Plastic strapping elongation recovery and retained tension

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.

For example:

• The rack testing method is designed to eliminate any variables. Plastic straps, each cut to 10" samples, were pulled on a rigid test rack to the upper levels of the working ranges:
  • - 300 pounds for polyester strapping
  • - 800 pounds for high-strength polyester strapping
  • - 200 pounds for polypropylene strapping.
• After reaching tensions at the upper levels of their working ranges, they were measured for length and left under tension for five days. The samples were then removed and allowed to "rest" for five days, and then again measured for length.
• The data in the graphs has been designed so that the 10-inch samples are treated as if they were 10 feet long. The latter measure offers an appropriate compromise between the 3-foot length of the strapping that might go around a small package and the 25-foot strap that might go around a large unit-load.

Retained Tension of Plastic Strapping

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:

• 700 pounds for steel strapping
• 300 pounds for polyester strapping
• 200 pounds for polypropylene strapping

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:

• Steel strapping - retained approximately 95% of tension
• Polyester strapping - retained approximately 70% of tension
• Polypropylene strapping - retained approximately 25% of tension

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:

• How much elongation recovery is required? (This is most important on loads that will shrink.)
• How much tension can be pulled on the strapping material without damaging the product?
• How much retained tension (creep) is allowed?

Keep in mind the following:

• Polyester strapping elongation recovery will be approximately 70%.
• Polypropylene strapping elongation recovery is 20 to 40% depending on the Initial Retained Tension.
• Where a modest amount of elongation recovery will suffice, and the package can take about 300 lbs of tension, polyester would be a more economical choice.
• If the load requires all of the elongation recovery achievable at about 200lbs, polypropylene strapping ranks first in choice, with polyester strapping second.
• Humidity has no effect on the elongation recovery of polyester strapping or polypropylene strapping.
• Temperature has an adverse effect on all plastic strapping materials, but significantly more on polypropylene strapping.