Introduction
In the evaluation of material durability, simulating the impact of environmental factors on plastic properties is a key part. As a common apparent deterioration phenomenon of plastics under the action of light, heat, oxygen, etc., its quantitative testing is of great significance for product quality control and life prediction. Laboratory accelerated aging tests conducted according to GB/T 16422.3 provide a standardized method for evaluating the yellowing behavior of plastics under specific temperature and humidity conditions. The Constant Temperature and Humidity Aging Chamber is the core equipment for this test, which creates a controlled and repeatable accelerated aging environment for plastic specimens by precisely controlling temperature and relative humidity.
Test Principle:
Plastic yellowing is essentially the result of oxidative degradation and other chemical reactions in its polymer chains under the combined action of heat, oxygen and possibly moisture, and the formation of chromogenic groups (such as carbonyl groups, conjugated double bonds, etc.). The GB/T 16422.3 standard specifies the aging test method for plastics exposed to laboratory light sources, which have clear requirements for the temperature and humidity conditions of the test environment. The constant temperature and humidity aging chamber acts as an environmental simulator here, which does not directly provide a light source but strictly maintains the constant temperature and humidity required for the test to investigate the accelerated or synergistic effects of specific humid and hot conditions on the yellowing process of plastics. The degree of yellowing is usually quantitatively characterized by measuring the color coordinate changes (e.g., Δb*, ΔYI values) before and after exposure of the specimen by a colorimeter.
Equipment requirements:
The temperature and humidity aging chamber that performs the test must meet the strict requirements of the standard for temperature and humidity control accuracy and uniformity. The equipment should have the ability to operate stably for a long time, ensuring that the temperature and humidity fluctuations and uniformity within the chamber are within the allowable deviation throughout the test cycle. The temperature control system usually adopts PID regulation, and the humidity control is mostly achieved by the dry and wet bulb method or the dew point method. The box structure should have good sealing and corrosion resistance, and the internal air circulation system should ensure the consistency of the surrounding environment of the specimen. Before testing, the working area of the device is verified using calibrated temperature and humidity sensors.
Testing process
The testing process strictly adheres to standard operating procedures. First, prepare a sufficient amount of plastic specimens according to the relevant sample preparation standards and clean their surfaces. Use a colorimeter to measure and record the initial color coordinates of each specimen. The specimen is then placed on the sample rack of the aging chamber in a non-overlapping, stress-free, and consistently exposed surface orientation. Set parameters according to product specifications or research purposes with reference to the relevant temperature and humidity conditions in GB/T 16422.3 (e.g., common test conditions may be set to specific temperatures and higher relative humidity). Start the device and start the timer. At the end of the scheduled exposure cycle (e.g., 24 hours, 48 hours, 96 hours, etc.), the specimen is removed, and its color coordinates are measured again after adjusting for the specified time under standard atmospheric conditions. Calculate and record the value of the color aberration.
Analysis of results
Data analysis is key to assessing the degree of yellowing. The change in the yellowness index (ΔYI) or the change in the b* value in the CIE Lab color space (Δb*, positive value indicates increased yellowing) is usually calculated for each specimen. The results can be expressed as the average of multiple measurements and their dispersion. The development kinetics of yellowing can be analyzed by plotting the curve of yellowing indicators (e.g., ΔYI) with exposure time. Comparing the yellowing data of plastic samples with different formulations and processes under the same conditions is helpful to evaluate the aging resistance of materials to moisture and heat. All data should be reported in full along with the test conditions (temperature, humidity, exposure time).
Notes:
To ensure the effectiveness and reliability of the test, the following points need to be paid attention to: Constant temperature and humidity aging chambers should be regularly calibrated, especially temperature and humidity sensors. The specimen should be placed to avoid obstructing the air duct in the box to ensure a uniform environment. The sensitivity of plastics with different chemical compositions to moisture and heat varies significantly, and the selection of test conditions should be targeted and clearly stated. Color measurement should be performed at the same location or area of the specimen, and attention should be paid to the effect of measuring the pore size on the results. The test report should document in detail any deviations from standard procedures.
Applications and prospects
Plastic yellowing test based on constant temperature and humidity aging box is widely used in many industries, such as outdoor building materials, automotive interiors, electronic and electrical shells, packaging materials, etc., to screen materials, optimize stabilizer systems, and predict the service life of products in hot and humid climates. With the development of materials science and the improvement of standard systems, testing methods are evolving towards higher accuracy, more coupling of environmental factors (such as temperature and humidity cycles), and more intelligent data collection, so as to more realistically simulate complex real environments and provide solid support for material research and development and quality assurance.
References
1. GB/T 16422.3-2022, Plastics - Test methods for exposure to light sources in laboratories - Part 3: Fluorescent ultraviolet lamps.
2. Review literature on related material aging and durability testing techniques.
3. Relevant literature on technical conditions and calibration specifications of constant temperature and humidity test equipment.