Isocyanate-based high performance photopolymers

Abstract

The development of radiation based curing has had a big impact on coating industry and additive manufacturing. It offers the possibility to manufacture technically advanced products in an economical way. Contrary to thermal curing, where heat exposure is mandatory to cure the formulation, radiation based curing typically does not utilize thermal treatment. Radiation-based curing triggers the curing process through light, which is an easy, fast and energy efficient possibility to obtain innovative products.Currently, industrially used materials for radiation-based curing are limited considering their mechanical properties, resulting in very brittle materials. Therefore, current research focuses on the toughening of these brittle networks. A possible improvement of the mechanical properties of the network offer the inclusion of urethane functionalities. Urethane groups inside the polymer network provide the formation of non-covalent inter- and intramolecular interactions, which further improve the modulus and the toughness of the formed network. Nowadays industrially used urethane containing radiation curable materials are (meth)acrylate which exhibit due to their flexible spacers a lower Tg and improved toughness compared to the pure (meth)acrylate polymers. The implementation of materials solely consisting of urethane bonds and accompanying the renunciation of other functionality forming monomers like (meth)acrylate to this selection would be a beneficial addition. In comparison, urethane based materials, also known as polyisocyanurates, provide impressive thermal and mechanical properties as a result of their structural characteristic of cyclotrimerized isocyanate and would offer a potential new approach to isocyanate-based high performance photopolymers.Aim of this work was to synthesize and investigate new isocyanate-based high performance materials via photopolymerization. To achieve this, the catalytic conditions for the cyclodimerization and cyclotrimerization of aliphatic and aromatic monofunctional isocyanates were investigated. The free bases 1,5-diazabicyclo(4.3.0)non-5-en, 1,1,3,3 tetramethylguanidin, triethylamine, tributylphosphine, triphenylphosphine, 1,2 dimethyl imidazole and 2-butyl-1-ethylhexyl imidazole were used for investigation of the cyclodimerization and cyclotrimerization of phenyl isocyanate and hexyl isocyanate. For successful cyclization experiments with the free bases, the corresponding photobase generators (PBGs) were synthesized to investigate the analogue light-activated reaction with respect to photopolymerization efficiency and thermal stability in the dark. A photobase generator is an organic compound, which generates upon irradiation with light an active species which further catalyzes the desired reaction. As a result of the high reactivity of isocyanates the majority of the synthesized PBGs exhibited dark reactivity in the presence of the isocyanates already at ambient temperatures. However, we identified a promising imidazole PBG, which afforded a stable reaction mixture. In future work, their solubility and reactivity will be optimized to afford cyclodimerization and cyclotrimerization of isocyanates suitable for coating applications and potentially even light-based 3D printing.