Functionalized Aramid Fibers and Composites for Protective

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Functionalized Aramid Fibers & Composites for Protective Applications: A Review Prakash Gore, and Balasubramanian Kandasubramanian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04903 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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Industrial & Engineering Chemistry Research

Functionalized Aramid Fibers & Composites for Protective Applications: A Review Prakash M. Gore,a Balasubramanian Kandasubramanian a* a

Structural Composite Fabrication Laboratory, Department of Metallurgical & Materials Engineering, Defence Institute of Advanced Technology (DU), Pune-411025, India. * Corresponding Author Email: [email protected]

Abstract Technological growth in advanced ammunition, and weapons has led to development of protective anti-ballistic composites, which are mostly based on Aramid fibers, as they absorb high impact energy, arising from penetrators. Enhanced performance of Aramid fibers (modulus~112,400MPa), is attributed to their compact molecular-structure, hydrogen-bonding, high-crystallinity, and high density (~1.44g/cm3). Methodologies like Layer-by-Layer, shearthickening, yarn-pullout, surface-functionalization via nanomaterials, etc. have been reported for modification of Aramid fibers, which are widely used with thermosets like Epoxy (due to process-friendliness). Recently, researchers are exploring thermoplastics with Aramids, due to their higher toughness, chemical-resistance, and thermal stability. Modification of Aramid fibers is mostly performed using nanomaterials e.g. carbon-nanotubes, graphene, silk fibroins, SiO2, ZnO, for enhancing their performance, and minimizing fiber-buckling under load. Following review presents advances in modification of Aramid fibers using nanomaterials with emphasis on thermoplastics for protective applications, their stress transfer mechanisms, Life Cycle Analysis, and concludes with their recycling/recovery methods. Keywords: Aramid Fibers; Nanomaterials; Composites; Life Cycle Assessment (LCA); AntiBallistic.

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For Table of Contents Only

1. Introduction The high impact resistant materials have become the prime necessity of the Police and Defence forces for countering the ballistic hazards, arising due to high velocity projectiles used in combat operations

1–5

. The impact resistant ballistic materials have been used for body

protection since medieval times, which have evolved from contemporary steel to present day highly efficient & light weight composite materials 2–4,6, as shown in the Timeline in Figure 1.


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Figure 1. Timeline of Armour Developments 2,4,7,8. Considering the high strength and exceptional mechanical stability, Steel was undoubtedly the first choice of material for high impact protection applications, however, in today’s technology driven combat fields, its use is limited due to its weight factor, because a soldier working in combat operations carries around 70 kg weight, out of which 12-20 kg belongs to body armour 2,5,9

. In recent decades composite materials have gained a prominent importance due to their

light-weight, stress transfer ability, high impact energy absorption capacity and ability to handle multi-hit objects simultaneously5,6. Advanced warfare systems coupled with technology-driven war tactics, have improved the striking capabilities of the projectiles, thereby causing a severe damage during combat operations. In advanced ballistic composite systems, researchers have widely used thermosetting resins based on Epoxy or Polyester due to ease in processing and stability after curing, however, the brittleness in thermosetting resins is not desirable for high impact applications 2. Researchers have explored various methods to ACS Paragon Plus Environment


overcome this drawback of thermosetting resins, but, increasing cost and complex synthesis routes are other limiting factors 2 Thermoplastic resins are another class of materials (e.g. PE, PP, PC, PTFE, PVC, PS, PU, etc.) which have been lately explored by Researchers for high impact protection applications, due to their selectivity based on amorphous and crystalline nature, and blending ability with thermoset and thermoplastic resins, and the energy dissipation ability [5,6]. The advanced composite based systems widely use high strength fabrics, as they leverage essentially required stress transfer ability after absorbing the shock energy3. Conventionally, Glass fiber and Carbon fiber have been widely used in the advanced composites systems which exhibit required stress transfer ability, however, however their performance efficiencies are limited for applications of ballistic protection, where high energy absorption, stabbing resistance, and strain deformation are necessarily required to counter the powerful shocks generated from the high velocity projectiles i.e. ~ 400 to 878 m/s 10. Since its invention in 1964, Aramid fibers have attracted the attention of Researchers in last few decades due to their high performance. These aramid fibers exhibit high-modulus (~112,400 MPa), and toughness & strength (~3,600 MPa), which is attributed to their close & pact molecular structure of polymeric chains, intrinsic strong Hydrogen bonding, eminent crystallinity (monoclinic structure), and elevated density (~1.44 g/cm3) 11. The higher density of the Aramid fibers helps in absorbing the shock waves generated from the high & low velocity impact projectiles. In modern technology driven combat operations, the advanced anti-ballistic composites require enhanced performance efficiency against multi-hit penetrators and fracture growth of the mode I, II & III type fractures 10. The multi-hit performance efficiency of these fabrics can be enhanced via efficient methodologies such as Layer-by-Layer, Shear Thickening Fluid (STF), Yarn Pullout, and functionalization via Nanomaterials e.g. CNT, Graphene Oxide, SiO2, ZnO, etc.,

12–16

. The above-mentioned techniques have been compared qualitatively as

shown in Table 1.


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Table 1. Fabric functionalization methods.

Methods

Refere nces

Advantages

Limitations

Improved mechanical strength due to functionalized layers, improved stress transfer High improvement in impact energy absorption, improves fabric deformability for shock absorption

Delamination can occur due to poor interfacial adhesion Rupture of fibers after critical shock, control over viscosity is required

Nanoparticle Functionalization

Uniform distribution of nanoparticles on fabric surface, strong interfacial adhesion

Complex Synthesis Route

18–20

Yarn Pullout

Improved higher impact energy absorption, Improved interyarn friction, improved tensile strength

Increased energy cost

15,21–23

Layer-by-Layer

Shear Thickening Fluid (STF)

2,11

1,17

The Layer-by-Layer method utilizes various functionalized layers of fabric, which renders improved mechanical strength and stress transfer, as reported by Laible et al. and Priyanka et al

2,11

. However, despite the enhanced mechanical strength, the poor interfacial adhesion is

sometimes a limiting factor

11

. The Shear Thickening Fluid (STF) method has been widely

used for the functionalization of Aramid fibers, it is a dense colloidal suspension, where a Polyethylene Glycol (PEG) and Silica are primarily utilized, where an abrupt change in viscosity occurs under increasing shear rate 1,17. STF facilitates improved absorption of impact energy (compared to untreated fabric) and fabric deformability required for sustaining the high shock impacts. However, the rupture of fibers under critical shock and viscosity control over suspension are sometimes the limiting factors

17

. The Yarn pull out is another widely used

method for improving the ballistic performance of Aramid fibers. The Yarn pull out method utilizes interyarn friction, where a frictional force between the individual yarns is increased for improving the alignment of fibers, and this interyarn friction directly governs the energy



dissipation in the fabric

15

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. The interyarn friction enhances the mechanical strength of the

fabric, which is attributed to the improved alignment of the yarns, however, it improves the Ballistic performance till threshold value, beyond this value, it can adversely affect the energy dissipation and further it may lead to failure of the composite during combat operations 23

15,21–

.

The functionalization via nanomaterials is another popular and advantageous method compared to other methods, the improved performance of the fabric via this method is attributed to the greater aspect-ratio of nanomaterials, which provides highly active-surface area, dispersibility at molecular level, this further helps in reducing fiber buckling under stress and enhances the interfacial adhesion with matrix and reinforcing fibers 3,11,18,18,18,24–41. Presently various review articles are available which discuss about the high impact performance of Aramid & other high strength fibers via various efficient methods, however, there is no literature, which specifically describes the performance of Aramid fibers under the influence of Nanomaterials via functionalization

18,19,21,34,41,42

. In this context, the present

review article gives state-of-the-art progress on fabrication and modification of Aramid fibers via nanomaterials with a specific emphasis on specialized high impact energy protection applications, and their progress. The review discusses the influence of Nanomaterials on Aramid fiber functionalization, the resulting Ballistic performance of Aramid fibers modified with various engineering polymers, and their stress transfer mechanisms. The review article further describes the important methods used for the fabrication of advanced protective antiballistic composites, and the challenges in future development of these high-performance materials. 2.

Fabric Functionalization Methods

(a) Shear Thickening Fluids (STF)


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Shear thickening fluid (STF) was first reported by Lee and co-workers for the functionalization of Kevlar fabric for practical Ballistic applications in year 2003

43

. The reported shear

thickening fluid was prepared using silica particles (450 nm) and ethylene glycol with a varying concentration i.e. 40%, 57%, and 62%, which was then impregnated on Kevlar fabric, the impregnated fabric was heated at 80oC for 20 min. The STF impregnated Kevlar fabric was tested a ballistic performance, where a projectile was fired at a speed of 244 m/s, their results showed more than 90% energy dissipation 43. Basically, the STF’s are a thick colloidal suspension with non-Newtonian behaviour, which exhibit a sudden increase in viscosity with increasing shear rate (Figure 2).

Figure 2. Shear Thickening behaviour of fluids. The main propitious characteristic of STF’s is their process reversible nature, where these fluids come back to their original liquid state, when the concentration is reduced from the



medium 17. The STF can be prepared using various solid shaped particles i.e. silica, calcium carbonate, etc. and inert carrier liquids such as water, ethylene glycol, polyethylene glycol, etc. In the STF, the carrier liquids demonstrate the Newtonian behaviour, however, the addition of particles change the nature of suspension to non-Newtonian state under the progressively increasing shear rate, as shown in figure 2. Initially, at the equilibrium the particles suspended in carrier liquid possess certain degree of randomness, however, under the action of shear rate, these particles get oriented in the form of layered structures, thereby showing a decrease in viscosity, i.e. a shear thinning nature. However, with progressively increasing shear rate, the particle suspended dispersion of STF exhibits abrupt increase in viscosity beyond critical shear rate, where the layered structure of particles is loses its orientation and gets transformed into particle groups known as ‘hydro-clusters’, thus demonstrating a shear thickening behaviour 16,17,43,44

.

The mechanism and structure of shear thickening fluids was studied by Hoffman in year 1972. The study underlined the principle working mechanism for the shear thickening fluids, which states that the particles in suspension are in a layered order below a critical shear rate. However, after crossing a critical shear rate, the particles lose their order and the acting hydrodynamic forces become powerful, due to this the layered particles become dis-organized and the transition from patterned to non-patterned structure causes sudden increase in viscosity 45.


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Figure 3. Effect of particle aspect ratio on STF’s. Reprinted with permission from ref 1. Copyright 2012 Taylor & Francis. The aspect-ratio of the particles is one of the major factors influencing the viscosity of STF’s (Figure 3). Some case studies have showed that, the particles with rod-like shape are efficient in improving the viscosity of the STF’s 1,46. In another study, Beazley et al., reported that the particles with enhanced aspect ratio are capable of increasing the viscosity of STF, which is attributed to the interlocking of particles and the rotational motion present in the flow field of STF

47

. Wetzel et al., reported the particles with lower loading in STF can still enhance the

viscosity, if it exhibited higher aspect ratio 48. They also observed that, the particles with higher aspect ratio have the more probability for getting in contact with their adjacent particles, thus they are more inclined to activate the thickening behaviour. In another study, Bossis et al. reported that the hydrodynamic forces manifesting in the STF, are directly proportional to the cube of higher sized hydro-clusters, that the clusters are more responsible for the thickening behaviour than the spherical particles. Considering individual particle as a sub-unit of the



hydro-clusters, it can be incurred that the particles with larger aspect ratio particles are more effective for improving the thickening mechanism of STF’s. Furthermore, the increase in the suspension viscosity, is also be attributed to the restricted particle motions, as a result of the entanglement of the particles 49. (b) Yarn Pullout: The Yarn pull out is a widely used technique employed for improving the ballistic performance of Aramid fibers. The Yarn pull out technique utilizes inter-yarn friction, where a frictional force between the individual yarns is increased for enhancing the alignment of fibers, and this inter-yarn friction directly governs the energy dissipation in the fabric 15. The inter-yarn friction enhances the mechanical strength of the fabric, which is attributed to the improved alignment of the yarns, however, it improves the Ballistic performance till threshold value, beyond this value, it can adversely affect the energy dissipation and further it may lead to failure of the composite during combat operations

15,21–23

. The fabric density, yarn directions in the fabric,

and the number of pulled yarn ends greatly influence the single and multiple pull-out forces. It has been observed that the single-yarn pull-out forces are lower than the multiple yarn pullout forces in para-aramid fabrics 50. It has been also been observed that the pull-out forces in the low-density fabric are lower than the countering high-density fabric. Additionally, it is also noted that the directional crimp ratios and the dimensions of the fabric significantly affect the yarn crimp extension 50. It is reported that the total extent of stick-slip force and manifesting retraction force in the multiple yarn pull-out method is highly irregular as compared to single-yarn pull-out. On other side, the stick-slip force is directly related to the total interlacement points present in the fabric, meanwhile the total manifesting retraction force is associated with the structural response of the fabric, as shown in Figure 4 (a). These factors are also associated with the fabric’s boundaries, and therefore the results based on the above-mentioned results are contemplated


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as crucial points for determining the energy absorption performance of soft ballistic structures 51

.

Figure 4. (a) Pictorial view of pull-out force components of the para-aramid fabric under yarn pull-out, (b) tow pullout test setup. In one of the study Hwang et al., have performed a yarn pull-out study on Aramid fibers, where they investigated the effect of ZnO nanowire interface on the ballistic performance of the aramid fabrics, further they measured the sliding friction between tows via the tow pullout test, where a controlled transverse tension was applied by pulling a single tow from a layer of fabric, as shown in Figure 4 (b)

15

. They performed the inter-yarn friction study using fabrics

reinforced with two distinct ceramic nanostructures i.e. ZnO nanoparticles, and ZnO nanowires, by generating changing inter-yarn friction intensities (Figure 5). Their study concluded that the yarn pull-out method improved the inter-yarn friction between aramid fabric



via ZnO nanowires, thus significant improvement energy absorption i.e. 23 times, and higher peak load i.e. 11 times, using the tow pullout test compared to bare aramid fabrics. They also reported that the inter-yarn friction can be tuned for Aramid fibers coated with by ZnO nanowires and nanoparticles, thus improving the performance for Ballistic applications. They also observed that, the failure of the tows can be delayed by ZnO Nanowires and nanoparticles, during the pullout test. Their study pointed out that the interlocking between ZnO nanowire arrays and the growth of ZnO nanowires at the junction of the tows are considered as the main contributing factors for improving the inter-yarn friction and thus, enhancing the resultant pullout energy 15.

Figure 5. SEM images of aramid fabrics with different reinforcement conditions after pullout test: (a and d) bare, (b and e) nanoparticle-coated, and (c and f) ZnO nanowire grown fabrics. Pullout direction and broken layers of ZnO nanowires are marked by red arrows and circles, respectively 15. Reprinted with permission from ref 15. Copyright 2015 Elsevier. 3.

Functionalization of Aramid Fibers via Nanomaterials ACS Paragon Plus Environment

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The functionalization via nanomaterials is one of the widely used method for enhancing the performance of Aramid fibers, which is attributed to the emergence of Nanomaterials in recent decades due to availability of new synthesis routes and advanced technologies. As evident from the results, the Nanomaterials facilitate enhanced aspect-ratio, significantly active-surface area, effective distribution at molecular level, thereby ameliorating the efficiency of Aramid fibers for high impact applications

3,6,24

. Following section describes various Nanomaterials

used for the functionalization of Nanomaterials. 3.1

Functionalization of Aramid fibers with Carbon Nanotubes (CNT)

In the recent study, Gonzalez-Chi et al. have reported a functionalization of Aramid fibers via carbon nanotubes with polypropylene (PP) as a matrix. In the experiments, they chemically modified the Multi Walled Carbon Nanotubes using a 3.0 M equimolar solution mixture of sulfuric acid and nitric acid under heating at 60°C for 15 min followed by ultrasonication and drying of slurry at 100°C 52. In the subsequent experimental study, the aramid fibers were functionalized via MWCNT’s using an immersion coating technique, followed by ultrasound, where 4 mg of chemically modified MWCNT’s were coated onto aramid fibers (550 mg) cylindrical frame rolling, and submerging into 100 mL of chloroform. The coating of MWCNT’s onto aramid fibers was achieved using an ultrasonication method for 1 h at a power of 165W and a frequency of 20 kHz, followed by drying of the coated aramid fibers in a convection oven. The performance of the MWCNT coated aramid fibers was evaluated using interfacial shear strength (IFSS) with PP microdroplet, via micro-bend test setup as shown in figure 6. They calculated the IFSS performance of the MWCNT coated aramid fibers via following equation 52

: 𝐼𝐹𝑆𝑆 =

𝐹 𝜋𝑑𝐿

Where, F = maximum pull-out force,


(1)


D = diameter of the fabric filament, L = Embedded length of the droplet.

Figure 6. Microbend Test Setup. The aramid fibers chemically surface modified with chlorosulfonic acid and functionalized with MWCNT’s, showed an enhancement in the IFSS strength from 5.86±0.9 MPa to 8.71±1.54 MPa 52. In one of the recent studies, Zhu et al. have fabricated a nanocomposite engineered with CNT functionalized aramid nanofibers (Figure 7) via vacuum-assisted flocculation and vacuumassisted layer-by-layer method 42. Their results demonstrated that the composite films prepared from MWCNT functionalized aramid nanofibers, reveal an ultimate strength of 383 MPa and a stiffness i.e. Young's modulus, up to 35 GPa. The thorough analysis of various imaging and spectroscopic methods, indicated an interfacial interactions at the interphase between CNT’s and aramid nanofibers, which further revealed the manifesting hydrogen bonding and π_π


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correlation, thus resulting in the improved mechanical properties. The developed aramid nanofiber based composites demonstrated a high thermal stability up to 520°C, and extremely low coefficients of thermal expansion i.e. 2 to 6 ppm. K-1 42.

Figure 7. Functionalization of Aramid nanofibers via MWCNT’s. In another study, Shah et al. have reported continuous procedure for functionalization of Aramid fiber via CNT’s (50 nm diameter) with uniform length and distribution. They claimed a process innovation, wherein CNT’s were uniformly infused into Aramid fibers for a matrices such as epoxy, polyester, vinyl ester, polyetherimide (PEI), polyether ether ketone (PEEK), polyphthalamide

(PPA),

polyetherketone

(PEK),

phenol-formaldehyde

(PF),

and

bismaleimide (BMI). They claimed that CNT infusion process exhibited a material residence time 5 to 600 seconds in a CNT growth chamber 53. In another study, Liu have reported a functionalization of Poly(p-phenylene terephthalamide) i.e. Aramid, fibers using MWCNT’s by polyanion synthesis route as described pictorially in Figure 8 54.



Figure 8. Aramid fiber functionalization via Polyanion Synthesis route. After polyanion synthesis route, the MWCNT-Aramid engineered nanocomposite membranes were fabricated using re-proton reaction and filtration process (Figure 8). Their results showed a formation of network structure was formed with interweaving of MWCNT’s which facilitated a stress transfer for applied tensile stress along with aramid matrix, thereby yielding a tensile strength of 327 MPa, i.e. 54.2% increase as compared to pristine sample. The MWCNTAramid nanocomposite films showed an enhancement in mechanical properties for a 2% loading of MWCNT’s. The in-situ Raman characterization revealed an interfacial stress transfer aramid matrix to MWCNT’s, due to induced crystallization of aramids around the network structure. They also claimed an accurate prediction for correlation between Young’s


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modulus and volume fraction loading of MWCNT’s (for a specific range) for developed composite films via Halphin-Tsai equation. Their results demonstrated an improved toughness of 8.05 ± 0.69 kJ/kg i.e. 208.4 % increase, as compared to pristine samples i.e. 2.61 ± 0.15 kJ/kg. Their study results also revealed that, the surplus addition of MWCNT’s forms a cluster structures, which leads to stress concentration phenomenon, thereby decreasing the enhancement of toughness 54. 3.2 Functionalization of Aramid Fibers with Silica & its Derivatives Silica i.e. SiO2, which is an oxide form of silicon, is widely used additive for enhancing the material properties. It is frequently found in the form of quartz, and in living creatures. It is a main element found in the sand all over the world 55. Silica possesses a complex structure and manifests in the form of various minerals. Based on the applications, it is used in the form of fumed silica, fused quartz, aerogels, and silica gel. It is widely used in polymer matrix composites for enhancing the mechanical properties for structural applications, conjointly it is also used is pharmaceuticals, food industry, and in microelectronics 55–57. Various researchers have explored the silica and its derivatives for the functionalization of Aramid fibers. In one of the study, Cheng it al. have used silica based sol-gel process for the surface functionalization of Benzimidazole engineered Aramid fibers, thereby improving its performance 58. The functionalization process has been represented pictorially in Figure 9.



Figure 9. Aramid Fiber functionalization via SiO2 Sol-Gel Process. They reported a grafting method for the surface functionalization of Aramid fiber engineered with Benzimidazole. In the functionalization process, the ‘NH’ groups present in Benzimidazole were used as an active sites, which facilitated the reaction with densely structured SiO2, thereby generating a rough surface, further it was treated with silane coupling agents via abundantly available NH2 and C=C groups present on Aramid fiber. The functionalized NH2/C=C groups present on the Aramid fiber facilitate its chemical bonding with matrices such as Bismaleimide (BMI), Elastomer, and Epoxy, further the SiO2 engineered rough surface helps in improving the mechanical interlocking of fiber with the matrix. Their experimental findings revealed an enhancement in interfacial shear strength for functionalized Aramid fiber with Epoxy, Natural Elastomer, and Bismaleimide (BMI) i.e. 43%, 117%, and 166%, respectively, as compared to non-functionalized Aramid fibers. In another study Li et al. have fabricated a silica derivative based composite (AF/aerogels), wherein a reinforced Aramid fiber was functionalized with silica aerogel matrix, which


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exhibited a highly enhanced thermal insulation characteristics, exceptional flexibility, and low density (Figure 10) 59,60.

Figure 10. Pictorial Depiction of Aramid Fiber Functionalization via Silica Aerogel Nanoparticles. The microstructural analysis of AF/aerogel composite revealed a well embedded Aramid fibers in aerogel matrix (Figure 11), thereby forming a main structural component for enhancing the mechanical strength of the composite.



Figure 11. Morphological analysis of AF/aerogel composites, (a) integral view of the composite, (b) Aramid fibers encompassed with silica aerogel fractions, (c) single Aramid fiber covered with silica aerogel nanoparticles, and (d) nano level porous structure of silica aerogel matrix. Reprinted with permission from ref 59. Copyright 2016 Elsevier. The flexural analysis performed via three point bending test revealed an enhancement in flexibility for ~5% loading of Aramid fibers, still maintaining thermal insulation efficiency, which could be attributed to the finely infused Aramid fibers in silica aerogel matrix. Their results revealed that, the progressive loading of Aramid fibers facilitated the enhancement in thermal conductivity i.e. in the range of 0.0221 to 0.0235 W.m-1.K-1, whereas the density decreased up to 0.142 g·cm−3. The thermal analysis performed via Thermogravimetry & Differential Scanning Calorimetry (DSC) analysis thermal stability of the composite up to 285 °C, which was attributed to the pure silica aerogel nanoparticles 59,60. 3.3 Functionalization of Aramid fibers with Graphene & its Derivatives ACS Paragon Plus Environment

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Graphene is an allotrope of carbon which exhibits single layered two-dimensional structure with hexagonally ordered sp2 hybridized carbon atoms in a honeycomb crystal lattice [Figure 12 (a)] 61,62. Considering the unique structure of Graphene which possesses carbon-carbon bond length of 0.142 nm, and an interplanar spacing of 0.335 nm, it exhibits extremely high mechanical strength i.e. tensile strength ~ 130.5 GPa, and a Young’s modulus ~ 1TPa

62,63

.

Further, Graphene is able to uniformly disseminate the impact load i.e. 10 times greater than the steel (per unit of weight), which could be attributed to its Hexagonally ordered Honeycomb type structure

61,62,64

.

Figure 12. Graphene Derivatives (a) Single-layer structure of Graphene, (b) Graphene Oxide. Considering the exceptional mechanical properties as mentioned earlier, various researchers have explored the utilization of Graphene for armor and ballistic applications. In one of the study, O’Masta et al. have fabricated 10 μm thick films of Polyvinyl alcohol (PVA) based composites engineered via liquid exfoliated multilayered (~30 layers) Graphene (MLG) sheets (volume loading~35%). They evaluated the mechanical and projectile impact performance of MLG/PVA composite films via various stress loading tests such as dynamic, transverse, quasistatic and edge-clamping, where they compared the results with films of equal mass of pure PVA and aluminum. The developed MLG/PVA composite films revealed a ballistic performance equivalent to aluminum i.e. Cuniff velocity ≥ 500 m.s-1 for a volume fraction of ~ 0.3 (Figure 13) 64.



Figure 13. (a) Effect of Graphene loading on MLG/PVA composites (a) Comparison of MLG/PVA composites with commercial composites for specific strength and stiffness. Reprinted with permission from ref 64. Copyright 2017 Elsevier. They observed a decrease in sensitivity for strain-rate, whereas it revealed an improved mechanical performance for reduced strain-rates as compared to Pure PVA matrix. The MLG/PVA composite revealed a toughness comparable to aluminum, with a reduced ductility compared Pure PVA. Further, the stiffness & strength of the composite films was found to be lower than the theoretical calculations, which is attributed to the irregular alignment of the platelets, coupled with the small size of Graphene. The Edge-clamped films of MLG/PVA composite revealed an improved i.e. double, ballistic limit for equal mass of aluminum foils for tested projectile masses, which was further supported by a membrane stretching analysis. Further, the membrane stretching analysis, showed that the highly ordered Graphene sheets in MLG/PVA composite can yield a theoretically calculated maximum mechanical properties, higher than three times, as compared to commercial composites, for a minimum ~10 vol% loading of Graphene sheets 64.


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In another study, Wetzel et al. have theoretically predicted the Ballistic performance i.e. penetration resistance, of the continuously ordered Graphene membranes via density functional theory. Their analysis revealed that the highly ordered and multilayered Graphene membranes can enable a penetration resistance for Ballistic mass of up to 100 times, greater than the existing commercial lighter barrier materials. They observed that the improved performance is mainly attributed to the highly increased speed of elastic wave coupled with the strain energy required for failure of the system. In one of the study Xie et al. have mimicked biological nacre structure in the fabrication of nanocomposite (Figure 14), engineered with graphene oxide (GO) [Figure 12 (b)] (o to 32 vol%) and silk fibroin (SF), via Layer-by-Layer technique using spin coating, which exhibited exceptionally high dynamic properties owing to the high mechanical properties of the components at nanoscale 24. They assessed the dynamic behavior of the GO/SF nanocomposites via micro-scaled ballistic test via 7.6 μm diameter silica based spheres moving at a speed of ~ 400 m/s. Further, they observed that specific penetration energy of the GO/SF composite increased rapidly, as the GO flakes were ordered from non-interacting, segregated sheets to a partly-lapped continuous GO sheets.



Figure 14. Graphene oxide (GO) & Silk Fibroin (SF) Engineered Nanocomposite. Their study further concluded that the continuous form of GO in the GO/SF layered assembly is the main driving factor for the advancement of the 2D nano-scaled composite armour systems 24. 3.4

Functionalization of Aramid fibers with Zinc Oxide (ZnO) Nanomaterials & its Derivatives

Zinc oxide (ZnO) is an inorganic white coloured water insoluble ceramic compound, which is mainly used as an additive in elastomers (60% of total produced ZnO), polymers, ceramic materials, construction industries, fasteners, nano-ferrites, as fire-retardant, first-aid tape for biomedical applications. Considering, the wide band gap and its ability to facilitate p & n type doping, it is widely used a semiconductor in electronic applications. It occurs in nature as a


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mineral i.e. zincite, which is treated synthetically to yield ZnO. Considering its ability to fasten with materials, it is also used in composites for enhancing the adhesion between the matrix and reinforcements, and for improving the interyarn friction required for Ballistic applications 15,65– 67

.

In one of the study, Patterson and Sodano have reported utilization of ZnO nanoparticles (synthesized via dilute colloidal solution of sodium hydroxide, ethanol, and zinc acetate) in Aramid fibers for enhancing the interfacial adhesion with matrix and the UV absorption for developing the advanced composites (Figure 15) 68. They used one-step deposition approach for ZnO nanoparticles on Aramid fibers, which resulted in an improved interfacial shear strength i.e. 18.9%, as analysed via single fiber pull-out method. Further, they oxidized the Aramid fiber surface via hydrolysis method in order to obtain the oxygen functionalized surface, as a result it showed 33.3% improvement for interface between fiber and ZnO nanoparticles. Their studies also revealed 75.9% and 93.7% retainment of tensile strength and modulus, respectively, when the ZnO nanoparticle coated Aramid fibers were treated in UV light source for 24 hours 68.

Figure 15. Aramid fiber function via ZnO Nanoparticles.



The enhancement of mechanical properties in ZnO/Aramid fibers is also attributed to the formation of C-O-Zn bond between the nanoparticles and fibers, and the resulting rough surface arising due to influence of ZnO nanoparticles 68. In one of the recent study, Hwang et al. have functionalized the Aramid fiber i.e. Kevlar, via ZnO Nanowires, thereby growing ZnO arrays on the fibers via inter-yarn friction technique, for enhancing the Ballistic performance (shown pictorially in Figure 16) 69. by One important mechanism is inter-yarn friction, which can be controlled through surface treatment of the fibers. Their results also demonstrated that the low aspect ratio of ZnO nanowires greatly improves the inter-yarn friction in the fibers. The incorporation of ZnO nanowires improved the tensile strength and elastic modulus of the functionalized fiber by 13% and 10%, respectively. Further, the ZnO nanowires improved the peak loading and fabric pull-out energy by approximately ~15 and ~23 times, respectively. They also claimed that the enhancement in the inter-yarn friction could be attributed to the interlocking of ZnO nanowires and fiber, coupled with manifesting yarn crossing-points 69.


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Figure 16. Aramid fiber functionalization via ZnO Nanowires and the yarn pull-out technique. In yet another study, Hazarika et al. have functionalized the Aramid fibers i.e. woven Kevlar fiber (WKF), via ZnO nanorods grown on fiber surface for improving its interfacial strength with matrix i.e. Polyester resin (PES) (Figure 17). The Aramid fiber functionalization with ZnO nanorods in was accomplished using low temperature assisted hydrothermal technique. of WKF using a low-temperature hydrothermal method and composite fabrication was achieved using Vacuum Assisted Resin Transfer Molding (VARTM) 70.

Figure 17. Aramid fiber functionalization with ZnO Nanorods and composite fabrication via VARTM Process. Prior to functionalization with ZnO nanorods, the Aramid fibers were surface treated via hydrolysis and ion exchange process Before growing the nanorods on the surface of the fibers, the WKF was subjected to surface hydrolysis and an ion-exchange process, for encompassing



of carboxylic acid group on the fiber surface for promoting the adhesion of ZnO nanorods with Aramid fibers. Their FTIR analysis revealed characteristic peaks at 460 cm-1 (Zn-O stretch ), 1100 cm-1 (C-N stretch), 3327 cm-1 and 1404 cm-1 (-OH stretch and bending from carboxylic acid group), which confirmed the hydrolysis of fiber surface, growth of ZnO nanorods on the fiber surface, and the bond formation between ZnO Nanorods, Aramid fibers, and Polyester resin. Further, their results were also supported with the XRD analysis which revealed peaks at 20.74° and 23.23° corresponding to 110 and 200 crystal planes, respectively, for Aramid fibers, and 100,101, and 002 crystal planes for ZnO nanorods, thus confirming the growth of the nanorods on Aramid fiber surface 70. The peak intensity for corresponding 002 crystal plane was found to be increasing with growth of ZnO nanorods in the direction of c-axis. The results were further confirmed with morphological analysis via SEM study, implying the appearance of ZnO nanorods with seeding cycles and the respective treatment times. They observed that, the ZnO nanorods developed effectively with respective seeding cycles and treatment times, with a maximum growth of ZnO nanorods at 10 hours. The thermogravimetric analysis showed thermal stability amount of ZnO nanorods with respective loadings. The impact analysis of the developed composites showed improved penetration limits for ZnO nanorod functionalized composites (with progressive loading) as compared to non-functionalized composites. The highest absorbed impact energy for ZnO nanorod functionalized composites was found to be 60.21% higher than non-functionalized composites. Further, their results also showed enhancements in mechanical strength i.e. tensile strength and modulus, and interfacial adhesion between ZnO functionalized Aramid fibers and the Polyester resin, along with the thermal improvement 70. 3.5

Functionalization of Aramid fibers with Silk Fibroin Based Materials


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Natural Silk fibers generated by the spiders are one of the excellent structural materials originating from nature. Presently, the silk fibers are produced from Bombyx mori, cocoon silks from silkmoths, and Antheraea pernvi. The silk fibers have exceptional mechanical strength as compared to known structural materials, which has been shown in Table 2 71,72. Table 2. Approximate mechanical properties of known structural materials. Energy to Break Materials

-2

Modulus (N.m )

References

-2

Strength (N.m ) (J.kg-1) 71,72

Spider Frame 10

9

5

1x10

1x10

1x10

Kevlar

1x1011

4x109

3x104

Cellulose Fibers

3x1010

8x108

9x103

2x1011

2x109

1x103

Tendon

1x109

1x108

5x103

Bone

2x1010

2x108

3x103

Rubber

Ca. 106

1x108

8x104

Viscid Silk

3x106

5x108

1x105

Silk

High Tensile Steel



Recently, researchers have discovered single long thread i.e. 25m length, of spider silk in Madagascar, which spanned across the river, spun by a spider. The Orb webs of the spider silk are able to sustain large kinetic impacts i.e. with respect to fiber diameter, arising from the spider prey. Spiders are able to form 5 divers silk fibers, whereas the female spider can form up to 7 diverse silk fibers. Spider silks are tailor-made with particular objective, thereby reflecting a diversity in mechanical strengths. for specific purposes and exhibit a great variation in mechanical properties. With each different type, the silk fibers exhibit toughness from 0.02 GPa to exceptionally high 1.7 GPa (greater than steel~1.5GPa), whereas the stretching varies from 10% to 500%. Large silk fibers have unusual blend of mechanical strength and elasticity facilitates exceptionally large toughness 72. Considering the exceptional properties of Silk fibers, researchers have tried to mimic its characteristics for the development of high performance composites. In one of the study, Lv et al. functionalized the silk fibers with Aramid fibers using biomimicking approach via synthesis of nanofibrous structures (Figure 18). They utilized the regenerated silk, engineered with inorganic nanomaterials such Graphene and CNT’s, where the Aramid fibers were functionalized with nanoengineered regenerated silk via nano-scaled fibrillation approach using hydrothermal surface treatment. The resultant silk fibroin functionalized composite films of Aramid fibers yielded enhanced mechanical strength i.e. ultimate tensile strength, and modulus, as compared to pristine regenerated silk fibroin films. The enhancement in mechanical properties was attributed to the nanofibrils of Aramid fibers, coupled with the increased β-sheet content of the silk fibroins 73.


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Figure 18. Functionalization Aramid fibers via Regenerated Silk Fibroins. Reprinted with permission from ref 73. Copyright 2017 American Chemical Society. They confirmed the β-sheet content present in the SF/ANF composite via FTIR and SEM analysis. They also claimed that the higher shear coupled with stretching could be responsible for the formation of β-sheets, which was further supported by FTIR peaks at 1625-1640 cm-1, 1510-1530 cm-1 and β-turn (1665-1690 cm-1). They also reported that, the high modulus SF/ANF composite after post-stretching process, exhibited higher alignment with adjacent silk fibroin, due to high shear. They concluded that the incorporation of the regenerated silk fibroin as low as 2 wt%, the SF/ANF composites yields the ultimate stress, Young’s modulus and toughness of ~210.43 MPa, ~6.25 GPa, & ~833.96 MJ m-3, respectively 73. Their observed mechanical properties were found to be higher than that of the ANF composite films engineered with CNT and other nanomaterials. Their studies also revealed that the reinforcing ability of the developed composite films also depended treatment time period of



the hydrothermal process. The shorter hydrothermal treatment times i.e.~1 hour, exhibit low colloidal stability in water, leading to inferior mechanical properties, whereas the higher treatment time i.e. > 3 hours, leads to diminishing of nanofibrous morphology, disintegrates into irregularly shaped pieces, thereby yielding poor mechanical properties. The optimized 2 hour hydrothermal treatment time provided nanofibrous morphology, along with solution stability in water, and compatibility with silk fibroin matrix, thus collectively enhancing the reinforcing effect in the silk fibroin functionalized ANF composite films 73. In another study, M.E. Messiry have enhanced the puncture i.e. stab, resistance of the composite fabric, which was engineered with silk fabric, para-Aramid fabric i.e. Kevlar-29, Polyester fabric. The developed composite fabrics of Silk fabric/para-Aramid fabric, and silk fabric/polyester fabric. They varied the number of layers of the each fabric and measured their performance with respect to puncher load (Figure 19). They reported that, the silk fabric (areal ~ 65 gsm) with five layers yields performance i.e. stab resistance, equivalent to para-Aramid fabric exhibiting 431 gsm areal mass 74.


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Figure 19. Silk-Aramid Fabric Based Multi-layered Composite and its Stab Resistance. They observed that, the composite engineered with five layers of silk fabric yielded a maximum performance i.e. puncher load of around 100 N. Further, they concluded that, the number of layers of silk fabric play a major role in determining the performance of the fabric. They also reported that, the force required for penetrating the multilayered engineered with Silk fabric and triaxially weaved para-aramid fabric can exhibit penetration resistance force of 50 N each. They also reported that the silk fabric engineered polyester fabric composite exhibits 0.122 N.m2/g of specific puncture resistance 74.



3.6

Functionalization of Aramid fibers with Miscellaneous Multifunctional Nanomaterials

Apart from the established nanomaterials, Researchers have also tried various multifunctional nanomaterials for improving the performance of the Aramid fibers. In one of the study, Fatema et al. reported a fabrication of a robust Aramid fiber i.e. Kevlar, functionalized with electroless plated Nickel, with iodine supported catalyzation process (free from palladium). They reported Aramid fiber treatment with an aqueous solution containing iodine–potassium iodide aqueous solution for transferring the iodide functional group onto fiber surface. Followed by the (Figure 20) iodide coating, the surface functionalized Aramid fiber was treated with silver nitrate solution, for converting iodide particles to silver iodide particles, which was further converted to silver metal particles. Finally, the electroless plated Aramid fiber was coated with a sleek layer of nickel which exhibited resistance to sonication, tape peeling, and corrosive alkaline solvents 75.


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Figure 20. Aramid fiber functionalization via iodination and electroless Nickel plating. The middle-layered silver particles produced on the Aramid fiber surface help in holding the electroless plated layer, and further acts a catalyst for electroless reaction. They also reported that the anionic surfactant employed in the catalyzation process improved the adhesion between the Aramid fiber surface and electroless plated layer. Their mechanical study showed an improvement in Young’s modulus i.e. from 76 GPa for pristine Aramid fiber to 99 GPa for electroless Nickel plated Aramid fiber. The enhancement in Young’s modulus was attributed to the high modulus of Nickel metal i.e. 211 GPa. Further, they observed a decrease in tensile strength i.e. 3.7 GPa for Pristine Aramid fiber to 2.7 GPa for Nickel plated Aramid fiber, the decrease was further observed for elongation at break i.e. 4.3% for pristine Aramid fiber, and 3% for Electroless Nickel plated Aramid fiber. The decrease in tensile strength and elongation at break, were attributed to iodination process followed by metal iodide formation i.e. tensile strength of Aramid fiber after iodination was 3.4 GPa, whereas after formation of metal iodides the strength was 3.1 GPa 75. In another study, Talib et al. have developed a composite based on weaved Aramid fibers (Kevlar-29), Alumina (Al2O3), and epoxy, whose high impact velocity performance was evaluated via experimental analysis coupled with theoretical calculations. The high velocity impact was performed using a cylindrical shaped projectile, where they evaluated wok done in elastic region, work done under radial and tangential stretching, and work done for plastic bending. The composites were fabricated with number of layers ranging from 6 to 10 76. Further, they performed the energy absorption analysis by presuming that the complete work done during deformation of plates is equal to entire loss of kinetic energy of projectile. They established a correlation between thickness of the composites and the limiting ballistic velocity. Their analysis revealed an enhancement in the target efficiency for bullet proof applications 76. They fabricated Kevlar-29/Al2O3/Epoxy composites with number of layers varying between 4 ACS Paragon Plus Environment


to 10, where they used Woven type Aramid fiber and 0/90 degree angled Aramid fiber. They compared the strength of composites engineered with woven type and 0/90 degree angled Aramid fibers. Their results revealed that the composite laminate with four layers having woven Aramid fiber exhibited tensile stress and Youngs modulus of 175.20 MPa, and 12.1 GPa, respectively, whereas the composite laminate with six layers having 0/90 degree angled Aramid fiber exhibited tensile stress and Youngs modulus of 270 MPa, and 11.30 GPa, respectively. 4. Thermoplastics Based Kevlar Reinforced Composites Conventionally, researchers have explored Thermoset based matrices e.g. Epoxy, with reinforcement of Aramid fibers, for manufacturing of the high impact resistant e.g. antiballistic, composites. Recently, researchers have been focusing on thermoplastic based matrices e.g. High Density Polyethylene (HDPE), Polypropylene (PP), Polyurethane (PU), etc. for high impact applications, which is attributed to their thermal stability, chemical resistance, and the ability to recycle matrix materials after service life 52,74,77–79. In one of the study, Memon et al. have fabricated a cut resistant protective composite by utilizing Polyethylene (PE) and Kevlar fabric (1:1) 77. They fabricated the PE/Kevlar protective composite using weaving and knitting processes. The composite fabric (with higher interfacings) developed using weaving process exhibited twice the cut resistance as compared composites developed using knitting process. They observed that the woven composite fabrics, and higher thickness of fabrics helps in improving the cut resistance as compared to knitted composite fabric. Further, they reported that the developed PE/Kevlar (50:50) composite fabric reveals twice the cut resistance as compared to pristine Kevlar (100%) and pristine Polyethylene (100%). The woven PE/Kevlar (50:50) composite fabric exhibited cut resistance up to 35 sharp strokes of steel cutter, whereas the same composite fabric developed using knitting process sustained only 20 strokes (Figure 21). They attributed the enhanced cut ACS Paragon Plus Environment

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resistance of the composite fabric to internal friction within the fabric, which increases as the number of interlayers increase, and the warp density, where the fabric yarns help in sustaining the cut resistance, thereby enhancing the work required to deform the fabric yarns. Further, they reported that the higher interlacements increase the tightness in fabric, thereby elevating the cut resistance performance 77.

Figure 21. Cut resistance performance of woven fabric samples (a) PE/Kevlar composite fabric, (b) Pure Kevlar fabric, (c) Pure PE fabric, (d) woven composite fabric, (e) knitted composite fabric. Reprinted with permission from ref 77. Copyright 2018 Springer Nature.



The puncture resistance analysis revealed higher performance for composite fabrics processed via knitting process, as compared to woven composite fabrics. Moreover, the pure Kevlar (100%) revealed highest puncture resistance, which they attributed to its higher tensile strength and increased cross-section. The lower puncture resistance in pure Polyethylene (100%), and the composite fabric was attributed to the low co-efficient of friction of Polyethylene 77. In other study, Ou et al. fabricated a composite based on Wood flour (WF), high density polyethylene (HDPE), and Kevlar fibers (reinforcement) by utilizing extrusion process. They reported that 2% to 3% loading of Kevlar fibers enhances the flexural, impact, and tensile properties of the developed composite 78.

Figure 22. SEM analysis of (a) WF-HDPE composite, (b) Kevlar Fiber-WF-HDPE, and (c) Grafted Kevlar Fiber-WF-HDPE composite 78. Reprinted with permission from ref 78. Copyright 2010 Elsevier.


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Further,

they

reported

that

the

surface

grafting

of

Kevlar

fibers

via

3-

chloropropyltrimethoxysilane and allyl chloride result in enhanced mechanical properties of the developed composite, which they attributed it to improved interfacial compatibilization between Kevlar fibers and HDPE (Figure 22). The impact performance of developed composite was evaluated using Izod impact tests, using notched and unnotched samples. They observed that the addition of Kevlar fibers helped in improving the impact strength. The addition of Kevlar fibers up to 3%, enhanced the impact strengths by 42.8% (notched sample) and 52.3% (unnotched sample). The results indicated that the grafting helped in improving the impact strength (maximum~6.5 kJ/m2) of the Kevlar fiber-WF-HDPE composite 78. Further, they reported that the crack initiation and propagation greatly influences the impact strength performance of the composite during impact testing. They claimed that the energy required to break is mainly related to the crack initiation in the fiber reinforced composites. They reported that the insufficient interfacial adhesion between the fiber and matrix, and the poor dispersion of filler i.e. wood flour, leads to reduced impact strength. They claimed that the generated cracks in such situations act as stress concertation points, thereby reducing the impact strength 78. In other study, Chen et al. have fabricated a composite of PVC foam reinforced with Chopped Aramid fiber, Glass fiber, and Carbon fiber via Vacuum Assisted Resin Transfer Molding (VARTM). The impregnation of chopped fibers was achieved using Epoxy resin in liquid state via VARTM process. The incorporation of aramid fibers improved the impact toughness of the composite from 150 kJ/m2 to around 230 kJ/m2 i.e. 43% improvement 80. In one of the study, Carrillo et al. reported fabrication of composite engineered with Polypropylene (PP) and Aramid fiber for Ballistic applications. They claimed that the incorporation of thermoplastic PP enhanced the ballistic strength of the composite for similar



areal density with less amount of Aramid fabric (Figure 23). They also claimed that the thermoplastic PP matrix facilitated the effective energy absorption mechanism via debonding and delamination with fabric. They reported that the overall ballistic performance of the developed composite was higher than the pristine Aramid fabric 79.

Figure 23. The impact performance of composite with four layers of (a) Aramid fibers, (b) Composite layers. (Impacted at a speed of 274.5 m/s). Reprinted with permission from ref 79. Copyright 2012 Elsevier. They claimed that the impact projectile was stopped by 3 and 2 layers in the composite, thus indicating the impact performance of 2 layered composite, which reduced the Aramid fabric amount by 40-60% when engineered with PP matrix. They claimed that the permanent deformation in the composite increases in linear fashion, with respect to increasing layers, which they attributed to the resulting enhanced energy absorption capacity of the composite


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(Figure 23). They reported that the thermoplastic PP matrix rendered increased energy absorption via failure mechanisms such as delamination and tension in the secondary yarns of the fabric. They claimed the composite containing only Aramid fabric layers, facilitate energy absorption via straining of the primary fabric yarns, which initially come in contact with projectile during impact. The composite containing PP matrix and Aramid fabric, facilitate energy absorption by debonding and delamination modes, along with the straining of secondary yarns due to even distribution energy by matrix 79. In other study, Nayak et al. fabricated composite laminate (thickness~15 mm) modified with PP matrix and Twaron Aramid fabric for Ballistic performance against 7.62 mm ArmorPiercing projectile (varying velocity)

81

. The developed composite was engineered with

Ceramic (with Zirconia toughened alumina (ZTA)) on front side (thickness~4 mm). They claimed that the incorporation of ZTA engineered ceramic enhanced the ballistic limit of the developed composite. Further, they reported that the absorption of impact energy was facilitated by ceramic fracture and elastic-plastic deformation of PP-Aramid fabric layers. They noticed that the breaking of the ceramic tiles was observed only for the impacted for area, which did not propagate further, as a result of PP-Aramid fabric layer. Their results showed ballistic limit of 575 m/s for 10mm thick PP-Aramid fiber composite, whereas showed the ballistic limit of 752 m/s for 15 mm thick PP-Aramid fiber composite 81. In one of the study, Stojanović et al. have fabricated a composite modified with Polyurethane (PU), para-Aramid and Polyvinylbutyral (PVB), along with nano silica particles (5 wt%). They treated the composite with coupling agent i.e. Silane, and Glutaraldehyde (GA) (for crosslinking). The developed nanocomposite cross-linked with Glutaraldehyde yielded an enhanced stabbing-resistance of 42.4 J, as compared to non-cross-linked composite, which revealed resistance of 21.3 J (Figure 24)

82

. Further, they also performed ballistic



characterization and analysis of the developed composites, via NIJ Standard of 0101.04 with a projectile (8 g) velocity of 358 m/s.

Figure 24. The stab tested composite panels showing (a) front side, (d) back side, (b), (c) and (b1) impact zone (front side). Reprinted with permission from ref 82. Copyright 2013 John Wiley and Sons. 5. Stress Transfer and Failure Mechanisms in Fiber Reinforced Composites In order to protect the object from ballistic impact, it needs to sustain the generated high impact energy shocks, exhibit cut-resistance, intact bonding of matrix and fibers, and simultaneously dissipate the energy in the composite by limited deformation of reinforcing fibers for energy absorption (Figure 25) 2,9,83–94.


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Figure 25. Projectile impact and its energy dissipation in the composite system. Xiaogang Chen have stated that the fiber reinforced composites materials can fail by two ways: (1) intra-ply breakage, where damage occurs at fiber-matrix inter face, polymer matrix, or fibers, and (2) inter-ply failure, where composite material fails via delamination of the plies.



While sustaining the ballistic impacts, the composite material can fail through cracking of matrix, fracture of fiber, delamination of plies/layers, and punching (Figure 26) 8.

Figure 26. Orientation of fibers in the composite laminate, and the failure modes during projectile impacts. Nevertheless, the ultimate aim of the anti-ballistic fibrous composites is to halt the projectiles from penetrating, therefore, the researchers have mainly focused on sustaining the ballistic impacts via absorption of the kinetic energy, for understanding the response of the composite via failure of fiber & matrix, delamination in plies/layers, and the growth of the stress waves, etc. 8,95. A projectile impact with varying velocity can cause generation of various stresses, which lead to failure of the developed materials i.e. composite system. Paul Hazel have suggested mechanisms through which a material can fail under ballistic impact 5. The composite engineered with materials such as glass or ceramic can undergo Brittle failure due to their lower


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fracture toughness. Such materials with brittle nature need kinetic energy for fracturing its surface during ballistic impact, nevertheless a small amount of kinetic energy is dissipated during this process. However, most of the kinetic energy is frequently transported to the generated fragments. Therefore, in some cases brittleness can be advantageous as it facilitates the formation of the fractured surface, thus enabling the ‘bulking’ in the material 5,8. While sustaining the stress transfer during ballistic impact, the metal engineered stiff materials undergo Grass cracking failure mechanism. In such processes, the propagation of cracks happen at high speeds i.e. close to sound speed in materials. The armor grade materials e.g. high content carbon steels, exhibiting weld joints are more prone to such types of failures during ballistic impacts, thus directly influencing the stress bearing capacity of the developed structures 3,5,8. Sometimes, the material can undergo delamination during high impacts, which is attributed to the wave reflections generated by stress, thus exceeding the tensile strength of the material under service. However, if this happens during planar impacts, then the material gets dragged by the initial stresses 5,8. Additionally, the polymer based composite materials can undergo melting at the hit points due to hard-pointed projectile, thus splitting the materials via viscous flow (due to melting) 5,9. Sastry et al. have reported that as the impacting projectile hits the material i.e. target, the shear and compression waves transmit forward from the impact location, and comes back after touching the back side. Subsequently, the shear and compression waves exhibiting certain motion, causes the target to generate their motion. Furthermore, the contact time of the projectile is greater than the time exhibited by the lower mode of target structure (in lower velocity impacts), whereas the same time is shorter in higher velocity impacts. It is observed that the boundary conditions also affect the target material during impacts with lower velocity, however the in the higher velocities, the impact gets localized at the impacted area, and it is



independent of the boundary conditions. A velocity with 20 m/s is considered as a threshold velocity, where a transition between higher and lower velocity impacts. The velocity greater than 3 km/s is considered as hyper velocity, and these velocities are exhibited by moving satellite (equipped with high strength composites) debris in the space, where they possess maximum chances of colliding i.e. impact, with each other at such high speeds 95. It has been observed that the kinetic energy generated by the impact projectiles is absorbed and dissipated in the target materials via various mechanisms, like, the kinetic energy absorption by the shifting cone on the back side of the target material, the energy absorption via primary yarns during tensile breakdown, the energy absorption via cracking of the matrix, the energy absorption by secondary yarns during elastic deformation, delamination of the plies/layers, the absorption of friction energy, and shear-plugging during projectile penetration. The thickness of the plies, volume fraction of the fibers, and the orientation in the fiber directly affect the performance of the fiber reinforces composites. Additionally, it has been observed that the thin composite laminates exhibit higher capacity for specific absorption of the energy, as compared to thick composite laminates. Furthermore, it is also observed that the different thin composite laminates connected with air-voids exhibit lower absorption capacity for specific energy, as compared to their total specific energy absorption capacity. This decrease in energy absorption is attributed to the small sized particles of the composite laminates which get impelled before the projectile, thereby producing the damage in the next adjacent plies/layers before the impact of the projectile 95. The ballistic impact performance of the reinforcing fabric in the composite is commonly evaluated via its ballistic limit and specific ballistic energy during service, which depicts fabric mass efficiency. The kinetic energy exhibited by the impacting projectile is given by following equation 3:


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𝑬𝒔 =

𝒎𝑽𝟐𝒃𝒍

(1)

𝟐𝑨𝝆

Where, Vbl = ballistic limit of projectile (m/s) (impact velocity of the projectile), m = mass (kg), Aρ = areal density (kg/m2), Es = specific ballistic energy of the impacting projectile (J.m2/kg). The composite material having n layers of reinforcing fabric exhibits areal density, which is evaluated using following equatiom: 𝑨𝝆 = ∑𝒏𝒊=𝟏 𝒕𝒊 𝝆𝒊

(2)

Where, ρi & ti = density and thickness of ith layer. The dissipated kinetic energy of the projectile is given by following equation: 𝟏

𝑬 = 𝟐 𝒎(𝑽𝟐𝒊 − 𝑽𝟐𝒓 )

(3)

Where, E = energy dissipated (joules), Vi = initial velocity of the projectile (m/s) (which is also Vbl), Vr = residual velocity of projectile after penetration (m/s) E = normalized initial velocity (of projectile). In the case of zero or partial penetration is achieved by the projectile i.e Vr = 0, then the above equation is modified as: 𝟏

𝑬 = 𝟐 𝒎𝑽𝟐𝒊

(4)

The NIJ Stadard of 0101.04-2001 states that, the baseline ballistic limit that is assigned to impact velocity of the projectile, where it completes penetration of the armour i.e. composite material, component with 50% of time i.e. V50 (different from the above-mentioned Vbl), should be evaluated staistically and experimentally 3. The composite material exhibits heterogeneity and anisotropic characteristics, which are given by complex equations, and hence, the equations for motion may be easily clarified by



Page 48 of 82

presuming the linear elastic characteristics of the stress–strain relation, is given by following equation 95,96: 𝝈𝟏𝟏 𝑪𝟏𝟏 𝝈𝟐𝟐 𝑪𝟐𝟏 𝝈𝟑𝟑 𝑪𝟑𝟏 = 𝝈𝟒𝟒 𝑪𝟒𝟏 𝝈𝟓𝟓 𝑪𝟓𝟏 {𝝈𝟔𝟔 } [𝑪𝟔𝟏

𝑪𝟏𝟐 𝑪𝟐𝟐 𝑪𝟑𝟐 𝑪𝟒𝟐 𝑪𝟓𝟐 𝑪𝟔𝟐

𝑪𝟏𝟑 𝑪𝟐𝟑 𝑪𝟑𝟑 𝑪𝟒𝟑 𝑪𝟓𝟑 𝑪𝟔𝟑

𝑪𝟏𝟒 𝑪𝟐𝟒 𝑪𝟑𝟒 𝑪𝟒𝟒 𝑪𝟓𝟒 𝑪𝟔𝟒

𝑪𝟏𝟓 𝑪𝟐𝟓 𝑪𝟑𝟓 𝑪𝟒𝟓 𝑪𝟓𝟓 𝑪𝟔𝟓

𝑪𝟏𝟔 𝝐𝟏𝟏 𝑪𝟐𝟔 𝝐𝟐𝟐 𝑪𝟑𝟔 𝝐𝟑𝟑 𝑪𝟒𝟔 𝝐𝟐𝟑 𝑪𝟓𝟔 𝝐𝟑𝟏 𝑪𝟔𝟔 ] {𝝐𝟏𝟐 }

(5)

Where, 1, 2, and 3 = co-ordinate axes, σ = stress, ϵ = strain, C = Material constants. The above-equation can be simplified as: {𝜎} = [𝐶]{𝜖}

(6)

The constant for material (C) i.e. matrix, becomes symmetrical after invoking compatibility conditions. Therefore, contemplating the compatibility and orthotropic characteristics of the composite material, the equation (5) can be simplified as: 𝝈𝟏𝟏 𝑪𝟏𝟏 𝑪𝟏𝟐 𝑪𝟏𝟑 𝝈𝟐𝟐 𝑪𝟐𝟐 𝑪𝟐𝟑 𝝈𝟑𝟑 𝑪𝟑𝟑 𝝈𝟐𝟑 = 𝝈𝟑𝟏 𝒔𝒚𝒎 {𝝈𝟏𝟐 } [

𝟎 𝟎 𝟎 𝑪𝟒𝟒

𝟎 𝟎 𝟎 𝟎 𝑪𝟓𝟓

𝝐𝟏𝟏 𝟎 𝝐𝟐𝟐 𝟎 𝝐𝟑𝟑 𝟎 𝝐𝟐𝟑 𝟎 𝝐𝟑𝟏 𝟎 𝑪𝟔𝟔 ] {𝝐𝟏𝟐 }

(7)

In the above equation, the overall number of material constants (C) has come to nine, hence, the composite material exhibiting orthotropic nature in three dimensional stress-state will give following simplified material constant (C) 95:


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(𝟏 − 𝒗)

[𝑪] =

𝒗

𝒗

𝟎

𝟎

𝟎

(𝟏 − 𝒗)

𝒗

𝟎

𝟎

𝟎

(𝟏 − 𝒗)

𝟎

𝟎

𝟎

𝑬

(𝟏−𝟐𝒗)

(𝟏+𝒗)(𝟏−𝟐𝒗)

𝟐

𝒔𝒚𝒎

𝟎

𝟎

(𝟏−𝟐𝒗)

𝟎

𝟐

[

(𝟏−𝟐𝒗) 𝟐

(8)

]

Where, E = Young’s modulus, m = Poisson’s ratio. Considering, the fiber structure, fiber orientations, type of resin utilized in the composite, intact packing of fiber-matrix, etc. can influence the stress transfer, and failure mechanisms. 6. Advanced Composite Fabrication Technologies Aramid fiber reinforced composites are primarily used in the high end engineering applications like protective composites for ballistics, Defence, aerospace, automobile, where it requires precision fabricated composite laminates having intact-fiber-matrix bonding, dimension specified designs, optimum quantity of resins, etc.

26,28,29,97–101

. Apart from aramid fiber

reinforce composites, researchers have explored various processes and techniques for the fabrication of composites in different engineering applications 102–114. However, for mitigating the critical requirements of ballistic, Defence, aerospace, etc. applications, the techniques such as Resin Film Infusion, Resin Transfer Molding have been necessarily used by the industries during the fabrication of such engineered composites. 6.1 Resin Film Infusion Resin Film Infusion (RFI) is a newly developed precision processing technique for development of specifically engineered composites, and hence it has emerged as a strong alternative to Resin Transfer Molding (RTM) technique. RFI utilizes the polymer resins in



semi-solid forms as films i.e. sheets, with specified thickness. The RFI has certain advantage over RTM like reduction of tooling cost, the applied vacuum ensures void free infusion of the fibers in the resins. As compared to thermoset resins, RFI also facilitates easy fabrication of thermoplastic based composites via semi-solid films (Figure 27) 26,29,97,98.

Figure 27. Resin Film Infusion (RFI) Setup. RFI technique utilizes sandwiched resin films supported with reinforcing fabric layers arranged in alternate fashion, after the arrangement of all components, the vacuum is applied to the setup. As observed from setup shown in Figure 27, the peel ply is used for the easier detachment of the composite laminate, whereas the bleeder helps in absorbing the excessive resin. The whole vacuum bagged setup is heated at specified temperature for the curing of the resin, simultaneously the required vacuum pressure is maintained constantly. Subsequently, the resin film starts to flow under the action of the heat, as a result of decreased viscosity. Further, the applied vacuum causes the infusion of resin in the adjacent reinforcing fibers, thus bringing the impregnation

matrix-fiber layers. The RFI process is suitable for all polymeric resins

(including thermoplastics & thermosetting resins) possessing film forming ability, it is generally carried out in autoclave chambers for maintaining the proper heat distribution for the curing of the resin, further the use of resin films avoids the use of solvent, thus making it solvent free process 26,29,97,98.


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6.2 Resin Transfer molding Resin Transfer Molding (RTM) is a widely used technique for the fabrication of high end composite structures. RTM process utilizes the preforms of fibers or reinforcing fibers or fabrics in the lower side half mold, which is later closed by the upper side half mold, during the composite fabrication 28,99–101. RTM setup consists of a mold with required shape and design, this mold is coated with a certain gel which facilitates easy detachment of composite laminates. In this process, the liquid resin along with catalyst is transferred to the mold using applied pressure, and the air is displaced via vents, till the mold gets completely filled up (Figure 28).

Figure 28. Resin Transfer Molding Setup. Further, RTM process can be integrated with vacuum, for attaining the uniform flow and distribution of resin. After, filling of the resin, the curing of the resin starts in the composite laminate under applied heat at certain temperature. The resin viscosity plays an important role in RTM process as it directly influence the resin transfer time. If the resin viscosity is higher, then the required pressure is also high, but this can also cause the dislocation of the fibers i.e.



resin wash, during mold filling, hence the recommended optimum viscosity for RTM process is between 100cP to 300cP 26,97. 6.3 Pre-impregnation (Prepreg) Technique Pre-impregnation (Prepreg) is a widely using processing technique for fabricating the high quality fiber-reinforced composites for critical engineering applications 4,95,97. Prepreg process involves impregnation of fiber reinforcements in resin solution (typically for thermosetting resins) or hot-melting (in film-form) of thermoplastic resins (Figure 29). In resin solution method, fiber impregnation is achieved by immersion (at a specified rate) of fibers in a resinsolution filled container, after which the impregnated fibers are sent either for curing or drying.

Figure 29. Prepreg Processing Method. In hot-melting process, resin-film (typically thermoplastic resins) and fiber reinforcement are interspersed i.e. sandwiching, in between release coatings i.e. films, accompanied with predetermined pressure and temperatures

97

. Prepreg technique can be effectively used for

fabricating the fiber reinforced composites based on Aramid, Carbon, Glass fiber, etc., with minimum defects and high dimensional accuracy. Processing of Prepreg is mostly carried out in compression or autoclave molding 4,43. An autoclave chamber contains well-stacked prepreg layers, arranged in a desired sequence, which are then spot-welded to improve dimensional stability and avoid displacement of stacked layers. Then, the vacuum-bagging is applied onto ACS Paragon Plus Environment

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stacked prepreg assembly for removing any entrapped air manifesting in the layers. The entire prepreg setup is then passed into an autoclave chamber for curing of the resin and integrating the whole assembly in one part, via application of optimized heat and pressure 4,97. Dixit et al. and Marsh have stated that aramid fiber based prepreg systems are mostly used in protective helmets and hand-held riot safeguards, which could be attributed to their ease of processing and dimensional stability of prepreg systems at small scales 115,116. 6.4 Filament Winding This technique utilizes strands of fibers, which are continuously/intermittently impregnated by passing them through a resin solution filled in container (Figure 30). Further, these impregnated fibers are winded on a mandrel which rotates at a specified speed with specific fiber orientation and tension. Mandrel rotation speed and the applied fiber tension are main factors for imparting the required strength in winded fibers.

Figure 30. Filament Winding Method. Optimized fiber tension applied in filament winding plays an important role, as it improves the compactness of impregnated fibers, and helps in reducing the porous voids in the composites. Finally, the mandrel containing impregnated fibers is transferred to heating chamber for curing, and thereafter it is removed as a completely fabricated composite product. Recently, Ma et al have studied energy absorption performance of filament winded composite based on Aramid



fiber, Carbon fiber, and Epoxy resin, via filament winding technique

Page 54 of 82

117

. They prepared

composite tubes of carbon-aramid and carbon-carbon fiber reinforced in epoxy matrix, for analyzing it’s energy absorption against various predefined crushing forces by compression method. Their developed aramid-carbon fiber composite revealed specific energy absorption of around 100 kJ/kg, they claimed it’s applicability for vehicle protection against external damages

117

. In another study, Wong et al have described fabrication of Aramid fiber and

Polyamide-6 blended yarns via Filament winding and compression mounding. They observed that the filament samples showed lesser void content i.e. 0.25%, as compared to compression molded samples 118. 6.5 Pultrusion Pultrusion is a composite processing methods, which involves continual drawing of reinforcing fibers through a specially designed die (heated at specified processing temperature) placed in closed heating chamber (Figure 31).

Figure 31. Pultrusion Process. In this process, reinforcing fibers (in the form of fabric or rovings) are opened (coming from creel stand) and passed to resin solution bath for impregnation. Further, the impregnated fibers are then passed to heated die (of specific temperature) for curing of resin and shaping of fibers


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97,119

. Finally, the cured profiles of composites are cut into specific patterns. Prior to curing, the

excess resin is removed by pre-reformer, in order to transfer the required resin in final composite profiles. In one of study, Motochika et al have fabricated fiber reinforced composite based on braded Aramid & Carbon fiber using Polyamide-6,6 as resin matrix via Pultrusion method

120

. Their results revealed that, the Aramid fibers acted as protective heat shield

(Thermal conductivity~2-4 W/mK) for Carbon fiber, further it showed an impact energies of 32.3 J and 28.4 J for braided fibers

120

. In another study, Wang and Chen have fabricated

Aramid fiber reinforced Epoxy composite Z-pins of via pultrusion method. Their results showed an improved elongation rate of 2.3%, along with tensile strength of 1.46 GPa

121

.

Goulouti et al fabricated Epoxy and Aramid-Glass fiber reinforced composite via pultrusion method. Their Aramid fiber composite revealed an improved heat protection with thermal transmittance of lower than 0.15 W/mK 122. 6.6 Mass Scale Scalability of Ballistic Composites Processing techniques like Resin Transfer Molding (RTM), Vacuum Bagging, Resin Film Infusion (RFI), Pultrusion, etc. have been widely used for large scale production of structural composites, where matrix resin content varies from 20% to 60%

4,116,123

. Above-mentioned

processing techniques are effectively used due to easy processing route, improved infusion of resin into fibers, and the lower processing costs. However, composites utilized in ballistic protection critically require matrix resin content below 20%, in order to transfer the maximum load to reinforcing fibers, and absorb maximum energy. Studies have shown in ballistic prepreg autoclave processing, changing pressures along with reduced resin content has lower effect on ballistic performance of thermoset composites. However, higher pressures are required for lower resin content, and reduce the wetting attributes of the ballistic composites. Further, higher pressures help in reducing the porosity of resins which exhibit inferior flow properties, and reduces the formation of wrinkles. Ballistic composite manufacturers Honeywell, ASC



Process Systems, Cytec Industries Inc. have stated that autoclave based prepreg molding is widely utilized for mass scale production of body armors and helmets. 123–125. 6.7 Nanofiber Processing for Composites With the advent of processing techniques, nanofibers 126 based on polymers have been widely explored for engineering and medical applications like protective clothing with breathable fabrics 127, effluent treatments 128–130, oil-water separation 131–133, scaffolds 126,134–136, nanofiber reinforcement in protective composites

42,61

, etc. Majority of thermoset resins utilized in

composites endure inferior delamination strength, lower impact performance, and fracture strength due to their brittle nature 97, and these drawbacks can be minimized by reinforcing the resin matrix with nanofibers which helps in improving the toughness of the composite without compromising mechanical properties

136–139

. In this context, researchers have explored

nanofibers of aramids for reinforcing of the composites e.g. thermosets and thermoplastics, 42,61,140,141

. Conventionally, electrospinning (most widely used), melt fiber drawing, phase-

separation, template synthesis, self-assembly,

12,140

etc. have been utilized for the production

of nanofibers. Considering the ease of processing, simple handling, and high efficiency, etc. made electrospinning as a widely used method for production of nanofibers, however, it suffers from limited production capacity at mass scale manufacturing. These limitations can be overcome by utilizing needleless electrospinning, and multi-needled electrospinning, but it leads to increased manufacturing cost and energy. Recently techniques like pressure gyration, infusion gyration, force spinning, rotary centrifugal spinning (based on centrifugal force) have been extensively explored by the researchers

126,134–136,138,139

, which is attributed to their

materials independent properties. Heseltine et al. and Hong et al. have reported the fabrication of nanofibers for mass scale production via novel “pressurized gyration spinning technique” for overcoming the drawbacks observed in conventional electrospinning. This method utilizes centrifugal force and pressure simultaneously for generating the nanofibers (Figure 32).


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Figure 32. Pressurized Gyration Technique. Reprinted with permission from ref 126. Copyright 2018 John Wiley and Sons. The setup involves centrally located rotating vessel (operated by DC powered motor) where orifices are placed at equal distance. In this setup, polymer based spinning solution is fed via syringe to vessel, till infusion of solution to bottom. Then, the pressurized nitrogen gas is passed in vessel to push the solution out of orifices under the action of centrifugal force during vessel rotation. Nanofibers having diameters in the range of 60 nm to 800 nm can be effectively generated for mass scale production at low-cost, by using pressurized gyration technique 126,139. 7. Life Cycle Assessment of Composites Since their introduction for various industrial, Defence, and engineering applications, fiber reinforced composites have been extensively utilized based on their performance criteria and their ability to be tailored with defined properties 26,28,97,100,102. In recent years, the technological advancements, rising environmental awareness, and cost management strategies, have forced the industries to consider the Life Cycle Assessment (LCA) of the products e.g. composite materials, from design stage to post use recycling of products 142–146. These LCA strategies help



in designing the efficient and durable products, with minimum burden on the ecosystem via recycling and reuse, post service operations 27,142,145–147. The life cycle analysis considers factors such as, efficiency of resources, closing of loop, life extension, and reduction of waste, thus it helps in the development of the integrated design methods, manufacturing, us, and recovery of the constituents from the used products e.g. composite materials (Figure 33) 142,144,146.

Figure 33. Life Cycle Assessment (LCA) of the materials/products.


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Here, the maximum attention is paid towards linking the design of the product to composite technology and environmental science. In these strategies the ‘recycling’ may not be ‘environmental friendly’, but can it serve the purpose of the waste minimization 144–146. Following points are considered during the Life Cycle Assessment, which in turn are applicable to composites as well: Goal Definition & Scope. The goal defining helps in determining the scope, aim, and functional unit of the study. The scope outlines the functional boundaries and type of impact, further it can also involve geographical and temporal boundaries (depends on the lifetime of product & their emissions) pertaining to product. The functional unit is considered as the main feature of the LCA studies, and is mainly associated with the utilization of the product. The functional unit can be understood using an example of fiber reinforced composite laminate (sandwiched structure) for ballistic protection applications. In this case the fibers and matrix can be functional unit which help in the absorption of the kinetic energy and transfers the stress throughout the composite. Another example is the utilization of composite panels for acoustic and thermal insulations e.g. in buildings, here the production unit consists of one m2 of the composite laminate, whose main function is to provide the support to structure via its stiffness. Here, the functional unit considers the service rendered by the composite laminate for given period of time e.g. years, thus it is expressed as m2/year 142,144,146. Analysis of inventory. The inventory outline the number of polluting emissions and consumption of resources associated with the production the forgoing stag. Commonly, it is time taking procedure, and is generally supported with the help of databases 145,146. Impact Analysis. The impact analysis assesses the impact on the environment and consists of following parts 146:



(a) Classification. It analyzes the emissions coming from the pollutant which are associated with impact categories e.g. emission of greenhouse gases, environmental toxicity, ecosensitivity, etc. (b) Characterization. It quantifies and measures the impacts of emissions for respective categories. (c) Valuation. It analyzes the related significance of respective categories by assessing their damages and their resulting impact on the society. Improvement Analysis. The aim of this study is to analyze the weaker sections in the production, and the available methods for further improvement. At this stage, the uncertainty and sensitivity analyzes are functioned together along with improvement analysis 142,144,146,148. Key points in Life Cycle Analysis. Before performing the detailed LCA study, the results with less than the total amount of the impact should be included in the study, along with the unambiguous definitions of the functional unit, boundaries, etc. Further, key points comprise of allotment processes, and the sensitivity assessment for analyzing the trustworthiness of the data. Lastly, the LCA study is found useful is interpretation stage, where it helps the choosing the environmental friendly materials and techniques 144,146,148. Active and Passive Applications. The environmentally active materials/products primarily impact the nature during their service lifetime, while the passive materials/products make the impact during production and post service life 27. The summarized impacts (of life cycle) of these i.e. active & passive, materials/products has been depicted in Figure 34 144,146,148.


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Figure 34. Environmental impacts of Active & Passive Materials/Products. 8. Recycling of Aramid Fibers The literature analysis reveals the abundant availability of publications for the recycling and reuse of commonly used reinforcing fibers such as Glass fiber, Carbon fiber, polyamides (e.g. Nylons), and the composites 149–151. However, considering the specific utilization of the Aramid fibers, their recycling has been limitedly explored, nevertheless, few researchers and industries have tried to recycle and reuse the Aramid fibers. Awais et al. have stated that, the recycling is an ability of the material where it retains its properties, which it exhibited in its original state 152

. Considering this, it has been observed that, various materials cannot undergo recycling, as

they fail to retain their original properties after service lifetime. In this context, Recycling Index (R) is an appropriate method for the comparison of the materials on similar scale, and it is inferred by considering the economic points. The Recycling Index (R) can be calculated using following equation 152:



𝑹=

𝑽𝒑 𝑽𝒎

Page 62 of 82

(9)

Where, Vm = lowest possible cost of a material (₹/kg), Vp = post-recycling cost of the material (₹/kg). It can be noticed that, if the value of R is close to 1, then the material is found to be recyclable. The manufacturer of high strength Twaron fibers i.e. Teijin Aramid, have described the recycling routes for the used Aramid fibers on their portal

153

. They have described the

utilization of the scrapped aramid fibers which are generally available in used, contaminated, processed, pure, and unprocessed forms (Figure 29). Their portal states that, they recycle the aramid fibers and convert them to pulp (solution form), which are further reused in the replacement of asbestos for various technology applications. The company portal states that, the Teijin Aramid collects waste Aramid fibers from used ballistic vests, leftovers of Yarns, scraps of fabric (e.g. Cut offs, salvages, etc.), scraps of aramid fibers produced during the production stage, aramid fiber reinforced optical fiber cables, used cut protective gloves, etc. 153

.

In other study, Awais et al. have performed the recycling of aramid fibers for the development of the cut resistant gloves for protective applications. The prepared cut resistant gloves exhibited higher performance as compared gloves prepared using virgin aramid fibers 152. They reported that the recycled aramid fibers exhibit 2 to 3 times lower cost as compared to their virgin counterparts. They reported that the virgin-recycled blend of aramid fiber yarns integrated with steel core in the gloves, revealed highest cut-resistance index i.e. 3.47, along the length , whereas it reported the cut resistance index of 5.90, along the width of the gloves. Also, the gloves with virgin-recycled aramid fiber blend (without steel core), demonstrated cut resistance index of 3.07, as compared to virgin aramid fibers i.e. 2.78


152

. Thus, they

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demonstrated the recycled aramid fibers (450 ₹/kg) are advantageous for cut resistant glove applications as compared to virgin aramid fibers (2200 ₹/kg). In another study, Loureiro et al. have described the reuse of recycled aramid fibers in polyamide (PA) 6,6 i.e. Nylon 6,6; matrix, for improving its interfacial adhesion and mechanical strength 154

. They prepared PA-6,6/aramid fiber composite, where the aramid fiber exhibited

concentrations of 5 wt%, and 10 wt%. The PA-6,6/aramid fiber composites were fabricated using via twin screw extruder (co-rotating) at temperatures from 255 °C to 275 °C. Further, they functionalized composites via following treatments: (1) washing of fibers with hexane (submerging at 65 °C for 3 h), followed by (2) surface treatment with NaOH solutions (submerging in 2 wt% & 6 wt% solutions for 30 & 45 min, respectively, at 100~105°C) with varied treatment parameters (NaOH solution concentration and treatment time were varied). Their SEM analysis revealed the improvement in the interfacial adhesion composite treated with NaoH. They claimed that, although the NaOH treatment improved the interfacial strength, it reduced the tensile strength of the composite. However, they reported that 10 wt% addition of aramid fibers in PA-6,6 matrix revealed improved mechanical properties. The tensile test analysis revealed that composite with 10 wt% of aramid fiber (surface treated with NaOH) showed enhanced tensile strength of 80.9 MPa, flexural strength of 1279 MPa, and the impact strength of 3.22 J/m2, as compared to pristine PA-6,6 matrix, which exhibited tensile resistance of 75.2 MPa, flexural strength of 1169 MPa, and the impact resistance of 3.22 J/m2 154. 9. Conclusion In the present review, we have discussed in detail about the functionalization of Aramid fibers via various nanomaterials such as CNT, SiO2, Silica Aerogels, Graphene, Graphene Oxide, Alumina (Al2O3), ZnO nanorods, Silk Fibroins, and electroless Nickel, etc. for the ballistic and protective applications. Further, the authors have discussed about aramid fiber engineered



thermoplastics such as High Density Polyethylene (HDPE), Polypropylene (PP), and Polyurethane (PU) for the protective and ballistic applications. Furthermore, the review discusses about the widely used composite fabrication technologies such as Resin Film Infusion (RFI), and Resin Transfer Molding (RTM), along with the stress transfer and failure mechanisms of aramid composites. Finally, the review concludes with the Life Cycle Assessment of composite based products along with their implications for product manufacturing to waste minimization, along with some research works on the recycling of Aramid fibers. Acknowledgement The authors would like to thank Dr. C. P. Ramanarayanan ,Vice-Chancellor of DIAT (DU), for encouragement and support. The authors are also thankful to Mr. Swaroop Gharde, for technical discussions on Life Cycle Analysis (LCA), and Ms. Kirti Thakur, for help with drawing of technical LCA diagrams and matrix equations. The authors are also thankful to anonymous reviewers for improving the quality of the manuscript by their valuable suggestions. The authors dedicate this review paper to Prof. (Dr.) L. M. Patnaik, former Vice-Chancellor of DIAT (DU), Pune. Conflicts of Interest Authors do not have any conflicts of interest. References (1)

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(2)

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(3)

David, N. V.; Gao, X.-L.; Zheng, J. Q. Ballistic Resistant Body Armor: Contemporary and Prospective Materials and Related Protection Mechanisms. Appl. Mech. Rev. 2009, 62 (5), 050802.

(4)

Bhatnagar, A. Lightweight Ballistic Composites: Military and Law-Enforcement Applications; Elsevier Science: San Diego, 2016.

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Hazell, P. Armour: Materials, Theory, and Design; CRC Press: Taylor & Francis Group, 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742, 2015.

(6)

Yadav, R.; Naebe, M.; Wang, X.; Kandasubramanian, B. Body Armour Materials: From Steel to Contemporary Biomimetic Systems. RSC Adv. 2016, 6 (116), 115145– 115174.

(7)

Williams, A. The Knight and the Blast Furnace: A History of the Metallurgy of Armour in the Middle Ages & the Early Modern Period; History of warfare; Brill: Leiden ; Boston, 2003.

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Advanced Fibrous Composite Materials for Ballistic Protection; Chen, X., Ed.; Woodhead Publishing series in composites science and engineering; Elsevier/WP, Woodhead Publishing: Amsterdam Boston Cambridge, 2016.

(9)

The Science of Armour Materials; Crouch, I. G., Ed.; Woodhead Publishing in materials; Elsevier: Amsterdam, 2016.

(10)

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