Fibers and Polymers 2010, Vol.11, No.5, 719-724
DOI 10.1007/s12221-010-0719-z
Preparation and Properties of Fumed Silica/Kevlar Composite Fabrics for Application of Stab Resistant Material *
Tae Jin Kang , Kyung Hwa Hong, and Mi Ran Yoo
Intelligent Textile System Research Center, Seoul National University, Seoul 151-742, Korea
(Received April 12, 2009; Revised May 12, 2010; Accepted May 19, 2010)
Abstract: The objective of this study is to develop an advanced stab proof material composed of shear thickening fluid (STF) and Kevlar fabric. In this study, silica/ethylene glycol suspension was prepared for the use as the STF, and it was analyzed by a rheometer, TEM and dynamic light scattering spectrophotometer. From the results, it was observed that the STF significantly showed the reversible liquid-solid transition at a certain shear rate. Also, we treated Kevlar plain fabric with the STF by 1 dip-1nip finishing method and investigated the mechanical and stab resistant properties. Through the investigation of the fumed silica/Kevlar composite fabric, we found that the STF impregnation significantly improved the stab resistance of Kevlar fabric against spike threats and so enhance the protection performance of Kevlar fabric as a stab proof material. Keywords: Shear thickening fluid (STF), Fumed silica, Kevlar composite, Stab resistance, Liquid body armor
Introduction
more freedom of movement [4,5]. Therefore, we prepared the STF impregnated fabric composites, and investigated their mechanical properties and stab resistance to evaluate if they are applicable for body armor materials. In particular, we used fumed silica suspension as a STF and Kevlar-KM2 woven fabric as a matrix in this study. On the other hand, recent trends have led to an increase in the number of applications for body armor with stab protection because the vast majority of assaults against correctional officers involve improvised stabbing weapons, rather than firearms [6]. Therefore, we only focused on the stab resistance of the fumed silica treated Kevlar composite in this study.
Personal body armor is protective covering used to prevent damage from being inflicted to an individual through use of direct contact weapons or projectiles usually during combat, or from damage caused by a potentionally dangerous environment or action [1]. The basic concept of body armor is simple; first, it should stop weapons or projectiles before they completely penetrate the armor and reach a wearer’s body. Second, it should spread the weapons’ energy over a larger portion of the body armor so that the final impact causes less damage. Therefore, over the years many products have been invented and tried in order to improve body armor. However, in spite of the improvements, the modern body armors still have some of the same drawbacks as the old ones. Regardless of whether the body armors are made of metal plates or fabric layers, they are mostly heavy, bulky and rigid. Thus, they may limit the wearer’s freedom of movement and so be impractical for use on the wearer’s arms, legs and neck [2]. Therefore, recently the U.S. Army Research Laboratory is developing a new technology to save more soldiers’ lives. This new technology is liquid body armor. The key component of the liquid armor is a shear thickening fluid (STF) composed of hard particles suspended in a liquid. Shear thickening is a non-Newtonian flow behavior observed as an increase in viscosity with increasing shear rate or applied stress [3]. Thus, STF is a flowable and deformable material in the ordinary conditions, however once a strong impact is applied to the STF, it turns into a rigid material. Therefore, STF can be used as a coating material to improve the functionality of body armor. Then, the body armor referred to liquid body armor becomes light and flexible, and consequently that makes the soldiers wearing the liquid body armor safer and also gives them
Materials
Experimental
The fumed silica used was Aerosil 200 (Degussa Corporation, Akron, OH) and Kevlar fabric was Kevlar-KM2 purchased from Barrday Inc (Charlotte, NC). More specifics on the fumed silica and Kevlar fabric used in this study were respectively displayed in Table 1 and Table 2. Ethylene glycol (EG) and other chemicals were purchased from Ducksan Pure Chemical Co. (Gyeonggi-do, Korea). All reagents were used as received without any further purification.
Table 1. Physical-chemical properties of Aerosil 200 Specific surface area (BET) Average primary particle Tapped density (approx.) SiO2-content (base on ignited material) pH (in 4 % dispersion)
*Corresponding author:
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200 ± 25 m2/g 12 nm 50 g/l ≥99.8 3.7~4.7
720 Fibers and Polymers 2010, Vol.11, No.5 Mechanical properties of Kevlar KM-2 Linear density (dtex) Tenacity (cN/tex) Breaking strength (N) Yarn property Elongation at break (%) Modulus (cN/tex) Moisture regain (%) Yarn count (yarns/inch) Area density (g/cm2) Fabric construction Thickness (mm) Fabric structure
Tae Jin Kang et al.
Table 2.
667 247.1 165 3.80 5560 7.0 28×28 149 0.2 Plain weave Figure 1.
Schematic diagram for fabric-spike friction test.
Figure 2.
Schematic diagram for flexibility test.
Figure 3.
Configuration of spike impactor for stab resistance test.
Preparation
Preparation of STF To improve the dispersibility of the silica particles in medium fluid EG, the fumed silica particles were predispersed in methanol and then blended with EG. Then, the dispersion was immediately treated by homogenization for 1 h and sonication for 10 h to further improve the dispersibility. Preparation of Fumed Silica/Kevlar Composite Fabrics The STF was applied to Kevlar fabrics by the following steps; Kevlar fabrics were immersed in the as-prepared STF. And then the wet fabrics were squeezed by a 2-roll mangle to set the specific wet pick up (ca. 50 %) and also to improve the infiltration of the STF. Lastly, the fabrics were dried in a vacuum oven at 65 C for 20 min. o
Characterization
The particle size of the fumed silica in EG was measured by Dynamic Light Scattering Spectrophotometer (Otsuka Electronics Co., Inc., Osaka, Japan). And transmission electron microscopy (TEM) images were obtained with a Philips CM 200 transmission electron microscope using the samples deposited on carbon coated copper grids. The rheological properties of the STF were investigated by a TA Instruments Rheometer-AR2000. The testing was carried out at a designed temperature in a steady state flow mode and shear rate ramp of 0-125 s using a peltier plate and a cone plate of size 40 mm and 4 angle. A field emission scanning electron microscope (FE-SEM) (JEOL JSM-630F, Japan) was used for high-magnification observation of neat Kevlar fabric and the fumed silica/Kevlar composite fabric. The tensile strengths of neat Kevlar and the fumed silica/ Kevlar composite sample were conducted in a single yarn form on MTS machine (Sintech 10/GL, MTS, Eden. Prairie, MN) with a 5 kN load cell. The cross-head speed was set at 3 mm/min, and the gauge length was 4 cm. To observe the frictional resistances between a spike impactor and fabrics, the maximum loads when the spike punctured the fabrics were measured by MTS system, as shown in Figure 1. Also, to measure the flexibility, neat Kevlar fabric and fumed silica/Kevlar composite fabric were each cut into 19×19 cm -1
o
square layers, 2 test specimens were prepared by stacking 10 layers for the first specimen and 15 layers of second specimen of the square layers. A 40 g weight was then attached to the test specimens according to the geometry shown in Figure 2 and the bending angle, θ was measured [7]. The quasi-static and dynamic stab testing of the fabric targets were carried out based on modified versions of NIJ standard 0115.00 [8]. The spike impactor and NIJ target backing used in this test are shown in Figure 3 and Figure 4, respectively. Quasi-
Fibers and Polymers 2010, Vol.11, No.5
STF/Kevlar Composite Fabric for Stab Resistant Material
Figure 4.
Schematic diagram for foam target backing.
static testing of neat Kevlar and the fumed silica/Kevlar composite fabrics was performed by mounting a spike impactor in the crosshead of a load frame (Sintech 10/GL, MTS, Eden. Prairie, MN) with a 5 kN load cell. The fabric target was positioned on top of the multi-layer foam target backing. The spike impactor was then driven into the fabric target at a rate of 20 mm/min. Load versus displacement curves were recorded and used for comparison of the fabric’s performance. Dynamic stab testing of the fabric targets was conducted with a Radmana ITC-2000 instrumented impact tester (McVan Instruments PTY Ltd., Victoria, Australia). The fabric target was positioned on top of the same multi-layer target backing that was used for quasi-static testing (see Figure 4). The force, velocity and displacement of the specimens were electrically monitored by the machine, and then the interrelations between stabbing speed and the total energy absorbed by the targets were obtained from the data. Results and Discussion
Preparation of STF
Fumed silica is a non-crystalline, fine-grain, low density and high surface area silica prepared by a flame hydrolysis process [9,10]. The primary structure of fumed silica consists of branched aggregates formed by the fusion of primary spherical particles of SiO . As a result of this complex aggregated structure, the silica system exhibit high surface area. The surface chemistry of fumed silica is hydrophilic due to the presence of hydroxyl groups on the surface. Therefore, when dispersed in a nonpolar liquid, aggregates of silica can interact through hydrogen bonding of surface hydroxyls. This gives rise to larger structures called flocs, which can be further connected into a three-dimensional network. The formation of these structures leads to large increases in viscosity and gel-like behavior [11]. However, if the liquid is polar, the liquid molecules preferentially form hydrogen bonds with the surface hydroxyls present on the silica aggregates [12]. Therefore, we used a polar liquid, EG as a continuous phase in this study. However, even though fumed silica particles were dispersed in EG, the fumed silica 2
Figure 5.
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TEM images of fumed silica in STF (scale bar: 100 nm).
Shear rate sweep of fumed silica/EG suspension at different silica concentration. Figure
6.
nanoparticles (mean diameter: ca. 12 nm) formed aggregates, as shown in Figure 5. According to dynamic light scattering measurement, the size range of the aggregates in 20 % STF suspension was observed from 50 to 200 nm. Figure 6 shows the rheological graph of the STF, fumed silica/EG suspension at different fumed silica concentration. It was observed that 18 wt% or more concentration of the fumed silica in EG revealed shear thinning behavior at low shear rates, and extreme shear thickening behavior at high shear rates (at above 200 s ). And also the critical shear thickening behavior showed more significantly as the fumed silica concentration increased. This phenomenon can be explained by the order-disorder transition [13-17] and the hydrocluster mechanism [18-23]. This critical transition from a flowing liquid to a solid-like material is due to the formation and percolation of shear induced transient aggregates, or hydroclusters, that dramatically increase the viscosity of the fluid. This hydrocluster mechanism has been supported by the demonstrations through rheological experiments as well as simulation [4]. In addition, we observed that the critical shear rate inducing the shear thickening behavior was changed by the STF temperature, as shown in Figure 7. This is because that the Brownian motion of fumed silica particles in EG is proportional to the temperature. Therefore, it was thought that the performance of the stab-proof products made of the STF and Kevlar -1
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SEM morphologies of neat Kevlar fabric (a) and fumed silica/Kevlar composite fabric (b). Figure 9.
Shear rate sweep of fumed silica/EG suspension at different temperature; STF: 20 wt% of fumed silica in EG. Figure
7.
SEM morphologies of fumed silica/Kevlar composite fabric at different magnification; (a) ×1000 and (b) ×7500. Figure 10.
Shear rate sweep of fumed silica/EG suspension with ascending shear rate followed immediately by descending shear rate; STF: 20 wt% of fumed silica in EG. Figure 8.
fabric would be changed by the environmental temperature. On the other hand, the shear response of the STF displayed reversible transition as shown in Figure 8.
Preparation of STF/Kevlar Composite Fabric
Figure 9 shows the pictures of neat Kevlar (Figure 9(a)) and fumed silica/Kevlar composite fabric (Figure 9(b)), and Figure 10 shows the surface morphologies of the silica/ Kevlar composite fabric at different magnification. The images of the silica/Kevlar composite fabric clearly show that STF is well dispersed over the entire surface on the Kevlar fabric, and particularly the STF is mostly incorporated between the fibers.
Mechanical Properties of STF/Kevlar Composite Fabric
Figure 11 exhibits the stress-strain curves of neat Kevlar fabric and fumed silica/Kevlar composite fabric. It was observed that the tenacity of the fumed silica/Kevlar composite fabric was a little greater than that of neat Kevlar fabric. It
Stress-strain curves of neat Kevlar yarn and fumed silica/Kevlar composite yarn. Figure 11.
Flexibility and thickness for pure Kevlar fabric and STF/ Kevlar fabric Weight Bending angle Thickness Description (g) θ (°) (cm) 10 layers of neat Kevlar 5.33 40 2.14 fabrics 10 layers of fumed silica/ 8.09 41 2.36 Kevlar composite fabrics Table 3.
was presumed that the STF filled in the interstices between Kevlar filaments kept the arrangement of the fibers in the yarn, which induced an increase in the endurance of the yarn
Fibers and Polymers 2010, Vol.11, No.5
STF/Kevlar Composite Fabric for Stab Resistant Material
under the pulling stress. The results of flexibility and thickness test for 10 layers of neat Kevlar fabric and 10 layers of fumed silica/Kevlar composite fabric are presented in Table 3. As seen in the table, the thicknesses of 10 layers of neat Kevlar fabric and 10 layers of the fumed silica/ Kevlar composite fabric are almost same, and the flexibility of the 10 layers of the fumed silica/Kevlar composite fabric is more identical to that of 10 layers of neat Kevlar fabric. Therefore, we thought that the fumed silica treatment does not damage to the wearer’s mobility in the stab-proof vest made of the fumed silica/Kevlar composite fabric.
Stab Resistant Properties of STF/Kevlar Composite Fabric
Figure 12 shows the quasi-static loading results for neat Kevlar and the fumed silica/Kevlar composite fabric targets against the spike impactor threats. As seen in the graph, the
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fumed silica composite fabric target supported significantly higher load ca. 350 N than neat Kevlar fabric target did ca. 100 N. This result correlates with the appearance of the targets after the testing, the fumed silica/Kevlar fabric target showed significantly less damage compared to neat Kevlar fabric target, as show in Figure 13. In addition, Figure 14 shows the interrelations between initial stabbing speed and the total energy absorbed by the fabric targets. As seen in the graph, the total energy absorbed by the fumed silica/Kevlar composite fabric increased as the impact speed increased. From this result, we confirmed that the shear thickening property of STF similarly appears in the STF treated Kevlar fabric samples. On the other hand, it was also observed that the frictional resistance between a spike impactor and the fumed silica/Kevlar composite fabric obviously increased with increasing the umber of fabric ply, however that of neat Kevlar fabric hardly changed, as shown in Figure 15. Therefore, we found that the fumed silica/EG suspension treatment administered to Kevlar fabric not only improves the shear thickening property at higher impact speed but also
Load-displacement curves for quasi-static stab testing of 10 layers of neat Kevlar fabric targets and 10 layers of fumed silica/Kevlar composite fabric target against spike impactor. Figure 12.
Initial impact speed versus maximum energy absorbed by neat Kevlar fabric and fumed silica/Kevlar composite fabric. Figure 14.
Photographs of neat Kevlar fabric and fumed silica/ Kevlar composite fabric after quasi-static stab testing. Figure 13.
Maximum load at friction between spike and fabrics; neat Kevlar fabric and fumed silica/Kevlar composite fabric. Figure 15.
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enhances the frictional resistance between a spike and fabric to the Kevlar composite.
Conclusion The stab resistant coating was conducted by treating one of shear thickening fluid (STF), fumed silica/ethylene glycol (EG) suspension on Kevlar fabric to improve the performance of body armor material. Through this study, dramatic improvements in puncture resistance are observed particularly under high speed loading condition, while slight increase in tenacity was observed. Also, it was found that the STF addition did not damage the flexibility of the STF treated Kevlar fabric compared with neat Kevlar fabric. Therefore, we expected that the fumed silica/Kevlar composite fabric would be an excellent candidate for body armor applications.
Acknowledgements The authors thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC program of MOST/KOSEF (R11-2005-065).
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