Paper whiteness is the ability of a paper to reflect completely after being illuminated, expressed as a percentage. From a technical point of view, the degree of whiteness of paper and board includes a variety of complex factors that are a function of the total spectral reflectance of the paper, the distribution of illumination energy, the viewing conditions, and the characteristics of the observer. In GB/T17749-1999 "Expression of Whiteness," whiteness is defined as the degree to which the object is colored white. The greater the value of whiteness, the greater the degree of whiteness. At present, there are many methods for expressing whiteness in various industries, and there are about 100 kinds. Regardless of the method used to express whiteness, the evaluation results of whiteness should be consistent with human visual perception.
1. Whiteness standards At present, the relevant national authorities have commissioned the China Pulp and Paper Research Institute to draft a whiteness standard concerning paper products. The standard will impose maximum limits on the whiteness (brightness) of paper and paperboard, exceeding the limit. The products are all unqualified.
Jiang Rongji, executive secretary of the Shanghai Papermaking Society, said, “An ordinary paper has a whiteness of about 100%, but from the comfort of the eyes, a whiteness of about 85% is enough.†And the concept of consumer spending exists. Deviation, as the paper is whiter, the better, but in fact the manufacturer has to add fluorescent bleach in order to increase the whiteness of the paper, which wastes resources and causes pollution. At the same time, paper with too high whiteness can also cause harm to human vision. The original paper with low whiteness is difficult to popularize because of the wrong consumption concept of the public. To solve the above problems, it is imperative to establish a standard for whiteness in order to help citizens establish a correct concept of consumption.
2. The representation of whiteness There are many ways to present whiteness, mainly ISO brightness, D65 brightness, and CIE whiteness.
The ISO brightness is defined in ISO 2470:1999 "Paper, paperboard and pulp - Determination of diffuse reflectance (ISO brightness)": Equipped with a filter or with corresponding functions to give a spectral characteristic with an effective wavelength of 457 nm, half width 44 nm, and the internal reflection factor determined by an instrument that adjusts the UV content of the light source irradiated to the sample in accordance with the CIE illuminant C.
The brightness defined in the national standard GB/T 7974-2002 "Determination of the brightness (whiteness) of paper, paperboard and pulp (diffusion vertical method)" is a simulated D65 light source of the reflective photometer specified in GB/T 7973 Under the condition, the sample has a blue light internal reflection factor of (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 indication of whiteness is the CIE whiteness, which is widely used internationally. ISO/TC6 has formulated corresponding test method standards. Some domestic companies adopt this whiteness representation method. CIE whiteness is divided into two lighting conditions: C light source and D65 light source. The corresponding ISO standards are ISO11475 "paper and paperboard - CIE whiteness measurement, D65/10° (outdoor light)" and ISO11476 "paper and paperboard - CIE. Whiteness measurement, C/2° (indoor light).
3. The national standard GB8940.1-88 for measuring the whiteness of paper stipulates: Based on the blue light of the D65 illuminator and the 45/0 illumination observation conditions, the paper whiteness is a measure of the blue light diffuse reflectance at 457 nm, and the whiteness reflects Brightness characteristics. Therefore, by detecting the brightness of the paper, the whiteness of the paper can be obtained, and the brightness and the whiteness are identical.
The reference value of the whiteness is based on the reflectivity of the 457 nm blue light irradiated onto the magnesia plate. The reflectance of the light on the paper surface is the percentage of the reflectance of the magnesia plate to indicate the paper whiteness.
The research shows that the area array CCD has better response sensitivity to 457nm blue light. After the color image acquired by the CCD is A/D converted, it is stored point by point in units of pixels in the computer. The amount of information stored at each point includes three kinds of R, G, and B at that point and the received radiation. Can be proportional to the component value. R, G, and B respectively represent the red, green, and blue components of the image; the luminance operation of the image is obtained by color conversion of the image, equation (1).
Where Y is the brightness information of the image and U and V are the color difference information of the image. After comparing the paper brightness value with the brightness value of the whiteness reference, the whiteness of the paper sample is obtained.
Problems to be aware of when testing paper whiteness:
1 When cutting the sample, it should be cut evenly on the paper banner. The total thickness of the test piece should be such that the reflection factor does not increase with the increase of the number of layers.
2 Each test piece shall be folded in a stack of positive and negative, horizontal and vertical directions.
3 If 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 agents, attention should be paid to the adjustment of the ultraviolet radiation quantity of the light source of the instrument. Excessively high or low levels may affect the test results.
The 5SBD-1 digital whiteness meter adopts a 45/0 geometric condition. The conditional test result is significantly affected by the directionality of the sample surface. Therefore, a directional test is required. In the test, the paper should be tested in the direction of the paper at a 45-degree angle (ie, the diagonal of the test piece is parallel to the front-rear direction of the instrument), and vertical and horizontal deviations of the test can be eliminated.
6 When using the SBD-1 digital whiteness meter for continuous sample testing, after testing several samples, the instrument should be recalibrated to eliminate the effect of instrument drift.
4. Improve paper whiteness 1 Slurry concentration. Assuming that the used slurry is sulfuric acid, adjusting the PH value of sulfuric acid in the range of 6.0 to 6.5, the whiteness of the pulp and the L, a, and b values ​​will be better, and the whiteness of the pulp will gradually stabilize.
2 Seeking a new type of surface sizing agent is also an effective way to improve the whiteness of the paper.
3 The whiteness of the filler. The whiteness of fillers varies greatly depending on their type and grade. If fillers with higher whiteness than pulp are used, the greater the amount of filler, the higher the whiteness of the paper.
4 The 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 coating amount. If the whiteness of the base paper is higher than that of the coating, some of the light that passes through the coating will be reflected back by the surface of the base paper and reflected again through the surface of the deep coating, and the whiteness may also be increased.
5. The effect of paper whiteness on printing color difference Whether it is through visual inspection or instrument measurement, we can all believe that the whiteness of the paper itself is always certain, and the difference in whiteness between different papers is also certain. From the available data, it can be seen that the post-print color difference of papers with different whiteness is not constant, and the time difference is large. For a given standard, the greater the difference in whiteness of the paper, the resulting color difference is not as large as one might expect, and sometimes it is smaller, and thus it can be seen that the difference between the print color and the whiteness of the paper is There is no positive relationship. Therefore, there is no positive relationship between the degree of similarity in whiteness of various papers and the similarity of the color after printing. In the case of printing out of paper, the practice of selecting a paper with similar whiteness by visual inspection is to replace the paper. Not advisable.
6. The relation between paper whiteness and color rendering effect The higher the whiteness of the paper, the more the surface of the paper can accurately express the characteristics of the ink color. This is because the white paper reflects the color light synthesized by subtracting the color of the clear ink layer back. Therefore, paper with a high degree of whiteness can reflect almost all the shades of light, making the inks look bright and pleasing and have good visual effects. The paper with low whiteness, because it absorbs only part of the color light, can neither express the contrast of the light and dark parts truthfully, but can easily cause color casts. Some appear yellowish, others appear reddish, some appear greenish, and others appear blueish. When the paper itself color cast, the color printed on the paper is the effect of both the ink and the paper integrated color, and this will inevitably appear some color cast situation, based on this situation, when printing is necessary on the white paper Degree and color cast were analyzed against the manuscript and corrective measures were taken to correct the color cast. For example, when the color of the paper is yellow, it is not appropriate to use dark yellow, peacock blue, and scarlet inks for printing. That is to say, on the one hand, ink should be used correctly to eliminate color casts according to the characteristics of paper color. 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. Then, printing paper is a paper base that is one of the main factors of color discrimination. It supports the whiteness of the white color of the color print, and presents the hue, lightness and saturation of the paper. Degree plays a decisive role. Therefore, in the printing, since the whiteness of the paper required by different prints is not the same, it is necessary to select the proper paper for printing in order to ensure the best print quality.
Analysis of paper whiteness
December 13 12:20:05, 2020
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