Determination of the Linear Thermal Expansion Coefficient of Plastics: Principles, Standards and Tes

The linear coefficient of thermal expansion (CLTE) is a key parameter governing the dimensional stability of plastic parts under varying service temperatures. This article explains how the linear thermal expansion coefficient of plastics is determined, covering test principles, international standards such as ISO 11359-2, ASTM D696, ASTM E831 and GB/T 1036, sample requirements, measurement procedures, and typical applications in automotive, electronics and precision engineering.

Technical Overview

The coefficient of linear thermal expansion (CLTE, symbol α) quantifies the fractional change in length of a material per unit change in temperature. It is calculated as the ratio of dimensional change to the original specimen length over a specified temperature interval, and is typically expressed in units of 10⁻⁶/K (µm/(m·K) or ppm/K). Accurate determination of this coefficient is essential for plastics because polymeric materials expand roughly five to ten times more than metals, which directly affects tolerance design, assembly fit and long-term dimensional stability.

During measurement, a specimen of defined geometry is subjected to a controlled temperature program while its length change is continuously recorded. Depending on the evaluation method, both the mean CLTE over a defined temperature range and the instantaneous (differential) CLTE as a function of temperature can be obtained. For amorphous plastics, the expansion behavior changes markedly at the glass transition, so results below and above Tg are usually reported separately. For reinforced, filled or injection-molded materials, the coefficient may be anisotropic, requiring measurements in both the flow (machine) direction and the transverse direction.

Testing Samples

Thermoplastics

Unfilled engineering and commodity thermoplastics such as PE, PP, PS, ABS, PMMA, PC, PA6, PA66, POM, PET, PBT, PPS and PEEK can all be tested. Specimens are commonly prepared as injection-molded strips, machined bars, or cut sections from finished parts.

Thermosets and Composites

Epoxy resins, phenolic molding compounds, polyurethanes and glass-fiber or carbon-fiber reinforced compounds are frequently measured. Because reinforcement fibers reduce and directionally alter expansion, testing in multiple orientations is recommended for these materials.

Films and Sheets

Packaging and technical films can be evaluated in the machine direction (MD) and transverse direction (TD) to characterize orientation-dependent expansion behavior.

Sample Requirements and Conditioning

• Specimen geometry: flat, parallel-ended strips with smooth, defect-free surfaces; typical dimensions of approximately 3–10 mm in length for thermomechanical analysis, or 50–125 mm in length for push-rod dilatometry.
• Cross-section: commonly 5–10 mm in width and 3–10 mm in thickness, with ends machined perpendicular to the longitudinal axis.
• Conditioning: prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618; hygroscopic materials such as polyamides may require drying or testing at a defined moisture content.
• Thermal history: when relevant, annealing state should be documented, since residual stresses and aging can influence measured expansion.

Testing Items

• Mean coefficient of linear thermal expansion over a specified temperature interval (for example −30 °C to 30 °C, 23 °C to 55 °C, or −40 °C to 85 °C).
• Temperature-dependent (instantaneous) CLTE curve across a wider range, typically from −100 °C up to 300 °C or beyond, depending on the material.
• Separate CLTE values below and above the glass transition temperature, where the expansion rate increases significantly.
• Glass transition temperature determined by thermomechanical analysis in accordance with ISO 11359-2.
• Directional CLTE for anisotropic materials: machine direction versus transverse direction for films, and flow direction versus cross-flow direction for molded parts.

Testing Methods

Push-Rod Dilatometry (ASTM D696, ASTM E228, GB/T 1036)

A vitreous silica dilatometer is used to measure the expansion of a relatively long specimen between −30 °C and 30 °C. The specimen is placed in a silica tube with a low-expansion push rod contacting its end; dimensional change is detected by a displacement transducer while the specimen is heated or cooled slowly in a liquid bath at approximately 1–2 °C per minute. The mean CLTE is calculated from the slope of the length-versus-temperature data over the specified interval. This classical method offers excellent accuracy for the standard room-temperature range.

Thermomechanical Analysis – TMA (ASTM E831, ISO 11359-1 and ISO 11359-2)

A small specimen is placed on a quartz platform and contacted by a probe applying a very low force, and its dimensional change is recorded while the furnace heats the specimen at a typical rate of 5 °C per minute. TMA requires only small specimens and allows measurement across wide temperature ranges, making it the preferred method for engineering design data, for evaluating expansion below and above Tg, and for simultaneous glass transition determination. Calibration is performed using certified reference materials such as fused silica and aluminum standards.

Interferometric Methods (ASTM E289)

Data Evaluation and Reporting

• A linear regression is performed on the length-change curve over each defined temperature interval, and the slope divided by the original specimen length yields the mean CLTE.
• The test report should state the standard applied, specimen dimensions and orientation, temperature interval and heating rate, mean CLTE values with units, and the complete expansion curve where required.
• Typical reference values for illustration: amorphous plastics such as PS and PMMA exhibit approximately 50–90 × 10⁻⁶/K, semi-crystalline polyolefins such as PP and PE approximately 100–200 × 10⁻⁶/K, engineering resins such as PC approximately 60–70 × 10⁻⁶/K, and 30% glass-fiber reinforced polyamide can be reduced to roughly 20–40 × 10⁻⁶/K.

Applications

• Automotive engineering: tolerance stack-up analysis for interior trim, exterior panels and under-hood components, where plastic parts are assembled with metal structures of much lower expansion.
• Electronics and electrical: thermal mismatch assessment between plastics, copper traces and silicon devices in connectors, housings and PCB-related applications, helping prevent warpage and solder joint failure.
• Precision instruments and optical systems: material selection for mounts and structural parts where dimensional stability directly affects alignment and focal performance.
• Building and construction: expansion joint design for PVC profiles, flooring and pipework exposed to seasonal temperature variation.
• Material development and quality control: verification of datasheet values, evaluation of filler and fiber loading effects, and generation of input data for finite element simulation of thermal stresses.

FAQ

Why is the CLTE of plastics so much higher than that of metals?

The weaker intermolecular forces and higher free volume of polymers lead to greater thermal vibration response, so unfilled plastics typically expand five to ten times more than steel or aluminum, making accurate CLTE data critical whenever plastics and metals are joined.

Which standard should be selected for testing?

ASTM D696 or GB/T 1036 is appropriate for the classical −30 °C to 30 °C range using a dilatometer, while ASTM E831 and ISO 11359-2 using TMA are preferred for wider temperature ranges, elevated-temperature design data and glass transition evaluation. Accredited third-party laboratories can advise on the most suitable method for a specific application.

How does the temperature range affect the result?

Plastics do not expand perfectly linearly with temperature; the coefficient generally increases at higher temperatures and rises sharply above the glass transition. For this reason, the measured CLTE is always a mean value tied to a stated temperature interval, and that interval should match the intended service conditions.

Can anisotropic materials be characterized?

Yes. Glass-fiber reinforced and highly oriented parts show significantly different expansion along versus across the fiber or flow direction, so parallel and perpendicular specimens are tested and reported separately.

How long does a typical test take?

Depending on the temperature program and conditioning requirements, a single CLTE determination generally takes from a few hours to one working day per specimen, with reporting available within three to seven working days including specimen preparation at accredited third-party laboratories.

What accuracy can be expected?

With properly calibrated equipment and conditioned specimens, repeatability within approximately ±5% or better is achievable for unfilled plastics; measurement uncertainty increases for very low-expansion or highly heterogeneous reinforced materials.