polyamide-6 blends: influence of compatibilizing agent on interface domains

Polymer Testing 21 (2002) 815–821 www.elsevier.com/locate/polytest

Material Characterisation

Polypropylene/polyamide-6 blends: influence of compatibilizing agent on interface domains J. Roeder a, R.V.B. Oliveira a, M.C. Gonc¸alves b, V. Soldi a, A.T.N. Pires a,∗ a

Grupo de Materiais Polime´ricos (Polimat), Departamento de Quı´mica, Universidade Federal de Santa Catarina, Campus Universitario–Trindade, 88040-900 Floriano´polis, SC, Brazil b Universidade Estadual de Campinas, Instituto de Quı´mica, Campinas, SP, Brazil Received 28 December 2001; accepted 7 February 2002

Abstract The influence of maleic anhydride grafted polypropylene (PP-g-MA) at the interface of immiscible polypropylene (PP)/polyamide-6 (PA6) blends was studied by infrared spectroscopy, differential scanning calorimetry and transmission and scanning electron microscopy. The formation of a new copolymer at the interface between the domains and the matrix was investigated. The characterization of the interfacial copolymer was accomplished by carbonyl vibration of the imide group at 1674 cm⫺1. The reduction of water absorption and the thermoanalysis experiments corroborated the imide linkage between the amino end groups of PA6 and the carboxylic groups of PP-g-MA. The modification of the PA6 particle size and the formation of the interfacial layer were characterized by microscopy and related to the reaction of the interfacial components.  2002 Published by Elsevier Science Ltd. Keywords: Blend; Polypropylene; Polyamide-6; Interface domains; Maleic anhydride grafted polypropylene

1. Introduction In recent decades, the properties of physical mixtures of polypropylene (PP)/polyamide-6 (PA6) have been studied. The mechanical properties of this immiscible blend, with poor interfacial adhesion and high interfacial tension between the dispersed and continuous phases, can be changed with the addition of a compatibilizer agent. Ide and Hasegawa [1] studied the effect of maleic anhydride grafted polypropylene (PP-g-MA) on PP/PA6 polymer blends. The structural stability and morphology of the blends were improved greatly by PP–PA6 grafted copolymers that were formed by the in situ reaction of anhydride groups with the amino end groups of PA6. Recently, Ohlsson et al. [2] studied the influence of the addition of polystyrene-block-poly(ethylene–stat-

∗ Corresponding author. Tel.: +55-48-331-9219; fax: +5548-331-9711. E-mail address: [email protected] (A.T.N. Pires).

butylene)-block-polystyrene (SEBS) or maleic anhydride grafted SEBS (SEBS-g-MA) on the PP/PA6 polymer blend. The interfacial copolymer formed by the reaction between PA6 and maleic anhydride graft polymers during melt mixing results in an effective compatibilization, with an influence on the micro- and macroscopic properties [2,3]. Analogous systems have been studied by Vocke and co-authors [4], using oxazoline grafted polyolefins or grafted SEBS as an effective compatibilizer in blends of polyolefins with engineering plastics. A reaction of the compatibilizer with polyesters and polyamides can occur between the oxazoline and the amino and carboxylic groups. The interfacial active block or graft copolymers can improve the blend morphology and the system compatibilization. Other examples of immiscible compatible blends are: PA6/polystyrene (PS)/copolymer (styrene methacrylic acid) [1], PA6/ABS/ copolymer (styrene–maleic anhydride) (SMA) [5], PA6/PP/ethylene propylene rubbers grafted with maleic anhydride (EPRg-MA) [6], and PA6/styrene–acrylonitrile (SAN) with an imidized acrylic (IA) polymer or

0142-9418/02/$ - see front matter  2002 Pulished by Elsevier Science Ltd. PII: S 0 1 4 2 - 9 4 1 8 ( 0 2 ) 0 0 0 1 6 - 8

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styrene/acrylonitrile/maleic anhydride terpolymer (SANMA) [7]. In recent years, more attention has been focused on reactive compatibilization of immiscible polymer blends giving rise to in situ generation of copolymers. The basic principle underlying reactive compatibilization is the capacity of the functional group present in one or both polymers to form graft or block copolymers in situ during melt processing. The effect of the chemical reaction at the interface on the morphological development of reactive blends depends on the manufacturing procedure [8]. For PP/PA6/PP-g-MA systems, there are three different ways of obtaining blends with reactive and non-reactive conditions, with single-step processing being more effective in improving phase adhesion than two-step blending [9]. Thomas and Groeninckx have shown the importance of the reactive process when EPR/PA6/EPR-g-MA blends are obtained by melting, with improved morphological stability and mechanical properties [10,11]. Different studies have reported the influence of processing conditions, synthesis of alternative compatibilizer agent and time dependence of the morphological development of specific blends [8,10,12,13]. In recent years, for PP/PA6 systems, PP-g-MA has been used as a compatibilizing agent and the best process is a singlestep [10] , but only a few attempts have been made to express the morphological development as a function of the reaction itself during polymer blending. In this work, we evaluate the reaction products formed during the single-step melt processing of PP/PA6 with PP-g-MA as a compatibilizer, with the formation of imide-coupled block copolymers that improve the compatibility of the PP and the PA6 and the morphological development.

2. Experimental 2.1. Material Isotatic polypropylene (semicrystalline polymer; melting temperature Tm ⫽ 169 ⴰC and glass transition temperature Tg ⫽ ⫺21 ⴰC) and polyamide-6 (semicrystaline polymer; Tm ⫽ 220 ⴰC and Tg ⫽ 40 ⴰC) were supplied by OPP Petroquı´mica S.A. and Petronyl, respectively. Polypropylene, grafted with 0.4% of maleic anhydride ( Tm ⫽ 148 ⴰC and Tg ⫽ 19 ⴰC), was produced by Exxon Chemical. All polymers were commercial products and were used without further purification. 2.2. Blend preparation Blends of different systems and compositions were prepared using a CSI Max Extruder, model CS-194 A, set to a length/diameter of 4, screw rotation rate of 70 rev/min, with two heating zones at 230 °C and simul-

taneous polymer addition. The PP/PA6 blend consisted of 70 wt% PP and 30 wt% of a dispersed PA6 phase. As the weight fraction of PP-g-MA compatibilizer increased, the PP weight fraction was reduced accordingly to maintain 30 wt% of PA6 and and 70 wt% total of PP+PP-g-MA. The process was repeated at least three times to obtain better dispersion of the components. Before blending, undiluted components were dried for 24 h at 80 °C under vacuum. 2.3. Polyamide-6 extraction The PA6 disperse phase was extracted with formic acid from compatible and incompatible polymer blends. Granules of the blends were placed in glass flasks with formic acid at room temperature for 120 h. After washing with fresh solvent, the samples were dried at 40 °C until constant weight was attained. 2.4. Infrared spectroscopy (FTIR) FTIR spectra of polymer blends and undiluted components were carried out on a 16 PC Perkin Elmer spectrophotometer, performing 20 scans, resolution 4 cm⫺1, using sample films prepared on ZnS plates. The spectra were fitted, to evaluate the real contribution of each, using Origin 6.0 software and Gaussian bases. 2.5. Differential scanning calorimetry (DSC) DSC thermograms were obtained on a Shimadzu 50 differential scanning calorimeter by heating from 0 to 250 °C at 10 °C/min in a nitrogen atmosphere (50 cm3/min). For all measurements, the samples were heated and maintained for 10 min at 250 °C, and then quenched in liquid nitrogen, after which a second heating was carried out at the same rate. 2.6. Water absorption test The amounts of water absorbed by the blend samples, with dimensions 20 mm × 10 mm × 1 mm, were determined following the ASTM D-570 procedure. In this experiment, polymer samples were dried for 24 h in a drying oven. Then they were immersed in distilled water at 25 °C and the weight gain was determined at different times. We measured the weight of the wet samples after wiping off all surface water with a dry cloth. Water absorption (%) was evaluated from the following equation: wt%absorption ⫽

wt%wet⫺wt%dry × 100. wt%dry

(1)

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2.7. Scanning electron microscopy (SEM) The polymer blend samples were fractured under liquid nitrogen and coated with gold to avoid charge by the electron beam. They were then analyzed by scanning electron microscopy (Philips XL 30). 2.8. Transmission electron microscopy (TEM) Transmission electron microscopy of the polymer blends was carried out on using a Zeiss CEM-902 electron microscope, operating at 80 kV. Thin sections (60 nm) were prepared by cryo-ultramicrotomy in a Leica ultramicrotome at ⫺80 °C. 3. Results and discussion 3.1. Infrared Fig. 1 shows infrared spectra in the range 1800 to 1550 cm⫺1 of (a) the PP/PA6 uncompatibilized blend at 70/30 wt% with a characteristic absorbance at 1640 cm⫺1, corresponding to the amide carbonyl group, and (b) the PP/PA6/PP-g-MA compatibilized blend at 60/30/10 wt%, which displays three other large peaks due to the formation of imide linkages. Vermeesch and co-workers [14] indicated imide group absorbances at 1779, 1726 and 1710 cm⫺1 that are attributed to in-phase, free and hydrogen-bonded out-of-phase carbonyl vibrations, respectively. However, other works have suggested the presence of carbonyl stretch vibrations at 1774 and 1703

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cm⫺1 [15], and ring absorptions at 1775 and 1730 cm⫺1 [16]. For the Gaussian deconvolution of the three large peaks of the compatibilized blend (see inset in Fig. 1), we assigned the peak at 1764 cm⫺1 to the carbonyl vibration of the imide group and the peak at 1710 cm⫺1 to the carbonyl vibration of the carboxylic acid from PPg-MA that had not reacted. The scheme of the predicted reaction between PA6 and PP-g-MA is shown in Fig. 2. To clarify this predicted reaction with the formation of grafted copolymer at the interface between component phases, the PA6 domains were removed with formic acid. The infrared spectra of (a) the uncompatibilized blend without PA6 and (b) the compatibilized blend after PA6 extraction are shown in Fig. 3. As may be expected, spectrum (a) shows only peaks due to groups of the PP molecules, which indicates the absence of interfacial interactions. On the other hand, spectrum (b) exhibits absorbances at 1640 and 1730 cm⫺1 due to vibrations of the PA6 carbonyl group and carbonyl imide linkage, respectively. This result indicates formation of the imide linkage by reaction at the interface. 3.2. Differential scanning calorimetry The DSC curves of the compatible blends evince the melting temperature corresponding to undiluted PA6 and only one melting temperature with an intermediate value between those of pure PP and PP-g-MA for the different compositions (Fig. 4). This shift of melting temperature indicates the miscibility of the PP and PP-g-MA, probably due to a small quantity of grafted maleic anhydride.

Fig. 1. Infrared spectra of blends: (a) PP/PA6 at 70/30 wt%; (b) PP/PA6/PP-g-MA at 60/30/10 wt%. The insert shows a Gaussian deconvolution for the spectrum of the compatibilized blend in the range 1850 to 1600 cm⫺1.

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Fig. 2.

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Scheme of interface grafting by reaction between a carboxyl group of maleic anhydride and a polyamide amino end group.

Fig. 3.

Fig. 4.

Infrared spectra for the blend with PA6 extraction: (a) uncompatibilized; (b) compatibilized.

DSC curves for PP/PA6/PP-g-MA (wt%): (a) 00/00/100, (b) 10/30/60, (c) 60/30/10 and (d) 100/00/00.

A high percentage of maleic anhydride can induce an immiscible phase in this mixture [17,18]. The extraction of PA6 domains was analyzed by DSC curves, as shown in Fig. 5. In the DSC curve of the uncompatibilized blend transitions corresponding to PA6 were not observed, indicating complete extraction of the

disperse phase in agreement with the infrared data. However, the DSC curve of the compatibilized blend supports the notion that imide linkage took place in the interface, since complete extraction of PA6 domains did not occur and we can observe transitions (Tm and crystallization temperature, Tc) attributed to PA6.

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Fig. 5.

DSC curves for the blend with PA6 extraction: (a) uncompatibilized; (b) compatibilized.

3.3. Water absorption Fig. 6 shows water absorption relative to polyamide content as a function of time for the blends. The uncompatibilized blend sample absorbed slightly more water than that corresponding to the equilibrium uptake of water in relation to the amount of pure polyamide. Ohlsson et al. suggest that the rise in absorption of water by the blend is probably due to filling of the fine capillary voids separating the phases [2]. However, the compatibilized blends absorbed less water than the corresponding uncompatibilized blends, probably related to the fewer free N–H groups due to imide linkage formation. The compatibilized blend can form a hydrogen bond that reduces the interfacial tension and the possibility of for-

Fig. 6.

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ming capillaries between domains and matrix. Then, the interfacial layer formed with the addition of PP-g-MA increases the adhesion between the phases and reduces voids and water uptake. 3.4. Scanning electron microscopy Fig. 7 shows SEM micrographs of cryo-fractured surfaces of PP/PA6/PP-g-MA blends. The 70/30/00 wt% blend [Fig. 7(a)] exhibits spherical phase domains of PA6 and weak interfacial adhesion, surrounded by the continuous PP phase, whereas for the 60/30/10 wt% blend [Fig. 7(b)] with homogeneous morphology the addition of PP-g-MA leads to a decrease in the diameter of PA6 phase domains and indicates greater interfacial

Water absorption relative to polyamide-6 content as a function of time for the uncompatibilized and compatibilized blends.

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Fig. 7. SEM micrographs of PP/PA6/PP-g-MA (wt%): (a) 70/30/00, (b) 60/30/10, (c) 70/30/00 with PA6 extraction and (d) 60/30/10 with PA6 extraction.

adhesion between the domains and matrix. The compatibilizing effect can be observed after solvent treatment of cryo-fractured surfaces where the SEM micrographs present spherical holes representing PA6 phase domains removed by extraction [Fig. 7(c) and (d)]. The small voids in the compatibilized blend, in comparison with the uncompatibilized blend, lead to increasingly homogeneous and dispersible PA6 domains. This behaviour is in agreement with the studies of Hosoda and co-workers [19]. The compatibility phenomenon can be induced by a third component that will interact chemically with both phases or will have specific interaction with one phase and physical contact with the other. The SEM micrographs demonstrate that the addition of the compatibilizer suppresses coalescence and is in accordance with the IR spectroscopy and DSC curves, suggesting the formation of an interfacial copolymer. Similar behaviour has been reported for the PA6/SEBS/SEBS-g-MA system [20]. 3.5. Transmission electron microscopy In the transmission electron micrograph of the uncompatibilized blend [Fig. 8(a)], the arrow points to voids

that separate the PA6 domains from the PP matrix, demonstrating the weak adhesion between the phases. With PP-g-MA addition to the binary blends, the domains decrease and are encapsulated in an interfacial layer as a thin shell [Fig. 8(b)]. This interfacial layer occurs by the formation of PP-g-MA-co-PA6 copolymer. Analogous results have been shown by Ro¨ sch and Mu¨ lhaupt [21] using maleic anhydride grafted elastomers to compatibilize PP/PA6 blends. Their domains formed agglomerates as core/shell-microparticles similar to honeycomb structures.

4. Conclusion The addition of small quantities of PP-g-MA to the incompatible blends of PP/PA6 increased the homogeneity of PA6 phase dispersion in the PP matrix with a reduction in the size of the domains and copolymer formation at the interface. This reaction was characterized by infrared spectroscopy, thermal analysis and microscopy methods that permitted a proper evaluation of the imide linkage. The present experimental results confirm earlier literature reports [1,2,15,19] according to

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Fig. 8.

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TEM micrographs from (a) uncompatibilized (b) compatibilized blends.

which a reaction between PA6 and PP-g-MA occurs and both components (PP and PA6) exhibit miscibility with the PP-g-MA.

Acknowledgements The authors wish to thank OPP Petroquı´mica S.A., Petronyl and CAPES for financial support.

References [1] F. Ide, A. Hasegawa, J. Appl. Polym. Sci. 18 (1974) 963. [2] B. Ohlsson, H. Hassander, B. To¨ rnell, Polymer 39 (1998) 6705. [3] I. Vieira, V. Severgnini, D.J. Mazera, M.S. Soldi, E.A. Pinheiro, A.T.N. Pires, V. Soldi, Polym. Degrad. Stab. 74 (2001) 151. [4] C. Vocke, U. Anttila, M. Heino, P. Hietaoja, J. Sepa¨ la¨ , J. Appl. Polym. Sci. 70 (1998) 1923. [5] B. Majumdar, H. Keskkula, D.R. Paul, Polymer 35 (1994) 3164. [6] A. Gonza´ lez-Montiel, H. Keskkula, D.R. Paul, Polymer 36 (1995) 4587.

[7] N. Kitayama, H. Keskkula, D.R. Paul, Polymer 41 (2000) 8041. [8] C.E. Scott, C.W. Macosko, Polymer 35 (1994) 5422. [9] J.-D. Lee, S.-M. Yang, Polym. Eng. Sci. 35 (1995) 1821. [10] S. Thomas, G. Groeninck, J. Appl. Polym. Sci. 7 (1999) 1405. [11] S. Thomas, G. Groeninck, Polymer 40 (1999) 5799. [12] A. Tedesco, R.V. Barbosa, S.M.B. Nachtigall, R.S. Mauler, Polym. Test. 21 (2002) 11. [13] G.M.O. Barra, J.S. Crespo, J.R. Bertolino, V. Soldi, A.T.N. Pires, J. Braz. Chem. Soc. 10 (1999) 31. [14] I.M. Vermeesch, G. Groeninckx, M.M. Coleman, Macromolecules 26 (1993) 6643. [15] T.T.M. Phan, A.J. Denicola Jr., L.S. Scahdler, J. Appl. Polym. Sci. 68 (1998) 1451. [16] M.F. Porto, R.V.A. Rowe, O. Santos, C.M. Garcia, L.T. Taylor, E.C. Muniz, A.F. Rubira, Polyimides Other High Temp. Polym. 1 (2001) 1. [17] J. Duvall, C. Selletti, C. Myers, A. Hiltner, E. Baer, J. Appl. Polym. Sci. 52 (1994) 195. [18] J. Duvall, C. Selletti, C. Myers, A. Hiltner, E. Baer, J. Appl. Polym. Sci. 52 (1994) 207. [19] S. Hosoda, K. Kojima, Y. Kanda, M. Aoyagi, Polym. Net. Blends 1 (1991) 51. [20] A.N. Wilkinson, L. Laugel, M.L. Clemens V, M. Harding, M. Marin, Polymer 40 (1999) 4971. [21] J. Ro¨ sch, R. Mu¨ lhaupt, Polym. Bull. 32 (1994) 697.