Thermal decomposition of nano-enabled thermoplastics: Possible environmental health and safety implications

Abstract

Nano-enabled products (NEPs) are currently part of our life prompting for detailed investigation of potential nano-release across their life-cycle. Particularly interesting is their end-of-life thermal decomposition scenario. Here, we examine the thermal decomposition of widely used NEPs, namely thermoplastic nanocomposites, and assess the properties of the byproducts (released aerosol and residual ash) and possible environmental health and safety implications. We focus on establishing a fundamental understanding on the effect of thermal decomposition parameters, such as polymer matrix, nanofiller properties, decomposition temperature, on the properties of byproducts using a recently-developed lab-based experimental integrated platform. Our results indicate that thermoplastic polymer matrix strongly influences size and morphology of released aerosol, while there was minimal but detectable nano-release, especially when inorganic nanofillers were used. The chemical composition of the released aerosol was found not to be strongly influenced by the presence of nanofiller at least for the low, industry-relevant loadings assessed here. Furthermore, the morphology and composition of residual ash was found to be strongly influenced by the presence of nanofiller. The findings presented here on thermal decomposition/incineration of NEPs raise important questions and concerns regarding the potential fate and transport of released engineered nanomaterials in environmental media and potential environmental health and safety implications.

Keywords: Aerosol; Incineration; Life cycle assessment; Nano-EHS; Nanotechnology.

Figure 1

Schematic of the integrated approach for the investigation of the thermal decomposition of nano-enabled products. The products consists of nanofillers (e.g. CNTs, Fe₂O₃). After the consumer use the products reaches its end-of-life by thermal decomposition generating two particulate byproducts, the released aerosol and the residual ash. Exposure to these byproducts might occur to both professionals and consumers. The residual ash might end up in landfills further raising questions regarding its fate and transport to the environment.

Figure 2

Released aerosol particle concentration and size. The particle concentration over time for PU-based (a,b) and PE-based (c,d) thermoplastics at two final decomposition temperatures T_d,final = 500 (a,c) and 800 °C (b,d). TEM images of in situ collected released aerosol particles from PU-CB (e), PU-CNT (f), PE-Fe₂O₃ (g) and PE-org. (h).

Figure 3

Chemical composition of released aerosols. TGA-FTIR spectra are at times just before the onset of characteristic CO peak for the PU-(a) and PE-based nanocomposites (b). (c) NMR spectra of pure PU decomposed at Td,final = 500 (red line) and 800 °C (green line), as well as PU-CNT decomposed at Td,final = 800 °C (blue line) along with their corresponding total organic H content and composition.

Figure 4

Residual ash morphology at T_d,final = 500 °C. SEM images of the residual ashes for PU (a), PU-CB (b), PU-CNT (c), PE (d), PE-Fe₂O₃ (e) and PE-org. (f). The presence of CNTs as well as pure Fe₂O₃ nanoparticles is detected. At T_d,final = 800 °C, only Fe₂O₃ nanoparticles, but no residual ash remains from all other materials.

Figure 5

XRD patterns of PE-Fe₂O₃ nanocomposite (black line), raw Fe₂O₃ nanofiller (red line) residual ash at T_d = 500 (green line) and 800 °C (blue line).