Yellowing refers to the phenomenon of whitish or light-colored materials turning yellow under conditions such as light, ultraviolet radiation, heat, oxygen, stress, chemicals, etc. It is also known as discoloration. The reasons for material yellowing vary, and it cannot be detected by a single method. It can only be identified based on the actual circumstances, investigating the causes of yellowing, and then solving them in a targeted manner.
Thermal yellowing, also known as high-temperature yellowing, is another type of yellowing. It refers to the phenomenon of the material’s surface turning yellow under high-temperature conditions. The cause of yellowing is due to high temperatures causing certain components in the material to decompose or undergo oxidation reactions with oxygen. Decomposition or oxidation reactions can lead to changes in the material’s structure, resulting not only in the yellowing phenomenon affecting the material’s appearance but also potentially impacting its physical and mechanical properties.
The occurrence of thermal yellowing, phenolic yellowing, and light-induced yellowing are different, with phenolic and light-induced yellowing being more common.
I. Detection of Thermal Yellowing
1）Thermal yellowing is often tested using accelerated thermal aging methods. Common testing standards include:
- ISO 20870
- GB/T 3903.7
- ISO 188
- GB/T 3512
- Set the specified temperature (e.g., 70°C) or temperature/humidity (e.g., 70°C temperature/100% humidity).
- Place the specimen in the oven, allowing all sides to be freely exposed to the air.
- After the specified time (e.g., 168 hours), remove the specimen and assess the degree of yellowing.
II. Causes and Countermeasures of Thermal Yellowing
The mechanism of thermal yellowing involves the thermal decomposition (degradation) of material components at high temperatures. The main reasons for yellowing are as follows:
- Substances formed directly from decomposition cause yellowing.
- Substances formed from decomposition undergo oxidation reactions with oxygen in the air, leading to yellowing.
The causes of thermal yellowing include insufficient thermal stability of the material itself, impurities in the material, and the influence of additives in the material (such as stabilizers, flame retardants), among others.
In light of these situations, the following preventative actions should be duly considered:
- Choose heat-resistant materials and optimize material formulations to enhance their inherent heat resistance.
- Use qualified raw materials and minimize components that cause yellowing in the formulation. Impurities in raw materials or poor later purification, containing chemical or mechanical impurities, can deteriorate material stability, and some impurities may catalyze degradation.
- Use antioxidant and heat-resistant stabilizers.
- Antioxidants and stabilizers help maintain material structure stability. Choose appropriate antioxidants and stabilizers based on the type of material. Antioxidants significantly weaken thermal oxygen reactions, while stabilizers effectively eliminate factors causing degradation.
- Improve material processing techniques and conditions.
- For example, use suitable processing temperatures and times to avoid material decomposition reactions. Strict drying treatment of materials before processing is essential.
- Use surface coatings or encapsulation to prevent yellowing.
The causes of material yellowing are varied, and so as the distinction between phenolic yellowing, light-induced yellowing, and thermal yellowing, all depending on ozone, impurities, moisture, acidity, and alkalinity. Therefore, unraveling the root cause of yellowing based on the type of material and external conditions is key to putting up a targeted and corrective approach to prevent the issue.