'Polyolefin elastomers' bow in.
Dow is commercializing seven initial Engage products in North America and Europe. They're intended for both crosslinked and thermoplastic use, and also for polyolefin impact modification. These materials will compete with flexible PVC, EVA, EMA, EEA, EPDM, EPR, and thermoplastic styrene block copolymers (SBCs). Not only do these new materials offer some superior performance properties, Dow says, but they "break the rules" of processability/performance tradeoffs experienced with conventional elastomers.
Like its Affinity plastomers, Dow's new POEs are all saturated ethylene octene copolymers, but POEs are distinguished by higher octene contents (over 20%), lower densities, and more rubbery properties. They also have a unique morphology that has just been experimentally verified for the first time.
As shown in the accompanying schematic, POEs consist of crystalline "hard blocks" surrounded by a "sea" of amorphous matter. This can be seen as one end of a continuum of polyolefin morphology. At the other end of the continuum are standard polyolefins, such as Dowlex LLDPE, consisting mainly of an ordered, "lamellar" crystalline structure of overlapping plates (lamellae). Dowlex LLDPE contains up to 10% octene and has relatively low amorphous content.
In Affinity plastomers with 10-20% octene comonomer, the flat crystalline lamellae are interspersed with amorphous domains, which accounts for plastomers' partly rubbery nature.
When it comes to POEs, even higher octene amorphous content yields a structure Dow calls "fringed micelles." Here, the crystalline segments of adjacent POE molecules clump together into a rigid domain surrounded by a "fringe" of amorphous side chains.
Dow's Engage family spans a density range from under 0.864 g/cc to 0.880 g/cc and Mooney viscosities (in standard rubber terminology) from under 5 to 35. Meanwhile, Exxon says it has produced Exact plastomers (some of them developmental) overlapping the Engage POE range--0.865-0.915 density and 520 Mooney viscosity. According to Fred Steininger, director of sales development for Exact, Exxon refers to the entire continuum of copolymers produced by its Exxpol metallocene technology as Exact plastomers, even those that will be marketed for many of the same applications as Dow's POEs.
The first seven grades in Dow's Engage POE family are the following:
* CL 8001 and CL 8002 are for cross linked low-voltage cable insulation. Both have 75 Shore A hardness and density around 0.87 g/cc. Mooney viscosities are 35 and 23, respectively, and melt indexes are 1 and 0.5 g/10 min. Dow says it has already received one Underwriters Laboratories approval for its POEs in wire and cable.
* EG 8100 is a general-purpose elastomer for crosslinked molded and extruded goods (e.g., gaskets, hoses, seals, belting, and electrical connections), as well as TPO impact modification. Shore hardness is 75A, density is 0.87, Mooney viscosity is 23, and M.I. is 1.
* EG 8150 is another g-p elastomer for much the same applications, though it provides even better impact modification. It has similar density and hardness; Mooney viscosity is 35; M.I. is 0.5.
* EG 8200 is a g-p elastomer for high-flow injection molding of crosslinked products, as well as polyolefin impact modification. It can also be used as a binder resin in additive dispersions. It has the same hardness and density as the preceding two grades, but M.I. is 5.
* EP 8500 is aimed at thermoplastic extrusion of clear tubing (e.g., medical and corrugated) and profiles such as appliance gaskets. It has 0.87 density, 5 M.I., and 75A hardness.
* LG 8005, presently available only in Europe, is for thermoplastic calendered fabric coatings. It has 0.87 density, 1 M.I., and 75A hardness.
These new products sell in a range of 80|cents~ to $1.50/lb.
WHAT THEY OFFER
For rubber applications, Engage POEs are said to offer advantages of free-flowing pellet form for convenient handling and faster mixing than baled materials. POEs are also reportedly compatible with typical elastomer compounding ingredients--oils and fillers--and accept high filler loadings easily. Their narrow molecular-weight and comonomer distributions (characteristic of all metallocene-catalyst polyolefins) are said to confer highly efficient crosslinking via peroxide, silane or radiation--but not sulfur as yet--and more consistent cured properties. Cured properties of POEs are very similar to those of EPDM, in terms of mechanical and electrical properties and resistance to chemicals and ozone. When it comes to heat aging, however, POEs show significantly better retention of properties than either EPDM or EPR, Dow says. The same goes for uv weathering resistance. POEs also reportedly show lower hot compression set than EPDM.
Processability is said to be another outstanding trait of POEs. In conventional elastomers, where typical Mooney viscosities typically range from 20 to 90, low Mooneys tend to indicate good processability but poor mechanical properties, and vice versa. POE's, however, "break the rules." With Mooneys of less than 5 to 35, POEs can process like a low-Mooney elastomer but perform like a higher Mooney material, Dow reports.
For thermoplastic applications, uncured POEs offer densities 10-20% lower than EVA and SBCs and 30% lower than flexible PVC. Versus EVA, POEs reportedly have better optics, thermal stability, and crack resistance. Unlike SBS, POEs have good weatherability. And compared with F-PVC, POEs boast similar clarity and gloss, but higher toughness and ductility at considerably lower temperatures. (Brittleness temperature for POEs is below -76 C.) Unlike PVC, POEs offer improved low-temperature flexibility at a broad range of Shore A hardnesses. Also unlike PVC, POEs exhibit good shear thinning and high thermal stability--both advantages for high-speed extrusion or molding. And POEs don't need plasticizers (or may use lesser amounts), thereby providing longer life.
The foregoing advantages, plus the ability to make POEs flame retardant without the use of corrosive halogens, are reasons why Dow plans future POE grades specifically to compete with PVC in building wire.
POEs are gamma-radiation sterilizable for medical applications. They are also extremely soft--under 2000 psi flexural modulus in some cases, which is well below Dow's Attane ULDPE--opening up applications in calendered goods such as coated fabrics for tablecloths (an area now being explored in Europe).
For impact modification, Engage POEs are said to offer significant advantages over EPDM and EPR. One key reason, Dow claims, is the superior dispersibility of POEs in PP, resulting in smaller, more uniform rubber domains in the PP matrix. This reportedly produces higher knit-line strength--better even than in-reactor TPOs. Dow says POEs also can provide better combinations of stiffness and toughness in a TPO than can be obtained with EPDM. Dow says POEs eliminate another previously unavoidable tradeoff: they permit using higher flow grades of PP to obtain easier molding with no loss of toughness.
Although most impact-modification work so far has been with uncured POEs, Dow is confident that these new resins have potential in PP-based thermoplastic vulcanizates as well. Soft touch handles and grips are applications of this type.
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|Author:||Naitove, Matthew H.|
|Date:||Mar 1, 1994|
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