Acrylate UV Curing System: Chemical Structure and Properties

The acrylate UV curing system is one of the most widely used and technically mature light-curing systems today. Its core lies in compounds (monomers or oligomers) that contain acrylate functional groups, which rapidly cure into films or form three-dimensional network structures through free radical polymerization under the influence of light (typically UV light) and photoinitiators.

1. Core Chemical Structure: Acrylate Groups

• Carbon-Carbon Double Bond (C=C): This is the reaction's active center. Under the attack of free radicals generated by the photoinitiator, the double bond opens, undergoing chain addition polymerization.
  
  
  
  

• Carbonyl Group (C=O): The carbonyl group forms a conjugated system with the double bond (-CH₂=CH-C=O-), which greatly activates the double bond, making it more susceptible to attack by free radicals, thus giving the acrylate system high reactivity and rapid curing properties.
  
  
  
  

• Ester Bond (-C-O-): This connects the acrylate group to the main part of the molecule (R group). The properties of the ester bond (polarity, flexibility, etc.) and the structure of the R group determine the overall performance of the molecule (such as viscosity, adhesion, flexibility, strength, durability, etc.).
  
  
  
  

2. Key Components and Their Chemical Structure and Properties

A complete acrylate UV curing system typically contains the following components:

1. Reactive Diluent (Monomer):

• Structure: These are low-molecular-weight compounds (<500 g/mol) containing one or more acrylate groups. They can be represented by the general formula: (Methyl)acrylate-R.
  
  
  
  

• Types and Structural Characteristics:
  
  
  
  

• Monofunctional Monomer: Contains one acrylate group (e.g., isobornyl acrylate - IBOA, hydroxyethyl acrylate - HEA, 2-ethylhexyl acrylate - EHA).
  
  
  
  

• Characteristics: Significantly reduces system viscosity, has relatively low curing shrinkage, but slower curing speed and lower hardness and crosslink density, offering good flexibility.
  
  
  
  

• Bifunctional Monomer: Contains two acrylate groups (e.g., hexanediol diacrylate - HDDA, dipropylene glycol diacrylate - DPGDA, neopentyl glycol diacrylate - NPGDA).
  
  
  
  

• Characteristics: Moderate viscosity, faster curing speed, higher curing film hardness, crosslink density, and resistance, with moderate shrinkage.
  
  
  
  

• Multifunctional Monomer (Three or More Functional Groups): Contains three or more acrylate groups (e.g., trimethylolpropane triacrylate - TMPTA, pentaerythritol triacrylate - PETA, dipentaerythritol penta/hexacrylate - DPPHA).
  
  
  
  

• Characteristics: High viscosity, significantly increases curing speed and crosslink density, resulting in high hardness, wear resistance, and chemical resistance, but with high curing shrinkage and poor flexibility, potentially causing skin irritation.
  
  
  
  

• Core Function: To dissolve and dilute the oligomers, adjust system viscosity for processes like coating, spraying, and printing. Participates in the curing reaction, becoming part of the cured network. By selecting different monomer structures and functionalities, the final cured film's properties (e.g., hardness, flexibility, shrinkage, reaction speed, and resistance) can be adjusted.
  
  
  
  

2. Oligomers (Resins):

• Structure: These have a higher molecular weight (typically 500-3000 g/mol), and contain two or more acrylate groups at the ends or side chains of their molecular backbone. They contribute mainly to the performance of the cured film. The backbone (R group) determines the oligomer's key characteristics.
  
  
  
  

• Main Types and Structural Features:
  
  
  
  

• Epoxy Acrylates: Obtained by reacting epoxy resins (e.g., bisphenol-A epoxy, phenolic epoxy) with acrylic acid.
  
  
  
  

• Characteristics: Fast curing speed, high hardness, good adhesion (especially to metals), excellent resistance to chemicals (acids, bases), but poor flexibility and moderate yellowing resistance (bisphenol-A type).
  
  
  
  

• Polyurethane Acrylates: Made by reacting isocyanates and polyols (e.g., polyester, polyether, polycarbonate) with hydroxyalkyl acrylates.
  
  
  
  

• Characteristics: Excellent overall performance, flexible design by selecting polyol types (polyester: strength, oil resistance; polyether: flexibility, hydrolysis resistance; polycarbonate: weathering, heat resistance) and isocyanate structures (aromatic: strength, hardness; aliphatic/aromatic: weathering, flexibility). Generally offers excellent flexibility, wear resistance, impact resistance, and elasticity. Curing speed is slower than epoxy acrylates.
  
  
  
  

• Polyester Acrylates: Formed by the condensation of polybasic acids/acid anhydrides with polyols, then esterified with acrylic acid.
  
  
  
  

• Characteristics: Low viscosity, good pigment wetting, and lower cost. The cured film is flexible, with better yellowing resistance (aliphatic). Hardness and strength are generally lower than epoxy and polyurethane types.
  
  
  
  

• Polyether Acrylates: Derived from polyether polyols (e.g., PPG, PTMEG) and acrylates.
  
  
  
  

• Characteristics: Low viscosity, excellent flexibility, hydrolysis resistance, and low-temperature resistance. The cured film is relatively soft with lower strength and heat resistance.
  
  
  
  

• Acrylic Esterified Acrylic Resin: Introduces acrylate groups into the side chains of acrylic resins (usually containing hydroxyl groups).
  
  
  
  

• Characteristics: Good compatibility with other acrylates, excellent weathering resistance, yellowing resistance, and flexibility.
  
  
  
  

• Core Function: Provide the key performance of the cured film, such as hardness, flexibility, strength, adhesion, chemical resistance, and weather resistance. This is the primary component of the formulation.
  
  
  
  

3. Photoinitiators:

• Structure: These compounds absorb light at specific wavelengths (mainly UV) and generate active free radicals (or cations, but in acrylate systems, primarily free radicals).
  
  
  
  

• Types and Structural Features (Radical Type):
  
  
  
  

• Cleavage Type: Absorbs light and breaks its internal chemical bonds to generate free radicals (e.g., benzoin derivatives, benzoin ketone derivatives).
  
  
  
  

• Hydrogen Abstraction Type: Absorbs light and excites the molecule to abstract hydrogen atoms from co-initiators (usually tertiary amines), generating two free radicals (e.g., benzoin ketone - BP, isopropylthioxanthone - ITX).
  
  
  
  

• Core Function: Absorb light energy, generate active free radicals, and initiate polymerization of the acrylate double bonds. Key factors in selection include absorption wavelength, initiation efficiency, odor, and yellowing resistance.
  
  
  
  

4. Additives:

• These include leveling agents, defoamers, wetting dispersants (used for color inks), stabilizers (heat stabilizers, inhibitors), pigments, fillers, etc. They contain little or no photocurable groups, mainly improving processing performance and the final film's appearance or specific properties without participating in the main crosslinking reaction.
  
  
  
  

3. Overall Properties of Acrylate UV Curing Systems

• Fast Curing Speed: Acrylate groups have high reactivity, and under the appropriate light intensity and photoinitiators, curing times can be as short as seconds or even milliseconds. This leads to high production efficiency.
  
  
  
  

• Environmentally Friendly: Uses reactive diluents instead of traditional volatile organic solvents, making it a low-VOC or solvent-free system.
  
  
  
  

• Energy Efficient: Cures at room temperature, consuming much less energy than thermal curing.
  
  
  
  

• Highly Customizable Performance: By selecting and combining different structures of oligomers (epoxy, polyurethane, polyester, etc.) and monomers with different functionalities/types, the final cured film's physical and chemical properties (e.g., hardness, flexibility, strength, adhesion) can be adjusted within a wide range.
  
  
  
  

• Suitable for Heat-Sensitive Substrates: Cures at room temperature, suitable for materials like plastics, paper, wood, and electronic components that are heat-sensitive.
  
  
  
  

• High Gloss and Good Decorative Qualities: Easily achieves a high-gloss, smooth surface.
  
  
  
  

4. Disadvantages and Challenges of Acrylate Systems

• Oxygen Inhibition: Oxygen in the air can quench free radicals generated by photoinitiators or react with chain radicals to form peroxide radicals, thereby terminating chain growth and causing surface tackiness. High light intensity, inert atmosphere protection (nitrogen), additives (such as amine co-initiators), or special initiators/monomers are required to overcome this.
  
  
  
  

• Shrinkage: When liquid monomers/oligomers turn into solid polymers, their molecular distances shorten (especially for multifunctional monomers/oligomers), leading to volume shrinkage (usually 5%-20%). This can cause internal stress, reduce adhesion, and lead to substrate warping.
  
  
  
  

• Skin Irritation and Odor: Many acrylate monomers (especially low molecular weight and multifunctional ones) are irritating to skin and mucous membranes and may have residual odors.
  
  
  
  

• Limited Deep Curing: UV light penetration is limited, posing challenges for deep curing of thick coatings, pigmented coatings, or complex-shaped components.
  
  
  
  

• Device Dependence: Requires specialized UV light source equipment.
  
  
  
  

• Yellowing Issues: Some photoinitiators (such as by-products of cleavage-type initiators) and oligomers (e.g., aromatic epoxy acrylates) may yellow under light or heat exposure.
  
  
  
  

• Flexibility/Brittleness Balance: High crosslink density systems (e.g., high proportions of multifunctional monomers/oligomers) may be hard but brittle.
  
  
  
  

The core of the acrylate UV curing system lies in its efficient acrylate groups and the free radical polymerization reaction mechanism. The chemical structure (monomer functionality and type, oligomer backbone structure) directly determines the viscosity, reactivity, and key performance properties of the final cured film, such as hardness, flexibility, strength, and durability. This system dominates in coatings, inks, adhesives, and 3D printing due to its ultra-fast curing speed, environmental friendliness, high energy efficiency, and customizable performance, but faces challenges like oxygen inhibition, shrinkage, irritation, and deep curing limitations. Ongoing research focuses on developing new monomers, oligomers, and initiators that overcome these challenges, offering even better performance and lower irritation.