4D printing and recyclable polymers Hytham Hassan , Veerle Vandeginste KU Leuven – Brugge, Department of Materials Engineering, Division Surface and Interface Engineered Materials, Belgium 4D printing is a promising branch of additive manufacturing and refers to the process of fabricating structures with shape memory properties from polymer filaments. Thus, 4D printing is similar to 3D printing, but with the added dimension of shape change over time 1 . The shape change is triggered by an external stimulus, which may be light, pH changes, electric and magnetic fields, as well as surrounding temperature. The filaments can be produced through melting and extrusion of recycled polymers, upon removal of impurities and addition of desired fillers and can result in 3D printable shape memory composites. Thermoplastic polymers are considered highly recyclable, thanks to their facile reheating and molding into new shapes. Moreover, some thermoplastic polymers feature excellent shape memory properties, thus providing eco-friendly solutions for 4D printed materials. These materials represent promising candidates in a wide range of applications, including soft actuators, sensor devices, artificial muscles, stents and catheters. New thermoset polymers with high strength, good recyclability, and excellent shape memory properties are also developed, and they can be 3D printed using direct light printing 2 , which has emerged as a new reliable 3D printing technique with high resolution, and high printing speed 3 . The properties of shape memory polymers may be largely modified, through the addition of fillers, which may help in lowering transition temperature, enhancing mechanical properties, and even changing the type of external stimuli that induce shape recovery. To conclude, 4D printing represents an effective solution for obtaining eco-friendly shape memory composites, with complex shapes and structures, tunable properties, and a large variety of shape memory applications. References 1. 1 P. Imrie and J. Jin, Polymer 4D printing: Advanced shape, change and beyond, J. Polym. Sci. , 2022, 60 , 149–174. 2. 2 A. Li, A. Challapalli and G. Li, 4D Printing of Recyclable Lightweight Architectures Using High Recovery Stress Shape Memory Polymer, Sci. Rep. , 2019, 9 , 7621. 3. 3 Z. Chen, M. Yang, M. Ji, X. Kuang, H. J. Qi and T. Wang, Recyclable thermosetting polymers for digital light processing 3D printing, Mater. Des. , 2021, 197 , 109189.
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