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Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 3)

Release time:

2026-10-08 23:39

Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 3)

Whitening or haze in coatings is a common defect in UV‑curable resin applications on difficult‑to‑adhere substrates, compromising optical performance. It manifests as reduced transparency in the cured film, resulting in a milky, whitish appearance. This issue is particularly critical in transparent coatings and optical applications, directly impacting both the visual quality and the optical functionality of the product. In coating systems for challenging substrates, the root causes of whitening share similarities with those observed in conventional UV resins; however, factors such as substrate surface treatment, the incorporation of adhesion promoters, and the use of modified resins introduce distinct nuances.

I. Water Contamination

Moisture ingress is a common cause of whitening in coatings, with effects that persist throughout the entire process—from storage and application to curing.

When the construction environment has excessively high humidity, the coating may absorb moisture from the air. Certain components in UV‑curable resins used on substrates with poor adhesion exhibit a degree of hydrophilicity and can absorb water when exposed to humid conditions for extended periods. The absorbed moisture may form microscopic vapor bubbles during curing, acting as light‑scattering centers and causing the coating to appear whitish or hazy. Additionally, moisture can impair the activity of photoinitiators, reducing curing efficiency and further exacerbating the whitening issue.

For substrates that are difficult to adhere to, the treated surface may absorb moisture from the environment during storage, further increasing the risk of whitening. Substrates subjected to corona or plasma treatment exhibit higher surface activity and are more prone to adsorbing water molecules from the air. Residual moisture on the substrate surface comes into contact with the coating during lamination; as the coating cures, this moisture vaporizes upon heating, forming microbubbles or acting as scattering centers within the coating. Additionally, surface moisture can impair the coating’s wetting and spreading, leading to localized application inconsistencies and the formation of hazy spots.

The construction environment’s relative humidity must be maintained within an appropriate range to ensure that the substrate surface is thoroughly dry. For systems sensitive to moisture, the substrate should be dried prior to coating, and application should be carried out in a low-humidity environment. Coated substrates should be finished as soon as possible to prevent prolonged storage, which could lead to moisture absorption on the surface.

II. Insufficient Curing

Insufficient curing can also cause the coating to turn white, a phenomenon whose mechanism is related to the scattering of light by residual monomers.

When the curing energy is insufficient, unreacted monomers remain within the coating. These low‑molecular‑weight substances act as light‑scattering centers, causing light to scatter inside the coating rather than pass through directly, which results in a hazy appearance. The higher the residual monomer content, the more pronounced the whitening effect. A poorly cured coating may exhibit tackiness on its surface, further compromising both its appearance and performance; moreover, it can attract dust, exacerbating the visual cloudiness.

The causes of insufficient curing energy can stem from multiple factors. A common issue is the attenuation of the light source’s output; UV lamps gradually age during use, leading to a steady decline in emitted energy. Insufficient irradiation time also results in inadequate curing—adjustments to production cycle times or operator oversight may cause workpieces to remain under the light for too short a duration. For dark‑colored systems, the opacity of pigments impedes UV penetration, making it even more challenging to achieve thorough deep‑layer curing.

Ensure that the curing energy is sufficient to achieve complete crosslinking of the coating. Regularly monitor the light source’s output power, and replace aging lamps promptly. For dark-colored systems or thick coatings, select a deep‑penetration photoinitiator and appropriately extend the curing time.

III. Compatibility Between Adhesion Promoters and Resin Systems

When the adhesion promoter exhibits poor compatibility with the resin system, phase separation may occur, leading to the formation of a microscopically dispersed scattering structure.

Adhesion promoters introduced into UV‑curable resins for difficult‑to‑adhere substrates—such as chlorinated polyolefins, phosphate esters, or organosilicon‑modified components—may fail to disperse uniformly in the coating if their compatibility with the base resin system is inadequate. In regions where their local concentration becomes excessively high, microscopic phase‑separation structures can form; when these structures attain a certain size, they scatter light, resulting in a hazy appearance of the coating.

Poorly compatible components may also gradually precipitate during storage, leading to changes in the coating’s appearance and performance. The introduction of certain fluorinated or organosilicon‑modified components can likewise affect the optical transparency of the coating. Fluorinated and organosilicon segments exhibit refractive indices that differ from that of the acrylic matrix; if these segments are not uniformly dispersed, they can give rise to optical inhomogeneities, thereby reducing the coating’s transparency.

When designing a formulation, it is essential to consider the compatibility of each component and select a raw-material system that exhibits mutual compatibility. If necessary, a compatibility enhancer may be added, or an appropriate amount of a co-solvent can be incorporated into the formulation to improve system homogeneity.

IV. Other Influencing Factors

In addition to the aforementioned primary causes, several other factors may also affect coating transparency.

The storage condition of the coating deserves close attention. Coatings that have expired or been stored improperly may undergo partial pre‑polymerization, leading to the formation of microscopic gel particles within the system. These particles act as scattering centers after application, resulting in whitening. The coating should be used within its shelf life and stored away from light as specified.

The surface condition of the substrate also affects the appearance. Scratches or defects on the substrate surface may be amplified after coating, resulting in a hazy visual effect. Differences in refractive index between the substrate and the coating likewise influence optical performance; systems with better refractive-index matching exhibit superior transparency.

V. Conclusion

Whitening or haze in coatings is a critical optical defect that requires close attention in UV‑curable resin applications on substrates with poor adhesion. Its causes include moisture ingress, insufficient curing, poor compatibility between adhesion promoters and the resin system, and improper storage conditions. Moisture incorporation creates microbubbles or scattering centers; inadequate curing leaves residual monomers that scatter light; poor compatibility leads to microscopic phase separation; and improper storage can result in gel particles. These issues are interrelated and must be systematically investigated during production. By controlling ambient humidity, ensuring thorough curing, optimizing formulation compatibility, and adhering to proper coating‑storage protocols, the occurrence of whitening can be effectively minimized, thereby safeguarding both the aesthetic quality and the optical performance of the final product.

Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.

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Typical Defects of UV Resins on Difficult-to-Adhere Substrates (Part 2)