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Suppose you just bought a brand spanking new copy of Titanic, or maybe a classic like Casablanca or Chinatown.  Once youíre home, settled in, bowl of popcorn at the ready, lights turned down low, you try to open the package.  And you try and try...  Much to your surprise the packaging appears to be adult-proof!  Only with the aid of a knife, or someone with more patience than you, do you get that package open.  If itís not a video cassette tape wrapping that has bedeviled you, then maybe it was the wrapping on a CD, a bottle of medicine, a jug of mustard, a jar of pasta sauce, or a box of any size and shape from any store on this planet.

Somewhere along the way youíve encountered shrink wrap plastic.  Okay, Iíve shown the bad side of shrink wrap, but it really is a good thing to have.  Not only will its attributes be pointed out, Iíll even explain how it shrinks!

Polymers: Au naturel

First we need to know a little bit about polymers in their natural state.  Polymers, as you know, are really long molecules. Pretend that one molecule is a string of yarn about 37 feet long.  Now let's give that ball of yarn to a little kitty cat. Heck, give Ďem two more lengths of yarn.  What the yarn looks like when kitty is through is how the individual polymer molecules in plastic look -- randomly oriented and tangled:

Shrink wrap plastic before it is shrunk looks different.  A thin film of a plastic, say poly(vinyl chloride) (PVC) or a polyolefin such as polyethylene, is oriented such that the polymer chains are stretched out:

Our rambunctious kitty never manages to roll the yarn back into a ball or stretch it out completely.  Statistics (and kittyís nature) dictate that the yarn will be in disarray.  The same goes for polymers:  the stretched out picture above is not statistically favorable and Mother Nature doesn't like it, but we force it into this awkward position anyway.  Notice how the oriented polymer appears larger, more extended.

Our extended, oriented polymer film can now be made into a bag or wrapped around a Christmas basket.  Then add a little heat and the film shrinks into place:

During orientation, the polymer is locked, or frozen, into its elongated state.  Excess energy increases molecular motion (wouldn't you move if a fire were placed under your backside?).  Now the elongated polymer recoils, or shrinks, back to itís preferred random and disordered conformation.

PVC bags can shrink 50% from their oriented size.  Their tensile strength also increases.  In addition to PVC, various polyesters are used for shrink wrap.  For large scale applications, such as wrapping boats or enclosing construction sites, a copolymer of ethylene and vinyl acetate (EVA) may be used.  This polymer retains some elasticity after shrinking, doesnít become brittle in cold weather, and is puncture and flame resistant.

Shrinking Benefits

What good is shrink wrap to you?  Hereís a brief list:  tamper resistance, protection from light, abrasion protection (no scratches on those jewel cases), translucence (no "pot luck" DVD selecting), and finally, Christmas baskets that can be made right in your own home and Aunt Greta will never know.  Oh, and by the way, was that video new, or was it a used copy shrink wrapped to look like a new copy?


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Copyright ©1998 | Department of Polymer Science | University of Southern Mississippi