**1. Introduction**

Nowadays, war migrates from the battlefields towards less predictable areas. All existing ammunition types and improvised explosive devices represent serious threats. The survival of defense, public order, or national security crews, and the vehicles that they use during a mission, is conditioned by several factors, including ballistic protection. Specific categories of threat require different levels of ballistic protection, regulated by the standards valid in each state.

New effective solutions for ballistic protection are offered by the evolution of composite materials, the synthesis of new materials, and the selection and correlation of performant materials [1]. These materials must be resistant to the action of a shock wave or the impact of a projectile [2–5], but they must also be lightweight so they do not overload the armor of a trooper or vehicle [6]. Composites reinforced with aramid fibers are described in the literature as one of the materials with the highest ratios between weight (mass) and level of

**Citation:** Toader, G.; Diacon, A.; Rusen, E.; Rizea, F.; Teodorescu, M.; Stanescu, P.O.; Damian, C.; Rotariu, A.; Trana, E.; Bucur, F.; et al. A Facile Synthesis Route of Hybrid Polyurea-Polyurethane-MWCNTs Nanocomposite Coatings for Ballistic Protection and Experimental Testing in Dynamic Regime. *Polymers* **2021**, *13*, 1618. https://doi.org/10.3390/ polym13101618

Academic Editor: Andrew B. Lowe

Received: 20 April 2021 Accepted: 14 May 2021 Published: 17 May 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/).

impact protection, but their cost is considered a disadvantage [6,7]. Another alternative to steel armor is the use of an aluminum substrate coated with composite materials [8]. Numerous studies have shown that the rigidity and hardness of a material can be improved using nanoparticles [7,9–12].

To obtain superior mechanical properties for a particular type of polymeric matrix, it is necessary to take into account the size, shape, and chemical properties of the nanometric filler particles [13]. Multiwall carbon nanotubes (MWCNTs) have been extensively used in nanocomposites due to their outstanding properties: high Young's modulus, stiffness, flexibility, and conductivity. Additionally, only a 1% MWCNTs content can lead to an increase of up to 36–42% of the modulus of elasticity for composite material [14–16]. Even though carbon nanotubes have great potential as nanofillers, a major concern related to the performance of MWCNTs-based nanocomposites is represented by the difficulty of achieving their homogenous dispersion. The dispersability of carbon nanotubes in the polymer matrix is conditioned by their chemical and physical compatibility. Therefore, in some cases, the functionalization of MWCNTs can become mandatory for their compatibilization with the organic substrate.

Polyurea has been widely reported as being an appropriate polymeric matrix for impulsive loading ballistic protection [17–22], due to its embedded reinforcing nanoscale hard domains, uniformly dispersed and chemically linked inside its soft elastic nanodomains [9,23]. Therefore, many researchers have investigated polyurea for their distinctive synergistic properties. Li et al. [24] employed Jeffamine® D2000 and two different types of isocyanates, isophorone diisocyanate and hexamethylene diisocyanate, to obtain self-healing elastomers. Even though they possess this self-healing property, these elastomers exhibit insufficient mechanical resistance (maximum tensile stress of approximately 3.5 MPa) for impulsive loading applications. Therefore, numerous studies [17,25,26] confirmed the beneficial contribution of polyurea coatings applied to metallic surfaces. Bai et al. [27] obtained a polyurea with improved mechanical resistance by employing diphenylmethane diisocynante and Unilink 4200 diamine, reaching a maximum true stress of approximately 13 MPa (in the quasi-static regime, at low strain rates) and 27 MPa under a dynamic regime. Li et al. [28] investigated the response of stainless steel plates coated with a commercial polyurea (LINE XS-350) to impulsive loadings. This commercial polyurea displayed a maximum quasi-static true stress of only 22.4 MPa. The use of polyurea coatings affords a better response of the coated metal sheet at the action of a shock wave or the impact of a projectile by suffering lower deformations than the neat metallic plates. Therefore, in terms of ballistic protection, the efficacy of polyurea is already validated by an important volume of data available in the literature. Polyurea-MWCNTs systems [29–31] were also demonstrated in multiple studies as good candidates for impulsive loading applications. However, one major inconvenience is that the chemical modification of the nanofiller sometimes involves significant costs that may not prove to be economically viable.

In many cases, a good dispersion of nanoparticles in the polymer matrix can be obtained only by prior functionalization. This process can sometimes be difficult to perform and expensive, as the synthesis process involves several steps. To simplify the manufacturing process and to reduce the production costs for polyurea nanocomposites designed for ballistic protection, a commercially available MWCNT product was used. This product consists of a concentrate of MWCNTs pre-dispersed in a polyester-polyol system, which ensures a good dispersion in the polyurea matrix. This simplified synthesis method could be more advantageous for coating extended areas specific for ballistic protection applications. Thus, the metal substrates can be protected against cracking/failure by being coated with this high-performance polyurea nanocomposite by simply spraying the premixed reactants onto the targeted surface.

Therefore, taking into consideration the unique set of properties of polyurea-polyurethane and the multiple advantages of using pre-dispersed MWCNTs, this study intended to provide a novel approach towards a new facile synthesis route for obtaining high-performing polyurea-polyurethane nanocomposite coatings. This study also comprises a section on the

experimental testing in a dynamic regime in order to prove the real contribution of this type of nanocomposite coating on the deformation mitigation of metallic plates for assessing the suitability of the materials' design in relation to ballistic protection applications. To improve the response of the metallic structures to impulsive loadings, we herein decided to employ our previously synthesized polyurea matrix [17], which was obtained from poly (propylene glycol) bis(2-aminopropyl ether) with Mn ≈ 2000 Da, diphenylmethane-4,4 diisocyanate, and 4 -diaminodiphenylmethane as chain extender. This polyurea possesses superior mechanical resistance, displaying a maximum quasi-static true stress of 33.76 MPa. Its unique set of properties established the premises for obtaining a performant nanocomposite with superior mechanical resistance (up to 40.84 MPa maximum quasi-static true stress) for ballistic protection applications. To the extent of our knowledge, this is the first paper that proposes an improved route for obtaining performant polyurea-polyurethane nanocomposites with pre-dispersed MWCNTs, conjoined with the evaluation of the mitigation effect brought by these performant materials and using the Hopkinson bar method. The novelty of this paper consists of both the straightforwardness of the polyurea coating fabrication method and the experimental set-up approach for the evaluation of the behavior of these materials at impact with a projectile.
