Common Issues and Solutions Regarding Hardness in UV Coatings

 Common Hardness Issues and Their Manifestations in UV Coatings

• Insufficient Hardness:​ After curing, the coating surface remains soft and is easily scratched or dented when tested with a fingernail or a hardness pencil. This issue is commonly seen in matte wood finishes and plastic coatings, failing to meet the abrasion resistance required for daily use.
• Uneven Local Hardness:​ The hardness across the surface of a single workpiece is inconsistent—some areas meet the standard while others remain soft. This often manifests as insufficient hardness at edges, corners, or in areas with a thicker coating application, compromising the product’s visual uniformity.
• Post-Curing Hardness Attenuation:​ The coating initially meets the hardness standard, but the hardness noticeably decreases after 1-3 days of storage or rapidly degrades under high temperature or humid conditions. This phenomenon is frequently observed in UV coatings intended for outdoor applications.
• Imbalance Between Hardness and Flexibility:​ Excessive pursuit of hardness leads to a brittle coating that chips or cracks upon minor impact. This problem is particularly prominent on curved areas of substrates like plastic or wood.

Analysis of the Core Causes of Hardness Issues

Formulation and System Level

• Improper Resin Selection: The use of resins with low functionality or a low glass transition temperature (Tg), such as certain monofunctional acrylic resins, results in insufficient crosslink density after curing, leading to inherently low hardness.
• Imbalanced Ratio of Reactive Diluents: Excessive use of low-hardness diluents (e.g., IBOA, TPGDA) or diluents with insufficient functionality leads to a loose crosslinked network, which compromises coating hardness.
• Unreasonable Photoinitiator Combination: Relying on a single type of photoinitiator or using an insufficient amount can cause incomplete deep-layer curing. The coating may appear dry on the surface but is not fully crosslinked internally, resulting in inadequate hardness.
• Additive Interference: Excessive addition of leveling agents, defoamers, or the use of additives with poor compatibility with the system can disrupt the resin crosslinking structure and reduce coating hardness.

Application and Curing Level

• Excessive Coating Thickness: UV coating curing depends on UV light penetration. An excessively thick coating (e.g., exceeding 100μm) leads to incomplete deep-layer curing, creating only a hard surface shell with a soft underlying layer.
• Mismatched Curing Parameters: Insufficient UV lamp power (e.g., power below 80mW/cm² for 365nm wavelength), excessively fast conveyor belt speed (curing time less than 1-3 seconds), or UV intensity degradation due to lamp aging all negatively affect the completeness of curing.
• Inadequate Substrate Preparation: Substrate surfaces contaminated with oil, moisture, dust, or other impurities, as well as insufficient sealing treatment of substrates like wood or plastic, can lead to poor adhesion. During hardness testing, this can manifest as “false softness” (coating detachment rather than inherent softness).

Environmental and Substrate Compatibility Level

Influence of Ambient Temperature and Humidity: Application conditions with temperatures below 15°C or humidity above 70% can slow down the UV coating curing speed, resulting in insufficient crosslinking reactions and decreased hardness.

Substrate Property Incompatibility: Flexible plastics (e.g., PVC) or soft woods (e.g., pine) have inherently high resilience. Using a high-hardness UV coating on such substrates can lead to a mismatch between coating hardness and substrate flexibility, manifesting as seemingly insufficient hardness or coating cracking.

Targeted Solutions and Practical Recommendations

Optimize the Formulation System to Strengthen the Hardness Foundation

• Precision Selection of Resins and Diluents:​ Prioritize high-functionality resins (e.g., trifunctional acrylic resins, epoxy acrylate resins) paired with high-Tg monomers (e.g., TMPTA, HDDA) to enhance crosslink density. Control the addition of low-functionality diluents, keeping their content below 20% is recommended.
• Scientific Combination of Photoinitiators:​ Employ a “primary + auxiliary photoinitiator” combination (e.g., 1173 + TPO), with the total dosage controlled at 3% to 5%. This ensures effective photoinitiation at different depths of the coating. For pigmented UV coatings, amine co-initiators can be added to counteract UV light blockage by the pigments.
• Reasonable Control of Additive Dosage:​ Limit the addition of leveling agents, defoamers, and other additives to no more than 1% of the system. Prefer additives with good compatibility, such as fluorine-modified or silicone-modified types, to avoid interfering with the crosslinking reaction. If necessary, 0.5% to 1% of wear-resistant fillers like nano-silica can be added to physically enhance coating hardness.

Standardize Application and Curing Processes to Ensure Complete Curing

• Control Coating Thickness:​ Adjust the application thickness based on substrate characteristics. For wood coatings, 30-60µm is recommended; for plastic coatings, 20-40µm. Use uniform, thin coats when spraying or rolling to avoid local buildup.
• Optimize Curing Parameters:​ Ensure UV lamp power meets standards (recommended ≥120 mW/cm²). Adjust conveyor speed based on coating thickness to guarantee a curing time of 1.5-3 seconds. Regularly measure lamp intensity (replace aging lamps every 3 months) and keep lamp surfaces clean to prevent dust from blocking UV light.
• Strengthen Substrate Pre-treatment:​ Thoroughly clean substrates to remove contaminants before application. Wood substrates require a sealing primer to block moisture and oils. Plastic substrates can undergo corona treatment or receive an adhesion promoter to enhance bonding, preventing misjudgment during hardness testing.

Adapt to Environment and Substrate, Balance Hardness and Practicality

• Control Application Environment:​ Maintain an environment temperature of 18-25°C and relative humidity of 40-60%. Dehumidification equipment should be used if ambient humidity is high to prevent moisture from affecting cure.
• Match Substrate Characteristics:​ For flexible substrates (e.g., soft plastics, pine wood), use UV coatings with “moderate hardness and high flexibility,” avoiding the blind pursuit of high hardness. For exterior-use UV coatings, antioxidants and UV absorbers must be added to prevent hardness degradation from prolonged sun exposure.

Post-Application Inspection and Optimization: Establish Closed-Loop Management

• Standardize Hardness Testing:​ Employ standardized methods like pencil hardness (GB/T 6739) or durometer testing. Ensure the coating is fully cured (e.g., after 24 hours at ambient temperature) before testing to prevent false readings due to premature testing.

• Targeted Fine-Tuning:​ If localized hardness is insufficient, check for coating buildup and adjust the application method. If hardness degrades after curing, optimize the photoinitiator ratio or add anti-aging agents. If hardness and flexibility are imbalanced, consider slightly reducing resin functionality or adding a small amount of flexible monomer (e.g., HEA).

Summary