Rubber is the classic example of an elastic material. You push on it, it deforms; you let go, it recovers. An elastic material responds to load almost immediately (the load travels through the material at the speed of sound). The amount of deformation is proportional to the load and independent of time.
Molasses is the classic viscous material. When a force is applied to a viscous material, it will flow. The longer the load is on the viscous material, the more it will flow. When the force is removed, it will stop flowing, and the material won’t recover.
When a viscoelastic (V-E) material is loaded, it will respond with a mixture of viscous and elastic behavior. Upon loading, a V-E material will immediately stretch (elastic behavior) and begin to flow (viscous behavior). When the load is removed from a V-E material, it will recover, some immediately (elastic behavior) and some will recover more over time (viscous behavior). Vinyl electrical tape is a classic and easily observed V-E material.
For short times of web handling (seconds, hours), viscoelasticity can be ignored in most materials. For long times of winding (hours, days, months), viscoelasticity can NOT be ignored. CD differences in creep is the cause of most bagginess.
Try this test yourself: Pull out a 2-3 foot length of electrical tape. Hang a 1-2 lb weight on it. Note the initial elongation and that the tape will continue to elongate. Take the weight off. Note the initial recovery and ongoing recovery.
Congratulations, you’ve just completed your first creep test.