Polyurethane Foams and Bio-Polyols from Liquefied Cotton Stalk Agricultural Waste

sustainability

Article Polyurethane Foams and Bio-Polyols from Liqueﬁed Cotton Stalk Agricultural Waste

Qingyue Wang * and Nuerjiamali Tuohedi Graduate School of Science and Engineering, Saitama University, Saitama City, Saitama 338-8570, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-48-858-3733; Fax: 048-858-3733

Received: 9 April 2020; Accepted: 19 May 2020; Published: 21 May 2020

Abstract: Cotton is planted on a large scale in China, especially in the Xinjiang Region. A large amount of agricultural waste from cotton plants is produced annually, and currently poses a disposal problem. In this study the product after liquefaction of cotton stalk powder was mixed with diphenylmethane diisocyanate to prepare polyurethane foams. The eﬀects of the liquefaction conditions on the properties of the polyols and polyurethane foams produced using cotton stalk were investigated. The optimal processing conditions for the liqueﬁed product, considering the quality of the polyurethane foams, were studied as a function of the residue fraction. Bio-polyols with promising material properties were produced using liquefaction conditions of 150 ◦C, reaction time of 90 min, catalyst content of 3 wt.%, and 20 w/w% cotton stalk loading. We investigated the optimal processing conditions for producing bio-foam materials with mechanical properties comparable to those of petroleum-based foam materials. This study demonstrated the potential of cotton stalk agricultural waste for use as a feedstock for producing polyols via liquefaction. It was shown that polyethylene glycol 400 (PEG400) and glycerin can be used as alternative solvents for liquefaction of lignocellulosic biomass, such as cotton stalk, to produce bio-polyol and polyurethane foams.

Keywords: liquefaction; cotton stalk; bio-polyol; polyurethane foams; biodegradable; thermal properties

1. Introduction The current concerns for the environment and limited availability of petrochemical resources have resulted in the use of lignocellulosic biomass attracting considerable attention, as it is biodegradable, readily available, and renewable [1]. Lignocellulosic biomass from forestry and agriculture could be an abundant feedstock for producing biofuels, such as fuel oil, and bio-based chemicals. Bio-polyols obtained using the thermochemical method, e.g., pyrolysis or liquefaction of lignocellulosic biomass, have promising properties for manufacturing formaldehyde resins [2,3] and bio-based polyurethane foams [4,5] with comparable attributes to their petroleum analogs. Liquefaction of biomass is an eﬀective method for processing biomass resources, which is classiﬁed into two categories based on the reaction conditions [6]: (1) a process requiring a high reaction temperature and pressure-proof reactor; and (2) a process performed using reactive solvents, i.e., phenols, polyhydric alcohols, and carbonates, in the presence of strong acid at relatively low temperature and atmospheric pressure. The liquefaction conditions for woody biomass at temperatures below 200 ◦C have been optimized in the second category [7–9]. These processes are based on solvolysis of lignocellulosic materials, where the liquefaction products can be used as raw materials for producing valuable polymer products. The pore size and density of foams produced from bio-based feedstocks can be easily manipulated to meet the needs of various applications. Polyols (polyether and polyester) and isocyanate are the two main raw materials for producing polyurethane foam, and are currently obtained from petroleum-based

Sustainability 2020, 12, 4214; doi:10.3390/su12104214 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 4214 2 of 13 products [10]. The major drawback of petroleum-based products is that they are non-renewable and not biodegradable. Due to increasing concerns over dwindling fossil resources and the environmental impact of their use, obtaining polyols from renewable biomass is an attractive alternative for the polyurethane industry. Most biomasses consist of cellulose, hemicellulose, and lignin, which contain more than one hydroxyl group in the molecular chains, which can be used as an alternative to petroleum-based polyols for preparing polyurethane materials [11]. Agricultural wastes, such as cornstalks and wheat straw, are eco-friendly and readily available feedstocks for sustainable production of energy and chemicals [12]. We previously achieved liquefaction of woody biomass using mixed solvents of alcohol and water, without requiring a catalyst [13]. The use of agricultural residues for producing high-value bio-products can help build a sustainable economy while having a signiﬁcant positive impact on the environment. To date, there has been little research on liquefaction of cotton stalks for bio-polyols and the subsequent production of polyurethane foams. Cotton stalks are rich in cellulose and have the potential to be used as a feedstock for the production of paper, industrial fuel, and composites. Currently, cotton stalks and other agricultural waste are generally burned, which wastes potential resources, pollutes the environment, and results in health problems for farmers. A large amount of energy and resources are being used for the current economic development in China. The increasing use of fossil fuels and burning of agricultural waste have exacerbated pollution problems, especially in urban areas. Therefore, it is meaningful to study methods for increasing the value of all biomass, while reducing environmental pollution. Previous studies have focused on the applications of cotton stalks as a renewable energy resource, which can be converted into bioethanol using available pretreatment methods to meet biofuel requirements. However, few studies have focused on cotton stalks as a feedstock for polyurethane materials. The aim of this study was to demonstrate a process for liquefying cotton stalk powder using polyethylene glycol (PEG) 400 and glycerin as liquefaction solvents.

2. Materials and Methods

2.1. Materials

Cotton stalk samples were obtained from Toksun farm (42.79◦ N, 88.83◦ E), which is a part of Turpan Agriculture, Xinjiang Province, China. The cotton stalk was air-dried, ground, and then sieved to give fractions with a particle size below 250 µm. It was then dried for 24 h in an oven (model: AS ONE ON-300) at 105 ◦C before use, and used in the way according to Wang et al. [12]. The chemical composition of the cotton stalk is shown in Table1. We measured the cellulose, hemicellulose and lignin of cotton stalk in the same way as a previous study [14]. Additives used in the polyurethane foaming process included polymeric methylene-4,40-diphenyl diisocyanate (PMDI; Japan Polyurethane Ind., Ltd., Tokyo, Japan), silicone surfactant (SH193; Toray Dow Corning Silicone, Ind. Ltd., Tokyo, Japan ), and dibutyltin dilaurate (Wako Pure Chemicals, Ind. Ltd., Japan). All other reagents were of analytical grade (Wako Pure Chemicals, Ind. Ltd., Japan). All chemicals used in this study were reagent grade and used without further puriﬁcation.

Table 1. Chemical composition and ultimate analysis of the raw cotton stalk material.

Methods Content Cotton Stalk Lignin 25.3 Hemicellulose 17.4 Composition analysis (wt.%) a-Cellulose 48.8 Ash 6.8 The mean values were calculated based on experiments repeated more than three times. Sustainability 2020, 12, 4214 3 of 13

2.2. Experimental Methods

2.2.1. Liquefaction of Cotton Stalk with PEG and Glycerin PEG and glycerin were used as the liquefaction reagents with a ratio of 4/1 (w/w%), with a solvent to cotton stalk ratio of 3/1– 6/1 (w/w%). The required amount of catalyst was added, and all reagents were mixed in a 500 mL three-neck reaction flask equipped with a stirrer and reflux condenser [14]. Then, the reaction flask was immersed in a preheated oil bath at five different temperatures: 120 ◦C, 130 ◦C, 140 ◦C, 150 ◦C, and 160 ◦C. After different liquefaction reaction times of 15, 30, 60, 90, 120, 150, 180 or 300 min, the liquefied cotton stalk samples in the flask were collected and placed in a vial for further analysis. The vial was immersed in cold water to stop the liquefaction reaction. To filter the residue, 10 mL of methanol was added to the vial and combined well using a shaking apparatus. Filtering was performed by using a pump (EVP-1000, KNF, EYELA, Japan) and filters (C5, ADVANTEC). Then, the filtered residue was dried for 24 h in an oven at 105 ◦C. The residue fraction was calculated by dividing the weight of the residue in the sampling vial by the initial weight of cotton stalk powder added to the reaction flask.

2.2.2. Characterization of Liquefaction Products The functional groups of liquefied cotton stalk samples were characterized using Fourier-transform infrared spectroscopy (FTIR; Jasco Co. Ltd., Japan) in the same way as a previous study. The liquefied sample was clipped thinly by potassium bromide (KBr) plates, and the pellet was placed in the FTIR instrument for measurement. The residue from liquefaction experiments was also characterized by FTIR after preparing pellets with 1 mg of sample and 100 mg of KBr. The infrared spectrum was 1 1 recorded in the 2000 to 500 cm− range by averaging 32 scans with a resolution of 4 cm− [12]. The average molecular weights of liquefied cotton stalk samples were measured using a gel permeation chromatography (GPC) system equipped with a column (KF-802, Shodex, Co. Ltd., Japan), an HPLC pump ( Pump PU-2080, JASCO Co. Ltd., Japan), a column thermostat (CO-2060, Jasco, Co. Ltd., Japan), and a refractive index-detector (RID) (RI-2031, Jasco, Co. Ltd., Japan). The analytical conditions are shown in Table2[ 12]. Exactly 10 mg of the liquefied cotton stalk samples was diluted with 10 mL of tetrahydrofuran (THF), and then analyzed by GPC [15].

Table 2. Gel permeation chromatography (GPC) analysis conditions.

GPC Conditions Mobile phase Tetrahydrofuran Flow rate 1 mL/min Dilute concentration of sample 0.1 w/v % Tetrahydrofuran column temp. 40 ◦C Volume of sample loop 100 µL

The viscosity of liqueﬁed cotton stalk was measured using a viscometer (SV-10/SV-100) and determined at 25 0.1 C. The vibrating viscometer used the tuning-fork vibration method to calibrate ± ◦ using standard puriﬁed water before measurement. For precise measurement, we left the sample at room temperature for 2 h, put 35 mL in the glass sample cup (AX-SV-35), then left the sample as is for a while, after placing the sensor plates and the temperature sensor, to ensure no changes to the sample temperature [14]. The hydroxyl value of the liquefied wood is an important factor as it determines the mechanical properties of the prepared resin. The acid number and hydroxyl values of liquefied wood were measured adapted from Erta¸set al. [16]. The hydroxyl value was calculated with acid values. The hydroxyl and acid value of liquefaction wood were determined by titration. Blank titration was conducted using the same procedure. Sustainability 2020, 12, 4214 4 of 13

2.2.3. Preparation of Polyurethane Foams Liqueﬁed cotton stalk (20.0 g), 1,4-diazabicyclo [2.2.2] octane (0.400 g), dibutyltin dilaurate (IV) (0.400 g), distilled water (0.400 g), and silicone foam stabilizer (0.800 g) were added to a polypropylene cylindrical container in this order. Then, the mixture was stirred at 500 rpm for 10 min using a stirrer. Then, the stirring speed was increased to 2000 rpm, and the required amount of polyphenyl methane polyisocyanate (PMDI) was quickly added with a syringe. After stirring for 25 s, the stirring rod was removed from the solution. After foaming, the materials were cured at room temperature for at least 2 d and used for various analyses.

2.2.4. Characterizations of Polyurethane Foams The thermal behavior of polyurethane foam is very important because it is mainly used as a thermal insulator. The thermal decomposition of polyurethane foams was analyzed by simultaneous thermogravimetry/differential thermal analysis (TG-DTA; DTG-60 Shimazu Corporation) to evaluate their thermolysis and combustion properties. The samples (10.00 mg) were tested at a heating rate of 20 ◦C/min from room temperature to 800 ◦C under a flow of Ar gas (200 mL/min) [17]. Cylindrical test specimens with a diameter of 35 mm and height of 20 mm were cut from three areas of the synthesized polyurethane foam. The samples were cured at 48 ◦C and a relative humidity of 50% for 48 h. Then, the volume and weight were measured to determine the bulk density (calculated ignoring surface and internal pores) following JIS-K7222 [18]. The compressive strength of a material is its ability to withstand a compressive load, where the maximum stress can be determined from such measurements. The compressive strength is obtained by dividing the original cross-sectional area of the test piece subjected to compression by the maximum load. The compressive strength of a material that is not crushed when compressed is determined by deforming the material by an arbitrary amount. The compressive strength (σ) (kPa) was measured according to JIS-K7220 [18]. F σ = m 103 (1) A0 × where Fm is the maximum force reached for a 10% deformation rate (N), and A0 is the initial cross-sectional area of test piece (mm2). Polyurethane foam specimens were immersed in liquid nitrogen and frozen, and then fractured into two pieces using pliers. The pieces were fixed to a sample holder using carbon tape, and then coated with Au–Pd using a vacuum sputter coater to impart conductivity to the sample surface. The coated sample was placed in the sample chamber of a scanning electron microscopy (SEM; Hitachi, S-2400) system and observed at an accelerating voltage of 15 kV.

2.2.5. Biodegradation of Polyurethane Foams Soil embedding tests were used to determine the rate of biodegradation of polyurethane foam following procedures given in the literature [17,19]. Samples were cut into specimens of 20 (width) 20 (length) 20 mm (thickness). The specimens were weighed and then embedded in × × soil with certain microorganisms for a period of up to one year. The PUF samples were put into the open space at intervals of 12 and 3 rows at a depth of about 5 cm from the ground at 20 cm intervals. The samples were unearthed at intervals of 1 month, then soil was washed with distilled water, sonicated for 10 min, and then dried in an oven at 105 ◦C. The weight was measured and the weight loss rate of PUF before and after embedding was calculated. The average weight loss of each type of sample was calculated from the average of 3 specimens. The average results are given within the experimental error of < 0.5 wt.% [17]. Eventually, the percent weight loss (WL) was calculated ± using the following equation: W WL = d 102% (2) W0 × Sustainability 2020, 12, 4214 5 of 13

where W0 is the oven dry weight of the specimen (g) before soil burial test, and Wd is the oven-dry weight of the sample after soil burial test (g).

3. Results and Discussion

3.1.Sustainability Effects of Liquefaction2020, 12, x FORConditions PEER REVIEW on the Polyol Properties 5 of 13 The residue fraction is frequently used as an index to quantify the optimal liquefaction conditions. 3.1. Effects of Liquefaction Conditions on the Polyol Properties The residue is the unreacted part of the biomass samples or the unresinified stuff generated when a condensationThe residue reaction fraction occurs inis thefrequently liquefied used samples as an [15 index,19,20 ].to Therefore,quantify the in thisoptimal study, liquefaction we examined theconditions. reaction conditions The residue that minimizeis the unreacted the amount part ofof residuethe biomass to identify samples the optimumor the unresinified conditions stuff for the generated when a condensation reaction occurs in the liquefied samples [15,19,20]. Therefore, in this liquefaction reaction, including the reaction temperature, reaction time, the amount of added catalyst. study, we examined the reaction conditions that minimize the amount of residue to identify the In addition, the amount of liquefied solvent was varied, and the change in the amount of residue in the optimum conditions for the liquefaction reaction, including the reaction temperature, reaction time, liquefaction reaction for reaction times of 0–300 min was investigated. the amount of added catalyst. In addition, the amount of liquefied solvent was varied, and the change 3.1.1.in the Eff ectamount of Biomass of residue Loading in the liquefaction reaction for reaction times of 0–300 min was investigated. 3.1.1.We examinedEffect of Biomass the eff ectLoading of the cotton stalk/solvent ratio on the amount of residue remaining after liquefaction,We examined and the resultsthe effect are of shown the cotton in Figure stalk/solven1. It showst ratio the on residuethe amount content of residue is increased remaining as reaction after timeliquefaction, increased afterand the 90 minresults except are shown with a cottonin Figure stalks: 1. It solventshows the ratio residue of 1/6, content and the is residueincreased content as decreasedreaction withtime increased increasing after amount 90 min of except added with solvent. a cotton It stalks: is thought solvent that ratio the of condensation 1/6, and the residue reaction betweencontent the decreased degradation with products increasing of theamount cotton of stalkadde samplesd solvent. was It is limited thought as thethat polyol the condensation concentration increasedreaction with between increasing the degradation amount of products solvent. of When the cotton the fraction stalk samples of cotton was stalk limited to solventas the polyol was 1 /6, a condensationconcentration reaction increased did with not occurincrea atsing alater amount reaction of solvent. stage, andWhen the th residuee fraction fraction of cotton was lowerstalk to than forsolvent other biomass was 1/6, loadings. a condensation However, reaction in terms did of not the occur effective at a utilizationlater reaction of biomass, stage, and it isthe ine residuefficient to usefraction very low was quantities lower than (16.6%) for other of biomass biomass loadings. in the liquefaction However, in product terms of as the more effective chemicals utilization are used,of resultingbiomass, in it a is higher inefficient capital to use investment. very low quantities Therefore, (16.6%) we used of biomass moderate in the raw liquefaction material concentration product as (20%)more and chemicals raw material are used,/solvent resulting ratio (1in/ 5)a higher as these capi conditionstal investment. gave aTherefore, residue fraction we used content moderate similar raw to thatmaterial of the 1 concentration/6 ratio. (20%) and raw material/solvent ratio (1/5) as these conditions gave a residue fraction content similar to that of the 1/6 ratio.

100 Cotton stalk: Solvent = 1:3 Cotton stalk: Solvent = 1:4 80 Cotton stalk: Solvent = 1:5 Cotton stalk: Solvent = 1:6 60

Residue content(%) Residue 20

0 0 60 120 180 240 300 Reaction time (min)

FigureFigure 1. Inﬂuence1. Influence of of solvent solvent ratio ratio and and reactionreaction time on on the the residue residue content content (here, (here, liquefaction liquefaction agent: agent: 80%80% PEG400 PEG400+20% +20% glycerin; glycerin; sulfuric sulfuric acid acid ratio:ratio: 3 wt.%; temperature: temperature: 150°C). 150 ◦C).

3.1.2.3.1.2. Eff Effectect of of Reaction Reaction Temperature Temperature FigureFigure2 shows 2 shows the the e ff effectect of of reaction reaction temperature temperature on the residue residue content content of of cotton cotton stalks stalks liquefied liquefied by 80%by 80% PEG400 PEG400+20% +20% glycerin, glycerin, using using 3 3 wt.%wt.% sulfuricsulfuric acid for for cotton cotton stalk. stalk. The The residue residue fraction fraction greatly greatly decreaseddecreased as theas the liquefaction liquefaction temperature temperature increased increased fromfrom 120120 to 160 160 °C.◦C. At At 120 120 °C,◦C, the the residue residue fraction fraction decreaseddecreased with with time time over over the the entire entire liquefaction liquefaction procedure,procedure, indicating indicating that that th thee condensation condensation reaction reaction waswas not not significant. significant. Conversely, Conversely, the the residue residue fractionfraction at 160 °C◦C was was the the lowest lowest after after a reaction a reaction time time of of 30 min; the effect of the condensation reaction was only visible after 60 min. At 160 °C, the residue fraction decreased at the early stage of the reaction, but then increased, indicating that both degradation and condensation occurred rapidly at high temperature. The lowest residue rate was achieved at 150 °C and reaction time of 90 min. Sustainability 2020, 12, 4214 6 of 13

30 min; the eﬀect of the condensation reaction was only visible after 60 min. At 160 ◦C, the residue fraction decreased at the early stage of the reaction, but then increased, indicating that both degradation and condensation occurred rapidly at high temperature. The lowest residue rate was achieved at 150 ◦C and reaction time of 90 min. Sustainability 2020, 12, x FOR PEER REVIEW 6 of 13 Sustainability 2020, 12, x FOR PEER REVIEW 6 of 13

100 120℃ 100 120℃ 130℃ 130℃ 80 140℃ ） 80 140℃

） 150℃ 150℃ 60 160℃ 60 160℃

40 40

Residue content (% Residue content 20 Residue content (% Residue content 20

0 0 0 60 120 180 240 300 0 60Reaction 120 time (min) 180 240 300 Reaction time (min) FigureFigure 2. 2.Influence Influence ofof liquefactionliquefaction temperature and and reaction reaction time time on on the the residue residue content content (here, (here, Figure 2. Influence of liquefaction temperature and reaction time on the residue content (here, liquefactionliquefaction agent: agent: 80% 80% PEG400 PEG400+ +20%20% glycerin;glycerin; sulfuric acid acid ratio: ratio: 3 3wt wt.%;.%; cotton cotton stalk: stalk: solvent solvent = 1:5).= 1:5). liquefaction agent: 80% PEG400 +20% glycerin; sulfuric acid ratio: 3 wt.%; cotton stalk: solvent = 1:5). 3.1.3. Effect of Catalyst Loading 3.1.3. Effect of Catalyst Loading 3.1.3. Effect of Catalyst Loading IncreasingIncreasing the the amount amount ofof catalystcatalyst cancan increase the the degree degree of of liquefaction liquefaction and and decrease decrease the the Increasing the amount of catalyst can increase the degree of liquefaction and decrease the liquefactionliquefaction temperature temperature and time.and time. However, However, high acid high concentrations acid concentrations can enhance can the enhance recondensation the liquefaction temperature and time. However, high acid concentrations can enhance the reactionsrecondensation of the liquefied reactions fragments, of the liquefied resulting fragment in an increases, resulting in the in amountan increase of insoluble in the amount residue of after recondensation reactions of the liquefied fragments, resulting in an increase in the amount of liquefactioninsoluble residue [18]. To after determine liquefaction the optimal [18]. To acid dete loadingrmine the for optimal liquefied acid cotton loading stalk, for several liquefied experiments cotton insoluble residue after liquefaction [18]. To determine the optimal acid loading for liquefied cotton werestalk, performed. several experiments Figure3 shows were performed. the e ffect Figure of acid 3 contentshows the on effect the residueof acid content content on of the cotton residue stalks stalk, several experiments were performed. Figure 3 shows the effect of acid content on the residue liquefiedcontent by of 80%cotton PEG400 stalks liquefied+20% glycerin; by 80% catalyst PEG400 content +20% glycerin; was changed catalyst into content 2 wt.% was to changed 6 wt.% for into cotton 2 content of cotton stalks liquefied by 80% PEG400 +20% glycerin; catalyst content was changed into 2 stalk.wt.% If theto 6 catalystwt.% for quantity cotton stalk. was If 2 wt.%the catalyst sulfuric qu acid,antity the was residue 2 wt.% fraction sulfuric wasacid, 38.5%, the residue while fraction increasing wt.% to 6 wt.% for cotton stalk. If the catalyst quantity was 2 wt.% sulfuric acid, the residue fraction thewas acid 38.5%, content while to increasing 3 wt.% resulted the acid in content a significant to 3 wt.% decrease resulted in in the a significant residue content decrease to in 15% the residue at 90 min. was 38.5%, while increasing the acid content to 3 wt.% resulted in a significant decrease in the residue content to 15% at 90 min. Further increasing the catalyst loading to 4 wt.% and 6 wt.% resulted in a Furthercontent increasing to 15% at the 90 catalyst min. Further loading increasing to 4 wt.% the and catalyst 6 wt.% loading resulted to 4 in wt.% a high and decrease 6 wt.% resulted in the residue in a high decrease in the residue content at 30 min. After 30 min, it resulted in a significant increase in the contenthigh atdecrease 30 min. in the After residue 30 min, content it resulted at 30 min. in After a significant 30 min, it increase resulted inin thea significant residue content.increase in Hence, the residue content. Hence, we concluded that 3 wt.% acid was the optimal catalyst loading for this we concludedresidue content. that 3Hence, wt.% acidwe concluded was the optimal that 3 catalystwt.% acid loading was the for optimal this system. catalyst loading for this system. system. 100 100 2wt% 2wt% 3wt% 3wt% 80 4wt% 4wt% 80 6wt% 6wt% 60 60

40 40 Residue content (%) content Residue

Residue content (%) 20 content Residue 20

0 0 0 60 120 180 240 300 0 60Reaction 120 time 180(min) 240 300 Reaction time (min) Figure 3. Influence of acid content and reaction time on the residue content (here, liquefaction agent: FigureFigure 3. Inﬂuence 3. Influence of of acid acid content content and and reaction reaction timetime onon the residue residue content content (here, (here, liquefaction liquefaction agent: agent: 80% PEG400 +20% glycerin; cotton stalk: solvent = 1:5; temperature: 150 °C). 80%80% PEG400 PEG400+20% +20% glycerin; glycerin; cotton cotton stalk: stalk: solventsolvent == 1:5; temperature: temperature: 150 150 °C).◦C). 3.1.4. Effects of Reaction Time 3.1.4. Effects of Reaction Time GPC data showed that each liquefaction product consisted of two major fractions with different GPC data showed that each liquefaction product consisted of two major fractions with different molecular weights. The molecular weight distribution of the liquefied cotton stalks for each reaction molecular weights. The molecular weight distribution of the liquefied cotton stalks for each reaction time at a reaction temperature of 150 °C, catalyst content of 3% and 1/5 fraction of cotton stalk to time at a reaction temperature of 150 °C, catalyst content of 3% and 1/5 fraction of cotton stalk to solvent are shown in Table 3. The chromatograms of the four products had a similar shape and solvent are shown in Table 3. The chromatograms of the four products had a similar shape and Sustainability 2020, 12, 4214 7 of 13

3.1.4. Effects of Reaction Time GPC data showed that each liquefaction product consisted of two major fractions with different molecular weights. The molecular weight distribution of the liquefied cotton stalks for each reaction time at a reaction temperature of 150 ◦C, catalyst content of 3% and 1/5 fraction of cotton stalk to solvent are shown in Table3. The chromatograms of the four products had a similar shape and position, indicating that they had similar molecular weight distributions. Here, Mp1 and Mp2 are the peak molecular weight of the high and low molecular weight fractions, respectively. The components with different molecular weights dissolved at a similar rate during liquefaction.

Table 3. Inﬂuence of reaction time on the molecular weights of liquefaction products.

Reaction Time (min) Mp1 Mp2 AreaP1: AreaP2 Mn Mw d = Mw/Mn 30 1340 1070 2.13 1020 1200 1.18 60 1400 1060 2.1 1030 1150 1.2 90 1450 1040 2.12 1040 740 1.22 300 1500 1035 2.13 1035 1280 1.23 Liquefaction agent: 80%PEG400+20% glycerin; sulfuric acid ratio: 3 wt.%; cotton stalk:solvent = 1:5; temperature: 150 ◦C.

Wood liquefaction in presence of acid leads to decomposition of cellulose into small compounds due to acid hydrolysis, and other oxidation reactions take place on cellulose. The intensity of carbonyl in these compounds may reflect the degree of cellulose decomposition. Therefore, Figure4 shows normalized FTIR spectra of the polyols obtained from liquefied cotton stalk at different liquefaction 1 times and under optimal conditions. The broad absorption band at 1720 cm− , that increased modestly with the reaction time, was attributed to the stretching vibration of carbonyl groups from levulinic acid (a decomposition product of cellulose). This confirms the occurrence of the reaction between the liquefaction solvents and the cotton stalk powder compounds or derivatives formed during 1 liquefaction. The C=C bands at 1640 cm− were from aromatic compounds generated by lignin 1 decomposition. The absorption bands at 1460–1330 cm− are attributable to CH2 in-plane bending 1 vibrations; a sharp band near 1120 cm− is attributed to C O stretching vibrations from an ether group, −1 and C–H stretching vibration bands occurred at 840 cm− . These peaks are considered important for resinification, and hence, their intensity values were compared [19]. The intensity of the carbonyl peak derived from the decomposition product of cellulose increased as the reaction proceeded, and reached its maximum at 90 min, after which the intensity did not change significantly. The viscosity is an important fluid property for studying the rheological behavior of the polyols and their suitability for the preparation of polyurethane foams. Figure5 shows the e ffect of liquefaction time on the viscosity at the reaction temperature of 150 ◦C, with a catalyst content of 3 wt.%, and a solvent volume of 1:5, for reaction times up to 300 min. The viscosity decreased drastically from 7950 to 950 mPa s over the first 90 min, then increased with reaction time after 120 min. It is worth noting · that this increase in viscosity followed the observed increase in residue fraction, which also showed a minimum at 90 min and then gradually increased due to the condensation reaction. The measured viscosity values were slightly larger than those of conventional polyols, but considered satisfactory for the preparation of polyurethane foams [19]. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 13

position, indicating that they had similar molecular weight distributions. Here, Mp1 and Mp2 are the peak molecular weight of the high and low molecular weight fractions, respectively. The components with different molecular weights dissolved at a similar rate during liquefaction.

Table 3. Influence of reaction time on the molecular weights of liquefaction products.

Reaction time (min) Mp1 Mp2 AreaP1: AreaP2 Mn Mw d =Mw/Mn 30 1340 1070 2.13 1020 1200 1.18 60 1400 1060 2.1 1030 1150 1.2 90 1450 1040 2.12 1040 740 1.22 300 1500 1035 2.13 1035 1280 1.23 Liquefaction agent: 80%PEG400+20% glycerin; sulfuric acid ratio: 3 wt.%; cotton stalk:solvent = 1:5; temperature: 150 °C.

15min

Absorbance 60min 90min 1720 1640 840 180min 1460 1360 1120 300min

2000 1600 1200 800 400 Wavenumber (cm-1) Sustainability 2020, 12, x FOR PEER REVIEW 8 of 13 FigureFigure 4. 4.Normalized Normalized FTIR FTIR spectra spectra ofof cottoncotton stalk polyols polyols prepared prepared with with different diﬀerent liquefaction liquefaction times times condensation(here,(here, liquefaction liquefaction reaction. agent: agent: The 80%measured 80% PEG400PEG400 viscosity +20%+20% glycerin; glycerin; values cottonwere cotton slightlystalk: stalk: solvent larger solvent = than1:5;= 1:5;sulfuric those sulfuric ofacid conventional acidratio: ratio:3 polyols,3 wt.%;wt.%; but temperature:temperature: considered 150 150 sati °C).◦sfactoryC). for the preparation of polyurethane foams [19].

1000 The viscosity is an important fluid property for studyingViscosity the rheological behavior of the polyols and their suitability for the preparation of polyurethane foams. Figure 5 shows the effect of liquefaction time on the viscosity900 at the reaction temperature of 150 °C, with a catalyst content of 3 s) wt.%, and a solvent volume･ of 1:5, for reaction times up to 300 min. The viscosity decreased drastically from 7950 to 950800 mPa·s over the first 90 min, then increased with reaction time after 120 min. It is worth noting that this increase in viscosity followed the observed increase in residue

fraction, which also showed700 a minimum at 90 min and then gradually increased due to the Viscosity (mPa

600

500 0 60 120 180 240 300 360 Reaction time (min) FigureFigure 5. E5.ﬀ Effectect of liquefactionof liquefaction time time on theon viscositythe viscosity (here, (here, liquefaction liquefaction agent: agent: 80% PEG40080% PEG400+20% +20% glycerin;

cottonglycerin; stalk: cotton solvent stalk:= 1:5;solvent sulfuric = 1:5; acid sulfuric ratio: acid 3 wt.%;ratio: 3 temperature: wt.%; temperature: 150 ◦C). 150 °C).

PolyurethanePolyurethane foams foams are are prepared prepared by theby the reaction reaction of a of polyisocyanate a polyisocyanate with with a polyol a polyol in the in presence the ofpresence a blowing of agent,a blowing surfactant, agent, surf andactant, catalyst. and Therefore,catalyst. Therefore, the change the inchange the acid in the number acid number and hydroxyl and numberhydroxyl of liquefied number of cotton liquefied stalk cotton as a function stalk as a of func reactiontion of time reaction were time investigated, were investigated, as shown as in shown Figure 6. Thein acidFigure number 6. The acid of the number bio-polyol of the increasedbio-polyol fromincreased 17 tofrom 24 17 mg to KOH 24 mg/g KOH/g with increasing with increasing reaction time.reaction This time. was attributedThis was attributed to the progress to the progress of the decomposition of the decomposition reaction; reaction; acidic acidic substances substances such as such as carboxylic acids from high-molecular-weight materials were formed, and a polyhydric carboxylic acids from high-molecular-weight materials were formed, and a polyhydric alcohol or alcohol or decomposition products could have been oxidized by the sulfuric acid [18]. Acidic decomposition products could have been oxidized by the sulfuric acid [18]. Acidic substances often substances often exist in the lignocellulosic components and can also be produced during the exist in the lignocellulosic components and can also be produced during the liquefaction by oxidation of liquefaction by oxidation of carbohydrates and lignin [21]. In contrast, the hydroxyl number tended carbohydratesto decrease (from and lignin 360 to [21 325]. Inmg contrast, KOH/g) the as hydroxylthe reaction number time increased. tended to This decrease was (fromattributed 360 toto 325the mg KOHdehydration/g) as the reactionand thermal-oxidative time increased. degradation This was attributedof the liquefaction to the dehydration solvent, along and with thermal-oxidative the reaction degradationbetween the of solvent the liquefaction and dissolved solvent, liquefied along components. with the reaction We conclu betweended that the the solvent polyols and prepared dissolved liquefiedfrom liquefied components. cotton stalks We concluded were suitable that for the preparation polyols prepared of polyurethanes. from liquefied cotton stalks were suitable for preparation of polyurethanes. 370 30 Hydroxyl number

Acid number 360

25 350

340 20 Acid number (mgKOH/g)Acid

Hydroxyl number (mgKOH/g) number Hydroxyl 330

320 15 0 60 120 180 240 300 Reaction time (min)

Figure 6. Hydroxyl number and acid number of liquefied cotton stalk as a function of time (here, liquefaction agent: 80% PEG400 +20% glycerin; cotton stalk: solvent = 1:5; sulfuric acid ratio: 3 wt.%; temperature: 150 °C). Sustainability 2020, 12, x FOR PEER REVIEW 8 of 13 condensation reaction. The measured viscosity values were slightly larger than those of conventional polyols, but considered satisfactory for the preparation of polyurethane foams [19].

1000 Viscosity

900 s) ･

800

700 Viscosity (mPa

600

500 0 60 120 180 240 300 360 Reaction time (min) Figure 5. Effect of liquefaction time on the viscosity (here, liquefaction agent: 80% PEG400 +20% glycerin; cotton stalk: solvent = 1:5; sulfuric acid ratio: 3 wt.%; temperature: 150 °C).

Polyurethane foams are prepared by the reaction of a polyisocyanate with a polyol in the presence of a blowing agent, surfactant, and catalyst. Therefore, the change in the acid number and hydroxyl number of liquefied cotton stalk as a function of reaction time were investigated, as shown in Figure 6. The acid number of the bio-polyol increased from 17 to 24 mg KOH/g with increasing reaction time. This was attributed to the progress of the decomposition reaction; acidic substances such as carboxylic acids from high-molecular-weight materials were formed, and a polyhydric alcohol or decomposition products could have been oxidized by the sulfuric acid [18]. Acidic substances often exist in the lignocellulosic components and can also be produced during the liquefaction by oxidation of carbohydrates and lignin [21]. In contrast, the hydroxyl number tended to decrease (from 360 to 325 mg KOH/g) as the reaction time increased. This was attributed to the dehydration and thermal-oxidative degradation of the liquefaction solvent, along with the reaction Sustainability 2020, 12, 4214 9 of 13 between the solvent and dissolved liquefied components. We concluded that the polyols prepared from liquefied cotton stalks were suitable for preparation of polyurethanes.

370 30 Hydroxyl number

Acid number 360

25 350

340 20 Acid number (mgKOH/g)Acid

Hydroxyl number (mgKOH/g) number Hydroxyl 330

320 15 0 60 120 180 240 300 Reaction time (min)

Figure 6. Hydroxyl number and acid number of liqueﬁed cotton stalk as a function of time (here, Figureliquefaction 6. Hydroxyl agent: number 80% PEG400 and acid+ 20%number glycerin; of liquefie cottond cotton stalk: stalk solvent as a= function1:5; sulfuric of time acid (here, ratio: 3 wt.%;

liquefactiontemperature: agent: 150 80%◦C). PEG400 +20% glycerin; cotton stalk: solvent = 1:5; sulfuric acid ratio: 3 wt.%; temperature: 150 °C). 3.2. Properties of Polyurethane Foam

3.2.1. Density and Compressive Strength The density of polyurethane foams synthesized from liquefaction products prepared at different reaction times is shown in Figure7. The density was higher for liquefied products with longer reaction times. This is consistent with the GPC results showing that the viscosity increased with increasing molecular weight of the liquefied product. Reaction mixtures with higher viscosities inhibited the foaming reaction of polyurethane foams and increased the density. Interestingly, there seems to be an inverse relationship between the density and hydroxyl value, which is thought to be due to the varied qualities of the foaming reaction. Specifically, the foaming reaction started in the moment when the isocyanate was added to the system. The bio-polyol with the highest hydroxyl value was mixed with the highest amount of isocyanate ([NCO]/[OH] = 1), which resulted in the largest volume of generated gas, and subsequently a more complete foaming reaction and good expansion. Therefore, a lower foam density was observed compared to samples with lower hydroxyl numbers. The compressive strength of the polyurethane foams was 28–267 kPa and decreased with increasing reaction time during liquefaction. The values plotted in the figures are mean values obtained from more than three repeated experiments, and errors are within 5%. In previous studies, prepared polyurethane foams from liquefied corn bran showed compressive strength between 70 and 142 kPa and density between 38 and 40 kg/m3)[10], while polyurethane foams prepared from liquefied wheat straw showed compressive strength and density varying from 169 to 212 kPa and from 30 to 56 kg/m3, respectively [19]. With increasing reaction time, the hydroxyl value decreased due to the formation of re-condensation molecules and the chain’s expansion reaction of polyurethane foam’s synthesis decrease, while the molecular weight of polyol that reacted with isocyanate increased. In this experiment, the isocyanate index was 1:1, where the polyol component was later used to prepare polyurethane foams due to the lower residue fraction. Therefore, the fraction of the soft component increased, and the compressive strength decreased accordingly. Sustainability 2020, 12, x FOR PEER REVIEW 9 of 13

3.2. Properties of Polyurethane Foam

3.2.1. Density and Compressive Strength The density of polyurethane foams synthesized from liquefaction products prepared at different reaction times is shown in Figure 7. The density was higher for liquefied products with longer reaction times. This is consistent with the GPC results showing that the viscosity increased with increasing molecular weight of the liquefied product. Reaction mixtures with higher viscosities inhibited the foaming reaction of polyurethane foams and increased the density. Interestingly, there seems to be an inverse relationship between the density and hydroxyl value, which is thought to be due to the varied qualities of the foaming reaction. Specifically, the foaming reaction started in the moment when the isocyanate was added to the system. The bio-polyol with the highest hydroxyl value was mixed with the highest amount of isocyanate ([NCO]/[OH] = 1), which resulted in the Sustainabilitylargest 2020volume, 12, 4214 of generated gas, and subsequently a more complete foaming reaction and good10 of 13 expansion. Therefore, a lower foam density was observed compared to samples with lower hydroxyl numbers. 160 (a)

120 ) 3

80 Density (kg/m Density 40

0 10 30 90 180 300 Reaction time (min) 300 (b)

200

100

Compressive Strength (kPa) Strength Compressive 0 10 30 90 180 300 Reaction time (min) FigureFigure 7. E ff7.ect Effect of reaction of reaction time time on the on (a the) density (a) density and ( band) compressive (b) compressive strength strength of polyurethane of polyurethane foams. foams. 3.2.2. Foam Morphology FigureThe8 showscompressive SEM imagesstrength of of the the fracture polyuretha surfacesne foams of polyurethane was 28–267 foams kPa and manufactured decreased with from liquefiedincreasing cotton reaction stalks. Thetime averageduring liquefaction. pore diameter Th ofe values the polyurethane plotted in the foams figures decreased are mean from values 350 to obtained from more than three repeated experiments, and errors are within 5%. In previous studies, 100 µm of foam as reaction time increased in the liquefaction process, and the cell wall became thicker. prepared polyurethane foams from liquefied corn bran showed compressive strength between 70 and This is consistent with the observation that the foaming reaction was inhibited by an increase in the 142 kPa and density between 38 and 40 kg/m3)[10], while polyurethane foams prepared from viscosity. One reason for the reduction in the compressive strength with increasing reaction time could liquefied wheat straw showed compressive strength and density varying from 169 to 212 kPa and not be clearly identified from these SEM images [22]. from 30 to 56 kg/m3, respectively [19]. With increasing reaction time, the hydroxyl value decreased 3.2.3. Thermal Stability The thermal behavior of polyurethane foams is very important because they are mainly used as thermal insulation materials. The thermal decomposition of the polyurethane foams was performed at a heating rate of 20 ◦C/min under an argon or air atmosphere. Figure9a,b shows the TG curve and time derivative of the weight loss of the polyurethane foam samples. The thermal decomposition of polyurethanes is a combination of random-chain scission, chain-end unzipping, and crosslinking reactions [23]. The urethane (200 ◦C) and urea (250 ◦C) linkages are the first to break and depolymerize into free isocyanate and alcohol. This is followed by degradation of the polyether polyols, and finally isocyanate, above 600 ◦C. Therefore, most of the weight loss occurred between 250 and 450 ◦C. Above this temperature, the weight loss was minimal as the structures were heavily crosslinked and resistant to degradation. Although there was no difference in the amount of decomposition residual, the decomposition start temperature tended to shift to a higher temperature as the reaction time of the liquefied material increased. The decomposition of the polyol main chain occurs at higher temperatures than that of urethane bonds [24]. The decomposition rate of the polyurethane foams derived from liquefaction was faster than that of PEG. This is because the polyol component of the liquefaction Sustainability 2020, 12, x FOR PEER REVIEW 10 of 13

due to the formation of re-condensation molecules and the chain’s expansion reaction of polyurethane foam’s synthesis decrease, while the molecular weight of polyol that reacted with isocyanate increased. In this experiment, the isocyanate index was 1:1, where the polyol component was later used to prepare polyurethane foams due to the lower residue fraction. Therefore, the fraction of the soft component increased, and the compressive strength decreased accordingly.

3.2.2. Foam Morphology Figure 8 shows SEM images of the fracture surfaces of polyurethane foams manufactured from Sustainabilityliquefied2020 cotton, 12, 4214stalks. The average pore diameter of the polyurethane foams decreased from 35011 to of 13 100 µm of foam as reaction time increased in the liquefaction process, and the cell wall became thicker. reactionThis is isconsistent non-uniform with the and observation is easily decomposed, that the foaming and reaction the polyurethane was inhibited foams’ by an combustibility increase in the has beenviscosity. improved. One reason for the reduction in the compressive strength with increasing reaction time could not be clearly identified from these SEM images [22].

FigureFigure 8. 8.SEM SEM images images ofof polyurethane foam foam samples samples prepar prepareded with with reaction reaction times times of (a) of15; ( a(b)) 15; 90; ((c)b) 90; Sustainability(c)180; 180; and and (d)2020 (d )300, 30012 ,min. x min. FOR PEER REVIEW 11 of 13

100 3.2.3. Thermal Stability Biofoam 30min Biofoam 30min Biofoam 90min Biofoam 90min The thermal behavior of polyurethaneBiofoam 300min foams is very important because they are mainly used as 80 (a) (b) Biofoam 300min thermal insulation materials. The thermalPegfoam decomposition of the polyurethane foams wasPegfoam performed

at a heating60 rate of 20 °C/min under an argon or air atmosphere. Figure 9a,b shows the TG curve and time derivative of the weight loss of the polyurethane foam samples. The thermal decomposition of

polyurethanes is a combination of random-chain scission,dX/dt chain-end unzipping, and crosslinking

Weight (%) Weight 40 reactions [23]. The urethane (200 °C) and urea (250 °C) linkages are the first to break and

depolymerize20 into free isocyanate and alcohol. This is followed by degradation of the polyether polyols, and finally isocyanate, above 600 °C. Therefore, most of the weight loss occurred between 250 and0 450 °C. Above this temperature, the weight loss was minimal as the structures were heavily crosslinked100 and 200 resistant 300 400 to degradation. 500 600 700 Althou 800 gh 100there 200was 300no difference 400 500 in 600 the 700amount 800 of Temperature (℃) Temperature (℃) decomposition residual, the decomposition start temperature tended to shift to a higher temperature as Figurethe reactionFigure 9. (a )9. TG time(a) curveTG of curve the and liquefied and (b) (b) time time material derivative derivative incr of of theeased. the weight we Theight loss lossdecomposition for for polyurethanepolyurethane of the foams foams polyol with with main different different chain occursprocessing processingat higher times. times.temperatures The The heating heating rate than rate of 20that of◦ 20C /of min°C/min urethane from from room roombonds temperature temperature [24]. The to to800decomposition 800◦C °C under under Ar Ar gasrate gas flow flowof ofthe polyurethane200 mLof /200min. mL/min. foams derived from liquefaction was faster than that of PEG. This is because the polyol component of the liquefaction reaction is non-uniform and is easily decomposed, and the 3.2.4.3.2.4. Biodegradation Biodegradation polyurethane foams’ combustibility has been improved. The degreeThe degree of biodegradation of biodegradation of theof the polyurethane polyurethane foam foam samples samples buried buried inin soil was assessed assessed each each monthmonth by measuring by measuring changes changes in thein the weight weight loss, loss, as as shown shown in in Figure Figure 10 10.. TheThe weightweight loss loss increased increased withwith time, time, giving giving a total a total loss loss of 15%of 15% over over one one year. year The. The average average values values ofof threethree samples and and error error of of samplessamples were were within within 5%. 5%. The The degradation degradation velocity velocity changed changed with with di differentfferent seasons.seasons. The The degradability degradability of polyurethaneof polyurethane foams foams derived derived from from liquefied liquefied matter matter and and those those derived derived fromfrom polyol showed showed a aclear clear differencedifference from from the initial the initial stage, stage, which which became became more more significant significant over over time. time. It is It clear is clear from from Figure Figure 10 that10 the weightthat the loss weight of the loss polyurethane of the polyurethane foam from foam liquefied from cottonliquefied stalk cotton was stalk faster was than faster that containingthan that polyolcontaining alone, which polyol indicated alone, which that the indicated wood component that the wood mainly component contributed mainly to the contributed biodegradability to the of biodegradability of the foams. the foams. 20 Biofoam Pegfoam

10 Weight loss(%) Weight

0 012345678910111213 Time (month) Figure 10. Weight loss of polyurethane samples over time in biodegradation experiments.

4. Conclusions The effects of the liquefaction conditions on the properties of the polyols and polyurethane foams produced using this feedstock were investigated. The optimal processing conditions for the liquefied product considering the quality of the polyurethane foams were studied as a function of the residue fraction. Bio-polyols with promising material properties were produced using liquefaction conditions of 150 ℃, reaction time of 90 min, catalyst content of 3 wt.%, and 20 w/w% cotton stalk loading. Bio-polyols produced under preferential conditions showed hydroxyl numbers of 325–360 Sustainability 2020, 12, x FOR PEER REVIEW 11 of 13

100 Biofoam 30min Biofoam 30min Biofoam 90min Biofoam 90min Biofoam 300min 80 (a) (b) Biofoam 300min Pegfoam Pegfoam

60 dX/dt

Weight (%) Weight 40

0 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Temperature (℃) Temperature (℃)

Figure 9. (a) TG curve and (b) time derivative of the weight loss for polyurethane foams with different processing times. The heating rate of 20 °C/min from room temperature to 800 °C under Ar gas flow of 200 mL/min.

3.2.4. Biodegradation The degree of biodegradation of the polyurethane foam samples buried in soil was assessed each month by measuring changes in the weight loss, as shown in Figure 10. The weight loss increased with time, giving a total loss of 15% over one year. The average values of three samples and error of samples were within 5%. The degradation velocity changed with different seasons. The degradability of polyurethane foams derived from liquefied matter and those derived from polyol showed a clear difference from the initial stage, which became more significant over time. It is clear from Figure 10 that the weight loss of the polyurethane foam from liquefied cotton stalk was faster than that Sustainabilitycontaining2020 polyol, 12, 4214 alone, which indicated that the wood component mainly contributed to 12the of 13 biodegradability of the foams.

20 Biofoam Pegfoam

10 Weight loss(%) Weight

0 012345678910111213 Time (month) FigureFigure 10. 10.Weight Weight loss loss of of polyurethane polyurethane samples ov overer time time in in biodegradation biodegradation experiments. experiments.

4. Conclusions4. Conclusions TheThe eﬀ ectseffects of theof the liquefaction liquefaction conditions conditions on theon the properties properties of theof the polyols polyols and and polyurethane polyurethane foams producedfoams produced using this using feedstock this feedstock were investigated. were investig Theated. optimal The optimal processing processing conditions conditions for the for liqueﬁed the productliquefied considering product considering the quality the of quality the polyurethane of the polyurethane foams were foams studied were studied as a function as a function of the of residue the fraction.residue Bio-polyols fraction. Bio-polyols with promising with promising material propertiesmaterial properties were produced were produced using liquefaction using liquefaction conditions of 150conditions°C, reaction of 150 time℃, reaction of 90 min,time catalystof 90 min, content catalyst of content 3 wt.%, of and3 wt.%, 20 wand/w% 20 w/w% cotton cotton stalk loading.stalk Bio-polyolsloading. Bio-polyols producedunder produced preferential under preferential conditions conditions showed hydroxyl showed hydroxyl numbers numbers of 325–360 of mg325–360 KOH /g, acid numbers below 5.00 mg KOH/g, and viscosities of 850–7950 mPa s. Polyurethane foams produced · under optimal conditions showed densities of 78–123 kg/m3 and compressive strength of 28–267 kPa. These results suggest that PEG–glycerin mixtures can be used as a liquefaction solvent to produce bio-polyols from agricultural waste biomass such as cotton stalk, which can subsequently be used to prepare polyurethane foams. The produced bio-polyols and polyurethane foams showed material properties comparable to their analogs from petroleum solvent-based liquefaction processes.

Author Contributions: Conceptualization, Q.W. and N.T.; methodology, Q.W. and N.T.; software, N.T.; validation, Q.W.; formal analysis, N.T.; investigation, N.T. and Q.W.; resources, Q.W.; data curation, N.T.; writing—original draft preparation, N.T.; writing—review and editing, Q.W.; visualization, N.T.; supervision, Q.W.; project administration, Q.W.; funding acquisition, Q.W. All authors have read and agreed to the published version of the manuscript. Funding: This study was partially supported by the special funds for Basic Researches (B) (No. 15H05119, 376 FY2015~FY2017) of Grant-in-Aid Scientific Research of the Japanese Ministry of Education, Culture, Sports, 377 Science and Technology (MEXT), Japan. Acknowledgments: N.T. would like to thank Japan Student Services Organizations for a grant that made it possible to complete this study. Conflicts of Interest: We declare no conflict of interest.

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