Christian Gorsche

Investigation of a cyanate ester based formulation and stereolithography-based additive manufacturing via Hot Lithography to produce polycyanurates with a glass transition temperature of 336 °C without any additives.

The 3D printing of polyesters is most commonly performed using polylactide filaments for material extrusion (formerly known as fused‐deposition modelling) or by introducing methacrylate end‐groups to polyesters, which can be polymerized by a radical mechanism. However, cyclic esters can also be polymerized via cationic ring‐opening photopolymerization, which opens up the possibility of using high‐resolution stereolithography as the method of choice to form polyester networks in situ during the printing process. Hereby, we present the first approach to 3D print polyesters starting directly from ε‐caprolactone via stereolithography at elevated temperatures, by introducing low amounts of crosslinks into the material to provide stability to the structure but maintain its degradable character and especially semicrystallinity. Photorheology tests showed that high temperatures and the introduction of crosslinkers led to increased reactivity and fast gelation. By performing tensile tests, dynamic mechanical thermal analysis and simultaneous thermal analysis the material properties were evaluated depending on the amount and type of crosslinker. It was shown that decreasing the amount of crosslinker significantly increased the crystallinity and influenced the mechanical properties as well as shape memory properties. Finally, successful 3D printing of a polyester was performed using 10 mol% of a crosslinker with oxetane moieties and 90 mol% ε‐caprolactone at 120 °C. © 2022 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

In lithography‐based additive manufacturing, mostly crosslinking monomers with acrylate functionality are applied, which yield brittle materials with inhomogeneous network architectures. The toughening of state‐of‐the‐art materials is an integral requirement for the advancement of photopolymer‐based three‐dimensional (3D) products. Here we show that the final material properties of acrylate networks can be adjusted through regulation of the radical curing mechanism using difunctional vinyl sulfonate esters to obtain toughened 3D structured materials. A substantial improvement of the thermomechanical behavior of resulting materials over materials regulated by state‐of‐the‐art reagents (e.g. thiols) is presented and first 3D parts have successfully been printed. Resulting materials exhibit reduced shrinkage stress, reduced warpage, higher overall conversion and higher glass transition temperatures compared with the pure acrylate network. © 2022 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Additive manufacturing of phenolic resins as a proof-of-concept for the first photo-chemical induced polycondensation by Hot Lithography. Through the dual use of a photoacidgenerator, the first pure 3D printing of Bakelite^© is investigated.

The terpolymer acrylonitrile‐butadiene‐styrene (ABS) is a widely used thermoplastic material due to its excellent mechanical properties, especially high toughness. However, the monomer system of ABS cannot be feasibly photopolymerized due to its reactivity, opacity and monomer volatility. We show the transfer of an ABS microstructure to photopolymers via monomer systems designed to mimic ABS while remaining photopolymerizable. Acrylonitrile was substituted by more reactive and less volatile maleimides, of which the N substituent influences crosslinking considerably. Instead of styrene, less volatile derivatives were utilized as comonomers. Poly(butadiene) was introduced as cheap, readily available and non‐volatile rubber. The resulting maleimide‐styrene‐poly(butadiene) networks exhibit varying microphase separations and simultaneous transparency. While optimized materials cannot quite exhibit the yield strain of hot‐pressed ABS filament, their toughness partly exceeds that of ABS. Superior thermal stabilities and glass transition temperatures up to 190 °C were observed. Finally, stereolithographic printing of one tuned monomer system was conducted. © 2021 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

In the last decades the importance of UV curable formulations has increased continuously. Their fast curing speed, solvent‐free polymerization conditions, and the formation of hard and highly crosslinked photopolymer networks represent major benefits. Commercial UV resins generally consist of multi‐functional vinyl oligomers, photoinitiators, additives, and reactive diluents. Mono‐ and multi‐functional reactive diluents serve as thinners to lower the overall resin viscosity and to improve processability. However, many monofunctional reactive diluents like isobornyl (meth)acrylate or benzyl (meth)acrylate exhibit high volatility, often already at room temperature. This causes adverse effects such as unpleasant odor, potential health risks, and changing resin composition during processing. A new group of monomers that show high potential for replacing traditional highly volatile reactive diluents are salicylate (meth)acrylates. In this work, salicylate‐based thinners are synthesized, polymerized, and characterized with respect to their viscosity, volatility, thermal stability, photoreactivity, and thermomechanical properties of their homopolymers. Additionally, a first example of their diluting effect in a highly viscous difunctional polyester urethane methacrylate is demonstrated with 30 wt% of a cycloaliphatically and an aromatically substituted salicylate methacrylate. The polymers of the diluted resin exhibit similarly high glass transition temperatures of 110 and 126 °C, which are in the range of the polymers of the undiluted resin.

Polymerization-induced shrinkage stress within bulk photopolymer networks represents one of the most pressing challenges for their application in 3D-printing, microelectronics and dentistry. The occurring shrinkage stress within commercial (meth)acrylate-based networks results from a combination of the covalent attachment of monomers and the gelation at early stages of the reaction. Alternatively, cyclic monomers (e.g. 1,1-disubstituted 2-vinylcyclopropanes, VCPs) have been reported as a possible monomer class yielding reduced polymerization shrinkage and consequently shrinkage stress due to a radical ring opening reaction. Another way to reduce the occurring shrinkage stress is to regulate the radical network formation via an addition fragmentation chain transfer (AFCT) concept. Herein, the light-induced radical polymerization of a combination of VCP monomers with variable amounts of an AFCT reagent (an ester-activated vinyl sulfonate ester EVS) was investigated. A high reactivity towards light-induced radical polymerization is confirmed for VCP/EVS mixtures via photoreactor, photo-DSC and real time (RT)-NIR-photorheology accompanied by a significant increase in CC double bond conversion with increasing amounts of EVS. Most importantly, both systems (VCPs and AFCT reagents) combined lead to an enhanced reduction of polymerization-induced shrinkage stress. The resulting materials showed, even at low concentrations of EVS, a high network homogeneity (indicated by a narrow loss factor plot in DMTA). Also, filled systems performed well with respect to reduced shrinkage force while maintaining sufficient E-modulus and flexural strength. The presented material concept has great potential for dental materials and lithography-based 3D-printing.