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Common Issues in UV Vacuum Plating (10)

Release time:

2026-06-04 07:08

In the production process of UV vacuum plating, poor weather resistance is a critical issue that compromises the long-term performance of the product. This deficiency manifests as yellowing, loss of gloss, chalking, or coating corrosion after a period of use—phenomena commonly observed in outdoor applications or products exposed to prolonged sunlight. Such defects are often not readily apparent at the time of shipment but gradually emerge during transportation, storage, or service, posing significant risks to product quality. Understanding the symptoms and underlying causes of poor weather resistance helps identify associated risks throughout the manufacturing process.

I. Manifestations of Poor Weather Resistance

Poor weather resistance manifests differently at various stages. In the early phase, the primary symptom is a decline in coating gloss, with the surface gradually losing its original high-gloss, mirror-like finish and becoming dull. For white or light-colored coated products, yellowing occurs: the once‑pristine white surface progressively turns yellow, and in severe cases develops a yellowish‑brown hue. In transparent topcoats, haze and cloudiness appear, reducing transparency and rendering the underlying coating layer indistinct and blurred.

As aging progresses, the coating surface may develop chalking. Rubbing the surface with a finger will cause fine powdery material to detach. When chalking is severe, the coating gradually thins, the protective film is lost, and corrosion spots begin to appear. After oxidation, the aluminum coating loses its metallic luster and exhibits grayish‑white or black discoloration. If corrosion continues to spread, it can lead to blistering, cracking, or flaking of the coating.

II. Insufficient Yellowing Resistance of the Topcoat

Insufficient yellowing resistance of the topcoat is the primary cause of yellowing and loss of gloss. Under UV irradiation, the resin system in the topcoat can undergo cleavage of unsaturated bonds or oxidative reactions, leading to the formation of chromophoric groups. In aromatic polyurethane acrylates, the benzene ring structure is prone to oxidation under UV light, generating quinone‑type chromophores that impart a yellow hue to the coating. Aliphatic polyurethane acrylates exhibit superior yellowing resistance compared to their aromatic counterparts, but they are more expensive; low‑quality topcoats often use resins with poor yellowing resistance to reduce costs.

Ether linkages, ester linkages, and other functional groups in the resin may undergo photodegradation under the combined action of ultraviolet light and moisture, leading to molecular chain scission. Following chain cleavage, the crosslinking density of the coating decreases, resulting in reduced surface hardness and diminished gloss. Small-molecule degradation products may migrate to the coating surface, forming hazy white spots.

III. The Issue of UV Absorbers

UV absorbers are critical additives for delaying the photodegradation of coatings. When the dosage of UV absorber is insufficient, it cannot effectively absorb incoming ultraviolet radiation, allowing a substantial portion of UV rays to penetrate the topcoat and directly interact with resin molecules and the coating layer. Moreover, if an inappropriate UV absorber is selected—such as one whose absorption wavelength range does not match the emission spectrum of the light source—the absorption efficiency will be low, failing to provide adequate protective performance.

Ultraviolet absorbers themselves undergo degradation and consumption under prolonged exposure to light; when their concentration is insufficient, the duration of their protective effect is shortened, and the coating will exhibit signs of aging within a relatively short period. When hindered amine light stabilizers are used in combination with ultraviolet absorbers, they can produce a synergistic effect; conversely, relying solely on ultraviolet absorbers without the complementary action of hindered amine light stabilizers will also limit weathering resistance.

IV. Insufficient Density of the Topcoat

The density of the topcoat directly affects its protective performance for the coating layer. A topcoat with insufficient density contains microscopic pores or defects, allowing water vapor and oxygen to penetrate into the coating layer through these pathways. Once inside, the water vapor and oxygen react with the metal: the aluminum coating oxidizes to form alumina, losing its metallic luster. Alumina appears gray‑white, causing the coating to take on a grayish, hazy appearance and resulting in a loss of metallic reflectivity.

The causes of insufficient sealant integrity in the topcoat include a low crosslinking density in the resin system, inadequate solvent evaporation during coating application resulting in micropores, and an excessively thin coating film. A topcoat with low crosslinking density exhibits larger intermolecular gaps, allowing water vapor and oxygen to penetrate more readily. Furthermore, a topcoat that has not been fully cured will have an even lower crosslinking density, leading to poorer barrier performance.

5. Insufficient curing of the topcoat

The impact of insufficient topcoat curing on weatherability is multifaceted. A poorly cured topcoat exhibits low crosslink density and inadequate coating hardness, making it more susceptible to degradation by external environmental factors. Unreacted resins and monomers may undergo secondary reactions under light exposure and humid‑heat conditions, leading to yellowing or embrittlement of the coating. Furthermore, residual photoinitiators in an incompletely cured topcoat can continue to decompose upon illumination, generating free radicals that initiate degradation processes.

Insufficient curing of the topcoat can also result in inadequate cohesive strength, making the coating prone to microcracking under thermal and moisture‑induced stresses. These microcracks provide rapid pathways for water vapor and oxygen to penetrate, accelerating corrosion of the coating layer. A poorly cured topcoat may exhibit a tacky surface, increasing its tendency to attract dust and contaminants and thereby hastening coating degradation.

6. The coating layer has limited oxidation resistance.

The limited oxidation resistance of the coating layer itself is a fundamental reason for its poor weatherability. Aluminum is a commonly used material in vacuum coating; due to its highly reactive chemical nature, it readily oxidizes in humid environments. Although the alumina film formed upon oxidation of the aluminum coating provides some protective effect, this oxide layer lacks metallic luster, and the volumetric expansion that occurs during oxidation can lead to cracking in the topcoat.

The thinner the coating, the poorer its oxidation resistance. A thin coating contains fewer metal atoms, requiring less oxygen for oxidation and thus becoming more susceptible to oxidation in a shorter time. Defects in the coating, such as pinholes and microcracks, also serve as initiation sites for oxidation; the metal at these defects oxidizes preferentially and the oxidation gradually spreads outward.

VII. Accelerated Aging in Humid and Hot Environments

Humid and hot environments accelerate the aging of coated products. High temperatures increase the rate of chemical reactions, including the thermal oxidative degradation of resins and the corrosion of metals. The presence of moisture is a prerequisite for the corrosion of coating layers; aluminum coatings oxidize very slowly in dry conditions but at a markedly faster rate in humid environments.

The alternating cycles of wet and hot conditions pose an even more severe challenge to the weathering resistance of coatings. Under high‑temperature, high‑humidity conditions, the coating absorbs moisture and swells; during drying, it shrinks. This repeated swelling and shrinking generates cyclic fatigue stresses, which can lead to cracking. Moreover, the condensation and evaporation of water vapor within the coating cause volumetric changes, further accelerating its degradation.

VIII. Synergistic Effects of Ultraviolet Radiation and Moisture

The synergistic effects of ultraviolet radiation and moisture are more severe than the aging impacts of either factor alone. Ultraviolet irradiation cleaves resin molecular chains, generating free radicals and hydrophilic groups, which render the coating surface more susceptible to water adsorption. The adsorbed moisture further accelerates the hydrolytic degradation of the resin and the electrochemical corrosion of the coating. Microcracks induced by UV radiation provide pathways for moisture ingress, while the presence of moisture, in turn, diminishes the resin’s photostability.

This synergistic effect is particularly pronounced in outdoor applications. Outdoor products are simultaneously exposed to multiple environmental factors—such as sunlight, rain, dew, and temperature fluctuations—resulting in an aging rate far faster than that of indoor products. UV vacuum‑plated components that have not been specially designed for weather resistance typically exhibit noticeable yellowing, loss of gloss, and coating corrosion within just a few months of outdoor use.

9. Differences in Weather Resistance Among Coatings of Different Colors

The color of the topcoat significantly affects weather resistance. Dark-colored topcoats absorb more ultraviolet radiation, leading to higher internal coating temperatures and faster aging. Black topcoats demand particularly high weather resistance; their pigments exhibit strong UV absorption, causing elevated coating temperatures and accelerated resin degradation. In contrast, white and light-colored topcoats reflect most of the UV radiation, resulting in lower coating temperatures and a relatively slower rate of aging.

The inherent light stability of pigments also affects the weatherability of the topcoat. Certain organic pigments may fade or change color under UV irradiation, resulting in a shift in coating hue that, when combined with resin yellowing, further degrades the appearance. Inorganic pigments such as titanium dioxide exhibit good light stability; however, high loadings of titanium dioxide can induce photocatalytic degradation of the resin, accelerating its breakdown.

X. Conclusion

Poor weather resistance is a quality issue that gradually emerges during the service life of UV‑vacuum‑plated products, with its root causes spanning multiple factors, including the topcoat formulation, curing conditions, coating‑layer characteristics, and environmental influences. Insufficient yellowing resistance in the topcoat, inadequate UV‑blocking capability of the resin system, or an insufficient or improperly selected amount of UV absorber can lead to rapid yellowing and loss of gloss under light exposure. Inadequate topcoat density and incomplete curing allow moisture and oxygen to penetrate the coating layer, resulting in oxidation and corrosion of the aluminum plating. The coating layer itself has limited antioxidant capacity, and thin coatings are even more susceptible to oxidation. Moreover, the combined effects of humid‑heat environments and the synergistic action of UV radiation and moisture accelerate aging. Finally, topcoats of different colors exhibit varying degrees of weather resistance. Taken together, these factors cause plated products to progressively lose their original metallic luster and aesthetic appeal when exposed outdoors or subjected to prolonged use. Understanding the manifestations and underlying causes of poor weather resistance is the essential prerequisite for identifying this defect.

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.

Bosheng’s Recommended Products – Vacuum Plating
Primer
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-113	Bisphenol A epoxy acrylate	High hardness, high gloss, high fullness, containing 20% TPGDA.
B-151	Modified epoxy acrylate	Low halogen, yellowing-resistant, excellent plating performance, and strong adhesion.
B-160D	Modified epoxy acrylate	Good flexibility, yellowing resistance, and excellent adhesion.
B-163	Modified epoxy acrylate	Good flexibility, excellent pigment wetting, and strong adhesion.
B-165	Modified epoxy acrylate	Good flexibility and strong adhesion
B-212A	Aromatic polyurethane acrylate	High cost-performance, excellent plating performance, good toughness, and resistant to boiling water.
B-221	Aliphatic polyurethane acrylate	Fast curing, resistant to boiling water
B-268M	Aliphatic polyurethane acrylate	Good flexibility, excellent adhesion, superior plating performance, and strong hiding power.
B-574C	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
Intermediate coat
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-374	Aliphatic polyurethane acrylate	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
B-601	Aromatic polyurethane acrylate	High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness.
B-6020	Special functional group acrylate	Resistant to boiling water, excellent color development, and strong interlayer adhesion.
Topcoat
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-221	Aliphatic polyurethane acrylate	Fast curing, resistant to boiling water
B-301	Aromatic polyurethane acrylate	Fast curing, excellent toughness, and good sandability.
B-302	Aromatic polyurethane acrylate	Fast curing, high strength, excellent toughness, and good grindability.
B-368	Aliphatic polyurethane acrylate	Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance.
B-374	Aliphatic polyurethane acrylate	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
B-574C	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
B-601	Aromatic polyurethane acrylate	High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness.
B-6016C	Special functional group acrylate	Easy to apply, resistant to yellowing and boiling water, and improves the appearance of the paint film.
B-6019	Special functional group acrylate	Good leveling, excellent wetting, resistant to boiling water, and superior color dispersion.
B-609	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch resistance, and chemical resistance.
B-615A	Aliphatic polyurethane acrylate	Fast curing, excellent toughness, wear resistance, and chemical resistance.
B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
B-6210	Aliphatic polyurethane acrylate	Low viscosity, chemical resistance, environmental resistance, and dual photothermal curing
B-6211	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch-resistant, and free of organotin.
B-919B	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, and superior chemical and wear resistance.
Monomer Recommendation
Product Model/English Abbreviation	Product Name/Product Type	Product Features
BM2223 (TPGDA)	Di(propylene glycol) diacrylate	Good flexibility and low volatility
BM3231 (TMPTA)	Trimethylolpropane triacrylate	High crosslink density, high hardness, high gloss, and excellent wear resistance.
BM3235 (PET3A)	Pentaerythritol triacrylate	Fast curing, high crosslink density, high hardness, and chemical resistance.
BM3380 (3EO-TMPTA)	Pentaerythritol triacrylate	More flexible and less irritating than TMPTA.
BM6261 (DPHA-80)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.
BM6263 (DPHA-90)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.

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Common Issues in UV Vacuum Plating (Part 11)

Common Issues in UV Vacuum Plating (Part 9)