Cystallinity: Gradient Column


This procedure determines a sample density and correlates that density to crystallinity using theoretical PET amorphous and crystalline densities according to ASTM D1505.


Five samples are prepared by cutting approximately 1/4″ samples from a preform or bottle without touching the samples to prevent oils from disturbing the testing. The entrapped air is removed from the surface of the sample and the samples are then dropped into a density gradient column and allowed to settle for approximately 15 minutes. The height of the samples is then carefully measured. Standard density balls are used to calibrate the density of the column. The sample density is then correlated to the theoretical amorphous (1.3331) and crystalline (1.45234) PET densities to determine the % crystallinity.