OMNSTAB Solution for high-end PU synthetic leather

PU synthetic leather is a type of artificial leather made from polyurethane resin coating and non-woven or woven fabric base material. Due to its smooth surface, light weight, good breathability, and wear resistance, it is widely used in sports equipment, shoes, clothing, and other fields. Compared with other types of artificial leather, PU synthetic leather is more environmentally friendly because its base material can be recycled and reused, and fewer additives are used in the production process.

The key coating processes for PU synthetic leather primarily include the following:  

(1) Dry Process  

Coating: Apply PU slurry evenly onto release paper (molded paper with leather texture) to form a surface layer.  

Drying: Heat in an oven to evaporate solvents and cure PU into a porous film.

Transfer: Peel the PU film from the release paper and bond it to the base fabric via heat pressing.

(2) Wet Process  

Immersion: Immerse the base fabric in PU slurry (containing DMF solvent).  

Solidification: Immerse in a water bath, where DMF reacts with water to displace it, causing the PU to solidify into a porous sponge-like structure.

Rinsing and drying: Remove residual solvent to form a breathable microporous layer.

(3) Wet + Dry Composite Process

Combine the breathable layer from the wet process with the smooth surface from the dry process to produce high-performance synthetic leather (e.g., ultra-fine PU leather).

What causes yellowing in polyurethane?

The yellowing of polyurethane is not simply a “color change,” but rather the result of a series of complex chemical reactions. The core mechanisms can be summarized into the following three types:

1. Oxidation of phenolic compounds and formation of quinones

During the production of polyurethane, if phenolic substances (such as catalysts, antioxidants, or raw material impurities) remain, these phenolic compounds are prone to undergo auto-oxidation reactions under conditions of oxygen, light exposure, or high temperatures:  

Step 1: The phenolic hydroxyl group (-OH) loses a hydrogen atom, generating highly reactive phenoxy radicals (PhO·).  

Step 2: The radical further reacts with oxygen to form peroxides (PhOO·), which gradually convert into quinone compounds (such as ortho-benzoquinone or para-benzoquinone).  

Result: Quinone compounds have a conjugated double bond structure, enabling them to absorb specific wavelengths of visible light (typically 400–500 nm), resulting in a yellow or brown color and causing yellowing of the material surface.  

2. Thermal degradation and catalytic action of nitrogen oxides (NOx)  

Under high-temperature conditions (e.g., processing, prolonged exposure to sunlight), the molecular chains of polyurethane may break down, releasing amine or phenol-based small molecules. When these substances come into contact with nitrogen oxides (NOx) in the air (such as vehicle exhaust or industrial emissions), they undergo photochemical yellowing:  

NOx (such as NO₂) reacts with amines to form nitrosamines, which are further oxidized into dark-colored products.  

Under NOx catalysis, the oxidation rate of phenolic compounds significantly accelerates, resulting in a substantial increase in quinone formation.  

3. Photochemical oxidation reactions  

Ultraviolet (UV) light is the primary factor triggering photochemical oxidation of polyurethane:  

① UV energy can break the C-H and C-O bonds in polyurethane molecules, initiating free radical chain reactions.  

② Photochemical oxidation not only accelerates the conversion of phenolic compounds but also causes surface cracking of the material, exposing more oxidizable groups and creating a vicious cycle.

How to address yellowing issues in high-end PU synthetic leather?

Select a proper primary antioxidant: Use phosphite-based antioxidants, which prioritize free radical scavenging and interrupt oxidative chain reactions.  

Choose a heat-resistant antioxidant: Select products with a thermal decomposition temperature >200°C to ensure stability during high-temperature processing.  

Especially for customers whose PU products are intended for outdoor use, we recommend adding a certain amount of UV absorbers to the PU resin. These can effectively absorb UV rays and prevent yellowing of the PU. Additionally, when UV absorbers, light stabilizers, and antioxidants are used together, they can produce a synergistic effect, achieving a result greater than the sum of their individual effects.

OMNISTAB Solutions for PU synthetic leather

OMNISTAB UV1

OMNISTAB UV571

OMNISTAB LS292

OMNISTAB AN1135

OMNISTAB AN6133

OMNISTAB HN130