Page 18 - The Inside Scoop Archive

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To: Inside Scoop Reader

What You Need To Know About Cling in Stretch Film

Welcome to another edition of the Inside Scoop. The purpose of the Inside Scoop is to provide you with
insights on the packaging industry from a semi-technical perspective that will help you in making good
bottom line decisions for your company. Each week we will explore another subject. This week we are
looking at cling in stretch film; what it is and how you use it.

Cling in stretch film is what makes the two layers of film adhere to each other when they are wrapped
around the pallet. Good cling is essential for pallet wrap because without it the film would unwind and fall
off the pallet during storage or transit. Film tails that come loose and interfere with automated wrapping
systems is also an issue caused by poor cling. All types of additives have been used at one time or another
to attempt to provide good cling in the film. I remember visiting a small stretch film manufacturer back in
the 70's who was mixing wax with his resin in an attempt to make the layers adhere to each other. He did
not stay in business very long and wax in film never caught on.

Cast stretch film is produced with a mirror like finish on the surface as it is extruded over a chill roller
during the manufacturing process which cools the film very quickly. Since both sides of the film have this
mirror like flat surface, cast film has some natural film adherence, or cling, as a result of the process,
particulary for film being stretched less than 100% and to a lesser extend at greater stretch levels. Blown
films on the other hand, have little to zero natural cling because the film cools slowly and the resins used
have less natural cling. Blown stretch film without any tackifier cling additives would be like trying to wrap
a pallet with two layers of can liners. Blown films are easily identifed as they are the ones that are noisy
and sticky.

The most widely used tackifier today is polyisobutylene or as it is commonly known, PIB. PIB is a good
tackifier and is widely used in blown and cast film formulations throughout the industry. The PIB is
injected into the extruder as the resin is melted down and mixed in with the resin melt. When the film is
wound onto the roll the film does not show any sign of tackiness as the PIB is inside the film structure.
Because PIB is not homogeneous with polyethylene, it separates from the structure and "blooms" to the
surface of the film providing the sticky cling on the outside of the stretch film. In many blown film plants,
the PIB enhanced stretch film is placed in a special heated room for a given number of days to allow the PIB
to bloom to the surface of the film. The film cannot be used to wrap pallets until the PIB has fully bloomed.

One of the problems with using PIB in blown stretch film is that it does not know much about direction and
can migrate or "swim" in the structure in different directions. If too much PIB is put into the film, or if
the temperatures in secondary storage are too hot, the PIB can do one or more of several bad things:

l The PIB can move toward the core and make the film closest to the core
very difficult to unwind causing film to be left on the end of the roll.

file://C:\Documents and Settings\William Jackson\Local Settings\Temp\tmp15E.htm 10/7/2006