Combustion Characteristics Analysis of PET Plastic Steel Strapping: Flammability Testing, Thermal De

PET plastic steel strapping is a high-strength polyester packaging material whose combustion characteristics directly influence warehouse fire safety, transport risk assessment, and regulatory compliance. This article presents a systematic analysis of the ignition temperature, limiting oxygen index, burning behavior, heat release, and smoke emission of PET strapping based on internationally recognized test methods. The findings help packaging engineers, fire safety managers, and logistics professionals evaluate fire risk and select the appropriate strapping grade for each application.

Technical Overview

PET plastic steel strapping, also known as polyester strapping band, is an extruded packaging material produced from polyethylene terephthalate (PET) resin. Because its breaking strength approaches that of steel strapping at equivalent cross-sections, it is widely used to secure palletized goods, cotton and fiber bales, bricks, timber, paper rolls, and heavy industrial products. As an organic polymer material, PET is combustible, and a quantitative understanding of its combustion characteristics is essential for fire load calculation, storage design, transport safety evaluation, and material selection.

The combustion performance of PET strapping is governed by the intrinsic chemistry of the polyester molecular chain. When exposed to a heat source, the material passes through several distinct stages:

• Softening stage (80–230 °C): the strap gradually loses rigidity and begins to relax under tension, reducing load-securing force well before any flaming occurs.
• Melting stage (250–260 °C): the crystalline phase melts completely; the strap can no longer maintain tension and molten polymer begins to flow.
• Thermal decomposition stage (350–400 °C): ester bonds in the polymer chain cleave, releasing combustible volatile fragments such as acetaldehyde, benzoic acid, and terephthalic acid derivatives.
• Ignition and flaming combustion (approximately 370–500 °C depending on ignition mode): the volatiles ignite, producing a yellow flame with a blue base, dense black soot, and melt dripping.
• Char formation: the aromatic ring structures in PET promote partial carbonization, leaving a carbonaceous residue that can slightly inhibit further burning.

Key indicative parameters for unmodified PET strapping include a limiting oxygen index (LOI) of 20–22%, a flash ignition temperature of approximately 360–390 °C, a self-ignition temperature of approximately 450–500 °C, and a net heat of combustion of roughly 21–23 MJ/kg. These values indicate that PET is moderately flammable: less flammable than polypropylene (PP) strapping, which has an LOI of only 17–18% and a heat of combustion near 43 MJ/kg, but far more combustible than steel strapping, which is non-combustible.

Testing Samples

Standard Virgin PET Strapping (Smooth Plane Type)

Extruded from virgin bottle-grade or strap-grade PET chips with an intrinsic viscosity of 0.80–0.85 dl/g. Typical dimensions are 16 mm × 0.8 mm or 19 mm × 1.0 mm, density 1.38–1.40 g/cm³, with breaking strengths of 4,000–8,000 N depending on cross-section. This sample represents the mainstream commercial product and serves as the baseline for combustion characterization.

Embossed PET Strapping

The embossed surface pattern reduces the effective load-bearing cross-section by approximately 5–10% while increasing the exposed surface area. The larger surface-to-volume ratio slightly accelerates radiant heat absorption and shortens the time to ignition compared with a smooth strap of equal mass.

Recycled-Content (Regenerated) PET Strapping

Produced from washed bottle flake or regenerated pellets with a lower intrinsic viscosity of 0.62–0.75 dl/g. Processing history, trace impurities, and residual moisture accelerate degradation, typically lowering the thermal decomposition onset temperature by 10–20 °C compared with virgin material and marginally increasing smoke yield.

Flame-Retardant Modified PET Strapping

Compounded with phosphorus-based, intumescent, or halogen-free flame retardant systems for applications with explicit flame-safety requirements, such as underground coal mines, petrochemical facilities, and certified export packaging. Target performance is an LOI of 26–30% or higher and a UL 94 V-0 classification.

Testing Items

A complete combustion characterization program for PET strapping covers the following items:

• Limiting oxygen index (LOI)
• Flash ignition temperature and self-ignition (autoignition) temperature
• Horizontal burning rate, after-flame time, and after-glow time (UL 94 HB)
• Vertical burning classification and flaming droplet behavior (UL 94 V-2 / V-0)
• Time to ignition and critical heat flux under piloted ignition
• Peak heat release rate (pHRR) and total heat release (THR)
• Mass loss rate, effective heat of combustion, and char residue
• Smoke production rate, total smoke production, and specific optical density
• Qualitative and quantitative identification of combustion gases, including carbon monoxide, carbon dioxide, aldehydes, and aromatic compounds
• Melt dripping and flaming droplet behavior as a secondary ignition risk indicator

Testing Methods

Limiting Oxygen Index (LOI) — ISO 4589-2 / ASTM D2863

Strap specimens are vertically supported and ignited from the top in a controlled oxygen–nitrogen atmosphere. Unmodified PET strapping yields an LOI of 20–22% by volume, with a mean value near 21%. Because normal air contains approximately 20.9% oxygen, PET strapping sits at the borderline of sustained candle-like combustion: it burns slowly in open air but shows a marginal self-extinguishing tendency in still or oxygen-depleted atmospheres. Thin strap geometry, heat feedback, and dripping allow sustained burning in practice. Embossed and high-recycled-content samples typically test 1–2 points lower, while flame-retardant grades exceed 26–30%.

Ignition Temperature Testing — ASTM D1929 (Setchkin Method)

Hot-air furnace testing determines the lowest temperature at which decomposition gases flash with a pilot flame (flash ignition) and the lowest temperature at which sustained combustion occurs without an external pilot (self-ignition). PET strapping shows a flash ignition temperature of approximately 360–390 °C and a self-ignition temperature of approximately 450–500 °C. Both values are meaningfully higher than those of polypropylene strapping (flash ignition approximately 330–350 °C), confirming PET's higher ignition threshold and greater tolerance of incidental heat exposure.

Horizontal and Vertical Burning Tests — ASTM D635 / ASTM D3801 / UL 94

Unmodified PET strapping generally meets the UL 94 HB classification, with horizontal burning rates typically below 40 mm/min for 0.8–1.0 mm thickness. In vertical testing, standard PET frequently exhibits flaming drips, which may produce a V-2 rating when the drips extinguish without igniting cotton indicators. Flame-retardant modified grades achieve V-0 performance, defined as after-flame times of no more than 10 seconds per ignition, no flaming drips, and a total after-flame time below 50 seconds across the specimen set.

Cone Calorimetry — ISO 5660-1 / ASTM E1354

At a radiant flux of 35 kW/m², PET strapping specimens show time-to-ignition values of roughly 60–150 seconds, peak heat release rates of 400–600 kW/m², and effective heats of combustion of 17–20 MJ/kg. At 50 kW/m², peak heat release can exceed 700 kW/m². Molten flow and flaming drips can relocate burning polymer beyond the specimen holder, so retention grids or frames should be used, and results should be interpreted together with the dripping observations.

Thermogravimetric Analysis (TGA) — ISO 11358

Under nitrogen, PET strapping shows onset of mass loss at 360–390 °C, a maximum decomposition rate at 445–470 °C, and a principal single-step mass loss of approximately 80–85%, leaving 10–18% carbonaceous char residue at 600 °C due to the aromatic terephthalate units. Under air, oxidative attack lowers the onset temperature and produces a more complex multi-step decomposition profile.

Differential Scanning Calorimetry (DSC) — ISO 11357

DSC confirms the thermal transitions that control early fire behavior: glass transition at 75–80 °C, cold crystallization at 130–150 °C for partially amorphous material, and a melting endotherm at 250–258 °C. These transitions explain why strapped loads lose restraint during the pre-ignition phase of a fire, well before visible flaming.

Smoke Density and Combustion Gas Analysis — ISO 5659-2, FTIR, and GC-MS

Because of its high aromatic content, PET generates dense, soot-laden smoke with significant optical obscuration. Evolved gas analysis identifies carbon dioxide, carbon monoxide, acetaldehyde, formaldehyde, benzene, toluene, trace styrene, benzoic acid, acetophenone, and terephthalic acid aerosols as principal products. Importantly, virgin PET contains no halogens, so combustion does not release hydrogen halide gases, giving it a smoke toxicity profile clearly more favorable than PVC-based materials.

Applications

The combustion characteristic data of PET strapping directly support the following engineering and commercial applications:

• Warehouse fire load calculation, commodity classification, and sprinkler system design for palletized goods secured with polyester strapping.
• Transport and export packaging risk assessment, including fire behavior inside closed freight containers and vessel holds.
• Selection of flame-retardant strapping grades for underground coal mines, petrochemical plants, electronics manufacturing, and offshore platforms where regulatory flame performance is mandatory.
• Thermal recovery and incineration planning, where the net calorific value of approximately 22 MJ/kg and halogen-free composition enable clean energy valorization.
• Insurance risk assessment and fire code compliance documentation for high-rack storage facilities.
• Quality control for strapping manufacturers verifying LOI and UL 94 performance of flame-retardant product lines.

A key practical conclusion is that PET strapping loses tension near its melting point of roughly 255 °C, so fire-exposed strapped loads may shift or collapse before flaming combustion fully develops. In rack storage, flaming melt drips falling onto lower levels constitute the dominant fire-spread mechanism and should be the primary focus of protective design.

FAQ

Is PET plastic steel strapping flammable?

Yes. PET strapping is a combustible thermoplastic with an LOI of 20–22%, a flash ignition temperature near 360–390 °C, and a self-ignition temperature near 450–500 °C. It is, however, less flammable than polypropylene strapping and ignites at substantially higher temperatures.

At what temperature does PET strapping ignite?

With a pilot flame, decomposition gases flash at approximately 360–390 °C. Without an external ignition source, self-ignition occurs at approximately 450–500 °C. Note that the strap melts and loses all tension at 250–260 °C, well before ignition.

What is the limiting oxygen index of PET strapping?

The LOI of standard PET strapping is 20–22%, which is marginally above the oxygen concentration of normal air. In practice, thin strap sections, heat feedback, and dripping allow slow sustained burning in open air, while the material shows a self-extinguishing tendency in still or oxygen-depleted conditions. Flame-retardant grades reach 26–30% or higher.

What smoke and gases does burning PET strapping produce?

Burning PET produces dense black soot together with carbon dioxide, carbon monoxide, acetaldehyde, formaldehyde, benzene, toluene, and aromatic acid aerosols. Virgin PET contains no chlorine, so corrosive or highly toxic hydrogen halide gases are not generated, unlike with PVC materials.

How does PET strapping compare with PP and steel strapping in a fire?

PET strapping is moderately flammable, with a higher LOI and roughly half the heat of combustion of PP strapping. Steel strapping is non-combustible but also relaxes at elevated temperatures. PET fails earlier in a fire because it melts near 255 °C, so for loads that must survive fire exposure for longer periods, additional containment measures should be considered.

Are flame-retardant PET strapping grades available?

Yes. Halogen-free flame-retardant compounds based on phosphorus or intumescent systems can raise the LOI above 26–30% and achieve a UL 94 V-0 classification, suitable for mining, petrochemical, and certified export applications.

What storage precautions are recommended for PET strapping?

Store strapping away from open flames, hot work zones, and high-temperature radiant sources; prohibit smoking in storage areas; minimize loose coil ends that can act as wick-like ignition paths; and align sprinkler protection with the actual commodity classification of the strapped goods, paying particular attention to dripping-driven fire spread in high-rack storage.