HOUSTON - Making stretch and specialty films from metallocene-based resins requires changes to processing equipment, but those changes apparently can be made easily and without great expense, according to two film makers. Jarred Tunison, director of product development for Roplast Industries Inc., and John Cook, technical manager for polyethylene for Huntsman Packaging Corp., spoke at the SPO '96 conference Sept. 25 in Houston.
Both Tunison and Cook noted that their companies had to adjust line speed to run metallocene linear low density polyethylene resins, and Tunison said nip pres-sures had to be increased slightly.
He said downstream equipment had to be adjusted to minimize defects and accommodate the softer films the materials produce.
Huntsman of Salt Lake City used metallocene LLDPE to make stretch films, Cook said. Huntsman experimented with blends of conventional and metallocene LLDPE, with metallocene resins making up 25, 50 and 75 percent of the content.
``Films with predominant [metallocene] LLDPE resin composition are overengineered,'' Cook said, noting that data showed the optimum performance of a blend for a film was 25-50 percent metallocene LLDPE.
Huntsman's performance criteria included puncture resistance, tear resistance, load holding force, stretch and resistance to restretching, and processability.
``Metallocene catalyzed LLDPE resins offer stretch film producers significantly higher load containment and significantly better puncture resistance,'' Cook said.
Roplast of Oroville, Calif., used low density polyolefin plastomer resins from Dow Chemical Co. of Midland, Mich., to produce monolayer blown films for specialty applications. In the first application, the company's goal was to maximize a package's oxygen transmission rate. In the second, the goal was to use blown film to reproduce a thick and highly elastic bag that was made through casting or injection molding.
Roplast succeeded in both applications, Tunison said.
In maximizing the oxygen transmission rate, the company found a practical limit for films, but he said it continues to seek higher oxygen transmission rates by using membrane patches whose functions can be controlled. The patches use different technologies.
Tunison said Roplast found that a blend of metallocene plastomer and LLDPE resins blocked severely at high processing temperatures, and became tacky at temperatures similar to those found in trucks and warehouses. Adjustments to machinery solved the blocking problems, he said.
Roplast was able to make the thick, elastic blown film bag with blends of 70 or 90 percent of a metallocene elastomer combined with LDPE. The 90/10 blend produced a blown film with a 9-mil thickness, and the 70/30 blend produced an 8-mil thickness, Tunison said.
Down stream, the oscillator on the turn bars, the haul off, cooling water and nip pressure had to be adjusted. Tunison said a rubber nip roller could be added to improve film quality.