Tuning the Properties of Biobased PU Coatings via Selective Lignin Fractionation and Partial Depolymerization

Abstract

Polyurethane (PU) coatings with high lignin content and tunable properties were made using a combination of fractionation and partial catalytic depolymerization as a novel strategy to tailor lignin molar mass and hydroxyl group reactivity, the key parameters for use in PU coatings. Acetone organosolv lignin obtained from pilot-scale fractionation of beech wood chips was processed at the kilogram scale to produce lignin fractions with specific molar mass ranges (M w 1000–6000 g/mol) and reduced polydispersity. Aliphatic hydroxyl groups were distributed relatively evenly over the lignin fractions, allowing detailed study of the correlation between lignin molar mass and hydroxyl group reactivity using an aliphatic polyisocyanate linker. As expected, the high molar mass fractions exhibited low cross-linking reactivity, yielding rigid coatings with a high glass transition temperature (T g). The lower M w fractions showed increased lignin reactivity, extent of cross-linking, and gave coatings with enhanced flexibility and lower T g. Lignin properties could be further tailored by lignin partial depolymerization by reduction (PDR) of the beech wood lignin and its high molar mass fractions; excellent translation of the PDR process was observed from laboratory to the pilot scale necessary for coating applications in prospective industrial scenarios. Lignin depolymerization significantly improved lignin reactivity, and coatings produced from PDR lignin showed the lowest T g values and highest coating flexibility. Overall, this study provides a powerful strategy for the production of PU coatings with tailored properties and high (>90%) biomass content, paving the path to the development of fully green and circular PU materials.

Life Cycle Environmental Impact Considerations in the Design of Novel Biobased Polyurethane Coatings

“…In all cases, target PU materials of comparable performance were considered, with tunable glass transition temperatures (T g ) ranging between 110 and 180 °C, high adhesion on different substrates (e.g., glass, metal, wood), and tunable surface energy as a function of composition and lignin type. 15 ■ MODELING SECTION: BUILDING THE LCA MODEL…”

“…As detailed in a separate contribution, partial depolymerization of OSL reduced its molar mass and polydispersity and significantly improved lignin reactivity toward the crosslinker. 15 As a result, coatings were produced showing low glass transition temperature (T g ) values and high coating flexibility. The addition of the lignin depolymerization step results in a PU coating production impact of 2.64 kg CO 2eq / kg coating (Figure 4).…”

“…For comparison with fossil-based PU coatings, four different methods of water-borne PU coating production with fossil-based building blocks were used as reference cases. In all cases, target PU materials of comparable performance were considered, with tunable glass transition temperatures ( T g ) ranging between 110 and 180 °C, high adhesion on different substrates (e.g., glass, metal, wood), and tunable surface energy as a function of composition and lignin type …”

“…13,14 Recent T h i s c o n t e n t i s research has also demonstrated that Fabiola organosolv lignin (OSL) can be further tailored via partial depolymerization by reduction (PDR), allowing tunability of the lignin characteristics to specific applications, including coatings. 15 Biobased isocyanates represent a particular challenge for PU coating production, and research and development has been mainly limited to isocyanate-free PUs as a means of defossilizing the process. 16 Recent research has led to the development of a fully biobased PU coating, including biobased isocyanates, at laboratory scale.…”

“…Organosolv processes are frequently used to fractionate lignocellulosic biomass into different streams, including lignin. One such method is the Fabiola process, which is an acetone organosolv process that has been successfully used on hardwood and herbaceous biomass feedstocks. , Recent research has also demonstrated that Fabiola organosolv lignin (OSL) can be further tailored via partial depolymerization by reduction (PDR), allowing tunability of the lignin characteristics to specific applications, including coatings …”