In photopolymer additive manufacturing systems, the synthesis method of 3D resins directly determines their curing behavior, mechanical properties, and applicable scenarios. As a polymer material with photosensitive polymerization as its core mechanism, its preparation process is not only a chemical reaction between monomers but also a process of precise molecular structure construction and performance regulation tailored to application needs. From the synthesis principle to process control, each step must consider reaction efficiency, product stability, and compatibility with end applications, thus forming a systematic preparation path.
The core of the synthesis principle lies in free radical or cationic polymerization. Mainstream 3D resins are based on acrylate monomers, achieving rapid curing through free radical polymerization. The essence of the reaction is that the photoinitiator decomposes under specific wavelength light to generate free radicals, which attack the acrylate double bonds, initiating chain growth and cross-linking, ultimately forming a three-dimensional network structure. For epoxy resins, cationic polymerization is often used. The protons or Lewis acids generated by the decomposition of the photoinitiator activate the epoxy groups, achieving low shrinkage and deep curing, but the reaction rate is relatively slow. The choice of synthetic route depends primarily on the target performance: acrylate systems are preferred for high hardness and fast curing; epoxy systems, or copolymerization with acrylates, are favored for low shrinkage and heat resistance, aiming for a balanced performance.
The construction of the monomer and resin backbone is the first step in synthesis. Commonly used matrix resins include epoxy acrylates, polyurethane acrylates, and polyester acrylates, whose preparation often combines prepolymer synthesis with monomer modification. For example, the synthesis of polyurethane acrylates typically uses isocyanates (such as HDI and TDI) and hydroxyl-containing acrylates (such as HEA and HPA) as raw materials, forming a prepolymer containing flexible urethane segments through stepwise polymerization, and then introducing acrylate end groups to impart photosensitivity. This process requires strict control of the molar ratio of isocyanate to hydroxyl groups, the reaction temperature (usually 60℃~80℃), and an inert atmosphere (nitrogen protection) to prevent side reactions such as the formation of urea bonds or gels, and to ensure a uniform molecular weight distribution. Polyester acrylates are produced by esterifying polyols (such as ethylene glycol and propylene glycol) with polycarboxylic acids (such as phthalic anhydride and adipic acid) to form polyesters, which are then reacted with acrylate esterifying agents (such as acrylic acid and methacrylic acid) to introduce double bonds. Their viscosity and flexibility can be adjusted by the alcohol-acid ratio and chain length.
The introduction and control of photoinitiators are crucial steps in the synthesis. Free radical photoinitiators (such as 1173, 819, and TPO) need to be added in the later stages of resin synthesis or during formulation, achieved through physical mixing. However, it is essential to ensure their compatibility with the matrix resin-poor compatibility may lead to phase separation or uneven curing. For special requirements (such as deep curing and low odor), photoinitiators can be grafted onto the resin backbone to form macromolecular photoinitiators, which improves compatibility and reduces migration. Cationic photoinitiators (such as iodonium salts and thiodonium salts) need to be co-designed with the epoxy group during synthesis to ensure effective activation of the epoxy group under light irradiation, while avoiding premature deactivation due to reaction with alkaline impurities in the system.
The integration and post-modification of functional additives endow resins with diverse properties. Additives added in the later stages of synthesis or formulation include leveling agents (such as organosilicones and fluorocarbons), defoamers (such as polyether-modified siloxanes), polymerization inhibitors (such as p-hydroxyanisole), and functional modifiers (such as heat-resistant monomers and toughening particles). For washable resins, water solubility needs to be improved through copolymerization of hydrophilic monomers (such as introducing hydroxyethyl acrylates) or graft modification (such as introducing polyethylene glycol segments onto the resin backbone); for flexible resins, the modulus is reduced by increasing the proportion of long-chain alkyl groups or flexible segments (such as polybutadiene). Such modifications require precise control of reaction conditions during synthesis to avoid damaging the original photosensitive structure or causing viscosity runaway.
Key control points in the synthesis process are crucial throughout. Regarding temperature control, free radical polymerization is significantly exothermic, requiring a cooling system to maintain a stable reaction temperature (typically not exceeding 90°C) to prevent explosive polymerization. An inert atmosphere (nitrogen or argon) eliminates the quenching effect of oxygen on free radicals, improving conversion rate. Reaction time must be determined based on monomer activity and conversion rate monitoring (e.g., FTIR tracking of double bond peak disappearance) to avoid underpolymerization or overpolymerization. Purification steps (e.g., vacuum distillation, thin-film evaporation) remove unreacted monomers, catalyst residues, and oligomers, ensuring resin purity and storage stability.
Overall, the synthesis method of 3D resins is a deep integration of molecular design, reaction engineering, and performance regulation: a basic framework is constructed by selecting polymerization mechanisms and monomer types; photosensitivity and functional properties are introduced through precise prepolymer synthesis and modification; and process adaptation and application expansion are achieved through the integration of additives. With the development of photopolymerization technology, synthesis methods are evolving towards low energy consumption, high controllability, and greening (such as bio-based monomer replacement and solvent-free synthesis), providing a more efficient path for the preparation of high-performance, multifunctional 3D resins and continuously empowering the refined and innovative development of additive manufacturing.
