The individual clay layers are separated in a continuous polymer matrix by an average distance that depends on clay loading. Usually, the clay content of an exfoliated nanocomposite is much lower wt. Figure 33 shows TEM images of intercalated and exfoliated polymer-layered silicates nanocomposites Let us now look at other physical properties such as thermal stability, ionic conductivity gas barrier, flame retardancy, etc. These improvements were due to the decrease in permeability, usually observed in exfoliated nanocomposites, which hinders the diffusion of volatile decomposition products.
In this context, the silicate layers act as a superior insulator and mass transport barrier to the volatile products generated during decomposition. Fire relevant properties, such as heat release rate HRR , peak heat release rate PHRR , smoke production and CO 2 yield are vital to the evaluation of the fire safety of the material. As a result, the decreased flammability of nanocomposites is one of their most important features. Table 20 shows cone calorimeter data for three different kinds of polymers and their respective nanocomposites with MMT.
It can be seen that all MMT-based nanocomposites exhibited reduced flammability. In general, the flame retardant mechanism of nanocomposites involves a high-performance carbonaceous-silicate char, which builds up on the surface during burning and serves as a barrier to both mass and energy transport.
Studies of the fire retardant properties of exfoliated Nylon 2 wt. Clays are believed to increase the barrier properties by creating a maze or 'tortuous path' Figure 34 that retards the progress of the gas molecules through the matrix This system, containing only a small fraction of OMLS organically modified layered silicates , exhibited reduction in the permeability of small molecule gases, e.
For example, at 2 wt.
The relative permeability coefficient value, i. Data were fitted with the Nielsen theoretical expression There is a significant decrease in the O 2 permeability for the nanocomposites, which is more pronounced at higher clay contents.
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Results of this study are given in Figure 36 , where one can observe a drastic decrease in relative permeability with the increasing length of the clay. In other words, the best gas barrier properties will be obtained by fully exfoliated rather than long layered silicates. Consequently, the presence of spherical, plate, cylindrical, etc.
The reduction of permeability arises from the longer diffusive path that the penetrants have to travel in the presence of reinforcements. Nanocomposites systems have also shown to be able to increase the ionic conductivity of polyethylene oxide PEO This improvement is due to the fact that PEO is not able to crystallize when intercalated. This eliminates the presence of crystallites, which are non-conductive in nature.
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Another outstanding feature of polymer nanocomposites prepared with OMLS as reinforcements is their noticeably improved biodegradability properties. Finally, the nanocomposite is completely degraded after 2 months. This behaviour is accounted for the presence of terminal hydroxylated edge groups in the silicate layers, which start heterogeneous hydrolysis of the PLA matrix after absorbing water.
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Another way of increasing the biodegradability of resulting materials is by addition of natural fibres. The use of renewable resources for making biodegradable polymers and reinforcements for nanocomposites may lead not only to the achievement of desirable properties, but also to the replacement, in the near future, of polymers obtained from non-renewable sources.
This may help minimizing environmental degradation and waste disposal problems associated with the extensive use of the synthetic polymers in the world. It is to be noted that a persistent stress on use of high technology reinforcements, such as CNTs, in the development of polymer nanocomposites may be a limiting factor in view of their high cost.
In this scenario, the possibility of using large quantities of inexpensive natural nanofibrous materials may be explored. Such materials include the natural clay-serpentine group chrysotile, antigorite, lizardite, amesite , nanolayers of the kaolin group kaolinite, dickite, nacrite , ribbons of the sepiolite-paligorskite group sepiolite, paligorskite , imogolite of volcanic origin and other minerals with graftable surfaces Further, synthetic materials made from the double hydroxide or hydroxyl salt groups with layered or fibrous structures, and even graftable single hydroxides having low cost and involving very common elements, will also play an important role in the future.
In addition, vegetable fibres disposed of as agricultural production waste can be also reduced to nanosize range materials and used as reinforcement agents Even in such non-graftable reinforcement surfaces, one can choose an appropriate chemical treatment and moulding temperature and carefully select the polymer to arrive at the best chemical compatibilization, leading to the optimized properties of the resulting materials. It is observed that these treatments usually improve the surface adhesive characteristics of the vegetable fibres through the removal of non-crystalline components such as lignin and hemicelluloses.
Carbon nanofibres, which are known to range from disordered "bamboo-like" to highly ordered "cup stacked" graphite structures, and have diameters in the range of nm, have been employed as reinforcing materials in a variety of polymer matrix compounds, including both thermoset polymers such as epoxy, polyimide and phenolic, and thermoplastic polymers, as polypropylene, polystyrene, PMMA, Nylon 12, and PEEK , Properties of nanotubes have already been given in Table Young's modulus and tensile strength of the PP nanocomposites were found to increase due to the good adhesion of oxidised carbon fibres, but stiffness showed a reverse trend with the increasing volume fraction of the fibres.
Even compressive strength and torsional moduli of the nanocomposites were found to be higher than that from the PEEK matrix. These scanty results on nanofibre-containing polymer composites reveal a need for more detailed and systematic studies on the dispersion and adhesion aspects, considering the varying morphologies of the fibres. Thostenson and Chou , have described the elastic properties of aligned MWCNTs in a polystyrene matrix nanocomposite system. Adopting 'micromechanics' approach, they have derived the axial elastic properties of CNTs considering equivalent effective fibre properties, viz.
This gives a relationship between Young's modulus and various dimensions of fibres, including their volume. Similar relationships of physical and mechanical properties would probably help in producing materials for specific applications. Significant toughening of polymer matrices after CNT incorporation has been reported. SEM images of brittle tensile fracture surfaces showed fairly uniform nanotube distribution and nanotube pullout In this system, the nanotubes were highly dispersed in the polymer matrix.
Finally, silicone elastomer-SWCNT reinforced composites have shown dramatic increases in both stiffness and strength Coming to the morphological studies of CNT-containing polymer nanocomposites, Figure 40 represents a TEM image of polystyerene containing 5 wt.
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Large-scale dispersion and alignment of carbon nanotubes in the polymer matrix can be observed from this image, which could explain the observed properties. Also, they remain curved in the nanocomposite as a result of their sharp flexibility To conclude, CNT-polymer composites, though may pose greater challenges for the Materials Scientists, will exhibit unique properties as mentioned above and hence offer greater potentials in terms of applications.
This is presented in the next Section.
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Applications of Nanocomposites 2,,,,, From the foregoing, it becomes evident that nanocomposites may provide many benefits such as enhanced properties, reduction of solid wastes [lower gauge thickness films and lower reinforcement usage] and improved manufacturing capability, particularly for packaging applications.
Tables 21 to 23 present potential applications of ceramic-, metal- and polymer-based nanocomposites, respectively. As it can be observed, the promising applications of nanocomposite systems are numerous, comprising both the generation of new materials and the performance enhancement of known devices such as fuel cells, sensors and coatings.
Although the use of nanocomposites in industry is not yet large, their massive switching from research to industry has already started and is expected to be extensive in the next few years. In this case, a novel method, which employs vacuum arc coating with a rotating cathode, is used for commercial production.
Similarly, one of the leading application areas is the automotive sector, with striking impact due to improved functionalities such as ecology, safety, comfort, etc. Details on the commercial usage of nanocomposites in automotives and future developments in this sector including CNT-based nanocomposites are now available For instance, there are reports on the current use of a number of nanocoatings in different parts of Audi, Evobus and Diamler Chrysler automobiles, as well as ongoing trials on fuel cells, porous filters foams and energy conversion components, which include nanoTiO 2 -containing paints.
Additionally, light weight bodies made of metal- or polymer-based nanocomposites with suitable reinforcements are reported to exhibit low density and very high strength e. Also, two-phase heterogeneous nanodielectrics, generally termed dielectric nanocomposites, have wide applications in electric and electronic industries Metal and ceramic nanocomposites are expected to generate a great impact over a wide variety of industries, including the aerospace, electronic and military , while polymer nanocomposites major impacts will probably appear in battery cathodes 6, , microelectronics , nonlinear optics , sensors , etc.
These are brought out mainly by the nanosize reinforcements used, which result in an appropriate morphology for the products. Tables 21 and 22 summarize the possible developments associated with these materials in catalysts, sensors, structural materials, electronic, optical, magnetic, mechanical and energy conversion devices suggested by researchers in the field. CNT-ceramic composites, on their turn, are reported to be potential candidates for aerospace and sports goods, composite mirrors and automotive spares requiring electrostatic painting.
One example is the Al 2 O 3 -CNT composite, which shows high contact damage resistance without a corresponding increase in toughness and hardness. It is reported 92 to be a candidate for engineering and biomedical applications. Despite these possibilities, there are only limited examples of industrial use of nanocomposite, mainly due to the challenges in processing and the cost involved, particularly for non-structural applications. In fact, one recent review deals with various methods for the preparation of super hard coatings with merits and demerits of each method.
However, the intense research in both metal- and ceramic-based nanocomposites suggests that the days are not far off when they will be actually in use. The cost factor may be a particularly serious problem for general engineering applications, while this may not be the case for specialized applications in electronics, aerospace, biomedical and other sectors, since the advantages might far outweigh costs and concerns in these sectors.
On the other hand, polymer-based nanocomposites are in the forefront of applications due to their more advanced development status compared to metal and ceramic counterparts, in addition to their unique properties. These include fold strength property increase, even with low reinforcement content wt. In this case, nanocomposites, which exhibit better gas barrier properties, can provide a longer shelf life. Such packaging, with different matrices and reinforcements, as well as different processing conditions, is being field tested by the US army since to arrive at an optimum combination.
Various types of polymer-based nanocomposites, containing insulating, semiconducting or metallic nanoparticles, have been developed to meet the requirements of specific applications. Recently, some PLS nanocomposites have become commercially available 18 , being applied as ablatives and as high performance biodegradable composites ,,,, , as well as in electronic and food packaging industries , These include Nylon-6 e. Durethan LDPU60 by Bayer Food Packages 18 and polypropylene for packaging and injection-molded articles, semi-crystalline nylon for ultra-high barrier containers and fuel systems, epoxy electrocoating primers and high voltage insulation, unsaturated polyester for watercraft lay-ups and outdoor advertising panels, and polyolefin fire-retardant cables, electrical enclosures and housings.
Table 24 shows some examples of commercially available polymer nanocomposites. Some of the products made from nanocomposites are shown in Figure Technological contributions in the areas of gas barrier, reinforcement and flame retardancy have also been extensively exploited , For example, heat-resistant polymer nanocomposites are used to make fire fighter protective clothing and lightweight components suitable to work in situations of high temperature and stress. This includes hoods of automobiles and skins of jet aircrafts, as opposed to heavier and costlier metal alloys. They can also replace corrosion-prone metals in the building of bridges and other large structures with potentially lighter and stronger capabilities , Also, unsaturated polyester UPE nanocomposites can be employed in fibre-reinforced products used in marine, transportation and construction industries Regarding the variety of applications of polymer nanocomposites, prominent impacts over the automotive industry can be highlighted, including their use in tyres, fuel systems, gas separation membranes in fuel cells and seat textiles, mirror housings on various vehicle types, door handles, engine covers, intake manifolds and timing belt covers , , with some of these already being exploited.
For example, a thermoplastic nanocomposite containing nanoflake reinforcements trade name Basell TPO-Nano is being employed for the development of stiff and light exterior parts, like the step-assists by GM Also, porous polymer nanocomposites can be employed for the development of pollution filters Other promising technological application in the horizon is in air bag sensors, where nano-optical platelets are kept inside the polymer outer layer for transmitting signals at speed of light gaining milliseconds to bring down the level of possible impact injuries These include impellers and blades for vacuum cleaners, power tool housings, and mower hoods and covers for portable electronic equipment, such as mobile phones and pagers Another example is the use of polymer nanocomposites in glues for the manufacturing of pressure moulds in the ceramic industry.
The development of environmentally friendly, non-foil and better packaging materials can reduce the amount of solid waste, improve package manufacturing capabilities, and reduce the overall logistics burden to users. In this context, the incorporation of nanoclay particles into thermoplastic resins has shown to be highly effective to improve barrier properties and package survivability Such excellent barrier characteristics have resulted in considerable interest in clay nanocomposites in food packaging applications, both flexible and rigid. Specific examples include packaging for processed meats, cheese, confectionery, cereals and boil-in-the-bag foods, also extrusion-coating applications in association with paperboard for fruit juice and dairy products, together with co-extrusion processes for the manufacture of beer and carbonated drinks bottles The use of nanocomposite formulations would be expected to enhance considerably the shelf life of many types of food.