Vibration, handling, time, temperature and humidity all cause some types of packages and loads to shrink or settle.
All plastic strapping materials elongate under tension and recover a portion of the elongation as stress is relieved. The latter quality - the elongation recovery of the plastic strapping - enables the straps to contract when the package shrinks. Steel strapping, on the other hand, elongates a small amount, but has 100% recovery below the yield point.
Plastic strapping materials will vary in their capacity to elongate and recover.
Loose straps provide little or no protection to a load. For this reason, strapping is applied under tension, and thus gives importance to the ability of the strapping material to retain that tension. The ability to retain tension differs with each type of strapping. Retaining applied tension to strapping material can be measured by pulling the strapping material to the desired tension on a rigid rack. This testing method ensures that the changes that occur are only from the changes within the strap itself.
The graph below illustrates how much tension was retained over a five day span after each strapping material had been applied at the upper level of its working range. This broke down to:
Loss of tension of plastic strapping is the result of the creep characteristic. Creep refers to the tendency to elongate under constant load. The elongation under constant load is referred to as tension decay.
The results of the test:
At 72F the majority of tension loss will occur within the first four hours after application on a rigid load. After the four hour window, tension decay is minimal. Heat will accentuate tension decay of plastic strapping. The tension losses are substantially increased at 140F. Humidity has no effect on the tension characteristics of steel, polyester or polypropylene strapping.
Therefore, for maximum retained tension on a rigid load, steel strapping is the choice, followed by polyester. If a limited amount of retained tension provides sufficient protection, such as on lightweight packages, polypropylene should be adequate.
Before choosing among the plastic strapping on the basis of elongation recovery, three questions must be asked:
Keep in mind the following: