PET Film Melting Temperature Determination: DSC Testing Methods, Standards, and Industrial Applicati

PET film melting temperature determination is a core thermal analysis procedure used to verify resin identity, quantify crystallinity, and define safe processing windows for polyethylene terephthalate films. Using differential scanning calorimetry (DSC) in accordance with ISO 11357 and ASTM D3418, laboratories measure melting peak temperature, melting enthalpy, and related thermal transitions with high precision. This technical guide covers sample types, testing items, test procedures, data evaluation, applications, and frequently asked questions for PET film melting temperature measurement.

Technical Overview

Polyethylene terephthalate (PET) film is a semicrystalline engineering polyester widely used in flexible packaging, electrical insulation, capacitor dielectrics, optical films, and industrial laminates. The melting temperature (Tm) of PET film, typically between 250°C and 260°C for homopolymer grades and spanning roughly 245°C to 265°C across modified formulations, is one of the most informative single parameters in thermal characterization of this material.

Melting temperature determination serves multiple purposes. It confirms that the film is manufactured from the specified resin rather than a substitute polymer, reveals the presence of copolymer content such as isophthalate (IPA) or cyclohexanedimethanol (CHDM), reflects the perfection and stability of the crystalline phase developed during orientation and heat setting, and establishes the upper temperature limit for downstream processing operations such as lamination, metallization, thermoforming, and heat sealing. Because PET film frequently operates close to its thermal limits in converting lines, accurate melting temperature data directly supports process safety and product reliability.

Differential scanning calorimetry (DSC) is the internationally recognized reference technique for this measurement. In a DSC experiment, a small film specimen and an inert reference are heated at a controlled rate, and the instrument records the endothermic heat flow associated with melting. The resulting curve yields the extrapolated onset temperature, the melting peak temperature, and the enthalpy of fusion, from which the degree of crystallinity can also be calculated.

Testing Samples

Biaxially Oriented PET Film (BOPET)

Biaxially oriented films represent the largest share of commercial PET film products. The stretching and heat-setting operations produce a highly oriented, strain-hardened crystalline structure, so the first-heating melting peak often appears slightly elevated or broadened relative to unoriented material. Specimens are typically punched as small discs from representative areas of the film roll.

Amorphous and Cast PET Films

Unoriented, rapidly quenched films show low initial crystallinity and frequently exhibit a cold crystallization exotherm at approximately 120°C to 140°C during the first heating scan, followed by the principal melting endotherm. Determining the melting temperature of these films requires careful separation of the overlapping thermal events.

Copolyester and Modified PET Films

Films containing comonomers such as IPA or CHDM melt at measurably lower temperatures than homopolymer PET, and high CHDM content grades (PETG-type copolyesters) may show a depressed and broadened melting transition or near-complete loss of crystallinity. Melting temperature measurement is the primary method for confirming modification level.

Recycled PET (rPET) Films

Films produced with post-consumer or post-industrial recycled content may display shifted peak temperatures, double melting peaks, or additional transitions caused by contamination, blend phases, or hydrolytic degradation. Melting temperature data are essential for qualifying rPET feedstock.

Coated, Metallized, and Laminated Films

Thin coatings, adhesives, aluminum metallization layers, and coextruded skins usually contribute little heat flow relative to the PET substrate and rarely interfere with the melting measurement. Where heavy coating loads or multiple polymer layers are present, the DSC curve may show additional transitions, which can be diagnostically valuable.

Testing Items

A complete PET film melting temperature study typically reports the following parameters:

• Extrapolated onset temperature of melting (Tm,onset), determined from the intersection of the extrapolated baseline and the leading edge of the melting endotherm
• Melting peak temperature (Tm,peak), the temperature at maximum endothermic heat flow, reported for both first and second heating scans
• Enthalpy of fusion (ΔHm), the integrated area of the melting peak in joules per gram
• Degree of crystallinity (Xc), calculated from the melting enthalpy using a reference enthalpy of approximately 140 J/g for fully crystalline PET
• Cold crystallization temperature (Tcc) and enthalpy (ΔHcc), where applicable for low-crystallinity films
• Crystallization temperature on cooling (Tc) from the melt, indicating nucleation and solidification behavior
• Glass transition temperature (Tg, approximately 76°C to 80°C for PET), frequently recorded in the same experiment at no additional cost

Testing Methods

Differential Scanning Calorimetry (DSC)

Heat-flux or power-compensation DSC instruments are used in accordance with ISO 11357-1 and ISO 11357-3, or ASTM D3418. The method offers temperature precision typically within ±0.5°C after proper calibration, requires only a few milligrams of material, and completes a full two-scan analysis in under one hour.

Sample Preparation

Specimens are punched or cut from the film, avoiding printed areas, heavy coatings, and edge zones when a clean resin measurement is required. Because thin films weigh little, several discs are usually stacked in a crimped aluminum pan to reach the target mass of 3 mg to 10 mg, with approximately 5 mg preferred for optimal peak resolution. Specimens are weighed to 0.01 mg on an analytical balance, and pan mass matching is recommended.

Instrument Calibration

The DSC cell is calibrated for temperature and enthalpy using certified reference materials bracketing the analytical range, most commonly indium (156.6°C, 28.45 J/g) together with tin or zinc. Baseline calibration and, where needed, heat capacity verification are performed before the measurement campaign. The cell is purged with high-purity nitrogen at approximately 50 mL/min to prevent oxidative degradation.

Test Procedure

• Equilibrate the specimen at 20°C to 30°C, below the glass transition
• Heat at 10°C/min to 280°C to 300°C (first heating scan), recording the as-received thermal behavior of the film
• Hold isothermally for 2 to 5 minutes to erase thermal and orientation history
• Cool at 10°C/min to 20°C to 40°C, recording the crystallization exotherm
• Reheat at 10°C/min to 300°C (second heating scan), recording the intrinsic melting behavior of the material

Data Evaluation and Reporting

The extrapolated onset and peak temperatures are constructed using the standard tangent constructions defined in ISO 11357-1. The melting enthalpy is obtained by integrating the endotherm against a sigmoidal or linear baseline. For partially amorphous films, the degree of crystallinity is calculated as Xc = (ΔHm − ΔHcc) / ΔHm° × 100%, where ΔHm° is taken as approximately 140 J/g for 100% crystalline PET. Reports present both first-heat and second-heat values, since the first heat reflects manufacturing and orientation history while the second heat characterizes the base resin.

Alternative Techniques

Hot-stage polarized microscopy can visually confirm the disappearance of birefringence at the melting point and is useful for oriented films, while capillary melting point apparatus provides a quick but less precise screening result. Neither technique supplies enthalpy or crystallinity data, so DSC remains the reference method for compliance testing.

Applications

Melting temperature determination is applied throughout the PET film value chain:

• Incoming material inspection and resin identity verification against supplier specifications and certificates of analysis
• Process window definition for heat sealing, thermal lamination, metallization, and shrinkage-controlled heat setting operations
• Quality control of biaxial orientation lines, where melting behavior tracks crystalline structure developed during stretching and annealing
• Qualification of recycled content, detection of polymer contamination, and verification of copolymer modification levels
• Failure analysis of films that have undergone unexpected thermal exposure, degradation, or dimensional instability in service
• Research and development support for new copolyester formulations, coating systems, and film structures
• Regulatory and customer compliance testing against automotive, electronics, and packaging material specifications

In every case, the measured melting temperature provides a rapid, quantitative link between molecular composition, processing history, and expected in-service thermal performance.

FAQ

What is the typical melting temperature of PET film?

Homopolymer PET films melt at approximately 250°C to 260°C, with well-processed BOPET typically showing a melting peak near 255°C. Comonomer modification shifts the peak downward, and heavily modified copolyesters may melt below 240°C.

Why do first and second heating scans give different results?

The first heating scan measures the film as manufactured, including orientation-induced crystallinity and residual stress, and is the appropriate value for processing decisions. The second heating scan, measured after controlled melting and cooling, characterizes the intrinsic resin and is preferred for material identification and lot-to-lot comparison.

How much sample is needed for the test?

A total specimen mass of 3 mg to 10 mg is sufficient. For films thinner than 25 micrometers, several stacked discs are cut and weighed together to reach this range.

Which standards govern the measurement?

The principal standards are ISO 11357-1 and ISO 11357-3, along with ASTM D3418 for transition temperatures and ASTM D3417 for enthalpy determination. Regional equivalents, such as the GB/T 19466 series, are technically aligned with ISO 11357.

How is degree of crystallinity calculated?

Crystallinity is derived by dividing the measured melting enthalpy (net of any cold crystallization exotherm) by the reference enthalpy of approximately 140 J/g for perfectly crystalline PET, then multiplying by 100%.

What does a double melting peak or broadened endotherm indicate?

Multiple peaks usually reflect populations of crystals with different perfection, typical of oriented or annealed films, while a broadened, depressed peak can indicate comonomer content, recycled blending, hydrolytic degradation, or incomplete crystallization.

Can coated or metallized films be tested without special preparation?

Yes. In most cases the PET substrate dominates the heat flow signal, and the melting temperature can be measured directly on punched specimens. Thick coatings may add small extra transitions that do not affect the PET peak assignment.

How long does a complete determination take?

A full two-scan DSC run with intermediate cooling takes roughly 45 to 60 minutes per specimen, and calibration-checked instruments routinely deliver reproducibility of better than ±1°C on peak temperature.