In two-component coating systems, the curing agent determines the durability, gloss, and service life of the coating film. However, yellowing often occurs after storage or use. The film becomes cloudy, loses transparency, and changes tone. The main cause lies in the oxidation process within the curing component. Understanding this mechanism and controlling the reaction helps maintain coating quality over time.
CAUSES OF YELLOWING IN CURING AGENTS
Oxidation reaction in the isocyanate group
In PU systems, the isocyanate group (–NCO) is highly reactive and easily reacts with oxygen and moisture in the air. This process forms by-products such as urea or amine, which appear as a yellowish-brown tint. After film formation, side reactions continue slowly, leading to discoloration over time. TDI and MDI-based hardeners are most affected due to their aromatic structure, while HDI and IPDI types show superior color stability.
Influence of UV light and temperature
UV light strongly accelerates polymer oxidation. When exposed to UV rays, polymer chains break down and generate free radicals that alter the chemical structure. These radicals react with oxygen to form yellow or light-brown compounds. At higher temperatures, the reaction rate increases significantly, especially in volatile solvent environments. Outdoor coatings or surfaces exposed to heat tend to yellow faster than indoor coatings.
Unstable catalysts and additives
Some catalysts such as tertiary amines, dibutyltin dilaurate, or tin accelerators can promote oxidation. When overdosed, they react with isocyanates to form colored by-products. Metallic impurities or solvents containing aldehydes also increase the yellowing rate. To minimize this, neutral catalysts should be used, additive levels must be controlled, and raw materials kept pure before blending.
EFFECTS OF YELLOWING IN COATINGS
Loss of clarity and gloss in the coating film
When the polymer oxidizes, its light-reflecting ability decreases sharply. The coating loses clarity, becomes dull, and takes on a yellow hue. For clear coatings, this causes visible base color distortion and reduces product aesthetics. In wood or high-end plastic coatings, transparency and gloss are key factors — even slight tone changes are considered quality defects.
Reduced surface protection
Oxidation breaks polymer chains in the coating network. Once the structure weakens, resistance to chemicals, moisture, and UV radiation declines. The coating becomes prone to chalking, cracking, or peeling under harsh environments. For exterior coatings, yellowing often comes with reduced hardness and adhesion, directly affecting long-term performance.
SOLUTIONS TO REMEDY YELLOWING IN CURING AGENTS
Use of non-yellowing polyisocyanate systems
The most effective solution is replacing aromatic systems with aliphatic polyisocyanates. HDI and IPDI-based curing agents have light-stable structures and resist oxidation under UV exposure. Desmodur N75 and Bayhydur from Covestro are prime examples. When reacted with polyols, they form a transparent, hard, and color-stable polymer network. The resulting coating maintains high gloss and long-term color retention under light.
Enhancing antioxidants and UV stabilizers
UV absorbers and HALS (hindered amine light stabilizers) are common additives in anti-yellowing coatings. UV absorbers capture harmful radiation before it breaks polymer bonds, while HALS neutralize free radicals formed during oxidation. The synergy between the two protects the film from photo-oxidative degradation. However, dosage must be precisely balanced, as excess can affect drying time and adhesion.
Controlling storage and application conditions
Curing agents should be stored in dry, cool areas away from sunlight. Containers must be airtight to prevent moisture absorption, as water reacts with isocyanate even in sealed drums. During mixing, the ratio must be accurate and the mixture thoroughly stirred for uniform dispersion. Applying the coating in low humidity and stable temperature reduces side reactions, keeping the film clear and colorfast for longer.
VICHEM – EXCLUSIVE DISTRIBUTOR OF COVESTRO CURING AGENTS AND RESINS IN VIETNAM
Vichem is the exclusive distributor of Covestro products such as Desmodur, Desmophen, and Uralac in Vietnam. We not only supply high-quality materials but also deliver international-standard technical services.
- Technical consulting: Helping customers select suitable grades for each coating system.
- Color testing: Vichem’s experts conduct lab trials to ensure accurate and consistent shade results.
- Application support: Assisting customers in integrating materials into production lines for optimal efficiency.
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