Polyurethane coating materials are prone to yellowing over time due to prolonged exposure to ultraviolet light or heat, affecting their appearance and service life. By introducing UV-320 and 2-hydroxyethyl thiophosphate into the chain extension of polyurethane, a nonionic water-based polyurethane with excellent resistance to yellowing was prepared and applied to leather coating. Through color difference, stability, scanning electron microscope, X-ray spectrum and other tests, it was found that the total color difference △E of the leather treated with 50 parts of the nonionic water-based polyurethane with excellent resistance to yellowing was 2.9, the color change grade was 1 grade, and there was only very slight color change. Combined with the basic performance indicators of leather tensile strength and wear resistance, it shows that the prepared yellowing-resistant polyurethane can improve the yellowing resistance of leather while maintaining its mechanical properties and wear resistance.

As people's living standards have improved, people have higher requirements for leather seat cushions, not only requiring them to be harmless to human health, but also requiring them to be aesthetically pleasing. Water-based polyurethane is widely used in leather coating agents due to its excellent safety and pollution-free performance, high gloss, and amino methylidynephosphonate structure similar to that of leather. However, water-based polyurethane is prone to yellowing under long-term influence of ultraviolet light or heat, affecting the service life of the material. For example, many white shoe polyurethane materials often appear yellow, or to a greater or lesser extent, there will be yellowing under the irradiation of sunlight. Therefore, it is imperative to study the resistance to yellowing of water-based polyurethane.

There are currently three ways to improve the yellowing resistance of polyurethane: adjusting the proportion of hard and soft segments and changing the raw materials from the root cause, adding organic additives and nanomaterials, and structural modification.

(a)Adjusting the proportion of hard and soft segments and changing the raw materials can only solve the problem of polyurethane itself being prone to yellowing, but cannot solve the influence of external environment on polyurethane and cannot meet the market requirements Through TG, DSC, abrasion resistance and tensile testing, it was found that the physical properties of the prepared weather-resistant polyurethane and the leather treated with pure polyurethane were consistent, indicating that the weather-resistant polyurethane can maintain the basic properties of leather while significantly improving its weather resistance.

(b)The addition of organic additives and nanomaterials also has problems such as high addition amounts and poor physical blending with polyurethane, resulting in a decrease in polyurethane mechanical properties.

(c)Disulfide bonds have strong dynamic reversibility, making their activation energy very low, and they can be broken and rebuilt multiple times. Due to the dynamic reversibility of disulfide bonds, these bonds are constantly broken and rebuilt under ultraviolet light irradiation, converting ultraviolet light energy into heat energy release. The yellowing of polyurethane is caused by ultraviolet light irradiation, which excites the chemical bonds in polyurethane materials and causes bond cleavage and reorganization reactions, leading to structural changes and yellowing of polyurethane. Therefore, by introducing disulfide bonds into the water-based polyurethane chain segments, the self-healing and yellowing resistance performance of polyurethane was tested. According to GB/T 1766-2008 test, △E was 4.68, and the color change grade was level 2, but since it used tetraphenylene disulfide, which has a certain color, it is not suitable for yellowing-resistant polyurethane.

Ultraviolet light absorbers and disulfides can convert the absorbed ultraviolet light into heat energy release to reduce the influence of ultraviolet light radiation on the polyurethane structure. By introducing the dynamic reversible substance 2-hydroxyethyl disulfide into the polyurethane synthesis expansion stage, it is introduced into the polyurethane structure, which is a disulfide compound containing hydroxyl groups that is easy to react with isocyanate. In addition, UV-320 ultraviolet absorber is introduced to cooperate with the improvement of polyurethane's yellow resistance. UV-320 containing hydroxyl groups, due to its characteristic of easily reacting with isocyanate groups, can also be introduced into the polyurethane chain segments and used in the middle coat of leather to improve the yellow resistance of polyurethane.

Through the color difference test, it was found that the yellow resistance of the yellow resistance polyureth Through TG, DSC, abrasion resistance and tensile testing, it was found that the physical properties of the prepared weather-resistant polyurethane and the leather treated with pure polyurethane were consistent, indicating that the weather-resistant polyurethane can maintain the basic properties of leather while significantly improving its weather resistance.