Development of a High-Density Nonwoven Structure to Improve the Stab Resistance of Protective Clothing Material

DEVELOPMENTIndustrial Health 2017, OF A 55NEW, 513–520 PROTECTIVE CLOTHING MATERIAL Original Article513

Development of a high-density nonwoven structure to improve the stab resistance of protective clothing material

Limin BAO1*, Yanling WANG1, Takeichiro BABA2, Yasuhiro FUKUDA2, Kaoru WAKATSUKI 1 and Hideaki MORIKAWA1

1Faculty of Textile Science and Technology, Shinshu University, Japan 2Japan Wool Textile Co., Ltd., Japan

Received July 15, 2017 and accepted August 29, 2017 Published online in J-STAGE October 5, 2017

Abstract: The purpose of this research was to enhance the stab resistance of protective clothing material by developing a new high-density nonwoven structure. Ice picks often injure Japanese police officers due to the strict regulation of swords in the country. Consequently, this study was designed to improve stab resistance against ice picks. Most existing anti-stab protective clothing research has focused on various fabrics impregnated with resin, an approach that brings with it problems of high cost and complicated processing. Seldom has research addressed the potential for improving stab resistance by using nonwoven structures, which exhibit better stab resistance than fabric. In this research, we prepared a series of nonwoven structures with densities ranging from about 0.14 g/cm3 to 0.46 g/cm3 by varying the number of stacked layers of Kevlar/polyester nonwoven under a hot press. We then proposed two methods for producing such hot-press nonwovens: the multilayer hot-press method and the monolayer hot-press method. Stab resistance was evaluated according to NIJ Standard-0115.00. We also investigated the relationship among nonwoven density, stab resistance, and flexural rigidity, and here we discuss the respective properties of the two proposed methods. Our results show that stab resistance and flexural rigidity increase with nonwoven density, but flexural rigidity of nonwovens prepared using the monolayer hot-press method only shows a slight change as nonwoven density increases. Though the two methods exhibit little difference in maximum load, the flexural rigidity of nonwovens prepared using the monolayer hotpress method is much lower, which contributes to superior wear comfort. Finally, we investigated the mechanism behind the stabbing process. Stabbing with an ice pick is a complicated process that involves many factors. Our findings indicate that nonwovens stop penetration primarily in two ways: nonwoven deformation and fiber fractures.

Key words: High-density, Nonwoven, Penetration energy, Stab resistance, Protective Clothing, Soft flexural rigidity

Introduction

Protective clothing, or special materials worn by police

*To whom correspondence should be addressed. officers for protective purposes, is designed to absorb or E-mail: [email protected] deflect slashing, bludgeoning, and penetrating attacks ©2017 National Institute of Occupational Safety and Health using weapons. Large numbers of Japanese police offi-

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. 514 L BAO et al. cers risk their lives to help keep society safe and secure. Table 1. Specifications of the nonwoven However, due to strict regulation of swords in the coun- Fiber Fiber diameter (dtex) Mass per unit area (g/m2) Kevlar: 2.25 try, police officers are most often injured by ice picks, Kevlar+10% PET 140 rather than by guns or knives. Consequently, police must PET: 4.4 wear stab-resistant clothing during routine duty, creating high demand for such clothing in order to protect officers from assault. Unfortunately, most stab-resistant clothing materials. Chang-sheng Lia researched the stab resistance incorporates a heavy, durable metal plate that imposes sig- of UHMWPE fiber composites impregnated with - ther nificant discomfort on the wearer. In recent years, some moplastics. In addition, thermoplastic films, particularly research has been carried out to improve wear comfort of polyethylene terephthalate and polypropylene films, can stab-resistant clothing by relying on flexible stab-resistant significantly improve the stab-resistance performance of materials utilizing soft Kevlar fabric. For example, Jessie composites9). When Silvio Leonardo Valença evaluated B. Mayo Jr., researched the puncture resistance of ther- the mechanical behavior of epoxy composite reinforced moplastic film-laminated aramid fabric, which exhibits with Kevlar plain fabric and glass/Kevlar hybrid fabric, he excellent properties, including flexibility1). M. J. Decker found that composites with a Kevlar/glass hybrid structure researched the stab resistance of treated Kevlar and nylon in the reinforcing fabric exhibited better performance10). fabrics treated with shear thickening fluid and found them When stabbed with an ice pick, the fibers near the point to exhibit significantly improved stab resistance and flex- of encroachment move away to create an increasingly large ibility2). The authors3) added microscopic particles to a tex- opening, allowing the pick to readily penetrate the mate- tile stab-proof material and verified the effectiveness of the rial11). To date, we have pursued the approach of impreg- resulting material in protecting against stabbing attacks. nating fabrics that have a large cover factor with elasto- Based on the results of a verification experiment in which mers to “lock” the yarn in place as a way of boosting stab fabrics were impregnated with particles, the authors were resistance. We have proposed a nonwoven fabric structure able to verify the mechanism responsible for increased that consists of dense, randomly positioned fibers as a stab-proof performance: the particles increased friction low-cost, stab-proof material, and the advantages of that drag, constraining movement of yarn strands and increas- structure have been verified12). The higher the nonwoven ing stab-proof performance. Xuhong Miao investigated density, the better the stab resistance. However, the biggest the stab resistance of warp knitted fabrics and found that issue is the lack of research dedicated to achieving high knitted fabric with a moderate density and longer under- density while retaining good flexibility. laps on the front guide bar performed better4). Yves In this research, we sought to develop a new low-cost, Termonia built a new model to analyze factors that influ- high-density structure made of fiber materials to improve ence stab resistance and found that puncture processes wear comfort and stab resistance. We also optimized the could be divided into four stages and that maximum force new structure and clarified the stabbing mechanism. Stab- occurs during friction against its conical section5). resistant body armor is classified according to the National In most recent research, stab impact testing focuses on Institute of Justice (NIJ) Standard 0115.0013). We evaluated knives or flat stabbing weapons. A. A. Levinsky compared stab resistance and flexibility using a machine developed the puncture resistance of two types of layered compos- by our laboratory and a bending machine, respectively. ite structure against knives and bullets6). Duong Tu Tien researched the anti-stabbing performance (vs. knives) of Experimental Method fabric layers woven with various hybrid yarns under different fabric conditions and developed an anti-stabbing Materials index capable of predicting or determining anti-stabbing The nonwoven used in this study, which was provided performance7). Magdi El Messiry investigated puncture by Japan Wool Textile Co., Ltd., consisted of staple Kevlar behavior (vs. knives) of flexible silk fabric composites in fiber (90 wt%) blended with low-melting polyester fiber soft body armor, finding that multi-layer silk fabric over (10 wt%) and manufactured into a nonwoven by the needle a para-aramid triaxial weave fabric (TWF) is superior to punching method. The mass per unit area of this nonwo- para-aramid plain-weave fabric8). Stab-resistant materi- ven was about 140 g/m2. Table 1 outlines the character- als usually incorporate woven fabrics, hybrid fabric, or istics of the nonwoven. In recent years, Kevlar fiber has uni-direction composites impregnated with thermoplastic been widely used for protective clothing due to its high

Industrial Health 2017, 55, 513–520 density, the better the stab resistance. However, the biggest issue is the lack of research dedicated to achieving high density while retaining good flexibility. In this research, we sought to develop a new low-cost, high-density structure made of fiber materials to improve wear comfort and stab resistance. We also optimized the new structure and clarified the stabbing mechanism. Stab- resistant body armor is classified according to the National Institute of Justice (NIJ) Standard 0115.0013). We evaluated stab resistance and flexibility using a machine developed by our laboratory and a bending machine, respectively.

2. Experimental method 2.1 Materials The nonwoven used in this study, which was provided by Japan Wool Textile Co., Ltd., consisted of staple Kevlar fiber (90 wt%) blended with low-melting polyester fiber (10 wt%) and manufactured into a nonwoven by the needle punching method. The mass per unit area of this nonwoven was about 140g/m2. Table 1 outlines the characteristics of the nonwoven. In recent years, Kevlar fiber has been widely used for protective clothing due to its high strength and flexibility. The melting point of polyester fiber is 110℃. Table 1. Specifications of the nonwoven Fiber Fiber diameter (dtex) Mass per unit area (g/m2) Kevlar: 2.25 Kevlar + 10% PET 140 PET: 4.4

2.2 Fabrication of a high-density nonwoven structure The density of traditional nonwovens is very low. In order to create high-density nonwovens, researchers in the past have hot-pressed pure Kevlar nonwovens to enhance their density. Fig. 1 illustrates this traditional method for

DEVELOPMENTproducing high-density OF A NEW nonwovens.PROTECTIVE CLOTHINGPressed Kevlar MATERIAL fibers are connected and spaces between fibers reduced to create515 a high-density nonwoven structure. However, pressed nonwovens revert to low density over time.

Normal after pressing over time

Fig. 1. Traditional Fig.method 1 Traditionalfor producing methodhigh-density for nonwovens. producing high-density nonwovens

To address this shortcoming, we proposed a new method in which Kevlar fibers are mixed with low-melted PET Normal and manufactured after pressing into a blended nonwoven over using time the needle-punching technique. Hot-pressing the low-density Fig. 2nonwoven Proposed methodto create for a producinghigh-density high-density nonwoven nonwovens melts the PET and bonds the Kevlar fibers. Fig. 2 illustrates this We hot-pressed nonwoven structuresmethod, which of varying allows density the nonwoven to invest igateto retain the relationshipits high density. between density and stab resistance. To do so, we utilized two methods to fabricate a high-density structure. The first was the multilayer press method, in which several layers of nonwoven are laminated and then hot-pressed into an 8 mm thick mold. Fig. 3 Normal after pressing over time illustrates this method. The second was the monolayer press method, in which a single hot-pressed layer is laminated. Fig. 2. Proposed methodFig. 2for Proposed producing high-densitymethod for nonwovens. producing3 high-density nonwovens The latter method embodies our proposal for improving the flexibility of high-density nonwovens. Fig. 4 illustrates We hot-pressed nonwoven structures of varying density to investigate the relationship between density and stab the monolayer press method. resistance. To do so, we utilized two methods to fabricate a high-density structure. The first was the multilayer press bonds the Kevlar fibers. Figure 2 illustrates this method, method, in which several layers of nonwoven are laminatedwhich and t allowshen hot-pressed the nonwoven into toan retain 8 mm its thick high mold.density. Fig. 3 illustrates this method. The second was the monolayer press metWehod, hot-pressed in which anonwoven single hot-pr structuresessed layer of varying is laminated. density to investigate the relationship between density and stab The latter method embodies our proposal for improving the flexibility of high-density nonwovens. Fig. 4 illustrates resistance. To do so, we utilized two methods to fabri- the monolayer press method. cate a high-density structure. The first was the multilayer

press method, in which several layers of nonwoven are Fig. 3. MultilayerFig. 3 Multilayerpress method. press method laminated and then hot-pressed into an 8 mm thick mold. Figure 3 illustrates this method. The second was the monolayer press method, in which a single hot-pressed layer is strength and flexibility. The melting point of polyester fiber laminated. The latter method embodies our proposal for is 110℃. improving the flexibility of high-density nonwovens. Fig-

ure 4 illustrates the monolayer press method. Fabrication of a high-density nonwoven structureFig. 3 Multilayer pressIn order method to confirm whether the polyester fibers melted The density of traditional nonwovens is very low. In as expected, we observed cross-section and surface speci- order to create high-density nonwovens, researchers in the mens of the hot-pressed nonwovens using a JSM-6010LA past have hot-pressedFig.4 Monolayer pure Kevlar press nonwovens method to enhance In Touch Scope SEM (Japan). The difference in the extent their density. Figure 1 illustrates this traditional method for of melting between the two methods was striking. Figure

producing high-density nonwovens. Pressed Kevlar fibers 5(a) illustrates a nonwoven without hot-pressing, while In order to confirm whetherare the connected polyester and fibers spaces melted between as fibers expected, reduced we observedto create cross-sectionFig. 5(b) depicts and surfacea nonwoven fabricated using the multi- specimens of the hot-pressed nonwovensa high-density using nonwoven a JSM-6010LA structure. In However, Touch Scope pressed SEM non (Japan).- layer The press difference method. in Figure the 5(c) shows a nonwoven fab- wovens revert to low density over time. ricated using the monolayer press method. Bonded fibers extent of melting between the two methods was striking. Fig.5(a) illustrates a nonwoven without hot-pressing, while To address this shortcoming, we proposed a Fig.new4 method Monolayer were press observed method in the sample produced using the proposed Fig.5(b) depicts a nonwoven fabricatedin which Kevlar using fibersthe multilayer are mixed pre withss method.low-melted Fig.5(c) PET andshows method. a nonwoven fabricated using the monolayer press method.manufactured Bonded fibers into awere blended observed nonwoven in the usingsample the produced needle- using Wethe proposedthen hot-pressed method. nonwoven structures of varying punchingIn order technique. to confirm Hot-pressing whether the low-densitypolyester fibers nonwo melted- densities as expe incted, order we to observed investigate cross-section the relationship and between surface We then hot-pressed nonwoven structures of varying densities in order to investigate the relationship between venspecimens to create of a thehigh-density hot-pressed nonwoven nonwovens melts using the PETa JSM-6010LA and density In Touch and stab Scope resistance. SEM (Japan). Figure T 6he shows difference nonwovens in the density and stab resistance. Fig. 6 shows nonwovens of varying densities. The nonwoven fabric machined by pre- extent of melting between the two methods was striking. Fig.5(a) illustrates a nonwoven without hot-pressing, while study12) is called high-density nonwoven fabric. The density of the nonwoven fabric produced by the proposed Fig.5(b) depicts a nonwoven fabricated using the multilayer press method. Fig.5(c) shows a nonwoven fabricated method becomes fairly high, and is called super high-density nonwoven fabric. The densest nonwoven has a density using the monolayer press method. Bonded fibers were observed in the sample produced using the proposed method. that is almost 6 times higher than the density of normal nonwovens. We then hot-pressed nonwoven structures of varying densities in order to investigate the relationship between density and stab resistance. Fig. 6 shows nonwovens of varying densities. The nonwoven fabric machined by pre- study12) is called high-density nonwoven fabric. The density of the nonwoven fabric produced by the proposed method becomes fairly high,4 and is called super high-density nonwoven fabric. The densest nonwoven has a density

that is almost 6 times higher than the density of normal nonwovens.

Normal after pressing over time Fig. 2 Proposed method for producing high-density nonwovens We hot-pressed nonwoven structures of varying density to investigate the relationship between density and stab resistance. To do so, we utilized two methods to fabricate a high-density structure. The first was the multilayer press method, in which several layers of nonwoven are laminated and then hot-pressed into an 8 mm thick mold. Fig. 3 illustrates this method. The second was the monolayer press method, in which a single hot-pressed layer is laminated. The latter method embodies our proposal for improving the flexibility of high-density nonwovens. Fig. 4 illustrates the monolayer press method.

516 L BAO et al. Fig. 3 Multilayer press method

Fig. 4. Monolayer press method. Fig.4 Monolayer press method

In order to confirm whether the polyester fibers melted as expected, we observed cross-section and surface specimens of the hot-pressed nonwovens using a JSM-6010LA In Touch Scope SEM (Japan). The difference in the extent of melting between the two methods was striking. Fig.5(a) illustrates a nonwoven without hot-pressing, while Fig.5(b) depicts a nonwoven fabricated using the multilayer press method. Fig.5(c) shows a nonwoven fabricated using the monolayer press method. Bonded fibers were observed in the sample produced using the proposed method. We then hot-pressed nonwoven structures of varying densities in order to investigate the relationship between density and stab resistance. Fig. 6 shows nonwovens of varying densities. The nonwoven fabric machined by pre- study12) is called high-density nonwoven fabric. The density of the nonwoven fabric produced by the proposed method becomes fairly high, and is called super high-density nonwoven fabric. The densest nonwoven has a density

that is almost 6 times higher than the density of normal (a)Nonwoven nonwov withoutens. hot-pressing (b) Using the multilayer press method

Fig. 5. The SEM photograph cross-section of the nonwoven fabric after a hot press.(c) Using the monolayer press method Fig. 5 The SEM photograph cross-section of the nonwoven fabric after a hot press. of varying densities. The nonwoven fabric machined by pre-study12) is called high-density nonwoven fabric. The density of the nonwoven fabric produced by the proposed method becomes fairly high, and is called super high-density nonwoven fabric. The densest nonwoven has a density that is almost 6 times higher than the density of normal nonwovens.

Observation of density We tested the density of hot-pressed nonwovens over a period of five days to confirm the stability of the nonwoven structure’s high density. The nonwoven structures produced using the two methods had the same densities. In order to confirm stable density, we tested the thickness of the hot-pressed nonwovens since their width, length, Fig. 6. NonwovensFig. 6 of Nonwovens varying densities. of varying densities

Industrial Health 2017, 55, 513–520 2.3 Observation of density

We tested the density of hot-pressed nonwovens over a period of five days to confirm the stability of the nonwoven structure’s high density. The nonwoven structures produced using the two methods had the same densities. In order to confirm stable density, we tested the thickness of the hot-pressed nonwovens since their width, length, and mass did not change. The thickness was tested in accordance with ISO standard 9073-2:199514). The testing method consisted of subjecting the nonwovens to additional pressure by placing a 300-gram iron plate on top of each sample and then testing the thickness. Fig. 7 illustrates this testing process. Fig. 8 provides thickness

variability curves plus the difference between hot-pressed PET-nonwoven (Kevlar + 10%PET fiber) and No-PET- nonwoven (only Kevlar fiber) after five days of testing. The thickness rate (i) refers to the ratio of present thickness to mold thickness. No-PET-nonwoven shows a change rate of about 54.8%, while PET-nonwoven shows a change rate of about 3.7%. It can be concluded that hot-pressed PET-nonwoven can maintain its high density perpetually. means worse flexibility. Flexibility can be seen to decline as the nonwovens’ density increases. However, nonwovens produced using the monolayer press method exhibit higher flexibility than nonwovens produced using the multilayer press method. The flexural rigidity obtained using the multilayer press method is about 1000 times higher than that from the monolayer press method, particularly in the case of the 28-layer nonwoven. The nonwoven layers in the samples produced using the multilayer press method can be seen to have fully adhered to one another. Normal nonwoven fabric (only KevlarHowever, fiber) nonwoven layers are Nonwoven only partially fab adheredric (Kevlar in the samples + 10%PET) produced using the new monolayer press 2 Fig. 7. Testing of nonwoven thickness.Fig.7method. Testing Compared of nonwoven with the traditional thickness method, flexural rigidity has been reduced by a factor of n (n: number of

layers). Normal nonwoven fabric (only Kevlar fiber) Nonwoven fabric (Kevlar + 10%PET) Fig.7 Testing of nonwoven thickness 4.0 200 4.0 3.5 Thickness before pressing 3.5 Thickness before pressing 100 3.0 3.0 With PET fiber Without PET fiber Withcm/cm) PET fiber 2.5 Without・ 0 PET fiber 2.0 Thickness after2.5 compression 1.5 Thickness after compressionM (gf -100

Thickness rate (i) Thickness 2.0 1.0 -200 0.5 1.5 -0.10 -0.05 0 0.05 0.10 0 20 40 rate (i) Thickness 60 80 100 120 Time (hours) K (cm-1) 1.0 Fig.Fig. 8 Thickness8. Thickness variability variability curves for nonwovens curves withfor and nonwovens without PET Fig. 9. Relationship between bending curvature and with and without PET. Fig. 9 Relationshipbending moment between (example). bending curvature and bending moment (example) 0.5 3. Results and discussion 0 20 40 60 80 100 120 )

3.1 Flexibility of high-density nonwovens 2 4 and mass did not change. The thickness was tested Time in resistance, (hours)10 we tested specimens’ flexibility using a In order to identify flexible materials with high stab resistance, we tested specimens’ flexibility using a KES-FB2- 14) multilayer hotpress S Multi-Purposeaccordance Bending Tester (Katowith Tech ISO Co., standard Ltd.). We used 9073-2:1995 10 samples for each. structure. The testing Fig. 9 illustrates KES-FB2-S the Nm Multi-Purpose Bending Tester (Kato Tech -6 103 monolayer hotpress relationship betweenmethod bending consisted curvature and ofbending subjectingFig. moment. 8 Thickness We the calculated nonwovens thevariability flexural rigidityto addicurves (curvature:- for Co.,-0.08 nonwovens Ltd., Japan). with We and used with 10out samples PET for each structure. 10 to 0.08cm-1) of the samples using a gradient of bending curvature and bending moment curve, and we investigated

tional pressure by placing a 300-gram iron plate on top Figure× 9 illustrates the relationship between bending cur- 2 the relationshipof between each flexibility sample and density.and thenThe differenc testinge between the thethickness. two methods inFigure terms of flexibility7 vature was 10 and bending moment. We calculated the flexural striking. illustrates3. Results this and testing discussion process. Figure 8 provides thickness rigidity (curvature: −0.08 to 0.08 cm −1) of the samples Fig. 10 illustrates the flexibility of nonwovens produced using the two different methods. Higher flexural rigidity 1 variability3.1 Flexibility curves of plushigh-density the difference nonwovens between hot-pressed using10 a gradient of bending curvature and bending moment PET-nonwoven (Kevlar+10%PET6 fiber) and No-PET- curve, and we investigated the relationship between flexi- In order to identify flexible materials with high stab resistance, 10we0 tested specimens’ flexibility using a KES-FB2- nonwoven (only Kevlar fiber) after five days of testing. bility and density. The difference between the 28two layers methods S Multi-Purpose Bending Tester (Kato Tech Co., Ltd.). We used 10 samples for each 16structure. 20 Fig. 24 9 illustrates the

The thickness rate (i) refers to the ratio of present thickness in ( rigidity Flexural terms of flexibility8 12 was striking. 10-1 torelationship mold thickness. between No-PET-nonwoven bending curvature shows and a bendingchange rate moment. Figure We calculated 100.1 illustrates the0.2 flexural the flexibility0.3 rigidity 0.4(curvature:of nonwovens0.5 -0.08 pro - × 6 3 ofto about0.08cm 54.8%,-1) of whilethe samples PET-nonwoven using a gradient shows a changeof bending rate curvatureduced and using bend theing two Densitymoment different (curve, 10 methods. and g/m we) investigated Higher flexural of about 3.7%. It can be concluded that hot-pressed PET- rigidity means worse flexibility. Flexibility can be seen the relationship between flexibility and density. The difference between the two methods in terms of flexibility was nonwoven can maintain its high density perpetually.Fig. 10 Flexibility of tononwovens decline produced as the usingnonwovens’ the multilayer density press method increases. and the However,monolayer press method striking. nonwovens produced using7 the monolayer press method

ResultsFig. 10 and illustrates Discussion the flexibility of nonwovens produced usingexhibit the higher two different flexibility methods. than nonwovens Higher flexural produced rigidity using the multilayer press method. The flexural rigidity obtained Flexibility of high-density nonwovens 6 using the multilayer press method is about 1,000 times In order to identify flexible materials with high stab higher than that from the monolayer press method, particu- means worse flexibility. Flexibility can be seen to decline as the nonwovens’ density increases. However, nonwovens produced using the monolayer press method exhibit higher flexibility than nonwovens produced using the multilayer press method. The flexural rigidity obtained using the multilayer press method is about 1000 times higher than that from the monolayer press method, particularly in the case of the 28-layer nonwoven. The nonwoven layers in the samples produced using the multilayer press method can be seen to have fully adhered to one another. However, nonwoven layers are only partially adhered in the samples produced using the new monolayer press method. Compared with the traditional method, flexural rigidity has been reduced by a factor of n2 (n: number of layers).

3.2 Stab resistance 200 To test the anti-stab performance of the samples, we performed stab testing using a machine designed by the authors in accordance with the applicable NIJ standard3, 11-12). The traditional testing method can only evaluate whether the 100 implement penetrates with a certain energy, and it is not capable of applying a load after striking. Therefore, it does not provide an understanding of the maximum penetrating load and penetration energy if the sample cannot be penetrated. Our lab developed a new evaluation machine to address these shortcomings. In this test apparatus, the ・ cm/cm) 0 ice pick is attached to the tip of an air cylinder that propels it so that it strikes the specimen. A regulator adjusted the

M (gf -100 maximum collision speed of this apparatus to 5m/s, but we also conducted tests at a collision speed of 2.83 m/s as specified by NIJ Standard-0115.00. In the load-displacement curves obtained from the experiment, the impact load increased after the ice pick struck -200 -0.10 -0.05 0 0.05 0.10the specimen and peaked when the maximum diameter of the ice pick penetrated the specimen. We assumed that -1 K (cm ) the maximum value at that moment was the maximum load. The integrated value of the load-displacement curve

518 from collision to the maximum load represents the energy required for penetration (penetrationL BAO et al energy),. which we Fig. 9 Relationship between bending curvature and bending momenusedt (example) as an index to evaluate the specimen’s stab protection.

) 2 104 1000 multilayer hotpress 900 Multilayer press Nm

-6 103 monolayer hotpress 800 Monolayer press 10 700 × 2 10 600 500 1

10 Maximumload(N) 400 300 100 28 layers 200 16 20 24

Flexural rigidity ( rigidity Flexural 8 12 10-1 100 0.1 0.2 0.3 0.4 0.5 0 6 3 Density (×10 g/m ) 0.1 0.2 0.3 0.4 0.5 Density(×10-6g/m3) Fig. 10. Flexibility of nonwovens produced using the multi- Fig. 10 Flexibility of nonwovens produced using the multilayer press method and the monolayer press method layer press method and the monolayer press method. (a) Maximum load 7

30 larly in the case of the 28-layer nonwoven. The nonwoven Multilayer press layers in the samples produced using the multilayer press 25 Monolayer press method can be seen to have fully adhered to one another. However, nonwoven layers are only partially adhered 20 in the samples produced using the new monolayer press 15 method. Compared with the traditional method, flexural 2 rigidity has been reduced by a factor of n (n: number of 10

layers). Penetration energy(J) 8 5 Stab resistance 0 To test the anti-stab performance of the samples, we per- 0.1 0.2 0.3 0.4 0.5 formed stab testing using a machine designed by the authors Density(×10-6g/m3) in accordance with the applicable NIJ standard3, 11–12). The traditional testing method can only evaluate whether the (b) Penetration energy implement penetrates with a certain energy, andFig. it 11is Stabnot resistanceFig. 11.curve Stab for nonwovensresistance curve produced for nonwovens using the monolayer produced press using and the multilayer mono- press methods capable of applying a load after striking. Therefore, it does layer press and multilayer press methods. not provide an understanding of the maximum penetratingFig. 11 illustrates the results of testing the two methods’ stab resistance. The density of the nonwoven is shown load and penetration energy if the sample cannoton the be horizontal pen- axis, while the maximum load (a) and penetration energy (b) are shown on the vertical axis. etrated. Our lab developed a new evaluation machineAlthough there to is a usedslight asdifference an index associated to evaluate with higher the specimen’s density, it can stab be concluded protection. that the stab resistance address these shortcomings. In this test apparatus,values the of theice high-densityFigure nonwovens 11 illustrates prepared the using results the two of met testinghods are the almost two themeth same.- The high-density pick is attached to the tip of an air cylinder thatnonwovens propels produced ods’ using stab the resistance.monolayer press The method density exhibit of theexc ellentnonwoven stab resistance is shown along with much better it so that it strikes the specimen. A regulator adjustedcomfortableness the at 3.1.on Higher-densitythe horizontal nonwovens axis, whileexhibited the bett maximumer stab resistance. load It (a)was andmisleading display. I maximum collision speed of this apparatus to correct5 m/s, as but follows. penetration The maximum energy load of(b) the are new shown high-density on the no nwovenvertical structure axis. is superior by a we also conducted tests at a collision speed of 2.83factor m/s of 3 as compared Although to the normalthere is nonwovens. a slight difference Fig. 12 shows associated the samples with after higherpenetration. Fibers in the specified by NIJ Standard-0115.00. low-density nonwovendensity, were displaced it can beduring concluded penetration, that whereas the stabfibers resistance in the high-density values nonwoven fractured, In the load-displacement curves obtainedwhich from increased the stabof resistance. the high-density In this way, we nonwovens proved the effectiveness prepared of theusing two methods.the two experiment, the impact load increased after the ice pick methods are almost the same. The high-density nonwo- struck the specimen and peaked when the maximum diam- vens produced using the monolayer press method exhibit eter of the ice pick penetrated the specimen. We assumed excellent stab resistance along with much better comfort- that the maximum value at that moment was the maximum ableness at 3.1. Higher-density nonwovens exhibited bet- load. The integrated value of the load-displacement curve ter stab resistance. It was misleading display. I correct as from collision to the maximum load represents the energy follows. The maximum load of the new high-density non- required for penetration (penetration energy), which we woven structure is superior by a factor of 3 compared to

Industrial Health 2017, 55, 513–520

Low-density nonwoven (0.144×106g/m3) High-density nonwoven (0.459×106g/m3) Fig. 12 Kevlar nonwovens after stabbing test

30 Multilayer press 25 Monolayer press

10 Penetration energy(J) 5

0 0.1 0.2 0.3 0.4 0.5 Density(×10-6g/m3)

(b) Penetration energy Fig. 11 Stab resistance curve for nonwovens produced using the monolayer press and multilayer press methods

Fig. 11 illustrates the results of testing the two methods’ stab resistance. The density of the nonwoven is shown on the horizontal axis, while the maximum load (a) and penetration energy (b) are shown on the vertical axis. Although there is a slight difference associated with higher density, it can be concluded that the stab resistance values of the high-density nonwovens prepared using the two methods are almost the same. The high-density nonwovens produced using the monolayer press method exhibit excellent stab resistance along with much better comfortableness at 3.1. Higher-density nonwovens exhibited better stab resistance. It was misleading display. I correct as follows. The maximum load of the new high-density nonwoven structure is superior by a factor of 3 compared to the normal nonwovens. Fig. 12 shows the samples after penetration. Fibers in the DEVELOPMENT OF A NEW PROTECTIVE CLOTHING MATERIAL low-density nonwoven were displaced during penetration, whereas fibers in the high-density nonwoven fractured, 519 which increased stab resistance. In this way, we proved the effectiveness of the two methods.

Low-density nonwoven (0.144×106g/m3) High-density nonwoven (0.459×106g/m3)

Fig. 12. Kevlar nonwovens after stabbingFig. 12test. Kevlar nonwovens after stabbing test

9 the normal nonwovens. Figure 12 shows the samples after subsidies provided by the Ministry of Education, Cul- penetration. Fibers in the low-density nonwoven were dis- ture, Sports, Science and Technology (JP26289005, placed during penetration, whereas fibers in the high-den- JP16K14113, and JP15H01789). sity nonwoven fractured, which increased stab resistance. In this way, we proved the effectiveness of the two methods. References 1) Mayo J Jr, Wetzel ED, Hosur MV, Jeelani S (2009) Stab and Conclusion puncture characterization of thermoplastic-impregnated aramid fabrics. Int J Impact Eng 36, 1095–105. [CrossRef] In this study, we focused on researching the stab resis- 2) Decker M, Halbach CJ, Nam CH, Wagner NJ, Wetzel ED tance of nonwovens against ice picks and developed a new (2007) Stab resistance of shear thickening fluid (STF) nonwoven structure that exhibits excellent stab resistance treated fabrics. Compos Sci Technol 67, 565–78. [Cross- Ref] and flexibility. Recognizing that numerous factors influ- 3) Bao L, Sato S, Wang Y, Wakatsuki K, Morikawa H (2017) ence nonwoven properties and final stab resistance, we Development of flexible stab-proof textiles impregnated carried out several different experiments and have pro- with microscopic particles. Journal of Textile Engineering vided their results. 63, 43–8. [CrossRef] After several trial experiments, we used Kevlar/polyes- 4) Miao X, Jiang G, Kong X, Zhao S (2014) Experimental ter nonwovens to create a high-density nonwoven structure Investigation on the Stab Resistance of Warp Knitted Fab- by hot-pressing the material. We found that increasing the rics. Fibres Text East Eur 22, 65–70. nonwoven density greatly improved stab resistance. 5) Termonia Y (2006) Puncture resistance of fibrous structures. Int J Impact Eng 32, 1512–20. [CrossRef] In order to produce high-density nonwovens, we hot- 6) Levinsky A, Sapozhnikov S, Grass T (2012) Development pressed nonwovens using the multilayer press method of knife and bullet resistant composite structures. Mech but found that the approach led to reduced flexibility. Compos Mater 48, 405–14. [CrossRef] To improve comfort, we proposed the monolayer press 7) Tien D, Kim J, Huh Y (2011) Evaluation of Anti-stabbing method. Our results showed that nonwovens produced Performance of Fabric Layers Woven with Various Hybrid using the monolayer press method were much more flex- Yarns under Different Fabric Conditions. Fibers Polym 12, ible than nonwovens produced using the multilayer press 808–15. [CrossRef] method. The stab resistance of the high-density nonwovens 8) Messiry M (2014) Investigation of Puncture Behaviour of Flexible Silk Fabric Composites for Soft Body Armour. prepared using the two methods were almost the same. Fibres Text East Eur 22, 71–6. 9) Lia C, Huang X, Li Y, Yang N, Shen Z, Fan X (2014) Stab Acknowledgments resistance of UHMWPE fiber composites impregnated with thermoplastics. Polym Adv Technol 25, 1014–9. [Cross- This study was conducted using scientific research Ref] 520 L BAO et al.

10) Valenca S, Griza Z, Oliveira V, Sussuchi E, Cunha F (2015) Stab-resistant Textile Materials with a Non-woven Fabric Evaluation of the mechanical behavior of epoxy composite Structure. Journal of Textile Engineering 62, 37–42. reinforced with Kevlar plain fabric and glass/Kevlar hybrid [CrossRef] fabric. Compos, Part B Eng 70, 1–8. [CrossRef] 13) NIJ Standard-0115.00, Stab resistance of personal body 11) Souma S, Bingo T, Morikawa H, Bao L. (2011) Proceedings armor (2000) of the 2nd Asian Protective Clothing Conference, 13–15. 14) ISO 9073-2: 1995 Textiles-Test methods for nonwovens- 12) Bao L, Sato S, Morikawa H, Soma S (2016) Improving Part 2: Determination of thickness

Industrial Health 2017, 55, 513–520