a coating for air knife coating machines. If it is used in a high-concentration blade coater, it will be used in coated papers.
4. Aluminum hydroxide
Has good whiteness, opacity and ink absorption.
5. Titanium dioxide
Titanium dioxide has a high opacity, so it is often used as a coating for micro-coated paper. However, due to its high price and high wear, it limits its use. The types are: rutile type and anatase type. Although the anatase type has a slightly poor opacity, it is relatively inexpensive and has a high whiteness, so it is widely used.
6. Plastic pigment
Currently developed plastic pigments can be generally divided into three types: binder type, solid type and hollow type, which are more commonly used in the latter. In the process of synthesis, different monomer ratios, initiator concentrations, agitation rates, and temperatures at the time of reaction can be utilized as a variation to control particle size, uniformity, stability, and reaction rate.
Regardless of the pigment, it is expected to have the following characteristics:
1. The particle size should be suitable. In general, the smaller the particle size, the better the whiteness, opacity and gloss of the paper. However, if the particle size is too small, the surface area will increase, resulting in an increase in the viscosity of the pigment coating, a decrease in fluidity, and an increase in the amount required for the agent.
2. The pigment particle type should also be suitable. The acicular pigment particles have poor fluidity, and the hexagonal and spherical pigment particles have better fluidity, and a porous coating layer can be formed to improve the ink absorbency of the paper.
3. High whiteness and opacity (strong shading ability).
4. It is easy to disperse in water and has good stability and fluidity.
5. With better chemical stability, the amount of adhesive is reduced as much as possible.
6. Lower wear resistance (reducing wear on coaters and printing plates).
(2). The adhesive
The agent is then divided into natural products and synthetic materials, and natural products include starch, cheese, soy protein and the like.
The composition has various synthetic latexes, polyvinyl alcohol and the like. The most widely used now is a mixture of starch-synthetic latex. The use ratio of synthetic latex is increasing, and recently all latex adhesives have been developed, which is consistent with the development of high concentration of coatings. The following requirements apply to any adhesive:
(1) It has strong adhesion to pigments.
(2) It has good adaptability and stability to pigments, but does not react chemically with pigments.
(3) It has suitable viscosity and film forming properties.
(4) Appropriate fluidity to facilitate coating operations and improve coating uniformity.
(5) The color should be light, free of foreign matter and good paint handling.
Due to the general adhesives, whether natural or synthetic, some properties cannot be improved. Therefore, in recent years, so-called co-Binders have appeared to supplement the water retention and fluidity of the adhesives. It is used in an amount of only about 10-30% of the total dose. There are now factories trying to use this to reduce the total amount of the subsequent dose. Due to the small amount, it can sometimes be regarded as an additive. Currently commonly used auxiliary adhesives are soy protein and rheoloe mdifier.
(3). Additives
There are dispersants, lubricants, preservatives, fluorescent agents, water resistance agents, as well as dyes, ink absorption and improvers, etc., which mainly play a role in the lack of pigments and adhesives, and reduce operational problems. When using, you should choose the variety and amount of use. Because there are many opposite things, you need to conduct pre-existing research to formulate the most suitable coating.
(4). Surface modification
calendering
To make a glossy coated paper, it is necessary to use a calender wheel for glossing. The equipment used in the past is a combination of a cotton or wool elastic wheel and a chilled roll. Calender (super calender). Recently, starting from Europe and the United States, Japan now also uses a soft calender containing a plastic enamel coated with a polymer.
Compared with ordinary paper machine pedestal and super calender, soft calender has the following characteristics:
1. Give the paper a higher degree of twist, stiffness, opacity and strength, and further improve the two-sidedness of the paper.
2. After calendering, the paper web has the same tightness, good printability and uniform ink penetration.
3. It can allow slightly higher water content of the paper, less paper breakage, less black spots, and good running performance.
4. Moderately reduce the proportion of wood pulp or reduce the basis weight of coated paper and newsprint without affecting the printing quality of the product.
5. It can be installed on the machine or used as a semi-wet calender. All cymbals can be individually driven and interchangeable for a wide range of applications.
6. Compared with the super calender, it has the advantages of low investment, small occupation, convenient operation and maintenance, low pressure zone and low power consumption.
However, if the number of pinch joints of the soft calender is too small, the high-gloss art coated paper can not be placed, and the super calender speed is too slow to limit the output, which has disadvantages, so it combines supercalendering. The Janus Concept calender, which combines the advantages of a machine with a soft calender, came into being in 1995 and is characterized by:
1. The number of nips can be selected according to the needs. The Janus Concept calender with 8 pinch joints is equivalent to the calendering effect of the traditional 12-kneading super calender.
2. Apply hot calendering, and the energy consumption is lower than that of the general soft calender.
Second, Coating
The method of paper coating can be divided into two types: on-machine coating and post-machine coating, each having its advantages and disadvantages, which are summarized as follows:
(1), On-board coating (ON-MACHINE COATING)
The so-called on-machine coating is to connect the paper machine to the coater, that is, a coater, which is a part of the paper machine, because the coating speed of the coater must be copied. The paper machine has the same speed and is suitable for producing a single type of coated paper with a large demand. The advantages and disadvantages of on-board coating are as follows:
Advantages:
1. The coating machine is coupled with the paper machine, so that paper loss at the end of the paper is not generated during the reel operation.
2. No special space is required to store the original paper roll to be used.
3. The space occupied by the machine is small.
4. It does not need to reel and transport the iron core, so it can reduce the need of labor.
5. There is no need for special drying equipment and paper holders and cutting machines, so the cost is relatively low.
6. The operation is simple, and it is suitable for mass production of coated paper with few types and large quantities.
7. The problem of the quality of the base paper can be identified early.
Disadvantages:
l. The change elasticity is small, and it is not suitable for producing coated paper with many varieties and few quantities.
2. If the coater or paper machine malfunctions, it will cause another machine to stop.
(2), After the machine coating (OFF-MACHINE COATING)
After the machine is coated, the paper machine is completely separated from the coating machine, and each is placed separately at a different place. The original paper roll copied by the paper machine is sent to the original paper holder of the coating machine, and the latter stage is required. At the time of processing, the coating machine is subjected to the coating work. Its advantages and disadvantages are summarized as follows:
Advantages:
1. The coater and the paper machine are independent of each other, so one coater can coat a variety of papers made by several paper machines.
2. The change elasticity is large, and it is suitable for producing various grades of coated products.
3. The speed of the coating machine is generally faster than that of the paper machine, so the scheduling is easy and the processing throughput is also large.
4. It is suitable for the production of coated paper with many kinds of finished products and a small amount.
5. Provide excellent coating quality.
6. The coating machine is separated from the paper machine, so when one of the two encounters a fault, it will not affect each other and cause the machine to stop.
Disadvantages:
1. When the original paper is in the reel operation, there will be paper damage at the end of the paper, and the paper loss rate is large.
2. The original paper roll and storage require high labor costs and plant space.
3. The coating machine needs to be equipped with equipment such as the original paper holder, paper cutting machine, paint drying, etc., so the machine occupies a large space and the equipment cost is relatively expensive.
It can be seen from the above that the on-board coating method has the advantage of lower cost, and the quality of the coated paper on the machine can be improved from various aspects, such as improving the coating machine, the drying method and the calendering equipment, or improving the hardware. A coating formulation such as a solid content and a reduced amount of an adhesive; this experiment is intended to improve the printability of the coated paper by opacity, gloss, printing gloss, etc., while adding an auxiliary adhesive to improve the water retention of the coating. Degree, high shear viscosity and other operability.
In the past, due to the speed of the coating machine and the speed of the paper machine, on-board coating usually only produces coated paper with a lower coating amount, such as LWC, or as a pre-process. Coated paper. After the improvement of the new generation of coating machine, a coating machine called HSM has been able to increase the coating amount. It is believed that after a short period of effort, the on-machine coated processing paper can also achieve the effect of similar coated paper.
Third, Conclusion
Whether the quality of coated paper is good or not, in addition to the great influence of the quality of the original paper, the composition of the coating is also an important factor. When different types of organic pigments are added to the coating to replace the white pigment commonly used, clay. The coated paper produced is proportional to the amount of organic pigment added in terms of whiteness, white gloss, printing gloss, opacity, smoothness, etc., which means that the organic pigment can improve the properties of the coated paper. .
When different types of organic pigments are added to the coating to replace titanium dioxide with high price, high whiteness and high refractive index, the coated paper produced is added with organic pigments in terms of white paper gloss, printing gloss and smoothness. The amount is proportional, but in terms of whiteness and opacity, it is inversely proportional to the amount of organic pigment added. This means that in some special applications, although organic pigments cannot completely replace titanium dioxide, the properties of coated paper can be maintained. A certain level does not differ too much.
When different types of organic pigments are added to the coating to replace the inexpensive and commonly used white pigment-calcium carbonate, the coated paper produced is whiteness, white gloss, printing gloss, opacity, smoothness, etc. The aspects are all proportional to the amount of organic pigment added, which means that the use of organic pigments is an inevitable trend to further enhance the properties of coated paper.
In the relationship between the coating amount and the quality of the coated paper, when the organic pigment is added to the coating, the properties of the coated paper are better than those without the organic pigment, and when the coated paper is close in nature, organic is added. The coated paper of the pigment can reduce the amount of coating. When the organic pigment is dense, the coating amount can be reduced by about 30%, and the organic pigment of the subsequent type can be reduced by about 40%. If the organic pigment is hollow, the coating amount can be reduced by about 45%. This means that in the trend of paper lightweighting, organic pigments can bring considerable help.
The quality of printed matter has a great relationship with the printed materials, and the quality of printed materials is closely related to the original paper, paint and surface treatment. Therefore, choosing a good paper as the printed material is a cost saving and improvement. The only way to print quality.