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BY-NC-ND 3.0 license Open Access Published by De Gruyter January 16, 2014

Impact of aspect ratio and CNT loading on the dynamic mechanical and flammability properties of polyethylene nanocomposites

Selvin P. Thomas, Mostafizur Rahaman and Ibnelwaleed A. Hussein
From the journal e-Polymers

Abstract

In this study, the effects of aspect ratio and loading of multiwalled carbon nanotubes (MWCNTs) on the dynamic mechanical, thermal, and flammability properties of low-density polyethylene (LDPE)/MWCNT nanocomposites prepared by the melt blending technique were investigated. At low CNT loading, CNT with low aspect ratio acted as a plasticizer in LDPE. The storage modulus of the nanocomposites increased with the increase in aspect ratio and CNT loading. The increase in scan rate for the composites results in the decrease in total crystallinity, crystallization peak temperature, and a late onset of crystallization. The flammability properties like heat release capacity, peak heat release rate, and total heat release decrease with the increase in both aspect ratio and loading of CNTs in the composites.

1 Introduction

The reinforcement effect of carbon nanotubes (CNTs) on thermoplastic and thermosetting polymeric matrices has been reported in the recent years (1–5). The reinforcing contribution of nanotubes depends on their dispersed filler content within the hosting matrix, the state of dispersion, and on the interfacial adhesion between the matrix and the filler (6–9). The potentiality of carbon nanotubes as reinforcement for polymer matrix is primarily due to their exceptional mechanical properties, very high aspect ratio, and specific surface-to-volume ratio compared to other traditional reinforcement fillers (10–14). It is established that the aspect ratio of CNT greatly affects the mechanical properties of the resultant composites (15).

The effect of CNTs on the dynamic mechanical properties of polymer nanocomposites was studied by several authors (16–18). They have shown that the modulus increases with the increase in CNT loading in the composites. Cipiriano et al. (19) studied the dynamic rheological and flammability behaviors of polystyrene/CNT composites and found that CNTs with large aspect ratio impart much higher storage moduli and viscosities, and greater reduction in flammability to the composites compared with those with low aspect ratio (19). A similar phenomenon was observed by Pötschke et al. (20) on the dynamic rheological properties of polycarbonate/CNT composites. Luo et al. (21) investigated the influence of aspect ratio and volume fraction of CNTs as well as the end gap between two coaxial CNTs using the homogenization theory and effective fiber models on macroscopic and microscopic mechanical properties and concluded that the numerical results are similar to those of conventional short-fiber reinforced composites (21).

There are many literatures on the flammability behavior of CNT-filled polymer nanocomposites (22, 23). Takashi et al. (24) studied the flammability of polypropylene/multiwalled carbon nanotube (MWCNT) nanocomposites and showed that the heat release rate is greatly reduced by the addition of CNT in polypropylene.

The effect of aspect ratio and CNT loading on the mechanical properties (tensile strength, tensile modulus, elongation at break), thermal properties (differential scanning calorimetry), non-isothermal crystallization kinetics, shear and extensional rheology, electrical and dielectric properties of low-density polyethylene (LDPE)/MWCNT nanocomposites has been studied earlier (25–28). Based on these studies, it is clear that the final properties of CNT-based polymeric composites are controlled by the loading and aspect ratio of CNTs.

Previously, our group studied the influence of aspect ratio and CNT loading on the crystallization kinetics as well as the mechanical properties of LDPE/CNT nanocomposites (25, 26). In the present work, the LDPE/CNT nanocomposites were prepared with three different aspect ratios of CNTs by the melt mixing technique. The influence of aspect ratio of CNTs and filler loading on the dynamic mechanical, thermal, and flammability behavior of the composites was studied.

2 Results and discussion

The variation of storage modulus with CNT loading for low-aspect-ratio (L), medium-aspect-ratio (M), and high-aspect-ratio (H) composites at different temperature steps is shown in Figure 1. It is observed from the figure that, for L and M composites, initially the storage modulus, G′, peculiarly decreases at low loading of CNTs and, thereafter, there is the increase in storage modulus with the increase in CNT loading in the composites. But the storage modulus value is seen to gradually increase with the increase in CNT loading for H composites. The increase in G′ is due to the reinforcing effect of CNT in the composites. The possible reason behind the initial decrease in G′ at low CNT loading for L and M composites is explained in the next section. However, G′ decreases with the increase in temperature steps for all the composite systems. This can be attributed to the softening of composites at high temperature. This results in the lowering of storage energy with the increase in temperature, and thus G′ is decreased. Figure 2 shows the variation of tan δ (G″/G′) with respect to CNT loading at different temperature steps. There is a great increase in tan δ when CNT is added to the neat LDPE and, thereafter, the effect of CNT loading on tan δ is marginal and irregular. It is observed from the figure that tan δ increases with the increase in temperature steps. This increment in tan δ with the increase in temperature is likely due to the strong influence of temperature on G′ in comparison to G″.

Figure 1 Storage modulus vs. CNT loading at different temperature steps for L, M, and H composites.

Figure 1

Storage modulus vs. CNT loading at different temperature steps for L, M, and H composites.

Figure 2 Tan δ vs. CNT loading at different temperature steps for L, M, and H composites.

Figure 2

Tan δ vs. CNT loading at different temperature steps for L, M, and H composites.

The effect of frequency (0.1, 1, 5, 11 and 20 rad/s) on the G′ of L, M, and H composites at different CNT loadings (0, 0.1, 0.5, 1, and 2 wt%), and measured at different temperatures (30°C, 50°C, 70°C, and 90°C), is presented in Figures 35, respectively. It is observed from the figures that G′ increases almost linearly with the increase in frequency. Furthermore, the aspect ratio of CNT has a peculiar behavior on the G′ of the three types of composites as also mentioned earlier. For the L composites (Figure 3), the G′ values for 0.1 and 0.5 wt% CNT-loaded composites are lower compared to the neat LDPE polymer (except 90°C). In contrast, the other higher CNT-loaded composites have higher values of G′ compared to the neat LDPE polymer at all frequency ranges. Actually, at lower loading, CNT acts as a plasticizer for LDPE. A plasticizer is an additive that increases the plasticity/fluidity of a material by increasing the free volume. This makes the material more flexible and soft. This plasticization effect may also be observed owing to the presence of voids in the composite due to very poor adhesion at low loading of CNT. This leads to the decrease in G′ values for 0.1 and 0.5 wt% CNT-loaded composites compared to the neat LDPE polymer (29). In the case of the M composite system (Figure 4), the 0.1 wt% CNT-loaded composite shows a lower G′ value compared to the neat polymer, while all other loadings show higher G′ values. For the H composite system (Figure 5), G′ values at all loadings show higher values compared to the neat polymer. Thus it can be mentioned that the plasticization effect of CNT is less pronounced for M composites and finally disappeared for H composites. This suggests a strong reinforcing role of aspect ratio on the dynamic mechanical properties of these composites. The reinforcement role of CNTs, in addition, relies on the efficiency of load transfer from the polymer matrix to the filler CNT, i.e., the interfacial strength between the polymer matrix and the CNTs. It has been mentioned in the literature that cylindrical particles with a small diameter, high-aspect-ratio particles, build stronger interfacial strength with the polymer matrix (30, 31). Thus it can be said that stronger interfacial strength results in better load transfer from polymer matrix LDPE to filler particle CNTs. This is why composites with higher aspect ratio of CNT exhibit better storage modulus property compared to composites having a low aspect ratio of CNT.

Figure 3 Storage modulus vs. frequency at different CNT loadings for L composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

Figure 3

Storage modulus vs. frequency at different CNT loadings for L composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

Figure 4 Storage modulus vs. frequency at different CNT loadings for M composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

Figure 4

Storage modulus vs. frequency at different CNT loadings for M composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

Figure 5 Storage modulus vs. frequency at different CNT loadings for H composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

Figure 5

Storage modulus vs. frequency at different CNT loadings for H composites measured at temperatures of 30°C, 50°C, 70°C, and 90°C.

The effect of scan rate (5, 10, 15, and 20°C/min) on melting and crystallization onset and peak temperature for neat LDPE and L1 composite is shown in Figures 6 and 7 and Table 1, respectively. The results show that the previously mentioned parameters are influenced by the scan rate. With the increase in scan rate, the onset and peak crystallization temperatures shifted to lower values. The total crystallinity is found to decrease with the increase in scan rate. An almost similar observation has been made for other composites. The total crystallinity has increased with the addition of CNT in the composite.

Figure 6 DSC melting plots of neat LDPE and L1 composite measured at different scan rates.

Figure 6

DSC melting plots of neat LDPE and L1 composite measured at different scan rates.

Figure 7 DSC crystallization plots of neat LDPE and L1 composite measured at different scan rates.

Figure 7

DSC crystallization plots of neat LDPE and L1 composite measured at different scan rates.

Table 1

DSC results of neat LDPE and L1 composite at different scan rates.

Rate (°C/min)Neat LDPEL1 Composite
Tc onset (°C)Tc peak (°C)Xc (%)Tc onset (°C)Tc peak (°C)Xc (%)
599.6497.0138.03101.5397.7939.86
1098.2895.7737.10100.1895.9438.44
1597.4294.6836.7699.2494.8037.81
2096.7494.3036.6298.5393.7437.27

Tc, crystallization temperature; Xc, crystallinity.

The flammability of composite samples was measured using a pyrolysis combustion flow calorimeter, also known as a microscale combustion calorimeter (MCC). Figure 8 shows the heat release rate (HRR) curves of L composites with respect to temperature. All samples present a single peak of HRR between temperatures of 497°C and 505°C. A similar type of single peak was also observed for M and H composites. The abstracted primary parameters obtained by MCC are heat release capacity (HRC), peak heat release rate (PHRR), total heat release (THR), and reduct-MCC (Table 2). The HRC is defined as the ratio of maximum heat release rate to the constant heating rate in the test. This is one of the measures of the fire hazard of a material (32, 33). It is seen from the table that the HRC is influenced by the addition of CNTs in the polymer composites. The HRC is found to decrease with the increase in CNT loading in the composites. In fact, the thermal conductivity of CNTs is greater than that of the polymer matrix.

Figure 8 Heat release rate (HRR) curves of L, M, and H composites with respect to temperature.

Figure 8

Heat release rate (HRR) curves of L, M, and H composites with respect to temperature.

Table 2

Flammability properties of L, M, and H composites.

HRC (J/g K)PHRR (W/g)THR (kJ/g)Reduct-MCC (%)
LMHLMHLMHLMH
01118±51071±542.58±0.220
0.11104±51065±41064±41061±51027±41001±443.91±0.1842.95±0.1942.88±0.21147
0.51098±41045±41033±41052±4997±4992±443.04±0.1242.57±0.2042.45±0.14279
11082±41040±51014±41033±5994±4959±442.87±0.1542.18±0.1342.01±0.134812
21041±41032±41006±4998±4962±4789±441.69±0.1841.54±0.1941.35±0.1371126

Thus the addition of CNT assists in conducting the heat throughout the polymer matrix. With the increase in CNT loading in the polymer matrix, this heat conduction throughout the matrix is increased, which minimizes the localized decomposition. This leads to the decrease in HRC with the increase in CNT loading in the composites. HRR is one of the most important parameters to characterize the fire hazard (34). The PHRR decreases with the increase in CNT loading in the composites. However, the total heat release, which is the integral of the HRR curve over the duration of the experiment, decreases marginally with the increase in CNT loading in the composites. Reduct-MCC (%) is the percent deduction in PHRR of the composites with respect to neat LDPE. Table 2 shows that the percent reduction in PHRR increases with the increase in CNT loading in the composites.

The HRC, PHRR, THR, and % reduct-MMC are also affected by the aspect ratio of CNTs. With the increase in aspect ratio of CNT in the composites at any particular concentration/composition, the HRC, PHRR, and THR are seen to decrease as is evident from Table 2. Actually, the high aspect ratio of CNT facilitates the heat conduction inside the composites more easily compared to the low aspect one. Therefore, localized decomposition, which results in the decrease of HRC, PHRR, and THR, is minimized with the increase in CNT aspect ratio in the composites. The percent reduction in PHRR (% reduct-MMC) is higher for high-aspect-ratio CNT-filled composites.

3 Conclusions

The increase in storage modulus with the increase in CNT loading is due to the reinforcing effect of CNT in the composites. However, the decrease in storage modulus with the increase in temperature steps can be attributed to the softening of composites at high temperature. It is observed that tan δ values increase with the increase in temperature steps, and the storage modulus increases almost linearly with the increase in frequency. The aspect ratio of CNT also affects the storage modulus of the composites. High storage modulus value is observed for composites having a high aspect ratio of CNT. With the increase in scan rate, both the total crystallinity and the crystallization peak temperature are found to decrease. The flammability properties decrease with the increase in both aspect ratio and loading of CNT in the composites.

4 Experimental

CNTs with different aspect ratios were purchased from Cheap Tubes Inc. (US). Energy dispersive X-ray spectroscopy data were provided by the supplier. The data showed that all the CNTs have compositions of 97.7% C, 0.21% Cl, 0.56% Fe, 1.87% Ni, and 0.02% S. The three types of CNTs, used for the study, were designated as L (low), M (medium), and H (high). Their characteristics are as follows: L had an outer diameter of 30–50 nm, an inner diameter of 5–15 nm, and a length of 0.5–2.0 μm; M had an outer diameter of 30–50 nm, an inner diameter of 5–15 nm, and a length of 10–20 μm; and H had an outer diameter of 20–30 nm, an inner diameter of 5–15 nm, and a length of 10–30 μm. The aspect ratio for the three types of CNTs (L, M, and H) was 31, 375, and 800, respectively (28). The LDPE was procured from Nova Chemicals (Canada). LDPE has a weight average molecular weight of 99.5 kg/mol, molecular weight distribution of 6.5, melt flow index of 0.75 g/10 min, and total short branch content of 22 branches/1000 C as determined by gel permeation chromatography and nuclear magnetic resonance, respectively (35). The as-received LDPE resin and LDPE/CNT nanocomposites were blended in a Plasti-Corder torque rheometer (Brabender). The processing temperature used was 190°C at 60 rpm, and time of blending was 10 min. The loading of CNT varied from 0.1 to 2 wt%.

Dynamic mechanical analysis (DMA) was used to measure the storage modulus, G′, and the loss modulus, G″, of polymer nanocomposites in the solid state as a function of temperature and frequency. The dynamic mechanical properties of the polymer samples were measured using a dynamic mechanical analyzer (DMA; Q800) from TA Instruments, USA. The DMA tests were run in single cantilever mode (length of fixture 8 mm). The testing samples/specimens were prepared in carver press using a fixed size mould of dimension 8 mm in length, 10.1 mm in width, and 0.70 mm in thickness. The temperature step and frequency sweep tests were run in strain-controlled mode, where frequency was in the range 0.1–100 Hz and the strain was 15 micron. The temperature was varied in the range 30–90°C with a step of 20°C per frequency sweep.

Differential scanning calorimetry (DSC) measurements were performed using a Q1000 model DSC (TA Instruments, US) equipped with a liquid nitrogen cooling system and all auto sampler. Nitrogen at a flow rate of 50 ml/min was used to purge the instrument to prevent degradation of the samples upon thermal treatments. The DSC was calibrated in terms of melting temperature and heat of fusion using a high-purity indium standard (156.6°C and 28.45 J/g, respectively). Composite samples (5–10 mg) were sliced and compressed into a non-hermetic aluminum pans. Initially, the samples were heated from room temperature to 180°C at a rate of 5°C/min, followed by a hold-up at 180°C for 2 min. The samples were cooled from 180°C to 20°C at a rate of 5°C/min, and the second heating was done from 20°C to 180°C at a rate of 5°C/min. The later heating-cooling cycles were done at a scan rate of 10°C/min, 15 °C/min, and 20°C/min.

Small-scale flammability tests were carried out on the Federal Aviation Administration’s Pyrolysis Combustion Flow Calorimeter, and samples were tested in triplicate according to ASTM D7309-07. Samples were 5 mg (±0.5 mg) in weight and were obtained from the centre of the composite plaques detailed below. The heating rate was 60°C/min in an 80 cm3/min stream of nitrogen; the maximum pyrolysis temperature was 900°C. The anaerobic thermal degradation products in the nitrogen gas stream were mixed with a 20 cm3/min stream of oxygen prior to entering the combustion furnace at 900°C. The heat release was determined by oxygen consumption calorimetry. PHRR data were reproducible within ±0.5%.


Corresponding author: Ibnelwaleed A. Hussein, Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia, e-mail:

The authors would like to acknowledge KFUPM for supporting this research project.

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Received: 2013-9-24
Accepted: 2013-10-22
Published Online: 2014-01-16
Published in Print: 2014-01-01

©2014 by Walter de Gruyter Berlin Boston

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