Density determination is one of the most fundamental and informative physical tests performed on plastic test specimens, offering direct insight into material identity, crystallinity, filler content, and molding quality. Accredited third-party laboratories rely on internationally recognized methods such as the immersion method, liquid pycnometer, density-gradient column, and gas pycnometer, governed by standards including ISO 1183 and ASTM D792. This technical article explains how precise density measurement of plastic specimens is carried out, which factors govern accuracy, and how the results are applied across the polymer industry.
Technical Overview
Density, defined as mass per unit volume and expressed in g/cm³ or kg/m³, is a primary physical property of plastics that reflects composition, molecular structure, and processing history. Because polymers are sensitive to crystallinity, orientation, void content, and thermal history, a density measurement performed on a molded specimen is far more than a simple identification number: it is a sensitive indicator of how the material was polymerized, compounded, and processed.
Precise density determination of plastic test specimens (such as tensile bars, compression-molded plaques, pellets, and films) is standardized under a coherent framework of international and national standards:
- ISO 1183-1 — Immersion method, liquid pycnometer method, and titration method for non-cellular plastics
- 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
- GB/T 1033 series — Chinese national standards technically aligned with ISO 1183
Typical density ranges of common thermoplastics illustrate the discriminating power of the measurement: polypropylene (0.90–0.91 g/cm³), low-density polyethylene (0.910–0.925 g/cm³), high-density polyethylene (0.941–0.965 g/cm³), polystyrene (1.04–1.06 g/cm³), ABS (1.03–1.07 g/cm³), polyamide 6 (1.12–1.15 g/cm³), polycarbonate (approximately 1.20 g/cm³), rigid PVC (1.35–1.45 g/cm³), polyoxymethylene (approximately 1.41 g/cm³), and PTFE (2.13–2.22 g/cm³).
Testing Samples
Molded Test Specimens (Tensile Bars)
Injection-molded or machined tensile bars, such as ISO 527-2 1A/1B multipurpose specimens or ASTM D638 Type I bars, are the most common samples for density testing. A section cut from the gauge length or the shoulder of the bar is typically used. Because injection molding introduces skin-core morphology, orientation, and possible residual stress, the measured density characterizes the specimen in its as-molded state and may differ slightly from the resin datasheet value, which is usually determined on compression-molded plaques.
Compression-Molded Plaques and Sheets
Compression molding under controlled cooling yields specimens with more uniform and reproducible morphology, which is why resin producers and standards such as ISO 1183 specify compression-molded samples for nominal density declaration. Plaques are cut into convenient pieces free of voids, blisters, and sink marks before testing.
Pellets, Granules, and Small Particles
Virgin resin in pellet or powder form cannot be conveniently weighed while suspended in a liquid. For such materials, the liquid pycnometer method or gas pycnometer method is applied, using several grams of conditioned pellets to obtain a representative average density.
Films and Finished Components
Flexible films, laminates, and cut sections of finished parts (pipe fittings, cable insulation, automotive trim) can also be measured. Care must be taken to remove surface contamination, and hygroscopic materials such as polyamide must be conditioned to a defined moisture state before weighing.
Testing Items
A typical density test report for plastic specimens includes the following measured or derived quantities:
- Density of the plastic at 23 °C, expressed in g/cm³ (typically reported to three or four decimal places)
- Specific gravity (relative density), the dimensionless ratio of the specimen density to the density of water at the reference temperature
- The test method applied (immersion, pycnometer, gradient column, or gas displacement) and the governing standard
- The immersion or displacement medium used and the test temperature
- Number of specimens tested and the arithmetic mean, together with the standard deviation
- Sample description: form, molding condition, conditioning state, and any surface preparation
- Optional derived results, such as estimated crystallinity index or comparison against a specified density limit
Testing Methods
Immersion Method (Hydrostatic Weighing)
The immersion method, described in ISO 1183-1 Method A and ASTM D792 Method A/B, is the most widely used technique for molded specimens. It is based on Archimedes' principle:
- The specimen is first weighed in air on an analytical balance (apparent mass a).
- The specimen is then weighed while fully immersed in a wetting liquid (apparent mass b), suspended from a fine wire; a sinker is attached when the specimen, such as polypropylene or polyethylene, floats on water.
- The density is calculated as ρ = a × ρliquid / (a − b), where ρliquid is the density of the immersion liquid at the test temperature (for pure water at 23 °C, 0.9976 g/cm³).
- A wetting agent (for example, 0.1 % of a non-ionic surfactant) is added to distilled water to prevent air bubbles from clinging to hydrophobic polymer surfaces, and the specimen is brushed or agitated to release entrained air.
With a balance resolution of 0.1 mg, a stable immersion platform, and temperature control within ±0.1 °C, repeatability on the order of ±0.001–0.002 g/cm³ is routinely achieved on single solid pieces.
Liquid Pycnometer Method
Specified in ISO 1183-1 Method B and ASTM D792 Method B, the liquid pycnometer method is preferred for pellets, granules, and powders. A calibrated glass pycnometer of known volume is weighed empty, filled with immersion liquid, and filled with liquid plus a known mass of sample. The displaced liquid volume yields the specimen volume, from which density is computed. The method averages out specimen-to-specimen variation but requires meticulous degassing and temperature equilibration.
Density Gradient Column Method
The density gradient technique per ISO 1183-2 and ASTM D1505 offers the highest sensitivity, with a working resolution down to approximately 0.0001 g/cm³. A tall column is filled with two miscible liquids (for example, isopropanol and water for the polyolefin range) in a continuously varying proportion, creating a stable linear vertical density gradient. Calibrated glass marker floats establish the density-versus-height calibration curve. When a small specimen is introduced, it sinks to the level where its density equals that of the surrounding liquid, and the equilibrium height is converted to density. This method is the reference procedure for classifying polyethylene grades, where the distinction between LDPE, LLDPE, MDPE, and HDPE is defined by narrow density windows.
Gas Pycnometer Method
ISO 1183-3 specifies a gas displacement technique in which helium or another inert gas fills a calibrated chamber, and the specimen volume is derived from pressure readings before and after expansion. Because no wetting liquid contacts the sample, the method is suitable for materials that absorb liquid, that contain open porosity, or that must remain dry, and it yields the true solid-phase density.
Factors Affecting Measurement Precision
- Temperature control: both the immersion liquid density and the polymer volume vary with temperature, so the bath must be held at 23 ± 0.5 °C or tighter, and specimens must be thermally equilibrated before weighing.
- Air bubbles and wetting: residual microbubbles are the dominant error source in the immersion method; surfactant addition, gentle brushing, and pre-soaking are essential.
- Specimen quality: internal voids, sink marks, moisture uptake in polyamides, and surface contamination bias the result; specimens should be void-free, clean, and conditioned per ISO 291 where applicable.
- Balance and buoyancy corrections: analytical balances of 0.1 mg resolution, calibrated sinkers and suspension wires, and air-buoyancy corrections are required for high-precision work.
- Morphological representativeness: injection-molded, compression-molded, and annealed specimens of the same resin can differ measurably in density, so the sample form must match the purpose of the test.
Applications
Precise density data on plastic specimens support a wide range of engineering and commercial decisions:
- Material identification and grade verification of unknown or incoming resin against supplier datasheets
- Polyethylene classification and specification compliance, where density defines the product class and correlates with stiffness, yield strength, and permeability
- Crystallinity estimation by comparing measured density with the densities of the fully amorphous and fully crystalline phases of the polymer
- Quality control of molding processes, since voids, insufficient packing, or excessive cooling rates produce measurable density deviations
- Filler and reinforcement content assessment in compounded materials such as glass-filled or mineral-filled compounds
- Crosslinking evaluation of XLPE cable insulation, where density trends complement gel-content and hot-set testing
- Support for regulatory declarations, customs classification, and input to mass and dimensional calculations in product design
FAQ
What accuracy can be expected from a density test on a plastic specimen?
The immersion method typically achieves repeatability of about ±0.001–0.002 g/cm³ on well-prepared solid specimens, while a properly calibrated density gradient column can resolve differences as small as 0.0001 g/cm³. Interlaboratory reproducibility is generally somewhat wider and depends strongly on consistent temperature control and bubble removal.
Which method should be used for polyethylene and polypropylene specimens?
For classification and tight specification limits, the density gradient column per ISO 1183-2 or ASTM D1505 is preferred. For routine quality control of solid molded pieces, the immersion method per ISO 1183-1 Method A with a sinker is fully adequate, since polyolefins float on water and must be forcibly submerged.
Why is temperature control so critical?
The density of the immersion liquid changes with temperature (water changes by roughly 0.0002–0.0003 g/cm³ per kelvin near room temperature), and polymers have comparatively high volumetric thermal expansion. A one-kelvin drift can therefore shift the reported density in the third or fourth decimal place, which is significant relative to specification windows.
Can density measurement indicate crystallinity?
Yes. When the densities of the perfectly crystalline and purely amorphous phases of a polymer are known, the measured specimen density can be converted into a crystallinity index. This relationship is widely used for polyethylene, polypropylene, and PET to track the effect of cooling rates and annealing.
How are specimens that absorb water handled?
Hygroscopic materials such as polyamides are dried or conditioned to a defined moisture state before testing, and the conditioning state is reported. Alternatively, a non-absorbing displacement liquid or the gas pycnometer method per ISO 1183-3 can be used to avoid liquid uptake altogether.
What is the difference between density and specific gravity?
Density is an absolute property with units of mass per volume, whereas specific gravity (relative density) is the dimensionless ratio of the specimen density to that of water at a reference temperature. Numerically the two are nearly identical for plastics, and both are reported under ISO 1183 and ASTM D792.
How many specimens should be tested?
Standards generally require testing at least three representative specimens and reporting the mean. For gradient column work, two to three drops of each specimen may be averaged, and for pellets the pycnometer charge should consist of sufficient granules to be statistically representative of the lot.