Introduction: UV curing “light” is the fourth mode of polymerization initiation.
Four ways of coating curing——(1) Thermal curing (room temperature——two-component epoxy/PU——heating——powder)——heat; (2) Moisture curing (one-component PU——reacts with water “air/moisture”); (3) Oxidative curing (alkyd——O₂ “slow drying/stepwise”); (4) UV/EB radiation curinglight radiation (UV——absorption——photoinitiator→free radicals——>within 1s-1min——liquid→solid)
“Light” is a “room temperature + extremely fast (1s-60s) + extremely low VOC” is the “greenest curing method”. The biggest technical obstacles of UV curing——(1)oxygen inhibition
(O₂——free radicals——peroxy ROO· “inactive” causing the coating surface tacky——cannot cure
“dark curing”/EB curing——can completely avoid——but equipment is expensive——UV——use amine co-initiation/wax floating layer/N₂ inerting——cost more controllable); (2) Yellowing from photoinitiator residue——coating over time——UV exposure + residual photoinitiator——coating turns yellow——(CQ (camphorquinone——dental/white)——yellowing——colorless——being phased out
——new type BAPO——low yellowing——is the focus of the latest photoinitiator research. The three major components of radiation curing raw materials (oligomer——reactive diluent——photoinitiator)——globally >500 types——how to choose “formula——corresponding application process and end-use requirements——requires scientific selection——not just any ‘UV paint’ will do”.
Raw material system of radiation-curable (UV/EB) coatings — composed of oligomers (Acrylated Oligomer — epoxy acrylate / polyester acrylate / PUA / amino acrylate — 40-60% — backbone — provides core physical properties), reactive diluent monomers (mono/di/multi-functional acrylates — 25-45% — dilution + crosslinking — reduces viscosity — increases curing speed — also introduces shrinkage) and photoinitiators (PI — Type I cleavage type — Type II hydrogen abstraction type — cationic type — 2-6% — absorbs UV — generates active centers “curing engine”) — three major components — together with additives — constitute the “UV/EB formulation’s four elements” — under UV irradiation or electron beam bombardment for seconds to tens of seconds — transforms from liquid resin into a solid coating with decorative, protective and functional properties.
I. The “Golden Triangle” of Chemical Structure and Properties of the Three Major Oligomers
| Oligomer | Chemical Structure | Tg(°C) | Flexibility | Weather Resistance | Curing Speed | Typical Applications |
|---|---|---|---|---|---|---|
| Epoxy Acrylate (EA——Bisphenol A Epoxy Acrylate) | Bisphenol A epoxy + acrylic acid——phenyl ring rigidity——diacrylate functionality | >80 (hard/brittle) | Poor (bending cracks) | Poor (phenyl ring——UV oxidation——yellowing) | Fast (acrylate——high double bond density) | Paper/woodwork/indoor “hard but brittle” |
| Polyester Acrylate (PEA——Polyester Acrylate) | Polyester polyol + acrylic acid——aliphatic——ester bond——flexible——medium polarity | <40 (soft/flexible) | Excellent | Good (aliphatic——no phenyl ring——less yellowing) | Medium | Plastics/flexible substrates/screen printing |
| Polyurethane Acrylate (PUA——PUA) | Polyester/polyether polyol + TDI/IPDI + HEA——urethane + acrylic | -40~+120 (wide adjustable range) | Excellent-superior (flexible chain segments) | Excellent (aliphatic HDI/IPDI type——excellent weather resistance——aromatic MDI type——yellowing) | Medium-fast (adjustable) | Automotive/3C/outdoor/flooring “three major scenarios——PUA——most versatile——most expensive” (>100 RMB/kg) |
FAQ
Q1: The decisive impact of oligomer “Tg” on the flexibility of UV coatings—why can the Tg of PUA be adjusted from -40 to +120°C?
Oligomer (Oligomer—molecular weight 500-5000 g/mol—in UV curing—provides the skeletal properties of the coating). Tg (glass transition temperature)—below Tg—coating is hard and brittle—above Tg—soft and flexible. EA (bisphenol A—benzene ring—rigid—Tg>80°C)—hard—cracks on bending—not suitable for flexible substrates (plastic/leather/thermoforming). PUA (polyurethane—soft segment—polyester/polyether/—provides low Tg120°C—soft segment/hard segment ratio—can adjust any Tg in the system from -40 to +120°C “Tg = ratio of soft segment/hard segment”)—this is PUA’s “full-range applicability” “soft segment—flexibility and adhesion—hard segment—hardness/chemical resistance—ratio—any Tg” so PUA is the “most widely used—most expensive—most formulation-flexible oligomer” in UV curing. PUA’s HDI type (aliphatic—yellowing resistant—outdoor—price >150 yuan/kg)—TDI type (aromatic—yellowing—price >80—indoor) selection—mainly depends on the trade-off between weather resistance and price.
Q2: How to balance the “functionality/dilution power/shrinkage/skin irritation” (four-dimensional trade-off) of reactive diluent monomers?
Monomers are classified by the number of acrylate double bonds—monofunctional (1 C=C—molecular weight >150-400—extremely strong dilution power—shrinkage di > poly”)—polyfunctional (3-6 C=C—molecular weight 20%—adhesion easily drops—high toxicity PII—e.g. TMPTA/DTMPTA “use in small amounts—improve crosslinking/hardness/curing speed”)—difunctional (2—balanced “general purpose—HDDA/TPGDA”). Four-dimensional trade-off—formulation “>50% monofunctional (dilution power)—>30% difunctional (balance)—<10% polyfunctional—crosslinking" optimal balance"“Any polyfunctional >15%—shrinkage—adhesion drops sharply—unacceptable”.
Q3: Oxygen Inhibition — Why do Type II photoinitiators (benzophenone/ITX) cure a non-tacky surface compared to Type I?
Type I (HCPK/184 — upon UV — homolytic cleavage — directly generates radicals) — radicals — at the coating surface — O₂ consumes radicals — generates peroxy radicals ROO· — no initiation activity “surface does not cure — tacky”. Type II (BP/ITX) — upon UV “hydrogen abstraction” from co-initiator (amine — e.g. EDAB — N — α-H abstracted — generates — amine radical (C· — high activity) + BP’s “ketyl” radical (moderate activity)) — this amine radical (C·)reacts with O₂ relatively slowly — can “before O₂ consumption — first react with acrylate C=C — cure — surface non-tacky”
— this is “Type II’s amine co-initiator increases the surface’s oxygen inhibition resistance”. Type I + Type II combination “Type I — deep/thick curing — Type II — surface/oxygen inhibition resistance — combination — surface layer + deep layer — full cure” this is a classic UV formulation combination “HCPK (184) + BP (benzophenone) + EDAB (amine co-initiator) (2:1:1) — suitable for >90% of general-purpose UV varnish systems”.
Q4: Matching of UV-LED (365/385/395nm) with photoinitiators — why is traditional HCPK (184) inefficient in LEDs?
Traditional mercury lamp (Hg) — broad spectrum — multiple spectral lines (254/313/365/405/436nm —> 254nm short wave — absorption for most PIs — effective). UV-LED — narrow spectrum (FWHM <20nm — central wavelength — 365/385/395/405nm) only "long-wave UV-A". HCPK (184) — maximum absorption at 250nm — almost no absorption at 365nm — extremely poor efficiency (<1/10 of mercury lamp) — no longer used in LED formulations. LED-matched PI TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide — absorption peak 350-410nm — 365nm/395nm — extremely strong absorption — is the “first choice” for LEDs plus >40% mercury lamp formulation — can replace HCPK — full cure
. Another LED-PI is BAPO (bisacylphosphine oxide — absorbs up to 440nm — “colorless/low yellowing” in visible region — but high price — used in dental and eyewear “high-end — LED-specific”.
Q5: Cationic UV curing—why can it continue curing after the “light is off” (“dark curing”)?
Cationic curing—(1)UV—cationic photoinitiator (sulfonium salt/iodonium salt)—generates superacid (H⁺/Brønsted superacid—e.g., HSbF₆—SbF₆⁻)
—superacid—immediately initiates cationic ring-opening polymerization of epoxy groups (or vinyl ethers)
—chain propagation—(2) the “new acid” generated after epoxy ring-opening (proton is not consumed—it is true catalysis—one H⁺ can open hundreds of epoxy rings
—therefore after “light off”—H⁺ remains—subsequent epoxy rings continue to open—this is “dark curing” until “H⁺ is terminated by base—or epoxy groups are exhausted”. Cationic dark curing—means thick coatings (>300μm—light penetration is limited—bottom layer lacks UV—but surface H⁺ can diffuse to the bottom layer—initiating deep curing—thick coatings—the advantage of dark curing is obvious—UV radical route has “no dark curing—bottom layer is forever—undercured—cationic solves this problem”
)—the weakness of cationic—moisture (water—base—consumes H⁺→curing stops—moisture sensitive—control ambient humidity <30%RH").
Q6: How to reduce the exposure risk to operators from the “Primary Irritation Index (PII)” of reactive diluents (HDDA/TPGDA)?
Acrylate monomers react with the -SH groups of skin proteins (Michael addition — acrylates are “electrophilic”) — triggering allergic contact dermatitis “permanent sensitization” — this is the core occupational risk of UV monomers
. PII (Primary Irritation Index) — (1) TPGDA (tripropylene glycol diacrylate — PII≈4-7 — severe sensitization “operation — PPE — closed system — absolutely — no open exposure — no skin contact”); (2) IBOA (isobornyl acrylate — PII≈1-3 — low sensitization “dilution power — mono-functional advantage — low PII — is a safer diluent monomer” “new formulations — prioritize IBOA — replace TPGDA — reduce PII”); (3) ethoxylated/propoxylated monomers (EO-TMPTA — propoxylation — reduces the “electrophilicity” of acrylates — reduces sensitization). PV (Peroxide Value — acrylic monomers — improper storage generates peroxides — stronger skin irritation — need to add polymerization inhibitor (MEHQ/p-methoxyphenol — >200ppm — during storage — inhibit oxidation into peroxides”.
Q7: Global UV raw materials — major global suppliers of oligomers/monomers/photoinitiators — Sartomer/Allnex/IGM/China — their capacity and positioning?
(1) Sartomer (Arkema — USA “King of Oligomers” >1000 types of oligomers and monomers — CN/SR series — industry benchmark — market share >30% — price >100 — supplies China — technical support — slow service “relies on agents”; (2) Allnex (formerly Cytec/Ciba “full range of radiation-curing products” oligomers + monomers (PETIA) — market share >25% — EBECRYL series — Asia-Pacific — technology and supply 60% PI market share — mainly produces — HCPK/TPO/907 — complete product range — stable — price — >30% higher than domestic”; (4) China (Tianjin Jiuri (UV oligomers — comprehensive development of photoinitiators — PI — TPO/184 — accounts for >30% of global supply — extremely low cost — quality varies by batch — is “cost-performance — first choice — but quality control — necessary evaluation”, Qiangli — mainly produces oligomers — gradually expanding). Global supply chain “Oligomers — Sartomer/Allnex — PI — IGM — China — ‘high-end stability’ vs ‘cost advantage'” formulation choice “quality reliability — or — cost competitiveness”
.
Q8: The contradiction between “shrinkage stress” of UV coating and substrate adhesion—are adhesion tests (cross-cut/pull-off) consistent?
UV curing—liquid→solidvolume shrinkage >10-20%
(each additional double bond—greater shrinkage—multifunctional—shrinkage >20%—linear shrinkage—causes tensile stress in coating)—coating tensile stress “tears the coating/substrate interface
“adhesion drops from >5MPa to 15%—poor adhesion—cross-cut <2 grade—pull-off <5MPa); (2) Low Tg/monofunctional—small shrinkage (5MPa)” This is why—monofunctional monomer is the “best” diluent for adhesion—while multifunctional is the “adhesion killer” because of excessive shrinkage.
Q9: How is the UV powder coating “oligomer + monomer + PI” three-component system “powdered”?
Traditional UV liquid——oligomer + monomer “liquid——contains >30% monomer (high-concentration liquid monomer)” how to become powder?——(1) The monomer uses solid acrylic monomer (solid——multifunctional——e.g. DTMPTA——melting point >40°C/solid——when mixed with resin——ground into powder——powder coating——at >90°C——melts and levels——monomer dissolves——dissolves oligomer——before UV curing cannot level——otherwise premature fusion——cannot be coated——this is the “contradiction” of UV powder, requiring precise control of the “melting→curing” window
; (2) Advantages of UV powder “zero VOC” + one-coat application——thick film——UV curing——low temperature (>100°C vs conventional powder 200°C——suitable for heat-sensitive substrates (wood/plastic/MDF/)”
——this is exactly “tailor-made” for UV powder “MDF (medium-density fiberboard) furniture”.
Q10: EB curing—no photoinitiator needed, “relying solely on electron bombardment” to directly generate free radicals—why are oligomers and monomers still needed—can’t acrylic resin be used alone?
EB curing—high-energy electrons (>150keV)—collide with acrylate (C=C, “electron knocks off an electron—generates radical cation—then reacts with another C=C—chain propagation” no PI needed—but still requires acrylate oligomers and monomers as the “C=C carriers” “oligomers—backbone + crosslinking—monomers—dilution + crosslinking—acrylic resin—can also be electron bombarded—but without oligomers/monomers—’backbone + C=C density—pure acrylic resin—may contain only a small amount of C=C—insufficient—low crosslink density—poor coating performance’
“EB formulation composition—same as UV—oligomers + monomers—only omit PI—maintain the same formulation skeleton—this stems from—EB “electrons—need C=C as the ‘target material’—no C=C—no reaction”.
Related Reading
Summary
Radiation curing (UV/EB) — composed of three parts: oligomers (EA for hardness, PEA for flexibility, PUA for full-range applications with adjustable Tg — most versatile, most expensive), reactive diluent monomers (monofunctional for dilution, difunctional for balance, polyfunctional for crosslinking), and photoinitiators (PI — Type I cleavage, Type II hydrogen abstraction, cationic acid) — plus LED matching and oxygen inhibition strategies — is one of the fastest-growing coating curing technologies worldwide (+8%). Global suppliers (Sartomer/Allnex/IGM — China (Jiuri “cost breakthrough”)) are making UV raw material selection and cost increasingly diversified. Kexin New Materials provides customers with full-set UV/EB formulation optimization, photoinitiator screening, and global supply resource services — “zero-pressure smart selection for you” — one-stop radiation curing solutions.