Coated paper is one of the fastest growing varieties in the paper industry in China in the past 10 years. Coated paper is used as a printing substrate, and it is a high-grade product in the category of cultural printing paper. It is a paper made by coating and finishing the printing base paper. In recent years, low-quantity coated papers that have emerged abroad have attracted a lot of interest from commercial printing, publishing and printing users because of their low basis weight (total amount not exceeding 60 g/m2), low cost, and good printing performance. , magazines, newspaper ads inserts, supermarket merchandise catalogs and other uses for printing.
In the past 10 years, the annual increase rate of coated paper consumption for coated paper and mechanical pulp in China was 18.8% and 25%, respectively. It is not difficult to predict that with the continuous adjustment and development of China’s economy and the improvement of people’s living standards, the use of coated paper in China will rise rapidly. Therefore, in the printing of coated papers, it is necessary to have a deep understanding of the printability of coated papers, especially the improvement of the printability of coated papers.
First, the basic situation of coated paper
The coated paper is coated on the surface of the base paper with a coating solution mainly composed of an auxiliary solvent such as a pigment, an adhesive, a tackifier, a fluorescent agent, a lubricant, etc., and dried and then subjected to a smoothing treatment. Excellent performance.
Pigment coated paper is the largest and most important category of coated paper. The pigment-coated paper is mainly composed of white pigments such as kaolin, calcium carbonate, barium sulfate, and titanium dioxide, plus a variety of adhesives and coatings formulated with various chemical adjuvants as required, and the coating is uniformly coated in a certain amount. Made of cardboard. Offset printing coated paper, commonly known as copperplate paper. The name of the copperplate paper has been long-standing. The material used to make the letterpress screen version was originally heavy metal copper. This type of printing plate is particularly suitable for printing such coated paper, so it is called copperplate paper. This kind of paper is a high-grade printing paper made by coating and finishing the base paper. Offset printing coated paper is used for printing various high-grade prints, such as calendars, pictorials, pictures, illustrations, high-end trademarks, product specifications, and so on.
Second, the printability of coated paper
The printability of paper includes workability and quality adaptability. Operational suitability mainly affects the printing efficiency, and quality suitability mainly affects the printing effect. As far as the operational suitability of paper is concerned, it is closely related to the tensile strength, toughness, flatness, and deformability of the paper. Closely related to quality suitability are paper whiteness, opacity, smoothness, surface strength, dimensional stability, and ink absorbency. It is customary to refer to the print quality applicability of paper as a narrow sense of printability.
Compressibility
When the paper is under certain pressure, it will be slightly compressed. After removing the excess force, the degree of paper returning to the original state is called compressibility. Compressibility is related to the size of the pressure. Paper compressibility is particularly important for printing. Because when printing, the paper is subjected to a certain pressure so that the ink is transferred from the plate to the paper. The paper with high compressibility can make the graphic image of the printing plate have sufficient and good elastic contact with the paper surface when the printing pressurization is appropriate, so that the graphic on the printing plate can be well transferred to the paper. Coated paper is generally less compressible due to the presence of the coating. The thicker the coating, the lower the compressibility. Under normal circumstances, coated paper with a large amount of coating has poor printability. Low coated paper due to low coating (applied around 6g/m2 per side), and the base paper contains mechanical pulp with good compressibility (usually the ratio is ≥25%), plus the use of soft calendering finishing, compressibility Coated paper is good, so its printing performance is good. In addition, the paper with good compressibility can also reduce the requirements for the smoothness of the paper and the flatness of the text surface.
2. Surface strength
The surface strength of paper is an important printability index. It refers to the degree of binding between paper fibrous tissue, fillers, and sizing, which determines the surface's ability to resist abrasion, powder loss, and lint resistance when printed.
For coated paper, the coating particles must adhere well to each other, the coating and the paper basically require adhesive bonding, and the paper-based fibers should have sufficient bonding strength. Otherwise, when the paper is brought into contact with the ink film on the plate at a relatively fast speed under the corresponding printing pressure, the ink film will peel off the paint particles and fine fibers, causing so-called hair loss and powder dropping, resulting in a paste plate . In offset printing coated paper, there is also the phenomenon of powder falling and hair loss due to the high viscosity of the offset ink used.
3. Whiteness
Coated paper, like other printed paper, has different requirements for whiteness depending on the purpose of the print. White paper can improve the contrast of text ink, and truly and objectively reflect the full color of the printed image, making the copy colorful and achieve the effect of illustration. The higher the paper whiteness, the more significant this effect is. However, the higher the whiteness of the paper, the stronger the reflected light energy and the greater the stimulation of the optic nerve. Inappropriate high brightness causes visual fatigue and is particularly disadvantageous for adolescents in their growing years. From the point of view of human physiological characteristics and usage, the whiteness value of printed paper for different purposes should not be the same, and the printed paper should have a suitable whiteness, rather than the higher the whiteness, the better.
4. Opacity
Opacity is also an important performance indicator for printing paper. The level of opacity directly affects the print penetration of printed products. Most printing papers need to have enough opacity, otherwise it will lead to after-printing. However, there are also individual patterns that need to be printed to show the effect of printing. This is a different story.
For the low-quantitative coated paper with double-sided printing, it is more practical to use printing opacity for evaluation because of its low basis weight and low coating amount. The opacity of a low basis weight coated paper is related to the opacity of the base paper, the coating formulation, and the hiding of the coating.
5. Gloss
The concept of gloss includes paper gloss and print gloss. The print glossiness of a paper refers to the fact that, under the specified conditions, the solid gloss ink is used to perform solid printing on a paper sample, and after drying, the glossiness of the blot area is measured and expressed as a percentage.
Gloss is a unique index of coated paper, which has a great influence on the aesthetic and three-dimensional impression of the printing effect. There is a very important relationship between the amount of coating and the integrity of the base paper coverage and gloss. The minimum requirements for paper gloss and print gloss for offset printing coated papers were 60% and 88%, respectively.
6. Absorptivity
Absorption of paper means the ability of the paper to absorb the binder and its solvent in the ink, which is usually expressed as a percentage. Absorption of paper is also called ink absorption. Its strength is very obvious on the printing hue and the gloss of the ink layer. In paper with strong ink absorption, the hue of printed ink tends to fade, and the ink layer also lacks gloss. In addition, since the binder in the ink is mostly absorbed by the fibrous tissue of the paper, the pigment particles are not sufficiently protected, the ink film on the print is not firm, and the phenomenon of “powdering†of the print is likely to occur. Conversely, if the ink absorption property of the paper is too weak, the ink layer of the print product will not be adhered securely, and the print will not be easily dried, which may cause the back of the print product to become dirty.
7. Smoothness
The smoothness of the paper is a technical indicator for evaluating the degree of surface roughness of the paper, and is a physical quantity indicating the level, smoothness, and uniformity of the surface of the paper. That is, under certain vacuum conditions, the volume of air, measured in seconds, is measured as the time it takes for the air to flow between the surface of the paper test specimen under pressure and the smooth glass surface. The higher the number of seconds, the better the smoothness. The better the paper smoothness, the more uniform and complete the contact between the paper and the printing plate at the time of printing. The more the ink layer on the layout (topography) or blanket is transferred, the better the color rendering effect of the print. On the other hand, paper with poor smoothness has uneven ink transfer and is insufficient, and the paper has a strong permeation to the ink, and the color rendering effect is also poor, and it is often easy to make the ink of the printed product fade and fade. The smoothness of the paper directly determines the print quality of the printed matter.
Third, improve the suitability of coated paper printing
As a high-grade printing paper, coated paper not only needs to be able to adapt to the printing requirements of various advanced printing machines, but also has bright colors, clear and vivid images after printing, and requires suitable prices, accurate product positioning, and high cost performance. The improvement of the printability of coated paper is not simply achievable from a certain point of improvement. It is necessary to work hard from the three aspects of the base paper, coating formulation, and on-site production management, from the raw materials (including the physical indicators of the base paper), the control of the coating formulation (including the improvement of the coating properties), to the subsequent sequence of finishing and a series of processes. There is a need for strict requirements, and only in this way is it possible to obtain coated paper with good quality and good printing properties.
Base paper
The non-coated paper used to produce coated paper is called base paper. The base paper has a great influence on the production and quality of coated paper. Therefore, there are special requirements for the composition, strength, surface properties and absorbency of the base paper.
1 raw paper composition
The coated paper base paper generally contains about 70% hardwood pulp, and the remainder is softwood pulp. It is generally required that the original paper has high tensile strength and surface strength. In order to increase the strength of the base paper, in addition to improving the composition of the slurry, it is sometimes possible to appropriately increase the degree of beating and surface sizing.
2 surface properties
The surface properties of coated paper base paper are relatively strict. The base paper must not be too rough or too smooth. Too rough raw paper can not meet the printing requirements even if the surface smoothness after coating is too smooth, the effective bonding area between the coating and the base paper is small, and the bond strength between the coating layer and the paper surface after coating is insufficient.
3 Absorption and uniformity
The attractiveness of the base paper is moderate, the absorption is too large, and the powder dropping phenomenon easily occurs after coating: Absorption is poor, and the base paper is not bonded to the coating layer after coating. The uniformity of the base paper is better to facilitate the uniform coating.
2. Control of coating properties
1 Coating
Coated paper must first have two major characteristics: that is, coating adaptability and printability. Coating adaptability means that a uniform coating amount can be achieved at the time of coating. The printability of printing is the effect of smooth printing quality.
The coating has a wide range of concentrations and can be between 30 and 70%. In general, the higher the concentration, the greater the viscosity and the poorer the flowability, which will affect the uniformity of the coating.
2 coating amount
When the coating amount becomes larger within a certain range, the smoothness of the paper is improved, the dot appearance effect is good, the ink absorption is uniform, and the ink receiving performance is good, and the ink is easily transferred to the coating surface uniformly, thereby achieving improved coating. The purpose of printing paper adaptability.
3. Development of new printing paper special printability improver
Combining the production process of coated paper, research and development of printability improver for coated paper is one of the important ways to improve the printability of coated paper in the future. For example, the “Printability Improvement Aid for Coated Paper†once developed by the Hangzhou Chemical Research Institute showed good results in use, resulting in a significant improvement in the printability of coated paper and a significant increase in the quality of coated paper products. In addition, the product has a reasonable manufacturing process and low cost, meets the requirements of environmental protection, and has good economic benefits.
In the past 10 years, the annual increase rate of coated paper consumption for coated paper and mechanical pulp in China was 18.8% and 25%, respectively. It is not difficult to predict that with the continuous adjustment and development of China’s economy and the improvement of people’s living standards, the use of coated paper in China will rise rapidly. Therefore, in the printing of coated papers, it is necessary to have a deep understanding of the printability of coated papers, especially the improvement of the printability of coated papers.
First, the basic situation of coated paper
The coated paper is coated on the surface of the base paper with a coating solution mainly composed of an auxiliary solvent such as a pigment, an adhesive, a tackifier, a fluorescent agent, a lubricant, etc., and dried and then subjected to a smoothing treatment. Excellent performance.
Pigment coated paper is the largest and most important category of coated paper. The pigment-coated paper is mainly composed of white pigments such as kaolin, calcium carbonate, barium sulfate, and titanium dioxide, plus a variety of adhesives and coatings formulated with various chemical adjuvants as required, and the coating is uniformly coated in a certain amount. Made of cardboard. Offset printing coated paper, commonly known as copperplate paper. The name of the copperplate paper has been long-standing. The material used to make the letterpress screen version was originally heavy metal copper. This type of printing plate is particularly suitable for printing such coated paper, so it is called copperplate paper. This kind of paper is a high-grade printing paper made by coating and finishing the base paper. Offset printing coated paper is used for printing various high-grade prints, such as calendars, pictorials, pictures, illustrations, high-end trademarks, product specifications, and so on.
Second, the printability of coated paper
The printability of paper includes workability and quality adaptability. Operational suitability mainly affects the printing efficiency, and quality suitability mainly affects the printing effect. As far as the operational suitability of paper is concerned, it is closely related to the tensile strength, toughness, flatness, and deformability of the paper. Closely related to quality suitability are paper whiteness, opacity, smoothness, surface strength, dimensional stability, and ink absorbency. It is customary to refer to the print quality applicability of paper as a narrow sense of printability.
Compressibility
When the paper is under certain pressure, it will be slightly compressed. After removing the excess force, the degree of paper returning to the original state is called compressibility. Compressibility is related to the size of the pressure. Paper compressibility is particularly important for printing. Because when printing, the paper is subjected to a certain pressure so that the ink is transferred from the plate to the paper. The paper with high compressibility can make the graphic image of the printing plate have sufficient and good elastic contact with the paper surface when the printing pressurization is appropriate, so that the graphic on the printing plate can be well transferred to the paper. Coated paper is generally less compressible due to the presence of the coating. The thicker the coating, the lower the compressibility. Under normal circumstances, coated paper with a large amount of coating has poor printability. Low coated paper due to low coating (applied around 6g/m2 per side), and the base paper contains mechanical pulp with good compressibility (usually the ratio is ≥25%), plus the use of soft calendering finishing, compressibility Coated paper is good, so its printing performance is good. In addition, the paper with good compressibility can also reduce the requirements for the smoothness of the paper and the flatness of the text surface.
2. Surface strength
The surface strength of paper is an important printability index. It refers to the degree of binding between paper fibrous tissue, fillers, and sizing, which determines the surface's ability to resist abrasion, powder loss, and lint resistance when printed.
For coated paper, the coating particles must adhere well to each other, the coating and the paper basically require adhesive bonding, and the paper-based fibers should have sufficient bonding strength. Otherwise, when the paper is brought into contact with the ink film on the plate at a relatively fast speed under the corresponding printing pressure, the ink film will peel off the paint particles and fine fibers, causing so-called hair loss and powder dropping, resulting in a paste plate . In offset printing coated paper, there is also the phenomenon of powder falling and hair loss due to the high viscosity of the offset ink used.
3. Whiteness
Coated paper, like other printed paper, has different requirements for whiteness depending on the purpose of the print. White paper can improve the contrast of text ink, and truly and objectively reflect the full color of the printed image, making the copy colorful and achieve the effect of illustration. The higher the paper whiteness, the more significant this effect is. However, the higher the whiteness of the paper, the stronger the reflected light energy and the greater the stimulation of the optic nerve. Inappropriate high brightness causes visual fatigue and is particularly disadvantageous for adolescents in their growing years. From the point of view of human physiological characteristics and usage, the whiteness value of printed paper for different purposes should not be the same, and the printed paper should have a suitable whiteness, rather than the higher the whiteness, the better.
4. Opacity
Opacity is also an important performance indicator for printing paper. The level of opacity directly affects the print penetration of printed products. Most printing papers need to have enough opacity, otherwise it will lead to after-printing. However, there are also individual patterns that need to be printed to show the effect of printing. This is a different story.
For the low-quantitative coated paper with double-sided printing, it is more practical to use printing opacity for evaluation because of its low basis weight and low coating amount. The opacity of a low basis weight coated paper is related to the opacity of the base paper, the coating formulation, and the hiding of the coating.
5. Gloss
The concept of gloss includes paper gloss and print gloss. The print glossiness of a paper refers to the fact that, under the specified conditions, the solid gloss ink is used to perform solid printing on a paper sample, and after drying, the glossiness of the blot area is measured and expressed as a percentage.
Gloss is a unique index of coated paper, which has a great influence on the aesthetic and three-dimensional impression of the printing effect. There is a very important relationship between the amount of coating and the integrity of the base paper coverage and gloss. The minimum requirements for paper gloss and print gloss for offset printing coated papers were 60% and 88%, respectively.
6. Absorptivity
Absorption of paper means the ability of the paper to absorb the binder and its solvent in the ink, which is usually expressed as a percentage. Absorption of paper is also called ink absorption. Its strength is very obvious on the printing hue and the gloss of the ink layer. In paper with strong ink absorption, the hue of printed ink tends to fade, and the ink layer also lacks gloss. In addition, since the binder in the ink is mostly absorbed by the fibrous tissue of the paper, the pigment particles are not sufficiently protected, the ink film on the print is not firm, and the phenomenon of “powdering†of the print is likely to occur. Conversely, if the ink absorption property of the paper is too weak, the ink layer of the print product will not be adhered securely, and the print will not be easily dried, which may cause the back of the print product to become dirty.
7. Smoothness
The smoothness of the paper is a technical indicator for evaluating the degree of surface roughness of the paper, and is a physical quantity indicating the level, smoothness, and uniformity of the surface of the paper. That is, under certain vacuum conditions, the volume of air, measured in seconds, is measured as the time it takes for the air to flow between the surface of the paper test specimen under pressure and the smooth glass surface. The higher the number of seconds, the better the smoothness. The better the paper smoothness, the more uniform and complete the contact between the paper and the printing plate at the time of printing. The more the ink layer on the layout (topography) or blanket is transferred, the better the color rendering effect of the print. On the other hand, paper with poor smoothness has uneven ink transfer and is insufficient, and the paper has a strong permeation to the ink, and the color rendering effect is also poor, and it is often easy to make the ink of the printed product fade and fade. The smoothness of the paper directly determines the print quality of the printed matter.
Third, improve the suitability of coated paper printing
As a high-grade printing paper, coated paper not only needs to be able to adapt to the printing requirements of various advanced printing machines, but also has bright colors, clear and vivid images after printing, and requires suitable prices, accurate product positioning, and high cost performance. The improvement of the printability of coated paper is not simply achievable from a certain point of improvement. It is necessary to work hard from the three aspects of the base paper, coating formulation, and on-site production management, from the raw materials (including the physical indicators of the base paper), the control of the coating formulation (including the improvement of the coating properties), to the subsequent sequence of finishing and a series of processes. There is a need for strict requirements, and only in this way is it possible to obtain coated paper with good quality and good printing properties.
Base paper
The non-coated paper used to produce coated paper is called base paper. The base paper has a great influence on the production and quality of coated paper. Therefore, there are special requirements for the composition, strength, surface properties and absorbency of the base paper.
1 raw paper composition
The coated paper base paper generally contains about 70% hardwood pulp, and the remainder is softwood pulp. It is generally required that the original paper has high tensile strength and surface strength. In order to increase the strength of the base paper, in addition to improving the composition of the slurry, it is sometimes possible to appropriately increase the degree of beating and surface sizing.
2 surface properties
The surface properties of coated paper base paper are relatively strict. The base paper must not be too rough or too smooth. Too rough raw paper can not meet the printing requirements even if the surface smoothness after coating is too smooth, the effective bonding area between the coating and the base paper is small, and the bond strength between the coating layer and the paper surface after coating is insufficient.
3 Absorption and uniformity
The attractiveness of the base paper is moderate, the absorption is too large, and the powder dropping phenomenon easily occurs after coating: Absorption is poor, and the base paper is not bonded to the coating layer after coating. The uniformity of the base paper is better to facilitate the uniform coating.
2. Control of coating properties
1 Coating
Coated paper must first have two major characteristics: that is, coating adaptability and printability. Coating adaptability means that a uniform coating amount can be achieved at the time of coating. The printability of printing is the effect of smooth printing quality.
The coating has a wide range of concentrations and can be between 30 and 70%. In general, the higher the concentration, the greater the viscosity and the poorer the flowability, which will affect the uniformity of the coating.
2 coating amount
When the coating amount becomes larger within a certain range, the smoothness of the paper is improved, the dot appearance effect is good, the ink absorption is uniform, and the ink receiving performance is good, and the ink is easily transferred to the coating surface uniformly, thereby achieving improved coating. The purpose of printing paper adaptability.
3. Development of new printing paper special printability improver
Combining the production process of coated paper, research and development of printability improver for coated paper is one of the important ways to improve the printability of coated paper in the future. For example, the “Printability Improvement Aid for Coated Paper†once developed by the Hangzhou Chemical Research Institute showed good results in use, resulting in a significant improvement in the printability of coated paper and a significant increase in the quality of coated paper products. In addition, the product has a reasonable manufacturing process and low cost, meets the requirements of environmental protection, and has good economic benefits.
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