Beta Phase Formation of Polypropylene with Bentonite Filler

In this paper, a study on Polypropylene (PP)/bentonite composites is presented. PP/bentonite composites were prepared using 5%, 10% and 25% of concentration in weight of unmodified and modified bentonite filler. The relative leve! of the beta phase of the samples was calculated using Wide Angle X-ray Diffraction (WAXD). Differential scanning calorimeter (DSC) was used to observe the melting and crystallization behaviour of the samples. The dispersion state of the composites was observed using optical rnicroscopy. The obtained results show the possibility of inducing the beta phase in PP material using bentonite as filler. However, only the composites of polypropylene with modified bentonite showed the beta signa! and higher content was at 10% of filler. Although at 10% of modified bentonite the dispersion is worse than in 5%, at 10% there are more nucleating sites for beta phase formation and consequently more beta content. However, as concentration ofthe filler increases to 25%, the nucleating sites also increase but its dispersion into the compound becomes worse which strongly decreases the beta phase


l. Introduction
Fillers with size in the micron scale are still used in polymers for a variety of reasons such as cost reduction, improved processing conditions, density control, improve the electrical properties, flame retardants and for the improvement of mechanical properties (Svab et al. 2008; Meng and Dou 2008). The most used fillers in thermoplastics are chalk, talcum, kaolin, mica and wollastonite. The choice of filler is based on the properties which are required for the intermediate or final product. Different properties ofthe final product are influenced by the composition, size, shape and structure of the chosen filler. The wollastonite has acicular structure (needle-like), chalk comprises irregular shaped particles, whereas the mica and talcum have platy or layered structures (Jarvela and Jarvela;1996). Bentonite filler could be interesting filler by its large amounts in Mexico, and by its price lower than chalk, kaolin, mica and wollastonite respectively.
The polypropylene (PP) is the choice polymer for home appliances, packaging and automotive industry due to their mechanical properties, simplicity of processing, the ability to incorporate varied types of fillers, and its relative low cost (Tjong et al., 1997). lt is well known that the PP exhibits severa! crystalline forms, namely the monoclinic alpha-phase, the hexagonal beta-phase and the orthorhombic gammaphase (Keith et al., 1959;Meille et al., 1990;Bruckner and Meille, 1989). Among all crystal structures, the alpha-phase, obtained under ordinary industrial processing conditions, is the most stable. The presence of beta-phase improves properties of PP, such as increasing the impact strength and the elongation at break (McGenity et al., 1992;Tjong et al., 1996;Tjong et al., 1997).
lt has been shown that the beta phase of polypropylene could be formed under specific conditions, e.g., by shear forces in the melt (Dragaun et al., 1977), by temperature gradient (Crissman, 1969), and in presence of a special nucleator (Jacoby et al., 1986), and by using fillers Liu et al. ( 1990).
Sorne filler have been reported to induce beta phase of polypropylene, i.e.: Liu et al. (1990) have studied compounds of a copolymer of ethylene-propylene with wollastonite. These authors detected beta phase of polypropylene in the copolymer induced by the incorporation ofwollastonite. However, to our knowledge, no study has been made concerning the formation of beta phase of polypropylene using bentonite filler. Therefore, in this paper, a study the on PP/bentonite composite is presented.

Materials and sample preparations
The materials used in this study were a polypropylene (PP) isotactic (Valtec trademark, HG009) supplied by Indelpro, Mexico and Bentonite mineral provided by BARMEX Mexico which was used as a filler. The mean diameter of the filler was 5 µm and the specific surface area was 12m 2 / g. Modified filler was prepared using the following stearic acid procedures proposed by Tabtiang and Venables (1999). The dried bentonite was loaded into the chamber of the Henschel blender with the stearic acid. The mixing was initiated at low speed of 1400rpm during 5 min and then increased to the high speed of 2800rpm for 15 min and in this speed the temperature ofthe blending was kept at 75 ºC. Finally, the chamber was cooled up to 40 ºC at low speed. Coated filler was stored at room temperature in desiccators until theirused.
PP/bentonite compounds were prepared using 5%, 10% and 25 % concentration of unrnodified and modified bentonite filler, based on a total sample weight of230g. Each sample was mixed during a time period of 12 minutes using a Brabender intemal mixer (DDRV-502 type) with a rotation speed of 50 RPM and a chamber temperature of 200ºC. A 0.2% concentration of antioxidant lrganox B225 from Ciba-Geigy was added to all samples in order to prevent thermal degradation ofthe polymer during the rnixing process.

Thermal analysis
A Perkin Elmer Diamond differential scanning calorimeter was used to observe the crystallization temperatures of the samples. The samples obtained directly from the mixing chamber were heated from 50 to 200ºC and then cooled to the initial temperature. Both heating and cooling processes were conducted at a scanning rate of 1 0ºC/min. The endothermic and the exothermic peaks were taken as the melting and crystallization temperature respectively.

Wide angle X-ray diffraction (WAXD)
The X-ray diffraction technique was used to quantify the relative level of the beta phase of polypropylene with the modified and unmodified bentonite. A Philips X-pert diffractometer with CuK. radiation at room temperature was used to collect the corresponding X-ray diffraction pattems. Radial scans of intensity vs. diffraction angle (20) were recorded ranging from 5° to 35º. The analysis was carried out directly on the flat part and smooth of the samples obtained directly from the mixing chamber.

Dispersion state
In order to characterize the dispersion state of the samples, thin films of approximately 50 µm thicknesses of the PP/bentonite composites were Enero -Marzo de 2012 prepared using compression molding at 200ºC. The dispersion state ofthe composites was observed on the thin films using a JEN CO optical microscope.

Results and Discussion
The melting behavior for the polypropylene samples and composites samples of polypropylene with unmodified and modified bentonite, are presented in Figures 1 and 2 respectively. In these behaviours, the transition between 144 and 152ºC corresponds to the melting of the beta phase and transition between 158 and 166ºC to the melting of alpha phase crystallites ofPP (Liu et al., 1990;Jacoby et al., 1986;Tjong et al., 1996b ). It can be seen that the compounds with unrnodified bentonite shows a slightly beta phase signa! only for samples with 10% of bentonite ( see Figure 1 ), whereas the compounds with modified bentonite all the samples exhibits beta phase signa! (see Figure 2). These behaviours clearly confirm that the used modified bentonite as filler in PP compounds increases the amount ofbeta phase. To quantify the relative level of beta phase in PP/bentonite samples, the X-ray technique was used. Figures 3 and 4 show the WAXD pattems for the composites samples of polypropylene with unmodified and modified bentonite, respectively. The ordinary PP shows the diffraction peaks at the 20 angles of 14.1°, 16.9° and 18.5° corresponding to the crystal planes (110), (040) and (130) respectively. The beta crystalline form is known to exhibit a strong peak ata 20 angle of 16° corresponding to the crystal plane (300) (Tumer-Jones et al., 1964; Samuels and Yee, 1972).
The relative amount of the beta phase is usually described in terms ofthe K value, which is defined by thefollowingrelation(Tumer Jonesetal., 1964): where Hp is the height of the strong single ~-form peak (300), and Ha1, Ha2, and Ha 3 are the heights of the three strong equatorial a-form peaks (110), (040) and (130), respectively. The obtained K values are present in Table l.  The beta signal in W AXD pattems for the cJ composites samples of polypropylene with unmodified bentonite it is not well defined therefore is impossible to calculate their content. Nevertheless, the composites samples of polypropylene with modified bentonite the beta signal it is clearly defined for the samples with 5% and 10% of bentonite and is absent for the compounds with 25% of bentonite. However, the beta content obtained for the sample with 10% of modified bentonite was higher than for the sample with 5% ofmodified bentonite (see table 1). At 10% there are more nucleating sites (more amount offiller particles) for beta phase formation and consequently more beta content. Figure 5, 6 and 7 show micrographs of the composites samples with 5%, 10% and 25% of bentonite respectively. From these figures can be observed that as the concentration of the filler increases, its dispersion into the compound becomes worse. However, the samples with modified bentonite showed smaller agglomerations for all concentrations ofthe filler. Rybnikar (1989) has studied crystallization of polypropylene filled with tale particles at various concentrations ofthe filler. He found that only a small fraction of the filler act as nucleator and there is a certain limit of concentration of the filler to reduce the spherulite size. On the other hand, Labour (2002) and Lozano et al. (2004) have reported that beta phase of PP is favored with a good dispersion state of the filler.
Therefore, from our results we can enface that at 1 O % of filler there are more nucleating si tes for PP than at 5% and consequently more beta level. However at

CONCLUSIONS
In the present work the beta phase induction in PP was achieved using bentonite as filler. However, only the composites samples of polypropylene with modified bentonite showed the beta signal. The beta content obtained for the sample with 10% of modified bentonite was higher than for the sample with 5% of modified bentonite. This behaviour was related with concentration of the filler and it can be shown that as the concentration of the filler increases, its dispersion into the compound becomes worse However, our results we can enface that at 1 O % of filler there are more nucleating sites for PP than at 5% and consequently more beta leve l. Although at 25% would have more sites, their dispersion into the compound becomes worse which decreases the beta phase formation.