Uniform quenching and pinning seems easy enough until you change polymers. As I said in Part 1, not all polymer melts will stick to a cool, chrome roll surface, or to other roll-surface metals or materials. In some instances, such as OPET, electrostatic pinning is sufficient to have the melt adhere to the chill roll and give an essentially amorphous sheet with cold, chromed surfaces. Note that the PET sheet is amorphous as this will be an important part of the understanding that comes from differences in quenching polypropylene and high-density polyethylene, which we will discuss now.
Pinning of polypropylene melts is relatively easy and is usually done with an air-knife pinning system, which does not supply a great deal of force to the melt curtain. Indeed, with OPP, it is common to have what would be considered poor pinning in the OPET manufacturing. Figure 6 shows a chill roll surface of a cast, air-knife pinned polypropylene sheet, which displays considerable air leakage past the pinning air knife. This would be unacceptable in OPET production. The difference is that OPET films are oftentimes sold for surface quality while OPP-packaging films are sold for area coverage. In actual fact, the OPP film surfaces often are molten in the tenter oven and much of the surface defects from the casting and MD stretching are essentially “erased,” yielding an acceptable surface for printing, heat sealing and laminations.
How hard can this be?
Now the fun and learning begin. One day at work, it was decided that we would produce biaxially oriented HDPE to see if it could replace OPP based on an assumed better and less expensive moisture barrier. How hard could it be, right? The extrusion was easy, and the die and feedblock were good enough to get started, so off we went. But then the HDPE would not quench; in fact, it just fell off the bottom of the cast roll. It was not being pinned. Electrostatic pinning worked, but for a company with a penchant against no water bath, it was a non-starter.
Coextruding copolymer PP on the surface helped, but the bare edges would not quench uniformly and would not work in the clips, causing all sorts of operability problems. You could not make the cast roll cold enough to quench the HDPE. The cast roll was finally abandoned and a three-roll stack was used to “carry” the 3-layer coextruded melt with copolymer PP skins around the roll and press the edges. This allowed some success with a low MD 1.2X stretch ratio and high TD 9X stretch ratio . But it was not possible to make a 100% HDPE film due to low- or no-melt adhesion to the cast roll.
1. US Patent 5725962, Bader et. al., “Oriented HDPE Films With Metallized Skin Layer," March 10, 1998.