How to alleviate polyurethane yellowing?

april 12, 2022
blog

In this article we will explain how to alleviate polyurethane yellowing. Polyurethane products 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 the 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 damage to its structure and properties. Therefore, investigating the mechanism and modification scheme of polyurethane yellowing has become a popular research topic in the polyurethane industry.

Mechanism of yellowing phenomenon

There are external and internal factors. Firstly, we will start with the internal factors of aromatic polyurethane. In order for the polyurethane to have good yellowing resistance, the correct selection of raw materials is an important consideration.

Internal factors:

  • UV light excited aromatic isocyanate to form hydrogen peroxide, and its photodegradation resulted in diquinonimide.
  • After Photo-Fries rearrangement, amine compounds are converted to colored azo compounds, resulting in yellow.
  • There is a certain degree of unsaturation on the molecular chain segment of the ordinary oligomer polyol, and the unsaturated bonds remaining in the polyurethane material prepared by it will be gradually oxidized to aldehydes, ketones and carboxylic acids under the influence of air, temperature, sunlight and other factors, resulting in yellowing of the polyurethane material.
  • In addition, polyether polyurethanes have poor stability and are prone to oxidation of molecular chemical bonds, leading to thermal oxidation degradation of polyurethanes.

External 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.

Omnistab solutions to alleviate polyurethane yellowing

UV absorbers; we have a benzotriazole UV absorber (Omnistab UV 571) which is a light yellow liquid that used for PUR coating and integral PU foam. It effectively improves the yellowing resistance level of the product. Secondly, the Omnistab UV 1, which is a phenyl methyl amidine. It’s also a light yellow liquid, with a broad spectrum UV absorption band with high cost performance.

Light stabilizers; The Omnistab LS 123 is a HALS which is clear pale yellow liquid and improves the thermal stability of the polyurethane. Secondly, the Omnistab LS 292, which is a colorless to pale yellow liquid. It effectively prevents cracking, loss of gloss and other paint deceases. It can significantly improve the products service life.

Antioxidants; Three different hindered phenol antioxidants, starting with Omnistab AN 1135. It is specially suitable for polyurethane soft foam to prevent polyols from generating peroxide during storage. It also prevents coking in the PU foaming process. It’s a colorless or pale yellow transparant liquid. Secondly, Omnistab AN 5057, which is a more yellow to reddish brown liquid. It has low volatility and good compatibility. It has excellent synergy in combination with Omnistab AN 1135. A third hindered phenol antioxidant is the Omnistab AN 245. A solid product which is white to off-white powder with low volatility and high extraction resistance. It can reduce PUR yellowing caused by NOx.

The last antioxidant to alleviate polyurethane yellowing is a alkyl phosphite ester, which is a colorless clear liquid. This is our Omnistab AN 6133, which is more eco-friendly & no phenol. It has low volatility as well as the others and anti-hydrolysis resistance. It can improve the thermal stability and aging resistance of PU.

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