This article studied the carboxylation of technical lignin and subsequent use as emulsion stabilizer. Oxidation was conducted with hydrogen peroxide under alkaline conditions. As both titration and Fourier-transform infrared spectroscopy (FTIR) showed, phenolic units were converted to carboxyl groups by oxidation. The treatment was most effective for soda lignin from Arkansas/straw, but also had significant effect on the softwood kraft lignin and softwood soda lignin. An increase in molecular weight by size-exclusion chromatography was further noted, which was less pronounced for the Arkansas/straw lignin. It was argued that one contributing mechanism was the monolignol composition, as the lignin from annual plants also contained S-units in addition to the G-units that mostly made up the softwood lignin. Moreover, purification prior to oxidation, i.e., removal of inorganic components in the lignin, showed no significant effect on the carboxylation process. Emulsion stabilization was studied with respect to the pH using three oxidized kraft lignins. Here, lower pH yielded better emulsion stabilization, unless the lignin precipitated, which switched the stabilization mechanism from interfacial adsorption to particle stabilization. It was argued that the degree of ionization played a key role, as a lower degree of ionization corresponded with better emulsion stability at the same ionic strength. At last, measurements of interfacial tension and interfacial rheology found that oxidized lignin behaved similar to water-soluble lignosulfonates and created viscoelastic interface layers. 

As society is rapidly converting from fossil-based materials to greener alternatives, the valorization of lignin through chemical modification has been given considerable attention. Characterizing this highly heterogeneous biopolymer is a constant challenge, and an emerging strategy for dealing with variations in material characteristics is combining traditional analytical techniques with chemometrics, such as Fourier-transform infrared (FTIR) spectroscopy with partial least squares regression (PLSR). Here, a calibration data set was built based on FTIR spectra and the total carbon-hydrogen bond (CHB) content of mixtures of technical lignins and alkanes, meant to emulate esterified samples. From this data, a PLSR model was built which predicted the CHB content of esterified lignin reaction products with an RMSECV=5.685 mmol/g and RMSEPred=5.827 mmol/g, and from which the weight percentage of ester-to-lignin was determined. When compared to wet-chemical analysis, good agreement between the techniques was found with an obtained RMSEPred=8.3 % and a R2Train=0.9752 for the degree of esterification. This indicates high model predictability and goodness of fit, and that the calibration data set successfully emulated esterified lignin samples. 

Barrier coatings on dry formed pulp were studied in this article, which were derived from Kraft lignin, stearic acid, and combinations thereof. Coating layers were applied by spray-coating with a solution of lignin or stearoyl chloride and subsequent heat treatment. Alternatively, lignin was esterified with stearoyl chloride on beforehand or combinations of lignin and stearoyl chloride on the air-laid mat were done. Since the treatments were applied prior to thermopressing, the coating agents permeated the top layers of each substrate. As our results show, coatings with lignin could improve the tensile strength and stiffness of the substrate. Grafting with stearic acid, on the other hand, affected the tensile properties negatively, which was argued to arise from worse binding of the cellulose fibers and degradation due to the presence of acid moieties. All treatments improved the barrier properties, as noted by a reduction in air permeation and water-vapor transmission rate (WVTR). The effect of spray coated lignin on WVTR was best, albeit showing less effect on the water absorption as measured by COBB1800. Stearic acid grafting yielded the opposite trend, i.e., reducing water absorption to a greater extent, while affecting WVTR less. Combinations of stearoyl chloride and lignin showed synergies and additive effects to some extent. In conclusion, various treatments for dry formed fibers were implemented and tested, which may promote the development of new barrier solutions for cellulose-based materials. 

This data article summarizes the material properties of some added-lignin thermoformed pulps (ALTPs). This type of molded pulp is particularly suited for replacing plastics in environments, where moisture is encountered, as the lignin reduces the transport and adsorption of water. The dataset was measured on wet formed substrates with either softwood chemi-thermomechanical pulp (CTMP) or northern bleached softwood Kraft pulp (NBSK). To the basis weight of 500 g/m2 pulp fibers (6 g pulp per 15 cm by 8 cm substrate area) were added 0.8 g of lignin, constituting a loading of 13.33 % added lignin per dry fibers. Right after forming, the sheets were pressed at 20 bar and room temperature between blotting paper to remove excess water, air dried overnight, and subsequently thermopressed at 175 °C and 200 bar. The resulting materials were measured and weighed, cut into test stripes with 15 mm width, and tensile tested. The absorption of water (water uptake) was measured after immersing the test pieces (15 mm by 15 mm) in distilled water for 24 h. The data shows slight increases in density after lignin addition and, on average, a reduction in water absorption. In particular, adding acetylated lignin significantly lowered the water uptake, as compared to the regular soda or Kraft lignin. The tensile strength and stiffness of CTMP was the same or lower after lignin addition, whereas NBSK showed increases in both stiffness and strength. 

Single-use plastic products have been identified as an environmental challenge. When such products are not recycled, they may end up in nature and thus cause, e.g., marine littering. Thermoformed wood pulp fibre products are gaining more interest to replace fossil plastic products. However, beverage caps made of wood pulp fibres are challenging due to the hygroscopic nature of wood fibres, i.e., they absorb water, deform and loose functionality. Hence, the purpose of this study was to develop a fibre-based beverage cap that could replace plastic tethered cap systems. Both unbleached and bleached Kraft pulp and chemo-thermo-mechanical pulp (CTMP) fibres were tested in thermoforming trials, using tailor-made metal moulds. The results showed that Kraft pulp fibres formed denser structures, with more limited water absorption, compared to CTMP. The mechanical properties of thermoformed specimens were suitable for the application, i.e., the strength, modulus and elongation were between 32 and 36 MPa, 4–4.9 GPa and 1.6–1.9%, respectively, depending on the type of pulp fibre. Additionally, in order to secure that the caps were functional in relevant conditions in contact with liquids (water or milk), the caps were surface modified by silylation and esterification to increase the liquid barrier. The results indicate that surface esterification increased the contact angle to 95°. On the other hand, the surface-modified caps could not entirely limit the liquid absorption over longer periods of time (>∼1 h) when the caps were directly exposed to liquid. However, the liquid barrier was satisfactory when the products were exposed to increased relative humidity in refrigerated conditions (relative humidity >76% and temperature <7 °C). 

This article provides a concise insight into the thermoforming of airlaid CTMP pulp. First, the airlaid process was studied, showing that fiber fractionation and the retention of fines occurred in the forming head. Then, the effect of temperature and pressure during thermoforming was investigated. Harsher conditions, i.e. higher temperature and pressure, yielded greater densification of the substrate and higher tensile strength. The maximum strength was found at the highest settings tested, that is, 100 MPa and 200°C. The screening of thermoforming conditions was also compared to previously published results on wetforming. Next, the effect of softwood CTMP pulp was delineated, which on average showed the best mechanical properties at elevated freeness and high degrees of bleaching. At last, a comparison between dry forming and wetforming was made for one selected pulp quality. Here, the dryformed substrates were stiffer at low elongation, yet the wetformed substrates yielded a greater extensibility and higher tensile strength. In conclusion, dryformed pulp mostly relies on temperature and pressure for bond formation during thermoforming, which produces materials that are distinctly different from wetformed molded pulp. 

In this paper, the potential of esterified Kraft lignin as a novel oil-soluble surfactant was examined. The lignin was chemically modified by esterification with lauric or stearic acid, making it soluble in solvents such as toluene or n-decane. Adsorption at the oil-water interface was then studied by the Du Noüy ring-method. The oil-soluble lignin behaved similar to water-soluble lignin surfactants, both the qualitative and quantitative progression of interfacial tension. Modeling revealed a surface excess of 7.5-9.0 × 10-7 mol/m2, area per molecule of 185-222 Å2, and a diffusion coefficient within the range 10-10 to 10-14 m2/s; all of which are in line with existing literature on water-soluble lignosulfonates. The data further suggested that the pendant alkyl chains were extended well into the paraffinic solvent. At last, bottle tests showed that the oil-soluble lignin was able to stabilize oil-in-water emulsions. The emulsion stability was affected by the concentration of lignin or NaCl as well as the oil phase composition. Aromatic oils exhibited lower emulsion stability in comparison to the aliphatic oil. In conclusion, a new type of surfactant was synthesized and studied, which may contribute to developing green surfactants and novel approaches to valorize technical lignin.

This article tested a novel concept for synthesizing green wax inhibitors. Four technical lignins were reacted with stearoyl chloride to produce esterified C18 esterified lignin. The effect of the reaction on the lignin molecular weight, characteristic FTIR spectra, and thermal degradation was surveyed. In addition, wax inhibition testing was performed by rheology on model waxy oils. The grafting reactions increased the mass-average molecular weight of the lignin and in some cases also the polydispersity index. FTIR analysis confirmed the success of esterification reactions as the O-H stretching band decreased, whereas the C-H and C═O stretching bands significantly increased. The thermal degradation was further found to occur at temperatures above 170 °C, indicating that the lignin wax inhibitors were thermally stable enough for crude oil production. The effect on waxy gelation was varied, showing that the low molecular weight waxes benefited more than the high molecular ones. A gelation point reduction of up to 6 °C was found after lignin addition. After the wax type, wax concentration, lignin concentration, and lignin type were varied, it was found that C18 esterified Kraft lignin exhibited the most beneficial effect. The results from viscometry agreed with the observations from the rheometric gelation point. Cross-polarized microscopy was used to map the effect on the wax crystal morphology. A difference was found only in the case of one esterified Kraft lignin, which yielded smaller and more finely dispersed wax crystals. In conclusion, a new wax inhibitor was synthesized by reacting technical lignin with stearoyl chloride. This lignin showed wax inhibitor activity in some of the tested cases. At this point, the length of the pendant alkyl chains (C18) is likely a limiting factor. However, this study attributes the potential for a new concept to synthesize green wax inhibitors. 

Purity of technical lignin is one of the main obstacles in the utilization of lignin to value-added chemicals, products, and materials. The objective of this study was to investigate and compare single and two stage purification methods for obtaining soda lignin with high purity. Extensive washing and extraction with water was found effective, increasing the abundance of acid insoluble lignin while reducing its ash content. Extraction with organic solvents was conducted with 2-propanol or blends of n-heptane/1-butanol or cyclohexane/acetone. These solvents were shown to have little effect on the total lignin content, as determined by wet-chemical methods. Two-stage treatments (washing with water followed by solvent extraction) were hence not better than single stage water extraction in terms of the lignin purity. Still, selective removal of low molecular weight components after solvent extraction was noted, reducing the overall polydispersity of the lignin. Evaporation at 40 °C also showed little effect, whereas calcination at 150 °C significantly increased the molecular weight of the soda lignin. The latter effect was explained by thermally induced cross-linking. In addition, the UV absorbance of the calcinated lignin increased, which is likely related to changes in the aromatic structure. Such effect also entailed that UV/vis spectrophotometry was found less reliable in determining the total lignin content. At last, a mathematical model was adapted to predict the total lignin content from FTIR spectrometry. In conclusion, the tested procedures can be used to purify soda lignin and adjust its molecular weight.

Lignin is classified as the second most abundantly available biopolymer after cellulose and as a main aromatic resource material. Lignin structure differs based on sources of origin and species of biomass with around 15–40% of lignin content based on dry weight. It is extracted from various types of lignocellulosic biomass through different pulping extraction methods. After extraction, lignin can be further functionalized through different chemical reactions to meet the requirements and specifications before being used in end products. Therefore, in this review paper, the details on extraction and the type of lignin, as well as chemical functionalization, are discussed. The chemical functionalization can be used to modify the lignin such through phenolic depolymerization or by other aromatic compounds, creating novel chemical active sites to impact a reactivity of lignin and through functionalization of hydroxyl functional group for enhancing its reactivity. Furthermore, the recent sustainable application of lignin was discussed in different fields such as nanocomposite, flame retardant, antioxidant, cosmetic, natural binder and emulsifier. This review hence provides a summary of the current stateoftheart in lignin technology and future outlook of potential application areas.