Abstract
A study was conducted on the puncture resistance of high-strength nonwoven natural rubber latex (NRL)-coated fabrics. High strength unidirectional (UD) polyethylene fabrics were coated using single dipping (SD), double dipping (DD), and triple dipping (TD) methods. The puncture resistance of the uncoated and coated fabrics was determined by measuring the force needed to penetrate the fabrics using a blunt probe. The mechanisms and modes of deformation involved during penetration were observed as well. In comparison with the uncoated fabrics, the SD, DD, and TD NRL coated fabrics gave higher puncture resistance of 39%, 47%, and 62%, respectively. The increments of the penetration force were proportional with the increase in the NRL-coated fabric thickness. For the uncoated fabric, the force given by the probe resulted in yarns stretching and yarn slippage upon penetration. However, the NRL-coated fabrics were not severely damaged but resulted in a curvature formed at the frontface of the fabric with minimal backface deformation. From the observations, the modes of failure were similar among the NRL-coated fabrics, that is, fiber stretching, delamination of NRL layer, and stretch marks on the fabric surface. The NRL layer acted as a protective sheath and increases the energy absorption and elasticity characteristics of the UD fabric.
Introduction
High-strength woven fabrics were introduced several decades ago starting from polyamides and followed by aramids, poly-phenylene benzobisoxazole (PBO), high-density polyethylene fiber, and many others. These fabrics are able to avoid penetration from high velocity projectiles and are commercially used for protective applications. Besides high-strength woven fabrics, nonwoven fabrics in the form of unidirectional (UD) arrangement such as those made from ultra high molecular weight polyethylene (UHMWPE) have also been extensively used as protective textiles. Popular UD fabrics are such as under the trademarks of Spectra Shield®, Dyneema®, and Tekmilon®. In comparison with woven fabrics in which yarns are interlaced between warp and weft (Figure 1), nonwoven UD fabrics have yarns that are arranged in parallel at 0°/90° angles together with a flexible resin as the binder (Figure 2). Others have thin polyethylene film laminated on top and bottom of the 0°/90° angle fabric layers. In terms of fabric properties, besides having excellent energy absorption characteristics, high-strength UD ultra-high molecular weight polyethylene fabrics have low specific weight as low as 0.97 and are thus more flexible in comparison with high-strength woven fabrics [1].
Structure of woven fabric. Structure of unidirectional fabric.
Currently, many research efforts are done on impregnating and coating high strength fabrics to enhance their performances and capabilities for protective textiles in particular against ballistic impact and stab threats [2–8]. Studies done by Chitrangad and Rodriguez-Parada [2] reported that impregnation of fluorinated finish on the aramid fabrics could enhance the ballistic properties of the fibers due to the relatively high fiber–fiber coefficient of friction. In another study, Tan et al. [3] reported that the ballistic penetration resistance of Kevlar fabric can be improved by impregnation of the fabric with colloidal shear thickening fluid (STF). Similarly, Kalman et al. [4] studied the STF intercalated into Kevlar fabrics by replacing hard silica particles STFs with Poly (methyl methacrylate) (PMMA). They found that the PMMA-STF-Kevlar gave better quasi-static spike resistance in comparison with silica-based STF (S-STF-Kevlar). Decker et al. [5] also employed STF into the high-strength fabric to explore the relationships between fabric architecture, STF treatment, and threat conditions. The results show that the intercalated STF into Kevlar fabric can enhance the penetration resistance and suggested that the modification of ballistic fabrics can provide improvement in stab resistance. In another study, Ahmad et al. [6] explored the application of high modulus natural rubber latex (NRL) on high-strength woven fabric that resulted in higher energy absorption characteristics with enhanced puncture resistance and ballistic impact performance. Works by Mayo et al. [7] found that the incorporation of thermoplastics to the aramid fabrics can provide some improvement in stab resistance. Lee et al. [8] provided a comprehensive review of previous studies on silica colloidal suspension (SCS) impregnated fabrics by comparing the particles size of SCS in silica. They concluded that 100 nm diameter of silica could lead to superior ballistic impact in comparisons with particle sizes of 300 nm and 500 nm.
The majority of these studies have attempted to predict that the incorporation of additional materials to high tenacity fabrics could enhance the penetration and impact performance of the fabrics due to the frictional effect. Cheeseman and Bogetti [9] reviewed that there are a few main factors that can increase the impact penetration of fabrics and part of them are the friction between yarns and the projectiles and friction between yarn–yarn in the fabric system. The effect of the friction played the main role in these cases, where the addition of coating or treatment materials particularly provide higher interyarn friction that could lead to superior performance during penetration and ballistic impact. Higher friction resulted in higher energy absorption because of the restricted movement of the yarns. In addition, many researchers [3–11] relate their studies with the effect of friction by using various methods such as quasi-static test, puncture resistance test, yarn pull-out test, and through modeling (LS–DYNA). A recent study by Sun et al. [12] reported the puncture behavior of woven fabric in plain, 2/2 twill, and 2/1 twill using finite element analysis (FEA).
The present study investigates the puncture resistance and the mechanisms of fabric failure of NRL-coated UD fabrics in comparisons with uncoated UD fabrics. The puncture resistance test is conducted using quasi-static mode to measure the load needed to penetrate a fabric sample and the amount of deflection that the fabric gains upon penetration. Principally, in a quasi-static puncture resistance test, a probe (indentor) of certain diameter and shape moves downward with the speed of between 50 and 100 mm/min and punched the fabric. This test is suitable to be used as a preliminary study to predict and simulate fabric deformation during ballistic impact tests because it is a simple, quick, and cheap mode of testing. According to Erlich et al. [13], puncture resistance test can give a clear picture of the fabric in terms of failure mechanisms. The results of the puncture resistance tests provided a useful indication of the penetration resistance upon ballistic impact as shown in several investigations [6,13]. The test could also provide some form of ranking the penetration resistance of fabrics.
Experimental
Materials
High-molecular-weight polyethylene (HMW-PE) Unidirectional bought from Compass International Corporation Ltd, China, with an areal density of 150 g/m2 and the average thickness of 0.16 mm was used in the study. The coating material is high-modulus prevulcanized NRL supplied by Revertex Malaysia Sdn. Bhd. The NRL has 60.5% total solid content (TSC) and viscosity of about 35 s (Ford cup 3 at 25°C).
Coating methods
Several dipping methods were conducted on the high-strength UD fabric: single dip (SD), double dip (DD), and triple dip (TD). For the SD method, the UD fabric sample was slowly impregnated into NRL placed in a container before hanging the samples to dry under room temperature. Similarly, the double and triple methods were performed after drying the fabric samples for about 20 min at each coating.
Puncture resistance test
The puncture resistance tests were conducted on the Testometric Tensile Tester following a modified puncture resistance tests according to the ASTM F1342, as described by Ahmad et al. [6]. The primary objective of the test is to identify the maximum loads needed to perforate the NRL-coated and uncoated fabrics. The samples were placed in a circular clamp with a diameter of 125 mm and tightly gripped to prevent slippage during the test. A 5 kN load cell was used at a constant speed of 100 mm/min. The test set-up for the penetration resistance test is shown in Figure 3 (a) and (b).
(a) Penetration resistance test set-up and (b) fabric sample for penetration resistance testing.
Results and discussion
Results of penetration resistance
Comparisons of neat and coated fabrics areal densities under puncture resistance
Mechanisms of penetration for uncoated fabric
Several failure modes were observed during penetration by the probe on the uncoated fabrics. Fibers at 0° in direct contact with the probe have high stress and tend to be stretched. Eventually, the fibers break at the edge of clamp upon reaching maximum penetration force and accompanied a sharp burst of sound. This resulted in the formation of a hole in the middle of the sample. Figure 4 (a) and (b) show the uncoated fabric sample (frontface and backface) after the puncture test. These results are however similar with that of woven fabrics, as reported by Ahmad et al. [6]: yarn breakage at maximum load and minimal yarn stretching. In another study, Sun et al. [12] indicates three stages of fabric failure, which are fabric tension, warp and weft yarn slippage, yarn breakage, and pull out.
The (a) frontface and (b) backface of uncoated fabric sample after the puncture test.
There is also evidence of fiber stretching marks on the fabric and delamination occurring between the two of the UD fabric (which are arranged at 0° and 90° with respect to each other; Figure 4 (a)). However, fibers located at 90° were delaminated and have been detached from their binder. These observations indicated the dissipation of high energy absorption over a large area has taken place by the fibers at contact point, thus minimal energy was received by the other fibers at 90°. It can be concluded that the fibers were severely damaged in the absence of the NRL coating layer.
Mechanisms of penetration for NRL-coated fabrics
In general, all the coated fabrics showed similar penetration failure mechanisms, as shown in Figure 5 (a)–(f). It can be seen that as the probe moves downward, high stress concentration occurred upon the area of NRL film that was directly in contact with probe. The coated fabrics also have similar stretched marks with the uncoated fabric. As the probe continued to penetrate into the coated fabric, the NRL film started to deform until the probe was not able to completely penetrate the NRL film upon reaching maximum penetration load. This might have been due to the high elongation at break of the NR latex film.
(a) SD Frontface, (b) SD Backface, (c) DD Frontface, (d) DD Backface, (e) TD Frontface, and (f) TD Backface.
In contrast with the uncoated fabric, the NRL coated fabrics did not show any delamination between the two layers of fabric, which are arranged at 0° and 90° with respect to each other. In addition, a circular region was formed on the coated fabric surface unlike the uncoated fabric in which a square hole was observed. There is also minimal rupture of the fibers as it was trapped in the NRL layer without much damage to the surface of NRL film (see Figure 6). Figure 7 shows the trend of load and displacement curves during perforation of the uncoated and coated fabrics. As can be seen, after reaching maximum force, the probe was allowed to perforate the sample until complete penetration occur or until maximum extension of 50 mm. Upon reaching the 50 mm mark, the uncoated sample was completely perforated. However, the NRL film on the coated samples prevents the samples from completely perforating. Figure 8 illustrates the extent of the probe move downwards, which was used as the standard needed to completely perforate both uncoated and coated fabric.
Fiber stretching and trapped within the natural rubber latex (NRL) layer at maximum force. Force–displacement trends for all samples. Schematic of fabric penetration.
Effect of friction on coated fabric during penetration
The investigations conducted above clearly indicates that the presence of NRL on the outermost layer of the UD fabric helps to protect the fabric and the fibers from further damage upon maximum force by the probe.
Although the puncture-resistant tests cannot be directly associated with that of ballistic impact, the results may give an indication of the strength of the NRL-coated fabrics in comparison with the uncoated fabric. The mechanisms of puncture may also suggest that the NRL film is able to absorb energy while preventing puncture by the probe and thus delaying the process of penetration. A study by Ahmad et al. [6] showed that NRL-coated fabrics have improved the ballistic impact performance of woven fabric systems.
Basically, friction was involved between the NRL-coated fabric with the probe and within the fibers. The strong and flexible NRL coating restricts the movement of the fibers in the fabric. The fibers were held tightly together and NRL coat may have significantly absorbed energy through the effect of friction and extension of the coating film. In order to investigate the effect of friction between the probe and the contact fabric, two different probe conditions were investigated (oily and dry). In general, the dry probe needed higher forces to penetrate the fabric in comparisons with the oily probe as can be seen in Figure 9. High contact area between the dry probe and fabric surface tends to increase the number of contact area, thus creating high frictional effects. In contrast, the oily probe may reduce the surface area with the fabric causing low frictional effects and allowing the probe to easily penetrate the fabric.
Load penetration at 50 mm displacement at two different probe conditions.
The Shirley Fabric Friction Tester (Figure 10) was used to measure friction on the uncoated and NRL-coated fabric’s surfaces. The coefficient of friction of the uncoated fabric was 0.4 µ according to the inclined plane method (Figure 11). However, the coefficient of friction of the NRL-coated fabrics was much greater than the uncoated fabrics, between 0.9 µ and 1 µ. The addition of NRL layer on the fabric’s surface contributed to the higher coefficient of friction due to their large contact surface area and thus increase the number of contact points.
The (a) front view and (b) side view of Shirley fabric friction tester. Schematic of inclined plane method.
Figure 11 shows a block of mass m initially resting on an inclined plane covered with the fabric to be tested. The frictional force F is equal to the component of the mass of the block parallel to the inclined plane [14].
The normal reaction N is equal to the component of the mass perpendicular to the inclined plane.
As the coefficient of friction, µ = F/N. Therefore:
Conclusions
High-strength polyethylene UD NRL-coated fabrics gave higher puncture resistance in comparisons with uncoated fabrics. Fibers in the uncoated fabric were severely damaged and the mechanisms of penetration were identified as fiber stretching, broken fiber at 0°, and fibers delamination at 90°. There was an obvious square hole at the contact point and fiber stretch marks on the fabric surface. The NRL-coated fabrics are able to restrict the movement of the fiber and protect the surface of the fabric from damages, with fibers trapped within both the top and bottom NRL layers. The modes of failure penetration of samples from three different dipping techniques are similar. There was a circular deformation on the frontface of fabric, small backface deformation, and fiber stretch mark. However, the damaged fibers were trapped within the NRL layer. Thus, the NRL-coated fabrics were not severely damaged due to the frictional effects involved between the fabric and probe and within the fibers. Therefore, the addition of NRL onto the fabric improved the performance of flexible UD fabric by offering higher frictional effects. The results from this study are not conclusive and it is suggested that further studies on the issues of NRL-coated UD fabric’s mechanical properties are needed.
Footnotes
Acknowledgment
The authors would like to thank Revertex Malaysia Sdn. Bhd. for the supply of NRL and the Research Management Institute (RMI) of Universiti Teknologi MARA Malaysia for the grant provided under the Research Excellence Fund.