Renewable High-Performance Polyurethane ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Renewable High-Performance Polyurethane Bioplastics Derived from Lignin−Poly(ε-caprolactone) Yan Zhang,*,† Jianjun Liao,† Xiangchen Fang,*,‡ Fudong Bai,‡ Kai Qiao,‡ and Lingmin Wang‡ †

Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡ Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, 31 Dandong Road, Fushun 113001, Liaoning, P. R. China S Supporting Information *

ABSTRACT: Here, we report novel lignin-poly(ε-caprolactone)-based polyurethane bioplastics with high performance. The poly(ε-caprolactone) (PCL) was incorporated as a biodegradable soft segment to the lignin by the bridge of hexamethylene diisocyanate (HDI) with long flexible aliphatic chains and high reactivity. The effects of -NCO/-OH molar ratio, content of lignin, and molecular weight of the PCL on the properties of the resultant polyurethane plastics were thoroughly evaluated. It is important that the polyurethane film still possessed high performance in the tensile strength, breaking elongation, and tear strength, which could reach 19.35 MPa, 188.36%, and 38.94 kN/m, respectively, when the content of lignin reached as high as 37.3%; moreover, it was very stable at 340.8 °C and presented excellent solvent-resistance. The results demonstrated that the modification of the lignin based on the urethane chemistry represents an effective strategy for developing lignin-based high-performance sustainable materials. KEYWORDS: Lignin, Polyurethane, Poly(ε-caprolactone), Bioplastics, High performance

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INTRODUCTION Lignin, as the second most abundant natural polymer and costeffective byproduct from the papermaking and biorefinery industries, has been considered as a valuable substitute for some petrochemical products. Currently, they are generally consumed as biorenewable fuel,1 as chemical reagents,2 as fertilizer,3 as fillers,4 as adhesives,5,6 and in other low-value applications.7−10 Thus, it is urgent to develop new strategies for the valorization of lignin as an abundant component for the production of high-value-added and sustainable materials. Lignin-containing recyclable plastics have recently arisen as an active field. For example, Naskar’s group reported a novel lignin-derived thermoplastic with telechelic polybutadiene as soft segments.11 Afterward, this group reported an improved strategy to synthesize polyurethane (PU) thermoplastics from lignin by changing the functional groups from carboxylic acids to isocyanates in the rubbery segment. The mechanical properties of the resultant copolymers were improved in comparison with the lignin due to the occurrence of the urethane covalent bonds.12 Also, lignin-based thermoset plastics have been reported by the Turri group.5 They presented novel thermoset PU coatings by cross-linking the commercial softwood kraft lignin with toluene diisocyanate (TDI) at different NCO/OH ratios. In addition, Sun’s group synthesized a novel biofoam by replacing commercial polyol with different © 2017 American Chemical Society

amounts of lignin (8.33−37.19% w/w) in traditional PU production, and its thermal stabilities were slightly improved.13 However, to the best of our knowledge, there are few reports on the lignin-based polymers uniting the high-performance and high-content lignin. Poly(ε-caprolactone) (PCL) is one of the main biodegradable polymers in the current and emerging bioplastics markets. The incorporation of PCL to the lignin has been studied by several groups by “graft to” or “graft from” methods. The boron end-functionalized PCL was grafted to the lignin by the formation of arylboronate ester linkages.14 In addition, the lignin-based polyurethane was prepared based on the lignin, PCL, and 4,4′-methylene-diphenylene diisocyanate (MDI).15 These pieces of research placed emphasis on the synthesis method while the mechanical properties have not been well studied. Furthermore, the lignin−PCL copolymer was synthesized by ring-opening polymerization (ROP) of ε-caprolactone (CL) initiated by the hydroxyl groups on the lignin.16 However, it is hard to strike a balance between the high content of lignin (>30 wt %) and maintaining the performance of the lignin− PCL copolymer. Received: January 27, 2017 Revised: March 10, 2017 Published: April 2, 2017 4276

DOI: 10.1021/acssuschemeng.7b00288 ACS Sustainable Chem. Eng. 2017, 5, 4276−4284

Research Article

ACS Sustainable Chemistry & Engineering It is well-known that lignin contains various functional groups including aliphatic and phenolic hydroxyl, methoxyl, carbonyl, and carboxyl moieties;17 these reactive groups have effectively enabled the preparation of the lignin-containing products, such as epoxies,18 phenolics,19 polyurethanes,20 etc. Herein, a novel renewable polyurethane film with a high content of lignin was successfully achieved based on the urethane chemistry; the hexamethylene diisocyanate (HDI) with long flexible aliphatic chains was used as activator for the hydroxyl groups of the lignin, and the PCL was incorporated as a biodegradable soft segment to improve the flexibility of the lignin-based polyurethane. The resultant lignin-derived polyurethane films exhibited excellent mechanical properties; moreover, the available biobased soft segment PCL and lignin could make the polyurethane film a completely renewable plastic.

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Table 1. Synthesis of the Different LPUs with the Corresponding Designations and Formulations

EXPERIMENTAL SECTION

Materials. Alkali lignin was purchased from Wuhan East China Chemical Co., Ltd. (Wuhan, China). Poly(ε-caprolactone) glycol (PCL, CAPA 2054, Mn = 500; CAPA 2100, Mn = 1000; CAPA 2201, Mn = 2000) was supplied by Perstorp (Sweden). Stannous octoate (Sn(Oct)2) and HDI were provided by Sigma-Aldrich. NaOH, HCl, and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. DMF was distilled over calcium hydride before use. Purification of the Lignin. The lignin was purified before use. Briefly, raw alkali lignin (50.0 g) was dispersed in water (500 mL) and then stirred. The pH value of the suspension was adjusted to 12 with 1 M NaOH aqueous solution. The heterogeneous mixture was centrifuged at the speed of 6000 rpm for 15 min, and the supernatant was decanted followed by filtration. One mole of aqueous HCl was slowly added to the supernatant until the pH value reached 3, and the mixture was centrifuged at the speed of 6000 rpm for 15 min. After that, the supernatant was decanted, the precipitation was resuspended in 500 mL of distilled water, and the procedure was repeated until the pH value of the filtrate reached neutral. The precipitated lignin was dried under vacuum at 70 °C for 24 h, and the yield of the purified lignin was about 20%. The content of the hydroxyl groups was determined using the method described by Kang.21 The batch of alkali lignin used in these studies had a total hydroxyl content of 1.99 mmol/ g. Synthesis of the Lignin−PCL Polyurethane (LPU). The hydroxyl groups of the lignin were activated by HDI, and then the PCL was incorporated by the reaction between the NCO groups of HDI and the OH groups of PCL. A series of lignin−PCL polyurethanes (LPUs) were prepared by adjusting -NCO/-OH (the molar ratio of isocyanate groups of HDI to hydroxyl groups of the lignin and PCL), the content of lignin, and the molecular weight of PCL. The -NCO/-OH ratio was calculated using eq 1:

‐NCO/‐OH =

WHDI × [NCO]HDI Wlignin × [OH]lignin + WPCL × [OH]PCL

a

Samples

-NCO/OH

Wlignin/WPCL (g/g)

Clignin(%)a

Molecular weight of PCL

LPU-1 LPU-2 LPU-3 LPU-4 LPU-5 LPU-6 LPU-7 LPU-8 LPU-9 LPU-10 LPU-11 LPU-12 LPU-13 LPU-14

0.5 1.0 1.2 1.35 1.5 1.75 2.0 1.35 1.35 1.35 1.35 1.35 1.35 1.35

5/10 5/10 5/10 5/10 5/10 5/10 5/10 5/10 5/10 5/0 5/3.75 5/5 5/6.25 5/7.5

29.24 26.03 24.94 24.18 23.47 22.36 21.36 27.16 28.95 81.49 43.15 37.30 32.84 29.34

500 500 500 500 500 500 500 1000 2000 500 500 500 500 500

C lignin =

Wlignin Wlignin + WPCL + WHDI

× 100%.

react for 3 h at 70 °C; then the PCL (10.00 g) was added to the reactor for another 10 h at 70 °C. Preparation of the LPU Film. The LPU film was fabricated by the spin-cast method. After the polymerization, the mixture prepared in the previous section was immediately spin-cast onto the PTFE board in air and allowed to cross-link in a ventilated oven at 70 °C for 24 h; then the LPU film was obtained. Characterization. Fourier-Transform Infrared Spectroscopy (FTIR). The infrared spectra of the purified lignin and LPU were determined using a Nicolet 5700 FTIR spectrometer. The LPU samples were measured by the KBr tablet method and scanned from 4000 to 400 cm−1 with a resolution of 4 cm−1. X-ray Diffraction (XRD). X-ray diffraction patterns were recorded by monitoring the diffraction angle 2θ from 10° to 60° on a Philips X’Pert PRO instrument under a voltage of 40 kV and a current of 40 mA. Water Contact Angle (WCA). The WCA measurements on the PU films were performed using a contact angle meter (POWEREACH, JC2000D2) at room temperature, and deionized water was used as probe liquid. Scanning Electron Microscopy (SEM). The surface features of the film were observed by SEM (Hitachi S-3400N, Hitachi High Technologies, Inc., Tokyo, Japan). The films placed on conductive tapes were sputter coated with gold and observed at an acceleration voltage of 15 kV. Thermogravimetric Analysis (TGA). Thermal gravimetric analysis (TGA) was performed with a STA449 F3 from Netzsch at a heating rate of 10 °C/min from 40 to 600 °C under nitrogen atmosphere. The weight of the samples was 5−7 mg; the rate of gas consumption was 40 mL/min. Tensile Test. The tensile tests of LPU films for the tensile strength and elongation at break were performed according to GB/T 1040 standard on a CMT 2206 (SANS) tensile tester. The right-angle tear strength was performed following the QB/T 1130 method on a CMT 2206 (SANS) tensile tester. All samples for tensile testing were conditioned at 25 °C and 50% humidity for 24 h prior to the test, and the tensile tests were carried out under similar conditions. Five specimens of each type were tested, and the average values were reported. Swelling Test. The swelling rate was conducted in DMF at a constant temperature of 25 °C for 72 h to reach equilibrium. After swelling, the excess solvent on the surface of each sample was removed by filter paper and the swollen films were weighed. The swelling rate was determined by the corresponding eq 2: m − m0 Swelling Rate = s × 100% m0 (2)

(1)

where WHDI, Wlignin, and WPCL represent the weights of HDI, lignin, and PCL, respectively; [NCO]HDI is the molar content of isocyanate groups in HDI; [OH]lignin and [OH]PCL are the molar contents of total hydroxyl groups in the lignin and PCL, respectively. The experimental conditions for preparing the LPU are given in Table 1. Typically, the purified alkali lignin (5.00 g) was dissolved in anhydrous DMF (50 mL) in the reaction flask at 70 °C for 3 h under magnetic stirring. A total of 45 μL of Sn(Oct)2 (0.025 g, 0.62 mmol, 5 wt‰ based on the mass of lignin) in dry toluene stock solution was added into the flask as a catalyst under the protection of argon by using a microliter syringe, and the exhausting−refilling process was repeated several times to remove toluene. Afterward, the HDI (5.00 mL, -NCO/-OH = 1.35) was added to the mixture and allowed to 4277


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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis Route of the LPU

where m0 is the mass of the dry LPU film before swelling (g) and ms is the mass of the LPU film after swelling (g). The cross-linking density of the LPU flims (νc/V0, unit: mol cm−3) was calculated by the Flory−Rethner equation: vc /V0 = − 2

the aromatic and aliphatic -OH stretching vibrations. The absorption peaks at 2932 and 2864 cm−1 were attributed to the symmetric and asymmetric -C−H stretching vibrations of the methyl and methylene groups, and the peak at 1697 cm−1 belonged to the -CO stretching of the carbonyl groups.23 More specifically, the sharp absorption peaks which located at 1607, 1512, and 1459 cm−1 corresponded to the existence of the aromatic ring.24 The main characteristic peaks of the guaiacyl and syringyl units of lignin appeared at 1275, 1038 cm−1 and 1219, 1120 cm−1, respectively.25 Compared with the spectrum of the purified lignin, the FTIR spectrum of the LPU displayed a narrow peak approximately at 3399 cm−1 and showed a new peak at 1536 cm−1, which were attributed to -NH stretching vibrations and bending vibrations. Meanwhile, two characteristic absorption peaks were found at 1733 cm−1 (-CO) and 1637 cm−1 (-NHCOO-), indicating the reaction between -OH groups and -NCO groups. The peak at 1258 cm−1 was associated with the -C−N stretching.26 In addition, the PCL segment in the LPU was identified by the increase of methylene -C−H bands (peak at 2862 cm−1). All the features clearly indicated that the LPU copolymer composed of PCL and lignin was successfully prepared. Crystallinity. The X-ray diffraction was performed for the purified lignin, PCL, and the LPU in an attempt to study the crystallinity. As shown in Figure 2, the purified lignin showed

v + χv 2 + ln(1 − v) V1 × (2v1/3 − v)

(3)

where νc is the effective number of cross-links (mol), V0 is the volume of dry polymer (cm3), V1 is molar volume of DMF (76.87 mL mol−1), χ is the polyurethane−DMF interaction parameter (0.40),22 ν is the volume fraction of polyurethane in the swollen gel (ν = V0/V), and V is the volume of the swollen gel at equilibrium (cm3).

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RESULTS AND DISCUSSION Synthesis of the LPU. The synthesis route of the LPU was depicted in Scheme 1. The LPU was prepared following two steps. First, the isocyanate groups of HDI were utilized to activate the hydroxyl groups in the lignin with Sn(Oct)2 as a catalyst. Second, PCL was introduced into the activated lignin via urethane linkages by a polyaddition reaction. The structure of the LPU was confirmed by FTIR spectral analysis. For the FTIR curve of the purified lignin in Figure 1, a typical and intense signal at 3500−3300 cm−1 was assigned to

Figure 2. XRD patterns of PCL, lignin, and the LPU with different molecular weights of PCL.

Figure 1. FTIR spectra of the purified lignin and the LPU. 4278


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alkyl−ether linkages, dehydration and decarboxylation reactions, the elimination of carbon dioxide and water, etc.29 While the second stage (>300 °C, Tmax is 353.7 °C) was related to the decomposition reaction and the condensation reaction of aromatic rings, and also related to the cleavage of carbon− carbon linkages in the lignin structure.30 For the LPUs, it also can be observed that the decomposition process of LPU occurred as two stages: the first stage (200− 375 °C) was associated with the decomposition of the urethane groups and the phenolic hydroxyl groups of lignin.31,32 The second stage (375−500 °C, Tmax is about 440 °C) was ascribed to the decomposition of ester bonds from the PCL and the rupture of carbon−carbon linkages between lignin structural units. As can be noticed, the decomposition temperature of the first stage (200−375 °C) was higher than that of the purified lignin; these results indicate that the incorporation of PCL into lignin bridged by HDI could improve the LPU thermal stability at low temperature (43.15%), the agglomeration of lignin would cause the drop of the mechanical properties and the LPU film was shown to be brittle. Moreover, the LPU had a hard, brittle nature and was difficult to be tested when there was no PCL incorporation. While the mechanical properties of the PU derived from the PCL-diol and HDI, such as the tensile strength, the elongation 4280


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Figure 4. SEM image of the surfaces of LPU films with different -NCO/-OH at 1.0 (a), 1.2 (b), and 2.0 (c); different contents of lignin at 43.15% (d), 32.84% (e), and 29.34% (f); and different molecular weights of PCL at 500 (g) and 2000 (h).

film showed a phase separation structure, which was related with the PCL segment crystallization. It is worth noting that the reason for the surface roughness is different, the grain size on the LPU surfaces decreased with the increase of -NCO/-OH, but the granular zone reduced with the decrease of content of lignin. Compared with the surface, the cross-sectional morphologies (Figure S2) of the LPU films are not porous but smooth. The reason for this difference could be attributed to the surface tension change during the drying process of preparation of the LPU film. Only when the lignin content of LPU is higher than 40 wt % will the cross-sectional morphology of the LPU film be rough, and that is caused by the imcomplete reaction of the lignin. Wettability. Water contact angle measurements were performed to determine the wettability properties of surfaces of the LPU films. As indicated in Table S1, the wettability of the LPU films was determined to some extent by the hydroxyl groups on the surface. The lignin molecules contain a large number of carboxyl and hydroxyl groups and show more hydrophilic behavior than the PCL. Thus, the water contact angles decreased with the increase of the content of lignin. The value of -NCO/-OH also affects the water contact angles. As shown in Figure 5, the LPU films exhibited adjustable wettability and the water contact angles of the LPU films increased from 64.4° to 94.6° due to the consumption of

Figure 5. Water contact angle of LPU films with different -NCO/-OH at 1.0 (a), 1.35 (b), and 2.0 (c).

hydroxyl groups caused by the increase of -NCO/-OH. The wettability property can be adjusted according to practical application to increase the value in use. Swelling Properties of the LPU Films. The effects of the -NCO/-OH, the content of lignin, and the molecular weight of PCL on the cross-link densities and swelling rate in DMF are shown in Figure 6. Figure 6a showed the effect of -NCO/-OH on the cross-link density and the degree of swelling. It is clear that the cross-linking densities increase from 3.14 to 45.10 × 10−3 mol/cm3 with the -NCO/-OH increase from 1.0 to 2.0. The swelling rate shows the opposite trend with the crosslinking density as expected. This phenomenon could be explained by the fact that there were more isocyanate groups to cross-link with the hydroxyl groups of PCL, and a more dense network structure formed, which would lead to a 4281


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Figure 6. Effect of -NCO/-OH (a), the content of lignin (b), and the molecular weight of PCL (c) on the swelling rate and cross-linking density of the LPU.

Scheme 2. Schematic Representation of the LPU Networks

The schematic presentation of the formation mechanism of the LPU nerwork structure was dipicted in Scheme 2. The network of the LPU was determined by the cross-link density and the crystallinity. The hydroxyl groups in lignin were activated by HDI, and the reactivity enhanced accordingly with the increase of -NCO/-OH, which resulted in an increased higher cross-link density, and thus, stronger intermolecular forces occurred. The moderate cross-linking increased the tensile strength and solvent-resistance of the films. However, the movements of the chain segments are confined, with an overincrease of -NCO/-OH, and this caused the negative effect on mechanical properties. The lignin is not only the highmodulus provider, but also the cross-linking agent. With the increase of the content of lignin, the role as a cross-linking agent became more important and led to the rise of the crosslink density. Meanwhile, lignin can produce the stress concentration points, but the tensile strength would not drop rapidly because the lignin can provide the higher modulus as well. However, the overage of lignin may cause the

decreased ability of polymer chains to accommodate a solvent molecule as the -NCO/-OH increases. Figure 6b illustrated the degree of swelling and the crosslinking densities as a function of the content of lignin in LPUs. The swelling rate decreased from 246.44 to 193.38% with the content of lignin increasing from 24.18 to 43.15%. However, the cross-linking densities increased from 4.25 to 5.08 × 10−3 mol/cm3 when the content of lignin increased from 24.18 to 43.15%. Lignin as a cross-linking agent would form much more cross-linking points and result in an increase of the crosslinking density; thus, the swelling rate decreased accordingly. Figure 6c displayed the effect of the molecular weight of PCL on the degree of swelling and the cross-linking densities. The swelling rate increased from 246.44 to 256.98% with the increase of the molecular weight of PCL. It is deduced that the increase of the molecular weight of PCL would decrease the density of the hydroxyl groups and increase the distance between the cross-linking points. 4282



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agglomeration, which led to a worse membrane performance. PCL was incorporated as a biodegradable soft segment to the lignin, and the increase of the molecular weight of PCL would decrease the density of the hydroxyl groups and resulted in a decreased cross-link density. Meanwhile, the grafted PCL chains formed a semicrystalline structure, and the PCL/lignin phase segregation occurred, which is responsible for the loss of the mechanical properties

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CONCLUSION

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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.7b00288. 1

H NMR spectra of acetylated lignin and lignin; crosssectional morphologies of LPU films; and water contact angles of the LPU films at varying formulations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel: +86-21-65243432. *E-mail: [email protected]; Tel: +86-2456389569. ORCID

Yan Zhang: 0000-0002-5022-362X Notes

The authors declare no competing financial interest.

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REFERENCES

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The high-performance LPU film with high lignin content was successfully achieved by the incorporation of PCL as a biodegradable soft segment to the lignin by the bridge of HDI with long flexible aliphatic chains and high reactivity. All films were characterized in terms of physical, thermal, and mechanical properties. The LPU film has better thermal stability compared with the purified lignin. Moreover, the adjustment of the content of lignin, -NCO/-OH, and the molecular weight of PCL were effective ways to control the mechanical properties of LPU films, and the properties of LPU were dependent on their network architecture. The LPU had excellent mechanical properties and could meet the daily use even when the lignin content reached as high as 37.3%. The incorporation of a soft segment of PCL into lignin could improve the strength of the lignin and expand the applications of lignin. The successful fabrication of the lignin-based polyurethane bioplastics represents an effective strategy to enhance the processability of lignin and develop sustainable materials in the bioplastic field, such as sealing materials, biodegradable packaging films, and bioadhesives.

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ACKNOWLEDGMENTS

Financial support from the National Natural Science Foundation of China (21274039), Shanghai Pujiang Program (14PJD014), 111 Project (B14018), Specialized Research Fund for the Doctoral Program of Higher Education (20130074110007), and National Key Research and Development Program (2016YFC1100700) is gratefully acknowledged. 4283


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