New developments in metallocene ethylene [alpha]-olefin elastomer blends with SBCs.

A sizeable fraction of thermoplastic resins produced today includes blends of two or more polymers, permitting materials scientists to economically create new materials with optimized properties (refs. 1 and 2). In the 1960s, Shell commenced to work with living polymerization, exploring the capability to add different monomers during polymerization to create real block structures (ref. 3). 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.

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 (ref. 1).

In the last decade, single site metallocene catalysts provided a myriad of new ethylene [alpha]-olefins (EAO) copolymers (refs. 4 and 5). These products had rapid growth in the TPE scenario as modifiers in polymer blends, as well as base materials for soft compounds. These products present a wide range of physical and mechanical properties and have some unique features for blending, such as pellet form for easy compounding, compatibility with other olefins, easy melting and improved processability (refs. 6-8).

A significant fraction of both SBCs and EAO 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 EAO based compounds have been used in a growing number of applications, often substituted for other materials to improve technical performance (refs. 9 and 10).

The present work focuses on the effect of blending ethylene [alpha]-olefin copolymers with SBC. In the first part, EAO/ SBS blends were studied with the weight ratios between polymers ranging from 40/60, 30/70 and 20/80, respectively. The benefits of using the metallocene based polyolefins as extenders in soft compounds are presented and discussed. In the second part, a typical SEBS compound for injection molding is modified with EAO, polypropylene and oil. The EAO/SEBS compounds are characterized for physical and mechanical properties. Melt rheology, chemical resistance and weathering performance of these compounds are compared with the non-extended compound.

Experimental

Blends and mixing procedure

The selected blend components are presented in table 1. The EAO copolymers include copolymers based on butene and octene monomer. The SBCs include styrene-butadiene-styrene and styrene-ethylene-butene styrene grades.

In the first part of this study, a range of blends was prepared using SBS as the base polymer and adding varying amounts for each of the four EAOs. For these EAO/SBS blends, the weight ratios of 20/80, 30/70 and 40/60, respectively, were used. All polymers were weighed to the nearest 0.1 gram in the preparation of a 200 gram batch and then blended using a Haake Rheocord 9000 equipped with cam blades. The conditions used for blending were: Mix temperature of 190[degrees]C, mixing time of about 3-4 minutes and rotor speed of 20 rpm. After blending, samples were compression molded into plaques for testing.

In the second part of this study, a typical SEBS compound containing SEBS, polypropylene and paraffinic oil was modified with both ethylene-octene (EO1) and ethylene-butene (EB1) copolymers, as shown in table 2. These EOA/SEBS blends were prepared on a ZSK-25 co-rotating twin-screw extruder at 190oC and 200 rpm. The processability and material properties were evaluated. Melt theology and weathering performance of these blends were determined.

Testing

For hardness, an average of five measurements (durometer A) was obtained for each sample plaque (ASTM D2240).

For tensile/elongation, tensile bars were die cut from compression molded plaques having a thickness of 3.175 mm. Elongation and force data were recorded during the stress/ strain test. An extensometer was used to measure elongation values at an extension rate of 500 mm per minute (ASTM D412).

For rubber processing analysis, the rheological properties of the samples were determined using frequency sweep and temperature sweep methods. Data were collected at 7% strain from 0.1 rad/s to 100 rad/s at an isothermal temperature of 190[degrees]C, and over a temperature range of 50[degrees]C to 190[degrees]C, in 5[degrees]C increments at 1% strain and 1 rad/s (ASTM D6204).

Mooney viscosity values were determined (ML 1 + 4 conditions) at a temperature of 125[degrees]C. The sample was placed in the Mooney instrument between two thin sheets of polyester film (ASTM D1646).

Results and discussion

The hardness results from the EAO/SBS blends are shown in figure 1. These data show that there is it reduction in the compound hardness as the amount of EAO copolymer is increased from 0 to 40 wt.%. Surprisingly, there does not appear to be a significant difference between the different EAO polymers evaluated. These data suggest that the EAO copolymers were dispersed in the SBS phase. Previous studies of 25 wt.% EAO copolymer in SBS have confirmed that the EAO copolymer is dispersed in a continuous SBS phase and that the dispersed phase has a domain size of less than 2 microns (ref. 8). The Mooney viscosity results from the same EAO/SBS blends are shown in figure 2. There was a substantial reduction in the compound Mooney viscosity, and a wide range of compound Mooney viscosities could be obtained. The reduction in Mooney viscosity of the EAO/SBS blends depends on the amount and Mooney viscosity of the EAO copolymer. The stress-strain curves for various EAO/SBS blends are shown in figures 3 and 4. The data in figure 3 can be used to compare the stress-strain behavior of the SBS block copolymer to those EAO/SBS blends containing 20, 30 and 40 wt.% ethylene-butene copolymer. These data show that the shape of the stress-strain curves for these EAO/SBS blends are very similar to the SBS block copolymer. The shape of these curves is characteristic of a thermoplastic elastomer compound(s) having a tough, rubbery behavior. In addition, these data show that the addition of the EAO copolymer, EB1, resulted in improved tensile strength of the EAO/SBS blends. Additional improvements in other mechanical properties, including tear strength, have also been observed. The data in figure 4 can be used to compare the stress-strain behavior of EAO/SBS blends containing different EAO copolymers. All of these blends contain 20 wt.% EAO copolymer. These stress-strain curves are all very similar and representative of compounds having a tough, rubbery behavior. These data also show that the EAO/SBS blends containing ethylene-octene copolymers exhibit higher tensile strengths at high elongations as compared to those EAO/SBS blends containing ethylene-butene copolymers. This difference could be due to the differences in strain hardening of octene based EAO copolymers versus butene based EAO copolymers, it has been postulated that this increase in strain hardening (i.e., slope of strain hardening) is due to differences in the amount of tie chain molecules. The dynamic mechanical rheology data shown in figure 5 can be used to compare the viscoelastic behavior of the EAO/SBS blends. These data show that the elasticity of the blends, as indicated by tan delta values (i.e., ratio of the viscous component to the elastic component), are very similar. These data show that the glass transition temperature of the styrene domains (at about 90[degrees]C) does not change upon the addition of EAO copolymers and does not shift with the type of EAO copolymer. This suggests that the EAO copolymers are not miscible with the styrene domains and are probably more closely associated with the hydrocarbon portion of the SBS copolymer. This is further demonstrated by examining the properties of SEBS/EAO blends.

[FIGURES 1-5 OMITTED]

The properties can be described as an additive relationship, where the upper limit is the linear rule of mixtures for the case of a perfect interracial adhesion and connectivity between blend components. The rigid domains create a barrier to shear flow, just as covalent crosslinking does, and the mechanisms of failure and flow in a stress field are related. They both arise from the pulling-out of bridging chain styrene blocks. For the case of the melt state, this energy is lower, however, in the solid state, it is comparable to that of reticulated elastomers.

The dynamic storage modulus versus temperature data for the SEBS-EAO compounds are presented in figure 6. All blends have an upper temperature transition at about the Tg of the polystyrene hard segments. The observed rubbery modulus plateau is the same order of magnitude for the studied compounds, suggesting a tri-dimensional structure with a load path continuity that distributes the stress applied. The RPA data shown in figures 5 and 6 validate that the glass transition temperature of the styrene domains did not change with the addition of the ethylene-1-butene and 1-octene copolymers. The plot in figure 7 illustrates the melt theology data of the SEBS-compounds. As expected, the SBC compound exhibits high shear sensitivity, because the two-phase domain configuration exists even in the melt state. At low shear rates, the styrenic domains remain intact, yielding a high zero shear viscosity. As the shear rate increases, these domains break up, lowering the viscosity. The two compounds extended with EOI and EBI showed similar behavior at high shear, lowering the viscosity at low rates due to a dilution effect on the polystyrene domains. Nonetheless, at high shear rates, the apparent viscosity of the compounds overlaps.

[FIGURES 6&7 OMITTED]

This behavior was also observed at even higher EAO levels (up to 50:50), suggesting an enhanced compatibility between the ethylene copolymer and the mid-block of SEBS. In addition to that, different loadings of EAO can provide compounds with a wide range of melt viscosities, depending on the relative viscosity difference between the ethylene copolymer and the SBC.

The processing rheology and mechanical properties are dependent on the level of energy holding the network together. Based on these results, the proposed blend structure for the SEBS-EAO 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 restrains the surrounding phase, and the yield stress prevents spontaneous de-mixing. Furthermore. the similarities between the mid-block of the SEBS and the olefinic nature of the EAO used as extender lowers the interfacial tension between the polymers and suggests a greater volume at the interface.

The known criteria for micro-phase separation into the domain network structure, interface volume and dissociation energy of the block copolymers are dependent on the interaction parameter between the polymers (related to the square of the difference in solubility parameter), and corroborates with this assumption.

Figure 8 shows some of the hidden benefits of blending EAO in SEBS based compounds, acting as a fusion promoter for the blend. The significant reduction in torque observed during mixing, as well as lower die pressure values at the same melt temperature and screw speed for both EO1 and EB1 blends, suggests an enhancement in processing stability over the conventional SEBS recipe. This feature allows the compounder either to increase production rates or get lower specific energy cost per weight unit of compound.

[FIGURE 8 OMITTED]

Conclusions

The data in this article demonstrate that ethylene [alpha]-olefin copolymers (EAO) can be blended with styrene block copolymers (SBCs) with little change in physical properties. This is especially evident for the SBS blends containing levels of about 20 to 30 wt. % EAO. At this level of EAO, the tensile/elongation data are essentially identical to the SBS reference sample. The stress/strain curves for the SBS blends are consistent with a thermoplastic elastomer having a tough, rubbery behavior, similar to vulcanized rubber. The observed reduction in Mooney viscosity and RPA complex viscosity for the blends are consistent with the relative molecular weight difference between the SBC and EAO copolymer. The rheological data demonstrate that a wide range of blend viscosities can be obtained. These rheological data also indicate that SBC based compounds can be optimized for rheological properties, depending on the relative molecular weight differences between the block polymer and ethylene copolymer. The tan delta data demonstrate that the addition of the EAO copolymer (at about 20 to 30 wt.%) has no effect on the elasticity of the SBC over a temperature range from about room temperature to well above the glass transition temperature (Tg) of the styrene domains. The RPA data show the glass transition temperature of the styrene domains does not change with the addition of the ethylene copolymers. This transition occurs at about 90[degrees]C and is typical for styrene based block polymers (i.e., SBS, SIS, SEBS, SEEBS, etc.).

The use of EO1 and EB1 as extender in a typical SEBS formulation confirmed the same trends observed on the simple SBS-EAO blends, as discussed previously. Rheological behavior and physical properties were analogous to the unmodified compound.

These results showed the great potential for these polymers, especially EAO produced via metallocene catalysis, to be used in real life situations to achieve a matchless equilibrium for cost and performance.

Table 1--description of EAO and SBC polymers
used in the study

Polymer type           Designatio    Densit     MFI      Mooney
                           n           y       (g/10'    (ML1+4
Ethylene-1-butene                    (g/cc)      )         )
Ethylene-1-butene         EB1        0.870      ~0.2       45
Ethylene-1-octene         EB2        0.865       5         6
Ethylene-1-octene         EO1        0.870      0.5        33
Styrene butadiene         EO2        0.870       5         6
  copolymer               SBS         0.94      <0.1      180
Styrene ethylene
  butene copolymer        SEBS

Polymer type             Wt. %
                        styrene
Ethylene-1-butene          --
Ethylene-1-butene          --
Ethylene-1-octene          --
Ethylene-1-octene          --
Styrene butadiene          31
  copolymer
Styrene ethylene           33
  butene copolymer

Table 2--SEBS typical injection molded compound
and blends with EO1 and EB1

(w/w) (a)         SEBS     SEBS-EO1    SEBS-EB1
SEBS               28         21          21
hPP (b)            21         21          21
EO1                --         7           --
EB1                --         --          7
Oil (c)            35         35          35
Filler (d)        15.9       15.9        15.9
Antioxidant (e)    0.1       0.1         0.1

a) Table entries in % weight

b) Polypropylene homopolymer - specific gravity 0.905 g/cc and MI
12g/10 min. at 230[degrees]C/2.16 Kg.

c) Paraffinic mineral oil used as softening agent, viscosity at
40[degrees]C 90-110 cSt.

d) Precipitated calcium carbonate

e) Irganox 1010, Ciba Specialty Chemicals

References

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