company news about Does a higher Tg make the material more brittle? The real causes of UV coating cracking and adhesion failure

In the field of UV curing, when engineers encounter issues such as “cracking” or ‘delamination’ in coatings, their first thought is often: “Could it be that the resin’s Tg (glass transition temperature) was set too high, making the film too brittle?” While it is true that Tg is a key indicator affecting flexibility, simply attributing cracking to a “high Tg” is actually a misconception. Today, we’ll start from the fundamental principles to identify the true “culprit” behind UV coating failure.

A high Tg does not necessarily mean the material is “brittle.” Tg is the critical temperature at which polymer chains transition from a frozen state to a mobile state. The advantages of a high Tg include high hardness, wear resistance, and good solvent resistance. The advantages of a low Tg include good flexibility and impact resistance. Why do materials with high Tg tend to crack? It is not due to the numerical value itself, but because a high Tg is often accompanied by high crosslinking density. When the crosslinking density is too high, the free volume within the material is severely compressed, preventing molecular chains from absorbing stress through microscopic displacement. However, this is merely a surface-level phenomenon; the real culprits are the following two core factors.

UV curing is a process that completes within a fraction of a second. As monomers and oligomers transition from a liquid to a solid state, the distance between molecules decreases abruptly, resulting in severe volumetric shrinkage (typically 5%–15%).

Accumulation of Shrinkage Stress: If the UV lamp power is too high or the initiator ratio is imbalanced, the internal stress generated by instantaneous curing cannot be released in time.

Result: When the internal stress exceeds the coating’s own cohesive strength, the coating cracks; when the internal stress exceeds the bond strength between the coating and the substrate, the coating peels off (adhesion failure).

This is a point many manufacturers tend to overlook: uneven curing.

Surface Overcuring: If the short-wavelength (around 254 nm) energy of the UV lamp is too strong, a dense, hard shell (a layer with an extremely high Tg) will form instantly on the surface.

Undercuring of the underlying layer: However, the long-wavelength light (365–395 nm) does not penetrate sufficiently, leaving the ink or coating at the bottom still “soft.”

Result: This “hard on top, soft on bottom” structure causes the surface hard shell to fail to deform synchronously when the substrate undergoes slight bending or temperature changes, instantly generating microcracks that spread rapidly.

Simply lowering the Tg will compromise wear resistance; the true optimization strategy lies in striking the right balance.

• Formulation Optimization: Introduce monofunctional monomers to reduce crosslinking density, or use flexible resins with an “internal plasticizing” effect.
• Shrinkage Control: Select oligomers with low shrinkage to minimize internal stress at the source.
• Precise Control of the UV Curing Process (Critical)
  • Stepwise curing: First, use low-energy pre-curing to release some stress, then use high-energy curing to fully cure the coating.
  • Spectral matching: Ensure the UV lamp’s spectrum can penetrate the coating. For example, for thick coatings or pigmented paint systems, UV LEDs in the 395 nm/405 nm bands—which offer greater penetration—must be used to ensure the underlying layer is thoroughly cured.
  • Heat Control: Excessive infrared heat accelerates substrate deformation, which in turn strains the coating. Selecting cold-light-source UV LEDs can significantly reduce the risk of cracking caused by thermal deformation of the substrate.

Tg is merely a reference point; stress balance is the true core of the process. As a professional supplier of UV curing equipment, we offer not only “light,” but also curing solutions featuring precise temperature control and spectral matching. If you encounter issues such as wrinkling, paint peeling, or cracking during printing or coating processes, please feel free to contact us. Our technical team is ready to provide you with customized, one-on-one testing services.