Yellowing is one of the common quality issues in UV coating processes, particularly prominent when applying UV varnish to plastic surfaces. Not only does yellowing affect the aesthetic appearance of the product, but it can also reduce the weather resistance and service life of the coating. This article analyzes the main causes of yellowing in UV coating and provides corresponding solutions.
I. Possible Causes of Yellowing
- Effect of Photoinitiators
Photoinitiators generate active free radicals under UV irradiation to initiate double-bond polymerization reactions. However, some active free radicals or degradation products may migrate to the coating surface, leading to yellowing. - Chromophores in Oligomers and Monomers
Oligomers and monomers may contain chromophores such as ester groups, carbonyl groups, and ether bonds, which are prone to oxidation or degradation under UV light, thereby causing yellowing. - Influence of Organic Amines
Organic amine compounds used in formulations are susceptible to photooxidation under UV irradiation, leading to coating yellowing. Therefore, the use of reactive amines in plastic UV varnish coatings should be minimized or avoided. - Effects of Film Thickness and Curing Time
Excessively thick coatings or overly long curing times may result in incomplete UV curing, leaving unreacted monomers or photoinitiators that exacerbate yellowing.
II. Feasible Solutions
- Selection of Low-Yellowing Oligomers and Monomers
During formulation design, prioritize the use of oligomers and monomers with excellent yellowing resistance, such as aliphatic polyurethane acrylates, to reduce the risk of yellowing. - Optimization of Photoinitiator Selection and Dosage
Choose photoinitiators with lower yellowing tendencies and control their dosage to avoid excessive accumulation of degradation products. - Reduction or Elimination of Organic Amines
In plastic UV varnish formulations, it is recommended to minimize or avoid the use of organic amine compounds to reduce the risk of photooxidation-induced yellowing. - Control of Coating Thickness and Curing Time
Reasonably control the spray coating thickness to avoid excessive thickness that leads to incomplete UV curing. At the same time, optimize the curing time to ensure sufficient curing of the coating and minimize the impact of residual photoinitiators.
III. Conclusion
Yellowing in UV coating involves multiple factors, including the chemical structures of photoinitiators, oligomers, and monomers, the use of organic amines, and process parameters. By optimizing formulation design, selecting yellowing-resistant materials, and controlling process parameters, yellowing can be effectively reduced, improving the weather resistance and appearance quality of UV coatings.
In practical production, it is recommended to conduct test validations based on specific application scenarios to identify the most suitable solution, ensuring the long-term stability and aesthetic quality of UV-coated products.
Post time: Jun-28-2025
