Abstract:

Traditional ink printing is convenient, but the excessive use of ink in this process has harmed both human health and the environment. Inkless printing technologies are mainly thermal and photosensitive in nature; however, they are susceptible to environmental impact, resulting in unstable printing and easy fading. In recent years, laser ablation printing technology has received considerable attention. However, the energy of the laser acting on the paper surface is affected by the surface roughness and tightness of the paper, which makes it difficult to obtain a uniform printing effect. Therefore, a complex and expensive paper-laser monitoring feedback system is required for modulating the laser parameters in real time to obtain uniform printing; however, such a system is not conducive to the popularization of laser ablation printing technology. To address this issue, a laser-induced inkless printing method, combined with micro-zone processing, is proposed. The associated high-energy characteristics and the thermal effect of laser are examined in this study. A multifunctional paper with an "organic-inorganic-organic" sandwich structure is constructed by vacuum filtration combined with thermocompression, using hydroxyapatite as the thermal insulating layer and wood fiber as the carbonized layer. The obtained functional papers display high flexibility, high tensile strength, and excellent flame resistance. Laser-induced inkless printing is realized via laser under-focusing or laser focusing onto hydroxyapatite sandwich paper. When the laser is irradiated on the surface of the functional paper, the surface-located cellulose fibers are carbonized due to the photothermal radiation effect of the laser, leaving graphitized carbon on the surface of hydroxyapatite layer. Notably, the hydroxyapatite in the interlayer of the functional paper is an outstanding thermal insulation material, which can effectively prevent further transfer of laser energy. Thus, owing to the sandwich structure of the multifunctional paper, laser inkless printing of both words and patterns is realized. This innovative approach offers a sustainable solution to traditional ink printing, while also providing a new avenue for micro-zone processing. The printed text or patterns can be preserved in harsh environments for long periods to be used for the storage of archival data. Meanwhile, the laser inkless printing method provides a new kind of touch reading material for acquired blindness, which avoids long-term contact with toxic printing ink. This study demonstrates that the hydroxyapatite sandwich paper is critical to realizing laser inkless printing without using expensive monitoring feedback systems, thus significantly reducing printing costs and effectively mitigating environmental pollution, marking it as an ideal technology for widespread adoption.

Key words: Laser, Hydroxyapatite, Sandwich structure paper, Ink-free printing