Thermal Effect of Ceramic Nanofiller AluminiumNitride on Polyethylene Properties

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


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.
Several studies have been made to improve the various properties of polyethylene by the addition of organic and inorganic fillers [9][10][11][12][13].Among these fillers, inorganic fillers increase the thermal properties of the polyethylene significantly [14][15][16][17][18].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

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.

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 cocatalyst.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.

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.

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.

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.

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

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

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).2. It can be seen that as the amount of nano-AlN increases, T m 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:

Differential Scanning Calorimeter (DSC) and Wide-Angle X-Ray Diffraction (WAXD)
where ΔH fus is the enthalpy of fusion of the polyethylene composites, and ΔH 0 fus is the enthalpy of fusion of the 100% crystalline polyethylene.ΔH 0 fus 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 [31][32][33].
Higher melting temperature in HDPE/15 mg AlN nanocomposite can be supported by the results of GPC (gel permeation chromatography).Molecular weight (M w ) 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.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].

Thermal Gravimetric Analysis (TGA)
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: 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: where W t is the mass at time t, W o 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.

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].Here, fibrous chains are formed making the material more crystalline.In the case of PE/30 mg 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/45 mg, 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].

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.

Figure 1 :
Figure 1: Activity of synthesized polyethylene nanocomposites at different experimental conditions.

Figure 2 (
a) displays the DSC thermograms (heating curve) of polyethylene composites as a function of the amount of AlN.The ΔH fus (heat of fusion) and T m (melting temperature) values along with the percentage of crystallinity for the blends as a function of AlN content are summarized in Table

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

Table 2 :
DSC, XRD and GPC results of AlN filled polyethylene composites.

Table 3 :
Micro-calorimeter results showing decomposition temperature ( o C) and heat release rate (W/g).