Low migration type I free radical polymerization photoinitiator
A recently published study focuses on novel acylphosphine oxides as high-performance and low migration type I photoinitiators for free radical polymerization.
Research Summary
Two low migration acylphosphine oxide photoinitiators containing unsaturated C double bonds (APO and DAPO) were designed and synthesized for free radical polymerization. The properties and performance of APO and DAPO in the free radical polymerization of trimethylolpropane triacrylate (TMPTA) were evaluated and compared with commercial photoinitiator 2,4,6-trimethyl (phenyl) diphenyl ether (TPO).
Development Trends
Photopolymerization is a green and developing technology that uses light sources to induce resin polymerization without the need for solvents, and has the advantages of high efficiency, safety, and environmental friendliness. Photoinitiator is the most important component in the photopolymerization system, which must effectively absorb light to generate active substances such as free radicals, cations, or anions. There are two reaction pathways for free radical photoinitiators: one is the alpha cleavage of chemical bonds under light irradiation (type I), and the other is the bimolecular hydrogen extraction reaction between the initiator and co initiator (type II). In the past decade, with the rapid development of light emitting diodes (LEDs), UV-LED radiators for photopolymerization have attracted great interest in many application fields such as ink, coating curing, adhesives, and 3D printing. UV-LED can provide monochromatic light with a near Gaussian distribution and relatively narrow bandwidth (typically 20-30 nm), which can be used at 365 nm, 385 nm, 395 nm, 405 nm, and 420 nm. Therefore, designing and synthesizing photoinitiators that absorb ultraviolet light in the wavelength range of 365-420 nm and effectively generate free radicals is a key issue in the field of UV-LED sensitized photopolymerization. According to reports, some acylphosphine oxide PIs can induce photopolymerization under LED irradiation.
2,4,6-trimethylphenyl diphenyl ether (TPO) is a commercial photoinitiator with a maximum absorption wavelength of 380nm, belonging to the free radical cleavage type (Type I) photoinitiator. TPO has high free radical yield, high induction efficiency, and excellent color stability, making it widely used in industrial fields such as UV-LED light curing coatings, inks, biomedical materials, etc. However, it also has some drawbacks, the most important of which is its high migration rate. In recent years, the high migration rate of TPO after curing ultimately poses a threat to human health, limiting its application in many industrial fields such as environmentally friendly UV curable coatings and food packaging inks. Therefore, inhibiting the migration of traditional photoinitiators is an unavoidable key issue.
At present, there are two types of photoinitiators that can achieve lower migration rates after photopolymerization: one is to increase the weight of the photoinitiator molecules, and the other is to design polymerizable photoinitiators containing unsaturated carbon carbon double bonds. Increasing molecular weight can be achieved by adding a selected higher molecular weight portion to the acyl group to obtain a phosphine oxide structure. Recently, research on acylphosphine oxide photoinitiators has mainly focused on the modification of phosphorus atoms to observe their effects on stability, initiation performance, and migration rate. However, there is little research on the migration rate of acylphosphine oxide derivatives containing carbon carbon unsaturated double bonds.
The possibility of industrial applications
The results showed that under the same reaction conditions, DAPO had the best double bond conversion rate (DC) for TMPTA among the tested photoinitiators; The migration rates of APO and DAPO are 2/3 and 1/3 of TPO, respectively, which are lower than those of conventional photoinitiators. Moreover, the solubility of DAPO in monomers is similar to that of TPO, providing the possibility for the industrial application of DAPO.
