The density of virgin, consumer and environmental plastics: An investigation using gas displacement pycnometry

Abstract

Density is a fundamental property of plastics and is particularly significant in determining the transport and fate of waste plastics that enter aquatic systems. However, densities are rarely determined in the environmental literature and values employed for modelling or impacts are often unsourced or derived from secondary databases. In this study, we employ helium displacement pycnometry to determine the skeletal densities of non-porous plastics whose polymer composition had been established from manufacturer's data, resin codes or Fourier transform infrared spectrometry. Two independent, collaborative laboratories, providing measurements within 3.5 % of each other and with precisions of <1 % (as relative standard deviation), analysed a total of 42 virgin, consumer and environmental plastics consisting of ten common polymer types. Measured densities of plastics in all categories, and most notably for polybutylene terephthalate, polyethylene terephthalate, polypropylene and polyvinyl chloride, were often outside the ranges reported by a comprehensive online resource. Possible reasons for discrepancies include the occurrence of dense additives (evaluated by X-ray fluorescence analysis), the presence of inaccessible microscopic pores below a laminated surface, contamination of the main polymer by a secondary one, and structural changes on weathering. Regardless of precise causes, most results suggest that individual polymers have a broader range of densities than is generally published or considered in the literature. Accordingly, and in particular where buoyancy is critical, more precise, sample-specific measurements are recommended.