Nanodispersions of carbon nanofiber for polyurethane foaming

Author's personal copyG. Harikrishnan et al. / Polymer 51 (2010) 3349e3353 3351a1000bG (Pa)Polyol-1%CNFMDI-1%CNFG (Pa)1000Polyol-3%CNFMDI-3%CNF10010010.0100ω (rad/s)100010.0100ω (rad/s)1000Fig. 2. Linear viscoelastic properties of CNF e component blends (a) 1% CNF in components (b) 3% CNF in components.3.3. Cell morphology and closed cell content of cured foamThe scanning electron micrographs in Fig. 3 show the cellmorphology of neat and CNF based foams. The cell size shows slightreduction by the addition of CNF. The corresponding cell densitiesare shown in Table 3. The potential nucleant density of filler duringheterogeneous bubble nucleation can be calculated by thefollowing equation [15].PND ¼ w r Pr compositeV P(1)where PND denotes potential nucleant density, w the weight fractionof the particle in the composite, r P is the density of the particle,r composite is the density of the polymer blend and V P is the volume ofindividual particle. According to above equation, the potentialnucleant density for 1% CNF in polyol (w 0.42% in foam) is1.17 10 12 /cm 3 . The observed cell density is 1.21 10 4 /cm 3 . In thecase of polystyrene foams, the potential nucleant density and theobserved cell density for 1% of CNF were 1.41 10 12 /cm 3 and2.78 10 10 /cm 3 , respectively [9]. This means that the nucleatingefficiency of CNF in reactive foaming of PU is significantly inferior tothat of non-reactive thermoplastic foaming. Fig. 3 (e) & (f) showsthe high magnification SEM micrographs of neat and nanocompositefoams. It is clear from Fig. 3(f) that the nanofibers arelocated on the cell windows of the foam.The closed cell content of both neat and nanocomposite foamsis essentially the same (Table 3). This indicates that CNF did notinduce cell opening and hence did not cause the loss of blowingagent. This is due to the fact that CNF particles are not antifoamingin nature, so that they do not induce film drainage andrupture. Particles which are phobic to polyurethane liquid matrixwill have de-wetting effect on cell window lamellae and thusaccelerates capillary drainage from the film resulting in coalescenceand rupture [16]. It has been shown that montmorillonitebased nano clays, owing to their antifoaming property, inducedpowerful cell opening in polyurethane foams, effectivelyincreasing the k-value and decreasing the thermal insulatingefficiency of the foam [6].3.4. Thermal conductivity, compressive modulus and fire resistanceThe thermal conductivity values shown in Table 3 fall in therange of k-values for commercial foams (0.01e0.02 W/mK). It isinferred that the thermal conductivity is reduced by 5.4% with theaddition of 1% CNF in MDI (w0.5% in foam), effectively increasingthe thermal insulating capacity of foam. This is much higher thanthe reported 2.8% reduction in k-value by the addition of up to10% of organically modified montmorillonite [3]. The reduction ink-value may be due to the combined effect of nucleation andreduction in radiative heat transfer by CNF. In a foam, the thermalconductivity comprises of contributions from various mechanismsof heat transfer such as conduction, convection and radiation. Inrigid polyurethane foam system, the contribution of convectiveheat transfer of blowing gas is shown to be insignificant [17,18].The effective thermal conductivity in a gas-solid two phase foamsystem k e (W/mK) could be represented as;k e ¼ k s þ k g þ 16sT3(2)33where, k s (W/mK) is the thermal conductivity of the solid, k g is thevapor phase thermal conductivity of gas phase, s is the StephaneBoltzmann’sconstant, T, the temperature (K), 3 (m 1 ) is theextinction coefficient. The third term of above equation denotes theradiative contribution to thermal conductivity (k r ). The rate ofradiative heat transfer contributes to about 25% of the total rate ofheat transfer in the foam [19]. Carbonaceous materials such ascarbon black have been shown to decrease the radiative heattransfer in PU foams by increasing the opacity to infrared radiationof the cell windows and struts and therefore decreasing the passageof infrared radiation through the foam [20].From Table 3, it is seen that the normalized (for density)compressive modulus of the nanocomposite foams increases. Sahaet al. also reported similar trend [11]. Regarding fire resistance ofthe foam, it is noted that the weight retention after the removal ofthe flame increases by 13% by the addition of 1% CNF in MDI (correspondingto 0.42% in foam) showing an enhanced fire resistanceof the nanocomposite foam.Table 2Kinetic profile of foaming.CNF (%wt) Gel time 2 (s) Rise time 3 (s)0 49 701% (Polyol) 47 711% (MDI) 46 684. ConclusionsNanodispersions of carbon nanofiber in polyurethane componentshave been prepared. Rheological estimations show thata filler loading as low as 3% in components is sufficient for makinga highly cross-linked network. Reactive foaming of these nanodispersionswas carried out to make polyurethane nanocomposite