Ghost fishing, caused by lost fishing lines and nets, has become a severe problem in marine environments. To eliminate ghost fishing in the ocean, the environmental degradation behavior of fishing lines must be understood. In this study, the environmental degradation of biodegradable nylon 4 fishing lines and commercial nylon 6, poly(ethylene terephthalate) (PET), and poly(vinylidene fluoride) (PVDF) fishing lines was simulated in the laboratory using an artificial weathering tester and biodegradation test in extracted seawater. To understand the degradation mechanism, the chemical and structural changes induced by photo-oxidation and biodegradation were investigated using tensile test, scanning electron microscopy, differential scanning calorimetry, gel permeation chromatography, infrared spectroscopy, and wide- and small-angle X-ray scattering. The results indicated that photo-oxidation occurred in the amorphous phase of the nylon 4, nylon 6, and PET fishing lines during ultraviolet (UV) exposure. The nylon 4 fishing lines exhibited excellent biodegradability, whereas the nylon 6, PET, and PVDF fishing lines could not be degraded by microorganisms in the extracted seawater. Both processes, i.e., photo-oxidation and biodegradation, were confined to the amorphous regions of nylon 4. Note that the PVDF fishing lines could not be degraded by UV exposure and biodegradation and, hence, should be recycled after use.
Microplastics have recently been identified as one of the major contributors to environmental pollution. To design and control the biodegradability of polymer materials, it is crucial to obtain a better understanding of the aggregation states and thermal molecular motion of polymer chains in aqueous environments. Here, we focus on melt-spun microfibers of a promising biodegradable plastic, polyamide 4 (PA4), with a relatively greater number density of hydrolyzable amide groups, which is regarded as an alternative to polyamide 6. Aggregation states and thermal molecular motion of PA4 microfibers without/with a post-heating drawing treatment under dry and wet conditions were examined by attenuated total reflectance-Fourier transform infrared spectroscopy and wide-angle X-ray diffraction analysis in conjunction with dynamic mechanical analysis. Sorbed water molecules in the microfibers induced the crystal transition from a meta-stable γ-form to a thermodynamically stable α-form via activation of the molecular motion of PA4 chains. Also, the post-drawing treatment caused a partial structural change of PA4 chains, from an amorphous phase to a crystalline phase. These findings should be useful for designing PA4-based structural materials applicable for use in marine environments.
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