The Effect of Talc on the Crystal Orientation in Polypropylene/Ethylene-Propylene Rubber/Talc Polymer Blends in Injection Molding.
A blend of polypropylene (PP), ethylene-propylene Arubber (EPR), and talc, the so-called reinforced polypropylene compound, is used in a variety of automotive materials. In particular, the newer PP/EPR/talc blends exhibit not only higher fluidity than that of the conventional blend but also excellent rigidity and impact resistance, These desirable characteristics were achieved by increasing EPR content to 20 wt% or greater and by controlling the primary structures of polypropylene, i.e., lower molecular weight and higher tacticity [1, 2]. The control of molecular structures of PP/EPR/talc blends resulted in the appearance of an extremely specific and interesting morphology. The morphological characteristics of this blends are as follows:
1) It has an "elastomeric matrix" morphology in which PP crystalline grains of the order of 10 nm are dispersed in an amorphous matrix that comprises the PP amorphous phase and EPR .
2) The shape of the crystalline grain is that of a quadrangular prism in which the b-axis of the PP crystal is preferentially oriented in the direction of thickness during injection molding .
Thus, this reinforced polypropylene compound was called "super-olefin polymer" (SOP). Previously, we investigated the relation between the morphology and resin composition and the relation between the morphology and primary structures of PP, i.e., molecular weight and tacticity, focusing in particular on the relation between PP and EPR . In this paper, we study the effect of talc in particular on the b-axis orientation of PP crystal during injection molding process.
2.1. Materials and Specimen Preparation
As for polypropylene, impact-resistant poly(propylene-ethylene) copolymer was used. This was made by first-stage polymerization of the propylene homopolymer portion (PP), followed by polymerization with the ethylene-propylene copolymer portion. The melt flow index and tacticity by using [C-NMR.sup.13] of homopolymer portion were 150 (g/10 mim.) and 98.5%, respectively. The weight of ethylene-propylene copolymer in PP was 8 wt%. As for EPR, the ethylene-propylene rubber (73 wt% ethylene, the density of 0.87g/[cm.sup.3]. Mooney viscosity of 70 at l21[degrees]C) was used. As for flaky fillers, three various talcs (average particle size 3.8 [sim] 16.6 [micro]m, Asada Milling Co., Ltd.), calcined talc (average particle size 6.3 [micro]m, Asada Milling Co., Ltd.), mica (the average particle size 6.3 [micro]m, Yamaguchi Mica Co., Ltd.), and glass flake (the average particle size 6.8 [micro]m, Nihon Sheet Glass Co. Ltd.) were used. Table 1 summarizes the composition of talc.
The original PP/EPR/filler blends were blended in the ratio of 65/15/20 according to their weight. In the case of changing the talc weight of the blends from 0 to 25 wt%, the ratio of PP/filler was varied, with the constant ratio of EPR. Kneading was carried out using a twin screw extruder (FCM, Kobe Steel Works Co., Ltd.) after mixing these materials with a supermixer. The flexural test pieces were made by using an injection molding machine ([alpha]-100B, Fanuc Co., Ltd.) under the following conditions: 1) 220[degrees]C cylinder temperature; 2) 45[degrees]C molding temperature; 3) 30 second cooling time.
The test pieces of flexural modulus were conditioned under the constant temperature of 23[degrees]C and humidity of 50% for more than one week after they were molded. The flexural modulus was measured with the autograph (ABM-Ec, ORIENTEC Co., Ltd.) according to ASTM D790.
2.2.1. Measurement of Particle Size Distribution of Filler
The particle size of various fillers was measured with a laser diffraction particle size analyzer (SALD-2000, Simadzu Co. Ltd.). Ion exchange water and sodium hexametaphosphate were used as the dispersing solvent of fillers and the dispersing agent, respectively.
2.2.2. Talc Composition Measurement
The weight of silicon dioxide in talc was edited in accordance with JIS G 1312-1998. The weight of aluminum oxide, diiron trioxide, and calcium oxide in talc was measured by the following procedures. Talc was dissolved in fusing mixture, which consists of nitric acid, hydrogen fluoride, sulfuric acid, and water. This solution was evaporated to dry on platinum dish. The residue was dissolved in hydrochloric acid solvent. Then the weight of aluminum, iron, and calcium was measured, respectively, by using the inductively coupled plasma atomic emission spectrometer (ICP, Seiko Instruments Inc. SPS 1500VR). The weight of aluminum oxide, diiron trioxide, and calcium oxide was calculated based on the weight of aluminum, iron, and calcium, respectively.
The weight of magnesium oxide in talc was measured as follows. The combined weight of magnesium and calcium was determined by titrating the solution prepared by using the ICP measurement with EDTA (0.05mol/l). The weight of magnesium was determined by subtracting the weight of calcium that was analyzed by the ICP measurement from that of this mixture. The weight of magnesium oxide was calculated based on this weight of magnesium.
2.2.3. Thermal Analysis
The heat of fusion was measured with a thermal analyzer (DSC-7, Perkin-Elmer Japan Co., Ltd.). SOP samples were heated from 50[degrees]C to 190[degrees]C at a heating rate of 10[degrees]C/min. The heat of fusion was calculated from the endothermic area at the range from 100[degrees]C to 175[degrees]C. All experiments were conducted in [N.sub.2] atmosphere. The samples were sliced from the central part of the test pieces parallel to the flow direction (MD)normal direction (ND) plane.
2.2.4. Observation of Polarizing Microscopy
Thin sections about 20 [micro]m thick were sliced from the central part of the flexural test pieces parallel to the MD-ND plane with a microtome, and were observed with a polarizing microscope (JSM-6100 Olympus Co. Ltd.) under a magnification of 40 X.
2.2.5. Observation of Scanning Electron Microscopy (SEM)
The surface of specimens was observed with Scanning Electron Microscopy (JSM-6100, JEOL Co., Ltd.). The specimens were the central portion of the flexural test pieces, which were obtained through breaking the specimens parallel to the MD-ND plane after immersion of the specimens in liquid nitrogen for 20 minutes. SEM photographs were taken under conventional secondary electron imaging conditions and with an accelerating voltage of 15kV. A thin layer of gold was sputtered on the sample before SEM observation.
2.2.6. X-ray Diffraction Measurement
The wide-angle X-ray diffraction diagram was recorded by a camera system connected to an imaging plate (IPR-420, Mac Science Co., Ltd.) using a graphite monochromatized Cu-K[alpha] radiation (1.542A) from an X-ray generator (Rotaflex RU-200, Rigaku Co. Ltd.) with a rotating anode. The collimator was a double pin hole type whose size was 0.9 mm[theta].
The wide-angle X-ray diffraction diagrams were taken at the central parts of the test pieces from three directions (Fig. 1). The index of the degree of the b-axis orientation of PP crystal parallel to the nomal direction (ND) of
specimen was calculated in the ratio of the intensity of (040) plane to the intensity of (110) plane. The intensities of (110) and (040) were measured from 2[theta] scan curves of the equatorial direction in EDGE view diagram.
3. RESULTS AND DISCUSSION
3.1.1. Effect of Flaky Fillers an the b-axis Orientation
PP lamellae, which are formed along the b-axis orientation of the PP crystal, are oriented along the direction of thickness in injection molded SOP, and this is one of the structural characters in the SOP (4). We studied the effect of adding talc to PP/EPR blends on the b-axis orientation of the PP crystal in SOP moldings. Talc is a type of flaky filler. A flaky filler in injection molding has the characteristic of being oriented along the MD-TD plane. We tried to determine whether the preferential b-axis orientation was caused only by the flaky shape of the filler or by the mutual interaction between the talc surface and the PP. First, the effect of adding flaky filler on the b-axis orientation was evaluated. We tested talc, calcined talc, mica, and glass flake. Figure 2 shows SEM photographs of these blended specimens taken parallel to the MD-TD plane. The fillers in the photograph are mostly oriented in the MD direction in qualitative orientation. Figure 3 shows polarized micrographs of the blended specim ens (sliced parallel to the MD- ND plane). The white area is the polarized area, that is, the oriented area along the MD direction. The degree of MD orientation is proportional to the intensity of whiteness. The degree of MD orientation in the talc blended specimen is very high and deep. The mica blended specimen is weakly oriented in the MD direction. The others are MD-oriented only on the thin skin layer. The polarized rnicrographs revealed the possible effect of the amorphous region on the orientation. in addition, this high birefringence might depend on the alignment of the crystalline fillers (talc, mica, and calcined talc). To evaluate the orientation of the PP crystal region, wide-angle X-ray diffraction patterns were obtained from THRU. EDGE and END views (Fig. 4). The strong intensities inside of the (110) reflection of PP, which were observed in the END and EDGE views in Fig. 4a and 4c, are the (002) reflections of talc and mica. These reflections were concentrated in the meridian direction. They we re not observed in the THRU view. As do the results of the SEM photographs, these results demonstrate that talc and mica particles were aligned parallel to the specimen surface. The THRU view of the talc blended specimen showed weak c-axis and [a.sup.*]-axis orientations in the MD direction (Fig. 4). The EDGE and END views showed that the (040) reflection of PP was concentrated in the meridian direction (ND), which meant that the b-axes were strongly oriented to the plane perpendicular to MD and TD, namely ND. The (110) reflection was concentrated in the equatorial direction, which meant that the [a.sup.*]-axes were oriented in the MD and TD directions. In the other blended specimens, the EDGE and END views showed that the b-axes were very weakly concentrated in the meridian direction (or ND). These results indicate that the b-axes of PP crystals in the talc-free blended specimens are not strongly oriented in the ND direction. Therefore, it is not only the shape factor of talc but also the structure of talc that leads to t he strong b-axis orientation of PP crystal to the ND direction in the blends.
3.1.2. Effect of Particle Size, Purity, and the Amount of Talc on the b-axis Orientation
Figure 5 shows the index of degree of b-axis orientation of PP crystal parallel to the ND direction (I(040)/I(110)) as a function of the content of talc A. I(040)/I(110) increased in proportion to the talc content except for talc content less than 0.5 wt%. The model illustrated in Fig. 6 can explain the effect of talc on the b-axis orientation. The model is composed of two regions: X and Y. In region X, the b-axes of crystals that are near talc particles orient themselves parallel to the ND direction. On the other hand, in region Y, the crystals that are not near any talc particles orient themselves randomly. If the talc content is increased, the number of region X should increase proportionally to the talc content. Thus, the proportionality of b-axis orientation to a talc content of more than 0.5 wt% can be understood to be the result of an increase in the number of region X, and hence, the increase of the total area of the talc surface. However, the discontinuous change at less than 0.5 wt% cannot be expla ined simply by the increase in the number of region X. One also has to consider that this material has an excellent capacity for orienting the crystal even if talc is not included. In this case, talc may have a superior ability to promote the orientation of the PP crystal, so described in 3.1.1. Therefore, it would be the combination of resin and talc that is responsible for the orientation behavior. In our case, the degree of orientation of the PP crystal strongly depended on the talc content of less than 0.5 wt%. Figure 7 shows the b-axis orientation (I(040)/I(110)) as a function of the talc particle size A, B, and C for a talc content of 20 wt%. For talc A, I(040)/I(110) decreased in proportion to the particle size, which indicated that the smaller the particle size, the higher the degree of orientation of b-axis. By decreasing the particle size to below the size of the nucleus of PP. the degree of orientation must decrease steeply to the value obtained for system without talc. Here, we limit the discussio n to the dependence of I(040)/I(110) on the talc content, which holds linearity. Since the weight fraction of talc is constant in these cases, the number of talc particles (N) depends on the radius of gyration of talc (r), and hence the total surface area (S) is given as S [sim] [Nr.sup.2] [sim] [r.sup.-1]. Thus, a decrease in particle size leads to an increase in the total surface area of talc, or the volume fraction of region X, and hence, the degree of orientation is increased.
As mentioned above, the interaction between PP chains and the talc surface is an important factor for the orientation of PP crystal. The functional group for the interaction on the talc surface is magnesium silicate hydroxide. Talc A, B, and C had different compositions, as shown in Table 1. The impurities contents were A [less than] B [less than] C. As seen from Fig. 7, the degree of orientation of PP crystal depended on the kind of talc even though their particle sizes were the same. We employed the wide-angle X-ray diffraction scattering (WAXS) method to investigate the differences in crystal structure among these tales. Figure 8 shows the WAXS profiles from talc A, B, and C. For talc A, the diffraction peaks  due to the magnesium silicate hydroxide crystal were clearly observed. WAXS profiles for talc B and C had other peaks that did not appear in the WAXS profile for talc A, e.g. peaks at 2[theta] = 21[degrees] and 32[degrees]. These extraordinary peaks are thought to arise from crystals of impurities . Comparing the WAXS profiles for talc B with C, the peaks due to impurities were stronger for talc C than for talc B, which meant that the impurity crystal contents were A [less than] B [less than] C. Therefore, the b-axis orientation of PP is affected by the impurities contained in the talc. In the context of our model, impurities decrease the effective surface area of talc and reduce the volume of Region X.
To summarize the above results, it is obvious that:
1) Talc blends among flaky filler blends show a peculiar strong b-axis orientation of PP crystal to the thickness direction in the injection molding.
2) The degree of b-axis orientation of PP crystal depends on the amount of the surface area of talc in contact with PP.
Figure 9 summarizes these results. In this Figure, region X and Y are strongly ND-oriented b-axes of PP crystals near talc particles and randomly oriented crystals, respectively. The orientation of the b-axis of PP depends on the talc content, the particle size of talc and impurities of talc. The degree of orientation of the b-axis can be increased by increasing the talc content, decreasing the particle size of talc, or by using pure talc, which contains only magnesium silicate hydroxide. Considering the orientation behavior based on our model, widening Region X increases the degrees of orientation of PP. That is to say, it is important to make as many PP crystals as possible grow from the talc surface, which can be accomplished by increasing of the surface area of talc.
In this study, we showed that talc in SOP has the b-axis of PP crystal oriented parallel to the thickness direction of the injection molding. However, in the case of SOP without talc, a weak orientation in the injection molding [7, 8] and a strong orientation in the film casted by the T-die method  have been reported. Therefore, SOP inherently tends to orient its b-axis parallel to the thickness direction of the moldings. The role of talc is only to enhance the nature of the b-axis orientation in the presence of flow. However, we do not yet know the reason why the b-axis of PP crystal tends to be oriented parallel to the thickness direction of the moldings. Although this peculiar crystal orientation was reported in the case of PP with high molecular weight/talc [10, 11], it is not clear why PP is likely to orient its b-axis parallel to the thickness direction of the moldings. If the mechanisms of crystallization and orientation of PP under the flow field are clarified in detail, they should provide a deep insight into the peculiar crystal orientation in SOP injection molding.
3.2. Effect of Talc on the Flexural Modulus of the SOP
The super-molecular structure with the peculiar orientation, described in Sec. 3.1. greatly influences the mechanical properties of SOP. Figure 10 shows the dependence of the flexural modulus (FM) on the content of talc A for the injection-molded pieces of SOP. Figure 11 shows the dependence of flexural modulus on the particle size of talc A, B and C at the content of 20 wt%. From Fig. 10, the was found to increase with increasing talc content. Figure 11 indicates that the FM increases with a decrease of the particle size, at the constant talc content. It should also be noted that the samples with talc containing fewer impurities have a higher FM even when the talc has the same content and particle size. This improvement in Fig. 10 is probably caused by the additive property of talc, and by some other factors.
In general, the FM depends on the crystallinity of polymeric matrix. Thus, it is possible to explain the increase of the FM as an increase in the crystallinity of PP. Figure 12 shows the fusion enthalpy ([delta]H) for the samples with talc of different particle sizes. The talc content of these samples is 20 wt%. Since the crystallinity of these samples is approximately the same as that in Fig. 10, the change of the FM shown in Fig. 11 did not originate from the crystallinity, but from the orientation of PP. As mentioned in Sec 3.1, the orientation of PP is closely related to the surface area of the talc, i.e., Region X in our model. Therefore, the FM of this material is controlled by Region X where the crystalline lamellae stick out of the talc surface. By increasing the orientation of PP crystals near talc, the volume fraction of Region X increases, which results in the improved rigidity.
The blend of PP/EPR/talc, which is called "superolefin polymer" (SOP), consists of PP with a molecular weight and high tacticity, EPR, which makes up 20 wt% or greater of the blends, and talc. One of the structural features of SOP material is that the b-axes of PP crystal lamellae are strongly oriented in the direction of thickness of the injection molding. We investigated the effect of the addition talc on PP crystal orientation and flexural modulus. Our results are as follows:
1) Among the flaky fillers, the talc blends are the most strongly oriented in the thickness direction of the b-axis of the PP crystal.
2) The degree of the orientation depends on the surface area of talc in contact with polypropylene.
These results make it clear that the flexural modulus of the injection molding varies depending on the surface area of talc. Increasing the orientation of the PP crystals around talc particles improves the rigidity of the blends.
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Composition of Various Talc Types. Sample Name Talc A Talc B Talc C Si[O.sub.2] 59.6% 54.7% 39.4% MgO 31.3 32.4 31.1 [Al.sub.2][O.sub.3] 0.39 0.13 2.4 [Fe.sub.2][O.sub.3] 0.49 0.20 1.9 CaO 2.0 1.51 5.0 others 6.22 11.6 20.2
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|Author:||OBATA, YUTAKA; SUMITOMO, TAKASHI; IJITSU, TOSHIKAZU; MATSUDA, MASATOSHI; NOMURA, TAKAO|
|Publication:||Polymer Engineering and Science|
|Date:||Mar 1, 2001|
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