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Comparison of dehydration methods for untreated lignin resole by hot air oven and vacuum rotary evaporator to synthesize lignin-based phenolic foam

We investigate two different dehydration methods to determine their suitability for preparing resoles for foam synthesis. A simplified process for synthesizing lignin foam (LF) from lignin resole (LR) dehydrated in a hot air oven (HAO) is compared with that dehydrated using a vacuum rotary evaporator (VRE). First, the LR formulation is prepared by mixing phenol with untreated lignin (0%–15% by weight), and subsequently, the prepared LRs are dehydrated using an HAO and a VRE. We find that for the same dehydration time, both techniques yield LRs with the same chemical compositions; however, the HAO technique affords a moisture removal of 13–17% by weight, whereas the VRE technique removes 9–12% moisture by weight. The LR obtained by the HAO is more viscous and maintains a circular shape after being dropped on a plate. In our experimental synthesis of LF containing VRE resole, biofoam is not formed owing to insufficient viscosity, whereas biofoam is obtained with the HAO resole. The synthesized LF exhibits a density range of 44.96–85.68 kg/m3 and a compressive strength of 103.28–152.27 kPa. Scanning electron microscopy investigations show that the morphology of the foam is a closed-cell structure. The simplified synthesis of LF from the HAO-treated resole offers significant advantages over the complexity of the conventional VRE approach in terms of equipment cost and energy consumption. The resulting foam exhibits a thermal stability and thermal performance comparable with the counterpart properties of phenolic foam.


Introduction
Phenolic foam (PF) is a popular polymer material that is used for thermal insulation. PFs are mostly applied in high-rise buildings, airports, and chemical transport pipelines because of their light weight, low thermal conductivity, and high thermal stability [1]. The general foaming of PF is usually realized using resin. The mixture is prepared in two steps [2]: first, synthetic phenolic resole (PR) is obtained by the reaction between phenol and formaldehyde. Water in the obtained resole solution is removed by reducing the pressure during evaporation (using a vacuum rotary evaporator, VRE) until 80% solid content is achieved [3]. While VRE-based resole dehydration has been preferred by researchers such as Li et al. (2017) and Wang et al. (2019) [4,5], this technique suffers from certain drawbacks when compared with simpler techniques such as hot air oven (HAO) drying. The equipment required for VRE is expensive, and VRE operation is complex and energy intensive because it requires pumping and pressure and temperature control mechanisms. In the second step of preparing PF, emulsifiers and other additives are added to the mixture, which is concurrently expanded and cured in an oven operated at a temperature of 50-80 C [6].
In this context, we note that the production of phenol from crude oil is also severely detrimental to the environment. The resulting phenol is nonrenewable and unsustainable, and the cost of production is also affected by the oil-price volatility [7,8]. Meanwhile, phenolic compounds from natural sources can be used as alternatives to reduce phenol use in the petroleum industry. Natural phenolic compounds such as phenolic acids from gallic acid, acetophenones from gallacetophenone, and phenolic compounds from tannin and lignin have been used in the development of a number of polymer products [9] such as wood composites [10], acoustic absorbers [11], resins [12,13,14,15], and foams [16].
Plant cells generally contain approximately 20-30% of lignin. Because lignin has phenolic structures, it can be used as a substitute for phenol [17]. In this regard, the depolymerization of lignin has been investigated toward increasing the number of reactive sites via catalysis [18,19] and varying the temperature [20] and reaction time to break the polymer structure [5]. The process can destroy lignin, thereby yielding 11.86-17.92% of monophenolic compounds [21,22]. The depolymerized lignin is widely applied to produce a range of products. Lignin foam (LF) is one of the products that makes use of treated lignin to replace~50% by weight of phenol [4,5]. The chemical structure of LF has been found to be similar to that of PF, with LF exhibiting densities in the range of 30.96-08 kg/m 3 and compressive strengths of 0.27-0.37 MPa [4]. The final foam product can be classified as a thermal insulator with a low thermal conductivity (0.035-0.038 W/m⋅K) [5]. However, the process of foam production is still complicated owing to the need for lignin depolymerization. In this regard, a previous study has proposed the use of untreated lignin to replace phenol in the production of foam, and reportedly, the resulting foam can be classified as a flame retardant and insulation material with a compressive strength of up to 1.01 MPa [23].
In this context, the purpose of this study was to compare the HAO and VRE methods for the dehydration of resoles for foam synthesis. Phenol was replaced by 0-15% by weight of untreated lignin to synthesize resole for use in LF synthesis. The resulting resole was monitored for its physical appearance, and the functional groups were determined. The biofoam properties such as the characteristic functional groups, density, compressive strength, morphology, thermal stability, and thermal performance were also investigated.

Synthesis of resole
Lignin resole (LR) was prepared as described by Li et al. (2017) [4] with slight modifications. The reactor setup included a three-necked flask equipped with a thermometer, additional chimney, cooling condenser, and preheated water bath operated at 85 C with a magnetic stirrer. The resole compositions with different lignin-to-phenol ratios (0:100, 5:95, 10:90, and 15:85 by weight) are listed in Table 1. The resoles with different lignin contents are designated as PR, LR-5, LR-10, and LR-15, where PR refers to the resole containing 0% lignin by weight, and LR-5, LR-10, and LR-15 refer to the resoles containing 5%, 10%, and 15% of lignin, respectively. The maximum lignin content was limited to 15% by weight because the addition of lignin beyond this amount led to an extremely high viscosity after HAO dehydration, and the resole could not be synthesized to form foam. As a representative example for formulation, to prepare LR-10, 4 g of lignin, 36 g of phenol, 10 g of distilled water, and 3.6 g of NaOH solution were loaded into the reactor and stirred for 30 min. Next, formaldehyde (62.4 g) was added dropwise into the reactor with the stirrer continuously working for 120 min. The LR was subsequently cooled to room temperature and neutralized by 5 M acetic acid to pH 5.5-6.5 with strong stirring. Two different methods were used to dehydrate the resin: VRE and HAO. The VRE was operated at 65 C and 0.08 bar, and the HAO was operated at 105 C. The dehydration time for both methods was 6 h.

Synthesis of lignin foam
The synthesis of the LF specimens from resoles LR-0 to LR-15 was carried out using the chemical compositions listed in Table 2. The synthesized LF was prepared with different lignin contents. For example, LF-15 was prepared from LR-15, which originally contained 15% lignin by weight. The LF was prepared in two stages. In the first stage, 20 g of resole, 0.8 g of surfactant, and 2 g of blowing agent were mixed in a paper cup using a stirrer. In the second stage, 2.8 g catalyst, 1.2 g water, and 1 g glycerol were manually blended in a beaker until the mixture formed a homo-crystal liquid. Subsequently, the crystal-liquid mixture was poured into a paper cup, vigorously stirred with a cream whipper for 1 min, and immediately placed in an oven operating at 80 C for 60 min.

Resole testing
The pH values of the resoles adjusted with acetic acid were measured using a pH meter (HI2020, Hanna, USA) at room temperature. In the study, we defined the solid content according to Eq. (1). The initial weight of the resole before dehydration by both methods was measured using a standard weight scale with a maximum capacity of 3100 g and accuracy of up to 0.01 g. After 6 h of dehydration, the resole weight was measured again. The moisture removed by dehydration was subsequently defined using Eq. (2). The moisture removed was determined at 2 h intervals to compare the drying rates of the two methods. The chemical functional groups of samples were determined using Fourier transform infrared spectroscopy (FTIR) (Tensor 27, Bruker, Germany). All spectra were acquired with 64 scans at a resolution of 4 cm À1 and a spectral range of 4000-650 cm À1 . The physical appearance of each resole specimen was monitored by dropping it on glass plate and leaving it undisturbed on the plate for~5 min before the visual examination of the forms of the drops with a camera (Fujifilm X-A3 digital camera, Fujifilm Japan).

Biofoam testing
The FTIR spectra of the LFs were analyzed as in the case of the resoles. All the samples were tested for the properties of interest. The density of the foams was tested as per ASTM D1622 [24], and the foam compressive strength with 10% strain was measured using a universal testing machine (UTM) (5567A, Instron, USA) with a crosshead speed of 2.5 mm/s as per ASTM D1621 [25]. A desktop scanning electron microscope (MiniSEM) (SNE-4500M, SEC, Korea) was used to examine the morphology of the samples at 50Â magnification. The thermal stability of the foam was tested using a thermogravimetric analyzer (TGA) (DTG-60H, Shimadzu, Japan) in the range of 50-650 C, a heating rate of 10 C/min, and a N 2 flow rate of 30 ml/min. The thermal performance was tested using an experimental method [26]. The PF and LF-15 samples were prepared with dimensions of 2 Â 2 Â 1 inch and placed in a wooden box with a thermal resistance test setup. Samples were heated by the radiation from a 150 W infrared lamp, and the temperature inside the box was recorded by a data logger for 120 min. Subsequently, the heat source was switched off, and the sample was left to cool at room temperature for 120 min. The porosity of foams, that is, the cumulative volume of pores between 1.70 nm and 300.00 nm, was measured using the nitrogen adsorption/desorption technique (ASPS2460, Micrometrics, USA) and calculated using the Barrett-Joyner-Halenda (BJH) method.

pH, solid content, and physical appearance of resoles
The pH, solid content, and physical appearance of the concentrated resoles prepared by the VRE and HAO methods are listed in Table 3. The pH values of the resoles were adjusted to suitable values and then were measured to be 5.31 for PR, and in the range of 5.20-5.23 for the LR-5-LR-15 samples. The solid contents of PR, LR-5, LR-10, and LR-15 after HAO dehydration were 86.36%, 85.31%, 82.88%, and 83.96%, respectively, and 90.86%, 88.42%, 90.11%, and 85.84%, respectively, after VRE dehydration. Here, we note that viscosity plays an important role in the formation of foam from the resole. Resoles with low viscosity do not have the capacity to retain the blowing agent during foam formation [23]. In contrast, resoles with excessive viscosity resist the expansion of air. Therefore, an appropriate viscosity must be chosen for foam preparation from the resoles. In this study, the viscosity of the resoles was not directly measured; it was inferred by monitoring their physical appearance ( Figure 1). We note from the table that the HAO resoles appear to more strongly maintain circular shapes than the VRE resoles. The HAO resoles exhibit a smoother texture and clearer and more well-defined rims. It can be seen from Figure 1, the VRE PR and LR-5 samples appear to contain excess phenol. Figure 2 shows the moisture removal from all the resoles, recorded every 2 h for 6 h. During the first two hours, the amounts of moisture removed by VRE from PR, LR-5, LR-10, and LR-15 are 7.07%, 10.89%, 8.17%, and 10.23% by weight, respectively, and the amounts by HAO are 6.84%, 6.24%, 9.08%, and 8.31% by weight, respectively. We note here that the moisture removed by VRE is higher than that by HAO because in the VRE case, the moisture is removed under vacuum (that is, at lower pressures), and because of the rotary action, the moisture evaporates more uniformly from the resole. In the case of HAO, water evaporates from the resole (placed in a container) only from the surface exposed to the drying environment. With continued dehydration for another two hours, the amounts of moisture removal from the PR, LR-5, LR-10, and LR-15 resoles are observed to be 8.35%, 11.09%, 9.00%, and 11.30% by weight, respectively, for VRE, and 12.16, 11.71, 16.14, and 13.67% by weight, respectively, for HAO. Between 2 h and 4 h, the amount of moisture removed by VRE changes only slightly with time, whereas the moisture removed by HAO continues to increase with time. After 6 h of drying, the amounts of water removed by VRE from PR, LR-5, LR-10, and LR-15 are 9.14%, 11.59%, 9.89%, and 12.16%, respectively, and 13.64%, 14.69%, 17.13%, and 16.05%, respectively, by HAO. Evidently, HAO affords a better performance over time, thereby indicating that this approach is more suitable for drying the resole.
Here, we note that resoles with greater amounts of lignin exhibit higher viscosity because lignin has a higher molecular weight (Mw ¼ 2000 g/mol) than phenol (Mw ¼ 94 g/mol) [4]. In our attempts to prepare foam from the VRE resole, we found that foam could not be formed from the resole owing to its low viscosity. However, a thin layer of foam was obtained and tested for functional groups for comparison with foam obtained from the HAO-treated resoles.

Functional groups of resole and foam
The functional groups and chemical bonds of the resoles and foams were determined by means of FTIR analysis. The infrared transmittance spectra of PR and samples LR-5-15, which were dehydrated by the HAO and VRE methods, are shown in Figure 3. The characteristic peaks of all the resoles show an absorption band of the hydroxyl group at 3320 cm À1 [27]. The signals between 2980 and 2805 cm À1 and at 1481 cm À1 indicate C-H stretching of the methylene bridge in the resole [28]. The peak at 1700 cm À1 can be attributed to the C¼O bonding of reactive acetic acid with hydroxyl groups by the esterification reaction [12]. The band at 1610 cm À1 corresponds to C-H bending, which can be attributed to the reaction of the resole with formaldehyde [29]. The characteristic bands of phenolic hydroxyls are observed at 1365 cm À1 , the C-O-C skeletal vibration of polysaccharide rings from formaldehyde phenolic bonds is observed at 1034 cm À1 , and C-O stretching is observed at 1005 cm À1 [30,31,32,33,34]. The adsorption peaks of C-H appear at 1112 cm À1 and between 918 and 665 cm À1 . The peak at 1112 cm À1 indicates C-H stretching vibrations in different groups of lignin [34]. C-H aliphatic bonds are detected at 880 cm À1 and the C-H aromatic bonds of phenol can be observed at 828 cm À1 , 758 cm À1 , and 694 cm À1 [27,33,35]. Figure 4 shows the infrared absorption spectra of LF-5, LF-10, and LF-15 (dehydrated by HAO and VRE); these spectra are very similar to the corresponding ones of the PF, which indicates that lignin biofoams exhibit a chemical structure similar to that of the PF. All the foams exhibit typical hydroxyl-group adsorption at 3358 cm À1 , C-H stretching in the range of 2980-2804 cm À1 and at 1472 cm À1 and 1112 cm À1 , and C¼O phenolic hydroxyl bonding at 1700 cm À1 . The bridge oxygen-carbon (C-O-C skeletal vibration) appears at 1034 cm À1 , C-O stretching at 1005 cm À1 , C-H aliphatic bonding at 880 cm À1 , and different groups related of C-H aromatic phenol bonding at 1365, 828, 758, and 694 cm À1 . All the spectra of the LRs and LFs are similar to the corresponding ones of the PR and PF, which suggests that the LRs exhibit a molecular chemical structure similar to that of pure PR, and different dehydration methods do not affect the chemical structure of the resole and foam. However, the peak at 600 cm À1 , attributed to S-C stretching, can be found only in the case of the LR, and the signal at 1219 cm À1 is assigned to the functional group C-O stretching of the syringyl ring [36,37]. This variation can be attributed to the syringyl ring being part of the lignin structure in the LR but not in the PR. Hence, the peak is not observed in the PR and PF.

Thermal stability of resole and foam
TGA analyses of PR and LR were performed to determine their thermal stabilities. The TGA curves under a N 2 atmosphere are shown in Figure 5; these curves can be divided into three stages. The first stage corresponds to moisture removal caused by the cross-linking and condensation reactions of the terminal groups below 300 C [38]. The second stage involves the second round of mass loss from PR in the temperature range of 300-450 C, attributable to the moisture loss resulting from the condensation reaction of methylene and phenolic groups with hydroxyl groups, as well as between two hydroxyl functional groups [39]. The third stage involves the degradation of phenol in the range of 450-600 C. This stage is the main weight-loss stage for resoles containing phenol and lignin owing to the further degradation of phenols into a carbonaceous structure [13,40]. We note from the figure that the mass residuals of PR, LR-5, LR-10, and LR-15 are 0.17%, 4.44%, 16.21%, and 5.22% by weight of the resoles, respectively. It is evident that at 650 C, the mass residual of LR is higher than that of PR. The addition of lignin is equivalent to adding aromatic groups to the resole, rendering the resole more thermally stable [13].
The thermal stabilities of PF, LF-5, LF-10, and LF-15 (HAO) are displayed in Figure 6. These results indicate a three-stage pyrolysis of the foams. In the first stage, foams are degraded in the temperature range of 0-130 C, during which the moisture in the samples is removed [28]. In the second stage, in the temperature range of 130-250 C, decarboxylation of air occurs in the pores [41,42]. In the third stage, the degradation of foams occurs under a nitrogen atmosphere at temperatures in the range of 250-550 C. In this temperature range, the terminal groups are destroyed, and the bridged methylene of phenols decomposes into carbonaceous structure [4,23]. The mass residuals of PF, LF-5, LF-10, and LF-15 at 550 C are 0.32%, 2.99%, 6.92%, and 2.29% by weight of the foams, respectively. A comparison of the carbon residuals of PF and LF-5-15 clearly reveals that the residual weight after the burning of PF is lower than that of the LF. This can be explained by the presence of lignin, which has a more stable aromatic structure than phenol. In addition, lignin is known to be a thermostable polymer [41,43].   to afford a phenol anion. The reaction between the phenol anion and formaldehyde yields a methylol anion by C-C bonding at the ortho position of phenol [27]. The methylol group can react with the phenol anion or other methylol anions by ether linkage as mono-, di-, and poly-nuclear methylol phenol, all of which have more than one CH 2 OH functional group [2]. Lignin, which contains phenolic, hydroxyl, methyl, and thiol groups, can be used to replace phenol in the synthesis of LR because its chemical structure is similar to that of phenol [44]. Figure 7(b) shows a possible mechanism pathway of LR synthesis based on our analysis and previous studies [4]. In the synthesis of LR, functional groups are crosslinked, as in the case of the PR. In this process, formaldehyde reacts with the phenolic hydroxyl group of lignin via a hydroxymethylation reaction to afford ortho-formaldehyde of the lignin phenolic compound. This compound combines with phenol anions and methylol anions from the PR via crosslinking between methylene and ether linkages to yield LR (Figure 7(c)) [27]. Table 4 lists the density and compressive strength of the LF and PF prepared with the HAO resole. The density values of the "control" PF, LF-5, LF-10, and LF-15 are 35.91, 44.96, 62.40, and 85.68 kg/m 3 , respectively. It should be noted that increased lignin addition correspondingly increases the LF density. Similarly, increased amounts of lignin also correspond to increased LR density. A higher LR density results in a higher LR viscosity. Because this high viscosity obstructs the expansion of air during LF synthesis, the obtained LF exhibits a high density [4]. The compressive strengths at 10% strain of PF, LF-5, LF-10, and LF-15 are 80, 103.28, 109.78, and 152.77 kPa, respectively; PF has a lower compressive strength than all the LF samples. It is noteworthy that the compressive strength of the foams increases with the amount of lignin because this strength is proportional to the density. Foams with lower densities exhibit greater porosity, with a corresponding reduction in compressive strength [45]. Moreover, we note here that the density and compressive strength of the LFs in this study are comparable with those of commercial foam and those reported in previous studies [46].

Morphology of phenolic foam
The SEM images of the PF and HAO-treated LF-5, LF-10, and LF-15 samples are shown in Figure 8. We note that the foam exhibits a closed-cell-type structure. Moreover, some of the pores are interconnected because the expanding air in the foam ruptures the thin walls between the pores or because of possible wall damage during foam cutting [23,47]. Our analysis of the pore sizes of PF, LF-5, and LF-10 revealed that the pore size increased with the amount of lignin in the foam. This result can be explained by the fact that the resole with higher lignin exhibits greater viscosity, and is hence more capable of retaining the blowing agent, which aids pore expansion in the foam [23]. The pore size of LF-15 is on average slightly smaller than that of LF-10 because  LR-15 exhibits the highest viscosity ( Figure 1). The surface tension of the resole increases with its viscosity, which resists the expansion of the pores in the foam [5]. That is, greater porosity (and hence improved thermal performance) can be achieved at the expense of compressive strength. Therefore, the LR must possess a suitable viscosity for foam synthesis.

Thermal resistance performance
Next, LF-15, whose compressive strength was the highest among all samples, was tested to determine its thermal performance with PF as the control in the temperature range of 31-66 C. The foams were alternately tested in the same reactor. The variation curves of the inner air temperature at the center of the wooden box are shown in Figure 9. The tested foams (LF-15 and PF) received heat from the source for 2 h, and the maximum air temperatures in the cases of LF-15 and PF were 34 C and 33.5 C, respectively. After 2 h of heating, the foams were left in the reactor to cool at room temperature for 2 h, and the air temperatures were measured to be 31.22 C and 30.58 C for LF-15 and PF, respectively. The air temperature differences after heating and after cooling were only 0.5 C, and 0.64 C, respectively, thereby indicating a comparable thermal performance between LF-15 and PF. The morphology tests of LF-15 revealed that an increase in the lignin amount increased the   pore size of the foam; however, the cumulative volumes of the porosity of LF-15 and PF were 0.010918 cm 3 /g and 0.037087 cm 3 /g, respectively. This result explains why PF has a greater thermal resistance than LF-15.

Conclusions
We experimentally synthesized LF from LR by substituting phenol with untreated lignin (0-15% by weight) using two different methods of resole dehydration, namely, the VRE and HAO methods. We found that the simple HAO method could remove greater amounts of moisture from the resole than the VRE method for the same operating time. The resole obtained by the VRE method could not be used to make foam because of its low viscosity resulting from the excess moisture retained in the resole. The two dehydration methods did not transform the chemical functional groups in the resoles and foams. The addition of lignin increased the density (44.96-85.68 kg/m 3 ) and compressive strength (103.28-152.27 kPa) of HAO-treated LF. Our SEM examinations of the LF morphology revealed the foam to possess a closed-cell-type structure with less porosity than the PF. The biofoam with lignin content exhibited thermal stability up to a temperature of~250 C. The thermal performance of LF-15 was comparable with that of PF. Our findings prove that the preparation of LR by HAO dehydration is a feasible alternative to the complex VRE dehydration process. This technique may be used for the production of lignin insulation foam for commercial purposes. In addition, the use of renewable lignin can form an alternative approach for producing biophenolics from natural resources. However, the foam obtained from LR should further be tested for properties such as thermal conductivity, flame resistance, and the limiting oxygenation index necessary for classification according to the current standards.

Declarations
Author contribution statement Pattaraporn Suttaphakdee: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Sutasinee Neramittagapong: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Somnuk Theerakulpisut: Analyzed and interpreted the data; Wrote the paper.
Arthit Neramittagapong: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.
Tinnakorn Kumsaen: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Funding statement
This work was supported by Khon Kaen University (The Incubation researcher project (2556)), the Faculty of Engineering (PhD.Che-1/2561) and the Research Center for Environmental and Hazardous Substance Management (EHSM) (2564).

Data availability statement
Data included in article/supplementary material/referenced in article.