ASTM D1505-10 Standard Test Method for Density of Plastics by Density Gradient Technique

The standard "ASTM D1505-10 Standard test method for density of plastics by density gradient technique" published by the American Society for Testing and Materials (ASTM) describes a test method for determining the density of solid plastics. This test method is based on observing the level at which a test specimen sinks in a liquid column that exhibits a density gradient compared to known density standards. This test method is equivalent to the standard "ISO 1183-2 Plastics - Methods for determining the density of noncellular plastics - Part 2: Density gradient column method" developed by the International Organization for Standardization (ISO). This ISO standard also describes a gradient column method for determining the density of noncellular molded or extruded plastics or pellets in void form. Density gradient columns are columns containing a mixture of two liquids, the density in the column increasing smoothly from top to bottom.

According to ASTM D1505-10, the density of a solid substance is a readily measurable property that is often useful as a means of tracking physical changes in a sample, indicating and identifying uniformity among samples. The test method described is designed to obtain results with an accuracy of better than 0,05 percent. When accuracy of 0,05 percent or better is desired, the gradient tube shall be configured so that vertical distances of 1 mm represent density differences of not greater than 0,0001 g/cm3. The sensitivity of the column is then 0,0001 g/cm3 mm. When less sensitivity is required, the gradient tube shall be configured to the required sensitivity.

The equipment used in these tests includes:

• Density gradient tube: A suitable graduated vessel with a ground glass stopper.
• Constant temperature bath: A means of controlling the temperature of the liquid in the tube within 23 degrees plus7minus 1 degree. A thermostatic water jacket around the tube is a satisfactory and convenient method of achieving this.
• Glass floats: A series of calibrated glass floats covering the density range to be studied and distributed approximately equally throughout this range.
• Pycnometer: It is an equipment used to determine the density of standard floats.
• Liquids suitable for the preparation of density gradients: Details of these liquids are given in the table given in the standard. It is very important that none of the liquids used in the tube create a solvent or chemical effect on the test samples during sample immersion.
• Hydrometers: A range of suitable hydrometers covering the range of density to be measured. These hydrometers should have a density graduation of 0,001.
• Analytical balance: has a precision of 0,0001 g or better.
• Siphon or pipette arrangement for filling the gradient tube: This piece of equipment should be designed so that the liquid flow rate can be adjusted to 10 6 5 mL/min.

The test specimen shall consist of a piece of the material under test. The piece shall be cut into any suitable shape for easy identification, but shall have dimensions which permit the most accurate measurement of the centre of volume of the suspended specimen. Care shall be taken when cutting the specimens to avoid changes in density resulting from compressive stress. The equilibrium positions of film specimens in the range of 0,025 to 0,051 mm thickness may be affected by interfacial tension. If this effect is suspected, films not less than 0,127 mm thick shall be tested. The specimen shall be free from foreign matter and voids, and shall have no voids or surface features which would cause bubbles to be trapped.

Glass floats are prepared by any suitable method to be completely annealed, approximately spherical, with a maximum diameter of less than one-fourth of the inside diameter of the column, and which will not interfere with the test specimens. A solution (400 to 600 mL) of the liquids to be used in the gradient tube is prepared so that the density of the solution is approximately equal to the lowest desired density. When the floats are at room temperature, they are slowly dropped into the solution. Floats that sink too slowly are stored and those that sink too rapidly are discarded or saved for another tube. If necessary to obtain a suitable float gap, selected floats are ground to the desired density by rubbing the head of the float on a glass plate over which a fine slurry of 400 or 500 mesh silicon carbide or other suitable abrasive has been spread. Progress should be monitored by dropping the float into the test solution at intervals and noting the change in sinking rate.

Test specimens in which the density change during conditioning is greater than the accuracy required for density determination shall be conditioned in accordance with the method listed in the applicable ASTM material specification prior to testing.

The densities of samples can be determined graphically or by calculation from the levels at which the samples settle by one of the methods given in the standard.

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