Polyurea for Blast and Impact Protection: History
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Contributor:
Rui Zhang
,
Weibo Huang
,
Ping Lyu
,
Shuai Yan
,
Xu Wang
,
Jiahui Ju

Polyurea has attracted extensive attention from researchers and engineers in the field of blast and impact protection due to its excellent quasi-static mechanical properties and dynamic mechanical properties. Its mechanical properties and energy absorption capacity have been tuned by means of formulation optimization, molecular dynamics (MD) simulation and the addition of reinforcing materials. Owing to the special molecular structure of polyurea, the mechanism of polyurea protection against blasts and impacts is the simultaneous effect of multiple properties. For different substrates and structures, polyurea needs to provide different performance characteristics, including adhesion, hardness, breaking elongation, etc., depending on the characteristics of the load to which it is subjected. 

  • blast protection
  • impact
  • polyurea

1. Introduction

Polyurea is a block copolymer synthesized from the rapid reaction between isocyanate prepolymers and polyamines [1][2][3][4][5][6][7]. The commercial polyurea formulation consists of two components, A and B, where the isocyanate prepolymer is component A, and component B consists of a homogeneous mixture of long polyether amines, chain extenders and additives.
A unique microphase separation can be observed in the polymer microstructure due to the existence of soft and hard segments [2][8][9][10]. The microstructure shows that the hard segments of the crosslinked mesh structure are uniformly distributed among the soft segments of the matrix, where the bright regions are the hard segments. Given this microphase-separated structure, polyurea can be considered a nano-composite, with the hard segments acting as reinforcements dispersed in a soft-segment matrix [11]. Such a special microstructure gives rise to excellent macroscopic properties, such as stability, high strength and aging resistance.

2. Protection Mechanisms

With blast and impact loading, the effect of polyurea on the dynamic response of the protected substrate is complicated. Both Liu [12] and Iqbal et al. [13] considered that shock-wave-induced hard domain ordering and crystallization, rearrangements and neutralization appear to be the dominant modes of energy dissipation in polyurea under blast loading conditions. This process is also accompanied by viscoelastic dissipation of the material, enhanced mechanical properties due to strain rate effects and the effect of impedance mismatch between the substrate and polyurea.

2.1. Soft and Hard Segment Rearrangement, Crystallization and Hardening

The high polarity of the hydrogen and oxygen atoms in the urea groups in the absence of loading facilitates the formation of hydrogen bonds between urea linkages, and the resulting copolymers show a hard-domain structural morphology characterized by high Tg embedded in a soft viscoelastic matrix with low Tg. For aromatic polyurea, hydrogen bonding also can be synergistically combined with π–π stacking interactions between the aromatic rings to promote the microphase separation of polyurea [14]. This particular structure constitutes an important mechanism in the impact protection of polyurea.
Under explosive loading, the material is subjected to the coupling effect of temperature and loading [2][15]. The disruption of the hydrogen bonds is affected by temperature in two stages, namely, slight dissociation and dramatic destruction corresponding to the coarsening process [2]. Due to the change in hydrogen bonds, it follows that the soft and hard segments in polyurea are structurally rearranged. In the range of 85–165 °C, dissociation of the bidentate hydrogen bonding occurs, the connection between the hard segments is lost and the microstructure of the material changes insignificantly. However, the loss factor decreases significantly, and the yield point of the material is missing. The hydrogen bonding begins to deconstruct dramatically from 165 °C, and the coarsening process begins to occur. The hard segments begin to self-assemble, the width and length of the band morphology become larger with increasing temperatures and the physical crosslinking density in the system decreases. The coarsening process promotes a microstructural transformation. In the second stage, the strength of the polyurea as well as the Young’s modulus undergoes a significant decrease. As the temperature increases, polyurea undergoes thermal decomposition. Lyu et al. [16] observed by thermogravimetric experiments that the thermal decomposition rate of polyurea reached the maximum at a temperature of 386 °C. At a temperature of 700 °C, the polyurea decomposed into carbon residue and lost its protective properties [16]. By comparing the FTIR spectra of polyurea before and after the explosion, Zhang et al. [17] found that the area of the carbonyl stretching region and imino stretching region was decreased. It was shown from the practical explosion data that the hydrogen bonds in polyurea were broken, and the microphase separation was also damaged after subjecting the polyurea to the explosion load.
In the process of the deformation of polyurea subjected to blast and impact loading, unlike under static loading, where cracks develop and propagate along weak zones (soft segments), the response of the hard segments of polyurea can more easily be activated, and material cracks propagate along with hard segments in the microstructure [18]. The hard segments tend to be ordered in the molecular structure in polyurea, and this tight arrangement of polar chain segments in turn promotes the formation of hydrogen bonds and the increased crystallization of the material. This also leads to the strain rate sensitivity (dependence) of polyurea [18]. Moreover, this process is accompanied by the breakage and reorganization of hydrogen bonds. The intermolecular hydrogen bonding state plays a critical role in dynamic hardening and strengthening [19].
Regarding the loading frequency, the stress caused by the blast or impact exhibits wide-ranging frequency components, and the optimal dissipative material is one in which the frequency of the stress wave matches the critical frequency and in which the optimal damping of the material matches the frequency [20][21]. It is usually associated with explosive loads of 400–500 Hz [22]. Iqbal et al. [13] found that their synthesized polyurea required at least 1013 Hz for the glass transition to occur at ambient temperatures, which is not enough to allow the glass transition of polyurea. The same phenomenon was found in concrete structures protected by polyurea by Liu et al. [12]. This indicates that blast loading puts the polyurea in a strain-hardened state when only the loading frequency is considered, which is not sufficient to induce several of the above polyureas to enter the glass state and undergo brittle failure.

2.2. Viscous Dissipation within the Material and Strain Rate Effects

In tension and compression experiments, polyurea has a significant nonlinear stress–strain relationship [22][23][24][25][26][27][28][29]. It is a typical viscoelastic material, which exhibits viscoelastic properties with respect to viscous dissipation to absorb impact energy and strain rate effects in its mechanical properties.
During material deformation, the soft segments connected in the material allow the hard segments to remain covalently connected to each other. The viscous dissipation of polyurea occurs mainly in the soft segments, with energy dissipation occurring through chain movement [30]. Perturbation by straining amplifies the strain energy by the phase separation morphology of the material. Through the development of a hybrid all-atom/coarse-grain (AA/CG) model, Zheng et al. [31] found that a strong correlation is created between the polyurea hard segments through hydrogen bonding, which reduces the mobility of the hard segments and leads to a substantial difference in mobility between the hard and soft segments. The difference in mobility causes the relative displacement of the polyurea molecular structure during deformation, which results in energy consumption by means of internal friction. High-frequency stress waves traveling through the viscoelastic material undergo multiple load–unload cycles, which can lead to a large amount of energy dissipation in a short period of time [20]. The process of energy dissipation is accompanied by an increase in temperature, which in turn affects the mechanical response [21][32][33]. Liu et al. [34] found that impact energy acting on polyurea is transformed into thermal energy under a high strain rate impact, resulting in thermal softening and stress reduction. Mott et al. [32] employed high-speed thermography to measure transient temperature changes in the deformed polyurea, and they found that the unusual viscoelasticity degree is due to a high Tg and large internal friction. The coupling causes the hard domains to move resonantly during shock propagation, thereby enhancing the energy dissipation [14]. The energy is absorbed by the resonance between the polymer segmental dynamics and the impact frequency. For higher frequencies, the hard domains become a resonator and consequently contribute to increase the overall material loss [8]. Moreover, as a result of the coating transient hardening, the reversible change in the material to a glassy state increased the modulus by about three orders of magnitude, which caused lateral spreading of the impact force, reducing the impact pressure [14][35][36].
The strain rate sensitivity (dependence) of polyurea contributes to its use in structural blast and impact resistance. The marked sensitivity of the strain rate, i.e., a shift from rubbery behavior towards glassy-like behavior upon high strain rate deformation, can be exploited to obtain substantial energy dissipation under particular conditions [19][25][36][37][38]. The strain rate sensitivity is closely related to the glass transition of the material [12]. In the study of Wu et al. [23], it was found that polyurea exhibits rubbery behavior at low strain rates and glassy behavior at high strain rates at room temperature. This is consistent with the dynamic thermo-mechanical properties of polyurea at different temperatures, which is fully in accordance with the law of the TTS. For the mechanical properties, Wu et al. [23] found that at high strain rates, unlike superelasticity at low strain rates, polyureas exhibit a significant yield slip, strain hardening properties and more pronounced strain rate effects. This is in agreement with Giller et al. [36], who suggested that the strain rate sensitivity of polyurea is greatest in its glass transition region. Gamache et al. [39] found that, in laminate structures, the mechanical stiffness of the coating is increased due to the strain hardening of the polyurea, which can laterally disperse the impact forces. This process allows for the additional work of deforming larger areas to enhance ballistic performance [39].

2.3. Impedance Mismatch between Substrate and Polyurea

The impedance mismatch between polyurea and the substrate during stress wave propagation is a major reason for the blast and impact resistance of polyurea from a macroscopic point of view [20][40][41]. Hailiang et al. [41] analyzed the propagation of stress waves in polyurea-coated steel plates with different structures and the energy dissipation mechanism through experimental and numerical methods. The propagation path of stress waves in composite plates was characterized based on the damage phenomena of the composite structures. They quantified the energy dissipation of composite plates with different structures at equal total area densities.
Wu et al. [23] also found the effect of impedance mismatch between polyurea and the substrate material on the blast resistance performance, and they found that, if the polyurea layer is not firmly bonded, the inertial deformation caused by polyurea at the interface joint makes the two separate, weakening the unloading effect of the reflected unloading wave and increasing the deformation deflection. With the polyurea located on the blast surface, polyurea is subjected to blast loading that causes a tensile wave, which results in bulge deformation, as described in ref. [42].

3. Research and Application of Polyurea in Blast/Impact Protection

Polyurea is a new protective material that has attracted a large number of researchers in different protective fields to study its characteristics [13][22][23][43][44][45][46][47][48][49][50][51]. According to the protection substrate types, the current research of polyurea in blast and impact protection can be divided into the protection of civil engineering structures, the protection of metal structures and protective applications for composite materials.

3.1. Protection of Civil Engineering Structures

The majority of existing building structures are not designed at the beginning to survive the effects of blast loads. Civil engineering structures are brittle, with low flexural strength, and are basically unable to absorb strain energy in the absence of reinforcement. Once subjected to the blast load, the effects are extremely serious. Most casualties in civil engineering structures during external explosions are caused by structural disintegration and debris resulting from the explosion [52][53][54][55][56][57][58][59]. Polyurea is highly applicable in structural reinforcement due to its special mechanical properties.
Wang et al. [54] found that sprayed polyurea can increase the ultimate blast resistance of clay masonry walls by a factor of 4.5–11, and the polyurea does not debond under the action of 8 kg TNT at a blast distance of 1 m. For gas explosion resistance in masonry walls, Gu et al. [56] studied masonry walls with polyurea sprayed on the front, back and both sides. It was found that the blast resistance of the front-side sprayed polyurea walls showed no significant increase, and the debris was still emitted from the backside of the walls. The blast resistance of the double-sided sprayed polyurea was optimal. Gu et al. [56] concluded that rear reinforcement is an acceptable option in the case of limited construction conditions.
For concrete protection, Shi et al. [52] compounded polyurea with woven glass fiber (WGF) based on the characteristics of polyurea and WGF. They found that the failure mode of the coating was changed from shear punching failure to tensile failure and caused a trend from local burst-induced perforation to overall plastic yielding of the reinforced concrete (RC) slab. Recently, Lyu et al. [16] proposed that the ability of polyurea-protected RC slab to maintain good integrity and stability was achieved by the combined effect of front and back blast surface coatings, i.e., the front surface coating resisted the high temperature generated by the blast and reflected the shock waves, and the back surface weakened the impact tensile waves and completely covered the concrete fragments. Through numerical simulations, they also demonstrated that polyurea coating increased the kinetic energy conversion rate of an RC slab, thereby improving the blast resistance of the RC slab [16].
Song et al. [53] used MG as reinforcing fibers, which were mixed with prepolymers and were sprayed onto the surface of the concrete beams along with the hardener to create the glass-fiber-reinforced polyurea (GFRPU). It was found that the GFRPU reinforcement can prevent the sudden spalling and debonding of concrete, and it retains the shear resistance capacity.
Fallon et al. [55] used a gas gun apparatus to conduct experiments on specimens with projectile velocities in the range of 45–150 m·s−1. It was found that the polyurea coating significantly reduced the impact damage of concrete through the tests and simulations of Fallon et al. [55], and the coating was found to have the effect of delocalizing the damage. Nevertheless, the study by Fallon et al. did not consider the role of adhesion between the polyurea and the substrate, and the polyurea coating they used was pre-sprayed and pasted on the impact surface of the specimen. In contrast, the adhesion between the coating and the substrate is considered by many researchers to be an important indicator in the blast and impact resistance of polyurea [41][56][60].
For autoclaved aerated concrete (AAC), Chen et al. [48] employed carbon-fiber-reinforced plastics (CFRP) and polyurea to provide a total thickness of 4 mm polyurea coating on the top, bottom and sides of AAC. It was shown that, although the ultimate loads of polyurea-coated AAC slabs under quasi-static loading were slightly lower than those of slabs reinforced with CFRP, the bottom and double-sided polyurea-coated AAC slabs were much more resistant to blasts than CFRP-reinforced AAC slabs. It was also found that polyurea can hardly improve the static load carrying capacity of reinforced concrete arches in the study by Liu et al. [12]. Notably, in the study by Sonoda et al. [61], they revealed that the polyurea protective coating is also not effective against RC structures for the first impact loading action at low and medium strain rates. With repeated impacts, however, the improvement of polyurea on the impact resistance of RC structures is significant and can significantly inhibit structural cracking and rigidity loss [61]. Among the many researchers studying the polyurea enhancement of concrete structures against blast and impact loads, the polyurea inhibition of structural crack evolution and spalling is considered an important macroscopic level of protection. It was also found by Wu et al. [59] in experimental and numerical simulations that the improved protection performance of polyurea becomes more prominent at smaller scale distances. Regarding the location of coating protection, same as the masonry wall, the backside of the blast side coated with polyurea has been found to be the most effective protection measure in concrete blast protection [48].
Although it is obvious that polyurea enhances the blast and impact resistance of concrete structures, the design thickness of polyurea coating needs to be synthetically designed considering the actual engineering needs and the economics of structural reinforcement. Wu et al. [59] noticed that the maximum mid-span displacement of RC slabs decreased significantly when the thickness of the polyurea layer increased from 1 mm to 6 mm, whereas the slab deformation was less affected when the thickness exceeded 6 mm. Therefore, Wu et al. [59] suggested that the valid thickness of polyurea for RC slabs should be approximately 1–6 mm.

3.2. Protection of Metal Structures

With regard to the protection of metal structures, a large number of studies have now shown that polyurea provides better protection for airborne explosions when the coating is sprayed on the backside of the metal structure [23][40][41][49][62][63]. The polyurea coating on the backside dissipates energy by tensile deformation and reduces the impact of shock waves on the metal structure, inhibiting structural deformation. On this basis, Zhang et al. [17] revealed that, under the combined effect of blast shock waves and fragmentation, the damage to the steel plate is reduced despite the fracture damage of the high-hardness polyurea on the backside. Chen [64] and Li et al. [40] found that full-area spraying on the backside offers far better protection than partial spraying because of the confined polyurea boundaries, which can absorb energy through tensile deformation and the peeling of the coating. Jiang et al. [62] found that the polyurea layer inside the metal tank can provide an additional bending moment to the structure to suppress the deformation during external blast loading. In a recent study by Jiang et al. [65] on the equilibrium equation of the tank, the polyurea coating increased the area density of the tank, thereby reducing the displacement of the tank under blast loading. Moreover, for the thickness and location of the coating, Jiang et al. [65] concluded that, when the total thickness of the polyurea is the same, it is more efficient to spray all the polyurea on one side than to spray the polyurea on both sides of the tank. The mass of the polyurea coating also has some influence on the blast resistance performance of the metal tank, as it increases the inertial force on the loaded surface [62]. Furthermore, Yongqing [40] and Hailiang et al. [41] suggested that the main energy-absorbing components during the action of the blast load are the metal plate in the front. However, Yongqing et al. [40] found that, when in contact with polyurea coating steel plates, the polyurea coating absorbs more kinetic energy than the steel plates.
The coating located on the blast/impact surface has no significant protection against the deformation of the structure. Remennikov et al. [45] revealed that the excessively high heat generated by the blast causes the polyurea layer to melt near the center of the steel plate, which may affect the overall effectiveness of the coating in reducing the deformation response of the steel plate to approaching blast loads. Most of the early studies (e.g., ref. [66]) thickened the polyurea coating to improve protection. Nevertheless, when influenced by the impact load, the impact surface polyurea can reduce the stress concentration of the steel plate, thus delaying or preventing the fracture of the plate [67].
However, Wu et al. [23] found that the protective performance of polyurea coating on the blast-face cannot be underestimated. The deformation of the circular tube protected by polyurea on the blast surface decreases and becomes progressively smaller as the coating thickness increases. By analyzing the microscopic morphology of the blast surface, Wu et al. [23] concluded that high temperature resistance and flame retardancy are essential properties of polyurea against blast loads. It was concluded by Zhang et al. [17] that the severe ablation of the blast surface by the detonation products substantially reduce the energy absorption efficiency of the coating. Comparing the energy absorption efficiency of the coating on the blast side and the back blast side, it is undeniable that the energy absorption efficiency of the coating on the blast side is lower.
Furthermore, similar to the blast protection of concrete structures by polyurea, it is of importance to have adhesion between polyurea and metal substrates. Rijensky et al. [68] revealed through simulations that the strength characteristics of the primer used prior to this coating should be considered in blast resistance protection. Premature cracking of the polyurea after the failure of adhesion to the substrate limits the effect of the polyurea on energy absorption [17].
With respect to reducing the impact of the explosion on the external environment, Matos et al. [43] used a combination of a semi-spherical pressure vessel and digital image correlation (DIC) to simulate the underwater implosion of polyurea-protected aluminum tubes and showed that the exterior and interior coated tubes release less energy after the implosion than the uncoated tube, with the interior coated releasing the least energy, and that doubling the volume of the coating does not significantly improve the mitigation effect. Subsequently, they also found that the polyurea causes a longer delay in the implosion pulse with this set of devices, which is sufficiently large to reduce the peak energy values by 35% for external coatings and 50% for internal coatings [69]. Remarkably, owing to the peculiarities of the fluid–solid coupling response to the underwater explosion (UNDEX), the protection and penetration mechanisms of metal structures coated with polyurea under the loads of UNDEX are vastly different from those of airborne explosions [63]. Li et al. [63] found that the aluminum plate coated with polyurea on the frontside ended up with the least severe plastic deformation when the underwater shock wave impacted, whereas the aluminum plate coated with polyurea on both sides showed the largest severe plastic deformation, which is contrary to the protection law of polyurea in the case of the airborne explosion.
Penetration is a special impact process with a high temperature, high pressure and high strain rate. Polyurea has been found to sharply increase the ballistic limit of the underlying steel substrates, with one significant mechanism being the impact-induced rubber-to-glass state transition [70][71]. Recently, polyurea was found to cause reductions in the residual velocity of the projectiles and to increase the ballistic performance of metal plates [30][70][72]. Metal plates coated on the frontside showed a more pronounced increase in ballistic limit and specific energy absorption than the backside [70]. Zhang et al. [70] also noticed a decrease in the total ballistic performance of the plates when the backside was sprayed with high-hardness polyurea. However, in early studies, researchers found that polyurea has no benefit in increasing the ballistic limit of steel plates [73]. This may be a result of the choice of rigid polyurea as a protective coating for the purpose of improving the strength of the composite structure. The coating underwent brittle damage at the time of penetration. However, for preventing the penetration of fragments, rigid polyurea such as AMMT-53 mentioned in ref. [42], although prone to cracking, local crushing and local collapse, provides a strong barrier to fragments due to its high hardness, which can effectively reduce the perforation rate.
With high strength and high break elongation, the position of the polyurea has a lesser effect on the protection, and the effect of back spraying is not higher than 20% compared to the front protection effect [74]. In this case, the underlying substrate maintains adequate bending stiffness to allow impact-induced polymer transition, which, in combination with the rupture and dissipation of the pressure wave due to impedance mismatch, contributes to large increases in ballistic penetration resistance [39][71]. Similar to the blast protection mechanism, it is also important that bonding strength contributes to the penetration resistance of polyurea/steel composite plates. Sun et al. [74] found that, if the bond strength between polyurea and the steel plate is enhanced, more of the kinetic energy of the projectile is converted into other forms during the penetration process. In laminate structures, the number of the layers of the laminate structure and the nature of the metal components were found to have very little effect on performance, which differs from blast/impact protection [39].
From the micro-protection scale, MD simulations of multilayer aluminum–polyurea nanostructures reported by Dewapriya et al. [72] have revealed that both the ballistic limiting velocity (V50) and specific penetration energy of the multilayer material and the aluminum nanofilms are remarkably higher than the experimental measurements for any material. In a further study, with an impact velocity of 1 km/s, the specific penetration energy of the polyurea–metal bilayers (5.39 MJ/kg) was found to be remarkably higher than the value reported for any nanomaterial at a similar velocity (i.e., 3.8 MJ/kg) [75]. Nevertheless, MD simulations at the nanoscale by Dewapriya et al. [76] showed that the polyurea at the impact face is more effective in mitigating the impact-induced damage, which is a result contrary to that obtained from experiments performed at the macroscale.
Moreover, since polyurea has a strong self-sealing property, it has the unique advantage of preventing the leakage of special containers such as water tanks and oil tanks from being penetrated [7][42]. Huang suggests that the self-sealing property is due to the high modulus of elasticity of polyurea, which results in the coating closing on itself after the coating suffers penetration damage [7]. However, not all polyureas exhibit this property, and rigid polyurea can only lead to less leakage Wu et al. [42] attributed this self-sealing property of polyurea to the thermal effect of high-speed fragments penetrating the polyurea layer to produce a grayish-white elastic filler to form the self-healing effect.

3.3. Protective Applications for Composite Materials

Given the mechanical performance characteristics and limitations of polyurea with respect to mechanical strength, numerous scholars have used polyurea in the field of composite impacts and blast and penetration resistance to reinforce the protective structure and have achieved relatively obvious protection. Tekalur et al. [22] found that combining polyurea as an interlayer material for E-glass vinylester (EVE) materials can increase the blast resistance of EVE by 100% for air blast loading. Leblanc et al. [77] demonstrated that the transient response of polyurea-coated E-glass plates improve with increasing coating thickness for UNDEX loading through conical impact tubes combined with DIC and simulations, with the polyurea coating located on the backside of the plates providing better performance than that located on the loading surface. In their further study, they found that both double thickness plates and coated plates of the same thickness (1.524 mm) are superior to single layer (0.762 mm) E-glass plates, and that the polyurea coating is superior to the thicker uncoated plate with respect to reducing material damage [78]. The damage of the coated composites decrease dramatically with increasing coating thickness compared to the baseline cylinders [79]. In the finite element simulation of the EVE-matrix-reinforced composite panel, Phuong et al. [80] developed a finite element model of polyurea-enhanced composite (PEC) panels and used the Mooney–Rivlin hyperelastic law to simulate the behavior of polyurea. They found that the polyurea coating applied to the backside of the panels improves the delamination problem of EVE multilayer panels, and both the fiber and the matrix are reduced.
However, the effect of polyurea on reducing delamination in multilayer panel structures is not applicable to this material. In multilayer fiber/vinylester composite panels, the polyurea coating as a backing layer improves the fiber damage resistance when subjected to blast loads, although it does not play a significant role in reducing interlayer delamination [81].
Polyurea composites against penetration are a hot research topic for non-metallic penetration-resistant materials in recent years. The penetration resistance tests reported by Petre [82] for MWCNT-OH-reinforced polyurea revealed that the reinforced composite coated protective ballistic plate was not penetrated, and it provided III-A protection against. 44 Magnum Semi Jacketed Hollow Point (SJHP) ammunition, whereas the standard ballistic protection plate was perforated on a single impact. Liu et al. [83] analyzed the morphological characteristics of polyurea CFRP composite panels after penetration damage in detail and summarized four points of failure modes, namely, (1) compression shear failure on the frontside of polyurea, (2) tension shear failure on the backside of polyurea, (3) petaling failure and (4) spallation and punching perforation failure. By comparing the residual velocity, energy absorption ratio, deformation and damage level of specimens at various coating locations, Liu et al. [83] concluded that spraying polyurea on the backside of CFRP composites can significantly improve the ballistic performance of composite panels, and backside spraying amplifies the damage area.
Multilayer polyurea/ceramic nanocomposite is a new protective material made through polyurea composite with silicon carbide (SiC), which has a high ballistic limit velocity and specific penetration energy. To accurately model the non-bonded interactions of the polyurea/SiC interface, Dewapriya et al. [84] used density functional theory (DFT) calculations to obtain an accurate set of Lennard–Jones parameters and found that the interfacial adhesion between the polymer and the ceramic is relatively weak. Subsequent MD simulations by Dewapriya et al. [84] showed that multilayer ballistic performance is significantly affected by interfacial adhesion.
Numerous scholars sought to improve existing helmets (e.g., advanced combat helmets (ACH)) by exploiting the shock-mitigation ability of polyurea to reduce traumatic brain injury (TBI) caused by intra-cranial cavity [85][86][87]. According to the simulations of Grujicic et al. [85][86], polyurea was found to significantly lower the peak load experienced by the brain, and these improvements in the helmet shock-mitigation performance were obtained only in the case of helmet-polyurea-based internal lining through the study of polyurea coating location. In considering the explosive charge standoff distance and the TBI-inducing intra-cranial metric, it is beneficial to spray polyurea on the exterior of the helmet [87].

This entry is adapted from the peer-reviewed paper 10.3390/polym14132670

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