Graphene nanoplatelets (GNPs) are carbon-based nanoparticles, commonly used as a nanofiller to improve the electrical conductivity, mechanical strength, and flame retardancy of polymers.
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Although a large number of GNP-reinforced products have been designed, the effects of combustion-generated emissions from these products in the lungs have not been significantly analyzed. Recently, Empa researchers sought to address this research gap and investigated the health risks associated with graphene residues. This study is available in NanoImpact.
Combustion of GNP-based Products
GNPs are a two-dimensional (2D) material that contains several layers of graphene sheets. As stated above, GNP is a popularly used nanofiller that enhances the properties of commercial composites. Hence, it is imperative to analyze the safety profiles of this material owing to its increased usage.
The combustion process is used for waste incineration of GNP-reinforced polymers, which involves the complete combustion of the composite material in a controlled manner. However, if there is accidental fire, only partial combustion occurs, generating significant soot; furthermore, it is challenging to completely elucidate the combustion process of nanomaterial-based polymers.
Epoxy (EP) combustion is a rapid process that generates a large amount of toxic gases (e.g., carbon monoxide) and soot. In contrast, polymers containing GNPs exhibit flame-retardant properties. For instance, when GNPs are added to EP resin, the combustion process slows down due to the barrier effect. This effect is attributed to the relocation of GNPs on the surface of the polymer, forming a protective layer.
At high temperatures, i.e., over 850°C, the transformation of GNPs occurs due to thermal oxidation. At the above-stated temperature, a hole in the graphitic layer is formed, or dislocation of GNPs from the matrix occurs. The char or airborne form of carbon produced during the combustion of GNPs poses health risks to humans and the environment.
Biological Effects of GNP on Humans
Several studies have reported the biological effects of GNPs, particularly those with 100 nm thickness and around 25 μm diameter. These GNPs with aerodynamic diameter can get deposited in the ciliated airway and alveolar structures. GNPs greater than 15 μm (diameter) cannot be phagocytosed by macrophages, which induces inflammatory cytokines. Frustrated phagocytosis can also lose cellular membrane integrity.
Different physicochemical features of GNPs, such as thickness, surface chemistry, lateral dimension, and surface area, influences GNP toxicity. The differential physicochemical characteristics lead to varied biological responses of GNPs.
During the combustion of carbon nanotube (CNT) reinforced polycarbonate (PC) and polyurethane (PU), an elevated polycyclic aromatic hydrocarbon (PAH) concentration adsorbed to the particles is generated. Several studies have indicated that PU-CNT increases the reactive oxygen species (ROS) formation risks, which can damage DNA. Similarly, compared to pure PC, PC-CNT exhibited increased cytotoxicity in human bronchial epithelial cells (BEAS-2B). Increased cellular exposure to the combustion emissions of PC-CNT showed higher intracellular ROS formation and DNA damage.
Effects of Combustion of GNP Polymer Composites
The cone calorimeter was used to investigate the effects of nanofillers on combustion. As stated above, temperatures above 800°C destroy the GNPs. The current study revealed the combustion characteristics of EP-GNP and EP. Each sample was subjected to combustion for 5 to 7 minutes and the combustion peak occurred at around 130–140 seconds.
GNPs marginally delayed the ignition time for epoxy composite due to thermal diffusivity. Compared to EP, EP-GNP reduced the peak heat release rate (pHRR) and marginally increased CO production. This finding could be due to the higher thermal conductivity of GNPs, compared to pure EP. A greater CO/CO2 ratio indicated incomplete combustion.
The current study demonstrated that GNPs did not affect the particulate number or size of combustion. It was observed that the released aerosols were in the range of 109 particles/cm3. Since the sizes of particulate emissions after EP and EP-GNP combustion were found to be smaller than 4 μm, it indicated the possibility of deposition in the alveolar region of the lung.
Raman spectroscopy and X-ray diffraction (XRD) of the particulate produced in the EP-GNP combustion process, revealed the absence of GNP in the airborne fraction, while GNPs were found in the residual ash.
The biological response of alveolar epithelial cells after 24 and 96 hours of exposure to graphene residues displayed acute cellular effects. Experimental results revealed that even at high particulate concentrations, GNPs did not cause any significant interference responses.
No apparent changes in cell morphology were observed when exposed to emissions from EP and EP-GNP. Although emissions from EP combustion did not affect cellular membrane integrity or mitochondrial activity, EP-GNP combustion led to a decrease in mitochondrial activity. The absence of lactate dehydrogenase (LDH) indicated that EP-GNP combustion did not cause cell death.
Scientists performed cytokine profiling to determine inflammatory responses to airborne emissions. A higher MCP-1 level was observed in cells exposed to emissions from both EP and EP-GNP combustion. In addition, EP-GNP triggered a higher production of GM-CSF. Notably, no prominent changes in oxidative stress-related genes (HMOX1 and SOD2) were observed when the cells were exposed to both EP or EP-GNP emissions.
Taken together, the current study indicated significant health risks associated with emissions of EP composites after combustion.
Reference
Netkueakul, W. et al. (2022) Airborne emissions from combustion of graphene nanoplatelet/epoxy composites and their cytotoxicity on lung cells via air-liquid interface cell exposure in vitro. NanoImpact, 27, p. 100414. https://doi.org/10.1016/j.impact.2022.100414.
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