CHAPTER 12 Advanced bulletproof and stab- and spike-resistant textiles Amirhossein Salehi Koohestani and Azadeh Bashari Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran 12.1 Introduction The history of the development of armor and weapons begins with human history. Humans have used various forms of guards to protect themselves from war injuries and other hazardous conditions. From the use of leather in the east to the use of chain armor in the west, people have never stopped trying to find better ways to protect against dangers (Fig. 12.1). During the development of body armors, it has always been desirable to use lighter and stronger materials to improve performance by reducing weight. Modern body armors can be divided into two categories: • Hard body armors are made of metal or ceramic plates. • Soft body armors are mainly made up of different layers of fabrics con- sisting of high-performance fibers. Conversely, the threat from edge or tip weapons is inherently variable because they are manually driven by a population who has different abili- ties and techniques. In addition, edging weapons may cover a wide range of knives, tools, swords, and other accessories that may have various degrees of sharpness and different types of cutting edges [2]. The easiest way of providing stab resistance is to use rigid sheets of metal or compos- ite. Such materials are sufficiently hard to defeat knives by resistance to indentation and to present a large resistance to further penetration [3]. The main disadvantages of rigid armor are in wearer comfort and cover- age. To be able to move the arms and waist properly, it is necessary to reduce the coverage or provide some space inside the plates. The best solution is to use several plates; these may be loosely held in different layers, or some form of hinges may be fitted to the edges of single plates. Therefore, plate edges provide weak points; multiple layers increase both weight and bulk, while effective and durable hinging appears to be Advances in Functional and Protective Textiles © 2020 Elsevier Ltd. DOI: https://doi.org/10.1016/B978-0-12-820257-9.00012-6 All rights reserved. 261 262 Advances in Functional and Protective Textiles Figure 12.1 Past to present of European body armor [1]. difficult to achieve in practice. Rigid armor has another important disad- vantage; it cannot undergo significant bend or large-scale deflection and must stop the threat within its own thickness [4]. Better solution for knife protection would be flexible textile armor and handgun protection. 12.2 The concept under using body armor The body armor is designed to protect the main body organs from damages due to bullet impact. If the bullet penetrates into the body, it will crush and displace the tissues to the organs while making temporary and permanent cavities. Since the human tissues act as a semi fluid, when the bullet creates pressure and shock waves, it damages the main organs and causes death. This is known as unsharpened trauma. More elastic and more cohesive tissues such as skeletal muscle, lung, empty intestine, nerve, blood vessel, and to some extent bone can survive from the temporary cavitation blunt trauma. However, less elastic, fewer cohesive organs, such Advanced bulletproof and stab- and spike-resistant textiles 263 as the liver, brain, and heart do not tolerate temporary cavitation blunt trauma well [5,6]. Factors that contribute to tissue damage are summarized in Table 12.1. The wound consists of three parts: • Entry wound: Usually smaller than the exit wound (Fig. 12.2A). Table 12.1 Factors that contribute to tissue damage [7]. Factors Description Bullet size The larger the bullet, the more resistance and the larger the permanent tract Bullet Hollow point and soft nose flatten out on impact, resulting in a deformity larger surface area involved Semi jacket The jacket expands and adds to the surface area Tumbling Tumbling of the bullet causes a wider path of destruction Yaw The bullet can oscillate vertically and horizontally (wobble) about its axis, resulting in a larger surface area presenting to the tissue Figure 12.2 Medium- and high-velocity wounds consisting of (A) the entry wound, (B) internal wound, and (C) exit wound [7]. 264 Advances in Functional and Protective Textiles Figure 12.3 Projectile impact into a ballistic fiber [9]. • Internal wound: Medium-velocity bullets inflict damage primarily by damaging tissue that the bullet contacts, and high-velocity bullets inflict damage by tissue contact and transfer of kinetic energy (the shock wave producing a temporary cavity) to surrounding tissues (Fig. 12.2B). • Exit wound: Not all gunshot wounds will have exit wounds, and on occasion, there be multiple exit wounds due to fragmentation of bone or the bullet (Fig. 12.2C). Generally, the exit wound is larger and has ragged edges [7]. A bullet-proof vest is designed to spread the energy throughout the material while deforming the bullet at the same time. This deformation exists in hard armor only. In soft armor, bullets are entrapped by the material. Once the bullet strikes the body armor, it is caught by the strong fiber structure known as a “web,” and the energy of the bullet is absorbed into the material until the bullet is stopped. In addition, hard materials that are made from ceramic or metal plates help to protect from nonpene- trating injuries (blunt trauma) to internal organs [5]. When a projectile strikes a fabric target, the response is a combination of global and local response. Global response indicates the behavior of the material away from the impact point, and local response refers to the behavior of material directly contacting the projectile, as shown in Fig. 12.3 [8]. 12.3 Material selection The bullet-resistant fabric has to avoid the bullet from penetrating and absorb its energy converting it into work of deformation. Therefore, strength, modulus, elongation at break, deformability of the bullet, and Advanced bulletproof and stab- and spike-resistant textiles 265 velocity of the transverse shock wave in the fiber are the main factors in selection of fibers for the soft body armor [10]. The details about the most commonly used fibers in bulletproof clothing are as follows. 12.3.1 Polyamide 66 Ballistic nylon fabric produced by DuPont was used as US body armor vests until 1983 and saved many lives. These types of fabric were effective against shrapnel but not effective for bullets. Therefore, they were good for aircrews because most injuries were due to shrapnel from antiaircraft fire. The early flak jackets comprised manganese steel plates sewn into a ballistic nylon vest covering the abdomen. They had a quick-release tab for emergency removal in the event of bailing out [11]. In 1945, ballistic nylon 66 (Fig. 12.4) flak jackets containing fiberglass plates were used as body armor for ground troops at the battle of Okinawa, making Okinawa possibly the primary military assault by a force of ground troops wearing body armor since the Middle Ages and certainly the initial for some centuries. Ballistic nylon vests with the Doron plates went on to be used to a significant extent in the Korean War. As of early 1953, 90,000 were dispatched to Korea. Historical evidence suggests that these ballistic nylon/ Doron vests stopped shrapnel, handgun projectiles and even grenade [12]. The US military continued to use ballistic nylon vests for its regular infantry forces until the 1980s. However, ballistic nylon is of little rele- vance to modern armor. For example, nylon usually absorbs energy more than p-aramids. In p- aramids, the transverse wave velocity is about three to four times higher than polyamide. Stress diffusion is more efficient in aramids, so the elon- gation of the fibers at impact area was measured around 10 µs after the shot was fired with 400 m/s speed [10]. It is evident that the strain in p-aramid fabric is spread in a larger area, and the elongation is much lower. Polyamide creeps under such high strain rates at which the ballistics operates, and melting and fusion at the interlacement points have also been noted [10]. Table 12.2 presents the properties of polyamide 66 versus other ballistic fibers. Figure 12.4 Polyamide 66 chemical structure. 266 Advances in Functional and Protective Textiles 12.3.2 Aramids Aramid fiber, composed of aromatic polyamides, has been a key material for use in various applications due to its diversified characteristics, includ- ing high strength, impact resistance, low density, good chemical resistance, high-heat resistance, and abrasion resistance. Aromatic polyamide fibers (Fig. 12.5) have been widely used in the development of lightweight soft body armor owing to their high performance-to-weight ratio [14]. The aramid fiber is containing poly (p-phenylene terephthalamide) (PPD- T) polymer: a classical poly condensation of p-phenylene diamine (PPD) and terephthaloyl chloride in the amide solvent [15]. The aramid solution is spun by a process called the dry-jet wet spinning (Fig. 12.6). In this process, an anisotropic solution of PPD-T is extruded through the air gap into a coagu- lated bath. The resultant yarn after coagulation is washed and dried [16]. High modulus and tenacity of the fiber are due to the high orientation and strong interchain bonding and high level of crystallization of mole- cules in aramid polymers. On the other hand, aramid fibers have trans- verse bands on the surface, and the pleated structure is formed during the coagulation process. First, the fiber skin is formed under stress, allowing the fiber core to relax during the crystallization and to form pleats at a uniform periodicity. The existence of pleats gives elasticity to the fibers, which increase the elongation before break and rupture strain.