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Journal of Nanomaterials
Volume 2012 (2012), Article ID 250364, 7 pages
http://dx.doi.org/10.1155/2012/250364
Research Article

Thermal Effect of Ceramic Nanofiller Aluminium Nitride on Polyethylene Properties

1Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, P.O. Box 5050, Dhahran 31261, Saudi Arabia
2Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
3Polymers Research, Operations & Maintenance, Saudi Basic Industries Corporation, Riyadh 13244, Saudi Arabia
4Center of Research Excellence in Petroleum Refining and Petrochemicals (CoRE-PRP), King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Received 2 June 2012; Accepted 2 July 2012

Academic Editor: Chunyi Zhi

Copyright © 2012 Omer Bin Sohail et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Ethylene polymerization was done to form polyethylene nano-composite with nanoaluminum nitride using zirconocene catalysts. Results show that the catalytic activity is maximum at a filler loading of 15 mg nanoaluminum nitride. Differential scanning calorimeter (DSC) and X-ray diffraction (XRD) results show that percentage crystallinity was also marginally higher at this amount of filler. Thermal behavior of polyethylene nanocomposites (0, 15, 30, and 45) mg was studied by DSC and thermal gravimetric analyzer (TGA). Morphology of the component with 15 mg aluminium nitride is more fibrous as compared to 0 mg aluminium nitride and higher filler loading as shown by SEM images. In order to understand combustibility behavior, tests were performed on microcalorimeter. Its results showed decrease in combustibility in polyethylene nanocomposites as the filler loading increases.

1. Introduction

In the world of thermoplastic, polyolefins find wide range of application because of its unique properties such as low cost, light weight, high strength, durability, noncorrosive nature, and ease in processability. Among the polyolefins, polyethylene is widely used for variety of applications.

In earlier studies, polyethylene (PE) was synthesized using metallocene catalysts [1]. Wherein methyl aluminoxane (MAO) was used as the cocatalyst [2]. In regard to catalytic activity in polymerization of ethylene, metallocene catalysts provided more flexibility than Ziegler-Natta catalysts, and are also in use for many industrial purposes [38].

Several studies have been made to improve the various properties of polyethylene by the addition of organic and inorganic fillers [913]. Among these fillers, inorganic fillers increase the thermal properties of the polyethylene significantly [1418]. In the last two decades, among various inorganic fillers, aluminium nitride (AlN) is of special interest due to its unique thermal properties [19]. Earlier Yu et al. [20] studied the thermal conductivity and thermal stability of the nano-AlN-filled cycloaliphatic epoxy/trimethacrylate. The effect of microsized (≤10 μm) AlN on the properties of polyether ether ketone (PEEK) prepared by solution blending was studied by Boey et al. [21] who reported that AlN can act as a good nucleating agent in the crystallinity of the polymer. Incorporation of AlN having (≤10 μm) in polystyrene also increased the thermal conductivity and high thermal conductivity was obtained for the composites having 20 wt.% filler [22].

The present work reports studies on the thermal properties of polyethylene nanocomposites prepared by using metallocene catalysts in the presence of AlN. Organic polymers are combustible, and flame retardants are used to suppress the combustible process [23, 24]. Since AlN provided better thermal behaviour within the polymer matrix, therefore in the present paper, effect on combustible behaviour was also noted with the help of microcalorimeter.

2. Experimental

2.1. Materials

AlN having size less than 100 nm, zirconocene (catalyst), Toluene, and cocatalyst MAO were purchased from Aldrich Chemicals and kept in oxygen-free environment to avoid any contamination.

2.2. Ethylene Polymerization

Polymerization of ethylene was performed in a 250 mL round-bottom flask equipped with a magnetic stirrer. The zirconocene catalyst (6 mg) and required amount of nano-AlN (15, 30, and 45) mg were added to the flask. The reactor was charged with toluene coming from solvent purification system so that any of the contaminations might not affect the catalysts and co-catalyst. MAO was used as a cocatalyst and as scavenger. Catalyst, cocatalyst, and solvent placement within the reactor were done inside a glove box. Then reactor was taken out from glove box and immersed in a constant temperature bath previously set to a desired temperature. After ensuring that bath temperature and reactor temperature were the same, ethylene was introduced into the reactor through an external chamber after evacuating nitrogen gas using vacuum. Polymerization was quenched after 30 minutes by adding methanol. The contact time between AlN and catalytic system prior to polymerization was 3 minutes. The polymer was washed with an excess amount of methanol and dried in an oven at 50°C. The conditions used for the study are given in Table 1.

tab1
Table 1: Experimental conditions used for the preparation of polyethylene through insitu polymerization by using zirconocene and MAO co-catalyst system.
2.2.1. Activity

Activity of the catalysts is measured by weighing the product and was noted as ratio of amount of product (polyethylene nanocomposite) to amount of catalyst consumed.

3. Characterization

3.1. Differential Scanning Calorimeter (DSC)

The melting and crystallization behavior of the composites were determined by using DSC-Q1000, TA instruments. To overcome the thermal history, heating and cooling was done for both first and second cycles under nitrogen atmosphere at the rate of 10°C/min and 5°C/min, respectively, from a temperature of −10°C to 200°C, and third cycle was performed at a rate of 10°C/min under nitrogen atmosphere and is analyzed in this study.

3.2. Wide-Angle X-Ray Diffraction (WAXD)

Measurements were obtained through Shimadzu X-ray diffractometer (40 kV, 40 mA) using Ni-filtered Cu K𝛼 radiation from 10 to 80 (in 2𝜃) with 15 s standing per step, and crystallinity was determined [25].

3.3. Thermal Gravimetric Analysis (TGA)

Thermal degradation studies were performed by thermogravimetric measurements using SDT Q600 (TA instruments). Samples weighing approximately 5 mg were heated in nitrogen atmosphere from 25° to 650°C at a heating rate of 10°C per minute.

3.4. Scanning Electron Microscopy (SEM)

The polymer surface was gold coated, and then surface morphology was studied by using scanning electron microscopy (LYRA3GM, TESCAN).

3.5. Microcalorimeter (MC)

Heat release rate (HRR) and ignition temperature were determined using micro-calorimeter (FTT Microcalorimeter), and sample size was kept at 3 mg.

4. Results and Discussions

4.1. Activity

Figure 1 represents the variation in the activity of synthesized polyethylene under various experimental conditions. It was found that the activity of polymerization reaction was higher for the composites having 15 mg of nanofiller (Table 1, entry no. 2) which were prepared using 5 mL of cocatalyst at 30°C. However, further increase in filler content does not improve the activity of polymerization compared to the control (Table 1, entry no. 3 and 4).This observation is similar to that observed earlier in case of Mn-doped-titania on the activity of metallocene catalyst by in situ ethylene polymerization [25]. In order to verify the influence of temperature on activity, reaction was conducted at 60°C (Table 1, entry no. 5 and 6). It is clear that at high temperature (60°C) activity of reaction increased (entry no. 6) when compared to its control (entry no. 5). However at 60°C, activity of the catalysts is less than the activity obtained for the reaction at 30°C (Table 1). Reduction in the amount of cocatalyst also increased the activity of the reaction with an incorporation of the same amount of filler (i.e., 15 mg, entry no. 8) as compared to its control (entry no 7).

250364.fig.001
Figure 1: Activity of synthesized polyethylene nanocomposites at different experimental conditions.
4.2. Differential Scanning Calorimeter (DSC) and Wide-Angle X-Ray Diffraction (WAXD)

Figure 2(a) displays the DSC thermograms (heating curve) of polyethylene composites as a function of the amount of AlN. The Δ𝐻fus (heat of fusion) and 𝑇𝑚 (melting temperature) values along with the percentage of crystallinity for the blends as a function of AlN content are summarized in Table 2. It can be seen that as the amount of nano-AlN increases, 𝑇𝑚 value shifts slightly towards the lower temperature. This can be attributed to the reduction in the lamellar thickness of crystallites imparted by the presence of AlN in the matrix as observed earlier in case of AlN-reinforced HDPE composites [26]. The percentage of crystallanity was calculated using the following expression as shown in Table 2: %ofcrystallinity=Δ𝐻fusΔ𝐻0fus×100,(1) where Δ𝐻fus is the enthalpy of fusion of the polyethylene composites, and Δ𝐻0fus is the enthalpy of fusion of the 100% crystalline polyethylene. Δ𝐻0fus of polyethylene was taken as 293 J/g [27]. Incorporation of 15 mg of AlN increased the percentage of crystallinity of the polyethylene, and at higher filler loading, it shows a deceasing trend. This increase is attributed to the heterogeneous nature imparted by the nanofiller, which results in an increase in the crystallinity of the composites [28, 29]. An increase in the crystal nucleation in the region surrounding the reinforced particles also attributes to increase in the crystallinity of the composites [30, 31]. However, at higher filler loading, agglomeration of the nanofiller may occur and a reduction in the mobility of the polymer chains with consequent decrease in the crystallite size and hence a reduction on the percentage of crystallanity [3133].

tab2
Table 2: DSC, XRD and GPC results of AlN filled polyethylene composites.
fig2
Figure 2: (a): DSC heating curves of nano-AlN-filled polyethylene composites. (b): X-ray diffraction (XRD) intensity versus 2-theta (deg).

Higher melting temperature in HDPE/15 mg AlN nano-composite can be supported by the results of GPC (gel permeation chromatography). Molecular weight (𝑀𝑤) of HDPE/15 mg AlN is higher as compared to HPDE (control) and higher filler loadings of polyethylene AlNnanocomposites (Table 1).

Crystallinity is determined through wide-angle X-ray diffraction (WAXD), and it showed the same trend as from DSC results as shown by Figure 2(b). Conventional method is used for measuring the percentage of crystallinity [34]. The imperfection of crystals in the presence of the AlN in homogeneities can also contribute to the decrease in crystallinity [35]. This observation has been corroborated with the results of TGA analysis.

4.3. Thermal Gravimetric Analysis (TGA)

Figure 3 displays the thermogravimetric (TG) curves for PE-AlN nanocomposites as a function of the amount of filler. From the TG curves, it is clear that degradation kinetics starts at a temperature of 300°C, and maximum degradation occurred in the range of 425 to 450°C, and there is no significant effect in the maximum degradation temperature of the composites. From Figure 3, it is also clear that addition of 15 mg of AlN in the polyethylene matrix increases the thermal stability of the composites however decreases at higher filler loading. This can be explained as follows: in the case of composites having 15 mg of AlN, due to its good dispersion in the polymer matrix, dissipation of heat between filler and the matrix occurs efficiently, thereby there is an increase in the thermal stability of the composites. Also the low heat capacity of AlN (0.738 J/g/°C) compared to HDPE (1.82009 J/g/C) causes to absorb heat rapidly which results in the degradation of polyethylene at higher temperature [36]. Even though, at higher filler concentration, interparticle distance between fillers decreases, thereby agglomeration and reduction in interfacial area between AlN and PE matrix occurs, and this results in the lowering of the thermal stability of the composites. The same trend has been observed by Goyal et al. in the AlN-reinforced PEEK composites [37]. The degradation kinetics of the composites is calculated by using the Broido method [38] with an assumption that the degradation follows a first-order reaction or a superposition of first-order process. This assumption of Broido leads to: []lnln(1𝛼)=ln𝐾Δ𝐸,𝑅𝑇(2) where 𝛼 is the amount of polymer degraded at time t, ΔE is the change in activation energy, 𝑅 is the universal gas constant, 𝐾 is apparent activation energy, and 𝑇 is the temperature in Kelvin scale. In this, 𝛼 can be calculated using the following equation: 𝑊𝛼=𝑜𝑊𝑡𝑊0𝑊,(3) where 𝑊𝑡 is the mass at time t, 𝑊𝑜 is the initial mass, and 𝑊 is the mass after infinite time. The advantage of Broido’s method of calculating the activation energy of thermal stability is that the result does not depend upon the value of heating rate and is independent of the value of temperature at which the reaction is maximum. The results obtained by using Broido’s method for the PE-AlN nanocomposites are given in Figure 4. It can be seen that composites having 15 mg of AlN show maximum activation energy as compared to other compositions because of high degradation temperature.

250364.fig.003
Figure 3: TG plots of nano-AlN-filled polyethylene composites.
250364.fig.004
Figure 4: Change in activation energy for the AlN/polyethylene composites.
4.4. Microcalorimeter (MC)

Combustibility test data as obtained through Microcalorimeter in terms of heat release rate and decomposition temperature are shown in Figure 5 and Table 3. Fire test data obtained through it can be correlated to the results obtained through Cone Calorimeter [39]. It is apparent that with increase in the content of filler, heat release rate decreases indicating an increase in thermal stability and decrease in combustibility [23, 24].

tab3
Table 3: Micro-calorimeter results showing decomposition temperature (oC) and heat release rate (W/g).
250364.fig.005
Figure 5: Microcalorimeter curves (heat release rate and temperature) of polyethylene AlN nanocomposites.
4.5. Scanning Electron Microscopy (SEM)

Figures (6(a)6(d)) show surface morphology; HDPE appears to be less fibrous (see Figure 6(a)) than HDPE/15mg AlN (see Figure 6(b)). Here, fibrous chains are formed making the material more crystalline. In the case of PE/30mg AlN (see Figure 6(c)), it appears that excess amount of AlN-nano particles restricted the growth of chains, and the structure is less fibrous. In PE/45mg, AlN fibrous surfaces became least prominent (see Figure 6(d)). The same type of fibrous morphology was also observed in another study where AlN nanoparticles were used [40]. Scanning electron microscopy (SEM) provided us with an advantage of exploring the surface morphology of PE (pure) and PE/AlN composites in a better way, and the same type of effect is also observed in previous studies [41].

fig6
Figure 6: Scanning electron microscopy of nano-AlN-filled polyethylene composites. (a) HDPE (control), (b) 15 mg AlN/HDPE, (c) 30 mg AlN/HDPE, and (d) 45 mg AlN/HDPE.

5. Conclusion

Ethylene polymerization was done to form polyethylene nanocomposites with nanoaluminum nitride. Catalyst activity was higher at 15 mg nanoaluminum nitride. Differential scanning calorimeter (DSC) results show that melting temperature is minutely affected by an increase in amount of filler. Whereas Percentage crystallinity data from differential scanning calorimeter (DSC) and X-ray diffraction (XRD) shows that at 15 mg AlN/percentage crystallinity increased marginally. It is also apparent from Micro calorimeter (MC) data that with increase in the content of filler, heat release rate decreases, indicating a decrease in combustibility. Surface morphology was observed through scanning electron microscopy (SEM); at 15 mg AlN/HDPE fibrous chains were formed, whereas at 30 mg AlN/HDPE and 45 mg AlN/HDPE, it became less prominent.

Acknowledgment

The authors wish to acknowledge the Deanship of Scientific Research, King Fahd University of Petroleum and Minerals for their funds under Project no. IN101018. Authors also wish to acknowledge the Center of Research Excellence in Petroleum Refining and Petrochemicals (CoRE-PRP), King Fahd University of Petroleum and Minerals for supporting this study.

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