An Overview of CNT—Kevlar Nanocomposites in the Application of Body Armor

Abstract The superlative mechanical properties of carbon nanotubes make them the filler material of choice for composite reinforcement. Carbon nanotubes have been considered to be one of the most researched materials of the 21st century. In this paper， carbon nanotubes reinforced Kevlar composite and its application in personal armor has been analyzed as well as the processing cost being evaluated. The advantages and disadvantages of the nanocomposite for the application were shortlisted. Current and future developmental trends for this application were discussed.

Key words Carbon Nanotubes；Kevlar；Reinforcement；Nanocomposite

中圖分類號：TQ050 文獻標識碼：A

Nanocomposite are multiphase solid materials where one of the phases has one， two or three dimensions of less than 100 nm， or structures having nanoscale repeat distance between the different phases. Nanocomposites are a wide range of materials having of two or more components. There is at least one component in the nm regime dimensions. The nanocomposites are always having outstanding high surface area to volume ratios. Furthermore， the interfaces act an important role in limiting or enhancing whole properties of a system. The mechanical， electrical， thermal， optical， electrochemical， catalytic properties of the nanocomposite will differ markedly from that of the component materials. During the recent years， one-dimensional nanocomposites are generally accepted as ideal systems for exploring a good number of novel phenomena at nanoscale and researching the size and dimensionality dependence of their functional properties. The field of one-dimensional nanocomposites such as nanotubes has attained a significant concern.

The carbon nanotube （CNT） is a famous member of one-dimensional nanocomposite family. It is an allotrope of carbon in cylindrical nanostructure with length-to-diameter ratio of up to 132，000，000：1 （Wang， 2009）. Nanotubes get their name for their long and hollow structure with walls formed by sheets of grapheme carbon. Nanotubes can be categorized as single-walled nanotubes （SWNTs） and multi-walled nanotubes （MWNTs）. Over last two decades， both of SWCNTs and MWCNTs have been researched extensively for their superior properties and wide range of applications than other materials. Current use and application of nanotubes has mostly been limited to the use of bulk nanotubes， which is a mass of rather unorganized fragments of nanotubes. Bulk nanotube materials may never achieve a tensile strength similar to that of individual tubes， but such composites may， nevertheless， yield strengths sufficient for many applications. Bulk carbon nanotubes have already been used as composite fibers in polymers to improve the mechanical， thermal and electrical properties of the bulk product. In this article， one of the special CNT reinforced material， CNT-Kevlar nanocomposite and its application in personal armor has been analyzed as well as the cost taken into consideration. The advantages and disadvantages of CNT-Kevlar nanocomposite for application in personal armor were identified. At last， the current and future developmental trends of CNT-Kevlar nanocomposite for application in personal armor were discussed.

1 The Application of CNT-Kevlar Composites in Personal Armor

Good stab-proof/bulletproof clothing could effectively stop the stab/bullet from penetrating the body. Therefore， personal armors demand a high mechanical strength. To build materials with superior strength and aw-tolerance， nanoscale reinforcements have natural advantages than their micrometer-sized counterparts because of their paucity of structural defects and high aspect ratio. A good example is carbon nanotube reinforced composites.

Using carbon nanotubes， there are possibilities for creating super-strong， high-performance polymer composite materials. Since carbon nanotubes are approximately 30 times stronger than steel but five times less dense （Gun'ko et al.， 2009）. This makes them the ultimate mechanical filler for reinforcing polymers， with very low densities， and Young moduli① superior to all other carbon fibers. However， a huge challenge still lies in the manufacturing of a high-performance nanocomposite because of the agglomeration tendency of the CNT llers and poor load transfer efficiency between the Kevlar matrix and the CNT reinforcements （Ma et al， 2009）. Essentially， the mechanical performance of CNT reinforced composites relies on the load-bearing status of the CNTs in the matrix. The two inherent problems shadow their promise as efficient load-bearers. It tends to self-agglomerate since by nature a multi-walled carbon nanotube with a diameter of 10 nm is always easier to be bent than a carbon fiber with a diameter of 10 m. The incorporation has poor load transfer efficiency because CNT atoms have smooth surfaces； their bonds to Kevlar are non-covalent which could result in ubiquitous interfacial slippages （Ma et al， 2009）.

To improve load-transfer efficiency， effort could be made on either strengthening CNT/polymer interface by grafting functional groups or design of load-transfer-favored structure （Ma et al， 2009）. The later measure has been applied in fabricating the CNT-Kevlar nanocomposite. Conventional techniques of this measure involve solution casting， melting， molding， extrusion， and in situ polymerization （Gun'ko et al， 2009）. In all of these techniques， CNT must either be incorporated into a polymer solution， molten polymer or mixed with the initial monomer before the formation of the final product （yarn etc.）. Thus， these cannot be applied in the case of insoluble or temperature sensitive polymers， which decompose without melting. Kevlar is not soluble in any common solvents， and without melting point decomposes above 400€癈 （Gun'ko et al， 2009）. As a result， Kevlar fibers must be produced by wet spinning from sulphuric acid solutions which is costly （Gun'ko et al， 2009）. The other way of incorporation is by swelling of Kevlar in a CNT suspension （Gun'ko et al， 2009）. Associate professor Gun'ko， a member of the College's Centre for Research on Adaptive Nanostructures and Nanodevices （CRANN）， together with his colleagues has discovered a stiff CNT-Kevlar composite could be produced in a CNT suspension in N-Methyl-2-Pyrrolidon.

By adopting the Gun'ko’s swelling method， the synthesizing cost could be lower down since it requires processing steps. In terms of the Carbon Nanotubes， the most common methods for CNTs synthesizing include CVD （Height， 2004）， laser vaporization and arc discharge method. Nowadays， purified CNTs and its composites of variable dimensions are already available from different manufacturers around the world in a reasonable price.

2 Summary of Merits Drawbacks

Advantages Disadvantages of CNT Reinforced Nanocomposites

3 Current and Future Developmental Trends

Traditional body armors take thick ceramic or metal plates as their panel， where the material is stiff enough that a bullet is not able to penetrate through. The ceramic or metal plates are used with a vest-shaped sheet of advanced plastic polymers made from Kevlar， Spectra Shielf or in other coutries， Twaron or Bynema. ②

The panel provides protection but not much comfort. It is placed inside of a fabric shell that is usually made from polyester or nylon. For bulletproof vest intended to be worn in especially dangerous situation， built-in pouches are sown to hold the plates. Typically this hard armor body offers more protection. However， it is more cumbersome due to its weight and rigidity.

In order to improve these body armors， number of methods by which nanocomposites are used include not only through Kevlar nano-coating carbon nanotubes but also using a nanocomposite based on tungsten disulfide nanotubes. Prior studies have reported that tungsten disulfide nanotubes were able to withstand shocks by a steel projectile up to 1.5km/s （Kurahatti et al.， 2010）. During the test， the material proved to be so strong that after the impact the samples remained essentially free from physical indentation. With the use of CNT reinforced materials， body armors providing proper protection and comfort would be produced. As nanotubes have very high strength due to its structure and double bonds that hold their carbon atoms together. Thus the use of carbon nanotubes as reinforcement of various matrixes such as Kevlar and tungsten is under further research. Future developments would result in producing light weight and strong materials to develop fabrics that could be used to produce stronger armors.

Some obstacles existing for practical applications of the technology includes the limiting factor of their length. By far manufacturers have only managed to create carbon nanotubes about one to two centimeters long. On the other hand， it is very costly to put into application and production of such body armors. Thus further research and development have to be conducted to increase tube-length and reduce the processing cost.

注釋

① Young moduli： the reflection of stiffness of a material.

② Bynema： A very strong and light synthetic yarn that is very tightly woven， layer upon layer （Kurahatti et al.， 2010）.

References

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