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Synthesis of Cardanol-based Polyols via Thiol-ene/thiol-epoxy Dual Click-reactions and Thermosetting Polyurethanes Therefrom Haoran Wang, and Qixin Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02423 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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ACS Sustainable Chemistry & Engineering

retention of the cardanol-based PU films. The thermal stability of cardanol-based PU films was improved after converting the phenolic hydroxyl to aliphatic hydroxyl. With increasing the hydroxyl number of cardanol-based polyols, the crosslink density, glass transition temperature, elastic modulus, tensile strength, and hardness of cardanol-based PU films were all improved. The results of this work confirmed that utilization of thiol-ene/thiol-epoxy dual click-reactions is an effective approach to synthesize cardanol-based polyols with relative high hydroxyl number. The significance of this work is believed to broaden the usage of cardanol in different PU formulations and applications. INTRODUCTION Considering the declining petroleum resources and the environmental problems caused by petroleum industry, developing polymers from renewable resources has received great attention both in academia and industry.1-3 Thermosetting polyurethanes (PUs) are one of the most important polymer materials which has been widely used in fabricating rigid foams, simulated woods, and protective coatings.4-5 Polyols and polyisocyanates are the typical components of thermosetting PUs. Currently, most of the thermosetting PUs are still prepared from petrochemical-based polyols. Thus, developing bio-based polyols is meaningful for sustainability in PU industry. Cardanol is extracted from cashew nut shell, which is the byproduct of cashew food industry and is annually renewable.6 As shown in Figure 1, the unsaturated alkyl phenolic structure makes cardanol be a unique and valuable building block for sustainable materials such as phenolic resins,7 epoxy resins,8-9 benzoxazine resins,10 and polyols.11-14 Generally, there are three reported strategies used for synthesizing cardanol-based polyols. The first one is based on the Mannich reaction of the phenol structure in cardanol with N-(2-hydroxyethyl)-1,3-oxazolidine.14 The

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hydroxyl number of the cardanol-based polyols produced by this strategy can be controlled by adjusting the molar ratio between cardanol and N-(2-hydroxyethyl)-1,3-oxazolidine. The main drawback of the cardanol-based polyols synthesized from this strategy is the yellowing problem due to the presence of carbon-carbon double bonds in the aliphatic side chain.

Figure 1. Chemical structure of cardanol. The second strategy is using the ring-opening reactions of epoxidized cardanol. Epoxidized cardanol can be prepared by using the reactions of epichlorohydrin and the phenolic hydroxyl in cardanol or epoxidizing the carbon-carbon double bonds in the aliphatic side chain of cardanol.8 Various ring-opening reactions have been reported to synthesize polyols from epoxidized biomass by using nucleophilic reagents such as amines, halides, alcohols, acids or water.13, 15-19 This is an effective strategy to prepare cardanol-based polyols with various hydroxyl number and some of the products have already commercialized. But all the ring-opening reactions that have been used to synthesize polyols need to be carried out at elevated temperatures. Thus, it is significant for the sustainability purpose to introduce novel effective ring-opening reactions that can be run at the room temperature to synthesize bio-based polyols. The third strategy (Scheme S1) is using the thiol-ene photo-click reaction of the carboncarbon double bonds in cardanol with the thiol group in 2-mercaptoethanol.11 This is a


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convenient and sustainable method to prepare bio-based polyols because thiol-ene reaction is classified as a click reaction. The features of a click reaction are atom economy, high yields, high selectivity, mild reaction condition, and simple purification step.20-23 One big disadvantage of this strategy is that the hydroxyl number of the synthesized polyols is limited by the amount of carbon-carbon double bonds. As the hydroxyl number of the polyols is a crucial factor in determining the crosslink density and the mechanical properties of the thermosetting PUs, polyols with relative high hydroxyl number are always demanded in formulating rigid PU foams and protective PU coatings.5, 12, 14, 18 In order to synthesize cardanol-based polyols with relative high hydroxyl number in a more sustainable approach, we introduced a novel thiol-based click reaction to synthesize polyols from epoxidized cardanol. To the best of authors’ knowledge, this is the first time that thiol-epoxy click reaction is used to synthesize bio-based polyols. Scheme S2 shows the proposed thiolepoxy reaction process. In the presence of base catalyst, the thiol group in 2-mercaptoethanol is firstly converted to thiolate anion which is a strong nucleophile. Then, the thiolate anion will attack the less hindered carbon in the epoxide ring to form alkoxide anion. Due to the acidity feature of the thiol molecular and the high basicity of the formed alkoxide anion, the homopolymerization of the epoxide group will not happen and the alkoxide anion will be protonated.24-26 Compared with other reported ring-opening reactions used to synthesize biobased polyols, the thiol-epoxy reaction can be carried out at room temperature with quantitative conversions which has great advantage in economizing the energy.25 Moreover, polyols with relative high hydroxyl number can be obtained by using thiol-epoxy reaction of epoxidized biomass and 2-mercaptoethanol due to the generation of additional secondary hydroxyl group.


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In this research, two novel cardanol-based polyols (Polyol B and Polyol C in Scheme 1) were synthesized from cardanol glycidyl ether or polyepoxide cardanol glycidyl ether by thiolene/thiol-epoxy dual click-reactions at room temperature, respectively. As a comparison, polyol A (Scheme 1) was synthesized from cardanol by thiol-ene reaction at room temperature. Three cardanol-based PU films were prepared by formulating the hexamethylene diisocyanate (HDI) trimers (Figure S1) with polyol A, polyol B, and polyol C, and named as PU-A, PU-B, and PUC, respectively. The thermal stability, viscoelastic properties, and mechanical properties of the prepared cardanol-based PU films were evaluated. In addition, the color retention of the prepared cardanol-based PU films under UV exposure were also studied.

Scheme 1. Synthetic route for the cardanol-based polyols. EXPERIMENTAL SECTION Materials. Cardanol (Ultra LITE 2023) was kindly provided by Cardolite Corporation (NJ, USA). HDI trimer (Desmodur N 3600, NCO equivalent weight = 183 g per equiv, solvent free) was kindly provided by Bayer Material Science (Pittsburgh, PA, USA). Anhydrous magnesium sulfate (≥98%), 2,2-dimethoxy-2-phenylacetophenone (99%), 2-mercaptoethanol (≥99%),


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lithium hydroxide (98%), ethanol (200 proof), dichloromethane (≥99.5%), methyl ethyl ketone (≥99%), dibutyltin dilaurate, dimethyl sulfoxide-d6 (DMSO-d6, 100%), chloroform-d (CDCl3, 100%), were all purchased from Sigma Aldrich. All chemicals were used without further purification. Pen-ray UV lamp (Model 11SC-1L) was purchased from UVP Inc (CA, USA). Synthesis of cardanol-based polyol A from cardanol by thiol-ene reaction. Cardanol (10.00 g, 0.034 mol), 2-mercaptoethanol (13.53 g, 0.173 mol), and 2,2-dimethoxy-2phenylacetophenone (0.07 g, 0.289 mmol) were charged into a pyrex glass tube equipped with a magnetic stirrer. While stirring at the room temperature, the reactants were irradiated by a penray UV lamp (365 nm, 1200 uw/cm2) for 24 h. After that, the reactants were washed with deionized water three times to remove the excess 2-mercaptoethanol and then extracted by dichloromethane. The dichloromethane phase was dried over anhydrous magnesium sulfate and filtered. Finally, 10.47 g cardanol-based polyol A was obtained after removing the dichloromethane by rotary evaporation. The hydroxyl number of cardanol-based polyol A is 324.35 mg KOH/g determined by titration according to ASTM D4274, as listed in Table 1. Synthesis of cardanol-based polyol B from cardanol glycidyl ether by thiol-ene/thiolepoxy dual reactions. The experimental details for synthesizing cardanol glycidyl ether can be found in the supporting information. Cardanol glycidyl ether (10.00 g, 0.028 mol), 2mercaptoethanol (11.14 g, 0.143 mol), and 2,2-dimethoxy-2-phenylacetophenone (0.06 g, 0.238 mmol) were charged into a pyrex glass tube equipped with a magnetic stirrer. While stirring at the room temperature, the reactants were irradiated by a pen-ray UV lamp (365 nm, 1200 uw/cm2). After 24 h of UV exposure, the mixture of lithium hydroxide (0.34 g, 0.014 mol) and ethanol (15.00 g, 0.326 mol) were added to the reactants. The reaction was carried out for 4 h at the room temperature. After removing the ethanol by rotary evaporation, the crude product was


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washed with deionized water three times and then extracted by dichloromethane. The dichloromethane phase was dried over anhydrous magnesium sulfate and filtered. Finally, 10.56 g cardanol-based polyol B was obtained after removing the dichloromethane by rotary evaporation. The hydroxyl number of cardanol-based polyol B is 357.94 mg KOH/g determined by titration according to ASTM D4274, as listed in Table 1. Synthesis of cardanol-based polyol C from polyepoxide cardanol glycidyl ether by thiolene/thiol-epoxy dual reactions. The experimental details for synthesizing polyepoxide cardanol glycidyl ether can be found in the supporting information. Polyepoxide cardanol glycidyl ether (10.00 g), 2-mercaptoethanol (11.14 g, 0.143 mol), and 2,2-dimethoxy-2-phenylacetophenone (0.02 g, 0.063 mmol) were charged into a pyrex glass tube equipped with a magnetic stirrer. After 24 h of UV irradiation at the room temperature, the mixture of lithium hydroxide (0.67 g, 0.028 mol) and ethanol (30.00 g, 0.978 mol) were added to the reactants. The reaction was carried out for another 12 h at the room temperature. After purification (the same procedure in synthesizing polyol B), 10.21 g cardanol-based polyol C was obtained. The hydroxyl number of cardanol-based polyol C is 440.24 mg KOH/g determined by titration according to ASTM D4274, as listed in Table 1. Table 1. Hydroxyl content of synthesized polyols.

Hydroxyl number

Hydroxyl equivalent

[mg KOH/g]

weight (HEW)*

Polyol A

324.35

172.96

Polyol B

357.94

156.73

Polyol C

440.24

127.43


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*: HEW = 56100/hydroxyl number Preparation of thermosetting PU films. The cardanol-based polyols were formulated with HDI trimer at an NCO/OH ratio of 1.1:1 to prepare the PU films. Methyl ethyl ketone was selected as the solvent and dibutyltin dilaurate was selected as the curing catalyst. The total formulation of each PU films includes 80.0 wt% polyol and HDI trimer, 19.5 wt% methyl ethyl ketone, and 0.5 wt% dibutyltin dilaurate. The films were cast onto cleaned steel panels (QD36, Q-Lab Corporation) and glass panels by a draw-down bar with a wet film thickness of 200 um. The PU films on glass panels were used to make free films. The wet films were kept at the room temperature for 6 h, followed by a thermally curing at 100 °C for 1 h. Characterization. A Varian Mercury 500 MHz spectrometer was used to take the 1H, DEPT135 13C NMR spectra. DMSO-d6 was used as the solvent for taking 1H NMR spectra and CDCl3 was used as the solvent for taking DEPT-135 13C NMR spectra. A HCT Ultra II quadrupole ion trap mass spectrometer (Bruker Daltonics, Billerica, MA) was employed to acquire the electrospray ionization-mass spectra (ESI-MS). Sodium trifluoroacetate was used as the ion source. The samples were dissolved in methanol with the concentration of 3 ug/ml. Thermogravimetric analysis (TGA) was acquired on thermal analyzer (TGA, Q500, TA instruments). The weight of the samples for the testing was in the range of 8-15 mg. The samples were heated from 45 °C to 800 °C in nitrogen atmosphere with a heating rate of 10 °C/min. Dynamic mechanical thermal analyzer (DMTA, Q800, TA instruments) was used to measure the viscoelastic properties of the films in tension mode with a constant frequency of 1 Hz. The size of the free film samples is 20 mm × 8 mm × 0.09-0.11 mm (Length × Width × Thickness).


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The films were cooled to -50 °C and held isothermally at -50 °C for 5 min, followed by heating to 150 °C with a rate of 3 °C/min. Instron 5567 (Instron Corp) was used to measure the tensile properties of the films at the room temperature with a crosshead speed of 3 mm/min. The size of the free film samples is 40 mm × 15 mm × 0.09-0.11 mm (Length × Width × Thickness). Three duplicates of each PU films were measured, and the average value and standard deviation was reported. The thickness of the films on the steel substrate was measured by a digital thickness gauge (Elcometer 415) and five duplicated measurements of each sample were used to calculate the average value and standard deviation. The Kӧing pendulum hardness, pencil hardness, and reverse impact resistance were measured following the standard of ASTM D4366, ASTM D3363, and ASTM D2794, respectively. A QUV aging chamber (Q-Lab Corp) was used to perform the UV aging test. The films on the steel substrate were exposed to UV radiation (340 nm, 0.89 w/cm2) at 60 °C. A spectro-guide spectrophotometer (BYK) was used to measure the color of the films on the steel substrate. The colorimetric values were recorded according to the CIELAB color space. RESULTS AND DISCUSSION Scheme 1 represents the synthetic route for the cardanol-based polyols. Cardanol glycidyl ether was synthesized by the reaction of phenolic hydroxyl in cardanol with epichlorohydrin. Polyepoxide cardanol glycidyl ether was synthesized by epoxidation of the carbon-carbon double bonds in the side chain of cardanol glycidyl ether. Polyol A, Polyol B, and Polyol C were synthesized by thiol-ene reaction or thiol-ene/thiol-epoxy dual reactions from cardanol, cardanol glycidyl ether, and polyepoxide cardanol glycidyl ether, respectively. Due to the additional secondary hydroxyl group generated from thiol-epoxy reaction, it is expecting that the hydroxyl


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number of the prepared cardanol-based polyols will increase as increasing the epoxide content of the cardanol-based reagent. Thus, the hydroxyl number of the prepared cardanol-based polyols should increase in the following order: Polyol A < Polyol B < Polyol C. Polyol A, Polyol B, and Polyol C were formulated with HDI trimer at an NCO/OH ratio of 1.1:1 to prepare the cardanol-based PU films. It is expecting that with increasing the hydroxyl number of the polyol, the cardanol-based PU films will have a more tightly crosslinked network structure and higher mechanical strength. Characterization of cardanol glycidyl ether. Figure 2a and Figure 2b show the 1H NMR spectra of cardanol and cardanol glycidyl ether, respectively. The disappearance of the resonance at 9.15 ppm corresponding to the proton of the phenolic hydroxyl in cardanol demonstrates the complete conversion of phenolic hydroxyl. Meanwhile, the new resonances at 4.27 ppm, 3.80 ppm, 3.29 ppm, 2.83 ppm, and 2.70 ppm are assigned to the protons in the glycidyl ether group.27 Figure S2a and S2b show the DEPT-135 ether, respectively. In DEPT-135

13

13

C NMR spectra of cardanol and cardanol glycidyl

C NMR spectra, the carbons attached with one or three

protons will show positive resonance; the carbons attached with two protons will show negative resonance; and the quaternary carbons will not be detected.28-29 Likewise, the new resonances at 68.66 ppm, 50.14 ppm, and 44.70 ppm are corresponded the carbon in the glycidyl ether group.30 All the results of NMR spectra suggest the successful preparation of cardanol glycidyl ether. Characterization of polyepoxide cardanol glycidyl ether. Figure 2c and Figure S2c show the 1H NMR and DEPT-135

13

C NMR spectra of the polyepoxide cardanol glycidyl ether,

respectively. The resonances at the range of 4.95-5.83 ppm in 1H NMR spectra are assigned to the proton of carbon-carbon double bonds, as shown in Figure 2a, 2b, and 2c. Based on 1H NMR integration, the conversion of carbon-carbon double bonds is around 73%. In the


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C NMR

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spectra, the new resonances around 56.91 ppm are corresponding to the carbons of the epoxide groups in the side aliphatic chain.8 Moreover, the epoxide equivalent weight of cardanol glycidyl ether and polyepoxide cardanol glycidyl ether is 365.20 and 177.80, respectively. The decreased epoxide equivalent weight further confirms the formation of epoxide group in the side aliphatic chain. Characterization of cardanol-based polyols. Figure 2e shows the 1H NMR spectra of the polyol A. The conversion of carbon-carbon double bond is estimated to be 96% by 1H NMR integration. The new resonance at 4.71 ppm corresponds to the proton in primary hydroxyl group (─S─CH2─CH2─OH) and no characteristic resonance (2.15 ppm) for the thiol group (─SH) was found. Based on 1H NMR integration, the average number of carbon-carbon double bond in one cardanol molecular is estimated to be 1.95. Theoretically, if one carbon-carbon double bond is added with one 2-mercaptoethanol, the integral of the resonance assigned to the proton in primary hydroxyl group (4.71 ppm) should be 1.95. However, the integral based on 1H NMR is 1.75 which indicated the occurrence of side reaction during thiol-ene addition. The side reaction might be the oligomerization which has been reported in synthesizing unsaturated triglyceridebased polyols via thiol-ene addition.20, 31 The formation of oligomer was further confirmed by ESI-MS (supporting information). Figure S2d shows the DEPT-135 13C NMR spectra of polyol A. The new resonance at 45.95 ppm corresponds to asymmetric carbon in α position of thiol ether (CH─S─CH2), which further confirms the thiol addition.20, 29 The hydroxyl number of the prepared polyol A is 324.35 mg KOH/g, as listed in Table 1. The 1H NMR spectra of polyol B and polyol C is shown in Figure 2f and Figure 2g, respectively. The DEPT-135

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C NMR of polyol B and polyol C is shown in Figure S2e and

Figure S2f, respectively. Similar to polyol A, thiol-ene addition of polyol B and polyol C can be


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confirmed by the conversion of the protons in carbon-carbon double bond, the occurrence of new resonance at 4.71 ppm for the protons in primary hydroxyl group, and the new resonance at 45.95 ppm corresponding to asymmetric carbon in α position of thiol ether group. Thiol-epoxy substitution of polyol B and polyol C can be verified by the disappearance of resonances at 2.83 ppm and 2.70 ppm corresponding to the protons in epoxide group, the disappearance of resonances at 50.14 ppm, 44.70 ppm, and 56.91 ppm (Figure S2c) corresponding to the carbons in epoxide group, and the appearance of new resonances at 5.14 ppm and 4.51 ppm corresponding to the protons in secondary hydroxyl group. The hydroxyl number of the prepared polyol B and polyol C is 357.94 and 440.24 mg KOH/g, respectively, as listed in Table 1. As expected, the cardanol-based polyols synthesized by thiol-ene/thiol-epoxy dual reactions (Polyol B and Polyol C) have higher hydroxyl number than the cardanol-based polyol synthesized by thiol-ene reaction (Polyol A).


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Figure 2. 1H NMR spectra (Solvent: DMSO-d6). Thermal stability of cardanol-based PU films. Figure 3 presents the TGA curves and their derivative curves of the PU films. All these three PU films show a two-stage degradation process.


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The first degradation stage is in the range of 200-400 °C, which is mainly the cleavage of urethane group.32-33 The second degradation stage is in the range of 400-500 °C, which is related to the radical decomposition of polymer chains.32 The temperatures at 5% mass loss (T5%), starting of second mass loss stage (TSEC), and maximum mass loss derivative temperature of second mass loss stage (TDTG-SEC) are summarized in Table 2. The main differences of the thermal degradation profile among these three PU samples are in the first stage. Generally, PU-B has the best thermal stability while PU-A is the worst. This phenomenon can be explained by the different hydroxyl compositions of the cardanolbased polyols. Polyol-A has a certain amount of phenolic hydroxyl, while the phenolic hydroxyl in polyol-B and polyol-C has been converted to aliphatic hydroxyl. Comparing with polyol-B, the ratio of the secondary aliphatic hydroxyl group to the primary aliphatic hydroxyl group in polyol-C is much higher. The main reason for polyol-B has a better thermal stability is due to the higher percentage of primary aliphatic hydroxyl group. It has been reported that the urethane group derived from primary aliphatic hydroxyl has a better thermal stability than that derived from secondary aliphatic hydroxyl.34-35 Moreover, the thermal stability of urethane group decreases in the order as follows: aliphatic isocyanate-aliphatic hydroxyl (∼250 °C) > aromatic isocyanate- aliphatic hydroxyl (∼200 °C) > aliphatic isocyanate-phenolic hydroxyl (∼180 °C) > aromatic isocyanate-phenolic hydroxyl (∼120 °C).32, 36 Therefore, it is reasonable to claim that converting the phenolic hydroxyl in cardanol to aliphatic hydroxyl can improve the thermal stability of the cardanol-based PUs.


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Figure 3. TGA curves of cardanol-based PU films. Table 2. Thermal and viscoelastic properties of cardanol-based PU films.

T5%

TSEC

TDTG-SEC

Tg

E' at Tg + 60 °C

ѵe

[°C]

[°C]

[°C]

[°C]

[MPa]

[mol/mm3]

PU-A

281.90

422.76

480.97

50.29

6.32

661

PU-B

322.10

428.82

488.25

65.37

7.98

803

PU-C

314.54

427.00

487.04

87.68

16.56

1578

Viscoelastic properties of cardanol-based PU films. Figure 4 shows the curves of storage modulus and loss factor (tan δ) as a function of temperature. The temperature at the maximum of tan δ is defined as the glass transition temperature (Tg). The crosslink density can be calculated by using equation (1):37 ѵ E ⁄3RT

(1)

Where ѵe is the number of moles of elastically effective network chains per volume, also called crosslink density; E' is the storage modulus of the thermoset polymer in the rubbery plateau


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region at Tg + 60 °C; R is the universal gas constant; and T is the absolute temperature. The values of Tg and ѵe are summarized in Table 2. The Tg values of the cardanol-based PUs are in the range of 50.29-87.68 °C and the ѵe values are in the range of 661-1578 mol/mm3. As expected, with increasing the hydroxyl number of the polyol, the crosslink density of PU films is increased. And with increasing the crosslink density, the Tg is increased due to the reduction of free volume.37 In addition, only one peak was found for each thermosetting PU in the curve of tan δ, which indicates the homogenous performance of the PU films.21

Figure 4. DMTA curves of cardanol-based PU films. Mechanical properties of cardanol-based PU films. Figure 5 shows the stress-strain curves of the cardanol-based PU films. PU-A shows yielding phenomenon, strain hardening behavior, and ductile fracture, while PU-B and PU-C show rigid performance and brittle fracture. The elastic modulus, tensile strength, elongation at break, hardness, and reverse impact resistance of the cardanol-based PU films are summarized in Table 3. The elastic modulus of the cardanolbased PU films is increased from 469.12 MPa (PU-A) to 985.89 MPa (PU-C) as the crosslink density is increased from PU-A to PU-C. The tensile strength has the similar trend as the elastic


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modulus, while the elongation at break is decreased with the increased crosslink density. From PU-A, PU-B to PU-C, the hardness is increased in both Kӧnig pendulum hardness and pencil hardness, while the reverse impact resistance is decreased. This demonstrates that the PU films become harder and more brittle with the increased crosslink density. The hardness and impact resistance results are consistent with and supported by the data in the tensile tests. In general, crosslink density plays a critical role in the mechanical properties of thermosetting polymers as the crosslinking controls the mobility of polymer chains. In this study, the crosslink density is varied by changing the hydroxyl number of cardanol-based polyols. Beyond that, the crosslink density can also be adjusted by controlling the ratio of isocyanate to hydroxyl and the type of isocyanate used during formulation.18 The significance of this study is not only able to synthesize cardanol-based polyols with high hydroxyl number to achieve a high hardness, elastic modulus, and tensile strength, but it is also able to achieve desirable mechanical properties by changing the hydroxyl number. This study provided approaches to generate cardanol-based PUs with different mechanical properties that will broaden the usage of the cardanol-based PUs in diverse applications.

Figure 5. Stress-strain curves of cardanol-based PU films.


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Table 3. Mechanical properties of cardanol-based PU films. Properties

PU-A

PU-B

PU-C

Thickness [um]

98.6 ± 9.7

95.3 ± 7.5

93.1 ± 10.3

Elastic modulus [MPa]

469.12 ± 30.22

697.96 ± 58.19

985.89 ± 45.68

Tensile strength [MPa]

26.78 ± 0.35

42.32 ± 1.46

44.30 ± 1.30

Elongation at break [%]

78.32 ± 6.37

18.90 ± 4.24

9.12 ± 1.23

Kӧnig pendulum hardness [s]

122

154

182

Pencil hardness

H

3H

4H

Reverse impact resistance [in lbs]

> 174

48

41

Color retention of cardanol-based PU films. Nowadays, a significant part of PUs are applied as a topcoat or base coat for the purpose of maintaining or providing color.38 Thus, it is worth to study the color retention of the prepared cardanol-based PU films. The color difference of the PU films is quantified by using CIELAB color system. In CIELAB color system, colors are described in three dimensions by using L*, a*, and b* which represents lightness, redness to greenness, and blueness to yellowness, respectively.39 Figure 6 presents the color difference of the PU films during the 10 days UV exposure. The change of b* is larger than that of L* and a* for PU-A and PU-B, while the color change of PU-C is negligible. As shown in Figure 7, the comparison of PU films before and after UV exposure, the yellowing of PU-A and PU-B is very obvious. Thus, the color measurement and the photo image confirmed that yellowing is the dominant change in the discoloration of PU-A and PU-B. However, PU-C did not exhibit that serious yellowing after UV exposure.


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The rapid yellowing could be caused by 2,2-dimethoxy-2-phenylacetophenone, which is used as the photoinitiator in the thiol-ene reaction during the process of synthesizing polyols. It has been

reported

that

yellow

compounds

will

be

generated

from

2,2-dimethoxy-2-

phenylacetophenone through bimolecular radical reactions.40 Comparing with polyol A and polyol B, the usage of 2,2-dimethoxy-2-phenylacetophenone during the process of synthesizing polyol C is much less because most of the carbo-carbon double bonds have already been converted to epoxide group. This might be the reason for the better color stability of PU-C than PU-A and PU-B. The yellowing problem caused by 2,2-dimethoxy-2-phenylacetophenone could be solved by using non-yellowing photoinitiator.40

Figure 6. Color difference of cardanol-based PU films during UV exposure.


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Figure 7. Photo images of cardanol-based PU films before and after UV exposure. CONCLUSIONS This study developed a novel effective and sustainable synthetic strategy for preparing cardanol-based polyols with relative high hydroxyl number. Benefiting from the highly thermodynamic favorable feature of the thiol-epoxy and thiol-ene click reactions, cardanol-based polyols were successfully synthesized at the room temperature with quantitative conversions. With increasing the hydroxyl number of cardanol-based polyol, the crosslink density of cardanol-based PUs was increased and therefore, the elastic modulus, tensile strength, and hardness of cardanol-based PUs were elevated. By converting the phenolic hydroxyl to aliphatic hydroxyl, the thermal stability of cardanol-based PUs was improved. This work provided approaches to generate cardanol-based PUs with different properties that will broaden the usage of cardanol in different PU formulations and applications to support the sustainability in PU industry. ASSOCIATED CONTENT Supporting Information. Scheme of thiol-ene and thiol-epoxy reaction; Chemical structure of HDI trimer; Experimental procedure for synthesizing cardanol glycidyl ether and polyepoxide cardanol glycidyl ether; DEPT-135

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C NMR spectra of cardanol, cardanol glycidyl ether,

polyepoxide cardanol glycidyl ether, polyol A, polyol B, and polyol C; ESI-MS of cardanolbased polyols; The peak assignment of cardanol-based polyols in ESI-MS. This material is available free of charge. (PDF) AUTHOR INFORMATION


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Corresponding Author *Qixin Zhou. Email: [email protected]. Tel: +1-330-972-7159 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the support from faculty start-up funding from The University of Akron. REFERENCES (1) Meier, M. A.; Metzger, J. O.; Schubert, U. S. Plant oil renewable resources as green alternatives in polymer science. Chem. Soc. Rev. 2007, 36 (11), 1788-1802. (2) Zhu, Y.; Romain, C.; Williams, C. K. Sustainable polymers from renewable resources. Nature 2016, 540 (7633), 354. (3) Miller, S. A. Sustainable Polymers: Opportunities for the Next Decade. ACS Macro Lett. 2013, 2 (6), 550-554. (4) Akindoyo, J. O.; Beg, M.; Ghazali, S.; Islam, M.; Jeyaratnam, N.; Yuvaraj, A. Polyurethane types, synthesis and applications–a review. RSC Adv. 2016, 6 (115), 114453-114482. (5) Chattopadhyay, D. K.; Raju, K. Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 2007, 32 (3), 352-418. (6) Balachandran, V. S.; Jadhav, S. R.; Vemula, P. K.; John, G. Recent advances in cardanol chemistry in a nutshell: from a nut to nanomaterials. Chem. Soc. Rev. 2013, 42 (2), 427-438.


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For Table of Contents Use Only

Synopsis: Cashew nut shell-derived polyols are presented here for applications as sustainable building blocks of polyurethanes.


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Figure 1. Chemical structure of cardanol. 48x28mm (600 x 600 DPI)


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Scheme 1. Synthetic route for the cardanol-based polyols. 78x32mm (600 x 600 DPI)


thiol-epoxy Dual

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