Development of High Performance Polyurethane Elastomers Using

Research Article pubs.acs.org/journal/ascecg

Development of High Performance Polyurethane Elastomers Using Vanillin-Based Green Polyol Chain Extender Originating from Lignocellulosic Biomass Haemin Gang,†,⊥ Daewoo Lee,‡,⊥ Kwon-Young Choi,§,⊥ Han-Na Kim,‡ Hoon Ryu,∥ Dai-Soo Lee,‡ and Byung-Gee Kim*,† †

School of Chemical and Biological Engineering, Seoul National University, Gwanak-ro 1, Gwanak-gu, 08826 Seoul, Republic of Korea ‡ School of Chemical Engineering, Chonbuk National University, Baekje-daero 567, Deokjin-gu, 54896 Jeonju-si, Jeollabuk-do, Republic of Korea § Department of Environmental Engineering, College of Engineering, Ajou University, World cup-ro 206, Yeongtong-gu, 16499 Suwon-si, Gyeonggi-do, Republic of Korea ∥ Industrial Biotechnology Program, Chemical R&D Center, Samyang Corporation, Daedeok-daero 730, Yuseong-gu, 34055 Daejeon, Republic of Korea S Supporting Information *

ABSTRACT: Vanillin can be obtained from waste of lignocellulosic bioresources with various methods.1−3 Such vanillin was used as chain extender [divanillin-ethanol amine conjugate (DVEA)] after its dimerization and further modification with ethanolamine in the synthesis of biobased polyurethane, thereby increasing wt % of biocontents in the final polymer. 1,4Butanediol often used as a general chain extender in polyurethane synthesis was replaced partially with DV-EA. The generated polyurethane hard segment consists of DV-EA polyol and MDI (methylene diisocyanate) units or 1,4-butanediol and MDI units, respectively. The properties of the DV-EA-based polyurethane were investigated with differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analyzer (DMA), X-ray diffraction spectroscopy (XRD), and universal testing machine (UTM). The results showed that this advanced polyurethane has 128% of Young’s modulus and 147% of increased strain compared to those of control, while its strength and thermal stability were maintained. It is expected that this new biobased tetraol may inspire a new perspective of vanillin application in biobased polyurethane synthesis. KEYWORDS: Polyurethane, Lignin, Chain extender, Vanillin

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INTRODUCTION Polyurethane (PU) is a very versatile polymer prepared by polyol and isocyanates and has the largest world market in volume of production. Various PUs have different properties for their purpose. Solid PUs are used as coatings, adhesives, and elastomers, whereas foams are used in automotive parts, furniture, and heat insulation.4,5 The properties of PUs are dependent on the chemical structures of polyol and isocyanate used, and can be optimized depending on their purposes such as cushioning, building insulation, sealants, and fibers.6,7 In spite of their versatile usages and market values, aromatic/ aliphatic isocyanates in the synthesis of PUs are reported to be generally toxic to human health by inducing severe allergic reaction or dyspnea. In addition, synthesis of isocyanate requires phosgene, which is one of the most hazardous chemicals.8−14 Therefore, phosgene free isocyanate synthesis © 2017 American Chemical Society

or isocyanate free polyurethane synthesis has attracted enormous attention to date.15−18 Ivan Javni, for example, synthesized soy-based PU by reacting carbonated soybean oil with different diamines.19,20 Da-Lei Sun successfully produced hexamethylene-1,6-diamine isocyanate by decomposition of dimethyl hexane-1,6-dicarbamate using zinc-incorporated berlinite as a catalyst.15 We were interested in developing a phosgene free process in PU synthesis by replacing MDIs and TDIs with new diisocyanates derived from lignin disruptive chemicals such as vanillin, vanillic acid, and ferulic acid. Since the introduction of isocyanate functional groups to carboxyl moiety of starting materials, the so-called Hoffmann rearrangeReceived: December 6, 2016 Revised: April 13, 2017 Published: April 14, 2017 4582

DOI: 10.1021/acssuschemeng.6b02960 ACS Sustainable Chem. Eng. 2017, 5, 4582−4588

Research Article

ACS Sustainable Chemistry & Engineering

MALDI-TOF analysis, 1 μL of super 2,5-dihydroxybenzoic acid (DHB) matrix solution (50 mg/mL in 70% acetonitrile/30% dH2O) was mixed with 1 μL of 10 mM divanillin in ethanol. The mixture was spotted on a stainless steel MALDI plate and dried at room temperature. Mass spectra were acquired with reflectron mode in positive ion mode under the following conditions (accelerating voltage = 20 kV, laser frequency = 60 Hz, ion source 1 voltage =19 kV, ion source 2 voltage = 16 kV, lens voltage = 9.8 kV, detector gain = 5.8 and laser power = 78%), and intensities of 1000 shots were accumulated to obtain a spectrum. Modification of Divanillin with Ethanol Amine. A 122 mg portion of ethanolamine was dissolved in 100 mL of DMF. A 302 mg portion of DV was dissolved into ethanolamine solution and heated at 70 °C for 5 h. The reaction mixture was filtered by vacuum filter and filtered cake dried in a 60 °C oven overnight. The dried cake was ground by mortar to obtain the powdered form of DV-EA. Synthesized DV-EA was characterized by MALDI-TOF and FT-IR. FT-IR spectra were analyzed in the wavenumber range (cm−1) 4000−650. For MALDI-TOF analysis, super 2,5-dihydroxybenzoic acid (DHB) was used as a matrix. A 1 μL portion of super DHB matrix solution (50 mg/mL in 70% acetonitrile/30% dH2O) was mixed with 1 μL of 10 mM DV-EA in DMF. The same MS analysis method and condition as that for divanillin analysis were used for DV-EA. Synthesis and Characterization of Vanillin-Based Polyurethane. A 2 mol portion of diisocyanate was dissolved in 1 mol of PTMEG. The reaction mixture was stirred at 60 °C under nitrogen atmosphere for 3 h. Chain extender of polyurethane was prepared by adding 0−20 mol % of DV-EA into 1,4-butanediol. Further addition of DV-EA resulted in phase separation. Stoichiometric amounts of the prepared chain extenders were mixed with PU prepolymer, prepared by mixing 2 mol of MDI and 1 mol of PTMEG-2000 and casted in glass mold and cured at 110 °C for 1 day. Overall synthesis process was shown in Figure 1.

ment, requires hazardous chemicals such as thionyl chloride and very harsh reaction conditions, it is not easy to do largescale production as well. Besides phosgene free isocyanate synthesis, another issue regarding PU synthesis is its biological contents with increasing concerns regarding eco-friendly polymer production and environmental regulations such as REACH (registration, evaluation, authorization, and restriction of chemicals). One of the efforts to increase biological contents in PU synthesis is to use natural oil polyol (NOP),21,22 which is usually derived from a plant oil like castor or crop with an aliphatic long chain structure. However, the rising costs of crops and feedstocks have created a demand for new sources of NOPs. Another approach demonstrated here is to use lignin derived phenolic compounds as a chain extender instead of 1,4-butanediol, which is generally used as a diol chain extender.23−25 One of the plausible candidates to replace a polyol extender unit is divanillin derived from lignin disruptive chemicals26−28 in the presence of radical initiators.29 It has aromaticity which can impose rigidity on the final PU and has the benefits of symmetrical difunctional groups of aldehyde and methoxyalcohol with ortho configuration on its aromatic body skeleton. The generated divanillin was easily converted to biobased polyol by two modification steps as it includes two alcohol and aldehyde functional groups at each end. We modified two aldehydes with monoethanol amine to introduce additional aliphatic alcohol groups on divanillin. This divanillin-ethanolamine (DV-EA) is a tetraol compound that has both aromaticity and an aliphatic alcohol, so that it can provide structural robustness to polyurethane. Herein, a vanillin-based polyol chain extender for PUs was successfully synthesized, and this DV-EA plays a role as a hard segment of PU. Here, DV-EA replaces the portion of 1,4-butanediol in PU to enhance the characteristics of PU. Thermal properties of DV-EA-based PU were further characterized.

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MATERIALS AND METHODS

Chemicals and Instruments. Vanillin (Daejung Chemicals, Daejeon, South Korea), ferric chloride hexahydrate (Kanto Chemicals), membrane filter (Millipore, 0.45 μm), and 4,4-diphenylmethane diisocyanate (MDI) (BASF) were purchased and used without any further purification. Poly(tetramethylene ether glycol) of molecular weight 2000 (PTMEG-2000), 1,4-butanediol, p-benzoquinone, and ethanolamine were purchased from Sigma-Aldrich and used without any further purification. Matrix-assisted laser desorption/ionizationtime-of-flight (MALDI-TOF) mass spectrometer (Bruker Daltonics Microflex LRF, Bruker), differential scanning calorimeter (Q-20, TA Instruments), dynamic mechanical analyzer (Q-800, TA Instruments), X-ray diffraction spectroscopy (D8 discover with GADDS, Bruker), and thermogravimetric analyzer (Q-50 from TA-instrument) were employed to investigate thermal properties of PUs. Tensile properties were measured with a universal testing machine (LR5K from Lloyd Instrument). FT-IR spectra were obtained with a Nicolet 6700 instrument (Thermo Scientific). Preparation of Divanillin. A 200 mmol portion of vanillin was dissolved into 500 mL of distilled water under 70 °C for full dissolution. A 200 mmol portion of ferric chloride was dissolved in 500 mL of distilled water and mixed with the prepared vanillin solution. After 1 h of stirring, the reaction mixture was cooled down in ice to terminate further reaction. The reaction mixture was filtered by vacuum filter with a 0.45 μm membrane filter. The filtered cake was washed with distilled water two times for elimination of ferrous chloride. After washing, the filtered cake was dried in a 60 °C oven overnight. The dried cake was ground by mortar to obtain the powdered form of divanillin. The synthesized divanillin was characterized by MALDI-TOF and 1H NMR (see Figure S1). For

Figure 1. Process flowchart of DV-EA polyurethane synthesis. Thermal stabilities of the PU elastomers were studied employing TGA with a temperature gradient of 20 °C/min, and changes of storage moduli and loss moduli of the PUs were measured with DMA with 5 °C/min in a N2 gas environment. Enthalpy changes for PUs were investigated employing DSC with 10 °C/min in N2 atmosphere. Infrared spectra of PUs were obtained with an FT-IR spectrometer (FT-IR 302 from Jasco). Stress−strain properties of the PU were measured employing a universal testing machine (UTM) at a pulling speed of 500 mm/min. Crystalline states of PUs were measured by XRD.

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RESULTS AND DISCUSSION Preparation of Divanillin-Ethanol Amine Conjugate. DV-EA was prepared by a two step reaction, i.e., dimerization 4583


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Figure 2. (A) FT-IR spectrum of DV-EA was obtained by pelleting. (B) MALDI-TOF spectrum of DV-EA.

four alcohol moieties in the divanillin backbone. However, since DV is only soluble under basic conditions in distilled water, and ethanol amine solution is a strong base (pH 11), only 61% of 20 mM of ethanolamine reacted with 10 mM of DV at 70 °C for 1 h. Under unoptimized conditions, a side product modified at one aldehyde moiety of DV was observed at the 346 (m/z) peak in MALDI-TOF spectra. To overcome such partial reaction of DV with ethanol amine, additional optimization of solvent screening was executed. Under dimethyl-formamide solution, ca. 92% yield of DV modification with ethanol-amine was achieved. DV-EA was characterized with FT-IR spectrum and MALDI-TOF in Figure 2. According to the FT-IR spectrum, no amine peak near 3400 cm−1 from ethanol amine was observed except the imine peak near 1653 cm−1, indicating that both aldehyde groups from reacted DV were completely modified. This was also confirmed by MALDI-TOF mass spectroscopy, and the observed m/z value of 388 was well-matched with the corresponding m/z value.

of vanillin and ethanolamine modification. First, dimerization of vanillin is C−C bond formation at C5 and C5′ position of the aromatic ring by radical polymerization. This oxidative radical dimerization of vanillin (100 mM) can occur using enzymatic methods such as 100 U of laccase (47% yield) and 1000 U of horseradish peroxidase (96% yield) under 1 L aqueous solution at room temperature (RT). However, the enzyme-based dimerization of vanillin has several disadvantages over chemical methods, i.e., low dimerization yield, poor solubility of vanillin in water, and high cost of enzyme used. To overcome these drawbacks, a synthetic FeCl3 catalyst dependent dimerization process was examined. FeCl3 provides low cost, an easy separation procedure, and mild reaction conditions even for a large-scale reaction. Yield of the dimerization in the presence of FeCl3 was ca. 60%, and the dimerized vanillin was characterized by 1H NMR and MALDI-TOF mass analysis as shown in Figure S1. Second, introduction of additional alcohol functional groups at the aldehyde of C1 and C1′ positions was investigated using monoethanol amine, thereby resulting in 4584


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Figure 3. Analysis of DV-EA polyurethane with TGA, DMA, DSC, and XRD with various compositions. (A) Thermal stability, (B) storage/loss modulus, (C) heat flow, and (D) XRD of DV-EA PUs and control.

Table 1. Result of Thermal Analysis of Control PU and DV-EA-Based PUs *soft segment Tg (°C)a **soft segment Tg (°C)b 5% Td (°C) a

control PU

DV-EA 5%

DV-EA 10%

DV-EA 15%

DV-EA 20%

−67.39 −61.61 329.59

−67.42 −61.73 334.95

−68.08 −63.03 336.56

−67.47 −62.34 338.12

−67.2 −61.96 341.54

* is measued by DSC . b** was measured by DMA.

Synthesis and Characterization of Vanillin-Based Polyurethane. Molecular weights of DV-EA PUs were measured by GPC chromatogram in Table S1. The TGA thermograms for the control MDI-(1,4-butanediol)-based PU and DV-EA-based PUs are compared in Figure 3A. Thermal stability of prepared PUs increases with increasing DV-EA content. According to TGA thermogram data, degradation temperature of 5% loss increased from 329.59 to 341.54 °C with increasing DV-EA contents. Ash contents of the PUs also increased with higher DV-EA contents. The improvement in the thermal stability of the PUs is attributable to a higher percentage of the replaced imine groups of DV-EA. Storage moduli and loss moduli of the PUs are shown in Figure 3B. Rubbery plateau regions were observed from 25 to 150 °C in Figure 3B. Tg’s of soft segment domain measured by the peak temperatures of loss moduli were not affected by the DV-EA in the PUs. It is of interest to note that the rubbery plateau modulus of the PUs examined was the highest, when 10% of 1,4-butanediol was replaced by DV-EA. The rubbery plateau moduli of PU elastomers were also determined according to changes in physical and chemical cross-link density. The highest modulus of PU elastomer means the highest physical cross-link density when the ratios of the PU elastomers to the chain extenders were stoichiometrically fixed.

The physical cross-link density of PU generally resulted from intermolecular hydrogen bonds among hard segments.30 The PU elastomers displaying the highest rubbery plateau moduli tend to form more intermolecular hydrogen bonds. Glass transition temperatures (Tg’s) of the PU were also observed employing DSC and shown in Figure 3C. Enthalpy changes according to the changes in the percentage of soft segment were observed near −62 °C. Thermal properties of the PUs are summarized in Table 1. It was confirmed that Tg of the PUs prepared by partially replacing 1,4-butanediol with DV-EA, i.e., 10 mol %, was the lowest among the PUs investigated. The lowest Tg of the PU is attributed to the highest microphase separation of soft segment domain and hard segment domain, which was confirmed by DMA data. The peak temperature of E″ is generally used to determine the glass transition temperature. The shifted data were presented to clearly distinguish the changes of Tg of each sample. X-ray diffraction spectroscopy was obtained to measure the crystalline state of PUs in Figure 3D. The X-ray source was Cu Kα radiation, powered at 40 kV and 30 mA with a radiation wavelength of 1.54 Å. The scattering angle (2θ) ranges from 5 to 40° and was scanned at 1°/min. In XRD spectra, all of the PUs showed a broad band centered around a diffraction angle (2θ) of 20°. According to Koberstein and Stein,31 dissociation of the hard 4585


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The degree of phase separation (DPS) can be obtained by the following equation.

segment is accompanied by endotherms observed around 180 °C in DSC thermograms due to the dissociation of interurethane hydrogen bonding within an ordered hard segment domain. On the basis of WAXS results, they suggested that the order is not truly crystalline, but composed of pseudolattice planes of the hard segment domains. It is postulated that the vanillin-based PUs showed the increase of hard segment melting temperature of the pseudocrystalline state due to the increase of microphase separation and hydrogen bonding. To clarify whether DV-EA PU is linear rather than crosslinked, PU specimens (0.5 g) were cut from each synthesized DV-EA PU sample. The specimens were immersed in 15 g of THF and DMF for 5 h at 70 °C, separately. After 5 h, all of the specimens were clearly dissolved in both solvents. This implies that the DV-EA PUs were linear PUs rather than cross-linked PUs (data not shown). Quantitative analysis of microphase separation was carried out by FT-IR spectra shown in Figure 4, and the ratios of

DPS =

C bonded R = C bonded + Cfree R+1

(2)

A hydrogen bonded NH stretching peak was observed at 3320 cm−1. However, a free hydrogen bonded NH stretching peak could not be observed at 3445 cm−1. In the case of DV-EA 10%, the hydrogen bonded NH stretching peak showed the highest intensity as compared to the others. This result can be explained by the strongest hydrogen bond in DV-EA-10% than in other PUs. The formation of hydrogen bonding by the urethane group can be determined by examining the hydrogen bonded carbonyl peak at 1703 cm−1 and free carbonyl peak at 1733 cm−1. It is worthwhile to note that DPS of the PUs showed the maximum when the rubbery plateau modulus of the PU was the highest while the soft segment Tg of the PU was the lowest among the PUs investigated. It is postulated that the partial replacement of 1,4-butanediol with DV-EA by 10 mol % resulted in the efficient hydrogen bonding and microphase separation of the PUs. Stress−strain behaviors of the PU elastomers are given in Figure 5 and summarized in Table 3. It was observed that

Figure 4. FT-IR spectra of DV-EA PUs and control.

absorbance peak intensity to calculated DPS were presented in Table 2. In general, microphase separation of PU accompanies intermolecular hydrogen bonds. Thus, the microphase separation can be studied with FT-IR spectra due to the hydrogen bonds.32,33 The degree of hydrogen bonds can be evaluated by FT-IR absorption peak intensities using eq 1 shown below A C ε R = bonded bonded = 1703 Cfreeεbonded A1733 (1)

Figure 5. Stress−strain curve of DV-EA PUs and control.

tensile strengths of the PU elastomers were comparable, while elongation at the break was increased with increasing DV-EA contents. It is of interest to note that Young’s modulus, i.e., a measure of chemical and physical cross-link density, showed the maximum with increasing the DV-EA contents according to the phenomenon discussed above.

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where A is the intensity of the characteristic absorbance, C is the concentration, and εbonded and εfree are the extinction coefficients of the hydrogen bonded carbonyl groups and the free carbonyl groups, respectively. In this study, the value of εbonded/εfree was taken as 1.0.

CONCLUSION We synthesized vanillin-based polyurethane with enhanced physical and chemical properties, compared to the properties

Table 2. Degree of Phase Separation by the Changes in the Percentage of DV-EA in Various PUs

Abonded/Afree DPS (%) DPM (%)

control PU

DV-EA 5%

DV-EA 10%

DV-EA 15%

DV-EA 20%

1.06 51.49 48.51

1.13 53.19 46.81

1.39 58.18 41.82

0.71 42.7 57.3

0.51 33.69 66.31

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ACS Sustainable Chemistry & Engineering Table 3. Mechanical Properties Control PU and Vanillin-Based PUs control PU

DV-EA 5%

DV-EA 10%

DV-EA 15%

DV-EA 20%

32.91 522.57 7.53

29.07 644.82 9.05

26.44 655.38 9.67

28.42 672.68 9.19

30.36 770.86 8.02

stress (MPa) strain (%) Young’s modulus (MPA)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02960. Characterization of divanillin and molecular weight of DV-EA PUs (PDF)

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ABBREVIATIONS DV-EA, divanillin-ethanolamine; MDI, methylene diphenyl diisocyanate; DSC, differential scanning calorimeter; DMA, dynamic mechanical analyzer; XRD, X-ray diffraction; PU, polyurethane; TGA, thermogravimetric analyzer; UTM, universal testing machine; NOP, natural oil polyol; DHB, 2,5dihydroxybenzoic acid; MALDI-TOF, matrix-assisted laser desorption/ionization-time-of-flight; NMR, nuclear magnetic resonance

for typical polyurethane, by changing DV-EA contents ranging from 5 to 20 mol %. DV-EA polyurethane was successfully characterized with DSC, DMA, TGA, FT-IR, and UTM analytical methods. DV-EA-based polyurethane maintained its thermal stability and strength, while heat flow was decreased and its ultimate strain was increased up to 147%. The enhanced polymeric properties might result from its polyol functional groups as well as DV-based aromatic ring structure. Aromatic rings from divanillin seem to stabilize the structure of the urethane polymers, and the four hydroxyl groups of DV-EA also facilitate additional hydrogen bonds between soft segments and hard segments of the polyurethane. Aliphatic amineethanol material with longer chain or aromatic amine-ethanol material may also be able to form polyurethane and affect polymeric and physical properties of modified biopolyurethane. This method of modification was inspired by the natural structures of the lignin complex to enhance the structural stability with polymeric materials. There have been several efforts to synthesize eco-friendly PUs such as bypassing phosgene dependent diisocyanate synthesis and incorporating eco-friendly oil-based bioresources instead of synthetic diols. Our results here demonstrate one kind of effort to facilitate functional eco-friendly PU synthesis without losing the desired polymeric properties. This new method may provide a novel modification method of biopolyurethane and inspire a new standpoint for vanillin application and polyurethane modification.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Byung-Gee Kim: 0000-0002-3776-1001 Author Contributions ⊥

H.G., D.L., and K.-Y.C. contributed equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No. 10049677, “BioIsosynatates and Alternative Biobased Materials for Polyurethane Using Green Carbon”, and we give special thanks to Han Gyu Park for the operation of the MALDI-TOF instrument. 4587


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