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Balancing hardness and flexibility is a common challenge when developing UV-curable coatings, inks, adhesives, and specialty finishes. A coating with high hardness may provide better scratch resistance but become brittle during bending or impact. Conversely, a flexible film may withstand deformation but lack the surface durability required for demanding applications.
Achieving the right balance requires more than selecting a UV resin based on a single performance label. Resin chemistry, monomer selection, curing conditions, substrate properties, and the complete formulation all influence the final coating performance.
Why Hardness and Flexibility Must Be Evaluated Together
UV resin is a primary film-forming component that affects curing behavior, viscosity, hardness, flexibility, adhesion, scratch resistance, chemical resistance, and surface properties. Resin functionality is particularly important because higher functionality can increase cross-link density and hardness after curing. However, excessive cross-linking may reduce flexibility and increase brittleness.
The ideal balance depends on the application and substrate. Glass, metal, and rigid plastics may require a harder surface, while leather, TPU, flexible films, and other deformable substrates need sufficient elasticity and adhesion. Monomers, photoinitiators, additives, pigments, and fillers can further change the coating’s viscosity, curing behavior, film formation, and mechanical properties.
Curing technology must also be considered. Mercury UV, UV LED, excimer, dual-cure, moisture + UV, and UV + thermal or 2K systems can produce different curing profiles and film properties. Incomplete curing, excessive shrinkage, or uneven energy distribution may affect the final hardness-flexibility balance.
Lencolo Resin Candidates for Balanced Performance
Guangdong Lencolo New Material Co., Ltd., operating under the Lencolo brand, offers a catalog of 167 UV resin grades across 16 product families. Several products are positioned around combinations of hardness, flexibility, adhesion, wear resistance, or film formation. These grades should be considered candidate materials for application-specific testing rather than universal recommendations.
Modified Epoxy Acrylate UV Resin
L-6113 is a low-viscosity modified epoxy acrylate described as offering fast curing, excellent film formation, soft elasticity, high hardness, and good scratch resistance. With a functionality of 4 and a viscosity of 800–1,500 CPS, it is indicated for wood, paper, plastic, and metal coatings, as well as inks, OPV, PVD, and 3D printing applications. It may be evaluated when a formulation requires a combination of surface hardness and film flexibility.
Two-Functional Polyurethane Acrylate
L-6207 is positioned around balanced wear resistance and flexibility, together with a fast curing rate and RCA resistance exceeding 200 times. Its functionality is 2, with a viscosity of 28,000–38,000 CPS. The product is indicated for 3C coatings, glass, plastic, and PVD applications, where wear resistance and mechanical balance are important considerations.
Four- and Six-Functional Polyurethane Acrylates
L-6390 offers a documented balance of flexibility and hardness, along with water and heat resistance, adhesion, and other application properties. It has a functionality of 4 and a viscosity of 4,000–8,000 CPS at 60°C. Its indicated applications include 3C coatings, DIY adhesives, SPC flooring, nail polish, and 3D printing.
L-6601A is described as providing a high curing rate, high hardness and flexibility, and good temperature resistance. It is used as a candidate for PVD coatings, gloss varnishes, inks, DIY adhesives, 3D printing, and plastic applications. L-6601D is a related organotin-free grade with high hardness and flexibility positioning, together with a low-viscosity profile for formulations with specific raw-material requirements.
High-Performance Polyurethane Acrylate
L-8403 is designed for hard-texture transfer coatings and combines low shrinkage, flexibility, hardness, and pigment wettability. With a functionality of 2 and a viscosity of 20,000–50,000 CPS, it is indicated for adhesives, inks, coating-film transfer, and nail gel applications.
Formulation Design and Application Testing
The performance of any selected UV resin depends on how it interacts with the rest of the formulation. Monomer functionality and concentration can influence cross-link density and flexibility. Photoinitiator selection affects curing completeness, while additives, pigments, and fillers may influence surface properties, adhesion, and mechanical performance.
Formulators should evaluate hardness and flexibility alongside scratch resistance, impact resistance, adhesion, shrinkage, chemical resistance, yellowing, and curing completeness. Testing should reflect the actual application method, film thickness, substrate, target viscosity, and curing equipment.
Lencolo provides technical reference formulations and process parameters marked as “Status: Verified” and “Usage: Reference only.” These materials can support preliminary development, but results from one formulation or substrate should not be assumed to apply directly to another project.
A Practical Approach to Resin Selection
When application details are incomplete, a specific resin should not be selected solely by its functionality or product description. Instead, formulators can first identify suitable candidate families, then compare grades according to viscosity, functionality, curing method, substrate compatibility, and required hardness-flexibility balance.
Ultimately, successful UV coating development depends on evaluating resin selection as part of a complete formulation system. By combining documented technical data with application-specific trials, formulators can develop coatings that provide the required balance of hardness, flexibility, adhesion, and long-term surface performance.
https://www.lencolo37.com/
Guangdong Lencolo New Material Co., Ltd.
