Novel rheological behavior claimed for new-tech polyolefins.

Wide-ranging benefits have been promised from new generations of ethylene copolymers born of so-called "single-site" metallocene catalyst technologies (see PT, Nov. '91, p. 15; Jan. '92, p. 13). The recent SPO '92 conference on specialty polyolefins in Houston aired additional details on performance benefits and also the first inklings of some unusual aspects of these resins' processing behavior. The greatest amount of new information was revealed in two papers by Dow Plastics, Midland, Mich., on its "Constrained Geometry Catalyst Technology," or CGCT for short. (Conference proceedings are available from the meeting's sponsor, Schotland Business Research, Inc., Princeton, N.J.)

VIRTUES OF NARROW MWD 

The most widely heralded characteristic of the new single-site linear polyolefins has been their narrow and precisely tailored molecular-weight distributions. The multiple benefits of narrow MWD were expounded by Dow project manger G.D. Schwank and by Dr. Douglas M. Selman, v.p. of Polymers Technology for Exxon Chemical Co., Houston. Exxon's "Exact" LLDPE's are the first of the new-technology resins to be commercialized in North America (PT, Sept. '92 , p. 13). Dow expects to field its first commercial new-technology resins--octene copolymers of 10-30% comonomer level--next year (see Industry Newsfocus).

Both firms noted the superior flexibility, clarity and cleanliness of these resins, as well as higher toughness and potential for downgauging. Increased flexibility tempts both Dow and Exxon to envision making inroads against flexible PVC in markets such as medical films and tubing, and perhaps even meat wrap.

Thermoplastic rubbers are another target. Dow's Schwank reported data suggesting a role for the new copolymers as replacements for EPR or EPDM in PP-based thermoplastic elastomers, or TPO's (see Technology Newsfocus).

High toughness and low catalyst residues (ash content) suggest advantages in wire and cable--a prime initial target for both Dow and Exxon. Dow also notes that absence of short-chain fragments increases the crosslinking efficiency of these resins.

Clarity, low heat-seal temperatures and high seal strength and toughness will also be advantages in packaging films. Exxon pointed out that absence of low-MW fractions eliminates "smoking" during processing and enhances taste and odor properties. Dow's Schwank reported that 2-mil film based on one resin in its new family has significantly higher gloss, lower haze, and greater puncture resistance and dart impact than similar films of Dow's Attane ULDPE, EVA or ionomer. Elmendorf tear was also higher than EVA or ionomer, but lower than ULDPE. And CGCT resin gave significantly higher seal strength than all the other resins at sealing temperatures down to 194 F.

'REWRITING THE RULES'

The most dramatic new information on Dow's CGCT resins was reported in Houston by R&D director Kurt W. Swogger. He noted that while narrow or "homogeneous" MWD imparts a number of performance benefits, it can also hurt extrudability. Referring to the new-tech resins that have been seen to date, Swogger said, "Unfortunately, such homogeneous polyolefin copolymers have a major processing disadvantage. That is, the narrow MWD greatly reduces melt processability." He compared rheology curves for a standard octene-based LLDPE and a "commercially available homogeneous polymer" (presumably one of Exxon's new Exact resins, the only such products yet on the domestic market). The new "homogeneous" copolymer appears less shear-sensitive than standard "heterogeneous" LLDPE of similar melt index. Thus, the homogeneous polymer has much higher melt viscosity under processing conditions.

This problem was acknowledged by Dr. Selman of Exxon. "Polymers made with single-site catalysts have narrow MWD and no long-chain branches. This makes them more viscous, and consequently, often require more time and energy when going through a processor's equipment." He noted that similar difficulties with flow characteristics were experienced when the first LLDPEs appeared. "The use of improved processing aids or multiple reactors and multiple catalyst systems are possible ways this problem will be economically solved." Multiple reactors or catalysts could be used to broaden the MWD and create bimodal resins such as have proved successful in LLDPE.

Greg McPike, v.p. of Exxon's Exxpol single-site technology venture, says these various approaches, as well as occasional equipment modifications, have succeeded in trials at selected customers without loss of physical properties. "We have been pleasantly surprised," McPike says, "to learn that processors who are experienced in molding and extruding LLDPE can rather quickly accommodate the new Exact resins."

Dow, however, lays claim to unique, patented technology for improving melt processability for without broadening the MWD at all, thus maintaining maximum mechanical properties. "Dow technology severs the link between MWD and processability." Swogger declared. Figure 2 compares a CGCT resin and a homogeneous polymer of similar melt index. The CGCT resin has higher melt viscosity at low-shear conditions--useful in blown film, extrusion coating, and blow molding--while dropping to lower viscosity at high-shear conditions.

Swogger showed that for a range of commercial homogeneous polyolefins, resins, both high-shear flowability and low-shear melt strength increase in linear proportion to the breadth of MWD. Dow's CGCT resins, however, can achieve a wide range of flow and melt strength at the same narrow MWD. Dow spokesmen say they will first reveal the polymer structure that accounts for this unique behavior at the SPE Polyolefins Div. RETEC in Houston next February.

Swogger showed data on two CGCT resins that process very much like its Dowlex LLDPE and Attane ULDPE resins of similar melt index and density. However, the CGCT resins manifest higher melt strength and greater resistance to melt fracture at high shear stress, an advantage for high-throughput processing.

Dow's Schwank further supported these claims with data on blown film co-extrusion runs of different heat-seal resins using the same extruder, rpm and output rate. The specific power (lb/hr/hp) for the CGCT resin was about the same as for Attane ULDPE and much lower than for EVA or ionomer.