Naphthyl-based acylphosphine oxide photoinitiators with high effiency and low migration

Highlights

• Naphthyl-based acylphosphine oxide photoinitiators (N1–N3) were designed and synthesized.

• N1–N3 possess red-shifted absorption spectra with high absorption molar absorption coefficients.

• In the light curing of TMPTA, N1–N3 exhibited the higher effiency than TPO.

• In the cured TMPTA, N1–N3 have lower migration than TPO.

Abstract

A series of new naphthyl-based acylphosphine oxides (N1–N3) have been designed and synthesized. The obtained compounds could be used in the photo-initiated free radical polymerization (FRP). The performances of N1–N3 in the FRP of trimethylolpropane triacrylate (TMPTA) and their UV–vis spectra were evaluated and compared with 2,4,6-trimethyl(phenyl)diphenyl oxide (TPO, a commercial photoinitiator). Results showed that N1–N3 possessed higher molar absorption coefficients and wider absorption wavelength than TPO. Under the same reaction conditions, N3 exhibited the best double bond conversion (DC) of TMPTA in the tested photoinitiators. Moreover, the calculated migration of N1–N3 of the solidified samples was 1/7, 1/3 and 1/23 of TPO, indicating the lower migration properties. Possible reasons of the performance of N1∼N3 were proposed.

Introduction

Light curing was widely used in coating, adhesive, ink, electronic packaging, 3D printing, food packaging and biomedical fields due to its efficiency, applicability, safety and environmental friendly properties [[1], [2], [3], [4], [5], [6], [7]]. Photoinitiators (PIs) are one of the most important components in the light curing system [8]. Among them, the acylphosphine oxide compounds exhibit excellent reactivity for UV and near-UV light irradiation [[9], [10], [11], [12]]. With the exposure of light, PIs (ground state) could convert to the transition state (radical or cation). And the latter is able to trigger certain monomer to polymer [13]. During the process, the matching of PIs and light is essential to achieve excellent conversion of monomer. Taking account of the safe usage and good light output, light-emitting diodes (LEDs) have been chosen as the light source. And the LEDs giving irradiation of near-UV or visible light [[14], [15], [16], [17]] are preferred. Accordingly, great efforts have been made in the design of PIs with long wavelength of light absorption. For example, Henne and Wolf et al. synthesized several acyl oxides and acyl sulfide with light absorption up to 600 nm [18,19]. In our previous work, by introducing of diethyl amino group into the acyl group, an acyl phosphine compound was synthesized. This compound exhibited red-shifted absorption spectrum with high absorption molar absorption coefficients [20].

Besides the absorption of long wavelength light, the migration of PIs is another issue that should be noticed. Migration means the transfer of unreacted species to the surface of materials. Such transfer is very important in food packaging and would confine the application of light curing in this field and other materials in direct contact with human body [21]. However, as reported by Cai et al., the newly synthesized initiating agents commonly possessed severe migration after photopolymerization [22]. Xiao et al. synthesized naphthalimide based methacrylated PIs for the photo-initiated polymerization under visible light [23]. Although the PIs exhibited high light curing efficiency, higher migration of PIs was also detected. From this point of view, reducing of the migration of PIs in light curing materials needs to be solved urgently.

In order to lower the migration and improve the absorption properties and curing efficiency of PIs, a series of monoacylphosphine oxides containing different naphthyl groups was designed and synthesized (N1-N4 in Fig. 1). The performances of these PIs in the FRP of TMPTA have been evaluated and compared with traditional photoinitiator (TPO). The migrations of PIs from the cured sample were also discussed.