Plastic density testing in accordance with Chinese national standards (the GB/T 1033 series) provides accurate, repeatable measurement of one of the most important physical properties of polymer materials. This guide explains the national standard framework for plastics density determination, covering applicable sample types, test items, the immersion, liquid pycnometer, titration, density gradient column, and gas pycnometer methods, typical industrial applications, and expert answers to frequently asked questions.
Technical Overview
Density, expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³), is defined as the mass of a material per unit volume at a specified temperature. For plastics, density serves as a primary identifier of resin type and grade, a sensitive indicator of crystallinity and filler content, and a routine quality control parameter in incoming inspection and finished product release. In China, the determination of density for non-cellular (solid) plastics is governed by the GB/T 1033 series of national standards, which is technically aligned with the ISO 1183 series and broadly comparable with ASTM D792, ASTM D1505, and JIS K 7112.
- GB/T 1033.1-2022 — Immersion method (Method A), liquid pycnometer method (Method B), and titration method (Method C); aligned with ISO 1183-1.
- GB/T 1033.2 — Density gradient column method; aligned with ISO 1183-2 and offering the highest measurement resolution.
- ISO 1183-3 — Gas pycnometer method for true (skeletal) density determination of powders and porous materials.
- GB/T 2918 — Standard atmosphere for conditioning and testing plastics (23 °C ± 2 °C, 50 % ± 10 % relative humidity).
- GB/T 6343 (equivalent to ISO 845) — Apparent density of cellular (foamed) plastics, applicable when rigid foam materials fall outside the scope of GB/T 1033.
- ASTM D792, ASTM D1505, and JIS K 7112 — Equivalent international procedures that enable cross-standard data comparison.
Unless otherwise agreed, density is reported at the reference temperature of 23 °C. Specimens must be free of voids, cracks, moisture, and surface contamination, because entrapped air and surface films are the dominant sources of error in hydrostatic weighing.
Testing Samples
Thermoplastic Resins, Granules, and Powders
Virgin and recycled resins such as PE, PP, PS, ABS, PVC, PET, PA, PC, PMMA, POM, and PTFE are commonly submitted in pellet, granule, or powder form. The titration and liquid pycnometer methods are well suited to these geometries, while the gas pycnometer method is preferred for fine powders and moisture-sensitive polymers.
Molded Specimens and Machined Test Pieces
Compression- or injection-molded plaques, offcuts from tensile dumbbells, and machined blocks with a mass of approximately 1 g to 5 g and a simple, closed shape are ideal for the immersion method. Specimens should be conditioned in the standard laboratory atmosphere before measurement.
Films and Sheets
Rigid films and sheets can be tested by stacking multiple clean, dry cut pieces to achieve an adequate mass, or by the density gradient column technique when very high resolution is required.
Filled, Reinforced, and Modified Compounds
Glass-fiber reinforced, mineral-filled, flame-retardant, and impact-modified compounds are tested to verify formulation consistency; density shifts reliably signal changes in filler loading, void content, or resin substitution between production lots.
Finished Products and Components
Cut sections from pipes, fittings, containers, profiles, and finished parts can be accepted provided the sampled region is representative, solid throughout, and can be cleaned of coatings, paints, and adhesives.
Testing Items
- Density of non-cellular plastics at 23 °C (g/cm³), determined per GB/T 1033.1 or GB/T 1033.2.
- Relative density (specific gravity) at 23/23 °C, i.e., the ratio of material density to the density of distilled water at the same temperature.
- True density by gas pycnometry (ISO 1183-3) for porous, granular, or hygroscopic materials.
- Apparent density of cellular plastics per GB/T 6343 / ISO 845 for foamed materials outside the scope of GB/T 1033.
- Data-sheet conformity checks and crystallinity estimation based on measured density versus amorphous and crystalline reference values.
Testing Methods
Selection among the national standard methods depends on sample geometry, sensitivity to water, and the precision required by the product specification. Accredited third-party laboratories typically apply the following procedures.
Method A — Immersion Method
Based on Archimedes' principle, the specimen is weighed in air and then reweighed while fully immersed in a liquid of known density, typically distilled water with a trace of wetting agent for hydrophilic surfaces. Density is calculated from the buoyancy difference, with corrections for the density of the immersion liquid and, where high accuracy is needed, for air buoyancy. For hygroscopic polymers such as polyamide, or for materials that swell in water, ethanol–water mixtures, kerosene, or other non-reactive liquids are substituted. With a balance of 0.1 mg resolution, accuracy on the order of 0.001 g/cm³ is routinely achieved.
Method B — Liquid Pycnometer Method
The specimen, such as pellets or irregular cuttings, is placed inside a calibrated pycnometer of known volume. The mass of liquid displaced by the immersed sample is determined by differential weighing of the filled pycnometer with and without the specimen. This method suits samples that are too small or too irregular to be suspended from a wire hook.
Method C — Titration Method
A quantity of granules is suspended in a liquid, and a miscible liquid of higher or lower density is added dropwise until the granules remain neutrally buoyant, neither sinking nor rising. The density of the resulting mixture, measured with a pycnometer or a precise hydrometer, equals the specimen density. The method is a rapid screening tool widely used for polyolefin pellets.
Density Gradient Column Method
A stable, linear density gradient is established in a tall column using two miscible liquids, and the column is calibrated with certified glass floats. Specimens are introduced and their equilibrium heights are converted into density values through the calibration curve. With resolution approaching 0.0001 g/cm³, this technique is the reference method for distinguishing polyethylene grades and for studying crystallinity distribution.
Gas Pycnometer Method
Helium or nitrogen displacement under controlled pressure measures the skeletal (true) volume of the sample, excluding open porosity and surface effects. This approach is ideal for powders, moisture-sensitive polymers, and materials whose surface chemistry makes reliable wetting in liquids difficult.
Applications
Density testing to Chinese national standards supports decision-making across the entire plastics value chain:
- Resin identification and grade confirmation — separating HDPE, LDPE, and LLDPE, or distinguishing PP homopolymer from copolymer families during incoming inspection.
- Formulation and quality control — monitoring filler or glass-fiber content in reinforced compounds, where each weight percent of mineral loading produces a measurable density shift.
- Crystallinity evaluation — calculating the crystalline fraction of PA, PP, or PET from measured density using two-phase reference densities.
- Process troubleshooting — detecting voids, incomplete fusion, or moisture-related defects in extruded and molded parts through anomalous density readings.
- Recycled content verification — checking lot-to-lot consistency of reclaimed feedstock before compounding.
- Regulatory and contractual compliance — issuing standardized test reports for product certification, customs declarations, and customer specifications.
Accredited third-party laboratories routinely perform these tests under GB/T 1033 and issue reports stating the method, immersion liquid, temperature, individual and mean results, and measurement uncertainty, so that data remain traceable and comparable between suppliers and buyers.
FAQ
What is the difference between density and specific gravity?
Density is an absolute quantity (mass per unit volume), whereas specific gravity, or relative density, is dimensionless: the ratio of the material's density to that of water at the reference temperature (23/23 °C). Numerically the two nearly coincide because the density of water at 23 °C is close to 1 g/cm³.
Which method should be chosen for routine testing?
For molded pieces, Method A (immersion) is the default because of its speed and precision. Pellets are best screened by Method C (titration) and confirmed by Method B (liquid pycnometer) or the density gradient column. Powders, porous, or water-sensitive materials are measured by the gas pycnometer method.
What are typical density values for common plastics?
- PP: 0.90–0.91 g/cm³; LDPE: 0.910–0.925 g/cm³; HDPE: 0.941–0.965 g/cm³
- PS: 1.04–1.06 g/cm³; ABS: 1.03–1.08 g/cm³; PMMA: about 1.18 g/cm³; PC: about 1.20 g/cm³
- PET: 1.34–1.40 g/cm³; PA6: 1.12–1.15 g/cm³; POM: about 1.41 g/cm³
- Rigid PVC: 1.35–1.45 g/cm³; PTFE: 2.1–2.3 g/cm³
Why must the test be performed at 23 °C?
The density of both the plastic and the immersion liquid changes with temperature — the density of water alone varies by roughly 0.0003 g/cm³ per degree near room temperature. GB/T 1033 fixes the reference temperature at 23 °C with tight thermostat control, typically ±0.5 °C for the immersion method, so that results from different laboratories are directly comparable.
Can hygroscopic plastics such as nylon be measured in water?
Direct water immersion risks water uptake and swelling, which distort the reading. Standard practice is to use a non-wetting liquid, such as an ethanol–water mixture or kerosene, or to apply the gas pycnometer method on a dried specimen.
How does density relate to crystallinity?
Because the crystalline phase of a semicrystalline polymer is denser than the amorphous phase, the measured density can be converted into a crystalline fraction using the two-phase mixing rule with literature values for fully amorphous and fully crystalline densities, providing a fast, non-destructive complement to thermal analysis.
What should be included in a test report?
A compliant report states the standard and method used, the immersion or gradient liquids, the test temperature, the conditioning history, individual and mean density values, and the measurement uncertainty, allowing the results to be defended in supplier audits and certification reviews.