# Bio-Adhesive Lignin-Reinforced Epoxy Acrylate (EA)-Based Composite as a DLP 3D Printing Material

## TL;DR

This paper introduces lignin as a bio-based additive to improve the strength of 3D-printed materials using DLP technology.

## Contribution

The study demonstrates that low concentrations of lignin can significantly enhance the mechanical properties of DLP-printed polymers.

## Key findings

- 0.025 wt% lignin increased tensile strength by ~39% while maintaining elongation at break.
- Ultrasonication provided the best lignin dispersion, leading to improved mechanical performance.
- Higher lignin loadings caused agglomeration and reduced printability and mechanical gains.

## Abstract

Digital light processing (DLP) 3D printing is a powerful additive manufacturing technique but is limited by the relatively low mechanical strength of cured neat resin parts. In this study, a renewable bio-adhesive lignin was introduced as a reinforcing filler into a bisphenol A-type epoxy acrylate (EA) photocurable resin to enhance the mechanical performance of DLP-printed components. Lignin was incorporated at low concentrations (0–0.5 wt%), and three dispersion methods—magnetic stirring, planetary mixing, and ultrasonication—were compared to optimize the filler distribution. Cure depth tests and optical microscopy confirmed that ultrasonication (40 kHz, 5 h) achieved the most homogeneous dispersion, yielding a cure depth nearly matching that of the neat resin. DLP printing of tensile specimens demonstrated that as little as 0.025 wt% lignin increased tensile strength by ~39% (from 44.9 MPa to 62.2 MPa) compared to the neat resin, while maintaining similar elongation at break. Surface hardness also improved by over 40% at this optimal lignin content. However, higher lignin loadings (≥0.05 wt%) led to particle agglomeration, resulting in diminished mechanical gains and impaired printability (e.g., distortion and incomplete curing at 1 wt%). Fractographic analysis of broken specimens revealed that well-dispersed lignin particles act to deflect and hinder crack propagation, thereby enhancing fracture resistance. Overall, this work demonstrates a simple and sustainable approach to reinforce DLP 3D-printed polymers using biopolymer lignin, achieving significant improvements in mechanical properties while highlighting the value of bio-derived additives for advanced photopolymer 3D printing applications.