2.1. Combustion of Nanomaterials and Nanocomposites

For thermal decomposition of nano-scaled filler material, TiO₂ (Aeroxide^® P25, Evonik, Essen, Germany), multi-wall CNTs (NC7000™, Nanocyl^®, Sambreville, Belgium), and CuO (product number 544868, Sigma-Aldrich^®, Munich, Germany) were used, each of which was applied as a suspension with deionized water. In each case, 4 g of the nanomaterial were dispersed in 1 L of deionized water and treated with ultrasound for one hour. In the case of CNTs a stable suspension with water was not achievable, therefore a stabilizer, 10 g/L gum arabic, was added to the suspension.

For the production of the nanocomposites, the nanomaterials were fed as delivered in the compounding process. In this investigation, TiO₂ compounds were produced in a Leistritz 27 HP extruder (Leistritz Extrusionstechnik GmbH, Nuremberg, Germany) with a 27 mm screw diameter and an L/D of 52. CNT and CuO compounds were produced in a Leistritz ZSE 18 MAXX extruder (Leistritz Extrusionstechnik GmbH, Nuremberg, Germany) with an 18 mm screw diameter and an L/D of 60. Both machines were equipped with a special encapsulated dosing technique for processing of nanoparticles, avoiding dust via a special sealing technique for refilling the gravimetric feeders. The produced compounds were characterized in view of mechanical properties on injection molded samples according to DIN EN ISO 527 and the filler content was checked via TGA.

For this study, a laboratory Bunsen-type burner (constructed by KIT, Karlsruhe, Germany) was used to represent the thermal decomposition of end-of-life nanocomposites. Either nanocomposite powders or nanoparticle suspensions were added to the feed gas stream of the burner. A rotating brush generator (RBG1000, Palas, Karlsruhe, Germany) was used for the dosage of the nanocomposite powders and an atomizer (ATM220, Topas, Dresden, Germany) was used for the experiments with pure nanoparticles. For a smooth operation of the rotating brush generator, the nanocomposites were sieved with a 315 µm sieve (Retsch, Haan, Germany) and the powder fraction with sizes smaller than 315 µm was used. The material reservoir was filled with approximately 4 g of NC powder and the feed rate was adjusted to 1 g/h. For the experiments with pure nanoparticles, suspensions with 4 g/L solid material and deionized water were prepared. The air volume flow for atomization was set at 1 l_N/min, which led to a dosing of about 4 g/h suspension.

The burner was operated with a premixed ethylene/air burning gas mixture with slightly over-stoichiometric conditions (λ_gas = 1.07). The dosage of nanocomposites reduced the air number by about 2%. The total airflow was set to 9.30 l_N/min and the ethylene flow to 0.61 l_N/min controlled via mass flow controllers (EL-Flow, Bronkhorst, Ruurlo, The Netherlands). At 430 mm above the burner, a sampling probe was installed, followed by a dilution stage (VKL10E, Palas, Karlsruhe, Germany). The dilution stage diluted the aerosol 10-fold on the one hand to decrease the temperature after the combustion and on the other hand to increase the available volume flow. Downstream of the dilution stage, the different systems for the aerosol characterization as well as human lung cell exposure were installed (Figure 1).

Setup for the thermal degradation of nanocomposite powders and nanoparticle suspensions with subsequent physicochemical and toxicological characterization.

For the aerosol measurement, an electrical low-pressure impactor (ELPI, Dekati, Kangasala, Finland) was installed downstream of the dilution stage. The ELPI measures charged particles in the size range of 6 nm to 10 µm and the deposited particles can be used for subsequent analyses. The ELPI is a low-pressure cascade impactor to which one electrometer per impactor stage is connected, which records the current of the impacted charged particles [31]. The particles are charged with a corona before entering the cascade impactor, then classified according to their inertia and their number is determined by the measured current per stage. The individual impactor stages on which the particles are deposited are, for example, occupied with aluminum foils, which can be used for imaging or chemical analysis after the measurement. The stage at which a particle impacts depends on its aerodynamic diameter, which is affected by particle size, shape, and density. The measurement results in a number size distribution.

The ELPI was equipped with aluminum foil substrates at each stage for the size-classified collection of particles that can be examined via scanning electron microscope (Zeiss Supra 55VP SEM with a field emission gun operating at 3 kV and 10 kV accelerating voltage and an aperture of 30 µm) (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany). Every second the complete particle size distribution was logged in dependence of the aerodynamic diameter and for further processing the time averaged value can be used. Since the ELPI measurement principle is based on the number concentration, the density of the particles is required to calculate the resulting mass concentration.

The characterization of the combustion gases from combustion of polyethylene (PE) fine granulate was carried out after adsorption of 2 L of the combustion-generated gaseous phase at 37 °C over 20 min on TENAX TA tubes (Merck KGaA, Darmstadt, Germany) using an air collection pump. The subsequent quantitative analysis followed in accordance with DIN ISO 16000-6: 2012-11 using thermal desorption combined with capillary gas chromatography and mass spectrometry. Using the external standard method, individual substances were quantified by including reference substances. Non-quantifiable substance peaks were compared with reference spectra from spectral libraries. Semi-volatile organic compounds (SVOC) were calculated as toluene equivalents (TE) and added up. The results are given as the sum of total volatile organic compounds (TVOC) and of SVOC.

At the reactor of the automated ALI exposure station, a scanning mobility particle sizer (SMPS + C, Grimm, Ainring, Germany) was installed to measure the generated aerosol as it was applied onto cell culture systems. With the SMPS, a number size distribution was measured in dependence of the mobility diameter, which has to be distinguished from the aerodynamic diameter the ELPI is measuring. The particles used for the determination of the particle size by ELPI were collected directly after dilution with dry air and were thus not affected by humidity. In addition, since none of the investigated particles were hygroscopic, no impact of humidity would be expected with respect to TEM images. In one of the exposure chambers of the ALI exposure station, Formvar film-coated copper grids with 200 mesh, type SF162 (Plano GmbH, Wetzlar, Germany), were installed to get an optical evaluation of the deposited particles via transmission electron microscopy (EM 109 (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany).

The occurrence of polycyclic aromatic hydrocarbons (PAHs), which presumably are condensed on the solid–particulate phase of the aerosol as a result of the combustion process of the (unfilled) PE-matrix, was evaluated exemplarily. Particulate matter (PMx) and condensates were sampled by filtering an aerosol volume of x m³ on a glass fiber plane filter at 37 °C. US EPA priority polycyclic aromatic hydrocarbons (16 PAHs) were quantified in total, placing untreated blank (pure glass fiber filters) and the PM sample in a glass vial containing 10 mL of dichloromethane (DCM) and kept in an ultrasonic bath for 30 min. The extract was then filtered through membrane filters (PTFE 45 µm) and again placed in 10 mL of DCM for ultrasonic extraction. Subsequently, 3 mL acetonitrile was added and the extract was again concentrated to 1 mL in a nitrogen stream. The PAH contents in the extract were determined via high-performance liquid chromatography (HPLC).

Usually, the dose can be determined via quartz crystal microbalance, but in this study the deposited particle masses were found to be below the detection limits. Kaur and colleagues state the detection limit of QCM to be 50 ng/cm² [32], but the doses in this study were lower. Therefore, the applied particulate dose was determined by using the measured number concentration obtained via an ELPI by calculating the mass concentration with an assumed value for the particle density and shape. The real NP density can differ significantly from the density value of the macroscopic material (bulk). Particles often do not exist individually but as agglomerates. Potential agglomerates are assumed to contain internal voids, which means that the agglomerate takes up a large volume with a comparably small mass, which decreases their density value. For the metal oxide particles alone subjected to thermal treatment, it can be seen from the TEM and SEM (Supplementary Figure S1) images that they are essentially primary particles with an approximately spherical shape, so the respective bulk densities were used for calculation (6.48 g/cm³ for CuO and 4.24 g/cm³ for TiO₂). In the case of nanocomposites, dose assessment is more complex because the aerosol consists of a mix of different degradation products after combustion. Carbon black is often given with a density of 2 g/cm³, while the unburned plastic has a density of 1 g/cm³. Therefore, a density of 1 g/cm³ and a dynamic shape factor of 1 was assumed for the thermoplastic matrix, nanocomposites, CNTs, and gum arabic (stabilizer).

The total mass of all impactor stages was added and related to the sample stream, resulting in a mass concentration. Using the duration of an experiment (t = 240 min), the area of the transwell membrane, the deposition efficiency, as well as the volume flow over the membrane, the area load could be calculated. The deposition efficiency was known through former studies [26,33] and was found to be approximately 2% by diffusional deposition and 10% by electrostatic deposition.