ISO Standard Test Specimen Density Testing: Methods, Standards, and Technical Guide

ISO standard test specimen density testing is a core quality control procedure used to determine the density of plastic and polymer test specimens in accordance with the ISO 1183 standard series. This technical guide explains the applicable specimen types, measured parameters, accepted testing methods, and practical applications of density measurement in materials testing laboratories.

Technical Overview

Density, defined as the mass of a material per unit volume and typically expressed in g/cm³ or kg/m³, is one of the most informative single-point properties available for characterizing plastic materials. ISO standard test specimen density testing refers to the determination of density on standardized specimens in accordance with the ISO 1183 series, the internationally recognized standard for measuring the density of non-cellular plastics. Because density is sensitive to crystallinity, molecular packing, void content, and additive loading, it serves as a reliable indicator of material identity, batch consistency, and processing history.

The ISO 1183 series comprises three parts: ISO 1183-1, which covers the immersion method, the liquid pycnometer method, and the titration method; ISO 1183-2, which specifies the density gradient column method; and ISO 1183-3, which describes the gas pycnometer method. Test specimens are commonly prepared and conditioned according to ISO 3167 for multipurpose specimens, ISO 293 for compression molding, or ISO 294 for injection molding. Conditioning is normally performed in the standard atmosphere of 23 °C and 50 % relative humidity as specified in ISO 291, and measurements are carried out at a controlled temperature of 23 °C, since polymer density varies measurably with temperature.

These methods are broadly equivalent to ASTM D792 and ASTM D1505, allowing laboratories to report results that are accepted in international trade, certification, and regulatory contexts. The choice of method depends primarily on the physical form of the specimen, the required precision, and whether the material absorbs or reacts with the test liquid.

Testing Samples

Specimen preparation has a direct influence on the measured density. Samples must be free of surface contamination, burrs, sink marks, and internal voids, and they must be conditioned to thermal and moisture equilibrium before measurement. The principal specimen categories covered by ISO-based density testing are described below.

Multipurpose Test Specimens (ISO 3167 Type A)

Multipurpose specimens, including Type A1A tensile bars and rectangular strips, are injection molded or compression molded under controlled conditions. Measuring density directly on these specimens allows density data to be correlated with tensile, flexural, and impact results obtained from the same molded batch, which is essential for process validation and material data sheets.

Compression-Molded Plaques and Sheets

Compression-molded plaques produced per ISO 293 are frequently used as reference specimens for resin characterization. The slow, low-stress molding cycle minimizes internal stresses and orientation, providing a density value that closely reflects the intrinsic crystallinity of the polymer. For crystalline resins such as polyethylene and polypropylene, cooling rate during molding must be reported because it strongly affects the measured density.

Film and Sheet Specimens

Thin films and sheets can be tested by cutting small pieces that sink freely in the immersion or gradient column liquid. Film specimens must be fully wetted on both surfaces, and the wetting agent concentration must be sufficient to prevent air bubbles from clinging to the surface, which would otherwise bias the result.

Pellets, Granules, and Powders

Raw resin in pellet, granule, or powder form is generally tested using the liquid pycnometer method (ISO 1183-1, Method B) or the gas pycnometer method (ISO 1183-3), because irregular particles cannot be suspended reliably for the immersion weighing procedure. Pellet density is widely used for incoming material inspection and resin classification.

Testing Items

Depending on the client requirements and the specimen form, ISO specimen density testing can include the following reported parameters:

• Density at 23 °C, expressed in g/cm³ or kg/m³, measured on the as-received or as-molded specimen.
• Relative density (specific gravity), the dimensionless ratio of specimen density to the density of water at the reference temperature.
• Apparent density of porous or irregularly shaped specimens, including the effect of surface-accessible voids.
• True skeletal density of solid material obtained by gas pycnometry, excluding open porosity.
• Crystallinity estimation derived from density using the two-phase crystalline-amorphous model for semicrystalline polymers such as PE and PP.
• Density distribution or conformity checks against material grade limits, such as the classification boundaries between LDPE, LLDPE, MDPE, and HDPE.

Testing Methods

ISO 1183 defines several measurement principles. The laboratory selects the appropriate method based on specimen geometry, required accuracy, and the chemical compatibility of the specimen with the working liquids. In all methods, an analytical balance with a resolution of 0.1 mg and temperature control of the liquid medium are required.

Immersion Method (ISO 1183-1, Method A)

The specimen is weighed in air and then weighed while fully immersed in a wetting liquid of known density, typically distilled water with a wetting agent, using a density determination kit mounted on an analytical balance. Density is calculated from the apparent loss of mass, with a correction applied for air buoyancy. A sinker and wire harness may be used for specimens lighter than the immersion liquid. The method applies to specimens with a mass of at least 1 g and achieves a practical accuracy of approximately 0.001 g/cm³.

Liquid Pycnometer Method (ISO 1183-1, Method B)

A calibrated glass pycnometer is weighed empty, filled with the working liquid, and then filled with liquid plus the specimen. The volume displaced by the specimen is derived from the mass difference, and density is calculated from the specimen mass and displaced volume. This method is preferred for pellets, granules, and small irregular pieces and achieves accuracy comparable to the immersion method.

Titration Method (ISO 1183-1, Method C)

Two miscible liquids of different densities are mixed, and the mixture ratio is adjusted by titration until the specimen remains suspended in neutral buoyancy. The density of the mixture at that point, measured with a hydrometer, equals the specimen density. The method is rapid and requires minimal equipment but provides lower precision, making it suitable only for screening purposes.

Density Gradient Column Method (ISO 1183-2)

A vertical column is filled with a stable, linear density gradient produced from two miscible liquids, and the column is calibrated using certified glass floats of known density. Small specimens introduced at the top sink to the height at which their density matches the surrounding liquid, and density is read from the calibrated height, typically to within 0.0001 g/cm³. This is the most precise method in the ISO 1183 series and is widely used for polyethylene resin grading, thin films, and research on crystallization behavior.

Gas Pycnometer Method (ISO 1183-3)

The specimen is placed in a sealed chamber of known volume, and an inert gas, usually helium, is admitted at a controlled pressure. The pressure change upon expansion into a reference volume allows the skeletal volume of the specimen to be calculated from the gas law. Because gas penetrates fine surface texture but not closed pores, the method yields true skeletal density and is ideal for powders, highly porous solids, and materials that swell or dissolve in liquids.

Applications

ISO specimen density testing supports decision-making across the entire polymer value chain, from resin production to finished part qualification:

• Resin classification and grade identification, including the density-based separation of LDPE (approximately 0.910 to 0.925 g/cm³), LLDPE and MDPE (approximately 0.926 to 0.940 g/cm³), and HDPE (approximately 0.941 g/cm³ and above).
• Incoming material inspection and supplier quality control, where density serves as a fast, sensitive conformity check against purchase specifications.
• Molding process validation, since mold temperature, melt temperature, hold pressure, and cooling rate all influence the crystallinity and therefore the density of molded specimens.
• Crystallinity assessment of semicrystalline polymers using the two-phase model, in which the measured density is interpolated between the densities of the purely amorphous and purely crystalline phases.
• Support of mechanical test programs, where density data from ISO 3167 specimens are reported alongside tensile and flexural properties on material datasheets.
• Failure analysis and research, including the study of aging, annealing, orientation, void formation, and the effect of fillers or reinforcements on composite density.

FAQ

Which ISO standard governs specimen density testing?

The governing standard is the ISO 1183 series. ISO 1183-1 covers the immersion, liquid pycnometer, and titration methods; ISO 1183-2 specifies the density gradient column method; and ISO 1183-3 specifies the gas pycnometer method. Specimen preparation and conditioning are governed by ISO 3167, ISO 293, ISO 294, and ISO 291.

At what temperature is the density measured?

Measurements are normally performed at 23 °C, consistent with the standard laboratory atmosphere defined in ISO 291. Because polymer density decreases with increasing temperature, the test temperature must be controlled, typically within ±0.5 °C, and reported together with the result.

Which method offers the highest accuracy?

The density gradient column method described in ISO 1183-2 provides the highest precision, with a resolution on the order of 0.0001 g/cm³, making it the preferred technique for resin grading and crystallinity studies. The immersion method is the most common routine method with an accuracy of approximately 0.001 g/cm³.

How is crystallinity calculated from density?

For a two-phase semicrystalline polymer, the mass fraction crystallinity is calculated as Xc = [ρc × (ρs − ρa)] / [ρs × (ρc − ρa)] × 100 %, where ρs is the measured specimen density and ρa and ρc are the densities of the fully amorphous and fully crystalline phases. For polyethylene, typical reference values are approximately 0.855 g/cm³ for the amorphous phase and 1.004 g/cm³ for the crystalline phase.

What specimen size and quantity are required?

For the immersion method, a specimen mass of at least 1 g is recommended, with a smooth surface free of voids and burrs. The liquid pycnometer and gas pycnometer methods accommodate smaller particles. Best practice is to test at least three replicate specimens and report the mean value along with the individual results.

Can ISO density results be compared with ASTM results?

Yes. ISO 1183-1 corresponds closely to ASTM D792, and ISO 1183-2 corresponds to ASTM D1505. When equivalent methods, temperatures, and conditioning procedures are used, results are generally comparable, although differences in liquid systems and calibration practice should be considered when evaluating small deviations.