Stress-induced phase separation in plastics drives the release of amorphous polymer micropollutants into water

Abstract

Residual stress is an intrinsic property of semicrystalline plastics such as polypropylene and polyethylene. However, there is no fundamental understanding of the role intrinsic residual stress plays in the generation of plastic pollutants that threaten the environment and human health. Here, we show that the processing-induced compressive residual stress typically found in polypropylene and polyethylene plastics forces internal nano and microscale segregation of low molecular weight (MW) amorphous polymer droplets onto the plastic's surface. Squeeze flow simulations reveal this stress-driven volumetric flow is consistent with that of a Bingham plastic material, with a temperature-dependent threshold yield stress. We confirm that flow is thermally activated and stress dependent, with a reduced energy barrier at higher compressive stresses. Transfer of surface segregated droplets into water generates amorphous polymer micropollutants (APMPs) that are denatured, with structure and composition different from that of traditional polycrystalline microplastics. Studies with water-containing plastic bottles show that the highly compressed bottle neck and mouth regions are predominantly responsible for the release of APMPs. Our findings reveal a stress-induced mechanism of plastic degradation and underscore the need to modify current plastic processing technologies to reduce residual stress levels and suppress phase separation of low MW APMPs in plastics.

Fig. 1. Surface stress drives amorphous polypropylene phase separation from semicrystalline polypropylene at 95 °C.

a Schematic of the cantilever test configuration and the associated stress distribution. Black box on cantilever surface indicates the location of the AFM test in Fig. 1b–g. b–g AFM in-situ tests at the same location (see arrow marker) on the compressive side of the cantilever around 5 mm from the clamped end; b, c before, d, e after 1 h and f, g 4 h in a 95 °C oven, respectively. c, e, g The red and blue lines correspond to L1 and L2, respectively, as indicated in b. h Raman spectra of bulk PP sheet (blue line), a-PP droplets from the surface of standard (std.) PP sheet (red line) and standard amorphous PP wax (pink line), respectively. i The GPC detector response as a function of elution time for the a-PP droplets. j The MW (red line) and MWD (blue line) of main peak in Fig. 1i. Source data are provided as a Source Data file.

Fig. 2. Quantitative analysis of surface stress driven a-PP phase separation.

a–g AFM images across the compressive surface of PP cantilever after 4 h of 95 °C oven heat. All images have the same scale bars. h Surface stress (blue line), a-PP droplet average height (pink line), and a-PP droplet volume per unit (red line) area across the compressive surface of PP cantilever after 4 h of 95 °C oven heat, respectively. i The normalized volumetric flow $\widehat{\mathbf{v}}$ of a-PP under different surface stress conditions and for oven temperatures of 60–95 °C, respectively. The linear fitting is consistent with the a-PP behaving like a Bingham plastic fluid. j The yield stress σ_y to initiate a-PP flow to the surface of bulk PP sheet under conditions of 60 to 95 °C oven heat, calculated from the intercepts of normalized volumetric flow, $\widehat{\mathbf{v}}=0$ in Fig. 2i. k Arrhenius plot of Eq. (2) at compressive surface stress conditions of −15 (pink line) to −26 MPa (red line). Data extracted from 5 temperatures in Fig. 2i (green dash line). The slope of linear fit at each stress is the flow activation energy. l The flow activation energy obtained from 5-temperature fit (Fig. 2k) and 3-temperature fit (Suppl. Fig. 13). Error bars indicate standard deviation. Source data are provided as a Source Data file.

Fig. 3. Amorphous polymer micropollutants release from a stressed polypropylene sheet.

a Protocol used to expose stressed PP sheets to 95 °C water for 4 h. b Optical image of the inner compressed surface of PP sheet after hot-water exposure. The blue box area has a high quantity of intact a-PP droplets, while the red box showed droplets deformed by air bubble at the hot-water-PP sheet interface. c SEM image of typical PP APMPs (red circles) captured on membrane filter surface (800 nm pore size). d Raman spectra of typical PP APMPs releases from stressed PP sheet (red line), compared to the spectra from bulk PP sheet (blue line), standard (std.) bulk amorphous PP wax (pink line), and droplets from the surface of the stressed PP sheet (dark yellow line), respectively. e The quantity of PP APMPs released from raw flat std PP sheet and stressed PP sheets with different diameters and compressive stresses, respectively. Error bars indicate standard deviation. Source data are provided as a Source Data file.

Fig. 4. Amorphous polymer micropollutants release from polypropylene bottle.

a SEM image of typical PP APMPs captured on membrane filter surface, marked by red circles. b The quantity of PP APMPs releases from the whole bottle, mouth/neck region, and body region, respectively. c AFM image of the released particles captured on membrane filter surface. d Raman mapping and identification of captured APMP particles in Fig. 4c. e, f SEM images of the inner surface of bottle’s mouth region before and after exposure to 95 °C water, respectively. Error bars indicate standard deviation. Source data are provided as a Source Data file.