Structural and optical properties of polypropylene–montmorillonite nanocomposites

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Materials Science and Engineering A 447 (2007) 261–265

Structural and optical properties of polypropylene–montmorillonite nanocomposites Lucilene Betega de Paiva a,∗ , Ana Rita Morales a , Thiago Ribeiro Guimar˜aes b a

Departamento de Tecnologia de Pol´ımeros, Faculdade de Engenharia Qu´ımica, Universidade Estadual de Campinas, Av. Albert Einstein, 500, Caixa Postal 6066, 13083-970 Campinas, S˜ao Paulo, Brazil b Departamento de Engenharia Metal´ urgica e de Materiais, Escola Polit´ecnica, Universidade de S˜ao Paulo, Av. Prof. Mello Moraes 2463, 05508-900 S˜ao Paulo, SP, Brazil Received 30 May 2006; received in revised form 21 September 2006; accepted 21 October 2006

Abstract In this work, polypropylene–commercial montmorillonite organophilic clay nanocomposites were prepared using a Werner Pfleiderer twin-screw extruder. Considering the nonpolar characteristic of the polypropylene, polypropylene-graft-maleic anhydride (PP-g-MA) was used as a coupling agent to improve the intercalation process into the layers of montmorillonite. The materials containing 2.5, 5.0, 7.5 and 10.0% of the clay (N2.5, N5, N7.5 and N10) and PP and two extra compositions containing only PP and 15.0 and 30.0% of PP-g-MA (PP, P15 and P30), respectively, were investigated upon the nanocomposites structures. The properties of materials were characterized by X-ray diffraction (XRD), SEM analysis and reflectance spectrophotometry. The X-ray diffraction showed exfoliated or intercalated structure for different concentrations; the SEM analysis showed a good dispersion of the clay in the PP matrix and the spectrophotometric analysis showed that the amount of clay used in the compositions resulted in different levels of opacity. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Montmorillonite; Polypropylene; Polypropylene-graft-maleic anhydride

1. Introduction Polymer–clay nanocomposites are a new class of composites, based on intercalation of polymer chains into organically modified clay. It is believed that the formation of nanocomposites offers improving flame retardancy, increasing heat distortion temperature, improving flexural modulus, decreasing in permeability and ionic conductivity at lower filler concentration, relative to the neat polymers and conventional composites [1,2]. These systems began to be studied in the early 1980s at Toyota Central Research Laboratories which developed a polyamide-6 nanocomposite containing 5% clay and showed an increase of 40% in tensile strength, 68% in tensile modulus, 60% in flexural strength, 126% in flexural modulus while the heat distortion temperature increased from 65 to 152 ◦ C and the impact strength was lowered by only 10% [3,4].

Corresponding author. Tel.: +55 19 3521 3907. E-mail addresses: [email protected] (L.B.d. Paiva), [email protected] (A.R. Morales). 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.066

The most commonly used clay for the polymer/clay nanocomposites is sodium montmorillonite, with a thickness of around 1 nm and lateral dimensions can vary up to several microns. The hydrophilic nature of the clay surfaces prevents homogeneous dispersion throughout the polymer phase, so it is necessary the exchange of the cations sodium for cations which are more organophilic, usually an ammonium or phosphonium cation of cationic surfactants. These modified clays are called organoclays, and are more compatible with polymers because they increase the spacing between the layers and reduce the surface energy of the filler [5,6]. Three main types of nanocomposites can be obtained when the clay is dispersed in a polymer: micro-composites in which the clay is not nanodispersed; intercalated structures, in which single or more polymer chains are intercalated between the clay layers, resulting in an ordered multilayer structure of alternating polymeric and inorganic layers, and the exfoliated structure when the silicates are completely and uniformly dispersed in the continuous polymer matrix [5,6]. Nowadays, several polymeric matrices like poly (methyl methacrylate), polyamide, polyethylene, polypropylene, poly


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(vinyl chloride) etc., have been used in nanocomposites. The main applications of nanocomposites include automotive, packaging, medical and textile areas. According to Bureau et al. [7] since the year 2000, specific efforts have been devoted to the development of nanocomposites based on thermoplastic polyolefins, especially polypropylene because of their wide and growing use in the automotive industry. However, the expected mechanical properties from these PPbased nanocomposites have yet to be presented. Full dispersion of organophilic clays in the nonpolar polymers such as polypropylene and polyethylene has remained difficult to achieve [1,8,9]. Thus, it is important to use a compatibilizer with polar groups, such as polypropylene-graft-maleic anhydride to obtain improvements in the properties of nanocomposites. This work describes the preparation of nanocomposites based polypropylene–clay using an organically modified montmorillonite by melt intercalation process in an extrusion process. The nanocomposites were characterized by X-ray diffraction, SEM analysis and reflectance spectrophotometry. 2. Experimental 2.1. Materials, preparation and characterization The materials used for the preparation of the samples were montmorillonite Cloisite 20A supplied by Southern Clay, polypropylene (PP) XM6150 K from Suzano and polypropylene-graft-maleic anhydride (PP-g-MA) Orevac CA100 from Atofina. First was prepared a masterbatch containing 25% of Cloisite 20A and 75% of PP-g-MA, and then the nanocomposites were prepared with four different amount of the clay, 2.5, 5.0, 7.5 and 10.0%, abbreviated as N2.5, N5, N7.5 and N10, respectively, by using a twin-screw extruder ZSK 25 Mega Compounder Werner Pfleiderer, L/D36, through five temperature zones, 230, 140, 140, 150 and 160 ◦ C and 300 rpm. The PP and two compositions containing 15 and 30% of PP-g-MA without clay were also processed as the same conditions to verify the influence of PP-g-MA in the PP matrix, abbreviated as PP, P15 and P30, respectively. The structures of the nanocomposites were evaluated by Xray diffraction in an equipment Philips, Model X’Pert. The ˚ between X-ray analysis has been carried out Cu, λ = 1.5406 A, 2θ = 2.0–10.0◦ . Scanning electronic microscopy (SEM) analysis was used to evaluate the morphological properties of PP, P15, P30, N2.5, N5, N7.5 and N10. All samples were fractured and covered with an alloy gold–palladium. The equipment used was LEO, model LEO 440i. The opacity of the system is a good indication of the dispersion of the clay in the matrix. The opacity degree was obtained by contrast ratio of reflectance spectrophotometric analysis using a Reflectance Spectrophotometer Datacolor Model Spectraflash SF 600 and “Colour Tools” Version 3.1.1 program. This technique consist in submit the injected bar samples with 3.2 mm to a source of light of 400–700 nm between a white background

and then between a black one. The contrast ratio that is one property controlled by the size and form of the particles and by difference of refraction index between particles and the environment, is associated with light scattering and is wavelength function. The white reflects all wavelengths and the black absorbs all wavelengths, so, the contrast ratio is a measure of percentage of light that goes across the material and is expressed by a 0–100 scale, where 0 corresponds to a transparent material and 100 to an opacous material. 3. Results and discussion 3.1. Structure of PP/PP-g-MA/clay nanocomposites Fig. 1 shows XRD results of PP, P15 and P30 and Fig. 2 shows XRD results of montmorillonite Cloisite 20A, masterbatch, N2.5, N5, N7.5 and N10. In all compositions are observed peaks in the region between 9◦ and 10◦ that are peaks of PP, showing that peaks between 2◦ and 7◦ are of the clay. In Fig. 2, the peaks of interest in the characterization of nanocomposites are between 2◦ and 3◦ that indicate a (0 0 1) basal spacing of the nanoclay and the peaks between 6◦ and 7◦ can be considered secondorder peaks or still the original peak of the montmorillonite before the modification. Lee et al. [10] also related the existence of second-order peaks in polypropylene–clay nanocomposites. The montmorillonite shows diffraction peak at 3.74◦ that correspond to a basal spacing of 2.36 nm. After the processing of masterbatch the peaks of Cloisite 20A were dislocated to 3.50◦ with basal spacing of 2.52 nm and 6.76◦ with spacing of 1.30 nm, showing that had occurred some intercalation of the PP-g-MA into the layers of the clay. In N2.5 can be seen a diffraction peak at 2.34◦ , basal spacing 3.77 nm, with an increase of 1.41 nm compared to a diffraction peak of montmorillonite Cloisite 20A, indicating that an intercalated structure was obtained, while in N5 the peak of diffraction at approximately 3◦ has not been

Fig. 1. XRD diffraction for PP, P15 and P30.

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containing 2% of Cloisite 15A showed basal spacing of 3.5 and 3.1 nm, what is characteristic of intercalated structures. In the nanocomposite containing 2% of Cloisite 30B was observed a collapse of the clay galleries from 2.0 to 1.2 nm after processing, showing that the use of side feeder was not able to promote a good dispersion of the clay how is expected. 3.2. Morphology of PP/PP-g-MA/clay nanocomposites Fig. 3 shows the SEM micrographs of the PP, P15, P30, N2.5, N5, N7.5 and N10 with an increase of 1000×. The dispersion of the clay in PP matrix was satisfactory for all compositions without the presence of large aggregates. The absence of large aggregates can be confirmed through micrographs of PP, P15 and P30. These micrographs are very similar when compared with the micrographs of the nanocomposites. The results suggest that the clay could be dispersed in PP with the processing conditions that were employed. 3.3. Reflectance spectrophotometric analysis

Fig. 2. XRD diffraction for Cloisite 20A, materbatch, N2.5, N5, N7.5 and N10.

observed, but a small peak at 1.33 nm appears, suggesting that an exfoliated structure was formed. N7.5 and N10 showed peaks at 3.0◦ and 3.66◦ with basal spacing of 2.94 nm and 2.41 nm, respectively, indicating structures intercalated too, maintaining the original structure of the masterbatch. The peaks between 6 and 7◦ were also observed in N7.5 and N10. The reduction observed in the basal spacing of N7.5 and N10 can be attributed to aggregation of the particles of the clay during the processing of the materials. The higher amounts of the clay and the process conditions employed made difficult the dispersion of the clay. It would be recommended to feed the clay into the molten polymer to promote a good exfoliation of the clay without compact the clay particles. This process can be done using an extruder with side feeder. The materials were processed without side feeder, but this isolated factor should not be used to explain the presence of both intercalated and exfoliated structures. It is important to consider many others parameters like, mixing conditions, kind of coupling agent, concentration of the clay, properties of the matrix, etc., to obtain exfoliated structures. Ton-That et al. [9] prepared polypropylene nanocomposites using montmorillonite Cloisite 15A and 30B from Southern Clay and two kinds of polypropylene-graft-maleic anhydride as coupling agent in an extruder with side feeder. The nanocomposites

This technique was used because transparency in nanocomposites is associated with a good level of exfoliation of the clay in the matrix. Table 1 shows contrast ratio values of reflectance spectrophotometric analysis. The contrast ratio increased with the addition of PP-g-MA in PP matrix in P15 and P30, from 29.35 to 35.36 and 39.71, respectively. A comparison between P15 and N5 that have the same concentration of PP-g-MA shows that the clay increases the contrast ratio from 35.36 to 48.23. Between P30 and N10 the same behavior is observed. In all nanocomposites, the contrast ratio also increased with increasing of concentration of PP-g-MA and clay. These results showed that one part of opacity is from PP-g-AM and the other is from the clay, but nanocomposites did not lose the transparency entirely with addition of the clay. The translucent characteristic can be attributed to dispersed phases of the clay in the matrix. The layers of montmorillonite are smaller than wavelengths of the light, so, do not deflect or reflect the light, and can produce transparent materials. A brown coloration was observed in the nanocomposites. According to Southern Clay nanocomposite color will vary depending on the polymeric matrix and the chemical treatment of the clay. The Southern Clay explains that for polypropylene nanocomposites containing Cloisite 20A and polypropyleneTable 1 Results of reflectance spectrophotometric analysis of PP, P15, P30 and PP/PPg-MA/clay nanocomposites Composition

Contrast ratio

PP P15 P30 N2.5 N5 N7.5 N10

29.35 35.36 39.71 35.85 48.23 51.50 56.53


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Fig. 3. Micrographs of PP, P15, P30, N2.5, N5, N7.5 and N10.

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graft-maleic anhydride as coupling agent the brown coloration and opaque characteristic is expected. But, for polypropylene nanocomposites containing only Cloisite 20A an off-white coloration is expected. Two hypotheses can be considered to the brown coloration obtained: (1) It is possible to be a result of interaction between maleic anhydride and the organic modifier of the clay Cloisite 20A (dimethyl, dihydrogenatedtallow, quaternary ammonium). (2) Or it is possible to be result of some degradation of the organo portion of the clay. These results can suggest a partial exfoliation of the clay, but could not be compared to others results because similar studies of opacity and color of the nanocomposites are not available in the recent literature. 4. Conclusion This work shows that nanocomposites with intercalated and exfoliated structures can be obtained, depending on the amount of the clay that was added in PP, using a twin screw extrusion processing. The results of X-ray diffraction analyses showed that the higher basal spacing, 3.77 nm, was obtained to N2.5 and an intercalated structure was formed. In N5 the peak of basal spacing disappeared suggesting that an exfoliated structure was formed, while in N7.5 and N10 the original structure of the masterbatch was maintained, with basal spacing of 2.94 and 2.41 nm, respectively, indicating structures intercalated also were formed. SEM analysis showed that the nanocomposites obtained a good dispersion of the clay in the PP and absence of large aggre-


gates of the clay, but could not show the extent of intercalation or exfoliation obtained. The reflectance spectrophotometric analysis showed that the transparency of nanocomposites is modified with increase of the amount of the clay and PP-g-MA, but it is not entirely lost. This is a good indication of dispersability and a partial exfoliation of the clay in the polymeric matrix. The techniques that are generally used to evaluate the intercalation and exfoliation of the clay in polymeric matrixes can give a good indication of the dispersion but it is known that for nanocomposites still there is large difficult to guarantee that one nanocomposite have the same state of dispersion the clay in all extension. Acknowledgment The authors would like to acknowledge the Cromex SA and CAPES for support of this project. References [1] S. Hambir, N. Bulakh, J.P. Jog, Polym. Eng. Sci. 42 (2002) 1800. [2] H. Yao, J. Zhu, A.B. Morgan, C.A. Wilkie, Polym. Eng. Sci. 42 (2002) 1808. [3] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1–2. [4] S. Bourbigot, E. Devaux, X. Flambard, Polym. Eng. Sci. 75 (2002) 397. [5] X. Zheng, C.A. Wilkie, Polym. Degrad. Stab. 81 (2003) 1. [6] G. Beyer, Plast. Add. Comp. (2002) 22–23. [7] M.N. Bureau, F. Perrin-Sarazin, T. Ton-That, Polym. Eng. Sci. 44 (2004) 1142. [8] S. Wang, Y. Hu, Q. Zhongkai, Z. Wang, Z. Chen, W. Fan, Mater. Lett. 57 (2003) 2675–2676. [9] M.-T. Ton-That, F. Perrin-Sarazin, K.C. Cole, M.N. Bureau, J. Denault, Polym. Eng. Sci. 44 (2004) 1212. [10] E.C. Lee, D.F. Mielewski, R.J. Baird, Polym. Eng. Sci. 44 (2004) 1773–1782.

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