Leire Lorenzo, Belén Maestro, Pablo Ortiz – TECNALIA / Walter Pitacco, AEP POLYMERS / Ibrahima Faye – ECOAT
Lignin, a major component of lignocellulosic biomass, is predominantly sourced from the pulp and paper industry, where it is often regarded as a low-value by-product. However, due to its abundance and unique chemical structure, lignin has garnered attention as a sustainable feedstock for producing polyols.
TECNALIA has recently developed an anionic ring-opening polymerization method to synthesize lignin polyols (LPO), enabling the production of aliphatic polyols with tailored properties (Scheme 1). This process has now been scaled up by AEP Polymers to 5L reactors, yielding up to 1.5 kg of product per batch, with the resulting polyols used for polyurethane dispersion (PUD) coatings by ECOAT.
This advancement paves the way for large-scale production of lignin polyols, offering a more sustainable alternative for the coatings industry, particularly in wood coating applications.
The research began at the lab scale by defining the target specifications for the polyols, which were set to match those of the current polyol used in the alkyd PUD.
The commercial polyols used by ECOAT for manufacturing PUD have an OH value between 60 and 200 mg KOH per g and a molecular mass ranging between 500 and 2000 g/mol. The relatively low molecular weight (Mw) of the target LPO and the high Mw (≈1300) of the starting kraft lignin (KL) (Stora Enso Lineo®) made this process challenging, considering that polyol synthesis involves grafting polyether chains onto lignin molecules, thus increasing their molecular weight by at least 3-fold and up to 30-fold. However, maintaining an appropriate molecular weight range is crucial to ensure the desired film formation and drying characteristics of the PUD.
Consequently, a fractionation step was introduced.
Fractionation with pure ethanol resulted in a soluble lignin with lower molecular weight, lower polydispersity (D), and higher OH value. Various polyols were synthesized by varying the reaction conditions (monomer ratio, concentration, amount of catalyst, etc.). The best resulting LPO had 32% lignin content, a Mw of 3075 Da, and an OH value of 103.
For the larger-scale synthesis of the LPOs, larger quantities of fractionated lignin were needed. Fractionated lignin was supplied by the Technical Research Centre of Finland (VTT), which performed it at a pilot scale. Unlike the method used at the lab scale in TECNALIA, the process implemented at VTT used a 65/35 ethanol: water mixture. The resulting lignin had a higher Mw than that fractionated with pure ethanol but lower than the starting kraft lignin. AEP Polymers undertook the upscaling of the previously optimized LPO synthesis. Due to the use of different lignin, some optimization was required to achieve an LPO with similar properties to that obtained at the lab scale. The synthesis was then improved through complete temperature control and process integration and finally scaled up to 2L and then to 5L reactors, demonstrating reproducibility.
With the obtained LPO, different percentages of substitution of the reference polyol in the PUD were attempted, and a 22.5% substitution was chosen since higher percentages led to gelation. The substitution of the reference polyol by the LPO resulted in a brown PUD, significantly more viscous and with higher gloss (Fig. 1).
The PUD was applied on wood (Fig. 2), and their properties were measured. The addition of LPO did not lead to significant differences in certain key performance parameters.
Specifically, the comparison showed similarity in terms of tack-free time, adhesion on metal, flexibility, and impact resistance. It is noteworthy that both the reference PUD and the PUD with LPO exhibited similar drying characteristics, with both formulations becoming tack-free after 30 minutes in the presence of a drier. Additionally, the results showed that both formulations achieved the same adhesion class (Class 0), indicating excellent adhesion to metal surfaces. Flexibility is a crucial property in coatings, especially in wood applications. In this case, the results suggest that both the reference PUD and the PUD with LPO exhibit a similar level of flexibility, indicating that the addition of LPO did not compromise this aspect.
Surprisingly, the impact resistance of both formulations was reported to be less than 5 cm, suggesting that the addition of LPO did not lead to an improvement in impact resistance. In summary, LIGNICOAT partners have demonstrated that ligninderived polyols can be produced at a kilogram scale, proving the viability of upscaling and confirming the robustness of the developed protocol. Lignin polyols could partially replace standard polyols used in alkyd-PUD systems. However, there was a significant limitation on the percentage that could be used due to the gelation effect caused by the LPO.
This research was funded by the Bio-Based Industries Joint Undertaking (JU) under the European Union’s Horizon 2020 research and innovation programme through the LIGNICOAT project (grant agreement No. 101023342). The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Bio-based Industries Consortium.
