Structure and Properties of BR Nanocomposites Reinforced with Organoclay

Transcription

1 Structure and Properties of BR Nanocomposites Reinforced with Organoclay Structure and Properties of BR Nanocomposites Reinforced with Organoclay Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang * Research Institute of Polymer Materials, Shanghai Jiao Tong University, Shanghai , People s Republic of China Received: 17 May 2004 Accepted: 27 September 2004 SUMMARY Butadiene rubber (BR)/organoclay nanocomposites were prepared by direct melt mixing of BR and clay modified with different primary and quaternary ammonium salts. BR/pristine clay composite and BR/organoclay nanocomposites were analysed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. The vulcanization characteristics and the mechanical properties of the BR/ pristine clay and BR/organoclay composites were investigated. The results showed that the interlayer distance of the organoclays was expanded, which indicated that intercalated BR/organoclay nanocomposites had been prepared. Organoclay effectively accelerated the vulcanization of BR, which was attributed to the intercalatant used to modify the clay. The tensile strength, elongation at break and tear strength of BR/organoclay nanocomposites are much higher than those of gum BR vulcanizate and BR/pristine clay composites. The organoclay modified with dimethyl dihydrogenated tallow ammonium chloride (DDAC) gave the best reinforcement effect in BR of all the organoclays. INTRODUCTION In recent years, layered silicates have generally been used to prepare polymer nanocomposites. Layered silicates such as montmorillonite clay and talc possess a unique 2:1 crystal structure 1. They have less compatibility with organic polymers because of their hydrophilicity. In order to render these hydrophilic silicates more organophilic, the hydrated cations of the interlayer can be replaced by cationic surfactants such as alkylammonium or alkylphosphonium. The modified silicates being organophilic, their interlayer distance is expanded and their surface energy is lowered. So modified silicates are more compatible with organic polymers 1. Up to now, only a few researches have been focused on rubber/clay nanocomposites. The research of Okada et al. 2 shows that the degree of reinforcement imparted by montmorillonite is about four times as large as that offered *Corresponding author. Tel.: ; fax: address: yxzhang@sjtu.edu.cn by carbon black in acrylonitrile-butadiene rubber (NBR). Nah et al. 3 found that the barrier performance of intercalated NBR/montmorillonite nanocomposite can be improved markedly by application of a small amount of montmorillonite modified with dimethyl distearyl ammonium bromide. Zheng et al. 4 also prepared exfoliated EPDM/ clay nanocomposite using clay modified with methyl bis(2-hydroxyethyl) cocoalkylamine. The tensile strength and elongation at break of the nanocomposites achieved 25 MPa and 640%, respectively, after adding 15 phr organoclay. Natural rubber/montmorillonite nanocomposites were prepared using montmorillonite modified with primary and quaternary ammonium salts via solution intercalation by Magaraphan et al. 5. They found that long primary amines gave better mechanical properties than the quaternary ones with the same carbon number. The longer the molecules of the organic modifying agents, the larger the interlayer distance of the montmorillonite becomes. This in turn means that more natural rubber molecules can intercalate into the galleries of the montmorillonite. Polymers & Polymer Composites, Vol. 13, No. 4,

2 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang Butadiene rubber (BR) is an important general purpose synthetic rubber, widely used in the tyre industry. Because the mechanical properties of BR are very poor, it has to be reinforced by fillers such as carbon black to acquire acceptable mechanical properties. Liao et al. 6, 7 prepared BR/montmorillonite nanocomposites by in situ polymerisation and found that the amount of 1,2-units in the BR increased with increasing montmorillonite content. Ganter 8 and Wang 9 et al. prepared BR/clay nanocomposites by solution intercalation. The tensile strength, elongation at break and tear strength of the nanocomposites were greatly improved compared with the gum BR vulcanizate. To our knowledge, there has been no report on the preparation of BR/clay nanocomposites by melt intercalation. In this work, pristine clay and organoclays modified with different primary and quaternary ammonium salts were used to prepare BR/pristine clay composites and BR/organoclay nanocomposites by melt compounding. The morphology of BR/ pristine clay composites and BR/organoclay nanocomposites were investigated by means of X- ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) was used to investigate the thermal degradation of BR vulcanizates filled with pristine clay and organoclay. The vulcanization characteristics and mechanical properties of the BR/pristine clay composite and BR/ organoclay nanocomposites were investigated. Materials EXPERIMENTAL Butadiene rubber (BR9000) was produced by Yanshan Petrochemical Co., Ltd., China; pristine clay (Na-MMT) and organoclays (DK1, DK2, DK3, DK4 and DK1N) were provided by Zhejiang Fenghong Clay Chemicals Co., Ltd., China. The intercalatants (modifiers) were trimethyl hydrogenated tallow ammonium chloride, methyl bis(2-hydroxyethyl) hydrogenated tallow ammonium chloride, dimethyl benzyl hydrogenated tallow ammonium chloride, dimethyl dihydrogenated tallow ammonium chloride and octadecylamine, respectively. All the other additives were industrial grade products. Compounding and Sample Preparation BR, pristine clay (30 phr) or organoclays (30 phr) were melt-mixed in a Haake rheometer Rheocord90 (Haake Co., Germany) at 90 C with a rotor speed of 90 rpm for 6 min. Then 2 phr stearic acid, 4 phr zinc oxide, 2.5 phr accelerator CZ (N-cyclohexylbenzothiazole-2-sulfenamide) and 1.5 phr sulfur were added to the above mixture at 30 C and 60 rpm in the Haake rheometer, and mixed for 5 min. The resulting mixtures were mixed further on a two-roll mill at ambient temperature for about 10 min. Finally the compounds were compressionmoulded at 150 C under 10 MPa for the optimum cure time (t 90 ) to yield vulcanizates. Measurement and Characterization Wide-angle X-ray diffraction (XRD) was used to study the expansion of the clay interlayer distance. The XRD patterns were obtained using a diffractometer (Dmax-rc, Japan) at the wavelength CuKα =1.541 nm with a generator voltage of 40KV and a generator current of 100mA. The diffractogram was scanned in the 2θ range from 1 to 20 at a rate of 4 /min.the fracture morphologies of tensile and tear samples were observed by scanning electron microscopy (SEM) (Model S-2150, HITACHI) after the fractural surfaces were sputter-coated with gold. Transmission electron microscopy (TEM) was performed on the ultra-thin films prepared by cryoultramicrotomy using a H-800 (HITACHI, Japan) instrument at an acceleration voltage of 200 kv. Thermogravimetric analysis (TGA) was carried out in a Perkin-Elmer TG analyser (Model TGA7) over a temperature range from room temperature to 800 C in a nitrogen flow at a heating rate of 20 C/min. The vulcanization characteristics were determined by an oscillating disk rheometer (UN 2030 China). The tensile properties were measured with dumbbell specimens (6 mm wide in the cross section) according to ASTM D a. The tear strength was tested according to ASTM D with an unnotched 90 angle test piece. Both tensile and tear tests were performed on an Instron series IX 4465 material tester (Instron Co., USA) at a crosshead speed of 500 mm/min. RESULTS AND DISCUSSION The Vulcanization Characteristics The effects of pristine clay (30 phr) and organoclay (30 phr) on the vulcanization characteristics of the BR composites are shown in Table 1. The scorch time 372 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

3 Structure and Properties of BR Nanocomposites Reinforced with Organoclay (T 2 ) of the BR composites decreased a great deal on adding pristine clay. However, pristine clay had little influence on the optimum vulcanization time (T 90 ). The maximum torque (MH) obviously decreased in the presence of pristine clay, and the scorch time and optimum vulcanization time were sharply reduced. The facts indicate that organoclays are effective accelerators for BR vulcanization reaction. Other researchers have found the same phenomenon in organoclay (modified with octadecyltrimethylamine and octadecylamine respectively) filled SBR 10 and NR 5,11,12. An interesting phenomenon can be observed if one compares the difference in scorch time and optimum vulcanization time between the gum BR and BR/organoclay compounds. That is, the vulcanization acceleration effect of organoclay on BR compounds should be attributed mainly to the greatly enhanced reaction rate in the induction period. It indicates that organoclay, or some functional group in the organoclay, may participate in the reaction during the induction period. The fact that the organoclay obviously increases the reaction rate in the induction period can be explained by the following facts. The following key accelerator species were formed during the induction period with the sulfur/sulfenamide 13, 14 vulcanization system This activated intermediate can react with amines and form the following chelate: In order to investigate the reason for the acceleration effect of organoclay on BR vulcanization, a further experiment was performed. The formulations and experimental results are shown in Table 1. Dimethyl dihydrogenated tallow ammonium chloride (DDAC) greatly accelerated the vulcanization of BR. An obvious vulcanization acceleration phenomenon can also be observed by directly adding pristine clay and DDAC to the BR compound. We can deduce that the acceleration effect of organoclay is attributable mainly to the amine functional groups in the organoclay. Moreover, the acceleration effect is enhanced by the intercalation of DDAC into the clay galleries. where the ligand denotes an amine. This chelate is more active than the sulfenamide accelerator 14. The scorch time and optimum vulcanization time of BR compounds filled with organoclay modified with a primary ammonium salt (DK1N) are longer than with other organoclays modified with different quaternary ammonium salts. This can be attributed to the stronger basicity of the amine functional group in quaternary ammonium salts than the primary ammonium salt. Generally the stronger the basicity Table 1. Effect of pristine clay, organoclays and DDAC on the vulcanization characteristics of BR compounds (150 C) Filler Content (phr) Scorch time (min.) T 2 Vulcanization time (min.) T 10 T 90 ML (dn m) MH (dn m) ΔM (dn m) None Pristine clay DK DK DK DK DK1N DDAC * Pristine clay/ddac 18/ * The DDAC content was determined by the practical content of DDAC in 30 phr DK4 analyzed via TGA Polymers & Polymer Composites, Vol. 13, No. 4,

4 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang of the amine functional group, the shorter the scorch time and optimum vulcanization time 15 are. Mechanical Properties The effects of pristine clay and organoclays (30 phr) on the mechanical properties of BR vulcanizates are shown in Figures 1, 2 and 3. The pristine clay has little reinforcement effect on BR, but the tensile strength and elongation at break are greatly improved when organoclays are incorporated in BR. Organoclay modified with DDAC (DK4) is the most effective in reinforcing BR. The tensile strength and elongation at break of the BR/DK4 vulcanizates were MPa and 698%, respectively. That is, the tensile strength and elongation at break of the BR/DK4 vulcanizate increased by 1400% and 750% in relation to those of gum BR vulcanizate, respectively. The order of reinforcement ability of pristine clay and different organoclays declines as follows: DK4> DK2> DK3> DK1N> DK1> Clay Figure 1. Effect of pristine clay and organoclays on tensile strength and elongation at break of BR vulcanizate Figure 2. Effect of pristine clay and organoclays on modulus at 100% and 300% of BR vulcanizate 374 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

5 Structure and Properties of BR Nanocomposites Reinforced with Organoclay Figure 3. Effect of pristine clay and organoclays on tear strength of BR vulcanizate The reason for the optimal mechanical properties for BR/DK4 vulcanizate among all the BR/organoclay vulcanizates may be that DK4 is modified with DDAC, which has two long alkyl chains. This renders the clay more compatible with nonpolar BR than other organoclays modified by intercalatants with only one long alkyl chain. Obviously, intercalation behaviour cannot completely explain the dependence of tensile properties of BR/organoclay nanocomposites on different intercalatants. As seen in Table 1, the obvious decrease in ΔM in relation to gum BR indicates that the degree of vulcanization of BR vulcanizates was obviously influenced by the different organoclays. It is well known that the crosslinking density has a profound influence on the tensile properties of rubber. So the degree of vulcanization is also an important factor influencing the tensile properties of BR/organoclay nanocomposites, besides the intercalation behaviour. The mechanism of the reinforcing effect of different intercalatants needs to be investigated further because of the complexity of the rubber/ingredients system. Organoclays obviously increase the tear strength of BR vulcanizates, although pristine clay has little effect on the tear strength of BR vulcanizates. The tear strength of BR/DK4 vulcanizates was the highest of all the BR/organoclay vulcanizates. It increased by 870% and 400% in relation to gum BR and BR/ pristine clay vulcanizates, respectively. Figures 4 ~ 6 show the dependence of the mechanical properties of BR vulcanizates on the DK4 content. First the tensile strength and elongation at break of BR vulcanizate increased slowly with increasing DK4 content. Then a sharp increase in tensile strength and elongation at break appeared when the DK4 content exceeded 10 phr. The tensile strength decreased with increasing DK4 content after achieving a maximum value at 30phr. The trend of elongation at break to increase became slow. In addition, the modulus at 100% and 300% of BR vulcanizates increased with increasing DK4 content. The highest 100% and 300% moduli were both achieved at 40 phr DK4. Another merit of BR/DK4 nanocomposite is the great improvement in the tear strength. Only a small amount of DK4 is enough to increase the tear strength of BR vulcanizates considerably. Moreover the tear strength increased further with increasing DK4 content. This trend became less evident when the DK4 content was high. Characterization TGA The TGA thermograms of pristine clay and organoclays are shown in Figure 7. From room temperature to 150 C the weight loss of pristine clay was about 6.73% that was associated with the removal of water molecules from the clay surface and interlayer. The weight loss in the temperature range Polymers & Polymer Composites, Vol. 13, No. 4,

6 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang Figure 4. Effect of organoclay content on the tensile strength and elongation at break of BR vulcanizates Figure 5. Effect of organoclay content on the modulus at 100% and modulus at 300% of BR Figure 6. Effect of organoclay content on the tear strength of BR vulcanizates 376 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

7 Structure and Properties of BR Nanocomposites Reinforced with Organoclay Figure 7. TGA thermograms of pristine clay and organoclays 150 C ~600 C for pristine clay was about 3.32%. This loss can be attributed to the decomposition of hydrogen-bonded water molecules and some of the OH groups from tetrahedral sheets 16. The weight loss of 5.27% in the range of 600 C ~800 C is attributed to the dehydroxylation of clay 16. The weight losses of different organoclays can be divided into four stages. Between room temperature and 150 C, The weight losses for all organoclays are 0~3% and this is attributed to the weight loss of the surface and interlayer-adsorbed water. In the 150~600 C range, the weight loss of different organoclays happens in two stages. The first stage originates mainly from the decomposition of intercalatant molecules in the extremes of the galleries and the second from the decomposition of those in the interior of the galleries 11. The weight loss for all organoclays in these two stages was from 20% to 45%. The last stage, from 600 to 800 C, can also be associated with the dehydroxylation of clay. The water and intercalatant content of pristine clay and the different organoclays are listed in Table 2. It is obvious that the water content of the organoclays decreases sharply in relation to the pristine clay. This indicates that the organophilic ability of clay can be improved by organic modification. The thermal decomposition behaviour of BR, BR/ pristine clay and BR/ DK4 vulcanizates was assessed by TGA and the results are shown in Figure 8. A slight increase in thermal decomposition temperature was observed for BR/ DK4 and BR/pristine clay vulcanizates compared to the gum BR vulcanizates. Table 2. Water and intercalatant content in pristine clay and organoclays Water content (%) (room temperature ~150 C) Intercalatant content (%) (150 C~600 C) Pristine clay Organoclay DK Organoclay DK1 N Organoclay DK Organoclay DK Organoclay DK The derivative thermogravimetric (DTG) curves of gum BR, BR/pristine clay and BR/DK4 vulcanizates (Figure 9) can provide more information about the effects of pristine clay and organoclays on the thermal decomposition behaviour of BR. The temperature of the degradation peak in the DTG curve is the temperature at which the weight loss rate was the fastest 17. It increased by 5 C when pristine clay was added to BR. And for organoclay DK4 this temperature increased by 16 C. This result was the same as that for the exfoliated BR/organoclay nanocomposite prepared by in situ intercalation polymerization 7. It is concluded that organoclay DK4 is much more effective in improving the thermal decomposition temperature of BR if one considers that the actual Polymers & Polymer Composites, Vol. 13, No. 4,

8 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang Figure 8. TGA thermograms of gum BR, BR/pristine clay and BR/organoclay vulcanizates Figure 9. DTG curves of gum BR, BR/pristine clay and BR/DK4 vulcanizates silicate content of DK4 was only about half that of pristine clay. This can be explained by the better dispersion of the organoclay in the rubber matrix. This can be proved by the morphology observations made using SEM, discussed in the following section. This better dispersion enhances the effective volume fraction of clay in the rubber matrix. The good thermal stability of BR/organoclay DK4 nanocomposites is attributed to hindered loss by diffusion of the volatile decomposition products 1. XRD The XRD patterns of pristine clay and organoclays are shown in Figure 10(a). The pristine clay showed a single (001) diffraction peak at 2θ=5.84, corresponding to an interlayer distance of 1.51 nm. The (001) diffraction peaks of organoclays obviously shifted towards the low-angle direction that indicates effective expansion of the interlayer distance in the clay (Table 3). This expansion of gallery height is 378 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

9 Structure and Properties of BR Nanocomposites Reinforced with Organoclay Figure 10. XRD spectra for (a) pristine clay and organoclays; (b) BR/pristine clay and BR/organoclay vulcanizates attributed to the intercalation of intercalatant chains by a cation exchange reaction. On the one hand, this intercalation of alkylammonium increases the hydrophobicity of the clay and the affinity of the clay for the BR matrix. On the other hand, it expands the interlayer distance and decreases the coulomb interaction and Van der Waals interaction between the clay layers. These effects facilitate the intercalation of polymer chains into the clay gallery. Figure 10(b) shows the XRD patterns of BR/pristine clay and BR/organoclay vulcanizates. The (001) diffraction peak of clay in the BR/ pristine clay vulcanizate is located at 2θ=6.02, corresponding to an interlayer distance of 1.47 nm, which is the same as pristine clay. This indicates that BR chains cannot intercalate into the galleries of pristine clay. The (001) diffraction peaks of BR/organoclay vulcanizates obviously shift towards low-angle direction in relation to the corresponding organoclay. This implies expansion of interlayer distance of clay (Table 3). The expansion of the interlayer distance is associated with the intercalation of BR molecules into the galleries of the organoclay. So intercalated BR/organoclay nanocomposites were prepared by melt intercalation. The intensity of the (001) diffraction peak of BR/organoclay DK2 vulcanizate decreased though an unexpected reduction in interlayer distance of clay in relation to organoclay DK2. Polymers & Polymer Composites, Vol. 13, No. 4,

10 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang Table 3. Interlayer distance of pristine clay, organoclays, BR/pristine clay and BR/organoclay vulcanizates Clay d 001 (nm) Vulcanizate d 001 (nm) Pristine clay 1.51 BR/pristine clay 1.47 DK BR/DK DK BR/DK DK BR/DK DK BR/DK DK1N 2.05 BR/DK1N 4.20 In addition, the (002) and (003) diffraction peaks appeared in the XRD patterns of organoclays and BR/organoclay vulcanizates. Joly et al. 18 observed the same phenomenon when they investigated the NR/montmorillonite (modified with dimethyl hydrogenated tallow (2-ethylhexyl) ammonium) nanocomposite. They explained that these harmonics are a strong indication of a very homogeneous swelling of the organo-modified montmorillonite, without exfoliation of the lamellae. The mechanical properties of the BR vulcanizate are greatly enhanced when BR molecules intercalate into the gallery of clay. However, only a slight improvement in the mechanical properties of BR/pristine clay vulcanizate in relation to gum BR vulcanizate was observed. The expansion of the interlayer distance of clay is propitious to improve the tensile strength, even though no strong linear relationship exists between the tensile strength of BR vulcanizate and the interlayer distance of organoclay (Figure 11). No evident relationship is observed between the tensile strength of BR/clay vulcanizate and interlayer distance of clay in BR/ clay vulcanizates (Figure 12). Fracture Morphologies of Tensile Samples The tensile fracture morphologies of gum BR, BR/ pristine clay and BR/DK4 vulcanizates are shown in Figure 13. In order to compare the size of clay particles before and after mixing with BR, the SEM photographs of pristine clay and organoclay DK4 are shown in Figure 13(a) and (b), respectively. The size of the pristine clay particles ranged from about 10µm to 100µm. This indicates that the clay particles had agglomerated 19. The size of the organoclay DK4 was a little smaller than that of pristine clay. The particle size distribution was more uniform than that of pristine clay. But the organoclay DK4 particles also belonged to agglomerates. The fracture surface of the gum BR vulcanizates (Figure 13(c)) was relatively smooth. The rupture is believed to have occurred because of brittle fracture arising from the low tensile strength and elongation at break. There were many larger clay particles on the fracture surface of the BR/pristine clay vulcanizate (Figure 13(d)). The size of the clay Figure 11. The dependence of tensile strength of BR/clay vulcanizates on interlayer distance of organoclay 380 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

11 Structure and Properties of BR Nanocomposites Reinforced with Organoclay Figure 12. The dependence of tensile strength of BR/clay vulcanizates on interlayer distance of clay in BR/clay vulcanizates Figure 13. SEM photographs of (a) pristine clay; (b) organoclay DK4 and tensile fracture surface of (c) gum BR vulcanizate; (d) BR/pristine clay (100/30) vulcanizate; (e) BR/DK4 (100/30) vulcanizate (a) (b) (c) (d) (e) Polymers & Polymer Composites, Vol. 13, No. 4,

12 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang particles was in the range between clay agglomerates and primary particles (whose size ranges from 1 to 10 µm 19 ) even though it was smaller than that of pristine clay itself because of the shearing effect during the mixing process. The interface between the clay particles and the rubber matrix was very clear. Moreover, cavities (attributed to the falling off of the clay particles from the rubber matrix under stress) were observed. These phenomena indicate that the compatibility and the interaction between clay phase and rubber phase are weak. So BR vulcanizate is not effectively reinforced by pristine clay. The fracture morphology of BR/organoclay nanocomposites (Figure 13(e)) is quite different from that of the above composites. Many parallel strips were distributed on the fracture surface. In addition, the size of the organoclay particles was much smaller than that of the clay particles in the BR/pristine clay composite. Moreover, the interface between organoclay and rubber was very blurry, indicating a stronger interface interaction. The high reinforcement capacity of organoclay is related to this morphology. Fracture Morphologies of Tear Samples SEM photographs of tear fracture surface of gum BR, BR/pristine clay and BR/DK4 vulcanizates are shown in Figure 14. The fracture morphology of the gum BR vulcanizates was typical of the tear morphology of rubbery materials, the so called cross-hatched pattern, which is composed of numerous webs and steps of different sizes 20. Many layered clay particles appeared on the fracture surfaces of BR/pristine clay vulcanizates. The size of the clay particles ranged from a few to a few tens of micrometres that is as large as those of particles on the tensile fracture surface. Moreover, the interface between the clay particles and the rubber matrix was also very clear. Stress concentrations formed at the weak interfaces between the clay and the rubber matrix when the stress was applied to it. Then the interfacial fracture occurred and the clay particles fell off the rubber matrix. This explains the low tear strength. Many strips parallel to the direction of tearing appeared on the fracture surface of the BR/DK4 vulcanizates; besides, no obvious clay particles were distributed on the fracture surface. Nah et al. 21 believe that the formation of this regularly spaced tear pattern is Figure 14. SEM photographs of tear fracture surface of (a) gum BR vulcanizate; (b) BR/pristine clay (100/30) vulcanizate; (c) BR/DK4 (100/30) vulcanizate (a) (b) (c) 382 Polymers & Polymer Composites, Vol. 13, No. 4, 2005

13 Structure and Properties of BR Nanocomposites Reinforced with Organoclay due to the evolution of cracks along the edges of the aggregates. Morphologies Observation from the TEM Figure 15(a, b) shows TEM photographs of a BR/pristine clay composite and a BR/DK4 nanocomposite, respectively. In Figure 15(a), the size of most of the clay particles exceeds 500 nm. The aspect ratio of clay particles is small, which means that these clay particles are formed by aggregation. Furthermore, many larger light areas are observed in these photographs, indicating that the distribution of clay particles is very heterogeneous. In Figure 15(b), the dark lines and the dark areas are the intersections of the clay layers and its aggregates. The light areas are the rubber matrix. Obvious orientation of clay layers in the rubber matrix is observed in the lower magnified TEM photograph. The structure of parallel alternating narrow, dark and light bands is observed in the higher magnified TEM photograph. The intercalated nanocomposite is confirmed by this result and that derived from XRD. Clay aggregates are dispersed in the rubber matrix with a size range from about 20 nm to 300 nm (perpendicular to the (001) direction). The lateral size of the clay particles Figure 15. TEM photographs of (a) BR/pristine clay (100/30) and (b) BR/DK4 (100/30) vulcanizates 500 nm (a) 100 nm 500 nm 50 nm (b) Polymers & Polymer Composites, Vol. 13, No. 4,

14 Shaohui Wang, Zonglin Peng, Yong Zhang and Yinxi Zhang is about 500 nm or less, which is the same as the clay crystallites 19. This indicates that the organoclay DK4 particles reduce into crystallites from original agglomerates as a result of the chemical interaction (originating from the modification of clay) and the shearing effect. CONCLUSIONS Butadiene rubber (BR)/organoclay nanocomposites were prepared by direct melt mixing of BR and clays modified with different primary and quaternary ammonium salts. XRD and TEM showed that the interlayer distance of the organoclays was expanded, indicating the intercalated structure of BR/organoclay nanocomposites. However, the BR molecules did not intercalate into the galleries of the clay in the case of BR/pristine clay composite. Organoclays effectively accelerate the vulcanization of BR, which is attributed to the intercalatant used to modify the clay. BR/organoclay vulcanizates have much higher tensile strength, elongation at break and tear strength than gum BR and BR/pristine clay vulcanizates. The organoclay modified with dimethyl dihydrogenated tallow ammonium chloride (DDAC) showed the best reinforcement effect on BR of all the organoclays. REFERENCES 1. Michael A. and Philippe D., Materials Science and Engineering, 28, (2000) Okada A., Fukumori K., Usuki A., Kojima Y., Sato N., Kurauchi T. and Kamigaito O., Polymer Preprints, 32, (1991) Changwoon N., Hyune J.R., Wan D.K. and Sung-Seen C., Polymers for Advanced Technologies, 13, (2002) Hua Z., Yong Z., Zonglin P. and Yinxi Z., Polymer Testing, 23, (2004) Rathanawan M., Woothchai T. and Ratree L., Rubber Chemistry and Technology, 76, (2003) Mingyi L., Jiedong Z., Chunqing Z., Xigao J., Yang L. and Aimin L., China Synthetic Rubber Industry, 25, (2002) Jiedong Z., Mingyi L., Yang L. and Hongde X., Acta Polymerica Sinica, (2003) Markus G., Wolfram G., Peter R. and Rolf M., Rubber Chemistry and Technology, 74, (2001) Yizhong W., Liqun Z., Chunhong T. and Dingsheng Y., Journal of Applied Polymer Science, 78, (2000) Mousa A. and Karger-Kocsis J., Macromolecular Materials and Engineering, 286, (2001) Arroyo M., Lopez-Manchado M.A. and Herrero B., Polymer, 44, (2003) Lopez-Manchado M.A., Arroyo M., Herrero B. and Biagiotti J., Journal of Applied Polymer Science, 89, (2003) Krejsa M.R. and Koenig J.L., Rubber Chemistry and Technology, 66, (1993) Qingzhi Y., Editor, Modern Rubber Technology, Beijing, China Petrochemical Press, 1997, Chapter Minzhuang Z., Editor, Rubber Technology, Guangzhou, South China University of Technology Press, 1993, Bala P., Samantaray B. K. and Srivastava S. K., Materials Research Bulletin, 35, (2000) Jisheng M., Zongneng Q. and Shufan Z., Acta Polymerica Sinica, (2001) Joly S., Garnaud G., Ollitrault R. and Bokobza L., Chemistry of Materials, 14, (2002) Richard A.V., Klaus D.J., Edward J.K. and Emmanuel P.G., Macromolecules, 28, (1995) Gent A.N., and Pulford C.T.R., Journal of Materials Science, 19, (1984) Changwoon N., Hyune J.R., Sang H.H., John M.R. and Myong-Hoon L., Polymer International, 50, (2001) Polymers & Polymer Composites, Vol. 13, No. 4, 2005