Composites: Part B 61 (2014) 41–48

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Composites: Part B

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Dispersion and roles of montmorillonite on structural, flammability,thermal and mechanical behaviours of electron beam irradiated flameretarded nanocomposite

http://dx.doi.org/10.1016/j.compositesb.2014.01.0351359-8368/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors: Address: Department of Chemical Engineering, Facultyof Engineering and Science, Universiti Tunku Abdul Rahman, 53300 Kuala Lumpur,Malaysia. Tel.: +60 3 4107 9802; fax: +60 3 4107 9803.

E-mail addresses: direct.beest@gmail.com (S.-T. Bee), direct.tinsin@gmail.com(L.T. Sin).

Soo-Tueen Bee a,b,⇑, A. Hassan b, C.T. Ratnam c, Tiam-Ting Tee a, Lee Tin Sin a,⇑, David Hui d

a Department of Chemical Engineering, Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, 53300 Kuala Lumpur, Malaysiab Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysiac Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor, Malaysiad Department of Mechanical Engineering, University of New Orleans, Lake Front, New Orleans, LA 70138, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 September 2013Received in revised form 15 January 2014Accepted 16 January 2014Available online 25 January 2014

Keywords:A. Polymer–matrix composites (PMC)B. Mechanical propertiesB. Thermal properties: Montmorillonite

In this work, the effects of montmorillonite (MMT) dispersion and electron beam irradiation onintercalation and flammability-thermal behaviours of alumina trihydrate (ATH) added low densitypolyethylene and ethylene vinyl acetate (LDPE–EVA) blends were investigated. MMT and ATH addedLDPE–EVA blends were compounded using Brabender mixer and compression moulded into sheets.The samples sheets were electron beam irradiated in the dosage range of 0 to 250 kGy. The dispersionand intercalation of nano-MMT in LDPE–EVA matrix were investigated through X-ray diffraction (XRD)analysis. The d-spacing measurements revealed that the addition of nano-MMT has effectively interca-lated into polymer matrix and this has enhanced the compatibility of ATH particles and LDPE–EVAmatrix. Limiting oxygen index test (LOI) revealed that the incorporation of MMT into ATH addedLDPE–EVA blends as improved the flame retardancy up to 26.5 LOI%. Besides, the application of electronbeam irradiation were also improved the flame retardancy of the blends by increasing the LOI% for about2% compared to non-irradiated samples. The application of irradiation dosage up to 250 kGy has rapidlyimproved the thermal stability of blends by delaying decomposition temperature and also promotingformation of char. The increasing of MMT loading level and irradiation dosage has effectively enhancedtensile strength and Young’s modulus by intercalating polymer matrix into interlayer galleries of MMTparticles. Beside, the formation of crosslinking networks in polymer matrix also could further enhancethe tensile strength and Young’s modulus. The intercalation effect of MMT particles and formation ofcrosslinking networks in polymer matrix could improve the thermal and mechanical properties. Conse-quently, this study has demonstrated that addition of MMT and electron beam irradiation into ATH addedLDPE–EVA blends could produce better flammability, thermal and physical properties of ATH addedLDPE–EVA blends.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Low density polyethylene (LDPE) and ethylene vinyl acetate(EVA) blends have been used in the wide range of engineering fielddue to their good physicomechanical properties. However, theapplication of LDPE–EVA blends as wire and cable insulation mate-rials are limited because of its poor flame resistance. The poor

flame resistance of LDPE–EVA blends is mainly attributed to theirhydrocarbon origin. The flame resistance of LDPE–EVA blendscould be improved by incorporating fire retardants [1–4]. Severalresearchers have reported that the incorporation of metal hydrox-ides type fire retardants (i.e. magnesium hydroxide, alumina trihy-drate (ATH), zinc borate, etc.) into LDPE–EVA matrix couldeffectively enhance the flammability of polymer blends [5,6]. Highloading levels (up to 60 wt.%) of the metal hydroxides type flameretardants are needed in order to increase the flame retardancyof the LDPE–EVA blends effectively [5]. However, the addition ofhigh loading levels of flame retardants such as magnesium hydrox-ide, ATH and zinc borate, in LDPE–EVA blends tend to cause dete-rioration on mechanical properties of the polymer blends [7]. ATH

42 S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48

is one of the most commonly used mineral flame retardants forpolymer materials due to its cheaper in price widely available inthe market [8]. Various techniques have been applied to enhancethe fire retardancy of LDPE–EVA blends without harming itsmechanical properties. Incorporation of layered nano-clay ormontmorillonite (MMT) into flame retarded polymer blends isone of the techniques to enhance the fire retardancy of polymerblends especially for LDPE–EVA blends. The polar group of EVA, vi-nyl acetate (VA) along the macromolecules chains of EVA couldhelp in improving the polymer matrix to intercalate into the lay-ered structure of clay or MMT particles [9,10]. The MMT intercala-tion effect in polymer matrix is usually detected through thechanges of interlayer spacing of MMT in polymer blends [1]. Thefinely dispersed MMT particles could significantly enhance theflame retardancy and mechanical properties of polymer blends.For instance, the addition of 5 wt% of montmorillonite was foundto improve the elongation at break of flame retarded LDPE–EVAblends by 10% to 30% [7]. On the other hand, synergistic combina-tions of flame retardants were also used to enhance the flameretardancy of polymer blends at lower loading while maintainingthe required mechanical properties. The combinations of alumin-ium hydroxides with zinc borate, magnesium hydroxides with zincborate and intumescent systems or silicates are the examples offlame retardants packages shown promising outcomes to enhancethe flame retardancy of polymer blends [7]. At the mean time, theaddition of combinations of montmorillonite (MMT) with fireretardants into polymer blends was also known to improve theflame resistance of polymer blends [1,11,12]. The incorporationof MMT particles could improve the flammability of polymerblends by delaying the thermal decomposition temperature whileincreasing the formation of char residue during combustion [12].

Besides, the poor mechanical properties of high amounts offlame retardant added polymer blends can be improved by usinghigh energy irradiation such as gamma-ray, X-ray or electron beamto modify the properties of polymer [13,14]. However, the applica-tion of irradiation dosage must be kept at an optimum level in orderto avoid excessive crosslinking in polymer blends. According to Sha-rif et al. [14,15], the application of electron beam irradiation dosageup to 250 kGy on LDPE–EVA blends has shown significant enhance-ment effect by the formation of crosslinking networks as indicatedin gel content results. The high degree of crosslinking networkstends to enhance the mechanical properties of LDPE–EVA blends.Nevertheless, when the application of electron beam beyond theoptimum level, the mechanical properties of polymer blends wouldseverely decrease and lost the value of applications [13,16]. In addi-tion, Sharif et al. [14] also found that excessive irradiation dosage isharmful to the elongation at break of LDPE–EVA blends due to theformation of high amounts of crosslinking networks in LDPE–EVAmatrix which restricted the mobility of molecular chains. Therestriction of molecular chains mobility in LDPE–EVA matrix couldalso reduce the ductility LDPE–EVA blends. Up to date, none of theseresearches focus on investigating the effects of montmorillonite(MMT) with irradiated flame retardant LDPE–EVA blends. The addi-tion of MMT is mainly to provide reinforcing effects while improv-ing the intumescent effect of polymer blends. Hence, the objectiveof this study is to investigate the dispersion roles of MMT on theintercalation and flammability-thermal properties of flame retar-dant added LDPE–EVA blends in relation to the irradiation effects.

2. Experimental

2.1. Materials

Low density polyethylene (LDPE) and ethylene vinyl acetatecopolymer (EVA) were used as polymer base in this study. LDPE

with grade Titanlene LDF200GG and EVA with grade of UE629were purchased from Titan Chemicals Corporation Sdn. Bhd.,Malaysia and USI Corporation, Taiwan, respectively. Alumina tri-hydrate, Al2O3�3H2O (ATH) with the grade of Micral 9400 wassupplied by J.M. Huber Corporation, United States. ATH wasadded into polymer compound as flame retardant in this study.Zinc borate was added into polymer compound as a smoke sup-pressant and it was supplied by ShanDong Chuan Jun Chemical,China. Trimethylolpropane trimethacrylate (TMPTMA) containing175 ppm monomethyl ether hydrquinone was supplied by Sig-ma–Aldrich (M) Sdn Bhd, Malaysia. TMPTMA was used as radia-tion sensitizer in this polymer compound to promote theformation of free radicals in polymer matrix during irradiationprocess. LDPE-grafted maleic anhydride (LDPEgMAH) with gradeof NG1201 was supplied by Shenghai Jianqio Plastic, China.LDPEgMAH grafted with 1% of maleic anhydride was used as acompatibilizer in this study to improve the compatibility betweenthe polymers matrix and fillers. Calcium stearate and Irganoxgrade of 1010 were supplied by Industrial Resins Malaysia Sdn.Bhd and Ciba Specialty Chemicals, respectively. Calcium stearatewas used as external lubricant to prevent the polymer melt fromsticking on processing machine during samples compounding.Irganox grade 1010 was added as antioxidant in polymer com-pound to prevent pre-mature degradation caused by either meltblending or irradiation stage. Montmorillonite (MMT) with gradeof Nanomer 1.3P was purchased from Nanocor, Arlington Heightand used as reinforcement filler in the polymer compound to im-prove the mechanical properties.

2.2. Samples preparation

All the LDPE–EVA blend formulations were containing a fixedamount of 50 phr LDPE, 50 phr EVA, 25 phr ATH, 6 phr LDPE-gMAH, 1 phr TMPTMA, 2.5 phr calcium stearate, 7.5 phr zinc borateand 0.05 phr Irganox 1010. Each formulation was added with dif-ferent amounts of MMT (i.e. 2.5 phr , 5 phr, 7.5 phr and 10 phr).The LDPE–EVA blends were compounded using the Brabender mix-er at the mixing temperature of 130 �C and rotor speed of 50 rpmfor 12 min. For samples compounding process, LDPE and EVA werefirstly melted in Brabender mixer at 130 �C for 4 min. Then, addi-tives were added into LDPE–EVA melts and mixed in Brabendermixer for another 8 min. The compounded samples were furtherhot pressed into 1 mm thickness sheet at the heating temperatureof 175 �C using a hot press machine. For the compression mouldingprocess, the compounded samples were preheated at the temper-ature of 175 �C for 8 min. The preheated samples were thenpressed at temperature of 175 �C and pressure of 10 MPa for5 min. The hot pressed samples were cooled down under pressureof 10 MPa for 2 min with the cooling rate of 15 �C/min. The 1-mmsheets were irradiated at room temperature to the irradiation dos-ages of 50 kGy, 150 kGy and 250 kGy with dose rate of 50 kGy perpass in an electron beam accelerator. The acceleration voltage ofelectron beam accelerator was set to 5 MeV.

2.3. Testing methods

2.3.1. X-ray diffraction (XRD) testThe X-ray diffraction (XRD) test was conducted using X-ray dif-

fractometer model XRD-6000 Shimadzu to evaluate the dispersionand intercalation pattern of MMT in LDPE–EVA matrix. The voltageand current of diffractometer were set at 40 kV and 30 mA. TheXRD spectra were recorded with the diffractometer in step-scanmode at room temperature by using Copper irradiation (wavelength of 0.1542 nm) generator at the scanning rate of 1� per minin the range of 0–2.52�.

Table 1The angle, d-spacing, change of d-spacing and inter-chain separation of 25 phr ATHadded LDPE–EVA blends filled with increasing of MMT loading level subjected tovarious electron beam irradiation doses.

Samples (irradiation dose) XRD analysis on curves (b) in Fig. 1–3

Angle, 2h d-spacing Dd R

MMT 1.285 6.86 � 8.582.5M-LE (0 kGy) 1.260 7.00 0.14 8.755.0M-LE (0 kGy) 1.261 6.99 0.13 8.747.5M-LE (0 kGy) 1.249 7.06 0.20 8.8310M-LE (0 kGy) 1.256 7.03 0.16 8.78

2.5M-LE (50 kGy) 1.257 7.02 0.15 8.775.0M-LE (50 kGy) 1.257 7.02 0.16 8.787.5M-LE (50 kGy) 1.244 7.09 0.23 8.8710M-LE (50 kGy) 1.264 6.98 0.12 8.73

2.5M-LE (150 kGy) 1.260 7.00 0.14 8.765.0M-LE (150 kGy) 1.247 7.08 0.21 8.857.5M-LE (150 kGy) 1.243 7.10 0.24 8.8810M-LE (150 kGy) 1.259 7.01 0.15 8.76

2.5M-LE (250 kGy) 1.264 6.98 0.12 8.735.0M-LE (250 kGy) 1.244 7.09 0.23 8.867.5M-LE (250 kGy) 1.254 7.04 0.17 8.8010M-LE (250 kGy) 1.252 7.05 0.18 8.81

0.0

0.4

0.8

1.2

1.6

2.0

Inte

nsity

, x10

6

0.0

0.4

0.8

1.2

1.6

2.0

Inte

nsity

, x10

6

0.0

0.4

0.8

1.2

1.6

2.0

Inte

nsity

, x10

6

0 0.4 0.8 1.2 1.6 2 2.4 2.8

2 Theta, o

0 0.4 0.8 1.2 1.6 2 2.4 2.8

2 Theta, o

0 0.4 0.8 1.2 1.6 2 2.4 2.8

2 Theta, o

10 phr MMT

7.5 phr MMT

5 phr MMT

2.5 phr MMT

MMT

2.5 phr MMT + 250 kGy

2.5 phr MMT + 150 kGy2.5 phr MMT + 50 kGy2.5 phr MMT + 0 kGy)

MMT

10 phr MMT + 250 kGy10 phr MMT + 150 kGy

10 phr MMT + 50 kGy10 phr MMT + 0 kGyMMT

(a)(b)

(a) (b)

(a) (b)

(a)

(b)

(c)Fig. 1. (a), (b) and (c) XRD curve of 25 phr ATH added LDPE–EVA blends filled with

S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48 43

2.3.2. Transmission electron microscopy (TEM) testTEM test was conducted to observe the dispersing and interca-

lation of MMT particles in polymer matrix. A transmission electronmicroscope (TEM) with the acceleration voltage of 100 kV wasused to study the morphologies of the nano-particles of MMTand the dispersion of MMT in LDPE/EVA matrix. The specimensof samples used for TEM test were in ultrathin form. The ultrathinspecimens were sectioned by using a cryogenic ultra-microtome.

2.3.3. Limiting oxygen index test (LOI)Limiting oxygen index (LOI) test was carried out using an appa-

ratus from Rheometer Scientific, United Kingdom in accordance toASTM D2863. The 1-mm sheets were cut into the dimensions of150 mm � 150 mm � 1 mm and the cut specimen was placed ver-tically in a transparent test column. A mixture of nitrogen and oxy-gen was purged into the transparent column for two minutes tocreate an oxygen–nitrogen atmosphere inside the test column.Then, the specimen was ignited with a burner at the top. The con-centration of oxygen in the mixture was increased until the con-centration level was sufficient enough to support the combustionof specimen. The LOI% of each formulation was measured with ninespecimens.

2.3.4. Thermogravimetric analysis (TGA)TGA test was carried out by using Mettler Toledo thermogravi-

metric analyzer with unit model of TGA/SDTA851e to analysis thethermal characteristics of the samples. The samples with theweight around 2 mg and 3 mg were firstly placed in a 150 ll silicacrucible. The samples were heated and scanned from temperature60 �C to 700 �C at a heating rate of 20 �C per min under a nitrogenatmosphere. The initial decomposition temperature and the ther-mal degradation weight loss (formation of char) of samples wererecorded and analyzed.

2.3.5. Tensile analysisTensile analysis was conducted by using Instron micro tester

model 5848 according to ASTM D1822. The 1 mm thickness com-pression moulded samples was cut into dumbbell shape by usingdumbbell sample cutter. The samples were tested under a cross-head speed of 50 mm per min under the load of 2 kN at room con-dition. The gauge length of samples was fixed at 14 cm. The resultsof elongation at break, tensile strength and Young’s modulus weretaken from the average of eight specimens.

3. Results and discussions

3.1. Crystallography-structural analysis

The X-ray diffraction analysis is usually performed to investi-gate the dispersion and intercalation state of MMT in polymer ma-trix. The positions of diffraction peaks on the XRD curves of LDPE–EVA blends could be observed between 2h = 0� to 2.52� to deter-mine the interlayer spacing (d-spacing) of silicate layers of nano-MMT. The d-spacing of the nano-MMT into ATH added LDPE–EVA blends were calculated through the Bragg’s equation as shownbelow [17]:

d ¼ k2 sin h

ð1Þ

where 2h is the angle of diffraction peak and k is the wavelength ofCu-irradiation and equals to 0.154 nm. The inter-chain separation, Rwas determined from Klug and Alexander equation as given below[17]:

R ¼ 5k8 sin h

ð2Þ

various MMT loading levels under increasing of irradiation doses (0–250 kGy).

44 S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48

where R is the inter-chain separation of the diffraction peak. The d-spacing, change of d-spacing and inter-chain separation of nano-MMT and all the samples were calculated from Eqs. (1) and (2) aspresented in Table 1.

According to Fig. 1, two diffraction peaks, which are peaks (a)and (b) were seen on the XRD curve of pristine nano-MMT. Fromthe XRD curves of the LDPE–EVA blends filled with increasing ofMMT loading level (as shown in Fig. 1(a)), the peak (a) was shiftedto lower angle and nearly disappear on the XRD curves. The peak(b) also found to slightly shift from 1.285� to lower angle around1.249� and 1.261�. Table 1 also presents the characteristic 2h, d-spacing, change of d-spacing and inter-chain separation of peak(b) for nano-MMT filler and MMT and ATH added LDPE–EVAblends. The d-spacing for peak (b) of nano-MMT added LDPE–EVA blends shows an increment within 0.13–0.20 nm comparedto pristine nano-MMT. This indicates that the nano-MMT particleswere homogeneously dispersed into the matrix of LDPE–EVAblends with increasing of MMT loading level. Besides, the dispers-ing of MMT particles in LDPE–EVA matrix was also depicted inFig. 2 under TEM observation. By observing Fig. 2(a), the MMT par-ticles were observed to disperse evenly in LDPE–EVA blends. TheLDPE–EVA matrix can also be observed to effectively intercalateinto the interlayer galleries of MMT particles as shown inFig. 2(b). The intercalation effect of MMT particles could increasethe distance between the interlayer galleries of MMT particles asindicated by the increment in d-spacing value of samples. Thiscan be explained where the hydrophobic section of nano-MMT en-ables the molten LDPE–EVA matrix to intercalate effectively intothe interlayer galleries of MMT particles with expanded d-spacing[17–19]. Such intercalation effect of the interlayer galleries of MMT

Fig. 2. The dispersing and intercalation effect of MMT particles in LDPE–EVA matrixfor non-irradiated, 25 phr ATH and 10 phr MMT added LDPE–EVA blends using TEMat 15,000 �magnification.

particles in LDPE–EVA matrix can be confirmed as depicted inFig. 2(b). It could be obviously seen that the MMT particles are con-sisted of multi layers galleries and the LDPE–EVA matrix was effec-tively intercalated into the interlayer galleries of MMT particles[20]. On the other hand, the typical hydrophilic characteristic atthe edge of MMT layers is compatible to the alumina trihydrate.This could further enhance the intercalation of ATH particles andLDPE–EVA matrix into nano-MMT interlayer galleries [12,17,18].Moreover, the intercalation effect of montmorillonite in polymermatrix could provide reinforcing effect to LDPE–EVA matrix byincreasing the surface area of interfacial adhesion between MMTparticles and polymer matrix. At low and moderate irradiationdoses (0–150 kGy), the d-spacing of peak (b) were slightly in-creased with increasing of MMT loading level from 2.5 to 7.5 phr.This also shows that low amounts of MMT particles in LDPE–EVAmatrix could promote the intercalation effect of LDPE–EVA matrixinto interlayer galleries of MMT particles under low and moderateirradiation doses. However, the d-spacing was slightly decreased asthe MMT loading level increased from 7.5 to 10 phr. This could bedue to the MMT particles in LDPE–EVA matrix are tended toagglomerate into larger particles when a high MMT loading levelis added. The agglomeration of MMT particles could reduce theeffectiveness of MMT particles intercalate and disperse in LDPE–EVA matrix. This is evidently observed with the reduction of d-spacing of MMT added in the blends. At high irradiation dosage(250 kGy), the d-spacing was observed to increase when MMTloading level increased from 2.5 to 5 phr. However, the d-spacingdecreased with subsequent increasing of MMT loading level. Thedecrement in d-spacing of 250 kGy irradiated samples withincreasing of MMT was observed to occur at low MMT loading levelin compared to lower irradiation dosages. The d-spacing of250 kGy irradiated samples was continued to decrease with furtherincreasing of MMT loading level from 5 to 10 phr.

At low MMT loading levels (2.5–7.5 phr), the d-spacing was ob-served to marginally increase with increasing of irradiation dosefrom 0 to 150 kGy. This indicates that the electron beam irradiationhas enhanced the dispersion and intercalation effect of MMT parti-cles in LDPE–EVA matrix by forming the crosslinking networks inpolymer matrix. The formation of crosslinking networks in poly-mer matrix could further enhance the intercalation effect of poly-mer matrix into the interlayer galleries of MMT particles. However,the d-spacing of all LDPE–EVA blends were observed to decreasewhen applied to high irradiation dosages (150 kGy and 250 kGy).This is attributable to the occurrence of chains scissioning inLDPE–EVA matrix [18]. At high loading level of MMT (10 phr),the d-spacing of ATH added LDPE–EVA blends was observed to re-duce marginally as the samples were irradiated to 50 kGy. How-ever, the d-spacing of ATH added LDPE–EVA blends wereincreased with further increasing of irradiation dosages (from 50to 250 kGy).

The inter-chain separation of ATH added LDPE–EVA blendswere increased with the addition of nano sized MMT. The MMTparticles added into matrix of ATH added LDPE–EVA blends couldfit into the cavities occur between the surface of ATH particlesand LDPE–EVA chains. This has led to increase the inter-chain sep-aration of LDPE–EVA blends. For non-irradiated, 50 and 150 kGyirradiated samples, the inter-chain separation was gradually in-creased as the loading level of MMT increased from 2.5 to7.5 phr. However, further increment of MMT loading level from7.5 to 10 phr was found to slightly decrease the inter-chain separa-tion. This might be attributed to occurrence of agglomerated parti-cles in LDPE–EVA matrix which could reduce the intercalationeffect of MMT particles into LDPE–EVA matrix. Thus, also reducethe inter-chain separation of LDPE–EVA blends. For 250 kGy irradi-ated samples, the inter-chain separation was increased from 8.73to 8.86 as MMT loading level had increased from 2.5 to 5 phr. Fur-

68

80

92

104

loss

, % 2.5 phr MMT

First stage degradation

S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48 45

ther increment in MMT loading level from 5 to 10 phr, the inter-chain separation was observed to decrease. This also indicates thatthe high density of crosslinking networks in polymer matrix couldenhance the inter-chain separation at higher loading levels.

20

32

44

56

60 160 260 360 460 560 660

Heating Temperature, oC

Wei

ght 5 phr MMT

7.5 phr MMT

10 phr MMT

(a)

(b)

20

30

40

50

60

70

80

90

100

60 120 180 240 300 360 420 480 540 600 660

Heating Temperature, oCW

eigh

t los

s, %

Non-irradiated

Irradiated at 50 kGy

Irradiated at 150 kGy

Irradiated at 250 kGy

Second stage degradation

First stage degradation

Second stage degradation

Fig. 3. (a) and (b) TGA curves of 25 phr ATH added LDPE–EVA blends filled withincreasing of MMT loading level subjected to increasing of irradiation doses.

3.2. Flammability-thermal analyses

Limiting oxygen index test (LOI) was conducted to investigatethe flame retardancy of polymer materials. It represents the mini-mum oxygen concentration in the mixture of oxygen and nitrogenrequired to support the combustion of materials. Table 2 shows theLOI% of the ATH added LDPE–EVA blends samples filled withincreasing of MMT loading level under various irradiation dosages.According to Table 2, the LOI% of all the samples were graduallyimprove with increasing of MMT loading level. Such phenomenonwas also observed by Chang et al. [12], Szép et al. [21] and Szusta-kiewicz et al. [22] who concluded that the addition of MMT canprovide the dripping and charring effects of the polymer blends.The increment of MMT loading level in the matrix of ATH addedLDPE–EVA blends could act as a char formation promoter by form-ing a protective layer on the surface of polymer matrix, subse-quently causing delay to the combustion process [7,12,23].Besides, the gas barrier properties of MMT filler in LDPE–EVA ma-trix could also impede the diffusion of flammable volatile gasesfrom mixing with free oxygen during combustion process, thusretarding the combustion process of ATH added LDPE–EVA blends[24,25].

By referring to Table 2, the LOI% of ATH added LDPE–EVA blendswere slightly improved with increasing of electron beam irradia-tion dosage from 0 to 250 kGy at all loading levels of MMT. Thisshowed that the formation of crosslinking networks in the matrixof ATH added LDPE–EVA blends could improve the fire resistanceof the ATH added LDPE–EVA blends. The crosslinked chains inATH added LDPE–EVA matrix could retard the combustion ofLDPE–EVA matrix by reducing the melt dripping [19]. Moreover,the reduction in the dripping of samples could increase thestrength of char formed during combustion and it would functionas a thermal insulation layer on the surface of the polymer matrix.The char covered on the polymer surface resist the heat transfer tothe inner region to vaporize the flammable polymer volatiles fromsupplying fuel for combustion process. Thus, it could further retardthe combustion of the polymer matrix.

TGA results of 25 phr added LDPE–EVA blends filled withincreasing of MMT loading levels under increasing of irradiationdoses were presented in Fig. 3 and Table 2. Based on Fig. 3, all

Table 2Limiting oxygen index (LOI), decomposition temperature and formation of char of25 phr ATH added LDPE–EVA blends filled with increasing of MMT loading levelsunder various irradiation dosages.

Samples 0 kGy 50 kGy 150 kGy 250 kGy

Limiting oxygen index (LOI), %2.5M-LE 22.2 ± 0.29 22.9 ± 0.23 23.8 ± 0.23 24.7 ± 0.175.0M-LE 23.1 ± 0.23 24.0 ± 0.23 24.6 ± 0.15 25.5 ± 0.177.5M-LE 24.0 ± 0.23 24.7 ± 0.23 25.3 ± 0.23 25.7 ± 0.2710M-LE 24.9 ± 0.23 25.3 ± 0.23 26.0 ± 0.26 26.5 ± 0.29

Mid decomposition temperature of first stage degradation (Td1), �C2.5M-LE 318.2 332.8 346.1 347.95.0M-LE 327.8 333.1 347.0 347.77.5M-LE 333.2 336.4 347.2 348.210M-LE 333.3 336.8 346.9 348.0

Formation of char, %2.5M-LE 21.4 22.5 22.9 23.15.0M-LE 22.0 22.6 23.0 23.27.5M-LE 22.5 22.8 23.1 23.310M-LE 22.6 23.0 23.1 23.2

the 25 phr ATH added LDPE–EVA blends were observed to demon-strate the degradation or decomposition in two degradation steps.The first step of degradation was taken place in the temperaturerange of 250–420 �C, while the second step of degradation was ob-served to occur in the range of 420–550 �C as shown in Fig. 2. Thefirst step degradation is attributed to the released of water vapourduring ATH combustion as well as elimination of acetic acid of EVAchains by formation of double bonds and also breakage of cross-linking networks for irradiated samples [1,7,9]. On the other hand,the second step of degradation is mainly involved the thermaldecomposition of ethylene backbone chains and also crosslinkedbackbone chains. The thermal decomposition of backbone chainin LDPE–EVA blends decreased the molecular mass with the forma-tion of combustible gases and char in small mass amount [1]. Byreferring to Table 2, the first stage decomposition temperaturewas gradually increased from 318.2 to 333.3 �C as the MMT loadinglevel increased from 2.5 to 10 phr. This also indicated that theincreasing of MMT loading level from 2.5 to 10 phr could enhancethe thermal stability of LDPE–EVA by delaying the decompositiontemperature with 15.1 �C. As mentioned earlier in LOI results, theintercalation of LDPE–EVA matrix into MMT particles galleriescould lead to prevent the diffusion of volatile gases produced dur-ing thermal degradation out from the polymer matrix [9,26]. TheMMT particles in LDPE–EVA matrix also could reduce the perme-ability of oxygen gas from LDPE–EVA surface into the LDPE–EVAmatrix. Hence, it has enhanced the thermal properties of LDPE–EVA blends. Besides that, the increasing of MMT loading level from2.5 to 10 phr was also observed to increase the formation of charresidue after the combustion process from 21.4% to 22.6% (as ob-served in Table 2). The char formed and covered on the LDPE–EVA blends could act as a protective layer that thermally insulatedthe LDPE–EVA matrix from combustion and also separated thepolymer surface from oxygen gas.

By referring to Table 2, the increasing of irradiation dosagesfrom 0 to 250 kGy gradually increased the first stage decomposi-

46 S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48

tion temperature of LDPE–EVA blends with increment in the rangeof 14.7–29.7 �C. It is obviously seen that the irradiation crosslink-ing has further improved the thermal stability of LDPE–EVA blendsby introducing the three dimensional crosslinking networks. Thiscould be due to the crosslinking networks formed in LDPE–EVAmatrix is more stable in resisting the formation of volatile gasesduring combustion [9]. Thus, the thermal stability of LDPE–EVAblends has been enhanced by the irradiation crosslinking. On theother hand, the formation of char residue after the thermal decom-position slightly increased as the irradiation dosages increasedfrom 0 to 250 kGy. This indicates that the formation of crosslinkingnetworks can promote the formation of char. Also, the char formedcould act as a thermal protective layer of LDPE–EVA blends by pre-venting the oxygen gas from contact with the surface of LDPE–EVAblends. Thus, the thermal stability could be enhanced with theapplication irradiation crosslinking process.

3.3. Mechanical properties

According to Fig. 4(a), the tensile strength of non-irradiatedLDPE–EVA blends was slightly increased as the loading level ofMMT increased from 2.5 to 7.5 phr. This might be attributed thedispersion of MMT particles in LDPE–EVA matrix could providereinforcement effect to the polymer matrix. The addition of MMTparticles into ATH added LDPE–EVA matrix could finely intercalateinto the cavities between the ATH particles and LDPE–EVA matrix.LDPE–EVA matrix can effectively intercalate into the interlayer gal-leries of MMT particles, while the ATH particles were attached tothe polar section of MMT particles [26,27]. The intercalation effectof MMT particles could improve the interaction between ATH and

8

10

12

14

16

18

20

2.5 5.0 7.5 10.0Loading level of MMT, phr

Ten

sile

Str

engt

h, M

Pa Non-irradiated Irradiated at 50 kGy

Irradiated at 150 kGy Irradiated at 250 kGy

(a)

350

400

450

500

550

600

2.5 5.0 7.5 10.0

Loading level of MMT, phr

Elo

ngat

ion

at B

reak

,

%

Non-irradiated Irradiated at 50 kGyIrradiated at 150 kGy Irradiated at 250 kGy

(b)

80

85

9095

100

105

110

2.5 5.0 7.5 10.0

Loading level of MMT, phr

You

ng's

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ulus

, M

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Non-irradiated Irradiated at 50 kGyIrradiated at 150 kGy Irradiated at 250 kGy

(c)Fig. 4. Effects of electron beam irradiation dosage on (a) tensile strength, (b)elongation at break and (c) Young’s modulus of 25 phr ATH added LDPE–EVA blendsunder various loading level of MMT.

LDPE–EVA matrix and promote stress transfer effectively frompolymer matrix to the ATH and MMT particles [14]. However, thetensile strength was slightly decreased with further increment inMMT loading level from 7.5 to 10 phr. This could be due to the highamounts of MMT particles were agglomerated into larger particlesand caused poor dispersion of MMT particles in polymer matrix.This also reduces the intercalation effect in polymer matrix andthus ruins the performance of tensile strength. On the other hand,the tensile strength of the LDPE–EVA blends was gradually in-creased with increasing of irradiation dosage at all loading levelsof MMT. This also indicated that the irradiation crosslinking couldfurther improve the reinforcement effect of MMT particles to theLDPE–EVA matrix [28]. The formation of crosslinking networkscould further enhance the interfacial adhesion between the poly-mer matrix and ATH and MMT particles while improving the com-patibility between the ATH and MMT particles and LDPE–EVAblends. This could enable the stress to be transferred effectivelyfrom polymer matrix to ATH and MMT particles, subsequently en-hances the tensile strength of LDPE–EVA blends [29].

Fig. 4(b) shows the effect of MMT loading level and irradiationdosages on elongation at break of 25 phr ATH added LDPE–EVAblends. The increasing of MMT loading level from 2.5 to 10 phrhas gradually decreased the elongation at break of all non-irradi-ated and irradiated LDPE–EVA blends. The intercalation of LDPE–EVA matrix into the interlayer galleries of MMT particles promotesformation of a restricted environment against the polymer chainsto move freely. This also reduces the ability of elongation for inter-calated LDPE–EVA blends. The agglomeration of MMT particles athigher loading level could significantly reduce the intercalation ef-fect of MMT particles in polymer matrix, subsequently the elonga-tion at break was reduced. On the other hand, the increment ofirradiation dosage from 0 to 150 kGy has gradually increased theelongation at break of LDPE–EVA blends at low MMT levels (2.5and 5 phr). This is due to the formation of crosslinking in polymermatrix could improve the intercalation effect of MMT particles andLDPE–EVA matrix. This also could further enhance the interfacialadhesion between ATH particles and LDPE–EVA matrix. However,the elongation at break of LDPE–EVA blends was significantly de-creased as the irradiation dosage increased up to 250 kGy. This isattributed to the high degree of crosslinking formed in LDPE–EVAmatrix which can resist the matrix reorganization and restrictthe mobility of polymer chains from slippage on each other whenunder stretching [14]. At high loading level of MMT (10 phr), theelongation at break of LDPE–EVA blends was gradually increasedwith increasing of irradiation dosage (from 0 to 250 kGy). Thisdue to the formation of crosslinking networks in LDPE–EVA matrixcould extent the continuities of polymer matrix. Therefore, theincrement of irradiation dosage has significantly increased theelongation at break of LDPE–EVA blends.

By referring to Fig. 4(c), the addition of MMT particles intoLDPE–EVA matrix has significantly increased the stiffness Young’smodulus of non-irradiated and irradiated LDPE–EVA blends. Theincrement of MMT loading level from 2.5 to 10 phr has graduallyincreased the Young’s modulus of LDPE–EVA blends under variousirradiation dosages. The incorporation of MMT particles into ATHadded LDPE–EVA matrix has effectively enhanced the Young’smodulus by intercalating of LDPE–EVA matrix into interlayer gal-leries of MMT particles. The intercalation of LDPE–EVA matrix intothe interlayer galleries of MMT particles and the attachment of po-lar edge section of MMT particle to ATH particles with high polarbehavior are played an important role in enhancing the Young’smodulus [27]. This also indicated that the MMT particles couldeffectively fit and intercalate into interfacial between ATH particlesand LDPE–EVA matrix and enhance the interfacial adhesion of ATHparticles in LDPE–EVA matrix. Therefore, the mobility of polymerchains has been restricted. At low loading level of MMT, the

S.-T. Bee et al. / Composites: Part B 61 (2014) 41–48 47

MMT particles could well distribute and disperse in LDPE–EVA ma-trix and this could help in enhancing the intercalation effect ofMMT particles in polymer matrix with minimum occurrence ofagglomerated MMT particles. However, the MMT particles tendedto agglomerate together into larger MMT aggregates particles athigh loading level of MMT. The agglomerated MMT particles couldreduce the intercalation effects of MMT particles in polymer ma-trix, thus reducing the Young’s modulus.

The effect of irradiation crosslinking on Young’s modulus ofLDPE–EVA blends has been investigated as shown in Fig. 4(c). Atlow MMT loading levels (2.5 and 5 phr), the increasing of irradia-tion dosages was found to significantly increase the Young’s mod-ulus of LDPE–EVA blends. This was attributed to the formation ofcrosslinking networks in LDPE–EVA matrix could highly restrictthe mobility of LDPE–EVA chains to slippage between each otherand thus enhance the Young’s modulus (stiffness) of LDPE–EVAblends. This indicated that the formation of crosslinking networksin ATH added LDPE–EVA matrix could further enhance the rein-forcement effect of ATH particles in LDPE–EVA matrix by enhanc-ing the interfacial adhesion (compatibility) within ATH particlesand LDPE–EVA matrix [28]. The high restriction in the mobility ofpolymer chains by the crosslinking networks formed in polymermatrix and has highly improved the Young’s modulus of ATHadded LDPE–EVA blends. Besides, the formation of crosslinkingnetworks in ATH added LDPE–EVA matrix also observed to lowerthe agglomeration of ATH and MMT particles in polymer matrix.Then, this also could increase the total effective MMT particles inpolymer matrix that reduce the continuities of polymer matrixand further enhance the Young’s modulus. However, the Young’smodulus of high MMT added LDPE–EVA blends was gradually de-creased with increasing of irradiation dose as shown in Fig. 4(c).This is due to the high MMT particles in LDPE–EVA matrix areagglomerated together into larger MMT particles. The formationof crosslinking networks in polymer matrix with MMT agglomer-ated particles was found unable to further improve the intercala-tion effects of MMT particles in LDPE–EVA matrix.

4. Conclusions

The dispersion roles of MMT on intercalation and flammability-thermal behaviours of electron beam irradiated ATH added LDPE–EVA blends were investigated in this study. The XRD analysisexhibited that the MMT particles were well dispersed and interca-lated in the matrix of LDPE–EVA blends. The intercalation effect ofMMT has significantly improved the compatibility between the fil-ler particles and polymer matrix producing samples with betterproperties. Moreover, the intercalation of MMT into LDPE–EVA ma-trix was effectively enhanced by electron beam irradiation with theformation of crosslinking networks. The addition of MMT has en-hanced the flame resistivity of ATH added LDPE–EVA blends wherethe LOI% for non-irradiated and irradiated samples was observed tothe highest at the loading level of 10 phr MMT. Meanwhile, theelectron beam irradiation also improved the flame resistivity ofATH added LDPE–EVA blends. The LOI value was found to increaseabout 2% as the irradiation dosage as the 10 phr MMT added sam-ples when subjected to 250 kGy compared to non-irradiatedsamples.

In addition, the increasing of MMT loading level also exhibitedthermal stability effect to the blends with the elevation of firststage of decomposition temperature when the MMT loading levelhas increased from 2.5 to 10 phr. This is due to the intercalationof LDPE–EVA matrix into MMT particles galleries can resist the per-meability of volatile gases out from polymer matrix. By increasingthe irradiation dosage from 0 to 250 kGy, the first stage decompo-sition temperature of ATH and MMT added LDPE–EVA blends were

also significantly improved. This indicates that the formation ofcrosslinking networks in matrix of ATH and MMT added LDPE–EVA blends is more stable in resisting the production of volatilegases when heating, thus enhancing the thermal stability ofLDPE–EVA matrix. The increasing of MMT loading level and cross-linking networks also found to induce the formation of char. Thechar formation could enhance the flame resistance and thermalstability of LDPE–EVA blends by reducing the permeability of vol-atile gases while acted as thermal insulation layer to reduce theheat transfer from vaporizing the flammable volatile polymercomponents.

The increment of MMT loading level has slightly increased thetensile strength of LDPE–EVA blends by intercalating the polymermatrix into the interlayer galleries of MMT particles. The effectiveintercalation of MMT particles in LDPE–EVA matrix could improvethe interaction between ATH particles and LDPE–EVA matrix byeffectively transferring stress from polymer matrix to MMT andATH particles. Thus, the tensile strength has been enhanced. Onthe other hand, the tensile strength of LDPE–EVA matrix also grad-ually increased with increasing of irradiation dosage by introduc-ing the formation of crosslinking networks in LDPE–EVA matrix.The increasing of MMT particles has exhibited an inferior effecton the elongation at break of LDPE–EVA blends. This is due theintercalation effect of MMT particles in LDPE–EVA matrix couldimprove the interfacial adhesion between ATH particles andLDPE–EVA matrix, thus reducing the elongation at break. Theincrement of irradiation dosage was significantly enhanced theelongation at break of LDPE–EVA blends. The formation of cross-linking networks in polymer matrix could extent the matrix conti-nuities of LDPE–EVA blends.

Acknowledgements

The authors are grateful for the financial support from ResearchUniversity Grant of Universiti Teknologi Malaysia. The authors arevery appreciating with the kindliness of Malaysian Nuclear Agency,Bangi, Selangor for allowing usage of their equipments in perform-ing this research. Special thanks to Nigel Foong from UniversitiTunku Abdul Rahman, Kampar Campus, on assisting the thermo-gravimetry analysis.

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