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Lignin-Based Thermosets with Tunable Mechanical and Morphological Properties: A Study of Structure-Property Relationships
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Coating Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. (Mats Johansson)ORCID iD: 0000-0002-8127-9183
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nowadays, there is an urgent need to decrease our dependence on fossilresources and shift towards the use of renewable resources for advancingsustainable development. Utilizing renewable and bio-based raw materials,such as lignocellulosic biomass, for designing new materials is a promisingapproach to promote this objective. The main components of lignocellulosicbiomass are cellulose, hemicellulose, and lignin. Lignin is the most abundantaromatic biopolymer in nature and it is produced on a large scale fromchemical pulping processes as technical lignin. Lignin has the potential as asustainable and renewable alternative to fossil-based aromatics in variousapplications, e.g. thermosetting resins.

Technical lignin has a complex and heterogeneous structure, with arelatively low chemical reactivity. It is characterized by a high dispersity, thepresence of various functional groups that are unevenly distributed along thelignin chains, and various interunit linkages between the monoaromatics. Toovercome the challenges associated with lignin heterogeneity, technicallignin can be fractionated and/or chemically modified.

In this work, LignoBoost Kraft lignin was used as a starting material toproduce lignin-based thiol-ene thermosets. Firstly, lignin was fractionatedusing two approaches: 1) sequential solvent fractionation, and 2) microwaveassistedextraction. These fractionation approaches enabled access to ligninfractions with unique and tunable properties. Subsequently, lignin waschemically modified, in particular through allylation. Two allylation reagentswere used: allyl chloride and diallyl carbonate. The use of allyl chlorideenables a selective allylation of the phenolic OH groups, leaving the aliphaticand carboxylic acid OH groups unmodified. On the other hand, diallylcarbonate can react with all the aforementioned OH groups, leading to ahigher degree of allylation. Subsequently, allylated lignin was thermallycross-linked with various polyfunctional thiols, leading to thiol-enethermosets. The structure-property relationships of the thermosets wereinvestigated by varying several parameters, including the lignin source,fractionation approach, chemical modification, and thiol cross-linker. Byadjusting these parameters, various thermosets with tunable mechanical andmorphological properties were produced. Understanding the structurepropertyrelationships of these bio-based materials is crucial for identifyingpotential applications.

Abstract [sv]

Nuförtiden finns det ett akut behov av att minska vårt beroende av fossilaresurser och övergå till användningen av förnybara resurser och därmedavancera den hållbara utvecklingen. Att använda förnybara och biobaseraderåvaror, såsom lignocellulosabiomassa, för att designa nya material är ettlovande tillvägagångssätt för att uppnå detta mål. Huvudkomponenterna ilignocellulosabiomassa är cellulosa, hemicellulosa och lignin. Lignin ärnaturens vanligaste aromatiska biopolymer och den produceras i stor skalafrån kemiska massaprocesser som tekniskt lignin. Lignin kan fungera som etthållbart och förnybart alternativ till fossilbaserade aromater i olikatillämpningar, t.ex. g. värmehärdande hartser.

Tekniskt lignin har en komplex och heterogen struktur, med en relativt lågkemisk reaktivitet. Det kännetecknas av en hög dispersitet, närvaron av olikafunktionella grupper som är ojämnt fördelade längs ligninkedjorna, och olikatyper av enhetsbindningar mellan monoaromaterna. För att övervinna deutmaningar som är förknippade med ligninets heterogenitet kan ligninfraktioneras och/eller kemiskt modifieras.

I detta arbete användes LignoBoost Kraft-lignin som utgångsmaterial föratt tillverka ligninbaserade tiol-en-härdplaster. Först har lignin fraktioneratsmed hjälp av två olika metoder: 1) sekventiell lösningsmedelsfraktionering,och 2) mikrovågsassisterad extraktion. Dessa fraktioneringsmetoder gjordedet möjligt att erhålla ligninfraktioner med unika och skräddarsyddaegenskaper. Därefter modifierades ligninet kemiskt genom allylering. Tvåallyleringsreagens användes: allylklorid och diallylkarbonat. Användningenav allylklorid möjliggör selektiv allylering av de fenoliska OH-grupperna,samtidigt som de alifatiska och karboxylsyra-OH-grupperna lämnasomodifierade. Diallylkarbonat kan å andra sidan reagera med alla de tidigarenämnda OH-grupperna, vilket leder till en högre grad av allylering. Däreftertvärbands härdades allylerat lignin termiskt med olika polyfunktionellatioler, för att ge härdplast med tiol-en-tvärbindingar. Strukturegenskapsförhållandenaför härdplasterna undersöktes genom att varieraflera parametrar, inklusive ligninkällan, fraktioneringsmetod, kemiskmodifiering och tioltvärbindare. Genom att justera dessa parametrarproducerades olika härdplaster med skräddarsydda mekaniska ochmorfologiska egenskaper. Att förstå relationerna mellan struktur ochegenskaper av dessa biobaserade material är avgörande för att identifierapotentiella tillämpningar.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2023. , p. 66
Series
TRITA-CBH-FOU ; 2023:19
Keywords [en]
Softwood, hardwood, technical lignin, solvent fractionation, allylation, thiol-ene thermosets, thioether linkage, wide-angle X-ray scattering, π-π stacking interactions
Keywords [sv]
Barrträ, lövträ, tekniskt lignin, lösningsmedelsfraktionering, allylering, tiol-en-härdplaster, tioeterbindning, vidvinkelröntgenspridning, π-π staplingsinteraktioner
National Category
Organic Chemistry Polymer Chemistry Polymer Technologies Materials Chemistry
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-327306ISBN: 978-91-8040-567-6 (print)OAI: oai:DiVA.org:kth-327306DiVA, id: diva2:1758728
Public defence
2023-06-15, F3, Lindstedtsvägen 26 & 28, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg Foundation
Note

QC 2023-05-23

Available from: 2023-05-23 Created: 2023-05-23 Last updated: 2023-05-25Bibliographically approved
List of papers
1. Exploring the Effects of Different Cross-Linkers on Lignin-Based Thermoset Properties and Morphologies
Open this publication in new window or tab >>Exploring the Effects of Different Cross-Linkers on Lignin-Based Thermoset Properties and Morphologies
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2021 (English)In: ACS Sustainable Chemistry and Engineering, E-ISSN 2168-0485, Vol. 9, no 4, p. 1692-1702Article in journal (Refereed) Published
Abstract [en]

The search for sustainable material solutions has put lignin as one of the prime candidates for aromatic building blocks in macromolecular materials. The present study aimed to demonstrate how lignin-based thermoset resins can be utilized in combination with different cross-linkers. Kraft lignin was used to produce thermosets with tunable mechanical and morphological properties. The lignin-based thermosets were obtained via a thermally induced thiol–ene reaction. The first part of this work was focused on Kraft lignin solvent fractionation and chemical modification of the ethanol soluble fraction. Chemical analysis indicated that the allylation process was selective toward phenolic hydroxyl groups. SAXS and SEM studies demonstrated that solvent fractionation and allylation processes affected the molecular and nanoscale morphological characteristics of lignin. The second part’s focus was on how the properties of thermosets can be tuned by using three different cross-linkers. The dynamic mechanical and morphological properties of three different thermosets were investigated via DMA, SAXS, and WAXS techniques. The three different thermosets exhibit similar molecular morphology but different storage modulus and glass transition temperature. In this work, it was shown that despite lignin’s heterogeneity it was possible to produce thermosetting materials with tunable properties.

Keywords
Kraft lignin, Solvent fractionation, Selective allylation, Thiol−ene thermoset, Mechanical properties, Small- and wide-angle X-ray scattering
National Category
Paper, Pulp and Fiber Technology Polymer Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-289222 (URN)10.1021/acssuschemeng.0c07580 (DOI)000617925200026 ()2-s2.0-85099956480 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation, CGFB 63343
Note

QC 20210217

Available from: 2021-01-22 Created: 2021-01-22 Last updated: 2024-03-18Bibliographically approved
2. Effect of Molecular Organization on the Properties of Fractionated Lignin-Based Thiol-Ene Thermoset Materials
Open this publication in new window or tab >>Effect of Molecular Organization on the Properties of Fractionated Lignin-Based Thiol-Ene Thermoset Materials
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this study, the combination of sequential solvent fractionation of technical Kraft lignin were followed by allylation of most OH functionalities to give highly functional thermoset resins. All lignin fractions were highly functionalized on the phenolic (≥95%) and carboxylic acid OH (≥85%), and to a significant extent on the aliphatic OH moieties (between 43 and 75%). The resins were subsequently cross-linked using thiol-ene chemistry. The high amount of allyl functionalities resulted in a high cross-link density. DMA measurements showed that thioether content dominates the performance of these thermosets with a glass transition temperature (Tg) between 73 and 99 °C and with a storage modulus between 1.9 and 3.8 GPa for all thermosets. The lignin fractions and lignin-based thermosets morphology, at nanoscale, was studied by wide angle X-ray scattering measurements (WAXS). Two π-π stacking interactions were observed: sandwich (≈4.1–4.7 Å) and T-shaped (≈5.5–7.2 Å). The introduction of allyl functionalities weakens the T-shaped π-π stacking interactions. A new signal corresponding to a distance of ≈3.5 Å was observed in lignin-based thermosets, which was attributed to a thioether organized structure. At the same time, a lignin superstructure, was observed with a distance/size corresponding to 7.9-17.5 Å in all samples.

Keywords
Softwood lignin, solvent fractionation, allylation, thiol-ene thermosets, thioether linkage, wide-angle X-ray scattering, π-π stacking interaction, superstructures
National Category
Organic Chemistry Polymer Chemistry Polymer Technologies Paper, Pulp and Fiber Technology
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-327224 (URN)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20230524

Available from: 2023-05-22 Created: 2023-05-22 Last updated: 2023-05-24Bibliographically approved
3. Impact of lignin source on the performance of thermoset resins
Open this publication in new window or tab >>Impact of lignin source on the performance of thermoset resins
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2023 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 194, p. 112141-112141, article id 112141Article in journal (Refereed) Published
Abstract [en]

A series of different technical hardwood lignin-based resins have been successfully synthesized, characterized, and utilised to produce thiol-ene thermoset polymers. Firstly, technical lignin was fractionated and allylated, whereafter it was crosslinked with a trifunctional thiol. Structural and morphological characteristics of the lignin fractions were studied by 1H NMR, 31P NMR, SEC, FTIR, DSC, TGA, and WAXS. The hardwood lignin fractions have a high content of C5-substituted OH groups. The WAXS studies on lignin fractions revealed the presence of two π-π stacking conformations, sandwiched (4.08–4.25 Å) and T-shaped (6.52–6.91 Å). The presence of lignin superstructures with distances/sizes between 10.5 and 12.8 Å was also identified. The curing reaction of the thermosets was investigated by RT-FTIR. Almost all thermosets (excepting one fraction) reached 95% of the thiol conversion in less than 17 h, revealing the enhanced reactivity of the allylated hardwood lignin samples.

The mechanical properties of the thermosets were investigated by DMA. The curing performance, as well as the final thermoset properties, have been correlated to variations in chemical composition and morphological differences of lignin fractions. The described results clearly demonstrate that technical hardwood lignins can be utilized for these applications, but also that significant differences compared to softwood lignins have to be considered for material design.

Place, publisher, year, edition, pages
Elsevier BV, 2023
Keywords
Hardwood lignin, Solvent fractionation, Allylation, Thiol-ene thermosets, Wide-angle X-ray scattering, π-π stacking interactions
National Category
Materials Chemistry Polymer Technologies Polymer Chemistry Paper, Pulp and Fiber Technology
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-327220 (URN)10.1016/j.eurpolymj.2023.112141 (DOI)001000190100001 ()2-s2.0-85159278832 (Scopus ID)
Funder
KTH Royal Institute of Technology
Note

QC 20230524

Available from: 2023-05-22 Created: 2023-05-22 Last updated: 2023-06-26Bibliographically approved
4. Microwave-Assisted Fractionation and Functionalization of Technical Lignin Towards Thermoset Resins
Open this publication in new window or tab >>Microwave-Assisted Fractionation and Functionalization of Technical Lignin Towards Thermoset Resins
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Lignin is the main source of aromatic natural polymers, with a potential to serve as a building block of rigid and thermally stable bio-based materials. However, because of its heterogeneity differences and not fully understood chemical structure it is still underutilized. Therefore, in this paper, technical softwood Kraft lignin is refined in to narrow-dispersity and relatively low molecular weight fractions by microwave-assisted processing. This process was carried out under non-catalytic conditions using a low boiling point solvent and higher pressure. The chemical properties of the retrieved fractions were investigated by 31P NMR, HSQC NMR, SEC, DSC, and TGA. One of the retrieved soluble lignin fractions was successfully chemically modified by microwave processing. The chemically modified lignin was characterized by 31P NMR and FTIR, which provided evidence of the introduction of the allyl moieties. Subsequently, the allylated lignin was cross-linked through thermally induced thiol-ene chemistry to produce lignin-based thermosets.

Keywords
Softwood lignin, microwave-assisted fractionation, microwave-assisted functionalization, allylation, thiol-ene thermosets.
National Category
Polymer Chemistry Polymer Technologies Paper, Pulp and Fiber Technology
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-327223 (URN)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20230524

Available from: 2023-05-22 Created: 2023-05-22 Last updated: 2024-05-23Bibliographically approved

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