Determination of the Coefficient of Thermal Expansion of PET Substrates

Testing Items

A comprehensive PET substrate CTE characterization program typically includes the following test items, each tied to a defined temperature interval and specimen orientation:

• Linear CTE in the machine direction (MD) below the glass transition temperature
• Linear CTE in the transverse direction (TD) below the glass transition temperature
• Through-thickness (Z-direction) CTE for sheet and stacked-film specimens
• Apparent CTE above the glass transition temperature, reported separately
• Glass transition temperature (Tg) determined from the expansion curve inflection point
• Dimensional shrinkage or irreversible expansion after the first heating cycle
• Anisotropy ratio between MD and TD expansion behavior
• Effective CTE of coated, metallized, or laminated PET constructions

Testing Methods

Thermomechanical Analysis (TMA) per ASTM E831 and ISO 11359-2

Thermomechanical analysis is the preferred method for PET film because it requires only a small specimen and delivers high displacement resolution, typically on the order of 0.1 micrometer. A flat quartz probe rests on the specimen under a small preload, commonly in the range of 0.01 to 0.05 N, and the specimen length is recorded continuously while the furnace heats at a controlled rate of 3 to 5 °C/min under a nitrogen purge. The CTE is calculated from the slope of the length-versus-temperature curve over the specified interval. ISO 11359-2 additionally allows the glass transition temperature to be determined from the same run by locating the point of slope change in the expansion curve.

In-Plane (MD and TD) Measurement of Thin Films

Because PET film used as a substrate is often only 12 to 250 micrometers thick, in-plane CTE is measured with a film or extension fixture that grips the strip specimen in tension while a minimal static force is applied. Paired specimens are cut along the machine direction and the transverse direction, and results are reported for each direction. This configuration is essential for correlating substrate expansion with printed pattern registration and lamination distortion in real converting processes.

Quartz Dilatometry per ASTM D696

For applications that require the CTE in a narrow, near-ambient interval, typically between -30 and +30 °C, a vitreous silica dilatometer per ASTM D696 offers excellent stability. The method is well suited to rigid PET sheet but less convenient for very thin, flexible films, where handling and specimen straightness limit accuracy.

Specimen Preparation, Conditioning, and Calibration

Specimens should be cut cleanly without burrs or edge defects, and at least three replicates per direction are recommended. Unless a specific conditioning state is being studied, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 prior to testing, since PET absorbs a small amount of moisture that can slightly influence dimensional behavior. The instrument is calibrated for temperature using the melting points of reference metals such as indium, and for length change using a certified reference material such as aluminum. To eliminate the influence of prior thermal history, many laboratories perform an initial conditioning heat run and report the CTE from a second heating run; the treatment applied must be stated in the test report together with the exact temperature range over which each CTE value was computed.

Applications

CTE data for PET substrates directly support material selection, process window definition, and reliability prediction across a broad range of industries:

• Flexible OLED and micro-LED displays, where thermal mismatch between the PET substrate and inorganic barrier or TFT layers drives film curl and layer cracking
• Flexible printed circuits and copper-clad laminates, where substrate expansion controls trace stress, solder joint fatigue, and dimensional registration
• Photovoltaic backsheets, where PET core film stability affects long-term module performance under thermal cycling
• Optical compensation and polarizing films, where dimensional change during processing alters optical retardation and cutting yield
• Touch sensors and membrane switches, where printed silver or ITO patterns must remain registered through lamination and service temperatures
• High-precision labels and printable electronics, where CTE governs multi-pass print registration and die-cutting accuracy
• Capacitor and insulating films, where dimensional stability influences winding quality and electrical performance

FAQ

What is the typical CTE value of PET film?

General-purpose biaxially oriented PET film typically shows an in-plane CTE of roughly 15 to 25 ppm/°C between room temperature and the glass transition region, with heat-stabilized, low-CTE display grades engineered to lower values. Above the glass transition temperature of about 76 to 80 °C, the apparent CTE rises substantially and the film may exhibit shrinkage, so values must always be quoted with their measurement temperature range.

Why do machine-direction and transverse-direction results differ?

Biaxial stretching aligns polymer chains preferentially along both film directions, but the degree of orientation is rarely identical in MD and TD. Because oriented chains expand less along their axis than perpendicular to it, the measured CTE differs between directions. Reporting both values, along with the draw ratio history and heat-setting conditions, is essential for accurate simulation and process design.

What temperature range should the test cover?

The range should mirror the intended service or processing environment. For labeling and display applications, an interval such as -40 to 60 °C is common. For process simulation involving lamination or curing, the range may be extended to 120 or 150 °C, in which case CTE values below and above the glass transition must be reported separately, since a single averaged slope would be misleading.

Which standards govern PET substrate CTE measurement?

The most widely applied standards are ASTM E831 and ISO 11359-2 for thermomechanical analysis, ASTM D696 for silica dilatometry near room temperature, and IPC-TM-650 methods for z-axis and in-plane expansion relevant to printed board and flexible circuit materials. The chosen standard, heating rate, preload, and evaluation interval must all be documented in the test report.

How is through-thickness CTE measured on very thin film?

When the film is too thin to give a reliable through-thickness signal, several layers are stacked to build a measurable height, and the effective CTE is determined with a flat probe in compression mode. Care must be taken to exclude trapped air and to verify that layer interfaces do not introduce artifacts, and the number of stacked plies is recorded in the report.

How many specimens are required and what precision can be expected?

A minimum of three valid specimens per direction is standard practice, and the mean, standard deviation, and evaluation interval are reported. With a well-calibrated TMA, reproducibility within approximately 5 to 10 percent is typically achievable for PET film, while anisotropy, thermal history, and moisture conditioning are the largest sources of variability between laboratories.