Determination of the Brittleness Temperature of PET Strapping Band

PET strapping band, widely used as a high-strength alternative to steel strap, must retain adequate flexibility and impact resistance when packages are exposed to cold environments during winter transport, outdoor storage, and cold-chain logistics. Brittleness temperature determination measures the lowest temperature at which PET strapping can withstand impact without cracking or fracturing, providing critical data for product qualification and quality control. This article explains the testing principles, applicable standards such as ASTM D746, ISO 974, and GB/T 5470, specimen requirements, test procedures, and typical applications of brittleness temperature testing for PET strapping band.

Technical Overview

PET strapping band, also known as polyester strapping or plastic steel band, is a high-tensile-strength packaging material extruded from polyethylene terephthalate resin. It combines the breaking strength and load-retention performance of steel strapping with the safety, rust resistance, and handling convenience of plastics, making it the preferred choice for securing steel coils, timber, bricks, machinery, and export cargoes. However, like all thermoplastic materials, PET progressively loses molecular mobility as the ambient temperature decreases, and below a critical point the strapping transitions from ductile to brittle behavior. In the brittle state, even a modest impact from a dropped coil, forklift jolt, or rail shunting shock can cause the strap to snap, resulting in load collapse and cargo damage.

The brittleness temperature is defined statistically as the temperature at which 50 percent of test specimens fail by cracking or complete fracture when struck under specified flexural impact conditions. Determining this temperature allows manufacturers, users, and third-party laboratories to verify that PET strapping will perform safely throughout its intended service temperature range. The test is performed in accordance with internationally recognized standards, most commonly ASTM D746 (Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact), ISO 974 (Plastics — Determination of the Brittleness Temperature by Impact), and GB/T 5470 (the corresponding Chinese national standard).

Testing Standards

• ASTM D746: Determines brittleness temperature of plastics by impact, using a chilled liquid bath and a pendulum-type striking arm.
• ISO 974: International standard for determination of brittleness temperature by impact, technically equivalent in principle to ASTM D746.
• GB/T 5470: Chinese national standard for plastics — determination of brittleness temperature by impact, widely referenced for domestic PET strapping certification.
• ASTM D618: Practice for conditioning plastics for testing, applied to specimen preparation before the brittleness test.

Testing Samples

Virgin PET Strapping

Strapping produced entirely from virgin polyethylene terephthalate resin typically exhibits a well-controlled molecular weight, crystallinity, and degree of axial orientation. Testing of virgin grades establishes the baseline brittleness temperature and confirms compliance with product datasheets, commonly targeting acceptable impact behavior at temperatures of -30 °C to -45 °C depending on the specification grade.

Recycled PET (rPET) Strapping

Strapping manufactured from recycled bottle flake or regranulated PET may contain residual moisture, contamination, or degraded molecular chains from hydrolysis, all of which can reduce low-temperature toughness. Brittleness temperature testing is particularly important for recycled-content strapping to confirm that cost savings do not compromise cold-weather performance.

Modified and Copolymer PET Strapping

Grades modified with impact modifiers, plasticizers, nucleating agents, or copolymer comonomers are engineered specifically for enhanced ductility at low temperature. Testing quantifies the effectiveness of these formulations and supports new product development and competitive benchmarking.

Specimen Preparation and Quantity

Specimens are machined or die-cut from the strapping along its length. Typical specimen dimensions are 6 ± 0.25 mm in width and 20 ± 0.25 mm in length, with a thickness of up to 2 mm, which accommodates most commercial PET strapping (0.5 mm to 1.5 mm thick, 12 mm to 32 mm wide). Edges must be smooth, free of nicks and burrs, because PET is notch-sensitive and surface defects bias results toward premature brittle failure. For a full statistical determination of the brittleness temperature, at least 20 to 30 specimens are recommended; specimens should be conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 24 hours, or 40 hours in accordance with ASTM D618, before testing.

Testing Items

• Brittleness temperature (T50) determination by impact method — the temperature at which 50 % of specimens fail.
• Single-point compliance test at a specified temperature (for example, no failures among specimens tested at -40 °C).
• Supplementary low-temperature tensile strength and elongation testing of the strapping.
• Low-temperature retention testing of seal or weld joint strength, where the strap joint is the critical weak point.
• Comparative evaluation between production batches, suppliers, or virgin versus recycled formulations.

Testing Methods

Test Principle

Specimens are clamped in a specimen holder and fully immersed in a temperature-controlled liquid bath, typically anhydrous ethanol or methanol chilled by dry ice or a liquid-nitrogen heat exchanger. After the specimens reach thermal equilibrium, a spring-driven or motor-driven striking arm with a defined edge radius impacts the specimens at a controlled velocity of approximately 2.0 ± 0.2 m/s while they are still immersed at the test temperature. The specimens are then removed, allowed to warm to room temperature, and inspected by bending for visible cracks, fissures, or complete fracture. Any visible crack is recorded as a failure.

Apparatus and Conditions

• Multi-station brittleness temperature tester with specimen holder and striking arm (edge radius 1.6 ± 0.1 mm).
• Temperature-controlled bath medium with uniformity maintained within ±0.5 °C, monitored by a calibrated thermocouple or thermometer with 0.1 °C resolution.
• Immersion (equilibration) time of 3.0 ± 0.5 minutes at each test temperature for specimens not exceeding 2 mm thickness.
• Conditioned laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for specimen handling before and after exposure.

Test Procedure

• Verify specimen dimensions, freedom from edge defects, and completion of conditioning.
• Set the bath to the first target temperature and allow the medium to stabilize.
• Mount five specimens in the holder, immerse for the specified equilibrium time, and trigger the impact while the specimens remain submerged.
• Remove the specimens, restore them to room temperature, flex each one, and record pass or fail based on crack presence.
• Repeat the sequence at additional temperatures selected so that failure fractions span from 0 % to 100 % across a span of roughly 10 °C, typically using at least four temperature points.

Data Processing and Expression of Results

The brittleness temperature is derived from the failure fractions observed at each temperature. The 50 % failure temperature is calculated either graphically by plotting failure percentage against temperature or arithmetically using the summation formula specified in the applicable standard. The result is reported as the brittleness temperature (T50) in degrees Celsius, together with the test standard used, specimen dimensions, number of specimens, temperature increment, and failure criterion. For single-point qualification tests, the report states the test temperature, number of specimens, and number of failures; zero failures at the specified temperature constitutes a pass.

Factors Influencing Accuracy

• Edge quality and notch sensitivity of specimens — poor cutting is the most common source of premature failure.
• Insufficient immersion time leading to incomplete thermal equilibration at the test temperature.
• Bath temperature gradients or calibration drift of the temperature sensor.
• Variation in striking velocity, clamp torque, or clearance between the striking edge and specimen holder.
• Material variables including crystallinity, degree of orientation, residual moisture, and production-line heat-treatment history.

Applications

• Quality control in PET strapping plants for routine batch release and process drift monitoring.
• Research and development of low-temperature grades, impact-modified formulations, and recycled-content compounds.
• Supplier qualification and incoming inspection by packaging users in steel, timber, textile, and building-material industries.
• Compliance verification for export shipments and winter logistics to cold-climate regions.
• Cold-chain and refrigerated-warehouse packaging qualification where straps are exposed to sustained sub-zero temperatures.
• Failure analysis of strapping breakage incidents during transport, handling, or storage.

FAQ

What brittleness temperature should PET strapping achieve?

Requirements depend on the end use. General-purpose grades are commonly qualified for no brittle failure at approximately -30 °C, while winter-grade and cold-chain strapping is typically specified at -40 °C to -45 °C or lower. The applicable value should always be agreed in the product specification or purchase contract.

Which standard should I choose: ASTM D746, ISO 974, or GB/T 5470?

The three standards share the same impact-based principle and yield comparable results. ASTM D746 and ISO 974 are preferred for international trade, while GB/T 5470 is commonly requested for the Chinese domestic market. Confirm the required standard with your customer or certification body before testing.

How much strapping sample must be submitted?

A length of at least 1 meter of the strapping is generally sufficient for a full T50 determination, providing more than 25 specimens plus spares. For single-point pass or fail testing, a shorter length may be acceptable.

Can the test be performed at only one temperature?

Yes. When the specification simply requires no failures at a defined temperature, for example -40 °C, a fixed-temperature screen with typically ten specimens is faster and more economical than a full multi-temperature determination of T50.

How does recycled content affect brittleness temperature?

Recycled PET often carries lower intrinsic viscosity, higher moisture history, and minor contamination, which can raise the brittleness temperature by several degrees. Careful drying, filtration, and formulation control in production usually keep rPET strapping within acceptable limits, but regular testing is recommended.

Is the brittleness temperature the same as the minimum service temperature?

No. The brittleness temperature is a laboratory statistical index of impact toughness at 50 % failure. In practice, engineers apply a safety margin so that the lowest expected service temperature remains comfortably above the measured brittleness temperature, particularly where shock loading is anticipated.

Conclusion

Brittleness temperature determination is a decisive quality gate for PET strapping band intended for demanding packaging and transport applications. Executed in accordance with ASTM D746, ISO 974, or GB/T 5470, the test quantifies the transition from ductile to brittle behavior, enabling manufacturers and users to match strapping grades to real-world thermal exposure. Careful specimen preparation, strict control of bath temperature and impact conditions, and statistically sound data evaluation together ensure that reported brittleness temperatures are accurate, reproducible, and suitable for certification, supplier approval, and safe cold-weather deployment of PET strapping.