  Citation: Kristiawan, R.B.; Rusdyanto, B.; Imaduddin, F.; Ariawan, D. Glass Powder Additive on Recycled Polypropylene Filaments: A Sustainable Material in 3D Printing. Polymers 2022, 14, 5. https://doi.org/10.3390/ polym14010005 Academic Editors: Arpan Biswas and Neha Tiwari Received: 17 November 2021 Accepted: 17 December 2021 Published: 21 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Glass Powder Additive on Recycled Polypropylene Filaments: A Sustainable Material in 3D Printing Ruben Bayu Kristiawan, Boby Rusdyanto, Fitrian Imaduddin * and Dody Ariawan Mechanical Engineering Program, Faculty of Engineering, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Surakarta 57126, Indonesia; yrbk.ruben@student.uns.ac.id (R.B.K.); bobyrusdyanto20@gmail.com (B.R.); dodyariawan@staff.uns.ac.id (D.A.) * Correspondence: fitrian@ft.uns.ac.id Abstract: This study aimed to characterize the effect of a glass powder additive on recycled polypropy- lene (rPP) materials from food packaging to be used as filaments in material extrusion (MEX) 3D printing applications. The composite filaments studied were rPP filaments with glass powder (GP) additive in the 2.5%, 5%, and 10% fractions. As a baseline, the filaments made of pure virgin PP and rPP without additive were used. The filament that has been successfully made is then printed into a tensile test specimen and an impact test to observe its mechanical properties. Fourier-transform infrared spectroscopy (FTIR) characterization was also carried out to determine the effect of chemical bonding and thermal characterization using thermogravimetric analysis (TGA) and differential scan- ning calorimetry (DSC). The results of FTIR characterization on the sample rPP + 10% do not show a typical peak shift of PP, but give rise to new peaks at wavenumbers of 1000 cm −1 (Si-O-Na), 890 cm −1 (Si-H) and 849 cm −1 (O-Si-O), which indicate the typical peaks of the glass constituent compounds. In the thermal characteristics, the addition of GP shows the improved stability of mass changes to heat and increases the melting temperature of rPP. The ultimate tensile strength and Young’s modulus for rPP-based specimens with 10% GP additive showed an increase of 38% and 42% compared to PP specimens. In addition to the improved mechanical strength, the addition of GP also reduces the bending deformation, which can be well controlled, and reduces curvature, which is a problem in semicrystalline polymer-based filaments. Keywords: material extrusion; 3D printing; glass powder; composite filament; recycle polypropylene 1. Introduction Since its discovery in 1954 and the start of commercial production by Montecatini in Ferrara in 1957, polypropylene (PP), a thermoplastic polymer, has become a successful commercial product [1,2]. Now, PP isthe fastest-growing commodity. Consumer demand for PP is very high, making it one of the most significant plastic commodities. PP is gener- ally used as a plastic packaging material due to its good chemical resistance and ease of processing methods. Unfortunately, PP packaging has a relatively short lifespan, making it one of the most found plastic waste, causing a harmful impact to the environment [3,4]. Recently, the growing trends of additive manufacturing (AM) methods present an oppor- tunity to recycle thermoplastics to be used as AM feedstocks. Nearly 30 years since its conception, AM has gradually overcome specialized applications and revolutionized all kinds of practices in various manufacturing industries. AM offers a potential solution when conventional manufacturing reaches its technological limits. These include a high degree of design freedom, lightweight design, functional integration, and rapid prototyping. These advantages have led to AM being adopted since its beginning in the aerospace and defense industry, especially the U.S. military, for test parts in drones and satellites [5–7]. The principle of AM processing for polymer materials according to ASTM 52900 is divided into two categories, thermal reaction bonding and chemical reaction bonding. In general, thermal reaction bonding is more widely used in material extrusion (MEX) 3D Polymers 2022, 14, 5. https://doi.org/10.3390/polym14010005 https://www.mdpi.com/journal/polymers