Plastic density testing by the immersion method is a high-precision gravimetric technique used to determine the density and specific gravity of non-cellular plastics based on Archimedes' buoyancy principle. Standardized under ISO 1183-1, ASTM D792, and GB/T 1033.1, the method is widely applied in material identification, incoming inspection, and quality control across the polymer industry. This article explains the measurement principle, sample requirements, operating procedure, calculation methods, and typical applications of immersion density testing.
Technical Overview
The immersion method is a gravimetric technique for determining the density of non-cellular (solid) plastics. It is based on Archimedes' principle: when a specimen is fully immersed in a liquid of known density, the buoyancy force acting on it equals the weight of the liquid displaced. By weighing the specimen separately in air and while immersed, the volume of the specimen can be derived, and its density calculated. Because the measurement relies on a mass difference rather than on dimensional measurement, the method delivers results with excellent repeatability and is suitable even for irregularly shaped parts.
The immersion method is standardized internationally in ISO 1183-1 (Plastics — Methods for determining the density of non-cellular plastics — Part 1: Immersion method, liquid pycnometer method and titration method), where it is designated as Method A. The corresponding North American standard is ASTM D792 (Standard Test Methods for Density and Specific Gravity of Plastics by Displacement), and equivalent national standards include GB/T 1033.1 and JIS K 7112. The method applies to thermoplastics and thermosets in solid form, including molded parts, machined pieces, pellets, films, and sheets, provided the material is free of open porosity.
A typical test setup consists of an analytical balance with a readability of 0.1 mg, a hydrostatic density determination kit with a specimen basket and immersion vessel, a wetting agent, and a temperature-controlled liquid bath. Tests are normally performed at 23 °C in accordance with ISO 291 standard atmosphere conditions, and results are reported in g/cm³ or kg/m³.
Testing Samples
Sample form and surface condition strongly influence the validity of immersion density results. Specimens must be solid, free of voids, cracks, and open pores, and their surfaces must be clean and smooth so that trapped air bubbles do not distort the buoyancy measurement. Common sample categories are described below.
Molded Specimens and Machined Pieces
Injection-molded, compression-molded, or machined specimens, such as dumbbell bars, plaques, and discs, are the preferred form because their compact, pore-free structure satisfies the assumptions of the method. ISO 1183-1 recommends a total specimen mass of at least 1 g to ensure adequate weighing accuracy, and specimens taken from different locations of a part may be tested individually to assess homogeneity.
Plastic Pellets and Granules
Virgin resin pellets and regranulated material can be tested by placing several pellets together in the immersion basket until a sufficient total mass is reached. Care must be taken to confirm that pellets are free of trapped gas bubbles, which are common in poorly degassed or recycled granulate and can cause erroneously low density readings.
Films and Sheets
Flexible films can be tested by folding or stacking strips so they remain fully submerged without floating, and by ensuring the wetting agent reaches all surfaces. Rigid sheets may be cut into convenient pieces and treated like molded specimens.
Finished Products and Cut Parts
Articles such as pipe fittings, housings, closures, and optical components can be tested directly in whole or in cut form, making the immersion method valuable for verifying finished-goods conformity. Painted, coated, or contaminated surfaces should be cleaned or removed where possible, since foreign layers alter the measured density.
Testing Items
The immersion method primarily delivers the following measurable quantities:
- Density at 23 °C, expressed in g/cm³ or kg/m³, determined to a resolution typically reaching 0.001 g/cm³ or better.
- Specific gravity (relative density), the dimensionless ratio of the specimen density to the density of water at the reference temperature.
- Conformance evaluation of the measured density against polymer datasheet ranges, purchase specifications, or regulatory limits.
- Comparative density checks before and after conditioning, annealing, sterilization, or exposure to chemicals, used to quantify physical change in the material.
- Density-based estimation of filler or reinforcement content in compounds containing mineral fillers or glass fiber.
Testing Methods
Test Principle and Calculation
The specimen is first weighed in air, giving the mass m1. It is then completely immersed in the liquid while still suspended from the balance, giving the apparent immersed mass m2. The difference m1 − m2 equals the mass of displaced liquid, from which the specimen volume follows. The density of the specimen is calculated as:
ρ(specimen) = m1 × ρ(liquid) / (m1 − m2)
where ρ(liquid) is the density of the immersion liquid at the test temperature (for water at 23 °C, approximately 0.9976 g/cm³). For the highest accuracy, corrections are applied for air buoyancy and for the immersed portion of the suspension wire or sinker, as specified in ISO 1183-1.
Equipment and Reagents
- Analytical balance with 0.1 mg readability, fitted with a hydrostatic weighing attachment or dedicated density kit.
- Immersion liquid, normally freshly boiled and cooled distilled or deionized water containing a small amount of wetting agent to prevent air bubbles from clinging to the specimen.
- A fine suspension wire, typically not more than 0.5 mm in diameter, and a sinker assembly for specimens less dense than the immersion liquid.
- A calibrated thermometer with 0.1 °C resolution and a thermostatic bath or temperature-stable environment maintaining the liquid at 23 °C ± 0.5 °C.
Operating Procedure
- Condition the specimen to equilibrium in the standard atmosphere of 23 °C and 50 % relative humidity unless otherwise specified.
- Clean the specimen to remove dust, mold release agents, oils, and other surface contaminants.
- Weigh the dry specimen in air on the density kit and record mass m1.
- Fully immerse the specimen in the temperature-stabilized liquid, ensuring complete wetting and the absence of adhering bubbles; brush or agitate gently if necessary.
- Record the apparent immersed mass m2 once the balance reading has stabilized.
- Measure and record the liquid temperature, obtain the corresponding liquid density, and calculate the specimen density with the required corrections.
- Perform at least two determinations on separate specimens or repeat measurements and report the mean value together with the test temperature and liquid used.
Special Handling Cases
- Specimens lighter than water, such as polypropylene and polyethylene, are tested using an attached sinker of known mass and volume, or with an alternative immersion liquid of lower density such as ethanol, with the calculation adjusted accordingly.
- Hygroscopic materials, including polyamide (nylon), should be dried to a defined state before testing, or a non-polar liquid may be used to prevent liquid uptake during measurement.
- Porous, foamed, or bubble-containing samples are outside the scope of the immersion method; the liquid pycnometer or titration procedures of ISO 1183-1, or the density-gradient column technique of ASTM D1505, are more appropriate alternatives.
Applications
Because density is a fundamental, composition-sensitive property, immersion testing serves a broad range of industrial and laboratory purposes:
- Polymer identification and grade differentiation, for example distinguishing LDPE, HDPE, and PP, or separating ABS from PS and HIPS.
- Incoming resin inspection and verification that delivered material matches the specified grade and datasheet density range.
- Process quality control in extrusion, injection molding, and compounding, where density shifts signal changes in filler loading, moisture, or processing conditions.
- Estimation of filler or glass fiber content in reinforced compounds by comparing measured density against calculated theoretical values.
- Crystallinity and annealing studies on semi-crystalline plastics, where density changes reflect the amorphous-to-crystalline phase balance.
- Failure analysis and research, including evaluation of degradation, void formation, or migration effects in aged components.
- Compliance testing for product specifications in accredited third-party laboratories and internal quality departments.
Representative density values at room temperature for orientation purposes include: PP approximately 0.90–0.91 g/cm³, HDPE approximately 0.94–0.97 g/cm³, PS approximately 1.04–1.06 g/cm³, ABS approximately 1.05–1.08 g/cm³, PC approximately 1.20 g/cm³, PVC approximately 1.35–1.45 g/cm³, PET approximately 1.38–1.41 g/cm³, and PTFE approximately 2.13–2.20 g/cm³.
FAQ
Why is the immersion method preferred for routine plastic density measurement?
It combines high accuracy, simple equipment, and applicability to irregularly shaped specimens. Since volume is derived from buoyancy rather than from geometry measurements, the method avoids the errors associated with dimensional measurement of curved or complex parts.
What accuracy and repeatability can be expected?
With a 0.1 mg balance, good temperature control, and careful bubble removal, results reproducible to within a few thousandths of a g/cm³ are routinely achieved. Precision data published in ISO 1183-1 indicate repeatability on the order of 0.003 g/cm³ for Method A under properly controlled conditions.
How are plastics that float in water tested?
Low-density materials such as PP and PE are measured with a non-wetting auxiliary sinker of known density and volume attached to the specimen, or by substituting a lower-density immersion liquid such as ethanol. In both cases the standard calculation is extended to account for the sinker contribution.
Why must bubbles and surface contamination be controlled?
Air bubbles adhering to the specimen increase buoyancy and produce a falsely low density, while dirt, oils, or absorbed liquids change the true mass of the specimen. Wetting agents, gentle agitation, and specimen cleaning are therefore essential steps of the procedure.
How does temperature influence the result?
The density of the immersion liquid changes measurably with temperature, and the specimen itself expands or contracts. The liquid temperature must therefore be recorded, its density at that temperature used in the calculation, and testing performed near the standard reference temperature of 23 °C for comparability.
What should be done if the sample contains voids or is porous?
The immersion method assumes a pore-free solid. Foamed, sintered, or void-containing specimens should be tested by the liquid pycnometer method, the titration method, or a density-gradient column, with the chosen method clearly stated in the test report.