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DEVELOPMENTS ON PROPYLENE-ETHYLENE COPOLYMERS BLENDS WITH STYRENE BLOCK

COPOLYMERS

Nei S. Domingues1, Carolina C. J. R. Bulhões2

1 *Softer Brasil Compostos Termoplasticos Ltda, Campo Bom-RS, Brazil and FEEVALE University Center – Science and Technology Institute - [email protected]

2 The Dow Chemical Company, São Paulo-SP, Brazil and Federal University of São Carlos – Materials Engineering Department – [email protected]

Developments on Propylene-Ethylene Copolymers blends with Styrene Block Copolymers

Thermoplastic Elastomers (TPEs) based on styrene block copolymers such as SBS and SEBS still draw technological and scientific interest due to characteristic low cost formulations combining the entropy-elasticity of elastomers with the processability of thermoplastics. This class of material plays an important role in replacing many traditional thermo-set rubber applications. Metallocene catalysts provided a broad range of new olefin based copolymers. Among them the propylene α -olefin copolymers had a fast growth in TPE scenario as modifier in polymer blends due to characteristic properties. In a previous paper we discussed the use of metallocene based ethylene-octene copolymers in blends with SBCs. It was observed the right balance of cost-performance when using ethylene-octene copolymers as elastomeric extenders. In this study we demonstrate the effect of different metallocene based propylene α-olefins copolymers (PAO) in blends with SBCs. The overal results indicate that such family of materials can be tailored to yield new TPEs with a combination of desirable softness, and mechanical properties, with improved processing.

Keywords: CBPol, Propylene copolymer, Styrene Block copolymer, Thermoplastic Elastomer

Introduction

A sizeable fraction of thermoplastic resins produced today are, in most cases, blends of two or more

polymers permitting materials scientists to economically create new materials with optimized

properties [1,2]. In the 60s, Shell commenced to work with living polymerization, exploring the

capability to add different monomers during polymerization creating real block structures. This

work resulted in a new class of Thermoplastic Elastomers (TPEs) being launched.

Styrene block copolymers (SBCs) such as SBS and SEBS present a two-phase morphology

combining the entropy-elasticity of elastomers with the processability of thermoplastics [3]. A

spatial domain and network pattern is formed due to phase segregation, and the network is

incapable of flow even when few inter-domain crosslinks are present, yielding the unique properties

observed for these materials.

In the last decade, single site metallocene catalysis provided a myriad of new olefins based

copolymers [4-8]. A new family of catalysts were also developed allowing the copolymerization of

propylene with various α -olefin comonomers over a broad range of compositions, high isotactic

content and high molecular weight [9]. These novel specialty propylene-ethylene copolymers are

unique in their performance balance due to their new microstructure. They feature narrow molecular

weight distribution and broad crystallinity distribution. The unique chain microstructure of these

new polymers provides films, fibers, and molded parts with excellent optics, sealing, and hot tack

perfomance, elasticity, flexibility, softness and adhesion to polyolefins [8,9]. The propylene-α-

olefins (PAO) had also a fast growth in TPE scenario as modifier in polymer blends because of their

wide range of physical and mechanical properties, as well as base materials for soft compounds.

These PAO have some unique features for blending such as pellet form for easy compounding,

compatibility with other olefins, easy melting and improved processability.

A significant fraction of both SBCs and PAO production is aimed at the manufacturing of

compounds. The main reason for this is their usefulness in compounding, leading to materials with

a broad scope of properties. Consequently, SBC and PAO based compounds have been used in a

growing number of applications, often substituting other materials to improve technical

performance [2].

Previous work has shown the effect of blending ethylene α-olefins (EAO) copolymers with SBC

[6]. The stress/ strain curves for SBS blends are a thermoplastic elastomer having a tough, rubbery

behavior, similar to vulcanized rubber. The rheological data indicate that SBC based compounds

can be optimized for rheological properties depending on the relative molecular weight differences

between the block copolymer and ethylene copolymer.

The purpose of this study was to determine the effect of different metallocene based propylene-α-

olefins (PAO) in blends with SBCs formulations normally used in the TPE industry. We are also

conducting a design of experiments (DoE) to better understand the PAO capabilities as modifier in

flexible SBCs compositions and map its properties for future publications.

Experimental

Anais do 10o Congresso Brasileiro de Polímeros – Foz do Iguaçu, PR – Outubro/2009

The selected blend components are presented in Table 1, PAO copolymers used in this study two

different comonomer levels as can be seen by the density.

Table 1. Description of PAO and SBC polymers used in the study

Polymer Type Designation Density [g/cm³] MFI [g/10’] Wt.% StyrenePropylene-α -olefin PC1 0.876 2.0 --Propylene-α -olefin PC2 0.859 2.0 --

Styrene-ethylene-butene-copolymer SEBS 0.91 < 0.1 33

For these PAO/SEBS blends a typical SEBS compound containing SEBS, polypropylene and

paraffinic oil is modified with two different PAO copolymers as shown in Table 2, and PAO/SEBS

ratio was set to 1.

Table 2. SEBS typical injection molded compound and blends with PAO(p.h.r.) a SEBS PAO1 PAO2

SEBS 100 50 50hPP b 30 30 30PC1 0 50 0PC2 0 0 50Oil c 100 100 100

Filler d 30 30 30a Table entries in phr (parts per hundred of rubber) b polypropylene homopolymer specific gravity 0.905 g/cc and MI 12g/10min at 230 C/2.16 Kgc paraffinic mineral oil used as softening agent, viscosity at 40 C 90-110 cStd precipitated calcium carbonate 325 mesh.

These PAO/SEBS blends were prepared on a Maris 45 co-rotating twin-screw extruder at 190 oC

and 300 rpm. Melt rheology, thermal and physical properties of these blends were determined.

Sample testing was performed using standard ASTM procedures, unless described otherwise.

Blends were examined in a Scanning Electronic Microscope (SEM) and captured on an FEI

Nano600 scanning electron microscope operated at an accelerating voltage of 10kV, a 5mm

working distance and spot size 5.0. Immersion mode was used with a solid state backscatter

detector to capture cryo-polished block faces core images which were stained with the vapor phase

of an aqueous ruthenium tetraoxide solution.

Results and Discussion

The main physical, thermal and rheological results from the PAO/SEBS blends are shown in Table

3.

Table 3. Summary of Properties obtained for the blends studied SEBS PAO1 PAO2

Hardness [Shore A] 55 70 43

Tensile @ Break [MPa] 11,5 12,6 4,5

Elongation @ Break [%] 818 824 731

Density [g/cm³] 0,978 0,968 0,966


Compression Set @ 23°C [%] 28 28 36

TMA – 900 µ m [ºC] 125,6 95,1 68,8

Tm [ºC] 148,2 150,2 148,8

Rheological Ratio (η 0.1 s-¹/η 100 s-¹)

203 25,7 25,8

These data show that there is a change in the compound hardness impacted by the PAO copolymer

density. These hardness data suggest that the PAO copolymers are associated with the

polypropylene phase. The results corroborate with the observed changes in melt temperature of

polypropylene component related to density changes on PAO. Our previous studies indicated that

the EAO copolymer is dispersed in a continuous SEBS phase associated with the olefinic mid-block

[6]. It is also clear from the data that level of comonomer in PAO strongly affects the overall

physical properties, when PAO density is reduced (higher comonomer content) there is a drastic

reduction on toughness and compound elasticity. The main properties of PAO with different

comonomer content were discussed in a previous study and are shown on Table 4 [10].

Evaluating the Table 3, the TMA (penetration temperature) temperature decreases when PAO is

added to the formulation, specially the lower density grade PC2, also pointing to a good interaction

between the PAO and the polypropylene component, by affecting its crystallization and introducing

defects in the crystalline region [7].

Table 4. Properties of PAO with different comonomer content.

Polymer

Designatio

n

Comonom

er Content Mol %

Mw kg/m

ol

MWD

Density

g/cm3

Crystallinit

yWt %

DSC

Tg°C

DMA

P/ E 0.0 0 316 2.7 0.903 57 12.5P/ E 4.4 4.4 329 2.2 0.891 45 6.9P/ E 8.2 8.2 296 2.2 0.888 34 -0.3P/ E 13.6 13.6 285 3.1 0.881 25 -6.4P/ E 15.7 15.7 262 2.2 0.871 14 -14.3P/ E 19.4 19.4 263 2.4 0.862 5 -19.5

Figure 2 compares the stress-strain characteristic of the SEBS block copolymer to those blends with

PAO/SEBS ratio equal to 1. These data show that the shape of the stress-strain curves for these

blends are very similar to the typical SEBS based compounds. The shape of these curves is

characteristic of a thermoplastic elastomer compound having a tough, rubbery behavior [5,6]. As

shown PAO1 compound exhibited higher toughness than the other two compounds, and similarly to

the SEBS compound both showed strain-hardening behavior, this was less evident on PAO2.


Figure 2. Effect of PAO on Stress-Strain behavior for the blends studiedThe ARES (Advanced Rheometric Expansion System) complex viscosity versus frequency is

plotted in Figure 3 and illustrates the melt rheology data on the compounds. As expected, the SBC

compound exhibits higher shear sensitivity, because the two-phase domain configuration even in

the melt state. At low shear rates the styrene domains remain intact, yielding a high zero shear

viscosity, as the shear rate increases these domains break up, lowering compounds’ viscosity [1].

Figure 3. Effect of PAO density on Complex Viscosity

The two compounds extended with PAO showed similar behavior across the entire frequency range,

with lower viscosity than of the SEBS compound, probably due to a dilution effect on the

polypropylene phase. Nonetheless at higher shear rates the apparent viscosity of all compounds is

similar. One can predict that different loadings of PAO will provide compounds with a wide range

of melt viscosities, depending on the relative viscosity difference between the propylene copolymer

and the SEBS. This behavior was also observed in EAO/SEBS compounds [6].

The comparison of core morphologies using SEM is shown in Figure 4. As can be seen PAO

addition induced a morphology comprised of a continuous PP matrix having dispersed SEBS

domains. Conversely higher SEBS content yielded a co-continuous morphology with both SEBS

and hPP in a continuous tridimensional network, which become more evident at higher

magnifications. Comparison between the PC1 and PC2 formulations revealed that altering the

density of propylene copolymer from 0.876 to 0.859 did not appear to influence the observed phase


morphology and dispersion of the SEBS phases. In both cases on can notice the discrete nature of

the SEBS domains. The SEBS domains were generally circular and ranged from approximately

0.2µm to 8µm in length.

Figure 4. SEM micrographs for studied blends a) SEBS; b) PAO1; c) PAO2.

Based on these results the proposed blend structure for the SEBS-PAO compounds consists of

interpenetrating co-continuous phases (IPN). The ability to form co-continuity is a result of the

network structure of the block copolymers in the melt, which restrain the surrounding phase and the

yield stress prevents spontaneous de-mixing. Furthermore, the similarities between the mid-block of

the SEBS, the polypropylene component and the olefinic nature of the PAOs used as extender,

lower the interfacial tension between the polymers and suggest a greater volume at the interface [5].


a)

b)

c)

Conclusion

Thermoplastic elastomers compounds based on SBCs can be modified with high level of propylene-

α -olefin to yield flexible TPEs which properties can be adjusted to commercially available

materials. The density of PAO affects physical-mechanical and thermal properties without

impairing flow characteristics. The results achieved for PAO containing compounds indicate that

such family of materials can be tailored to yield new TPEs with a combination of desirable softness,

and mechanical properties, with improved processing.

Acknowledgments

The authors wish to thank Softer Brasil Compostos Termoplasticos and Dow Chemical Co. for the

permission to publish this paper, and V. dos Reis for mixing the various blends.

References

1. Legge NR; Holden G; Quirk R; Schroeder H E Thermoplastic Elastomers; 2nd ed. Hanser Publisher: New York, 1996.2. De SK; Bhowmick AK Thermoplastic Elastomers From Rubber-Plastic Blends; Ellis Horwood: New York, 1990.3. To Shell Internationale Research Maatschappij N.V. NL 64006862 , 1964.4. Cady LC; Denton DP. FLEXPO'97; SPE: 1997.5. Parikh DR; et. al. 151st Rubber Division Meeting; ACS: 1997.6. Domingues N.S., Hughes MM. 164th Rubber Division Meeting; ACS: 2003. 7. Walton KL; Clayfield T. SPE Annual Technical Conference; 2000.8. Parikh DR; Guest M; Patel R; Ahmed W; Betso S; Ho T; Guerra R; Allen J. SPE Annual Technical Conference; 1996.9. D. Sexton; O. Ansems; L. Hazlitt; International Polyolefins; 2004.10. S. Chum; Ch. Stephens; B. Poon; P. Ansems; A. Hiltner; E. Baer. SPE Antec Conference; 2003.


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