Modification of PO-TPEs with a fully paraffinic liquid modifier.

Polyolefin-based thermoplastic elastomers (PO-TPEs) are a versatile and commercially important class of thermoplastic elastomers, finding utility in a wide variety of applications that require moderate softness and elasticity. They are often the TPE of choice to address market drivers such as cost and recyclability. This popularity leads to a global market estimated at 2,500 KTA, with an annual growth rate expected to exceed 7% over the next five years. The bulk of this volume is attributable to three main categories of PO-TPE (ref. 1):

* Semi-crystalline random copolymers;

* microphase separated block copolymers; and

* dynamically vulcanized thermoplastic/elastomer blends (also called thermoplastic vulcanizates, TPVs).

The first category includes very low-crystallinity random copolymers, such as the ethylene/octene (E/O) and propylene/ ethylene (P/E) copolymers available from ExxonMobil Chemical, Dow Chemical, Mitsui Chemicals and others (refs. 2 and 3). The second category is most prominently represented by styrenic triblock copolymers like SEBS (styrene-ethylene/butene-styrene) and SEPS (styrene-ethylene/propylene-styrene), in which the mid block is a polyolefin copolymer (ref. 4). The third category is dominated by TPVs derived from polypropylene and EPDM (ref. 5). Other possibilities for PO-TPEs exist (e.g., stereoblock polypropylene [ref. 6] and polyoletin block copolymers [refs. 7 and 8]), but these do not have a significant market presence to date.

All these materials combine a majority "soft" (low modulus, mostly to completely amorphous) phase with a minority "hard" (high modulus, crystalline or glassy) phase. For the random and block copolymer materials, the hard domains act as thermally reversible physical crosslinks for the soft phase network, giving rise to an elastic response to deformation. TPVs have a very different morphology, involving a majority elastomer phase that is dispersed into domains surrounded by a matrix of a minority thermoplastic phase; consequently, the mechanism responsible for their elastomeric nature is different than for copolymer-based PO-TPEs.

In either case, the complex morphology provides many degrees of freedom that can be adjusted during synthesis, including soft/hard ratio, molecular weight, size scale of the hard domains, modulus of the soft domains, etc. As a result, there is a wide range of property combinations that can be made available for a given base PO-TPE. Even so, it is not unusual to add a secondary polymer to the base polymer, often to supplement the hard domains or strengthen the soft domains, which only further broadens this property envelope and allows for fine adjustments to meet specific objectives. For PO-TPEs, this secondary polymer is frequently high-crystallinity polypropylene.

Low molecular weight liquids are also common blend components in PO-TPE compounds. These may be only process aids, or they may be utilized as plasticizers to vary properties. Processing improvements, for the most part, are simply derived from a reduction in compound viscosity, although proper viscosity control can be critical to morphology optimization in TPVs. Plasticization, on the other hand, is a more complex topic, because it influences so many physical properties that are key to the overall performance of a fabricated article, including softness, flexibility and elasticity. There are three primary mechanisms at work, including dilution of the hard domains, reduction of the modulus of the soft phase and depression of the glass transition temperature (Tg) of that soft phase. Ideally, there is no effect on the hard domains, beyond dilution. While all three mechanisms must be considered when tailoring performance attributes through a liquid modifier, it is the extent of [T.sub.g] depression for the soft phase that is most often critical to determining the plasticization efficiency and how far those attributes can extend to low temperatures. Of course, permanence is also an important consideration, since the liquid modifier must be retained for it to act as a plasticizer.

By far, the most common liquid modifiers for PO-TPEs are mineral oils, and more specifically, the so-called paraffinic mineral (or process) oils (ref. 9). Despite the paraffinic name, these refinery-produced oils always contain a substantial fraction of naphthenic and, in some cases, aromatic moieties, owing to their barrel of oil heritage. This also limits their molecular weight range. While the final composition and molecular weight of a mineral oil is determined by how it is refined (via solvent extraction, hydroprocessing, etc.), its utility as a plasticizer for polyolefins is certainly limited by the fact that it is not fully paraffinic in nature. For example, its [T.sub.g] may not be low enough to effectively plasticize the soft phase, or it may adversely affect the hard phase. Its molecular weight may be insufficiently high to ensure adequate long-term permanence. For some applications, it may be not be clean enough, in terms of color, odor or food compatibility. Finally, its compatibility with a PO-TPE may be poor enough to cause oil bleed and/or fogging problems.

These limitations can be substantially reduced through the use of a fully paraffinic, amorphous liquid modifier with a low [T.sub.g]. Such materials have been recently introduced by ExxonMobil Chemical as Elevast polymer modifiers. Elevast moditiers are highly compatible with polyolefins, available over a wide range of molecular weights (from about 500 to 5,000 g/ mol) and exhibit step-out PO-TPE plasticizer properties:

* Exceptionally low pour points;

* extremely low volatility and fogging characteristics;

* water-white color;

* low odor;

* negligible interference with other additives (including grafting and partial crosslinking agents); and

* indirect food contact approval.

In addition, direct food contact approvals have already been obtained for certain applications, and this list is growing.

Materials

To probe the key effects involved with PO-TPE plasticization by Elevast polymer modifiers, we have used the set of copolymers described in table 1 and a single grade of Elevast (R170). This particular grade has a viscosity on the high end of the range for paraffinic process oils, hut lower and even higher viscosity grades are available. Grade selection will he dependent on polymer type and targeted property attributes.

The basic properties of Elevast R170 are compared to a few common paraffinic process oils in table 2. Note the unique combination of low pour point, high molecular weight and low color for Elevast. The low pour point, which is due to the low [T.sub.g] of Elevast (less than 70[degrees]C), translates into superior depression of the [T.sub.g] of the soft phase in modified PO-TPEs. This is demonstrated in figure 1 for representative propylene-based and ethylene-based random copolymers. A high molecular weight generally translates into high permanence. In fact, fogging tests consistently give zero (unmeasurable) fogging for polyolefins plasticized with Elevast R170. The low color means Elevast is unlikely to contribute to discoloration of transparent goods.

Surprisingly, the high molecular weight of Elevast does not give rise to a high viscosity. In fact, Elevast has a markedly lower viscosity at a given molecular weight than a paraffinic process oil. One consequence of this is a larger decrease in melt viscosity, which maximizes the processing credits associated with plasticization. Figure 2 shows an example of how the melt flow rate (MFR), a common descriptor of processibility used for polyolefins, increases exponentially with Elevast concentration. Another consequence is that a lower viscosity can be chosen, to ease liquid handling, without sacrificing a high level of permanence.

[FIGURE 1 OMITTED]

Methods

Physical properties were measured following ASTM methods; test specimens were cut from compression-molded plaques. Tension set was determined from a 100% strain hysteresis test. Glass transition temperature was determined by dynamical-mechanical thermal analysis, while melting point was determined by differential scanning calorimetry. Blends were prepared using either a small-scale batch mixer, with Elevast added after the polymer components were first masticated, or continuous extrusion with direct injection of E1evast into the melt section of the extruder.

Changes in PO-TPE properties upon plasticization

Many of the changes in properties resulting from modification of a PO-TPE with Elevast are directionally similar to those achievable with high-quality paraffinic process oils. However, the quantitative differences, as reflected in both the range and balance of performance attributes, are often significant and unexpected, allowing Elevast to be used successfully in applications where conventional process oils have come up short.

In general, plasticization makes a polymer softer and more flexible. With Elevast, the extent can be significant, as shown in figure 3. Durometer hardness and flexural modulus both decrease steadily with increasing plasticization, to the extent that a 68 durometer A/13 MPa modulus material is transformed into a 40 durometer A/5 MPa modulus material. At the same time, processibility increases dramatically.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The effects of plasticization with Elevast are also manifested in the tensile properties. Not only is the initial tensile modulus depressed to a similar extent as the flexural modulus, but the stress at all strains is reduced. For PO-TPEs with sufficient crystallinity to cause plastic yield under tensile deformation, the addition of Elevast reduces the yield stress, increases the yield strain, and eventually eliminates the yield point; this is demonstrated in figure 4 for c(P/E)-1. With increasing concentration, the stress-strain curve looks more and more elastomer-like (e.g., no yield and continuous strain hardening). Tensile strength progressively decreases and ultimate elongation increases somewhat.

As shown in figure 3, tension set also decreases, but there appears to be a limit to how much this measure of elasticity can be improved. The reason is likely related to the underlying hard/soft domain morphology in semi-crystalline random copolymers, wherein the crystallites act as physical crosslinks. These crystallites are diluted, but not otherwise substantially altered upon addition of modifier. Softening the network may initially reduce the stress on the crystalline domains, allowing improved recovery after deformation, but eventually the small volume of crystallites cannot support deformation of the swollen network without some level of disruption that results in a permanent set. Introduction of some modest level of chemical cross-linking, which is quite efficient in the presence of Elevast due to its pure paraffinic character, could be one way to push tension (and compression) set beyond this apparent limit.

[FIGURE 5 OMITTED]

The data in figure 3 also hint at a subtle, but important aspect of using Elevast to modify a PO-TPE: the ability to tailor properties along a continuum towards a much softer, more flexible version of the starting material. This is more clearly exemplified in figure 5. Here, data from figure 3 are combined with similar data for c(P/E)-1 and plotted to expose basic property tradeoff relationships (e.g., softness--flexibility). Blue and green symbols represent the neat P/E random copolymers, while red and black symbols represent plasticized blends that become softer and more flexible in a smooth progression with increasing Elevast concentration. In this way, it is easy to see that plasticization with Elevast provides options both in between and beyond the available grades of a given PO-TPE.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Compatibility with blend-based PO-TPEs

Plasticization with Elevast is also effective when used in conjunction with a polymer blending approach to tailor PO-TPE properties. In particular, Elevast can be used to counteract the inherent stiffening and hardening effects when a small amount of high-crystallinity polyolefin is added to a soft PO-TPE (e.g., PP in P/E copolymers). In this case, Elevast actually plasticizes both blend components, or, more specifically, the amorphous phases in both components, and does not significantly affect the melting point ([T.sub.m]) of the PP, as shown in figure 6. Consequently, the properties of the TPE benefit from the presence of the high-melting PP crystallites, while the effect of the relatively high [T.sub.g] of PP on low-temperature properties is reduced.

Moreover, while use of Elevast in combination with PP certainly provides an additional degree of freedom in designing a PO-TPE formulation, it also often allows for a different balance of properties than can be achieved using either PP or Elevast alone. This versatility is demonstrated in figure 7 for blends of P/E copolymer, PP and Elevast. Addition of modest amounts of both modifiers allows a material to be formulated with essentially the same hardness as the neat copolymer, but with a higher flexural modulus, higher Vicat softening temperature and higher MFR. More significant modification yields a completely different material, with a Vicat softening temperature roughly the same as the base copolymer, but significantly softer, more flexible, and with a much better processibility. All these materials have a modest tension set (recovery after 100% elongation), as well as an ultimate elongation of greater than 1,000%.

Another reason to utilize Elevast in conjunction with a polymeric modifier is to improve optical properties. More often than not, polymer blends are hazy, due to a phase-separated heterogeneous morphology and refractive index mismatch between the phases. In some cases, addition of Elevast reduces this effect--by either reducing the size of the heterogeneous structures or reducing the refractive index mismatch--and therefore clarity can improve. Figure 8 demonstrates just such a trend for a blend of P/E copolymer and a nucleated PP.

Other copolymer-based PO-TPEs

The trends observed for Elevast-modified P/E random copolymers are generally valid for other types of copolymer PO-TPEs as well. For example, a similar continuous variation in properties, and in the balance of those properties, occurs when Elevast is added to a low-density ethylene/octene plastomer. This is shown in figure 9 where, as before, flexural modulus and hardness both decrease, while MFR increases.

[FIGURE 8 OMITTED]

An example of the utility of Elevast to modify SEBS is provided in figure 10 (tensile behavior) and table 3 (corresponding physical properties). It is seen that a high-styrene SEBS can be softened to look very much like a low-styrene SEBS in terms of both stress-strain behavior (no yield, greater than 800% ultimate elongation) and hardness (~ 54 durometer A), but with a higher Vicat softening temperature (79[degrees] vs. 69[degrees]C). This effect is due to the fact that, while Elevast certainly depresses the [T.sub.g] of the soft E/B phase, it does not seem to interact with the polystyrene domains (presumably due to its purely paraffinic composition), and so the [T.sub.g] of the hard phase remains high. The results in table 3 also demonstrate that Elevast can be used to augment the common practice of modifying SEPS with PP, which provides a means to achieve property combinations not readily available from a single grade.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Successful plasticization of a TPV requires more sophistication than the simple blending approach illustrated here, since the presence of a low molecular weight liquid during the dynamic vulcanization step can have a profound influence on the final morphology. Thus, a plasticizer will affect properties both directly and indirectly. Qualitatively, however, the impact of a low molecular weight liquid on the properties of a TPV are expected to be similar to that for a copolymer-based PO-TPE.

Permanence

Elevast modifiers are sufficiently compatible with polyolefms that they generally do not migrate to the surface of PO-TPEs. Nor are they particularly volatile. Thus, permanence is typically not a concern. For example, none of the Elevast-modified TPEs in this article showed signs of oil bleed or weight loss, even after two weeks of aging.

However, it is possible to induce oil bleed and weight loss by using a loading of Elevast that exceeds the "saturation limit" for a given PO-TPE system. This limit is primarily determined by thermodynamics of mixing. The most important factors are the volume fraction of the soft phase, the chemical makeup of the soft phase, the soft/hard phase morphology and the specific grade of Elevast. In practice, the upper loading limit for Elevast in PO-TPEs is much more likely to be determined by the point where property modification suffers from diminishing returns, rather than this thermodynamic limit.

Conclusions

ExxonMobil Chemical has introduced a new family of purely paraffinic liquid modifiers under the Elevast tradename. These materials exhibit high compatibility with polyolefins and a unique combination of attributes that address many of the challenges (e.g., bleeding, fogging, interference with other ingredients) currently faced by TPE formulators. With a plasticizing efficiency that is superior to even the "best" paraffinic mineral oils, Elevast widens the range of properties, and changes the balance among those properties, for polyolefin-based thermoplastic elastomers. The end result is the potential for dramatic improvements in both processibility and performance.

References

(1.) E.N. Kresge, Chapter 5 of Thermoplastic Elastomers, 3rd Edition, G. Holden, H.R. Kricheldorf and R.P. Quirk, Eds., Hanser Gardner, 2004.

(2.) M. Varma-Nair, M.G. Williams and N.R. Dharmarajan, ANTEC 2004 Conference Proceedings, Society of Plastics Engineers, 2,178.

(3.) S. Datta, S. Srinivas, C.Y. Cheng, W. Hu, A. Tsou and D.J. Lohse, Rubber World 229 (1), 55 (October 2003).

(4.) G. Holden and D.R. Hansen, Chapter 3 of Thermoplastic Elastomers, 3rd Edition, G. Holden, H.R. Kricheldorf and R. P. Quirk, Eds., Hanser Gardner, 2004.

(5.) A.Y. Coran and R.P. Patel, Chapter 7 of Thermoplastic Elastomers, 3rd Edition, G. Holden, H.R. Kricheldorf and R. P. Quirk, Eds., Hanser Gardner, 2004.

(6.) G.W. Coates and R.M. Waymouth, Science 267, 217 (1995).

(7.) D.J. Arriola, E.M. Carnahan, P.D. Hustad, R.L. Kuhlman, and T.T. Wenzel, Science 312 (2006).

(8.) A. Hotta, E. Cochran, J. Ruokolainen, V. Khanna, G.H. Fredrickson, E.J. Kramer, Y.W. Shin, F. Shimizu, A.E. Cherian, P.D. Hustad, J.M. Rose, and G.W. Coates, Proc. Nat. Acad. Sci. 103, 15,327 (2006).

(9.) B.J. Gedeon, M. Martin and N.L. Yenni, Thermoplastic Elastomers 2000 Conference Proceedings, Society of Plastics Engineers, 157.

(Copyright 2007 ExxonMobil. All rights reserved)

Bryan R. Chapman and David B. Dunaway, ExxonMobil

Chemical

Table 1--PO-TPEs used in this study

           PO-TPE type                       Hard domains    Molecular
                                                             weight
c(P/E)-1   Propylene/ethylene random         Polypropylene   160 kg/mol
           copolymer (18% crystallinity)     crystals

c(P/E)-2   Propylene/ethylene random         Polypropylene   250 kg/mol
           copolymer (4% crystallinity)      crystals

c(E/O)     Ethylene/octane random            Polyethylene    130 kg/mol
           copolymer (6% crystallinity)      crystals

SEES-1     Styrene/ethylene-butene/styrene   Polystyrene     120 kg/mol
           block copolymer (28% styrene)     glass

SEES-1     Styrene/ethylene-butene/styrene   Polystyrene     80 kg/mol
           block copolymer (14% styrene)     glass

           Density

c(P/E)-1   0.88 g/[cm.sup.3]

c(P/E)-2   0.86 g/[cm.sup.3]

c(E/O)     0.87 g/[cm.sup.3]

SEES-1     0.91 g/[cm.sup.3]

SEES-1     0.90 g/[cm.sup.3]

Table 2--comparison of conventional paraffinic process oils with
Elevast (approximate properties)

                           Sunpar 2280      Rexon 815   Paralux 6001
                              (Sunoco)   (ExxonMobil)      (Chevron)

Viscosity (kinematic) @            480            460            120
  40[degrees]C, cSt
Molecular weight, kg/mol           0.8            0.8            0.6
Pour point, [degrees]C             -10            -10            -20
Color
  ASTM                               4              3           <0.5
  Saybolt                         <-30           <-30            +20
Carbon type,
  Paraffinic                        70             70             70
  Aromatic + naphthenic             30             30             30

                           Drakeol 34   Elevast R170
                            (Penreco)   (ExxonMobil)

Viscosity (kinematic) @            75             70
  40[degrees]C, cSt
Molecular weight, kg/mol          0.5            0.7
Pour point, [degrees]C            -10            -50
Color
  ASTM                           <0.5           <0.5
  Saybolt                         +30            +29
Carbon type,
  Paraffinic                       70            100
  Aromatic + naphthenic            30

Table 3--plasticization of styrenic block-copolymer PO-TPE

Composition                      SEBS-1        SEBS-2

  SEBS (wt. %)                  100     64    80   100
  hPP (wt. %)                    --     16    --    --
  Elevast R170 (wt. %)           --     20    20    --
Hardness
  Durometer A                    81     64    55    53
Softening temp.
  Vicat (200 g), [degrees]C     112     93    79    69
Tensile
  100% modulus, MPa             2.8    2.1   1.2   1.2
  Tensile strength, MPa        19.3   13.1   7.3   8.5
  Ultimate elongation, %        630    730   920   870
Thermal
  Tg (EB block), [degrees]C     -45    -53   -52
  Tg (S block), [degrees]C      100     97   100