Light-Curing Technology Series — Monomers

Depending on the type of active species, photopolymerization technologies can be classified into radical photopolymerization, cationic photopolymerization, anionic photopolymerization, and others. At present, radical photopolymerization is the dominant technology among all photopolymerization methods. The following section primarily introduces monomers suitable for radical photopolymerization.

Photocuring technology can achieve 100% solids content and contains no solvents, because photocurable monomers can serve as solvents in conventional formulations, known as reactive diluents. Photocurable monomers can be classified according to their functional groups into mono‑functional, di‑functional, and polyfunctional monomers. Here, the functional group primarily refers to acrylate double bonds or acrylamide double bonds: a monomer containing one double bond is called a mono‑functional monomer; one containing two double bonds is called a di‑functional monomer; and one containing three or more double bonds is called a polyfunctional monomer.

Representative monofunctional monomers include THFA, IBOA, ACMO (containing an acrylamide double bond), EOEOEA, LA, LMA, HEA, HEMA, HPA, and HPMA. In general, these monofunctional monomers exhibit low viscosity, strong diluting power, and minimal volume shrinkage. Beyond serving as diluents, they can also enhance the formulation’s wetting of the substrate; some even exert a mild etching effect on the substrate, thereby improving adhesion. For example, THFA, when used with plastic substrates, can boost adhesion to such materials. However, their low viscosity and high permeability often result in significant skin irritation. Most monofunctional monomers have very low viscosities and readily penetrate the skin, so adequate protective measures are essential during handling. Due to their relatively low crosslinking density, cured monofunctional monomers generally possess limited mechanical strength, making them suitable primarily for inks or adhesives where high mechanical performance is not required—particularly low-viscosity inks. Their application in coatings that demand substantial mechanical strength is comparatively limited. Furthermore, because monofunctional monomers typically have lower molecular weights and boiling points, they can volatilize easily on production lines involving heating, potentially contaminating equipment or the environment. Therefore, when formulating, it is crucial to balance curing performance with the specific processing conditions. If the process operates at elevated temperatures, it is advisable to select monomers with higher boiling points as reactive diluents whenever possible.

Representative difunctional monomers include TPGDA, HDDA, DPGDA, NPGDA, PO2‑NPGDA, TEGDA, PEG (200/400/600) DA, PDDA, BDDA, and others. These monomers exhibit slightly higher viscosity than monofunctional counterparts. Owing to the presence of an additional double bond, they impart a relatively higher crosslink density upon UV curing, resulting in enhanced mechanical strength. By tuning the length of the intermediate chain segments, their cured films can be tailored from flexible to rigid, enabling broad applications across various UV‑curable systems.

Representative polyfunctional monomers include TMPTA, TMP(EO3)TA, PETA, PET4A, DPHA, Di‑TMP4A, THEICTA, and others. These monomers typically exhibit relatively high viscosities; owing to their multiple reactive double bonds, they undergo extensive crosslinking upon photopolymerization, yielding materials with high hardness and pronounced brittleness—though these drawbacks can be mitigated through ethoxylation modifications. Moreover, polyfunctional monomers tend to display significant volumetric shrinkage upon curing, which limits their use in 3D printing. Due to their large molecular weights and the resulting high crosslink density after curing, they are less volatile, leading to finished products with superior gloss, excellent corrosion resistance, and reduced odor. In practical applications, the cure rate of the formulation can be fine-tuned as needed, and selecting appropriate monomers may even allow them to function as resins, thereby reducing the overall resin content and facilitating more straightforward optimization of the formulation’s viscosity.

In addition to the conventional monomers described above, there are also certain monomers that not only possess an acrylate double bond but also feature other reactive functional groups, such as epoxy groups (GMA). These monomers can be used not only in radical photopolymerization systems but also in cationic curing formulations, enabling dual-cure performance. Although the number of such monomers is relatively limited, custom synthesis can be arranged with specialized manufacturers based on specific application requirements.

Different chemical structures have a significant impact on the final material properties. The table below lists the effects of several common monomer structures on these properties:

In formulations, the resin remains the primary structural backbone, particularly in applications that demand mechanical performance. Since resins used in photopolymerization are typically oligomers, the distinction between resins and monomers can sometimes be blurred. To meet target performance requirements while achieving cost-effectiveness, certain higher‑molecular‑weight monomers may be employed as resins, and highly reactive, low‑viscosity resins can be treated as monomers. Consequently, in practical applications, formulation engineers must have a thorough understanding of each monomer’s properties and the relevant regulatory requirements, and, based on specific performance needs and cost considerations, make the most appropriate selection.