Yellowing refers to the phenomenon in which white or light-colored materials turn yellow on the surface under various conditions such as light, ultraviolet radiation, heat, oxygen, stress, chemicals, and more. It is also known as “phenolic yellowing.” The causes of material yellowing vary and cannot be detected using a single method. It is necessary to investigate the reasons for yellowing based on the specific circumstances and then address them with targeted solutions.
Phenolic yellowing is one type of yellowing, which occurs due to the use of phenolic compounds as antioxidants during the production and processing of materials. Among these compounds, 2,6-di-tert-butyl-4-methylphenol (BHT) is the most widely used antioxidant due to its strong antioxidant properties, stability, and cost-effectiveness. However, BHT can react with nitrogen oxides to form new compounds, leading to material yellowing.
1. Detection of Phenolic Yellowing
1) ISO and GB standards provide identical testing methods for detecting phenolic yellowing.
- ISO 105-X18
- GB/T 29778
2) Testing Procedure:
- Prepare a sealed test package, with seven glass plates holding six test papers, five samples, and one control fabric in each package.
- Fold the test paper along its 100mm edge and place each sample and the control fabric separately in the middle of the folded test paper, resulting in six combination samples. These combination samples are separated by glass plates.
- Use three layers of polyethylene film without BHT to wrap the glass plates, test papers, samples, and control fabric tightly and seal them with adhesive tape.
- Place the test package on a testing device and apply a pressure of (5±0.1) kg. You can stack up to three test packages on a single testing device.
- Place the testing device in a constant temperature chamber/oven at (50±3)°C and maintain it for 16 hours ±5 minutes.
- Remove the testing device from the constant temperature chamber/oven, take out the test package, and allow it to cool.
- Evaluate the samples within 30 minutes after opening the test package; otherwise, some samples may fade when exposed to air. Start by examining the control fabric, and if the control fabric turns yellow to at least level 3 according to the gray scale, the test is considered normal. If the control fabric doesn’t reach level 3, repeat the test with new samples and materials.
picture: phenolic yellowing testing equipment
2. Causes and Countermeasures of Phenolic Yellowing
Phenolic yellowing results from a chemical reaction between the antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT) in the material and nitrogen oxides, producing 2,6-di-tert-butyl-p-nitrophenol (DTNP). DTNP sublimates easily and can migrate onto other materials even at room temperature. DTNP is colorless under acidic conditions but turns yellow immediately under alkaline conditions.
Considering the characteristics of BHT, the following points should be noted:
- Avoid contact with materials containing BHT. BHT is widely used in various industries, so products should avoid contact with materials containing BHT during processing, packaging, and transportation, especially with packaging materials. The presence of yellow DTNP substances on packaging materials can transfer to the product, causing it to yellow. This is the most common cause of phenolic yellowing.
- Implement self-protection for products. If it is difficult to avoid contact with materials containing BHT during packaging and transportation, it is possible to apply anti-phenolic yellowing treatment to the product in advance. By adding appropriate additives, materials can effectively prevent the absorption of derivatives. Reduction powders can also be used to prevent or treat partial yellowing.
- Avoid using BHT as an antioxidant. BHT is a cost-effective antioxidant widely used in various fields, including the synthetic rubber and plastics industry, although it may not be the primary antioxidant. Therefore, in the footwear industry, phenolic yellowing may occur not only in textile products such as shoelaces and shoe uppers but also in components of molded shoes. If yellowing occurs in molded shoe components stored in warehouses without exposure to light and under normal temperature conditions, it is necessary to investigate whether phenolic yellowing is the cause. It could be due to contact with materials containing BHT or the molded shoe material itself containing BHT. Poor air circulation in the warehouse and high nitrogen oxide content can also lead to chemical reactions.
- Consider environmental effects. In addition to providing good ventilation, it is essential to avoid placing products in an alkaline environment, given the characteristics of DTNP.
Phenolic yellowing is preventable and controllable, and the chemical reaction itself is reversible. Therefore, by giving this issue due attention, it can be avoided.