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R. Taschner, P. Gauss, P. Knaack, R. Liska:
"α-Ketoesters as non-aromatic photoinitiators for radical polymerization of (meth)acrylates";
Poster: European Polymer Congress 2019 (EPF 2019), Crete, Greece; 09.06.2019 - 14.06.2019; in: "European Polymer Congress 2019 (EPF 2019)", (2019), S. 468.

Photopolymerization of (meth)acrylate-based formulations has become a widespread method for many industry sectors due to the high energy efficiency and low curing times of this technology. Various products, from simple coatings to more complex applications are based on this method. Common industrial radical photoinitiators are generally based on aromatic ketones with the benzoyl-chromophore as the key constituent. In medical or food packaging applications, residual photoinitiator or photoproducts migrating into the packaged product have to be avoided, particularly of toxicological reasons. Therefore a new generation of non-aromatic initiator systems was developed. Besides their high reactivity in (meth)acrylic formulations, they show good bleaching properties and high biocompatibility.


Fig 1: pathway of aromatic ring exchange in the molecule to achieve α-ketoester structure
The benzoyl chromophores of cleavable photoinitiators are generally also problematic as various photoproducts are generated during the curing reaction. Especially volatile and odorous compounds such as benzaldehyde can be problematic at the production site or when it comes to food packaging. [1] Degradation and recombination products of aromatic initiators are potentially mutagenic or toxic to the human body. [2] Therefore, even safe initiators can lead to substances migrating out of the resulting polymer network and becoming hazardous. So non-aromatic photoinitiators are of high interest for industrial applications.
α-Ketoglutaric acid is a metabolite in the human body, therefore a highly biocompatible, non-volatile photoinitiator based on the ketoester concept. Additional there are two main approaches to limit the migration of those initiators after curing, by synthesizing macromolecular or polymerizable derivatives. Compared to benzophenone-amine systems, the α-ketoesters show better mechanical properties in terms of glass transition temperature increase, raised storage modulus at elevated temperatures, enhanced tensile strength and higher rates of polymerization, which corresponds to the reactivity of a photoinitiator, in an (meth)acrylate based monomer system.

Fig. 2: α-ketoglutaric acid based initiators
The photoproducts of α-ketoesters underwent toxicological tests and were considered harmless to humans [3]