Influence of γ-ray modified MWCNTs on the structural and thermal properties of high-density polyethylene

Author’s Accepted Manuscript Influence of γ-ray modified MWCNTs on the structural and thermal properties of high-density polyethylene Bilal Ghafoor, Malik Sajjad Mehmood, Umair Shahid, Mansoor A. Baluch, Tariq Yasin www.elsevier.com/locate/radphyschem

PII: DOI: Reference:

S0969-806X(16)30114-1 http://dx.doi.org/10.1016/j.radphyschem.2016.04.004 RPC7118

To appear in: Radiation Physics and Chemistry Received date: 29 January 2016 Revised date: 1 April 2016 Accepted date: 7 April 2016 Cite this article as: Bilal Ghafoor, Malik Sajjad Mehmood, Umair Shahid, Mansoor A. Baluch and Tariq Yasin, Influence of γ-ray modified MWCNTs on the structural and thermal properties of high-density polyethylene, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2016.04.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Influence of γ-ray modified MWCNTs on the structural and thermal properties of high-density polyethylene Bilal Ghafoor1, Malik Sajjad Mehmood2, 3*, Umair Shahid1, Mansoor A. Baluch3, Tariq Yasin2 1

Materials Science and Engineering Department, Institute of Space Technology, 44000 Islamabad, Pakistan 2 Advance Polymer Laboratory, Pakistan Institute of Engineering and Applied Sciences, 45650, Islamabad, Pakistan 3 University of Engineering and Technology, 47050, Taxila, Pakistan *

Corresponding Author: Name: Dr Malik Sajjad Mehmood Email address: [email protected][email protected] Phone #: 0092 51 9047884 Fax #: 0092 51 9047420

Abstract This study aims to investigate the influence of adding 100 kGy γ-irradiated multi wall carbon nano tubes (MWCNTs) on the structural and thermal properties of high-density polyethylene (HDPE). The effects of further γ-irradiation in the presence of γ-MWCNTs on aforementioned properties have also been investigated. FTIR spectroscopic measurements of HDPE and HDPE/γ-MWCNTs composites reveal that modification of MWCNTs with ≤100 kGy of γ-dose reduces its efficiency as free radical quencher. This behavior is found to increase further with the increase in the concentration of γ-MWCNTs. Wide angle X-ray diffraction (WAXD) data shows a decrease in percent crystallinity and shifting of crystalline peaks toward lower values of 2θ angles. This behavior is mainly attributed to the oxidation induced due to residual free radicals. Thermal analysis reveals that addition of γ-MWCNTs decreases the thermal stability as far as onset thermal degradation temperature, percent crystallinity, and melting temperature of UHMWPE/ γ-MWCNTs. In addition to this, gel content measurements show that insoluble percnetage of UHMWPE is higher with the incorporation γ-MWCNTs and further irradiation. The gel contents are found to improve up to 29 % and 60 %, respectively with the incorporation of γ-MWCNTs and further irradiation. Keywords: γ- irradiation; γ- ray modified MWCNTs; HDPE; FTIR; DSC/TGA; nanocomposites

1.0.

Introduction

Various chemical and physical properties have made high density polyethylene (HDPE) a material of choice for many industrial applications such as packaging, wire and cable insulation, etc. These properties include high mechanical strength, high crystallinity, light weight and minimum costs for final product. However, the thermal and mechanical properties (more specifically impact strength) of HDPE are of major concern for its use and service life in many industrial applications (Chowdhury et al., 2012; Shafiq et al., 2013).In this respect, composites of this polymer have been formulated by using various techniques including self-reinforcement, reinforcement with carbon fibers, with fillers of micron size, and with fillers of nano size to enhance properties more specifically mechanical properties (Barus et al., 2010; Dintcheva et al., 2009; Grigoriadou et al., 2011; Roumeli et al., 2014; Subramaniam et al., 2012). The use of nano scale fillers is preferable because of their larger surface area as compared to micron size fillers. Other notable advantages of using nano fillers for polymer reinforcement are improving or adjusting the optical, thermal and flame retardant properties (Barus et al., 2010; Grigoriadou et al., 2011; Gutiérrez and Palza, 2015). Composites based on carbon nano tubes (CNTs) have gained much attention because of their improved chemical stability, and mechanical, optical, thermal, and electrical properties. The outstanding results of incorporating CNTs in polymer matrices have lead researchers to focus on CNT-reinforced polyethylene (PE), including composites of HDPE (Chaudhari et al., 2011; Dubey et al., 2012; Kingston et al., 2014; Park et al., 2008; Wang et al., 2009). Lee et al. incorporated MWCNTs in HDPE/carbon black blend and reported an improvement in the positive thermal coefficient of HDPE/carbon black/MWCNTs blend (Lee et al., 2006). In other relevant studies, Kumar and other researchers have placed huge emphasis on the effects of MWCNTs incorporation on thermal and mechanical properties of HDPE/MWCNTs composites. They have found that thermal degradation of these composites started at lower temperature as compared to unformulated HDPE; however, improved mechanical properties have been reported by the incorporation of CNTs (Kumar et al., 2013; Martínez-Morlanes et al., 2012; Rama Sreekanth et al., 2012). MWCNTs have proved to be potential filler for HDPE. In order to completely explore and utilize the potential of MWCNTs, compatibility among the polymer and MWCNTs has to be improved during the physical and chemical techniques and/or methods. MWCNTs modifications involving

chemical methods endow them with various surface functionalities which consequently, are responsible to enhance the compatibility among CNTs and polymer matrix. Use of toxic chemicals and tiresome time consuming procedures with chemical modifications of CNTs have lead researchers to adopt alternate technique or method that should be relatively environmentfriendly, simple and cost-effective. Ionizing radiation such as gamma rays and electron beams has been used for the modification of MWCNTs. Jung et al. has dispersed MWCNTs in hydrogen peroxide and irradiated the resulting dispersion using an electron beam to obtain CNTs of shorter length (Jung et al., 2008). Safibonab and other researchers evaluated the effect of γrays on the surface area of MWCNTs and reported larger surface area and pore volume along with carbonyl functional groups for 100 kGy irradiated MWCNTs (Murakami et al., 2015; Safibonab et al., 2011). Morlanes et al. (Martínez-Morlanes et al., 2011) reported the improved interaction between MWCNTs and polyethylene on irradiation which is responsible for enhancing the mechanical properties of composites. Ionizing radiation has also been used to modify phyisco-chemical and structural properties of HDPE. For comprehensive details of radiation induced structural and chemical modifications of HDPE, its composites and the composites of other members of the polyethylene (PE) family, see:(Mehmood et al., 2014; Mehmood et al., 2013b; Richaud, 2015). In this study, the effects of various γ-doses on the structural, morphological and thermal properties of pure HDPE and HDPE/γ-MWCNTs were investigated.

Wide angle X-ray

diffraction (WAXD),thermal gravimetric analysis (TGA), differential scanning calorimetric (DSC), Brunauer–Emmett–Teller (BET),

Gel contents analysis, and Fourier

transform

infrared (FTIR) spectroscopic measurements were performed to have more conclusive results of HDPE nano-composites with modified/ less defective MWCNTs.

2.0. Experimental Procedure 2.1. Materials HDPE in powder form (F00952; density = 0.952 g/cm3; MFI= 0.05 g/10min) was purchased from ExxonMobil chemical (Riyadh, Saudi Arabia). The antioxidants, Irganox-1010 (AO-1) and Irgafos-168 (AO-2) were purchased from Ciba Specialty Chemicals (Basel, Switzerland). MWCNTs were gifted by Korea Advanced Institute of Science and Technology. All other chemicals such as stearic acid, acetone etc. were used as such without any further purification.

2.2. Melt blending and irradiation Composites of HDPE were prepared by using method of melt blending in a Thermo Haake PolyLab Rheomix 600 (Thermo Electron Corp, Karlsruhe, Germany). HDPE powder was melted at 170°C at constant rotor speed of 60 rpm, and during melting of HDPE, stearic acid (1 phr), Irganox-1010 (0.2 phr) and Irgafos-168 (0.1 phr) were added. After next couple of minutes, 0.5 % and 1.0 % of gamma irradiated MWCNTs (γ-MWCNTs) were added in HDPE containing stearic acid and AO-1 and AO-2. To obtain homogeneous dispersion of γ-MWCNTs, admixing of composition was continued for additional 10 minutes at the same temperature and rotor speed. The molten compositions containing 0.5 % and 1.0 % of MWCNTs were then brought to room temperature and compressed into sheets of 1 mm thickness. The compression process was carried out using hot press available at Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan) for 10-15 min at 180 °C under the pressure of 200 bar, followed by cooling in air down to room temperature (25 °C) under the same pressure. These nano-composites were assigned identification codes as C, H, and F throughout the text of this manuscript representing the samples containing 0.0%, 0.5%, and 1.0% of γ-MWCNTs, respectively. MWCNTs were irradiated using gamma rays in open air at an absorbed dose of 100 kGy to obtained γ-MWCNTs prior to incorporation in HDPE. Sheets of HDPE and its composites were also irradiated in open air with γ-rays at an absorbed dose ranging from 25 kGy to 150 kGy with at increments of 25 kGy. The subscript with identification codes (i.e. C, H, and F) were used to represent the amount of absorbed dose within the text of manuscript. Irradiation services were provided by Pakistan Radiation Services (PARAS), Lahore, Pakistan while using

60

Co gamma

irradiator (Model JS-7900, IR-148, and ATCOP) operating at a constant dose rate of1.2 kGy/h and ambient temperature.

2.3. Characterization To determine the effective surface area of the γ-MWCNTs, a high performance volumetric physiorption apparatus at 77 K i.e. BET was used prior to incorporation of these γ-MWCNTs in HDPE. Chemical compositions of un-irradiated HDPE/γ-MWCNTs composites and irradiated HDPE/γ-MWCNTs were determined by Fourier Transform Infrared spectroscopy. The spectra of samples were collected using Nicolet 6700 FTIR spectrophotometer (Thermo Electron Corp,

Waltham, Massachusetts, USA) in attenuated total reflectance mode. Spectra were collected at a constant spectral resolution of 6 cm-1 in the range of 4000–500 cm-1 by averaging of 216 scans. Thermal decomposition behaviors of the composites have been assessed by using MettlerToledo TGA/SDTA851 thermo-gravimetric analyzer (Schwerzen- bach, Switzerland). The analyses were performed by heating the samples (approximately having 7 mg weight each and 3 samples of each group) from ambient temperature to 550 °C under continuous purge of nitrogen (40 mL/min) at a rate of 20°C/min. Calorimetric analyses were carried out by using Q100 DSC from TA Instruments (New Castle, DE, USA) at a heating and cooling rates of 10 °C/min in Aluminum pan containing approximately 5mg sample for each test. These scans were performed under nitrogen environment and registered between 20 °C to 200 °C. The melting and recrystallization temperatures were measured from maximum temperatures of the endothermic and exothermic peaks, respectively. To have in depth analyses of the effects of γ-MWCNTs incorporation on the crystal structure, average crystallite size, and percentage crystallinity, wide angle X-ray diffraction (WAXD) pattern of all the composites were recorded using X-ray diffractometer (Model X’ TRA48 Thermo ARL) operating at 45 kV and 40 mA. For all samples, radial scans were recorded in reflection scanning mode from 5º to 60º at a scanning rate of 11/min at room temperature. In order to investigate the degree of crosslinking of HDPE and HDPE/γ-MWCNTs, gel contents measurements were performed by using Soxhlet extractor and xylene. The percentage values of gel content were calculated by using the relation:

Where Wo and Wa are the weights of sample before and after extraction. It is important to mention here that all tests were performed while following the standard protocols defined by American Society for Testing and Materials (ASTM, D2765-11 2006, D3418-15 2015, E1131-03 2003, E1426-14 2014, F2102-13 2013)

3.0. Results and Discussion 3.1. Influence of γ-rays on the properties of MWCNTs In order to have better dispersion and compatibility of MWCNTs in HDPE for composite preparation, MWCNTs were irradiated in powder form with γ-rays with a dose of 100 kGy

because it was reported by Safibonab et al (Safibonab et al., 2011) that quality of MWCNTs can be improved by irradiating them with a dose value of 100 kGy. To evaluate the influence of γirradiation on the surface and structural properties, BET and FTIR analysis of raw and γ irradiated MWCNTs (in open air) was performed. The surface analysis while using BET analysis shows that the surface area of un-irradiated MWCNTs and γ-MWCNTs are 16.57 m2/g and 247.99 m2/g, respectively. The significant increase in the surface area of MWCNTs upon irradiation can be explained by the fact that irradiation results in smaller pore sizes and larger pore volumes on the surface of MWCNTs. This increase in micro porosity of MWCNTs is mainly responsible for larger specific surface area of γ-MWCNTs as compared to un-irradiated MWCNTs. In addition to increase in the specific surface area of MWCNTs upon irradiation, significant structural modifications are also evident. These modifications includes the significant increase in the absorption bands at 3444 cm-1(related to the stretching vibrations of the isolated surface of OH in the carboxyl group), at 1635 cm-1and 1715 cm-1(related to the stretching of MWCNTs backbone and C=O groups), at around 2933 cm-1(related to C-H stretching mode of H-C=O), as shown in Figure 1. Moreover, the relative decrease of absorption bands at 2933 cm-1 and 1715 cm-1 in comparison to the peak at 1635 cm-1further confirms the decrease in the defective structures and improvement in the quality of MWCNTs upon irradiating them with 100 kGy dose in open air. Comprehensive details on these structural modifications in MWCNTs upon irradiation has been covered in the literature (Jung et al., 2008; Kingston et al., 2014; Martínez-Morlanes et al., 2012; Murakami et al., 2015; Safibonab et al., 2011).

3.2. Influence of γ-rays on the properties of HDPE/γ-MWCNTs nano composites 3.2.1. Structural Analysis 3.2.1.1 FTIR spectroscopy In order to evaluate the influence of γ-MWCNTs incorporation and high energy irradiations on the structural properties of HDPE, FTIR spectroscopy has been performed.

The scans as

described in section 2.3 were registered and shown in Figure 2 for samples containing 1.0 % of γ-MWCNTs to highlight the significant structural changes. FTIR spectra of HDPE and its composites are shown in Figure 1. Un-irradiated composite (F0) spectrum exhibited absorption bands corresponding to C-H asymmetric and symmetric stretching, and bending vibrations. In

addition, bands can also be observed at 1596 cm-1, 1262 cm-1 and in the range of 1100-1000 cm-1. The bands at 1596 cm-1, 1262, and in the range of 1100-1000 cm-1 were attributed to the COO-, C-O stretching and C-C stretching absorption, respectively. The irradiated composites also exhibited above mentioned absorption bands. However, particular area of interest was the tie-line of CH2/CH3 peaks (extended from 1650-1750 cm-1, and 1330-1390 cm-1) which corresponds to oxidation index (OI) of these nanocomposites. The values of OI represent the degradation of polyethylene due to oxidation quantitatively on relative scale (Khan et al.).The values of oxidation index for samples containing 1.0 % of γ-MWCNTs is shown in Figure 1 and it can be seen that values of OI increases with absorbed dose due to chain scission and oxidation reaction by radiation induced free radicals. Although, it has been recently reported that MWCNTs acts as radical scavengers (Martínez-Morlanes et al., 2012) but action of MWCNTs as scavenging agent is less prominent here. The irradiation of MWCNTs before composite formulation might be reason for this behavior because, it has also been reported that irradiating MWCNTs with ≤100 KGy of γ-dose resulted in improving their qualities via reducing the defects already present in them and these defective sites are responsible for quenching the free radicals(Jung et al., 2008; Safibonab et al., 2011).

Figure 1 3.2.1.2. Wide angle X-ray Diffraction (WAXD) Analysis Wide angle X-ray diffraction (WAXD) study has been conducted to perform detailed investigation of HDPE and its composites with the addition of γ-irradiated MWCNTs. Shown in figure 2, are the WAXD patterns of HDPE/γ-MWCNTs containing 1.0 % of γ-MWCNTs as nano additives. The figure clearly indicates that these patterns exhibit the diffraction peaks at 21.6° and 24.0°, which correspond to the (110) and (200) planes of polyethylene (PE). However, these peaks are found to be less intense and shifted toward the lower values of 2θ (see Figure 2) i.e. for the composites irradiated with 50 kGy and 150 kGy of γ-dose, and the peak at L(110) were shifted from 21.50º to 21.30º along with a significant decrease in intensity. The reduction in intensity shows the decrease in percent crystallinity of nano-composites with irradiation. This decrease in crystallinity may be due to the scissioning of polymer chains in the boundaries of crystalline region, whereas the chain scission is mainly due to the free radicals induced by the oxidation reactions of radiation induce free radicals which are also responsible for

the reduction of average crystallite sizes. The shifting of peak positions towards lower values of 2θ is the experimental evidence for this reduction in average crystallite sizes with irradiation treatment of these HDPE/γ-MWCNTs. Other plausible reasons for decrease in percentage values of crystallinity and average crystallite sizes also include: 

Enhancement of MWCNTs quality by reducing/eliminating the defects inside MWCNTs by treating them with ≤100 kGy of γ-dose (Jung et al., 2008; Safibonab et al., 2011)



Higher chain mobility of PE with MWCNTs incorporation which results in slightly boosted migration of trapped free radicals inside the crystalline core, causes chain scission at the boundaries of crystalline core by reacting with diffused oxygen (Mehmood et al., 2013a).

The results obtained from WAXD study i.e. decrease in the percentage crystallinity and average crystallite sizes affects the thermal characteristics of these composites (as explained in section 3.0) and also support our claim regarding the higher values of O.I of these HDPE/γ-MWCNTs.

Figure 2 3.2.2. Thermal Analysis 3.2.2.1. Thermo gravimetric Analysis The thermal stabilities of HDPE and HDPE/γ-MWCNTs composites were investigated in inert atmosphere. The results which include the Tonset, T10, T30, T50, T70, (temperature at which 10%, 30%, 50%, and 70%, weight has been lost) are summarized in Table 1. HDPE and HDPE/γMWCNTs composites show the single step mass loss and almost the similar degradation profile with an abrupt single step mass loss and final volatilization. This single step mass loss is ascribed to the degradation of polyethylene back bone while the 5 % mass loss is selected as a criterion for onset degradation temperature, and is found to have slightly lower values with the incorporation of MWCNTs and irradiation. This behavior of composite is attributed to higher chain mobility due to plasticity induced by MWCNTs incorporation along with the radiation induced oxidation degradation(Oral and Muratoglu, 2011). However, for 30 %, 50 %, and 70 % weight loss there is no significant difference (see Table 1).

Table 1

3.2.2.2. Differential Scanning Calorimetery (DSC) For further investigation of the thermal characteristics of HDPE and HDPE/γ-MWCNTs composites, DSC has been performed and the results of second heating and cooling run are shown in Figure 3. The control sample i.e. HDPE shows its characteristics melting and recrystallization temperatures which are affected with γ-irradiation. A significant decrease in the heat of fusion (which is calculated as the area under the endothermic peak) with γ-irradiation, is found. Moreover, further decrease in the heat of fusion values has been visualized by increasing the γ-dose (see Figure 4 below). The plausible elucidation for this behavior is the chain scissions close to crystalline lamella immediately after irradiation due to radiation induced free radicals. The decrease in heat of fusion with irradiation reveals that percentage crystallinity of HDPE is also decreased with irradiation. The results of DSC are in agreement with WAXD data presented in section 3.2.2. In addition, the re-crystallization behavior also follows the same trend as that of melting as far as radiation of HDPE is concerned. The melting temperature (Tm) is increased slightly with the incorporation of γ-MWCNTs. On the other hand considering the heat of fusion, initially it decreases up to 50 kGy of γ-dose and increases for the composites irradiated with the dose level of 100 kGy). The increase in heat of fusion and percent crystallinity for 100 kGy irradiated composite is due to the fact that exposure of MWCNTs with further 100 kGy induces the defective sites which serves as free radical quenchers (Martínez-Morlanes et al., 2012; Park et al., 2008). Occurrence of crystallization at lower temperature in irradiated nanocomposites compared with the un-irradiated ones is due to the degradation effect. The exposure of PE to higher doses give rise to the degradation and hence, shorting of polymer chains. These shorter polymer chains consequently entangle and disentangle easily as compared to the longer chains. The reduction in melting temperature and crystallinity of nanocomposites was monotonic with increase in γ-dose. Figure 3

3.2.3. Gel contents Measurements

Gel content of irradiated samples is presented in Figure 4. The graph showed improvement in the gel content of HDPE with absorbed dose. The increase in gel content is due to increase in crosslinking of polymer chains with γ-irradiation. For nanocomposites, gel content is increased up to 100 kGy dose. As can be noticed from the figure, the nanocomposites possessed higher gel content than that of HDPE. This might be due to the higher crosslinking effect of γ-MWCNTs. The defects produced during the radiation treatment with >100 kGy of MWCNTs have contributed to crosslinking by the generation of radicals during gamma irradiation of nanocomposites. The nanocomposites irradiated at 150 kGy absorbed dose showed low gel content. This is mainly due to the defects induced in MWCNTS by irradiating the HDPE/γMWCNTs with >100 kGy (as MWCNTs are already irradiated with 100 kGy of γ-dose) of γdose serves as radical quenchers which results in reduction of crosslink density. Figure-4

Conclusion HDPE/γ-MWCNTs nano composites were prepared and these composites were irradiated with various γ-doses. The infrared spectra revealed that modification of MWCNTs with γ-rays reduces its efficacy as free radical quencher and as a result of which radiation induced oxidation is increased. The percentage crystallinity, thermal stability, and crystalline lamellae thicknesses of the composites were observed to decrease with the addition of γ-MWNCTs while the behavior of HDPE/γ-MWCNTs composites with irradiation was not that different than the irradiated HDPE. Gel content measurements revealed that the irradiation of HDPE in the presence of γMWCNTs increased the crosslink density up to 60 %. A significant increase in crosslink density is of particular importance for industrial application of the composites. Acknowledgement Dr Malik Sajjad Mehmood acknowledges the financial support provided by Directorate of ASR&TD, UET, Taxila Pakistan for conducting this study. The technical and moral support by Mr. Muhammad Shafiq from PIEAS, Islamabad and Anna Sanawaar from UET, Taxila is also acknowledged here.

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List of Figures Figure 1: FTIR spectra of HDPE/γ-MWCNTs un-irradiated and irradiated composites with values of oxidation index (OI). Figure 2: Wide angle X-ray diffraction patterns of composites containing 1.0 % of γ-MWCNTs. Figure 3: Representative DSC heating curves of HDPE/γ-MWCNTs composites containing 1.0 % of γ-MWCNTs Figure 4: Percentage values of gel contents of HDPE and its composites as a function of absorbed dose

Figure-1

Figure-2

Figure-3

Figure-4

List of Tables Table 1: TGA data of pristine, HDPE/MWCNTs, and HDPE/γ-MWCNTs Sample

TOnset(°C)

T10(°C)

T30(°C)

T50(°C)

T70(°C)

HDPE, 0% γ-MWCNTs,0kGy

428.0

445.0

467

476.0

482.6

HDPE, 0% γ-MWCNTs,50kGy

425.0

447.7

468.1

478.4

484.0

HDPE, 0% γ-MWCNTs,150kGy

420.5

443.2

465.9

477.2

481.8

HDPE, 0.5% γ-MWCNTs,0kGy

402.0

436.9

463.0

476.0

482.6

HDPE, 0.5% γ-MWCNTs,50kGy

413.6

439.8

465.9

476.1

481.8

HDPE, 0.5% γ-MWCNTs,150kGy

406.5

434.4

462.5

473.4

479.6

HDPE, 1.0% γ-MWCNTs,0kGy

416.0

441.0

465.9

475.0

481.8

HDPE, 1.0% γ-MWCNTs,50kGy

398.8

438.8

467.0

476.1

482.9

HDPE, 1.0% γ-MWCNTs,150kGy

388.6

428.4

459.0

472.7

479.5

*

TOnset = temperature at 5% mass loss; T10 = temperature at 10 % mass loss; T30 = temperature at

30 % mass loss; T50 = temperature at 50 % mass loss; T70 = temperature at 70 % mass loss

Highlights 

Nano composites of HDPE with 100-kGy irradiated MWCNTs are made



The effect of incorporation of γ-ray modified MWCNTs in HDPE is studied



Structural and thermal stability of HDPE/γ-MWCNTs nano composites is studied