A Guide to ISO and ASTM Test Standards for Polyolefins

ISO (International Organization for Standardization) and ASTM (American Society for Testing and Materials) are two major standards bodies used for testing plastic materials. ISO standards are generally more common in Europe and international trade, while ASTM standards appear more often in technical data sheets originating from North America — two standards measuring the same property can have different test conditions (specimen size, speed, temperature), so results may not be directly comparable.

A value on a spec sheet must always state which standard it was measured under; a value given without a standard is considered incomplete information.

The most commonly used standards for MFI are ASTM D1238 and ISO 1133 (see the 'What Is MFI' article). For density, ASTM D792 and ISO 1183 are common references (see the 'What Is Density' article).

Tensile strength and elongation at break are generally measured according to ASTM D638 or ISO 527, by pulling a specimen at a set speed until it breaks.

The two most common methods for impact strength are Izod (ASTM D256 / ISO 180) and Charpy (ISO 179) — both measure the energy consumed to break a notched specimen with a pendulum impact, but specimen clamping and impact geometry differ, so numerical values from the two methods cannot be converted into one another for the same material.

Flexural modulus is generally measured according to ASTM D790 or ISO 178 and indicates the material's resistance to bending/stiffness.

Melting temperature is generally measured by differential scanning calorimetry (DSC) according to ASTM D3418 or ISO 11357. Heat deflection temperature (HDT) indicates the temperature at which a material begins to deform under a specific load, and is typically tested according to ASTM D648 or ISO 75.

When comparing technical data sheets from different suppliers, you need to look not just at the numerical value but also at which standard and test condition it was measured under — otherwise there is a risk of making an incorrect comparison. In uncertain cases, referring to the manufacturer's current TDS document is recommended.