In the paint and chemical industry, solvents play a critical role. They affect solubility, evaporation rate, and product stability. However, a common issue is solvent yellowing during storage or use.
This phenomenon not only reduces aesthetic quality but can also impact final product performance. Especially for light-colored paint systems, color deviation poses a significant risk. Therefore, understanding the causes and solutions is essential for businesses.
OVERVIEW
Solvent yellowing is a color change phenomenon in which a clear solvent turns light yellow or dark yellow. This typically occurs during storage but may also happen when the solvent is exposed to air or light.
The degree of yellowing can vary. In some cases, the change is slight. In others, the color shift is significant. In the paint industry, this is a serious issue because solvents can directly affect the color of the coating.
MAIN CAUSES OF SOLVENT YELLOWING
Aldehyde and Peroxide Impurities in Aromatic Solvents
Aromatic hydrocarbon solvents—such as Toluene, Xylene, and Solvent Naphtha—are the most commonly used in industrial paint production. These solvents offer high solvency and are widely used in alkyd, phenolic, and epoxy systems.
However, the benzene ring in their molecular structure creates a weakness. When exposed to air under improper storage conditions, aromatic hydrocarbons undergo partial oxidation. This reaction forms carbonyl compounds—especially aldehydes and peroxides—which exhibit a characteristic pale yellow color. Even at very low concentrations (a few tens of ppm), these impurities can cause noticeable discoloration.
Contamination from Storage Tanks and Pipelines
Carbon steel tanks that are not cleaned periodically are the most common source of contamination in plants. Residual resins from previous batches, iron oxide layers inside the tank, or accumulated water at the bottom can all dissolve into fresh solvent. Iron (Fe) acts as a strong catalyst for oxidation reactions. Even a few ppm of iron ions can significantly accelerate yellowing.
Pipelines made from unsuitable materials can cause similar issues. Aromatic solvents like Xylene can dissolve inner coatings of low-quality plastic pipes, generating colored impurities.
Improper Storage Conditions (Temperature and Light)
High temperatures accelerate chemical reactions. Solvent storage areas can reach 38–42°C in summer—much higher than standard storage conditions recommended by manufacturers (typically below 30°C).
Direct sunlight exposure increases localized temperature and provides energy to initiate photo-oxidation reactions. Dark steel drums help minimize this effect, while white or transparent plastic containers stored outdoors without cover present the highest risk.
Unsealed Containers — Oxygen and Moisture Ingress
This is the simplest yet most common cause. Partially opened drums, improperly sealed discharge valves, or damaged gaskets allow continuous exposure to oxygen and moisture.
In Vietnam’s humid climate, moisture readily interacts with impurities. As a result, aldehydes promote further reactions, accelerating yellowing. For solvents used in isocyanate (PU) systems, moisture ingress can also trigger side reactions with NCO groups, leading to white precipitation or gelation directly in the container.
TESTING AND EVALUATING THE DEGREE OF YELLOWING
Testing should be conducted periodically to detect issues early. A common method is visual color assessment, though it is only qualitative.
In practice, companies often use standard color scales, such as the Hazen (APHA) scale. Spectrophotometric analysis can also be used for more accurate results. For deeper investigation, laboratory analysis of chemical composition can help identify specific causes.
TREATMENT METHODS
Light Yellowing, APHA 20–50
The solvent can still be used if intended for equipment cleaning or dilution in dark-colored paints (black, brown, dark blue). In these cases, the solvent’s base color does not affect the final paint color. However, it must not be used for white paints, pastel paints, or clear PU systems.
Severe Yellowing, APHA > 50 or with Sediment
Do not use for production. The batch should be isolated. Samples should be sent to the laboratory for root cause analysis before deciding on disposal or reprocessing.
Entire Supplier Batch Shows Yellowing
This issue must be addressed with the supplier. Request the Certificate of Analysis (CoA) for the corresponding production batch. Compare actual laboratory results with the provided CoA specifications. Any discrepancy serves as a basis for claims and replacement.
TRENDS TO REPLACE AROMATIC SOLVENTS IN THE PAINT INDUSTRY
A notable trend is the gradual replacement of aromatic solvents in many paint plants due to their tendency to yellow. As a result, alternative solvents are increasingly preferred, especially those with lower VOC levels.
Among them, glycol ethers and esters are widely used. These solvent groups are less prone to oxidation, resulting in more stable color—especially during long-term storage.
CONCLUSION
Solvent yellowing is not a minor issue. It directly affects paint color and quality, particularly for white and light-colored coatings. The causes are typically related to impurities in raw materials, improper storage, and contaminated storage systems.
Sustainable solutions focus on three key aspects. First, strict control at incoming inspection. Second, proper storage conditions. Third, selecting suppliers with transparent CoA documentation. This is not an additional cost but a preventive measure against greater risks—especially costs incurred at the final stage of production.
VICHEM – AUTHORIZED DISTRIBUTOR OF CHEMICAL ADDITIVES FROM MAJOR BRANDS SUCH AS COVESTRO, SINOPEC, BUHLER,… IN VIETNAM
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