JOURNAL OF COMPOSITE M AT E R I A L S

Article

Effect of the ethylene-acrylic acid melt index on the structural characteristics and properties of high-density polyethylene/ layered double hydroxide nanocomposites prepared via the master-batch method

Journal of Composite Materials 2014, Vol 48(2) 245–256 ! The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998312470153 jcm.sagepub.com

Yaozhu Tian1,2, Huan Zhang2, Jun Qin2,3, Jie Yu2, Liping Cheng1 and Qing Lv3

Abstract High-density polyethylene/Mg2Al-layered double hydroxide nanocomposites with different ethylene-acrylic acid random copolymer/Mg2Al-layered double hydroxide master batches were prepared by melt-mixing. The effect of the ethyleneacrylic acid random copolymer melt index on the exfoliated/intercalated nanocomposite structures was investigated by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. The crystallization behavior and thermal property of the nanocomposites were determined via differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis and cone calorimetry. The Mg2Al-layered double hydroxide layers were well-dispersed at the nanometer level, providing direct evidence of the formation of intercalated/exfoliated nanocomposites, as well as the important effect of ethylene-acrylic acid random copolymer melt index on these structures and their properties. Lower ethylene-acrylic acid random copolymer melt index resulted in higher exfoliation and better dispersion of layered double hydroxide in the high-density polyethylene matrix. Reductions in the heat release rate, total heat release and the carbonic oxide and carbon dioxide production of the nanocomposites were also observed. These phenomena led to higher thermal stability and changes in the storage modulus, tan and loss modulus of the nanocomposites, as well as improvement in the heterogeneous nucleation effect of layered double hydroxide on high-density polyethylene.

Keywords High-density polyethylene, layered double hydroxide, nanocomposites, melt index, ethylene-acrylic acid

Introduction Layered double hydroxides (LDHs) are typical anionic clay minerals composed of positively charged metal hydroxide sheets with intercalated anions and water molecules in the interlayer region. They are also known as hydrotalcite-like compounds. The general III chemical formula for LDHs is ½MII 1x Mx II ðOHÞ2 xþ An  mH O, where M is a divalent metal x=n 2 ion, MIII is a trivalent metal ion, and An x=n is an anion.1–4 One of the research hotspots concerning LDHs is their application in polymers. The addition of LDH to polymers and its subsequent intercalation and exfoliation improves the ionic conductivity, sharply decreases the heat release rate (HRR), reinforces the optical property and increases the composite tension, gas permeability and selectivity of the resulting

products.5–9 However, the high surface charges and inherent large polar groups of LDH limit its exfoliation in nonpolar polyolefin.10–12 Among the many methods used to prepare polyolefin/LDH nanocomposites, the melt compounding technique is the most challenging.2

1 The Material and Metallurgy College, Guizhou University, Guiyang, China 2 National Engineering Research Center for Compounding and Modification of Polymer Materials, Guiyang, China 3 Key Laboratory of Karst Environment and Geohazard Prevention (Guizhou University), Ministry of Education, Guiyang, China

Corresponding author: Jun Qin, Caijiaguan campus, Guizhou University Guiyang 550003, China. Email: [email protected]

246 Pristine LDH is not suitable for intercalation by polymer chains because of its short intergallery space. The two most common melt compounding methods that improve LDH dispersion in meltcompound polyolefin nanocomposites involve organic modification of the LDH, which increases the average spacing between the LDH layers, and compatibilization of the components, either by functionalizing the polyolefin or through the addition of a separate compatibilizing polymer.13–16 Costa et al.2,10–17 modified LDH with sodium dodecylobenzene sulfonate using maleic anhydride-grafted polyethylene as a compatibilizer to prepare polyolefin/Mg-Al-LDH nanocomposites through the melt-mixing technique, obtaining better thermal and flammability properties. Some studies have reported the preparation and characterization of nanocomposites based on High-density polyethylene (HDPE) and LDHs.9,10,17–20 In our earlier work,18 pristine LDH was directly blended with ethylene acrylic acid (EAA) to prepare a master batch in which LDH was well dispersed in the matrix. EAA can replace the carbonate ion and water molecules in the interlayer regions and mix with polyolefin, thereby resulting in higher LDH dispersion in the nanocomposites. However, the effects of the properties of the compatibilizers or polymers (as the carrier resin in the LDH master batch) on the nanocomposite structure and properties have not been fully described in the literature. Determination of these effects would be of great technological significance and it is highly important in improving the mechanical and other properties of HDPE. To the best of our knowledge, only a few studies of the effects of the properties of the compatibilizer (or as a carrier resin in the LDH master batch) on HDPE/ LDH nanocomposite structure and properties have been published. The current study prepares HDPE/ Mg2-Al LDH nanocomposites using different master batches and describes their related properties. The effect of EAA melt index (MI) on the dispersion of LDH layers and their interaction in HDPE matrices were characterized by Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WXRD). In addition, the crystallization behavior and thermal property of the nanocomposites were determined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and cone calorimetry.

Materials and experimental methods Materials HDPE (5200B), with density of 0.968 g/cm3 and MI of 1.01 g/10 min, was supplied by Beijing Yanshan

Journal of Composite Materials 48(2) Petrochemical Co., Ltd. Ethylene-acrylic acid random copolymers EAA1 (acrylic acid content, 9.7%; MI, 5.0 g/10 min) and EAA2 (acrylic acid content, 9.7%; MI, 20.0 g/10 min) were purchased from Dow Chemical Company and used to prepare the EAA/ LDH master batch. Mg2Al-LDH (Xiangfan Petrochemical Co., Ltd.) has a MgO:Al2O3 molar ratio of 4.2, which corresponds to an Mg: Al ratio of approximately 2 and LDH layer space of 0.77 nm (d003). All materials were dried at 80 C under vacuum prior to mixing.

Nanocomposites preparation HDPE/Mg2Al–LDH nanocomposites were prepared via two melt-dispersion steps. Firstly, EAA was mixed with Mg2Al-LDH at a mass proportion of 20:6 using a twin-screw extruder (Type TSE-40A/400-44-22, Nanjing, China) at a screw speed of 150 r/min. Nanocomposites containing different content of Mg2Al–LDH nanoparticles were then prepared through melt-mixing with HDPE and the EAA/ Mg2Al–LDH master batch, in which the mass ratio of HDPE, EAA and LDH was 100/20/6. The temperatures at six zones, from the hopper to the die, were 160, 170, 180, 190, 200 and 210 C, and the screw speed was 400 r/min. Specified amounts of the EAA/ Mg2Al–LDH master batch were added to HDPE, and the product containing EAA1 was labeled as HPEAA1 and that containing EAA2 was labeled as HPEAA2.

Characterization and measurements The FTIR spectra of the LDH materials were obtained using a Nicolet NEXUS-670 FTIR spectrometer. The degree of swelling and the interlayer distance in the layer structure of Mg2Al–LDH were determined by XRD using a Rigaku (Japan) D/max-RA XRD. The Cu-Ka radiation of the rotating anode generator operated at a voltage of 40 kV and at a current of 40 mA; the scanning rate was 2 /min at an interval of 0.01 . TEM was performed on a JEM-200 CX transmission electron microscope operating at an accelerating voltage of 200 kV. Nonisothermal crystallization kinetic measurements were conducted using a TA Q10 DSC. Approximately 7 mg of the sample was heated from 50 C to 200 C at a rate of 10 C/min under a nitrogen atmosphere and then kept at 200 C for 5 min to eliminate any nuclei that could act as seed crystals. The samples were then cooled to 50 C at a cooling rate of 10 C/min. DMA testing was performed on 35  12.72  3.17 mm3 specimens using a TA DMA between 100 and 130 C at a heating rate of 5 C/min, and a frequency of 1 Hz. The thermal stability of the materials was determined by TGA on a TA Q-50 instrument.

Tian et al. The samples were heated from 30 C to 800 C at a rate of 15 C/min and under air atmosphere with a flow rate of 20 mL/min. Cone calorimetry was performed on a cone calorimeter (Stanton Redcroft, UK) according to ISO 5660 standard procedures. Each 100  100  3 mm3 specimen was wrapped in aluminum foil and exposed horizontally to an external heat flux of 35 kW/m2.

Results and discussion FTIR analysis of the EAA/Mg2Al-LDH master batch Figure 1 shows the FTIR spectra of the carbonate anion-containing raw LDH, the EAA/Mg2Al-LDH master batch and pure EAA. The broad band in the 3700 cm1 to 3200 cm1 range corresponds to the anhydride groups of hydroxide layers and interlayer water. The –OH stretching intensity in the EAA/ Mg2Al-LDH master batch decreases. At the same time, the –CH2 stretching vibration in the 2850 cm1 to 2965 cm1range and the -C ¼ O vibration in the 1630 cm1 to 1720 cm1 range for EAA and their master batches (Figure 1(b) to (d)) indicate that EAA was absorbed into the interlayers and on the surfaces of the hydroxide layers during the melting process. In the FTIR spectrum of EAA/Mg2Al-LDH (Figure 1(c) and (d)), the strong absorbance of EAA at1630 cm1 to 1720 cm1, as well as those of LDH at 1367 cm-1 (g3), 781 cm1 (g2) and 565 cm1 (g3), significantly weakens and almost disappears. The new band at 1600 cm1 (corresponding to the C ¼ O stretching vibration of the metal carboxylate salt) may be due to the reaction

Figure 1. Fourier transform infrared (FTIR) spectra of raw layered double hydroxide (LDH) containing carbonate anion and the ethylene acrylic acid (EAA)/Mg2Al-LDH master-batches. a: LDH; b: EAA; c: EAA1/Mg2Al-LDH; d: EAA2/Mg2Al-LDH.

247 of EAA and LDH with anhydride groups and may indicate the occurrence of intercalation/exfoliation.

WXRD analysis of the EAA/Mg2Al-LDH master-batch Figure 2 shows the XRD patterns of the master batchbased EAA/Mg2Al-LDH in the 2y ¼ 5 to 40 range. The comparison of the two composites shows that the basal position peak (003) of the EAA2/Mg2Al-LDH master batch (Figure 3(b) and (c)) is smaller than that of the EAA1 master batch and has almost disappeared, indicating the occurrence of exfoliation and the increase of interlayer distance. On the other hand, the higher basal position peak (003) of the EAA1 master batch (Figure 1(b) and (c)) indicates less exfoliation and the decrease of interlayer distance. The complete disappearance of the XRD peaks suggests a high degree of exfoliation or the presence of a small diffracting volume. This result may be due to a high degree of anion exchange with EAA2. When the content of EAA is certain, the greater the MI is, the lower the plastic raw material viscosity and the smaller the molecular weight is. Therefore, at the same mass fraction, the EAA with the higher MI has more molecules and reacts more easily with LDH. This condition is beneficial to insert EAA into the interlamination of LDH and exfoliate LDH.

TEM of the master batches Figure 3 shows TEM images of the EAA1/Mg2Al-LDH and EAA2/Mg2Al-LDH master batches. The images

Figure 2. X-ray diffraction (XRD) curves of ethylene acrylic acid (EAA)/Mg2Al-LDH master-batches. a: layered double hydroxide (LDH); b: EAA1/Mg2Al-LDH (20/6); c: EAA2/Mg2AlLDH (20/6).

248 show large exfoliated and intercalated LDH in the EAA master batch. Moreover, the scattered density contributes to the increase in the LDH content. However, there are more intercalated LDH in the EAA1/Mg2Al-LDH master batch than in EAA2/ Mg2Al-LDH. Figure 3(c) and (d) shows that most LDH molecules, which exhibit the 2 d nanoflake form, are tiled in the EAA, exhibiting thin nanolayers, nearly uniform thickness, and close distribution. In addition, these LDH molecules nearly fill all EAA particles, likely due to the improved adhesive properties and processing liquidity of EAA at certain AA contents and higher amounts of added EAA MI. In turn, the contact between EAA and LDH is improved, and the carboxyl group in the EAA molecular chain reacts more easily with the hydroxyl on the LDH surface during melt grafting at high temperature. The formation of an ionic bond between Mg2þ and Al3þ thus becomes more facile. When more ionic bonds are formed, more CO2 is produced between the LDH layers instead of EAA molecular chain inserts. In addition, the use of an extruder screw with strong shear promotes the exfoliation of LDH.

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Effect of EAA MI on the composite structure and properties Effect of EAA MI on the composite structure. Figure 4 shows the XRD pattern of the HDPE/EAA/Mg2Al-LDH (HDPE: EAA: LDH ¼ 100:20:6) nanocomposites. Both nanocomposites do not exhibit the (003) crystal diffraction peak of LDH, suggesting that the LDH in these nanocomposites may have been completely exfoliated. Figure 5 shows SEM images of the nanocomposites. When EAA was added, fibrous connections between the LDH particles and the HDPE matrix are formed, resulting in the formation of a regular network structure and indicating the higher compatibility and dispersion of LDH in the matrix. On the other hand, composites without EAA exhibit a smooth and clear interface, which is indicative of lower compatibility and dispersion of LDH in the matrix.18 The two composite materials display a neat EAA/Mg2Al-LDH network structure, which is consistent with the FTIR result for the master batch. After LDH was exfoliated in the system, the ionic bonds of the EAA/Mg2Al-LDH

Figure 3. Transmission electron microscopy (TEM) photomicrographs of the ethylene acrylic acid (EAA)/Mg2Al-LDH masterbatches. a: EAA1/Mg2Al-LDH (15000); b: EAA1/Mg2Al-LDH (100,000); c: EAA2/Mg2Al-LDH (15,000); d: EAA2/Mg2Al-LDH (100,000).

Tian et al. master batch form a mesh structure in the composite material. Figure 6 shows TEM photomicrographs of the HDPE/EAA/Mg2Al-LDH nanocomposites. The LDH in the two composite materials is small, uniform, and

Figure 4. Wide-angle X-ray diffraction (WXRD) scans of high-density polyethylene (HDPE)/ethylene acrylic acid (EAA)/ Mg2Al-LDH nanocomposites. a: layered double hydroxide (LDH); b: HPEAA1; c: HPEAA2.

249 well distributed, indicating higher compatibility with the HDPE matrix. In addition, most of the LDH particles are in an exfoliated state, which is consistent with the SEM and XRD findings. The LDH particles in the nanocomposites are more highly exfoliated than those in the master batches. Moreover, the LDH particles in the EAA2 master batch are more highly exfoliated than those in the EAA1 master batch (Figures 3 to 5). The LDH in the HPEAA1 nanocomposites is smaller, more uniform and shows greater distribution than that in the HPEAA2 nanocomposites, indicating that at a certain AA content, the closer correlation between the MI of the EAA and the HDPE matrix increases the compatibility between the two molecules and results in increasing EAA scattering in the matrix. In turn, the dispersion of LDH in the matrix becomes more uniform. At high temperature, the insertion of molecular chains of the matrix into the LDH layers is promoted by a secondary shear, which results in further exfoliation of LDH. Furthermore, the molecular chains increase as the EAA MI decreases, thereby increasing the shear strength and promoting the exfoliation of LDH.

Figure 5. Scanning electron microscope (SEM) photomicrographs of high-density polyethylene (HDPE)/ethylene acrylic acid (EAA)/ Mg2Al-LDH nanocomposites. a: HPEAA1 (2000); b: HPEAA1 (6000); c: HPEAA2 (2000); d: HPEAA2 (6000).

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Figure 6. Transmission electron microscopy (TEM) photomicrographs of high-density polyethylene (HDPE)/ethylene acrylic acid (EAA)/Mg2Al-LDH nanocomposites. a: HPEAA1 (15,000); b: HPEAA1 (100,000); c: HPEAA2 (15,000); d: HPEAA2 (100,000). (a) cooling and (b) heating.

(a)

(b)

Figure 7. Differential scanning calorimetry (DSC) curves of the high-density polyethylene (HDPE)/ethylene acrylic acid (EAA)/ Mg2Al-LDH nanocomposites. a: pure HDPE; b: HPEAA1; c: HPEAA2. (a) Cooling and (b) Heating.

Effect of EAA MI on the nonisothermal crystallization. Figure 7 shows the DSC curves of pure HDPE and HDPE/ EAA/Mg2Al-LDH nanocomposites. The degree of crystallinity, the total crystalline rate and other crystalline parameters obtained from the DSC curves are listed in Table 1. The crystallization temperature and

total crystallization rate of the composite material are higher than those of pure HDPE, while the melting temperature is lower. These results are attributed to the synergistic effects of EAA and LDH. As heterogeneous HDPE nucleating agents, EAA and LDH can both increase the crystallization temperature and

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Table 1. The molting and crystallization parameters.

Pure HDPE HPEAA1 HPEAA2

HDPE/EAA/LDH

Tc ( C)

Tm ( C)


T ( C)


H (Jg-1)

Xc (%)

100/0/0 100/20/6 100/20/6

116.74 119.16 118.31

136.02 135.63 135.3

19.28 16.47 16.99

207.4 158.8 156.6

70.9 68.4 67.37

EAA: ethylene acrylic acid; HDPE: high-density polyethylene; LDH: layered double hydroxide.

Figure 8. Storage modulus of (a) pure high-density polyethylene (HDPE), (b) HPEEAA1 and (c) HPEAA2.

crystallization rate of the composite material. Moreover, the high EAA content can reduce the melting temperature of the composite materials because of the lower melting temperature of EAA. As the LDH content increases, the relative crystallinity of HPEAA1 improves. However, the relative crystallinity of the HPEAA2 composite materials initially increases before decreasing; eventually reaching a level that is less than that of the HPEAA1 composite at 6 phr LDH. This result is primarily due to the improved liquidity, as well as the increased contact and reactivity between the carboxyl groups of the EAA acrylic chain segment, as the EAA MI increases. In turn, the adverse heterogeneous nucleation effect of the acrylic chain segments on HDPE is hindered, and the relative crystallinity initially increases at lower LDH content. However, at higher LDH content, the LDH particles in the HPEAA1 composite are well exfoliated and evenly dispersed, thus promoting the heterogeneous nucleation effect of LDH on HDPE and resulting in high crystallinity.21,22

Effect of the EAA MI on DMA. Figure 8 shows the storage modulus of pure HDPE and HDPE/EAA/Mg2Al-LDH nanocomposites. When the temperature is between 100 and 0 C, the storage modulus of HPEAA2 is higher than that of HDPE and HPEAA1 and that of HPEAA1 is the lowest. At the same AA content, the EAA with the higher MI promotes intercalation of EAA or HDPE into the LDH layers (Figure 6), thereby improving the stiffness of the composite materials, as well as its resistance to deformation. The storage modulus also increases. As the temperature increases, the storage modulus of pure HDPE, as well as that of the nanocomposites, decreases. The storage modulus of HPEAA2 sharply decreases, mainly because at high temperature, the higher MI of EAA can increase the molecular chain motion of the composite materials, which weakens the physical interaction between the molecules and LDH and results in a sharp decline in the storage modulus. Figure 9 shows the loss modulus of pure HDPE and the HDPE/EAA/Mg2Al-LDH nanocomposites.

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Figure 9. Loss modulus of (a) pure HDPE, (b) HPEAA1 and (c) HPEAA2.

Figure 10. Tan d of pure high-density polyethylene (HDPE), (b) HPEAA1 and (c) HPEAA2. EAA: ethylene acrylic acid.

Three kinds of materials in the near 50 C region have a significant relaxation peak, which corresponds to their a transformation. When the temperature is between 100 and 20 C, the order of the loss modulus is

HPEAA2 > HPEAA1 > pure HDPE. The loss modulus of HPEAA2 is higher than that of HPEAA1 is mainly because of the higher EAA MI, which promotes the connection with the LDH surface. However,

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Figure 11. Thermogravimetric analysis (TGA) curves of (a) pure high-density polyethylene (HDPE), (b) HPEAA1 and (c) HPEAA2.

Table 2. Thermogravimetric analysis (TGA) parameters of pure HDPE, HDPE/EAA1-LDH and HDPE/EAA2-LDH.

Pure HDPE HPEAA1 HPEAA2

HDPE/EAA/LDH

T0.05 ( C)

T0.1 ( C)

T0.5 ( C)

Tf ( C)

Rmax (%/ C)

100/0/0 100/20/6 100/20/6

319.48 416.32 381.44

370.66 442.10 398.12

419.92 469.39 443.61

449.38 472.19 399.96

2.668 2.632 2.013

T0.05, T0.1 and T0.5: temperatures at the rate of weight loss of 5%,10% and 50%, respectively; Tf: rapid decomposition temperature; Rmax: rate of rapid decomposition.

at 20 C or above, the loss modulus of HPEAA1 is higher than that of HPEAA2. This result is due to the higher LDH layer exfoliation rate in the HDPE matrix when the MI of EAA is low. The LDH layers exhibit a strong interface interaction with the matrix, thereby limiting the molecular freedom of movement. At low EAA MI, the ‘‘crank movement’’ of molecular chain segments occurs more frequently, and the loss modulus slightly increases. However, both comprehensive results are limited. An increased EAA MI inhibits the dispersion of LDH, weakens the interface reactions and increases the loss modulus. On the other hand, a high EAA MI also enhances the freedom of movement of the short molecular chains that ‘‘hang’’ in the LDH layers, thereby increasing the loss modulus. Figure 10 shows the tan d of pure HDPE and the HDPE/EAA/Mg2Al-LDH nanocomposites. When the temperature is between 100 and 22.22 C, the loss moduli of the three materials are low; however, the

tan  of the two nanocomposites are higher than that of pure HDPE. This result is due to the decrease in internal friction when the molecular chain sections are frozen, resulting in smaller tan d. However, for nanocomposite materials, the higher energy loss due to the stronger interaction between LDH and EAA indicates a higher tan d compared with that of pure HDPE. When the temperature ranges from 30 C to 70 C, the order of tan  obtained from the three materials is HPEAA2 > HPEAA1 > pure HDPE. At this temperature range, pure HDPE and the HPEAA1 nanocomposite have larger molecular chains compared with the HPEAA2 nanocomposite. At the same temperature, the restricted movement of the larger molecular chains results in a decreased tan d. In addition, the effective exfoliation and dispersion of the LDH layers further restricts the movement of the large molecules, which also contributes to the decrease of tan d.

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(b)

Figure 12. Heat release rate (HRR) and total heat releases (THR) of (a) pure high-density polyethylene (HDPE), (b) HPEAA1 and (c) HPEAA2.

(a)

(b)

Figure 13. The CO and CO2 production of (a) pure high-density polyethylene (HDPE), (b) HPEAA1 and (c) HPEAA2.

Effect of EAA MI on thermal stability. Figure 11 shows the TGA curves of pure HDPE and the HDPE/EAA/ Mg2Al-LDH nanocomposites. Table 2 lists the parameters of these materials. Compared with pure HDPE, the T0.1, T0.05, T0.5 and Tf of the nanocomposite materials are clearly improved, whereas the Rmax of HPEAA1 is higher than that of the HPEAA2 nanocomposite. This result is mainly attributed to the high heat resistance of the LDH layers, as well as the greater compatibility effect of EAA. When the LDH dosage is fixed at 6 phr, the lower MI EAA increases the thermal stability more than the higher MI EAA, mainly because the lower MI EAA can better improve the exfoliation degree of LDH in HDPE than a higher MI EAA, whereas the higher MI EAA increases the dispersion of intercalated LDH in the HDPE matrix (Figure 6).23 Figures 12 and 13 show the combustion performance curves of the different EAA nanocomposites (100/20/6).

Table 3 lists the experimental data that corresponds to the combustion performance. The HRR, total heat releases (THR) and CO and CO2 production of the nanocomposites are clearly reduced because of the improved compatibility of EAA and the inhibition effect of the exfoliated LDH layers on oxygen dispersion. A higher number of exfoliated LDH layers enhance the inhibition effect of the exfoliated LDH layers on oxygen dispersion, which improves the thermal stability and reduces the release of harmful gases.24

Conclusions In this study, HDPE/Mg2Al–LDH hybrid nanocomposites with different EAA/Mg2Al–LDH master batches were prepared through the twin-screw extrusion compounding process. The effects of the EAA MI on the exfoliated/intercalated structure of nanocomposites

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Table 3. Some cone parameters of pure HDPE, HPEAA1 and HPEAA2.

Pure HDPE HPEAA1 HPEAA2

PHRR (kW/m2)

EHC (MJ/kg)

SEA (m2/kg)

THR (MJ/m2)

TTI (s)

1095.31 926.07 1033.94

45.17 44.85 45.92

302.62 331.93 301.98

258.6 250.8 254.3

58 41 50

PHRR: peak value of heat release rate; EHC: effective heat of combusion; SEA: specific extinction area; THR: total heat release; TTI: time to ignition.

were investigated by XRD, TEM, FTIR and SEM. The crystallization behavior and thermal property were determined by DSC, TGA, DMA and cone calorimetry. The Mg2Al–LDH layers are well-dispersed at the nanometer level, both in the master batches and nanocomposites, thereby providing direct evidence of the formation of intercalated/exfoliated nanocomposites. The EAA MI significantly affects the exfoliated/ intercalated structure and properties of the nanocomposites. Although a higher EAA MI can promote the formation of exfoliated/intercalated structures in the master batch, a lower EAA MI promotes the exfoliation of the LDH in the nanocomposites, indicating that a lower EAA MI is more advantageous in improving the exfoliation rate and dispersion of the LDH in the HDPE matrix. Lower EAA MI also results in the reduction of the HRR, THR and the CO and CO2 production of the nanocomposites. In turn, the thermal stability is improved. A lower storage modulus and loss modulus, as well as a higher tan d, are obtained at higher temperature. These phenomena enhance the heterogeneous nucleation effect of LDH on HDPE and result in high crystallinity. Acknowledgement The authors are grateful for the financial supports of this research from National Science and Technology Supporting Project Foundation of China, Major Science and Technology Projects of Guizhou Province as well as from Major Science and Technology Projects of Guizhou Province.

Conflict of interest None declared.

Funding This work were supported by National Science and Technology Supporting Project Foundation of China (2007BAB08B05), Major Science and Technology Projects of Guizhou Province ([2010]6003), and Major Science and Technology Projects of Guizhou Province ([2011]6023).

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