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Thermoplastic elastomers based on ionomeric polyblends of zinc salts of maleated polypropylene and maleated EPDM rubber.

INTRODUCTION

Ionomers contain up to about 10 mole percent of ionic groups pendent to a hydrocarbon chain (1, 2), and the ionic aggregates show profound influence on polymer properties and melt viscosity (3, 4). The melt viscosity of the ionomers decreases on addition of polar additives such as zinc stearate (5-7).

It has been shown that the blends of two ionomers or an ionomer with another polymer can be made compatible by either ion-ion or ion-dipole interactions (8-14). Eisenberg and co-workers have studied the miscibility of urethane elastomers with styrene-styrene sulfonic acid copolymer (15, 16). Duvdevani and co-workers have compared the tensile strength, melt viscosity and morphology of zinc salt of sulfonated EPDM/polypropylene (PP) blends with blends of unmodified EPDM and PP (17, 18). Weiss and co-workers have studied the miscible blends of nylon-6 and zinc sulfonated polystyrene ionomer (19). De and co-workers have developed ionic thermoplastic elastomers from ionomeric polyblends (20-25). It has also been reported that the miscibility of PP/EPDM binary blends can be controlled by adding ethylene-co-methacrylic acid ionomer and by applying dynamic vulcanization (26). Zinc salts of sulfonated EPDM polymers are thermally stable, strongly associating ionomers with properties approaching those of covalently crosslinked EPDM at very low levels of suffonate groups (27, 28). Datta et al. (7) have developed an ionic elastomer based on maleated EPDM rubber.

The objective of the present work was the development of an ionic thermoplastic elastomer based on the ionomeric polyblends of zinc oxide neutralized maleated EPDM rubber (Zn-mEPDM) and zinc-oxide neutralized maleated polypropylene (Zn-mPP). Studies include measurement of physical properties, infrared spectroscopic studies, dynamic mechanical thermal analyses, X-ray studies, and processability studies.

EXPERIMENTAL

Details of the materials used and their characteristics are given in Table 1.

Preparation of Ionomeric Polyblends of mPP and mEPDM

Formulations used for the preparation of ionomeric polyblends are given in Table 2. Ionomeric polyblends based on mPP and mEPDM were prepared in a Brabender Plasticorder, model PLE-330, at 190 [degrees] C and at a rotor speed of 60 rpm. First mPP was added and allowed to melt for 5 min. Then mEPDM was added and mixed for another 2 min. Finally stearic acid and zinc oxide were added and mixed for another 3 min. Preliminary studies showed that 10 phr of ZnO was sufficient for the complete neutralization of the carboxylic acid groups in the polymers. We have studied the infrared spectra of maleated EPDM rubber and the blends at different loadings of ZnO and it was [TABULAR DATA FOR TABLE 1 OMITTED] found that below 10 phr of ZnO the neutralization was partial, as indicated by the appearance of both carboxylic acid band at 1708 [cm.sup.-1] and metal carboxylate band at 1534 [cm.sup.-1]. Furthermore, at 10 phr of ZnO loading, it was found that bands due to carboxylic acid groups at 1779 and 1708 [cm.sup.-1] totally disappeared and a new band due to metal carboxylate vibration appeared at 1534 [cm.sup.-1] with a shoulder at 1588 [cm.sup.-1]. It was also found from the measurement of physical properties that as the ZnO loading increases there is an increase in physical properties, which reach a maximum at 10 phr of ZnO and then remains constant. Reaction occurring in the heterophase (molten polyblend-solid ZnO) might be responsible for the substantial excess of ZnO. Stearic acid reacts with ZnO with the liberation of water, which hydrolyzes the maleic arthydride groups present in the polymer backbone to maleic acid groups and the resultant zinc stearate increases the rate and extent of neutralization by improving the solubility of ZnO in the polymer matrix (7). After the mixing was over, the hot material was sheeted out in a two-roll mill. The mixes were then molded at 200 [degrees] C for 20 min in an electrically heated hydraulic press at a pressure of 5 MPa. After molding, the mixes were cooled to room temperature by circulation of cold water through the platens. For the preparation of mix M8, first mEPDM was materbatched with zinc stearate in a two-roll mill at room temperature for 4 min and the other procedures were similar to that explained earlier.

Measurement of Physical Properties

The stress-strain properties of the samples were measured with dumb-bell samples according to ASTM D412 (1987) in a Zwick Universal Testing Machine (UTM), model 1445, at a cross-head speed of 500 mm/min. Tear strength was measured in a Zwick UTM, model 1445, using a 90 [degrees] nick-cut crescent samples according to ASTM D624 (1986). Hardness was determined as per ASTM D2240 (1986) and expressed in Shore A units. The tension set at 100% extension was determined as per ASTM D412 (1987).

Infrared Spectroscopic Studies

Infrared spectroscopic studies on the compression moulded thin films of the samples were carried out using a Perkin-Elmer 843 spectrophotometer with a resolution of 3.2 [cm.sup.-1].

Dynamic Mechanical Thermal Analysis

Dynamic mechanical thermal analyses were done using a Dynamic Mechanical Thermal Analyser (DMTA MK-II, Polymer Laboratory, UK). The testing was performed in bending mode at a frequency of 3 Hz over a temperature range of -100 [degrees] C to + 150 [degrees] C and at a heating rate of 2 [degrees] C/min.

Processability Studies

The processability studies were carried out using Monsanto Processability Tester (MPT), model 83077, at the shear rates of 61.5, 122.9, 245.8 and 491.6 [s.sup.-1] and at a temperature of 190 [degrees] C. The capillary length (30 mm) to diameter (1 mm) ratio (L/D) was 30. The preheat time for each sample was 5 min.
Table 2. Formulations of the Mixes.

Mix Number

Ingredients M0 M1 M2 M3 M4 M5 M6 M7 M8

mEPDM 100 90 80 70 60 50 0 60 60
mPP 0 10 20 30 40 50 100 40 40
Zinc oxide 10 10 10 10 10 10 10 0 10
Stearic acid 1 1 1 1 1 1 1 0 1
Zinc stearate 0 0 0 0 0 0 0 0 10


X-Ray Studies

X-ray studies of the samples were performed with Philips X-ray diffractometer (type PW1840) using a nickel filtered CuKa radiation from Philips X-ray generator (type PW1729). Accelerating voltage and current were 40 KV and 20 mA, respectively.

Reprocessability Studies

The reprocessability studies of 60/40 Zn-mEPDM/Zn-mPP blend was studied by extruding the sample through MPT at 190 [degrees] C, using a die of L/D ratio 30 and at a shear rate of 122.9 [s.sup.-1]. The extrudate was re-extruded under similar conditions and the process was repeated for up to three cycles. The preheat time for the sample before each extrusion was 10 min. The tensile strength of the extrudate from each cycle was measured after 24 hrs.

RESULTS AND DISCUSSION

Physical Properties

The physical properties of the neat ionomers and the blends are summarized in Table 3. Figure 1 shows the variation of tensile strength, tear strength and hardness with blend composition. It can be seen that the ionomeric polyblends show synergism in tensile strength, tear strength and hardness, in the sense that the experimental values are higher than that predicted by the additivity rule. The synergism in physical properties of the blends, as discussed later, may be due to the formation of strong interfacial ionic aggregates or ionic crosslinks, which decreases the interfacial free energy in the blend and thus act as a compatibilizer (20-24). The elongation at break of the blend is found to decrease with increase in mPP content. The 50/50 blend exhibits very low elongation at break and behaves like a thermoplastic. Furthermore, tension set and modulus of the blend increase with increases in mPP content in the blend.

The role of interfacial ionic aggregates acting as compatibilizer in the polyblends is evident from the comparison of the physical properties of the 60/40 ionomeric polyblend (mix M4) against the corresponding non-ionomeric polyblend (mix M7). Figure 2 shows the stress-strain plots of mix M4 and mix M7. It is interesting [TABULAR DATA FOR TABLE 3 OMITTED] to note that mix 4 exhibits higher physical properties than mix M7. The tensile strength and tear strength of mix M4 is higher by a factor of more than two as compared to mix M7. The addition of 10 phr zinc stearate to the 60/40 ionomeric polyblend causes slight reduction in physical properties of the blend. This may be due to the poor compatibility of zinc stearate with polypropylene portion in the blend. It is also believed that zinc stearate would interact much less with ZnOOC-groups present in the polymers as compared to Zn[O.sub.3]S-groups and contribute to poorer properties.

Infrared Spectroscopic Studies

Infrared spectra of the two neat ionomers in the range 1800-1200 [cm.sup.-1] are shown in Fig. 3. Figure 3a shows the spectrum of Zn-mPP. The formation of ionomer is evident from the broad band centered at 1571 [cm.sup.-1] in the asymmetric carboxylate stretching region. This band is due to the presence of metal carboxylate ions in the polymer (29, 30). The strong and intense band at 1451 [cm.sup.-1] accounts for the coupled vibration of both -C[H.sub.2]- bending and -C[H.sub.3] asymmetric deformation (29-32). An intense and sharp band at 1375 [cm.sup.-1] is assigned to the -C[H.sub.3] symmetric deformation (29, 30). Figure 3b shows the infrared spectrum of Zn-mEPDM. As observed in the spectrum of Zn-mPP, this spectrum also shows a broad band centered at 1560 [cm.sup.-1] due to the asymmetric carboxylate stretching of zinc carboxylate ions. The strong and intense band at 1463 [cm.sup.-1] is attributed to the -C[H.sub.2]bending vibration and a sharp band at 1376 [cm.sup.-1] accounts for the -C[H.sub.3] symmetric deformation. Figure 3c shows the spectrum 80/20 ionomeric polyblend (mix M2). The blend spectrum also shows a band in the asymmetric carboxylate stretching region, but the band profile is altered. Instead of broad band, as observed in the case of neat ionomers, the blend shows a peak at 1533 [cm.sup.-1] with a shoulder at 1587 [cm.sup.-1]. The changes in spectral profile may be due to the mutual interaction of zinc carboxylate ions present in the neat ionomers.

Figure 4 shows the spectra of 60/40 non-ionomeric polyblend (mix M7), 60/40 ionomeric polyblend (mix M4) and 60/40 ionomeric polyblend with 10 phr zinc stearate (mix M8). Figure 4a shows the spectrum of mix M7. A weak band observed at 1779 [cm.sup.-1] indicates the [greater than]C = O symmetric stretching of cyclic nonconjugated five membered ring (maleic anhydride group) [30]. The band at 1708 [cm.sup.-1] is assigned to the hydrogen bonded carboxylic acid pairs (29, 30). It is evident from the spectrum of the ionomeric polyblend [ILLUSTRATION FOR FIGURE 4B OMITTED] that formation of ionomer results in disappearance of these bands and causes appearance of a new band in the asymmetric carboxylate stretching region. The spectral features of this band is similar to that of mix M2 and shows a peak at 1534 [cm.sup.-1] and a shoulder at 1588 [cm.sup.-1]. The spectrum of zinc stearate filled ionomeric polyblend Fig. 4c shows a more intense peak at 1533 [cm.sup.-1] and shoulder band at 1592 [cm.sup.-1], implying the interaction between zinc carboxylate ions present in the zinc stearate and zinc carboxylate ions in the blend (7).

The interaction of the two ionomers in the blend was studied with the help of the difference spectra. The sum of the spectra of the neat ionomers, obtained by weighted addition of the spectra of the neat ionomers, were subtracted from the corresponding blends to obtain the difference spectra. The difference spectra obtained for the 80/20 and 60/40 ionomeric polyblends (mixes M2 and M4) are shown in Fig. 5. The difference spectra show a positive absorption band at 1534 [cm.sup.-1]. The positive absorption in the asymmetric carboxylate stretching region is attributed to the strong ionic interaction of zinc carboxylate ions in the blend. It is also apparent from the difference spectra that the band centered at 1578 [cm.sup.-1] in the case of mix M4 and the band at 1598 [cm.sup.-1] in the case of mix M2 show negative absorption. This reveals the changes in spectral features in the asymmetric carboxylate stretching region of the blends due to the interaction between the two neat ionomers. The intermolecular ionic interaction changes the environment of zinc ions around the carboxylate ions (7). Because the same amounts of ZnO and stearic acid were used in the neat polymers and the blends, the effect of resultant zinc stearate on the infrared difference spectra is cancelled.

Figure 6 shows the schematic representation of the formation of interfacial ionic crosslinks in the ZnmEPDM/Zn-mPP blend by the neutralization of carboxylic acid groups present in the neat polymers by zinc oxide in presence of stearic acid.

Dynamic Mechanical Thermal Analysis

Figure 7 shows the plots of tan[Delta] versus temperature of the neat ionomers and the ionomeric polyblends. The results of dynamic mechanical thermal analyses are summarized in Table 4. Zn-mEPDM shows a glass-rubber transition at -31 [degrees] C and the corresponding temperature is abbreviated as [Tg.sub.1]. Zn-mPP shows a glass-rubber transition at 12 [degrees] C and the corresponding temperature is abbreviated as [Tg.sub.2]. There occurs insignificant changes in Tg's of the corresponding ionomers in the blends. The blends exhibit two Tg's corresponding to the neat ionomers. This reveals that the blends are immiscible at all compositions. Besides glass-rubber transitions, both the neat ionomers and the ionomeric polyblends show a weak diffuse transition at high temperature. It is known that in the case of ion-containing polymers ion pairs segregate to form ionic aggregates, which restrict the mobility of the adjacent segments of the polymer chains and form a separate rigid phase (33, 34). This immobile segments of the polymer chains undergoes relaxation at high temperature. This transition is called ionic transition and the temperature corresponding to this transition is abbreviated as Ti. The magnitude of tan[Delta] at Ti depends on the amount of ion content in the polymer (33). It has been reported that polypropylene shows a [Alpha]-relaxation around 80 [degrees] C (35). Therefore, it is anticipated that ionic transition in Zn-mPP and the ionomeric polyblends are also masked by the [Alpha]-relaxation of mPP. It is also noted that at lower concentration of mPP in the blend, the transition at [Tg.sub.2] is overshadowed by the transition at Ti.

Figure 8 shows the plots of log E[prime] versus temperature of the neat ionomers and the ionomeric polyblends. As expected, Zn-mPP shows relatively high modulus, presumably due to its high crystallinity. ZnmEPDM exhibits comparatively low modulus because of its amorphous nature and the existence of the rubbery plateau over a temperature span of 150 [degrees] C is the consequence of ionic crosslinks, which restrict the melt flow. Furthermore, increase in mPP content in the blend increases the modulus. The plots of log E[prime] at 30 [degrees] C versus blend composition are shown in Fig. 9. It is seen that the ionomeric polyblends exhibit synergism in modulus. The higher modulus of the ionomeric polyblends than that predicted by the additivity rule may be the result of stronger ionic crosslinks in the blends than that in the neat ionomers. It is known that at ambient temperature the ionic domains act as reinforcing filler (1).

Figure 10 compares the plots of tan[Delta] and log E[prime] versus temperature of the ionomeric polyblend (mix M4) and the corresponding non-ionomeric polyblend (mix M7). It is interesting to note that mix M4 shows a slightly higher modulus than mix M7 and the magnitude of tan[Delta] at [Tg.sub.1] of mix M4 is much less than mix M7. The reduction in tan[Delta] at [Tg.sub.1] of mix M4 is possibly due to the stiffness arising out of the intermolecular ionic interactions. At the same time, it is also seen that mix M4 shows a weak transition at 61 [degrees] C where s the same is absent in mix M7. This is again attributed to the occurrence of the biphasic structure in the ionomers, as discussed earlier.

Processability Studies

Figure 11a shows the log-log plots of apparent viscosity versus shear rate of the ionomeric polyblend (mix M4), ionomeric polyblend with 10 phr zinc stearate (mix M8) and the non-ionomeric polyblend (mix M7). It can be seen that the apparent viscosity varies nonlinearly with shear rate, indicating the non-Newtonian behaviour of the polymer melts. The apparent viscosity of the mixes decreases with increasing shear rate. It is also interesting to note that the melt viscosity of the ionomeric polyblend is higher than the corresponding non-ionomeric polyblend at all shear rates studied. The high melt viscosity of the ionomeric polyblend is believed to be due to the formation of strong interfacial ionic network, which restrict the melt flow. The addition of zinc stearate causes marked reduction in the melt viscosity of the ionomeric polyblend. The extrudates of ionomeric polyblend and zinc stearate filled ionomeric polyblend show no melt fracture at the shear rates studied. The reduction in melt viscosity of the ionomeric polyblend by the addition of zinc stearate is due to the fact that at temperature beyond [TABULAR DATA FOR TABLE 4 OMITTED] its melting point ([greater than] 128 [degrees] C), zinc stearate solvates the ionic aggregates (5-7). Figure 11b shows the log-log plots of shear stress versus shear rate. Shear stress is found to increase with increasing shear rate and the ionomeric polyblend exhibits higher shear stress than the corresponding non-ionomeric polyblend at all shear rates.

X-Ray Studies

The percent crystallinity of the blends obtained by X-ray studies is summarized in Table 5. It is seen that the non-ionomeric polyblend (mix M7) exhibits a higher degree of crystallinity than the corresponding ionomeric polyblend (mix M4). The ionic aggregates in the crystalline polymer is known to cause reduction in crystallinity (1). Furthermore, the percent crystallinity of the blend increases with increase in mPP content.

Reprocessability Studies

Figure 12 shows the variation of apparent viscosity and tensile strength of the extrudate of the 60/40 ZnmEPDM/Zn-mPP blend (mix M4) with number of cycles of extrusion. It is seen that apparent viscosity of the blend and tensile strength of the extrudate almost remain unchanged at all cycles. The reprocessability is due to the thermolabile nature of both ionic domain and crystalline domain present in the blend. The retention in strength even after three cycles of extrusion shows the thermoplastic elastomeric nature of the blend.

A schematic representation of the morphological structure of the ionomeric polyblend is shown in Fig. 13. As is normally observed in the case of thermoplastic elastomers it is a combination of hard domains and soft segments. The hard domains consist of a crystalline domain due to polypropylene blocks and an ionic domain due to zinc carboxylate ions.

CONCLUSIONS

Ionomeric polyblends of Zn-mEPDM and Zn-mPP in the compositions of 90/10, 80/20, 70/30, 60/40, parts by weight, behave as ionic thermoplastic elastomers (ITPE). The blends show synergism in tensile strength, tear strength and hardness, which is believed to be due to the formation of strong interfacial ionic aggregates. The ionomeric polyblend shows higher physical properties than the corresponding non-ionomeric polyblend. Infrared spectroscopic studies reveal the occurrence of intermolecular ionic interaction between the neat ionomers in the blend. Zinc stearate acts as an ionic plasticizer in the melt but causes slight reduction in the physical properties. Processability studies [TABULAR DATA FOR TABLE 5 OMITTED] show that the ionomeric polyblend exhibits higher melt viscosity than the corresponding non-ionomeric polyblend. Reprocessability studies reveal that the blends can be reprocessed like thermoplastics without deterioration in properties.

ACKNOWLEDGMENT

The authors are thankful to Uniroyal Chemical Co., USA, for supplying the materials and also to University Grants Commission, New Delhi, for providing the financial support for this work.

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Author:Antony, Prince; Bandyopadhyay, S.; De, S.K.
Publication:Polymer Engineering and Science
Date:May 1, 1999
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