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Q: Why is balancing hardness and flexibility so difficult in UV-curable coatings?

A: Hardness and flexibility are often competing requirements in UV curing. A highly cross-linked coating can provide good hardness, scratch resistance, and surface durability, but excessive cross-linking may make the cured film brittle. On the other hand, a highly flexible film can tolerate bending and impact but may have lower hardness or wear resistance.

UV resin is one of the main film-forming components in a UV formulation, so resin chemistry and functionality can strongly influence this balance. However, the final properties are not determined by the resin alone. UV monomers, photoinitiators, additives, pigments or fillers, substrate, film thickness, and curing conditions all contribute to the final performance.

Q: Does higher functionality always mean higher hardness?

A: Not necessarily. Higher-functionality UV resins can form a denser cross-linked network during curing, which may improve hardness and surface durability. However, increasing functionality without considering flexibility and adhesion can also increase brittleness.

The required balance depends on the substrate and application. Glass, metal, and some PVD coatings may require greater surface hardness, while plastics, leather, flooring materials, and other flexible substrates may require greater elasticity and adhesion.

Therefore, functionality should be treated as one formulation variable rather than a standalone indicator of final coating performance.

Q: Which UV resin grades can be evaluated for a hardness-flexibility balance?

A: Several grades in the Lencolo UV resin portfolio can be used as candidate materials for comparative testing.

L-6113 is a modified epoxy acrylate with functionality 4 and viscosity of 800–1,500 CPS. It is described with fast curing, good film formation, soft elasticity, high hardness, and scratch resistance. Its indicated applications include wood, paper, plastic, and metal coatings, inks, OPV, PVD, and 3D printing.

L-6207 is a two-functional polyurethane acrylate with a viscosity of 28,000–38,000 CPS. It combines wear resistance and flexibility with fast curing and RCA resistance above 200 times, with applications in 3C coatings, glass, plastic, and PVD.

L-6390 is a four-functional polyurethane acrylate offering water and heat resistance, good adhesion, and a documented balance of flexibility and hardness. It can be evaluated for 3C coatings, SPC flooring, DIY adhesives, nail products, and 3D printing.

Q: Are there higher-functionality options for applications requiring both hardness and flexibility?

A: L-6601A is a six-functional polyurethane acrylate positioned around high curing speed, high hardness and flexibility, and temperature resistance. Its viscosity is 2,500–5,000 CPS at 60°C, with indicated applications including PVD, gloss varnish, inks, DIY adhesives, 3D printing, and plastics.

L-6601D is a related organotin-free grade with high hardness and flexibility and a viscosity of 20,000–36,000 CPS. It can be included in formulation screening where an organotin-free resin is required.

For transfer-coating applications, L-8403 is a two-functional polyurethane acrylate characterized by low shrinkage, good flexibility, good hardness, and pigment wettability. Its viscosity is 20,000–50,000 CPS, with applications including adhesives, inks, coating-film transfer, and nail gel.

These grades should be treated as candidate materials rather than universal solutions.

Q: How do curing conditions affect the hardness-flexibility balance?

A: Curing conditions can significantly change the final properties of a UV coating. Mercury UV, UV LED, Excimer + UV, Dual Cure, Moisture + UV, and UV + Thermal/2K systems can produce different curing profiles.

Photoinitiator selection, UV wavelength, curing energy, exposure time, and film thickness should therefore be evaluated together with the resin. Incomplete or uneven curing may result in lower surface hardness or inconsistent mechanical properties, even when the resin has suitable technical characteristics.

The application method also matters. Spray coating, roller coating, curtain coating, flexographic printing, offset printing, screen printing, inkjet, and adhesive applications can require different viscosity and film-forming behavior.

Q: What should be tested before selecting the final UV resin?

A: Technical data is useful for identifying candidate materials, but final selection should be based on application testing. Important evaluation points include:

• Hardness and scratch resistance

• Flexibility and bending resistance

• Adhesion to the actual substrate

• Abrasion resistance

• Curing speed and completeness

• Chemical and water resistance

• Gloss and surface appearance

• Yellowing and weather resistance

• Viscosity and application behavior

A reference formulation that performs well under one set of conditions should not automatically be assumed to perform identically on another substrate or with a different curing system.

A practical approach is to first define the substrate, coating or printing method, curing technology, film thickness, and required hardness-flexibility balance. Candidate UV resins can then be compared through controlled formulation trials.

For formulators evaluating UV coatings, inks, adhesives, flooring, PVD, plastics, or 3D printing applications, grades such as L-6113, L-6207, L-6390, L-6601A, L-6601D, and L-8403 provide different starting points for hardness-flexibility evaluation. Final suitability should be confirmed under the actual formulation, substrate, application, and curing conditions.

https://www.lencolo37.com/
Guangdong Lencolo New Material Co., Ltd.

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