Plastic Density Testing to International Standards: A Technical Guide to ISO 1183, ASTM D792, and Re

Plastic density testing is one of the most widely performed physical analyses in the polymer industry, providing a fast and reliable indicator of material identity, purity, crystallinity, and process consistency. This guide explains how accredited third-party laboratories measure the density and specific gravity of plastics in accordance with international standards such as ISO 1183-1, ISO 1183-2, ISO 1183-3, ASTM D792, and ASTM D1505. It covers applicable sample types, test methods, accuracy expectations, and typical industrial applications.

Technical Overview

Density, defined as mass per unit volume and typically expressed in g/cm³ or kg/m³, is a fundamental physical property of plastic materials. Because it correlates closely with molecular structure, crystallinity, filler content, and the presence of voids or contaminants, density measurement serves as a primary tool for material identification, incoming inspection, and quality control. A closely related quantity is specific gravity (relative density), the dimensionless ratio of a material's density to that of water at a specified reference temperature, commonly reported as 23 °C/23 °C.

Density determination for non-cellular plastics is governed by a family of harmonized international and national standards, principally:

• ISO 1183-1 — immersion method, liquid pycnometer method, and titration method
• ISO 1183-2 — density gradient column method
• ISO 1183-3 — gas pycnometer method
• ASTM D792 — density and specific gravity of plastics by displacement
• ASTM D1505 — density of plastics by the density-gradient technique
• ASTM D1895 — apparent density (bulk density) of plastic materials
• GB/T 1033 series and JIS K 7112 — Chinese and Japanese national standards aligned with ISO 1183

Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 before testing. Because thermal expansion causes polymer density to change measurably with temperature, all measurements are performed at a controlled temperature, normally 23 °C, and the test temperature is reported together with the result. Note that cellular (foamed) plastics fall outside the scope of ISO 1183; their apparent density is determined in accordance with ISO 845 or ASTM D1622.

Typical density ranges of common plastics, which make density a useful fingerprint for material identification, include:

• PP (polypropylene): approximately 0.90 g/cm³
• PE (polyethylene): approximately 0.91–0.96 g/cm³, increasing from LDPE to HDPE
• PS and ABS: approximately 1.04–1.06 g/cm³
• PC (polycarbonate): approximately 1.20 g/cm³
• Rigid PVC: approximately 1.35–1.45 g/cm³
• PTFE: approximately 2.1 g/cm³
• Mineral-filled or glass-reinforced compounds: correspondingly higher values

Testing Samples

Density can be measured on virtually any solid plastic form, but the choice of specimen type influences both the applicable method and the interpretation of results.

Molded Specimens

Compression- or injection-molded plaques, bars, and dumbbell sections are the preferred reference specimens. They should be free of internal voids and surface defects, and should have a mass of at least 1 g for hydrostatic weighing. Molded specimens best represent the solid-state density of a compound, and supplier datasheets are typically based on compression-molded plaques.

Pellets and Granules

Virgin resin pellets and granules can be tested by the pycnometer, titration, or gradient column methods. Pellet density may deviate slightly from molded-part density due to pelletization conditions, trapped microvoids, and cooling-rate-dependent crystallinity, so the specimen form should always be stated in comparison studies.

Films and Sheets

Flexible films and thin sheets are cut into small pieces and tested in groups to obtain sufficient mass. Care must be taken to remove adhering air bubbles, often with the aid of a wetting agent or a brief immersion under reduced pressure. Density gradient columns are particularly effective for thin films.

Finished Parts and Components

Sections cut from production parts are tested to verify batch-to-batch consistency, detect voids or contamination, and confirm that molding conditions produced the expected degree of crystallinity. The immersion method can be applied non-destructively to many parts, provided they can be fully dried and weighed accurately afterwards.

Powders and Small Particles

Fine powders, regrind, and micropellets are unsuitable for hydrostatic weighing and are instead characterized by the titration method, gas pycnometry, or apparent density measurement.

Testing Items

Standard reporting items for a density test campaign include:

• Density at 23 °C, expressed in g/cm³ or kg/m³
• Specific gravity (relative density) at a stated temperature pair, for example 23 °C/23 °C
• Apparent (bulk) density for powders, pellets, and free-flowing molding materials
• True skeletal density by gas pycnometry for powders and porous or irregular specimens
• Comparison of measured values against supplier datasheets and internal specification limits
• Derived evaluations, such as void content of fiber-reinforced plastics in accordance with ASTM D2734, and estimation of crystallinity for semi-crystalline polymers

Testing Methods

Five principal method families are used, each with distinct equipment, specimen requirements, and accuracy levels.

Immersion Method (Hydrostatic Weighing) — ISO 1183-1 Method A, ASTM D792

The specimen is weighed in air on an analytical balance and then re-weighed while fully immersed in a liquid of known density, typically distilled water with a small amount of wetting agent. According to Archimedes' principle, the apparent loss of mass upon immersion equals the mass of displaced liquid, from which the specimen volume is calculated and the density derived. For materials less dense than the immersion liquid, such as polyethylene and polypropylene, a sinker and the corresponding suspension corrections are applied. This is the fastest and most widely used routine method, achieving accuracy typically within ±0.001 g/cm³, or better with high-resolution balances and strict temperature control.

Liquid Pycnometer Method — ISO 1183-1 Method B

A calibrated glass pycnometer is weighed empty, filled with the displacement liquid, and then weighed again containing both liquid and specimen. The density is calculated from the mass of liquid displaced by the specimen. The method is well suited to pellets, film offcuts, and small or irregular pieces that are difficult to mount on a hydrostatic balance, and delivers accuracy comparable to the immersion method when the temperature is tightly controlled.

Titration Method — ISO 1183-1 Method C

The specimen is placed in a column of liquid, and a second, miscible liquid of different density is titrated in until the specimen reaches neutral buoyancy, remaining suspended without sinking or floating. The density of the resulting mixture, measured with a pycnometer, equals the density of the specimen. This method is intended for powders, small granules, and film trimmings, and can reach a resolution of approximately ±0.0005 g/cm³.

Density Gradient Column — ISO 1183-2, ASTM D1505

Two miscible liquids of different densities are combined to produce a stable, linear vertical density gradient inside a thermostatted column. Calibrated glass floats of known density serve as position markers, allowing the density at any height to be read from the column scale. Small specimens are introduced at the top and settle at the height where the liquid density equals their own. The gradient column is the highest-precision routine method, with a capability on the order of 0.0001 g/cm³ (0.1 kg/m³), and is the reference technique for fine differentiation of polyethylene grades and for research applications. Specimens must be bubble-free, and equilibration typically requires several hours.

Gas Pycnometer Method — ISO 1183-3

A gas pycnometer, usually employing helium, determines the true skeletal volume of a specimen from gas displacement in a calibrated chamber. Because the gas penetrates fine open porosity, the result represents the true solid density rather than an envelope volume. This method is ideal for powders, materials with complex geometry, moisture-sensitive polymers, and specimens for which liquid immersion is impractical.

Apparent (Bulk) Density — ASTM D1895

Apparent density is the mass per unit volume of a pourable material, including the voids between particles, measured by letting the material flow freely into a calibrated cup. It is used for powder handling, hopper design, and incoming inspection of molding materials, and must not be confused with the solid density measured by ISO 1183.

Typical method selection guidance is summarized below:

• Solid molded or machined piece for routine QC — immersion method, ISO 1183-1 Method A or ASTM D792
• Pellets and small irregular pieces — liquid pycnometer, ISO 1183-1 Method B
• Powders and film trimmings — titration method, ISO 1183-1 Method C
• Highest precision, resin grade differentiation, research — density gradient column, ISO 1183-2 or ASTM D1505
• Powders, porous or liquid-sensitive materials — gas pycnometer, ISO 1183-3
• Pourability and bulk handling studies — apparent density, ASTM D1895

Applications

Density testing according to international standards supports a wide range of industrial objectives:

• Incoming resin verification and supplier qualification against datasheet values
• Material identification and differentiation, for example separating LDPE, LLDPE, and HDPE, or distinguishing PP homopolymer from impact copolymer
• Crystallinity monitoring for semi-crystalline resins, supporting molding and extrusion cooling optimization
• Void and defect screening in molded parts, extrusions, and films
• Estimation of filler or reinforcement content in compounds containing talc, calcium carbonate, or glass fiber
• Support for polyethylene pressure-pipe grade classification and specification compliance
• Recycling operations, where density supports polymer sorting and contaminant detection
• Generation of compliance data for automotive, packaging, electrical, and medical device documentation

In many quality systems, density is measured at every receiving inspection because a deviation of even 0.005 g/cm³ from the reference value can indicate contamination, an incorrect resin lot, or a significant change in crystallinity or formulation.

FAQ

What is the difference between density and specific gravity?

Density is an absolute quantity expressed in mass per unit volume, such as g/cm³. Specific gravity, or relative density, is a dimensionless ratio between the density of the material and the density of water at stated temperatures, for example 23 °C/23 °C. The two values are numerically close for plastics because water has a density near 1 g/cm³, but they are distinct quantities and the reference temperature should always be reported.

Which method should be selected for my material?

Selection depends on specimen form, required accuracy, and the specification invoked by the customer or datasheet. Solid parts and plaques are usually tested by the immersion method; pellets by pycnometry or titration; powders by titration or gas pycnometry; and the highest-precision work, such as polyethylene grade differentiation, by density gradient column. Accredited third-party laboratories can recommend the most appropriate method for a given material and objective.

How much sample is required?

For hydrostatic weighing, a single piece or combined pieces with a total mass of at least 1 g, preferably 1 g to 5 g, are recommended. The pycnometer and titration methods require only a few grams of small pieces or powder, and the gradient column can work with a few pellet slivers. Testing multiple specimens and reporting the mean improves representativeness.

Why does the density of pellets sometimes differ from that of molded parts?

Pelletization and molding impose different cooling histories on a semi-crystalline polymer, producing different degrees of crystallinity, and pellets may also contain trapped microvoids. Differences on the order of 0.001 g/cm³ to 0.01 g/cm³ are therefore possible. Datasheet values generally refer to compression-molded specimens, so comparisons should be made on an equivalent specimen basis.

What accuracy and repeatability can be expected?

The immersion and liquid pycnometer methods typically achieve accuracy within about ±0.001 g/cm³, the titration method about ±0.0005 g/cm³, and the density gradient column up to approximately ±0.0001 g/cm³ under well-controlled conditions. Repeatability depends on temperature stability, complete removal of surface bubbles, balance calibration, and, for gradient columns, regular recalibration with reference floats.

Do moisture and temperature affect the results?

Yes. Specimens are conditioned per ISO 291 or ASTM D618, and hygroscopic polymers may require drying before testing. Since polymer density decreases with rising temperature at a rate of roughly a few hundredths of a percent per degree Celsius, measurements are made at 23 °C with the immersion liquid temperature recorded and corrected as required.

How long does a typical test take?

Immersion, pycnometer, and gas pycnometer measurements can usually be completed within one working day once conditioning is finished. Density gradient columns require several hours of preparation and specimen equilibration, and results may be issued the following day. A compliant test report states the standard and method used, the immersion or displacement medium, the test temperature, the specimen form, individual and mean results, and the measurement uncertainty where applicable.