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Free radicals: Light generated free radicals, acrylic ester double bond addition polymerization; Afraid of oxygen, quick drying, large shrinkage, stop when the lights are turned off. Cation: Light induced superacid, epoxy/oxetane/vinyl ether ring opening polymerization; Not afraid of oxygen, with dark curing, minimal shrinkage, afraid of water and alkaline impurities

1、 Mechanism comparison

✅  UV free radical curing photoinitiators (1173, TPO, 184, etc.) absorb and decompose light to generate free radicals; Free radicals attack the C=C double bond, leading to chain addition polymerization; Termination method: Free radicals combine with each other, undergo dismutation, or be quenched by oxygen; Stop the light, the reaction immediately terminates

✅  UV cationic curing onium salt photoinitiators (iodonium salt, thionium salt PAG) absorb light and release proton strong acid (H ⁺); Strong acid catalyzed ring opening of epoxides, oxetanes, and vinyl ethers; Characteristics: Active acids are difficult to self terminate, and even after turning off the lights, the acid continues to catalyze cross-linking, which is known as dark curing/post curing; Oxygen does not quench acids, and there is no oxygen blocking polymerization

Comparison item: UV radical curing, UV cation curing

Base resin/monomer acrylic ester (epoxy acrylic ester, PUA, polyester acrylic ester, acrylic monomer), cycloaliphatic epoxy, oxetane, vinyl ether

Photoinitiator α - hydroxyketone, acylphosphine oxide, iodonium salts such as benzophenone, and triarylthionium salts (photoacid generator PAG)

Sensitive and strong oxygen blocking polymerization! The surface is prone to sticking and poor surface dryness, requiring amine enhancement, high-energy LED, inert protection from oxygen, and perfect surface dryness even in air

Water/alkali sensitivity is basically not afraid of water; A small amount of water does not affect curing. It is sensitive to moisture, alkaline fillers, and amine additives! Water/alkali will neutralize strong acids and directly stop curing (formula pit)

The curing speed is extremely fast: milliseconds to a few seconds, the production line is running slowly at high speed, and the light only produces acid; Complete curing takes tens of seconds to several hours, gradually perfected through dark curing

Large volume shrinkage: 3%~10%, high internal stress, easy warping and detachment of thick coatings. Small shrinkage: 1%~4%, low internal stress, excellent adhesion, suitable for substrate bonding

Dark curing (continue to react after turning off the lights) ❌  Almost none, the light stops and the reaction stops ✅  Very strong, thick film, shaded areas, and opaque areas can also slowly solidify

Low raw material cost, wide selection of monomers and resins, and flexible formula; Acyclic epoxides and onium salt initiators are expensive, and there are few options for raw materials

The characteristics of the paint film are high hardness, wear resistance, and a wide range of adjustable formulas; High internal stress, strong adhesion, chemical resistance, good resistance to yellowing, and low shrinkage; Slightly tough, average rigidity

The restrictions on pigments/fillers are loose, but most fillers can be used strictly: alkaline fillers (calcium carbonate, some talc powder) cannot be used, as they will quench the acid and directly fail to cure; Priority neutral fillers: barium sulfate, quartz, alpha alumina (as you previously researched)

3、 Advantages and disadvantages&applicable scenarios

Advantages:

Very fast curing, suitable for high-speed production lines;

The resin, monomer, and additive systems are extremely mature, with adjustable hardness, elasticity, and wear resistance;

High tolerance for fillers and easy formulation; Disadvantages:

Oxygen blocking polymerization, thin coating surface is prone to sticking to hands;

Shrinkage is large, and the adhesion of metal and glass substrates is prone to overturning, resulting in high stress on thick coatings; Suitable for: wood varnish, plastic UV topcoat, paper varnish, screen ink, 3D printing ordinary resin

Advantages:

Anaerobic polymerization inhibition, excellent surface dryness, no need for amine to enhance efficiency;

Low shrinkage, low stress, very strong adhesion to metals, glass, and ceramics;

Dark curing: Thick coatings, colored deep paints, and shaded areas can also slowly cure;

Resistant to chemicals, hydrolysis, low yellowing, suitable for optical components; Disadvantages:

Extremely sensitive to humidity and alkaline substances, with poor curing due to high humidity in the workshop;

Cannot be paired with amine or alkaline fillers;

Long curing cycle and slow production line rhythm; Expensive raw materials; Suitable for: metal UV primer, optical coating, UV glue, thick film protection, glass coating, electronic packaging, precision parts requiring low shrinkage