The Benefits of Using PU Anti-yellowing Agent

We launched two anti yellowing agents, Omnistab HN 130 & Omnistab HN 150. Both are highly suitable for polyurethane applications. In this article we introduce how they fix yellowing polyurethane.

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Polymers are prone to turn naturally yellow during processing. Moreover, the most fundamental reason of yellowing is the stability of the polymer structure. For example, oxidation and high temperature will effect the structure (polymer degradation).

Firstly, some polymers are sensitive to UV rays and light stabilizers will protect them against yellow over time during UV exposure. Secondly, some polymers are sensitive to oxidation and anti-oxidants will protect them. That sounds logical, right? So why do you need an anti-yellowing agent for polyurethane?

What is polyurethane?

Polyurethane is a plastic material, which exists in different forms. It can be tailored to be either flexible or rigid, and is the material of choice for a wide range of end-user applications.

Polyurethanes are formed by combining polyols alcohol with more than two reactive hydroxyl groups per molecule) with a diisocyanates (TDI and MDI) or polymeric isocyanates in the presence of suitable catalysts and additives.

There is a large variety of different types of polyurethanes and each is made in a slightly different way:

• Carbon dioxide is used as a blowing agent to create the soft, comfortable feel of a mattress or sofa. The more blowing agent is used, the softer the resulting foam.
• In rigid foams, a gas such as pentane is 'trapped' in the closed cells of the foam, optimizing its insulation capacity.
• Rollerblade wheels, on the other hand, do not require a blowing agent and instead have a dense and hardwearing consistency.

Characteristics of Polyurethane

Polyurethane contains a urethane group in the main chain of the macromolecule. Because the groups, contained in the macromolecule of the polyurethane, are all high polar groups. And it also contains a polyether or a polyester soft segment, polyurethane has the following characteristics:

1. High mechanical strength and oxidation stability
2. High flexibility and resilience
3. Excellent resistance against oil, solvents, water and fire

We emphasize three reasons for polyurethane yellowing. Yellowing of hot oxygen aging caused by high temperature during processing. Further, yellowing caused by exposure to nitrogen oxides (NOx) in the air.

Likewise, yellowing caused by exposure to UV rays. The polyurethane structure contains an amino group, which is easily oxidized and yellowed. Next to that, an unsaturated bond, which is prone to ultraviolet yellowing.

Use of polyurethane

To sum up, the polyurethane product is highly susceptible to yellowing during use. With these characteristics, polyurethane has a wide range of uses;

1. polyurethane synthetic leather
2. polyurethane foam
3. polyurethane coating (white paint, milk paint, other oil based paints)
4. polyurethane adhesive
5. urethane rubber (elastomer)
6. polyurethane fiber

Polyurethane is used in civil engineering, address drilling, mining and petroleum engineering to block water, stabilize buildings or subgrades; as paving materials, for runways in sports fields, indoor floors of buildings.

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New anti-yellowing agents: Omnistab HN 130 and Omnistab HN 150

Omnistab HN 130 and Omnistab HN 150 are amine anti-yellowing agents, which can be widely used in the production of polyurethane applications. For example, PU profiles, shoe materials, plastics and rolled paper fibers to prevent yellowing during processing.

The products are also commonly used as antioxidants, as a stabilizer and can participate in polymerization. It is also used as an anti-yellowing agent in polyurethane coatings. The main difference between the two products is the molecular weight and melting range.

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Frequently asked questions

A great number of manufacturers, including those that make high-grade sponge, have tried different solutions. Different antioxidants and light stabilizers have been added to improve the resistance of polyurethane flexible foam to yellowing, but so far to no avail.

Reasons why polyurethane turns yellow

The yellowing is caused by an oxidation reaction in the backbone of the polymer, triggered by antioxidants. UV light accelerates this oxidation process. Because it is the polymer itself which is being oxidized, the yellow color cannot be extracted.

When making urethane polymer, a polyol, water and an aromatic isocyanate are reacting. When an amine forms on one of the isocyanate groups instead of a new urethane linkage, this resulting aromatic amine is capable of being oxidized to a quinone. As the oxidation proceeds over time, the yellow color of the quinones are making the foam more yellow.

Yellowing of polyurethane occurs because of one of these reasons:

• High temperature (hot oxygen aging) during the manufacturing process or if the foam is exposed to heat in end use;
• Reaction with the nitrogen oxide (NOx) in the air, typically caused by from tow motor emissions, gas fired furnaces or pollution;
• Ultraviolet light;
• The presence of butylated hydroxy toluene (BHT), a commonly used antioxidant;

Fixing the problem

Antioxidants have both positive and negative effects. Normally polyol manufacturer will add antioxidants in the polyol to guarantee the safe production in downstream foam manufacturers. The domestic polyether producers typically use compound antioxidant system including BHT and amine antioxidants or phenol cyanogen antioxidants. But international producers mainly choose some big molecular weight hindered phenolic antioxidant, such as Omnistab AN .

We recommend to add 0.05-0.50% Omnistab AN to improve the ability of thermal pressure resistance.

Omnistab AN is a new development and efficiently addresses the issue of polyurethane flexible foam turning yellow, we expect this product will be widely used.

Are you interested in learning more about PU Anti-yellowing Agent? Contact us today to secure an expert consultation!