Cationic photopolymerization of cyclic esters at elevated temperatures and their application in hot lithography

Abstract

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.

Evaluation of Sulfonium Borate Initiators for Cationic Photopolymerization and Their Application in Hot Lithography

“…The acid scavenger’s purpose is to prevent overpolymerization during the printing process by neutralizing the migrating superacid. This acid–base mechanism of the sulfonium salts’ superacid and EDAB enhances resolution of the printed part significantly . The following photo-DSC study was performed to obtain the best suiting printing temperature for hot lithography.…”

“…In addition, the anthracene-derivative DBA was used at 0.35 mol %, and the superacid scavenger EDAB was used at 0.12 mol % based on epoxy groups according to literature. 10 The dibutoxylated anthracene shows a tail-out region up to around 425 nm, making it perfectly suitable for the printers laser source. The acid scavenger's purpose is to prevent overpolymerization during the printing process by neutralizing the migrating superacid.…”

“…This acid−base mechanism of the sulfonium salts' superacid and EDAB enhances resolution of the printed part significantly. 10 The following photo-DSC study was performed to obtain the best suiting printing temperature for hot lithography. Therefore, reactivity of the sulfonium salts was measured at 25, 50, 70, 90, and 110 °C.…”

“…The sulfonium salts were used as 1.0 mol % referred to epoxy groups of BADGE. Additionally, in some studies, the anthracene-derivative DBA was used as 0.35 mol % and the superacid scavenger EDAB was used as 0.12 mol % based on epoxy groups according to the literature . The mixtures were heated to 80 °C, homogenized in a vortex device, followed by 30 min of ultrasonic bath treatment at around 50 °C, and vortexed again to achieve a homogeneous formulation.…”

“…For example, hot lithography allows for 3D-printing of low reactive monomers and high viscosity resins such as the pure bisphenol A-diglycidyl ether BADGE, which cannot be printed at room temperature by standard SLA or digital light processing (DLP) . Hot lithography is especially suitable for temperature dependent cationic photopolymerization and enables for example the efficient printing of epoxy resins, oxazolins, and cyclic esters …”