Transformation of lignin into biobased thermoset

MASTER THESIS IN FIBRE AND POLYMER SCIENCE Stockholm, Sweden 2018

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH

Transformation of lignin into biobased thermoset

LINNEA CEDERHOLM

A ^BSTRACT

Combined microwave assisted extraction/degradation of technical lignin in green solvents was successfully employed to generate polyphenolic oligomers with lower M w than the starting material.

For Lignoboost, the highest liquid yield (65 %) was obtained in 20 min at 160 °C using ethanol as solvent. This is an increase in ethanol soluble yield with 38 % compared to solvent extraction. The highest yield for Lignosulfonate was obtained with methanol as solvent, at 160 °C for 20 min. Obtained liquid fractions were analysed by SEC, FT-IR, DSC, TGA, ³¹ P-NMR and 2D-HSQC NMR in order to explain the mechanism of the increased yield, and to study the structural changes after microwave extraction/degradation. 2D-NMR indicates cleavage of β-O-4 inter-unit linkages, but also that some modification around the bond could take place. Lignin based thermosets were synthesised employing the polyesterification between lignin, citric acid and poly(ethylene glycol) (PEG). It was concluded that introduction of PEG into the system was crucial for a homogenous thermoset synthesis with a high gel content. From TGA analysis it could be concluded that the thermoset based on original Lignoboost had a lower thermal stability than the counterparts prepared from lower molecular weight fractions. This implies that the esterification reaction between original Lignoboost and the other co-monomers is obstruct by sterically hindrance, which means that pre-conditioning is positive for the final material properties.

Keywords: lignin, microwave heating, solvolysis, biobased thermoset

F RÅN LIGNIN TILL BIOBASERADE HÄRDPLASTER - S AMMANFATTNING

I denna studie utnyttjades en mikrovågsbaserad teknik, för att framgångsrikt extrahera och bryta ner lignin till polyfunktionella oligomerer med lägre molekylvikt än ursprungsmaterialet. Både lignin extraherat genom sulfat- och sulfitprocessen, d.v.s. kraft lignin (Lignoboost) och lignosulfonat, undersöktes. Det högsta lösliga utbytet för Lignoboost (67 %) kunde uppnås efter 20 min vid 160 °C genom att använda etanol som lösningsmedel, vilket är en ökning med 38 % jämfört med enbart extraktion i etanol. Under samma förhållanden uppnåddes även det högsta lösliga utbytet för Lignosulfonat, fast genom att använda metanol som lösningsmedel. De erhållna lösliga fraktionerna analyserades med hjälp av SEC, FT-IR, DSC, TGA, ³¹ P-NMR samt 2D-HSQC NMR, med syftet att förklara ökning i lösligt utbyte samt studera eventuella strukturella förändringar efter bearbetning i mikrovågsugnen. Resultat från 2D-NMR indikerar på nedbrytning av β-O-4 bindningar, men även på att modifikationer kring bindningen kan ha uppkommit. Tvärbundna, ligninbaserade material syntetiserades genom att nyttja polykondensationsreaktionen mellan lignin, citronsyra och polyetylenglykol (PEG), vilket resulterade i esterbindningar. Det var möjligt att dra slutsatsen att introducering av PEG in i systemet var avgörande för att nå homogena material med hög andel tvärbindningar. Genom TGA analyser kunde det fastslås att tvärbundna material baserade obehandlad Lignoboost hade lägre termisk stabilitet än dess motsvarigheter baserade på fraktioner med lägre molekylvikt. Detta tyder på att esterreaktionen mellan obearbetad Lignoboost och de två andra monomererna försvåras genom steriskhindring, vilket innebär att bearbetning av ligninet medför positiva effekter på egenskaperna hos det slutgiltiga materialet.

Nyckelord: lignin, mikrovågsbaserad teknik, solvolys, biobaserad härdplast

A BBREVIATIONS

CA Citric acid

Ð Dispersity (M w /M n )

DSC Differential scanning calorimetry

EtOH Ethanol

FTIR Fourier transform infrared spectroscopy

GC Gel content

GCT Gel content test

LB Lignoboost

LS Lignosulfonate

MeOH Methanol

MW Microwave

M w Weight-average molecular weight NMR Nuclear magnetic resonance

PEG Polyethylene glycol

SEC Size exclusion chromatography TGA Thermogravimetric analysis

TS Thermoset

T ABLE OF C ONTENTS

1 I NTRODUCTION ... 1

2 B ACKGROUND ... 2

2.1 S TRUCTURE AND CHEMISTRY OF LIGNIN ... 2

2.1.1 E XTRACTION PROCESSES ... 3

2.2 V ALORISATION OF TECHNICAL LIGNIN ... 3

2.2.1 R EFINING OF LIGNIN INTO MACROMOLECULAR PRECURSORS ... 4

2.2.2 M ICROWAVE ASSISTED DEGRADATION ... 5

2.3 L IGNIN BASED THERMOSETS ... 5

3 P URPOSE OF THE STUDY ... 7

4 E XPERIMENTAL ... 8

4.1 M ATERIALS ... 8

4.2 M ICROWAVE ASSISTED SOLVOLYSIS OF LIGNIN ... 8

4.2.1 S OLVENT SCREENING ... 8

4.1.2 E FFECT OF TIME AND TEMPERATURE ... 9

4.2 T HERMOSET SYNTHESIS ... 10

4.2.1 D ETERMINATION OF REACTION CONDITIONS ... 10

4.2.2 S YNTHESIS OF LB-CA-PEG- BASED THERMOSETS ... 11

4.3 M ATERIALS CHARACTERISATION ... 11

4.3.1 S IZE EXCLUSION CHROMATOGRAPHY (SEC) ... 11

4.3.2 F OURIER -T RANSFORM I NFRARED S PECTROSCOPY (FT-IR) ... 12

4.3.3 D IFFERENTIAL SCANNING CALORIMETRY (DSC) ... 12

4.3.4 T HERMOGRAVIMETRIC ANALYSIS (TGA) ... 12

4.3.5. N UCLEAR MAGNETIC R ESONANCE (NMR) ... 12

5 R ESULTS AND D ISCUSSION ... 14

5.1 M ICROWAVE ASSISTED DEGRADATION OF LIGNIN ... 14

5.1.1 S OLVENT SCREENING ... 15

5.1.2 O PTIMISATION OF TIME AND TEMPERATURE ... 17

5.1.3 T HERMAL ANALYSIS ... 19

5.1.4 I NVESTIGATION OF DEGRADATION REACTION ... 21

5.2 T HERMOSET SYNTHESIS ... 34

5.2.1 D ETERMINATION OF REACTION CONDITIONS ... 34

5.2.2 S YNTHESIS OF LB-CA-PEG- BASED THERMOSETS ... 36

6 C ONCLUSIONS ... 41

7 F UTURE WORK ... 42

8 A CKNOWLEDGMENTS ... 43

9 R EFERENCES ... 44

1

1 I NTRODUCTION

The world of today is a heavily fossil resource dependent one. That has however not always been the case. Going back no further than 200 years ago, the dominating feedstocks for energy, organic chemicals and fibres were plant-based ¹ . Over the last half a century the situation has been drastically changed though – today oil is the major organic chemical feedstock ² . Even though the use of petroleum for chemicals and plastic production only count for approximately 9 % ³ and 4 % ⁴ of the world’s total production respectively, the demand is estimated to increase rapidly with a rate outpacing that of energy from the same resource ² . The fact that around 97 % ³ of all chemical products today are derived from oil or natural gas may partly be ascribed the petrochemicals industry’s co-evolution with fuel-oriented oil refining. This have resulted in highly efficient processes and optimized technologies where the majority of the oil is used for energy (fuels), with only some fractions (by-products) being used to make products of higher value, like plastics ² . But with increasing concerns about climate change and human’s impact on the environment, alternative feedstocks have the recent years gained larger attention.

One possible model for switching the chemical industry to use of renewable resources is the biorefinery, where the concept of the oil refinery should be translated into the use of a biomass source. This means that after processing its biomass, the refinery should deliver a range of products from fuels to value- added chemicals in conformity with an oil refinery, where the added-value products makes it possible to produce fuels at a competitive cost ⁵ . One of most important matters though, in the debate regarding chemical and material production from renewable resources, is to find abundant feedstocks that do not compete with food production, but from which the building blocks can be efficiently derived. The first- generation refinery concept were focusing on edible corps like maize and oil seeds ⁶ , which opened up for the fuel-versus-food debate ² . Another option is however, the now more emerging second-generation biorefinery concept, which instead utilizes lignocellulosic biomass (e.g. forest) that do not directly compete with food production ⁶ . But this not is totally risk-free either, and up-scaling needs to be done with caution. There are therefore voices calling for that the biobased plastic industry maybe should be developed independent of biofuels ⁷ . Since the scale differences between fuel and plastic industry are large, and the potential in developing an environmental and societal sustainable chemical and material production is greater if uncommitted from the fuel industry ⁷ . One way to handle this challenge is to address waste products from already existing industries as important feedstocks ^2,6–8 .

One resource that for a long time has been considered as waste generated by the pulp and paper industry is lignin, and it has mainly been used for energy recovery at the paper mills ⁹ . Lignin is one of three major constituents in lignocellulosic biomass, and it is the most abundant natural source of aromatic structures ⁹ . Today, the interest in development of biobased materials is large, and a lot of research and development is going on utilising e.g. fatty acids, terpenes and carbohydrates as feedstocks ^6,8 . Some of the most well-known examples are polylactic acid and biobased polyethylene, both derived from carbohydrates. But they are, like the majority of the bio-derived polymers, aliphatic in character. There are examples of aromatic monomers though, that have been converted from the non-aromatic sugars, like terephthalic acid and furan dicarboxylic acid ⁶ . However, during the past year’s lignin has received growing attention as a renewable, abundant and low-cost source of aromatic structures for chemical and material production. But its potential is not yet fully employed ¹⁰ .

The purpose of this study was therefore to investigate the potential in valorisation of lignin by combined

microwave extraction/degradation into polyphenolic oligomers, and in turn using these as precursors

for thermoset synthesis.

2

2 B ACKGROUND

2.1 S TRUCTURE AND CHEMISTRY OF LIGNIN

As mentioned earlier, lignin is the most abundant natural source of aromatic structures ⁹ . It is found in association to cellulose and hemicellulose in woody plants, like trees and grasses. There it aids as a

“glue” that keeps the fibres together, provides stiffness to the stem and protects it form microbial attack

11,12 . Furthermore, the hydrophobicity of lignin enables transport of water and nutrition throughout the plant ¹² .

The chemical structure of lignin is rather complex and differs quite a lot from other biologically synthesised molecules by being irregular and racemic, which may be addressed to the biosynthesis mechanism – radical polymerization ¹² . The precursors are essentially three different kinds of phenylpropene’s (monolignols) – p-coumaryl-, coniferyl- and sinapyl alcohol (Figure 2.1). These primary units are connected by a large variety of linkages including ether (C-O-C) and carbon-carbon (C-C, condensed) bonds. Figure 2.2 illustrates some typical inter-monolignol linkages, where β-O-4 (between the β-carbon the aliphatic chain and the oxygen on the aromatic carbon-4) is the most common one. ^9,11,12 The monolignol composition, but also the frequency of different inter-monolignol linkages, differs from species to species – e.g. softwood lignins are mainly composed by coniferyl alcohol with small amounts of p-coumaryl alcohol, whereas hardwood and grass lignins comprise both coniferyl- and sinapyl alcohol (to different degrees) and plus small amounts of p-coumaryl alcohol. ⁹ The monolignols are however not only connected to each other, but also covalently cross-linked to carbohydrates (LCC-bonds) by ether or phenyl glycoside bonds ¹¹ .

Figure 2.1. Structure of the primary units (monolignols) of lignin.

Figure 2.2. Some typical inter-monolignol linkages.

3 2.1.1 E XTRACTION PROCESSES

Lignin may be isolated from the biomass by several different chemical processes. The most common objective of these methods is to separate lignin from the cellulosic fibre pulp, where the latter is used for making e.g. paper products ⁹ . The residual technical lignins possess different properties – in means of molecular weight, polydispersity, solubility, functional groups (e.g. alcohol, carboxylic acid, sulfonic acid, thiols) and types of inter-monolignol linkages – depending on pulping technique. The extraction processes can be roughly divided into sulfur processes and sulfur-free processes, where the two sulfur processes – kraft and sulfite pulping – are the ones generating the largest production volumes of technical lignins – kraft lignin and lignosulfonates ¹² . Kraft lignins are extracted through an alkali process using sodium hydroxide (NaOH) and sodium sufide (Na 2 S) at 170 °C. This results in technical lignin that are soluble in alkali solution and in highly polar organic solvents, with a molecular weight of 1,000-3,000 Da and a polydispersity of 2-4 ¹² . Their structures contain a high degree of condensed bonds and large amounts of phenolic groups ⁹ . This extraction process has been further improved by the so called Lignoboost process, developed by Inventia (Sweden) ¹³ . Sulfite pulping, on the other hand, is an acidic process using a solution of bisulfite salts (e.g. NaHSO 3 ) at 140 °C. ⁹ The resulting lignin is water soluble, due to the considerable amount of sulfonate groups that has been attached to the aliphatic chains ⁹ . The molecular weight as well as polydispersity is larger than that of kraft lignin – 36,000- 61,000 g/mol and Ð = 4-9 ¹² .

One reason to why transforming technical lignins into value-added products is a challenging task, is their complex composition of diverse structures with different molecular weights and several inter- monolignol linkages. As described, in previous section this is a result of the biosynthesis pathway, but also the extraction process is influencing. The current pulping technologies were developed with only the carbohydrate valorisation in mind, where no attention being paid to the residual lignins as long as they were sufficiently enough isolated from the pulp. A different approach for future second generation biorefineries could instead be the “lignin-first” strategy, meaning direct valorisation of lignin from the lignocellulosic biomass rather than looking at it as a raw material set by the pulp and paper industry ¹⁰ . This could potentially reduce the heterogeneity, simplifying the following refining process. However, since this is yet no fully developed technology, there are still motives for looking at the lignin fractions generated by existing industries (like pulp and paper) for further valorisation.

2.2 V ALORISATION OF TECHNICAL LIGNIN

Several different approaches towards valorisation of lignin have been investigated in the literature. As unmodified, lignin has the potential of being used for industrial applications as e.g. UV stabilizer ¹⁴ , antioxidant ^15–17 and filler ¹⁸ . These are however low-value applications, and due to its thermal degradation etc. only a small amount of lignin can be incorporated directly into polymer blends. Hence, in 2010 only 2 % of the total extracted lignin was used commercially ⁹ . There is therefore a need of finding other ways in which technical lignin can be used in a larger scale and for high value applications.

Lignin could e.g. be fragmented by pyrolysis, oxidation and hydro processing in order to reach low

molecular weight aromatic compounds ^9,19 , or by gasification giving syngas ⁹ . Another approach could

be using lignin as a macromolecular precursor for material synthesis. However, the reactivity of

unrefined technical lignins are generally low and uneven, ascribed i) the large degree of etherification

of phenolic hydroxyl groups ²⁰ , ii) steric hindrance of the present hydroxyl groups (aliphatic or aromatic)

due to lignin’s molecular structure ^21,22 and iii) its heterogeneity ¹⁰ . This makes technical lignin an

4

inconsistent and low reactive precursor for material synthesis, why preconditioning is considered necessary 10,20,23–25 . For this, different strategies may be adopted. The key of them all though, is to increase the reactivity of lignin and refining it into a more predictable system, which in turn can be used for material synthesis.

To summarize, one interesting but challenging approach in valorisation of lignin for value-added products could be by complete depolymerisation into well-defined, low molecular weight, aromatic structures that could replace petroleum based analogues in conventional processes ^10,22 . There is however also need for methods where lignin can be utilised more directly as a macromolecule/oligomer.

If well designed, those routes could have the benefits of being more cost effective and environmentally compatible due to a shorter reaction path and use of less energy, reagents and solvent ²² . Both strategies are identified as intriguing, though this study only will stress the later.

2.2.1 R EFINING OF LIGNIN INTO MACROMOLECULAR PRECURSORS

Sequential membrane filtration and solvent extraction are examples of refining methods that do not include chemical modification, where both separate lignin according to molecular weight. The membrane technologies are based on mechanical separation, where controlled molecular mass fractions are obtained by sequential filtration through membranes with decreasing cut-off ^26–28 . They have the advantage of consuming low amounts of energy, but comes with the issue of quite rapid fouling which results in need of continuous cleaning ¹⁰ . For solvent extraction on the other hand, the isolation is based on solubility. Duval et.al. ²⁹ have developed a sequential solvent fractionation process using ethyl acetate, ethanol, methanol and acetone which generates fractions of increasing molecular weight and lower polydispersity compared to the original lignin. Obtained fractions can be used as they are, or being functionalized by chemical modifications, in which new active sites are being introduced to the structure – e.g. NH 2 (amination); Br, Cl, I (halogenation); NO 2 (nitration); OH (demethylation) ^9,22 . This could be made in order to increase lignin’s reactivity, increase the flexibility of lignin derived polymers or to increase its solubility in certain solvents ⁹ .

Another approach is solvolysis of lignin. A lot of the work done on degradation of technical lignins have been aiming on achieving monophenols for either chemical or fuel production ^30–32 , but there are also examples in the literature where precursors from material production have been the main focus

23,33 . Depending on the goal, different reaction conditions have been adopted - using acid ^30,33,34 -/base ^31,32

catalysis or non-catalytic ^35,36 conditions, at temperatures from 100-400 °C. Often conventional

heatining ^23,30–32 have been employed, but another emerging technology – popular due to i.e. its energy

efficiency – is microwave heating 33,34,36–38 . Minami et.al. ³⁹ have studied the reaction behaviour of lignin

model compounds in supercritical methanol (MeOH), reporting rapid cleavage of β-O-4 and α-O-4

linkages, whereas the condensed 5-5 and β-1 linkages were stable. Miller et.al. ³² studied the base

catalysed depolymerisation of lignin and model compounds in supercritical MeOH and ethanol (EtOH)

at 290 °C, where lowest amount of ether-insoluble residues was achieved using EtOH as solvent. Also

Erdocia et.al. ³⁵ have reported supercritical MeOH being less effective than supercritical EtOH in

reaching high monomer yields. Huang et.al. ⁴⁰ ascribes this difference to the capping agent and

formaldehyde scavenging properties of EtOH, which hinders repolymerisation to a larger extent than

MeOH. The formaldehyde is reported to be formed during the decomposition of lignin, which also has

been observed in water solvent media ⁴¹ .

5 2.2.2 M ICROWAVE ASSISTED DEGRADATION

Microwaves are electromagnetic radiation with a frequency of 300 MHz to 300 GHz, which irradiation forces have the capability of aligning polar molecules in the radiation field ⁴² . With an oscillating microwave field, the polar molecules will try to direct themselves in the direction of the field, which causes friction and hence heat generation. In this way, the heat is produced directly from inside of the material, in contrary to conventional heating where the heat is produced from outside the material and then being transmitted by conduction and convection ⁴² . This gives microwave assisted heating technologies some advantages by enabling shorter reaction times, better control of the heating rate, higher energy efficiency and by preventing surface overheating, but it has also shown to be able to enhance reaction rates and to result in high yields also at milder conditions ^38,42,43 . However, if the material is inhomogeneous (in e.g. composition, geometry or size) this will lead to uneven distribution of the microwave energy, which in turn may generate hot spots – i.e. local overheating ^42,43 .

Microwave assisted degradation of lignin have been performed in several different solvents – e.g.

isopropanol ³⁶ , ethylene glycol ³⁷ , poly(ethylene glycol) and glycerol ³³ , but also in ionic liquids ⁴⁴ . Since the microwaves only affect polar molecules, it can be assumed that the polarity are of importance. From a study using three different solvents of different polarity (DMF, DMSO and ethylene glycol) by Dhar et.al. ³⁸ it was concluded that at high temperatures the degradation of lignin was more efficient in a more microwave absorbing and polar solvent. However, at lower temperatures the differences were more equalized. Sequeiros et.al. ³³ studied the liquefaction of lignin under MW heating using poly(ethylene glycol) (average M n 400) and glycerol as solvents. The solvents were found to be incorporated to the structure of lignin, which led to a high content of reactive hydroxyl groups. The optimal product was reached after just 5 min microwave treatment at 155 °C using 1 % of sulphuric acid as catalyst, and resulted in 99.1 % of dioxane/water (80/20 w/w) soluble product.

Since the microwave technology has shown to be able to enhance reaction rates ⁴³ , it would be interesting to study the combined extraction/degradation of lignin under rather mild and un-catalysed conditions.

In the first part of this study, the microwave assisted extraction/degradation of lignin will therefore be investigated using three different green solvents – water, methanol and ethanol – which all are polar, microwave absorbing solvents. The influence of reaction time and temperature will also be evaluated, with the aim of reaching a high lignin yield in the soluble fraction, with a reduced molecular weight and dispersity compared to the starting material.

2.3 L IGNIN BASED THERMOSETS

During the past year’s lignin has received growing attention as a renewable, abundant and low-cost source of aromatic structures for chemical and material production. As described earlier in section 2.2, lignin could be valorised as a platform chemical, replacing petroleum based analogues in conventional thermoplastics. But it could also be used as a macromolecular pre-cursor in thermoset synthesis.

Recently there has been a lot of articles published regarding lignin based thermoset synthesis. Some

examples are unsaturated polyester-styrene resins ⁴⁵ , polyurethane coatings ²¹ and lignin-epoxy based

networks ^24,46 . But also thiol chemistry have been exploited ^47,48 . The approach is often a two-step

synthesis, starting from functionalisation of lignin fractions followed by a curing reaction. The benefit

with this is that the chemical behaviour of lignin could more easily be investigated, but – if efficient –

a simple one-pot reaction would be preferable in order to reduce the amount of solvent and less waste

6

generation. Furthermore, thermosets cannot be reshaped or reprocessed by heat – as the case for physically cross-linked materials and thermoplastics – which complicates recycling of those materials ^49,50 . There are therefore motives for development of recyclable thermosets. Moreover, the crosslinking agent should be economical, non-toxic and preferably bio based.

On example of linkages that can provide degradability to polymers are ester bonds, which can be hydrolysed in basic or acidic solutions ⁴⁹ . Since the most common functionality in lignin is hydroxyl groups, which easily can from ester linkages by condensation reaction with e.g. carboxylic acids ⁵¹ (Scheme 2.1), this seems like a good starting point. In Figure 2.3 are six examples of naturally occurring acids are presented, which could be acting as co-monomer in a lignin-polyester based thermoset. It is believed that a functionality greater than two could be favourable in yielding a high degree of cross-linking. Moreover, if the melting temperature is below the curing temperature this would probably have positive effect on the esterification reaction, due to higher movability of the reactants. These are aspects taken into consideration when choosing a suitable co-monomer.

Citric acid mp: 156 °C

Tartaric acid mp: 171-174 °C

Malic acid mp: 130 °C

Oxalic acid mp: 189-191 °C

Malonic acid mp: 135-137 °C

Glutaric acid mp: 95-98 °C Figure 2.3 Examples of naturally occurring acids and their melting points.

Scheme 2.1. Esterification reaction between hydroxyl and carboxylic acid groups.

7

3 P URPOSE OF THE STUDY

In the first part of this study, the potential of using MW assisted extraction/degradation in green solvents to transform technical lignins into more defined polyphenolic oligomers with lower dispersity, will be investigated. The molecular weights of the obtained solid and liquid fractions will be analysed. The reaction conditions will be optimised, towards degradation and high liquid yield, in regard to solvent (water, methanol and ethanol), reaction time and temperature. The degradation reaction will be investigated with the assistance of FT-IR, ³¹ P-NMR and 2D-NMR analysis, in order to explain the mechanism of the increased yield and to study the structural changes after MW extraction/degradation.

DSC and TGA analysis will be utilized to analyse the thermal properties of obtained lignin fractions.

The technical lignins that will be used in this study are Lignosulfonate and Lignoboost kraft lignin, which are the ones generating the largest production volumes from the industry ⁵² .

In the second part of this study, lignin based thermosets will be synthesised employing the esterification

reaction mechanism between hydroxyl and carboxylic groups of lignin and citric acid in a one-pot

reaction. The curing reaction will be adapted regarding e.g. catalyst and curing time and temperature,

aiming at a thermoset with a high gel content. Both MW degraded lignin and the original lignin material,

as well as solvent extracted lignin, will be used in order to investigate the structure-property relationship

between the MW degraded lignin and the two other fractions. For this FT-IR, TGA and DMA analysis

will be employed.

8

4 E XPERIMENTAL 4.1 M ATERIALS

Lignosulfonate (LS) (alkaline lignin, average M w =10 000 g/mol, 4% sulfur, Sigma Aldrich), softwood kraft lignin obtained from the Lignoboost process (LB) (dried in fume hood for 24 h), methanol (98.5

%, VWR), ethanol (96 %, VWR), acetone (99.9 %, VWR), citric acid (monohydrate, ≥99.5 %, Fulka), poly(ethylene glycol) (PEG ⁴⁰⁰ ) (average M n 400 Da, Sigma Aldirch), p-toluenesulfonic acid (p-TSA) (monohydrate, 98.5 %, Sigma Aldrich), 4-(dimethylamino)pyridine (DMAP) (≥99.0 %, Fulka), 1- methylimidazole (MIZ) (99 %, Sigma Aldrich), N,N-dimethylformamide (DMF) (anhydrous, 99.8 %, Sigma Aldrich), pyridine (puriss., absolute, over molecular sieve (H 2 O ≤0.005 %), ≥99.0 % (GC), Sigma Aldrich), N-hydroxy-5-norbornene-2,3-dicarboxylic acid imide (HONB) (97 %, Sigma Aldrich), chromium(III) acetylacetonate (99.99 % trace metals basis, Sigma Aldrich), 2-chloro-4,4,5,5- tetramethyl-1,3,2-dioxaphospholane (95 %, Sigma Aldrich), chloroform-d (CDCl 3 -d) (99.8 % + silver foil, Cambridge Isotope Laboratories, Inc.) and dimethyl sulfoxide-d 6 (DMSO-d 6 ) (99.9 %, Cambridge Isotope Laboratories, Inc.) was used without further purification. All water used was purified from Milli-pore Milli-Q plus water purification system.

4.2 M ICROWAVE ASSISTED SOLVOLYSIS OF LIGNIN

All microwave assisted experiments were performed in Milestone FlexiWave MA186, at an effect of 800 W. Before put in the microwave, all samples were sonicated for 10 min.

4.2.1 S OLVENT SCREENING

Solvent screening experiment of LB as well as LS was carried out according to a fractional factorial experimental design, see Table 4.1. Here the influence of solvent, temperature and time on liquid fraction yield and molecular weight were investigated. The solvent was treated as a qualitative factor using water, methanol and ethanol respectively. Temperature and time were treated as independent quantitative factors, and were tested at three levels – 80-120-180°C and 20-40-60 min.

0.5 g lignin was transferred to a 50 mL E-flask, hereafter the solvent (20 mL) was added. The mixture/solution was sonicated for 10 min and the transferred to a microwave vial. Treated in microwave with a ramping time of 20 min and cooling time of 20 min. The isotherm conditions are specified in Table 4.1. For each run, both LB and LS were treated at the same time in separate vials, always having the thermometer stick in the one containing LB.

The solid residues were filtered of and the vial was washed 2-3 times with solvent. The filtrate was collected in a 100 mL beaker and let to evaporate in room temperature. The reduced solution was transferred to a sample vial for final evaporation under a gentle air flow. Both solid and liquid fraction finally dried in the vacuum oven at 23°C for at least 12 h. The yields for both solid and liquid fractions (Y ^solid and Y ^liquid ) were calculated according to (1), were m lignin is the exact mass of lignin that was weight and put in an E-flask and m fraction is the mass of specified fraction after drying. The molecular weights of both fractions were determined separately by SEC (3.3.1). The results were statistically analysed using MODDE 9.0 software.

𝑌 ^{𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛} = ^{𝑚 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛}

𝑚 _{𝑙𝑖𝑔𝑛𝑖𝑛} (1)

9

Sample Lignin Solvent

Temperature (°C) /level

Time (min) /level

Sample Lignin Solvent

Temperature (°C) /level

Time (min) /level

LB W11 LB Water 80/1 20/1 LS W11 LS Water 80/1 20/1

LB W22 LB Water 120/2 40/2 LS W22 LS Water 120/2 40/2

LB W33 LB Water 160/3 60/3 LS W33 LS Water 160/3 60/3

LB M12 LB Methanol 80/1 40/2 LS M12 LS Methanol 80/1 40/2

LB M23 LB Methanol 120/2 60/3 LS M23 LS Methanol 120/2 60/3

LB M31 LB Methanol 160/3 20/1 LS M31 LS Methanol 160/3 20/1

LB E13 LB Ethanol 80/1 60/3 LS E13 LS Ethanol 80/1 60/3

LB E21 LB Ethanol 120/2 20/1 LS E21 LS Ethanol 120/2 20/1

LB E32 LB Ethanol 160/3 40/2 LS E32 LS Ethanol 160/3 40/2

LB W22 LB Water 120/2 40/2 LS W22 LS Water 120/2 40/2

LB W22 LB Water 120/2 40/2 LS W22 LS Water 120/2 40/2

LB W22 LB Water 120/2 40/2 LS W22 LS Water 120/2 40/2

4.1.2 E FFECT OF TIME AND TEMPERATURE

Degradation experiments of LB as well as LS were continued using ethanol as solvent for LB and methanol for LS (see Table 4.2), treated in separate runs due to the use of different solvents. For each run, one vial containing only solvent was also put in the microwave in order to keep the same amount of vials as in previous experiments. Otherwise, the procedure was the same as describe in the section 3.2.1. Yields were calculated according to (1).

Sample Lignin Solvent

Temperature (°C) /level

Time (min) /level

Sample Lignin Solvent

Temperature (°C) /level

Time (min) /level

LB E10 LB Ethanol 80/1 0/0 LS M10 LS Methanol 80/1 0/0

LB E11 LB Ethanol 80/1 20/1 LS M11 LS Methanol 80/1 20/1

LB E12 LB Ethanol 80/1 40/2 LS M12 LS Methanol 80/1 40/2

LB E13 LB Ethanol 80/1 60/3 LS M13 LS Methanol 80/1 60/3

LB E20 LB Ethanol 120/2 0/0 LS M20 LS Methanol 120/2 0/0

LB E21 LB Ethanol 120/2 20/1 LS M21 LS Methanol 120/2 20/1

LB E22 LB Ethanol 120/2 40/2 LS M22 LS Methanol 120/2 40/2

LB E22 LB Ethanol 120/2 40/2 LS M22 LS Methanol 120/2 40/2

LB E23 LB Ethanol 120/2 60/3 LS M23 LS Methanol 120/2 60/3

LB E30 LB Ethanol 160/3 0/0 LS M30 LS Methanol 160/3 0/0

LB E31 LB Ethanol 160/3 20/1 LS M31 LS Methanol 160/3 20/1

LB E32 LB Ethanol 160/3 40/2 LS M32 LS Methanol 160/3 40/2

LB E33 LB Ethanol 160/3 60/3 LS M33 LS Methanol 160/3 60/3

Table 4.1. Experimental details of solvent screening experiment.

Table 4.2. Experimental details of degradation experiment evaluating the effect of time and temperature.

10

A solvent extraction experiment was also carried out using ethanol for Lignoboost (LB EtOH ) and methanol for Lignosulfonate (LS MeOH ). 0.5 g lignin was transferred to a 50 mL E-flask. Hereafter the solvent (20 mL) was added. The mixture/solution was stirred with magnetic stirrer for 2 h. The solids were filtered of and transferred back to the E-flask. Further 20 mL solvent was added and left under stirring for 2 h before filtered again. The filtrates were collected in 100 mL beakers and let to evaporate in room temperature (the two fractions obtained from the 1 ^st and 2 ^nd extraction were added into one).

The reduced solutions were transferred to separate sample vials for final evaporation under a gentle air flow. Both solid and liquid fractions were finally dried in the vacuum oven at 23°C for at least 24 h.

4.2 T HERMOSET SYNTHESIS

4.2.1 D ETERMINATION OF REACTION CONDITIONS

In order determine suitable reaction conditions, seven thermoset (TS) synthesis experiments were carried out according to Table 4.3, using LB EtOH together with citric acid (CA) as cross linker. Three different catalysts were tested – p-toluenesulfonic acid (p-TSA), 4-(dimethylamino)pyridine (DMAP) and 1-methylimidazole (MIZ). In all cases but one, the reactants were pre-dissolved in acetone, which was let to evaporate in the fume hood for 1h before curing. The curing took place in a heating oven at 160 °C for 3h or at 110 °C for 24 h. All cured samples were analysed by FT-IR and through a gel content test (GCT) in acetone. It was performed by adding 17-18 mg of the sample to 10 mL of acetone. The solids were filtered off after three days, and the gel content (GC) was calculated according to (2).

𝐺𝐶 = ^{𝑚 𝑠𝑜𝑙𝑢𝑏𝑙𝑒}

𝑚 _{𝑖𝑛𝑖𝑡𝑖𝑎𝑙} (2)

Curing conditions Sample Lignin

(mg)

Citric acid

(mg) Catalyst Pre-dissolution in acetone

Time (h)

Temperature (°C)

TS-1 39.8 26.1 p-TSA Yes 3 160

TS-2 40.5 26.1 - Yes 3 160

TS-3 40.0 26.0 - No 3 160

TS-4 40.1 26.2 - Yes 24 110

TS-5 40.4 26.3 p-TSA Yes 24 110

TS-6 40.1 26.1 DMAP Yes 24 110

TS-7 40.2 26.1 MIZ Yes 24 110

LB-ref. 39.8 - - Yes 3 160

CA-ref. - 25.9 - Yes 3 160

Table 4.3. Experimental details of TS synthesis experiments evaluating the effect of different reaction conditions.

11 4.2.2 S YNTHESIS OF LB-CA-PEG- BASED THERMOSETS

Six thermoset (TS) synthesis experiments were carried out using lignin together with CA and PEG ⁴⁰⁰ as cross linkers, according to Table 4.4, using 1 wt% of DMAP as catalyst. Four different feed ratios lignin:PEG – 4:1; 2:1; 1:1; and 1:2 – were tested and the OH:COOH ratio was set to 1:1.

All reactants were pre-dissolved in acetone (3 mL) under stirring for 1h. The solution was thereafter casted onto an aluminium mould, and the acetone was let to evaporate under 1h. The samples were first let to cure in oven at 160 °C for 3h. The temperature was thereafter raised to 200 °C and kept isothermal for 1h, before the samples were taken out. The obtained films were released from the aluminium by putting them in separate acetone baths for 2 days. The insoluble films were taken out and dried in the vacuum oven for 3 days. The acetone from the baths were transferred to vials and let to evaporate, before also the vials were put in the vacuum oven 2 days, in order to see whether some unreacted reactants had been extracted. The masses of the dried soluble and insoluble samples were used to calculate the TS gel content, according to (3).

𝐺𝐶 = ^{𝑚 𝑖𝑛𝑠𝑜𝑙𝑢𝑏𝑙𝑒}

𝑚 _{𝑠𝑜𝑙𝑢𝑏𝑙𝑒} + 𝑚 _{𝑖𝑛𝑠𝑜𝑙𝑢𝑏𝑙𝑒} (3)

Name Lignin

Sample (mg)

PEG ⁴⁰⁰ (mg)

Citric acid (mg)

Lignin : PEG ⁴⁰⁰ w/w ratio

Lignin content (%)

TS 4:1 -LB EtOH LB EtOH 340 85 208.5 4:1 48

TS 2:1 -LB EtOH LB EtOH 280 140 273 2:1 40

TS 1:1 -LB EtOH LB EtOH 220 220 269.5 1:1 31

TS 1:2 -LB EtOH LB EtOH 148 296 260.9 1:2 21

TS 1:1 -LB E31 LB E31 220 220 269.5 1:1 31

TS 1:1 -LB original LB original 220 220 269.5 1:1 31

TS CA+PEG - - 456 240 - 0

4.3 M ATERIALS CHARACTERISATION

4.3.1 S IZE EXCLUSION CHROMATOGRAPHY (SEC)

The molecular weight of both solid and liquid fractions after microwave treatment was estimated by size exclusion chromatography (SEC). Two different systems were used depending on the solubility of the samples – one with DMSO (0.5 % w/w LiBr) and one with water (10 mM NaOH) as mobile phase.

For the water system, the analysis was carried out on a Dionex Ultimate-3000 HPLC system (Dionex, Sunnvale, CA, USA), at 40 °C and with a flow rate of 1 mL/min. The samples (4 mg/mL in 10 mM NaOH) were first filtered with a 0.2 μm nylon syringe filter, and thereafter injected into a system of three PSS suprema columns (300 x 8 mm; 10 μm particle size; 30 Å, 1 000 Å and 1 000 Å pore sizes) connected in series, together with a guard column (50 x 8 mm, 10 μm particle size). The system was endued with an LPG-3400SD gradient pump, a WPA-3000SL autosampler and a DAD-3000 UV/vis detector (Dionex, Sunnvale, CA, USA) besides a Waster-410 refractive index (RI) detector (Wasters,

Table 4.4. Experimental details for thermoset synthesis.

12

Milford, MA, USA). Linear pullulan standards with a peak molecular weight in the rage of 342 Da to 708 kDa (Polymer Standard Services, Germany) were used for calibration.

For the DMSO system, the analysis was performed on a PSS SECurity 1260 at 60 °C and with a flow rate of 0.5 mL/min. The samples (4 mg/mL in DMSO + 0.5 % w/w LiBr) were first filtered with, and thereafter injected into a system of three PSS GRAM columns (10 μm particle size; one precolum and the two with pore sizes of 100 Å and 10 000 Å) connected in series. The system was endued with PSS SECurity 1260 refractive index (RI) detector, which had a temperature of 40 °C. Linear pullulan standards in the rage of 342 Da to 708 kDa (Polymer Standard Services, Germany) were used for calibration. The data was processed using PSS WinGPC UniChrom V8.10 software.

4.3.2 F OURIER -T RANSFORM I NFRARED S PECTROSCOPY (FT-IR)

A Perkin-Elmer Spectrum 2000 FT-IR spectrometer (Norwalk, CT) was used for Fourier-Transform Infrared Spectroscopy (FT-IR) analysis. The instrument was equipped with a MKII Golden Gate single reflection ATR system (Grasbey Specec, Kent, UK). All spectra were recorded at room temperature, in the rage of 600-4000 cm-1, using 16 scans with an average resolution of 4.0 cm ^-1 . The data were analysed using PerkinElmer Spectrum software V10.5.1.

4.3.3 D IFFERENTIAL SCANNING CALORIMETRY (DSC)

The glass transition temperature (T g ) of some selected LB microwave treated liquid samples was determined using a Mettler-Toledo Differential Scanning Calorimeter (DSC), endued with a sample robot and cryo-cooler. The data were evaluated using Mettler Toledo STARe software V15.00a. 3-5 mg of each sample was placed in an aluminium sample holder (100 μL). The experiment began with heating ramp from 25 °C to 150 °C at a rate of 10 K/min. This was followed by an isotherm at 150 °C for 2 min, before being cooled down to -20 °C (-10 K/min). After a 2 min isotherm, the sample was finally heated to 240 °C (10 K/min). The entire experiment was performed under nitrogen gas flow (50 mL/min).

To determine Tg of the lignin-CA-PEG-based TS, a programme were the sample was cooled down to - 60 °C in the second cycle and with a heating rate in the third cycle of 40 K/min, were adopted.

4.3.4 T HERMOGRAVIMETRIC ANALYSIS (TGA)

A Mettler-Toledo TGA/DSC, equipped with a sample robot, was used for thermogravimetric analysis of some selected LB microwave treated liquid samples and the lignin-CA-PEG-based TS. The data were evaluated using Mettler Toledo STARe software V15.00a. 7-8 mg for each sample was placed in an alumina oxide cup (70 μL), which was heated from 25 °C to 650 °C, with a heating rate of 10 K/min, under 50 mL/min nitrogen flow.

4.3.5. N UCLEAR MAGNETIC R ^ESONANCE (NMR)

Both ³¹ P-NMR and 2D hetero nuclear single quantum coherence (HSQC) were recorded at room temperature utilizing a Bruker Advance III HD 400 MHz instrument with BBFO probe equipped with a Z-gradient coil.

31 P-NMR was used for hydroxyl group quantification of some lignin sample, according to procedure

reported by Argyropolous in 1994 ⁵³ . The sample (30 mg) was first dissolved in 100 μL DMF and 100

μL pyridine under stirring for 30 min at 50 °C. Thereafter, 100 μL internal standard (HNOB at a

13

concentration of 70 mg/mL using chromium acetylacetonate as relaxation agent (5 mg/mL)) was added to the solution. After further 30 min stirring at room temperature, 100 μL of the derivatization agent (2- chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane) was added together with 400 μL CDCl 3 -d. The reaction was let to proceed for 30 min at room temperature. The NMR analysis was then performed with 256 scans, 4 dummy scans and a relaxation time of 5 sec. The data was processed MestreNova (Mestrelab Reseach), and internal standard (δ P = 151.3 ppm) was use as internal reference. The integral 150-144.5 ppm was assigned aliphatic OH, 144.5-136.5 ppm to phenolics and 136-133 ppm to COOH.

2D-NMR analysis (HSQC), the sample (100 mg) was dissolved in 750 μL DMSO-d 6 . The experiment

was carried out with 100 scans ( ¹³ C), 16 dummyscans and a relaxation time of 1.5 sec. The data was

processed MestreNova (Mestrelab Reseach) using 90° shifted sine-bell apodization window. Phase

correction was applied in both directions, and the central DMSO (δ C /δ H = 39.5/2.5 ppm) was use as

internal reference. For semi-quantitative structural study, the unsubstituted carbon 2 the aromatic groups

(Ar-2) was used as internal standard, as reported elsewhere ⁵⁴ . The ¹³ C- ¹ H correlation signals were

according to previous work by others ^46,55 and analyses of model compounds ⁵⁶ .

14

5 R ESULTS AND D ISCUSSION

5.1 M ICROWAVE ASSISTED DEGRADATION OF LIGNIN

After all experiments of the MW assisted extraction/degradation were performed, it was discovered in

31 P-NMR that all liquid fractions contained residuals of solvent after only being dried in vacuum oven.

Since this would affect the result in calculation of Y liquid , a moisture content test were performed by picking three samples per solvent and lignin randomly (all with reaction different conditions). The selected samples were put in a heating oven at 60 °C for 6 days. Thereafter the average weight losses were used to calculate the moisture content (MC) (Table 5.1), according to (3).

𝑀𝐶 = ^{𝑚 ̅ 𝑙𝑜𝑠𝑠}

𝑚 _{𝑖𝑛𝑖𝑡𝑖𝑎𝑙} ∗ 100 % (3)

31 P-NMR were performed on one of the samples to see whether all solvent was removed or not. In Figure 5.1 it is possible to see that the treatment in heating oven resulted in a large decrease of the peak representing ethanol compared to the same sample before heat treatment, even though there is still some residues of ethanol left. The relative intensity decreased considerably though - from around 50 % of the total aliphatic OH to 10 %, corresponding to an 80 % decrease. Hence, (1) could be re-written for calculation of Y ^liquid , assuming that MC represented all residual solvent. All presented values of Y ^liquid in the following sections have thus been adjusted by (4).

150 148 146 144 142 140 138 136 134

After

Before

EtOH

EtOH

ppm

Lignin Solvent MC (%)

LB Water 5.71 ± 1.49

LB Methanol 1.89 ± 0.82

LB Ethanol 6.27 ± 1.87

LS Water 2.03 ±0.47

LS Methanol 3.54 ± 0.77

LS Ethanol 6.86 ± 3.47

Figure 5.1. ³¹ P-NMR of LB E32 before and after moisture content test.

Table 5.1. Results from moisture content test. Reported

values are mean values with standard deviations calculated

from three different samples per lignin and solvent.

15 𝑌 ^{𝑙𝑖𝑞𝑢𝑖𝑑} = ^{𝑚 𝑙𝑖𝑞𝑢𝑖𝑑}

𝑚 ^{𝑙𝑖𝑔𝑛𝑖𝑛} ∗ (1 − ^𝑀𝐶

100 ) (3)

5.1.1 S OLVENT SCREENING

All results from the solvent screening experiment are presented in Table 5.2 and 4.3. Through statistical analysis of Y ^liquid , M w liquid and Ð ^liquid in MODDE 9.0 it was shown that the choice of solvent had the greatest significant effect on Y ^liquid , considering both types of lignin. This is illustrated in Figure 5.2, where a similar conclusion can be made only by observing how the colour is shifting, due to lignin concentration, throughout a couple of the liquid samples (after microwave degradation, before being dried). All liquid fractions appeared as solutions but the fraction of LB in water, which more seemed like a cloudy mixture water and insoluble lignin.

Water Methanol Ethanol

0 10 20 30 40 50 60 70 80 90 100

b) Y liqu id (%)

Lignoboost Lignosulfonate a)

The highest yields for LB were obtained using MeOH or EtOH as solvent. As seen in Figure 5.2 the calculated effect for MeOH was somewhat higher than for EtOH. The difference is however within the confidence intervals. Disregarding the effect of the microwave treatment, a solvent extraction study by Duval et.al. ²⁹ have demonstrated a greater solubility of LB in MeOH than in EtOH. However, the difference between their reported yields (61.2 % for MeOH and 46.8 % for EtOH) are much greater than what the solvent screening experiment indicated (54.9 % for MeOH and 51.9 % for EtOH). In a depolymerisation study of lignin by Huang et.al. ⁴⁰ it was shown that EtOH generates higher monomer yield than MeOH, due to its capability of hindering repolymerisation reactions by acting as capping agent and formaldehyde scavenger. Considering the molecular weights of the obtained solid fractions (M w solid ) – where EtOH generates a M w solid that is bit larger that of LB original , whereas using MeOH or water results in a highly increased M w solid (Table 5.2) – it implies that repolymerization reactions occur to a much lower extent in EtOH as against to the two other solvents. This could therefore also be an explanation to why the observed difference in Y ^liquid between EtOH and MeOH is lower than that of only solvent extraction. Hence, it cannot be claimed that the larger increase in Y ^liquid for EtOH than

Figure 5.2. a) The effect of solvent on Y ^liquid with

corresponding confidence intervals (conf. level 0.95),

calculated in MODDE 9.0 b) Picture of some liquid

fraction before drying. From left to right: LB W22 , LS W22 ,

LB M12 , LS M12 , LB E21 , LS E13 .

16

MeOH is due to more frequent depolymerisation reactions, since it rather depends on the rate difference between depolymerisation and repolymerisation. Furthermore, the molecular weights of the obtained liquid fractions indicates that EtOH generates a lower M w liquid than MeOH. These results could be ascribed solubility, and correspond quit well to the solvent extraction study performed by Duval et.al. ²⁹ .

In the case of LS (Table 5.3), much higher yield was obtained in water compared to the other two solvents. This could be explained by the fact that LS original is completely soluble in water, while only partly soluble in MeOH and EtOH. Turning to the M w liquid , it is also possible to see that the molecular weight was more or less unchanged using water as solvent. MeOH and EtOH showed on the other hand a large decrease in M w liquid

, however with fairly low yields. Interestingly, there is no big increase in M w solid

compared to LS original , which was observed for LB. In addition, the Y ^liquid for methanol is significantly larger than for ethanol, implying that the capping agent and formaldehyde scavenger effect of ethanol ⁴⁰ is much lower in this system compared to LB. This suggests that the depolymerisation reaction takes place though another mechanism, and that repolymerization occurs to a lower extent.

Based on these results ethanol was chosen as solvent for LB – because of the high yield, low molecular weight and possibly lower degree of repolymerization – and methanol for LS – since the other two gave either a lower yield or almost unchanged molecular weight - for further investigating the effect of time and temperature.

Yield (%)

Molcular weight ^a

(g/mol) Dispersity ^a Sample Y ^liquid Y ^solid M w liquid M w solid Ð ^liquid Ð ^solid

LB original - 100 - 7 660 - 7.8

LB W11 2.9 70 4 450 6 110 9.5 5.5

LB W22 6.3 69 12 000 32 900 12 28

LB W33 3.6 79 7 590 69 600 14 53

LB M12 51 33 2 170 19 500 3.0 7.5

LB M23 53 30 2 920 43 100 3.3 14

LB M31 57 30 2 710 46 500 2.9 15

LB E13 51 41 1 850 11 600 2.8 5.8

LB E21 48 32 2 130 14 700 2.1 3.7

LB E32 62 21 2 330 10 900 3.1 4.5

LB W22 4.0 70 18 000 9 240 2.1 7.7

LB W22 5.9 65 8 270 18 800 11 16

LB W22 5.3 62 6 860 9000 10 7.3

Table 5.2. Result from the solvent screening experiment of LB. In the table are yields of obtained liquid (Y ^liquid ) and solic (Y ^solid ) fractions presented, as well as determined weight molecular weights and dispersity.

a Determined by DMSO SEC

17 5.1.2 O PTIMISATION OF TIME AND TEMPERATURE

After the solvent screening experiment, the influence of time and temperature were investigated. From that it could be concluded that a shorter time at a higher temperature was the most effective condition to receive a high yield, concerning both LB as well as LS. In Figure 5.3, the results for LB are presented.

It is possible to see that during the ramping time of 20 min the temperature has a very little influence on Y ^liquid , and the result is similar to that of only solvent extraction, LB EtOH (47 %). During the following isotherm the yield is slowly increasing with time at the two lowest temperature. At 160 °C though, a rapid increase is observed during the first 20 min isotherm, followed by a decrease as the reaction time is prolonged. This indicates that repolymerisation takes place to a larger extent at a high temperature, which will outpace the depolymerisation with time. Under all conditions, a distinct decrease in M w liquid

and Ð ^liquid could be observed compared to LB original , rather similar to what others have reported regarding ethanol extraction in the literature ^29,46 However, no large difference when varying the time and temperature could be observed, other than a small increase in M w liquid

with increasing temperature – which also signals a larger degree of repolymerisation at elevated temperatures. It should however be noted that a higher temperature also helps dissolving molecules of larger molecular weight than at room temperature. Altogether, this indicates though that mainly the fractions of larger molecular weights are affected by the microwave treatment, and degraded into lower molecular weight fractions that can be dissolved in ethanol. Hence, at the most effective reaction conditions – 160 °C for 20 min (LB 31 ) – the

Y ^liquid has been increased with 38 % compared to only solvent extraction. An explanation to this could

be that the higher molecular weight fractions contain more ether linkages that are easier to break during microwave treatment. This is supported by recent published work by Gioia et.al. ⁴⁶ , where four refined

Yield (%)

Molcular weight ^a

(g/mol) Dispersity ^a Sample Y ^liquid Y ^solid M w liquid M w solid Ð ^liquid Ð ^solid

LS original - 100 - 11 500 ^b 1.2 ^b

LS W11 70 - 11 800 ^b - 1.2 -

LS W22 74 - 11 400 ^b - 1.2 -

LS W33 80 - 9 050 ^b - 1.1 -

LS M12 8.7 75 1 560 11 600 ^b 2.7 1.1 ^b

LS M23 23 61 1 980 13 300 ^b 2.4 1.2 ^b

LS M31 24 62 1 660 7 850 ^b 2.0 1.1 ^b

LS E13 2.1 86 1 850 11 900 ^b 2.8 1.2 ^b

LS E21 9.0 82 1 160 12 500 ^b 2.1 1.2 ^b

LS E32 12 75 1 190 11 380 ^b 2.1 1.1 ^b

LS W22 78 - 1 500 - 1.1 -

LS W22 72 - 11 300 - 1.2 -

LS W22 74 - 10 000 - 1.2 -

Table 5.3. Result from the solvent screening experiment of LS. In the table are yields of obtained liquid (Y ^liquid ) and solic (Y ^solid ) fractions presented, as well as determined weight molecular weights and dispersity.

a Determined by DMSO SEC

b Determined by water SEC

18

fractions of LB obtained by solvent extraction were thoroughly characterized, and where the fraction with the highest molecular weight (5 400 g/mol) contained over four times more β-O-4 inter-unit linkages that the lowest molecular weight fraction (1 000 g/mol). If the fractions of low molecular weight would have been affected to a similar extent, this would probably be indicated by a lower molecular weight and/or increasing dispersity compared to solvent extraction.

0 10 20 30 40 50 60 70 80 90 100

LB _EtOH a)

20+20

0 20 20+40

Y l iqu id (%)

Time (min)

80 ^o C 120 ^o C 160 ^o C

20+60

0 1000 2000 3000 4000 5000 6000 7000 8000

M l iqu id w (g /mo l)

Time (min)

80 ^o C 120 ^o C 160 ^o C b)

0 20 20+20 20+40 20+60

1 2 3 4 5 6 7 8

Ð

Time (min)

80 ^o C 120 ^o C 160 ^o C c)

0 20 20+20 20+40 20+60

The MW assisted extraction/degradation of LS in MeOH resulted in a large decrease in M w liquid

, but an increase in Ð ^liquid , compared to original LS (Figure 5.4). Here it should be kept in mind though that all data for liquid fractions in MeOH were obtained by SEC with DMSO as solvent, whereas the data for LS original were determined by water SEC. This could obstruct the comparability. Furthermore it is clear that temperature had a much greater influence than time on the Y ^liquid , within the analysed intervals. The highest yield (24 %) was reached under the same conditions as for LB – 160 °C for 20 min – which is a doubling compared to only solvent extraction (LS MeOH ) (12 %). Nevertheless, for being an effective way to degrade LS, the process has to be more optimized.

To summarize, combined MW assisted extraction/degradation of LB in EtOH showed that it was possible to reach a high yield (65 %) with a low molecular weight (2 700 g/mol) and dispersity (3.07)

Figure 5.3. Illustrates a) how Y ^liquid b) M w liquid c) Ð ^liquid for liquid fractions of LB in ethanol is influenced by time and temperature

in the microwave treatment. All experiments were performed with a ramping time of 20 min and followed by an isotherm for

20, 40 or 60 min at specified temperature. Values for LB original : M w = 7660 g/mol, Ð = 7.8.

19

after only 20 min treatment at 160 °C. It was therefore decided to focus on LB for further characterization and thermoset synthesis.

0 10 20 30 40 50 60 70 80 90 a) 100

20+60 20+40

20+20 Y liqu id (%)

Time (min)

80 ^o C 120 ^o C 160 ^o C

20

LS MeOH

0 500 1000 1500 2000 2500 3000 11200 11400 11600 11800

M l iqu id w (g /mo l)

Time (min)

80 ^o C 120 ^o C 160 ^o C b)

20 20+20 20+40 20+60

0 1 2 3 4

Ð

Time (min)

80 ^o C 120 ^o C 160 ^o C c)

20 20+20 20+40 20+60

5.1.3 T HERMAL ANALYSIS

DSC analysis of some selected LB samples (Figure 5.5 a) were performed in order to evaluate how time and temperature could affect the thermal properties of obtained liquid fractions. It appeared that the T g of the MW degradation products was increased with increasing MW temperature (see Table 5.3 for data). This corresponds well to the M w liquid earlier determined by SEC, and is believed to be due to a larger extent of repolymerisation at higher temperatures. In Table 5.4 are also thermal stability data presented. Thermal degradation temperature at 5 % (T 5% ), 30 % (T 30% ) and maximum weight loss temperature (T max ) were obtained from TGA measurements (Figure 5.5 b) and c)). To define the thermal stability of the samples, the statistic heat resistance index (T s ) was used, determined from (4) ^57,58 . It can be noted that the overall thermal stability are lowered for all microwave treated samples as well as the ethanol extracted, compared to LB original . This could be explained by the lower M w of all MW degradation product, as against LB original . Around 100 °C an initial weight loss of around 3 % is observed, which is believed to be due to residual solvent.

𝑇 _𝑆 = 0.49(𝑇 _5% + 0.6(𝑇 _30% − 𝑇 _5% )) (4)

Figure 5.4. Illustrates a) how Y ^liquid b) M w liquid c) Ð ^liquid for liquid fractions of LS in methanol is influenced by time and

temperature in the microwave treatment. All experiments were performed with a ramping time of 20 min and followed by an

isotherm for 20, 40 or 60 min at specified temperature. Values for original LB: M w = 11 500 g/mol, Ð = 1.2.

20

Glass transition ^a (°C) Thermal stability data ^b (°C)

Lignin T g T max T 5% T 30% T S

LB original 145.67 ± 0.61 391 236 388 160

LB M11 119.71 ± 1.18 389 237 370 155

LB E11 122.03 ± 1.68 375 165 365 140

LB E21 125.28 ± 3.46 390 166 369 141

LB E31 128.34 ± 6.07 386 136 373 136

LB E33 117.84 ± 5.99 391 166 371 142

LB EtOH 119.06 ± 2.63 385 193 366 145

Table 5.4. T g values and thermal degradation stability data for some selected microwave treated LB liquid samples.

a Determined by DSC. All values are means out of three repeated analyses presented with calculated standard deviations.

b Determined by TGA

Figure 5.5. Thermal analysis of some selected microwave treated LB liquid samples: a) DSC scans; b) TGA curves were weight loss (%)) are plotted against temperature; c) TGA curves were weight loss rate (%/min) are plotted against temperature.

100 200 300 400 500 600

0 20 40 60 80 100

Wei ght (%)

Temperature (

C) LB

LB

LB

LB

LB

LB

LB

b)

-50 0 50 100 150 200 250 300

Temperature ( ^o C) LB _M11

a)

End oth ermi c he at f lo w

LB _M11 LB _E11 LB _E21 LB _E31 LB _E33 LB _EtOH

100 200 300 400 500 600

-5 -4 -3 -2 -1 c ) 0

LB

LB

LB

LB

LB

LB

LB

Weig ht loss rate ( %/min)

Temperature (oC)