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A Contradictory Concurrence: Why Do "Slow Cure" and "Excessive Cracking" Occur Simultaneously?

Traditionally, "slow cure" typically indicates insufficient crosslink density, while "cracking" indicates excessive crosslink density and excessive internal stress. These two seem to be at opposite ends of the spectrum, so how can they coexist in the same formulation? This is the crux of the matter. It reveals an overlooked truth: what we pursue isn't a simple "average functionality" value, but rather a "properly distributed crosslinked network structure."

When a formulation is mismatched, the following disastrous polymerization processes are most common:

1. "Island" formation (a source of high stress): To achieve high hardness or reactivity, high-functionality monomers (such as trifunctional TMPTA and hexafunctional DPHA) are often added to the formulation. Under UV light, these highly reactive, multi-armed molecules will initially "cluster," instantly forming extremely dense and rigid "polymer islands" in localized areas.
2. The slow "ocean" (the appearance of slow curing): Meanwhile, the main resin in the formula (such as high-molecular-weight polyurethane acrylate (PUA) or epoxy acrylate (EA)) and low-functionality monomers lag far behind in polymerization due to steric hindrance, viscosity, or differences in reactivity. Together, they resemble a "viscous ocean" surrounding the "isolated island."
3. Disastrous consequences: Cracking (>20%): The first "islands" to form experience a huge volume shrinkage during the polymerization process. Because they are extremely rigid and cannot release stress through deformation, these stresses will accumulate and tear at the weak boundary between the "islands" and the "ocean", forming microcracks and eventually leading to macroscopic cracking. Slow curing: From an overall perspective, although some parts have been "over-cured" and cracked, the vast "ocean" area (main resin and low-activity monomers) has not yet fully reacted. This results in insufficient apparent hardness of the coating, incomplete surface drying, and even stickiness - this is the appearance of "slow curing".

High functionality does not equal high crosslink density. Formulators often mistakenly use high-functionality monomers to build crosslink density. The correct combination is a "homogeneous network" constructed from a balanced mix of high-, medium-, and low-functionality raw materials. Within this network, flexible segments (derived from PUAs or difunctional groups) are interspersed between rigid nodes (derived from high-functionality groups), creating a "rigid and flexible" structure.

• Reducing the average functionality: This is the most direct means. Replace some trifunctional and above monomers (such as TMPTA) with difunctional monomers (such as DPGDA, TPGDA).
• Introduce "flexible chain segments" - increase toughness: Add flexible polyurethane acrylate (PUA) polymers to the formula, especially aliphatic PUA. Their long chain structure is like a spring, which can effectively absorb and disperse the stress caused by curing shrinkage.
• Use "low Tg value" monomers: Introduce monomers such as IBOA (isobornyl ester). Although it is a single functional group, its large alicyclic structure can effectively improve the toughness of the coating and prevent cracking while providing good dilution.

• Optimize the "reactivity gradient": Make sure that the reactivity in your formula is "synergistic". If the reactivity of your main resin (oligomer) is relatively low, you need to use a highly active diluent monomer (such as TPGDA) to "drive" the entire reaction, rather than letting the high-functional group monomer "take the lead" and cause an "island" effect.
• "Speed ​​up" rather than "build up": Slow curing is sometimes not due to an insufficient total amount of functional groups, but rather insufficient "effective collisions". Properly increasing the reactivity of the system (for example, replacing the slower HDDA with TPGDA), or choosing an oligomer that is insensitive to oxygen inhibition, is more effective than simply increasing the number of functional groups.
• Matching the photoinitiator (auxiliary means): Under the premise of a reasonable network structure, ensure that the absorption peak of the photoinitiator matches the emission spectrum of the UV lamp. For colored or thick coating systems, use a long-wavelength initiator (such as TPO, 819) to ensure deep curing, which can effectively improve the problem of "drying on the surface but not drying on the inside" or slow overall curing.

UV curing formula design is a precise "material architecture". Cracking rates exceeding 20% ​​or slow curing are just manifestations of a building's "collapse". As formulators, our responsibility is not to "patch" the cracks (such as adding additives), but to return to the design blueprint and examine the load-bearing structure of this "building" - that is, the ratio of functionality and the distribution of cross-linking density. A successful UV formula must have a "homogeneous" and "strong" cross-linking network. It is not achieved by the "violent stacking" of one or two high-functionality raw materials, but by the perfect synergy of high, medium and low-functionality components in reaction kinetics.