A New Thermoplastic Vulcanizate (TPV)/Organoclay Nanocomposite: Preparation, Characterization, and Properties

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1 A New Thermoplastic Vulcanizate (TPV)/Organoclay Nanocomposite: Preparation, Characterization, and Properties JOY K. MISHRA, 1 GUE-HYUN KIM, 2 IL KIM, 1 IN-JAE CHUNG, 3 CHANG-SIK HA 1 1 Department of Polymer Science and Engineering Pusan National University, Pusan, , Korea 2 Department of Chemical Engineering, Applied Engineering, Division, Dongseo University, Pusan, , Korea 3 Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong -Dong, Daejon, Korea Received 9 December 2003; revised 21 April 2004; accepted 9 May 2004 DOI: /polb Published online in Wiley InterScience ( ABSTRACT: The preparation, characterization, and properties of the new thermoplastic vulcanizate (TPV)/organoclay nanocomposites are reported in this article. The nanocomposites were prepared by the melt intercalation method. The organoclay was first treated with glycidyl methacrylate, which acts as a swelling agent for organoclays, as well as a grafting agent for TPV (in the presence of dicumyl peroxide) during the melt mixing. The nanocomposite was intercalated, as evidenced by X-ray diffraction. The tensile modulus of the 5% TPV/organoclay nanocomposite was higher than that of the 20% talc-filled microcomposite. The storage modulus of the nanocomposite was higher than that of the pristine TPV. The most important observation is obtained from dynamic mechanical analysis, which reveals that the glass-transition temperature of the polypropylene phase of the nanocomposite increases (as compared to virgin TPV), whereas the ethylene propylene diene monomer phase remains almost the same Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: , 2004 Keywords: dynamic mechanical analysis; organoclay; thermoplastic vulcanizates (TPV); X-ray diffraction INTRODUCTION A large number of inorganic materials, such as glass fiber, talc, calcium carbonate, and clay minerals have been successfully used as additives to improve the strength of polymers. The extent of property enhancement depends on many factors, including the aspect ratio of fillers, their degree of dispersion and orientation in the matrix, and the adhesion of the fillers to the matrix interface. Correspondence to: C.-S. Ha ( csha@pusan.ac.kr) Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, (2004) 2004 Wiley Periodicals, Inc. Polymer layered silicate nanocomposites exhibit outstanding properties that are synergistically derived from organic and inorganic components. They also exhibit superior mechanical properties, 1 3 improved solvent resistance, 4 enhanced ionic conductivity, 5 reduced flammability, 6 and enhance the biodegradability of biodegradable polymers. 7 These materials are important model systems for study of the structure and properties of polymers in confined environments In an intercalated nanocomposite, extended polymer chains remain between the host layers, whereas in an exfoliated hybrid silicate layer they are randomly dispersed in a continuous polymer matrix, 2900

2 NEW TPV/ORGANOCLAY NANOCOMPOSITE 2901 such that the interlayer distance is comparable to the radius of the gyration of polymers. 13 The incorporation of organoclays in polymer matrices has been a well-known process for 50 years. In 1950, Carter et al. 14 prepared organoclays with several organic onium bases to reinforce latex-based elastomers. The incorporation of organoclay into a thermoplastic polyolefin matrix was disclosed in 1963 by Nahin and Backlund of the Union Oil Co. 15 However, practical exploitation began with the production of a nylon-based nanocomposite developed by Okada et al. 1 in a Toyota laboratory. Numerous research groups have already highlighted clay nanocomposites based on a variety of polymers, including polypropylene (PP), polyethylene, polystyrene, epoxy resin, poly(methyl methacrylate), nylon, polyurethane, N-vinyl carbazole, 45 polyaniline, phenolic resin, 49 liquid-crystalline polymer, polyimide, natural rubber, styrene butadiene rubber, 57 and ethylene propylene diene monomer (EPDM) rubber, 58 among others. Thermoplastic elastomer compositions based on the blends of uncured ethylene propylene diene monomer (EPDM) rubber and PP are referred to as thermoplastic polyolefin (TPO), whereas the blends of PP and dynamically vulcanized EPDM rubber are referred to as thermoplastic vulcanizates (TPV). Both TPO and TPV are currently being widely used in the automotive industry. Reinforcing fillers are generally mixed with TPO and TPV to increase stiffness. However, conventional fillers, such as talc, mica, and calcium carbonate are used to increase stiffness while also increasing weight and melt viscosity, but decreasing toughness and optical clarity. Glass-fiber reinforcement provides a high degree of stiffness with a corresponding increased fabrication cost. The traditional reinforcements and fillers must be used at high loading levels, which increase the weight and cause an adverse effect in the automotive applications area, because 90% of the total energy used by an automobile during its life cycle is from fuel consumed by its own weight. 59 A small amount of nanofiller (2 5%) is generally sufficient to increase the stiffness of the material, and is thus cost-effective for automotive applications. 59 Moreover, they are easily recyclable, since the additive weight is lower. Very few articles on PP-based and EPDM-based nanocomposite/hybrids are available Although there is very little information available regarding TPO-based nanocomposites, 64 no article has been available on TPV-based nanocomposites until now. In this article we report on the preparation, characterization, and properties of a TPV-based nanocomposite. Various properties of a particular nanocomposite depend on the successful dispersion of the clay inside the polymer matrix. Because TPV is not polar, it is not sufficiently compatible with the organoclay. To improve the dispersibility of the clay, the following technique is used. 63 The organoclay is first treated with glycidyl methacrylate, which increases the gallery spacing of the organoclay via intercalation with it. The glycidyl methacrylate-treated clay is then melt-blended with TPV in the presence of dicumyl peroxide (DCP). During melt blending, the glycidyl methacrylate will tether itself onto the TPV backbone by virtue of a grafting reaction. It is well known that in the presence of DCP, both PP and EPDM undergo a grafting reaction with glycidyl methacrylate The enlarged interlayer spacing and strong interaction caused by the grafting reaction improve the dispersion of silicate layers in the TPV matrix. EXPERIMENTAL The TPV used was Santoprene ( ), with a specific gravity of 0.97 and a hardness of 80 A. DCP and glycidyl methacrylate (GMA) were purchased from Aldrich Chemical Company. Talc filler was also purchased from Aldrich. An organically modified clay, Cloisite 20 A, was procured from Southern Clay in Texas. The structure of the organic modifier is shown below; in which HT represents the hydrogenated tallow ( 65% C18, 30% C16, and 5% C14). CH 3 P CH 3 O N O HT P HT The glycidyl methacrylate-treated 20 A is prepared by mixing 50 g of Cloisite 20 A and 15 cc of glycidyl methacryalate in a Haake rheocoder at 75 rpm for 1 h. The chemical structure of the glycidyl methacrylate is shown below: CH 2 AC(CH 3 )OCOOOCH 2 OCHOCH 2 { } O

3 2902 MISHRA ET AL. Table 1. Sample code Composition of Various Samples TPV (gm) GMA-treated Cloisite 20A (gm) DCP (gm) Talc (gm) TPV 50 TVN TVN TVN TVTM Three nanocomposites with clay contents of 2.5%, 5%, and 7.5% were prepared and designated as TVN2.5, TVN5, and TVN7.5, respectively. The mixing formulation is shown in Table 1. The samples were prepared with the melt intercalation method invented by Vaia, Ishi, and Giannelis. 67 TPV was first melted in a Haake rheocorder at 190 C and 50-rpm rotor speed. DCP was then added; after 2 min glycidyl methacrylate-treated organoclay was added. The mixing was allowed to continue for 10 min. For comparison purposes (mechanical properties), 20% talcfilled TPV microcomposite, designated as TVTM20, was also prepared by mixing 40 g of TPV and 10 g of talc in a Haake rheocorder at 190 C and 50-rpm rotor speed. Fourier transform infrared (FTIR) spectra were recorded on a JASCO FTIR-460 infrared spectrometer using thin films of the samples prepared by compression molding. X-ray diffraction (XRD) studies of the samples were carried out with a Rigaku D/max 2200 H X-ray diffractometer operating at 40 kv and 50 ma (CuK ). The scanning rate was 0.5 / min. The tensile properties of the samples were measured with the help of a Universal Tester (H. T. E, H 25 km and 500 Lm extensometer) with a strain rate of 100 mm/min at room temperature. At least six dog bone-shaped replicas of each sample were used. Dynamic mechanical analysis of the sample was carried out with a dynamic mechanical analyzer (Rheovibron DDV-25 F; Orientec Co.) in the tension mode under the following test conditions: frequency 1 Hz and scanning rate 5 C/min, within the temperature range of 100 to 150 C. The morphology of the nanocomposite was analyzed by a Phillips CM-20 transmission electron microscope (TEM), with an accelerating voltage of 120 kv. RESULTS AND DISCUSSION Evidence of Grafting from IR Spectroscopy The IR spectra of the pure TPV and 5% organoclay-based nanocomposite (TVN5) are shown in Figure 1. FTIR spectra of TPV and 5% organoclay-based TPV (TVN5) nanocomposite.

4 NEW TPV/ORGANOCLAY NANOCOMPOSITE 2903 Figure 2. XRD patterns of TPV/oraganoclay nanocomposites. Figure 1, and reveal a peak at 1729 cm 1 that is assigned as the carbonyl stretching frequency of the glycidyl methacrylate molecule. This clearly proves that the glycidyl methacrylate molecule was grafted to the TPV during melt mixing of TPV and GMA-treated organoclay. The IR absorption bands were located at 944 cm 1 and 850 cm 1, which are characteristic of the oxirane ring of the grafted GMA molecule overlapped with the polypropylene bands of the TPV. 68 Moreover, the peak that appears in Figure 1 at 3629 cm 1 is because of the OOH stretching frequency of Cloisite 20 A (OOH group of the montmorillonite), whereas the peak at 522 cm 1 is because of the AlOO stretching frequency of the Cloisite 20 A. Dispersibility of Organoclay in the Polymer Matrix Direct evidence of the intercalation of polymer molecules is obtained from the XRD patterns of the nanocomposite in the range of , which is shown in Figure 2. The peak corresponding to the basal spacing (d 001 ) of the organically modified clay (Cloisite 20 A) appears at (corresponding to d nm). The glycidyl methacrylate-treated Cloisite 20 A shows a peak at (corresponding d spacing is 3.1 nm) with a higher intensity than 20 A. This is because of the ordered intercalation of the glycidyl methacrylate molecule inside the silicate layer. The d 001 peak (the peak corresponding to the 001 plane of the silicate layer) of the clay has been shifted to the low angle, corresponding to an increase in d spacing from 2.5 nm to 4.09 nm for TVN2.5, 4.14 nm for TVN5, and 3.68 nm for TVN7.5, respectively. It is also observed from Figure 2 that there is a large decrease in peak intensity in all the TPV/clay nanocomposites as compared to 20 A. This decrease in peak intensity is due to the decrease in coherent layer scattering The intensity of the peak is substantially lower for TVN2.5, whereas for TVN5 and TVN7.5 the intensity increases (as compared to TVN2.5). The result of the XRD analysis can be explained by the intercalation of the polymer inside the clay.

5 2904 MISHRA ET AL. the silicate layers, with an average thickness of 1 nm. The distance between the two silicate layers is found to be 4.1 nm. Figure 3. TEM micrograph of the 5% organoclaybased TPV (TVN5) clay nanocomposite. The glycidyl methacrylate treatment of the organoclay increases the gallery spacing of the clay, which results in a decrease in the cohesive energy of the clay platelet, thereby facilitating the intercalation of the polymer chains inside the clay gallery. Reaction heat, when the grafting reaction of the glycidyl methacrylate in the TPV backbone occurs, is a driving force for the layer separation, which also helps the intercalation process. 63 At low clay contents, the intercalation of the polymer led to the disordering of the layered structure, thus, a decrease in the XRD intensity is observed. However, with further addition of clay, the peak intensity increased, confirming the intercalation of the polymer layers without disruption of the ordered structure. The process of polymer melt intercalation is governed by two factors: the interaction between the polymer chains and the host silicate layers and the transport of the polymer chains in the silicate gallery. The confinement of the polymeric chains inside the silicate galleries results in a decrease in the overall entropy of the polymer chains, and the increased conformational freedom of the tethered ammonium cations compensate for the entropy loss as the silicate layers separate from each other. The dispersibility of the glycidyl methacrylate treated organoclay (Cloisite 20 A) is also confirmed by TEM, which is shown in Figure 3. The dark line in the TEM image is the intersection of Dynamic Mechanical Properties Figure 4 depicts the storage modulus of TPV and TPV-based nanocomposites over a temperature range of 100 to 150 C. At 99 C, the storage modulus of TPV is Pa, which decreases with the increasing temperature; at 120 C it drops to Pa. This is attributed to insufficient thermal energy to overcome the potential barriers for transitional and rotational motions of segments of the polymer molecules in the glassy region, whereas above the glass-transition temperature (T g ), the thermal energy becomes comparable to the potential energy barriers to the segmental motions. It is clear from Figure 4 that the reinforcement effect is prominent above the T g of the polypropylene phase (in the rubbery plateau) when the material is soft and flexible, thus causing a significant improvement in the storage modulus above the T g. At room temperature, the enhancement of the storage modulus (as compared to virgin TPV) is 97% for TVN2.5, 144% for TVN5, and 233% for TVN7.5, respectively. The enhancement of the storage modulus depends on the degree of intercalation and the aspect ratio of the dispersed clay particles. The intercalation of the polymer chains increases the active surface area of the filler. Polymer chains confined in the silicate layer are immobilized and the effective Figure 4. Dynamic storage modulus of TPV/organoclay nanocomposites as a function of temperature.

6 NEW TPV/ORGANOCLAY NANOCOMPOSITE 2905 Tan behavior of TPV/organoclay nanocomposites as a function of temper- Figure 5. ature. immobilization of these chains accounts for the increase in the hydrodynamic storage modulus. 71 The variation of tan with the temperature of the virgin TPV and TPV/organoclay nanocomposites is shown in Figure 5. In the virgin TPV, two peaks appear at 31.5 C and 0.6 C, and are attributed to the T g of the EPDM and PP phase, respectively. In all the TPV/organoclay nanocomposites, the T g of the EPDM phase remains almost the same, but the T g of the PP phase has been shifted to a higher temperature. The increase in the T g of PP (hard phase) is because of the hindered cooperative motion of the polymer chains in the constrained environment. 72 Laus et al. 73 studied the thermoplastic elastomer nanocomposite system, wherein a thermoplastic elastomer is generated from poly(styrene-b-butadiene) block copolymer. From dynamic mechanical analysis, they observed that the T g of the hard polystyrene phase increases, whereas the T g of the polybutadiene phase remains the same. In our case, the thermoplastic elastomer system is generated from an elastomer plastic blend: in this nanocomposite system, the T g of the hard PP phase increases, whereas the soft EPDM phase remains same. However, selective intercalation of the PP phase into the organoclay is not possible because the surface energy of EPDM and PP are almost the same. The reason why the T g of EPDM remains the same is not clear at this time and more experimentation of the various thermoplastic elastomer systems is needed (concentrating only on the glass-transition phenomena). Tensile Properties The tensile properties of the samples are summarized in Table 2. The tensile modulus of all the TPV/organoclay nanocomposites is higher than that of the pristine TPV. The tensile modulus of 2.5% organoclay-based TPV (TVN2.5) nanocomposite is 80% higher than that of the pristine TPV, and the tensile modulus of the 5% organo- Table 2. Sample code Mechanical Properties of Various Samples Tensile Modulus (kgf/cm 2 ) Tensile Strength (kgf/cm 2 ) Elongation at break (%) TPV TVN TVN TVN TVTM

7 2906 MISHRA ET AL. clay-based nanocomposite (TVN5) is 16% higher than the 20% talc-filled TPV-based microcomposite (TVTM20). The tensile modulus of the nanocomposites also increases with increased clay loading. The tensile modulus expresses the stiffness of a material at the start of a tensile test and, in general, strongly increases when a nanocomposite is formed. The intercalation of the polymer chains inside the silicate layers leads to a tremendous increase in the surface area when interaction between the clay and polymer matrix occurs, resulting in a dramatic increase of modulus in the entire prepared nanocomposite. However, in the case of the talc-filled microcomposite, the surface area of interaction is less; thus, to improve stiffness, a very high talc loading (or other conventional filler) is necessary, which is not cost-effective in automotive applications, where minimization of weight (of the material part) is the prime concern for material selection. The tensile strength of the entire prepared TPV/organoclay nanocomposite is found to be higher than that of pure TPV. The tensile strength of the nanocomposite depends on several factors, such as the dispersion of organoclay inside the TPV matrix, interaction of the clay with the matrix, compatibility of the PP phase and the EPDM phase of the TPV, and the filler-filler interaction. 74 The increased tensile strength of the nanocomposite originates from the interaction of the polymer matrix and fillers. Intercalation of the polymer matrix inside the clay layers facilitates the polymer filler interaction.the high aspect ratio of organoclay also enhances the tensile strength of the nanocomposite by increasing the nanofiller contact surface area with the polymer matrix. It is also observed from Table 2 that the tensile strength of TPV/organoclay nanocomposites decreases with increased organoclay loading. This is because of the fact that as the filler concentration increases agglomeration among filler particles inside the polymer matrix also increases. This agglomeration results in a reduction of the organoclay aspect ratio, thereby decreasing the contact surface of the organoclay and the matrix polymer. However, the agglomeration of organoclay also induces a local stress concentration inside the composite; thus, during tensile deformation nanocomposites containing higher amounts of organoclay deform in a brittle manner and have relatively lower tensile strength. Table 3. Sample code Solvent Resistance Properties The solvent uptake measurement of the nanocomposite was conducted with toluene as a solvent. The sample (a circular film prepared by compression molding) was immersed in a solvent for two days at 25 C and 1 h for 100 C. The results are shown in Table 3. The solvent uptake rate was calculated with the following formula: 75 Solvent uptake rate W 2 W 1 W 1 100%. where W 2 is the weight of the wet sample and W 1 is the weight of the dry sample. It is observed from Table 3 that the solvent uptake rate decreases with increased filler content. The solvent uptake rate (at 25 C) of pure TPV is 48.7%, whereas the 7.5% clay-based nanocomposite has a solvent uptake rate of 22.4%. The solvent uptake rate of all samples is higher at 100 C than at 25 C. This decrease in solvent uptake with increased filler content occurs because of the interaction between the filler (Cloisite 20 A) and thermoplastic vulcanizates. The interaction leads to the formation of a bound polymer in close proximity to the reinforcing filler, which restricts the solvent uptake. As the amount of filler loading increases, the amount of the bound polymer is lower and, consequently, the solvent uptake is lower. CONCLUSIONS Solvent Uptake Rate of Nanocomposites Solvent uptake rate at 25 C (%) Solvent uptake rate at 100 C (%) TPV TVN TVN TVN We have successfully prepared a new TPV/organoclay nanocomposite with the melt intercalation method. The organoclay is treated with glycidyl methacrylate, which acts as a swelling agent for organoclay as well as acting as the grafting agent for TPV during melt mixing in the preparation of the nanocomposite. The nanocomposite

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