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Q: I want to measure the density of a mineral-filled composite polymer mixture by the ASTM D1505 method. What liquid will be used for density measurement (density >1)? The products to be tested are made of either polypropylene or polyethylene with calcium carbonate and additives. We will be adding 50%–70% CaCo3 to the resin.
A: ASTM D1505, Standard Test Method for Density of Plastics by the Density-Gradient Technique, uses a graduated glass column which is charged with two solutions, such that one is greater than and one is less than the density of the material to be tested. The liquids are charged into the column by slowly adding varying amounts of each using a tremie tube. The liquid at the bottom of the column is of greater density and, as the column is filled, the density decreases until the top of the column contains only the lighter density liquid. The density gradient will be uniform through the length of the column. Standard glass spheres of known but varying density are introduced into the column along with the specimens using a charging basket. This basket is slowly lowered through the column without disturbing the density gradient, distributing the glass spheres and specimens according to their relative densities. Using the glass spheres as reference points, the density at the locations where specimens are neutrally buoyant is calculated by interpolation. ASTM D1505 is probably the most sensitive, but also painstakingly difficult to run and expensive test method. Values to four-to-five-decimal-place accuracy are routinely obtained.
This method can be extended to high-density materials greater than one if suitably dense liquids can be selected. I have heard of people using surfactants and cornstarch instead of alcohol for this purpose. However, preparing the column is difficult (requiring a constant temperature) and column stability is also a challenge. Have you tried ASTM D792 specific gravity with a density measurement kit for an analytical balance? Although not as accurate as ASTM D1505, this test would lend itself to much less uncertainty and is a lot easier to set up and run.