UV Resin Classification, Synthesis, Properties, and Application Scenarios

I. Introduction

UV resin, often referred to as photocurable resin, is the core matrix raw material for UV?curable products such as UV coatings, UV inks, and UV adhesives. It belongs to the category of low?molecular?weight photosensitive resins.

These resin molecules carry reactive groups (unsaturated C=C double bonds, epoxy groups, etc.) that can undergo photopolymerisation. Without the need for high?temperature heating, they can complete crosslinking curing in just a few seconds under ultraviolet irradiation. They are currently the core functional materials for energy?saving, environmentally friendly, and high?efficiency mass production in the industrial field.

In the entire UV?curing formulation system, the UV resin is the key factor determining the final performance of the coating film. Core properties such as hardness, flexibility, adhesion, weather resistance, and chemical resistance are all dominated by the resin; photoinitiators, reactive diluents, fillers, and pigments serve only as auxiliary modifiers.

Therefore, a thorough understanding of the synthesis principles, performance shortcomings, and suitable application scenarios of different types of UV resins is the essential prerequisite for formulation development and process optimisation in UV curing.

From the perspective of the photopolymerisation mechanism, mainstream UV resins on the market can be divided into two major systems: free?radical photocurable resins and cationic photocurable resins. The reaction mechanisms, molecular structures, and application fields differ significantly; the following sections will analyse each in detail with reference to actual industrial applications.

II. General Overview of the Two Photocuring Systems

2.1 Free?Radical Photocuring System Resins

The core reactive structures are carbon?carbon unsaturated double bonds, mainly including four types: acryloyloxy, methacryloyloxy, vinyl, and allyl groups.

Based on industrial curing speed measurements, the polymerisation reactivity of these double bonds ranks as follows:
acryloyloxy > methacryloyloxy > vinyl > allyl.

Due to these reactivity differences, more than 90% of free?radical UV resins on the market today are acrylate?modified resins. The main categories include epoxy acrylates, polyurethane acrylates, polyester acrylates, and polyether acrylates. Among these, epoxy acrylates and polyurethane acrylates are the two matrix resins with the largest industrial consumption and the broadest application coverage.

2.2 Cationic Photocuring System Resins

Completely different from the free?radical system, cationic UV resins do not contain C=C double bonds; they rely on epoxy groups or vinyl ether groups for photo?induced ring?opening polymerisation. Common commercial types are general?purpose epoxy resins and vinyl ether resins.

Key advantages: no oxygen inhibition, extremely low curing shrinkage, and the ability to undergo deep?section dark curing.

Key drawbacks: relatively slow curing speed and higher raw material costs.

Therefore, this system is rarely used in ordinary civil coatings or inks; it is more suitable for high?end niche applications such as optical coatings, precision electronic encapsulation, and thick?film curing.

III. Detailed Description of Mainstream Free?Radical UV Resins (Synthesis + Performance + Practical Applications)

3.1 Epoxy Acrylate (EA) – The Largest?Volume, Most Cost?Effective Resin in the Industry

Epoxy acrylate is an essential resin in the UV?curing industry, accounting for more than 60% of total UV oligomer consumption.

Synthesis principle: The epoxy group on the epoxy resin molecule undergoes ring?opening esterification with (meth)acrylic acid, introducing acrylate reactive double bonds at the chain ends, thereby endowing the resin with UV?curing capability.

Based on the structure of the base epoxy resin, it can be divided into four main categories:

3.1.1 Bisphenol A Epoxy Acrylate (General?Purpose Basic Grade)

Synthesised by direct esterification of bisphenol A epoxy resin with acrylic acid, this is the most fundamental and versatile EA resin. The molecule contains rigid benzene rings and pendant hydroxyl groups, which determine its performance.

? Advantages

• Fastest curing speed among all UV oligomers.

• High coating hardness and excellent surface gloss.

• Outstanding chemical resistance (acids, alkalis, solvents).

• Pendant hydroxyl groups improve substrate adhesion; good heat resistance and electrical insulation.

? Obvious Shortcomings

• Excessive molecular rigidity leads to brittle films with poor flexibility – prone to cracking under bending or impact.

• Rapid crosslinking causes many double bonds to become locked before reacting, leaving many residual reactive groups.

• Tends to yellow over time; poor ageing resistance.

?? Practical Applications
Widely used in screen?printing inks, general?purpose UV adhesives, and wood topcoats where high flexibility is not required. In practice, they are usually blended with flexible PUA resins and low?viscosity flexible diluents to improve film brittleness.

3.1.2 Novolac Epoxy Acrylate (Electronics?Grade)

Synthesised from novolac epoxy resin, this type has a very high density of benzene rings and much higher functionality than the bisphenol A system, leading to stronger crosslinking.

The cured film exhibits extremely high crosslink density, excellent heat resistance, and outstanding solder resistance, with further improved hardness and corrosion resistance – but even greater brittleness.

?? Practical Applications
Not used for general topcoats; it is the dedicated matrix resin for PCB solder?resist inks, suited for the high?temperature soldering conditions of circuit boards.

3.1.3 Modified Epoxy Acrylates (Performance?Optimised Grades)

To address the common drawbacks of ordinary EA – brittleness, weak adhesion, and yellowing – the industry has developed five major chemical modification routes, all of which are commercially mature:

• Amine modification: aliphatic amine pre?extension reduces curing shrinkage and improves adhesion; quaternary ammonium salt grafting optimises substrate wetting, reduces pinholes and craters, and is suitable for corona?treated plastic substrates.

• Phosphate modification: introduction of phosphate groups forms a complex layer on metal surfaces, enhancing anchoring on metals – exclusively used for metal UV primers.

• Polyacid anhydride modification: chain extension and carboxyl group grafting reduce shrinkage and improve heat resistance; long?chain anhydrides can enhance molecular flexibility to balance brittleness.

• Long?chain fatty acid modification: introduction of flexible carbon chains improves bending performance and enhances pigment/filler wetting and dispersion – suitable for high?pigment?loading colour coating systems.

• Halogen modification: brominated EA provides flame retardancy (self?extinguishing) for fire?retardant UV coatings; fluorinated EA has a high refractive index, suitable for optical lens and optical film coatings.

3.1.4 Epoxidised Oil Acrylates (Auxiliary Flexible Grades)

Using natural drying oils such as soybean oil or linseed oil as raw materials, the double bonds in the oil are first epoxidised and then esterified with acrylic acid.

? Advantages: excellent molecular flexibility, outstanding pigment dispersion, and very low viscosity.
? Disadvantages: poor curing reactivity; low film hardness and mechanical strength.
?? Application: not used alone; employed as an auxiliary resin blended with high?activity EA and PUA to reduce system viscosity, improve levelling, and enhance pigment dispersion.

3.2 Polyurethane Acrylate (PUA) – Best Overall Performance, First Choice for Flexibility

3.2.1 Basic Definition and Core Performance

PUA is synthesised by the addition reaction of polyisocyanates, long?chain diols, and hydroxyalkyl acrylates. The molecule contains numerous urethane bonds that can form inter?molecular hydrogen bonds. The molecular chain segments can be freely designed, making PUA the UV resin with the most commercial grades and the widest tunable performance window.

? Key Advantages: perfectly overcomes the brittleness of EA; excellent film flexibility, impact resistance, and abrasion resistance; does not crack or become brittle under high?low temperature cycles; outstanding adhesion to difficult substrates such as plastics, leather, and films.

? Key Shortcomings: relatively slower curing speed, high resin viscosity, and higher raw material costs.

3.2.2 Two Main Synthetic Routes

• Chain extension first, then end?capping: excess isocyanate reacts with long?chain diol to form an NCO?terminated prepolymer, which is then end?capped with hydroxyalkyl acrylate to introduce reactive double bonds – suitable for preparing high?molecular?weight, highly flexible PUA.

• End?capping first, then chain extension: isocyanate first reacts with hydroxyalkyl acrylate, then the diol is added for chain extension – the reaction exotherm is milder, viscosity is easier to control, and it is suitable for large?scale industrial production.

3.2.3 Three Industrial Synthesis Methods

• Solution polymerisation: uses solvents for temperature control, yielding smooth reaction and low product viscosity, but solvent residues lead to high VOC – gradually being phased out.

• Bulk polymerisation: solvent?free, zero VOC, high purity, meets environmental requirements – currently the mainstream process; drawback is concentrated exotherm, requiring careful temperature control to avoid gelation.

• Sol?gel polymerisation: introduces nano?silica as an inorganic phase to prepare organic?inorganic hybrid PUA, significantly improving heat resistance and hardness – a hot R&D direction for high?performance protective coatings.

3.2.4 Advanced Modified PUA Resins

• Waterborne PUA: water replaces organic diluents – zero VOC, no skin irritation, highly environmentally friendly; however, it requires drying to remove water, increasing curing energy consumption.

• Silicone/fluorine?modified PUA: improves weatherability, hydrophobicity, anti?soiling, and release properties – suitable for outdoor weather?resistant coatings and anti?stick coatings.

• Hyperbranched/nano?modified PUA: reduces intrinsic viscosity, decreases the need for reactive diluents, while improving mechanical strength – ideal for low?viscosity, high?solids high?end formulations.

3.3 Polyester Acrylate (PEA) – Low?Odour, High?Wetting Auxiliary Resin

Prepared by esterification of low?molecular?weight polyester diols with acrylic acid, PEA has very low viscosity and can serve either as a matrix resin or as a reactive diluent.

? Advantages: extremely low odour, low skin irritation, excellent pigment wetting and dispersion, and moderate film flexibility.
? Disadvantages: moderate curing speed and relatively weak mechanical strength.
?? Application: mostly used in pigmented UV topcoats and printing colour inks; amine?modified PEA can alleviate oxygen inhibition and improve surface cure.

3.4 Polyether Acrylate – Specialised Resin for Flexibility and Viscosity Reduction

Synthesised from polyethylene glycol or polypropylene glycol backbones, this resin has excellent molecular flexibility and far better yellowing resistance than epoxy?based resins.

Obvious shortcomings: very poor hardness, solvent resistance, and acid/alkali resistance – cannot be used alone as a primary film?former.

Core use: viscosity reduction and flexibility adjustment in formulations; amine?modified grades are often used in low?odour, environmentally friendly ink systems.

3.5 Pure Acrylate Oligomers – Auxiliary Resins for Weathering and Anti?Yellowing

Prepared by grafting acrylate reactive double bonds onto polyacrylate side chains, the standout feature is outstanding yellowing resistance, excellent weatherability, and broad substrate compatibility.

Shortcomings: very low film hardness, poor abrasion and chemical resistance – cannot form films alone; used only as auxiliary resins to enhance weathering and anti?yellowing properties of coatings.

3.6 Silicone Acrylate Oligomers – Specialised for Functional Coatings

With Si?O siloxane backbones, these resins have extremely low surface tension, providing excellent release properties, hydrophobicity, high?low temperature resistance, and resistance to濕熱 ageing.

Mechanical properties are relatively weak; not for general topcoats. Exclusive applications include release coatings for release paper, fibre optic protective coatings, and moisture?sealing coatings for electronic circuits.

IV. Comprehensive Comparison and Selection Guidelines for the Two Photocuring Systems

Based on the curing mechanisms and actual industrial performance, the key differences and selection logic between the free?radical and cationic UV resin systems are summarised as follows:

V. Conclusion

In current mass?production applications of UV curing, free?radical acrylate resins remain the absolute mainstream, with a clear division of roles:

• Epoxy Acrylate (EA): delivers high hardness and cost?effectiveness – suited for general hard coatings and inks.

• Polyurethane Acrylate (PUA): offers high flexibility, excellent adhesion, and outstanding weatherability – ideal for high?end flexible substrates and protective coatings.

• Polyester, polyether, silicone, and other resins: function as auxiliary blending resins to complement the performance gaps of single resins.

Meanwhile, cationic UV resins, with their unique advantages of low shrinkage and no oxygen inhibition, precisely cover the high?precision, thick?film, and high?flatness applications where free?radical resins fall short.

In actual formulation development, there is no need to pursue a single resin that does everything. Instead, the optimal product design approach is to scientifically combine resins based on substrate characteristics, curing process requirements, and cost constraints.