Specific gravity is defined as the ratio of the weight of the given volume of a material to that of an equal volume of water at a stated temperature. The temperature selected for determining the specific gravity of plastic parts is 23°C. Specific gravity values represent the main advantage of plastics over other materials, namely, light weight. All plastics are sold today on a cost per pound basis and not on a cost per unit volume basis. Such a practice increases the significance of the specific gravity considerably in both purchasing and production control. Two basic methods have been developed to determine specific gravity of plastics depending upon the form of plastic material. Method A is used for a specimen in forms such as sheet, rods, tubes, or molded articles. Method B is developed mainly for material in the form of molding powder, flakes, or pellets.
This method requires the use of a precision analytical balance equipped with a stationary support for an immersion vessel above or below the balance pan. A corrosion-resistant wire for suspending the specimen and a sinker for lighter specimens with a specific gravity of less than 1.00 is employed. A beaker is used as an immersion vessel. The test specimen of any convenient size is weighted in air. Next, the specimen is suspended from a fine wire attached to the balance and immersed completely in distilled water. The weight of a specimen in water (and sinker, if used) is determined. The specific gravity of the specimen is calculated as follows:
Specific gravity = a (a + w) / b
a = weight of specimen in air;
b = weight of specimen (sinker, if used) and wire in water;
w = weight of totally immersed sinker (if used) and partially immersed wire.
This method, which suitable for pellets, flakes, or powder, requires the use of an analytical balance, a pycnometer, a vacuum pump, and a vacuum desiccator. The test is started by first weighing the empty pycnometer. The pycnometer is filled with water and placed in a water bath until temperature equilibrium with the bath is attained. The weight of the pycnometer filled with water is determined. After cleaning and drying the pycnometer, 1–5 g of material is added and the weight of the specimen plus the pycnometer is determined. The pycnometer is filled with water and placed in a vacuum desiccator. The vacuum is applied until all the air has been removed from between the particles of the specimen. Last, the weight of the pycnometer filled with water and the specimen is recorded. The specific gravity is calculated as follows:
Specific gravity = a (b + a / m)
a = weight of the specimen;
b = weight of the pycnometer filled with water;
m = weight of the pycnometer containing the specimen and filled with water.If another suitable immersion liquid for the water is substituted, the specific gravity of the immersion liquid must be determined and taken into account in calculating the specific gravity.