Polypropylene-(ethylene-propylene) heterophasic copolymer [PP-EP]/EVA systems.
Poly(propylene) (PP) and poly(ethylene vinylacetate) (EVA) are two important commercial polymers. The former is a semicrystalline thermoplastic, whereas the latter is considered an elastomer. In the last few years, it has been argued that PP and EVA are immiscible in each other (1-5); nevertheless, the existence of particular interfacial interactions between these two polymers has suggested the existence of compatible blends (1, 6). in a previous study on the morphology and mechanical properties of PP/EVA mixtures, two distinct regions and one transition zone were observed as a function of EVA concentration (1). At concentrations below 40% EVA, the PP/EVA blends were considered non-compatible in spite of the small changes in properties of the mixture. However, at concentrations above 60% EVA, this blend was classified as compatible because some properties of PP were enhanced. A parallel transition in morphology was observed at EVA concentrations between 40% and 60% in agreement with the reported results (1). In summary, differences in compatibility in PP/EVA blends as a function of EVA concentration were attributed to the existence of local interfacial interactions and changes in morphology.
The copolymerization of ethylene with propylene into a PP matrix results in a heterophasic copolymer of PP. This is the case of polypropylene(ethylene-propylene) heterophasic copolymers (hereafter called PP-EP copolymers). PP-EP copolymers are usually named "block copolymers." However, this term is not accurate (7-11) since ethylene-propylene are randomly copolymerized into a PP backbone. For PPEP/EVA blends, enhanced interpolymer interactions are expected to occur because of the presence of ethylene segments in both PP-EP and EVA. These interactions could lead to more compatible blends than the PP homopolymer/EVA blends. Therefore, molecular interactions and mechanical properties of a series of PP-EP/EVA blends were studied in a wide range of concentrations. The results are compared with those previously reported (1).
Materials and Blend Preparation
The PP-EP copolymer was obtained from Himont Inc. (melt flow index of 8.0 dg/min at 230[degrees]C and density of 0.90 17 g/[cm.sup.3]) and the EVA copolymer from Alcudia Inc. (28% of VA, melt flow index of 7.0 dg/min at (190[degrees]C) and density of 0.9451 g/[cm.sup.3]).
Thin films and thick probes were prepared. PP-EP and EVA were first mixed at 180[degrees]C in a Brabender mixing device (Plasticorder PL-2000) in the following PP-EP/EVA ratios: 100/0, 80/20, 70/30, 60/40, 40/60, 30/70, 20/80, 0/100. A series of thin films (40 [mu]m thickness) and thicker probes (2 mm thickness) were prepared by compression molding at 190[degrees]C. An extra set of thin films (20 [mu]m thickness) was prepared by casting from xylene solutions. For preparation of the thick probes, the PP-EP and EVA copolymers were first mixed in a Werner & Pfleiderer (ZSK-30) twin-screw extruder. The mixtures were then injected into molds (with ASTM specifications) using a Battenfeld (BA 750 CDK) injection molding machine. Thick plaques (1 mm thickness) were prepared in the following ratios: 100/0, 80/20, 60/40 and 40/60. Details of process conditions and dimensions of samples were reported in a previous paper (1).
Techniques and Methods
The ethylene content in the PP-EP copolymer was determined by infrared spectroscopy using a Nicolet 550 FTIR spectrometer. The resolution was 4 [cm.sup.-1] for 30 scans. A calibration curve was generated with a series of i-PP-LLDPE thin films (40 [mu]m taking as a reference the absorbances at 720 [cm.sup.-1] (LLDPE) and 974 [cm.sup.-1] (i-PP).
Differential scanning calorimetry (DSC) traces of thin films and thick probes were obtained in a TA-DuPont 1090 calorimeter. Thermal scans were made from 0[degrees]C to 188[degrees]C at a heating rate of 10[degrees]C/min using nitrogen atmosphere. For these experiments, all starting materials (PP, PP-EP and EVA) and blends were first heated, in order to erase thermal history. from -20[degrees]C to the equilibrium temperature of PP (188[degrees]C) at a heating rate of 10[degrees]C/min (using nitrogen atmosphere). Samples were then cooled to -20[degrees]C at a rate of 10[degrees]C/min. In a second run, samples were heated to 188[degrees]C at a heating rate of 10[degrees]C/min (using nitrogen atmosphere) in order to register the melting process after constant non-isothermal crystallization.
Dynamical mechanical analyses (DMA) of thick probes were carried out in a TA DuPont 983 analyzer. The experimental conditions were a heating rate of 3[degrees]C/min, a temperature range within -100--80[degrees]C and 0.1 Hertz of frequency. Injected probes were used as obtained from the injection machine. However, in our experiments, all sample preparation conditions were maintained strictly constant and particularly at zone 4 (nozzle) at 210[degrees]C. The actual ejection temperature was 65[degrees]C, injection time, 6 sec; injection rate, 40 mm/sec; cooling time, 25 sec and total injection cycle, 33 sec. This gives a constant non-isothermal cooling rate of approximately 264[degrees]C/min.
Wide-angle X-ray diffraction (WAXD) patterns of thin films and thick probes were recorded using an X-ray Siemens D-5000 diffractometer with Ni-filtered CuK[alpha] radiation generator. Patterns were recorded at 4[degrees]/min within the 2[theta] range 8[degrees]C-30[degrees].
Small-angle X-ray scattering (SAXS) measurements of PP-EP/EVA thick plaques (1 mm) were performed in the 10-meter-length instrument at Oak Ridge National Laboratory, USA (12, 13). Two sample-to-detector distances, namely, 1.119 and 5.119 m, were used to obtain a wide range of dispersion data (14, 15). Absolute intensity measurements were obtained for each distance and then both curves were joined together. The collected data were radially (azimuthally) averaged in the range of the scattering vector 0.1 [less than or equal to] q [less than or equal to] 4.9, where q = (4[pi]/[lambda])sin [theta], and [lambda] = 1.54 A. The scattering data were converted to absolute differential cross section by means of pre-calibrated secondary standards. The absolute intensity axis was in [cm.sup.-1].
The spherulitic morphology of PP-EP/EVA cast films (20 [mu]m) was registered with an Olympus BH-2 polarizing optical microscope (POM) under cross polars.
The Izod impact strength of PP-EP/EVA thick probes was measured in a Custom Scientific Instrument 137C following the ASTM-D256-93 method. Samples were ice-water cooled immediately before any measurement. Tensile strength and elongation data were obtained with a United CCF-45 universal machine following the ASTM-D638 method. Details on the characterization techniques and methods are given elsewhere (1).
RESULTS AND DISCUSSION
The determination of the ethylene content in the PP-EP copolymer required a series of compositions of i-PP and LLDPE (3, 6, 8, 16 y 22% of LLDPE). These samples were prepared by casting from xylene and the i-PP/LLDPE films were analyzed by FTIR. The absorbances at 720 [cm.sup.-1] for the LLDPE ([A.sub.720]) and 974 [cm.sup.-1] for the i-PP ([A.sub.974]) were used to obtain the ratio [A.sub.720]/[A.sub.974] versus percent LLDPE. A least square linear fit of this data resulted in the following relationship:
%LLDPE = ([A.sub.720]/[A.sub.974]) + 0.0327/0.029 (1)
In a similar manner, PP-EP films were prepared by casting and then characterized by FTIR. The [A.sub.720]/[A.sub.974] ratio for this copolymer was 0.455. By introducing this value into Eq 1, it was determined that the PP-EP copolymer contained approximately 16% ethylene. The validity of Eq 1 was verified with a commercial 8%-ethylene content PP-EP copolymer. The ethylene content of the copolymer is important considering that interfacial interactions, morphology and mechanical properties, all related to compatibility, are discussed in terms of this comonomer. The PP-EP is actually a complex mixture containing a PP homopolymer, a branched copolymer consisting of a PP backbone and random ethylene propylene branches, and a random ethylene propylene copolymer. Therefore, the 16% ethylene is randomly distributed in the whole system.
Small changes in terms of EVA concentration in both the crystallization temperature and the melting point have been reported (1, 16). The results were explained in terms of molecular interactions in crystalline zones particularly at high EVA compositions. Such interactions were also considered in amorphous zones, and they were associated with changes in [T.sub.g] of both polymers at low EVA compositions and particularly between 40% and 60% EVA. DSC heating traces of PPEP/EVA blends with different EVA content are shown in Fig. 1. These traces mainly consist of multiple melting endotherms such as those commonly displayed by polypropylene in the [alpha] a and [beta]-crystalline forms (17). Also, there is a melting endotherm around 120-125[degrees]C associated with the ethylene-propylene of the PP-EP copolymer. The EVA copolymer only shows a broad melting endotherm around 8000 associated with melting of ethylene sequences of the copolymer. The intensity of the PP-EP and EVA signals depended on blend composition. It is also observed that the melting point of the PP is slighty affected as the EVA content increases. This behavior is similar to that reported for PP homopolymer in PP/EVA blends (1).
The glass transition temperatures of PP ([T.sub.g(PP)]) and EVA([T.sub.g(EVA)]) of both pure polymers and blends were measured by DMA as shown in Fig. 2. The [T.sub.g(EVA)] in PP-EP/EVA blends increased by 5[degrees]C in respect to the pure EVA and then it shifted slightly and continuously to higher temperatures as the EVA concentration increased. It can be also noticed that for the PP/EVA blends, the increase in the [T.sub.g(EVA)], in respect to the pure EVA, was only 2[degrees]C, and then it increased in about two more degrees for an EVA concentration of 60%. As for the glass transition temperature associated to PP in PP-EP/EVA blends, difficulties were encountered in resolving it. Figure 2 shows, for example, that the shift of the [T.sub.g(PP)] to lower temperatures involves, from the beginning, an overlapping of signals. In other words, the shape of the DMA thermograms corresponds to a broad signal in which the [T.sub.g] of both EVA and PP overlap. Therefore, these results suggest high interfacial interactions between PPE P/EVA, an effect that can produce a higher degree of compatibility, in contrast to the PP/EVA system.
In PRP/EVA blends sain et al. (18) used the [T.sub.g], determined by DMA, to study interaction between phases. In spite of the presence of the ethylene monomer in both PRP and EVA. no significant changes in [T.sub.g(PP)], for concentrations between 1.5% and 15% EVA (42% VA), were observed. A possible reason was that the EVA concentrations used by these authors was too low as to induce significant changes in [T.sub.g]. In a different study (5), the thermal properties of the PRP/ EVA blends were compared with those of PEP/EVA compatibilized with PP-MAH. For the compatibilized system a sudden drop in the storage modulus E' (close to -50[degrees]C), associated to the [T.sub.g] of the random ethylenepropylene blocks, was observed. The [T.sub.g] of this block was shifted to lower temperatures indicating chemical interactions between PEP/EVA and PP-MAN. U et al. (19) reported the thermal properties of chlorinated PP(CAPP)/EVA blends. For this case it was observed that the chlorination of PP allows a strong interaction between CHC1 and the carbonyl groups of CAPP and EVA respectively. The presence of a single [T.sub.g] in the whole range of concentrations indicated typical behavior of a miscible blend. Thomas and George (2) reported PP/EVA blends mechanically crosslinked. They observed a broadening of the Tan signals, which was associated with interfacial bonding between phases (compatibility). An improvement in the interaction between PP and EVA has been also tried by chemical modification of the EVA copolymer. This is the case of the PP/EVASH blends reported by Dutra et al. (20). These authors proposed partial miscibility on the basis of a small decrease in the melting temperature at low EVA concentrations.
In order to determine the characteristic unit cell of i-PP blended with EVA. the wide angle X-ray diffraction patterns of the PP-EP/EVA blend were obtained from thin films and thick probes. Figure 3a shows the WAXD patterns of PP-EP/EVA thin films with different EVA concentration. In this figure, i-PP shows the characteristic peaks of the [alpha] and [beta] hexagonal structures. There is a also a diffraction peak at 21.5[degrees] related to the ethylene moiety of the PP-EP component. It also observed that as the EVA concentration increases, the peak intensities, associated to the [alpha] form, decrease. The peak at 2[theta] = 16.1[degrees] related to the [beta] form tends to disappear as the EVA concentration increases. The lower intensities are basically due to an increase of the amorphous character of this blend. The [alpha] form predominates over the [beta] form for thick probes as shown in Fig. 3b. Therefore, process conditions affect the resulting crystalline structure of PP-EP/EVA blends.
In general, the [alpha] form of the i-PP is preferentially developed at high isothermal crystallization temperatures or at low cooling rates, when crystallization is made from the melt (21). It has been reported that the formation of [beta] crystals is promoted by several factors; colorants (22), by quenching (23, 24), low isothermal crystallization temperatures (17) and the use of reinforcements such as glass fiber (21). In this study, we conclude that the decrease of the [beta] form with a parallel increase of the [alpha] form (the most stable of both), observed in PP-EP/EVA thin films as the EVA concentration increases, is due to the interaction between PP-EP and EVA.
The crystallinity ([X.sub.c]) of blends was measured and found related to morphological changes. This parameter was calculated considering both the total area under the entire diffraction pattern and the area of peaks in the crystalline regions. The results shown in Fig. 4 indicate that, as expected, crystallinity in PP-EP/EVA blends decreases as the amount of EVA in the blend increases. This tendency was similar to that observed for PP/EVA blends (1).
Being the purpose to obtain quantitative morphological information, the PP-EP/EVA blends were studied by small angle X-ray scattering. The interface distribution function [g.sub.1](r) (25), calculated from absolute intensity experimental data as a function of the scattering vector, is shown in Fig. 5. For PP-EP, this function shows three peaks, two positive and one negative. SAXS theory and calculations involved in obtaining the [g.sub.1](r) function have been reviewed elsewhere (16). Theoretically (25), the two positive peaks are related to both the amorphous ([1.sub.a]) and the crystalline ([1.sub.c]) thickness, and the negative peak to the long period (L). Assuming an infinite lamellar stacking model (26) the correlation distances [r.sub.1], [r.sub.2] and [r.sub.3] can be associated to the lamellar thicknesses and long period. Because of the high crystallinity of PP-EP, [r.sub.1] was assigned to the amorphous lamellar thickness ([1.sub.a]), [r.sub.2] to the crystalline thickness ([1.sub.c]) and [r.sub.3] t o the long period (L). The results are shown in Fig. 6. The value [1.sub.c] = 10.6 nm obtained for pure PP-EP is within the range of the reported values (9-11 nm) for PP crystallized under similar conditions (27, 28).
Variations in lamellar thickness In semicrystalline/amorphous polymeric blends have been used to determine the interaction between phases. Cheung et al. (29) observed a strong increase in the amorphous thickness in ([epsilon]-caprolactone/polycarbonate) (PCL/PC) blends and the effect was associated to the incorporation of PCL units in interlameller zones. This behavior was particularly observed at high PC concentrations. In the present study, interactions between PP-EP and EVA were also associated with changes in interlamellar zones. This was based on the fact that the crystalline thickness decreased monotonically in a nonlinear fashion, while the amorphous thickness increased linearly as a function of EVA concentration as shown in Fig. 6. The nonlinear decrease in the crystalline thickness could be indicative of a continuous thinning process of lamellae, generating as a consequence higher amorphous regions. Therefore, an expected behavior would be an increase of physical interactions in the amorphous state b etween PP-EP and EVA.
The spherulitic morphology of PP-EP/EVA thin films observed with the optical microscope and cross polars is shown in Fig. 7. It is observed that PP-EP shows well-defined crystallization patterns with average spherulitic dimensions of 100 [mu]m. The crystalline texture of the PP-EP/EVA blends at low EVA concentration (20%) is similar to the one obtained for PP-EP but with some imperfections. Increasing the EVA concentration at 40%. the spherulitic pattern is still maintained; however, dark spotted areas inside the spherulites are observed. At 60% EVA the spherulitic pattern is not well defined and the amorphous zones predominate. This morphological evolution is in agreement with the explanations given so far that intraspherulitic amorphous areas are generated as the EVA content increases giving place to a higher amount of molecular interactions in this phase.
We have demonstrated that the ethylene fraction of the PP-EP copolymer promotes interactions between PP-EP and EVA, therefore, we should expect the mechanical properties of the corresponding blends to be somehow affected. The elongation-at-break and the impact and tensile strengths of PP/EVA and PPEP/EVA blends are shown in Table 1. Both the elongation at break and the impact strength of the PP-EP copolymer showed higher values; 65% and 43 J/m respectively, than those corresponding to i-PP (16); 25% and 17 J/m respectively. This is an indication that the ethylene-propylene sequences contribute to the elastomeric nature of the PP-EP blends. When the PP-EP copolymer was blended with 20% EVA, the elongation at break slightly increased as shown in Table 1. However, when the EVA concentration increased from 20% to 40% a marked increase occurred. As for the tensile strength results shown in Table 1, there was a drastic decrease of this property with the addition of EVA in the PP/EVA system. However, above 20% EVA i t decreased only slightly. Regarding PP-EP/EVA blends, the decrease in strength was not as marked and the tensile strength remained below PP/EVA although with the same tendency.
Blending often results in the improvement of some mechanical properties at the expense of some others. Sain et al. (18) reported PRP/EVA blends (with concentrations between 0% a and 47% EVA) in which the elongation at break of the PRP copolymer was high at low EVA content. However, at high EVA concentrations it displayed a marked decrease. On the other hand, at low EVA contents the tensile strength remained almost constant but at high EVA contents it decreased. This contradicts our results, and some other studies (1, 3), in which the elastomeric materials improved the elongation properties with a minimum effect on the tensile strength. It should be noticed, however, that Sain et al. (18) used EVA with a much higher vinyl acetate concentration (42%), different blending conditions, and a different processing method (compression molding) in order to obtain thick probes. Hudec et al. (5) studied PRP/EVA blends compatibilized with MAH-PP. They found that at low EVA content (8%), a small concentration of MAR-PP (le ss than 5 wt%) enhanced both the brittle transition and the impact strength of blends, while elongation and tensile strength did not change significantly. This enhancement was attributed to a better dispersion of the elastomeric phase into the matrix and probably to an increase in the interfacial interactions promoted by the modifier. In a different study with PP/EVASH blends (20), the increase in the elastic modulus at low EVASH contents (1% to 5%) was associated with an improved adhesion between PP and EVA. In summary, the PP-EP/EVA system is a complex one whose mechanical behavior depends on blend composition.
PP-EP/EVA blends were studied using different experimental techniques and the results were compared to those of PP/EVA blends. It was shown that ethylene-propylene (EP) sequences significantly improve physical interactions in amorphous zones between PP-EP and EVA. DMA analyses of PP-EP/EVA blends indicated that, as the concentration of EVA increased, the [T.sub.g] of the EVA copolymer shifted to higher temperatures while the [T.sub.g] of the PP shifted to lower temperatures. These changes in opposite directions were taken as an indication of enhancement of interfacial interactions in amorphous zones. The elongation at break and impact strength of PP-EP/EVA blends were improved compared to both pure PP-EP and the corresponding PP/EVA blends. There was a drastic decrease in tensile strength with the addition of EVA in the PP/EVA, although at high EVA concentrations, it decreased only slightly. The decrease in tensile strength of PP-EP/EVA was not as marked and it remained below PP/EVA at high EVA concentrations.
The results were associated to the existence of physical interactions in the amorphous zones (promoted by "coalescence"), to the good distribution of dispersed particles in the matrix, and to the elastomeric character of blends. In general, PP-EP/EVA blends have better compatibility than PP/EVA blends owing to the favorable interfacial interactions between the ethylene groups contained in both PP-EP and EVA.
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Table 1 Mechanical Properties of PP/EVA Blends Thick Probes. Composition Elongation at Impact Strength Tensile Strength break (%) (J/m) (MPs) i-PP * 25 17 35 PP-EP 65 43 20 PP-EP/EVA (80/20) 75 49 15 PP-EP/EVA (60/40) 250 72 15 PP-EP/EVA (40/60) 525 200 13 * Ramfrez Vargas et al. (1)
We would like to thank the Mexican National Council of Science and Technology (CONACYT) for the grant 28554-U. We also thank Blanca Huerta, Josefina Zamora and Roberto Benavides for their help in obtaining and processing experimental data.
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E. RAMIREZ-VARGAS (1), F.J. MEDELLIN-RODRIGUEZ (2), D. NAVARRO-RODRIGUEZ (1), C. A. AVILA-ORTA (1), S. G. SOLIS-ROSALES (1) and J. S. LIN (3)
(1) Centro de Investigacion en Quimica Aplicada Blvd. Enrique Reyna 140 Saltillo, Coahuila, 25100, Mexico
(2) CIEP-FCQ, UASLP Au. Dr. Manuel Nava 6, Zona Universitaria San Luis Potosi, S. L. P., 78210, Mexico
(3) Solid State Division Oak Ridge National Laboratory Oak Ridge, Tennessee, 37831
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|Title Annotation:||Morphological and mechanical properties of polypropylene [PP]/poly(ethylene vinyl acetate) [EVA] blends, part 2|
|Author:||Ramirez-Vargas, E.; Medellin-Rodriguez, F.J.; Navarro-Rodriguez, D.; Avila-Orta, C.A.; Solis-Rosales|
|Publication:||Polymer Engineering and Science|
|Article Type:||Statistical Data Included|
|Date:||Jun 1, 2002|
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