Water Soluble Initiators

Due to ecological reasons water based resin formulation were introduced during the last decades and are nowadays state of the art. Nevertheless, several inadequate characteristics of radiation curable formulations have to be considered. Beside insufficient mechanical and chemical characteristics of the cured film, photoinitiators are often poorly soluble or they are evaporated during the pre-drying step. Improved solubility of PIs is often achieved by the introduction of ionic groups, as found in various commercially available products. Application of these initiators can lead to decreased stability of emulsion type resins, as well as poor compatibility of the PI after evaporation of the water. Recent developments are dealing with polyethylene oxide residues. Also renewable resources such as glucose or glucamine are not only environmentally friendly, they also give high reactivity and good compatibility.

Especially in the field of acidic dental primer formulations special requirements are given. Bimolecular initiators suffer from low reactivity due to the solvent cage effect, which can be avoided by phenyl glycine based coinitiators that undergo spontaneous decarboxylation. [2] Monomolecular acylphosphine oxides with hydrophilic residues have been introduced recently as highly reactive alternative. [3] Very recently, Li and Na-salts of mono- and bis-acylphosphine oxides have been introduced that show high water-solubility and reactivity in water-based formulations. Low cytotoxicity makes them suitable for biomedical applications [4].

[1] R. Liska: "Photoinitiators with functional groups: V. New water-soluble photoinitiators containing carbohydrate residues and copolymerizable derivates thereof"; Journal of Polymer Science Part A: Polymer Chemistry, 40 (2002), S. 1504 - 1518.

[2] G. Ullrich, P. Burtscher, U. Salz, N. Moszner, R. Liska: "Phenylglycine Derivatives as Coinitiators for the Radical Photopolymerization of Acidic Aqueous Formulations"; Journal of Polymer Science Part A: Polymer Chemistry, 44 (2006), S. 115 - 125.

[3] G. Ullrich, B. Ganster, U. Salz, N. Moszner, R. Liska: "Photoinitiators With Functional Groups. IX. Hydrophylic Bisacylphosphine Oxides for Acidic Aqueous Formulations"; Journal of Polymer Science Part A: Polymer Chemistry, 44 (2006), S. 1686 - 1700.

[4] S. Benedikt, J. Wang, M. Markovic, N. Moszner, K. Dietliker, A. Ovsianikov, H. Grützmacher, R. Liska: "Highly efficient water-soluble visible light photoinitiators"; Journal of Polymer Science Part A: Polymer Chemistry, 53 (2015), 24.