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Compounding process was carried out on a Brabender Measuring Mixer N50 (Duisburg, Germany). The composites containing 100 phr (parts per hundred part of rubber) ethylene-norbornene copolymer were filled with 1 phr or 3 phr of a spinel pigment. The EN/spinel pigment composites were prepared at a rotor speed of 50 rpm with a chamber temperature of 110 °C. Afterwards, the EN/pigments composites were molded into films with a thickness of 1 mm using a hydraulic press for 5 min at 110 °C. The thermal properties of the EN/pigments blends were measured using a Mettler Toledo Thermogravimetric Analyzer TGA (Columbus, OH, USA). Composite samples of approximately 10 mg were placed in an aluminum oxide crucible and heated from 25 °C to 600 °C in an argon atmosphere, with a heating rate of 10 °C/min. Microscale combustion calorimetry was used to measure the flammability of the EN/spinel pigment composites. Similar fragments of each composite weighing approximately 2.5 mg were measured on an MCC micro-calorimeter (Fire Testing Technology Limited). Flammability tests were performed with the following parameters: pyrolyzer temperature 750 °C and combustor temperature 900 °C. Mechanical properties were measured using a universal tensile testing machine Zwick/Roell 1435 (Zwick Roell Group, Ulm, Germany) at a uniform crosshead speed of 500 mm/min. Testing was carried out following ISO 37 standard guidelines. Elongation at break (EB) and tensile strength (TS) were calculated as the average of five measurements (the error of measurement was around ± 10%). The aging coefficient K was estimated according to the formula [36]:

Diffuse UV-Vis spectra of the pigment powders were recorded on an Evolution 201/220 UV–Visible Spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The samples were stored under normal conditions and the spectra were recorded in the range of 200–1100 nm, directly in air with no further pretreatment. The accuracy of the apparatus was ±0.8 nm and the repeatability was ≤0.05 nm. Color changes were recorded in the wavelength range of 360–740 nm with a CM-3600d spectrophotometer (Konica Minolta Sensing Inc., Osaka, Japan) and represented in the CIELAB color space. Obtained data were calculated as the average of five measurements from the different places of sample. The measurement error was around ±10%. Total color change ΔE was calculated according to the following equation [37]:

where ∆L, ∆a, and ∆b represent the differences between the initial and final values for L (brightness), a (red-green color coordinate), and b (yellow-blue color coordinate), respectively.

Infrared absorbance spectra were obtained using a Thermo Scientific Nicolet 6700 (Waltham, MA, USA) for attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The ATR-FTIR technique was applied in the wavenumber range of 4000–500 cm−1 to analyze the formation of the oxidation products. To calculate the carbonyl index (CI), the changes in the relative absorbance intensity of the ketone group A>C=O (corresponding to 1800–1680 cm−1) were compared to those of methylene group A-CH2- (at 3000–2800 cm−1), according to the formula [38]:

The UV aging process was simulated in an Atlas UV 2000 (Ametek Atlas, Linsengericht, Germany) apparatus. Accelerated aging was performed at wavelength λ = 343 nm over 900 h. The procedure combined consecutively repeating segments: a daily segment (radiation intensity 0.7 W/m2, temperature 60 °C, duration 8 h) and a night segment (no UV radiation, temperature 50 °C, duration 4 h). The morphology of the EN copolymer samples colored with different two spinel pigments was evaluated based on scanning electron microscopy (SEM). SEM images were taken using a scanning electron microscope (SEM, Zeiss, ULTRA Plus, Oberchoken, Germany) at magnifications of 5000× and 1000×. Prior to SEM observations, liquid nitrogen-fractured surfaces of the composites were carbon sputtered.

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