Free-Radical Curing Mechanism
Used for: Acrylates, methacrylates, unsaturated polyesters, styrenics, vinyl monomers.
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Initiation:
Free-radical initiators (e.g., peroxides, azo compounds, or UV photoinitiators) generate radicals under heat or light.
Example:R−O−O−R→ 2R⋅R-O-O-R
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Propagation:
The radical adds to a vinyl group (C=C), creating a growing chain radical.R⋅+CH2=CHX→R−CH2−CHX⋅R
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Termination:
Two radicals combine or disproportionate, stopping growth.R⋅+R⋅→R−RR
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Very fast polymerization.
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Works with a wide variety of monomers.
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Common in UV-cure adhesives and 3D printing resins.
Limitations:
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Oxygen inhibition → oxygen scavenges radicals, slowing surface cure.
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Shrinkage and stress during cure.
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Less tolerant to impurities.
Cationic Curing Mechanism
Used for: Epoxies, vinyl ethers, oxetanes, cyclic ethers.
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Initiation:
Strong cationic photoinitiators (like diaryliodonium or triarylsulfonium salts) generate a superacid upon UV exposure.
Example: -
Propagation:
The proton (or Lewis acid) opens a strained ring (like epoxy or oxetane), creating a positively charged active center that continues to open other rings.H++R−CH−CH2O→R−CH+−CH2OHH^+ + R-CH-CH_2O
- Termination:
Termination is less common; polymerization can keep going as long as active centers exist (“living” character). Moisture or nucleophiles can quench it.
Advantages:
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No oxygen inhibition (unlike free radicals).
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Lower shrinkage.
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Good adhesion, toughness, chemical resistance.
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“Dark cure”: reaction continues after UV/heat stops, since active centers persist.
Limitations:
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Sensitive to moisture (water quenches cations).
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Slower than free-radical curing.
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Fewer monomer types compatible (mainly epoxies, vinyl ethers, oxetanes).
Key Differences at a Glance
| Feature | Free-Radical Curing | Cationic Curing |
|---|---|---|
| Reactive species | Free radicals (•) | Carbocations (H⁺, Lewis acids) |
| Typical monomers | Acrylates, methacrylates, styrene | Epoxies, vinyl ethers, oxetanes |
| Oxygen sensitivity | Strong (inhibited) | Not inhibited |
| Speed | Very fast | Slower |
| Shrinkage | Higher | Lower |
| After-light curing | Stops when light stops | Continues (“dark cure”) |
| Moisture sensitivity | Low | High |
In short:
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Free-radical curing = fast, versatile, but oxygen-sensitive.
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Cationic curing = slower, oxygen-insensitive, continues curing after initiation, excellent for high-performance epoxies and coatings.
