Composites MWNT/epoxy

Two case studies are presented in which polymer nanotube composites are proposed as replacements for conventional materials. We evaluate the technical and economic feasibility of using them as smart materials for strain gauges thus, exploiting their electrical properties, and as structural materials for aircraft panels bringing into play their mechanical properties. Our analysis shows that as new strain gauge materials, polymer nanotube composites offer many advantages. As a possible replacement for aluminum in an aircraft panel, it is found that a hybrid composite of (Epoxy 33% carbon fabric + 30% carbon fibers + 3% CVD-MWNT) is an attractive candidate. 

Similar trends are also observed for other functional groups. For instance, triethylene-tetramine (TETA)-functionalised MWNT-epoxy composites showed an increase in the Young s modulus of 38% and about 30%i in the tensile strength at very low nanotube loadings, which corresponds to dI7dFf of 355 GPa. In another work, MWNTs coated with silica and then functionalised with 3-methacryloxypropyltrimethoxysilane (3-MPTS) were added to polypropylene (PP). As expected, the 3-MPTS-functionalised MWNT-PP composite has a higher tensile strength than the pristine MWNT-PP... 

The first true mechanical study was made by Schadler et al. in 1998. They measured the stress-strain properties of a MWNT-epoxy composite during both tension and compression. In tension, the modulus increased from 3.1 GPa to 3.71 GPa on the addition of 5 wt% nanotubes, a reinforcement of d I7d Vf = 18 GPa. However, better results were seen in compression, with an increase in the modulus from 3.63 to 4.5 GPa, which corresponds to a reinforcement of 26 GPa. No significant increases in the strength of toughness were observed. The difference between tension and compression was explained by Raman studies which showed significantly better stress transfer to the nanotubes in compression than in tension. This can be explained by the fact that load transfer in compression can be thought of as a hydrostatic pressure effect, whereas load transfer in tension relies on the matrix-nanotube bond. However, it should be pointed out that later studies showed the reverse to be true, Le. load transfer in tension but none in compression.In further contrast, work by Wood et has shown that the mechanical response of SWNTs in tension and compression are identical. 

Alloui, A., Bai, S., Cheng, H., Bai, J. - Mechanical and electrical properties of a MWNT/epoxy composite . Composites Sci. Technol. 62 (2002) 1993-1998 Ruoff, R., Lorents, D. - Mechanical and thermal properties of carbon nanotubes . 

The filler route has proved to be very efficient to obtain isotropic composites with relatively large improvements in matrix properties at small mass (volume) fractions of nanocarbon. For example, electrical percolation in epoxy has been obtained with only 0.0025 wt% of multi-wall nanotubes (MWNTs) [12]. Similarly, a 2.7-fold increase in matrix modulus has been observed on addition of 0.6 vol% MWNTs to polyvinyl alcohol (PVA) [13]. Although more modest compared to the previous two examples, a... 

The results of Table 15.7 show that the hybrid composite (Epoxy 33% carbon fabric + 30% carbon fibers + 3% CVD-MWNT) gives the maximum cost saving and is, therefore, given top ranking. Of the two second best materials (Epoxy+20% CVDMWNT and Epoxy+33%carbon fabric+30% carbon fibers) the latter is a more... 

In a more recent work, MWNTs have been incorporated into surface-modified, reactive P(St-co-GMA) nanofibres by electrospinning. Then resulting nanofibres have been functionalised with epoxide groups and added to the epoxy matrix producing reinforced epoxy resins. The polymer composites have demonstrated over a 20% increase in flexural modulus, when compared with neat epoxy, despite a very low composite fibre weight fraction (at approximately 0.2% by a single-layer fibrous mat). The increase is attributed to the combined effect of the well-dispersed MWNTs and the surface chemistry of the electrospun fibres that enabled an effective cross-linking between the polymer matrix and the nanofibres. 

Figure 9.33 Frequency dependence of the reflection loss for PANI/MWNT-2/epoxy at various sample thickness (d = 1.0,2.0, 3.0,4.0, and 5.0 mm) in (a) 2-18 GHz and (b) 18-40 GHz range. Reprinted from Ref [60] with permission from RSC. Frequency dependence of the reflection loss for PANl/BaTiO epoxy composites in (c) 2-18 GHz and (d) 18-40 GHz range. Reprinted from Ref [121] with permission from Springer.

A trend of CTE similar to the latter results was obtained by TMA measurements performed on the MWCNTs infused through and between glass fiber tows along the through-thickness direction [74]. Both pristine and functionalized MWNTs were used in fabricating multiscale glass fiber-reinforced epoxy composites. The CTEs of the resin and the resin-fiber system were tested by TMA with a ramp rate of 5 °C/min. 

The lap-shear strength of carbon-fiber composites bonded using an epoxy adhesive was increased by 45% with the addition of 5% of MWNT (Hsiao et al. 2003). However, the locus of failure was also altered - from interfacial for the control specimens to cohesive for the nanotube-modified materials. 

Barrau and co-workers have found that amphiphilic palmitic add could be favorable for an efSdent dispersion of CNTs in an epoxy matrix. The hydrophobic part of palmitic acid was absorbed onto the surface of CNTs, whereas the hydrophilic head group induced electrostatic repulsions between CNTs, effeaively preventing their aggregation. The cosolvent has also been found to affect the dispersion of CNTs in polymer matrix. Very recently, Camponeschi a reported the use of trifluoroacetic acid as a cosolvent for the dispersion of MWNTs in a conjugated polymer poly (3-hexylthiophene) and PMMA via a solution process. SEM, optical microscopy, and light transmittance studies indicated that a better dispersion of CNTs in polymer matrices was obtained by using trifluoroacetic acid. Many other polymer composites such as polyurethane/CNT, PS/CNT, epoxy/ CNT, poly(vinyl alcohol)/CNT, " P(MMA-co-EMA)/ CNT, polyacrylonitrile/CNT, and polyethylene/CNT have also been fabricated by this method. ... 

As is well known to all, chemical functionalization disrupts the extended n conjugation of CNTs and consequently reduces the electrical conductivity of functionalized CNTs. Silane-functionalized CNT/epoxy nanocomposites exhibited lower electrical conductivity than did untreated CNT composites at the same content of CNTs. In 2005, Cho and co-workers observed that the electrical conductivity of the surface-functionalized MWNT composites was lower than that of the untreated MWNT composites with the identical content of CNTs. This is attributed to the increased defects in the lattice stmaure of carbon-carbon bonds on the surface of CNTs due to the acid treatment. Generally, the severe functionalization of CNTs can sigrtificantly lower the conductivity of the composites. However, several researchers have foimd that the fimaionalization of CNTs could improve the electrical conductivity of the composites. In 2005, Tambutri and co-workers reported that the functionalization of SWNTs with -COOH and -OH groups enhanced the conductivity of composites compared to untreated SWNTs. 

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