Laboratory-Simulated Photoirradiation Reveals Strong Resistance of Primary Macroplastics to Weathering

Abstract

The photodegradation of macroplastics in the marine environment remains poorly understood. Here, we investigated the weathering of commercially available plastics (tabs 1.3 × 4.4 × 0.16 cm), including high-density polyethylene, low-density polyethylene, polypropylene, polystyrene, and polycarbonate, in seawater under laboratory-simulated ultraviolet A radiation for 3-9 months, equivalent to 25-75 years of natural sunlight exposure without considering other confounding factors. After the exposure, the physical integrity and thermal stability of the tabs remained relatively intact, suggesting that the bulk polymer chains were not severely altered despite strong irradiation, likely due to their low specific surface area. In contrast, the surface layer (∼1 μm) of the tabs was highly oxidized and eroded after 9 months of accelerated weathering. Several antioxidant additives were identified in the plastics through low temperature pyrolysis coupled with gas chromatography/mass spectrometry (Pyr-GC/MS) analysis. The Pyr-GC/MS results also revealed many new oxygen-containing compounds formed during photodegradation, and these compounds indicated the dominance of chain scission reactions during weathering. These findings highlight the strong resistance of industrial macroplastics to weathering, emphasizing the need for a broader range of plastics with varying properties and sizes to accurately estimate plastic degradation in the marine environment.

Keywords: FTIR; Norrish reactions; additive; macroplastic; photodegradation; pyrolysis-GC/MS.

Figure 1

Oxidation indices of HDPE (A), LDPE (B), PP (C), PS (D), and PC (E) from laboratory weathering experiments. T₀–T₃ represents 0–9 months of weathering, and indices of different functional groups were from the FTIR analysis. Data are the averages of six replicate measurements; error bars represent the standard deviations.

Figure 2

SEM images of surface morphology with magnification of 250× by increasing weathering exposure days for HDPE (A,B,C,D at T₀, T₁, T₂, and T₃, respectively), LDPE (E,F,G,H at T₀, T₁, T₂, and T₃, respectively), PC (I,J,K,L at T₀, T₁, T₂, and T₃, respectively), PP (M,N,O,P at T₀, T₁, T₂, and T₃, respectively), and PS (Q,R,S,T at T₀, T₁, T₂, and T₃, respectively).

Figure 3

Weight normalized composition of pyrolysis product groups identified through low-temperature pyrolysis of HDPE (A), LDPE (B), PP (C), PS (D), and PC (E) from laboratory weathering experiments. The total peak areas of different chemical classes were normalized by the sample weight. Data are the averages of triplicate normalized peak areas; error bars represent the standard deviations.

Figure 4

Hypothetical photochemical pathways of plastics under laboratory weathering conditions.