Managing Annealing Condition Variables in Polyethylene Specific Gravity Arbitration
Polyethylene specific gravity arbitration requires converting disputed density claims to a standardized thermal baseline using ISO 17855-2 Method B annealed plaques.
Buoyant Displacement
Meaning
Measurement of solid density relies on the weight change of a sample when it is submerged in a liquid of known density. Utilizing buoyant displacement allows quality control labs to verify the crystallinity of polyethylene or the filler loading in glass-reinforced resins. This technique is more accurate than direct measurement of part volume because it accounts for irregular geometries.
Immersion Liquid
Selection of the fluid depends on the density of the polymer and its chemical resistance. Distilled water is common for materials denser than one gram per cubic centimeter, while alcohols or silicon oils are required for polyolefins that would otherwise float. The liquid must not cause the resin to swell or dissolve during the test.
Calculation Procedure
Results come from the difference between the dry weight and the submerged weight divided by the density of the medium. Corrections for temperature are necessary because the density of the immersion liquid varies with the heat of the room. Precise balances equipped with a density kit are standard in extrusion plants for checking regrind ratios.
Variation in the fourth decimal place can indicate a change in the molecular weight or the presence of voids.
Error Control
Surface tension and air bubbles are the primary sources of inaccuracy in these measurements. Wetting agents are sometimes added to the liquid to prevent bubbles from clinging to the part surface and creating false buoyancy. An air bubble as small as one millimeter can noticeably alter the recorded density of a small test plaque.
Careful handling ensures that the sample is fully wetted and that no part of the suspension wire touches the sides of the container. The lab environment must be free from vibration and drafts that could disturb the balance reading.