High potential of new dyeing photoinitiators for fast curing of (meth)acrylate compositions under low intensity UV–Vis light

Abstract

The present work describes novel two-component photoinitiating systems for radical polymerization of (meth)acrylate-based compositions. The proposed photoinitiators consist of a squaraine dye as a photosensitizer and various types of onium salts, which play the role of a co-initiator. Two novel symmetrical squaraines, namely, 1,3-bis(6-chlorobenzothiazolamino)squaraine (SQN1) and 1,3-bis(2-methylbenzothiazolamino)squaraine (SQN2), were synthesized and used in combination with two commercially available co-initiators, diphenyliodonium chloride (I1) and N-methoxy-4-phenylpyridinium tetrafluoroborate (NO), and the newly synthesized N-alkoxypyridinium salt, i.e. N-methoxy-4-(3-phenylpropyl)pyridinium tetrafluoroborate (NO1). These systems were used as highly active photoinitiators for radical polymerization of (meth)acrylates activated by UV-Vis light. The physicochemical properties of the new photoinitiators were evaluated. The great features of squaraines, i.e. high molar extinction coefficients and sharp and broad absorption bands, make them good light absorbers in the UV–Vis region. The proposed sensitizers absorb light in the range of 300–500 nm. The activity of bimolecular photoinitiators was studied via photodifferential scanning calorimetry (photo-DSC). On the basis of the results obtained, the kinetic parameters, such as the rate of polymerization (R_p) and the final monomer conversion (FC), were estimated. The polymerization experiments were carried out using methacrylate monomers, namely, 2-hydroxyethyl methacrylate (HEMA) and triethylene glycol dimethacrylate (TEGDMA), and acrylates with different numbers of reactive groups, namely, 1,6-hexanediol diacrylate (HDDA), 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) and pentaerythritol tetraacrylate (PETRA). It was found that squaraine derivatives in the presence of onium salts I1, NO and NO1 effectively initiated the radical polymerization of methacrylate and acrylate monomers. A final conversion between 40% and 100% for methacrylates and in the range from 30% to about 100% for acrylates was observed.