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The control of miscibility of PP/EPDM blends by adding ionomers and applying dynamic vulcanization.

INTRODUCTION

The vulcanization of rubber under shear is called "dynamic vulcanization." Dynamically vulcanized thermoplastic elastomer blends have been widely used in the plastics industry because of their advantages in processing; even with the presence of crosslinked elastomers, a thermoplastic nature can be obtained by the dynamic vulcanization. This prevents the formation of a three-dimensional infinite network in the elastomer phase.

Ionomers, which have a small number of ionic groups ([less than or equal to] 15 mol %) along nonionic backbone chains (1-3), have attracted interest because of their ability to compatibilize certain blends (4-7). One of the common ionomers is poly(ethylene-co-methacrylic acid) (EMA) ionomer, where the acid groups are partially or fully neutralized by metal ions. The EMA ionomers have been blended to improve the toughness of polyamides or poly(ethylene terephthalate) even though the role of the ionomers in those systems is not fully understood (6-9). Recently, blends of ionomer with polyethyelene or ethylene-propylene-diene terpolymer (EPDM) have been studied (7, 8, 10).

Blends of polypropylene (PP) and EPDM have been extensively studied, because a wide range of properties can be obtained by changing their composition (11-16). It was generally considered, however, that PP and EPDM are incompatible though their molecular structures are similar. Thus, many attempts have been made to enhance the miscibility of these two components, which significantly affects the mechanical properties, such as impact strength, of the final blends (17-19).

The object of this work is to control the miscibility of PP/EPDM binary blends by adding EMA ionomer and by applying the dynamic vulcanization. We focus our investigation on the effect of the metal used for neutralization in the EMA ionomer, and next, on the contents of ionomer and morphology of the PP/EPDM/ionomer ternary blends. The composition of the blend was fixed as 1:1 weight ratio of PP/EPDM.

EXPERIMENTAL

Materials

In Table 1 the characteristics of the polymers used are summarized. The isotactic polypropylene (PP) was PP4017 by Korea Petrochemical Co. Ethylene-propylene-diene terpolymer (EPDM) with ethylidene-2-norbornene (ENB) as a termonomer was Royalene 521 by Uniroyal. Two kinds of EMA ionomer were supplied by DuPont. Dicumyl peroxide (DCP) was used as a vulcanizing agent.
Table 1. Materials and Their Characteristics.


Material Properties Source


PP Mn = 2.83 x [10.sup.4] Korea Petrochemical
 Mw = 2.02 x [10.sup.5] Co. Ltd
 MFI(a) = 6.0 (PP4017)
 Mw/Mn = 7.14


EPDM [[Eta].sub.inh](dl/g)(b) = 1.22 Uniroyal
 I.V.(c): 15.2 (Roy.521)
 PE/PP(mol%)(d) = 52.0/48.0
 ENB Type


Ionomer A (Na-neutralized)
 Cation Type: [Na.sup.+] DuPont
 Ethylene/Methacrylic Surlyn 8528)
 Acid = 91/9
 % Neutralization: 50%
 Sp.gr = 0.94
 MFI(a) = 1,3


Ionomer B (Zn-neutralized)
 Cation Type: [Zn.sup.++] DuPont
 Ethylene/Methacrylic (Surlyn 9520)
 Acid = 91/9
 % Neutralization: 50%
 Sp.gr = 0.95
 MFI(a) = 1.1


DCP Granule Type Mitsui Chemical Co.


a Melt flow index.


b 0.5 g/dl xylene solution at 70 [degrees] C.


c By ICI titration method.


d By IR analysis.


Preparation of Blends

EPDM and peroxide were preblended by a 3 inch x 7 inch research mill (Farrel Co.) at 80 [degrees] C for 10 min. DCP content was 0.33 phr or 1.0 phr, based on the amount of EPDM. Dried ionomers, at 63 [degrees] C under vacuum for three days, were used. EPDM was dynamically vulcanized with PP and ionomers at constant shear conditions; the roll-milled strands of EPDM were blended with PP and ionomer in a Brabender roller mixer (Type w50H) at 190 [degrees] C for 15 min. The composition of PP and EPDM was fixed in a 1:1 ratio. For the ternary blends, ionomer contents were varied from 5 to 20 parts based on the total amount of PP and EPDM. Shear intensity was controlled at 60 rpm. Under such processing conditions, the analysis by dynamic DSC showed that EPDM was fully vulcanized during mixing, whereas ionomer was not crosslinked. After that, the blends were placed in a vacuum oven at 63 [degrees] C for three days and then stored in desiccators prior to blending or testing. The notations for blends are summarized in Table 2.

Measurements

The rheological properties of blends were measured with a Rheometrics dynamic spectrometer (RDS 7700). The test temperature was 200 [degrees] C and the cone-and-plate mode was used. Cone geometry was 0.1 rad in angle and 1.2 cm in radius. The size of sample discs was 2.5 cm diameter and 1 mm thickness. Strain was maintained at 15% for all the samples. The kinetics for crystallization were studied by the dynamic DSC method with six different scan rates: 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 [degrees] C/min. Details of the dynamic DSC method are described elsewhere (18). The weight fraction [X.sub.t] of the crystals at time t was calculated from the ratio of generated heat to total heat. Scanning electron microscopy (SEM, JEOL 35-CF) was used for morphological analysis. Liquid nitrogen was used for the fracture of samples and followed by gold coating.
Table 2. Composition of Blended Materials.


 EPDM Ionomer
Notation PP Content Content Content


PP50-0.33DEP 50.0 wt% 50 wt% no ionomers


PP50-0.33DEP/IA5 47.5 47.5 5.0
PP50-0.33DEP/IA10 45.0 45.0 10.0
PP50-0.33DEP/IA15 42.5 42.5 15.0
PP50-0.33DEP/IA20 40.0 40.0 20.0


PP50-0.33DEP/IB5 47.5 47.5 5.0
PP50-0.33DEP/IB10 45.0 45.0 10.0
PP50-0.33DEP/IB15 42.5 42.5 15.0
PP50-0.33DEP/IB20 40.0 40.0 20.0


PP50-1.0DEP 50.0 wt% 50 wt% no ionomers


PP50-1.0DEP/IA5 47.5 47.5 5.0
PP50-1.0DEP/IA10 45.0 45.0 10.0
PP50-1.0DEP/IA15 42.5 42.5 15.0
PP50-1.0DEP/IA20 40.0 40.0 20.0


PP50-1.0DEP/IB5 47.5 47.5 5.0
PP50-1.0DEP/IB10 45.0 45.0 10.0
PP50-1.0DEP/IB15 42.5 42.5 15.0
PP50-1.0DEP/IB20 40.0 40.0 20.0


RESULTS AND DISCUSSION Rheological Properties

Typical complex viscosities of the dynamically vulcanized EPDM and PP/ionomer ternary blends are shown as a function of frequency in Fig. 1. Up to 15% of ionomer content, the viscosities of the ternary blends were increased by increasing the content of ionomers. But the effect of ionomer content was slightly different depending on the types of ionomer, i.e., whether it is Na-neutralized ionomer A or Zn-neutralized ionomer B. When EPDM was dynamically vulcanized with 0.33 phr of DCP, the viscosity behavior was similar to that of 1.0 phr of DCP.

The effects of the type and content of ionomer and the content of DCP are shown, at a frequency of 0.1 [sec.sup.-1], in Figs. 2 and 3. The viscosities of ternary blends were higher than those of binary PP/EPDM blends, and maximum values appeared around 15 wt% of ionomer, regardless of the type of ionomer and the content of DCP. Maximum values are often observed when there are strong interactions between droplets, or when the blends are intimately mixed (20-23). It can be implied that compatibilization was achieved, to some degree, in the presence of ionomers, and the compatibilizing effect was maximum at [approximately] 15 wt% of ionomers for the PP and EPDM binary blend.

In Fig. 2, when EPDM was dynamically vulcanized with 0.33 phr of DCP, the values of viscosity for ionomer A-containing ternary blends were higher than those of ionomer B-containing ternary blends [less than or equal to] 15 wt% of ionomer. In Fig. 3, the reverse is true when EPDM was dynamically vulcanized with 1.0 phr of DCP. The results suggest that the effects of the types of ionomer on the rheology are strongly dependent on the content of DCP. When the crosslinking density of EPDM is low, ionomer A showed a better compatibilizing effect than ionomer B. When the crosslinking density is high, then ionomer B showed a better compatibilizing effect than ionomer A. The phase behavior of the highly vulcanized blends (1.0 phr of DCP) may be governed by the inherent crosslinking tendency of the metal cation. It is thought that the ionomer neutralized by a divalent metal cation leads preferentially to an interpenetrating polymer network (IPN) structure with another crosslinked network, rather than with that neutralized by a monovalent metal cation (24-27). (More details will be discussed later.) The multiphase behavior of the slightly vulcanized ternary blends (0.33 phr of DCP) is similar to that of the linear ternary blends, as has been reported (20). It has been established that ionic domains in ionomers act as thermo-reversible crosslinking points, even though the size and properties of the ionic micro-domains are governed by the content of carboxylic acid, the degree of neutralization of the acid group, and the type of metal cation for neutralization, etc. (28-30).

The storage moduli (G[prime]) of the highly vulcanized EPDM and PP/ionomer ternary blends are shown in Fig. 4. Similar behavior for G[prime] was observed for the slightly vulcanized EPDM and PP/ionomer ternary blends, as shown in Fig. 5. As in the case of complex viscosities, the storage moduli of the ternary blends are higher than those of the PP/EPDM binary blend over an extended range of frequency. The storage moduli increased with the ionomer content [less than or equal to] 15 wt%, but decreased with further increases in ionomer content. In both cases, the storage moduli showed maxima 15 wt% of ionomer. For all the dynamically vulcanized ternary blends as well as the dynamically vulcanized PP/EPDM binary blend, plateaus began to appear at low frequencies. The tendency of G[prime] to approach a plateau at low frequencies is what was found in other composite systems such as ABS, acrylonitrile-butadiene copolymer, and filled polymers (31, 32), i.e., this implies the multiphase morphology of the PP/EPDM/ionomer ternary blends.

The plot of tan [Delta] as a function of frequency is shown in Figs. 6 and 7. In Fig. 6, the value of tan [Delta] was increased with frequency, but maximum values are shown at intermediate frequency [approximately] 10 [sec.sup.-1] for all the dynamically vulcanized blends. The monotonic decrease of tan [Delta] was usually found in simple polymers. The increase and the occurrence of peak values in tan [Delta] are due to the inherent incompatibility between the three components, i.e., PP/EPDM/ionomer. Note that the G[prime]-G[double prime] crossover point (i.e. tan [Delta] = 1.0) was not observed for all the dynamically vulcanized blends regardless of the type of ionomer and the contents of DCP. This means that the response of G[prime] dominates over the entire range of frequencies.

Harrell and Nakajima (33) reported that the plot of G[prime] and G[double prime] vs. frequency can be used for analyzing the degree of chain branching. According to them, the storage modulus increased with long chain branching, and thus, for the extreme case, the response of G[prime] dominates over the entire range of observed frequences; i.e., the G[prime]-G[double prime] crossover (i.e., tan [Delta] = 1) point was not observed. Ionomer-added ternary blend systems showed no crossover point regardless of ionomer type. This suggests that the ternary blend systems have long chain branches that formed during melt processing (34). i.e., it implies that an interpenetrating network could occur among the three components owing to the inherent ionic character of the ionomers for crosslinking. Although the Na-neutralized ionomer A can form an interpenetrating network between PP and EPDM, the possibility is less than that of Zn-neutralized ionomer B because of the monovalent nature of [Na.sup.+].

Hirasawa and Hamazaki (26) reported that an amine salt complex was formed between a Zn ionomer and a polyamide oligomer by coordination of the amino end group with the zinc carboxylate of the former, by compounding 5-20 wt% of polyamide oligomer with a primary amino end group into the ionomer in a molten state. They also reported a complex can be formed from ionomers with other divalent cations, whereas one is not formed from alkaline metal ions.

The dynamically vulcanized ternary blends showed more elastic properties than viscous properties because the values of tan [Delta] were less than 1 over all the frequency ranges, regardless of the type of ionomer and the content of DCP. The effect of ionomer addition was pronounced when 15 wt% of ionomers were added. However, when the content of DCP was 1.0 phr, as shown in Fig. 7, the values of tan [Delta] were increased monotonically by increasing frequency, but peak values at intermediate frequencies were not shown for all the dynamically vulcanized ternary blends.

Crystallization Behavior and Morphology

The rates of crystallization for the dynamically vulcanized ternary blends are shown in Figs. 8 and 9. The crystallization temperature was 117 [degrees] C where the PP phase only can be crystallized. The crystallization rate of the ternary blends is slower than that of the binary blends, and the ternary blends, which included ionomer B, showed slower crystallization rate than ternary blends that included ionomer A.

The results suggest that the dynamically vulcanized ternary blends may have strong interpentrating characteristics. It has been reported that the characteristics of slow crystallization of a polymer blend are indirect evidence of the presence of interpenetrating polymer networks (10, 35). According to the literature, the interpentrating polymer networks (IPNs), which possess physical interlocking at interfaces, strongly restrict crystallization (35, 36). This IPN structure is postulated for the dynamically vulcanized EPDM and ionomers, especially for the blends containing Zn-neutralized ionomer B.

When the DCP content is high (1.0 phr), the crystallization rate of the ionomer containing ternary blends is much slower, and the tendency to form the IPN structure is more favored than when it is low (0.33 phr). SEM micrographs of the dynamically vulcanized ternary blends that contain 1.0 phr of DCP are shown in Fig. 10. One can see that when the ionomer content was 5 wt%, the PP and EPDM blends are incompatible, i.e., their phases are separated and the domain of EPDM was peeled off from the continuous matrix of PP. For the dynamically vulcanized EPDM and PP/ionomer ternary blends with 15 wt% ionomer, compatibilization was achieved between the PP and EPDM phases. The Zn-neutralized ionomer B showed a much better compatibilizing effect than Na-neutralized ionomer A.

The results of morphology analysis agreed well with those of viscosity analysis, shown in Fig. 1. The morphology of the dynamically vulcanized EPDM/PP/Zn-neutralized ionomer B blends was presumed as a thermoplastic IPN by analyzing the morphology.

The "Thermoplastic IPN" has been designated as a new class of IPNs by Sperling for the combinations of physically crosslinked polymers, including multi-block copolymers, semicrystalline polymers, and ionomers, especially when some degree of dual-phase continuity was attached for both polymers (37-40). It can be stated that a blend of crosslinked EPDM of microgel size, ionomer, and semicrystalline PP can be categorized as one of the thermoplastic IPNs. In this vein, the authors already found that the dynamically vulcanized EPDM and Zn-neutralized ionomer binary blends exhibited the thermoplastic IPN structure (10).

From the crystallization behavior and morphology it may be considered that the dynamically vulcanized EPDM/PP/Zn-neutralized ionomer B blend exhibits behavior similar to that of a thermoplastic IPN, even though more definitive evidence would be required to draw conclusions.

Fairley and Prud'homme found the presence of one phase during their studies on the dynamic melt properties of low-density polyethylene/EMA ionomer blends (7). By considering that polyethylene is a parent polymer for EPDM, molecular intermixing between EPDM and ionomer can occur to some extent in the molten state, i.e., a compatibilizing effect of the added ionomer in a PP/EPDM binary blend can be expected. Thus, the inherent partial miscibility between EPDM and ionomer plays a synergetic effect for the improvement of poor miscibility between PP and EPDM along with the formation of a thermoplastic IPN structure by dynamic vulcanization.

CONCLUSIONS

From rheological measurements of dynamically vulcanized EPDM, PP and ionomer ternary blends, it has been found that these ternary blends show thermoplasticity even though they contain chemically crosslinked EPDM. It was presumed that the thermoplastic nature of the blends can be ascribed to dynamic vulcanization, which prevents the formation of a three-dimensional infinite network in the EPDM phase. Even though further analysis may be required, it has been shown that the ternary blends of divalent ionomer, PP, and microgel EPDM show the behavior of a thermoplastic IPN, based on rheological properties, crystallization behavior, and morphology. It is concluded that addition of the ionomer and application of the dynamic vulcanization can show synergy, to improve the miscibility of PP and EPDM. The types of metal ion and the contents of DCP significantly affect to the properties of the ternary blends. The dynamically vulcanized EPDM and PP blend, with 15 wt% of Zn-neutralized ionomer and 1.0 phr DCP content, showed the most prominent thermoplastic IPN structure and enhanced miscibility of all the blends studied in this work.

ACKNOWLEDGMENT

This work was financially supported by the Korea Science and Engineering Foundation (grant No. 931-1100-011-2), Korea.

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Title Annotation:isotactic polypropylene and ethylene-propylene-diene terpolymer
Author:Kim, Youngkyoo; Cho, Won-Jei; Ha, Chang-Sik
Publication:Polymer Engineering and Science
Date:Oct 15, 1995
Words:3545
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