(413e) Upgrading Lignin Side-Streams into Sustainable Ligniox Plasticizers and Dispersants

Lignin is extensively studied and currently underutilized source for sustainable biochemicals, biomaterials and biofuels. Large quantities of lignin are available from kraft pulp production, even if lignin is still mainly burnt to provide process energy. Recently, also emerging biorefineries are entering the technical lignin market providing different lignin qualities, such as hydrolysis lignin from bioethanol production, and lignins from soda and organosolv pulp production.

Dispersants are widely used for suspending colloidal particles in cosmetics, paints, pharmaceuticals, oil drilling mud, cement, and ceramic applications. Besides lignosulfonates from sulfite pulp production, mainly synthetic polymers are used as dispersants in these applications. For substituting the current non-biodegrable polymers with bio-based (less expensive) materials, technical lignins have been modified in several ways, e.g. by sulfomethylation, oxidation, carboxymethylation, grafting with PEG, hydroxymethylation/propylation and sulfonation to convert them into water-soluble dispersants. Despite the encouraging results, the modification procedures seemed to be industrially unattractive and the property-performance relationships for lignin-based dispersants case dependent [1].

LigniOx technology is a simple and cost-efficient alkali-O₂ oxidation process for conversion of technical lignins for concrete plasticization, i.e. cement dispersing, or versatile dispersants [2;3]. The LigniOx process can be integrated into the biorefineries or operated in stand-alone units (e.g. by chemical industry). The oxidation is active toward phenolic hydroxyls and introduces acidic groups into the lignin polymer, while retaining its polymeric structure. Depending on the oxidation conditions, especially pH, the negative charge and molar mass of lignin can be adjusted in a controlled way. The previous studies on mortar and concrete plasticization have demonstrated that the soda and kraft LigniOx lignins have great potential to compete with lignosulfonates in their traditional markets, and even with synthetic products. LigniOx lignins have also shown high dispersing performance of several inorganic pigments, such as TiO₂, color pigment, and CaCO₃, used in paints, for instance [4;5]. Currently, the oxidation technology is optimized for several different lignin raw materials primarily to produce concrete plasticizers [6;7].

This study presents the very recent results of applying kraft, organosolv, and hydrolysis lignin based LigniOx lignins as plasticizers in cement mortar. In addition, the dispersing performance of LigniOx lignins in special carbon black, used e.g. in paints, coatings, inks, textile printing pastes, and aqueous graphite dispersions, is demonstrated. The potential environmental impacts of the LigniOx plasticizer production in kraft pulp mill were also studied and compared to benchmark synthetic superplasticizer from a life cycle assessment (LCA) perspective.

Experimental

Industrial lignin raw materials provided by Metsä Fibre, CIMV and St1 were used. Oxidations of different lignin raw materials were performed under the range of conditions reported in [2;3].

The plasticizing performance of oxidized lignin solutions was evaluated in mortar prepared using a standard sand mixture, Portland cement (CEM I 52.5N), and water. Plasticizer dosages 0.3 - 0.6 wt% (active matter) of cement were used. Few drops of de-airing agent were used in all tests. The fluidity of the mortar was measured using Haegermann flow table. Commercial polycarboxylate ether and sulfonated naphthalene based superplasticizers and lignosulfonate based plasticizers were used as references.

For the LCA, only the production phases of the kraft lignin based LigniOx plasticizer and the synthetic concrete superplasticizer admixture [9] were analyzed, taking out the transport, usage phases and end-of-life. Production of LigniOx plasticizer was conducted integrated to a modern kraft pulp mill [8]. The production of 1 kg of concrete plasticizer was considered as a functional unit. Five potential impact categories were assessed: global warming, acidification, eutrophication, abiotic depletion, and ozone layer depletion. The method used was CML-IA baseline.

The dispersing performance of the oxidized kraft, organosolv and hydrolysis lignin based samples was evaluated in Special Carbon Black (Orion Engineered Carbons). The dispersion prepared using high shear mixing contained 10 wt% of pigment in water. Dispersant dosages 0.25 - 2.5 wt% (active matter) of pigment were used. The viscosity of the pastes was measured as a function of shear rate at 25°C. Commercial polyacrylic acid and lignosulfonate based dispersants were used as references.

Results and discussion

Strong dispersion capabilities of the LigniOx solutions were verified in cementitious mortar. Kraft and organosolv lignin based LigniOx solutions provided equal or even better plasticization performance compared to the commercial synthetic references. The performance of hydrolysis lignin based LigniOx was a bit lower than synthetic references. Optimization of process conditions for hydrolysis lignin continues. For developing a LigniOx based concrete plasticizer, the next phases will include product formulation, verification of the plasticizing performance in fresh concrete and determination of early and matured strength of the hardened product. Field-testing will follow the laboratory scale investigations.

According to the LCA, the production of kraft lignin based LigniOx plasticizer presented the best environmental performance, showing 50-60% lower impacts in eutrophication and acidification categories, around 70-80% lower in global warming and abiotic depletion indicators and about 50% lower impacts in ozone layer depletion indicator. The main contributors to the total environmental impacts of LigniOx plasticizer produced integrated in kraft pulp mill were the use of sodium hydroxide, and chlorine dioxide (due to allocation of part of bleaching chemicals to LigniOx product).

All the tested LigniOx lignin solutions showed high dispersing performance in special carbon black when using dosing of 2.5%. The viscosity values were measured just after preparing the dispersion and after 7d. There was no changes seen after the storage period. Better dispersing performance of LigniOx lignins over commercial polyacrylic acid reference (2.5%) was supported also by microscopy and particle size analyses. Differences in the performance between the tested LigniOx samples, varying in terms of molar mass, were seen when decreasing dosing. Kraft lignin based sample (Mw of 7 kDa) was still showing high performance, while the other LigniOx samples as well as the lignosulfonate or polyacrylic acid reference could not retain the viscosity as low. The results highlight that the oxidation conditions must be optimized according to the specific end-uses to provide high-performance LigniOx products for each lignin type. Regarding the new end-uses of LigniOx lignin, performance in the actual formulated end-product (paints, coatings or inks), and the co-effect with all the other ingredients should be demonstrated.

Conclusions

LigniOx oxidation technology converts various industrial lignin raw materials to high-performing plasticizers and versatile dispersant. In addition to kraft lignin, it was shown that LigniOx solutions based on organosolv and hydrolysis lignin have high dispersing performance in cementitious material. The results of life cycle assessment of the LigniOx production integrated to a kraft pulp mill indicated better environmental performance when comparing to production of commercial synthetic concrete plasticizer admixture. LigniOx solutions also efficiently dispersed special carbon black in water, which further opens new application possibilities for oxidized lignins. LigniOx lignins are seen as a promising, bio-based and environmentally friendly alternative for several applications areas with significant market volume.

Acknowledgements

Bio Based Industries Joint Undertaking (BBI JU) under the European Union‘s Horizon 2020 Research and Innovation program is acknowledged for funding the project LigniOx (Lignin oxidation technology for versatile lignin dispersants) under grant agreement No 745246.

References

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