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Posted: Aug 30th, 2007
Flame-retardant materials with more nanotechnology and less toxic chemicals
(Nanowerk Spotlight) Firefighters and stuntmen certainly appreciate the fire resistant capabilities of modern textiles. Going far beyond such niche use, flame retardant materials have become a major business for the chemical industry and can be found practically everywhere in modern society. If you live in a country where houses are mostly built from wood (like in the U.S.; where, on the other hand, the things that used to be wood are now plastic - like christmas trees; flame retardant ones of course) most structural timber and wood elements such as paneling are treated to make them more fire resistant. Plastic materials are replacing traditional materials like wood and metal - just look at the toys you played with and the ones your kids have today. Unfortunately, the synthetic polymeric materials we group under the term 'plastic' are flammable. To decrease their flammability they require the addition of flame-retardant chemical compounds. The plastic casings, circuit boards and cables of your computers, electrical appliances or car are flame retardant. So is practically every material in airplanes, trains and ships from the fabric of seats to every kind of plastic structure found onboard. Name any plastic product and chances are it has been made flame retardant. Conventional methods for making plastic flame retardant involve a range of not exactly harmless chemicals. Improving the flame retardancy of polymeric materials without the use of toxic chemicals could now become possible thanks to the synergistic effect of carbon nanotubes and clay.
ABS resins (acrylonitrile-butadiene-styrene) are a good example of an ubiquitous industrial plastic material used in a variety of diverse applications such as appliances (where 23% of all produced ABS is used), transportation (21%), piping (13%), electrical and electronic components (11%), medical applications (4%), and miscellaneous applications including, toys, luggage, lawn and garden products, shower stalls, furniture and ABS resin blends with other polymers (28%). ABS is the largest-volume engineering thermoplastic resin. It is positioned between commodity plastics (e.g., polypropylene or polystyrene) and higher-performing engineering thermoplastics (e.g., polycarbonate or polyurethane). Global demand for ABS resins is estimated to be almost four million tons per year.
Flame retardants can either be incorporated in the manufacture of structural plastics such as ABS but also foams and textile fibers or impregnated into timber, textile yarns etc. They can also be added as protective coatings. The main chemical families of flame retardants (FRs) are inorganic chemicals (including antimony, aluminum and tin compounds), bromine and chlorine based FRs, phosphorus based FRs, and nitrogen (melamine) based FRs. The use of some of these chemicals in FRs has come under increased scrutiny due to their suspected negative health and environmental impact. Especially the use of brominated flame retardants in polymer formulations has come under a lot of fire (so to speak). Firefighters, for instance, are at risk from the potentially toxic chemicals that are created when products containing FRs burn.
In response, individual European countries, such as Norway, Germany and Sweden started to require companies to replace brominated FRs with safer alternatives. To harmonize efforts in Europe, the European Union banned the use of all polybrominated diphenyl ethers (PBDEs) and polybrominated biphenyls (PBBs) in electronic products starting in 2006. The chemical industry lobby in the U.S. so far has successfully prevented any nation-wide legislation on this issue. In April, Washington has become the first state in the nation to ban PBDEs in specific situations. PBDEs are facing bans in California by 2008.
Therefore it has come handy that researchers have been able to modify the flammability properties of polymers with carbon nanotubes (CNTs). Previous research has shown that nanoparticle fillers are highly attractive for the purpose of making a material more flame retardant, because they can simultaneously improve both the physical and flammability properties of the polymer nanocomposite. It also has been shown that CNTs can surpass nanoclays as effective flame-retardant additives if they form a jammed network structure in the polymer matrix, such that the material as a whole behaves rheologically like a gel ("Nanoparticle networks reduce the flammability of polymer nanocomposites").
Both CNTs and clay can improve the flame retardancy of plastics. Their flame retarding mechanisms are different and their effect for improving the flame retardancy is limited when they are used alone. However, when they are used together, a significant synergism happens.
"The common explanation of the synergistic effect between clay and carbon nanotubes on improving flame retardancy of polymers is that carbon nanotubes act as a sealing agent in final chars with network structure after combustion" Prof. Zhengping Fang explains to Nanowerk. The flame retardancy of polymer nanocomposites is strongly affected by the formation of a network structure.
"But how is this network formed?" asks Fang "If formed, does it form during the first stage or during combustion? How does the network structure affect the flammability properties of nanocomposites?"
Scanning electron microscopic images of the chars of ABS nanocomposites after cone calorimeter tests: ABS/2 wt% clay (A), ABS/2 wt% MWCNTs (B), ABS/1 wt% clay/1 wt% MWCNTs (C). A thick, relatively intact char layer with lower crack density was formed for ABS/clay/MWCNTs nanocomposites (C) than the individual filled samples (A) and (B). For chars of ABS/clay/MWCNTs nanocomposites, it is found that many MWCNTs run across between clay layers, indicating a strong interaction between clay and MWCNTs. (Reprinted with permission from IOP Publishing)
Fang points out that in previous studies a synergistic effect has been found on improving flame retardancy in polymer nanocomposites. "The existence of clay enhances the graphitization degree of multiwalled CNTs (MWCNTs) during combustion. Clay assists in the elimination of dislocations and defects and the rearrangement of crystallites. Al2O3, one of the components of clay, acts as the catalyst of graphitization. Besides, the co-existence of clay and carbon nanotubes can enhance the network structure in polymer matrixes."
"The coexistence of clay and MWCNTs in the composites can form a more effective confined space and enhances the network structure, which can be responsible for the improved flame retardancy for polymer nanocomposites in our study" says Fang. " Basically, because coexistence of clay and MWCNTs can form more effective confined geometry, the ABS/clay/MWCNTs nanocomposite can form network structures in lower temperature and protect the polymer matrix more efficiently."
What will be interesting to industrial manufacturers of ABS and other synthetic polymeric materials is that the synergistic effect between clay and carbon nanotubes can reduce the amount of flame retardant chemicals added to flame retarded materials. "Due to the excellent barrier properties of clay and tensile strength of carbon nanotubes we expect to obtain flame retardand materials with high performance characteristics and with much less use of potentially toxic chemicals" says Fang.
Traditionally, to improve the dispersion of nanofillers (such as clay and CNTs) in a polymer matrix, a surfactant was grafted onto the nanofillers. This of course reduces the flame retarding effect of the nanofillers because the surfactants themselves are usually flammable. Thus, one of the future directions of Fang's research could focus on improving the dispersion of nanofillers in polymer matrix without reducing their flame retarding effect.
As with so many other interesting findings we introduce you to here at Nanowerk, Fang's research about this new nanocomposite (with ABS as the matrix) is in its initial phase and much more work needs to be done before such materials get close to industrial use.