**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 propulsion thrust [1]. The main aspects that influence the combustion behavior are the constituent elements and the configuration of the grains. Structurally, solid rocket composite propellants are typically based on a heterogeneous combination of distinct compounds that

**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

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**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/).

serve as fuel, oxidizer, burn-rate modifiers, and binder [1,2]. Usually, the applicability of polyurethane or polyurea matrices lies in the field of thermal insulations, hydro-isolations, adhesives, and ballistic protection [3–6]. In the case of energetic materials, they have a dual purpose, as a binding agent and as an organic fuel [1,2]. These polymeric matrices are used for binding other solid components of the rocket propellants, to protect them against environmental agents and to confer a certain geometry and mechanical strength. In state-ofthe-art composite rocket propellants, ammonium perchlorate (AP) is the preferred oxidizer, almost without exception, due to its outstanding properties [1,2,7,8]. However, in the last decade, environmental impact issues have urged research activities for a replacement with "green" oxidizers, such as phase-stabilized ammonium nitrate (PSAN) or ammonium dinitramide (ADN) [2,9–11]. A common practice in rocket propellant technology consists in adding metallic additives to increase the heat output, the density, and the specific impulse of the energetic material. Aluminum fine powder, magnesium, and boron are the main candidates as fuels in rocket propellant formulations. Together with the oxidizers and additives, the solid fuel is introduced in the polymeric matrix to ensure homogeneity, protection, and proper mechanical characteristics of the energetic mixture [2,8,12–16].

Historically, the beginning of the use of polymers in the development of solid rocket propellants is marked by the introduction of nitrocellulose (NC). NC was mainly employed in homogeneous (colloidal) rocket propellants, providing the necessary structural integrity to the propellants so that they could be molded into different geometric configurations, according to the type of launching system [1,2,17]. In heterogeneous propellants, polysulfide polymers were initially used as the binding agent. The drawback of this polymer was the high incompatibility with metallic fuels, causing safety problems occasionally leading to autoignition during long-term storage [2,18]. In this context, the necessity of developing a new class of binders became imperative. The polybutadiene chain was found to be a more suitable binder for the energetic composites, due to its high elasticity and low glass temperature. The copolymer of butadiene and acrylic acid (PBAA) was the first one to be used from this class [1,2,18,19]. The low viscosity of these binders allowed significant solid loading ratios of oxidizer and metallic fuel in the energetic composites. However, these materials exhibited poor mechanical properties. This aspect was overcome using a polybutadiene acrylonitrile copolymer (PBAN). The low cost of production, the low viscosity, and the impulse increase made PBAN, at that time, the ideal candidate as a rocket propellant binder [1,2,18–21]. However, the high temperature required to cure these propellants led to the development of other types of binders, such as carboxyl-terminated polybutadiene (CTPB). CTPB-based composites displayed significantly enhanced mechanical properties, especially at lower temperatures, in comparison with PBAA or PBAN binders, without affecting the specific impulse, density, or solid loading ratios [1,2,18,19]. The curative agents used for these types of polymers employed as binders are usually based on di- or tri-functional epoxides or aziridines [18]. Modern solid rocket propellants use hydroxylterminated polybutadiene (HTPB), an inert binder from the same class as CTPB, which ensures the optimal combination of thermodynamic and mechanical properties [7,8,18,20]. The HTPB binders exhibit superior elongation capacity at low temperatures and better ageing properties in contrast with CTPB [18,19]. The curing process of HTPB is quicker due to the use of isocyanates [19,22,23]. Despite the advantages of HTPB binders, the problem of using this type of inert binder in a composite rocket propellant is represented by the necessity of adding an extra amount of oxidizer to balance the oxygen score and achieve the appropriate performances. Consequently, it is desirable to develop more versatile polymers with substantial energetic character. The integration of azido- or nitro-functional groups on the polymeric chain is conducted for a significant performance enhancement with lower amounts of energetic materials. Among the polymers employed as energetic binders in solid rocket propellants, two worth mentioning are polyglycidyl nitrates (PGN) and glycidyl azido polymers (GAP) [10,18]. GAP are considered excellent energetic polymers due to their unique structure, which ensures a high heat of formation. These polymers are recommended for rocket propellant composites that comprise eco-friendly oxidizers

with weaker energetic characteristics, such as ammonium nitrate, to balance the energy loss caused by replacing classical oxidizers such as ammonium perchlorate [10].

In recent times, the pollution of ecosystems due to the inappropriate disposal of PET waste is widespread. Moreover, the high level of carbon dioxide emissions released into the air because of their combustion has led to the application of preventive measures. As a countermeasure to these ecological issues, interest in the development of materials based on recycled PET has achieved substantial growth [24]. As a result, in December 2015, the European Commission adopted a plan based on a circular economy strategy that will be applied in the context of reusing plastic wastes [24–27]. For obtaining eco-friendly polyols, with extensive applicability, PET waste can be recycled by undergoing a controlled degradation procedure that aims to generate oligomers that can be subsequently utilized for specific applications [27–30].

In this paper, a new group of flexible polyurethanes was synthesized from polyols obtained through the recycling of PET waste. A commercial polyol and a commercial aromatic polyisocyanate were also employed for the development of polyurethane formulations. These polyurethanes were subsequently used as binders in various rocket propellant composites, also comprising an eco-friendly oxidizer. The polyurethane networks were softened with an energetic plasticizer (triethylene glycol dinitrate, TEGDN) to boost the exothermic decomposition of these rocket propellants and to obtain composites with appropriate mechanical behavior suitable for this type of application. The synthesized eco-friendly composites can be successfully employed as rocket propellants, an aspect that was demonstrated through the subscale rocket motor experimental investigations that we performed.

Therefore, the novelty of this work consists in the innovative path of demonstrating the applicability of novel synthesized polyurethanes as binders for new environmentally friendly rocket propellants, herein reported. These original energetic mixtures, comprising newly synthesized components such as polyurethanes (from recycled PET) as the binder, PSAN as the eco-friendly oxidizer, TEGDN as the energetic plasticizer, together with aluminum for fuel and Fe2O3 as the catalyst, achieved remarkable performances as rocket propellants, demonstrable by the results obtained through the measurements performed using the subscale rocket motor.

Furthermore, the experimental data revealed that these new "green" binders could successfully replace classical binders from existing rocket propellants, since they possess comparable performances (similar thermal stability, up to 300 ◦C [31], similar mechanical properties [32]), but multiple advantages in comparison with HTPB binders [33], emphasized in the experimental section. The new rocket propellant composites, besides their environmentally friendly character, displayed better performances: up to a 30% improvement of the specific volume (in comparison with HTPB-based rocked propellant from consecrated aviation missiles). The heat of combustion (≈1000 cal/g) and Tg values (−60 ◦C to −32 ◦C) were comparable with nitrocellulose double base propellants (≈1020 cal/g and −35.5 ◦C) [34,35], which ensure that they maintain their performances even at lower environmental temperatures. The polyols obtained from PET degradation, the polyurethanes, and the composite rocket propellant formulations were investigated through specific analytical tools. Moreover, the feasibility of these novel energetic composites was demonstrated through the evaluation of the combustion rate and maximum pressure, proving that these new materials are suitable for rocket propellant applications.

Polyurethane networks, mainly those based on HTPB, are currently considered stateof-the-art propellant binder systems, being extensively used for composite solid propellants. Unfortunately, as already described above, they possess several disadvantages, among which the drawbacks of their inadequate life-cycle assessment can also be mentioned. In contrast, the materials developed in this study are especially designed to be environmentally friendly, while ensuring improved performances as new "green" alternatives for rocket propellants.

### **2. Materials and Methods**

*2.1. Materials*

2.1.1. Materials for the Catalytic Degradation of Polyethylene Terephthalate (PET)

PET (Mn ≈ 40,000–70,000 Da, melting range: 254–260 ◦C) was obtained from postconsumer bottles cut into square shaped flakes (5 mm × 5 mm). The PET flakes were washed with distilled water and dried at 100 ◦C for 6 h before being introduced into the degradation reactor. Poly(ethylene glycol) of an average Mn≈ 570–630 Da (PEG600, Sigma Aldrich, St. Louis, MO, USA), titanium(IV) isopropoxide (TTIP, 97%, Sigma Aldrich, St. Louis, MO, USA), adipic acid (AA, 99%, Sigma Aldrich, St. Louis, MO, USA), sebacic acid (SA, 99%, Sigma Aldrich, St. Louis, MO, USA), and tin(II) 2-ethylhexanoate (C16H30O4Sn, 92.5–100%, Sigma Aldrich, St. Louis, MO, USA) were used as received.
