Synthesis of Reprocessable Lignin-based Non-isocyanate Poly(imine-hydroxyurethane)s Networks

Danwei Xue,Bailiang Xue,2,*,Rui Tang,Chao Shen,Xinping Li,Wei Zhao

1.College of Bioresources Chemical and Materials Engineering,Shaanxi University of Science and Technology,Xi'an,Shaanxi Province,710021,China

2.Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education,Qilu University of Technology(Shandong Academy of Sciences),Jinan,Shandong Province,250353,China

Abstract:In this study,an environmentally friendly and non-toxic route to synthesize lignin-based non-isocyanate poly(imine-hydroxyurethane)s networks was explored.Specifically,the NH2-terminated polyhydroxyurethanes(NPHUs)prepolymer was first synthesized from bis(6-membered cyclic carbonate)(BCC)and diamine via the ring-opening reaction.Subsequently,the corresponding ligninbased non-isocyanate polyurethanes(NIPUs)with tunable properties were synthesized from NPHUs and levulinate lignin derivatives containing ketone groups via the Schiff base reaction.The structural,mechanical,and thermal properties of NIPUs with different stoichiometric feed ratios of BCC and levulinate lignin were characterized by Fourier transform infrared spectroscopy(FT-IR),nuclear magnetic resonance(NMR),differential scanning calorimetry(DSC),dynamic mechanical analysis(DMA),and thermogravimetric analysis(TGA).The results indicated that the tensile strength,Young's modulus,toughness,storage modulus,glass transition temperature,and thermal stability of lignin-based NIPUs gradually increased with increasing lignin content,and the highest Young's modulus of 41.1 MPa was obtained when lignin content reached 45.53%.With good reprocessing properties,this synthetic framework of ligninbased NIPUs also provides sustainable non-isocyanate-based substitutions to traditional polyurethane networks.

Keywords:lignin;non-isocyanate polyurethane;Schiff base reaction;reprocessing

1 Introduction

Lignocellulose mainly consists of cellulose,hemicellulose,and lignin,among which lignin is the most abundant natural aromatic compound with high availability and a very complicated polyphenolic structure.Lignin,a byproduct of pulp and paper refining,shows great potential as a desirable alternative raw material to fossil resources.To take advantage of its renewability and backbone rigidity,lignin-based thermosets,such as polyurethanes[1],epoxy[2],and phenol resin[3],have been extensively studied.However,less than 2% of the millions of tons of industrial lignin are used as natural stabilizers and surfactants for high value-added products[4-6],while the rest has been disposed of or burned as a low-value fuel,due to lack of appropriate conversion technologies.Consequently,this under-utilization spurs the need to develop lignin-based products.

Conventional polyurethanes(PUs)are usually synthesized through the polycondensation reaction between diisocyanate and a diol,and have been widely used in various applications,such as rigid and flexible foams,coatings,elastomers,adhesives,and sealants[7].However,isocyanate precursors are very harmful at each stage of the life cycle of the polymers.The health effects of isocyanate exposure include long-term asthma and other lung problems[8].Thus,the use of environmentally friendly non-isocyanate polyurethanes(NIPUs)as an alternative synthetic route has attracted increasing attention due to their use of green materials and procedures[9-10].Compared with the conventional isocyanate-based PUs,the most popular alternative pathway is the polyaddition between cyclic carbonates and diamines,resulting in polyhydroxyurethanes(PHUs)containing hydroxyl groups on the repeating units in their backbones with unique properties.The introduction of hydroxyl groups leads to the formation of hydrogen bonding,which endows the NIPUs with improved mechanical,thermal,self-healing,and reprocessing properties[11-13].

The use of lignin as one of the raw materials to partially replace commercial polyols in the synthesis of conventional isocyanate-based PUs has been widely reported.So far,the utilization of lignin in PU formulation can be divided into direct use or modification[1,14].Only a few studies have reported lignin-based NIPUs with reasonable mechanical properties.Currently,the application of lignin or ligninderived monomers in the synthesis of NIPUs has been modified to produce cyclic carbonate-functionalized lignin derivatives,which were mostly obtained via glycidylation of lignin,followed by cycloaddition with CO2

[15-17].However,this strategy requires highly toxic epichlorohydrin and high-pressure equipment to achieve insertion of CO2.Furthermore,Kühnel et al[18]reported that cyclic carbonate-functionalized lignin can be prepared by oxyalkylation with glycerol carbonate to insert adjacent hydroxyl groups,and then reacted with dimethyl carbonate via transesterification.However,this method would lead to high amounts of the coproducts associated with oxyalkylation(CO2)and transesterification(MeOH).Recently,Huang et al[19]reported a facile strategy for lignin modification with levulinic acid via esterification.These lignin derivatives containing ketone groups could be reacted with diamines to produce polyimine thermosets via the Schiff base reaction.Inspired by this work,the ligninbased NIPUs synthesized from NH2-terminated polyhydroxyurethanes(NPHUs)and levulinate lignin derivatives obtained via Schiff base reactions may be a promising synthetic strategy.To the best of our knowledge,the utilization of lignin as a macromonomer in the synthesis of non-isocyanate poly(imine-hydroxyurethane)s(PIHUs)has not yet been explored.

In this work,the goal was to design and synthesize innovative lignin-based NIPUs with reasonable mechanical and physical properties via a two-step synthesis.We first synthesized the NPHUs prepolymer using bis(6-membered cyclic carbonate)(BCC)and diamines via the ring-opening reaction.After the synthesis of the NPHUs prepolymer,lignin-based NIPUs with tunable properties were further synthesized via the Schiff base reaction using different lignin derivatives containing ketone groups.The effects of different stoichiometric feed ratios of BCC and levulinate lignin on the structural,mechanical,and thermal properties of lignin-based NIPUs were investigated by Fourier-transform infrared spectroscopy(FT-IR),nuclear magnetic resonance(NMR),dynamic mechanical analysis(DMA),differential scanning calorimetry (DSC),thermogravimetric analysis(TGA),and mechanical testing machines.

2 Experimental

2.1 Materials

Enzymatic hydrolysis lignin(EHL)was purchased from Longlive Biological Technology Co.,Ltd.(Shandong Province, China). The chemical characteristics of lignin have been reported in our previous study[20].Levulinic acid(LA),N,N'-dicyclohexylcarbodiimide(DCC),JEFFAMINE®D-400 polyoxypropylenediamine(D-400),4-dimethylaminopyridine(DMAP),and pentafluorobenzaldehyde were purchased from Aladdin(Shanghai,China).Di(trimethylopropane)(98%)was purchased from J&K Scientific(Beijing,China),and used as received.Diphenyl carbonate was purchased from Sinopharm(Beijing,China).All reagents were of chemical grade.

2.2 Methods

2.2.1 Synthesis of levulinate enzymatic hydrolysis lignin(LEHL)

LEHL was synthesized according to a previously reported method with modifications[19].A typical procedure is as follows:DMAP(11.22 g,0.092 mol),DCC(208.8 g,1.012 mol),EHL(220 g,containing 0.920 mol hydroxyl groups),and LA(160 g,1.38 mol)were dissolved in dimethylformamide(DMF,500 mL).Subsequently,the mixtures were magnetically stirred at 25℃for 48 h.When the reaction was complete,the mixture was vacuum filtered to remove the by-product N,N'-dicyclohexylurea,and condensed using a rotary evaporator.We poured the obtained mixtures into 200 mL distilled water and methanol(1∶1,V/V),and stirred it for a further 0.5 h.After filtration,the obtained solid was washed three times with deionized water,followed by drying at 70℃for 24 h in a vacuum oven.Ultimately,200 g light brown powder was obtained with a yield of 61.3%.

2.2.2 Synthesis of BCC

BCC was synthesized according to a previously reported method[21].The following is a detailed description of the synthesis of BCC.Di(trimethylopropane)(10 g,39.9 mmol)and diphenyl carbonate(85.7 g,400 mmol)were added to a round bottom flask.The flask was placed in an oil bath set at 140℃.After 48 h,the mixture was cooled to 25℃and washed with diethyl ether to remove the excess diphenyl carbonate.The resultant white solid powder was then recrystallized in tetrahydrofuran three times,and the resulting white crystals(BCC,6 g,yield of 60%)were placed in a vacuum drying oven overnight at 35℃.

2.2.3 Preparation of NPHUs

The formulations of the NPHUs prepolymer were calculated depending on the different molar ratios of[BCC]∶[LEHL]with 6.25 mol% DMAP on D-400(Table 1).The molar ratio of cyclic carbonate groups from BCC and ketone groups from LEHL to the amine functional groups from D-400 was fixed at 1.To obtain the NPHUs prepolymer,D-400 was added in excess to ensure the complete consumption of cyclocarbonate.In a typical experiment,1.28 g D-400 and different contents of BCC(0.9676,0.9192,0.8708,0.6774,and 0.4838 g)were mixed in 3 mL DMF for 48 h at 50℃.

2.2.4 Preparation of lignin-based NIPUs

After prepolymerization,in order to react with the free amino groups of the different NPHUs prepolymers,the corresponding solution of LEHL(0.07,0.15,0.89,and 1.47 g)in 5 mL DMF was added to the prepolymer solution and stirred for 4 h.After the reaction,the solution was poured into a 20 cm×20 cm×1 cm polytetrafluoroethylene mold,and placed in a fume hood overnight to evaporate the solvent,followed by further reaction in a blow oven at 70℃for 12 h.In order to ensure complete polymerization,the pre-cured polymer was hot-pressed at 160℃for 30 min and postcured at 180℃for 2 h in a vacuum oven to obtain lignin-based NIPUs.

2.2.5 General characterizations

1HNMR spectra were obtained on a Bruker NMR spectrometer(AVANCE III HD 400 MHz,Germany)using 30 mg sample;5µL pentafluorobenzaldehyde was dissolved in 1 mL dimethyl sulfoxide(DMSO-d6).FT-IR spectra were recorded on a Bruker TENSOR II spectrometer(Bruker,VERTEX 70,Germany).The sample was scanned 32 times from 400 to 4000 cm−1with a resolution of 4.0 cm−1.Uniaxial tensile testing was performed with an AI-7000-AGD testing machine(Gotwell,China)at a cross-head speed of 5 mm/min.The samples were cut into a dumbbell-shaped spline with approximate dimensions of 25 mm×5 mm.The measurement was performed three times for each sample.Dynamic mechanical properties were measured using a TA Instruments DMA Q800(TA instrument,USA)in tension mode.Samples with dimensions of 20 mm×5 mm×0.3 mm were measured at a constant frequency of 1 Hz,an amplitude of 2μm,and a heating rate of 5℃/min,over a temperature range from−60℃to-55℃.DSC analyses were carried out using a NETZSCH DSC200F3 (Netzsch, Germany)calorimeter at 20℃/min in the range from−40℃to 40℃under nitrogen.TGA were performed using a TGA Q50(TA instrument,USA)at a heating rate of 10℃/min from 50℃to 600℃under a nitrogen atmosphere(50 mL/min).The reprocessing properties of the samples were characterized by measuring changes in the mechanical properties before and after processing.The sample was completely cut into tiny fragments and the fragments were hot-repressed in a mold for shaping.The mechanical properties of the reprocessed samples were tested using the AI-7000-AGD testing machine.

3 Results and discussion

3.1 Synthesis of lignin-based NIPUs

In this study,the investigated lignin-based NIPUs were based on the dynamic chemistry between BCC,diamine,and LEHL through transcarbamoylation exchange reactions.The BCC monomer can be easily prepared via the ring-closing reaction of polyols using diphenyl carbonate as the ring-closing reagent[22].The lignin derivative containing ketone groups can be obtained via an esterification reaction with levulinic acid[19].As shown in Fig.1,lignin-based NIPUs were synthesized by a two-step method.First,BCC reacts with excess polyetheramine D-400 at 50℃to form the NPHUs prepolymer via the ring-opening reaction,and then the lignin derivative containing ketone groups was reacted with the unconsumed amino group through the Schiff base reaction.By varying the stoichiometric feed ratios of BCC and LEHL in Table 1,we prepared a series of lignin-based NIPUs,BLx:y(x∶y=BCC∶LEHL=100∶0,95∶5,90∶10,70∶30,and 50∶50,the molar ratio of cyclic carbonate groups from BCC to ketone groups from LEHL).The lignin content were 0,3.24%,6.42%,31.13%,and 45.53% in the lignin-based NIPUs for the samples BL100:0,BL95:5,BL90:10,BL70:30,and BL50:50,respectively.

Table 1 The feeding formulations of different lignin-based NIPUs

3.2 Characterization of lignin-based NIPUs

The chemical structure of LEHL was determined by1H NMR and FT-IR spectroscopy.In the1H NMR spectrum of LEHL(Fig.2(b)),the newly appeared peak at 2.1 ppm is assigned to the methyl protons of the acetylpropyl terminal in LEHL.Pentafluorobenzaldehyde was used as the internal standard to determine the ketone group content of LEHL,which was found to be 2.17 mmol/g.In the1H NMR spectrum of BCC(Fig.2(c)),the peak at 4.25 ppm corresponds to the methylene protons from the cyclocarbonate groups.In the1H NMR analysis of the NPHUs prepolymer prepared by the aminolysis reaction between BCC and D-400(Fig.2(d)),the peak at 4.25 ppm attributable to the cyclocarbonate groups was not present,which signified that the reaction was completed.

The FT-IR spectrum(Fig.3(a))revealed that the absorption peak at 1702 cm−1,which is attributable to the carboxyl group of lignin,was not present on LEHL,and the newly appeared absorption peaks at 1716 and 1740 cm−1on LEHL were related to the stretching vibration of C=O bond on the ketone and ester groups,respectively.Furthermore,the intensity of the absorption peak at 3443 cm−1,which is attributable to the hydroxyl group of LEHL,was weaker than that of EHL.The intensity of the absorption peak at 2936 and 2859 cm−1related to—CH2—and—CH3of LEHL was stronger than that of EHL.These results indicate the successful graft of the ketone group and the formation of an ester bond[19].In order to ensure complete polymerization via the Schiff base reaction,the chemical structure of lignin-based NIPUs was investigated by FT-IR spectroscopy.As shown in Fig.3(b),the absorption peak at 1716 cm−1related to the ketone group was not present,and the absorption band at around 1650 cm−1ascribed to C=N stretching vibration appeared on BL70:30and BL50:50,indicating that the NPHUs prepolymer reacted to form imine groups.The absorption peak at 1700 cm−1,which is associated with carbamate,represents the formation of ligninbased NIPU networks,which is consistent with previous reports[23-24].

3.3 Mechanical properties of lignin-based NIPUs

The mechanical properties of the resultant lignin-based NIPUs were studied by a static uniaxial tensile test at 25℃,and their stress-strain curves are presented in Fig.4.Compared with the control sample without lignin,the lignin-based NIPUs obtained possess enhanced mechanical properties,which were probably due to the imine-based network and the aromatic structure of lignin.With the increase of lignin content from 3.24%to 45.53%,the tensile strengths of the lignin-based NIPUs gradually increased from 0.47 MPa to 2.86 MPa,whereas their corresponding elongation-at-break values decreased from 330% to 151%,which is superior to most lignin-based NIPUs that have been reported.Lee et al[25]showed that carbonated soybean oil could be cured with a coupling agent,3-aminopropyltriethoxysilane,to form urethane bridges,and when lignin was introduced,a sustainable polyurethane with a low tensile strength(~1 MPa)was obtained.Salanti et al[17,26]reported that cyclic carbonated lignin derivatives cured with diaminododecane were too brittle for mechanical property measurements.Mimini et al[27]reported on lignosulfonate-based NIPUs that used organic carbonates in the cyclocarbonation reaction;however,no mechanical testing was performed.Recently,Sternberg et al[28]synthesized cyclocarbonated lignin through oxyalklyation and subsequent cyclocarbonation,which was then cured with dimer diamine to obtain highperformance lignin-based NIPUs.However,lignin oxyalkylation was performed at higher reaction temperatures(>170℃),and their reprocessing properties were not reported.

The Young's modulus of lignin-based NIPUs continuously increased with increasing lignin content,and the highest Young's modulus of 41.1 MPa was obtained when the lignin content reached 45.53%.These results were attributed to the incorporation of larger proportions of lignin-based rigid segments in the polymeric structure.However,for samples with lower lignin content due to the relatively extensive use of polyetheramine D-400 as the raw material,a decrease in tensile strengths and Young's modulus was observed;this was due to a large number of ether chain soft segments in the lignin-based NIPUs.In addition,owing to the rigid structure of lignin,the fracture absorption energy of NIPUs increased with increasing lignin content,indicating that the increase in lignin content contributes to the improvement of the toughness of NIPUs(Fig.4(c)).All these results indicate that the incorporation of lignin contributes to the improvement of the mechanical performance of lignin-based NIPUs.

3.4 Dynamic mechanical properties of lignin-based NIPUs

DMA was further performed to investigate the effects of the[BCC]∶[LEHL]ratios on the mechanical properties of the final lignin-based NIPUs.As shown in Fig.5(a),the storage modulus(E')gradually increased with an increase in lignin content from 3.24% to 45.53% in lignin-based NIPUs,which is consistent with the trend of Young's modulus due to the rigid structure of lignin.The glass transition temperature(Tg)was measured from the slope change present in the heating curve of the DSC thermograms.As shown in Fig.5(b),higher loadings of the aliphatic polyetheramine D-400 created a more dramatic transition from the glassy state,while a higherTgwas obtained with increasing lignin content.These results are associated with the complex macromolecular structure of lignin[29],which limited the chain transfer in the polymeric structure.

3.5 Thermal stability of lignin-based NIPUs

TGA and differential thermogravimetry(DTG)were used to examine the thermostabilities of all ligninbased NIPUs in Fig.6,and the characteristic parameters are summarized in Table 2.

The weight loss curves of all lignin-based NIPUs can be roughly divided into two stages of decomposition.The first stage between 150℃and 350℃was mainly related to the side chain of ligninand urethane bonds[30],and the second stage between 350℃and 550℃was attributed to the polyether chain segment of the diamine and the cross-linked aromatic structure of the lignin component[31].From the TGA curves(Fig.6(a)),the 5% weight loss temperatures(Td5%)of lignin-based NIPUs was reduced with increasing lignin content,and the corresponding temperature at the maximum degradation rate(Tmax)was slightly decreased.However,it is emphasized that the 50% weight loss temperatures(Td50%)and final residue of lignin-based NIPUs were clearly higher than those of the control sample,demonstrating that the thermal stabilities of lignin-based NIPUs were improved with the increase in lignin content.Furthermore,the maximum rate of weight loss of all lignin-based NIPUs occurred at approximately 320℃-400℃(Fig.6(b)),which is mainly due to the massive degradation of lignin,as well as the dissociation of the urethane bond and lignin components at around 350℃,followed by the rapid breakdown of the diamine constituents[28].It can be seen that the maximum rate of weight loss of all lignin-based NIPUs gradually decreased with an increase in lignin content,which was probably due to the cross-linked aromatic structure of the lignin component.

Table 2 Thermal properties of lignin-based NIPUs

3.6 Reprocessing properties

Conventional crosslinked PUs cannot be recycled once a permanent crosslinked network is formed during the curing process.However,introducing DMAP may enable the recycling of PU thermosets.Therefore,a reprocessing test was performed to examine the recyclability of the lignin-based NIPUs with the DMAP catalyst by cutting the material into pieces and treating it at 130℃and 10 MPa for 1 h.The fragmentized pieces of lignin-based NIPUs can be reprocessed into shapes after hot-press treatment in a mold(Fig.7(a)).The ester bonds underwent a transcarbamoylation exchange reaction at elevated temperatures in the presence of DMAP catalyst,and led to the rearrangement of chemical cross-linking networks.These reactions endow lignin-based NIPUs with reprocessability.As shown in Fig.7(b),the samples can be reprocessed in a short period of time and retain more than 75% tensile strength.These results confirmed the reprocessability of lignin-based NIPUs.

4 Conclusions

In this study,an environmentally friendly and non-toxic route to synthesize lignin-based non-isocyanate polyurethanes(NIPUs)is presented.Bis(6-membered cyclic carbonate)(BCC)was first used to prepare NH2-terminated polyhydroxyurethanes under mild conditions,which subsequently reacted with highly reactive ketone groups on levulinate enzymatic hydrolysis lignin(LEHL)to prepare lignin-based NIPUs.The tensile strength,Young's modulus,toughness,storage modulus,glass transition temperature,and thermal stability of lignin-based NIPUs gradually increased with increasing lignin content,and the highest Young's modulus of 41.1 MPa was obtained when the lignin content reached 45.53%.Lignin-based NIPUs exhibit good reprocessing properties.The dynamic mechanical and reprocessing properties of lignin-based NIPUs will be investigated in future research.This study represents a new strategy for the synthesis of value-added lignin-based NIPU networks,which may be considered good candidates for green materials.

Acknowledgments

The authors wish to express their gratitude for the grants from the Natural Science Foundation of China(21706154),the National Key Research and Development Program of China(2017YFB0307903),the Foundation of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education of China(KF201916),and the Natural Science Foundation of Shaanxi Province,China(2019JQ-277).