Determination of Relative Density of Plastics: Principles, Methods, and Standards

Relative density determination is one of the most fundamental physical tests for plastic materials, providing essential data for material identification, quality control, and engineering design. This technical article explains the principles, standard test methods, sample requirements, and practical applications of plastic relative density measurement in accordance with internationally recognized standards such as ISO 1183 and ASTM D792.

Technical Overview

Relative density, also known as specific gravity, is defined as the ratio of the density of a material to the density of a reference substance, typically water, measured under specified conditions. For plastic materials, relative density is a dimensionless quantity that reflects the mass-to-volume relationship of the polymer and is widely used for material identification, batch-to-batch consistency verification, and estimation of filler content or crystallinity.

The determination of relative density for plastics is governed by several internationally recognized standards, including ISO 1183-1, ISO 1183-2, ASTM D792, ASTM D1505, and the Chinese national standard GB/T 1033. These standards describe multiple test methods based on Archimedes' buoyancy principle, liquid displacement, or density gradient techniques. The choice of method depends on the shape, size, and nature of the specimen, as well as the required precision.

Testing is typically performed at a standard laboratory temperature of 23°C ± 2°C, as density is temperature-dependent. Accurate control of temperature, immersion liquid purity, and surface bubble removal are critical factors that influence the reliability of test results. Accredited third-party laboratories routinely perform these measurements as part of material qualification, incoming inspection, and research programs.

Testing Samples

Molded or Machined Solid Specimens

Solid specimens in the form of bars, discs, plaques, or cubes are the most common sample type for relative density testing. Specimens should be free from voids, cracks, sink marks, and surface contamination. A typical specimen mass between 1 g and 50 g is suitable for the immersion method, provided it can be conveniently handled with the weighing assembly.

Pellets and Granules

Raw polymer pellets and granules can be tested using the pycnometer method or by enclosing multiple pellets in a cage or basket for immersion weighing. Pellet samples should be representative of the lot and free from moisture, dust, and processing aids that could alter the apparent density.

Films and Sheets

Thin films and sheets may be tested by stacking multiple layers or cutting pieces of sufficient total mass. Care must be taken to avoid entrapped air between layers. Films with thickness below approximately 100 micrometers are often better suited to density gradient column techniques.

Finished Products and Components

Sections cut from finished parts, such as pipes, fittings, containers, and injection-molded components, may be tested directly. Samples should be taken from regions of uniform wall thickness, away from gates, weld lines, and highly stressed areas where density gradients may exist.

Powders and Small Particles

Fine powders, such as polyethylene resin powders, are typically measured using the pycnometer (density bottle) method or a gas displacement pycnometer, since individual particles are too small for direct immersion weighing.

Testing Items

Relative density testing of plastics typically encompasses the following measurement items and derived parameters:

• Relative density (specific gravity) at 23°C relative to water
• Absolute density expressed in g/cm³ or kg/m³
• Density of amorphous and semi-crystalline thermoplastics
• Density of thermosetting materials and cured compounds
• Density of filled and reinforced compounds, including glass-fiber and mineral-filled grades
• Bulk density and apparent density of powders and pellets, when required
• Density variation across a component or along an extruded profile
• Comparison of specimen density against datasheet or specification limits

Testing Methods

Immersion Method (Hydrostatic Weighing)

The immersion method, described in ISO 1183-1 Method A and ASTM D792 Method A, is the most widely used technique. The specimen is first weighed in air using an analytical balance. It is then fully immersed in a wetting liquid, usually distilled water containing a small amount of wetting agent, and weighed again while fully submerged. According to Archimedes' principle, the buoyancy force equals the weight of the displaced liquid, allowing the specimen volume to be calculated. The relative density is obtained from the ratio of the two weighings, corrected for the density of the immersion liquid at the test temperature. A density determination kit with a wire basket and immersion vessel mounted on the balance is standard equipment for this method.

Pycnometer Method

Described in ISO 1183-1 Method B, the pycnometer method is suitable for pellets, powders, and small particles. A calibrated pycnometer is weighed empty, filled with immersion liquid, and then filled with the specimen plus the displaced liquid. The specimen density is calculated from the mass differences and the known density of the liquid. This method requires careful degassing to eliminate trapped air bubbles.

Titration (Flotation) Method

ISO 1183-1 Method C describes a flotation technique in which the specimen is suspended in a liquid mixture whose density is gradually adjusted by titration until the specimen neither sinks nor floats, reaching neutral buoyancy. The density of the liquid at that point, measured with a precision hydrometer or oscillator, equals the density of the specimen. This method is useful for small specimens or waterproof materials.

Density Gradient Column Technique

ISO 1183-2 and ASTM D1505 describe the density gradient column method, which offers the highest precision among routine techniques. A tall column contains two miscible liquids arranged so that a continuous, calibrated density gradient exists from top to bottom. Calibrated glass floats of known density are positioned in the column, and the test specimen is dropped into the column; the level at which it settles indicates its density. This method can resolve density differences as small as 0.0001 g/cm³ and is commonly used for polyethylene classification and crystallinity studies.

Hydrometer Method

ISO 1183-1 Method D applies to plastics in the form of powder or void-free shaped objects that can be tested by observing flotation in liquids of known density. Although less precise than other methods, it provides a rapid screening technique for routine quality control.

Ultrasonic and Non-Destructive Methods

For in-process monitoring and finished part verification, ultrasonic velocity measurement can be correlated with density. These non-destructive techniques complement classical gravimetric methods and are increasingly used in production environments, although they generally require calibration against standard reference methods.

Applications

Relative density determination serves numerous practical purposes across the plastics industry:

• Material identification and differentiation of polyolefin grades, such as distinguishing high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE)
• Quality control of incoming resin and verification of supplier material certificates
• Estimation of filler or reinforcement content in compounded materials by comparing measured density with calculated theoretical values
• Assessment of crystallinity in semi-crystalline polymers, since density increases with crystalline content
• Detection of porosity, voids, and incomplete consolidation in molded or extruded products
• Support of failure analysis investigations, where density deviations may reveal contamination, degradation, or processing anomalies
• Input data for engineering calculations, including part weight estimation, buoyancy design, and mold shrinkage prediction
• Compliance verification against material specifications in automotive, packaging, piping, medical device, and electrical industries
• Research and development of novel compounds, foams, and polymer blends

FAQ

What is the difference between density and relative density?

Density is an absolute quantity expressed as mass per unit volume, typically in g/cm³, while relative density is the ratio of the material density to the density of a reference substance, usually water at a specified temperature. Because water has a density close to 1 g/cm³ at room temperature, the numerical values of density and relative density for plastics are often similar, but relative density is dimensionless.

Which standard should be used: ISO 1183 or ASTM D792?

Both standards describe comparable immersion-based methods, and results are generally consistent when test conditions are matched. ISO 1183-1 is commonly requested in Europe and Asia, whereas ASTM D792 is widely used in North America. The choice typically depends on customer requirements, product specifications, and regulatory jurisdiction. GB/T 1033 is the Chinese national standard equivalent.

Why is temperature control important in density testing?

Both the specimen and the immersion liquid change volume with temperature, and water density itself varies with temperature. A difference of only a few degrees can introduce measurable error, particularly for high-precision measurements. For this reason, tests are standardized at 23°C and thermometers with 0.1°C resolution are used to monitor the immersion liquid.

How are air bubbles on the specimen surface removed?

A small amount of wetting agent, such as a drop of surfactant, is added to the immersion liquid to reduce surface tension. The specimen may also be gently agitated or pre-wetted with the liquid before measurement. Residual bubbles must be eliminated because they add apparent buoyancy and cause underestimated density values.

What accuracy can be expected from these methods?

The immersion method typically achieves repeatability within approximately 0.001 g/cm³ when performed with a calibrated analytical balance. The pycnometer method offers similar precision for pellets and powders, while the density gradient column can achieve precision on the order of 0.0001 g/cm³, making it the preferred method for fine discrimination among polyethylene grades.

Can foamed or porous plastics be tested by immersion?

Open-cell foams absorb the immersion liquid, which leads to erroneous results, so they require special procedures, sealed-coating techniques, or gas pycnometry instead. Closed-cell foams with intact cell structures may be tested by immersion, provided no liquid penetration occurs during the measurement period.

How long does a typical relative density test take?

A single immersion measurement, including conditioning and equilibrium weighing, is usually completed within 30 minutes. Density gradient columns require initial setup and calibration, and specimens may need several hours to reach their equilibrium position, though multiple specimens can be measured simultaneously once the column is established.