Overview
The molecular structure of transparent resin may change under long-term use or under specific environmental conditions, resulting in yellowing of the material. This yellowing will directly affect the appearance quality and optical properties of the product. The yellowing index is a key parameter to quantify the degree of color change, which is calculated by comparing the initial state of the material with the color difference after aging. Accurate evaluation of yellowing index is important for material development, quality control, and life prediction.
Measurement principle
A colorimeter is an instrument that works based on a standard chromaticity system set by the International Illumination Commission. It irradiates the sample by simulating a standard light source, and the detector receives the light signal reflected or transmitted by the sample and converts it into chromaticity coordinates. For clear resins, transmission measurement mode is usually used. The main chromaticity parameters output by the instrument include luminosity L* value and color coordinates a* and b*. Among them, the b* value represents the color shift in the yellow-blue direction, and the positive value represents the yellowish tint, which is one of the core data for calculating the yellowing index.
The calculation of the yellowing index usually follows the relevant criteria, and its basic formula can be expressed as: YI = (C * b*) / (L* + D), where C and D are constants based on the chosen criterion. For example, a commonly used standard definition formula is: YI = (100 * (1.28 * X - 1.06 * Z)) / Y. where X, Y, and Z are the CIE three stimuli values of the sample. By measuring the YI value before and after aging, the change ΔYI can intuitively reflect the degree of yellowing.
Operational points
To establish a reliable yellowing index evaluation process, it is necessary to control the whole process from sample preparation, instrument calibration to data analysis. First, the transparent resin sample to be tested needs to be prepared into a specimen with a specified thickness and surface condition, and ensure that it is clean and free of scratches. Before measurement, the colorimeter must be calibrated using a standard whiteboard and blackboard, and a standard light source and observer perspective must be selected that matches the actual environment in which the product will be used.
When measuring, the sample should be closely attached to the measuring port of the instrument to avoid interference from external light. Each sample should be measured multiple times at different locations to obtain an average value, ensuring that the data is representative. Record the chromaticity value in the initial state and calculate the initial yellowing index. After the sample has undergone accelerated tests such as photoaging and thermal aging or actual use for a period of time, the above steps are repeated under exactly the same measurement conditions to calculate the yellowing index and change value ΔYI after aging.
| Key Action Steps | Core purpose and precautions |
| Sample preparation and processing | Ensure uniform thickness, consistent surface, and clear markings. |
| Instrument calibration and setup | Regular calibration, fixed light source, viewing angle and other measurement conditions. |
| Background and mode selection | Transmission mode, using a standard background, for consistency. |
| Multi-point measurement and recording | The average value is taken to reduce the error caused by local inhomogeneity. |
| Data calculation and analysis | YI and ΔYI were calculated according to the selected standard formula, and the change trend was evaluated. |
Influencing factors
The accuracy and repeatability of measurement results are affected by a variety of factors. The uniformity, thickness, and surface cleanliness of the sample itself are the primary factors. In terms of instruments, the stability of the light source, the sensitivity of the detector, and the calibration status are crucial. Environmental conditions such as ambient light and temperature fluctuations can also introduce errors. Therefore, strict control of experimental conditions and clarification in the report are the basis for data comparability.
The interpretation of ΔYI results needs to be combined with specific application scenarios and industry standards. Normally, an increase in the positive value of ΔYI indicates increased yellowing. It is necessary to establish internal quality acceptance standards or refer to industry general norms to determine whether the degree of yellowing is within the acceptable range. Long-term monitoring of the change trend of ΔYI can also be used to study the effects of different formulations, processes or protective coatings on the yellowing resistance of resins, and provide data support for material improvement.
Summary
Using colorimeter to evaluate the yellowing index changes of transparent resin is an objective and quantitative effective means. Reliable and reproducible yellowing data can be obtained through standardized measurement processes, strict environmental controls, and adequate awareness of the influencing factors. This method not only serves the offline inspection of product quality, but can also be deeply applied to the research of material aging mechanism, new product development and life evaluation, providing a key technical basis for improving the durability and market competitiveness of transparent resin products.
References
International Illumination Commission. CIE 015:2018 Colorimetry.
American Society for Testing and Materials. ASTM E313 Practice for Calculating Yellowness Index and Whiteness Index of White and Near-White Opaque Materials.
National Standardization Administration of China. GB/T 7921 Uniform color space and chromatic aberration formula.