Thermal stability and curing behavior of acrylate photopolymers for additive manufacturing

Abstract

Next-generation additive manufacturing processes based on UV-curing acrylate photopolymers extend the barriers of functional part production by deploying rapid processing speeds, complex geometries with high resolutions, and an extended material spectrum. Many technologies introduce temperature-dependent curing and decomposition behavior of acrylates to the list of process-related challenges. This investigation targets a comprehensive analysis of the curing behavior and thermal stability of acrylic photopolymers by implementing analyzing techniques like thermogravimetric analysis, Fourier-transform infrared spectroscopy, and differential scanning calorimetry. Significant parameters such as the UV intensity and the isothermal temperature are varied based on the design of experiments. Characteristic evolving gases at elevated temperatures are identified and discussed towards their relevance for the curing process. The calorimetric results demonstrate increasing reaction speeds with elevated UV intensities as well as a restricted accelerating effect of increasing temperatures. The reaction enthalpy proves to be strongly dependent on the chosen temperature. The results represent the base for the comparison of different kinetic curing models in future investigations.

Modeling of the curing kinetics of acrylate photopolymers for additive manufacturing

“…2,3 Compared with state-of-the-art photopolymer-based AM, the temperature experienced by the acrylate during processing represents a critical factor regarding the curing behavior for both technologies. The temperature along with the UV intensity can be utilized to increase or decrease the extent and speed of the curing reaction of the photopolymer, which was demonstrated experimentally in Reference [4]. Further, high temperatures proved challenging for the thermal stability of specific components of the examined acrylate systems.…”

“…The models are fitted toward experimentally determined UV-DSC data points. All experimental results were gathered and discussed in Reference [4]. The curve-fitting procedures follow two different approaches: single measurement analysis and clustered measurement analysis.…”

“…The kinetic models of this investigation are based on results from differential scanning calorimetry measurements with a UV-light source (UV-DSC). A detailed description of the experimental setup as well as a detailed analysis of the measurement results can be found in Reference [4]. The experiment parameters are summarized in Table 1.…”

“…Heating rate is 5 K/min and the cooling rate of 20 K/min is used with a five-second delay before the UV treatment. 4…”

“…However, since the curing reactions of this investigation are triggered by photon interaction and not by temperature increase, the activation energy can technically not be considered as an energy threshold provided by temperature increase. 4 Accordingly, instead of an Arrhenius-based reaction rate constant k T ð Þ (compare Equation ( 3)), a substituted version of an Arrhenius-like exponential decay function is used to describe the reaction rate constant k I,T ð Þ in dependence of the UV intensity I and the temperature T. The clustering can be performed in two ways for k I, T ð Þ, which are summarized in Table 2. In case of clustering regarding the UV intensity, k I, T ð Þ can be described by Equation ( 9):…”