HIGHLIGHT The Discovery of Polymer-Clay Hybrids MASAYA KAWASUMI Toyota Central Research and Development Laboratories, Incorporated, Ngakute, Aich 4801192, Japan Received 5 August 2003; accepted 21 August 2003 ABSTRACT: The first successful example of a polymer-clay hybrid was nylon-clay hybrid (NCH), which is a nano-meter-sized composite of nylon-6 and 1-nm-thick exfoliated aluminosilicate layers of the clay mineral. NCH was found and developed at Toyota Central Research and Development Laboratories over 17 years ago. The NCH containing a few weight percentages of clay exhibits superior properties such as high modulus, high strength, and good gas-barrier properties. The key for the discovery of NCH was the polymerization of a nylon monomer in the interlayer space of the clay. This highlight presents the development of NCH from its discovery to its commercialization. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 819 824, 2004 Keywords: nylon-clay hybrid; clay mineral; aluminosilicate; nylon-6; -caprolactum; 12-aminolauric acid Masaya Kawasumi is a senior researcher at Toyota Central Research and Development Laboratories, Inc. (TCRDL). He holds B.S. and M.S. degrees from Nagoya University (Japan). He joined TCRDL in 1985 as a polymer chemist. He obtained a Ph.D. in Macromolecular Science from Case Western Reserve University (Cleveland, OH), under the supervision of Professor Virgil Percec. His main research interests are in the development of polymeric materials that include clay hybrid materials, liquidcrystalline polymers, and proton-exchange membranes for fuel cells. M. KAWASUMI Correspondence to: M. Kawasumi (E-mail: masakawa@ katch.ne.jp) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 819 824 (2004) 2004 Wiley Periodicals, Inc. 819
820 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) Figure 1. Structure of montmorillonite. INTRODUCTION The origin of polymer-clay hybrids starts with the creation of nylon-6-clay hybrid (NCH) developed in 1986 under Toyota Central Research and Development Laboratories, Inc. s (TCRDL) exploratory project. The creation of NCH was accomplished by one man s inspiration and several important findings. Of course, the excellent project management and the efforts of many other researchers led to its complete success. In the development stage of NCH, significant technological contributions were also made by Toyota Motor Corp. (TMC) and Ube Industries, Ltd. (Ube). I mainly describe the highlights of the discovery of NCH from the early stage of the project. Since I joined the project after it had already started, I experienced only a part of NCH developmental history. Therefore, the stories described here are not only what I experienced but also the experiences of other coworkers. THE CREATION You may wonder what the starting point of NCH was and why the strange combination of nylon and clay mineral arises. The answer is in the inspiration of Dr. Kamigaito, who was the director of our division at TCRDL. One day, he broke an old fossil to pieces and believed that it smelled like the beach. He thought that the smell came from organic compounds such as amino acids derived from extinct animals. He was surprised at the stability of fragile organic compounds in the mineral. From this finding, he was inspired by what will happen if amino acids are intercalated into clay minerals. This was the starting point of NCHs. Let me explain the structures of clay minerals. As shown in Figure 1, montmorillonite, which is one type of layered clay mineral, consists of aluminosilicate layers, about 0.2 m in length and 1 nm in thickness. It has exchangeable cations, such as the sodium cation, between its layers. Montmorillonites, exchanged with alkyl ammoniums, intercalated various kinds of organic compounds such as toluene between the layers. Dr. Kurauchi, who was the manager of the polymer materials laboratory, proposed and started a new project for the development of NCH. The image of the target material, NCH, was already clear at this stage. The NCH should be a nanometer composite composed of nylon-6, which is a polymer of amino acids, and a layered clay mineral. Typically, in NCH 1-nm-thick aluminosilicate layers of the clay mineral should be fully exfoliated and dispersed in a nylon-6 matrix as shown in Figure 2. However, in the beginning no one knew how to produce it and what kinds of properties NCH would exhibit. The first trial was made by a simple melt mixing of nylon-6 with a clay mineral intercalated with alkyl ammonium. The alkyl ammonium was used to add hydrophobicity to the clay. However, the clay did not disperse at all, only producing an inhomogeneous composite material in which we could see large particles of the clay in the polymer. Then, Dr. Usuki, an organic chemist, started intercalation experiments of montmorillonite with -caprolactum as a nylon monomer. 1 The first significant finding in the creation of NCH was this experiment. He measured the interlayer distance of the montmorillonite interca-
HIGHLIGHT 821 Figure 2. Conceptual figure of NCH synthesis. lated with amino acids with various alkylene chain lengths by X-ray diffraction (XRD), both in the absence of and in the presence of -caprolactum. In the case of the montmorillonite intercalated with short amino acids less than (CH 2 ) 8, the interlayer distances were almost identical regardless of the presence of -caprolactum. However, in the case of the montmorillonite intercalated with 12-aminolauric acid or longer, the interlayer distance increased significantly by the presence of -caprolactum. This meant that the nylon monomer was spontaneously intercalated into the interlayer of montmorillonites. I joined the NCH project in the polymer material laboratory after finishing my first-year education from our company. The supervisor was Dr. Okada. The given theme was the synthesis of processable NCH by injection molding. At this time, not only the structural concept but also the synthetic process designed for NCHs were already proposed, namely, the in situ polymerization of nylon monomer intercalated into montmorillonite to produce the NCH s structure, as depicted in Figure 2. First, Dr. Okada thought that the key point was the polymerization method. Because several synthetic routes of nylon-6 are known, that is, anionic and cationic ringopening polymerizations of -caprolactum and condensation polymerization of, -aminocaproic acid, we first chose an anionic polymerization method. We anionically polymerized -caprolactum in the presence of montmorillonite intercalated with 12-aminolauric acid. Although the polymerization proceeded rapidly, the interlayer distance of montmorillonite did not increase. After several months of experimental failure, we decided to thermally polymerize it by just heating the mixture. 2 This idea came from the basic experiments by Dr. Fukushima and Dr. Inagaki, who were in the inorganic materials group. They were investigating various kinds of intercalated montmorillonites as a catalyst for the polymerization of -caprolactum. They reported that some of them act as catalysts for its polymerization. This was the second key point leading to the creation of NCH. We tried to polymerize the mixture of -caprolactum and montmorillonite intercalated with 12-aminolauric acid at various contents from 2 to 70 wt % to produce a variety of composites. By measuring interlayer distances of the composites by XRD, we discovered that the interlayer distance of the clay in the composites increased with a decrease in the clay content. However, I was not sure if the exfoliated dispersion of montmorillonite in the nylon matrix had truly been achieved. Therefore, I asked Mr. Noritake, who was the transmission electron microscopist, to observe the composites by transmission electron microscopy (TEM). After several days, he finished the TEM analysis and said to me, I observed nothing but dark lines. Figure 3 shows the micrograph. I said to him, Dark lines? I thought that we should have observed at least platelike structures from the aluminosilicate layers dispersed in the matrix. I thought I had failed to produce the exfoliated composites. However, Mr. Noritake said to Figure 3. Transmission electron micrograph of NCH (clay content: 5 wt %).
822 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) me, This should be what you want. The dark lines are intersects of the aluminosilicate layers. To confirm his comments, I measured the interlayer distances in the micrographs. I observed that the distances were almost identical to those measured by XRD. We did it! I showed the micrographs of the first NCHs to Dr. Kurauchi and Dr. Okada. They were surprised to see them. I remember the words of Dr. Kurauchi: How can aluminosilicates exist like this in a nylon matrix? This was the moment when we confirmed the creation of NCH. It was very interesting that the heat-induced polymerization led to NCH, but the anionic polymerization did not. We speculated that under the heat polymerization conditions, the ammonium cations in the interlayer acted as an cationic catalyst for the ring-opening polymerization of -caprolactum. Because the polymerization occurred in the interlayer space, the nylon-6 was produced in the interlayer, and this resulted in the increase of the interlayer distance. However, the anionic polymerization only occurred outside of the interlayer spaces and resulted in no increase of the interlayer distance. Then, we decided to produce NCH on a larger scale to make samples by injection molding. First, we had to synthesize several kilograms of NCH for injection molding. However, we needed to decide one thing. What should be the range of montmorillonite content? It is typical to use 10 40 wt % of glass fibers or inorganic fillers to reinforce nylons. However, from the previous experiments, we knew if the clay content was too high, the hybrids were generally brittle powders and could not be injection-molded. We decided to produce NCHs starting at a rather small content range of the clay, from 2 to 8 wt %. This decision was the third step in the success of NCH creation as an industrial material. To extract the outstanding feature of polymer-clay hybrids, a small percentage of clay content is sufficient. In other words, one of the surprising features is that a big change in physical properties can be realized by a small amount of clay. If we had produced NCH with 10 wt % or more of the clay, we might have concluded that these were nothing but useless materials because of their brittleness. The first injection-molded NCH was just handmade. I synthesized NCHs in glass-separable flasks followed by grinding with a mill and washing with hot water. It took more than 1 month for me to obtain enough of the NCHs. I injection-molded the first NCHs into dumbbell specimens with Dr. Kurauchi and Dr. Okada because their melt viscosities were low enough to be injection-molded. We made specimens one by one and noticed two things during molding. The first molded samples were really rigid. We could feel them by bending them with our hands. The samples looked like just another resin from nylon. The second point we noticed was that they were Figure 4. nylon-6. Comparison of transparency between NCH and rather transparent as shown in Figure 4. That was due to the suppression of spherulite growth. We measured various physical properties of the NCH specimens and compared them with nylon-6 specimens. 3 Soon, we found many outstanding properties of these new materials. They exhibited superior heat-distortion temperatures and higher moduli than nylon-6 in the stretching experiments. The dynamic storage moduli of the NCHs exceeded those of nylon-6 especially above its glass-transition temperature. Even the NCHs with only 5% of the clay exhibited moduli three times higher at 120 C as compared with nylon-6. 3 Other properties of NCHs such as the water-absorption rate, thermal-expansion coefficients, and gas-barrier properties 4 were also different from those of nylon-6. The characterizations revealed interesting structural aspects of the NCHs. and forms of crystals exist in nylon-6. Nylon-6, without the clay, exhibited mainly the form, whereas the NCHs were mainly of the form. A more detailed characterization demonstrated that the plane including hydrogen bonding in the form of nylon-6 crystals was parallel to the layer of aluminosilicates in NCH. 5 This result apparently indicated that the dispersed aluminosilicates layers controlled the crystal structure of nylon-6. From the quantitative analyses of the terminal groups in the NCHs by titration, it was speculated that the amino terminal groups of nylon-6 polymers were ammonium, which were ionically bonded to the aluminosilicate layers. The origin of superior properties of the NCHs was thought to originate not only from their molecular level dispersion of 1-nm-thick aluminosilicate layers in a nylon matrix but also from the strong ionic interactions between nylon and the layers. This was demonstrated in other experiments. The NCHs exhibited different mechanical properties with various types of clay minerals
HIGHLIGHT 823 such as synthetic mica, saponite, and hectorite. The strength of the ionic interactions between clay and nylon, which were estimated by solid-state 15 N NMR with model compounds, were consistent with the hybrid mechanical properties. 6 The polymerization methods were improved to enhance the polymerization rate by adding 10 wt % of, -aminocaproic acid. Also, sodium montmorillonite could be used in the syntheses of NCHs by just adding a small amount of an acid. 7 With these advances, we were able to produce the same quality of NCH in 6 h. Previously, the time used to be 48 h. New members, Dr. Kojima and Ms. Sasaki, joined our project, and they made many advances in the NCH developments not only in the syntheses but also in the characterizations. DEVELOPMENT Figure 5. Timing belt cover injection-molded with NCH. For the next step of the project, TMC suggested that we collaborate with Ube, which was one of Japan s nylon-6 producers. We proposed the NCH development project to Ube, and they accepted the offer without hesitation. We disclosed to them our synthetic procedures and the materials properties. They traced our experiments and reproduced in just one month what took us more than one year to complete. I was amazed at the speed of their experiments. During the development of the NCH as a commercial material, many problems arose. We were able to solve each problem through the excellent collaborations between TMC and Ube. I mention two examples. The first example is the improvement of the brittleness in NCHs. I found a clue to solve this problem through other unrelated experiments. The purpose was to make NCHs stronger by ionically crosslinking it through the interaction between nylon molecules and clays. To achieve this structure, both terminal groups of nylon-6 should only be ammonium ions. I thought this could be done by adding diamine to the carboxylic acid terminal group to form an amine terminal group. As I performed the polymerization with a small amount of diamine, I was not able to obtain strong NCHs but tough NCHs. 8 The improvement of toughness was rather drastic. From the analyses of the diamine-modified NCHs, we found that the diamines preferentially intercalated into the aluminosilicate layers and hindered the exfoliation. By this mechanism, in the diamine-modified NCHs, several stacked aluminosilicates were dispersed into the nylon matrix. This stacking level could be controlled by the amount of diamine leading to an improvement and control of their toughness. This mechanism was the reason why we could hardly obtain the hybrid structure for nylon-66. In nylon-66, diamines were used as comonomers and preferentially intercalated and hindered exfoliation. The second example was similar to the first example. Nylon is normally used with additives such as antioxidants. When we used antioxidants composed of copper during injection molding of NCHs, the obtained NCHs always exhibited very poor mechanical properties. TEM and XRD analyses demonstrated that the clay layers were not at all dispersed. Of course, they perfectly dispersed before the injection molding. The only reason for this phenomenon was that copper induced the aggregation of the dispersed clay during the injection molding. This was rather surprising that such huge aluminosilicate layers could move so quickly in a viscous nylon matrix and stack into layers within a few minutes. We were luckily able to avoid this problem by changing the type of antioxidant, which was found by Ube. The synthetic method was eventually changed from the polymerization method to the melt-blending method with special techniques, which led to a jump in its productivity that was initially accomplished by Ube. In 1990, the first commercialized product of NCHs was in an automotive timing belt cover (Fig. 5) used in automobile engines. 9 With the NCH cover, 25% weight reduction was achieved as compared with that of glass fiber-reinforced nylon. These hybrid techniques were successfully applied to various polymer systems including polyimide 10 and polypropylenes, 11 although each polymer system needed a different method to achieve a hybrid structure.
824 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 42 (2004) PUBLICATION Dr. Okada first presented NCHs at the American Chemical Society Meeting in 1987. 12 After our international introduction of NCHs, much research related to polymerclay hybrids had begun throughout the world. Chemical Abstracts listed 238 journal articles related to polymerclay hybrids up to now. Polymer-clay hybrids have quickly led to the development of a new field of research known as the nanocomposites materials. The number of related articles continues to increase even after 2000. TCRDL is sometimes called the birth place of polymerclay hybrids and has been one of the leading research institutes to date. New challenges are continuing to establish the clay hybrid technology in a large variety of polymer systems. REFERENCES AND NOTES 1. Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamigaito, O. J Mater Res 1993, 8, 1174. 2. Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J Mater Res 1993, 8, 1179. 3. Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J Mater Res 1993, 8, 1185. 4. Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. Mater Life 1993, 5, 13. 5. Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O.; Kaji, K. J Polym Sci Part B: Polym Phys 1994, 32, 625. 6. Usuki, A.; Koiwai, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J Appl Polym Sci 1995, 55, 119. 7. Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J Polym Sci Part A: Polym Chem 1993, 31, 1755. 8. Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T. Kobunshi Ronbunshu 1995, 52, 440. 9. Kurauchi, T.; Okada, A.; Nomura, T.; Nishio, T.; Saegusa, S.; Deguchi, R. SAE Technical Paper; 1991; Series No. 910584. 10. Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J Polym Sci Part A: Polym Chem 1993, 31, 2493. 11. Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Macromolecules 1997, 30, 6333. 12. Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito, O. Polym Prepr (Am Chem Soc Div Polym Chem) 1987, 28(2), 447.