Yellowing refers to the phenomenon where white or light-colored materials turn yellow on the surface under conditions such as light, ultraviolet radiation, heat, oxygen, stress, chemicals, etc. It is also known as discoloration. The varying cause for material yellowing can only be identified based on the actual situation and it could then be solved in a targeted manner.
Light yellowing is the phenomenon of the surface color of white or light-colored materials turning yellow due to exposure to ultraviolet radiation.
Current national and international methods for testing resistance to light yellowing use incandescent lamps and heating control devices to simulate natural environments or expose specimens to ultraviolet radiation. The color changes on the surface of white or light-colored samples are observed within a specified time to determine the degree of discoloration, thus assessing the material’s ability to resist yellowing under solar radiation.
I. Testing for Resistance to Light Yellowing:
1) Testing Methods
- HG/T 3689-2014 “Test Method for Yellowing Resistance of Footwear”
- QB/T 4672-2014 “Test Method for Yellowing Resistance of Synthetic Leather and Artificial Leather”
- GB/T 30669-2014 “Textiles – Color Fastness Test – Resistance to Light Yellowing”
- ASTM D1148-13(R2018) “Standard Test Method for Rubber Deterioration-Discoloration from Ultraviolet (UV) or UV/Visible Radiation and Heat Exposure of Light-Colored Surfaces”
Note 1: ISO standards do not have a specific test method for resistance to light yellowing. Some customers may modify and use the ISO 105-B02 method for light fastness testing to conduct resistance to light yellowing tests.
2) Testing Principles and Procedures:
Note 2: Due to different light sources, there is no comparability between the methods.
a) Sunlight Method: Simulates natural environments using a solar lamp and a heating control device to assess the color changes on the sample’s surface.
Prepared samples, as per standard specifications, are placed into the instrument. The light source and temperature conditions used in the instrument should meet the requirements. Set the distance between the sample and the light source, chamber temperature, and speed according to standard requirements. Start the instrument to continuously irradiate the sample under simulated sunlight. After reaching the specified time, remove the sample to determine the extent of discoloration.
b) Ultraviolet Method: Evaluates the color changes on the sample’s surface by exposing it to ultraviolet radiation.
Prepared samples, according to standard specifications, are placed into the instrument. The light source (and temperature conditions) used in the instrument should meet the requirements. Set the distance between the sample and the light source (and chamber temperature) according to standard requirements. Start the instrument to continuously irradiate the sample under ultraviolet light. After reaching the specified time, remove the sample to determine the extent of discoloration.
c) Xenon Arc Lamp Method: Illuminates the sample with a xenon lamp equipped with a visible light filter to evaluate the color changes on the sample’s surface.
II. Causes and Countermeasures of Light Yellowing
Light is a form of energy, and its energy varies with different wavelengths. Typically, ultraviolet light below 400nm has the most significant destructive effect on footwear materials. The mechanism behind light yellowing is the oxidative degradation of materials by light.
For some materials, when they absorb light energy, the molecular chains at the absorption sites may break, causing the material to turn yellow, as seen in diene-type elastomeric materials. Polyurethanes with aromatic (benzene ring) structures exhibit poor color stability because the quinone structures in their composition produce quinone imine chromophores that cause yellowing under ultraviolet light exposure.
SBS copolymers used in TPR/TPE products contain polybutadiene unsaturated bonds in their molecular structure. Under the action of light and oxygen, significant photo-degradation occurs, leading to the formation of numerous degradation or cross-linking products with visible chromophores (conjugated π bonds) within the material. This manifests as the characteristic yellow appearance of aging.
To enhance resistance to light yellowing, the following methods can be considered:
- Choose materials with better light resistance. Aliphatic PU (polyurethane) is more stable in color than aromatic counterparts.
- Add UV stabilizers to reduce UV absorption and minimize its impact on the material.
- Add stabilizers targeted at fixing material structures and strengthening material stability.
Light yellowing is a crucial cause of yellowing, and it is an irreversible process that requires special attention.