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Development of a new production method for a polypropylene-clay nanocomposite.

INTRODUCTION

The clay mineral is a potential nanoscale additive because it is composed of silicate layers in which the fundamental unit is a 1-nm-thick planar structure. If a dispersion of these nanometer-scale silicate layers could be achieved in a polymer matrix, the interfacial area might be larger (>700 [m.sup.2]/g for the clay mineral) than for micrometer-scale dispersions and the interaction between the silicate layer and the polymer might be a maximum. Also, in this condition, the mechanical properties might be further improved and new and/or unexpected hybrid properties might be derived synergistically from the two components. In our previous work we synthesized a nylon 6-clay hybrid (nanocomposite) (NCH) in which 1-nm thick silicate layers of clay minerals were exfoliated and homogeneously dispersed in the nylon 6 matrix (1). The NCH exhibits various superior properties, such as high strength, high modulus and high heat resistance compared to conventional nylon 6 (2). Since then, polymer-clay nanocomposites have attracted the attention of a great number of researchers, and other polymer-clay nanocomposites such as polyimide (3), epoxy resin (4-8), polystyrene (9-13), polycaprolactone (14), acrylic polymer (15), polyurethane (16), poly (ethylene terephthalate) (17), polypropylene (18-21) and polyethylene (22) have been reported.

Polypropylene is one of the most widely used polymers in automotive parts, so the polypropylene-clay nanocomposite (PPCN) is a most attractive material. If PPCN could be used in automotive parts, the weight of the car might be reduced and the design might be changed in ways that are currently only dreamed of. PPCN has already been prepared by several methods. We have demonstrated the preparation of PPCN by a melted-intercalation process (a melt compounding process) using maleic anhydride-modified polypropylene and organo-clay for the first time, to the best of our knowledge (20). Recently, Bergman and co-workers reported the synthesis of a polyethylene- and a polypropylene-clay (fluorohectorite) nanocomposite by an in-situ polymerization process with a Pd catalyst (23). However, they found that the turnover frequency [TOF = mol polypropylene/(mol Pd h)] of the catalyst in the preparation of the PPCN was much slower than the TOF in that of the polyethylene-clay nanocomposite. Therefore, the melt-compounding process for preparing PPCN is also a useful method from an industrial standpoint. In the melt-compounding process it is necessary for the clay mineral to be pretreated with an organic cation such as the alkyl ammonium ion in order to achieve a nanometer-scale dispersion of the silicate layers. This type of pretreatment process is usually known as organo-modification. The organo-modification must be carried out separately from the melt compounding in an extruder. First, a granular clay mineral is dispersed in the water. The organic cation is then added to the dispersion, followed by filtration, drying, and milling. This process is cumbersome and also costly. If the organo-modification could be eliminated, the procedure might be simplified and the production cost of PPCN might be lower than the conventional process.

We have already made attempts to eliminate the organo-modification and have reported a compounding process using the clay mineral without any organo-modification (Na-montmorillonite) for a nylon 6-clay nanocomposite. In this process, a clay mineral water slurry (clay slurry) was prepared by dispersing Na-montmorillonite in water, and the clay slurry was injected into melted nylon 6 in a twin-screw extruder (24). However, there are two problems with this clay slurry process. The first problem is that when the clay slurry includes over 5 wt% of clay mineral it shows a very high viscosity. Therefore, it is difficult to supply the clay slurry into the twin-screw extruder. The second problem is that a large amount of water is necessary to reduce the high viscosity of the clay slurry. The excess water can become vaporized and the water vapor may jet out through any open vent port unless the screw design is of a high specification.

In this paper we tried to develop a new production method for PPCN using non-pretreated clay and also without the requirement for a large amount of water. In this method we focused our attention on the nature of the clay mineral when it was exfoliated in the water. The basic concept of this method was that a clay slurry was achieved in the twin-screw extruder by adding the clay mineral and the water separately.

EXPERIMENTAL

Materials

The materials used in this study were montmorillonite, polypropylene, two types of compatibilizers, and water. The montmorillonite was purified Na-type montmorillonite (Na-Mt, brand name: Kunipia-F, cation exchange capacity C.E.C = 115 mmeq/100 g, d-space = 1.21 nm) purchased from Kunimine Industries Co., Ltd. The polypropylene was homo-polypropylene (MA2, brand name: Novatec MA2, melt flow rate 16 g/min, JIS K 6758) purchased from Japan Polychem Corporation. One of the compatibilizers was octadecyl trimethyl ammonium chloride (OTM), which we used to stabilize the interface between the silicate layers in the clay mineral and the polypropylene. The OTM was purchased from Tokyo Kasei Kogyo Co., Ltd. The other compatibilizer was maleic anhydride-modified polypropylene (Ma-g-PP, brand name: U-mex 1001, acid vale 26 mg KOH/g) purchased from Sanyo Chemical Industries, Ltd., which was used to modify the dispersibility of the silicate layers of the clay mineral. The water that we used was deionized.

Equipment and Extruder Screw Design for the New Production Method

A twin-screw extruder was used for the new PPCN production method. The extruder was a co-rotating intermeshing extruder, TEX30 77BW-20V from Japan Steel Works, Ltd. The screw diameters were 32 mm and the length-to-diameter ratio (L/D) was 77:1. The long screw-length was an important characteristic of this twin extruder. In order to prevent the use of a large amount of water and the consequent venting of water vapor, the screw had four geometrical sections. The first section consisted of some kneading screw elements and many full-flight screw elements. The polypropylene was melted and the polypropylene, the clay mineral, the Ma-g-PP and the OTM were premixed in this section. The second section was the most important zone, where the water was injected into the extruder. The key feature of this method is to supply the clay mineral and the water separately. This section has two sealing rings before and after it to maintain the water vapor pressure and to achieve a clay slurry in the extruder. The other role of the sealing rings was to prevent venting of the water. In this second section there were multiple kneading screw elements to disperse the silicate layers of the clay mineral into a slurry state in the polypropylene. The third section had an open vent port to release the water vapor. The fourth section had a vacuum vent port to remove any remaining water from the sample.

Preparation of PPCN by the New Production Method

Ten types of samples, with different compositions of PP, the clay mineral (Na-Mt). Ma-g-PP and OTM were prepared in this study, as shown in Table 1. Seven of the samples (PPCN1 through PPCN7) contained Na-Mt, MA2 was raw polypropylene, while MA2(70)/Ma-g-PP(30) was a mixture of MA2 and Ma-g-PP. These two samples were prepared for comparison. PPCN-ORG was a conventional sample with organo-clay (Na-Mt treated with the octadecyl ammonium ion). In order to make clear the effect of Ma-g-PP, samples including 30 parts of Ma-g-PP and samples without Ma-g-PP were prepared. From the viewpoint of stabilizing the PP/silicate-layer interface by carrying out ion exchange of OTM with the sodium ion between the layers of clay mineral, OTM was added in quantities equivalent to 0, 0.25, 1 eq of C.E.C of Na-Mt.

PP and Ma-g-PP were supplied so that the sum totals were set to 10 kg/h. Na-Mt and OTM were supplied in quantities corresponding to the PP and Ma-g-PP by using a loss-in-weight feeder. Water was injected into the extruder at a rate of 2 kg/h using a pump. The rotation speed of the screw was 300 rpm. The extrusion temperature was set as 180[degrees]C-200[degrees]C, though the actual temperature of the resin was 200[degrees]C-220[degrees]C because of the generation of heat by shearing. When the water was injected, compared with the case where water was not injected, the pressure was measured as 1 MPa. Since the saturated vapor pressure of water at 200[degrees]C is calculated to be 1.7 MPa, it was considered that the water vapor pressure was maintained in the twin-screw extruder. Pale-yellow and/or pale-brown strands of the PPCNs were obtained. The residence time was about 6 minutes. The strands that were obtained were cooled in a water bath, pelletized with a cutter and dried under vacuum at 80[degrees]C. The inorganic content derived from the clay mineral was measured by burning the organic matter of the sample. The results are listed in Table 1.

Evaluation of the Dispersibility of the Clay Mineral in the Polypropylene Matrix

The dispersibility of the silicate layers in the PPCNs was evaluated by using an X-ray diffractometer and a transmission electron microscope (TEM). X-ray diffraction (XRD) patterns of thin films of the PPCNs and related samples were obtained using a Rigaku X-ray diffractometer, RINT-TTR, with Cu-K[alpha] radiation at 50 kV and 300 mA. TEM observations of the PPCNs were performed on ultra-thin sections of the films using a JEM-2010 TEM with an acceleration voltage of 200 kV. These ultra-thin sections were produced by the freeze-cutting method using a microtome. Thin films for both the XRD and the TEM were obtained by press-molding at a temperature of 230[degrees]C.

Measurement of Flexural Properties

The flexural test was performed at 23[degrees]C according to ISO 178. The flexural test was performed on an Instron Universal Material Testing System (Type 4302). The speed of the crosshead was 5 mm/min. Test pieces for the flexural test were obtained by injection molding using a Nissei Plastic Industrial Co. PS40E2ASE injection molder. The temperature of the cylinder was 200[degrees]C-220[degrees]C, and that of the mold was 40[degrees]C.

RESULTS AND DISCUSSION

Evaluation of the Dispersibility of the Clay Layers in the Nanocomposites

The XRD patterns of the PPCN-ORG, MA2 are shown in Fig. 1. In the XRD pattern of the PPCN there was no peak and the diffraction strength (signal intensity) gradually decreased from the low angle to the high angle side. This shows that the silicate layers of the clay mineral were exfoliated, and is a characteristic XRD pattern of a clay-exfoliated type of polymer-clay nanocomposite. In addition, the signal intensity of the MA2 XRD pattern decreased rapidly until the angle was 1[degrees], after which it tended asymptotically toward a zero value. This shows there is nothing from which X-rays are reflected.

In order to clarify the effect of OTM, the XRD patterns of PPCN1, PPCN2 and PPCN3 were compared, as shown in Fig. 2. These samples were lacking Ma-g-PP. There was a peak in the XRD pattern of PPCN1, originating from Na-Mt swelled slightly with water. By contrast, there are three peaks in the XRD pattern of PPCN3. These peaks indicate distances of 6.1 nm, 1.9 nm and 1.4 nm between silicate layers, respectively. The peak for the 1.4 nm separation again originates from Na-Mt swelled slightly with water. The two remaining peaks for 6.1 nm and 1.9 nm separation originate from OTM inserted between the layers. Therefore, the layer structure was changed by the addition of OTM. The interlayer distance of the organo-clay, which was Na-Mt treated with OTM (OTM-Mt), was 2.2 nm. The 6.1 nm interlayer distance was larger than that of the OTM-Mt. This was considered to be because the dispersing silicate layers were stabilized by the OTM. On the other hand, PPCN1 was considered to have recondensed with the removal of water since the silicate layers were not stabilized without the OTM. In addition, since only a small amount of OTM was added in the PPCN2, it was considered that the silicate layers were not dispersed.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Next, to demonstrate the effect of Ma-g-PP, the XRD patterns of PPCN4, PPCN5 and PPCN6 were compared, as shown in Fig. 3. These samples included 30 parts of Ma-g-PP and the quantity of OTM was changed from 0, 0.25, and 1 eq respectively. As the amount of added OTM increased, the dispersion of the silicate layers steadily improved. In the case of the PPCN6, it was thought that most of the clay had exfoliated and dispersed, and the dispersion state of the silicate layers in the PPCN6 was the best of all the samples that we produced at this time. From these results, we can say that the effect of the Ma-g-PP is that the silicate layers that are interface-stabilized by the OTM are dispersed more finely.

[FIGURE 3 OMITTED]

To confirm the dispersibility of the silicate layers in PPCN6, the silicate layers were observed using TEM. A TEM micrograph of PPCN6 is shown in Fig. 4, where the dark lines are cross sections of the silicate layers in the clay (1 nm thickness). The silicate layers are dispersed homogeneously in the polypropylene matrix. The dispersion state of the silicate layers in PPCN6 was the same as that of the PPCN-ORG, as shown in Fig. 5. From these results we can say that a polypropylene-clay nanocomposite can be produced by the new production method without pretreating the clay mineral.

The XRD pattern of PPCN7 is shown in Fig. 6. There are two peaks, which indicate distances between the silicate layers of 6.8 nm and 1.4 nm, respectively. The peak for the 1.4 nm distance is due to Na-Mt swelled slightly with water. The other peak for 6.8 nm separation originates from OTM and Ma-g-PP inserted between the layers. From this result, even though the addition of clay in excess of 5 wt% was difficult using the clay slurry method, it is possible to produce a polypropylene-clay nanocomposite with more than 5 wt% clay by the new production method.

[FIGURE 4 OMITTED]

Figure 7 shows schematic figures depicting the dispersion of the silicate layers of the clay in the polypropylene during compounding by the extruder. We consider that the mechanism for the dispersion of the silicate layers into polypropylene is as follows. The clay is kneaded into the PP in the first section of the extruder. In the second section, the injected water is taken in by the hydrophilic clay rather than by the hydrophobic PP. The water swells the clay and a clay slurry is achieved in the extruder. The silicate layers of the clay mineral are dispersed in the polypropylene matrix under a strong shear field. At the same time, a cation exchange reaction occurs between the OTM and the Na cation binding with silicate layers, and the hydrophilic silicate layers are stabilized in the hydrophobic PP. Further-more, the silicate layers that are stabilized by the OTM are dispersed finely by the MA-g-PP. In the third and fourth sections, although the water is evaporated, the silicate layers are fixed by being stabilized and they maintain the state they are in, without any aggregation. The key point about the silicate layer dispersion using this method is that it is quite different from that of the conventional method using the organo-clay. In the conventional method, the polymer chains intercalate into the galleries of the stacked layered silicates and the silicate layers exfoliate into the PP matrix. On the other hand, in this method, the exfoliated silicate layers in water are fixed into the polymer matrix without aggregation of the silicate layers.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Study of the Flexural Properties of the PPCNs

The flexural properties of the PPCNs and related samples are summarized in Table 2. From the results of the XRD and TEM observations, PPCN6 was considered to have the best dispersibility, which was equivalent to that of conventional material (PPCN-ORG). The flexural properties of PPCN6 were compared with the PPCN-ORG. The flexural strength, the flexural elongation at break, and the flexural modulus of the PPCN6 were almost the same as those of the PPCN-ORG. These flexural properties therefore also suggest that a viable polypropylene-clay nanocomposite can be produced by the new production method without pretreating the clay mineral.

With regard to the flexural properties, we then compared the series for which the amount of added OTM was varied, i.e., the series PPCN4, PPCN5 and PPCN6. The flexural strengths and the flexural moduli of these samples are plotted in Fig. 8. The addition of 5 parts of OTM improved the flexural strength and the flexural modulus significantly. However, when the quantity of added OTM was increased to 30 parts, these properties were improved even more dramatically compared with the addition of 5 parts of OTM. These results suggest that PPCN with good flexural properties is obtained, even if we reduce the amount of added OTM and Ma-g-PP. Therefore, these results provide useful data for polypropylene clay nanocomposites from an industrial standpoint.

[FIGURE 8 OMITTED]

CONCLUSION

We have successfully developed a new production method for polypropylene-clay nanocomposites. This method did not require any pretreatment of the clay mineral with an organo-cation. In this method we focused our attention on the nature of the clay mineral, which was exfoliated in water. By controlling the pressure of the water vapor, a clay slurry state was achieved in the twin-screw extruder. Two compatibilizers were added to the mixture of the clay mineral and the polypropylene to prevent aggregation of the clay mineral, and a polypropylene nanocomposite was successfully prepared. In this polypropylene nanocomposite, the silicate layers of the clay mineral were exfoliated and dispersed uniformly in the polypropylene matrix. The polypropylene nanocomposite had almost the same excellent properties as a conventional polypropylene-clay nanocomposite.
Table 1. Compositions of the Polypropylene Nanocomposites Prepared by
the New Production Method and of the Related Samples.

 Composition
 PP (MA2) Ma-g-PP Na-Mt OTM
Sample (part) (part) (part) (eq.)

PPCN1 100 0 5 0
PPCN2 100 0 5 0.25
PPCN3 100 0 5 1
PPCN4 70 30 5 0
PPCN5 70 30 5 0.25
PPCN6 70 30 5 1
PPCN7 70 30 10 1
MA2 100 0 0 0
MA2(70)/Ma-g-PP(30) 70 30 0 0
PPCN-ORG 70 30 7 (Organo-clay)

 Water Inorganic Content
Sample (kg/h) (wt%)

PPCN1 2 4.6
PPCN2 2 4.7
PPCN3 2 5.0
PPCN4 2 4.4
PPCN5 2 4.2
PPCN6 2 4.4
PPCN7 2 9.1
MA2 2 0
MA2(70)/Ma-g-PP(30) 2 0
PPCN-ORG 0 4.2

Table 2. Flexural Properties of the Polypropylene Nanocomposites and
Related Samples.

 Flexural Property
 Flexural Flexural Elongation Flexural Modulus
 Strength at Break
Sample (MPa) std (%) std (GPa) std

PPCN1 50.5 1.2 >11.5 -- 1.94 0.09
PPCN2 56.2 0.3 >11.5 -- 2.27 0.02
PPCN3 47.9 0.4 >11.5 -- 2.18 0.06
PPCN4 53.5 0.4 >11.5 -- 2.12 0.02
PPCN5 60.5 0.5 7.3 0.4 2.45 0.09
PPCN6 60.5 0.8 3.8 0.2 2.76 0.02
PPCN7 59.8 0.9 2.8 0.1 3.54 0.05
MA2 48.6 0.5 >11.5 -- 1.67 0.09
MA2(70)/Ma-g-PP(30) 49.7 0.5 >11.5 -- 1.68 0.06
PPCN-ORG 61.2 1.0 2.9 0.2 2.88 0.11


[c] 2004 Society of Plastics Engineers

Published online in Wiley InterScience (www.interscience.wiley.com).

DOI: 10.1002/pen.20115

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MAKOTO KATO*, MITSUMASA MATSUSHITA, and KENZO FUKUMORI

TOYOTA Central R & D Labs., Inc.

Nagakute, Aichi, 480-1192, Japan

*To whom correspondence should be addressed.

E-mail: makoto@mosk.tytlabs.co.jp
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Author:Kato, Makoto; Matsushita, Mitsumasa; Fukumori, Kenzo
Publication:Polymer Engineering and Science
Date:Jul 1, 2004
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