The printing viscosity index (PVI) is referred to as the printing viscoity index (PVI) value, which is the ratio of the viscosity of the same raw paste at two different shear rates (usually a shear ratio of 10:1) to measure the rheology of the paste. The viscosity can be measured with a NDJ.1 rotary viscometer. The rotary viscometer is composed of two cylinders inside and outside. The sample is placed between the two cylinders. The outer cylinder remains stationary. The inner cylinder (rotor) rotates at a certain angular velocity. The moment of rotation is calculated from the torsion angle of the suspension spring. Out fluid viscosity. The inner cylinder is divided into five numbers, and its rotation speed is 60r/min and 6r/min or 100r/min and 10r/min, that is, the PVI value is 60 or 7700, respectively. . . The structural viscosity of the paste can be expressed by the decrease in viscosity after the increase of the shear rate. The PVI value of the general printing raw paste is 0.1-1.0; when the Pw value is equal to 1, the raw material is a Newtonian fluid. The PVI value is related to the structural viscosity index. The smaller the PVI value is, the larger the structural viscosity index of the original paste is.
With a large PVI paste, the viscosity changes little when the shear rate changes. Therefore, the printing paste containing such original paste has little change in viscosity when the printing speed changes, and can maintain a relatively uniform printing effect. On the contrary, the original paste with smaller PVI value changes significantly with printing speed, and the printing effect is not easy to control. At this time, the solid content of the paste must be appropriately increased.
Printing and dyeing workers in Europe and the United States study the rheology of the original paste by printing the ink; while countries such as Japan and India, by printing the value of the printing index to solve some of the printing process problems.
1 Mixing of various raw pastes Using a single paste makes it difficult to meet various requirements in printing production because of the single nature of macromolecules. For this reason, two or more kinds of mixed pastes are often used in printing to compensate for each other's deficiencies.
When pastes are used together, the most prominent problem is the change of viscosity and the resulting rheological changes. Usually with low solids, high viscosity paste, and high solids, low viscosity paste mixed with, for example, the emulsion paste and low degree of polymerization of alginate blended. In order to adapt to different printing requirements, various processed pastes sold on the market are mixed slurries that have been premixed, for example, blending various degrees of etherification of Locust bean gum.
After the pastes are mixed together, the viscosity generally decreases, but the rheological properties have improved, and the paste's printing viscosity index also changes accordingly. Generally, the printing viscosity index of mixed paste can be calculated by the following formula:
In the formula, Wa and WB are the blending weights of the original paste A and the original paste B, respectively. The more you mix the original paste, the better the linear relationship of PVI values. However, the change in the viscosity coefficient of the original paste after blending cannot be reflected from the PVI value, but must be seen from the original paste diagram.
2 The original paste's diagram shows the original paste's PVI as the abscissa, and the apparent viscosity's logarithm (lgn) as the ordinate, which is the original paste's diagram. The original paste PVI value on the right side of the diagram is large. The apparent viscosity of the original paste is measured with a rotary viscometer. When the rotation speed is different by 10 times, the enthalpy change is smaller (the vertical line is shorter), indicating the speed of the printing machine. Changes have little effect on the original paste; the original paste on the left side of the diagram is the opposite. The characterization of commonly used pastes is shown in Figure 1.
3 The relation between the printing method and the PVI value The shear stress of the flat screen printing and the roller printing is different, so the rheological requirement for the printing paste is not the same. Colorants for flat screen printing are not necessarily suitable for roller printing. Tests on commonly used printing pastes have shown that different viscosity index PVI values ​​of the chart are suitable for different printing methods.
The PVI value of the color paste used for flat screen printing is smaller (ie, has a smaller structural viscosity index), and the viscosity is higher. The original paste suitable for flat screen printing should be a pseudoplastic fluid, ie flat screen printing is the extrusion of a scraper (or magnet bar) and the paste is pressed through the mesh at a lower speed (this shear stress is smaller). The color paste printed on the fabric is free to penetrate and diffuse. Therefore, in order to maintain the smoothness of the outline, the color paste must have both high viscosity and self-uniformity, and must have a certain rheology. When printing thin lines, a high thixotropy is required, which is different from printing a large surface pattern.
When the roller prints, the color paste stored in the copper roller pattern or slashed lines is scraped and squeegeeed and transferred to the fabric in the presence of pressure. Usually, after the color paste is transferred, it is squeezed by the following copper roller, and then it is quickly dried. Therefore, the fluidity of the color paste and the permeability of the semi-finished product are particularly important.
Rotary screen printing is to inject the colorant into the inner wall of the nickel round net, and then squeeze the color paste through a scraper (or magnet bar) and print it on the fabric from the mesh wall of the circular net. As the rotary screen printing is continuous printing, the color paste is printed by a squeegee to complete the printing process. Therefore, the original printing paste property of the printing is between the flat screen printing and the roller printing, that is, the solid content is high, and Lower viscosity.
4 Relationship between fiber material and PVI value When the hydrophobic fabric is printed, the paste Pw value and the field must reach the minimum saturation value of the color paste, in order to obtain a satisfactory printing effect in this area. However, when the hydrophilic fabric is printed, the paste concentration and enthalpy should not exceed the maximum saturated value of the color paste, otherwise it is difficult to achieve a satisfactory printing effect.
Of course, the above is not absolute. In actual use, the printing effect is related to factors such as screen size, screen opening rate, copper roller engraving method, permeability of printing paste, level dyeing, It depends on the pattern, the color paste, and the colorant on the fabric. Although several indicators can not be measured at present to predict the actual printing effect, according to the performance test data of various pastes, in combination with actual production conditions, adjustments can be made in the printing process to obtain satisfactory printing. System effect.
With a large PVI paste, the viscosity changes little when the shear rate changes. Therefore, the printing paste containing such original paste has little change in viscosity when the printing speed changes, and can maintain a relatively uniform printing effect. On the contrary, the original paste with smaller PVI value changes significantly with printing speed, and the printing effect is not easy to control. At this time, the solid content of the paste must be appropriately increased.
Printing and dyeing workers in Europe and the United States study the rheology of the original paste by printing the ink; while countries such as Japan and India, by printing the value of the printing index to solve some of the printing process problems.
1 Mixing of various raw pastes Using a single paste makes it difficult to meet various requirements in printing production because of the single nature of macromolecules. For this reason, two or more kinds of mixed pastes are often used in printing to compensate for each other's deficiencies.
When pastes are used together, the most prominent problem is the change of viscosity and the resulting rheological changes. Usually with low solids, high viscosity paste, and high solids, low viscosity paste mixed with, for example, the emulsion paste and low degree of polymerization of alginate blended. In order to adapt to different printing requirements, various processed pastes sold on the market are mixed slurries that have been premixed, for example, blending various degrees of etherification of Locust bean gum.
After the pastes are mixed together, the viscosity generally decreases, but the rheological properties have improved, and the paste's printing viscosity index also changes accordingly. Generally, the printing viscosity index of mixed paste can be calculated by the following formula:
In the formula, Wa and WB are the blending weights of the original paste A and the original paste B, respectively. The more you mix the original paste, the better the linear relationship of PVI values. However, the change in the viscosity coefficient of the original paste after blending cannot be reflected from the PVI value, but must be seen from the original paste diagram.
2 The original paste's diagram shows the original paste's PVI as the abscissa, and the apparent viscosity's logarithm (lgn) as the ordinate, which is the original paste's diagram. The original paste PVI value on the right side of the diagram is large. The apparent viscosity of the original paste is measured with a rotary viscometer. When the rotation speed is different by 10 times, the enthalpy change is smaller (the vertical line is shorter), indicating the speed of the printing machine. Changes have little effect on the original paste; the original paste on the left side of the diagram is the opposite. The characterization of commonly used pastes is shown in Figure 1.
3 The relation between the printing method and the PVI value The shear stress of the flat screen printing and the roller printing is different, so the rheological requirement for the printing paste is not the same. Colorants for flat screen printing are not necessarily suitable for roller printing. Tests on commonly used printing pastes have shown that different viscosity index PVI values ​​of the chart are suitable for different printing methods.
The PVI value of the color paste used for flat screen printing is smaller (ie, has a smaller structural viscosity index), and the viscosity is higher. The original paste suitable for flat screen printing should be a pseudoplastic fluid, ie flat screen printing is the extrusion of a scraper (or magnet bar) and the paste is pressed through the mesh at a lower speed (this shear stress is smaller). The color paste printed on the fabric is free to penetrate and diffuse. Therefore, in order to maintain the smoothness of the outline, the color paste must have both high viscosity and self-uniformity, and must have a certain rheology. When printing thin lines, a high thixotropy is required, which is different from printing a large surface pattern.
When the roller prints, the color paste stored in the copper roller pattern or slashed lines is scraped and squeegeeed and transferred to the fabric in the presence of pressure. Usually, after the color paste is transferred, it is squeezed by the following copper roller, and then it is quickly dried. Therefore, the fluidity of the color paste and the permeability of the semi-finished product are particularly important.
Rotary screen printing is to inject the colorant into the inner wall of the nickel round net, and then squeeze the color paste through a scraper (or magnet bar) and print it on the fabric from the mesh wall of the circular net. As the rotary screen printing is continuous printing, the color paste is printed by a squeegee to complete the printing process. Therefore, the original printing paste property of the printing is between the flat screen printing and the roller printing, that is, the solid content is high, and Lower viscosity.
4 Relationship between fiber material and PVI value When the hydrophobic fabric is printed, the paste Pw value and the field must reach the minimum saturation value of the color paste, in order to obtain a satisfactory printing effect in this area. However, when the hydrophilic fabric is printed, the paste concentration and enthalpy should not exceed the maximum saturated value of the color paste, otherwise it is difficult to achieve a satisfactory printing effect.
Of course, the above is not absolute. In actual use, the printing effect is related to factors such as screen size, screen opening rate, copper roller engraving method, permeability of printing paste, level dyeing, It depends on the pattern, the color paste, and the colorant on the fabric. Although several indicators can not be measured at present to predict the actual printing effect, according to the performance test data of various pastes, in combination with actual production conditions, adjustments can be made in the printing process to obtain satisfactory printing. System effect.
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