ASTM D 792 Testing Plastic Density and Relative Density

Scope:

These ASTM D 792 Standard test methods describe the specific gravity or density of a solid as a valuable measure for material identification, tracking physical changes, ensuring uniformity, and estimating average density. Density variations stem from factors like crystallinity, plasticizer loss, solvent absorption, and composition differences. It aids in determining strength-weight and cost-weight ratios.

Test Procedure:

The specimen is weighed in the air with precision. For specimens weighing 1 to 10 g and having a density < 1.00 g/cm³, the weight is recorded with 0.1 mg precision. For others, it is recorded with 1 mg precision. If necessary, a long wire is attached from the hook to the vessel support on the balance. The specimen is suspended about 25 mm above the vessel support on the wire.

The immersion vessel is mounted on the support, and the suspended specimen (and sinkers if used) is fully submerged in water at 23±2°C. Care is taken to ensure that the vessel does not touch the sample holder or specimen. Adhering bubbles are removed by gently rubbing with wire, especially in specimen and sinker holes. If bubbles persist due to dissolved gases, a vacuum is recommended. The mass of the suspended specimen is determined precisely, and this apparent mass is recorded as ‘b,’ including the specimen, sinker (if used), and partly immersed wire. Unless specified, the weighing is done quickly to minimize water absorption by the specimen.

The sample holder (and sinker, if applicable) is weighed in water, immersing it to the same depth as in the prior step. This weight is noted as ‘w’ (mass of the sample holder in liquid). The process is repeated for the necessary specimens, and it is recommended to have two specimens per sample.

Video 01: Density and Specific Gravity (Relative Density)

Test Specimen:

This ASTM D 792 test method requires that the specimen for testing be a smooth single piece, sized for the apparatus, with a volume ≥ 1 cm3. The thickness should be ≥ 1 mm per 1 g weight. Ideally, specimens should weigh 1-5 g, but up to around 50 g is acceptable.

Data:

Standard categorization ASTM D 788 Determine the plastic’s specific gravity and density using the formula below:

                   specific gravity 23/23℃ =aa+ w-b

where:

a = apparent mass of the specimen without a sinker or wire in the air, 

b = the apparent mass of the specimen fully submerged in liquid, and

 W = apparent mass of the fully submerged sinker (if employed) and partially submerged wire.

                       Plastic desnsityD23℃,kg/m3 =specific gravity 23/23℃ 997.5

Conclusion:

This ASTM  D 792 test method covers the density and specific gravity (Relative Density) of Plastics by Displacement. We can use this method to calculate  the specific gravity of the plastic and its density.