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Influence of phase modifiers on morphology and properties of thermoplastic elastomers prepared from ethylene propylene diene rubber and isotactic polypropylene.

INTRODUCTION

There have been substantial developments in the field of polymer blends in the last three decades or so [1]. In such two-phase or multiphase polymeric materials, there is a strong relation between phase structure and end-use properties [1]. Polymer blends offer convenient and less expensive alternative to developing entirely new type of polymers.

Polymer blends are generally categorized into two main classes: immiscible and miscible blends. Immiscible blends are those, which exist in two different phases. On the other hand, miscible blends are those, which exist in a single homogeneous phase. Apart from these two types of blends, there is a third category, which comprises of "technologically compatible blends" or alloys [2]. The later are those which exist in two or more different phases on microscale, but exhibit macroscopic properties similar to those of a single phase material [3-5]. Most of the polymer blends are thermodynamically immiscible, resulting in an insignificant interfacial adhesion and a gross phase-separated morphology, consequently leading to low mechanical properties.

Blending an elastomer with a thermoplastic polymer can result in materials called thermoplastic elastomers (TPE) [1-4, 6]. The elastic property of rubber is combined with the processability of thermoplastic polymers in the resultant TPEs [4-11]. The combination of these properties makes these blends good alternative to crosslinked rubbers. The use of TPEs has significantly increased since they were first produced about 35 years ago. Global demand for TPEs will grow 6.2% annually through 2009 [12].

TPEs based on ethylene propylene diene rubber (EPDM) and polypropylene (PP) have increased dramatically in popularity in recent years [13, 14]. These blends, commonly referred to as thermoplastic olefins (TPOs), are partially well-suited for the applications requiring outstanding aging and weathering characteristics, ozone and heat resistance, ease of fabrication, and low cost. Various types of TPEs based on rubber-plastic blends [15-23] have been reported from our laboratory.

Earlier studies mostly concern biphasic EPDM-PP blends for developing TPEs and thermoplastic vulcanizates (TPVs) [24-26]. Many attempts have been made to enhance the interfacial adhesion between different components of a blend [27-29] by interfacial agents which influence the phase morphology of immiscible polymer blends by compatibilizing effect. In situ grafting (reactive blending) of maleic anhydride (MAH) into the blend of PP/EPDM improves the mechanical properties and morphology of the blend by improving the interfacial adhesion [30] between PP and EPDM. This has also been improved by using a compatibilizer, i.e. block copolymers. The compatibilizer acts as emulsifier which causes a finer dispersion of phases and improves physico-mechanical properties and morphology of the blends [31, 32]. A detailed study, however, is lacking in this field.

The aim of this research was to investigate the effect of phase modifiers on the morphology and properties of thermoplastic elastomer from EPDM and isotactic polypropylene (iPP) blend. Two classes of modifiers were chosen for this study. Polar modifiers were selected in such a way that part of the structure has similarity with either the plastic or the rubber phase and the other part will have potential to interact with the counterpart rubber or the plastic phase. These were maleated ethylene propylene diene rubber (MAEPDM), sulphonated ethylene propylene diene rubber (SEPDM), maleated polypropylene (MAPP), and acrylated polypropylene (AAPP). These modifiers were also selected with an aim to lower the viscosity of the plastic phase or to increase the viscosity of the rubber phase to provide a viscosity gradient. These different modifiers were taken at 5, 8, and 10 phr loadings and their effects on mechanical and dynamic mechanical thermal properties, interaction between the components, and morphology were studied. Also, nonpolar polymer such as poly styrene-ethylene-butylene-styrene (SEBS), which is known to compatibilize such rubber-plastic systems, was chosen in this investigation. Very little work has been reported on the use of theoretical model for studying the dynamic mechanical property of TPE systems. In this work, applicability of a few models to the unmodified and the modified EPDM-iPP blends has also been examined.

EXPERIMENTAL

Materials

The basic characteristics of the materials used in this study are listed in Table 1. EPDM (Buna EPG 2470), was obtained from Lanxess, Germany. Maleated EPDM rubber (Royaltuf 465), (low ethylene content), supplied by Crompton-Uniroyal Chemical Co., Naugatuck, CT, was used. Sulphonated EPDM (Ionomer 2590) was supplied by Uniroyal Chemical, Naugatuck, CT. iPP (homopolymer) (Koylene ADL) was supplied by IPCL. Acrylic acid grafted PP (Polybond 1001) and MAH grafted PP (Polybond 3200) was supplied by Crompton Uniroyal Chemical, Naugatuck, CT. Poly [styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) triblock copolymer was supplied by Shell Chemical Co. (Washington Blvd., OH; presently Kraton Polymers, Houston, TX). LR grade heptane was purchased from Nice Chemical Co., Cochin, India.

Preparation of the Blends

All the blends were prepared by a batch process in a Sigma blender (S.C. Dey & Co., Kolkata, India), having a mixing chamber volume of 100 ml. The blending was performed with a speed of 40 rpm using sigma type rotor at a temperature of 190[degrees]C. The blending sequence used was as follows:

First, EPDM was allowed to soften for 2 min. Modified EPDM (or SEBS) was added to molten EPDM and melt-mixed for next 2 min. Then iPP was added gradually and melt-mixed with the resultant blend for another 4 min. In the case of the blends with modified PP, after 2 min of EPDM addition, iPP was incorporated and melt-mixed for 2 min. Finally, modified PP was added and the mixing was carried out for another 4 min of a total mixing time of 8 min. The blend was removed immediately after mixing from the mixer and passed once through a two-roll mill (Schwabenthan, Berlin) to get a sheet of about 2 mm thick in molten condition. After giving 8 h maturation time, the sheets were compression-molded (Moore press, Birmingham, UK) at 180[degrees]C, for 4 min at 5 MPa pressure in between Teflon foils. The sheets were then cooled down to room temperature under the same pressure. To focus the study on the effect of modifiers only, a constant rubber-plastic ratio of 74:26 was chosen to get maximum elastomer property with the blend, beyond which no thermoplastic elastomeric property was achieved even with phase modifiers with regular morphology.

Recycling of the Blends

The first step of recycling, sheets of the binary and the ternary blends were taken and cut into small pieces. The small pieces of the blends were then put into the Sigma blender and remixed for 4 min at 190[degrees]C and 40 rpm rotor speed. The blend was removed immediately after remixing from the mixer and the same procedure as discussed in the previous section was followed for sample preparation and subsequent testing.

Measurement of Mechanical Properties

Mechanical properties were measured as per the ASTM D 412-98 in a universal tensile testing machine (UTM) Zwick/Roell Z010 (Zwick GmbH and Co., Ulm, Germany) at a crosshead speed of 500 mm/min at room temperature after the specimens were die-punched from 2 mm thick sheets with ASTM Die-C. The average of three results was reported in each case. The TestXpert[R]II software was used for data acquisition and analysis. Tension set was measured at 25[degrees]C after stretching the samples for 10 min at 100% elongation, according to ASTM D412-98.

Dynamic Mechanical Thermal Analysis

Viscoelastic properties of the blends alongwith the control polymers were carried out with a dynamic mechanical thermal analyzer (DMTA IV) (Rheometric Scientific, Piscataway, NJ) operated in tension compression mode within the temperature range of -100 to 130[degrees]C (with heating rate of 2[degrees]C/min, constant input sinusoidal frequency of 1 Hz and 0.01% strain amplitude). Storage modulus (E'), loss modulus (E"), and damping coefficient (loss factor, tan [delta]) were measured as a function of temperature for all the representative samples under identical testing conditions.

Morphological Characteristics

Scanning Electron Microscopy. Bulk morphology studies by scanning electron microscopy (SEM) of different blend compositions were performed using JSM5800, by JEOL, Japan operated at 20 kV of acceleration voltage at room temperature. The elastomeric component of the blends was etched out by using heptane solvent for 3 h. The extraction of the elastomer was nearly quantitative and left a polypropylene matrix that was somewhat swollen but recovered after proper drying. The samples were sputter coated by gold prior to scanning.

Atomic Force Microscopy. For detecting surface phase morphology, scanning and analysis of the samples were carried out using a multimode atomic force microscope (AFM) with a nanoscope IIIa controller by Digital Instruments, Santa Barbara, CA. Phase images were taken for surface morphologies of different blends in air at ambient conditions (25[degrees]C temperature, 60% humidity) with a tapping mode Si probe (TESP) having spring constant (K) of 20-40 N/m.

Theoretical Models

The theoretical models were used for predicting dynamic mechanical properties versus morphological aspects of polymer blends.

Kerner Model. The assumptions underlying this model [33] are that spherical inclusions of various sizes are randomly distributed in the volume of the matrix.

The Kerner equation for the shear modulus for a multi-component system is:

G/[G.sub.m] = [[n.summation over (i=1)][[[[G.sub.i] x [[PHI].sub.i]]/[(7 - 5[v.sub.m])[G.sub.m] + (8 - 10[v.sub.m])[G.sub.i]]] + [[[PHI].sub.m]/[15(1 - [v.sub.m])]]]]/[[n.summation over (i=1)][[[[G.sub.m] x [[PHI].sub.i]]/[(7 - 5[v.sub.m])[G.sub.m] + (8 - 10[v.sub.m])[G.sub.i]]] + [[[PHI].sub.m]/[15(1 - [v.sub.m])]]]] (1)

where, i = 1, 2, 3,... n (number of inclusions), G, shear modulus of the blend; [G.sub.m] and [G.sub.i], shear modulus of the matrix and the inclusion, respectively; [v.sub.m], Poisson ratio of the matrix; [[PHI].sub.i] and [[PHI].sub.m], volume fraction of the inclusion and the matrix, respectively. For a binary blend of viscoelastic materials, Eq. 1 can be adapted for the complex modulus (E*) through the correspondence principle and the relation E* = 2(1 + v*)G*, where v*(v* = v' + iv") is the Poisson's ratio.

The Paul Model. In the Paul model [34], the constituents are assumed to be in a state of macroscopically homogeneous stress. Adhesion is assumed to be maintained at the interface of a cubic inclusion embedded in a cubic matrix. When a uniform stress is applied at the boundary, the elastic modulus of the composite is given by

[E.sub.c] = [E.sub.m] [[1 + (m - 1)[[phi].sub.i.sup.2/3]]/[1 + (m - 1)([[phi].sub.i.sup.2/3] - [[phi].sub.i])]] (2)

Using the same model, for uniform displacement at the boundary Ishai and Cohen [35] obtained the following equation:

[E.sub.c] = [E.sub.m] [1 + [[[phi].sub.i]/[m/(m - 1) - [[phi].sub.i.sup.1/3]]]] (3)

where, m = [E.sub.c]/[E.sub.m], [[PHI].sub.i], volume fraction of inclusion; [E.sub.c], tensile modulus of composite; [E.sub.m], tensile modulus of matrix.

The Counto Model. The simpler model, for two-phase system proposed by Counto [36] assumes perfect bonding between the particle and the matrix. The modulus of the composite is given by:

1/[E.sub.c] = [[1 - [[phi].sub.i.sup.1/2]]/[E.sub.m]] + [1/[(1 - [[phi].sub.i.sup.1/2])/[[phi].sub.i.sup.1/2][E.sub.m] + [E.sub.i]]] (4)

where, [E.sub.i], tensile modulus of inclusion; [E.sub.c], tensile modulus of composite; [E.sub.m], tensile modulus of matrix; [[PHI].sub.i], volume fraction of inclusion.

Fourier Transform Infrared Spectroscopy (FTIR)

Transmission mode Perkin Elmer fourier transform infrared spectroscopy (FTIR, LX185256, USA) was used within the range of 4,000 [cm.sup.-1] to 400 [cm.sup.-1] for the characterization of film samples with a resolution of 4 [cm.sup.-1]. All the spectra were taken after an average of 16 scans for each specimen. The results were analyzed by using Spectrum[TM] software.

Wide Angle X-ray Diffraction (WAXD)

X-ray diffraction of the blends and the neat polymers were performed using PW 1820 X-ray diffractometer (PHILIPS, Holland) at acceleration voltage of 20 kV and 20 mA with Cu [k.sub.[alpha]] ([lambda] = 0.1542 nm) radiation source. The scans were carried out in the range of 10-60[degrees] (2[theta]) at a scanning rate of 3[degrees] [min.sup.-1]. The area under the crystalline and amorphous portions was determined in arbitrary units and the percent crystallinity [37] was calculated using the following equation:

% Crystallinity = [[I.sub.c]/([I.sub.c] + [I.sub.a])] x 100 (5)

where [I.sub.c] and [I.sub.a] are the integrated intensity of peaks corresponding to the crystalline and amorphous phases of the polymer, respectively. Area under the curve was calculated using ORIGIN 7.0[TM] software by Gaussian method and graphical plotting.

Measurement of Melt Viscosity

Melt viscosity of the neat polymers was measured with an automated capillary rheometer (Monsanto processability tester, Monsanto, USA) having die diameter of 1 mm (L/D = 30:1) with a barrel radius of 9.53 mm. Viscosity was measured [38] at a temperature of 190[degrees]C, at a shear rate of 122.5 [s.sup.-1] by using following equation.

[[eta].sub.app] = [[tau].sub.app]/[dot.[gamma].sub.app] (6)

where [[eta].sub.app], apparent viscosity; [dot.[gamma].sub.app], apparent shear rate; [[tau].sub.app], apparent shear stress.

RESULTS AND DISCUSSION

Effect of the Phase Modifier Concentration on the Mechanical Properties

Variation of mechanical properties of EPDM-iPP blends with different modifiers as the third component at different concentrations is depicted in Table 2. The sample without the modifier breaks at 84% elongation with maximum stress, [[sigma].sub.max] of 4.64 MPa, as shown in Table 2. With the addition of the phase modifiers, the mechanical properties of many compositions are improved with respect to the control sample.

In the case of ternary EPDM-iPP blends containing either SEPDM ([B.sub.5] to [B.sub.10]) or SEBS ([E.sub.5] to [E.sub.10]) modifier, maximum stress ([[sigma].sub.max]) increases with the increase in the modifier concentrations, whereas in the case of PP modified systems ([F.sub.5]-[F.sub.10], [G.sub.5]-[G.sub.10]), [[sigma].sub.max] increases initially with the modifier concentration up to 8 phr followed by a decrease at further loading of 10 phr. In these two cases, maximum value of [[sigma].sub.max] is observed at 8 phr of the respective modifier concentrations. It is also evident from Table 2 that the moduli at 50% and 100% elongation and elongation at break values of the ternary blends increase significantly up to 8 phr of concentration of the modifiers except for [C.sub.8] and [G.sub.8].

[FIGURE 1 OMITTED]

Lowest tension set value is observed at 8 phr rubber modified EPDM-iPP blends, whereas 10 phr plastic modified EPDM-iPP blends give the best result. In all cases, tension set values fall within the TPEs specification.

It is also interesting to note that the phase modifiers can convert an apparently non-thermoplastic elastomer (<100% elongation at break for initial blend EPDM-iPP) to a thermoplastic elastomer. The actual affinity of these modifiers has been investigated and is discussed in the subsequent sections.

From Table 2, it is clear that EPDM-iPP blends with 8 phr modifier concentration gives the best compromise of physical properties ([[sigma].sub.max], elongation at break (%), tension set (%), and modulus values) when overall results are taken into consideration. Hence, EPDM-iPP blends with optimized modifier concentration of 8 phr have been taken up for dynamic mechanical thermal analysis, spectroscopic, and microscopic analyses.

DMTA Analysis

Thermomechanical studies by dynamic mechanical analyzer has been used to study the viscoelastic behavior of the unmodified and the modified EPDM-iPP blends. The tan [delta] plots (Fig. 1a and c) indicate that the blends have two glass transitions (one for EPDM, [T.sub.g1] and another for iPP, [T.sub.g2]), reflecting the thermodynamically immiscible nature. In the case of EPDM-iPP blends, [T.sub.g1] of the rubber decreases from -50.9[degrees]C to -54.7[degrees]C and the peak for iPP is shifted to higher temperature by 3.9[degrees]C (Fig. 1a). Because of the intensive mixing of the rubber with the plastics, the high entanglements in EPDM breaks down, as a result of which, [T.sub.g] of the rubbery phase in the blend decreases to a lower value. Similarly, [T.sub.g2] of iPP increases due to mechanical shearing and subsequent thermal cross-linking. However, the storage modulus of the blend A (ie. the unmodified EPDM-iPP binary blend) remains in between those of the EPDM and the iPP (Fig. 1b).

Inclusion of the rubber phase modifier has prominent effect in bringing up the [T.sub.g] for the elastomer phase modified TPEs, (from -54.7[degrees]C for A to -52.6[degrees]C for [E.sub.8] and to -41.2[degrees]C for [B.sub.8]) as revealed in tan [delta] versus temperature plot (Fig. 1c), probably due to the higher molecular weight of the modified EPDMs (SEPDM, MAEPDM) and SEBS and compatibilization effect. tan [delta] peak height is reduced and approches towards higher temperature for the rubber modified systems. These are given in Table 3. This effect is most prominent in the case of MAEPDM as phase modifier which may be due to the viscosity difference, as explained later. The difference between the two [T.sub.g]s is also reduced considerably on phase modification at the interface. PP modified phase modifiers show a similar trend in reducing the tan [delta] peak height and bringing the [T.sub.g] towards higher temperature region, indicating compatibilization effect in the blend (Table 3). All these ternary EPDM-iPP blends have more or less tendency of in situ compatibilization upon use of phase modifiers. The corresponding loss factor values at the [T.sub.g]s of rubbery and plastic phases are lowered.

When different modifiers are used, it is observed that the tan [delta] peak of EPDM and iPP approaches towards each other. This indicates that there must be some interaction between EPDM and iPP due to presence of the modifiers, which increase the compatibility of EPDM and iPP blends. This was also evidenced from the interaction parameter ([[chi].sub.AB]) of iPP with different modifiers in the resultant blends at room temperature, as discussed later.

The earlier results of mechanical and dynamic mechanical properties of the blends can be explained with the help of morphology, infrared spectroscopy, crystallinity, melt viscosity, and interaction parameter. These are now discussed as follows.

Analysis of Blend Morphology

Morphological Investigation by Scanning Electron Microscopy. Scanning electron microphotographs (SEM) of the binary and the ternary blends are shown in Fig. 2. Etched out rubber phases in heptane solvent (good solvent for EPDM, but not for iPP) can be viewed as dark holes or patches in the dark-bright contrast of SEM bulk morphologies for the blends.

The type of morphology varies both with the concentration of the components and with the viscosity of the two major components. In this case, the rubber is more found to be particulate even in the case of 74/26 rubber-plastic blends. It can be seen from the SEM photomicrographs that the dispersed rubber particle size in the polymer blends is influenced by the interfacial tension of the components. Initially for EPDM-iPP blend, no clear morphology was generated even after solvent etching of the rubber phase; a few scattered holes can be seen in Fig. 2a indicating absence of (dispersing medium-dispersed phase) proper TPE morphology. This corroborates with the low mechanical properties and elongation at break (%), reduced storage modulus of the control EPDM-iPP blend (Table 2). On blending the phase modifiers, well distributed dispersed phases are formed, as shown in Fig. 2b and c for SEPDM (3.3-26.6 [micro]m domain size) and MAEPDM (0.9-3.6 [micro]m domain size) modified ternary blends, respectively. The interaction parameter mismatch, FTIR peak shifts, and viscosity difference are reasons for the generation of this wonderful morphology (discussed later). Elastomeric modifiers in these cases show more adhesive affinity for EPDM phase in the respective ternary blends. SEBS modified EPDM-iPP blend registers near-perfect dispersed phase morphology having domain size of dispersed rubber phases in the range of 0.5-4.8 [micro]m (Fig. 2d). With higher loading of SEBS, morphology takes even better shape in the context of equal distribution of dispersed rubbery phases. Some debris can be observed on the surface which may be due to etched SEBS part sticking to continuous iPP matrix. For iPP modifiers, ca. MAPP and AAPP, SEM pictures display (Fig. 2e and f) respective morphologies for these ternary blends. The average domain size of rubbery dispersed phase ranges from 7 to 30 [micro]m for these blends. Hard domains on the surface of the continuous iPP matrix in MAPP and AAPP modified blends are resulted from the preferential affinity of these modified PPs with the iPP matrix.

Surface Morphology by Atomic Force Microscopy. After going inside the bulk morphology by SEM, the next approach has been to investigate the surface phase morphology of the respective blends in the nanometer level by atomic force microscopy (AFM) in tapping mode. EPDM-iPP has not shown any clear and uniform TPE morphology (not shown). For the SEPDM modified ternary blend, alternate continuous lamellae of hard and soft phases of iPP and EPDM/SEPDM (respectively) can be observed with lamella size ranging from 300-700 nm for continuous iPP phase and 1.2-2.0 [micro]m for soft rubber dispersed phases on softer tapping (Fig. 3a and b). On harder tapping, onto the rubbery region, a distributed morphology of soft segments (having domain sizes of 20-50 nm) in the dispersing iPP matrix can easily be observed in a small scan area of 1 [micro]m.

In the case of MAEPDM modified sample surface, alternate lamellae of iPP and rubbery phase is evident from Fig. 3c, where occasional circular rubbery patches are also observed both in larger (Fig. 3c) and smaller scan area (Fig. 3d).

Proper dispersion of rubbery phases in continuous iPP matrix as evidenced from SEM and AFM studies are behind the better physical properties of these phase modified EPDM-iPP TPEs.

Incorporation of SEBS into EPDM-iPP as phase modifier shows an interesting observation in AFM surface morphology for EPDM-iPP-SEBS ternary blends. Since SEBS is a triblock copolymer, it exhibits a lamellar morphology all throughout its surface when scanned in its pristine condition. When this SEBS is blended in situ with the EPDM-iPP blend, the SEBS blocks migrate to the dispersing PP matrix (as evidenced from Fig. 3e and f). The dispersed EPDM rubbery phases (85-500 nm) are surrounded by SEBS block copolymer modifier which is further held by iPP phase as matrix for more interaction towards PP matrix due to very less difference in interaction parameter mismatch value between iPP and SEBS (discussed later).

[FIGURE 2 OMITTED]

Correlation of Experimental Values With the Theoretical Models

The experimentally obtained storage modulus value in tension mode has been correlated with a few dynamic mechanical models as discussed in the earlier section.

Figure 4 shows the comparative traces from experimental and model calculated values. Good deviation is observed from the Counto model, while Kerner, Paul, and Ishai-Cohen models have shown close agreement with the experimental E' values specifically at the plastic region.

[FIGURE 3 OMITTED]

Now, ternary systems, EPDM-iPP-MAEPD[M.sub.8], which has shown improved mechanical properties, better morphology, and better compatibility with recyclability due to interaction among the components has been chosen as a representative system. The experimental values are plotted along with the theoretical values (Fig. 4b). Morphology obtained with these TPE systems can now be correlated with the predicted ones from Pual, Kerner, and Ishai-Cohen models. This analysis provides that the trend of experimental results is in line with the calculated values obtained from these models. These models predict right morphology with right modulus for the binary and ternary blends having TPE properties.

[FIGURE 4 OMITTED]

FTIR Analysis

The structural changes on the films were analyzed using Fourier transform infrared spectroscopy (FTIR) [39]. The summary of the peaks obtained from the different materials is given in Table 4. In the case of MAEPDM (Fig. 5) a sharp characteristic peak is observed at 1,715 [cm.sup.-1] for the -C=O stretching vibration, but in EPDM-iPP-MAEPDM blend, there is no peak at 1,715 [cm.sup.-1]. In this blend, the peak for -C-H deformation for -C[H.sub.3] group at 1,375 [cm.sup.-1] is shifted to 1,372 [cm.sup.-1]. Similarly, in the case of SEPDM, some characteristics peaks are obtained at 1,154 [cm.sup.-1], 1,027 [cm.sup.-1], 967 [cm.sup.-1], and 611 [cm.sup.-1] for sulphonation of EPDM. But in EPDM-iPP-SEPDM blend, there is no peak at 1,154 [cm.sup.-1] and 1,027 [cm.sup.-1], which may be due to presence of very small amount of sulphonated group or reaction of sulphonated group. The peak at 1,154 [cm.sup.-1] in SEPDM is shifted to 1,166 [cm.sup.-1] in the blend ([B.sub.8], not shown). This indicates that there might be some interaction between the modifier and EPDM-iPP blend at higher temperature. The same thing happens in the case of MAPP modified EPDM-iPP blend i.e the peak at 1,713 [cm.sup.-1] disappears in the blend. In MAPP modified blend, -C-H deformation peak is shifted from 1,459 [cm.sup.-1] to 1,462 [cm.sup.-1]. This indicates that at higher temperature MAH group is converted into carboxylic acid group and interacts with the adjacent phases. In the case of AAPP modified EPDM-iPP blend (Fig. 5), it is observed that characteristic peak at 1,709 [cm.sup.-1] is not found in the blend mostly due to very low concentration of the modifier present. Like MAH group in MAEPDM and MAPP, acrylic acid group also interacts with different phases of the blend at higher temperature as evident from the fact that peak at 1,375 [cm.sup.-1] for -C-H deformation of -C[H.sub.3] group is shifted to 1,377 [cm.sup.-1]. SEBS gives characteristic peaks at 3,025 [cm.sup.-1], 2,852 [cm.sup.-1], 1,601 [cm.sup.-1], and 698 [cm.sup.-1]. In the SEBS modified blend, no peak shift indicates almost any chemical interaction between SEBS and EPDM-iPP blend. The enhanced physical properties of this TPE is mainly due to the physical interaction only which is discussed further in later sections.

[FIGURE 5 OMITTED]

Wide Angle X-ray Diffraction

Crystallinity of neat polymer and polymer blends has been calculated by using Eq. 5 from the wide angle (2[theta], 10[degrees]-60[degrees]) X-ray diffraction plots (Fig. 6). Crystallinity of iPP is calculated to be 39% (Table 5).

From the XRD data, it is observed that crystallinity of iPP decreases to about 2/3rd of its original value when it is blended with EPDM rubber, as provided in Table 5. Modification of EPDM-iPP blend by 8 phr (optimized) of SEPDM, MAEPDM, and SEBS imparts slight decrement (5-7%) in crystallinity from the control EPDM-iPP blend (Table 5).

Maximum drop in crystallinity from EPDM-iPP blend can be observed for the SEBS modified ternary blend. SEBS shows preferential affinity towards PP phase. This fact is evident from the morphological analysis of the blend (EPDM-iPP-SEBS). Table 5 shows an increasing trend in % crystallinity of PP modified ternary blends as compared to EPDM-iPP and EPDM modified blends. This is probably the reason for the improvement of the mechanical properties (Table 2) of EPDM-iPP blend when it is compatibilized by the modified PPs.

[FIGURE 6 OMITTED]

Effect of Melt Viscosity

At the temperature of blending (190[degrees]C), melt viscosity (calculated from Eq. 6) of neat iPP is found to be much lower (0.23 kPa s) as compared to neat EPDM (1.21 kPa s) as provided in Table 6.

As the melt viscosity values of the modified EPDMs (SEPDM and MAEPDM) and SEBS are higher than that of neat EPDM and iPP, and melt viscosity of modified PP is lower than that of iPP (Table 6), variation in mechanical and dynamic mechanical properties can be ascribed partly due to this viscosity changes during the melt reactive blending. The increased viscosity difference between EPDM and iPP by using modified EPDM and modified PP results in improved morphology of EPDM-iPP blends (Fig. 2) which gives rise to better mechanical properties.

Calculated Theoretical Interaction Parameter

Compatibilization with the help of the third component is one of the various possible ways to explain the improvement in physico-mechanical properties and morphologies of binary polymer blends. The interaction parameter can be estimated from the solubility parameter of the individual components.

The solubility parameter ([delta]) has been thermodynamically calculated using group contribution methods, taking additive nature of contributions of different functional groups [40, 41] to the thermodynamic property. Using the values of group contributions as given by Small, the solubility parameter of a homopolymer has been measured [42] as follows:

[delta] = [[rho][summation][F.sub.i]]/M (7)

where, [rho], density of the component; M, the molecular weight of the repeat unit of the polymer; [summation][F.sub.i], the sum of the group contribution of all the chemical groups in the repeat unit.

Similarly, the Hilderbrand solubility parameter of a copolymer is calculated from,

[[delta].sub.c] = [summation][[delta].sub.i] x [W.sub.i] (8)

where, [[delta].sub.i] is the solubility parameter of the ith component and [W.sub.i] is weight fraction of ith component.

For the binary blends, the Flory-Huggins parameter ([chi], the interaction parameter) at a particular temperature is calculated according to the following equation.

[[chi].sub.AB] = [[V.sub.r]/RT] ([[delta].sub.A] - [[delta].sub.B])[.sup.2] (9)

where, [[chi].sub.AB] is the interaction parameter, [V.sub.r] is reference volume, conveniently taken to be 100 c[m.sup.3]/mole, [[delta].sub.A] and [[delta].sub.B] are the solubility parameters of the polymers A and B. R, the universal gas constant and T, reference temperature (K).

Using Eq. 9, the interaction parameters ([chi]) of different components has been theoretically calculated at room temperature and at temperature of blending (190[degrees]C). Table 7 summarizes the theoretical calculated mismatch values of [chi] between two constituting components at room temperature and at temperature of melt blending. It is known that the lower the interaction parameter mismatch, [DELTA][chi] (difference-value of [chi]), the higher the tendency of the compatibilizer to remain at the blend-interface and the higher is the compatibility between two polymers at the interface and the better is the physical property [43].

It is observed from Table 7 that interaction between EPDM rubber and modified EPDM is more compared to the interaction between iPP and modified EPDM, whereas interaction between iPP and modified PP is more than interaction between EPDM and the modified PP. This is due to the structural similarity between the modified EPDM-EPDM and the modified PP-iPP pairs. The low interaction parameter mismatch values may be the reason for the improved mechanical properties with respect to the control EPDM-iPP blend for EP or PP modified systems. For SEBS, interaction parameter mismatch is much lower with iPP than with EPDM (Table 7). The interfacial tension of highly incompatible homopolymers is greatly reduced by SEBS block copolymer by physical bondage. Phase separation into uniformly dispersed microdomains is therefore facilitated for this reason. A number of factors determine the state of the SEBS in a phase-separated homopolymer system. First, the entropy of mixing of this SEBS with the mixture of homopolymers favors a random distribution; while on the other hand, localization of the SEBS at the interface displaces the EPDM and iPP away from each other and lowers their enthalpy of mixing. At the same time, the block copolymer-homopolymer enthalpy of mixing is lowered as each block of the homopolymer prefers to extend into its compatible homopolymer. The entropy and enthalpy of mixing SEBS favor micellar aggregation rather than random distribution in the bulk of the homopolymers. As a result, the micelles may compete with the interfacial region for copolymer chains which is dependant on the relative reduction in the free energy, as well as the surface area [44].

Recycling of Blends

Table 8 illustrates the effect of recyclability on physico-mechanical properties of modified EPDM-iPP blends at a representative 8 phr modifier concentration. On recycling, elongation at break value has a tendency to reduce, but remains well within the limit for TPEs (all EB values are more than 100%) [2]. For the blends containing MAEPDM and SEPDM as modifier, reprocessing imparts even more strength (~22% and ~16% increase in [[sigma].sub.max] value after 3rd recycling, respectively) due to better morphology or better interaction at the blending conditions. Because of the presence of these modifiers, reprocessing breaks down the dispersed rubber domain sizes into smaller ones. Hence, the resultant recycled blends show improved modulus and [[sigma].sub.max] values (Table 8). Another phase modifier, SEBS, being a block copolymer, follow order-disorder transition (ODT) at the temperature (190[degrees]C) and high shear (40 rpm with sigma rotors) of blending. Repeated blendings transfer the ordered SEBS into disordered state. This is the reason for the reduction in mechanical properties (55% reduction in [[sigma].sub.max] value after 3rd recycling) for disordered-state-SEBS phase blended system in subsequent processing at same temperature (Table 8). This part of recyclability study is subject of further detailed investigation. After the 1st, 2nd, and 3rd recycling [[sigma].sub.max], elongation at break and moduli values show that these modified blends can truly be termed as TPEs.

CONCLUSIONS

Thermoplastics elastomers (TPE) based on EPDM-iPP blend were prepared by using different modifier systems. Initially, the neat EPDM-iPP binary blend did not have the properties of a thermoplastic elastomer. Application of suitable phase modifiers (modified EPDMs and PPs) significantly improved the physical properties of the blends. It was also observed that mechanical properties of the blends were a function of the concentration of the modifiers. With 8 phr of the phase modifier, 50% improvement in [[sigma].sub.max] and 200% in elongation at break was registered for some blends. The blends also showed good recycling behavior, indicating formation of TPEs.

Good morphology with well distributed dispersed rubbery phases were observed in the case of the modified EP-iPP ternary blend systems as revealed from scanning electron and atomic force microscopic studies.

SEPDM and MAEPDM showed better affinity for the EPDM phases, whereas the MAPP, AAPP, and SEBS have more interaction towards iPP phase, as revealed from interaction parameter [[chi].sub.AB] mismatch values calculated theoretically at room temperature and at the temperature of blend preparation. This is also confirmed by the slight decrease in crystallinity value of the EPDM modified ternary blends and increase of the same with PP modified systems. As SEBS showed more affinity towards PP phase, it enhanced the crystallinity of the ternary blend system as compared to the binary one. Chemical interaction was observed in the case of SEPDM, MAEPDM, MAPP, and AAPP modified EPDM-iPP blends, whereas only physical interaction was observed in the SEBS modified EPDM-iPP blend leading to formation of TPEs.

Theoretical models by Kerner, Ishai-Cohen, and Paul showed good agreement of the trend of dynamic mechanical modulus with morphological aspects of the TPEs prepared.

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Pinka Chakraborty, Anirban Ganguly, Suman Mitra, Anil K. Bhowmick

Rubber Technology Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

Correspondence to: Anil K. Bhowmick; e-mail: anilkb@rtc.iitkgp.ernet.in
TABLE 1. Characteristics of the materials taken.

Materials Characteristics details

EPDM Mooney viscosity [ML.sub.(1 + 4)] at 125[degrees]C = 24,
 ethylene content = 69%, Termonomer (ENB) content = 4%,
 specific gravity = 0.86.
MAEPDM Maleic acid/maleic anhydride monomer = 1 wt%, Mooney
 viscosity [ML.sub.(1 + 4)] at 125[degrees]C = 60, ethylene
 content = 55%, specific gravity = 0.89.
SEPDM Percent by weight of ionic group = 2.7, Mooney viscosity
 [ML.sub.(1 + 4)] at 100[degrees]C = 45, avg. no of
 S[O.sub.3.sup.-] groups/molecule = 13, specific gravity =
 1.12.
SEBS Styrene content = 30%, MFI @230[degrees]C, 2.16 kg = 5 g/10
 min, specific gravity = 0.91.
iPP MFI @ 230[degrees]C, 2.16 kg = 5.6 g/10 min. specific
 gravity = 0.905.
MAPP Maleic anhydride = 1 wt%, MFI @ 190[degrees]C, 2.16 kg =
 110 g/10 min, specific gravity = 0.91.
AAPP Acrylic acid content = 6 wt%, MFI @230[degrees]C, 2.16 kg =
 20 g/10 min, specific gravity = 0.91.
Heptane Boiling range (90%) = 90-100[degrees]C, specific gravity =
 0.70.

TABLE 2. Physical properties of EPDM-iPP blend and effect of modifiers
on these blends.
 Elongation at
 Maximum stress, break (%)
Sample (a) Modifiers (phr) [[sigma].sub.max] (MPa) (with % change)

A Nil 4.64 84 (00%)
[B.sub.5] SEPDM (5) (b) 4.50 152 (+81%) (c)
[B.sub.8] SEPDM (8) 4.88 177 (+111%)
[B.sub.10] SEPDM (10) 5.00 118 (+40%)
[C.sub.5] MAEPDM (5) 3.70 145 (+73%)
[C.sub.8] MAEPDM (8) 3.68 138 (+64%)
[C.sub.10] MAEPDM (10) 4.70 171 (+104%)
[E.sub.5] SEBS (5) 5.15 226 (+169%)
[E.sub.8] SEBS (8) 6.90 252 (+200%)
[E.sub.10] SEBS (10) 7.07 227 (+170%)
[F.sub.5] MAPP (5) 5.14 161 (+92%)
[F.sub.8] MAPP (8) 5.39 178 (+112%)
[F.sub.10] MAPP (10) 3.94 122 (+45%)
[G.sub.5] AAPP (5) 4.78 214 (+155%)
[G.sub.8] AAPP (8) 6.24 194 (+131%)
[G.sub.10] AAPP (10) 4.33 107 (+27%)

 Modulus (MPa) at elongation of
Sample (a) 50% 100% Tension set (%)

A 4.5 -- --
[B.sub.5] 4.4 4.6 20
[B.sub.8] 4.5 4.7 18
[B.sub.10] 4.8 4.9 20
[C.sub.5] 3.5 3.8 30
[C.sub.8] 3.5 3.6 14
[C.sub.10] 4.1 4.5 18
[E.sub.5] 4.3 4.7 20
[E.sub.8] 5.6 6.0 18
[E.sub.10] 5.8 6.3 13
[F.sub.5] 4.7 4.9 14
[F.sub.8] 4.8 5.1 22
[F.sub.10] 3.6 3.9 14
[G.sub.5] 4.0 4.4 20
[G.sub.8] 5.3 5.7 22
[G.sub.10] 4.2 4.0 14

(a) Effective rubber-plastic blend ratio = 74:26.
(b) The values in the parenthesis indicate modifier concentration, eg.
5, 8, 10 parts per 100 parts of respective polymer.
(c) The values in the parenthesis indicate percent changes with respect
to value for the unmodified blend (A).

TABLE 3. Dynamic mechanical properties of EPDM, iPP, modified and
unmodified EPDM-iPP blends.

 Log E' (Pa) @
 temperature
 ([degrees]C) [T.sub.g] ([degrees]C)
Sample 0 25 75 [T.sub.g1] [T.sub.g2]

EPDM 6.72 6.21 5.73 -50.1 --
iPP 9.38 9.20 8.72 -- -4.3
A 8.06 7.76 7.25 -54.7 0.1
[B.sub.8] 8.38 8.09 7.58 -41.2 3.6
[C.sub.8] 7.67 7.23 6.97 -37.4 10.9
[E.sub.8] 7.64 7.57 6.99 -52.6 -18.5
[F.sub.8] 8.22 7.96 7.36 -53.7 0.5
[G.sub.8] 8.36 8.07 7.47 -42.5 3.6

 Tan delta max.
 [T.sub.g1] [T.sub.g2]
Sample Max 1 Max 2

EPDM 0.485 --
iPP -- 0.052
A 0.203 0.067
[B.sub.8] 0.169 0.063
[C.sub.8] 0.160 0.079
[E.sub.8] 0.145 0.061
[F.sub.8] 0.193 0.065
[G.sub.8] 0.174 0.063

TABLE 4. Identification of different IR peaks.

 Wave number
Sample name ([cm.sup.-1]) Peak assignment

EPDM 1,687 Isolated C=C stretching
 1,450-1,459 -C-H deformation for -C[H.sub.3]
 1,375 -C-H deformation for -C[H.sub.3]
 1,303 -C[H.sub.2]- wagging
 1,154 C-C asymmetric stretching
 807 C-H bending
 720 -(CH)[.sub.2]-, n > 3 rocking
SEPDM 1,154 Sulphonic acid salts, S[O.sub.3] stretching
 1,027 Sulphonic acid salts, S[O.sub.3.sup.-]
 [M.sup.+], S[O.sub.3] stretching
 967 Free inorganic ions S[O.sub.3.sup.2-]
 611 C-S stretching of S[O.sub.3.sup.-]
MAEPDM 1,715 -C=O stretching
 1,426 Carboxylic acid dimmer due to C-O stretching
 950 C-O-C stretching, cyclic 5 member ring acid
 anhydride
SEBS 1,601 Styrene, C=C stretching
 1,461 C=C bond
 698 Styrene, monosubstituded benzene ring
iPP 1,457-1,453 -C-H deformation for -C[H.sub.3]
 1,379 -C-H deformation for -C[H.sub.3]
AAPP 1,709 -C=O stretching of AA
 1,371 O-H deformation for -COOH gr., -C-H
 deformation for -C[H.sub.3]
MAPP 1,713 -C=O stretching of MA
 1,375 O-H deformation for -COOH gr., -C-H
 deformation for -C[H.sub.3]

TABLE 5. Percent crystallinity of iPP and different blends.

 [DELTA] %
 crystallinity [DELTA] % crystallinity
 % crystallinity with respect with respect to EPDM-
Sample ([X.sub.C]) to iPP iPP

iPP 39 -- --
EPDM-iPP 13 -67 --
EPDM-iPP-SEPDM 12.5 -68 -4
EPDM-iPP-MAEPDM 12 -69 -7
EPDM-iPP-SEBS 12 -69 7
EPDM-iPP-MAPP 14 -64 7
EPDM-iPP-AAPP 19 -51 46

TABLE 6. Melt viscosity of all virgin materials at 190[degrees]C.

Sample App. viscosity (kPa s)

EPDM 1.21
SEPDM 2.67
MAEPDM 2.01
SEBS 1.48
iPP 0.23
MAPP 0.02
AAPP 0.14

TABLE 7. Interaction parameter ([[chi].sub.AB]) of various components at
room temperature and at temperature of blending.

 [DELTA][chi] values
 at 25[degrees]C [DELTA][chi] values at
Binary components (X [10.sup.-3]) 190[degrees]C (X [10.sup.-3])

[chi]PP-EP 42.2 27.2
[chi]PP-SEP 116.3 74.9
[chi]EP-SEP 18.3 11.8
[chi]PP-MAEP 49.2 31.7
[chi]EP-MAEP 00.2 00.1
[chi]PP-SEBS 00.6 00.3
[chi]EP-SEBS 32.6 21.0
[chi]PP-MAPP 08.9 05.7
[chi]EP-MAPP 12.3 07.9
[chi]PP-AAPP 03.3 17.5
[chi]EP-AAPP 27.2 88.4

Table 8. Effect of recycling on physical properties of 8 phr phase
modified EPDM-iPP blends.

 % change in
Samples with [[sigma].sub.max] [[sigma].sub.max] Elongation at
recycling cycles (MPa) value break (%)

[B.sub.8] 4.88 -- 177
 R1 4.16 -15 229
 R2 5.83 +19 153
 R3 5.67 +16 198
[C.sub.8] 3.68 -- 138
 R1 3.45 -6 139
 R2 4.06 +10 137
 R3 4.48 +22 123
[E.sub.8] 6.90 -- 252
 R1 5.70 -17 133
 R2 4.46 -35 158
 R3 3.08 -55 132
[F.sub.8] 5.39 -- 178
 R1 3.64 -32 145
 R2 4.14 -23 108
 R3 3.35 -38 115
[G.sub.8] 6.24 -- 194
 R1 4.76 -24 175
 R2 5.32 -15 150
 R3 4.24 -32 175

 Modulus (MPa) at
Samples with elongation of
recycling cycles 50% 100% Tension set (%)

[B.sub.8] 4.44 4.74 18
 R1 3.60 3.87 22
 R2 5.54 5.73 32
 R3 5.16 5.46 44
[C.sub.8] 3.50 3.64 14
 R1 3.05 3.40 28
 R2 3.65 4.05 26
 R3 3.91 4.44 24
[E.sub.8] 5.64 6.03 18
 R1 5.40 5.65 --
 R2 4.05 4.38 --
 R3 2.81 3.07 20
[F.sub.8] 4.79 5.14 22
 R1 3.45 3.63 26
 R2 3.96 4.10 38
 R3 3.01 3.34 16
[G.sub.8] 5.32 5.73 22
 R1 4.44 4.68 18
 R2 5.09 5.28 --
 R3 3.70 4.07 40

R1, R2 and R3--after 1st, 2nd, and 3rd recycling, respectively.
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Author:Chakraborty, Pinka; Ganguly, Anirban; Mitra, Suman; Bhowmick, Anil K.
Publication:Polymer Engineering and Science
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Date:Mar 1, 2008
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