PAPERmaking! Vol10 Nr3 2024

3$3(5 making! The e-magazine for the Fibrous Forest Products Sector

Produced by: The Paper Industry Technical Association

Publishers of: Paper Technology International ®

Volume 10 / Number 3 / 2024

PAPER making! FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL ® FROM THE PUBLISHERS OF PAPER TEC Volume 10, Number 3, 2024 CONTENTS:

FEATURE ARTICLES: 1. Coatings : Functional surfaces, films and coatings with lignin – a review. 2. Nanocellulose : Fit-for-use nanofibrillated cellulose from recovered paper. 3. Chemistry : Boosting inorganic filler retention. 4. Tissue : Evaluation of special pulps for greener tissue paper. 5. Water Treatment : Degradation of lignin-containing wastewaters with bacteria. 6. Sustainability : Which wastepaper should not be processed? 7. Wood Panel : Wood-based panels from recycled wood – a review. 8. Packaging : Recent advances in fibre-based packaging for food applications. 9. Food Contact : Critical review of test methods. 10. Heart Health : Heart-heathy diet – 8 steps to prevent heart disease. 11. Negotiation : Ten simple tips to improve negotiating skills. 12. Computers : Keyboard shortcuts.

SUPPLIERS NEWS SECTION: News / Products / Services : x

Section 1 – PITA CORPORATE MEMBERS AFT / PILZ / VALMET Section 2 – PITA NON-CORPORATE MEMBERS ANDRITZ / VOITH

Section 3 – NON-PITA SUPPLIER MEMBERS BTG / POWER ADHESIVES / SIEGWERK / VISION ENGINEERING Advertisers: ABB / FINBOW / VAKUO

DATA COMPILATION: Events : PITA Courses & International Conferences / Exhibitions & Gold Medal Awards Installations : Overview of equipment orders and installations between Jul. and Nov. Research Articles : Recent peer-reviewed articles from the technical paper press. Technical Abstracts : Recent peer-reviewed articles from the general scientific press. Career Opportunity : Production Engineer at Holmen, Workington Mill The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International ® and the PITA Annual Review , both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating .

Contents

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PAPER making! FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL ® FROM THE PUBLISHERS OF PAPER TEC Volume 10, Number 3, 2024

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REVIEW

Functional surfaces, fi lms, and coatings with lignin – a critical review

Cite this: RSCAdv. , 2023, 13 , 12529

Jost Ruwoldt,* Fredrik Heen Blindheim and Gary Chinga-Carrasco

Lignin is the most abundant polyaromatic biopolymer. Due to its rich and versatile chemistry, many applications have been proposed, which include the formulation of functional coatings and fi lms. In addition to replacing fossil-based polymers, the lignin biopolymer can be part of new material solutions. Functionalities may be added, such as UV-blocking, oxygen scavenging, antimicrobial, and barrier properties, which draw on lignin's intrinsic and unique features. As a result, various applications have been proposed, including polymer coatings, adsorbents, paper-sizing additives, wood veneers, food packaging, biomaterials, fertilizers, corrosion inhibitors, and antifouling membranes. Today, technical lignin is produced in large volumes in the pulp and paper industry, whereas even more diverse products are prospected to be available from future biore fi neries. Developing new applications for lignin is hence paramount – both from a technological and economic point of view. This review article is therefore summarizing and discussing the current research-state of functional surfaces, fi lms, and coatings with lignin, where emphasis is put on the formulation and application of such solutions.

Received 22nd December 2022 Accepted 3rd March 2023

DOI: 10.1039/d2ra08179b

rsc.li/rsc-advances

Technical lignin is the product of biomass separation processes and hence di ff ers from natural or pristine lignin, as it is found in lignocellulose biomass. 3 The composition and properties of technical lignin are largely determined by their botanical origin, extraction process, puri  cation, and potential chemical modi  cation. 4 Presently, there are some 50 – 70million tons technical lignin available from pulping or biore  nery operations. Most is burned to produce energy in biore  nery processes and only approx. 2% is sold commercially. 5 Technical lignin isolated from pulping processes includes Kra  and soda lignin from alkali pulping, lignosulfonates from sul  te pulping, and organosolv lignin from solvent pulping. 6 The two main types of technical lignin are lignosulfonates (approx. 1 million Dr Fredrik Heen Blindheim is a Postdoctoral researcher at RISE PFI. He received a PhD in Organic Chemistry at the Norwegian University of Science and Technology, specializing in medicinal chemistry and the development of small-molecule bacterial kinase inhibitors. In his current position, he works with chemical modi  cation, quanti  cation, and character- ization of technical lignins for green applications in industry. His main interests are in organic synthesis and spectroscopic analysis.

1. Introduction Lignin is the second most abundant biopolymer on earth, a  er cellulose. Natural lignin is synthesized from the three mono- lignol precursors, namely p -hydroxyphenyl (H unit), guaiacyl (G unit), and syringyl (S unit) phenylpropanoid. 1 Lignin from so  wood consists primarily of G units, whereas hardwood lignin contains both G and S units. 2 Moreover, lignin from annual plants, such as grass or straw, can contain all three monolignol units.

RISE PFI AS, Høgskoleringen 6B, Trondheim 7491, Norway. E-mail: jostru.chemeng@ gmail.com

Dr Jost Ruwoldt is a research scientist at RISE PFI, Norway. He graduated with a PhD in Chemical Engineering from the Norwegian University of Science and Technology (NTNU) in 2018, and an MSc in Chemical and Bioprocess Engineering from Hamburg University of Technology (TUHH) in 2015. His current work includes lignin technology, thermoforming of wood pulp, and biomass

conversion and utilization. In addition to his work at RISE PFI, he is a visiting researcher and lecturer at TU Berlin, Germany.

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tons per year) and kra  lignin (<100 000 tons per year). In addition, the advent of hydrolysis and steam-explosion lignin have created new types of technical lignin. 7,8 The use of ionic- liquids or supercritical solvents have furthermore yielded the products ionosolv lignin and aquasolv lignin, respectively, with new and interesting features. 9,10 Lignin is polyaromatic and due to this structure, it is less hydrophilic than polysaccharidic biopolymers, e.g. , cellulose, hemicellulose, starch, alginate or chitosan. 11 It is hence a promising candidate in various applications, including: (i) reduction of wettability of hydrophilic materials, (ii) addition of functionalities, such as protection from UV light, antioxidant and antimicrobial properties, and (iii) tailoring of materials and formulations, e.g. , for controlled substance release, adsorption, or antifouling mechanisms. 12 – 16 However, chemical modi  ca- tion is required for most applications of lignin. Such modi  - cations frequently make use of lignin's hydroxyl groups, for example, by gra  ing reactions during phosphorylation, sulfo- methylation, esteri  cation, or amination. 17 The aromatic moieties in lignin can furthermore be targeted for, e.g. , replac- ing phenol in formaldehyde resins. 18 At last, the carboxyl groups in lignin may also serve as reactive sites for polyesters. 19 Interest has also been strong for the use of technical lignin in polymeric materials, e.g. , for thermoplastics or thermosets. 20 Processability of lignin in thermoplastics can be done without modi  cation, as lignin is an inherently thermoplastic mate- rial. 21,22 Lignin's glass transition temperature can range from about 60 – 190 °C and may depend on many factors, including the botanical origin and pulping type, moisture content, and chemical modi  cation. 23,24 Lignin can also be chemically modi  ed to improve the application of lignin as specialty chemicals or in polymeric materials. 25 – 27 Additionally, the utilization of lignin as macromonomer, i.e. , thermoset precursor, can be done as part of polyurethanes, polyesters, epoxide resins, and phenolic resins. 11 End-uses include the production of rigid or elastic foams, rigid and self-healing materials, adhesives, biocomposites, and coatings. 19,28 – 33

One long-held belief is that lignin provides water-proo  ng in the wood cell wall to support water-transport. 34 Despite yielding a contact angle below 90°, which would be required to pose as a hydrophobic material, various researchers have shown that lignin can reduce the wettability and water-uptake of wood and pulp products. 18,35 – 37 Hence, both technical and chemically modi  ed lignin have been proposed as additives for packaging materials. 38 Reduction of wettability of  ber-based packing is a particularly interesting application, considering environ- mental and societal drivers regarding reduction of single-use plastics and environmental pollution. Lignin could thus form the basis of coatings or impregnation blends, provided that the lignin-coating complies with food contact requirements. One example for lignin-blends is the combination with starch during surface-sizing of paper, which can improve extensibility and reduce wetting of the starch-matrix. 35,39 Layer-by-layer assembly with multivalent cations or polycationic polymers has also been done, which can improve the strength and hydrophobicity of cellulose. 40,41 Other applications of lignin, its derivatives and mixtures include the use for controlled-release fertilizers, antifouling membranes,  re retardancy, dye sorption, wastewater treat- ment, and corrosion inhibitors. 14 – 16,42 – 44 One publication even reported an unintentional but yet advantageous coating of coir  bers, where the lignin delayed oxidation and thermal degra- dation of the  bers in a polypropylene composite. 45 Major drivers for using lignin are economical aspects by attributing value to a by-product from pulping or biore  nery operations, and sustainability by replacing fossil-based mate- rials with biopolymers. Many applications can thus bene  t from the inclusion of lignin in functional surfaces,  lms, and coat- ings. The mechanism of action and application mode can hereby di ff er greatly. This review therefore represents an e ff ort to structure and summarize recent progress, where emphasis is put on both the process and  nal use for lignin in surfaces and coatings.

2. Fundamentals 2.1. Structure and composition of natural lignin

Lignin is part of the lignin-carbohydrate complexes (LCC) that are found in cell walls of plants and woody materials, as illus- trated in Fig. 1. The cellulose  bers are tightly bound to a complex network of hemicellulose and lignin, and the three biopolymers provide strength and stability to the cell walls. In addition to providing structural integrity, lignin helps building hydrophobic surfaces which are important in transport chan- nels for water and nutrients. 46 The complex lignin network consists of the three 4-hydrox- yphenyl propylene units, or monolignols, formed from the parent compounds p -coumaryl- ( p -hydroxyphenyl, H-unit), coniferyl- (guaiacyl, G-unit) and sinapyl alcohol (syringyl, S- unit), see Fig. 2. 47 The monolignols di ff er only in the presence or absence of one or two aromatic methoxy groups ortho to the hydroxyl group. These are synthesized invivo from the aromatic amino acid phenylalanine, formed in the shikimic acid pathway in plants. 48 The resultant monolignols undergo a variety of

Dr Gary Chinga Carrasco was born in Chile and moved to Nor- way in 1987. He graduated with a Cand. scient. degree in cell biology (1997) and Dr ing in chemical engineering (2002). He was one of two recipients of the Norwegian Wood Processing Association Award 2019 for nanocellulose research and winner of the 2021 – TAPPI's International Nanotechnology Division Mid-Career Award. He is

Associate Editor of the Bioengineering Journal, and Editor-in-Chief of the Section – Nanotechnology Applications in Bioengineering. Currently, he is lead scientist at RISE PFI in the Biopolymers and Biocomposites area.

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Fig. 1 Composition of lignocellulosic biomass and the structural roles of cellulose, hemicellulose, and lignin. 46

Fig. 2 Monolignol structure and positions indicated by blue numbers and letters.

so  woods and hardwoods, respectively. As this is the most common linkage, many deligni  cation processes target this speci  c linkage. For so  woods, the 5 – 5 linkage is also impor- tant, and has an abundance of 18 – 25% per 100 C 9 units, while this linkage occurs only around 3 – 9% in hardwoods. 52 In the process of isolating technical lignins, both the labile aryl-alkyl and b -O-4 bonds are most prone to cleavage. 53 This results in technical lignins having more condensed and variable structures than native lignin, and a wide variety in molecular weight ( M w ). Mass average values ( M w ) of 1000 – 15000 gmol − 1 for soda lignin, 1500 – 25000gmol − 1 forKra  lignin, and 1000 – 150 000 g mol − 1 for lignosulfonates have been reported, depending on botanical origin and process conditions. 54 Native lignin is a virtually in  nite macromer that is both randomly- and poly-branched. 55 The bonds between the lignin and surrounding hemicellulose and cellulose found in LCC have recently been reviewed. 56 All so  wood lignin, and 47 – 66% of hardwood lignin, is reportedly bound covalently to carbohy- drates, and mainly to hemicellulose. The most common types of linkages found in LCCs are benzyl ether-, benzyl ester-, ferulate ester-, phenyl glycosidic- and diferulate ester bonds. 57 Note that due to the high degree of variability in inter-unit and LCC

radical cross-coupling reactions which results in the complex, and varied, lignin network. The ratios of the three monolignols in lignin from di ff erent sources can vary quite signi  cantly, hardwood lignins contain G- (25 – 50%) and S-units (50 – 70%), so  wood lignins contain mostly G-units (80 – 90%), while grass lignin contains mixtures of S- (25 – 50%), G- (25 – 50%) and H- units (10 – 25%). 49 The monolignol composition (H : G : S ratio) can also vary between tissue types in the same organism, which has been illustrated in the cork oak, Quercus suber . Lignin from the xylem (1 : 45 : 55) and phloem (1 : 58 : 41) di ff er less in composition than the two compared to the phellem (cork-part, 2 : 85 : 13). These di ff erences a ff ect the occurrence of speci  c interunit linkages, where an increase in S-units lead to an increase in alkyl – aryl ether ( b -O-4) bonds: 68% in cork, 71% in phloem, 77% in xylem. 50 The di ff erence in abundance of the three monolignols lead tomany di ff erent types of interunit linkages in lignin, speci  - cally between angiosperm (hardwood and grass) and gymno- sperm(so  wood) lignin. 51 The most common interunit linkage is the b -O-4alkyl – aryl ether bond (Fig. 3), which occurs between 45 – 50% or 60 – 62% of phenyl propylene unit (C 9 units) in

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Fig. 3 Common linkages between monolignols identi fi ed in lignins. 51,52

linkages, Fig. 4 should only be taken as an illustrative example. The lignin macromolecule is polydisperse and may exhibit various linkages and functional groups. 4 In other words, lignin should be considered as statistical entities rather than distinct polymers.

2.2. Isolation of technical lignin Lignocellulosic biomass consists of cellulose (30 – 50%), hemi- cellulose (20 – 35%) and lignin (15 – 30%), where the lignin acts as a “ glue ” within the LCCs. 58,59 The actual lignin content of the biomass is highly in  uenced by its botanical origin e.g. , 28 – 32%

Fig. 4 Adaptation of Adler's representation of softwood (spruce) lignin with color-coded monolignols: p -hydroxyphenyl (H-unit) in black, guaiacyl (G-unit) in blue and syringyl (S-unit) in red. 55

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Fig. 5 Lignin extraction processes and their products. 3

sulfonation at the a -carbon, which leads to cleavage of aryl- ether bonds and subsequent crosslinking. 68 Both Kra  and sul  te black liquor typically contain signi  cant amounts of carbohydrate and inorganic impurities. 64 The soda- anthraquinone process is mostly applied in the paper industry on non-woody materials like sugarcane bagasse or straw. 64 The material is treated with an NaOH solution (13 – 16 wt%) at high pressures and temperatures of 140 – 170 °C, where anthraqui- none is added to stabilize hydrocelluloses. 64,69 The resulting soda lignin is sulfur-free and contains little hemicellulose or oxidized moieties. Organosolv lignin is produced in an extrac- tion process using organic solvents and results in separation of dissolved and depolymerized hemicellulose, cellulose as residual solids, and lignin that can be precipitated from the cooking liquor. 65 Various solvent combinations are possible, such as ethanol/water (Alcell process) or methanol followed by methanol and NaOH and antraquinone (Organocell process), which will a ff ect structure of the resultant materials. 51 Common for all organosolv lignins is that their structures are closer to that of natural lignins, in particular compared to Kra  ligninor lignosulfonates. They are additionally sulfur-free and tend to contain less than 1% carbohydrates. 65 Several other methods of biomass processing have been developed that are targeted at lignin extraction, rather than producing cellulose  bers, where lignin is as a byproduct. 64 Milled wood lignin (MWL) can be produced to closely emulate native lignin, but at the expense of process yields. 70 This method is considered gentle but time consuming, o  en requiring weeks of processing, making it viable only in a laboratory setting. 58 Other techniques that aim to produce native lignin analogues include cellulolytic enzymatic lignin (CEL) and enzymatic mild acidolysis lignin (EMAL). The CEL procedure was developed as an improvement of the MWL process, where higher yields were obtained without increasing milling duration. 46 By adding an additional acidolysis step, Guerra et al. were able to again improve on the yield, while still producing lignin that closely resembled the native structure. 70 The physicochemical pretreatments aim to reduce lignin particle size through mechanical force, extrusion, or other. These techniques include

is found in pine and eucalyptus wood, while switchgrass contains only 17 – 18% lignin, 60 and less than 15% is typically found in annual plants. 61 The  rst step in lignin valorization is biomass fractionation, where the cellulose, hemicellulose, and lignin are separated from each other. Several techniques have been developed, which can be grouped into sulfur and sulfur- free pulping from the paper and pulp industry, and bio- re  nery processes that aim to produce of materials, chemicals, and energy from biomass. 59 The latter may speci  cally be designed to isolate lignin of high purity and reactivity, whereas pulping originally produced lignin as a by- or waste-product. An overview is given in Fig. 5. The three industrial extraction methods for lignin are kra  , sul  te and soda pulping. In addition, organosolv pulping has been developed to extract lignin and separate the pulp  bers. Commercialization of this process has not yet been done, but interest has risen recently in this technology, as organosolv pulping produces a technical lignin of high purity and reac- tivity. Several other methods also exist, but these are mainly used in lab-scale and are referred to as biore  nery concepts, or “ pretreatments ” . 62 In the Kra  pulping process, the lignocellu- losic biomass is mixed with a highly alkaline cooking liquid containing sodium hydroxide (NaOH) and sodium sul  te (Na 2 S), at elevated temperatures of 150 – 180 °C. From the resulting black liquor, kra  lignin can be precipitated out by lowering the pH to around 5 – 7.5. 46 In the LignoBoost process, this precipitation is done by adding  rst CO 2 and then sulfuric acid. Kra  lignin has a sulfur content of 1 – 3%, is highly condensed, contains low amounts of b -O-4 linkages, and is frequently burned for energy and chemical recovery at the mills. 63 – 65 In the Kra  process, lignin is fragmented through a - aryl ether or b -aryl ether bonds, which results in increased phenolic OH content in the resultant lignin. 66 The sul  te process is another specialized pulping technique, which utilizes a cooking liquor containing sodium, calcium, magnesium or ammonium sul  te and bisul  te salts. 65 Treatments are typically conducted at 120 – 180 °C under high pressures, which gives lignosulfonates that contain 2.1 – 9.4% sulfur, mostly in the benzylic position. 67 Lignosulfonates are cleaved mainly through

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hydroxyl groups with TBDMS-Cl, and the resulting material could be incorporated into low-density polyethylene (LDPE) blends forming a hydrophobic polymer matrix. 75 Lignin is a versatile sca ff oldfordi ff erentmodi  cations depending on the desired application. For the production of epoxy resins, epoxi- dation with epichlorohydrin is a common technique. This approach has also been combined with CO 2  xation resulting in cyclic carbonates being incorporated in the lignin. 76 2.4. Analysis techniques Techniques to assess lignins and lignocellulosic biomass have long been a topic of great interest, both for quantitative and qualitative characterization. Such techniques are also critical to probe and assess chemical modi  cations. A summary of common methods is given in Table 1. Di ff erent techniques are o  en combined to provide a better overall picture. For example, chemical modi  cation of lignin may be probed in terms of molecular weight, i.e. , by using size- exclusion chromatography, and abundance of functional groups, as determined by FTIR or 2D NMR analysis. The tech- nique of choice can depend on factors such as the target groups of interest, but also on availability and cost. The polydisperse nature of technical lignin can sometimes make accurate measurements di ffi cult. This is manifested, for example, in the incomplete ionization of phenolic moieties during titration or UV spectrophotometry, as the con  guration and side chains of phenolic moieties induce varying degrees of resonance stabilization.

steam explosion, CO 2 explosion, ammonia  ber expansion (AFEX) and liquid hot water (LHW) pretreatments. 46 Ionic liquids have also been successfully used for lignin isolation. Five cations with good solubilizing abilities were identi  ed: the imidazolium, pyridinium, ammonium and phosphonium cations, while the two large and non-coordinating anions [BF 4 ] − and [PF 6 ] − were found to disrupt dissolution of the lignin. 46 The chosen extractive method will not only a ff ect the characteristics of the resulting lignin, but also the amount that is extracted. Several methods have been developed for the deligni  cationof sugarcane bagasse, e.g. , milling, alkaline or ionic liquid extraction, where yields of 17 – 32% were obtained depending on the method of choice. 71 2.3. Chemical modi  cation Chemical modi  cation of technical lignins is well explored and include a huge variety of techniques (see Fig. 6 for illustrative examples). Technical lignins have been modi  ed by a myriad of techniques, such as esteri  cation, phenolation and ether- i  cation. 6 Urethanization with isocyanates has been explored towards polyurethan production, 72 and allylation of phenolic OH groups enabled Claisen rearrangement into the ortho -allyl regioisomer which is of interest for its thermoplastic proper- ties. 73 The solubility and charge density of technical lignins can be a ff ected by sulfomethylation or sulfonation, 17,74 and meth- ylation of the phenolic OH groups have led to lignin with an increased resistance to self-polymerization 17 The thermal stability of lignins has also been improved by silylating the

Fig. 6 Examples of chemical modi fi cations of technical lignin.

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Table 1 Characterization techniques to assess lignins quantitatively and qualitatively

Method

Description

Limitations

Ref.

76 and 77

Calibration required with samples of known concentrations. Large dataset (training and test sets) needed for reliable quanti  cation. Training samples and prediction samples cannot di ff er greatly. Analyses are sensitive to sample preparation techniques NMR experiments are expensive, instruments found at specialized institutions and universities. Both experiments and data processing can be highly time-consuming Full derivatization of OH-groups is essential for proper quanti  cation. Inverse gated decoupling pulse sequence needed for quanti  cation: reduced sensitivity and increases relaxation time of analysis Less accurate than 31 PNMR. A ff ected by incomplete ionization of functional groups. Presence of other ionizable groups can a ff ect results Heterogeneity of COOH- and OH- groups distorts in  ection point. Limited to quanti  cation of COOH- and OH-groups (and possibly other ionizable groups) Time consuming calibration required. Samples must be within linear range. Acetylation is o  en used for increased solubility prior to analysis Variations in inherent metal contents greatly a ff ects the pyrolysis reaction of the biomass Interference from other aromatic compounds (phenylalanine, tyrosine) can distort image. Not suited for imaging all tissue types Prone to artefact-generation arising from sample preparation. Fragmentation of lignin during imaging due to high-energy ion bombardment

FTIR spectroscopy

Popular technique to combine with chemometric methods such as principal component regression (PCR) or partial least squares (PLS) regression. Have been used for successfully determining lignin content in biomass samples Extremely detailed information about inter-unit linkages can be obtained. Has allowed for the assignment and quanti  cation of over 80% of linkages in lignin oil from reductive catalytic fractionation of pine wood Di ff erentiation of the phenolic OH content of the three monolignols is possible from experiments a  er derivatization of the OH groups Cruder determination of phenolic OH content is possible by comparing the di ff erences in absorption at speci  cmaxima between neutral and alkaline solutions A fast and cheap alternative to wet- chemical methods for determining both phenolic OH group and carboxylic acid contents Popular technique for obtaining weight and number average molecular weights, M w and M n , and for further calculating the polydispersity index (PI) of samples Analysis of biomass composition, quanti  cation of volatiles, bio-oil and biochar. Can be coupled with TGA and FTIR. Label-free method with high sensitivity and chemical selectivity for imaging of lignin in e.g. plant cell walls Visualization of monolignol distribution on plant sample cross- sections

1 H/ 13 C 2DNMR

78 and 79

31 PNMR

78 and 80

UV-vis spectroscopy

81 and 82

Simultaneous conductometric and acid – base titrations

Size exclusion chromatography (SEC)

78 and 83

84 and 85

Gas chromatography – pyrolysis (GC-Py)

Coherent anti-Stokes Raman scattering (CARS) microscopy

58 and 86

Time-of-  ight secondary ion mass spectrometry (ToF-SIMS)

hydrophobization are frequently mentioned for lignin, 34,77,78 which would normally fall into the second category, unless the purpose is to protect the underlying substrate from degradation by water. The di ff erent applications will be discussed more in detail in this chapter. The end-use usually determines the manner, in which mixtures and coatings must be formulated. In principle, four di ff erent approaches can be distinguished, which are (1) application of neat lignin, (2) blends of lignin with other active

3. Formulations and applications of lignin-based surfaces and coatings The coatings and surface modi  cations in this review most o  en ful  ll one of two purposes. Firstly, they may seek to protect the underlying substrate, e.g. , from mechanical wear, chemical attack (corrosion), or UV radiation. Secondly, they add functionality such as antioxidant, controlled substance release, or antimicrobial properties. Reduced wetting and

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Fig. 7 Overview of di ff erent application modes for producing functional surfaces and coatings with lignin.

angles ranging from 40 – 60°. Despite widely diverse composi- tions, the solubility in water was found to be the parameter governing the properties of the thin  lms. Similar results were obtained by Notley and Norgren, who found that lignin coatings prepared from diiomethane or formamide yielded even lower contact angles at about 20 – 30°. 34 The approach was further re  ned by Souza et al. , who treated the spin-coated lignin  lms via UV radiation or SF6 plasma treatment in addition. 84 While the UV treatment reduced the contact angle from about 90° to 40°, the plasma treatment produced superhydrophobic surfaces with contact angles exceeding 160°. The latter was also shown to induce major surface restructuring with a strong incorporation of CF x and CH x groups, which would account for the large increase in contact angle. Coatings with lignin-based nano- particles can also be made by evaporation-induced self- assembly, whose properties and morphology are strongly gov- erned by the drying conditions and evaporation rate. 85 An example of the obtained morphologies is shown in Fig. 8. Based on these reports, research is generally concurring on the fact that lignin by itself is not a hydrophobic substance. Harsh treatments, chemical modi  cations, or  ne-tuning of surface morphology are necessary to invoke hydrophobicity. Spin-coated  lms of milled-wood lignin have furthermore been investigated for enzyme adsorption. 86 Similarly, the adsorption of proteins on colloidal lignin has been studied by Leskinen et al. , who produced protein coronas on the lignin particles via self-assembly. 87 The authors further showed that this deposition was governed by the amino acid composition of the protein, as well as environmental parameters such as the pH and ionic strength. The use of lignin for protein-adsorption is an interesting implementation, as it can provide di ff erent surface chemistries than its lignocellulosic counterparts. Still, the compatibility with in vivo environments is questionable, as biodegradation is not given here.

or inert materials, (3) the blending of lignin in thermoplastic materials, and (4) the use of lignin as a precursor for synthe- sizing thermoset polymers. An overview of the di ff erent approaches for formulation and application is given in Fig. 7. These will be discussed in more detail further on. While surface layer or coating are usually applied onto another material, there are also implementations that include lignin as part of the overall base-matrix. Examples for the latter include lignin-derived biocarbon particles for CO 2 capture or wastewater treatment, polyurethane foams, and lignin as an internal sizing agent in pulp products. 36,43,79,80 The predominant way of using lignin in functional surfaces is by blending with other substances. Such formulations o  en include agents, which are established for a particular application, e.g. , starch for paper sizing or clay for controlled-release urea fertilizers. 81,82 Formulations in polymer synthesis usually draw on speci  c functional groups that are found in lignin, for example, the hydroxyl groups as polyol replacement in polyurethane or the aromatic moieties as phenol replacement in phenol- formaldehyde resins. 18,33 3.1. Surfaces and coatings with neat lignin Applying technical lignin by itself is a simple approach, as no co-agents are required. While some degree of adhesion to the substrate is o  en given, pressure and heat may be applied in addition. Publications pertaining to this topic can be grouped into two categories, i.e. , fundamental research studying the formation and properties of lignin-based  lms and coatings, as well as applied research, which is usually focused on a speci  c end-use. 3.1.1. Fundamental research. A fundamental study was performed by Borrega et al. , who prepared thin spin-coated  lms from six di ff erent lignin samples in aqueous ammo- nium media. 83 The  lms exhibited hydrophilicity with contact

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Fig. 8 Coatings comprising lignin particles produced by evaporation-induced self-assembly (a) and vertical cross-section of the obtained layers (b). 85

3.2.1. Lignin-ester derivatives. Esteri  cation of lignin with fatty acids has been investigated by several authors. This approach bears potential, as it combined two bio-derived (macro-)molecules. The lignin contributes a backbone for gra  ing and may improve dispersibility and adhesion of the fatty acids on lipophobic surfaces. The fatty acids can in turn render the lignin more hydrophobic, improving the water barrier, e.g. , on paper substrates. To improve the reaction yield, reactive intermediates are frequently used. Several publications have studied the use of lignin esteri  ed with fatty acid-chlorides as hydrophobization agents for paper and pulp products. 78,93 The coating a ff ected both the surface chemistry and morphology, as illustrated in Fig. 9. The result is usually a decrease in water-vapor transmission rate (WVTR), oxygen transmission rate (OTR), and an increase in aqueous contact angle. Oxypropylation with propylene carbonate has been used as an alternative esteri  cation approach, which yielded a similar hydrophobization and barrier e ff ect on recycled paper. 94 A downside of oxypropylation is the use of toxic reac- tants, i.e. , propylene oxide, and the requirement for high pres- sure during the reaction. While fatty acid chlorides do not need high pressures, these chemicals are highly corrosive and require the absence of water. All mentioned aspects can stand in the way of commercial implementation. Hua et al. reacted so  wood Kra  lignin with ethylene carbonate to convert phenolic hydroxyl units to aliphatic ones, 95 as these are considered more reactive. The samples were further esteri  ed with oleic acid and spin- or spray-coated onto glass, wood, and Kra  pulp sheets. The authors showed that hydro- phobic surfaces with contact angles ranging from 95 – 147°were possible. The pulp boards furthermore showed a more uniform surface a  er the coating. Esteri  cation with lauroyl chloride was also used by Gordobil et al. , who studied their application as wood veneer by press-molding and dip-coating. 96 While the feasibility to treat wood and wood-based products was demonstrated on a technological level, the comparison to established treatment agents is frequently lacking. For example, linseed oil is an established wood-treatment agent, which undergoes self-polymerization in the presence of air. Paper- sizing agents can be based on compounds that are similar in

3.1.2. Applied research. An example for applied research would be paper and pulp products, which can be rendered less hydrophilic by surface-sizing. Application of the lignin can be done via an aqueous dispersion or alternatively by impregna- tion a  er dissolution in a solvent. 35,88 A similar approach was used to treat beech wood with lignin nanoparticle via dip- coating, which improved the weathering resistance of the wood. 89 Such dip-coating may preserve breathability of the substrate due to the porous structure. In this context, the patent application WO2015054736A1 should be mentioned, which discloses a waterproof coating on a range of substrates including paper. 90 In this invention, the lignin is coated onto the substrate a  er at least partial dissolution, followed by heat or acid treatment. However, as discussed above, the lignin by itself is not a hydrophobic material. While lignin-nanoparticles may alter the surface morphology of pulp products, an improvement in long-term water-resistance may be mostly determined by a ff ecting mass-transfer kinetics. Deposition of lignosulfonates on nylon has been demon- strated, which improved the ultraviolet protection ability of the fabric. 91 This deposition took place from aqueous solution and under heating, reportedly yielding a chemical bonding of lignin's OH groups to the NH groups of nylon 6. Such bonding would indeed be necessary, as the lignosulfonate would other- wise be easily washed away. Zheng et al. coated micro  brillated cellulose with Kra  lignin and sulfonate Kra  lignin, which promoted  re retard- ancy of the material. 42 At last, iron-phosphated steel was rendered more resistant to corrosion a  er spray coating with lignin, which was  rst dissolved in DMSO and other commer- cial lignin-solvents. 92 While proven in the lab, these two appli- cations must be considered with care, as unmodi  ed lignin is a brittle material, which can limit the long-term durability of such products. 3.2. The use of chemically modi  ed lignin Chemical modi  cation of lignin is frequently done to improve or enable the processability in blends with materials. In addi- tion, chemical modi  cation may add or alter functionalities as required in speci  c applications.

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Fig. 9 SEM images of (a) uncoated paperboard and (b) paperboard coated with lignin-fatty acid ester. This fi gure has been adapted/reproduced from ref. 78 with permission from Elsevier B.V., copyright 2013.

used in wood-varnish formulations with a higher technological maturity. 3.2.3. Other approaches. A variety of other modi  cations has been proposed to develop coatings from lignin. For example, Dastpak et al. reacted lignin with triethyl phosphate to spray-coat iron-phosphated steel for corrosion protection. 44 Coating of aminosilica gel with oxidated Kra  lignin was per- formed by electrostatic deposition, which improved the adsorption capacity for dyes from wastewater. 102 Wang et al. phenolated lignosulfonate, followed by Mannich reaction with ethylene diamine and formaldehyde to produce slow-release nitrogen fertilizers. 103 The  nal product exhibited elevated contact angles, however, an increased surface roughness likely also contributed to this e ff ect, as the phenolated and aminated lignin exhibited nanoparticle structures. A di ff erent approach was taken by Behin and Sadeghi, who acetylated lignin with acetic acid to coat urea particles in a rotary drum coater. 104 The use of lignin in slow-release fertilizers can be useful, as lignin can have a soil-conditioning e ff ect. However, biodegradability also must be considered, which can be negatively a ff ected by chemical modi  cation. Self-healing elastomers were synthesized by Cui et al. , who gra  ed lignin with poly(ethylene glycol) (PEG) terminated with epoxy groups. 31 The authors concluded that a new material was developed with potential application for adhesives, but the ultimate stress was comparably low at 10 – 12 MPa. The material was named as a self-healing elastomer; however, the appear- ance and rheological properties suggest a thixotropic gel instead. 3.3. Blends of lignin with other substances In the context of this review, the largest number of publications was found for lignin-blends with other substances. The advan- tage of this approach lies in the ease of implementation,  exi- bility for later adjustments, and potential synergies with other co-agents. The lignin and other additives may be mixed right before or during surface modi  cation, hence not requiring lengthy preparations such as the synthesis of chemically modi  ed lignin or a pre-polymer. To facilitate better overview, this section was subdivided into several sub-section, which were distinguished by the application area or formulation-approach. 3.3.1. Cellulose  bers and other wood-based products. The use of lignin in combination with cellulose  bers,  brils, or derivatives has received considerable attention, as this can yield

function to fatty acids, such as resin acids or alkenyl succinic anhydride. Considering these examples, the questions arises whether modifying lignin bears an advantage over using established coatings or sizing agents. In the light of this discussion, the acid-catalyzed transesteri  cation of lignin with linseed oil should be mentioned. 77 According to the authors, a suberin-like lignin-derivative was produced, which introduced hydrophobicity on mechanical pulp sheets, while being more compatible with the  bers than linseed oil alone. The proposed process is simple in setup and reactants, which facilitates ease of implementation. In addition, the lignin is prescribed a key function, i.e. , acting as a compatibilizer between the  bers and the triglycerides. At last, controlled-release fertilizers with lignin-fatty acid gra  polymers have been proposed. Wei et al. crosslinked sodium lignosulfonate with epichlorohydrin, followed by esteri  cation with lauroyl chloride. 97 Sadehi et al. reacted lignosulfonate with oxalic acid, proprionic acid, adipic acid, oleic acid, and stearic acid. 98 The modi  ed lignin was further used to spray-coat urea granules. Both implementations showed enhanced hydrophobicity and the ability to coat urea for slower release of nitrogen. Still, it would be important to compare such approaches with established coating or blends of lignin and natural waxes or triglycerides, which do not require an elaborated synthesis. 3.2.2. Enzymatic modi  cation. Enzymatic modi  cation of lignin has the advantage of comparably mild reactions condi- tions, which can have a positive impact on process economics. On the downside, enzymes are comparably expensive and imposes higher technological demands. In addition, the variety of lignin-compatible enzymes is somewhat limited. Enzymatic treatment can induce a number of changes to lignin, such as oxidation, depolymerization, polymerization, and gra  ingwith other components. 99 For example, Mayr et al. coupled ligno- sulfonates with 4-[4-(tri  uoromethyl)phenoxy]phenol using laccase enzymes. 100 A  er successful coupling, the lignosulfo- nate  lms exhibited reduced swelling and an increase in aqueous contact angle. Fernandez-Costas et al. performed laccase-mediated gra  ing of Kra  lignin on wood as a preser- vative treatment. 101 While the reaction itself was deemed a success, the desired antifungal e ff ect was only obtained a  er inclusion of additional treatment agents, such as copper. It is hence questionable if enzymatically coupled lignin poses as a competitive wood-treatment agent, as the lignin could also be

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onto cellulose nanocrystals. 110 The product was cast into thin  lms, which showed nanostructured morphologies with increased water resistance and the ability to form self- supported hydrogel-  lms. In another publication, Hambard- zumyan et al. simply mixed the cellulose nanocrystals with lignin in solution, a  erwhich  lms were cast onto quartz slides and dried by evaporation. 111 The authors found that optically transparent  lms with UV-blocking ability could be produced. It was concluded that increasing the CNF concentration allowed for better dispersion of the lignin macromolecules, dislocating the p – p aromatic aggregates and hence yielding a higher extinction coe ffi cient. An elaborate work on lignin-starch composite  lms was conducted by Baumberger. 39 The  lms were produced via oneof two methods: (1) powder blending of thermoplastic starch and lignin, followed by heat pressing and rapid cooling, and (2) dissolution in water or dimethyl sulfoxide followed by solvent- casting and solvent evaporation. The author concluded that the lignin acted either as  ller or as extender of the starch matrix, where the compatibility was favored by medium relative humidity, high amylopectin/amylose ratios, and low molecular weight lignin. Lignosulfonates formed good blends and imparted a higher extensibility onto the starch  lms, likely due tobene  cial interactions between sulfonic and hydroxyl groups. Non-sulfonated lignin, on the other hand, improved water- resistance to a greater extent. Three recent studies have found that incorporating lignin into a molded pulp materials can reduce the wettability of the material, as witnessed by an increase in contact angle or a decrease in water-uptake. 8,36,88 The advantage of such imple- mentation is that high temperature and pressure will promote densi  cation, as the lignin can  ow into cavities. High densities of up to 1200 kg m − 3 were reported, where the uptake of water is hindered not only by limiting mass-transport, but also by con  ning the swelling of cellulose  bers. 88 Various researchers have included lignin in the formulation of paper-sizing agents. In one implementation, Javed et al. blended Kra  lignin with starch, glycerol, and ammonium zirconium carbonate to produce self-supporting  lms and paperboard coatings. 112 The mechanical  lm stability was better

all-biobased materials and coatings. For example, eucalyptus Kra  lignin and cellulose acetate were combined in solution and cast onto beech-wood, which produced a protective coating similar to bark. 37 However, the authors did not determine the mechanical properties of the product, which would be impor- tant to address, as the potential brittleness could impart prac- tical use. On the other hand, the biodegradation of lignin is indeed more challenging than that of cellulose and hemi- cellulose, 105 which may hence contribute to an improved resis- tance against certain fungi and bacteria. In addition, the lignin- based veneer may add functionalities such as water-repellence, UV-protection, and improved abrasion resistance, 106 but still a comparison with established treatment agents is lacking. Cellulose nano  brils (CNF) and (cationic) colloidal lignin particles was cast into  lms by Farooq et al. , yielding improved mechanical strength as compared to the CNF alone. 107 A sche- matic of the proposed interactions is given in Fig. 10. The authors concluded that the lignin particles acted as lubricating and stress transferring agents, which additionally improved the barrier properties. The discussed e ff ects could also be induced by the lignin acting as a binder, hence  lling gaps and providing an overall tighter network. 36,88 Riviere et al. combined lignin- nanoparticles and cationic lignin with CNF, however, the oxygen barrier and mechanical strength were lower than the CNF without added lignin. 108 This e ff ect was likely due to a disruption of the binding between CNF networks. The poly- phenolic backbone of lignin generally provides less opportuni- ties for hydrogen bonding than compared to the cellulose macromolecule. The authors work on solvent extraction of lignin from hydrolysis residues is noteworthy, however, and the work showed promising potential for antioxidant use. LCC were combined with hydroxyethyl cellulose, producing free-standing composite  lms. 109 In this study, the addition of LCC enhanced the oxygen barrier properties and could also improve the mechanical stability and rigidity. A better e ff ect of LCC was noted than combining lignosulfonates with hydrox- yethyl cellulose alone. Synergies could hence arise from carbo- hydrates that are covalently bond onto the lignin. An interesting approach was taken by Hambardzumyan et al. , who Fenton's reagent to partially gra  organosolv lignin

Fig. 10 Schematic illustration of proposed interaction between CNF and di ff erent lignin morphologies. 107

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