PET Strapping in Humid Environments: Test Report

Steel rusts. PP creeps. What happens to PET at 95% RH?

We tested it.

For procurement managers sourcing strapping for export packaging, outdoor storage, or Southeast Asian distribution hubs, humidity is the silent killer of load security. Steel corrodes. Polypropylene absorbs moisture and relaxes. But polyester (PET)?

The data is finally in.

Test Protocol: 168 Hours at 95% RH

Standard: GB/T 22344-2008, GB/T 19817-2005

Sample: 12mm × 0.7mm PET strapping, virgin grade

Condition: 95% relative humidity, 23°C, 168 hours (7 days)

Control: Same batch stored at 50% RH

Key Findings

1. Tensile Strength Retention: 92–94%

After 7 days in near-saturation humidity, PET strapping retained over 92% of its original breaking load .

Compare that to steel: 24 hours in marine air, and rust blooms appear. Compare to PP: moisture absorption triggers creep—straps loosen, pallets shift, claims follow.

Bottom line: Humidity does not soften PET.

2. No Corrosion. No Staining.

Steel leaves orange trails on white goods, paper reels, and furniture. PET leaves nothing.

Microscopic inspection after testing showed zero surface oxidation. Cargo arrives looking exactly as it left—no cleanup, no customer complaints about rust marks .

3. Dimensional Stability: ±0.1%

Swelling is the enemy of automated strapping heads. Wet PP expands, jams feed wheels, and triggers downtime.

Our measurement: After 168 hours at 95% RH, PET width variation remained within ±0.1mm. Thickness variation: under 0.02mm .

Translation: Your automatic machine runs the same Monday morning as Friday afternoon. Rain or shine.

4. Bond Strength: Unaffected

Heat-welded joints were pull-tested post-exposure. Failure mode? Strap break, not seal separation.

Humidity does not attack the molecular structure of the weld interface. If your joint is strong on Day 1, it remains strong on Day 30 .

The "Banana Boat" Case

A Thai fruit exporter shipped fresh produce to Europe in reefer containers. Condensation inside the box—inevitable. Steel straps rusted within 48 hours. PP straps stretched. Pallets arrived collapsed.

They switched to 12mm PET.

Result:

Zero rust complaints

Zero re-strapping at destination

$26,000/year saved in damaged goods and labor

The Science Behind It

Unlike nylon or cellulosic fibers, PET does not contain hygroscopic groups that bond with water molecules . Moisture sits on the surface—it does not penetrate the crystalline structure.

Stress relaxation in high humidity? PET's decay curve follows a power function, not an exponential collapse . Translation: It holds tension. Even when the air is thick enough to drink.

Industry Standard Reference

Qualified PET strapping meets:

Tensile strength: ≥300 N/mm²

Humidity resistance: 95% RH, 168h, strength retention ≥90%

Temperature range: -40°C to +60°C continuous

The Verdict

If your cargo crosses oceans, sits in open yards, or breathes refrigerated air:

Steel is a liability.

PP is a gamble.

PET is the insurance policy that pays for itself.

Need certified PET strapping for your humid lane?

Visit: www.strappackage.com

Email: winnie@adtooo.com