Polyurethane materials are widely used in various industries, such as plastics, rubber, fibers, adhesives, textiles and thermal insulation materials. MDI is the main aromatic polyisocyanate used in polyurethane industry. However, the polyurethane materials prepared from aromatic isocyanate are prone to yellowing under UV light. Yellowing not only affects the appearance of polyurethane material, but also predicts the damage of its structure and properties.
Therefore, exploring the mechanism and modification scheme of polyurethane yellowing has become a hot research topic among polyurethane industry.
Mechanism of Yellowing phenomenon：
- The internal factors of aromatic polyurethane
In order to ensure that the polyurethane has good yellowing resistance, the proper selection of raw materials is an important consideration.
① UV light excited aromatic isocyanate to form hydrogen peroxide, and its photodegradation resulted in diquinone imide.
② After Photo-Fries rearrangement, amine compounds are converted to colored azo compounds, resulting in yellow.
③ There is a certain degree of unsaturation on the molecular chain segment of the common oligomer polyol, and the unsaturated bonds left in the polyurethane material prepared by it will be gradually oxidized into aldehydes, ketones and carboxylic acids under the influence of air, temperature, sunlight and other factors, resulting in the yellowing of the polyurethane material.
④ In addition, polyether polyurethanes have poor stability and are prone to molecular chemical bond oxidation, leading to thermal oxidation degradation of polyurethanes.
Environmental factors
Polyurethane materials are sensitive to UV radiation, temperature and humidity, and photothermal oxidation, hydrolysis and degradation caused by heat and light accelerate the yellowing of polyurethane products.
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