**3. Results and Discussion**

Polyurea and MWCNT nanocomposites designed for ballistic protection applications must meet certain performance criteria related to their thermal and mechanical strength. Materials exposed to the impact with a projectile or at the action of a shock wave can suffer both considerable deformation (or failure of the structure) and thermal degradation. Thus, polyurea coatings have the role of reducing these devastating effects.

The first step in our study consisted in the morphology characterization of the nanocomposites using SEM analysis. In Figure 2 SEM images of the PU-NC4 sample are presented. The nanocomposite consists of a continuous film; however, it can be noticed that, in some areas (highlighted with a white ring), there is a lack of homogeneity. The explanation could be due to the MWCNTs' aggregation in the polymer matrices and their escape at the surface of the polymer films, respectively.

To strengthen the hypothesis about the aggregation of the MWCNTs in sample PU-NC4, the second step of this study involved an investigation of the distribution of the MWCNTs inside the polyurea-polyurethane matrix through a micro-CT technique. CTVol® Micro-CT Surface Rendering Software allowed a realistic visualization of the samples (Figures 3a–d and 4) and displayed a homogenous spatial distribution of the nanofiller inside the polymeric matrix. Reconstruction of the collected images was performed by the NRecon reconstruction software (local version), and the full set of reconstruction results

were visualized via DataViewer® 2D/3D Micro-CT Slice Visualization(Micro Photonics Inc., Allentown, PA, USA). Representative 2D cross-section slices of each reconstructed sample were illustrated in Figure 3e–h. As can be observed from Figure 3, the MWCNTs are well dispersed inside the polyurea-polyurethane matrix. Moreover, the increase of MWCNTs concentration is well distinguishable in the four micro-tomographs.

**Figure 3.** Micro-CT images (70 μm voxel size) of polyurea-polyurethane-MWCNTs nanocomposites comprising: (**a**,**e**) 0.05 wt.% MWCNTs, (**b**,**f**) 0.1 wt.% MWCNTs, (**c**,**g**) 0.2 wt.% MWCNTs, (**d**,**h**) 0.3 wt.% MWCNTs.

**Figure 4.** Micro-CT image (10 μm voxel size) of PU-NC4 sample.

Even if the nanocomposite samples containing 0.3 wt.% MWCNTs (PU-NC4) exhibited an apparent macro-homogeneity, there are regions where these nanoparticles tend to agglomerate, as can be observed in Figure 4.

Thermogravimetric analysis showed that the synthesized polyurea-polyurethane-MWCNTs films displayed a high thermal resistance, as can be observed in Figure 5 and Tables 2 and 3.

According to the TGA analysis, it appears that the introduction of carbon nanotubes into polyurea films had a slightly positive effect on thermal resistance, as their degradation process began later compared to the control sample (PU). However, as the concentration of carbon nanotubes increased, this effect decreased. The temperature corresponding to the maximum decomposition rate was also higher in the case of nanocomposites. Starting at 354 ◦C (in the case of PU samples) and at 366 ◦C, respectively (in the case of the nanocomposite samples), a significant weight loss process could be observed due to the decomposition process of the polymeric matrix (Figure 5).

**Figure 5.** TGA and DTG analyses of the polyurea-polyurethane-MWCNTs nanocomposites.


**Table 2.** Thermal properties of polyurea-polyurethane-MWCNTs nanocomposites.

T10% = decomposition onset temperature (measured at 10% weight loss); Tmax = the maximum decomposition temperature, corresponding to the maximum of DTG peak (the first derivative of the thermogravimetric curve).



Another important aspect related to the performance of polyurea-based materials designed for ballistic protection is related to their glass transition temperature. At lower temperatures, but still above the value of the glass transition temperature, polyurea tends to pass into the glass phase due to the energy dissipation phenomenon generated during a deformation caused by the action of a shock wave or impact with a projectile [17,20,31]. As can be observed in Figure 6, the synthesized materials exhibited two glass transition temperatures, each of them corresponding to the flexible and rigid nanodomains of the

polyurea nanocomposite, respectively. Table 4 summarizes the two Tg values obtained for the synthesized materials. It can be noticed that the values obtained for the glass transition temperatures associated with the flexible nanodomains (Tg1) were quite similar to the blank sample (PU) because there are no significant differences between the aliphatic chains situated at a considerable distance from the rigid nanodomains, but there is a visible difference between the Tg2 values obtained for the reference sample (PU) and the nanocomposite samples (PU-NC) because the carbon nanofiller joins the rigid nanodomains, thus reducing the mobility of the polymeric chains situated in their proximity.

**Figure 6.** DSC curves obtained for polyurea-polyurethane-MWCNTs nanocomposites.

**Table 4.** Glass transition temperature obtained for polyurea-polyurethane-MWCNTs nanocomposites (Tg1–glass transition corresponding to the flexible nanodomains; Tg2–glass transition corresponding to the hard nanodomains).


To evaluate the behavior of polyurea-polyurethane films under the mechanical stress action, the samples were subjected to tensile tests. Table 5 and Figure 7 illustrate the obtained results. As can be noticed, the introduction of carbon nanotubes into the polyurea matrix led to an increase of Young's modulus, which means that they have become more rigid than the reference sample.

Although these materials were stiffer than the control sample, they showed higher deformation energy, which suggests that these materials can dissipate more energy. Since these materials were specially designed for ballistic protection, their high capacity to absorb and dissipate energy represents a remarkable advantage.

The pre-dispersed MWCNTs achieved a good dispersion in the polymer matrix until it reached a concentration of 0.2% MWCNTs but, at higher concentrations, the nanofiller tends to agglomerate. This situation leads to a decrease in the tensile strength of the samples containing 0.3% MWCNTs (PU-NC4). Since PU-NC3 displayed the highest value of tensile true strain (σT) and the highest deformation energy (Figure 7 and Table 5), we

can affirm that these maximum values indicate the concentration of 0.2% MWCNTs as representing the optimal composition (Figure 8) for this type of polyurea nanocomposite. The results are also sustained by SEM and Micro-CT analyses through the aggregation of the MWCNTs in the case of PU-NC4.


**Table 5.** Mechanical properties of polyurea-polyurethane- MWCNTs nanocomposites.

\* Young's Modulus was calculated according to the mathematical model described by Xue and Hutchinson [32].

**Figure 7.** True stress–true strain plots for the synthesized polyurea-polyurethane nanocomposite films.

**Figure 8.** Variation of the maximum of true stress, depending on the concentration of the nanofiller.

The impact strength of a material is an index of its toughness. When a load is applied to a polymer, a part of the energy is dissipated throughout the polymer mass, while another part is stored in the material and will be released after the load is removed. The DMA analysis aimed to follow the evolution of storage (E ) and loss (E") modulus of the synthesized nanocomposites to evaluate their potential to be used as coatings for the structures that need improved ballistic protection. Figure 9 illustrates a comparative plot of the E and E" values for the blank sample (PU), and for the nanocomposite which displayed the best tensile test results (PU-NC3).

**Figure 9.** Comparative E and E" plot between polyurea and polyurea-polyurethane nanocomposites.

Both E and E" were higher in the case of PU-NC3, showing that this nanocomposite possesses the ability to store a significant amount of energy, and it can also dissipate a higher quantity of energy in comparison with the neat polyurea (PU). The peaks corresponding to the loss modulus can allow the estimation of the glass transition temperature at around −50 ◦C, indicating that this value obtained through DMA technique is following DSC analysis.

The characterization of these materials specially designed for ballistic protection would not be complete without experimental measurements which aim to evaluate their behavior at the impact with a projectile or the action of a shock wave. Therefore, for evaluating the efficiency of the obtained materials, round-headed projectiles were employed for impacting aluminum plates coated with the synthesized polyurea-polyurethane nanocomposites on the backside.

For the experimental testing in the dynamic regime, we used a Hopkinson bar set-up, with every sample being fixed and aligned for axial symmetry. An air gun, which allows the variation of pressure, launches the striker (the spherical head projectile in our case) towards the direction of the center of the tested specimen. Thus, the sample is rapidly impacted by the compressive stress wave generated. At impact, the samples undergo rapid and permanent deformations or fractures. Tables 6 and 7 and Figure 10 display the results obtained with the Hopkinson bar tests. As can be noticed in Table 7, at an initial projectile acceleration pressure of 0.2 bars, all the samples underwent deformations, but none of them suffered fissures. Starting with 0.3 bars, small cracks appeared in the aluminum plate, but the samples covered with polyurea, even if they displayed higher deformations, did not suffer fractures, meaning that neat polyurea already offers significant improvement. At a higher initial projectile acceleration pressure of 0.4 bars, the aluminum plate underwent a higher fracture, but the polyurea-coated specimens did not exhibit fissures. Only at 0.5 bars of initial projectile acceleration pressure could a small fissure be observed on the aluminum plate coated with polyurea, but the polyurea film remained unbroken due to its higher elasticity, and the specimens coated with the nanocomposite suffered only low

deformations and no fractures. At a higher initial projectile acceleration pressure of 0.6 bars, polyurea films PU and the nanocomposite containing the lowest MWNCTs concentration PU-NC2 started to crack. At 0.6 bars, the aluminum plate coated with PU-NC3 suffered low fractures, but this polymeric film resisted up to 0.8 bars. Therefore, we can affirm that, only at values higher than 0.8 bars for the initial projectile acceleration pressure, did both layers (metallic layer and polymeric nanocomposite coating) undergo visible fractures; at lower values, the metallic layer was the only one that cracked. In the case of PU-NC4 specimens, the metallic layer exhibited low fissures at 0.7 bars, and both layers (metallic plate and polymeric nanocomposite coating) were fractured at values higher than 0.8 bars. The increase in the resistance of the aluminum plates covered with PU and PU-NC can also be highlighted by the recordings made with the help of the pressure transducer (Table 6). From the values displayed in Table 6, it can be noticed that, for the tests performed on the uncoated metallic plates, the maximum value of the force was 2.6 kN at the pressure of 0.3 bar. This characteristic displayed a decreasing trend for the tests performed at higher pressures, and could be associated with the loss of the integrity of the plate (fracturing phenomenon). On the other hand, for the specimens coated with polyurea, the values measured with the pressure transducer were almost double for the maximum force, more precisely 5 kN for PU-NC2 at 0.5 bars. For higher initial projectile acceleration pressure, all specimens, including the ones coated with the nanocomposite, followed the same trend: after reaching a maximum value of the force, there was a decrease, a phenomenon associated with the loss of the integrity of the metal plates.

**Table 6.** Maximum force values obtained during the impact tests with Hopkinson bar.


**Figure 10.** Maximum force values obtained during the impact tests with Hopkinson bar.

**Table 7.** Images of the tested specimens, captured after the impact tests with Hopkinson bar (pressure in bar and pattern of deformation).

#### **Table 7.** *Cont.*

#### **4. Conclusions**

Polyurea-polyurethane and MWCNTs nanocomposites were obtained to be used for ballistic protection applications through a facile synthesis approach involving MWCNTs pre-dispersed in a polyester polyol-based resin. To demonstrate that these materials are suitable for this type of application, they were subjected to thermal and mechanical characterization using different analysis techniques (SEM, μCT, TGA, DSC, DMA, tensile tests), and they were also subjected to ballistic tests using a Hopkinson bar system. SEM and micro-CT analyses confirmed the homogenous dispersion of the MWCNTs inside the polyurea-polyurethane matrix for the samples containing 0.05%, 0.1%, and 0.2% MWC-NTs. In the nanocomposites containing 0.3% MWCNTs, it can be noticed that, in some regions, there is a lack of homogeneity due to the tendency to form aggregates at higher concentrations of this nanofiller. TGA showed that the nanocomposite films have good thermal stability (up to about 300 ◦C). The presence of MWCNTs delayed the onset of the decomposition process of polyurea-polyurethane films, and the maximum degradation temperature was about 10 ◦C higher than that of the reference polyurea sample (PU). DSC curves displayed two glass transition temperatures due to the coexistence of the two segregated nanodomains from the componence of polyurea matrix (flexible and rigid nanodomains). According to tensile test results, it turned out that, from all the synthesized materials, the polyurea-polyurethane nanocomposite film containing 0.2% MWCNTs is the optimal option for ballistic protection applications, since it possesses the highest deformation energy. DMA analysis also demonstrated that PU-NC3 samples had a remarkable capacity for absorbing and dissipating energy. Experimental testing in a dynamic regime of the polyurea-coated aluminum plates showed that the polymeric layer allows the metal plate to maintain its integrity at an acceleration pressure value that is almost three times higher than the one for the uncoated metallic specimen. Synthesized nanocomposites possess unique properties that recommend them to be used in the modernization of ballistic protection equipment and devices. Thus, military vehicles or bulletproof vests could be safer, but also lighter and less expensive.

We can conclude that the synthesized polyurea-polyurethane-MWCNTs nanocomposites possessing this exceptional combination of properties and advantages are suitable for employment as complementary materials for ballistic protection.

**Author Contributions:** Conceptualization, G.T., A.D. and E.R.; methodology, E.R., M.T. and A.R.; software, R.G.; validation, G.T., A.D., E.R., F.R., M.T. and E.T.; formal analysis, G.T., A.D., E.R. and R.G.; investigation, G.T., A.D., F.R., M.T., P.O.S., C.D., A.R., E.T., F.B. and R.G.; resources, F.R. and E.T.; data curation, G.T., A.D., P.O.S., C.D., A.R., F.B. and R.G.; writing—original draft preparation, G.T., A.D. and E.R.; writing—review and editing, G.T., A.D. and E.R.; visualization, G.T., A.D., E.R., F.R., P.O.S., C.D., E.T. and F.B.; supervision, A.D., E.R., M.T., and A.R.; project administration, E.R. and A.R.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by the National Authority for Scientific Research from the Ministry of Education, Research and Youth of Romania through the National Project PN-III-P2- 2.1-PTE-2019-0400 Ctr. No. 49PTE/2020. The authors would also like to thank for the financial support provided by the National Authority for Scientific Research from the Ministry of Education, Research and Youth of Romania through the National Project PN-II-PT-PCCA-2013 No. 278/2014.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
