Valorisation of crude glycerol in the production of liquefied lignin bio-polyols for polyurethane formulations

International Journal of Biological Macromolecules 247 (2023) 125855 Available online 17 July 2023 0141-8130/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Valorisation of crude glycerol in the production of liquefied lignin bio-polyols for polyurethane formulations Fabio Hern´ andez-Ramos a , * , María Gonz´ alez Alriols a , M. Mirari Antxustegi b , Jalel Labidi a , Xabier Erdocia c a Biorefinery Processes Research Group (BioRP), Chemical and Environmental Engineering Department, University of the Basque Country (UPV/EHU), Plaza Europa 1, 20018 San Sebastian, Spain b Biorefinery Processes Research Group (BioRP), Chemical and Environmental Engineering Department, University of the Basque Country (UPV/EHU), Avda. Otaola 29, 20600 Eibar, Spain c Biorefinery Processes Research Group (BioRP), Department of Applied Mathematics, University of the Basque Country (UPV/EHU), Rafael Moreno “Pichichi” 3, Bilbao 48013, Spain ARTICLE INFO Keywords: Bio-polyol Organosolv lignin Crude glycerol Liquefaction ABSTRACT Bio-polyols, produced by liquefying lignin with polyhydric alcohols, offer a promising alternative to conventional polyols for polyurethane production. To enhance the sustainability on the production of these bio-polyols, this study proposes the use of crude glycerol and microwave-assisted liquefaction as substitutes for conventional methods and commercial glycerol. This approach reduces the energy requirements of the reaction while also adding value to this by-product. The synthesis of bio-polyols with suitable properties to produce elastic and rigid polyurethane was carried out using previously optimised reaction conditions. Organosolv lignins obtained from Eucalyptus globulus and Pinus radiata were employed, using polyethylene glycol and crude glycerol as solvents and sulphuric acid as a catalyst. Several parameters of the bio-polyols were analysed, including hydroxyl number (I OH ), acid number (A n ), and functionality (f), suggesting that the bio-polyols were suitable for polyurethane synthesis. Bio-polyols formulated to produce rigid polyurethanes exhibited I OH values of 554 and 383 (mg KOH/ g), A n values of 1.91 and 4.21 (mg KOH/g), and functionalities of 4.16 and 3.14 for Eucalyptus globulus and Pinus radiata lignin. In the case of bio-polyols for elastic polyurethanes, the values were 228 and 173 (mg KOH/g) (I OH ), 20.94 and 25.09 (mg KOH/g) (A n ), and functionalities of 3.51 and 2.08. 1. Introduction The impact of human activity has become so relevant that the Nobel Prize winner, Paul J. Crutzen, proposed a new term to place it in a geological context [1]. This new term, the Anthropocene, even though it has not been officially established by academia, is now in vogue. However, the mere fact that the scientific community has acquired and maintained it so far, makes it clear how important the human footprint on the environment is. The use of petroleum, not only as fuel but also as a feedstock for the manufacture of different materials, represents one of the main causes of the degradation of the environment. For this reason, the scientific community is looking for new sources of raw materials that could totally or partially substitute the use of petroleum. Among petroleum derived materials, PUs, first synthesised by Dr. Otto Bayer in 1937, have become one of the most versatile man-made synthetic materials [2]. Such versatility lies in their excellent mechanical, chemical, and physical properties, such as abrasion resistance, elasticity, biocompatibility, durability, or toughness [3]. PUs can be used for the manufacture of a variety of products, such as foams, elastomers, paints, coatings, adhesives or medical applications [4]. PUs are cross-linked materials formed by a poly-addition reaction between isocyanates (with more than one isocyanate group per molecule) and molecules with two or more OH reactive groups called polyols [5]. In 2019, the polyol market generated around USD 26.2 billion and it is expected to reach USD 34.4 billion by 2024 [6]. Although the polyols employed nowadays to synthesise PUs are generally petroleum-derived compounds, it is possible to substitute them with lignocellulosic biomass-derived materials to produce more eco-friendly PUs that satisfy the requirements of new and more rigorous legislation [7]. Lignocellulosic biomass is formed mainly by cellulose, * Corresponding author. E-mail address: [email protected] (F. Hern´ andez-Ramos). Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac https://doi.org/10.1016/j.ijbiomac.2023.125855 Received 24 April 2023; Received in revised form 26 June 2023; Accepted 14 July 2023 International Journal of Biological Macromolecules 247 (2023) 125855 2 hemicellulose, and lignin forming a composite matrix [8], and it is considered a sustainable raw material source to produce high valuedadded commodities [9]. Cellulose, the earth’s most abundant renewable biopolymer, can be used in different industrial fields. Probably its most well-known application, apart from pulp production, is the conversion to bioethanol to be used as biofuel. Nevertheless, its remarkable properties, such as high abrasion resistance, biocompatibility, biodegradability and chemical stability, make it suitable to be used for the manufacture of composites [10], in medical applications, food packaging and photoelectric materials, among others [11]. Meanwhile, hemicelluloses can be used to produce xylooligosaccharides and furfural [12]. Finally, lignin, which is considered as the most abundant renewable phenolic polymer [13], is usually burned to supply energy to paper mills where it is generated as waste [14]. However, due to its phenolic nature, high availability, low cost, and the presence of aliphatic and phenolic OH groups, lignin can be used to create high value-added biobased products, including bio-based polyols [15]. Nevertheless, despite the large number of OH group present in the lignin molecule, chemical modifications of its structure are usually necessary, as many of these groups are not accessible because of being sterically hindered, which significantly decreases the reactivity of this molecule [16]. In this work, different lignin modification strategies have been tested to obtain these polyols, of which, oxyalkylation and liquefaction with polyhydric alcohols are the most relevant ones [17]. Lignin liquefaction reaction is typically carried out under atmospheric pressure and acidic conditions (H 2 SO 4 ) employing mild temperatures(150–170 ◦C), a reaction time around 90 min, and using PEG and CG as solvents [18]. Hence, as conventional lignin liquefaction methods entail high energy expenses attributed to lengthy residence times and elevated temperatures, it becomes imperative to investigate alternative approaches that are more eco-friendly. The aim is to diminish operational costs and optimise the aforementioned factors to enhance the industrial feasibility of the process. In this regard, microwave-assisted irradiation presents itself as a compelling substitute for traditional lignin liquefaction techniques. By enabling rapid and uniform heating, it reduces reaction time and subsequently lowers energy consumption [19]. Generally, technical grade petroleum derivedglycerol is used for the liquefaction process. There is, however, a large amount of glycerol that is generated as a by-product in the biodiesel industry. In fact, for every 10 t of generated biodiesel, 1 t of crude glycerol is produced. Once purified, this glycerol can be used in the food, pharmaceutical and cosmetic industries [20]. Nevertheless, due to the large surplus of CG caused mainly by the booming of biodiesel industry, the price of the refined product decreased in recent years [21], thus making it unprofitable for small plants [22]. It is therefore necessary to find an industrial use for this waste, such as reactant it in the liquefaction of lignin to produce polyols. Different studies have been carried out in this field, where CG was successfully employed to synthesise biopolyols from lignocellulosic biomass [18,23,24] or lignin [25–29]. Hence, the combination of using the application of both microwaveassisted reaction and the utilisation of crude glycerol in the lignin liquefaction process can be a significant step forward in incorporating lignin into the polyurethane industry. However, unlike conventional methods where temperatures, times, and reactant concentrations are well established, it is necessary to continue research in order to establish suitable reaction conditions. For this reason, in the present study, Eucalyptus globulus and Pinus radiata organosolv lignin samples were liquefied employing PEG and CG through microwave irradiation technology. The optimised reaction conditions were obtained from a previous work [30]. The produced bio-polyols were characterised to evaluate relevant parameters for their use as polyurethane precursors. Thus, hydroxyl number index (I OH ), acid number (A n ), molecular weight (M w ), polydispersity index (PI), functionality (f) as well as liquefaction yield and rheological behaviour were determined. 2. Materials and methods 2.1. Materials Eucalyptus globulus and Pinus radiata were kindly supplied by Papelera Guipuzcoana Ziku˜ naga S.A. and Ebaki XXI S.A. The vegetable oil employed to obtain the CG was sunflower oil used for cooking and collected from the canteen of the Gipuzkoa Engineering School in Eibar, University of the Basque Country, UPV/EHU. Sulphuric acid (96 %), KOH (85 %) and PEG400 were purchased from Panreac. Ethanol and methanol were obtained from Scharlab S.L. The rest of technical reagents were supplied by Fisher Scientific, i.e., sodium sulphate anhydrous (≥99 %), dimethyl-formamide (DMF, for HPLC ≥%99.9), ethyl acetate (HPLC grade), lithium bromide, 1,4-dioxane, pyridine (analytical grade) and phthalic anhydride (%98). 2.2. Lignin obtention procedure Lignin samples from Eucalyptus globulus and Pinus radiata sawdust were obtained through an organosolv delignification treatment employing a 1.5 L stainless steel 5500 Parr reactor equipped with a 4848 Parr controller. After the delignification process, the resulting black liquors were treated by an ultrasonication process using a HD 3100 Sonoplus ultrasonic homogenizer. Finally, the black liquors were acidified to precipitate the lignin, obtaining an aqueous phase and a solid phase (lignin) which were separated by membrane filtration in a 2 L stainless steel holder employing a 0.22 μ m pore diameter nylon filter. The reaction conditions for the organosolv and ultrasonication processes were defined in a previous study [31]. The molecular weight distribution of the resulting ultrasonicated organosolv lignin samples from Eucalyptus globulus and Pinus radiata (EUL and PUL respectively) are listed in Table 1 and were determined by gel permeation chromatography (details described in Section 2.4.2). 2.3. Synthesis of bio-polyols through microwave assisted liquefaction Table 2 summarises the reaction conditions used to synthesise the bio-polyols. These reaction settings were established as optimal in a previous work [30]. The reaction was carried out employing a CEM Microwave Discover System Model with a temperature control instrument and an internal temperature sensor. The reactions were performed as follows: the solid liquid ratio was 1:6 in all cases and the reaction time was 5 min, under constant stirring. Previously established quantities of reagents were weighed into a quartz vessel and (4 g in total) introduced into the reactor. As soon as the reaction was completed, the vessel was cooled down to a safe temperature for handling. Acetone was employed to dilute the obtained product to facilitate filtration to separate the biopolyol from the solids. Finally, a rotary evaporator was used to remove the acetone from the bio-polyol. 2.4. Transesterification of vegetable oil to obtain crude glycerol CG was obtained by the transesterification reaction of used vegetable oil (sunflower oil for cooking) with methanol in a molar ratio of 6:1 (methanol:oil). The reaction was catalysed by KOH (1 % wt. of oil). Oleic acid with a molecular weight of 884 g/mol was assumed as the predominant triglyceride for the calculations. The reaction was performed as follows: firstly, the oil was filtered to remove the impurities; then it Table 1 Molecular weight distribution of lignin samples. Sample M w (g/mol) M n (g/mol) PI EUL 2837 888 3.196 PUL 2924 911 3.209 F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 3 was heated to 60 ◦C in a volumetric flask employing a heating plate with magnetic stirring (600 rpm). Once the temperature of 60 ◦C was reached, a previously prepared methanol/KOH mixture was added. The reaction was kept for 120 min under reflux to maximise the conversion. The reaction was considered finished as soon as a good phase separation of the mixture was observed, and it was left for 24 h in a separation funnel to separate the biodiesel and glycerol. 2.4.1. Characterisation of crude glycerol Physical properties of CG, such as density, pH and viscosity were determined. The density was calculated by measuring the weight of a known volume of crude glycerol at room temperature. The pH of the CG was determined at room temperature employing a pH meter Crison basic 20 by dissolving 1.00 ±0.1 g of CG in 50 mL of deionised water. Ash content was analysed following the ISO 2098-1972 Standard method, which consist in burning at 750 ◦C for 3 h 1 g of CG in a muffle furnace. The elemental analysis was carried out using a Leco TruSpec HCNS micro elemental analyser at 1050 ◦C. Both carrier gas (pure Helium 3×) and test gas (extra pure Oxygen 4×) were supplied by Nippon Gases. The calibration was performed using Leco Sulfamethazine (C =51.78 %; H =5.07 %; N =20.1 %; O =11.5 %; S =11.5 %). Triplicate assays were performed using 2 mg samples. Oxygen was calculated by difference. The chemical composition of CG was determined through GC–MS analysis. 0.2 g of CG were dissolved into 25 mL of methanol (HPLC grade), and the solution was injected in a GC (7890)-MS (5975C inert MSD with Triple-Axis Detector) Agilent equipped with a HP-5MS ((5 %-Phenyl)-methylpolysiloxane, 30 m ×0.25 mm) capillary column with Helium as carrier gas. The temperature program is as follows: the program started at 50 ◦C; then, it was raised to 120 ◦C at an 8 ◦C/min heat rate; this temperature was kept for 5 min; then, it was increased to 280 ◦C at 8 ◦C/min and held for 8 min; finally, the temperature was raised to 300 ◦C at 10 ◦C/min and held for 2 min. FTIR analysis was performed to analyse and compare the chemical structure between CG and commercial glycerol. A PerkinElmer Spectrum Two FT-IR Spectrometer equipped with a Universal Attenuated Total Reflectance accessory provided with an internal reflection diamond crystal lens was employed. 20 scans in transmission mode were collected with a resolution of 4 cm −1 in the range of 4000–400 cm −1 . 2.4.2. Characterisation of the obtained bio-polyols Bio-polyols that were obtained through the liquefaction of lignin employing PEG400 and CG were characterised to determine important parameters, such as molecular weight distribution (M w , M n and PI), I OH , A n , and f. A gel permeation chromatography (GPC) analysis was used to determine the molecular weight distribution of the bio-polyols. To this end, a JASCO instrument equipped with an LC.Netll/ADC interface, two columns in series (PolarGel-M 300 mm ×7.5 mm) and a RI-2031Plus refractive index detector was used. N,N-dimethylformamide with 1 % lithium bromide was employed as mobile phase with a flow rate of 700 mm 3 /min and a temperature of 40 ◦C was used. The calibration curve was made employing polystyrene standards with molecular weight from 266 to 70,000 g/mol (Sigma-Aldrich). I OH (mg KOH/g) was calculated following the ASTM D4274 standard, as follows: 0.5–1 g of bio-polyol was dissolved into 25 mL of the phtalation reagent consisting of 115 g of phthalic anhydride dissolved in 700 mL of pyridine. The reaction was carried out at 115 ◦C for 1 h under constant stirring. Afterwards, 50 mL of pure pyridine was added through the condenser. The resulting solution was back titrated employing a NaOH solution (O.5 M). The acid number (A n ) was determined according to ASTM D974 standard by dissolving 0.4 g of bio-polyol in 50 mL of a 4:1 (v/v) solution of 1,4-dioxane in water. Due to the dark colour of the resulting solution, it was not possible to perform a titration using phenolphthalein as indicator. Therefore, a potentiometric titration was done using an automatic titrator (888 Titrando Metrohm) through Tiamo 2.5 software. The thermal degradation of bio-polyols was studied through a thermogravimetric analysis (TGA). 5 mg of bio-polyol were heated under inert atmosphere (N 2 10mL⋅min −1 ) from 25 ◦C to 800 ◦C. The heat rate was 10 ◦C⋅min −1 . The equipment employed was a TGA/SDTA RSI analyser (Mettler Toledo). The rheological behaviour of the biopolyols were analysed through both oscillatory and rotational tests. The former was carried out to determine the storage modulus (G ′ ), while the latter was used to study the viscosity and shear stress as a function of the shear rate. A Haake Viscotester IQ (Thermo Fisher Scientific) rheometer was employed using a coaxial cylinders geometry (CC 25 DIN/Ti adapter) with a piston radius of 12.54 mm and a ring gap of 1.00 mm. The frequency sweep for the oscillatory test was from 0.1 to 100 rad⋅s −1 at a fixed strain of 10 %, while for the rotational test a shear rate sweep from 0.02 to 120 s −1 was used. The measurements were collected at room temperature. 3. Results and discussion 3.1. Crude glycerol characterisation This section contains the physical properties and composition of CG. The pH of the obtained crude glycerol was 10.55 ±0.02, which indicates the presence of residual KOH catalyst and potassium salts formed during the transesterification reaction. This value is in agreement with those obtained by other authors who characterised crude glycerol from the transesterification reaction of vegetable oils with NaOH or KOH as catalyst [32–34]. As it was expected, the density of CG (1.03 ±0.07 g/ cm 3 ) resulted lower than that of pure glycerol (1.259 g/cm 3 ) due to the presence of lighter impurities such as fatty acids, fatty acids methyl esters (FAMEs), water and methanol traces [33]. The water content in CG significantly varies depending on the manufacturing industry, from a 3.6 % in the case of the soap industry to a 55.3 % in CGs from Stearin production. CGs obtained from transesterification reaction present water contents from 8.16 % to 43.2 % [35], although a maximum of 12 % is recommended to reduce purification costs [36]. Therefore, the water content of the CG obtained in this work (11.64 ±1.61 %) is within the specifications of a crude glycerol obtained through transesterification reaction. Such water, can hydrolyse the triglycerides to form free fatty acids (FFA) which results in soaps decreasing the reaction yield [35]. The elemental analysis of CG showed that the 46.00 ±1.29 % of the organic matter corresponded to Carbon (C). This high value can be explained by the high presence of impurities, such as, soaps, FAMEs and glycerides, which have higher C content than glycerol. Hu et al. [33] reported similar C values for CG obtained from different soy and vegetable oil wastes. In addition, the obtained nitrogen (0.15 ±0.01 %), hydrogen (8.17 ±0.33 %) and oxygen (35.6 ±1.58 %) percentages were in accordance with the values reported for different CG obtained from different vegetable oils [20,33,37]. Nevertheless, the measured sulphur concentration (1.33 ±0.04) was higher than the values obtained in the mentioned studies, ranging from ppms to a maximum of 0.078 %. The chemical composition of CG was determined through GC–MS. In addition to glycerol (41.84 ±0.17 %), CG was found to be rich in other compounds which include fatty acids (11.46 ±6.01 %) and FAMEs (26.31 ±7.68 %), among others (Fig. 1). The chemical structure of the CG was determined through FTIR analysis and it was compared with a commercial glycerol sample Table 2 Liquefaction reaction conditions (data pending publication). Bio-polyol Rigid bio-polyol Elastic bio-polyol EPR CG PPR CG EPE CG PPE CG Cat (% wt.) 0 0 5 3.86 Temperature (◦C) 161 159 180 160 PEG/CG (% wt.) 3/1 3/1 7.57/1 7.34/1 F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 4 (Fig. 2). CG showed the main functional groups of commercial grade glycerol: O – H stretching and bending (3300 cm −1 and 920 cm −1 respectively), C – H asymmetric and symmetric stretching (2920 cm −1 and 2851 cm −1 respectively), C – O stretching of primary alcohol (1456 cm −1 ) and secondary alcohol (1110 cm −1 ) as well as H 2 O blending (1650 cm −1 ) [32,38]. Moreover, characteristic peaks of CG were also observed. The first one, a small peak associated to C – – C stretching (3015 cm −1 ) related to unsaturated compounds [32]; the second one, associated with the presence of carbonyl groups (C – – O) of esters or carboxylic acids of fatty acids (1745 cm −1 ) [39]. Finally, a signal related to the presence of carboxylate ions COO − was observed (1560 cm −1 ), indicating the presence of soap in the CG sample [32,39]. 3.2. Characterisation of the bio-polyols Table 3 summarises the data obtained from the characterisation of the bio-polyols. An adequate M w of polyols is essential to obtain PUs with the desired soft segment properties. Depending on the final application, the PUs’ molecular weight should be between 300 and 1000 (g/ mol) for rigid PU, and between 2000 and 10,000 (g/mol) for elastic PU [40]. As expected, the higher the acid concentration, the higher the M w . Thus, bio-polyols for rigid PU (EPR CG , PPR CG ) where no catalyst was used, showed lower M w than the bio-polyols for elastic PU (EPE CG , PPE CG ) (Fig. 3). This increase in the M w is a consequence of the repolymerisation reactions which are favoured in the presence of an acid catalyst [41]. In addition, EPE CG bio-polyol showed a significantly higher M w than PPE CG since, as the catalyst concentration increased above 3 %, the repolymerisation reactions also increase [18]. On the other hand, it has been documented that the polymerisation reactions between glycerol, FFA and FAMEs of CG can also increase the M w of biopolyols [42]. However, since the bio-polyols with higher CG content but without catalyst showed the lowest molecular weight, it could be concluded that these reactions were of lesser importance than the repolymerisation reactions caused by the acid catalyst. The polydispersity index is also crucial for the final application of the PU, as it is related to the chain length variation and, depending on the chain length of the polymer, the PU could show an undesired behaviour [43]. It was observed that the polydispersity index was also affected by repolymerisation reactions caused by an increase in catalyst concentration. Thus, the bio-polyol with the highest catalyst concentration (EPE CG ) exhibited the highest M w and polydispersity index, followed by PPE CG . As stated above, EPE CG and PPE CG bio-polyols’ M w values fitted in the range for the manufacture of elastic PUs. On the other hand, although the molecular weights of EPR CG and PPR CG bio-polyols were slightly higher than the required for the manufacture of rigid PUs, they could be considered suitable for the manufacture of this kind of PUs. The I OH required for the synthesis of rigid PUs ranges between 200 and 1000 mg KOH/g, while for elastic PUs it is between 28 and 160 mg KOH/g [44]. It is also well known that an elevated A n can decrease the efficiency of the reaction, so a low A n value is desired [45]. These two Fig. 1. GC–MS chromatogram of crude glycerol. Fig. 2. FTIR spectra of commercial glycerol and crude glycerol. Table 3 Molecular weight distribution, I OH , A n , functionality (f), equivalent weight (EW) and yield of bio-polyols. Sample M n (g/mol) M w (g/mol) PDI I OH (mg KOH/g) A n (mg KOH/g) f EW Yield (%) EPR CG 442 ±34 1742 ±275 4.25 ±0.55 554 ±4 1.91 ±0.06 4.16 ±0.10 101.20 ±0.66 93.55 ±3.00 EPE CG 941 ±30 8818 ±127 9.38 ±0.16 228 ±36 20.94 ±2.75 3.51 ±0.68 248.98 ±38.77 70.75 ±0.47 PPR CG 453 ±24 1431 ±362 3.14 ±0.63 383 ±8 4.21 ±0.90 3.14 ±0.16 146.68 ±3.23 90.60 ±0.56 PPE CG 780 ±20 5530 ±131 7.10 ±0.31 173 ±16 25.09 ±2.59 2.08 ±0.27 325.36 ±30.16 79.35 ±0.83 Fig. 3. Molecular weight distribution of the liquefied bio-polyols. F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 5 parameters are closely related, since an increase of the acid number decreases the hydroxyl number of the polyol [46]. This correlation is observed for the obtained bio-polyols, where those with the highest I OH index (EPR CG and PPR CG ) presented the lowest A n value. The higher A n index observed in EPE CG and PPE CG compared to EPR CG and PPR CG resulted from the use of sulphuric acid as reaction catalyst. The highest I OH values of EPR CG and PPR CG compared to EPE CG and PPE CG can be explained, firstly, by the absence of catalyst in the reaction, which decreased the I OH index [47]. On the other hand, the higher amount of glycerol used in the formulation of EPR CG and PPR CG bio-polyols contributed to the increase of the I OH index [48]. The synthesis of PUs requires different functionalities depending on the final application, as shown in Scheme 1. Thus, for the synthesis of rigid PUs, high functionalities between 3 and 8 are preferred to produce crosslinks that reinforce the structure, while for elastic PUs, such as flexible foams, elastomers or adhesives, among others, the desired functionalities are between 2 and 3. Such low functionalities result in materials with low crosslink density that allow the mobility of the polymer chains [40]. Consequently, the bio-polyols synthesised for rigid PU applications showed appropriate functionalities of 4.16 in the case of EPR CG and 3.14 for PPR CG . In the case of the bio-polyols for elastic PU applications, while PPE CG had an adequate functionality of 2.08, EPE CG was slightly above 3. However, considering its I OH and M w values, it could be considered suitable for the manufacture of elastic PUs. The chain derived from a hydroxyl group, or the equivalent weight of the polyol, is also a relevant parameter to be considered (Eq. (1)). A short chain implies a higher density of urethane bonds and therefore more cohesion between them, mainly through secondary hydrogen bonds. This, together with the high functionality, results in a rigid structure. On the other hand, a long chain decreases the concentration of urethane bonds, decreasing the cohesion between them, and together with a low functionality and high mobility of the main polyol chain, resulting is an elastic PU [49]. EW =56.1⋅100 Corrected IOH (1) As expected, according to the data summarised in Table 3, the biopolyols EPR CG and PPR CG , showed a low EW, which is adequate for the synthesis of rigid PUs, while bio-polyols with lower I OH value, EPE CG and PPE CG , presented a higher EW more suitable for polyurethanes with a more flexible structure. While all the studied properties are relevant to obtain bio-polyols with the appropriate characteristics for PU production, the yield constitutes another key factor and is equally important to make the process industrially feasible. In the lignin liquefaction process, the catalyst has a significant impact on the reaction yield, since in the presence of an acid catalyst the lignin repolymerisation reactions are increased, generating a higher amount of solid residue, and therefore reducing the yield [18]. The presence of water in the reaction can promote the fragmentation of lignin into smaller and more reactive molecules through hydrolysis increasing the reaction yield [25]. This water can be formed as a byproduct of the condensation of glycerol into polyglycerol during the liquefaction reaction, therefore, the more glycerol in the medium, the more water and the higher the yield [50]. According to the results obtained and summarised in Table 3, EPR CG and PPR CG bio-polyols with a higher amount of crude glycerol and without catalyst showed higher yield than EPE CG and PPE CG bio-polyols where catalyst and lower concentration of glycerol were used. The use of CG in liquefaction reactions can reduce the yield of such reactions. This is due to the presence of impurities such as FA and FAMEs and a lower amount of glycerol in the reaction medium [45]. However, the presence of acid catalyst in the medium showed more influence on the reduction of the liquefaction yield. Thus, EPE CG and PPE CG with a lesser CG content showed a substantially lower yield than EPR CG and PPR CG , which can only be explained by the greater influence of the acid catalyst. Furthermore, it was also observed that with a very similar PEG/CG ratio, but with a higher catalyst concentration, EPE CG showed lower yield than PPE CG . Thermogravimetric analysis of the resulting bio-polyols was performed to determine the relationship between chemical structure and degradation. The TGA thermograms and their corresponding derivative thermogravimetric curves (DTG) are shown in Fig. 4. Based on the DTG curves, it was concluded that EPE CG and PPE CG bio-polyols showed four degradation zones, while only three degradation zones were observed in EPR CG and PPR CG bio-polyols. In addition, EPE CG , PPE CG and PPR CG showed an unidentified degradation zone which was not observed in EPR CG sample. This degradation stages are summarised in Table 4. The first one, between 30 and 110 ◦C, is associated with moisture or the presence of solvent in the sample. It could be observed a weight loss in all cases in this degradation zone. However, it was clearly visible a higher weight loss in PPR CG and PPE CG samples, which could be due to the presence of the acetone employed in the process. The second degradation region (115–270 ◦C) corresponds to the degradation of glycerol [51]. The third degradation region takes place between 275 and Scheme 1. Hypothetical crosslinking in lignin-based rigid and elastic polyurethanes. F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 6 335 ◦C for EPE CG and PPE CG and it is related to the degradation of PEG [51]. It should be noted that the degradation of the β – O – 4 and C – C bonds of lignin occurs between 250 and 400 ◦C. Nevertheless, depending on the M w and the repolymerisation degree of the lignin molecule, this degradation could happen at different temperatures [52]. Thus, in the case of EPE CG and PPE CG , with higher M w , lignin degradation (4th degradation area) was observed between 315 and 440 ◦C, whereas in bio-polyols with lower M w (EPR CG and PPR CG ), this degradation takes place at lower temperatures. This can be caused because, in the latter, the degradation zone of PEG and lignin overlap, showing a single degradation zone, i.e. the third degradation region for these bio-polyols. Finally, between 690 and 775 ◦C, a small weight loss was observed, possibly due to the presence of inorganic impurities in the samples. However, the origin of this peak could not be clearly determined. Finally, to determine the viscoelastic properties and the fluid behaviour of the bio-polyols, a rheological study of the samples was performed. To study the viscoelastic behaviour, an oscillatory test was carried out comparing the storage module (G ′ ) with the loss module (G ″ ). Based on the analysis of this test, illustrated in Fig. 5, it was concluded that the bio-polyols exhibited a liquid behaviour, since in all cases the G ″ was higher than the G ′ over the whole frequency range. Furthermore, as expected, the value of the modules increased with increasing molecular weights, EPE CG and PPE CG showed higher modules than EPR CG and PPR CG [53] . The fluid behaviour of bio-polyols, as well as their viscosity, was studied through a rotational test analysing the relation between the viscosity ( η ), shear stress ( τ ) and shear rate (˙γ). These parameters were fitted to the Ostwald-de Waele power-law equation (Eq. (2)), where the fitting parameters (n and k) are dependent on the nature of the fluid and the measurement conditions. τ =k⋅˙γn(2) Thus, the fluid can be Newtonian, pseudo plastic or dilatant when the flow index parameter (n) is n =1, n <1 and n >1, respectively. The value of the parameter k, known as the consistency index, which is associated with the apparent viscosity of the fluid at a shear rate of 1 s −1 , increases with increasing viscosity. A summary of the data provided by the software is presented in Table 5, and the obtained flow curves are shown in Fig. 6a and b. R 2 values >0.99 were obtained in all cases, indicating that the rheograms were well adjusted, and, therefore, the selected model to evaluate the rheological behaviour was adequate. In all cases, since the flow index (n) values were equal or very close to unity, it was concluded that the bio-polyols were Newtonian-type. This behaviour was observed in Fig. 6a, where the viscosity remained constant regardless the applied shear rate (˙γ) and was confirmed by studying the behaviour between the shear stress ( τ ) and shear rate (˙γ) (Fig. 6b). It was observed that, in all cases, the graphical representation of these parameters resulted in a straight line which passed through the origin and whose slopes were equal to the k value of each bio-polyol. Furthermore, as expected, the viscosity of the bio-polyols with higher M w were higher than those with lower molecular weights [54]. Thus, the viscosity of each bio-polyol was in concordance to its consistency index (k) value [55], being 1.4299 Pa⋅s for EPE CG , 0.7290 Pa⋅s for EPR CG , and 0.9352 Pa⋅s and 0.7927 Pa⋅s for PPE CG and PPR CG . Therefore, as these values were lower than 300 Pa⋅s, the bio-polyols were suitable for PU production. Fig. 4. TGA thermograms and DTG curves of liquefied bio-polyols. Table 4 Degradation stages on of the TGA-DTG curves of the analysed bio-polyols. Sample 1st 2nd 3rd 4th 5th Tint Tm Tint Tm Tint Tm Tint Tm Tint Tm EPE CG 30–100 72 115–270 233 270–315 283 315–440 391 705–775 758 EPR CG 30–110 70 125–255 214 214–513 360 – – – – PPE CG 30–110 70 120–270 235 270–335 291 335–450 403 705–775 758 PPR CG 30–110 56 120–250 205 250–512 361 – – 690–770 740 Tint: Temperature interval; Tm: Maximum degradation temperature. Fig. 5. Storage module (G ′ ) and loss module (G ″ ) (Pa) as function of ω (rad/s) of bio-polyols. Table 5 Power-Law linear functions based on the rheological data obtained from the studied bio-polyols. Sample Function k (Pa⋅s n ) n R 2 EPE CG τ =1.4299⋅˙γ1.0176 1.4299 1.0176 0.9985 EPR CG τ =0.7290⋅˙γ0.9911 0.7290 0.9911 0.9995 PPE CG τ =0.9352⋅˙γ0.9978 0.9352 0.9978 0.9988 PPR CG τ =0.7927⋅˙γ1.0011 0.7927 1.0011 0.9939 F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 7 3.3. Effect of crude glycerol in bio-polyols parameters The results obtained by other authors by liquefying lignocellulosic biomass employing, among others, glycerol and CG as solvents are summarised in Table 6. Nonetheless, since the reaction conditions employed in this study were the optimal conditions established in our previous work, the results obtained in this work (Table 3) were first compared with the results obtained in our previous work (Table 6) [30]. The reaction yields employing CG and technical grade glycerol were very similar, however a decrease was observed when CG was used due to the presence of impurities and lower glycerol content, which decreased the reaction efficiency [45]. In addition, the lower amount of glycerol molecules in crude glycerol and the consumption of hydroxyl groups due to competitive reactions caused by impurities are responsible for the decrease of the I OH index in bio-polyols comparing to those obtained when commercial glycerol was employed [23]. The A n exhibited the same behaviour as that observed in our previous study, i.e., the higher the catalyst concentration, the higher the number of acids. Nonetheless, using crude glycerol as solvent, the A n values were slightly lower since organic impurities in the crude glycerol could lead to the consumption of acidic compounds [23]. Comparing the molecular weight obtained in this work with those obtained in our previous work, where a technical Fig. 6. (a) Viscosity ( η ) as a function of shear rate (˙γ); (b) shear stress ( τ ) as a function of shear rate (˙γ). Table 6 Different studies of lignocellulosic biomass and lignin liquefaction employing commercial or crude glycerol. Raw material Solvent Yield (%) I OH A n Mw PI f Catalyst H 2 SO 4 USE Ref. Eucalyptus globulus organosolv lignin PEG +G 98.63 ± 0.71 595.15 ± 33.92 2.74 ± 0.00 1394 ±12 3.69 ± 0.08 4.03 ± 0.15 – Rigid PU [30] 71.98 ± 1.41 253.84 ± 60.59 33.01 ± 0.00 4895 ± 325 9.37 ± 1.12 2.36 ± 0.44 5 % Elastic PU Pinus radiata organosolv lignin 98.93 ± 0.14 514.28 ± 42.70 5.63 ± 0.23 1383 ±43 3.58 ± 0.08 3.55 ± 0.26 – Rigid PU 87.56 ± 3.30 209.67 ± 3.70 30.56 ± 0.15 5408 ± 765 6.95 ± 0.91 2.91 ± 0.08 3.86 % Elastic PU Eucalyptus globulus Kraft lignin PEG +G 95.27 537.95 – 1775 3.51 – 3 % – [54] Olive tree pruning organosolv lignin PEG +G 99.07 811.8 – – – – 1 % – [57] Eucalyptus globulus kraft lignin PEG +G >86 100–660.08 0.8–10.70 1459–1990 – – 3, 6, 9 (organic acids) – [58] Enzymatic hydrolysis lignin of cornstalk PEG +G >90 191–409 – – – – 15 % – [47] Alkaline corncob lignin PEG +G 97.47 484.03 – 525 1.13 – 1.5 % PU foam [59] Kraft lignin (softwood) CG >0.61 (g/g CG) 412 5088 2.2 – – PU foam [27] Organosolv lignin (sugarcane bagasse) 224 7867 4.9 – Lignosulphonate (hardwood) 592 7384 3.3 – Acetone soluble lignin CG +1,4BDO 72.64 ≈1100 ≈4 – – – – PU foam [29] Kraft pine lignin CG +1,4BDO 93 670 – – – – – PU foam [25] Rapeseed cake PEG +G 84 586 – – – – 80:20:3 (PEG: G: H 2 SO 4 ) – [51] Data seeds 96 395 – – – – Olive stone 92 496 – – – – Corncob 91 504 – – – – Apple pomace 97 428 – – – – Wheat straw – 350 28 1270 1.22 – PU foam [48] Taiwan acacia PEG +G 95.2 310 25.6 – – – 3:1:0.09 (solvent: biomass: H 2 SO 4 ) Adhesives [60] China fir 98.4 287 38.0 – – – Norway spruce EG 99.7 825–623 48.2–47.8 – – – 4.5 g Adhesives [61] 1,4-Butanediol (1,4-BDO), glycerol (G), crude glycerol (CG), ethylene glycol (EG). F. Hern´ andez-Ramos et al. International Journal of Biological Macromolecules 247 (2023) 125855 8 grade glycerol was used for the liquefaction process, it is worth noting that there was an increase on the molecular weight in all cases, particularly in EPE CG bio-polyol. This can be explained by the polymerisation reactions between glycerol, FFA and FAMEs [56]. Consequently, the polydispersity indexes of the CG synthesised bio-polyols were higher than those obtained with technical grade glycerol. This increase in the M w and PI was in agreement with that reported by Hu et al. [45]. It could also be concluded that the functionalities were also similar in all cases and appropriate to the class of PUs for which they were synthesised. However, in the case of EPE CG , the functionality was slightly above the limit value for the synthesis of elastic PU. As for the viscosities, it should be noted that, as expected, with the increase in the molecular weight of the samples obtained with crude glycerol, the viscosities also increased or remained practically equal, as in the case of PPE CG . EPR CG and PPR CG bio-polyols yields were within the typical values obtained for the liquefaction of lignocellulosic biomass or lignin, despite using CG instead of technical grade glycerol (Table 6). However, the yields obtained by Tran et al. [29] liquefying soluble acetone lignin using CG and 1,4-BDO in the presence of sulphuric acid were very similar to those obtained for EPE CG and PPE CG . It is noteworthy that, in most of the studies that are summarised in Table 6, polyols were used for the manufacture of foams. In such studies, the I OH varied between 100 mg KOH/g and 811.8 mg KOH/g. Therefore, the I OH values obtained for EPR CG and PPR CG bio-polyols, which are intended for rigid PUs, were found to be in good concordance with the values of the literature. Few of the studies summarised in Table 6 indicated the PI of polyols, and those that did so, referred to polyols for the synthesis of PU foams. The PI obtained for EPR CG and PPR CG in this work are in line with those reported by other authors in such studies. Lee et al. [60] liquefied Taiwan acacia and China fir employing PEG and glycerol with sulphuric acid as catalyst to produce polyols. Such polyols, which showed a I OH of 310 and 287 (mg KOH/g), were used to synthesise PU adhesives. EPE CG and PPE CG bio-polyols, synthesised to be used in the elaboration of elastic PUs, showed a very similar I OH to those obtained in that study. Similarly, Jiang et al. [61] synthesised PU adhesives employing polyols from liquefied lignocellulosic biomass (Norway spruce). However, in this case the I OH was significantly higher (825 and 623 mg KOH/g). As for PI, unfortunately, to the best of our knowledge, no studies were found indicating the PI value for polyols obtained through the liquefaction of lignin for PU adhesive synthesis. Likewise, no papers were found indicating the functionality of polyols obtained through this process. Finally, the A n value was, in all cases, within the usual range (0–40 mg KOH/g) for this type of process. 4. Conclusions In this paper, bio-polyols suitable for the manufacture of rigid and elastic PUs were successfully prepared through the liquefaction of organosolv lignin from Eucalyptus globulus and Pinus radiata. The liquefaction reaction was carried out employing a microwave reactor using the optimal reaction conditions established in a previous study. Polyethylene glycol and crude glycerol, which was obtained from used vegetable oil, were utilised as solvents for this process. The effect of crude glycerol is obvious when comparing the results obtained with those obtained in our previous work. The liquefaction yields decreased in all cases due to the lower amount of glycerol in the mixture. Thus, the yields obtained for bio-polyols produced for rigid PUs where no catalyst was employed were higher than 90 %, whereas for bio-polyols intended for elastic PUs, where sulphuric acid was used as catalyst, the yields were significantly lower, between 70 and 80 %. The I OH of bio-polyols were also affected by the lower glycerol content when crude glycerol was used. The I OH values obtained in this study were 554 and 383 (mg KOH/g) for EPR CG and PPR CG bio-polyols respectively and 228 and 173 (mg KOH/g) for EPE CG and PPE CG. The A n value of the samples was in the expected range for this type of polyols. Regarding the functionalities, although in the case of EPE CG the obtained value was slightly above the desired one, it could be said that the functionalities were adequate for the manufacture of the type of PU for which they were intended. This assumption was confirmed by taking into account these results together with the rest of the parameters that were studied. Overall, even though the results of some parameters have worsened, it can be said that crude glycerol recovered from used oil is suitable for the manufacture of biopolyols to produce elastic and rigid PUs. CRediT authorship contribution statement Fabio Hern´ andez-Ramos: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Validation, Writing – original draft, Writing – review & editing. María Gonz´ alez Alriols: Supervision, Validation, Data curation, Visualization. M. Mirari Antxustegi: Supervision, Validation, Data curation, Visualization. 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