ASTM D696 Standard Test Method for Coefficient of Linear Thermal Expansion of Plastics Between −30°C and 30°C with a Vitreous Silica Dilatometer

Get Certified ASTM D696 Testing for Reliable Thermal Expansion Evaluation

ASTM D696 Coefficient of linear thermal expansion of plastic materials is the method used to measure the coefficient of linear thermal expansion of plastic materials in the temperature range of −30 °C to +30 °C, by means of a vitreous silica dilatometer. ASTM D696 testing assesses changes in dimensions as a result of temperature and helps characterize, develop, and compare different plastic materials. The method applies to plastics with coefficients of expansion greater than 1 μm/(m·°C), while materials with very low expansion require alternative measurement techniques.

• Materials: Plastic materials with coefficients of linear thermal expansion greater than 1 μm/(m·°C).
• Measurement: Determines the coefficient of linear thermal expansion from dimensional change.
• Temperature Range: Covers −30 °C to +30 °C under the standard test conditions.
• Apparatus: Uses a vitreous silica dilatometer with a controlled temperature environment.
• Specimen Condition: Requires negligible creep or elastic strain that could significantly affect measurement accuracy.
• Low Expansion Materials: Does not apply to materials with coefficients below 1 μm/(m·°C).
• Temperature Extension: Requires another appropriate method for temperatures outside the ASTM D696 range.
• Thermal Effects: Attempts to minimize influences from moisture, curing, plasticizer loss, stress release, and phase changes.

What are the Uses of ASTM D696 Testing?

The coefficient of linear thermal expansion of plastic materials can be determined using the controlled procedure outlined in ASTM D696. Consequently, engineers can use the results to evaluate dimensional changes caused by temperature and support material selection, product development, and dimensional design. The ASTM D696 standard-

• Determines the coefficient of linear thermal expansion of plastics.
• Checks dimensional changes from −30°C to +30°C.
• Helps choose materials for temperature-sensitive applications.
• Supports engineers in the evaluation of thermal dimensional stability.
• Offers design and engineering expansion information.
• Facilitates comparison of thermal expansion properties of plastic materials.
• Assists in estimating possible dimensional changes due to temperature.
• Supplies information for applications that need to be controlled for dimensional tolerances.

Which materials can be tested under ASTM D696?

ASTM D696 is applicable to plastic materials that have a coefficient of linear thermal expansion in excess of 1 μm/(m·°C) and an elastic strain or creep that is sufficiently low for the conditions of the test. This method can be used to assess molded, machined, or cast specimens prepared to minimize strain and anisotropy. Very low expansion coefficients need more sensitive techniques like interferometric or capacitance measurements.

Why is ASTM D696 Important?

ASTM D696 is significant because it allows one to measure the change in length of plastics in a controlled manner. The measured coefficient is used for dimensional design and to aid engineers in considering thermal expansion of components and assemblies. Moisture changes, curing, loss of plasticizer or solvent, stress release, phase transition, specimen preparation, temperature homogeneity, and creep may affect the measured expansion, and it is necessary to control these as far as possible.

ASTM D696 Testing Process and Data Collection

In ASTM D696 testing, the sample is placed within the dilatometer, which is made of vitreous silica, and is subjected to controlled temperatures within the range. The length of the specimens is measured by the measuring device, which records changes in length due to changes in temperature. The initial and final lengths, along with the temperature difference, are used to calculate the coefficient of linear thermal expansion.

Common Challenges and Troubleshooting

Typical problems in ASTM D696 are specimen creep, incorrect initial specimen length measurement, temperature gradient, specimen indentation, high friction in the dilatometer, and poor thermal equilibrium. Before taking measurements, technicians should check the dimensions of the specimens, the accuracy of the measuring devices, the uniformity of the bath temperature, the fit of the specimens, and the stabilization of the temperature. Results may not be purely reversible thermal expansion in materials where moisture changes, curing, loss of plasticizer, stress release, or phase transitions are occurring.

• The laboratory documents the length of the specimens at the start and at the end of the test and the test temperature.
• The coefficient of linear thermal expansion is computed by the analyst as the change in length per unit length over the temperature change.
• Technicians ensure that the dimensional change measured is not greatly influenced by specimen deformation, creep, or indentation.
• Engineers consider moisture, curing, plasticizer or solvent loss, stress release, and phase changes when interpreting thermal expansion results.
• The resulting coefficient is suitable for comparing plastic materials and for determining the dimensional changes due to temperature variation.

Link to ASTM D696