The usage of photopolymers for Additive Manufacturing (AM) technology Vat Photopolymerization (VP) has increased in recent years. Reasons for this increase include the successful efforts to advance materials’ mechanical, optical, chemical and thermal properties. Digital Light Synthesis (DLS) is one of most recent developments in this field.
Learn more about DLS, also known as Continuous Liquid Interface Production (CLIP), in our video series Material Science in Additive Manufacturing. Click here to get to the video!
What are dual curing resins?
Not only did the California-based company Carbon, Inc. develop the DLS process, but also pioneered a two-part resin system.
Part A and part B of the resin system “are initially mixed in a predefined mixing ratio and attain their dimensional stability through ultraviolet (UV)-curing during DLS. However, their final mechanical properties are reached by the sequential thermal curing in a convection oven.” 
While this dual curing mechanism makes a wide range of materials suitable for DLS, the total processing time is increased by additional curing in the convection oven that may take several hours to achieve complete curing.
The challenge of AM photopolymers
The major challenge of photopolymers is their brittleness. Materials only achieve the status as a manufacturing standard if highly consistent and reproducible properties as well as process stability are guaranteed. “Thermal curing and thus temperature is a decisive factor for dual curing systems. However, little is known so far about the consequences of increased temperatures, e.g., due to the strong exothermic reaction heat caused by the radical photopolymerization during DLS or higher room temperatures.” 
Research question and objectives
The research paper “Investigation of the temperature influence on the dual curing urethane-methacrylate resin Rigid Polyurethane 70 (RPU 70) in digital light synthesis (DLS)” was written by J. Bachmann, E. Gleis, G. Fruhmann, J. Riedelbauch, S. Schmölzer and O. Hinrichsen. It aims to “characterize the influence of temperature on a dual-curing resin […] and to provide a deeper understanding of the chemical reaction mechanism of the RPU 70 in the dual curing process.” 
The full paper is available here: https://doi.org/10.1016/j.addma.2020.101677
Tracking the complete reaction and the thermal conversion with Photo-DSC
“The thermal conversion of the liquid resin into an elastomer was analyzed with viscosity measurements, Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and Photo-DSC.” 
The analysis with the NETZSCH Photo-DSC 204 F1 Phoenix® was the most effective method in tracking the thermal conversion as well as to identify optimal exposure times and exposure intensity for UV reactive resins.
The Photo-DSC is therefore suitable to track the complete photopolymerization process of dual curing resins in DLS and additionally detect thermal post-curing.