GALVESTON, TEXAS - Twenty-six machinery company officials learned how metallocene catalyst resins will process while making blown and cast film during a Film Machinery Manufacturers Sympo-sium sponsored by Exxon Chemi-cal Co. Marsha Arvedson, head of polyethylene volume products at Exxon, said Exxon will provide limited quantities of its metallocene resins free-of-charge, with no need for secrecy agreements.
Thirty-five Exxon employees attended the symposium, held April 15-16 in Galveston.
Nearly all modern extruders designed for linear low density polyethylene can process Exxon's Exceed metallocene-based LLDPE without difficulty, but the process can be optimized with some adjustments to the screw, air ring and die, according to a presentation by two Exxon officials, Craig Erderly, senior development engineer, and Werner Kurzbuch, consultant.
The company's Exact Plasto-mers are a bit harder to process in monolayer structures, al-though coextrusion presents few problems, they said.
Because they have a narrower molecular weight distribution than conventional LLDPE, metallocene resins are slightly less shear-sensitive. They display a more viscous behavior and greater heat generation when using LLDPE screws, and require the extruder to operate at higher motor torque levels. Metallocene LLDPE resins also have slightly lower melt strength and bubble stability than traditional materials. Metallocenes are easier to draw down to thin gauges from wide die gaps.
For cast film, the new materials can be made 10-15 percent thinner when run through typical die gaps of 20-30 mils, without draw resonance.
The preferred screws for metallocene PE extrusion are dual-channel, low-shear barrier mixing screws in smooth-bore extruders, the Exxon officials said.
Exxon recommends that chilled air be as cool as possible without causing condensation. Higher tower heights, if feasible, also are preferred. Exxon also recommends the lowest possible nip pressure without air loss and, if available, nip cooling.
Arvedson said machinery ad-justments to process metallocene will result in broader improvements, improving the processing of any linear PE copolymers.