A Solvent-Free and Scalable Method To Prepare Soybean-Oil-Based

Research Article pubs.acs.org/journal/ascecg

A Solvent-Free and Scalable Method To Prepare Soybean-Oil-Based Polyols by Thiol−Ene Photo-Click Reaction and Biobased Polyurethanes Therefrom Yechang Feng,† Haiyan Liang,† Ziming Yang,‡ Teng Yuan,† Ying Luo,† Puwang Li,‡ Zhuohong Yang,*,† and Chaoqun Zhang*,† †

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China Products Processing Research Institute, Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524001, P. R. China

‡

ABSTRACT: A solvent-free and scalable method was developed for the preparation of soybean-oil-based polyols by a thiol−ene photo-click reaction with a homemade photochemical reactor. The effect of reaction parameters, including photoinitiators, reaction time, molar ratios of thiols to carbon−carbon double bonds, and power of the mercury lamp, on the structures of the resulting polyols was investigated. The mechanism of the thiol−ene photo-click reaction was also discussed. On the basis of these novel polyols, several polyurethanes were prepared using different diisocyanates (aliphatic, cycloaliphatic, and aromatic isocyanate) and characterized. The resulting polyurethane films possess good performance, including the highest glass transition temperature of 41.3 °C, tensile strength of 15.7 MPa, and elongation at break of 471.0%. KEYWORDS: Soybean oil, Polyols, Thiol−ene reaction, Polyurethane

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INTRODUCTION

Vegetable oils are esters formed by glycerol and different fatty acids containing 8−18 carbon atoms that can be either poly-, mono-, or unsaturated, depending on the plant type and climatic conditions of their growth.12 Although some vegetable oils inherently contain reactive functional groups in their fatty acid chains, such as hydroxyls (castor oil) or epoxies (vernonia oil), most vegetable oils require the introduction of hydroxyl groups on the reactive sites (carbon−carbon double bonds) prior to their use for polyurethanes. Currently, there are four methods for the transformation of these inexpensive and renewable natural feedstocks into polyols.13 Epoxidation of carbon−carbon double bonds followed by ring opening with nucleophilic reagents (amines, halides, or alcohols) is an effective way to produce polyols.14,15 However, the resulting hydroxyl groups are usually secondary which demonstrates low activity toward isocyanates. Hydroformylation with CO and H2 can prepare polyols with primary

Polyurethanes (PUs) are one of the most important polymers and have been widely used in a variety of applications, such as coatings,1,2 sealants,3,4 adhesives,5,6 foams,7,8 and composites,9,10 because of their excellent properties. Generally, PU is prepared by the polyaddition of isocyanate with at least two NCO groups and polyol with two or more hydroxyl groups. The structures and properties of the resulting polyurethanes can be tuned by the selection of appropriate polyols and diisocyanates and mass ratios between these two. Only a few diisocyanates are commercially available while a variety of polyols are designed and supplied. Thus, the design and production of novel polyols become the dominating factor for the determination of the performance of PUs. So far, most of the polyols used industrially are derived from petroleum. With the depletion of the world’s crude oil stock and increasing environmental concerns, efforts on a global scale are dedicated to find a renewable resource (such as cellulose, natural oils, and so on) for novel polyols to replace petroleum based counterparts.11 © 2017 American Chemical Society

Received: May 27, 2017 Revised: June 17, 2017 Published: July 11, 2017 7365

DOI: 10.1021/acssuschemeng.7b01672 ACS Sustainable Chem. Eng. 2017, 5, 7365−7373

Research Article

ACS Sustainable Chemistry & Engineering

diisocyanate (TDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI) were purchased from Jining Baiyi Chemical Co., Ltd. All materials were used as received without further purification. Preparation of Soybean-Oil-Based Polyols. The homemade photochemical reactor is shown in Figure 1. The 100−1000 W

hydroxyls, which are far more reactive than the secondary hydroxyls with isocyanates.16,17 However, expensive rhodium catalysts and hazardous gases are needed. Partial transesterification of vegetable oils with alcohols or amine at an elevated temperature can also produce a mixture of mono- and diglycerides with primary hydroxyls.18,19 However, the absence of hydroxyl groups in the middle of fatty acid chains (unless castor oil is used) results in these chains acting as unreactive plasticizers. Ozonolysis of vegetable oil followed by reduction can produce polyols with a terminal primary hydroxyl, but the maximum hydroxyl number per molecule of the resulting polyols is limited to 3.20 It can be expected that more and more efforts are dedicated for the development of novel polyols for high-performance polyurethanes. Recently, there is increasing interest in the application of the thiol−ene click reaction for the development of a new route for polyols because of their virtues of high yields, simple reaction conditions, short reaction times, and easy purifications.21−23 For example, Desroches et al. have studied the parameters and side effects of the thiol−ene reaction by a model study of oleic acid with 2-mercaptoethanol, and successfully prepared rapeseed-oil-based polyols.24 The polyurethanes therefrom had the same thermal resistance as a commercial polyol. Alagi et al. optimized the reaction conditions to prepare castor-oil- and soybean-oil-based polyols with more than 99% carbon−carbon double-bond conversion.25 The resulting thermoplastic polyurethanes showed high toughness and the maximum tensile strength of 13.07 MPa. Ionescu et al. reported high-functional polyols with a hydroxyl number of 286 mg KOH g−1 by the thiol−ene reaction of castor oil with mercaptoethanol.26 The rigid polyurethane foams therefrom had a compression strength of 127 kPa. However, the mechanism of the thiol−ene reaction for the vegetable system is rarely reported. In addition, the scalable production of polyols by the thiol−ene reaction remains a challenge. In this paper, a solvent-free and scalable method was developed for the preparation of soybean-oil-based polyols by the thiol−ene click reaction with a homemade photochemical reactor. The effect of reaction parameters on the structures of the resulting polyols was thoroughly investigated, including types of photoinitiators, reaction time, molar ratios of thiols to carbon−carbon double bonds, and power of the mercury lamp. Proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), and Fourier transform infrared spectroscopy (FTIR) were used for the characterization of the structures of the resulting polyols and optimization of the reaction conditions. The mechanism of the thiol−ene click reaction was also elucidated. Polyurethanes from these soybeanoil-based polyols were prepared and characterized. The effect of different diisocyanates on the thermophysical properties, thermal resistance, and mechanical properties was discussed.

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Figure 1. Photochemical reactor for the thiol−ene photo-click reaction. adjustable mercury lamp is set in the central part and surrounded by eight quartz tubes. A magnetic stirrer is placed beneath the tubes for homogeneous mixing during the reaction. Appropriate ratios of SO, ME, and photoinitiator were added into the quartz tubes which were placed into the photochemical reactor. After the appropriate time, the product was diluted by ethyl acetate, washed with saturated NaCl solution at least 4 times, dried by anhydrous MgSO4, and filtered. Finally, the polyols derived from soybean oil were obtained after removal of the organic solvent by rotary evaporation and drying in the vacuum oven overnight. Preparation of Polyurethanes. Soybean-oil-based polyol (SOP) was thoroughly mixed with different diisocyanates (MDI, TDI, IPDI, and HDI), dibutyltindilaurate (DBTDL), and methyl ethyl ketone. The molar ratio of isocyanate groups to hydroxyl is 1.05:1. The mixtures first reacted at 70 °C for 3 h before being cast into the silicone molds and heated at 80 °C overnight (at least 12 h) in an oven for acquisition of the PU films. The resulting PU films were cut into specific dimensions for thermomechanical testing. Characterization. A Bruker AV600 spectrometer was used for recording of the proton nuclear magnetic resonance spectra of SO and SOPs. CDCl3 was used as solvent in all the measurements. Fourier transform infrared spectroscopy (FTIR) analysis was conducted over the wavenumber range 400−4000 cm−1 using a Thermo-Nicolet Nexus 670 FT-IR spectrometer. The molecular weight was measured by a THF-eluted Shimadzu Prominence GPC system equipped with an RID-10A refractive index detector, and Shodex KF804L and KF802.5 columns. The column flow rate and temperature were 1.0 mL min−1 and 40 °C. The standard sample used was polystyrene. The hydroxyl value of the polyols was determined using the Unilever method. A 10 g mixture of acetic anhydride and pyridine (the weight ratio is 1:9) was added to a round-bottom flask with 1.0 g of polyol under vigorous stirring. After the mixture reacted at 90−100 °C for 1 h, 25 mL of pyridine and 10 mL of distilled water were added. After the reaction continued another 25 min, the final products were titrated with a 0.5 M potassium hydroxide solution with phenolphthalein as an indicator. A blank determination was carried out with a similar procedure. A dynamic mechanical analysis (DMA) of PU films was conducted using a Netzsch DMA 242C dynamic mechanical analyzer with a filmtension mode of 1 Hz. Rectangular specimens of 20 mm × 6 mm × 0.5 mm (length × width × thickness) were used for the analysis. The samples were cooled by liquid nitrogen and held isothermally at −60

MATERIALS AND METHODS

Materials. Soybean oil (SO) was purchased from Jiangxi Yipusheng Pharmaceutical Co., Ltd. 2-Mercaptoethanol (ME) (>98%) was purchased from Alfa Aesar (China) Chemicals Co., Ltd. 2-Hydroxy2,2-dimethylacetophenone (1173) was purchased from Ryoji Organic Chemical Co., Ltd. Isopropyl thioxanthone (ITX) was purchased from Shanghai Curease Chemical Co., Ltd. Ethyl acetate was purchased from Shanghai Titan Scientific Co., Ltd. Magnesium sulfate was purchased from Tianjin Damao Chemical Reagents Factory. Sodium chloride was purchased from Tianjin Fuyu Chemicals Co., Ltd. Huntsman Suprasec 2642 diphenylmethane diisocyanate (MDI) was supplied by Guangzhou Xinye Commercial Co., Ltd. Toluene 7366


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ACS Sustainable Chemistry & Engineering Scheme 1. Preparation of Soybean-Oil-Based Polyols by the Thiol−Ene Photo-Click Reaction

Figure 2. 1H NMR spectra of soybean-oil-based polyols with and without a photoinitiator.

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°C for 3 min, and then heated to 120 °C at a rate of 5 °C min−1. The glass transition temperatures (Tg’s) of the films were generated from the peaks of tan δ curves. Thermogravimetric analysis (TGA) was conducted on a NetzschSTA 449C thermal analyzer. The films were heated from room temperature to 700 °C at a rate of 20 °C min−1 in nitrogen. The tensile properties were measured by a Shenzhen Suns UTM 5000 universal testing machine with a crosshead speed of 100 mm min−1. Rectangular films of 50 mm × 10 mm (length × width) were used for the test. The final tensile strength and elongation at break were obtained from average values of at least three replicates of each sample.

RESULTS AND DISCUSSION Preparation and Properties of Soybean-Oil-Based Polyol. The preparation of soybean-oil-based polyols by the thiol−ene photo-click reaction is shown in Scheme 1. The effect of several reaction parameters including photoinitiators, reaction time, molar ratios of thiols to carbon−carbon double bonds, and power of the mercury lamp on the structures of the resulting polyols was studied. The reaction conditions were optimized for acquirement of the maximum yield and doublebond conversion. Furthermore, the mechanism of the thiol− ene photo-click reaction is also discussed. 7367


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ACS Sustainable Chemistry & Engineering Effect of Photoinitiators. Two types of photoinitiators (1173 and ITX) were studied. Photoinitiator 1173 is cleavagetype, which produces free radicals by cracking bonds under the UV light while ITX belongs to hydrogen-capturing-type photoinitiators, which capture the hydrogen from other compounds for the production of radicals. A reaction time of 5 h, molar ratios of thiols to carbon−carbon double bonds of 3:1, and power of the mercury lamp of 300 W were used. The polyols prepared with 1173, ITX, and without a photoinitiator are identified as SOP-1173, SOP-ITX and SOP-NP, respectively. The 1H NMR spectra of polyols with and without a photoinitiator are shown in Figure 2. The hydrogen number of the functional groups was obtained with the use of the area under the peaks at 4.3−4.4 ppm for normalization corresponding to methylene of the glycerol chain. Compared to those for soybean oil (SO), peaks at 5.3−5.5 ppm for the resulting polyols, corresponding to carbon−carbon double bonds of fatty acids, decrease after UV irradiation for 5 h. In addition, new peaks at 2.7−2.8 and 3.7−3.8 ppm appear, which were, respectively, assigned to the two methylenes of ME grafting to SO. The results show that ME was successfully grafted into the carbon chains of SO by the reaction with carbon−carbon double bonds on the chains of fatty acids. Although the click reaction can be performed with or without a photoinitiator, the efficiency varies considerably under different conditions according to the carbon−carbon double-bond conversion as shown in Table 1.

presented in Scheme 2 b−d. The efficiency and yield are proportional to the content of the thiyl radicals that participated in the propagation step. That is why 1173 demonstrates much more efficiency than ITX in the thiol− ene click reaction of vegetable oils. Effect of Reaction Time. The effect of reaction time on the structures and double-bond conversion of the polyols is systematically studied. Four reaction time variables (0.5, 1, 3, and 5 h) were studied. Molar ratios of thiols to carbon−carbon double bonds of 3:1 and power of the mercury lamp of 300 W with 2 wt % 1173 were used. The corresponding polyols are identified as SOP-0.5h, SOP-1h, SOP-3h, SOP-5h, respectively. As shown in Figure 3a, peaks at 5.3−5.5 ppm, corresponding to carbon−carbon double bonds, decline with the increase of reaction time while peaks at 2.7−2.8 and 3.7−3.8 ppm (representing the methylenes of ME) increase. The GPC chromatograms of polyols prepared with different reaction times are shown in Figure 3b. Compared with those of SO, which is the starting material, the peaks of the polyols shifted to shorter times and became broadened, indicating increasing molecular weights. The peaks in the retention time of 15.5− 16.5 min correspond to the expected polyols. The peaks in the retention time of 14.5−15.5 min are assigned to the trimers and dimers of SO, respectively.24 Under UV irradiation, hydroperoxides and cyclic peroxides are generated followed by decomposition and radical coupling, resulting in the formation of oligomers by the self-oxidation mechanism.24,31 The existence of oligomers in the polyols may provide the resulting polyurethanes with a hyperbranched structure.31 FTIR spectra of the resulting polyols are shown in Figure 3c. The bands at 1750 cm−1 are used for normalization corresponding to esters of the vegetable-oil chain. The vibration of CH−S bonds at 600−700 cm−1 is too weak to be detected.24 An evident increase in the bands of 3416 cm−1 could be observed with the extension of reaction time, which is assigned to the stretching vibration of hydroxyl groups. At the same time, bands at 3008 cm−1, corresponding to the stretching vibration of carbon− carbon double bonds, decrease as the reaction time increases and completely disappear after 3 h. Therefore, a reaction time of 3 h will be used in the following study. Effect of Molar Ratios of Thiols to Carbon−Carbon Double Bonds. The effect of molar ratios of thiols to carbon− carbon double bonds on the structures and double-bond conversion of the polyols is systematically investigated. Four molar ratio variables (1:1, 2:1, 3:1, and 4:1) were studied. A 1173 wt % of 2, a reaction time of 3 h, and power of the mercury lamp of 300 W were used. These samples are identified as SOP-1:1, SOP-2:1, SOP-3:1, and SOP-4:1, respectively. In Figure 4a, peaks between 5.3 and 5.5 ppm, representing carbon−carbon double bonds, decrease with the increase of thiol content and disappear when the molar ratio reaches 4:1. Obviously, excessive thiols are needed for the complete consumption of carbon−carbon double bonds, because the carbon−carbon double bonds of SO are located in the middle of carbon chains, which shows low reactivity. A similar conclusion can be obtained from the GPC results in Figure 4b. When the molar ratio is lower than 2:1, a wide peak corresponding to residual SO remains at 16.5 min, and a shoulder, representing the polyols, appears at 16 min. Once the molar ratio rises to 3:1, the peak at 16.5 min disappears, indicating the total consumption of SO. FTIR results are shown in Figure 4c. Bands at 3416 cm−1, corresponding to the

Table 1. Properties of Soybean-Oil-Based Polyols double-bond conversiona (%) SO SOP-1173 SOP-NP SOP-ITX SOP-0.5h SOP-1h SOP-3h SOP-5h SOP-1:1 SOP-2:1 SOP-3:1 SOP-4:1 SOP-100W SOP-200W SOP-300W SOP-400W

78.54 30.63 10.21 36.78 59.86 79.00 79.54 35.96 35.96 79.93 94.55 79.93 92.81 91.42 89.44

number-average molecular weight

weight-average molecular weight

PDIb

1288

1296

1.01

1550 1681 1771 1779 1464 1485 1785 1859 1834 1872 1832 1847

1587 1723 1810 1819 1501 1531 1823 1888 1870 1901 1863 1876

1.02 1.02 1.02 1.02 1.02 1.03 1.02 1.02 1.02 1.02 1.02 1.02

a Calculated on the basis of 1H NMR spectra by the method previously reported.32 bPolydispersity index (PDI).

Photoinitiator 1173 possesses the highest efficiency (80.30%) while ITX demonstrates the lowest (21.56%). According to the general mechanism of the thiol−ene reaction, three steps including initiation, propagation, and chain transfer occur as shown in Scheme 2a.27,28 In the initiation step, each molecular 1173 produces two radicals under the UV light followed by the transfer to thiol, leading to the formation of two thiyl radicals.29 However, only one radical is generated by the hydrogencapturing of ITX or direct cleavage of the labile S−H bonds without a photoinitiator.30 The detailed mechanism is 7368


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Scheme 2. Mechanism of the Thiol−Ene Photo-Click Reaction (a) and the Formation of Thiyl Radicals Initiated with 1173 (b), ITX (c), and without a Photoinitiator (d)

Figure 3. 1H NMR spectra (a), GPC curves (b), and FTIR spectra (c) of soybean-oil-based polyols with different reaction times.

hydroxyl groups, increase with the increase of thiol content. The disappearance of bands at 3008 cm−1, corresponding to the carbon−carbon double bonds, occurs when the molar ratio reaches 3:1. However, trace carbon−carbon double bonds of SOP-3:1 are still observed in the 1H NMR spectrum. Therefore, 4:1 is chosen as the optimal molar ratio of thiols to carbon− carbon double bonds for the total conversion of carbon− carbon double bonds. Effect of Power of the Mercury Lamp. An analysis of the effect of power (100, 200, 300, and 400 W) of the mercury

lamp on the structures and double-bond conversion of the polyols was carried out. These polyols are identified as SOP100W, SOP-200W, SOP-300W, and SOP-400W, respectively. As presented in Figure 5a, peaks at 5.3−5.5 ppm, assigned to the carbon−carbon double bond, disappear when the power is more than 200 W. The intensities of peaks at 2.7−2.8 and 3.7− 3.8 ppm, corresponding to the methylenes of ME, show a tendency of increasing from 100 to 200 W and then declining slightly from 200 to 400 W. In addition, carbon−carbon double-bond conversions of SOP-200W, SOP-300W, and SOP7369


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Figure 4. 1H NMR spectra (a), GPC curves (b), and FTIR spectra (c) of soybean-oil-based polyols with different molar ratios.

Figure 5. 1H NMR spectra (a), GPC curves (b), and FTIR spectra (c) of soybean-oil-based polyols with different powers of the mercury lamp.

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certain value, a rapid decrease in storage modulus is observed; finally, a rubbery plateau appears in all curves at high temperature. In the curves of tan δ, only one peak can be observed, indicating the homogeneous properties of PU films. Tg values of different polyurethanes are obtained from the peak of tan δ curves and summarized in Table 2. PU-HDI shows the lowest Tg because of the linear aliphatic structure of HDI,33 while PU-MDI has the highest Tg resulting from the double rigid benzene rings of MDI. Thermal Stability. TGA curves and their derivative curves of PU films are shown in Figure 7. These curves show a similar

400W show no significant difference, indicating that power of the mercury lamp is not a main factor for the thiol−ene reaction. This is consistent with the observation from Myriam Desroches.24 In addition, there is no significant difference in the GPC curves and FTIR spectra for the polyols prepared with different powers of the mercury lamp. For a guarantee of the full conversion of carbon−carbon double bonds into hydroxyl groups, 300 W is chosen as the optimal power. As shown in Table 1, the conversion of carbon−carbon double bonds into hydroxyl groups varies intensively with the increase of reaction time and molar ratio. The power of the mercury lamp has little effect on the conversion. Thus, reaction time and molar ratio of thiols to carbon−carbon double bonds are the main factors affecting the reaction efficiency. According to the above results and discussion, the optimal conditions for the thiol−ene reaction of SO are obtained as follows: 2 wt % 1173 as photoinitiator, reaction time of 3 h, molar ratio of thiols to carbon−carbon double bonds of 4:1, and power of the mercury lamp of 300 W. The polyol prepared with these reaction conditions possesses a hydroxyl value of 199 mg KOH g−1 (from a titration test). This novel polyol is used for the preparation of polyurethanes with different diisocyanates in the following section. Properties of Polyurethanes Prepared from SOP. Thermomechanical Properties. PU films were prepared by the reaction of SOP with different diisocyanates at a molar ratio of NCO to hydroxyl of 1.05:1. The SOP with primary hydroxyl groups shows high reactivity toward aromatic and aliphatic diisocyanates, especially MDI. The mixture of SOP with MDI in the presence of DBTDL as a catalyst gelled in less than 1 min at 60 °C. Figure 6 shows the storage moduli and loss factor as functions of temperature for PU films with different

Figure 7. TGA curves (a) and their derivative curves (b) for PU films based on SOP with different diisocyanates.

trend and can be divided into two degradation phases: 200− 350 and 350−500 °C. The first phase is attributed to the disassociation of unstable urethane groups.34 The order of the thermal stability from high to low is PU-TDI, PU-IPDI, PUHDI, and PU-MDI resulting from the difference in the thermal stability of the corresponding urethane segments, which is consistent with the observation from the Hablot group.35 The second phase corresponds to the chain scission of the corresponding polyols. Upon application of the same polyol, no significant difference of the thermal stability between polyurethanes is found in this phase. Furthermore, temperatures of 10% degradation (T10), 50% degradation (T50), and maximum decomposition rates (Tmax) are summarized in Table

Figure 6. Storage moduli and loss factor (tan δ) as functions of temperature for PU films with different diisocyanates.

diisocyanates from −60 to 120 °C. The storage modulus (E′) values of all films show similar trends: a slow decrease is observed at low temperature; once temperature surpasses a

Table 2. Thermal and Mechanical Properties of PUs Based on Different Diisocyanates TGA in nitrogen (°C) PU-HDI PU-IPDI PU-MDI PU-TDI

DMA Tg (°C)

T10

T50

Tmax (first/second)

−1.6 33.2 41.3 33.2

319 319 313 297

379 369 379 365

349/381 339/417 359/462 315/375

tensile strength (MPa) 0.8 8.8 15.7 10.9 7371

± ± ± ±

0.2 0.2 0.6 0.7

elongation at break (%) 52.9 147.2 471.0 101.8

± ± ± ±

0.7 10.8 65.0 8.8

Young’s modulus (MPa) 1.6 6.0 3.4 10.7

± ± ± ±

0.5 0.3 0.6 1.4


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ACS Sustainable Chemistry & Engineering Notes

2. PU-TDI demonstrates the lowest values of the above degradation temperatures while PU-MDI shows the highest Tmax values. Mechanical Properties. Figure 8 shows the stress−strain curves of PU films based on SOP. In the curves of PU-MDI and

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51673075), the Guangdong Province Science & Technology Program (2016A080200004, 2016A090600004, 2016A090400002, 2017A010103015, and 2015A010105023), Program of the Science and Technology Department of Guangdong, China (2014A010105038), and the Central Public-Interest Scientific Institution Basal Research Fund for Innovative Research Team Program of CATAS (17CXTD-30).

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(1) Thakur, S.; Karak, N. Castor oil-based hyperbranched polyurethanes as advanced surface coating materials. Prog. Org. Coat. 2013, 76 (1), 157−164. (2) Wynne, J. H.; Fulmer, P. A.; McCluskey, D. M.; Mackey, N. M.; Buchanan, J. P. Synthesis and Development of a Multifunctional SelfDecontaminating Polyurethane Coating. ACS Appl. Mater. Interfaces 2011, 3 (6), 2005−2011. (3) Ding, H. Y.; Xia, C. L.; Wang, J. F.; Wang, C. P.; Chu, F. X. Inherently flame-retardant flexible bio-based polyurethane sealant with phosphorus and nitrogen-containing polyurethane prepolymer. J. Mater. Sci. 2016, 51 (10), 5008−5018. (4) Ding, H. Y.; Wang, J. F.; Wang, C. P.; Chu, F. X. Synthesis of a novel phosphorus and nitrogen-containing bio-based polyols and its application in flame retardant polyurethane sealant. Polym. Degrad. Stab. 2016, 124, 43−50. (5) Li, Z.; Zhang, R. W.; Moon, K. S.; Liu, Y.; Hansen, K.; Le, T. R.; Wong, C. P. Highly Conductive, Flexible, Polyurethane-Based Adhesives for Flexible and Printed Electronics. Adv. Funct. Mater. 2013, 23 (11), 1459−1465. (6) Udagama, R.; Degrandi-Contraires, E.; Creton, C.; Graillat, C.; McKenna, T. F. L.; Bourgeat-Lami, E. Synthesis of Acrylic-Polyurethane Hybrid Latexes by Miniemulsion Polymerization and Their Pressure-Sensitive Adhesive Applications. Macromolecules 2011, 44 (8), 2632−2642. (7) Hu, S. J.; Wan, C. X.; Li, Y. B. Production and characterization of biopolyols and polyurethane foams from crude glycerol based liquefaction of soybean straw. Bioresour. Technol. 2012, 103 (1), 227−233. (8) Tan, S. Q.; Abraham, T.; Ference, D.; Macosko, C. W. Rigid polyurethane foams from a soybean oil-based Polyol. Polymer 2011, 52 (13), 2840−2846. (9) Gupta, T. K.; Singh, B. P.; Dhakate, S. R.; Singh, V. N.; Mathur, R. B. Improved nanoindentation and microwave shielding properties of modified MWCNT reinforced polyurethane composites. J. Mater. Chem. A 2013, 1 (32), 9138−9149. (10) Wu, C.; Huang, X. Y.; Wang, G. L.; Wu, X. F.; Yang, K.; Li, S. T.; Jiang, P. K. Hyperbranched-polymer functionalization of graphene sheets for enhanced mechanical and dielectric properties of polyurethane composites. J. Mater. Chem. 2012, 22 (14), 7010−7019. (11) Nohra, B.; Candy, L.; Blanco, J.-F.; Guerin, C.; Raoul, Y.; Mouloungui, Z. From Petrochemical Polyurethanes to Biobased Polyhydroxyurethanes. Macromolecules 2013, 46 (10), 3771−3792. (12) Zhang, C.; Garrison, T. F.; Madbouly, S. A.; Kessler, M. R. Recent advances in vegetable oil-based polymers and their composites. Prog. Polym. Sci. 2017, 71, 91. (13) Desroches, M.; Escouvois, M.; Auvergne, R.; Caillol, S.; Boutevin, B. From Vegetable Oils to Polyurethanes: Synthetic Routes to Polyols and Main Industrial Products. Polym. Rev. 2012, 52 (1), 38−79. (14) Ji, D.; Fang, Z.; He, W.; Luo, Z. Y.; Jiang, X. B.; Wang, T. W.; Guo, K. Polyurethane rigid foams formed from different soy-based polyols by the ring opening of epoxidised soybean oil with methanol, phenol, and cyclohexanol. Ind. Crops Prod. 2015, 74, 76−82.

Figure 8. Stress−strain curves of PU films based on SOP with different diisocyanates.

PU-IPDI, elastic regions are initially observed, followed by yield points and plastic regions.36 PU-TDI exhibits a brittle nature with low elongation at break, but its modulus and tensile strength are high. The tensile strength, elongation at break, and Young’s modulus of PU films are summarized in Table 2. PUMDI, PU-TDI, and PU-IPDI that were prepared with the rigid ring structure of diisocyanates show higher tensile strength and Young’s modulus than PU-HDI. PU-MDI has the highest tensile strength of 15.7 MPa and highest elongation at break of 471.0%. This may be due to the special structure of this commercial isocyanate. Furthermore, the elongation at break of PU-IPDI is higher than that of PU-TDI, which could be explained by the higher volume fraction and higher average molecular weight between the cross-linking points.35

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CONCLUSIONS A solvent-free and scalable method for the preparation of soybean-oil-based polyols by the thiol−ene photo-click reaction was developed in a homemade reactor. The reaction conditions were optimized as follows: 2 wt % 1173 as photoinitiator, a reaction time of 3 h, molar ratio of thiols to carbon−carbon double bonds of 4:1, and power of the mercury lamp of 300 W. The resulting soybean-oil-based polyol shows high reactivity toward diisocyanates because of its characteristic of primary hydroxyl groups. The polyurethanes from aromatic and cycloaliphatic diisocyanates show better thermal and mechanical properties than those from aliphatic isocyanate in terms of a higher Tg, thermal stability, tensile strength, and Young’s modulus. PU film prepared from MDI possesses the highest Tg of 41.3 °C, tensile strength of 15.7 MPa, and elongation at break of 471.0%. The thiol−ene photo-click reaction provides a green and efficient platform for the transformation of vegetable oils into polyols for high-performance polyurethanes.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Zhuohong Yang). *E-mail: [email protected] (Chaoqun Zhang). ORCID

Chaoqun Zhang: 0000-0001-5754-8729 7372


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A Solvent-Free and Scalable Method To Prepare Soybean-Oil-Based

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