A wide range of studies have shown that carbon nanotubes (CNTs) either single-walled (SWCNTs) or multi-walled (MWCNTs) are among the most promising alternatives for traditional flame retardants to be used in different polymers, such as, PP, PE, PLA, lignocellulose, epoxy resin.84-88 These studies have demonstrated that adding a small amount of well-dispersed CNTs (<5 wt%) to polymer composites can significantly improve the fire behavior due to their fascinating chemical and physical properties.
CNTs have a highly elongated structure with a large aspect ratio. Their specific geometry enables them to create a strong protective network in the condensed phase to protect the underlying polymer from heat. This behavior could result in suppression of heat release rate (pHRR reduction in cone calorimetry) and weight loss rate during the combustion.85, 90 Moreover, low flame spread rate, smoke-suppression, and anti-dripping properties have been reported while incorporating CNTs in different polymers.
Improvement in fire retardancy of polymer composites is strongly dependent on uniform dispersion of CNTs and their interactions with the polymer matrix.84, 89 There are different surface modification methods and fabrication techniques, supporting CNTs homogenous distribution in polymers. Moreover, hybridization of CNTs with inorganic compounds or covalently bonded organic flame retardants has been investigated to improve their thermal stability. It is worth noting that polymers with aromatic rings in their structures, such as, polycarbonates are known to have a good interfacial adhesion with CNTs.
Chemical treatment of CNTs with concentrated sulfuric acid and nitric acid has proven to be successful when incorporated in different polymers, such as, polylactic acid or lignocellulose.86, 89 It was found that the incorporation of 2 wt% of acid-treated MWCNTs to lignocellulose/chitosan composites (LC/GC/MWCNT) can decrease the total heat release and the total smoke production by 10.7% and 45.5% compared with those of virgin polymer during cone calorimetry test. LC/GC/MWCNT composite also exhibited higher LOI values compared to neat polymers. Moreover, Yang et al showed that addition of 1 wt% acid-treated CNTs to PLA/calcium magnesium phytate composite promoted a significant reduction in peak heat release rate up to 35% and a char residue of 18.4%.
There are some studies that introduce melt-blending preparation methods without any primary treatments or modifications. Kashiwagi et al has reported a significant reduction in pHRR of polypropylene (PP) by addition of 1 vol% MWCNTs. PP/MWCNTs composites were prepared using direct melt blending without any primary modifications or additional additives.85 Rahatekar et al demonstrated the flame retardancy effect of MWCNTs in epoxy resin through uniform dispersion of nanofillers via high shear mixing.88 Almost 50% reduction in mass loss rate (MLR) was observed for Epoxy/MWCNTs composites compared with neat epoxy, using the gasification apparatus.88 In another work, Kashiwagi et al has investigated the importance of uniform dispersion of CNTs in PMMA matrix in fire performance of the resulting composite.92 SWCNTs were added to PMMA and dimethylformamide (DMF) suspension and permitted to disperse by bath sonication for 24 h. Uniform dispersion of SACNTs were controlled by their concentration in DMF. Cone calorimetric test results showed a significant reduction in pHRR for samples with 0.5 wt% uniformly dispersed SWCNTs in PMMA. On the other hand, PMMA containing 0.5 wt% poorly dispersed SWCNTs proved to have similar fire performance as the virgin PMMA.
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