Analysis of paper whiteness
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
Paper whiteness is the ability of a paper to be completely reflected after exposure to light, expressed as a percentage. From a technical point of view, the whiteness of paper and board includes a variety of complex factors that are a function of the total spectral reflectance of the paper, the illumination energy distribution, the viewing conditions, and the observer characteristics. In GB/T17749–1999 “Representation of Whiteness”, whiteness is defined as the degree to which the object is whitened. The greater the whiteness value, the greater the whiteness. At present, there are many methods for expressing whiteness in various industries, and there are about 100 kinds of methods. Regardless of the method used to express whiteness, the evaluation results of whiteness should be consistent with human visual perception.
1. Whiteness standard
At present, the relevant state departments have commissioned the China Pulp and Paper Research Institute to draft a whiteness standard for paper products, which will set the maximum limit on the whiteness (brightness) of paper and paperboard. qualified.
Jiang Rongzhen, executive secretary of the Shanghai Papermaking Society, said, “The whiteness of an ordinary printing paper is about 100%, but from the comfort of the eyes, the whiteness is about 85%.” The concept of citizen consumption exists. The deviation is that the whiter the paper is, the better, but in fact, the manufacturer has to add fluorescent bleach to increase the whiteness of the paper, which wastes resources and causes pollution. At the same time, the white paper with too much whiteness will also cause damage to human vision. The original recycled paper with low whiteness is difficult to promote because of the wrong consumption concept of the public. In order to solve the above problems, in order to help the public to establish a correct concept of consumption, it is imperative to establish a white standard.
2. Method of expressing whiteness
There are many ways to express whiteness, mainly ISO brightness, D65 brightness and CIE whiteness.
ISO brightness is defined in ISO 2470:1999 "Paper, Paperboard and Pulp - Determination of Diffuse Reflectance Factor (ISO Brightness)": with a filter or with corresponding function to make the spectral characteristic effective wavelength 457 nm, half The internal reflection factor measured by an instrument having a wave width of 44 nm and adjusting the UV content of the light source irradiated to the sample in accordance with the CIE illuminator C.
The brightness defined in China's national standard GB/T 7974-2002 "Determination of brightness, whiteness of paper, paperboard and pulp (diffuse vertical method)" is: Simulation D65 of the reflectance photometer specified in GB/T7973 Under the condition of light source, the sample has a reflection coefficient of blue light at the dominant wavelength (457±0.5) nm. This definition is what we call whiteness in everyday use and is called D65 brightness in the ISO 2470:1999 standard.
Another way to express whiteness is CIE whiteness, which is widely used internationally. ISO/TC6 has established corresponding test method standards. Some domestic enterprises also use this whiteness representation method. CIE whiteness is divided into two lighting conditions: C light source and D65 light source. The corresponding ISO standards are ISO 11475 "paper and cardboard - CIE whiteness measurement, D65/10o (outdoor light)" and ISO 11476 "paper and cardboard - Determination of CIE whiteness, C/2o (indoor light).
3. Measuring paper whiteness
The national standard GB8940.1-88 stipulates that the whiteness of the paper is a measure of the diffuse reflectance of the 457 nm blue light, and the whiteness reflects the brightness characteristic, based on the blue light of the D65 illuminant and the 45/0 illumination observation condition. Therefore, by detecting the brightness of the paper, the whiteness of the paper can be obtained, and the brightness and whiteness are identical.
The whiteness reference value is based on the reflectance of the 457 nm blue light irradiated onto the magnesium oxide plate, and the whiteness of the paper is expressed as a percentage of the reflectance of the magnesium oxide plate.
Studies have shown that the area CCD has better response sensitivity to blue light at 457 nm. After A/D conversion, the color image acquired by CCD is stored point by point in pixels in the computer. The amount of information stored at each point contains R, G, B and the received radiation at that point. A component value that can be proportional. R, G, and B respectively represent the red, green, and blue components of the image; the luminance calculation of the image is obtained by image color conversion, Equation (1).
Where Y is the luminance information of the image, and U and V are the image color difference information. After comparing the brightness value of the paper with the brightness value of the whiteness reference, the whiteness of the paper sample is obtained.
Points to be aware of when testing paper whiteness:
① When cutting the sample, it should be evenly cut on the paper banner. The total thickness of the test piece should reach the extent that the reflection factor no longer increases with the increase of the number of layers.
② Each test piece should be stacked in a stack of front and back, vertical and horizontal.
③ The instrument has function conversion, the function handwheel conversion must be in place, otherwise it will affect the accuracy of the test results.
④ For the determination of paper containing fluorescent whitening agent, attention should be paid to the adjustment of the ultraviolet radiation amount of the instrument source. Too high or too low will affect the test results.
⑤ The 5SBD-1 digital whiteness meter adopts the geometric condition of 45/0. The test results of this condition are significantly affected by the directionality of the surface of the sample, so the orientation test is required. In the test, the paper shall be tested at a 45-degree angle in the longitudinal and lateral directions of the paper (ie, the diagonal of the test piece is parallel to the front-rear direction of the instrument) to eliminate different test deviations in the vertical and horizontal directions.
⑥ When the sample is continuously tested with the SBD-1 digital whiteness meter, after testing several samples, the instrument should be recalibrated to eliminate the influence of instrument drift.
4. Improve paper whiteness
① concentration of the slurry. Assuming that the slurry used is sulfuric acid, the pH value of the sulfuric acid is adjusted in the range of 6.0-6.5, the whiteness of the pulp and the L, a, and b values are better improved, and the whiteness of the pulp gradually becomes stable.
② Seeking a new type of surface sizing agent is also an effective way to improve the whiteness of paper.
③ whiteness of the filler. The whiteness of the filler varies greatly depending on its type and grade. If a filler with a higher whiteness than the slurry is used, the more the filling amount, the higher the whiteness of the paper.
④ whiteness of the paint. The whiteness of the paint printing paper is determined by the whiteness of the base paper, the whiteness of the paint, and the amount of coating. If the whiteness of the base paper is higher than the coating, some of the light that passes through the coating will be reflected back from the surface of the base paper and reflected again through the surface of the deep coating, and the whiteness will also increase.
5. The effect of paper whiteness on printing chromatic aberration
Whether through visual inspection or instrumental measurement, we can believe that the whiteness of the paper itself is always certain, and the whiteness difference between different papers is also certain. It can be seen from the existing data that the color difference of the printed paper with different whiteness is not constant, and the time is large and small. For a given standard, the greater the whiteness difference of the paper, the resulting color difference is not as large as people expected, and sometimes it is smaller, so it can be seen that between the printing chromatic aberration and the whiteness difference of the paper There is no positive relationship. Therefore, there is no positive relationship between the similarity of whiteness of various papers and the similarity of postpress color. In the case of printing out of paper, it is a visual method to select a paper with similar whiteness instead. Not advisable.
6. The relationship between paper whiteness and coloring effect
The higher the whiteness of the paper, the more the surface can accurately represent the color of the ink. This is because the white paper reflects the color light synthesized by the color reduction of the transparent ink layer. Therefore, the white paper with high whiteness can reflect almost all the color light, making the printed ink color bright and pleasing, and the visual effect is good. Paper with low whiteness, because it only absorbs part of the color light, can not faithfully express the contrast between the light and dark parts, and easily cause color cast. If some appear yellowish, some appear reddish, some appear greenish, some appear blue and so on. When the paper itself is color cast, the color printed on the paper is the effect of the combination of ink and paper. This will inevitably lead to some color cast. Based on this situation, it is necessary to white the paper when printing. The degree and color cast are analyzed against the original, and the purpose of correcting the color cast is achieved by taking appropriate measures. For example, when the color of the paper is yellowish, it is not suitable to use inks of dark yellow, peacock blue and red, etc., that is to say, on the one hand, according to the characteristics of paper color, the ink should be correctly selected to eliminate the color cast. On the other hand, the color shift can be corrected by the ink adjustment process.
7. Summary
Paper whiteness is the fundamental source of accurate color rendering. Printing paper, especially as one of the main factors of color discrimination, carries the whiteness of the white paper, which plays a role in the hue, brightness and saturation. Decisive role. Therefore, in printing, since the whiteness of the paper required for different printed matter is different, it is necessary to select a suitable paper for printing to ensure the best printing quality.