Yellowing Factors of Sponge

Analysis of Core Factors Affecting the Yellowing of Polyurethane Sponge

The yellowing of polyurethane soft foam has long been a challenge for both foam manufacturers and polyol suppliers. Many foam producers, particularly those specializing in high-end products, have attempted to enhance the anti-yellowing performance of foam by adding antioxidants and light stabilizers, but the actual improvement results have often been unsatisfactory.

Generally, from the perspective of additives, the yellowing of sponge includes the following four types:

Four Major Types of Yellowing and Their Mechanisms

1. Thermal Oxidative Yellowing

Cause: During foam formation (reaction temperatures can exceed 100°C) and subsequent processing (such as hot pressing or cutting), the polyurethane molecular chains undergo oxidative degradation under high temperature and oxygen, generating chromophores (e.g., quinone structures) that lead to yellowing.

Countermeasure: This is where antioxidants primarily play a positive role by interrupting the oxidative chain reaction to prevent degradation.

2. Gas Fume Yellowing

Cause: When sponge is exposed to air containing nitrogen oxides (NOx, mainly from vehicle exhaust and industrial emissions), amine compounds (particularly from amine-based antioxidants or catalysts) react with NOx to form yellow nitrosamines or azo compounds.

Countermeasure: Avoid or reduce the use of amine substances that readily react with NOx.

3. Fabric Staining

Cause: Certain small molecular substances (mainly volatile ones) in the sponge migrate to light-colored fabrics in contact with it (such as sofa covers or mattress fabrics), causing localized yellowing of the fabric.

Primary Cause: Data clearly indicates that the antioxidant BHT is the main cause of fabric staining. BHT itself can oxidize to form yellow substances, and its volatility facilitates migration to the fabric surface.

4. UV Aging Yellowing

Cause: Certain structures in the polyurethane molecular chains (such as urethane groups formed by aromatic isocyanates MDI/TDI) undergo photodegradation under ultraviolet light, forming chromophores.

Countermeasure: Adding light stabilizers (such as UV absorbers or hindered amine light stabilizers) is necessary to mitigate this, which goes beyond the capability of ordinary antioxidants.

Core Contradiction: The "Dual Role" of Antioxidants

Positive Effect: Prevents thermal oxidative yellowing during processing. Without antioxidants, the sponge may already degrade and yellow during production. This is the fundamental reason why polyol manufacturers must add antioxidants.

Negative Effects: Some antioxidants themselves act as "catalysts" or "reactants" for other types of yellowing.

Amine-based antioxidants: While effective against thermal oxidation, they exacerbate gas fume yellowing and light-induced yellowing.

BHT-based antioxidants: Although low-cost, they are the primary culprit behind fabric staining.

Root Cause and Solution Directions

Foam manufacturers often overlook the profound impact of the preset antioxidant system in upstream polyols on the yellowing performance of the final product.

Solution Directions:

Start with Raw Material (Polyol) Selection:

Inquire with polyol suppliers about the antioxidant systems they use. For high-end foam products with high anti-yellowing requirements, prioritize polyols with "low-volatility, high-durability" antioxidant systems that are non-BHT and non-amine.

Modern high-performance antioxidants, such as "hindered phenol + phosphite" blended systems, can provide good thermal oxidative stability while avoiding BHT's volatility issues and amine-related gas fume yellowing.

Optimize Formulation and Processing:

Carefully consider post-treatment antioxidants/light stabilizers: If the base polyol's antioxidant system is incompatible, simply adding additives later is often ineffective and may introduce new problems (e.g., adding amine-based antioxidants worsening gas fume yellowing).

Consider the type of isocyanate used: For outdoor products requiring extreme UV resistance, consider using aliphatic isocyanates (e.g., HDI) as the base for polyurethane. Their yellowing resistance is far superior to commonly used aromatic isocyanates (TDI/MDI), though at a significantly higher cost.

In summary, solving the yellowing issue cannot rely solely on "remedial" additive measures during foam production. Instead, it requires a systematic review of the entire supply chain from the perspective of materials science. Selecting polyols with antioxidant systems that match the yellowing resistance requirements of the final product is the most fundamental and effective approach to resolving this challenge.