Shrinkwrap is a material made up of polymer plastic, usually PVC with a mix of polyesters. When heat is applied to this material it decreases in size so that it forms a seal over whatever it was covering. Shrinkwrap is commonly found on CDs, DVDs and software packages, but can also be found as safety sealant on jars and bottles as well as to seal off exposed, soldered joints in electric wiring.
Commercial shrink wrap usually takes the form of a clear film applied to products to protect them from dust, pests, surface contamination and moisture. The shrinkwrap provides a tamper-evident seal that helps ensure freshness and discourage pilfering.
How shrinkwrap works
The molecules of plastic that make up shrinkwrap are in forced into a polymer state during manufacturing. At a microscopic level the material will look like a series of long, thin molecules lined up neatly in rows. Applying heat to these polymer molecules will increase molecular motion (meaning that they will vibrate against each other) and cause them to return to their natural state. The long, thin lines of molecules of the polymer will tangle tightly with each other, decreasing the volume of the material - producing the 'shrink' effect.