Design of Phosphate-based Dynamic Covalent Network for Fused Deposition Modeling
Q.S. Cornelissen (TU Delft - Mechanical Engineering)
S. Kumar – Coach (TU Delft - Mechanical Engineering)
Rint P. Sijbesma – Mentor
J.P.A. Heuts – Graduation committee member
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Abstract
Additive manufacturing (AM) is considered an environmentally friendly manufacturing process that builds solid 3D structures layer-by-layer from a computer-aided design (CAD), resulting in reduced waste compared to conventional subtractive manufacturing. However, a significant challenge in AM is the extensive use of plastics, which lack a standardized recycling process. Insufficient adhesion between printed layers contributes to waste generated with AM. Therefore, it is important to prioritize sustainability as a design parameter for future AM materials.
Dynamic Covalent Networks (DCNs) offer a potential solution by combining the desirable characteristics of thermosets with the recyclability of thermoplastics. DCNs undergo bond rearrangement reactions influenced by external stimuli such as heat or light, leading to changes in their topology. Bond exchange reactions can occur through two mechanisms: dissociative, involving separate steps of bond breaking and forming, or associative, involving simultaneous bond breaking and forming.
Recently, a phosphate triester-based DCN was developed, obtaining a neighboring β-hydroxyl group which could perform transesterification exchange reactions within the network via the formation of a cyclic phosphate triester intermediate in a dissociative manner. Based on this network rearrangement, we synthesized a network by reacting phosphoric acid with a diglycidyl ether to perform ring-opening reactions, creating a pendent β-hydroxyl functional group that assists in neighboring group transesterification.
The reversibility of the bond exchange reaction within the network was investigated using a heating and cooling cycle with variable temperature (VT) ³¹P solid-state NMR (SSNMR). Furthermore, the network was reprocessable using compression molding at 130 °C. Fast relaxation times of 5 seconds at 200 °C were observed. Additionally, frequency sweep and dynamic mechanical temperature analysis (DMTA) experiments showed profiles that were expected for a dissociative bond exchange mechanism.
However, an increase in the storage modulus at 150 °C was observed, indicating a curing process of the network. Subsequently, similar experiments were performed on the cured network, in which a reduction of the dynamic properties of the network was noted, with a stress relaxation time of 114 seconds at 200 °C. DMTA and frequency sweep experiments confirmed its increased storage modulus as well.
Nevertheless, the uncured network was reprocessable via extrusion at 200 °C. However, it required at least 50 minutes for the network to obtain a viscous flow behavior suitable for optimal extrusion.