DUSSELDORF, GERMANY - Plastic resins made with metallocene catalyst technologies require different processing techniques than conventional resins. While resin producers have stated repeatedly that metallocene resins require processing techniques that differ only slightly from conventional resins, William Hellmuth said each metallocene resin requires a distinctive technique.
Hellmuth is senior product manager for blown film systems for Battenfeld Gloucester Eng-ineering Co. Inc. of Gloucester, Mass.
Hellmuth was one of 26 speakers at Metallocenes '96, an international conference on metallocene catalyst technologies. He based his comments on information developed at Battenfeld Gloucester's Technical Center and from resin producers.
``The loose term `metallocenes' is not sufficient for machinery suppliers,'' Hellmuth said. ``It is like saying, `I am going to run polyethylene, polypropylene, barrier resins and seal resins in my structure.'
``It is important to be as explicit as possible when specifying resins to be run on extrusion equipment.''
In discussing metallocene-based linear low density PE, Hellmuth pointed out that a molecular weight distribution narrower than conventional resins could mean an extruder would operate at higher temperatures and motor torque levels, while decreasing bubble stability and easing tensions on winding and draw down.
A metallocene resin with less chain branching would result in faster melt relaxation and less draw resonance. One with lower density would have greater elasticity, decreased specific rate in a grooved feed machine and in-creased specific rate in a smooth-bore machine, while being harder to wind.
While processors using LLDPE are likely to have extruders with drive lines and gears designed for increased motor torque, Hell-muth noted that processors using conventional LDPE will have difficulty using LLDPE resins.
``The issue of increased motor torque depends on one's operational base line. In areas where LDPE is still the prominent commodity resin, these transition issues may become bigger than life,'' Hellmuth said.
Also, he noted that processors that use smooth-bore extruders can expect to see increased output when they use metallocene resins, while processors using grooved-bore extruders may see a decrease.
``Plastomer pellets tend to be softer and tackier than standard LLDPEs. Soft pellets cannot withstand the shearing forces developed [in a grooved-bore extruder] and therefore, are not as efficient in conveyance [through the extruder]. This is why the specific rates drop as pellets get softer,'' Hellmuth said.
Higher melt viscosity becomes higher flow resistance in a die, and Hellmuth said die designers must increase spiral clearances and maintain a wide gap to reduce pressure and melt fracture. Web handling also must be adjusted with metallocene resins, especially with films made thinner because of down gauging allowed by stronger, tougher resins, Hellmuth said.
``Since metallocenes behave like pallet stretch films, the most successful winders should be speed-controlled center/surface drive winders. The complication that plastomers bring is the sensitivity to straight surface winding,'' he said.
``These films prefer low lay-on pressure or even gap winding to avoid the air entrapment that causes winding ridges, knots or bruises. Turret winders offer the flexibility to handle these requirements.''
Hellmuth also said high winding tension should be avoided because the high elastic recovery properties of metallocene resins can cause reel hardness.