Weather resistance testing of PET composite strapping (polyester plastic steel strap) evaluates how ultraviolet radiation, humidity, and temperature extremes affect the tensile strength, elongation, and surface integrity of this high-strength packaging material. This article explains the applicable international standards, sample preparation requirements, laboratory exposure methods, and acceptance criteria used to verify the long-term outdoor durability of PET strapping in demanding industrial packaging applications.
Technical Overview
PET composite strapping, widely known as PET plastic steel strap or polyester strapping band, is a high-tensile packaging material extruded from polyethylene terephthalate. It combines breaking strength comparable to lightweight steel strapping with advantages such as rust resistance, absence of sharp edges, lower weight, and elastic shock absorption during transit. Because PET strapping is frequently used for outdoor storage, open-yard palletizing, and long ocean shipments, its long-term durability under sunlight, rain, heat, and cold must be verified before it is qualified for demanding applications.
Polyester polymers degrade outdoors through two principal mechanisms. The first is photodegradation: ultraviolet photons in the 290 to 400 nm wavelength range break ester bonds in the polymer backbone, causing chain scission, surface chalking, micro-cracking, and a progressive loss of tensile strength. The second is hydrolysis: combined heat and moisture attack ester linkages and reduce the average molecular weight of the material, which further embrittles the strap. Weather resistance testing accelerates both mechanisms under controlled laboratory conditions so that years of field exposure can be simulated in weeks or months.
The most widely applied test standards include ISO 4892 (Plastics — Methods of exposure to laboratory light sources), ASTM G155 and ASTM G154 for xenon arc and fluorescent UV exposure, ASTM D4329 for accelerated weathering of plastics, and product-level specifications such as ASTM D3950 for nonmetallic strapping and GB/T 22344 for polyester strapping bands. Performance is typically quantified by comparing the tensile breaking strength and elongation of the strap before and after exposure, expressed as a percentage strength retention rate.
Testing Samples
Test specimens are taken from production rolls of finished strapping in the width and thickness to be qualified, for example 16 mm, 19 mm, or 25 mm widths. Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours in accordance with ISO 291 or ASTM D618 before baseline measurement. A statistically valid sample group, typically a minimum of five specimens per exposure interval, is required to reduce scatter in the results. Sample types commonly evaluated include the following:
Natural (Transparent) PET Strapping
Unmodified, translucent PET strap contains no UV stabilizer and serves as the control or baseline material. It is the most sensitive to photodegradation and is generally suitable only for indoor or short-term outdoor use. Testing this grade establishes the reference degradation curve against which stabilized products are compared.
UV-Stabilized PET Strapping
This grade incorporates hindered amine light stabilizers (HALS) and UV absorbers such as benzotriazole or benzophenone compounds during extrusion. These additives intercept free radicals and absorb harmful radiation before it reaches the polymer chains. Weathering tests verify that the stabilizer package delivers the expected service life in outdoor environments.
Black Carbon-Filled PET Strapping
Black strap contains roughly 1.5 to 3 percent carbon black pigment, which functions as a highly effective UV screen by absorbing and converting ultraviolet energy. This grade normally shows the highest strength retention after accelerated exposure and is the preferred choice for extended outdoor storage of bricks, timber, and steel products.
Testing Items
A complete weather resistance evaluation program for PET strapping covers the following measurable parameters:
- Breaking strength before and after exposure, determined by tensile testing in accordance with ASTM D3950 or GB/T 22344 procedures, expressed in kilonewtons.
- Strength retention rate, calculated as the ratio of aged breaking strength to initial breaking strength multiplied by 100 percent, the primary acceptance index.
- Elongation at break and residual elastic recovery, which reveal embrittlement before catastrophic strength loss occurs.
- Visual appearance changes including discoloration, chalking, gloss loss, surface crazing, and macroscopic cracking, assessed visually and with low-magnification microscopy.
- Color difference measurement expressed as delta E, quantified with a colorimeter for colored or black grades.
- Joint or system strength retention, testing friction-welded joints or buckle connections after aging, since the sealing interface often degrades faster than the strap body.
- Intrinsic viscosity or molecular weight analysis, used as an optional diagnostic to quantify the degree of chain scission caused by photolysis and hydrolysis.
Testing Methods
Weather resistance testing combines several accelerated laboratory exposure methods and, for full qualification, natural outdoor weathering to validate the correlation between accelerated results and real service life.
Xenon Arc Lamp Exposure
Xenon arc testing per ISO 4892-2 or ASTM G155 provides the best spectral match to full-spectrum terrestrial sunlight. A typical cycle uses daylight filters with an irradiance of 0.35 W/m² at 340 nm, a black panel temperature of 63 °C, a chamber temperature near 38 °C, 50 percent relative humidity, and periodic water spray such as 102 minutes of light followed by 18 minutes of light plus spray. Common exposure durations range from 500 to 2000 hours, with tensile specimens pulled at intermediate checkpoints to plot the degradation curve.
Fluorescent UV-Condensation Exposure
Fluorescent UV testing per ISO 4892-3 or ASTM G154 uses UVA-340 lamps in a repeating cycle of 8 hours of UV irradiation at 60 °C followed by 4 hours of condensation at 50 °C. This method emphasizes the UV and dew components of weathering, is economical to operate, and is widely used for screening and quality control comparisons between material lots.
Damp Heat and Hydrothermal Aging
Steady-state damp heat exposure at 85 °C and 85 percent relative humidity, or 40 °C and 95 percent relative humidity, per IEC 60068-2-78 or GB/T 2423 standards isolates the hydrolytic degradation mechanism. Duration is typically 500 to 1000 hours. This test is particularly important for strapping destined for ocean containers and humid tropical climates where moisture-driven embrittlement dominates.
Thermal Cycling and Low-Temperature Flexibility
Cycling between minus 40 °C and plus 70 °C per IEC 60068-2-14 or GB/T 2423.22 subjects the strap to repeated thermal shock. Post-exposure bending tests around a mandrel and tensile tests confirm that the aged strap does not crack or embrittle in cold climates, which is essential for winter outdoor use and cold-chain logistics.
Salt Fog Exposure
Neutral salt spray testing per ISO 9227 or ASTM B117 for 48 to 96 hours evaluates performance in marine and coastal atmospheres. While PET itself is corrosion resistant, this test verifies that the strap surface, printed markings, and any metal buckle connections do not suffer accelerated degradation in the presence of chloride-laden moisture.
Natural Outdoor Weathering
Reference specimens mounted on 45-degree south-facing exposure racks per ISO 877, ASTM D1435, or ASTM G7 in benchmark climates such as humid subtropical Florida and dry, high-radiation Arizona provide the correlation baseline. Exposure periods of 6 to 24 months are used to validate the acceleration factor of the laboratory methods and to support long-life product claims.
Applications
Weather-resistant PET strapping qualified through the test program above is used across industries where packaged goods face outdoor storage or harsh transit environments:
- Steel service centers for bundling coils, rebar, tubes, and profiles stored in open yards.
- Brick, block, tile, and precast concrete manufacturers whose palletized products remain outdoors for months.
- Sawmills and timber exporters packaging lumber, boards, and panel packs exposed to sun and rain.
- Cotton gins, fiber processors, and recyclers producing high-density bales stored in the field.
- Export packaging and container lashing where straps endure salt-laden humid air during multi-week ocean voyages.
- Heavy machinery, glass, and ceramics packaging requiring both high strength and safe handling without sharp edges.
In these applications, a verified weathering test report assures end users that the strap will retain sufficient tension and breaking strength throughout the intended storage and shipping window rather than failing prematurely under UV load.
FAQ
How many hours of xenon arc exposure equal one year outdoors?
There is no universal conversion factor because degradation depends on climate, latitude, season, and rack orientation. As a practical guideline, 1000 hours under ISO 4892-2 conditions is often roughly comparable to one to two years of temperate outdoor exposure, but every product qualification should be confirmed against natural weathering data.
What strength retention value is considered a passing result?
Acceptance limits are set by the purchaser or product specification. A common industrial requirement is at least 90 percent breaking strength retention after 500 hours of UV exposure and at least 85 percent after 1000 hours, with no visible surface cracking. UV-stabilized and black carbon-filled grades typically exceed these thresholds.
Why does transparent PET strapping fail quickly outdoors?
Unstabilized PET has no protection against UV photons, which cleave the polymer chains at the strap surface. Within a few months of direct sunlight, the surface chalks, crazes, and cracks, and breaking strength can drop below half of its original value. This is why natural strap is restricted to indoor or short-term use.
Does black PET strapping really resist UV better than colored strap?
Yes. Carbon black absorbs across the UV spectrum and dissipates the energy as negligible heat, shielding the polymer. Black and properly UV-stabilized grades consistently deliver the highest retention rates in accelerated and natural weathering trials.
Can weathered strapping still be used after long storage?
Only after re-evaluation. If chalking, discoloration, or cracking is visible, or if a retest shows breaking strength below the specification limit, the strap should be downgraded or discarded. Periodic retesting of stock rotated slowly through inventory is a recommended quality practice.
Which standards should a weather resistance test report reference?
A complete report normally cites the exposure standard such as ISO 4892-2, ASTM G155, or ASTM G154, the tensile evaluation standard such as ASTM D3950 or GB/T 22344, the conditioning standard ISO 291, and, where applicable, natural weathering standards ISO 877 or ASTM D1435, together with all exposure parameters, durations, and calculated retention rates.