Introduction
The color of pulp is one of the key indicators to evaluate its appearance quality and subsequent application performance, especially in the paper, packaging and special paper industries, the consistency of color directly affects the visual performance and market acceptance of the final product. As an objective and accurate optical measurement technique, spectrophotometry provides a reliable means for the standardized determination of pulp coloriness by quantifying the reflection or transmission characteristics of samples to specific wavelengths of light. Based on the internationally accepted color space theory, this method can effectively avoid the deviation of the subjective judgment of the human eye, and is widely used in production quality control and R&D processes.
Rationale
Spectrophotometry determines chromaticity based on the principle of light-matter interaction. When a beam of light hits the surface of the pulp sample, part of the light is absorbed and part of the light is reflected. The grating or interferometer built into the instrument will reflect the light and divide the light, and the detector will measure the reflectance at each wavelength, and then calculate the chromaticity value. Commonly used color spaces include the CIELAB system, where L* represents luminosity and a* and b* represent chromaticity coordinates, representing red-green and yellow-blue directions, respectively. The color difference value ΔE can be used to compare the color difference between the sample and the standard.
The calculation formula is as follows:
Luminosity L* = 116 × (Y/Yn)1/3 - 16
Chromaticity coordinates a* = 500 × [(X/Xn)1/3 - (Y/Yn)1/3]
Chromaticity coordinate b* = 200 × [(Y/Yn)1/3 - (Z/Zn)1/3]
where X, Y, and Z are the three stimuli values of the sample, Xn、Yn、ZnThe three stimuli values of the reference white.
Instruments and reagents
The measurement uses a spectrophotometer equipped with an integrating sphere to ensure the diffuse reflection measurement mode. The instrument should be calibrated regularly with a standard whiteboard and blackboard to ensure measurement accuracy. The pulp sample should be prepared into a uniform sheet according to the standard method in advance, and the thickness and density should be consistent to avoid the influence of light transmission. The laboratory environment should control temperature and humidity and avoid direct light exposure. No special chemical reagents are required, but a standard chroma plate is prepared as a reference.
Assay steps
First, the spectrophotometer is turned on to warm up, and the instrument is self-tested and calibrated. The prepared pulp sample is placed flat in the measuring hole to ensure complete coverage. Select Specular Reflection (SCI) or Exclude Specular Reflection (SCE) mode, set according to standard requirements. The measurement wavelength range is typically 400-700nm with intervals of 10nm or 20nm. Each sample measures at least three different locations, averaging them. Record L*, a*, b* values, and calculate chromatic aberration ΔE if necessary. Clean the sample stage after measurement to avoid contamination.
Result analysis and precautions
The data obtained can be used directly to evaluate the chromatic properties of the pulp. A high L* value indicates high brightness, close to white; a* positive value is red, negative value is green; b* positive value is yellow, negative value is blue. The influence of pulp type, slurry source and pulping process should be considered in the analysis. Precautions include: sample preparation should be uniform and flawless, instrument calibration should be strict, measurement environment should be stable, and the color of the operator's clothing should be interfered with the reflected light. Regularly participate in inter-laboratory comparisons to ensure method consistency.
Application and standard reference
This method is suitable for the determination of color of various chemical pulps, mechanical pulps and waste pulps, and provides a basis for bleaching control, dye addition and quality grading in the production process. Relevant standards include ISO 5631, TAPPI T527, etc., which specify specific measurement conditions and reporting formats. In practical applications, the optical properties of pulp can be comprehensively evaluated in combination with other indicators such as whiteness and opacity.
Summary
Spectrophotometry is an efficient and objective technical method based on international color space theory, which can provide accurate quantitative data. Through standardized operation and calibration, this method helps to improve the reliability of pulp product quality control, promote industry technical exchanges and trade facilitation. In the future, with the development of instrument technology, measurement efficiency and applicability are expected to be further expanded.
Cited Literature
ISO 5631: Paper and board — Determination of colour by diffuse reflectance method.
TAPPI T527: Colour of paper and paperboard by spectrophotometry.
CIELAB colour space: CIE Publication No. 15.