polymers Article Novel Polyurethanes Based on Recycled Polyethylene Terephthalate: Synthesis, Characterization, and Formulation of Binders for Environmentally Responsible Rocket Propellants Florin Marian Dîrloman 1,† , Gabriela Toader 1,† , Traian Rotariu 1, *, Tudor Viorel T , igănescu 1,2 , Raluca Elena Ginghină 3 ,Răzvan Petre 3 , Florentina Alexe 3 , Mihai Ionut , Ungureanu 1 , Edina Rusen 4 , Aurel Diacon 4 , Adi Ghebaur 4 , Monica Duldner 5 , Alina Elena Coman 5 and Robert T , incu 6   Citation: Dîrloman, F.M.; Toader, G.; Rotariu, T.; T , ig˘ anescu, T.V.; Ginghin˘ a, R.E.; Petre, R.; Alexe, F.; Ungureanu, M.I.; Rusen, E.; Diacon, A.; et al. Novel Polyurethanes Based on Recycled Polyethylene Terephthalate: Synthesis, Characterization, and Formulation of Binders for Environmentally Responsible Rocket Propellants. Polymers 2021, 13, 3828. https://doi.org/10.3390/ polym13213828 Academic Editor: Sándor Kéki Received: 30 September 2021 Accepted: 28 October 2021 Published: 5 November 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/). 1 Military Technical Academy “Ferdinand I”, 39-49 George Cosbuc Boulevard, 050141 Bucharest, Romania; florin.dirloman@mta.ro (F.M.D.); gabriela.toader@mta.ro (G.T.); viorel.tiganescu@mta.ro (T.V.T , .); mihai.ungureanu@mta.ro (M.I.U.) 2 Military Equipment and Technologies Research Agency, 16 Aeroportului Street, Clinceni, 077025 Ilfov, Romania 3 Research and Innovation Center for CBRN Defense and Ecology, 225 Soseaua Oltenitei, 041327 Bucharest, Romania; raluca.ginghina@nbce.ro (R.E.G.); razvan.petre@nbce.ro (R.P.); florentina.alexe@nbce.ro (F.A.) 4 Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania; edina.rusen@upb.ro (E.R.); aurel.diacon@upb.ro (A.D.); adi.ghebaur@upb.ro (A.G.) 5 National Institute of Research and Development for Chemistry and Petrochemistry, 202 Splaiul Independentei, 060041 Bucharest, Romania; monica.duldner@icechim.ro (M.D.); alina-elena.coman@icechim.ro (A.E.C.) 6 Center for Organic Chemistry “C.D. Nenitescu” of the Romanian Academy, 202B Splaiul Independentei, 060023 Bucharest, Romania; tincurobert@gmail.com * Correspondence: traian.rotariu@mta.ro † Co-first authors, equally contributed to this work. Abstract: Novel polyurethane-based binders, specifically designed for environmentally responsible rocket propellant composites, were obtained by employing the polyester-polyols that resulted from the degradation of polyethylene terephthalate waste. A new class of “greener” rocket propellants, comprising polyurethanes (based on recycled PET) as the binder, phase stabilized ammonium nitrate (PSAN) as the eco-friendly oxidizer, and triethylene glycol dinitrate (TEGDN) as the energetic plasticizer, together with aluminum as fuel and Fe 2 O 3 as the catalyst, is herein reported. The components of the energetic mixtures were investigated (individually and as composite materials) through specific analytical tools: 1 H-NMR, FT-IR, SEM-EDX, DTA and TGA, tensile and compression tests, DMA, and micro-CT. Moreover, the feasibility of this innovative solution is sustained by the ballistic performances exhibited by these composite materials in a subscale rocket motor, proving that these new formulations are suitable for rocket propellant applications. Keywords: polyethylene terephthalate recycling; polyurethane; binder; polymeric composite; rocket propellant; mechanical properties; subscale rocket motor 1. Introduction Solid rocket propellants are a particular class of energetic materials that are developed to ensure the propulsion of spacecrafts, missiles, and rockets toward a target. The main energetic transformation is represented by steady combustion in an enclosed environment, specifically designed to release hot gases at high speeds to obtain the appropriate propul- sion thrust [1]. The main aspects that influence the combustion behavior are the constituent elements and the configuration of the grains. Structurally, solid rocket composite pro- pellants are typically based on a heterogeneous combination of distinct compounds that Polymers 2021, 13, 3828. https://doi.org/10.3390/polym13213828 https://www.mdpi.com/journal/polymers