Carbon nanotubes are excellent additives to impart electrical conductivity in plastics. Their high aspect ratio (length divided by diameter) of 1000 means that a very low loading is needed to form a percolating mixture in a polymer compared to materials with lower aspect ratios, such as carbon black, chopped carbon fiber, or stainless steel fiber. The use of a low loading of a very small nanotube as a conductive additive gives greater retention of the base polymers' toughness. It is important for automotive applications like fuel system components or exterior body panels that must be static dissipative but must also be able to withstand low temperature impacts without a catastrophic failure. For the electronics industry, the low loading of a very small nanotube gives a static dissipative part with a smooth surface that has much lower particulate sloughing. Particulates and static are very bad things in the manufacture of computer hard disc drives or in the manufacture of computer chips.
Work by Hyperion has shown that certain classes of what is typically thought of as a thermosetting elastomer can be compounded with our nanotubes to make a masterbatch. This masterbatch can then be let down and curing agents added to make a conductive elastomer. The principle advantage of these thermosetting elastomers is that they have better chemical resistance and a lower surface hardness than thermoplastic elastomers. O-rings for quick-connectors used in automotive fuel lines have a need for chemical resistance, low permeability to minimize evaporative losses and low surface hardness for good sealing and low insertion force. They are frequently made from a class of fluoroelastomers called FKMs. There is a growing mandate from the car manufacturers for a continuous conductive pathway in the fuel system from fuel tank to engine. Even if the fuel lines and the connectors on each end of the fuel line are conductive, an insulating O-ring will break the conductive pathway and possibly allow the accumulation of a static charge. Up until recently the issue has been that to make an O-ring conductive with carbon black involved unacceptable sacrifices in the part's performance.
Precix Inc., a major manufacturer of sealing systems for automotive, aircraft and industrial uses, has developed a nanotube-based conductive O-ring for automotive quick connects. Of particular interest is the improved resistance of the O–ring to gasoline permeation. Automobiles sold in America have been mandated to not only lower the emissions out the exhaust pipe, but also have to lower the total evaporative losses of gasoline from the entire car. A better sealing O-ring, with better barrier properties is critical to meeting these increased performance targets. Nanotube based material provides excellent gas barrier resistance. A surprising benefit of the use of nanotubes as the conductive additive is the change in resistivity with compression. Typically, as a conductive O-ring made with carbon black is compressed, the resistivity increases. It is thought that this is due to the breakage of the carbon black structure under compression, due to the strength of nanotubes.
The control of plastic's combustion with flame retardant (FR) additives is essential in many industries such as aircraft, building/construction, public transport, and electrical/electronics equipment. FR additives work by breaking one of the links that produce and support combustion: heat, fuel and air. The control of the toxic byproducts and smoke is also becoming a factor in assessing flame retardant additives. Increasingly, FR additives are used in combination, often with a synergistic effect. The search for non-halogenated FRs has led to nanoclays, one nm thick by 1000 nm. diameter. Initial research showed that the addition of as little as 5% of nano-sized clay particles could produce a 63% reduction in the flammability of polyamide 6. More recent studies have shown that flame retardancy in many other polymers can be boosted by dispersing clay at the molecular level.
Multi walled carbon nanotubes (MWCNTs) are the most reasonable ones for commercialization as a composite additive or coating element, as per Krzysztof Grzybowski . They offer less attractive properties, but they can be produced on a high scale at a relatively low price. In the past, their application area was limited to a few specific solutions, in which cost was not a driving factor (such as sport equipment). However, over the last two years, intensive scaling up of MWCNTs production has led to the dramatic price decrease down to below US$100/kg (depending on the shipment conditions). Current technical developments in nanotubes fabrication promise further increase in the production capacity and price reduction. In effect, one of the key players–Bayer Materials, has announced plans of increasing production capacity of MWCNTs from 60 tpa (status for the end of 2007) to 3000 tpa by the end of 2012. Despite of the fact that some of the market experts are skeptical with this prognosis, the planned 50 times increase in production capacity should be taken into account as one of the signals depicting the changes that are currently taking place in chemical industry mostly related to the application of nanomaterials.
While discussing about potential applications of MWCNTs we can depict some of them such as their applicability in polymeric composites in order to boost material mechanical endurance and thermal or electric conductivity. It was reported that a loading of 1% of MWCNTs distributed within polyethylene increased the material strain energy density by approximately 150% and the ductility by approximately 140%). Some scientists have recently proposed application of CNTs in fiber composites. They found that the tensile strength of a poly (vinyl alcohol) film tripled with the addition of 1 wt% of single wall nanotubes. According to the experiments, incorporation of 1 wt% of CNTs in polyacrylonitrile increased the tensile strength and modulus of the resulting material by 64% and 49% respectively). In scientists' opinion the reinforced material reduced its shrinkage by 50% and possessed raised temperature of melting by 40oC in comparison to the material without additive. Moreover, mechanical analysis of CNTs based composites suggests that embedded particles may effectively hamper the formation of cracks that can propagate and lead to fatigue failure. In addition, there are even unique applications such as an atomic force microscope cantilever tip material or antenna and tuner in fully functional radio. However, the potential usage of MWNTs as a flame retardant additive is an application that is becoming to be commercially and technically possible. Thus, CNTs-reinforced polymer or (metallic) is seen as a potentially fruitful area of new, tougher, and fatigue resistant material. Other additives like carbon nanofibers are few times (or one order bigger) than CNTs and can affect design material with its relatively huge sizes. Moreover, their price is also leveraged due to the increasing demand from aerospace sector.
Besides, the CNTs additive is also beneficial for materials' electrical properties by formation of conductive chains in the composites. Application of these particles in polyaniline impacts its electrical characteristics. During experiments with the metal-semiconductor devices fabricated from the polyaniline and carbon single wall nanotubes composites it was found that the current-voltage characteristic was significantly increased due to the nanotubes additive. The term of flame retardant additives refers to the components, which enable various materials to meet fire-safety requirements.
Current regulations are forcing producers from various industries (such as the household sector, electronics, building and construction, transportation, etc) to meet the more and more stringent standards referred to the products' usage safety and described by the normalized tests like UL-94 and many others. The need for fire resistant material is undoubted when fire statistic data is analyzed. For instance, in the US, the number of fires increased about 6% from 2004 right up to 2006. In addition, during this time the loss caused by these fires increased more than 15% up to a value of 11307 of millions of dollars. Many of the popular products that benefit from flame retardants include plastic enclosures for consumer electronics, printed circuit boards, wire and cable, electrical connectors, foam insulation, foam seating in furniture, automobiles, and textiles. Flame retardants can either be incorporated in the manufacture of structural plastics, such as ABS (acrylonitrile-butadiene-styrene) resins and also in foams and textile fibers or impregnated into timber and textile yarns. The global flame retardant chemicals market was 1.8 mln metric tons (as of 2006) and its value is now estimated below US$3 bln. These numbers include the major flame retardants groups: inorganic, bromine-based, chlorine-based, phosphorus-based, and nitrogen-based. In addition, this market still exhibits increasing potential. The compound annual growth rate is estimated at 4-5% for the total market up to 2015, with the highest growth seen in developing countries especially in the Asia-Pacific region, including China, India, and South Korea. There will be stable demand in established markets such as North America and Europe. As of 2006, the North American market was valued at US$780 mln, and the European market was valued at US$762 mln. This data confirms the market potential of carbon nanotube application in flame retardant systems.
One of the frequent questions about CNTs' usage as flame retardant is: why to use them for this purpose instead of other cheaper additives (such as carbon nanofibers). This could be because it was found that these particles provide material nonflammability with relatively small concentration compared to the other fillers. Carbon multi walled nanotubes usually possess diameters in the range from 50 to 150 nm and are of the length of few micrometers to few tens of micrometers. CNTs themselves are not flame retardants–they don't put out the fire. CNTs are less visible and less noticeable by the end user in the resultant material. Thus, they do not affect the material's design and should less influence its price. Besides, they often provide enhanced functionality to the material.
There are a number of studies investigating this material as a flame suppressing additive. So far, they have examined various sets of polymeric composites together with multi walled nanotubes composition. It was proved that 1-2% addition of these nanotubes results in the heat release rates decrease for polypropylene from 2800 kW/m2 to 700 to 800 kW/m2. In case of EVA polymer, the additive of 4.8% of MWCNT provided a decrease in the peak heat release rate from 475 to 400 kw/m2.
The carbon nanotube dispersed in the bulk composite acts as a flame retardant additive in several ways. Firstly, during a fire network of carbon nanotubes inside material is acting as a barrier for chemicals diffusion and thermal transport. During a fire carbon nanotubes provide a protective char on the material surface. Char of carbon nanotubes decrease the out of the material diffusion of evaporated chemicals and simultaneously the diffusion of oxygen from close environment toward the surface of the material. Moreover, the char made of carbon nanotubes provides the heat thermal barrier that decreases the rate of the heat transport. In effect, the melting of the material and its evaporization undergoes slower than in comparison to the unprotected one. During a fire rate of evaporized chemicals is lower and the fire intensity becomes significantly decreased.
Besides, in case of the intensive fire melting of the material and its transport toward the fire is also hampered due to the presence of the nanotubes. Scaffold of carbon nanotubes causes that convective movement that appear during a fire in molten material is reduced and the relevant mass and heat transfer ratios are also decreased.
Professor Kashiwagi depicted the importance of aspect ratio of CNTs particles on the composite's (polystyrene with MWCNTs) flammability. Generally, the higher aspect ratio of additive results in better flame retardant performance of resultant material (lower heat release rates and mass loss rates). According to the experimental results the effect of the size of MWCNTs is more dominant than the total number of nanotubes or their total surface area. For flame retardant applications, the nanotubes used in the composites should possess at least 50 to 100 aspect ratio values. In addition, the carbon nanotubes will also boost the properties of the composites such as mechanical strength, electrostatic discharge feature, electrical conductivity, and thermal conductivity. These features have a significant impact on the product performance and quality. Addition of MWCNTs can not only increase nonflammability of a nonhalogenated flame retardant polymer, but it can also enhance its thermal conductivity or provide electrostatic discharge effect. The potential additional benefits of CNTs application treated as added value properties cannot be ignored and often decides about the solution attractiveness and clients' acceptance. According to the popular among developers opinion such unique set of improved parameters is highly attractive for product market attractiveness and could help in making decision about the CNTs implementation within the product.
For flame retardant applications, the efficient concentration of nanotubes within designed material depends mostly on the particles structure, surface modifications, and state of additive dispersion and resin properties. The percolation threshold for multi walled carbon nanotubes was found to be in the range of approximately 0.0025%4). However, according to the experimental results we can say that the necessary concentration of nanotubes is in the range of 1-4%. Increasing popularity of carbon nanotubes result in their decreasing price. Current price of multi walled carbon nanotubes placed below US$100/kg should go down to the level of US$10-20/kg within next few years. At that level price, this additive will be highly attractive for various applications.
On the other hand, application of single walled carbon nanotubes is now rather not realistic for flame retardant nanocomposites, because of their low dispersive properties and high price. However, some latest experiments prove possibility of their high scale production.
The key issue for CNTs potential application in flame retardant solution refers to the proper homogenous dispersion. It was found and experimentally proved that material with bundles of CNTs is able to form, so called, empty “islands” that are free of these particles. Experiments proved that material failure as a flame retardant is mostly resulted due to its poor dispersion. However, this problem should be classified as a “know how” limitation and many producers have overcome it in their technologies. Besides, current CNTs producers are often selling them in the convenient to implement form of masterbatches that help end users to fully take advantage of CNTs' beneficial features. |
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