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Applications of auxetic textiles
Applications of auxetic textiles
10.1
10
Introduction
As described in previous chapters, the auxetic foams, textiles and composites exhibit enhanced properties in many ways. With all those enhanced properties and auxetic behaviour, the auxetic materials can be applied in many areas [1 3]. Although auxetic materials have not yet been widely used today, they have great potential to revolute the materials currently used in the world. It has been demonstrated that the auxetic materials are attractive for many applications such as filtration, protection, functional garments, sensors and sports equipment [1 5]. The applications of the auxetic materials are summarised and listed in Table 10.1. This chapter introduces the potential applications of the auxetic foams, textiles and composites in different fields.
10.2
Auxetic textiles for clothing applications
One of the most important applications of the auxetic textiles is to fabricate garments. Both auxetic knitted fabric and auxetic woven fabric have in-plane auxetic behaviour as described in previous chapters, which indicates that these Table 10.1 Applications of the auxetic materials. Areas
Applications
Clothing
Maternity dress, children’s wear, lingerie and fashion wear, sportswear etc. Smart wound dressing, dental floss, suture, surgical implants etc. Bulletproof vest, antiblast curtain, antivibration gloves, helmet, body armour, protective sportswear etc. Packaging materials Seat belt, antiexplosion structure etc. Vanes for aircraft gas turbine engines, aeroplane wings, fuselage etc. Composites, smart fastener etc. Sensors, filtration materials etc.
Healthcare Protection Package Automotive Aeroplane and aerospace Industry Others
Auxetic Textiles. DOI: https://doi.org/10.1016/B978-0-08-102211-5.00010-3 © 2019 Elsevier Ltd. All rights reserved.
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fabrics have lateral expansion property [1,3,4]. Therefore the garments made from the auxetic fabrics will show size expansion and pore-opening effects when stretched [4]. The size expansion effect and pore-opening effect of the garment can be very useful in many situations. With size expansion effect the garment made from auxetic fabrics is able to have size fitting and size growing properties, which is desirable for people in a period of growth such as children and pregnant women [1,3,4]. Using auxetic fabrics to make children’s wear can be an economic choice for parents and a safety choice for children. Clearly, children grow up so fast that parents have to keep buying new garments for their children frequently. Especially for children who are under 3 years old, they may require new garments with bigger size every month. Meanwhile, those old small garments will never be used again and will be disposed. This costs a great amount of money and is so wasteful. Normally, many parents may buy clothes with larger size for their children to let them grow up. However, these kinds of clothes are too loose, which causes children get injured while playing. This problem can be solved using those clothes which are made by auxetic fabrics named ‘growth clothes’. The original size of the ‘growth clothes’ is designed for small babies and it can be stretched to a large degree to suit the size required by children. This type of trousers is made from the fabric with foldable structures which are typically auxetic for manufacturing auxetic fabric introduced in Chapter 5, Auxetic fabrics based on knitted structures. The auxetic fabric made of foldable structures can easily expand in both length and width directions, making the clothes capable of adapting to the body size change in children for a long time. Therefore parents do not need to frequently buy new clothes for their children, and this type of clothes also provides better comfortability. As the auxetic wear can achieve good fitting, children do not need to wear poor-fitted clothes anymore. Using auxetic wear can also save money and reduces waste. In addition to children’s wear, the auxetic fabrics can be used to manufacture closing wear to enhance the shape fit and comfort. As shown in Fig. 10.1A, when a closing wear made of conventional fabric is put on an elbow, the shrinkage of the fabric in the direction perpendicular to the stretching direction not only restricts the movements of muscles and joints but also produces additional pressure on the skin, causing discomfort of the body. However, the situation can be different if an auxetic fabric is used. As shown in Fig. 10.1B, an auxetic fabric will laterally expand instead of shrinking when stretched. Since the deformation of the auxetic fabric adapts to that of the body, there will be no additional pressure. Therefore the muscular and joint movements will not be restricted. It should be pointed out that in addition to the unusual deformation behaviour, other properties such as air and moisture permeability are also enhanced due to open-up of auxetic fabric structures when stretched. The maternity dress is another example of using auxetic fabric to provide pregnant women a comfortable pregnancy period. Currently, conventional elastic materials such as spandex are commonly used for the belly or adjustable waist in maternity dresses. These types of maternity dresses can expand at the belly with the expansion of the belly to some extent. However, the fabric stretches gradually with
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Figure 10.1 Deformation behaviour of sportswear: (A) conventional fabric, (B) auxetic fabric.
Figure 10.2 Maternity dress: (A) conventional maternity dress; (B) auxetic maternity dress.
the growth of the belly. As a result, increasing pressure is applied on the belly by the fabric due to its elastic nature, which causes severe discomfort. Besides, wrinkles may also develop on the dress due to the shrinkage of the fabric, which affects the shape of the garment. In order to solve these problems, the auxetic fabrics can be used for making maternity dresses. Fig. 10.2 shows that the maternity dress made with auxetic fabric has excellent size-adjustability and does not exhibit wrinkles on the fabric surface. When the belly grows, the dress expands in both the waist direction and the direction perpendicular to it. Hence, the belly cannot bear too much pressure caused by the dress as the auxetic dress can naturally expand and forms a dome shape, which perfectly fits the changing shapes of the belly
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throughout the pregnancy period. Also, as the fabric does not shrink in the perpendicular direction, the garment will remain smooth and tidy. There are still many other potential applications of auxetic textile materials in clothing. As introduced in Chapter 5, Auxetic fabrics based on knitted structures, the auxetic spacer fabrics have excellent shape-fitting ability and formability. They can perfectly fit the hemispherical surface without too much force applied. In addition, they also provide good air permeability and soft handle. These advantages enable the auxetic spacer fabrics to be a perfect material for manufacturing bras for female [3]. The bras made by auxetic spacer fabrics can both protect the breast of female and provide comfortability. In addition to giving ease to the female bras, the auxetic fabrics normally exhibit extremely high porosity and increase air permeability due to its special structure design. Therefore the auxetic fabrics are desirable for manufacturing functional sportswear with enhanced comfortability as the auxetic fabrics are able to transform the sweat from the interior surface to the exterior surface quickly to keep the inner surface and skin surface dry [3]. The fabrics made from helical auxetic yarns (HAYs) are a good choice for fashion designing due to their special arrangements of yarn and pore-opening effect of the fabric. Conventionally, garments come designed with certain colours and patterns. The colours are either printed or dyed, and the patterns can be printed or woven on the fabric. In most situations, once the clothes are manufactured, the patterns or colours of the fabric cannot be changed unless the dyes are washed away. For the HAYs, Wright et al. [6] and Miller et al. [7] proved that when the HAYs are used to manufacture fabric woven with a special design, the fabric can achieve a colour-change effect under tension. As having been presented in Chapter 6, Auxetic fabrics based on woven structures, this fabric is manufactured with two layers, including an auxetic layer formed by the HAYs and an inserted layer with colour. When the fabric is stretched, it will expand in the direction perpendicular to the stretching direction and open the pores showing the layer beneath. This process changes the original colour of the fabric and creates a special aesthetics of the garment. It is believed that the colour-change effect of the HAY-made fabrics can be a promising material in the fashion industry for indicated purpose. Although the auxetic fabrics have not been largely applied in the garment industry, with all the potential applications in clothing introduced above, developing auxetic clothes can be a significant direction and the auxetic garments are possible to become a huge market in the future.
10.3
Auxetic textiles for medical and healthcare applications
The auxetic textiles can also find a number of applications in the medical and healthcare area. With the pore-opening effect and the expansion in both directions under tension, the auxetic fabrics can be used to manufacture medical textiles such as smart bandage, and the auxetic yarn can be used to manufacture dental floss and
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sutures [3,8,9]. By using auxetic materials, these medical textiles can obtain better effect than the conventional products. In general, there are four categories for medical textiles, including nonimplantable materials, extra-corporeal devices, implantable materials and healthcare/hygiene products. It is suggested that the auxetic textiles can be used to fabricate nonimplantable materials, healthcare products and even implantable materials [3,4,8,9]. The nonimplantable materials are used outside the human body to assist the recovery of wound. These include wound dressing and bandages etc. The conventional wound dressings have functions of (1) providing protection against infection, (2) absorbing blood and exudate from the wound, (3) promoting wound healing and (4) applying medication to the wound when required. The wound dressings are formed by three layers: a contact layer, an absorbent pad and a base material. The contact layer prevents adherence of the dressing to the wound and guarantees that the dressing can be removed from wound easily. The absorbent pad is put in the middle of the wound dressing to absorb blood or liquids and to provide a cushioning effect to protect the wound. The conventional bandages are used to hold dressings in place over wounds and to support and comfort human body. In some situations the compression bandages are applied to exert additional compression to the wound to prevent thrombosis and leg ulceration etc. Also, the orthopaedic cushion bandages can be used under compression bandages to provide padding and prevent discomfort. These conventional wound dressings and bandages can help wound healing to some extent, but they also have some disadvantages. For example, the medicine is applied to the wound with a certain amount and it requires changing the wound dressings regularly to apply new medicine to the wound. In order to solve the problems of the conventional wound dressings and bandages and to make patients more comfortable during healing period, a series of high-tech wound dressings such as alginate wound dressings are developed. The high-tech wound dressings need to remove excess exudate and toxic component to maintain a high humidity at wound/dressing interface, to allow gaseous exchange, to provide thermal insulation, to offer protection against secondary infection, to be free from particulate or toxic contaminants, to allow removal without trauma at dressing change. By using auxetic textiles (auxetic fabric or auxetic microporous fibres) the wound dressings and bandages can control the drug release and the speed of drug release and also adjust the permeability of the wound dressing accordingly. These auxetic wound dressings and bandages which can meet the criteria of high-tech wound dressings and bandages are also called as smart wound dressings and bandages [3,8,9]. In these products the wound recovery medicine can be stored inside of them or in the fibres. As shown in Fig. 10.3, when the wound bandage is stretched due to swelling of the wound, the fabric structure or micropores of the fibres will open and release the medicine to heal the wound due to the poreopening effect of the auxetic textiles. During the wound healing the delivery speed of the medicine can be controlled by the size of the opening pores of the structures or the fibres through controlling the degree of stretching force. If the wound gets infected and is becoming worse, the wound will swell and the wound bandage will be stretched more leading to more open of the pore and the increase of the delivery
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Figure 10.3 Smart bandage.
speed of the medicine. When the wound gets better, the bump of the wound will decrease causing less stretching force, smaller pores and slower medicine delivery. After the wound is healed the swelling will gradually disappear resulting in the closure of the structure or the pores and stopping the delivery of the medicine. The textile-made implantable materials are the textile structure that can be used inside the human body for different purposes such as closure and replacement. The most common implantable materials are sutures, vascular grafts, artificial valve, artificial ligaments and so on. It has been proven that textile materials are suitable for manufacturing soft-tissue implants. These artificial implants are used to replace the malfunctioning body organs, which is critical for saving human life in many situations. With proper biocompatibility the auxetic materials can be a better option for spinal implant, vascular grafts and artificial valve because the auxetic tubular fabrics can expand in the radial direction when stretched. The expansion of the blood vessel is good for blood circulation which is necessary for thrombosis patients [4]. Regarding healthcare products, the auxetic yarns can be used to manufacture dental floss [3]. The conventional dental floss uses conventional plied yarn which becomes thinner under tension. Therefore when the dental floss is moving between two teeth, the string becomes thinner so that the cleaning effect of the dental floss can be reduced. If the auxetic yarns are used to replace the conventional yarn, the expansion effect of the auxetic yarn can make the dental floss to fill the gap between the teeth and contact teeth more intensively reaching a better cleaning effect of the dental floss.
10.4
Auxetic textiles in protective applications
Up to date, many researches have shown great potential of utilising auxetic materials in protections because the auxetic materials exhibit excellent energy absorption, indentation resistance, energy dissipation and increased fracture toughness
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properties [1 5]. As introduced in Chapter 3, Auxetic polymers, compared with conventional foams, the mechanical properties of the auxetic foams show a distinctive improvement, which lead to utilising the auxetic foam to replace the use of conventional foams in many areas [10]. Also, as mentioned in Chapter 5, Auxetic fabrics based on knitted structures, the auxetic knitted fabric is able to be produced with high-performance fibre and 3D structures. These auxetic knitted fabrics have fantastic energy absorption and shape-fitting ability, which enables the auxetic knitted fabrics to have many applications in protective clothing and equipment [3,4]. The auxetic composites described in Chapter 9, Auxetic fibre reinforced composites, also have enhanced mechanical properties and can be used for protective applications [11,12]. The auxetic foam, fabrics and composites all can be used to or combined to manufacture bulletproof vest, explosion-proof curtain, antivibration gloves and protective clothes and equipment for sports etc. By using the auxetic materials, these protective clothes or equipment can provide improved protections to human body. One of the most common protective textiles is the body armour (bulletproof vest, ballistic protective helmet, antistab vest etc.). The body armour is used to protect people from being hurt by projectiles and knives etc. They are commonly used by solders, policemen, celebrities and politicians. Currently, there are two types of body armours: hard and soft. The hard body armour provides protection against high velocity projectiles but is far less comfortable. It is usually made from composite material incorporating a ceramic/plastic/metal plate. The soft body armour is manufactured from high-performance fibres such as Kevlar, Dyneema and Spectra. Since soft body armour is based on textile materials, it is more comfortable but only provides low-to-medium protections against low-to-medium velocity projectiles or slashes. The requirements for modern body armour are more than simply providing protection against projectile penetration or knife slash. It is important to protect human body from the blunt trauma caused by the sudden conversion of the kinetic energy. To realise the protection the body armour needs to have rapid conversion and dissipation of the kinetic energy from a striking bullet or other projectiles to stain energy of the body armour system. Nowadays, when manufacturing soft body armour from conventional high-performance fibres, to improve the protection effect, normally fibres with higher modulus and interyarn friction will be used and the armour will be produced with tighter fabric structures and have more fabric layers etc. For hard body armour, it uses thicker and harder composites to obtain better performance. These methods provide better protection to some extent but reduce the comfortability while wearing body armour. Steffens et al. [13,14] have demonstrated how to manufacture auxetic fabrics by using high-performance fibres and fabricate high-performance composite based on the auxetic knitted fabric. They believed that when the auxetic knitted fabrics were made from high-performance fibre such as Kevlar and Nomex, the fabrics could find potential application in bulletproof vest, cut resistance fabrics, helmets etc. The combination of the high indentation resistance of the auxetic materials and high modulus of the high-performance fibre can easily dissipate the kinetic energy of the bullet or the cutting object throughout the whole fabric structure. Therefore by
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using auxetic knitted fabric, the soft body armour is able to have better protection, enhanced comfortability and lower weight. In addition, the auxetic foams and composites have shown great energy absorption, indentation resistance and fracture toughness performances. Taking advantages of these properties, these two materials can be used to manufacture hard body armour for high-level protection. The hard body armour made from auxetic foam or composite is able to exhibit enhanced protection and reduced weight, which is important for military use. The auxetic fabric, foam and composite can also be combined to make bulletproof product. For instance, Mitsubishi has patented a design for bulletproof application. This design has one auxetic component and one conventional component. The overall Poisson ratio of this product is close to zero. Another example of the protective application of the auxetic textile is the auxetic blast-proof curtain [15]. The blast-proof curtain is used in those buildings such as government building, military base and high-profile commercial properties which are potential terrorist targets. The blast-proof curtains currently in use are fixed over the inside of windows in those buildings and formed with two layers. They consist of one thick net-curtain fabric and one antishatter film. The conjunction of fabric and film can prevent both explosion’s shock wave and fragments of glass from damaging the inside of the building and tearing the material. The auxetic blast-proof curtain aims to remove the use of antishatter films and to provide better protection by using auxetic yarns which are woven into a well-designed and carefully controlled textile structure. The auxetic blast-proof curtain is designed to have high endurance and to capture fragments such as flying glass when windows are blown in. Such fragments can cause severe damages to the building and injuries to people working in the building. The auxetic blast-proof curtain is produced on a craft loom using the HAYs which have been introduced in Chapter 4, Auxetic fibres and yarns [15]. To manufacture this type of curtain, the HAYs are carefully arranged in the fabric to reach the through-the-thickness auxetic effect of the fabric making the fabric become thicker when stretched [15]. Also, the HAYs used are designed with different wrapping angles to obtain different auxetic effects and to prevent different levels of explosion. The curtain can have pore-opening effect under tension due to the auxetic yarn and special design. Hence, it will have small pores to allow the pass of some of the blast wave and these small pores will be much smaller than the flying fragments. This effect can help to reduce the pressure that the curtain has to bear with. Since the auxetic blast-proof curtain can have the excellent protection with 1 2 mm thick, it can let in a certain amount of natural light, which is important for people’s health and mood. The auxetic foam and spacer fabric can find an important application in fabricating one type of antivibration gloves which can provide a better distribution of the pressure and less compressive stress at the interface between the glove and the human hand, resulting in less damage to skin tissue and capillaries when hands are under severe shaking or striking [3,16]. The antivibration gloves are useful for those workers who need to use electric drills, big hammers and so on. Scarpa et al. [16] fabricated a series of polyurethane (PU) foams and tested their mechanical properties for antivibration gloves application. Five auxetic foams were
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manufactured by using heat compression method with different compression ratios in axial and radial directions. All the five foams exhibited auxetic behaviour and increased stiffness under compression. The transmissibility of these samples was also tested to show a good result. These auxetic foams are believed to be the promising material for antivibration application. However, Wang and Hu [3] identified that the gloves made from the auxetic foams may not be comfortable and people may become allergic to them after a long-time wearing. Using auxetic spacer fabric to replace the PU foam can be a good way to enhance the comfortability of using antivibration gloves. Auxetic materials are also promising for manufacturing protective clothing and equipment for sports due to their excellent energy absorption and shape-fitting ability [1 5]. They can be used for producing kneecap, wrist protector, helmet etc. to protect vulnerable parts of human body in dangerous sports such as car racing, horse racing, cycling, skiing and snowboarding [1 5,17]. Several literatures have presented the use of auxetic foams and fabrics in manufacturing sportive safety devices and patents describing the way to produce sportswear with auxetic materials can be found. Here, two examples of utilising auxetic foams to make sportswear are briefly introduced. Allen et al. [17,18] used auxetic foam to make snow-sport safety devices which are some types of composite pad made from the auxetic foam covered by a semirigid polypropylene (PP) shell. The conventional open-cell PU foam is first conversed to the auxetic foam through thermal mechanical method with a volume compression ratio of 3. The PP shell is placed unbonded on the top of the foam for testing. Fig. 10.4 shows the concentrated load compression testing results of the auxetic foam and the conventional foam. It can be seen that the auxetic samples almost have four times higher peak force and three times higher energy absorbed, which indicates that, when this type of combination is used in the sportswear, it can provide better energy absorption and greater resistance to bottoming out. Another example is the conjunction of auxetic foam and shear-thickening gel named ‘armourgel’ material. This material has been used to manufacture cycling protection equipment. The cells of the armourgel are made into reentrant geometry as shown in Fig. 2.1. When the surface of the armourgel is under force, either compression or indentation, the reentrant cell will shrink in both directions of the material creating auxetic effect. The shear-thickening effect will also be created to prevent indentation and absorb energy. This material is able to control the protection level through adjusting the thickness of the cell wall. The cell walls can increase in thickness from 1 5 to 3 15 mm, which are proportional to the impact force applied.
10.5
Auxetic textiles for packaging applications
Both conventional foams and textiles are largely applied in package. The packaging materials require good abrasion resistance and durability. In addition, for the packaging of fragile products, the packaging materials need to provide good protection to them. The auxetic textiles and foams can find applications in packaging as well.
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Figure 10.4 Compression testing results: (A) peak force, (B) energy absorbed [17].
Uzun and Patel [19] compared the tribological properties of the auxetic PP fibres with conventional PP fibres and pointed out that the auxetic PP fibres had much better abrasion resistance (15% 35%). The conventional PP fibres are manufactured using normal melt-spinning technique, while the auxetic PP fibres are produced using the modified melt spinning described in Chapter 4, Auxetic fibres and yarns. Both auxetic PP fibres and conventional PP fibres are used to knit plain
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weft-knitted fabrics and then the fabrics are subjected to the Martindale abrasion tester for testing. The conventional PP fibre fabric and the auxetic PP fibre are rubbed 20,000 times, respectively. The experimental results suggest that the outstanding abrasion resistance of the auxetic PP fibres makes them as a promising material for packaging. The auxetic foams can be used to replace the conventional foams as the inside packaging materials to provide necessary protection as the auxetic foams exhibit better energy absorption and indentation resistance. It is believed that using auxetic foams can be a better choice for packaging than conventional foams.
10.6
Auxetic textiles for automotive applications
It is possible to use auxetic textiles in automobiles as the seat belt [1 3]. The seat belt is one of the most crucial safety devices in automobiles, which can save lives in some extreme situations such as car accident. The seat belt prevents the forward movement of the wearer in a controlled manner during sudden deceleration of the vehicle. There are three types of seat belt currently being used in different situations, including lap and chest seat belt for car, lap seat belt for airplane and lap and shoulders seat belt for racing cars. The seat belts must meet several strict requirements to guarantee their function through the lifespan, for instance, a seat belt should be able to carry a static load of 1500 kg or 14.7 kN with a maximum extension of 25% 30% and needs to have excellent abrasion resistance, heat resistance, light resistance, light weight and flexibility of easy use. Conventionally, the seat belts are manufactured with high tenacity polyester and nylon continuous filament yarns. The conventional seat belts are useful and save lives for many years, but they have one disadvantage. In order to make a seat belt work properly, it has to be fastened tightly, which makes people feel uncomfortable especially when the seat belt is functioning. Due to the sudden decrease of the speed, the human body will be subjected to a huge pressure from the seat belt. This force could easily hurt human body. In some extreme cases, people’s ribs may be broken. If the seat belt is produced with high-performance auxetic fabrics, this problem may be resolved. As the auxetic fabric will be wider under stretching, the contact area between seat belt and human body will be increased. Hence, the impact pressure that the seat belt applies to human body will be decreased resulting in less possibility of hurting the human body.
10.7
Auxetic textiles for industrial applications
One of the most important industrial applications of the auxetic textiles is manufacturing composite materials which have been specifically introduced in Chapter 9, Auxetic fibre reinforced composites. The auxetic composites
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manufactured from auxetic fabrics or auxetic composites realised from structural design show enhanced mechanical properties in many areas, indicating that these composites can find various applications in industry such as making sensors, engine fine blades and vanes for gas turbine engine. In addition, there are other applications of auxetic materials in industry. One example is the auxetic fastener designed by Choi and Lakes [20]. The auxetic fastener can be installed more easily, but removal is more difficult than conventional materials. This auxetic fastener is designed a little larger than the hole within a tolerance in the radial dimension. When the fastener is being inserted to the hole, it will be pressed in axial direction; thus due to the auxetic effect the fastener will become smaller in the diameter, which makes it easy to be installed. After the fastener is installed and the compression force is removed from the fastener, the fastener will tend to expand in both axial and radial directions. This expansion effect locks the fastener in position tightly. When the fastener is needed to be removed from the hole, compressing it to make it decrease in the radial direction to achieve easy removal. The auxetic fastener has the advantage of simple press-fit insertion and easy removal.
10.8
Auxetic textiles for other applications
The auxetic textiles can also find applications in other areas. For example, the auxetic foams and fabrics can be used to create one type of smart filtration system [21 23]. This smart filtration system can be utilised in both air and water filtration. It allows the passing through the particles with certain size and the size can be changed through changing the loading level due to the pore-opening effect of the auxetic materials under tension. By using this smart filtration system the filtration level could automatically be changed through simply changing the loading level rather than changing the filtration pad [21 23]. The auxetic fabrics can be used as geotextiles as well because of the filtration function. The filtration function of geotextile may be the most widely known and used function [24]. The use of geotextiles can prevent aggregation of the particles under the road, which damages the road surface in raining days [24]. As shown in Fig. 10.5, the geotextile is used as a filter between the layers of the particles under the road so that liquids can pass through the plane of the geotextile while preventing the passage of soil particles [24]. Therefore the geotextile must have enough permeability to let the liquid pass through and controlled even pore size and pore size distribution to control the migration of the particles. The materials cross-plane permeability and pore size characteristics are the two most important properties of the geotextiles. If the auxetic fabrics are used as the geotextiles, clearly they can meet both requirements. The auxetic geotextiles can even adjust the speed of the liquid flow. Taking advantage of the pore-opening effect, in those extreme weather conditions such as rainstorm and flood, the filter will let more water pass through to reduce water on the road surface, which can reduce potential dangers to pedestrians.
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Figure 10.5 Filtration function of the geotextiles.
References [1] Hu H, Zulifqar A. Auxetic textile materials a review. J Text Eng Fashion Technol 2016;1(1):00002. [2] Alderson A, Alderson K. Auxetic materials. Proc Inst Mech Eng Part G J Aerosp Eng 2007;221(4):565 75. [3] Wang Z, Hu H. Auxetic materials and their potential applications in textiles. Text Res J 2014;84(15):1600 11. [4] Ma P, Chang Y, Boakye A, Jiang G. Review on the knitted structures with auxetic effect. J Text Inst 2017;108(6):947 61. [5] Critchley R, Corni I, Wharton JA, Walsh FC, Wood RJ, Stokes KR. A review of the manufacture, mechanical properties and potential applications of auxetic foams. Phys Status Solidi B 2013;250(10):1963 82. [6] Wright JR, Burns MK, James E, Sloan MR, Evans KE. On the design and characterisation of low-stiffness auxetic yarns and fabrics. Text Res J 2012;82(7):645 54. [7] Miller W, Hook P, Smith CW, Wang X, Evans KE. The manufacture and characterisation of a novel, low modulus, negative Poisson’s ratio composite. Compos Sci Technol 2009;69(5):651 5. [8] Alderson A, Alderson K. Expanding materials and applications: exploiting auxetic textiles. Tech Text Int 2005;14(6):29 34. [9] Evans KE, Alderson A. Auxetic materials: functional materials and structures from lateral thinking!. Adv Mater. 2000;12(9):617 28.
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[10] Yang W, Li Z-M, Shi W, Xie B-H, Yang M-B. Review on auxetic materials. J Mater Sci 2004;39(10):3269 79. [11] Alderson K, Simkins V, Coenen V, Davies P, Alderson A, Evans K. How to make auxetic fibre reinforced composites. Phys Status Solidi B 2005;242(3):509 18. [12] Jiang L, Gu B, Hu H. Auxetic composite made with multilayer orthogonal structural reinforcement. Compos Struct. 2016;135:23 9. [13] Steffens F, Rana S, Fangueiro R. Development of novel auxetic textile structures using high performance fibres. Mater Des 2016;106:81 9. [14] Steffens F, Oliveira FR, Mota C, Fangueiro R. High-performance composite with negative Poisson’s ratio. J Mater Res 2017;32(18):3477 84. [15] Engineering and Physical Sciences Research Council. Expanding blast-proof curtain will reduce impact of bomb explosions, ,http://www.epsrc.ac.uk/newsevents/news/ 2010/Pages/blastproofcurtain.aspx.; 2010 [accessed 10.02.12]. [16] Scarpa F, Giacomin J, Zhang Y, Pastorino P. Mechanical performance of auxetic polyurethane foam for antivibration glove applications. Cell Polym. 2005;24(5):253 68. [17] Allen T, Duncan O, Foster L, Senior T, Zampieri D, Edeh V, et al. Auxetic foam for snow-sport safety devices. Snow sports trauma and safety. Springer; 2017. p. 145 59. [18] Allen T, Martinello N, Zampieri D, Hewage T, Senior T, Foster L, et al. Auxetic foams for sport safety applications. Procedia Eng 2015;112:104 9. [19] Uzun M, Patel I. Tribological properties of auxetic and conventional polypropylene weft knitted fabrics. Arch Mater Sci Eng 2010;44(2):120 5. [20] Choi J, Lakes R. Design of a fastener based on negative Poisson’s ratio foam. Cell Polym. 1991;10(3):205 12. [21] Rasburn J, Mullarkey P, Evans KE, Alderson A, Ameer-Beg S, Perrie W. Auxetic structures for variable permeability systems. AIChE J 2001;47(11):2623 6. [22] Alderson A, Rasburn J, Evans K. Mass transport properties of auxetic (negative Poisson’s ratio) foams. Phys Status Solidi B 2007;244(3):817 27. [23] Alderson A, Rasburn J, Ameer-Beg S, Mullarkey PG, Perrie W, Evans KE. An auxetic filter: a tuneable filter displaying enhanced size selectivity or defouling properties. Ind Eng Chem Res 2000;39(3):654 65. [24] Hudson K, East G. Geotextiles; 1991, No. 6, ARRB.
10.1
10
Introduction
As described in previous chapters, the auxetic foams, textiles and composites exhibit enhanced properties in many ways. With all those enhanced properties and auxetic behaviour, the auxetic materials can be applied in many areas [1 3]. Although auxetic materials have not yet been widely used today, they have great potential to revolute the materials currently used in the world. It has been demonstrated that the auxetic materials are attractive for many applications such as filtration, protection, functional garments, sensors and sports equipment [1 5]. The applications of the auxetic materials are summarised and listed in Table 10.1. This chapter introduces the potential applications of the auxetic foams, textiles and composites in different fields.
10.2
Auxetic textiles for clothing applications
One of the most important applications of the auxetic textiles is to fabricate garments. Both auxetic knitted fabric and auxetic woven fabric have in-plane auxetic behaviour as described in previous chapters, which indicates that these Table 10.1 Applications of the auxetic materials. Areas
Applications
Clothing
Maternity dress, children’s wear, lingerie and fashion wear, sportswear etc. Smart wound dressing, dental floss, suture, surgical implants etc. Bulletproof vest, antiblast curtain, antivibration gloves, helmet, body armour, protective sportswear etc. Packaging materials Seat belt, antiexplosion structure etc. Vanes for aircraft gas turbine engines, aeroplane wings, fuselage etc. Composites, smart fastener etc. Sensors, filtration materials etc.
Healthcare Protection Package Automotive Aeroplane and aerospace Industry Others
Auxetic Textiles. DOI: https://doi.org/10.1016/B978-0-08-102211-5.00010-3 © 2019 Elsevier Ltd. All rights reserved.
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fabrics have lateral expansion property [1,3,4]. Therefore the garments made from the auxetic fabrics will show size expansion and pore-opening effects when stretched [4]. The size expansion effect and pore-opening effect of the garment can be very useful in many situations. With size expansion effect the garment made from auxetic fabrics is able to have size fitting and size growing properties, which is desirable for people in a period of growth such as children and pregnant women [1,3,4]. Using auxetic fabrics to make children’s wear can be an economic choice for parents and a safety choice for children. Clearly, children grow up so fast that parents have to keep buying new garments for their children frequently. Especially for children who are under 3 years old, they may require new garments with bigger size every month. Meanwhile, those old small garments will never be used again and will be disposed. This costs a great amount of money and is so wasteful. Normally, many parents may buy clothes with larger size for their children to let them grow up. However, these kinds of clothes are too loose, which causes children get injured while playing. This problem can be solved using those clothes which are made by auxetic fabrics named ‘growth clothes’. The original size of the ‘growth clothes’ is designed for small babies and it can be stretched to a large degree to suit the size required by children. This type of trousers is made from the fabric with foldable structures which are typically auxetic for manufacturing auxetic fabric introduced in Chapter 5, Auxetic fabrics based on knitted structures. The auxetic fabric made of foldable structures can easily expand in both length and width directions, making the clothes capable of adapting to the body size change in children for a long time. Therefore parents do not need to frequently buy new clothes for their children, and this type of clothes also provides better comfortability. As the auxetic wear can achieve good fitting, children do not need to wear poor-fitted clothes anymore. Using auxetic wear can also save money and reduces waste. In addition to children’s wear, the auxetic fabrics can be used to manufacture closing wear to enhance the shape fit and comfort. As shown in Fig. 10.1A, when a closing wear made of conventional fabric is put on an elbow, the shrinkage of the fabric in the direction perpendicular to the stretching direction not only restricts the movements of muscles and joints but also produces additional pressure on the skin, causing discomfort of the body. However, the situation can be different if an auxetic fabric is used. As shown in Fig. 10.1B, an auxetic fabric will laterally expand instead of shrinking when stretched. Since the deformation of the auxetic fabric adapts to that of the body, there will be no additional pressure. Therefore the muscular and joint movements will not be restricted. It should be pointed out that in addition to the unusual deformation behaviour, other properties such as air and moisture permeability are also enhanced due to open-up of auxetic fabric structures when stretched. The maternity dress is another example of using auxetic fabric to provide pregnant women a comfortable pregnancy period. Currently, conventional elastic materials such as spandex are commonly used for the belly or adjustable waist in maternity dresses. These types of maternity dresses can expand at the belly with the expansion of the belly to some extent. However, the fabric stretches gradually with
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Figure 10.1 Deformation behaviour of sportswear: (A) conventional fabric, (B) auxetic fabric.
Figure 10.2 Maternity dress: (A) conventional maternity dress; (B) auxetic maternity dress.
the growth of the belly. As a result, increasing pressure is applied on the belly by the fabric due to its elastic nature, which causes severe discomfort. Besides, wrinkles may also develop on the dress due to the shrinkage of the fabric, which affects the shape of the garment. In order to solve these problems, the auxetic fabrics can be used for making maternity dresses. Fig. 10.2 shows that the maternity dress made with auxetic fabric has excellent size-adjustability and does not exhibit wrinkles on the fabric surface. When the belly grows, the dress expands in both the waist direction and the direction perpendicular to it. Hence, the belly cannot bear too much pressure caused by the dress as the auxetic dress can naturally expand and forms a dome shape, which perfectly fits the changing shapes of the belly
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throughout the pregnancy period. Also, as the fabric does not shrink in the perpendicular direction, the garment will remain smooth and tidy. There are still many other potential applications of auxetic textile materials in clothing. As introduced in Chapter 5, Auxetic fabrics based on knitted structures, the auxetic spacer fabrics have excellent shape-fitting ability and formability. They can perfectly fit the hemispherical surface without too much force applied. In addition, they also provide good air permeability and soft handle. These advantages enable the auxetic spacer fabrics to be a perfect material for manufacturing bras for female [3]. The bras made by auxetic spacer fabrics can both protect the breast of female and provide comfortability. In addition to giving ease to the female bras, the auxetic fabrics normally exhibit extremely high porosity and increase air permeability due to its special structure design. Therefore the auxetic fabrics are desirable for manufacturing functional sportswear with enhanced comfortability as the auxetic fabrics are able to transform the sweat from the interior surface to the exterior surface quickly to keep the inner surface and skin surface dry [3]. The fabrics made from helical auxetic yarns (HAYs) are a good choice for fashion designing due to their special arrangements of yarn and pore-opening effect of the fabric. Conventionally, garments come designed with certain colours and patterns. The colours are either printed or dyed, and the patterns can be printed or woven on the fabric. In most situations, once the clothes are manufactured, the patterns or colours of the fabric cannot be changed unless the dyes are washed away. For the HAYs, Wright et al. [6] and Miller et al. [7] proved that when the HAYs are used to manufacture fabric woven with a special design, the fabric can achieve a colour-change effect under tension. As having been presented in Chapter 6, Auxetic fabrics based on woven structures, this fabric is manufactured with two layers, including an auxetic layer formed by the HAYs and an inserted layer with colour. When the fabric is stretched, it will expand in the direction perpendicular to the stretching direction and open the pores showing the layer beneath. This process changes the original colour of the fabric and creates a special aesthetics of the garment. It is believed that the colour-change effect of the HAY-made fabrics can be a promising material in the fashion industry for indicated purpose. Although the auxetic fabrics have not been largely applied in the garment industry, with all the potential applications in clothing introduced above, developing auxetic clothes can be a significant direction and the auxetic garments are possible to become a huge market in the future.
10.3
Auxetic textiles for medical and healthcare applications
The auxetic textiles can also find a number of applications in the medical and healthcare area. With the pore-opening effect and the expansion in both directions under tension, the auxetic fabrics can be used to manufacture medical textiles such as smart bandage, and the auxetic yarn can be used to manufacture dental floss and
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sutures [3,8,9]. By using auxetic materials, these medical textiles can obtain better effect than the conventional products. In general, there are four categories for medical textiles, including nonimplantable materials, extra-corporeal devices, implantable materials and healthcare/hygiene products. It is suggested that the auxetic textiles can be used to fabricate nonimplantable materials, healthcare products and even implantable materials [3,4,8,9]. The nonimplantable materials are used outside the human body to assist the recovery of wound. These include wound dressing and bandages etc. The conventional wound dressings have functions of (1) providing protection against infection, (2) absorbing blood and exudate from the wound, (3) promoting wound healing and (4) applying medication to the wound when required. The wound dressings are formed by three layers: a contact layer, an absorbent pad and a base material. The contact layer prevents adherence of the dressing to the wound and guarantees that the dressing can be removed from wound easily. The absorbent pad is put in the middle of the wound dressing to absorb blood or liquids and to provide a cushioning effect to protect the wound. The conventional bandages are used to hold dressings in place over wounds and to support and comfort human body. In some situations the compression bandages are applied to exert additional compression to the wound to prevent thrombosis and leg ulceration etc. Also, the orthopaedic cushion bandages can be used under compression bandages to provide padding and prevent discomfort. These conventional wound dressings and bandages can help wound healing to some extent, but they also have some disadvantages. For example, the medicine is applied to the wound with a certain amount and it requires changing the wound dressings regularly to apply new medicine to the wound. In order to solve the problems of the conventional wound dressings and bandages and to make patients more comfortable during healing period, a series of high-tech wound dressings such as alginate wound dressings are developed. The high-tech wound dressings need to remove excess exudate and toxic component to maintain a high humidity at wound/dressing interface, to allow gaseous exchange, to provide thermal insulation, to offer protection against secondary infection, to be free from particulate or toxic contaminants, to allow removal without trauma at dressing change. By using auxetic textiles (auxetic fabric or auxetic microporous fibres) the wound dressings and bandages can control the drug release and the speed of drug release and also adjust the permeability of the wound dressing accordingly. These auxetic wound dressings and bandages which can meet the criteria of high-tech wound dressings and bandages are also called as smart wound dressings and bandages [3,8,9]. In these products the wound recovery medicine can be stored inside of them or in the fibres. As shown in Fig. 10.3, when the wound bandage is stretched due to swelling of the wound, the fabric structure or micropores of the fibres will open and release the medicine to heal the wound due to the poreopening effect of the auxetic textiles. During the wound healing the delivery speed of the medicine can be controlled by the size of the opening pores of the structures or the fibres through controlling the degree of stretching force. If the wound gets infected and is becoming worse, the wound will swell and the wound bandage will be stretched more leading to more open of the pore and the increase of the delivery
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Figure 10.3 Smart bandage.
speed of the medicine. When the wound gets better, the bump of the wound will decrease causing less stretching force, smaller pores and slower medicine delivery. After the wound is healed the swelling will gradually disappear resulting in the closure of the structure or the pores and stopping the delivery of the medicine. The textile-made implantable materials are the textile structure that can be used inside the human body for different purposes such as closure and replacement. The most common implantable materials are sutures, vascular grafts, artificial valve, artificial ligaments and so on. It has been proven that textile materials are suitable for manufacturing soft-tissue implants. These artificial implants are used to replace the malfunctioning body organs, which is critical for saving human life in many situations. With proper biocompatibility the auxetic materials can be a better option for spinal implant, vascular grafts and artificial valve because the auxetic tubular fabrics can expand in the radial direction when stretched. The expansion of the blood vessel is good for blood circulation which is necessary for thrombosis patients [4]. Regarding healthcare products, the auxetic yarns can be used to manufacture dental floss [3]. The conventional dental floss uses conventional plied yarn which becomes thinner under tension. Therefore when the dental floss is moving between two teeth, the string becomes thinner so that the cleaning effect of the dental floss can be reduced. If the auxetic yarns are used to replace the conventional yarn, the expansion effect of the auxetic yarn can make the dental floss to fill the gap between the teeth and contact teeth more intensively reaching a better cleaning effect of the dental floss.
10.4
Auxetic textiles in protective applications
Up to date, many researches have shown great potential of utilising auxetic materials in protections because the auxetic materials exhibit excellent energy absorption, indentation resistance, energy dissipation and increased fracture toughness
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properties [1 5]. As introduced in Chapter 3, Auxetic polymers, compared with conventional foams, the mechanical properties of the auxetic foams show a distinctive improvement, which lead to utilising the auxetic foam to replace the use of conventional foams in many areas [10]. Also, as mentioned in Chapter 5, Auxetic fabrics based on knitted structures, the auxetic knitted fabric is able to be produced with high-performance fibre and 3D structures. These auxetic knitted fabrics have fantastic energy absorption and shape-fitting ability, which enables the auxetic knitted fabrics to have many applications in protective clothing and equipment [3,4]. The auxetic composites described in Chapter 9, Auxetic fibre reinforced composites, also have enhanced mechanical properties and can be used for protective applications [11,12]. The auxetic foam, fabrics and composites all can be used to or combined to manufacture bulletproof vest, explosion-proof curtain, antivibration gloves and protective clothes and equipment for sports etc. By using the auxetic materials, these protective clothes or equipment can provide improved protections to human body. One of the most common protective textiles is the body armour (bulletproof vest, ballistic protective helmet, antistab vest etc.). The body armour is used to protect people from being hurt by projectiles and knives etc. They are commonly used by solders, policemen, celebrities and politicians. Currently, there are two types of body armours: hard and soft. The hard body armour provides protection against high velocity projectiles but is far less comfortable. It is usually made from composite material incorporating a ceramic/plastic/metal plate. The soft body armour is manufactured from high-performance fibres such as Kevlar, Dyneema and Spectra. Since soft body armour is based on textile materials, it is more comfortable but only provides low-to-medium protections against low-to-medium velocity projectiles or slashes. The requirements for modern body armour are more than simply providing protection against projectile penetration or knife slash. It is important to protect human body from the blunt trauma caused by the sudden conversion of the kinetic energy. To realise the protection the body armour needs to have rapid conversion and dissipation of the kinetic energy from a striking bullet or other projectiles to stain energy of the body armour system. Nowadays, when manufacturing soft body armour from conventional high-performance fibres, to improve the protection effect, normally fibres with higher modulus and interyarn friction will be used and the armour will be produced with tighter fabric structures and have more fabric layers etc. For hard body armour, it uses thicker and harder composites to obtain better performance. These methods provide better protection to some extent but reduce the comfortability while wearing body armour. Steffens et al. [13,14] have demonstrated how to manufacture auxetic fabrics by using high-performance fibres and fabricate high-performance composite based on the auxetic knitted fabric. They believed that when the auxetic knitted fabrics were made from high-performance fibre such as Kevlar and Nomex, the fabrics could find potential application in bulletproof vest, cut resistance fabrics, helmets etc. The combination of the high indentation resistance of the auxetic materials and high modulus of the high-performance fibre can easily dissipate the kinetic energy of the bullet or the cutting object throughout the whole fabric structure. Therefore by
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using auxetic knitted fabric, the soft body armour is able to have better protection, enhanced comfortability and lower weight. In addition, the auxetic foams and composites have shown great energy absorption, indentation resistance and fracture toughness performances. Taking advantages of these properties, these two materials can be used to manufacture hard body armour for high-level protection. The hard body armour made from auxetic foam or composite is able to exhibit enhanced protection and reduced weight, which is important for military use. The auxetic fabric, foam and composite can also be combined to make bulletproof product. For instance, Mitsubishi has patented a design for bulletproof application. This design has one auxetic component and one conventional component. The overall Poisson ratio of this product is close to zero. Another example of the protective application of the auxetic textile is the auxetic blast-proof curtain [15]. The blast-proof curtain is used in those buildings such as government building, military base and high-profile commercial properties which are potential terrorist targets. The blast-proof curtains currently in use are fixed over the inside of windows in those buildings and formed with two layers. They consist of one thick net-curtain fabric and one antishatter film. The conjunction of fabric and film can prevent both explosion’s shock wave and fragments of glass from damaging the inside of the building and tearing the material. The auxetic blast-proof curtain aims to remove the use of antishatter films and to provide better protection by using auxetic yarns which are woven into a well-designed and carefully controlled textile structure. The auxetic blast-proof curtain is designed to have high endurance and to capture fragments such as flying glass when windows are blown in. Such fragments can cause severe damages to the building and injuries to people working in the building. The auxetic blast-proof curtain is produced on a craft loom using the HAYs which have been introduced in Chapter 4, Auxetic fibres and yarns [15]. To manufacture this type of curtain, the HAYs are carefully arranged in the fabric to reach the through-the-thickness auxetic effect of the fabric making the fabric become thicker when stretched [15]. Also, the HAYs used are designed with different wrapping angles to obtain different auxetic effects and to prevent different levels of explosion. The curtain can have pore-opening effect under tension due to the auxetic yarn and special design. Hence, it will have small pores to allow the pass of some of the blast wave and these small pores will be much smaller than the flying fragments. This effect can help to reduce the pressure that the curtain has to bear with. Since the auxetic blast-proof curtain can have the excellent protection with 1 2 mm thick, it can let in a certain amount of natural light, which is important for people’s health and mood. The auxetic foam and spacer fabric can find an important application in fabricating one type of antivibration gloves which can provide a better distribution of the pressure and less compressive stress at the interface between the glove and the human hand, resulting in less damage to skin tissue and capillaries when hands are under severe shaking or striking [3,16]. The antivibration gloves are useful for those workers who need to use electric drills, big hammers and so on. Scarpa et al. [16] fabricated a series of polyurethane (PU) foams and tested their mechanical properties for antivibration gloves application. Five auxetic foams were
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manufactured by using heat compression method with different compression ratios in axial and radial directions. All the five foams exhibited auxetic behaviour and increased stiffness under compression. The transmissibility of these samples was also tested to show a good result. These auxetic foams are believed to be the promising material for antivibration application. However, Wang and Hu [3] identified that the gloves made from the auxetic foams may not be comfortable and people may become allergic to them after a long-time wearing. Using auxetic spacer fabric to replace the PU foam can be a good way to enhance the comfortability of using antivibration gloves. Auxetic materials are also promising for manufacturing protective clothing and equipment for sports due to their excellent energy absorption and shape-fitting ability [1 5]. They can be used for producing kneecap, wrist protector, helmet etc. to protect vulnerable parts of human body in dangerous sports such as car racing, horse racing, cycling, skiing and snowboarding [1 5,17]. Several literatures have presented the use of auxetic foams and fabrics in manufacturing sportive safety devices and patents describing the way to produce sportswear with auxetic materials can be found. Here, two examples of utilising auxetic foams to make sportswear are briefly introduced. Allen et al. [17,18] used auxetic foam to make snow-sport safety devices which are some types of composite pad made from the auxetic foam covered by a semirigid polypropylene (PP) shell. The conventional open-cell PU foam is first conversed to the auxetic foam through thermal mechanical method with a volume compression ratio of 3. The PP shell is placed unbonded on the top of the foam for testing. Fig. 10.4 shows the concentrated load compression testing results of the auxetic foam and the conventional foam. It can be seen that the auxetic samples almost have four times higher peak force and three times higher energy absorbed, which indicates that, when this type of combination is used in the sportswear, it can provide better energy absorption and greater resistance to bottoming out. Another example is the conjunction of auxetic foam and shear-thickening gel named ‘armourgel’ material. This material has been used to manufacture cycling protection equipment. The cells of the armourgel are made into reentrant geometry as shown in Fig. 2.1. When the surface of the armourgel is under force, either compression or indentation, the reentrant cell will shrink in both directions of the material creating auxetic effect. The shear-thickening effect will also be created to prevent indentation and absorb energy. This material is able to control the protection level through adjusting the thickness of the cell wall. The cell walls can increase in thickness from 1 5 to 3 15 mm, which are proportional to the impact force applied.
10.5
Auxetic textiles for packaging applications
Both conventional foams and textiles are largely applied in package. The packaging materials require good abrasion resistance and durability. In addition, for the packaging of fragile products, the packaging materials need to provide good protection to them. The auxetic textiles and foams can find applications in packaging as well.
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Figure 10.4 Compression testing results: (A) peak force, (B) energy absorbed [17].
Uzun and Patel [19] compared the tribological properties of the auxetic PP fibres with conventional PP fibres and pointed out that the auxetic PP fibres had much better abrasion resistance (15% 35%). The conventional PP fibres are manufactured using normal melt-spinning technique, while the auxetic PP fibres are produced using the modified melt spinning described in Chapter 4, Auxetic fibres and yarns. Both auxetic PP fibres and conventional PP fibres are used to knit plain
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weft-knitted fabrics and then the fabrics are subjected to the Martindale abrasion tester for testing. The conventional PP fibre fabric and the auxetic PP fibre are rubbed 20,000 times, respectively. The experimental results suggest that the outstanding abrasion resistance of the auxetic PP fibres makes them as a promising material for packaging. The auxetic foams can be used to replace the conventional foams as the inside packaging materials to provide necessary protection as the auxetic foams exhibit better energy absorption and indentation resistance. It is believed that using auxetic foams can be a better choice for packaging than conventional foams.
10.6
Auxetic textiles for automotive applications
It is possible to use auxetic textiles in automobiles as the seat belt [1 3]. The seat belt is one of the most crucial safety devices in automobiles, which can save lives in some extreme situations such as car accident. The seat belt prevents the forward movement of the wearer in a controlled manner during sudden deceleration of the vehicle. There are three types of seat belt currently being used in different situations, including lap and chest seat belt for car, lap seat belt for airplane and lap and shoulders seat belt for racing cars. The seat belts must meet several strict requirements to guarantee their function through the lifespan, for instance, a seat belt should be able to carry a static load of 1500 kg or 14.7 kN with a maximum extension of 25% 30% and needs to have excellent abrasion resistance, heat resistance, light resistance, light weight and flexibility of easy use. Conventionally, the seat belts are manufactured with high tenacity polyester and nylon continuous filament yarns. The conventional seat belts are useful and save lives for many years, but they have one disadvantage. In order to make a seat belt work properly, it has to be fastened tightly, which makes people feel uncomfortable especially when the seat belt is functioning. Due to the sudden decrease of the speed, the human body will be subjected to a huge pressure from the seat belt. This force could easily hurt human body. In some extreme cases, people’s ribs may be broken. If the seat belt is produced with high-performance auxetic fabrics, this problem may be resolved. As the auxetic fabric will be wider under stretching, the contact area between seat belt and human body will be increased. Hence, the impact pressure that the seat belt applies to human body will be decreased resulting in less possibility of hurting the human body.
10.7
Auxetic textiles for industrial applications
One of the most important industrial applications of the auxetic textiles is manufacturing composite materials which have been specifically introduced in Chapter 9, Auxetic fibre reinforced composites. The auxetic composites
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manufactured from auxetic fabrics or auxetic composites realised from structural design show enhanced mechanical properties in many areas, indicating that these composites can find various applications in industry such as making sensors, engine fine blades and vanes for gas turbine engine. In addition, there are other applications of auxetic materials in industry. One example is the auxetic fastener designed by Choi and Lakes [20]. The auxetic fastener can be installed more easily, but removal is more difficult than conventional materials. This auxetic fastener is designed a little larger than the hole within a tolerance in the radial dimension. When the fastener is being inserted to the hole, it will be pressed in axial direction; thus due to the auxetic effect the fastener will become smaller in the diameter, which makes it easy to be installed. After the fastener is installed and the compression force is removed from the fastener, the fastener will tend to expand in both axial and radial directions. This expansion effect locks the fastener in position tightly. When the fastener is needed to be removed from the hole, compressing it to make it decrease in the radial direction to achieve easy removal. The auxetic fastener has the advantage of simple press-fit insertion and easy removal.
10.8
Auxetic textiles for other applications
The auxetic textiles can also find applications in other areas. For example, the auxetic foams and fabrics can be used to create one type of smart filtration system [21 23]. This smart filtration system can be utilised in both air and water filtration. It allows the passing through the particles with certain size and the size can be changed through changing the loading level due to the pore-opening effect of the auxetic materials under tension. By using this smart filtration system the filtration level could automatically be changed through simply changing the loading level rather than changing the filtration pad [21 23]. The auxetic fabrics can be used as geotextiles as well because of the filtration function. The filtration function of geotextile may be the most widely known and used function [24]. The use of geotextiles can prevent aggregation of the particles under the road, which damages the road surface in raining days [24]. As shown in Fig. 10.5, the geotextile is used as a filter between the layers of the particles under the road so that liquids can pass through the plane of the geotextile while preventing the passage of soil particles [24]. Therefore the geotextile must have enough permeability to let the liquid pass through and controlled even pore size and pore size distribution to control the migration of the particles. The materials cross-plane permeability and pore size characteristics are the two most important properties of the geotextiles. If the auxetic fabrics are used as the geotextiles, clearly they can meet both requirements. The auxetic geotextiles can even adjust the speed of the liquid flow. Taking advantage of the pore-opening effect, in those extreme weather conditions such as rainstorm and flood, the filter will let more water pass through to reduce water on the road surface, which can reduce potential dangers to pedestrians.
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Figure 10.5 Filtration function of the geotextiles.
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[10] Yang W, Li Z-M, Shi W, Xie B-H, Yang M-B. Review on auxetic materials. J Mater Sci 2004;39(10):3269 79. [11] Alderson K, Simkins V, Coenen V, Davies P, Alderson A, Evans K. How to make auxetic fibre reinforced composites. Phys Status Solidi B 2005;242(3):509 18. [12] Jiang L, Gu B, Hu H. Auxetic composite made with multilayer orthogonal structural reinforcement. Compos Struct. 2016;135:23 9. [13] Steffens F, Rana S, Fangueiro R. Development of novel auxetic textile structures using high performance fibres. Mater Des 2016;106:81 9. [14] Steffens F, Oliveira FR, Mota C, Fangueiro R. High-performance composite with negative Poisson’s ratio. J Mater Res 2017;32(18):3477 84. [15] Engineering and Physical Sciences Research Council. Expanding blast-proof curtain will reduce impact of bomb explosions, ,http://www.epsrc.ac.uk/newsevents/news/ 2010/Pages/blastproofcurtain.aspx.; 2010 [accessed 10.02.12]. [16] Scarpa F, Giacomin J, Zhang Y, Pastorino P. Mechanical performance of auxetic polyurethane foam for antivibration glove applications. Cell Polym. 2005;24(5):253 68. [17] Allen T, Duncan O, Foster L, Senior T, Zampieri D, Edeh V, et al. Auxetic foam for snow-sport safety devices. Snow sports trauma and safety. Springer; 2017. p. 145 59. [18] Allen T, Martinello N, Zampieri D, Hewage T, Senior T, Foster L, et al. Auxetic foams for sport safety applications. Procedia Eng 2015;112:104 9. [19] Uzun M, Patel I. Tribological properties of auxetic and conventional polypropylene weft knitted fabrics. Arch Mater Sci Eng 2010;44(2):120 5. [20] Choi J, Lakes R. Design of a fastener based on negative Poisson’s ratio foam. Cell Polym. 1991;10(3):205 12. [21] Rasburn J, Mullarkey P, Evans KE, Alderson A, Ameer-Beg S, Perrie W. Auxetic structures for variable permeability systems. AIChE J 2001;47(11):2623 6. [22] Alderson A, Rasburn J, Evans K. Mass transport properties of auxetic (negative Poisson’s ratio) foams. Phys Status Solidi B 2007;244(3):817 27. [23] Alderson A, Rasburn J, Ameer-Beg S, Mullarkey PG, Perrie W, Evans KE. An auxetic filter: a tuneable filter displaying enhanced size selectivity or defouling properties. Ind Eng Chem Res 2000;39(3):654 65. [24] Hudson K, East G. Geotextiles; 1991, No. 6, ARRB.