Density

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Density

General information

To determine the density, the mass and the volume must generally be determined. The density ρ of a homogeneous body is defined as the ratio of the mass m to the total volume V, i.e.

\rho \,=\,{\frac {m}{V}}

where V is the volume of the total amount of substance, i.e. including any pores. The values determined in this way are also referred to as gross density, but if one refers to the volume of the solid alone, one speaks of pure density. The gross density of poured substances is also called bulk density. The density has the unit of measurement g cm^-3 or kg m^-3. The known physical methods are used to determine the density experimentally:

Buoyancy method,
Floating method,
Pycnometer method and
Density gradient column.

Compared to other materials, plastics have a relatively low density of around 0.9 g cm^-3 for the polyolefins polypropylene (abbreviation: PP) and polyethylene (abbreviation: PE) to around 2.3 g cm^-3 for polytetrafluoroethylene (abbreviation: PTFE). Polyolefins swim in water due to their lower density and for this reason it is also possible to separate them from other plastics, e.g. during recycling.

As some physical properties are closely linked to density, e.g. crystallinity, a change in these properties can be monitored using density measurements.

**Table**: The density of plastics compared to other material classes
Materials	Density (g/cm³)
Natural rubber (NR)	0.92 – 1.0
Polyethylene (PE)	0.92 – 0.96
Polypropylene (PP)	0.9 – 1.0
Polystyrene (PS)	1.05
Polycarbonate (PC)	1.0 – 1.2
Polyamide (PA)	1.0 – 1.2
Polymethyl methacrylate (PMMA)	1.16 – 1.20
Polyvinylchloride (PVC)	1.2 – 1.4
Polybutylene terephthalate (PBT)	1.30 – 1.32
Polyoxymethylene (POM)	1.34 – 1.43
Polyvinylchloride, post chlorinated (PVC-C)	1.47 – 1.55
Polytetrafluorethylene (PTFE)	> 1.8
Foamed plastics	0.005 – 0.100
Integral plastics	to 1.0
Gold	19.3
Steel	7.8
Aluminium	2.7
Wood	0.2 – 0.95
Water	1.0

The Table shows that the density of various materials is in some cases many times higher than that of plastics. The higher density of other materials can be attributed to two causes:

With a density of 7.8 g cm^-3 and 2.7 g cm^-3 respectively, the individual atoms of iron and aluminum are significantly heavier than the elementary components of polymers, such as carbon, nitrogen, oxygen and hydrogen atoms.
The average distance between the atoms in plastics is sometimes greater than in metals.

References

DIN 1306 (1984-06): Density – Concepts, Presentation of Values
ISO 1183-1 (2025-06): Plastics – Methods for Determining the Density of non-cellular Plastics – Part 1: Immersion Method, Liquid Pycnometer Method and Titration Method
ISO 2781 (2018-06): Rubber, Vulcanized or Thermoplastic – Determination of Density
ISO 845 (2006-12): Cellular Plastics and Rubber – Determination of Apparent Density
Michaeli, W., Greif, H., Kaufmann, H., Vossebürger, F.-Z.: Technologie der Kunststoffe. Carl Hanser, Munich Vienna (1992) (ISBN 3-446-15821-9; see AMK-Library under H 11)
Woebcken, W. (Hrsg.): Stoeckhert – Kunststofflexikon. Carl Hanser, Munich Vienna (1998), 9th Edition (ISBN 978-3-446-17969-1; see AMK-Library under G 3)
Hellerich, W. Harsch, G. Haenle, S.: Werkstoff-Führer Kunststoffe – Eigenschaften, Prüfungen, Kennwerte. Carl Hanser, Munich Vienna (1996), 7th Edition (ISBN 3-446-17617-9; see AMK-Library under G 1)

Density determination

ISO 1183 [1] specifies three methods for determining the density of non-foamed plastics that are in the form of bubble-free moulded or extruded products. Cavities such as holes or gas bubbles distort the measurement result. The methods listed in the standard are

Determination of density using the immersion method for semi-finished products and moulded parts (buoyancy method) – method A
Determination of density using the liquid pycnometer for particles, powders, flakes, granules or comminuted finished parts – method B
Titration method for bubble-free plastics in any shape – method C

Immersion method

Determination of density using the immersion method (method A)

The test is carried out on analytical balances or special devices designed to determine the density with a measuring accuracy of 0.1 mg. In addition to the container for the immersion liquid, a pycnometer with a lateral overflow capillary is required to accurately determine the density of the test liquid, usually freshly distilled water or deionized water or another suitable liquid with a mass fraction of no more than 0.1 % of a wetting agent to assist in the separation of air bubbles.

First the mass m_A of the sample is determined by weighing in air, then the mass m_IL of the sample is determined by weighing in the test liquid. The temperature of the immersion liquid must be 23 ± 2 °C or 27 ± 2 °C.

The density ρ_S of the sample at 23 °C (or 27 °C), in grams per cubic centimeter, is calculated using the following equation:

\rho _{S}\,=\,{\frac {m_{A}\cdot \rho _{IL}}{m_{A}-m_{IL}}}

where

m_A		is the mass of the sample in air (in grams)
m_IL		is the mass of the sample in the immersion liquid (in grams)
ρ_IL		is the density of the immersion liquid at 23 °C (or 27 °C) certified by the supplier or determined experimentally (in grams per cubic centimeter)

For samples whose density is less than that of the immersion liquid, the test may be carried out in exactly the same way as described above, with the following exception: a sinker of lead or other high-density material is attached to a wire in such a way that the sinker rests below the liquid level, as does the sample when immersed. The sinker may be considered as part of the suspension wire. In this case, the density of the sample must be calculated using the following equation:

\rho _{S}\,=\,{\frac {m_{A}\cdot \rho _{IL}}{m_{A}+m_{K,IL}-m_{S+K,IL}}}

where

m_K,IL		is the mass of the sinker when immersed in the liquid, in grams;
m_S+K,IL		is the mass of the specimen and the sinker when immersed in the liquid, in grams

The indices are derived from the English and stand for S for specimen, IL for immersion liquid and A for air.

The density of casting resins, moulded materials and filled thermoplastics is strongly influenced by the type of filler and its content. It is therefore not possible to make a statement about the density of the base material for such moulding materials, unless the residues are completely captured after ashing. The influence of colorant additives (approx. 0.5 to 1.5 %) is relatively small and has an effect on density up to a maximum of 0.01 g/cm³ [2].

References

[1]	ISO 1183-1 (2025-06): Plastics – Methods for Determining the Density of Non-cellular Plastics – Part 1: Immersion Method, Liquid Pycnometer Method and Titration Method
[2]	Hellerich, W., Harsch, G., Haenle, S.: Werkstoff-Führer Kunststoff–Eigenschaften, Prüfungen, Kennwerte. Carl Hanser, Munich Vienna (1996) 7th Edition (ISBN 3-446-17617-9; see AMK-Library under G 1)

Titration method

Determination of density using the titration method (method C)

In the titration method, density is determined by limiting density differences in test liquids [1]. The smaller the differences in density, the more accurately the density can be determined using this method [2].

The test is carried out in a glass measuring cylinder with a nominal volume of 250 ml. A bath thermostat, a thermometer with a measuring accuracy of 0.1 °C and aerometer spindles with a measuring accuracy of 0.001 g cm^-3 are also required.

The most important thing is the test liquids of different densities, which may contain up to 0.1 m.-% wetting agent to avoid air bubbles, as is the case when determining density using the immersion method.

**Table**: Liquid systems for method C
System	Density (g cm^-3)
Methanol / benzyl alcohol	0.79 – 1.05
Isopropanol / water	0.79 – 1.00
Isopropanol / diethylene glycol	0.79 – 1.11
Ethanol / water	0.79 – 1.00
Toluene / carbon tetrachloride	0.87 – 1.60
Water / aqueous sodium bromide solution^a	1.00 – 1.41
Water / aqueous calcium nitrate solution	1.00 – 1.60
Ethanol / aqueous zinc chloride solution^b	0.79 – 1.70
Carbon tetrachloride / 1,3-dibromopropane	1.60 – 1.99
1.3-dibrompropan / ethylenbromid	1.99 – 2.18
Ethylenbromide / bromoform	2.18 – 2.89
Carbon trachloride / bromoform	1.60 – 2.89
Isopropanol / methyl glycol acetate	0.79 – 1.00
^a A density of 1.41 corresponds approximately to a mass fraction of 40 % sodium bromide ^b A density of 1.70 corresponds approximately to a mass fraction of 67 % zinc chloride

The following chemicals may also be used in the various liquid mixtures:

Chemicals	Density (g cm^-3)
n-octane	0.70
Dimethylformamide	0.94
Tetrachlorethane	1.60
Ethyl iodide	1.93
Methylene iodide	3.33

To carry out method C, two miscible, freshly distilled liquids of different densities are required. One of them must have a density lower than that of the test material, the other must have a density higher than that of the test material. The density values given in the table for the different liquids can be used as a guide.

The liquid that comes into contact with the sample during the measurement must not have any effect on the sample.

Measure exactly 100 ml of the immersion liquid with the lower density using the volumetric flask and insert it into the clean, dry measuring cylinder with a nominal volume of 250 ml. The measuring cylinder is placed in the liquid bath set to (23 ± 0.5) °C [or (27 ± 0.5) °C] and controlled by the thermostat.

The samples are then placed in the measuring cylinder. They must fall to the bottom and be free of air bubbles. The measuring cylinder and its contents are placed in the liquid bath to equalize the temperature, stirring at regular intervals.

When the temperature of the liquid is (23 ± 0.5) °C [or (27 ± 0.5) °C], the immersion liquid with the higher density is added millimeter by millimeter from the automatic burette. After each addition, the liquid is stirred vertically with a flat-tipped glass rod, avoiding the formation of air bubbles.

After each addition of the second liquid and mixing, the behaviour of the samples is observed.

Initially, they fall quickly to the bottom, but as the proportion of the second liquid increases, the falling movement of the samples slows down. At this point, the liquid with the higher density is added in 0.1 ml portions. The total amount of the second liquid added is recorded as soon as the lightest samples begin to suspend in the liquid at the level to which they were agitated without moving up or down within 1 min. At this point in the titration, the density of the liquid should be determined using a pycnometer. At this point, the density of the liquid corresponds to the lower density limit of the samples.

Further portions of the liquid with the higher density are added until the heaviest samples remain at a constant height (float) for at least 1 min. The required amount of liquid with the higher density is recorded.

For each pair of liquids, the functional relationship between the added quantity of the liquid with the higher density and the density must be determined and represented graphically.

At each point of the graphical representation of the functional relationship, the density of the liquid mixture can be determined exactly using the pycnometer method.

ISO 1183 points out that some of the chemicals listed in the table can be hazardous and that the regulations of the Technical Guidelines for Hazardous Substances (TRGS) must be observed when working with these media.

References

[1]	ISO 1183-1 (2025-06): Plastics – Methods for Determining the Density of Non-cellular Plastics – Part 1: Immersion Method, Liquid Pycnometer Method and Titration Method
[2]	Hellerich, W., Harsch, G., Haenle, S.: Werkstoff-Führer Kunststoffe –Eigenschaften, Prüfungen, Kennwerte. Carl Hanser, Munich Vienna (1996) 7th Edition, (ISBN 3-446-17617-9; see AMK-Library under G 1)

Liquid pycnometer method

Determination of density with the liquid pycnometer (method B)

An analytical balance with a measuring accuracy of 0.1 mg, a liquid bath with immersion liquid, a pycnometer and a vacuum desiccator are required to carry out the test.

The aim of the method is to determine the density of samples in the form of powder, granules or flakes, which must be weighed as delivered. The samples must be in the range of 1 g to 5 g.

First, the empty and dry pycnometer is weighed. An appropriate amount of plastic material is weighed into the pycnometer. The sample is covered with the immersion liquid and all adhering air bubbles are removed by placing the pycnometer in a desiccator and applying a vacuum. The vacuum is released and the pycnometer is filled with the immersion liquid. The pycnometer is set to a constant temperature [(23 ± 0.5) °C (or (27 ± 0.5) °C)] in the liquid bath and then filled to the limit of its capacity.

The pycnometer is filled with degassed (deaerated), distilled or deionized water, the air is removed as described above and the mass of the pycnometer including contents is determined at the test temperature.

The density ρ_S of the sample at 23 °C (or 27 °C), in grams per cubic centimeter, is calculated using the following equation:

\rho _{S}\,=\,{\frac {m_{S}\cdot \rho _{IL}}{m_{1}-m_{2}}}

where

m_{_S}		is the mass of the sample (in grams)
m_₁		is the mass of the liquid required to fill the empty pycnometer (in grams)
m_₂		is the mass of the liquid required to fill the pycnometer with the sample inside (in grams)
ρ_{_IL}		is the density of the immersion liquid at 23 °C (or 27 °C), in grams per cubic centimeter, as determined by the supplier or experimentally.

General information

Density determination

Immersion method

Titration method

Liquid pycnometer method

See also