Compatibilizing effect of SEBS-g-MA on the ...

Composites: Part A 39 (2008) 1802–1814

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Compatibilizing effect of SEBS-g-MA on the mechanical properties of different types of OMMT filled polyamide 6/polypropylene nanocomposites Kusmono a, Z.A. Mohd Ishak a,*, W.S. Chow a, T. Takeichi b, Rochmadi c a

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia Department of Materials Science, Toyohashi University of Technology, Toyohashi 441-8580, Japan c Department of Chemical Engineering, Gadjah Mada University, Jln. Grafika 2 Yogyakarta, Indonesia b

a r t i c l e

i n f o

Article history: Received 19 April 2008 Received in revised form 27 July 2008 Accepted 28 August 2008

Keywords: A. Nano structures B. Mechanical Properties B. Strength E. Injection Molding

a b s t r a c t Nanocomposites based on polyamide 6/polypropylene (PA6/PP) blends containing organophilic montmorillonite (OMMT) and maleic anhydride grafted styrene–ethylene/butylene–styrene (SEBS-g-MA) were prepared by melt compounding method followed by injection molding. Four different types of OMMT were used in this work, i.e. dodecylamine modified MMT (D-MMT), 12-aminolauric acid modified MMT (A-MMT), stearylamine modified MMT (S-MMT), and commercial organo-MMT (C-MMT). X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed the presence of SEBS-g-MA did not produce any apparent effect on the dispersion of OMMT in the PA6/PP matrix. Incorporation of SEBS-g-MA into the nanocomposites enhanced strength, ductility, and impact strength but slightly reduced stiffness. A good balance of strength, stiffness, and toughness was obtained for PA6/PP/OMMT nanocomposites in the presence of SEBS-g-MA. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Polymer/clay nanocomposites (PCNs) have received special attention in recent years because of their improved properties at very low clay loading compared with conventional filler composites. Among these improved properties are mechanical, dimensional, barrier to different gases, thermal stability, and flame retardant enhancements with respect to the pristine polymer. These improved properties were attributed to the strong synergistic effects between the polymer and the silicate platelets on both the molecular or nanometric scale [1,2]. Since pristine clay is not compatible with most polymers because of its hydrophilic nature, it needs to be chemically modified to render its surface more hydrophobic. The most popular surface treatment is via ion exchange of the clay with organic ammonium cations, which not only render its surface more hydrophobic, but also expand the spaces between the silicate layers [3]. Despite of the significant improvement of properties as mentioned before, a major drawback of PCNs that limits their vast range of potential engineering applications is reduced toughness, especially when the organoclay content was above 5 wt.% [4]. Recently, considerable efforts were devoted to improve the toughness of PCNs by the addition of thermoplastic elastomer. González et al. [5] reported that super-tough PA6/clay * Corresponding author. Tel.: + 60 4 5995262; fax: + 60 4 5941011. E-mail address: zarifi[email protected] (Z.A. Mohd Ishak). 1359-835X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2008.08.009

nanocomposites could be obtained with an incorporation of 30% SEBS-g-MA. An improvement in the toughness with the presence of SEBS-g-MA was also reported by Tjong et al. [6,7] for nanocomposites based on PA6 and PP. Chiu et al. [8] reported that the addition of maleated polyethylene-octene elastomer (POEg-MA) to PA6/organoclay nanocomposites led to decrease in the tensile strength and modulus, however, the elongation at break and impact strength increased significantly. More recently, Chow et al. [9] and Wahit et al. [10] demonstrated that the ductility and toughness of PA6/PP/ organoclay nanocomposites were improved by the addition of maleated ethylene-propylene rubber (EPR-g-MA) and POE-g-MA, respectively. Based on our previous study [11], modification of clay using various different types of organic modifiers plays an important role towards the enhancement of stiffness and strength. However, the ductility of the various OMMT/PA6/PP nanocomposites has been adversely affected. Thus, the incorporation of SEBS-g-MA is expected to produce PA6/PP/OMMT nanocomposites with a good balance of stiffness, strength, and toughness. In this study, PA6/PP/organoclay nanocomposites were prepared by melt compounding. Four different types of organically modified MMT were used in this work, i.e., dodecylamine modified MMT (D-MMT), 12 aminolauric acid modified MMT (A-MMT), stearylamine modified MMT (S-MMT), and commercial organoMMT (C-MMT). The aim of this work was to evaluate the effect of SEBS-g-MA on the morphological, mechanical, and thermal properties of PA6/PP/OMMT nanocomposites.

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2. Experimental 2.1. Materials PA6 (Amilan CM 1017) used in this study was a commercial product of Toray Nilon Resin AMILAN, Japan. The melt flow index or MFI and density of PA6 were 35 g/10 min (at 230 °C and 2.16 kg load) and 1.14 g/cm3, respectively. PP (Pro-Fax SM-240) was purchased from Titan Himont Polymer (M) Sdn. Bhd., Malaysia. MFI and density of PP were 25 g/10 min (at 230 °C and 2.16 kg load) and 0.9 g/cm3, respectively. Styrene–ethylene/butylene–styrene triblock copolymer grafted with 1.84 wt.% of maleic anhydride (SEBS-g-MA) was supplied by Kraton Polymer, USA under the trade name of KratonTM FG 1901X. It has been reported that the ratio of styrene to ethylene/butylene in the triblock copolymer was 30/70 by wt.% and its MFI value of 22 g/10 min (at 230 °C and 5 kg load). The molecular weights of the polystyrene and poly-(ethylene/butylene) copolymer blocks were 7000 and 37,500 g/mol, respectively. Kunipia-F, Na-montmorillonite (Na-MMT) clay with cation exchange capacity (CEC) of 119 mEq/100 g, was supplied by Kunimine Industry Co., Japan. Three different types of alkyl ammonium salts used to modify Na-MMT clay were dodecylamine [NH2 CH3 (CH2)11], 12 aminolauric acid [NH2 (CH2)11COOH], and stearylamine [NH2 CH3 (CH2)16 CH2], respectively. 12-aminolauric acid was used as received from Tokyo Kasei Kogyo Co., whereas dodecylamine and stearylamine from Kishida Chemical Co., Osaka Japan. A commercial organoclay (Nanomer I.30TC) supplied by Nanocor, Inc., USA was also used in this study. This organoclay was a white powder containing montmorillonite (70 wt.%) intercalated by octadecylamine (30 wt.%). The designation, composition, and the MFI values of samples tested are given in Table 1. 2.2. Preparation of organically modified montomorillonite Organophilic montmorillonite (OMMT) was prepared by cation exchange of MMT with various alkyl ammonium salts according to a method reported by Agag and Takeichi [12] and has been reported in our previous work [11]. Three different types of OMMT prepared in this work were dodecylamine modified MMT (D-MMT), 12 aminolauric acid modified MMT (A-MMT), and stearylamine modified MMT (S-MMT), respectively. In addition, commercial organoclay (Nanomer I.30TC) was also used and termed C-MMT. 2.3. Preparation of PA6/PP/OMMT nanocomposites PA6/PP (70/30), OMMT (4 phr), and SEBS-g-MA (5 phr) were melt-mixed in a co-rotating intermeshing twin-screw extruder (BERSTORFF ZE 25) at temperature ranged from 230 to 240 °C and a screw speed of 70 rpm. Prior to melt-mixing, PA6 pellets

and organoclay were dried using an oven at 80 °C for 15 h. The extrudates were then injection-molded into standard tensile (ASTM D638 type I) and flexural specimens (ASTM D790) using an injection molding machine (Haitian HTF160X). The barrel zone temperatures were set at 190, 235, 250, 255, 260, and 250 °C and a mold temperature of 110 °C. Prior to injection molding, all pellets were dried in an oven at 80 °C for 15 h. 2.4. Melt flow index (MFI) Melt flow index of the samples were measured by using a Dynisco Melt Flow Indexer (at 230 °C, load 2.16 kg) according to ASTM D 1238. 2.5. X-ray diffraction (XRD) X-ray diffraction measurements of the nanocomposites were carried out on bars. All these experiments were performed in reflection mode with a D5000 diffractometer (Siemens) using CuKa radiation at a scan rate of 0.3°/min in a 2h range of 2–10°, and operated at 30 kV and 20 mA. 2.6. Transmission electron microscopy (TEM) TEM measurements were carried out with a JEOL JEM-200CX TEM operating at an accelerating voltage of 200 kV. The specimens were prepared using a Leica Ultracut UCT ultramicrotome. Ultrathin sections of about 60 nm in thickness were cut with a diatome diamond knife (35°) at room temperature. 2.7. Differential scanning calorimetry (DSC) The melting and crystallization behaviors of the PA6/PP blend and its nanocomposites were studied by differential scanning calorimetry (DSC; Perkin Elmer DSC-6) in nitrogen atmosphere. The sample (ca. 7 mg) sealed in aluminum pan was heated from 50 to 250 °C at 10 °C/min, held at 250 °C for 5 min, cooled back to 50 °C at 10 °C/min, held at 50 °C for 5 min, and then the second heating and cooling similar to the first were performed in order to erase the thermal history during processing. The melting and crystallization thermograms were recorded from the second heating and cooling. The melting temperature (Tm), crystallization temperature (Tc), and degree of crystallinity (Xc) of PA6 and PP components were determined from the DSC thermograms. The fusion enthalpies DHf (PP) and DHf (PA6) were measured and the degree of crystallinity Xc (PP) and Xc (PA6) were calculated from Eqs. (1) and (2) [13]:

X C ðPPÞ ¼

DHf ðPPÞ 1 100% DH0f ðPPÞ wPP

X c ðPA6Þ ¼

ð1Þ

DHf ðPA6Þ 1 100% DH0f ðPA6Þ wPA6

ð2Þ

Table 1 Designation, composition, and MFI values of samples Sample designation

Composition

Parts (phr)

MFI (g/10 min)

PA6/PP PA6/PP/4D-MMT PA6/PP/4A-MMT PA6/PP/4S-MMT PA6/PP/4C-MMT PA6/PP/5S/4D-MMT PA6/PP/5S/4A-MMT PA6/PP/5S/4S-MMT PA6/PP/5S/4C-MMT

PA6/PP PA6/PP/Dodecylamine-MMT PA6/PP/Aminolauric acid-MMT PA6/PP/Stearylamine-MMT PA6/PP/Commercial organo-MMT PA6/PP/SEBS-g-MA/Dodecylamine-MMT PA6/PP/SEBS-g-MA/Aminolauric acid-MMT PA6/PP/SEBS-g-MA/Stearylamine-MMT PA6/PP/SEBS-g-MA/Commercial organo-MMT

70/30 70/30/4 70/30/4 70/30/4 70/30/4 70/30/5/4 70/30/5/4 70/30/5/4 70/30/5/4

74.96 43.11 48.76 27.28 42.50 6.44 8.90 2.83 16.71

1804


where, DH0f ðPPÞ ¼ 209:2 J/g and DH0f ðPA6Þ ¼ 190:8 J/g are the fusion enthalpies of 100% crystalline PP and PA6 [14], respectively. wPP and wPA6 are the weight fractions of PP and PA6, respectively.

2θ = 3.48°; d = 2.54 nm C-MMT

2.8. Mechanical properties

Relative Intensity

Tensile and flexural tests were carried out with a universal testing machine (Instron 3366) at room temperature according to ASTM D638 type I and ASTM D790, respectively. Tensile test was performed at a crosshead speed of 50 mm/min. For flexural test, a three-point bending configuration was selected with a support span length of 50 mm and a crosshead speed of 3 mm/min. Izod impact test was carried out on notched specimens using a Pendulum Hammer Impact 25 S/N V67R (Galdabini) according to ASTM 256-02 with a impact speed of 3.46 m/s.

2θ = 3.98°; d = 2.22 nm

S-MMT 2θ = 5.02°; d = 1.76 nm A-MMT 2θ = 5.44°; d = 1.62 nm D-MMT

2.9. Fractography studies The fracture surface of impact specimens was investigated using Field Emission Scanning Electron Microscopy (Supra 35VP24-58 FE SEM) at an acceleration voltage of 15 kV. The fracture surface was sputter-coated a thin gold–palladium layer in vacuum chamber for conductivity before examination. 2.10. Dynamic mechanical analysis (DMA) The dynamic mechanical properties of PA6/PP blend and its nanocomposites were performed under dual cantilever mode on a dynamic mechanical analyzer (Mettler Toledo Model DMA 861e). Measurements of storage modulus (E0 ) and tan d as a function of temperature (T) were recorded in the range of 100 to 150 °C at a frequency of 1 Hz and a heating rate of 10 °C/min in a nitrogen atmosphere. The dimension of the specimen was 55 12.5 3.0 mm3.

2

4

6

8

10

2θ (°) Fig. 1. XRD patterns of OMMT.

found for the PA6/PP/5S/4S-MMT nanocomposite. The better exfoliated structure in the PA6/PP/5S/4S-MMT nanocomposite as evidenced by the XRD and TEM results shown later is believed to be responsible for the observed trend. In exfoliated nanocomposites, individual clay platelets with high aspect ratio are dispersed homogenously in polymer matrix. This leads to a high contact surface area and gives rise to a strong interaction between organoclay and polymer matrix. López-Quintanilla et al. [2] reported that PP nanocomposites with exfoliated structure have a lower MFI than one with intercalated structure. 3.2. X-ray diffraction (XRD)

2.11. Head distortion temperature (HDT) Heat distortion temperature (HDT) of PA6/PP blend and its nanocomposites were measured using specimens having dimensions 125 12.50 3.0 mm3 according to ASTM D 648. The test was conducted at a heating rate of 2 °C/min and a fiber stress of 1.8 MPa using a four Station Advanced HDT/Vicat Softening Point Apparatus (Ray-Ryan Test Equipment, Ltd).

3. Results and discussion 3.1. Melt flow index (MFI) Table 1 shows the MFI values of uncompatibilized and SEBS-gMA compatibilized PA6/PP/OMMT nanocomposites. It can be seen that the addition of OMMT into PA6/PP matrix resulted in a decrease in its MFI value. This may be attributed to the interaction between the amine groups of organic modifier in the organoclay and amide groups in the PA6. Among all the uncompatibilized nanocomposites, PA6/PP/4S-MMT exhibited the lowest MFI value due to better exfoliated structure in this nanocomposite. A significant reduction in MFI values of PA6/PP/OMMT nanocomposites was observed in the presence of SEBS-g-MA. This may be due to the interaction between the amine groups of organic modifier within organoclay and maleic anhydride groups of SEBS-g-MA. Another possible interaction is between the amine groups of organic modifier within organoclay and amide groups of PA6 via hydrogen bonding. A similar observation was reported by Chow et al. [9,14] for PA6/PP/organoclay nanocomposites compatibilized with both EPR-g-MA and PP-g-MA. The lowest MFI value was

Fig. 1 reveals the XRD patterns of OMMT. The D-MMT, A-MMT, S-MMT, and C-MMT patterns shows the characteristic diffraction peaks at 2h = 5.44°, 5.02°, 3.98°, and 3.48° respectively, corresponding to basal spacing (d001) of 1.62 nm, 1.76 nm, 2.22 nm, and 2.54 nm respectively. The higher basal spacing observed for both S-MMT and C-MMT compared to both D-MMT and A-MMT may be attributed to longer alkyl chains of S-MMT and C-MMT. Both S-MMT and C-MMT composed of 18 C atoms whereas both D-MMT and A-MMT possessed 12 C atoms. Reichert et al. [15] reported a similar observation where the silicate interlayer distance was increased with increasing alkyl chain length of the alkyl-substituted primary amine of organoclay. Furthermore, the basal spacing of A-MMT was greater than that of D-MMT. This could be due to larger dimension of the –COOH group in A-MMT than –CH3 in the D-MMT [16]. There are the two functional groups in the A-MMT, namely amine and carboxylic acid groups. Fig. 2 shows the XRD patterns of PA6/PP blend and PA6/PP/ OMMT nanocomposites. Both PA6/PP/4 D-MMT and PA6/PP/4AMMT exhibited a broad shoulder peak at 2h = 2.99° and 3.89°, respectively, indicating the formation of a mixture of intercalated and exfoliated structures in the nanocomposites. On other hand, PA6/PP/4S-MMT and PA6/PP/4C-MMT did not show any diffraction peak of organoclay in the range of 2h = 2–10°, suggesting the formation of exfoliated structure in both nanocomposites. Fig. 3 displays the XRD patterns of SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites. In case of PA6/PP/5S/4D-MMT, a broad shoulder peak at 2h = 3.93° appeared in the XRD pattern, indicating a mixture of intercalated and exfoliated structures was formed in this nanocomposite. Thus, the presence of SEBS-g-MA in PA6/PP/4D-MMT did not promote dispersion of organoclay in

1805


2θ = 3.24° PA6/PP/5S/4C-MMT

Relative intensity

Relative intensity

PA6/PP/4C-MMT

PA6/PP/4S-MMT

2θ = 3.89°

PA6/PP/5S/4S-MMT

PA6/PP/4A-MMT

PA6/PP/5S/4A-MMT 2θ = 3.93°

2θ = 2.99°

PA6/PP/5S/4D-MMT

PA6/PP/4D-MMT

PA6/PP

2

4

6

8

10

2θ (°) 2

4

6

8

10

Fig. 3. XRD patterns nanocomposites.

of

the

SEBS-g-MA

compatibilized

PA6/PP/OMMT

2θ (°) Fig. 2. XRD patterns of the PA6/PP and PA6/PP/OMMT nanocomposites.

the PA6/PP matrix. The diffraction peak of organoclay disappeared in the XRD pattern of PA6/PP/5S/4S-MMT nanocomposite. The absence of diffraction peak probably suggests that organoclay was exfoliated and dispersed in the PA6/PP matrix and the presence of SEBS-g-MA did not affect dispersion and exfoliation of organoclay. For PA6/PP/5S/4C-MMT, a broad shoulder peak at 2h = 3.24° appeared in the XRD pattern, suggesting the formation

of a mixture of intercalated and exfoliated structures in the nanocomposite. Accordingly, the addition of SEBS-g-MA into PA6/ PP/4C-MMT reduced the degree of exfoliation of organoclay in the PA6/PP matrix. 3.3. Transmission electron microscopy (TEM) Fig. 4 displays the TEM images of PA6/PP/4S-MMT, PA6/PP/5S/ 4S-MMT, PA6/PP/4C-MMT, and PA6/PP/5S/4C-MMT nanocomposites. The dark lines represent the intersection of individual silicate

Fig. 4. TEM images: (a) PA6/PP/4S-MMT; (b) PA6/PP/5S/4S-MMT; (c) PA6/PP/4C-MMT; (d) PA6/PP/5S/4C-MMT.


3.4. Differential scanning calorimetry (DSC) Figs. 5 and 6 show the DSC heating and cooling scans of the uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites. The melting temperature (Tm), crystallization temperature (Tc), and the degree of crystallinity (Xc) of PP and PA6 components are summarized in Table 2. From Fig. 5, it can be seen that all samples exhibited two strong peaks, i.e., at around 161–166 °C and 220–223 °C in which correspond to the melting temperatures of PP and the a-form crystals of PA6 [19], respectively. In addition, a weak peak at around 214 °C attributed the melting temperature of c crystal form of PA6 phase was also observed. With the exception of PA6/PP/4S-MMT, the incorporation of OMMT slightly increased the Tm of PP and PA6. This result indicates that the addition of clays did not influence the structure and stability of the PP and PA6 crystals formed [20]. However, the addition of S-MMT into PA6/PP matrix drastically decreased the Tm of PA6 to 214.5 °C, corresponding to the melting temperature of c crystal form of PA6 phase [21]. This suggests that the presence of

Tm(PA6)

Tm(PP)

PA6/PP/5S/4C-MMT PA6/PP/5S/4S-MMT PA6/PP/5S/A-MMT

Heat flow endo

layer while the gray base corresponds to the PA6 phase. In case of PA6/PP/4S-MMT (cf. Fig. 4a), OMMT was well exfoliated into individual silicate layer in the PA6 phase; indicating exfoliated structure was achieved as confirmed by XRD results (cf. Fig. 2). The strong tendency of the OMMT to be located in the PA6 phase could be attributed to the fact that the OMMT has a higher affinity to more polar PA6 phase than PP phase. This is in agreement with our previous work [17] on PA6/PP nanocomposites whereby AFM was used to prove the affinity of nanoclays to PA6 phase. The presence of exfoliated structure could also be observed in the SEBS-gMA compatibilized PA6/PP/4S-MMT (PA6/PP/5S/4S-MMT) (cf. Fig. 4b). The presence of SEBS-g-MA did not produce any apparent affect on the exfoliation of OMMT in the polymer matrix, which is consistent with the XRD results as discussed earlier. The formation of exfoliated structure could also be obtained in the TEM image of PA6/PP/4C-MMT (cf. Fig. 4c). It is interesting to note that in the case of PA6/PP/5S/4C-MMT, apart from the presence of exfoliated structure, a few elliptical particles without any silicate layer were observed (cf. Fig. 4d). The elliptical particles may represent the SEBS-g-MA phase. The silicate layers are predominantly located in the PA6 phase and not in the SEBS-g-MA phase. This provides a clear indication that the polar substituents of OMMT have a strong affinity to PA6 instead of SEBS-g-MA. A similar observation was reported by González et al. [5] in case of the PA6/OMMT/SEBSg-MA nanocomposites. They found that OMMT was not present both in the SEBS-g-MA phase and in the interphase. For PA6/PP/ 5S/4C-MMT, the XRD and TEM results are not consistent each other where a mixture of exfoliated and intercalated structure is formed (XRD results) and the exfoliated structure (TEM image). According to Dasari et al. [18], XRD is not a reliable tool for analyzing the complex dispersion of clay layers in ternary nanocomposites, particularly, polymer–rubber–clay systems.

PA6/PP/5S/4D-MMT PA6/PP/4C-MMT PA6/PP/4S-MMT PA6/PP/4A-MMT PA6/PP/4D-MMT

PA6/PP

50

100

150

200

250

Temperature (°C) Fig. 5. DSC heating scans of the uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites.

PA6/PP/5S/4C-MMT PA6/PP/5S/4S-MMT PA6/PP/5S/4A-MMT PA6/PP/5S/4D-MMT

Heat flow endo

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PA6/PP/4C-MMT PA6/PP/4S-MMT PA6/PP/4A-MMT PA6/PP/4D-MMT PA6/PP TC (PA6)

TC (PP)

50

100

150

200

250

Temperature (°C) Fig. 6. DSC cooling scans of the uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites.

S-MMT promoted the phase transformation from a to c for PA6 phase. The may be attributed to better dispersion and exfoliation of S-MMT in the PA6/PP matrix. The introduction of nanoclay filler into the polymer matrix had a strong heterogeneous nucleation effect, which was favorable for the formation of the less stable c-form crystals of PA6 [16,19]. From Fig. 5 and Table 2, it can also

Table 2 DSC results of the uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites Sample designation

Tm (PP) (°C)

Hm (PP) (J/g)

Tc (PP) (°C)

Xc (PP) (%)

Tm (PA6) (°C)

Hm (PA6) (J/g)

Tc (PA6) (°C)

Xc (PA6) (%)

PA6/PP PA6/PP/4D-MMT PA6/PP/4A-MMT PA6/PP/4S-MMT PA6/PP/4C-MMT PA6/PP/5S/4D-MMT PA6/PP/5S/4A-MMT PA6/PP/5S/4S-MMT PA6/PP/5S/4C-MMT

161.5 163.5 163.8 162.5 164.4 164.1 163.1 164.1 166.1

16.9 11.9 13.0 7.1 16.5 11.2 9.5 11.2 7.1

118.9 118.5 119.9 115.5 116.4 72.2 75.8 78.5 76.5

26.9 19.7 21.5 11.8 27.3 19.5 16.5 19.5 12.3

220.7 221.1 221.7 214.5 220.4 221.8 220.5 221.5 223.1

34.8 23.1 24.8 14.6 34.4 21.1 21.1 24.6 15.4

188.2 183.2 183.9 183.2 187.4 184.9 185.2 185.5 184.2

26.1 18.0 19.3 11.4 26.8 17.2 17.2 20.1 12.6

1807


3 without SEBS-g-MA with SEBS-g-MA

Tensile modulus (GPa)

2.5

2

1.5

1

0.5

0 PA6/PP

PA6/PP/4D-MMT PA6/PP/4A-MMT PA6/PP/4S-MMT PA6/PP/4C-MMT

Sample designation Fig. 7. Effect of SEBS-g-MA on the tensile modulus of PA6/PP/OMMT nanocomposites.

be seen that both melting temperatures of PP and PA6 in the nanocomposites remained unchanged by the addition of SEBS-gMA, with the exception of PA6/PP/4S-MMT. The melting temperature of PA6 was significantly increased from 214.5 to 221.5 °C when the PA6/PP/4SMMT nanocomposite was compatibilized with SEBS-g-MA. This suggests that the addition of SEBS-g-MA into PA6/ PP/4S-MMT nanocomposite led to the phase transformation from c to a-form crystals of PA6 phase. In other words, the presence of SEBS-g-MA into PA6/PP/4S-MMT induced the formation of a form crystal of PA6 phase. It can also be noted from Fig. 6 and Table 2 that the crystallization temperature of PP was decreased for all samples, with the exception of PA6/PP/4A-MMT nanocomposite. Furthermore, the presence of clay led to a decrease in the crystallization temperature of PA6 in the nanocomposites. This could be attributed to the retarding effect of the silicate layers on the PP and PA6 crystals growth [22]. Furthermore, the addition of SEBS-g-MA into the nanocomposites drastically decreased the crystallization temperature of PP but slightly increased the crystallization temperature of PA6. The reduction in the crystallization temperature of PP could be attributed to a compatibilzation phenomenon generated by an increase in interactions between PP and PA6 resulting from the formation of a SEBS-g-PA6 copolymer. This SEBS-g-PA6 copolymer situated at the PP/PA6 interface could retard the crystallization of PP [23]. With the exception of PA6/PP/4C-MMT, the addition OMMT decreased the degree of crystallinity of both PP and PA6. This could be attributed to the physical hindrance of MMT layers to the motion of polymer molecular chains tends to retard the crystallization of polymer phase of nanocomposites reinforced with MMT clays, leading to a decrease in the degree of crystallinity [16]. A similar observation was reported for PA6/clay nanocomposites [24]. The lowest crystallinity obtained for the PA6/PP/4S-MMT may be attributed better exfoliated structure in the nanocomposite. With the exception of PA6/PP/4S-MMT, the addition of SEBS-g-MA into the nanocomposites decreased the degree of crystallinity of both PP and PA6 components (cf. Table 2). This may be attributed to the formation of a SEBS-g-PA6 during melt-mixing which could hinder the polymer chain packing, resulting into the reduced percent crystallinity [25]. Tjong et al. [7] reported a similar observation on PP/SEBS-gMA/organoclay nanocomposites. They found that the incorporation

of SEBS-g-MA into PP and PP/organoclay nanocomposites reduced the degree of crystallinity. On the contrary, the addition of SEBS-gMA into the PA6/PP/4S-MMT increased the degree of crystallinity. This suggests that the presence of SEBS-g-MA favored the crystallization of both PP and PA6 in the nanocomposites. This may be due to the better dispersion and exfoliation of silicate layers in the PA6/PP/5S/4S-MMT nanocomposite. 3.5. Mechanical properties 3.5.1. Tensile properties Fig. 7 shows the effect of SEBS-g-MA on the tensile modulus of PA6/PP/OMMT nanocomposites. It can be seen that the incorporation of OMMT into PA6/PP matrix significantly increased its tensile modulus. The improvement in stiffness may be caused by the reinforcement effect of the rigid inorganic clay and the constraining effect of silicate layers on molecular motion of polymer molecular chains [4,26]. The nanocomposite containing stearylamine modified MMT (S-MMT) showed the highest tensile modulus followed by those containing A-MMT, D-MMT, and then C-MMT. From Fig. 7, it can also be observed that the addition of SEBS-g-MA into the nanocomposites slightly decreased the tensile modulus, i.e. 6– 10%. The slight decrease in stiffness may be attributed to the low modulus elastomeric phase of SEBS-g-MA. A similar observation was reported for SEBS-g-MA toughened PA6/clay nanocomposites [5,6]. Tjong and Bao [6] found slight reduction in stiffness in the PA6/clay nanocomposites due to the elastomeric nature of SEBSg-MA. Fig. 8 displays the effect of SEBS-g-MA on the tensile strength of nanocomposites. For both PA6/PP/4D-MMT and PA6/PP/4A-MMT, the lower tensile strength than PA6/PP blend may be due to the poor dispersion of OMMT in the PA6/PP matrix. On other hand, the presence of S-MMT and C-MMT increased the tensile strength of PA6/PP matrix. The highest value was obtained for the nanocomposite containing S-MMT followed by those containing C-MMT, A-MMT, and then D-MMT. The better exfoliated structure in the PA6/PP/4S-MMT nanocomposite as shown by the XRD and TEM results discussed earlier is believed to be responsible for the higher tensile strength. With the exception of PA6/PP/5S/4C-MMT, the presence of SEBS-g-MA resulted in an increase in the tensile strength of nanocomposites. The improved interfacial adhesion

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60

without SEBS-g-MA with SEBS-g-MA

Tensile strength (MPa)

50

40

30

20

10

0 PA6/PP


Sample designation Fig. 8. Effect of SEBS-g-MA on the tensile strength of PA6/PP/OMMT nanocomposites.

resulting from the formation of a SEBS-g-PA6 copolymer is believed to be responsible for the improvement in tensile strength. Moreover, the presence of SEBS-g-MA facilitates expansion of interlayer spacing by inclusion of some polar groups (maleic anhydride of SEBS-g-MA) to intercalate between the silicate layers through hydrogen bonding to the amine terminal group of organic modifier within organoclay. This enhanced interlayer spacing of stacked nanolayers, which in turn are separated nanolayers and dispersed homogenously (exfoliated structure). The miscibility of SEBS-g-MA with polar groups of the nanoparticles and the PA6/ PP matrix mediates between the surface chemistry of the polymer and the organoclay at the interphase, which contributes to the increment in the tensile strength [27]. The formation of SEBS-gPA6 and possible interaction between SEBS-g-MA, clay, and PA6/ PP have been reported in our previous study and proved by FTIR analysis [28]. Physical interaction (physical entanglement) also possible occurs between PP and SEBS-g-MA because the chemical structure of PP is similar to the ethylene–butylene (EB) midblock of SEBS-g-MA [29]. As a result the formation of SEBS-g-PA6,

possible interfacial interaction via hydrogen bonding between SEBS-g-PA6 copolymer and the amine group of organic modifier within the organoclay, and the physical entanglement PP/SEBS contributed to an improvement in the compatibility between PA6, PP, SEBS-g-MA, and organoclay. The highest tensile strength was observed for the SEBS-g-MA compatibilized nanocomposite containing S-MMT followed by those containing A-MMT, D-MMT, and C-MMT, respectively. This could be due to the better dispersion and exfoliation of silicate layers in the PA6/PP matrix for PA6/PP/5S/4S-MMT nanocomposite as confirmed by the XRD results earlier. A high aspect ratio of clay particles which are dispersed more finely (exfoliated) coupled with a good interfacial interaction with the polymer matrix will facilitate a good stress transfer to the silicate layers; this consequently leads to improvement of the tensile strength [30]. The elongation at break of PA6/PP matrix was drastically decreased in the presence of OMMT (cf. Fig. 9). This may be due to the restraints on mobility of the polymer chains caused by the intercalated/exfoliated clay platelets [5]. On the contrast,


Elongation at break (%)

12 10 8 6 4 2 0 PA6/PP


Sample designation Fig. 9. Effect of SEBS-g-MA on the elongation at break of PA6/PP/OMMT nanocomposites.

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the incorporation of SEBS-g-MA into the PA6/PP/OMMT nanocomposites increased the elongation at break. The increase in ductility could be attributed to the formation of a SEBS-gPA6 copolymer which improved interfacial adhesion between PA6, PP, and organoclay. Moreover, the reduction in degree of crystallinity is also believed to be responsible for the improved ductility [31]. It was found that the presence of SEBS-g-MA reduced the degree crystallinity (cf. Table 2). However, the elongation at break value of SEBS-g-MA compatibilized nanocomposites was still lower than that of PA6/PP blend. In other words, the addition of SEBS-g-MA was not able to compensate fully for the embrittlement caused by the OMMT. The highest value in elongation at break was obtained for the SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposite containing commercial organoclay (PA6/PP/5S/4C-MMT). This could be attributed to high content of organic modifier (34%) within the commercial organoclay which may serve as plasticizer and then increased ductility. From TGA results [11], it was found that the content of organic modifier within D-MMT, A-MMT, S-MMT, and C-MMT was 22%, 19%, 33%, 34%, respectively.

3.5.2. Flexural properties Figs. 10 and 11 illustrates the effect of SEBS-g-MA on the flexural modulus and strength of PA6/PP/OMMT nanocomposites, respectively. The addition of OMMT into PA6/PP blend remarkably increased its flexural modulus (cf. Fig. 10) and slightly increased its flexural strength (cf. Fig. 11). These improvements could be attributed to high stiffness and aspect ratio of silicate layers in the OMMT [27]. The flexural modulus and strength of PA6/PP/4SMMT nanocomposite were the highest among all of the uncompatibilized nanocomposites. This may again be due to better exfoliated structure in the PA6/PP/4S-MMT nanocomposite. Furthermore, the presence of SEBS-g-MA in the nanocomposites decreased the flexural modulus and did not produce any significant effect on the flexural strength. This trend resembles to that of the tensile properties. The reduction in flexural modulus could again be attributed to the low modulus elastomeric phase of SEBS-g-MA. Similar observations were reported by Hasan et al. [32] on maleated polyethylene-octene elastomer (POE-g-MA) compatibilized PA6/PP/ organoclay nanocomposites and Dasari et al. [33] on SEBS-g-MA compatibilized PA6,6/organoclay nanocomposites.

3.5 without SEBS-g-MA with SEBS-g-MA

Flexural modulus (GPa)

3

2.5

2

1.5

1

0.5

0 PA6/PP


Sample designation Fig. 10. Effect of SEBS-g-MA on the flexural modulus of PA6/PP/OMMT nanocomposites.

90 without SEBS-g-MA

Flexural strength (MPa)

80

with SEBS-g-MA

70 60 50 40 30 20 10 0 PA6/PP


Sample designation Fig. 11. Effect of SEBS-g-MA on the flexural strength of PA6/PP/OMMT nanocomposites.

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8 without SEBS-g-MA

Impact strength (kJ/m2)

7

with SEBS-g-MA

6 5 4 3 2 1 0 PA6/PP


Sample designation Fig. 12. Effect of SEBS-g-MA on the impact strength of PA6/PP/OMMT nanocomposites.

3.5.3. Impact strength Fig. 12 demonstrates the effect of SEBS-g-MA on the impact strength of PA6/PP/OMMT nanocomposites. It can be seen that the addition of OMMT into PA6/PP blend drastically decreased its impact strength. A similar trend to that of the elongation at break was observed. The reduction in impact strength could be attributed to the immobilization of the macromolecular chains by the clay particles, which limited their ability to adapt to the deformation and make the material more brittle. In addition, each silicate layer or aggregates of silicate layers was the site of stress concentration and could act as a micro crack initiator [34]. On the contrary, the incorporation of SEBS-g-MA into the nanocomposites significantly improved the impact strength. This could be attributed to the improved interfacial adhesion between PA6, PP and organoclay resulting from the formation of a SEBS-g-PA6 copolymer. In addition, the improved impact strength could also be caused by the reduced degree of crystallinity [35]. From DSC results discussed earlier (cf. Table 2), it was found that the addition of SEBS-g-MA reduced the degree of crystallinity, leading to the improved impact strength. According to Ahn and Paul [36], rubber particles dispersed within a neat PA6 matrix increase toughness via cavitation which relieves the triaxial stress state ahead of the advancing crack trip and allows the PA6 matrix to shear yielding and thereby dissipate more energy and enhance toughness. Kelnar et al. [37] also found that the elastomer particles increased toughness by both acting as stress concentrators (by initiating energy absorbing micro deformations) and influencing the clay-induced matrix crystalline structure. The improvement in impact strength by the addition of elastomer was also reported for PA6/SEBS-g-MA/clay nanocomposites [5], PA6/POE-g-MA/clay nanocomposites [8], PA6/ABS/POE-g-

MA/clay nanocomposites [38], and for PA6/PP/POE-g-MA nanocomposites [32]. Chiu et al. [8] reported that the chemical interaction between the maleic anhydride groups of POE-g-MA and the amine end group of PA6 was believed to be responsible for the improved impact strength of POE-g-MA compatibilized PA6/clay nanocomposites. Among all of the SEBS-g-MA compatibilized nanomposites, the PA6/PP/5S/4A-MMT showed the highest impact strength. This may be attributed to the presence of carboxylic acid group in A-MMT allowing the formation of hydrogen bonding between the polymer chains and the surfactant molecules, leading to a strong interface between the polymer matrix and the clay platelets. A similar observation was reported by Xie et al. [39] for the PA6/organoclay nanocomposites, in which used organoclay with two different types of groups in the surfactant, namely amine and hydroxyl groups. They found that the PA6 nanocomposites containing the hydroxyl group within the surfactant of organoclay exhibited higher ductility than one containing amine group within the surfactant of organoclay. The significant improvement in impact strength reflects SEBS-g-MA as an effective toughening agent of PA6/PP/OMMT nanocomposites. 3.6. Fractography studies Fig. 13a and b show the SEM micrographs of impact fracture surface of PA6/PP/4A-MMT and PA6/PP/5S/4A-MMT, respectively. The PA6/PP/4A-MMT exhibited a very inhomogeneous fracture surface and a two-phase morphology, i.e. the spherical phase domains of PP surrounded by the continuous PA6 phase (cf. Fig. 13a). This indicates weak interfacial adhesion between PA6 and PP owing to both was the immiscible polymers. No evidence

Fig. 13. SEM micrographs of impact fracture surface of (a) PA6/PP/4A-MMT and (b) PA6/PP/5S/4A-MMT nanocomposites.

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of plastic deformation on the fracture surface of PA6/PP/4A-MMT explains low impact strength (cf. Fig. 12). Furthermore, the incorporation of SEBS-g-MA into PA6/PP/4A-MMT produced a more fine, uniform, and stable phase morphology (cf. Fig. 13b). This indicates a strong interaction and adhesion between PA6, PP, and organoclay. This may be attributed to the interfacial chemical reaction between the maleic anhydride groups of SEBS-g-MA and the amine terminal groups of PA6, leading to the formation of a SEBS-g-PA6 copolymer which contributes to decrease the interfacial tension and to enhance PP phase dispersion and interfacial adhesion between PA6 and PP [40]. Moreover, the presence of micro void can be observed in fracture surface of PA6/PP/5S/4A-MMT. This may result from the debonding and cavitation of SEBS-g-MA particles from PA6/PP matrix. The improved interfacial adhesion due to the formation of the copolymer and cavitation of elastomeric particles are believed to be responsible for an increase in impact strength in the PA6/PP/5S/4A-MMT (cf. Fig. 12).

a

3.7. Dynamic mechanical analysis (DMA) Fig. 14a shows the storage modulus (E0 ) as a function of temperature for uncompatibilized and SEBS-g-MA compatibilized PA6/PP/ OMMT nanocomposites. It can be seen that all the uncompatibilized nanocomposites exhibited higher values in E0 than the PA6/ PP blend over the temperature range examined. This indicates that the addition of OMMT significantly increased the storage modulus of PA6/PP matrix. This improvement may be attributed to the stiffness of the MMT layers, the constraining effect of these layers on molecular motion of polymer chains, and also by the combined effect of aspect ratio and degree of dispersion of MMT layers [4,26]. Contrarily, the storage modulus of SEBS-g-MA compatibilized nanocomposites was lower than that of uncompatibilized nanocomposites. This indicates that the presence of SEBS-g-MA reduced the stiffness of nanocomposites. This is consistent with the flexural modulus obtained from the static flexural tests

2500 PA6/PP PA6/PP/4D-MMT PA6/PP/4A-MMT

Storage Modulus (MPa)

2000

PA6/PP/4S-MMT PA6/PP/4C-MMT PA6/PP/5S/4D-MMT PA6/PP/5S/4A-MMT

1500

PA6/PP/5S/4S-MMT PA6/PP/5S/4C-MMT

1000

500

0 -100

-50

0

50

100

150

Temperature (°C) 0.14

b

PA6/PP PA6/PP/4D-MMT

0.12

PA6/PP/4A-MMT PA6/PP/4S-MMT

0.1

PA6/PP/4C-MMT PA6/PP/5S/4D-MMT

Tan δ

PA6/PP/5S/4A-MMT

0.08

PA6/PP/5S/4S-MMT PA6/PP/5S/4C-MMT

0.06

0.04

0.02

0

-100

-50

0

50

100

150

Temperature (°C) Fig. 14. (a) Storage modulus (b) tan d as a function of temperature for uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites.

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70

without SEBS-g-MA with SEBS-g-MA

60

Tg (°C)

50

40

30

20

10

0 PA6/PP


Sample designation Fig. 15. Effect of SEBS-g-MA on the Tg of PA6/PP/OMMT nanocomposites.

(cf. Fig. 10). The reduction in storage modulus could be correlated to the elastomeric nature of SEBS-g-MA. A similar observation was also reported by Chiu et al. [8] for POE-g-MA compatibilized PA6/ organoclay nanocomposites. The lowest storage modulus obtained for PA6/PP/5S/4C-MMT among all the compatibilized nanocomposites may be attributed to the plasticizing effect stemming from the elastomeric nature of SEBS-g-MA and high content of organic modifier (octadecylamine) within commercial organoclay, i.e. 34% from TGA data [11]. Fig. 14b reveals the tan d as a function of temperature for uncompatibilized and SEBS-g-MA compatibilized PA6/PP/OMMT nanocomposites. Two dynamic relaxation peaks were observed at around 42–57 °C and 29 to 50 °C, which are referred to as a and b relaxation peaks of PA6 [41], respectively. The a relaxation peak is believed to be related to the breakage of hydrogen bonding between polymer chain which induces long range segmental chain movement in the amorphous area. This is assigned to the glass transition temperature (Tg) of PA6 and shown in Fig. 15. The

b relaxation is related to the segmental amide group in the amorphous area which un-attached to the other amide group by hydrogen bonding. With the exception of D-MMT, it can be seen in Fig. 15 that the addition of OMMT into PA6/PP matrix slightly reduced the Tg of PA6. The slight reduction in the Tg may be attributed to high content of organic modifier (19–34%) within OMMT from the TGA data [11] which probably remained unintercalated within the clay galleries and served as a plasticizer during compounding of the clays with PA6/PP matrix. Moreover, the alkyl ammonium surfactant in the OMMT, having less affinity towards the matrix polymer rendered a hydrophobic environment near the clay surface, which shielded the thermodynamic interactions between OMMT and PA6 [24]. It is interesting to note that the presence of SEBS-g-MA in PA6/PP/OMMT nanocomposites led to a larger decrease in the Tg (cf. Fig. 15). This may be caused by the plasticizing effect from the elastomeric nature of SEBS-g-MA. The PA6/PP/5S/4C-MMT exhibited the lowest value in Tg among all samples. This could again be attributed to the plasticizing effect


HDT (°C)

100

50

0 PA6/PP


Sample designation Fig. 16. Effect of SEBS-g-MA on the HDT of PA6/PP/OMMT nanocomposites.


stemming from both the elastomeric nature of SEBS-g-MA and the high content (34%) of surfactant (octadecylamine) in the commercial organoclay. From Fig. 14b, it can also be seen that the addition of OMMT resulted in a decrease in the intensity of the a-relaxation peak. The presence of intercalated/exfoliated structure and the strong interactions between OMMT and PA6 leads to the restricted mobility of polymer chains [8]. On the contrary, the incorporation of SEBS-g-MA into PA6/PP/OMMT nanocomposites yielded a considerable increase in the intensity of the a-relaxation peak. This was again probably caused by the elastomeric contribution of SEBS-g-MA. Note that SEBS-g-MA should be located in the amorphous phase and in addition, it reacts with PA6 by forming an interphase of amorphous nature. This increases the intensity of the a-relaxation peak. A similar observation was also reported by Chow et al. [9] for EPR-g-MA compatibilized PA6/PP/organoclay nanocomposites. 3.8. Head distortion temperature (HDT) Fig. 16 depicts the effect of SEBS-g-MA on the HDT of PA6/PP/ OMMT nanocomposites. It can be seen that the HDT of PA6/PP matrix was significantly increased in the presence of OMMT. This may be attributed to the presence of strong hydrogen bonds between the polymer matrix and organoclay surface [42]. The nanocomposite prepared using stearylamine modified montmorillonite (PA6/PP/4S-MMT) displayed the highest HDT value among all nanocomposites. This may be caused by the better exfoliated structure in the PA6/PP/4S-MMT nanocomposite as confirmed by XRD and TEM results discussed earlier. It is also interesting to note from Fig. 16 that the HDT values of SEBS-g-MA compatibilized PA6/ PP/OMMT nanocomposites were lower than those of uncompatibilized ones. This indicates the presence of SEBS-g-MA reduced the thermal stability of nanocomposites. This is in agreement with the DMA and flexural modulus results discussed earlier. The elastomeric nature of SEBS-g-MA was believed to be responsible for the reduced HDT values. A similar observation was reported by Lai et al. [38] for POE-g-MA compatibilized PA6/ABS nanocomposites. The lowest HDT value was found for PA6/PP/5S/4C-MMT, which is in accordance with the storage modulus results (cf. Fig. 14a). This was probably caused by the plasticizing effect stemming from the elastomeric nature of SEBS-g-MA and high content (34%) of organic modifier within commercial organoclay. 4. Conclusion PA6/PP/OMMT nanocomposites were prepared by melt compounding using co-rotating twin-screw extruder followed by injection molding. XRD and TEM results revealed that the presence of SEBS-g-MA did not produce any apparent effect on the exfoliation and dispersion of OMMT in the PA6/PP matrix. The incorporation of SEBS-g-MA into the PA6/PP/OMMT nanocomposites enhanced strength, ductility, and impact strength but slightly reduced stiffness. The addition of both OMMT and SEBS-g-MA into PA6/PP blend produced the balanced properties between strength, stiffness, and toughness. The best reinforcement effect was played by S-MMT due to its longest alkyl chains, largest basal spacing, and better exfoliation of organoclay in the PA6/PP matrix. The addition of SEBS-g-MA was found to reduce both storage modulus and HDT of the PA6/PP/OMMT nanocomposites due to the elastomeric nature of SEBS-g-MA. Acknowledgement The financial support of AUN/SEED-Net JICA Project grant no: 6050071 is gratefully acknowledged.

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