Addressing Carbon Black Challenge: Preliminary Near-Infrared Optical Screening of Bio-Derived Black Flexography Ink

Abstract

The compatibility of dark printing inks with near-infrared (NIR) automated sorting systems is an important consideration in plastic packaging recyclability. While bulk carbon black pigmentation is known to suppress NIR signals and render packaging difficult to detect, the spectral behavior of thin flexographic ink films, particularly novel biogenic alternatives, remains underexplored. This study presents a laboratory-scale comparison of the NIR spectral behavior of a conventional petroleum-derived furnace carbon black ink (Standard Black Ink, SBI) and a novel bio-based flexographic ink containing micronized biochar pigment and a bio-based binder (Bio Black Ink, BBI). The spectral reflectance of both systems was measured across the 700–2500 nm range. Both printed ink–photo-paper systems exhibited measurable NIR reflectance within the sorting-relevant spectral region, although the measured spectra represent the combined response of the ink films and the underlying photo-paper substrate. Over the 1400–2200 nm range, the SBI exhibited a mean reflectance of 12.74 ± 0.56%, exceeding the numerical 10% reflectance benchmark referenced by the Association of Plastic Recyclers (APR), and reached a peak reflectance of approximately 0.18 near 1950 nm. The BBI exhibited moderately lower reflectance, with values of approximately 0.08–0.16 within the same window and a mean reflectance of 10.54 ± 0.62%, slightly above the same numerical benchmark. The APR protocol itself was not performed. Below approximately 1150 nm, the BBI radiance fell below that of the lowest available 2% calibration standard; these data were therefore designated as below the validated quantification limit and were not used for quantitative interpretation. The results identify the biochar-based ink as a candidate for further investigation on representative packaging polymers and whole plastic articles. The findings constitute laboratory-scale optical screening of ink–photo-paper systems and do not demonstrate polymer classification, APR protocol compliance, or commercial sorting performance.