Cationic photopolymerization in hot lithography

Abstract

In comparison to thermal polymerization, photopolymerization of resins exhibits various advantages such as solvent-free formulations, rapid curing, low energy consumption and the ability to cure with high spatial resolution. Such photopolymerizable material systems find application in lithography-based additive manufacturing technologies (L-AMTs), giving superior precision as well as smooth object surfaces and a relatively cheap setup. Since their introduction, these methods have exhibited steady growth, with applications in a variety of fields (i.e. tooling, biomedical and dental). Radical photopolymerization of conventional (meth)acrylates is currently the dominating chemistry used in L-AMTs due to their extraordinarily high reaction rates. However, drawbacks of these radical systems include inhibition by oxygen, high shrinkage upon cure and the potential risk of cytotoxicity in vivo, which greatly limits their applications. In order to overcome these difficulties, cationic photopolymerization of vinyl ethers, epoxides and oxetanes was introduced. One drawback, however, is that cationic systems need to be polymerized at short wavelengths (< 365 nm) to achieve high reaction rates, which limits the curing thickness and applications in vivo. At higher wavelengths (> 375 nm), low curing speeds of commercially available monomers are observed, hence only very few and expensive monomers can be used in L-AMTs. The aim of this work was to find ways to increase reaction rates of low-cost epoxy and oxetane monomers to make them more accessible for 3D-printing at longer wavelengths. One way to accelerate cationic photopolymerization is to increase the reaction temperature. Therefore, stereolithography at elevated temperatures, so called Hot Lithography, was applied. This enables systems containing commercially available monomers (i.e. epoxides, oxetanes and vinyl ethers) with high reaction rates. These systems were thoroughly examined via laser exposure tests to gain initial information on their processability via the Hot Lithography technology. The most promising cationic systems were then tested toward their reactivity (by means of photo DSC, RT-NIR-photorheology) and the subsequently 3D-printed parts were tested for their (thermo)mechanical performance (by means of tensile testing, DMTA and Dynstat impact test).