The single-site polyolefin story continues to unfold.

Resin companies are stepping up the race to develop improved polyolefins made with new single-site, metallocene catalysts. Last month's Specialty Polyolefins conference in Houston (sponsored by Schotland Business Research Inc., Princeton, N.J.) introduced a new entrant in the race, bringing a major new addition to the plethora of catalyst technologies whose potential is just beginning to be explored. Also revealed at the conference was tantalizing new information on the processing and performance of resins emerging from these technologies--including the first "low-density polypropylene." SPO '93 also gave a clearer picture of what these new materials may cost. (Proceedings are available from the sponsor for 8350.)

The biggest news of the conference was the announcement by U.K.-based BP Chemicals Ltd. that it had successfully used supported metallocene catalysts in its fluid-bed process to make LLDPE in large-scale pilot-plant trials. BP implied this development opened up the vast potential of metallocene technology to its existing gas-phase polyethylene licensees.

Developing a silica support for single-site metallocene catalysts allows them to be used in gas-phase processes. All commercial production of metallocene-catalyst PE in the U.S. (by Dow Plastics and Exxon Chemical) uses an unsupported catalyst in solution. Although Mobil Chemical Co. earlier announced a successful gas-phase metallocene process for PE (thought to use a supported catalyst), it has been practiced only on a pilot-plant scale.

BP's new catalysts reportedly produce PE resins with extraordinary dart impact strength--up to 900 g, or about four times tougher than conventional Ziegler-type LLDPE. According to BP catalyst research manager Peter Williams, the new resins also show "improved MD tear strength and marginally better balance between MD and TD tear strength". He added, "These resins can exhibit improved process-ability, including good bubble stability in film or low sag in blow molding. It's possible to obtain enhanced product performance without increasing extruder die pressure or motor torque."

HEXENE COPOLYMER FROM BP's METALLOCENE CATALYST VS. CONVENTIONAL LLDPE

Catalyst Type        Single-Site   Typical Ziegler-TYPE
FILM PROPERTIES                    A        B
Density(1)           916           918      918
MI, g/10 min         2.3           1        2.1
Dart Drop, g/mil     900           240      180
Tear Str., g/mil
MD                   357           250      210
TD                   473           480      470
Secant Mod., psi
MD                   19,140        21,750   21,750
TD                   23,345        26,100   23,200
Hexane Extract., %   0.8           2        3.5

PROCESSING CONDITIONS(2)
(2:1 BUR at 1.5 mil)

Melt Temp., F        338           356      356
Bubble Stability     Good          Good     Good

1 Unannealed. 2 Kiefel R040, 25:1 L/D; 100-mm die; 2.5-mm die gap

BP Chemicals announced a second new metallocene catalyst development: "multi-site" mixed-catalyst systems, combining metallocene and conventional Ziegler titanium catalysts on the same silica support. These multi-component catalysts could be used by retrofitting existing reactors. BP has four U.S. PE Excensees that could potentially make such a retrofit: Chevron Chemical Co., Quantum Chemical Co., Eastman Chemical Co., and Formosa Plastics Corp.

BP's multi-site mixed catalyst can be tailored with various proportions of either catalyst to produce "bimodal high-strength film grades that are easy to process," Williams said. Furthermore, these bimodal resins, with a variety of possible molecular-weight distributions, are made in a single gas-phase reactor--a big potential cost saving over multi-reactor approaches. And mixed catalysts could also be more economical than all-metallocene processes because they would use less of the high-cost metal-locene and its required cocatalyst.

BP isn't the first to pursue mixed metallocene and Ziegler catalyst. Quantum gave a technical paper at the MetCon conference in Houston in May, which reported the development of supported, hybrid metallocene-Ziegler catalyst to make HDPE and LLDPE. (Quantum is sampling experimental grades of both types, made in a gas-phase pilot plant.) Meanwhile, Union Carbide Chemicals and Plastics Co. has said that its new Unipol II gas-phase technology can use both metallocene and conventional catalysts.

SYNDIOTACTIC LOW-DENSITY PP

Another unique product of metallocene catalysts is syndiotactic PP (sPP). This resin has methyl groups on two sides of the polymer backbone and is therefore inherently less crystalline than standard isotactic PP (iPP), which has pendant methyls on one side only. At the SPO '93 conference, Fina Oil and Chemical Co., Dallas, offered the first details on how sPP processes in both injection molding and cast film extrusion.

Fina's manager of polymer R&D Dr. Edwar Shamshoum said, "Longer cycle time for sPP is inevitable but can be improved to some extent by choosing the proper activity (stereoregularity) level, processing conditions, and nucleators." For example, Fina tested three experimental sPP grades and found that highertacticity grades crystallize faster than lower tacticity ones, as might be expected. Use of nucleating agents also speeds crystallization. But there ends any similarity between sPP and iPP.

Processing conditions for sPP are definitely out of the ordinary. Whereas cooling a mold will normally speed up cycle time and part removal, Fina found that with sPP, mold cooling has the opposite effect. Cooler molds make soft, sticky parts that take longer to demold. Hotter mold temperatures, on the other hand, allow faster part removal, though cycle time is still longer than with iPP.

Slow cooling from the melt also lowers density, compared with fast cooling or quenching. 'This is the first time in which the preparation of low-density PP has been reported in the literature," Shamshoum said. He reported on one low-tacticity grade with a density of 0.8703 g/cc. Conventional iPP homopolymer is in the 0.904).91 density range.

Die swell is also lower for sPP than for iPP. Fina's three sPP grades have die swells of 1.21, 1.13 and 1.26, compared with 2.4-2.6 for iPP. And sPP's narrower MWD causes less shear thinning in the melt than with iPP, which could lead to longer injection boost times.

Impact strength and clarity of sPP are extraordinary. Fina reports notched Izod impact strengths of 12.2 to 14.3 fi-lb/in. for three developmental sPP grades vs. 0.5 ft-lb/in. for iPP. Haze values for injection molded plaques of the three sPP grades were 20-46% vs. 63-77% for iPP.

Cast-film extrusion of sPP also shows odd cooling behavior. When Fina varied chill-roll temperatures from 45 F to 90 F, lower roll temperatures only marginally improved film tensile strength and elongation. But cooler rolls greatly improved dart impact. Using a 45 F chill roll, dart impact was 140 g (for 0.3-mil film), vs. 100 g with a 90 F chill roll.

Higher melt temperatures even more dramatically improved dart impact. Processing one sPP resin at 410 F, for instance, produced film with a dart impact of 75 g. Raising melt temperature to 489 F improved dart impact to 140 g. A hotter melt also reduced backpressure. A melt temperature of 410 F created backpressure in the extruder of 3880 psi. When melt temperature was raised to 489 F, backpressure dropped to 2200 psi. Optics also improved when sheet was made at higher melt temperatures, but there was little effect on tensile and elongation properties.

RESIN COST WILL DROP

How much will these metallocene-catalyst PE and PP resins cost? Consulting firm Chem Systems Inc., Tarrytown, N.Y., predicts metallocene catalysts will come down in cost gradually. Single-site catalyst now adds 46c/lb more than does conventional catalyst to the cost of LLDPE, said Chem Systems technical director Don Bari. By 1997, he expects single-site catalysts to add only 2-3.5c/lb. Then metallocene resins could compete in commodity markets. Except for the catalyst, production costs are comparable to Ziegler processes.

Dr. Norman Brockmeier of Argonne National Laboratory, Argonne, Ill., analyzed what metallocenes will add to the cost of homopolymer PP and impact copolymers. He predicted that catalyst cost would contribute 3.7|cents~/lb to overall PP manufacturing costs, vs. 1.1|cents~/lb for Ziegler catalyst. However, Brockmeier said that premium will drop 30-50% as metallocene production increases. By 2002, he said, metallocenes will make 16% of the world's PP and by 2010 a third of it.

Metallocenes leave different amounts of residues from high-activity Ziegler catalysts, he said. Aluminum residues will be nearly three-fold higher, but chlorine will be far less.