Tailored urethane methacrylates for tough, thermoplastic-like photopolymers via hot lithography

Abstract

Additive manufacturing technologies (AMTs) or 3D-printing is a fast growing industrial sector for processing different materials, such as polymers, metals and ceramics. This industry enables the manufacturing of customized products with complex geometries. Especially, lithography-based additive manufacturing technologies (L-AMTs) offer the advantage of high resolution and high accuracy. Usually, L-AMTs process liquid photoresins that are cured upon UV/Vis irradiation, yielding highly cross-linked networks with high hardness and heat deflection. However, the high hardness of photopolymers comes along with high brittleness that limits the field of application. Consequently, toughening of photopolymers represents a point of main effort in research and industry. In order to develop tougher photopolymers, this work focuses on the synthesis and the analysis of different high molecular weight (15-35 kDa) urethane dimethacrylate toughness-modifiers (TNMs), consisting of flexible polyether soft segments and rigid urethane hard blocks. Their strong hydrogen bonds and the high molecular weights lead to increased elongation at break and toughness. First, the necessity of the methacrylate end group of the TNMs was tested. Furthermore, the influence of arrangement of the hard and soft segments within the toughness-modifiers was investigated. All measurements were carried out in formulations containing a crosslinker and a reactive diluent. They were compared considering their photoreactivity, leaching behavior and (thermo)mechanical properties for example with tensile tests and DMTA measurements. Photopolymers consisting of these compounds lead to tough thermoplastic-like materials with high hardness and heat deflection. However, the resins show a high viscosity, due the strong intermolecular forces and the high molecular weights of the TNMs. Thus, they are processed at elevated temperature. To use additive manufacturing for such viscous resins, Cubicure GmbH developed Hot Lithography as potential processing method.