Researchers from Helmholtz-Zentrum Hereon, a German materials research institute, in collaboration with its Institute of Active Polymers, BTU Cottbus-Senftenberg’s Institute of Materials Chemistry, and Berlin-based VESC Studio, have developed a water-based 3D printing ink composed of 70 wt % lignosulfonate, an industrial byproduct of the pulping industry. Published in ACS Publications, the study describes a direct ink writing formulation processed at room temperature without organic solvents, chemical cross-linkers, freeze-drying, or thermal postcuring. Printed structures can be recycled through grinding and rehydration while maintaining stiffness and thermal degradation behavior across nine reuse cycles.
Most commercial 3D printing inks rely on fossil-derived thermoplastics and irreversible cross-linking chemistries. Biomass-derived alternatives based on lignin have previously required organic solvents, thermal curing, or limited lignin loadings below 50 wt % due to rheological instability. Lignosulfonate, which accounts for approximately 88% of lignin waste streams and is water soluble due to its sulfonate groups, was selected as the primary feedstock. Methyl cellulose, a cellulose derivative, functions as a reversible physical binder, while glycerol serves as a plasticizer to tune mechanical response. All components are water soluble and mixed at a 1:1 dry mass-to-water ratio.
Rheological measurements show time-dependent viscosity increases from 2000 Pa·s at 3 minutes after preparation to 6500 Pa·s at 60 minutes (0.1 s⁻¹ shear rate), attributed to hydrophobic interactions within methyl cellulose and hydrogen bonding among components. Under shear, viscosity decreases from approximately 6000 Pa·s to 50 Pa·s as shear rate rises from 0.1 to 16 s⁻¹, demonstrating shear-thinning behavior required for extrusion. Oscillation testing identifies a yield stress at approximately 14 Pa, marking a transition from solid-like (G′ > G″) to liquid-like (G″ > G′) behavior. FTIR analysis confirms hydrogen bonding in the 3700–3000 cm⁻¹ region, with peak shifts from 3336 cm⁻¹ to 3319 cm⁻¹ as glycerol content increases. Atomistic modeling estimates hydrogen bond density rising from approximately 50 to 65 bonds per 10 nm³ when glycerol increases from 10 wt % to 18 wt %.
Mechanical properties vary with methyl cellulose-to-glycerol ratio. Young’s modulus ranges from 2.4 ± 0.6 MPa at 18 wt % glycerol to 106.9 ± 17.3 MPa at 10 wt % glycerol. One-way ANOVA confirms statistically significant differences (p < 0.001). Elongation at break remains between 18% and 26%. Volumetric shrinkage during drying depends on formulation despite identical water content. L(70/18/12) exhibits 16 ± 2% shrinkage, while L(70/20/10) shows 35 ± 3%. Drying of single filaments completes within approximately 6 hours; complex geometries require 48 hours at room temperature. Differential scanning calorimetry detects no glass transition or melting transition up to 200 °C. Thermogravimetric analysis indicates initial mass loss of 4.6% at 100 °C due to residual moisture, followed by degradation events near 250 °C and 340 °C. Printed samples heated to 160 °C display brittle behavior, with elongation at break below 0.5%.
Microcomputed tomography reveals porosity between 14% and 19%, with pore diameters ranging from 50 to 400 μm. SEM imaging confirms layer fusion at nozzle diameters between 0.8 mm and 0.2 mm. Humidity exposure between 30% and 90% relative humidity increases water uptake above 70%, reducing stiffness and increasing ductility, though dimensional stability remains intact under indoor conditions (40–60%). Plant-watering tests conducted over four weeks show no structural collapse under repeated water exposure.
Recycling experiments involved nine cycles of grinding, rehydration, and reprinting at a 1:1 mass ratio of dry material to water. Thermogravimetric curves show no shift in degradation peaks across cycles. Young’s modulus remains within approximately ±15% of the original value. Variations in elongation at break, ranging from 10% to 37%, are attributed to pore morphology and residual water content rather than chemical degradation.
Life-cycle data cited in the study report greenhouse gas emissions for lignin-based materials between 0.15 and 1.1 kg CO₂-equivalent per kilogram, compared to 0.5–1.4 kg for PLA and 1.1–2.6 kg for ABS. Material cost is calculated at €0.05 per gram, comparable to common thermoplastic filaments. Room-temperature processing eliminates melt extrusion and postcuring energy demands.
Results demonstrate a lignin-based DIW ink with high waste content, tunable modulus in the MPa range, thermal shape stability to 200 °C, and repeated recyclability enabled by reversible physical cross-linking. Further life-cycle analysis would be required to quantify full environmental impact across production and reuse stages.
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Featured image shows Printed geometries, humidity testing, and plant study. Image via ACS Publications.
