Experimental Study of Effect of Coating and Calendering

The calender is the last step in the production of coated paperboards. The calendering improves the smoothness of the paper by increasing the average gloss. At the same time, the distribution of surface porosity and gloss of the coated paper is also improved. However, the high distribution of these surface properties is often not used as an indication of the improvement of the paper surface properties.

The purpose of the completed intermediate trial study was to explore the surface properties of the paper and the appearance of spotting on the paper surface caused by coating and post-calendering in offset printing paper. A single experiment was conducted with a coating roll and a coating squeegee, and the wet-on-wet secondary coating was performed on a metered size press (MSP) coater. The coating of low-quantitative ink-containing wood pulp paper is only one-sided coating. Coated paper was calendered with a supercalender. In calendering experiments, the temperature of the steel roll and the number of nips used were studied. The calendered paper surface was analyzed, and a sheet-fed offset printing test was conducted to measure the printed piebald spots.

In order to identify the quality of the coated paper, the uncalendered paper was subjected to a burning test to determine the distribution of the coating on the surface of the original paper. Measurements show that the wet-to-wet coating is consistent with the distribution of the coating's equivalent curve and weight distribution curve. On the other hand, a single blade test shows that it has more "fill pit" effect. In this case, the material distribution of the coating is not uniform.

When calendering is performed, the coated surface is compressed and the porosity of the coating is reduced. Because the peak of the coating is compressed more than the valley, resulting in uneven distribution of the coating's porosity, the wet-on-wet coating non-calendering test shows that the coating porosity exhibits a more prominent non-uniformity.

When calendering is performed at a low temperature, the gloss and smoothness of the coated paper do not depend on the coating process. As the calendering humidity increases, the surface quality of the coated paper is significantly affected by the distribution of the coating amount. However, the increase in the number of nips in calendering, the difference in this coating process does not exist.

test

Coating

The coating test was conducted on a CTP intermediate test coater, which included knife coating and metered foot press coating. In blade coating, the coating is applied using a coating roll.

Double-sided wet versus wet coating was performed on a metered size press coater. The running of the base paper was improved and a rotary roller was used. The paint is fed on the first transfer roll of a metered size press coater and the paint is transferred from its surface to the paper in the usual way. As the base paper is wound on another transfer roller, this roller is facing the transfer stick that has already been painted. When this paint quickly achieves a short dwell (SDTA), it is transferred to paper. The distance between the first size press application and the second coating was only 3.5 meters.

Printing test

The calendered paper was made on a Roland Favourite RVT model sheet-fed offset printing press. The printing was done on a single side of the plate at a printing speed of 5000 sheets/hour. The grid lines are 20, 50, 70, and 100%. Paper for each printing unit is printed on these grid lines. All four kinds of printing test ink use dark blue ink. The spotted prints printed on 100% squared lines in printing units 2 and 4 were examined with an image analysis device.

2. Coating amount analysis

The distribution of the coating material was analyzed using uncalendered paper and determined by the indirect method of burning ash. In this measurement, the coated paper was first carbonized and blackened, and the coating layer turned into a gray material. The average blackness and its variation were analyzed with an imaging analyzer. The blackness of the measurement result is then converted into the material distribution per unit area (g/m2). The variation was analyzed by frequency and the material distribution was characterized by the material frequency in the wavelength range.

3. The distribution of porosity in the coating

When a coated paper is calendered, the porosity of its coating becomes evenly distributed, because the high convex portion of the paper surface is subjected to greater pressure than the concave portion, and Engstrom determines the porosity of the coating by the following method. Distribution. For ordinary white coatings, the absorption coefficient of light is smaller than the scattering coefficient of light, and the reflection factor Rx at a certain point x, for the paper subjected to burning test, the value depends on the scattering coefficient Sx of light, and the coating amount at that point. Wx. According to the Kulbeka-Munk relationship, the base paper under the coating has a constant reflection factor.

If the coating has the same porosity and pore size, in this hypothetical case, the coating will not be squeezed, Sx will be a constant, because Sx depends on the above two factors. Variations in reflectance can be used indirectly to determine variation in the amount of coating. In this way, the distribution of the gray and gray variability of burning paper can be converted into the distribution of materials and materials.

In calendering, the porosity of the coating and the scattering coefficient of light decrease simultaneously. The porosity becomes non-uniform, which means that the scattering coefficient of light becomes not constant. For such a coating, the variation of the reflection factor cannot be used to reflect the variation of the coating amount, but it can be used to reflect the variation of the light scattering ability, that is, the product of Sx·Wx. Under the assumption that Sx is a constant, the formula can be used to determine the amount of coating. This distribution of performance material that is more valuable than the true material distribution can be measured. We can make the assumption that the distribution of the coating material is not affected by calendering. Comparing the ratio of the distribution V*(s)/V(s) of the calendered coating material indicates that the greater the ratio, the greater the heterogeneity of the porosity distribution.

4. Gloss distribution

Gloss (both gloss and uniformity) is very important for coated paper. The uniformity of the gloss strongly affects the appearance quality of all coated papers. However, the uniformity of gloss is now widely measured.

The distribution of the glossiness or the micro-glossiness of the coated paper here is determined using imaging calculations. The reported value of the micro gloss is expressed by an index, which relates to the interval of the wavelength, and the larger the value, the more uneven the gloss.

conclusion and discussion

The surface quality of coated paper can be finished by calendering. As the temperature of the steel roll increases and the number of nips increases, the surface smoothness and gloss can be improved.

Calendering temperatures of 20 degrees Celsius and 90 degrees Celsius have little effect on the microscopic properties (gloss and smoothness) of the wet-on-wet coated paper and blade coated paper. When the calendering temperature reached 140 degrees Celsius, the gloss and smoothness of blade coated paper was significantly higher than that of wet-on-wet coated paper at 2 and 3 nips. However, when the nip is as high as 5 and 9, the gloss and smoothness of the two coating methods are almost indistinguishable.

Figures 1 and 2 also clearly show the effect of calender conditions (calender nip and temperature) on the surface quality. The thickness of paper is another parameter that is affected in calendering. The relationship between the glossiness and the thickness of paper is described from FIG. 3 . All calendered papers have a very close relationship.

The distribution of the coating on the paper surface depends on different coating processes. The distribution of the coating amount can be verified by burning an uncalendered paper.

Figure 4 shows the variation of burned and noncalendered burned samples at different calendering temperatures in the 2-4mm wavelength range. From the results of the non-calendered paper, it is not surprising that the wet-to-wet coating has a more uniform coating distribution than the doctor blade. This is not surprising because wet-on-wet coating provides limited transfer on the first coat. The paint on the paper is a prominent part of the paper. In the case of a surface coating, the surface pit portions of the base paper have been filled with the first coating, and thus the distribution of the coating amount is better than that of a single-layered doctor blade.

In all cases, the brilliance increases the variation of coated paper. The increase in calendering temperature caused an increase in the variation, and the effect of temperature was more pronounced in the fifth nip and the ninth nip. Therefore, the calendering has a great influence on the properties of the coated paper. The increase in the variation is inseparable from the fact that the light scattering intensity of the coated surface will be more and more inhomogeneous; the pressure acting on the entire surface of the coating layer may not be uniform. This phenomenon can be seen both wet-to-wet and blade-coated paper.

However, the increased ratio between the variation of the calendered paper burning test and the variation of the non-calendered paper burning test is greater for wet vs. wet coated paper. Even at the lowest calendering conditions (2 nips), the difference between the two coated papers is large; the wet-to-wet coated paper is 1.31 and the doctor-coated paper is 1.06 (see Figure 5). .

This result further confirms the hypothesis that the more evenly distributed the coating amount, the more uneven the distribution of porosity obtained after calendering. The peak of the coating layer is subjected to more pressure than the valley. For both coating methods, the variation and the ratio measured in the coated paper burning test are always different, regardless of the calendering conditions. The variation of blade coated paper is always larger, and the ratio of wet to wet coated paper is always larger. Although in the two coating methods, we cannot observe macroscopic differences in the system properties such as smoothness and gloss. For example, when the calendering temperature is 140 degrees Celsius, the variation and the ratio of the wet-to-wet coated paper burning test increase with the number of nips, increasing uniformly from the five nip to the nine nip, and the gloss and smoothness are in this domain. Stays the same.

The microscopic gloss depends on the distribution of the coating amount, and Figure 6 is a test report with a wavelength of 1-2 mm. The wet-to-wet coated paper has a high microscopic gloss, that is, the gloss of the coated paper is even more uneven, and the gloss difference measured with a very short wavelength is even more so that it can be seen with the eyes. To. Very small damage to the fiber level is visible, and for the macro level, he only reduces the uniformity of the gloss without damaging the surface properties.

The printing effect was slightly disappointing, with very small differences (such as poor printing performance) only seen on the spotted between the two coated papers. The effect of porosity distribution on spotting is less obvious. This may mean that other parameters must be considered before directly considering the distribution of porosity. When printing, a lot of work needs to be done to understand what happened.

In spite of this, there seems to be a phenomenon where only a greater amount of uninked smut is applied to the wet coated paper cloth doctor blade. On the other hand, scraper coated paper has more ink spots. If this is the case, it still requires a lot of experimentation to confirm it.

in conclusion

The detailed analysis of this article shows that the interactions that occur during coating and calendering are complex.

Source: China Printed Material Network