High value-added use of waste photosensitive polyimide to develop low-shrinkage UV-curable inks for digital light projection technology

Abstract

In the mass industrial production process of polyimide photoresists, waste generation is inevitable due to unqualified molecular weights, inappropriate ionic content of resins, or storage expiration. Consequently, the recycling and reuse of waste photosensitive polyimide (w-PSPI) is a growing concern in both scientific and industrial communities. In this work, w-PSPI was explored to prepare photocurable, fully crosslinked inks for digital light projection technology (DLP). For this purpose, suitable photoactive solvents and crosslinking agents were first screened to overcome the issue of large shrinkage caused by solvent volatilization. The formation of cross-linked network structures during DLP processing and subsequent imidization was also investigated. As a result, a polyimide DLP ink with minimum size shrinkage (less than 5 %) was proposed. For a given active solvent, the mechanical behavior and heat resistance of DLP-formed objects were significantly influenced by the ratio of short-chain and long-chain crosslinkers and the imidization temperature. By optimizing ink composition and imidization temperature, the DLP-formed objects showed high heat resistance (Td5 > 385 °C, Tg > 150 °C) and balanced mechanical properties of strength and toughness (Modulus ≥1 GPa, elongation at break >5 %). This w-PSPI ink has been successfully utilized to manufacture several complex 3D structures and vertical 3D microarrays with 100-µm on a limited-resolution DLP printer. This study not only points the direction for the high-added utilization of w-PSPI but also demonstrates the potential for expanding PSPI application from 2D lithography to 3D micro-manufacturing.