Effect of surface modified kaolin on properties of polypropylene grafted maleic anhydride

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Effect of surface modified kaolin on properties of polypropylene grafted maleic anhydride Ni Yang a,1, Zuo-Cai Zhang a,1, Ning Ma a, Huan-Li Liu a, Xue-Qing Zhan a, Bing Li a, Wei Gao a, Fang-Chang Tsai a,⇑, Tao Jiang a, Chang-Jung Chang b, Tai-Chin Chiang c, Dean Shi a,⇑ a Hubei Key Laboratory of Polymer Materials, Key Laboratory for the Green Preparation and Application of Functional Materials (Ministry of Education), Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China b Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, ROC c Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 8 January 2017 Received in revised form 18 February 2017 Accepted 18 February 2017 Available online xxxx Keywords: PP-g-MAH Kaolin Modification Thermal stability

a b s t r a c t To achieve reinforcement of mechanical and thermal performances of polypropylene (PP) product, this work aimed at fabrication of surface modified kaolin (M-kaolin) filled polypropylene grafted maleic anhydride (PP-g-MAH) composites with varying contents of fillers and investigation of their mechanical and thermal properties. And the prepared PP-g-MAH/M-kaolin composites were characterized by means of Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Fracture analysis by SEM showed M-kaolin particles were well dispersed in the PP-g-MAH matrix. Mechanical behaviors were determined by tensile strength, tensile strain at break and impact strength analysis. Impact strength of PP-g-MAH/2 wt% M-kaolin composites was improved up to 30% comparing with unfilled composites. Thermostability had been found enhanced when M-kaolin added. The results revealed PP-g-MAH/M-kaolin composites showed the optimal thermal and mechanical properties when 2 wt% of M-kaolin was added. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Introduction

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Polypropylene (PP), which possesses the advantages of rich raw materials, nontoxicity and easy processing, etc., has become widely used polymer material. It has been applied in various areas such as plastic parts, automotive industry and additional industrial applications [1]. However, the poor mechanical and thermal properties of PP have limited its further development. Recently, fillersreinforcement polymer composites have received considerable attentions, as properties of composites can be improved by adding fillers [2–5]. Fillers such as calcium carbonate [6], hydrotalcite [7], fiberglass [8] and bentonite [9], etc. have been commonly used in fillers-reinforcement composites. Liu and Liang [10] reported polypropylene (PP)/montmorillonite (MMT) nanocomposites, grafting with surfactants gave rise to a considerable improvement of the mechanical property. N. Othman [11] prepared bentonite filled polypropylene composites and discussed effect of compati-

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⇑ Corresponding authors at: Hubei University, 368 Youyi Avenue, Wuchang, Wuhan, Hubei 430062, People’s Republic of China. E-mail addresses: [email protected] (F.-C. Tsai), [email protected] (D. Shi). 1 Co-First Authors: Ni Yang and Zuo-Cai Zhang.

bilisers on mechanical and thermal properties. S. Nekhlaoni [12] used Talc and Moroccan clay as reinforcements to improve the inherent properties of PP-SEBS-g-MA matrix. However, there is still rare research on kaolin. Mareri reported that the particle size of kaolin impact the properties of the kaolin/PP composite [13]. Kaolin, nonmetallic mineral filler [14], is used widely in ceramic, papermaking and coating, etc., and its content is impressively sufficient in our country. Meanwhile, as rigid particles, kaolin can not only improve the toughening effects but also not lose rigidity, thermostability and processability of materials. So, in order to expand applied range of PP, an effective and economical method is to compound PP with kaolin. Nevertheless, because of polarity difference between PP and kaolin, they can’t get good compatibility with each other. In this way, it has negative influence on the performance and quality of PP-based products. The effective way is to make PP grafted with maleic anhydride (MAH) [15], which gives the PP-based materials polarity and charge. On the other hand, coating treatment using KH550 promotes the adhesion between inorganic particles and organic polymers [16]. Therefore, the coupling agent modifying kaolin (M-kaolin) could be better compatible with PP. In this paper, we modified kaolin through using KH-550 (i.e., amine functionalized silanes), and prepared PP-g-MAH/M-kaolin

http://dx.doi.org/10.1016/j.rinp.2017.02.030 2211-3797/Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Yang N et al. Effect of surface modified kaolin on properties of polypropylene grafted maleic anhydride. Results Phys (2017), http://dx.doi.org/10.1016/j.rinp.2017.02.030

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composites by melt blending of PP-g-MAH and M-kaolin. Then we investigated the effects of M-kaolin on mechanical and thermal properties of PP-g-MAH composites. It was expected that the modified kaolin could be dispersed well in PP as it had superior dispersibility and interface bonding force with PP, and the mechanical and thermal properties of the resultant composite materials were significantly improved. The preparation mechanism and chemical reaction mechanism are shown in Fig. 1. The main purpose of this research was to improve the mechanical and thermal properties of polypropylene. Our results may be useful in the automotive industry and household necessities for daily use.

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Experimental part

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Materials

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The available polypropylene with the trade name of T300 was supplied from SINOPEC Shanghai Petrochemical Co., Ltd. It was characterized by a melt flow index (MFI) of 3.0 ± 1.0 g/10 min. Kaolin (the particle size was between 15 lm and 25 lm) was provided by Yulin PuBai kaolin clay Co., Ltd. The coupling agent 3-aminopropyltriethoxysilane (KH-550) (99% Purity) was purchased from Nanjing YouPu Chemical Industry Co., Ltd. Maleic anhydride (MAH) (
99.5% purity) was offered by Tianjing BoDi Chemical Industry Co., Ltd.

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Modification of kaolin particles

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In order to modify the surfaces of kaolin particles by the wet processing, 25 g kaolin powder was suspended in 100 mL ethanol

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by ultra-sonication for 20 min, with the temperature rising to 70 °C progressively. 0.45 mol KH-550 was dissolved in 4 mL ethanol, and then glacial acetic acid was added until the pH value is between 3.0 and 5.0. We take out 50 mL of the specimen and added the 1 mL deionized water in solution to measure the pH value. Then the mixture solution was dropped into the ethanol and kaolin suspension under vigorous stirring by a peristaltic pump at a constant flow rate of 1 mL/min. After stirring for 1 h, the modified kaolin was collected by filtration, then washed with deionized water and ethanol for several times and dried in vacuum at 70 °C.

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Preparation of PP-g-MAH and PP-g-MAH/M-kaolin composites

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PP, grafted with MAH with mass ratio 100:3 in a twin-screw extruder (TE-34, Nanjing KeYa Chemical equipment Co., Ltd, feeding temperature = 140 °C, extrusion temperature = 180 °C, screw speed = 9.5 Hz). Subsequently, PP-g-MAH were cooled and pelletized. After that, they were vacuum dried in oven at 70 °C for 24 h. PP-g-MAH and M-kaolin were prepared in a Plastograph-Mixer machine (SU-70ML, Changzhou source of plastic rubber and plastic technology Co., Ltd, China), at a screw speed of 60 r/min, with the temperature of 175 °C, and blended for 6 min. The melt-mixing procedure of PP-g-MAH/M-kaolin composites was performed by blending with better combination. Then the pelletized composites were dried in 70 °C for 12 h and injection molded in an injection molding machine (Babyplast/6/10p, BABYPLAST Co., Ltd, Italy). After its ejection from the mould, specimens (six dumbbellshaped splines and six impact splines according to GB/T 1040.52006 for each sample) were cooled. Samples were listed in Table 1.

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Fig. 1. Preparation of PP-g-MAH/M-kaolin.

Table 1 Formulations for various PP composites. Sample

Material

PP (%, w/v)

PP-g-MAH (%, w/v)

M-kaolin (%, w/w)

kaolin (%, w/w)

#1 #2 #3 #4 #5 #6 #7 #8

Pure PP PP-g-MAH PP/kaolin PP-g-MAH/1 wt% PP-g-MAH/2 wt% PP-g-MAH/3 wt% PP-g-MAH/4 wt% PP-g-MAH/5 wt%

100 – 98 – – – – –

– 100 – 99 98 97 96 95

– – – 1 2 3 4 5

– – 2 – – – – –

M-kaolin M-kaolin M-kaolin M-kaolin M-kaolin

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Characterization

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Mechanical analysis

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The notched impact testing was carried out on a XJU-22J (Chengde testing machine Co., Ltd, China) radial-boom impact tester. The tests were conducted according to GB/T 1043-93. Six specimens were tested for each sample. The tensile strength and the tensile strain at break were measured with an electronic universal testing machine (Shimadzu Corporation, Japan) according to GB/T 1040-92. All test specimens were molded in the same way as impact specimens. Six specimens were tested for each sample.

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Infrared spectroscopic analysis

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Fourier transform infrared spectroscopy (FTIR) was recorded in the transmission mode with a Nicolet FT-IR spectrometer (Thermo Fisher Scientific, USA) using KBr pellets. The spectra were collected at 2 cm1 resolution between 400 and 4000 cm1 with collection times of approximately 1 min.

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Morphology analysis

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The morphology was examined under a JEOL-JSM6510lv scanning electron microscopy (SEM, Electronic Co., Ltd, Japan) with an accelerating voltage of 20 kV.

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Thermal analysis

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The tests of thermogravimetric/differential thermal analysis (TGA/DTA) were investigated by TGA–DTA instrument (PERKIN ELMER Instrument Co., Ltd, USA) with the heating rate of 10 °C/min, ranging from 35 to 600 °C under nitrogen condition. Approximately 6 mg of samples were used in each test.

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Results and discussion

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Chemical structure of the PP-g-MAH/M-kaolin composites

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To verify native kaolin was successfully modified by KH-550, Fig. 2(a) with (b) showed the FTIR spectra of kaolin and M-kaolin particles: the 1092 cm1 band was ascribed to the SiAO stretching vibration and 843 cm1 was ascribed to the SiAOAAl stretching vibration. The two curves were similar except in the peaks of 3416 cm1 (absorption peak of H2O) and 3475 cm1 (absorption peak of KH-550), which were due to the stretching vibrations of the OAH stretching and NAH stretching, so it confirmed the presence of KH-550 on the Kaolin particles. The detailed assignments of IR absorption peaks were shown in Table 2. By comparing Fig. 2(c) with (d), there were some new absorption peaks appearing in Fig. 2(d). New absorptions at 1104, 1668 and 1728 cm1 were characteristic for the stretching vibration of carbonyl from MAH, which could indicate MAH was exactly grafted onto PP. The FTIR spectra of PP/kaolin, and PP-g-MAH/ 2 wt% M-kaolin were shown in Fig. 2(e) and (f), obviously, the 1096 cm1 band was ascribed to the SiAO stretching vibration. And there were new absorption peak at 3345 and 3358 cm1 in Fig. 2(f). The appearance of new peaks were H2O and imino stretching vibration peak which was amide formation derived from KH-550 and MAH reaction product, respectively. However, the above mentioned 1728 and 1668 cm1 absorption bands shown on the FT-IR spectra of PP-g-MAH specimens almost red shifted and disappeared completely, respectively. Presumably, this disappearance is due to the formation of PP-g-MAH/ 2 wt% M-kaolin

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copolymers through the reaction of maleic anhydrides groups of PP-g-MAH with the terminal amine groups of M-kaolin molecules during the reactive extrusion of PP-g-MAH/2 wt% M-kaolin resins.

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Composite morphology

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SEM images of pure PP and different amount of M-kaolin filled composites were shown in Fig. 3. Fig. 3(a) showed a smooth fracture surface of the fractured cross-section after mechanical testing, indicating that the pure PP was characteristics of brittle fracture.

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Fig. 2. FTIR spectra of (a) kaolin, (b) M-kaolin, (c) PP, (d) PP-g-MAH, (e) PP/kaolin and (f)PP-g-MAH/ 2 wt% M-kaolin.

Table 2 Assignments of IR absorption peaks. Wavenumber (cm1)

Assignment

References

1096 1105 1668 1728 3345 3416–3475

Stretching vibration of SiAO Stretching vibration of C@O in MAH Stretching vibration of C@O in MAH Stretching vibration of C@O in MAH Stretching vibration of NAH Valence vibration of OAH and/or NAH issued by KH-550 and polyphosphate chains Saturated and unsaturated CAH

[17] [18] [18] [18] [17] [18,19]

H2O or NH3

[20]

3078, 2965, 2922 1457, 1378, 890 4000–3200

[20]

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Fig. 3. SEM images of (a) PP and the PP-g-MAH specimen sections with different M-kaolin content: (b) 1 wt%; (c) 2 wt%; (d) 3 wt%; (e) 4 wt%; (f) 5 wt%.

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Fig. 4. The values of tensile strength (h) and the tensile strain at break (d) of PP-gMAH specimens with varying contents of M-kaolin powders.

Fig. 5. The values of impact strength (h) of PP-g-MAH specimens with varying contents of M-kaolin powders.

Compared with the M-kaolin particle size distribution in Fig. 3(b) with (c), Fig. 3(b) and (c) displayed obvious ‘‘sea-island” structure which was characteristics of ductile fracture. And this apparent ductile fracture enhanced the impact strength of PP-g-MAH matrix. On the one hand, the surface energy of composites was reduced after KH-550 had been well-distributed coating with M-kaolin particles; On the other hand, the interfacial strength of composites was enhanced as amide formation derived from KH-550 and MAH reaction. That indicated that M-kaolin could both improve mechanical toughness and strength of composites. In Fig. 3(c), significant agglomeration was observed when the M-kaolin loading exceeded 2 wt%, which resulted in a decrease in the mechanical properties of PP-g-MAH/M-kaolin composites. At M-kaolin contents higher than 3 wt%, the particle size distribution of these ‘‘sea-island” structure agglomerate significantly as

the M-kaolin contents continue to increase. In fact, almost no ‘‘sea-island” structure can be found in 4 wt% and 5 wt% specimens, wherein the M-kaolin particle sizes were arrived about 50 to 100 mm (see Fig. 3e–f). Presumably, the interesting M-kaolin morphologies described above is attributed to the increase and/ or surplus of sufficient M-kaolin particle for overlapping and agglomerate, since the possibility of overlapping the oriented, dispersed M-kaolin particle size in PP-g-MAH/M-kaolin specimens augment with increasing the M-kaolin contents.

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Mechanical properties

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Figs. 4 and 5 and Table 3 showed the mechanical properties of PP-g-MAH/M-kaolin samples with different M-kaolin contents. It showed optimal loading of M-kaolin was found to be 2 wt% and

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N. Yang et al. / Results in Physics xxx (2017) xxx–xxx Table 3 Mechanical properties of pure PP and PP-g-MAH/M-kaolin composites. Material

Tensile strength (MPa)

Tensile strain at break (%)

Impact strength (kJ/m2)

Pure PP PP-g-MAH PP-g-MAH/1 wt% PP-g-MAH/2 wt% PP-g-MAH/3 wt% PP-g-MAH/4 wt% PP-g-MAH/5 wt%

41.32 41.85 45.14 46.89 45.48 43.25 42.50

773.52 747.03 792.17 847.96 788.51 767.99 758.84

2.50 4.05 4.81 5.28 4.72 4.27 4.11

M-kaolin M-kaolin M-kaolin M-kaolin M-kaolin

Fig. 6. (a)TGA and (b) DTG curves of pure PP, PP-g-MAH and its M-kaolin filled composites. 226 227 228 229

higher M-kaolin loading led to the detrimental tensile strength of composites. The average tensile strain at break value reached a maximum when the content of M-kaolin was 2 wt%. And the similar trend could be found in impact strength, which was given in

Fig. 5. Compared with pure PP, the impact strength of PP-g-MAH/ M-kaolin samples were enhanced. It increased to 5.28 kJ/m2 when 2 wt% of M-kaolin was added. Results showed that, although kaolin particles were uniformly embedded in the coupling agent KH-550, the reducing surface energy made the dispersibility increased. Moreover, PP macromolecular main chain was bridged with proper polar side chain after it was grafted onto MAH. And MAH reacted with KH-550 to form amido bond which increased interfacial compatibility between M-kaolin particles and PP-g-MAH matrix, thus improving interfacial binding force between kaolin particles and PP. These improvements would minimize some harmful effects of the inorganic additive on composite mechanical properties. Hence, the modified kaolin particles could improve toughness and strength of the PP composite material. These results clearly support the ideas of ‘‘Chemical structure of the PP-g-MAH/M-kaolin composites” and ‘‘Composite morphology” that PP-g-MAH molecules are miscible with M-kaolin molecules to some extents in the molecular level as the M-kaolin contents of PP-g-MAH/ M-kaolin specimens are less than 2 wt%. Based on these premises, it is reasonable to believe that the mechanical properties of PP-g-MAH/M-kaolin specimens are better than PP and PP-g-MAH specimen. However, the gradually grown overlapping and agglomerate M-kaolin phase make the mechanical properties of PP-g-MAH/M-kaolin specimens to behave more like those of the PP-g-MAH specimen as their M-kaolin contents reach more than 2 wt%.

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Thermal properties

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The thermal stability of pure PP, PP-g-MAH and PP-g-MAH/ M-kaolin composites samples with different content of M-kaolin were evaluated using TGA and DTG analysis, as shown in Fig. 6. Decomposition of pure PP occurred in a single step. From Fig. 6 and Table 4, we obtained PP degraded sharply with the increasing temperature after T5% under nitrogen condition [21], and the maximum decomposition rate was appeared at 375 °C. No any residue left at 410 °C. PP-g-MAH compound showed that the first peak of degradation was appeared at 140 °C and the second peak of degradation was appeared at 382 °C with the maximum mass loss rate was appeared at 353 °C (T50%). By providing initiator and residue of maleic anhydride, the thermal decomposition property was

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Table 4 TGA data of pure PP, PP-g-MAH and its M-kaolin filled composites. Material

Pure PP PP-g-MAH PP-g-MAH/1 wt% PP-g-MAH/2 wt% PP-g-MAH/3 wt% PP-g-MAH/4 wt% PP-g-MAH/5 wt%

Temperature(°C)

M-kaolin M-kaolin M-kaolin M-kaolin M-kaolin

T5%

T1max

T50%

T2max

4T (T2maxT1max)

283 244 365 372 209 319 289

402 140 143 144 141 142 141

375 353 446 449 433 436 428

N/A 382 451 454 438 441 438

N/A 242 308 310 297 299 297

*

T5% and T50% - Temperatures at 5% and 50% weight loss, T 1max and T 2max– first and second mass loss peak temperatures obtained from the DTG curve, N/A – Not applicable.

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decreased when compared with PP. PP-g-MAH and M-kaolin based composites also had two peaks, and their first peaks were similar to that of PP-g-MAH because of the dissociation of maleic anhydride and initiator as well as moisture evaporation. And those composites showed over 1 wt% of remaining residue after 550 °C, as M-kaolin hadn’t completely decomposed. TGA results in Table 4 showed the thermostability of PP-gMAH/M-kaolin composites was increased firstly and then declined. The PP-g-MAH composite with 2 wt% M-kaolin had the best thermostability. From Table 4, the highest peak for PP-g-MAH/ 2 wt% M-kaolin was appeared at 454 °C, which enhanced by 14% compared with that of pure PP. And the maximum mass loss rate for PP-g-MAH/2 wt% M-kaolin occurred at 449 °C, which increased by 74 °C compared with that of pure PP. These changes could confirm that the combination of M-kaolin particles effectively blocked molecular thermal motions, and made free-radicals tighter to enhance thermostability of PP-g-MAH/M-kaolin composites. However, when the content of M-kaolin was greater than 2 wt%, agglomeration of M-kaolin in PP-g-MAH matrix would decrease the thermostability of PP-g-MAH/M-kaolin composites. These results clearly support the ideas of ‘‘Chemical structure of the PP-g-MAH/M-kaolin composites”, ‘‘Composite morphology” and ‘‘Mechanical properties” that PP-g-MAH molecules are miscible with M-kaolin molecules to some extents in the molecular level as the M-kaolin contents of PP-g-MAH/M-kaolin specimens are less than 2 wt%.

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Conclusion

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In this article, a series of PP-g-MAH/M-kaolin composite materials were successfully prepared through the melt-blending of PPg-MAH and M-kaolin. M-kaolin was modified by a silane coupling agent KH-550 and then added into PP-g-MAH composites. We found when the content of M-kaolin was 2 wt% in the composites, it showed an apparent improvement of mechanical and thermal properties. In the mechanical test, PP-g-MAH/M-kaolin composite with 2 wt% of M-kaolin had the best impact toughness, increased by 30% when compared with PP-g-MAH. And the tensile strength and tensile strain at break of composites also reached maximum values, increased by 12% and 14% respectively compared with PP-g-MAH. It confirmed that the interface between M-kaolin and PP was optimized, which caused the strength and toughness of composites improved. Furthermore, when the M-kaolin content in PP-g-MAH/M-kaolin composites was 2 wt%, it could effectually restrain molecular thermal movements, thus improving thermostability of PP-g-MAH/M-kaolin composites. Based on these premises, it is reasonable to believe that PP-g-MAH molecules are miscible with M-kaolin molecules to some extents in the molecular level as the M-kaolin contents of PP-g-MAH/M-kaolin specimens are less than 2 wt%.

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Author contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Acknowledgements

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This study has been supported by the National High Technology Research and Development Program (863 program) of China No.2012AA06A111; Hubei Provincial Natural Science Foundation of China No.2014CFB552; Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials of China No.00001647909.

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