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Materials Science and Engineering A 529 (2011) 224– 229

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A

journa l h o me pa ge: www.elsev ier .com/ locate /msea

mproving the impact resistance performance of Kevlar fabrics using silica basedhear thickening fluid

nkita Srivastava, Abhijit Majumdar ∗, Bhupendra Singh Butolaepartment of Textile Technology, Indian Institute of Technology, Delhi 110 016, India

r t i c l e i n f o

rticle history:eceived 24 April 2011eceived in revised form 13 July 2011ccepted 8 September 2011vailable online 16 September 2011

a b s t r a c t

Impact energy absorption capacity of Kevlar woven fabrics has been improved using silica particle basedshear thickening fluid (STF). The influence of padding pressure and silica concentration in STF on add-on%,yarn pull out force and impact energy absorption has been studied. Higher padding pressure reduces theSTF add-on%, but increases the impact energy absorption. Therefore, it is possible to achieve enhancedimpact energy absorption performance at lower STF add-on%, i.e. lower fabric weight. It is also understood

eywords:mpact resistanceevlarhear thickening fluidilica particle

that the yarn to yarn friction, as measured by the quasi-static yarn pull-out force, only partially influencesthe impact performance of STF treated Kevlar woven fabrics.

© 2011 Elsevier B.V. All rights reserved.

oven fabrics

. Introduction

Kevlar woven fabrics are often used as the base material forhe soft body armours. Generally 20–50 layers of Kevlar fabrics areequired to stop a bullet fired by a shotgun or revolver. Use of multi-ayer Kevlar fabrics not only makes the body armour heavy but alsoimits the flexibility and mobility of the wearer. As a result, protec-ive body armours are primarily used to prevent lethal injuries toorso and head. Therefore, development of materials which wouldrovide better impact resistance at lower weight has remained

perennial challenge in body armour research. In recent years,he application of shear thickening fluid (STF) on body armour

aterials has gained the attention of materials scientists by demon-trating promising results in terms of impact energy absorption.

STFs are non-Newtonian fluids, which show drastic rise in vis-osity beyond a critical shear rate. Considering the fact that bulletmpact takes place at a very high speed (300–1000 m/s), STF can beriggered to the higher level of viscosity during impact. This maynduce a liquid to solid like transition in STF during impact andhus the mobility of the threads in the fabrics become restrictednd energy dissipation improves. Besides, the solidification of STFlso facilitates the absorption of energy during impact. Theoretical

nvestigation on the causes of shear thickening was conducted byoffman [1–3]. Hoffman’s original hypothesis was that the particlesrder into layers at low rates of shear, but at some critical point an

∗ Corresponding author. Tel.: +91 011 26591405; fax: +91 011 26581103.E-mail address: majumdar@textile.iitd.ac.in (A. Majumdar).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.09.021

instability develops, which causes them to break out of their layersand jam into one another. Windhab [4] explained that particle char-acteristics, which are expected to have an impact on the structuringbehaviour in shear flows are particle size, size distribution, shape,particle morphology and interfacial properties between solid par-ticle and surrounding liquid phase. Several other researchers [5,6]have investigated the shear thickening phenomenon of colloidalsuspensions.

Applied researches on the development of body armour mate-rials using STF began only in the last decade. Lee et al. [7,8]demonstrated that the impact performance of Kevlar fabrics canbe improved significantly by the addition of colloidal STF. Impreg-nated STF–fabric composites showed improved ballistic protectionas compared with simple stacks of untreated fabrics and STF. Wet-zel and Wagner [9] compared the ballistic performance of fullyimpregnated Kevlar with the patterned impregnated Kevlar (bothhaving six layers) and found that the patterning has little or noeffect on the impact resistance. Wetzel et al. [10] also investigatedthe effect of rheological parameters of STF on the performance oftreated Kevlar fabrics. Researches by Tan et al. [11] showed that theballistic limits of single, double and quadruple ply fabric systemsincrease with the concentration of silica particles in silica watersuspension (SWS) up to an optimum level of 40%. Beyond 40% con-centration, the ballistic limit starts to decrease. Kalman et al. [12]investigated the effect of particle hardness on the impact perfor-

mance of STF treated fabrics by synthesizing a dispersion of polymethyl methacrylate (PMMA) particles. SEM micrographs of thedamaged zone showed that, unlike the silica STF, the softer PMMAparticles did not abrade the Kevlar. The ballistic results showed that

A. Srivastava et al. / Materials Science and

Table 1Specification of Kevlar fabrics.

Parameters Fabric 1 Fabric 2

Weave Plain PlainAreal density (g/m2) 200 465Yarn denier 1000 3100

tsaioimp

aasimmSf

2

2

diDes2cf

Fabric sett (ends and picks per inch) 22 × 22 18 × 18Finish Water repellent –

he softer PMMA particles led to reduced performance in compari-on to the harder silica dispersions. Some researchers [13–16] havelso reported the synthesis and rheological characterization of sil-ca particle based STF and spike and stab resistance performancef Kevlar fabrics treated with the STF. In recent years, some excit-ng researches have been conducted using finite element analysis

ethod to understand the deformation and failure of textile andolymeric materials during impact [17–19].

Although a lot of interest has been shown by researchers inpplication of STF on body armour materials, some of the keypplication parameters like padding mangle pressure, paddingpeed and silica concentration in STF, etc. have not been stud-ed methodically. Hence in this research, an effort has been

ade to study the effect of two parameters namely paddingangle pressure and silica concentration in STF (referred to as

TF concentration), on the impact performance of treated Kevlarabrics.

. Materials and methods

.1. Materials

Two types of Kevlar fabrics were used in this research. Theetails of the fabrics are given in Table 1. Silica particles (R-972),

n powder form, were obtained from Evonik (formerly known asegussa, Germany). Polyethylene glycol or PEG (MW 200) andthanol were supplied by Merck Limited. Polyvinyl alcohol (PVA)

upplied by Fine Chem was also used in this study as an additive. The00 g/m2 fabrics used in the study were pretreated with a fluoro-arbon (PTFE) based water repellent surface finish while the otherabric (465 g/m2) had no such finish.

Fig. 1. Shear thickening behaviou

Engineering A 529 (2011) 224– 229 225

2.2. Treatment of Kevlar fabrics with STF and polyvinyl alcohol(PVA)

STF was prepared by dispersing silica particles in PEG with vary-ing amount of silica loadings (40%, 50% and 60%, w/w). Ethanol wasadded to decrease the surface tension of the dispersion to enable itto wet the Kevlar fabric (STF:ethanol = 1:6). A high speed homoge-nizer at 17,800 rpm was used to make the dispersion. Kevlar fabricsamples were padded with the STF with varying padding manglepressures of 0.5 bar, 1 bar and 2 bar. Following padding, the fabricswere dried at 80 ◦C for 40 min in a hot air oven to evaporate ethanolfrom the samples.

Polyvinyl alcohol was also applied on Kevlar fabrics to analyzethe effect of yarn to yarn friction in improving the impact resistanceperformance. PVA powder (4%) was mixed in boiling distilled waterand stirred continuously for 15 min. This solution was kept for 12 hand then Kevlar fabrics were padded using padding mangle pres-sure of 1 bar. After treatment, fabric was dried at 80 ◦C for 30 minin a hot air oven.

2.3. Rheological test of STF

The rheological test of the STF was done using Anton Parr PhysicaMCR 101 stress controlled rheometer using parallel plate geome-try. The gap between the two parallel plates was kept at 0.5 mm.The viscosity was measured at three different levels of tempera-ture (0 ◦C, 25 ◦C and 50 ◦C) using Peltier temperature control deviceof the equipment. For a given temperature, the shear rate wasincreased from 0 to 200 s−1 and corresponding viscosity was mea-sured. The rheological results have been shown in Fig. 1.

2.4. Particle size analysis

Particle size analysis of silica was carried out on Beckman Coul-ter Delsa Nano C particle size analyzer. This analysis was done bytaking 0.1% silica particle dispersed in ethanol. The average particlesize was found to be 278 nm for silica R-972.

r of silica particle based STF.

226 A. Srivastava et al. / Materials Science and Engineering A 529 (2011) 224– 229

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Table 2Add-on% of silica at different levels of padding pressure and concentration.

Paddingpressure (Bar)

Silicaconcentration (%)

Add on (%)

200 g/m2 fabrics 465 g/m2 fabrics

0.5 40 4.4 6.150 5.2 5.960 6.4 6.4

1.0 40 4.1 5.050 4.7 5.760 5.9 5.8

ig. 2. Scanning electron micrograph of STF treated Kevlar filament in 200 g/m2

abrics.

.5. SEM image analysis

The surface characteristics of various STF treated Kevlar fab-ics were studied with the help of scanning electron microscopeZEISS, model: EVO 50). The samples were first coated with

silver layer to provide surface conduction before their scan-ing. The micrographs of Kevlar filaments extracted from the STFreated 200 g/m2 and 465 g/m2 fabrics are shown in Figs. 2 and 3,espectively.

.6. Yarn pull-out force

Yarn pull-out is a quasi-static test and it represents the frictionetween the yarns in a woven fabric. During ballistic impact, impactnergy is dissipated by various modes and yarn to yarn friction isonsidered to be one of them [20,21]. One end of the fabric strip iseld by the fixed jaw of Instron tensile tester (Model 3365). Then,ne free end of the yarn is pulled out from the fabric by the movableaw at a constant velocity. The peak force required to pull-out thearn from the fabric is noted which gives the idea of yarn to yarnriction in the fabrics.

.7. Dynamic impact resistance test

Dynamic impact tests were conducted on treated as well asntreated Kevlar fabrics by using drop-weight testing instrumentCEAST, Model: FRACTOVIS PLUS). The speed of impact was 8 m/s.

ig. 3. Scanning electron micrograph of STF treated Kevlar filament in 465 g/m2

abrics.

2.0 40 1.9 2.550 2.2 2.660 2.1 2.9

The diameter of the impactor, which has a hemispherical head, is1.3 cm. The fabric samples were firmly held between two circularjaws with pneumatic pressure. The inner diameter of the circularjaws is 7.6 cm. The instrument measures the impact energy interms of Joules.

3. Results and discussion

3.1. Rheological analysis

The rheological results of STF having 60% concentration areshown in Fig. 1. It is observed that shear thickening commencesbetween the shear rate of 20–50 s−1 depending on the temper-ature. It is evident that at higher temperature, shear thickeningoccurs at higher shear rate. The shear thickening behaviour ismore prominent at 0 ◦C temperature as the viscosity rises up to185 Pa s. However for 25 ◦C and 50 ◦C temperature viscosity risesup to 160 Pa s and 100 Pa s, respectively. This implies that the tem-perature has a significant effect on shear thickening behaviour ofSTF. As shear thickening plays some role in determining the impactperformance of STF treated Kevlar fabrics, this performance islikely to vary with temperature. Hence while engineering soft bodyarmors having specific performance levels, the effect of tempera-ture should be kept in consideration. Otherwise, armors designedfor low temperature regions may fail if used at warmer locations.

3.2. Influence of padding pressure and STF concentration onadd-on%

The effect of padding pressure and STF concentration on weightadd-on% is shown in Table 2. The add-on% is generally higher for the465 g/m2 fabrics as compared to that of 200 g/m2 fabrics. The rea-son may be attributed to the fact that the former does not have anyfluoro-carbon finish which ensures better penetration and distribu-tion of STF within the fabric structure resulting in higher add-on%although the areal density of the fabric, based on which the add-on% is calculated, is high. Scanning electron micrographs shownin Figs. 2 and 3 also depicts that silica particles are more uni-formly distributed in case of 465 g/m2 fabrics. From Table 2, it isalso observed that as the padding pressure increases, for the samelevel of STF concentration, the weight add-on% reduces. This is dueto the fact that higher padding pressure squeezes out more fluidfrom the fabric during the padding process. For the same paddingpressure, weight add-on% generally increases with the increase inSTF concentration. Figs. 4 and 5 depict the simultaneous effect ofpadding pressure and STF concentration on the weight add-on%for the 200 g/m2 and 465 g/m2 fabrics, respectively. From the sur-

face plots, it is evident that padding pressure has strong influenceon the weight add-on%. The effect of concentration on add-on% isless pronounced as compared to that of padding pressure. For boththe fabrics, the maximum add-on% is attained when the padding

A. Srivastava et al. / Materials Science and Engineering A 529 (2011) 224– 229 227

Table 3Yarn pull-out force of Kevlar fabrics.

Padding pressure (Bar) Silica concentration (%) 200 g/m2 fabrics 465 g/m2 fabrics

Yarn pull out force (kg) % Increase Yarn pull out force (kg) % Increase

Untreated – 0.39 – 0.54 –0.5 40 0.68 74.4 2.18 303.7

50 0.75 92.3 2.62 385.260 0.81 107.7 2.65 390.7

1.0 40 0.68 74.4 2.16 300.050 0.70 79.5 2.04 277.860 0.71 82.1 2.10 288.9

2.0 40 0.62 59.0 1.83 238.950 0.69

60 0.74

1.0 PVA (4%) 0.40

Fig. 4. Weight add-on% for 200 g/m2 fabrics.

Fig. 5. Weight add-on% for 465 g/m2 fabrics.

76.9 1.96 263.089.7 2.01 272.2

2.6 2.79 416.7

pressure is minimum (0.5 bar) and STF concentration is maximum(60%).

3.3. Influence of padding pressure and STF concentration on yarnpull-out force

Table 3 shows the results of yarn pull-out test at various levels ofpadding pressure and STF concentration. In general, the yarn pull-out force is higher for 465 g/m2 fabrics than that of 200 g/m2 fabricsas the yarn diameter is coarser in case of the former and thereforeit has greater area of contact with the other yarns.

It is observed from Table 3 that as the padding pressureincreases, for the same STF concentration, yarn pull-out force showsconcomitant reduction. However, for the same padding pressure,higher concentration of STF results in higher yarn pull-out force.Figs. 6 and 7 depict the surface plots of yarn pull-out force againstdifferent levels of STF concentration and padding pressure. It isobserved that padding pressure and STF concentration both influ-ence the yarn pull-out force. For both the fabrics, highest yarnpull-out force was attained at 0.5 bar padding pressure and 60%STF concentration. On the other hand, for both the fabrics, lowestyarn pull-out force was attained at 2 bar padding pressure and 40%

STF concentration. Therefore, it can be inferred that there is goodassociation between the trends of add-on% and yarn pull-out force.Higher add-on% of silica particles on the Kevlar fabrics enhances

Fig. 6. Yarn pull-out force for 200 g/m2 fabrics.

228 A. Srivastava et al. / Materials Science and Engineering A 529 (2011) 224– 229

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Fig. 8. Impact energy absorption by 200 g/m2 fabrics.

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Fig. 7. Yarn pull-out force for 465 g/m2 fabrics.

he friction between the yarns and thereby produces higher yarnull-out force.

It is also noted from Table 3 that the treatment of Kevlar fabricsith 4% PVA solution increases the yarn pull-out force drastically

or 465 g/m2 fabrics. However, the increase in yarn pull-out force isnly marginal for the 200 g/m2 fabrics. This may be due to the facthat 465 g/m2 Kevlar fabrics are without water repellent treatmentnd therefore PVA film is formed uniformly in case of 465 g/m2

abrics and it increases yarn to yarn friction significantly. However,s the 200 g/m2 fabrics have water repellent coating, the wettingnd penetration of PVA solution was poor causing only marginalncrease in yarn pull-out force.

.4. Influence of padding pressure and STF concentration onmpact energy absorption

Table 4 shows the impact energy absorption results for 200 g/m2

nd 465 g/m2 Kevlar fabrics, respectively. In general, for the sameTF concentration, increase in padding pressure increases thempact energy. Besides, for the same padding pressure, highermpact energy absorption is noticed with higher STF concentration.igs. 8 and 9 depict the surface plots of impact energy absorption

y the Kevlar fabrics at different levels of padding pressure and STFoncentration. It is observed that the highest energy was absorbedhen the padding pressure is maximum (2 bar) and STF concen-

ration is also maximum (60%). In the preceding part, it was foundFig. 9. Impact energy absorption by 465 g/m2 fabrics.

able 4mpact energy absorbed by Kevlar fabrics.

Padding pressure (bar) Silica concentration (%) 200 g/m2 fabrics 465 g/m2 fabrics

Impact energy (J) % Increase Impact energy (J) % Increase

Untreated – 25.3 – 41.4 –0.5 40 30.9 21.9 114.4 176.6

50 35.7 40.9 114.4 176.760 40.4 59.4 119.2 188.3

1.0 40 32.8 29.6 87.8 112.250 44.3 74.8 115.7 179.860 44.6 76.0 123.2 197.8

2.0 40 35.0 38.0 104.5 152.750 48.8 92.8 122.2 195.560 51.6 103.6 129.3 212.8

1.0 PVA (4%) 32.5 28.3 60.7 46.7

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A. Srivastava et al. / Materials Scienc

hat the maximum weight add-on% and the maximum yarn pull-ut force were achieved when padding pressure was minimum0.5 bar) and STF concentration maximum (60%). Therefore, it cane inferred that higher add-on% and higher yarn pull-out force doot necessarily ensure better impact performance. One of the pos-ible causes of such observation may be that increase in paddingressure forces the silica particles deeper into the yarn structurend results in better distribution on individual yarns and filaments.herefore, in spite of having lower add-on% at higher pressure, bet-er impact performance is attained due to more uniform applicationf STF.

From Table 4, it is observed that PVA treated 200 g/m2 and65 g/m2 Kevlar fabrics absorbed 28.2% and 46.7% more impactnergy, respectively, than the corresponding untreated Kevlar fab-ics. This is interesting as the yarn pull-out force increased by16.7% in case of the latter after treatment with PVA. However,he improved yarn pull-out result has not been translated intompact performance. This bolsters the perception that STF treatedevlar fabrics show enhanced impact performance only partly due

o increased friction between the constituent filaments and yarns.owever, the shear thickening effect seems to play a more pivotal

ole in the energy absorption mechanism. This factor would play aore important role if STF were distributed more uniformly in the

abric structure.The other important inference that can be drawn from the quasi-

tatic yarn-pull out and dynamic impact results is that the differentypes of energy absorption mechanisms are at work at differentesting speeds. The testing speed for dynamic impact was 8 m/shile it was only 0.3 m/min for yarn pull-out method. It has already

een discussed that while dynamic impact performance exhib-ted an upward trend with increasing padding pressure (decreasingdd-on%), it was just the opposite in case of low speed yarn pull-ut test. It implies that predicting impact performance of materialseant for high speed impact situations would give erroneous

esults if evaluation is done with lower velocity tests.

. Conclusions

The effect of STF concentration and padding pressure on theerformance of STF impregnated Kevlar woven fabrics have beentudied. Lower padding pressure and higher STF concentration

ncreases the weight add-on%. Weight add-on% has good associ-tion with the yarn pull-out force results. The maximum weightdd-on% and maximum yarn pull-out force occur at minimumadding pressure (0.5 bar) and maximum STF concentration (60%).

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Engineering A 529 (2011) 224– 229 229

However, yarn pull-out force results do not agree well with theimpact energy absorption results as the maximum value of thelatter is achieved at maximum padding pressure (2 bar) and max-imum STF concentration (60%). This indicates that increased yarnto yarn friction, represented by higher yarn pull-out force, is onlypartially responsible for the improved energy absorption of STFtreated Kevlar fabrics. Although higher padding pressure reducesthe weight add-on%, it ensures better penetration and uniformdistribution of the STF within the yarn structure. Therefore, it ispossible to develop STF treated Kevlar fabrics which is light-weightyet better from impact resistance viewpoint.

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