Hardness and flexibility are often treated as competing requirements in UV-curable coatings. A coating with high hardness may provide better scratch and wear resistance but become brittle when the substrate bends or experiences impact. A highly flexible film can tolerate deformation more effectively but may lack the surface durability required for demanding applications.

This balance is particularly important for coatings used on plastics, glass, metal, wood, 3C electronics, PVD components, flooring, and other substrates where both surface performance and mechanical durability are required.

Why Hardness and Flexibility Are Difficult to Balance

UV resin is one of the primary film-forming components in a UV-curable formulation. Its functionality, chemical structure, viscosity, curing response, hardness, flexibility, adhesion, and surface properties can all influence the final coating.

However, resin selection alone does not determine the final result. UV monomers, photoinitiators, additives, pigments or fillers, substrate characteristics, film thickness, application method, and curing conditions can all affect the hardness-flexibility balance.

Functionality and Cross-Link Density

Resin functionality is closely related to the cross-linked network formed during UV curing. Higher-functionality oligomers can generate denser networks and contribute to higher hardness.

At the same time, excessive cross-link density may reduce flexibility or increase brittleness. For this reason, functionality should be selected according to the required mechanical balance rather than treated as a simple indicator of better performance.

The Lencolo UV resin portfolio includes grades ranging from difunctional products to resins with functionality up to 15, allowing different approaches to cross-link density and mechanical performance.

Substrate and Application Conditions

The required balance changes with the substrate.

Rigid materials such as glass and metal may accommodate harder films, while plastics, flexible films, leather, or soft-touch surfaces may require greater elasticity and toughness. Flooring coatings may need both flexibility and abrasion resistance because the cured film experiences repeated mechanical stress.

Application method also matters. Spray, roller, curtain, flexographic, offset, screen, inkjet, and adhesive applications can require different viscosity and film-forming characteristics.

Curing Method

Mercury UV, UV LED, Excimer + UV, Dual Cure, Moisture + UV, and UV + Thermal/2K systems can produce different curing profiles.

The resin must therefore be considered together with the photoinitiator system, UV wavelength, curing energy, exposure time, and any secondary curing process. A formulation that provides the desired hardness under one curing condition may behave differently under another.

Lencolo UV Resins for Hardness-Flexibility Balance

Several Lencolo UV resin grades are documented with properties related to both hardness and flexibility. They can serve as starting points for formulation evaluation.

L-6113 Modified Epoxy Acrylate

L-6113 is a low-viscosity modified epoxy acrylate designed around a balance of soft elasticity and high hardness. It is also described with fast curing, good film formation, and good scratch resistance.

The resin has a functionality of 4, an acid value of 4–8, and a viscosity of 800–1,500 CPS. Its indicated applications include wood, paper, plastic, and metal coatings, inks, OPV, PVD, and 3D printing.

L-6207 Two-Functional Polyurethane Acrylate

L-6207 combines wear resistance and flexibility with a fast curing rate and RCA resistance of more than 200 times.

It has a functionality of 2, an acid value of ≤2, and a viscosity of 28,000–38,000 CPS. Its applications include 3C coatings, glass, plastics, and PVD coatings.

This type of resin can be considered when surface durability needs to be combined with a certain degree of flexibility.

L-6390 Four-Functional Polyurethane Acrylate

L-6390 provides a documented balance of flexibility and hardness together with good adhesion, water resistance, and heat resistance.

It has a functionality of 4, an acid value of ≤1, and a viscosity of 4,000–8,000 CPS at 60°C. Application areas include 3C coatings, DIY adhesives, SPC flooring, nail polish, and 3D printing.

L-6601A and L-6601D Six-Functional Grades

L-6601A is positioned around a high cure rate, high hardness, and flexibility, together with temperature resistance. It has a functionality of 6 and a viscosity of 2,500–5,000 CPS at 60°C.

Its applications include PVD coatings, gloss varnishes, inks, DIY adhesives, 3D printing, and plastics.

L-6601D, an organotin-free related grade, also provides high hardness and flexibility with a viscosity of 20,000–36,000 CPS. It can be considered when organotin-related formulation restrictions need to be addressed.

L-8403 High-Performance Polyurethane Acrylate

L-8403 is a two-functional polyurethane acrylate positioned for hard-texture transfer coatings. It combines low shrinkage, flexibility, hardness, and good pigment wettability.

Its viscosity is 20,000–50,000 CPS, with applications including adhesives, inks, coating-film transfer, and nail gel.

Why Formulation Testing Is Still Necessary

A resin described as having good hardness and flexibility does not guarantee the same balance in every formulation. Monomer selection can change flexibility and viscosity, while photoinitiators influence curing response. Additives, pigments, fillers, substrate treatment, and coating thickness can further change the final film properties.

For this reason, candidate resins should be evaluated under the actual formulation and processing conditions.

Useful evaluation parameters may include:

• Surface hardness

• Scratch and abrasion resistance

• Flexibility or bending resistance

• Adhesion

• Chemical and water resistance

• Curing speed

• Gloss and appearance

• Yellowing and weather resistance

Where several grades meet the basic requirements, comparative testing under identical conditions can provide a clearer basis for final selection.

A Practical Approach to UV Resin Selection

Hardness-flexibility balancing is ultimately an application-specific formulation problem. The substrate, target mechanical properties, curing technology, viscosity requirements, and complete formulation system should be considered together.

Guangdong Lencolo New Material Co., Ltd., operating under the Lencolo brand, offers 167 UV resin grades across 16 product families, with product-level technical information covering functionality, viscosity, key characteristics, and typical applications.

Grades such as L-6113, L-6207, L-6390, L-6601A, L-6601D, and L-8403 provide different starting points for formulations requiring a controlled combination of hardness and flexibility. Final suitability should be confirmed through formulation trials and application testing under the intended substrate and curing conditions.

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Guangdong Lencolo New Material Co., Ltd.