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Sheet-Like Lignin Particles as Multifunctional Fillers in Polypropylene Fenggui Chen, Wanshuang Liu, Seyed Ismail Seyed Shahabadi, Jianwei Xu, and Xuehong Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b01369 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Sustainable Chemistry & Engineering

Sheet-Like Lignin Particles as Multifunctional Fillers in Polypropylene Fenggui Chen1, Wanshuang Liu1, Seyed Ismail Seyed Shahabadi1, Jianwei Xu2, Xuehong Lu1,*

1

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798, Singapore 2

Institute of Materials Research and Engineering, 2 Fusionopolis Way, #08-03, Innovis,

Singapore 138634, Singapore

*

Corresponding author: Tel: +65 6790 4585

E-mail: [email protected] (X. Lu)

ACS Paragon Plus Environment

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ABSTRACT: Lignin is an attractive renewable reinforcing agent for polyolefins, and also a promising low-cost antioxidant for polymers. It, however, exhibits poor compatibility with nonpolar polymers. In this work, alkali lignin was freeze-dried to achieve sheet-like morphology and then incorporated into polypropylene (PP) by melt compounding. Owing to the significantly increased interfacial area and improved dispersion, with the addition of only 2 wt% freeze-dried lignin, the PP/lignin composites show much enhanced tensile mechanical properties, including moderately improved Young’s modulus and almost doubled elongation at break compared with those of neat PP. The enhancements brought by the sheet-like lignin are far more impressive than that achieved with the same amount of as-received lignin. The composites with the freeze-dried lignin also have rough fractured surfaces with fiber pull-out near the interface, revealing significant toughening effect of the lignin, which can be attributed to the crazing near the interface, and enhanced relaxation in PP-lignin interphase as evidenced by the reduced Tg. Furthermore, the large interfacial area also drastically improves the antioxidant effect of lignin, greatly slowing down the UV-induced and thermo-oxidative degradation of PP. After two weeks of intense UV exposure, neat PP becomes very brittle with its yield strain reduced to about 37% of its original value, whereas the yield strain of the composite with 2 wt% sheet-like lignin is almost unchanged, demonstrating the excellent free-radical scavenger effect of the lignin.

KEYWORDS: lignin, antioxidant, toughness, polypropylene, freeze-drying, sheet-like


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INTRODUCTION Lignin is a natural macromolecular material that, with cellulose, forms the chief part of woody plant tissues. It accounts for nearly 30% of the organic carbon on earth, second only to cellulose.1-2 As a by-product from paper production industry, lignin is readily available at low cost, with a reported annual world production volume of about 50 million tons. In contrast to lignin’s abundance in nature, humans have scarcely made full use of lignin.3-4 Currently lignin is mainly burned as fuels, which gives it very low value. Since lignin is one of the few renewable sources of aromatic macromolecules and possesses rich phenolic and aliphatic hydroxyl groups, enormous efforts have been devoted to the development of value-added applications for lignin.5-7 For example, lignin has been used as a macro-monomer to synthesize thermosetting polymers, such as phenol-formaldehyde resins,8-9 epoxy resins,10 and polyurethane.11-12 It can also be incorporated into thermoplastic polymers as reinforcing or toughening agents.13-16 It has been widely reported that the addition of lignin into polymers can modify mechanical and other physical properties of the polymers.17-19 For example, thermal stability of epoxy/lignin composites could be enhanced by the incorporation of lignin.20-21 In comparison with inorganic fillers, the advantages of lignin as fillers in polymers mainly lie in its light weight, low cost, and renewable nature.4, 22 In addition, owing to its abundant phenolic groups, lignin is also a freeradical scavenger, and can be used as a natural antioxidant. It has been employed as low-cost sustainable stabilizers in thermoplastics, such as polyolefins, to improve the oxidative, thermal, and light stability of the polymers.23-28 However, lignin has a large number of polar groups. It has, therefore, poor compatibility with nonpolar polymers like polyethylene and polypropylene (PP), causing poor dispersion of lignin in these polymers.24, 29-31 As a result, so far the great potential of lignin as efficient free-radical scavenger and reinforcing/toughening agents in


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nonpolar polymer systems has not been adequately demonstrated. In this connection, classically important factors such as phenol content and intrinsic reactivity of the lignin molecules are less important than physical factors that can facilitate their dispersion in polymers. For instance, when modified lignin was employed to replace pristine lignin, the dispersion of lignin in nonpolar polymers is typically not improved significantly and hence the enhancement in material performances can hardly be achieved.27, 32 To address this issue, recently greater efforts have been devoted to improve the dispersion of lignin in polymer matrices via physical means.15, 24 In particular, there is a high demand for improving the dispersion of this fascinating low-cost multifunctional sustainable filler in high-volume nonpolar polymers commonly used for outdoor applications. Freeze-drying (FD) has been widely used to prepare highly porous materials, such as carbon and polymer aerogels.33-36 In a high-rate freezing process, the solute molecules in the solutions do not have enough diffusion time to form large aggregates, giving rise to micron- or even nanometer-sized aggregates separated by small ice crystals.37 The aggregates could be in sheet or even fiber forms owing to the templating effect of the ice crystals. Thus in the subsequent FD step, the sublimation of the ice crystals would lead to a highly porous material with high specific surface area. Since alkali lignin has three-dimensionally branched macromolecular structure and could be well dispersed in water to form colloidal solutions, it was hypothesized that FD of lignin aqueous solutions would give smaller lignin particles in the form of sheets or fibers; the reduced size and increased aspect ratio of the particles may greatly facilitate the dispersion of lignin in polymers by melt compounding, while the high specific surface area of the lignin sheets or fibers would provide larger interfacial area, benefiting the functions of lignin as antioxidant and reinforcement/toughening agents.38 To verify the hypothesis, in this work, alkali lignin


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particles prepared via FD were incorporated into PP, which is frequently used for outdoor applications, whereas is completely nonpolar and prone to UV and thermo-oxidative degradation. Herein, for the first time we report impressively enhanced tensile mechanical and UV-resistant properties of the PP/lignin composites. Our results show that the enlarged interfacial area between lignin and PP as well as the improved dispersion of lignin in PP not only enhance reinforcing effect of the lignin in PP, but also drastically enhance UV and thermal-oxidant stabilities. The improved lignin dispersion also enables us to demonstrate the great potential of lignin as a toughening agent in PP and reveal the corresponding toughening mechanism.

EXPERIMENTAL SECTION Materials Alkali lignin (AL) was purchased from TCI America (USA, TCI product number: L0082, softwood lignin, Elemental composition from elemental analysis: C=49.1%, H=4.5%, S=2.1%) and used as received. PP (Cosmoplene H101E, melt flow index = 3.5 g/10 min, density = 0.9 g/cm3) was supplied by Polyolefin Company (Singapore) Pte. Ltd. All other chemicals were purchased from Sigma-Aldrich Chemicals Incorporation (USA) and used without further purification. All solutions were prepared using deionized (DI) water. Preparation of PP/lignin composites Lignin aqueous colloidal solutions with concentration of 100 and 50 mg/mL, respectively, were prepared by dissolving lignin in DI water and subsequently placing the solution bottle in an ultrasonic bath. The solutions were freezed at -196°C using liquid N2, then freeze-dried in vacuum for 3 days.


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A certain amount of as-received and freeze-dried lignin were mixed with PP pellets, respectively, by melt compounding using a Hakke MiniLab twin screw micro extruder (Germany) at 190 °C. Prior to compounding, PP pellets were dried in a vacuum oven at 60ºC for 24 hours. Characterization The morphologies of the lignin samples were examined using a field emission scanning electron microscope (FESEM, JEOL JSM 6340F) after coating the samples with gold. The same SEM was also used to examine the fractured surface of the polymer and polymer/lignin blend samples. The dispersion state of lignin in PP was examined using a transmission electron microscope (TEM, JEOL 2100) at 200 kV, as well as an Olympus BX53 polarizing optical microscope (POM) at a magnification of 1×100. The Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore volume of the samples were determined by nitrogen adsorption measurements using a Tristar-3000 surface area analyser. The tensile properties were measured using an Instron 5567 machine according to ISO standard 527 at a crosshead speed of 50 mm/min using dumbbell-shaped specimens (ISO 527-2-Typer 5A, 500 N load cell). More than five specimens were tested for each set of variation. The dumbbell-shaped specimens were prepared using Thermo Hakke microinjector (Germany) at 210 °C with a pressure of 800 bar. Differential scanning calorimetry (DSC) was recorded on a TA Instruments DSC Q10 using N2 as purge gas at heating and cooling rates of 10 ºC/min. The samples were firstly heated to 190 oC and maintained for 2 min to remove any previous thermal history before cooling down. The melting temperature (Tm) and crystallization temperature (Tc) were determined from the first heating and the subsequent cooling cycle, respectively, while the glass transition temperature (Tg) was determined from the second heating cycle. The heat of


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fusion (∆Hm) and crystallization enthalpy (∆Hc) were determined from the areas of the melting peaks and crystallization peaks, respectively. Thermo-gravimetric analysis (TGA) was conducted using a TA Q500 TGA at a heating rate of 10oC/min from room temperature to 650 ºC in N2 (sample purge rate = 60 mL/min). Degradation temperature (Td) is defined as the temperature at 5 wt % weight loss. Fourier transform infrared spectroscopic (FTIR) measurements were performed using a Perkine-Elmer Instruments Spectrum GX FTIR spectrometer at room temperature from 500 to 4000 cm-1. Each sample was scanned 16 times at a resolution of 4 cm-1. UV and thermo-oxidative stability tests To study the light stability of the samples, the dumbbell-shaped test specimens were placed in an Atlas Suntest XXL+ UV lamp chamber, which had a layer of reflective aluminium foil inside to ensure that all the specimens would have an even exposure to UV. The UV irradiation was conducted at 35 °C for 2 weeks with light intensity of 50 W/m2. Isothermal TGA in air at 200 oC was also measured to disclose the thermos-oxidative stability.

RESULTS AND DISCUSSION Sheet-like morphology of the freeze-dried lignin In order to study the effect of FD on morphology of the obtained lignin particles, aqueous solutions containing different amounts of alkali lignin were freeze-dried using liquid nitrogen. As shown in Figure 1, the as-received lignin consists of spherical or irregular shaped particles with sizes of tens of microns (Figure 1a; the size distribution is shown in Figure S1), whereas the freeze-dried lignin samples exhibit sheet-like morphology (Figures 1b and 1c) owing to the template effect of ice crystals. Moreover, when the lignin concentration of the aqueous solution used for FD is reduced from 100 to 50 mg/mL, the lignin sheets have some micron-sized pores (Figure 1c), and the typical thickness is reduced to ∼200 nm, as shown in Figure 1c inset. Such


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thin lignin sheets have larger specific surface area (SSA) than the as-received lignin but have almost no nanometer-sized pores, as shown by their BET N2-adsorption isotherms in Figure 2 and BET analysis results in Table S1. The dependence of the morphology of the freeze-dried lignin particles on the concentration of the lignin aqueous solution used for FD is because when the lignin concentration is above 5 mg/mL, the total volume of the lignin bulk structure (including pore volume) would not shrink significantly in the FD process (Figure 2 inset). Thus, the initial lignin concentration dominates the density and morphology of the obtained lignin aerogel.33, 35 With the reduced lignin concentration, to form a sheet-like particle of the same thickness, some lignin molecules would need to diffuse for a longer distance to aggregate, which is impossible under very high freezing rates. Dispersion state of the lignin in PP It is well known that the performance of polymer composites strongly depends on the dispersion state of the fillers in the polymer matrices. For instance, a better dispersion may result in enhanced interactions between the polymers and fillers,39 and the mechanical properties of the composites may thereby be improved owing to the more effective stress transfer from the matrices to fillers. To investigate the dispersion state of the freeze-dried lignin in PP, the asreceived and freeze-dried lignin were incorporated into PP, respectively, via melt compounding under the same conditions for comparison. The OPM images in Figure 3 show that the sizes and shapes of the as-received lignin particles (the dark regions) are not changed significantly after the melt compounding, i.e., they are still spherical or irregular shaped particles with sizes of tens of microns (Figure 3a). By contrast, many short fine dark lines are observed for the composites of the freeze-dried lignin and PP (Figure 3b), implying that the freeze-dried lignin are in the form of small sheets in the PP matrix. Obviously, the thin lignin sheets obtained by freeze-drying could


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be broken down into small pieces more easily than as-received lignin particles, facilitating their dispersion in the PP matrix. The relatively good dispersion of the freeze-dried lignin in PP is confirmed by TEM images given in Figure S2. Thus, compared with as-received lignin, the freeze-dried lignin would provide much larger interfacial area and also be dispersed more uniformly in the PP matrix.40 Tensile mechanical properties of the PP/lignin composites As lignin consists of rigid aromatic macromolecules, it is recognized as an abundant and inexpensive reinforcing agent in relatively flexible polymers such as polyolefins. To investigate the reinforcement effect of the freeze-dried lignin, a series of PP/lignin composites were prepared for tensile tests. Typical tensile stress–strain curves of the PP/lignin composite samples are presented in Figure 4 for easy comparison with that of neat PP. The compositions and preparation conditions for the composite samples are summarized in Table 1. For example, PP/lignin2%-noFD refers to the composite containing 2 wt% of as-received lignin, while PP/lignin6%-FD@50 represents the composite containing 6 wt% of freeze-dried lignin obtained from the solution with lignin concentration of 50 mg/mL. The tensile properties of the composites are also compared with those of neat PP in this table. From Figure 4, it is clear that the addition of lignin into PP can increase tensile modulus, whereas at the same time, it slightly lowers the yield stress and strain of the composites. The freeze-dried PP/lignin composites exhibit higher modulus than their counterpart with the same amount of as-received lignin, and for the former, as the lignin content increases, the reinforcement effect becomes more prominent. Moreover, as the lignin concentration used for FD reduces from 100 mg/mL to 50 mg/mL, the resultant freeze-dried PP/lignin composite exhibits a further increase of modulus. Obviously, the reinforcement effect is closely related to


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the specific surface area of the lignin fillers. The enhanced tensile modulus may be attributed to the aromatic and rigid structure of lignin, and the larger interfacial area of the freeze-dried lignin also leads to improved filler–matrix interactions.39 Similarly, it is easy to understand that similar to other rigid fillers, lignin also induces a decrease in yield strain, and the extent of the decrease increases slightly upon the improvement of dispersion state of lignin in the PP matrix as well as the increase of lignin content. Different from the changes in modulus, yield stress and yield strain, the change in elongation at break brought by the addition of freeze-dried lignin is more impressive, as shown in Figure 4 and Table 1. The composite containing 2wt% as-received lignin (PP/lignin2%-noFD) possesses a much smaller elongation at break than neat PP, indicating that the addition of lignin makes the composite much more brittle. On the contrary, the incorporation of freeze-dried lignin into PP gives rise to significantly increased elongation at break. In particular, PP/lignin2%-FD@50 has the longest elongation at break, 95.5%, which is nearly double that of neat PP. Obviously, the drastically increased elongation at break given by the freeze-dried lignin is due to the much increased interfacial area between the lignin fillers and polymer matrix, whereas without FD, the much smaller interaction area between PP and large lignin particles results in very limited interfacial adhesion, and therefore the elongation at break drops drastically. In addition, PP/lignin2%-FD@50 exhibits a significantly larger elongation than PP/lignin2%-FD@100. This demonstrates that thinner, smaller and more porous sheet morphology of the freeze-dried lignin obtained from a more dilute lignin solution gives larger interfacial area and hence enhanced interactions with the polymer matrix. However, the increase of the content of the freeze-dried lignin to 6 wt% will result in a reduction of the elongation at break, presumably because at a higher content, the lignin is more difficult to be dispersed uniformly. Therefore, both the


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morphology of the lignin fillers and their dispersion state in the matrix play an important role in determining the overall mechanical properties of the composites.40 The remarkable increase of the elongation at break implies that the freeze-dried lignin may act as an effective toughening agent to improve the toughness of the composites, which can also be demonstrated by the much rougher fractured surface of the composite specimens. The SEM images in Figure 5 show the morphologies of the fractured surfaces of neat PP and the freezedried PP/lignin composites. The fractured surfaces were obtained by breaking the specimens using a Resil Impactor under the same condition. Clearly, the fracture surface of neat PP (Figure 5a) is smooth except for some river-like lines, indicating a brittle failure mode without much ductility. By contrast, the PP with freeze-dried lignin displays a rougher fractured surface, and the pull-out of fibers can be observed in a high-magnification image (insets in Figure 5a). The different features of the fractured surfaces confirm the toughening effect induced by the incorporation of freeze-dried lignin, which is probably due to crazing near PP-lignin interface. The enhanced relaxation in PP-lignin interphase regions may also contribute to the toughening mechanism significantly,14, 41-42 which will be further discussed in a latter section. UV and thermo-oxidative stabilities of the PP/lignin composites It is well known that PP is prone to free radical-induced degradation, which can therefore easily degrade in outdoor environments, resulting in a severe decrease of material toughness.26-27 In general, the PP degradation in outdoor environments is mainly due to its exposure to UV light combined with heat. Since lignin contains abundant phenolic groups, it has been employed as an antioxidant to scavenge free radicals generated by UV-induced degradation.27 In this study, the composites of PP with freeze-dried lignin possess much larger contacting areas between the


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lignin and PP. Thus, it is expected that the lignin may act as a more efficient free radical scavenger to improve the UV stability of PP. In order to investigate the antioxidant function of lignin in PP, tensile test bars of the composites were exposed to UV light with light intensity of 50 w/m2 at 35 °C for 14 days. Figures 6 and 7 show the comparison of tensile properties of the composites as well as neat PP before and after the UV irradiation (the data are also summarized in Table S2). It is observed that the yield stress, yield strain and elongation at break of neat PP reduce by 22%, 63% and 87%, respectively, whereas its tensile modulus increases slightly after the UV irradiation. The deterioration of mechanical properties of neat PP is due to UV-induced degradation of polymer chains, which is evidenced by the appearance of the characteristic absorption band of carboxylic group at 1711 cm-1 in the FTIR spectra shown in Figure 8.25 Furthermore, broad absorption in the regions of ~3150 to ~3600 cm-1 and ~1050 to ~1260 indicate the existence of hydroxyl groups (OH) and C-O bonds, respectively. By contrast, the PP/lignin composites all exhibit much smaller or even no negative changes in tensile properties after the UV irradiation, revealing the excellent antioxidant function of lignin. For instance, the tensile modulus, yield stress, and yield strain of PP/lignin2%-FD@50 remain the same after the UV exposure, while its elongation at break reduces by only about 29%. Furthermore, compared with the composite with the asreceived lignin, the composites with the freeze-dried lignin show less deterioration of the tensile properties after the UV irradiation. For example, PP/lignin2%-noFD exhibits a 24% decrease of the yield strain, whereas the yield strain of PP/lignin2%-FD@100, PP/lignin2%-FD@50 and PP/lignin6%-FD@50 remain almost unchanged after the UV irradiation. More impressively, PP/lignin2%-FD@50 retains about 68% of its original elongation at break value after the UV irradiation, which is much larger than that of the composite with the as-received lignin (12.2%),


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implying that PP/lignin2%-FD@50 is still fairly tough after the UV radiation. As discussed above, the FD process, especially when using the solution with relatively low lignin concentration, significantly increase the specific surface area of the lignin. Thus, PP/lignin2%FD@50 would have the largest contacting area between the antioxidant, lignin, and PP, allowing the lignin to act as a free-radical scavenger more efficiently. To further confirm the excellent free radical scavenging function of freeze-dried lignin, FTIR analysis was conducted and the results are shown in Figure 8. It can be seen that for neat PP, an intense band at ~1711 cm–1, which can be attributed to stretching vibration of carbonyl (C=O) group, appears after the UV irradiation. The principal reaction occurring in UV-irradiated PP is chain scission or side-group abstraction. During the reactions, the radicals generated by UV irradiation oxidize the C-H bonds on the PP chains to form the carbonyl groups, which induce chain scission. Thus the intense carbonyl characteristic band indicates that neat PP has under gone severe oxidative degradation. By contrast, PP/lignin2%-FD@50 shows extremely weak carbonyl characteristic band after the UV irradiation, demonstrating that the freeze-dried lignin are able to scavenge the free radicals efficiently. Besides UV stability, the thermos-oxidative behavior of the composites was also investigated to further demonstrate the antioxidant function of the freeze-dried lignin in PP. Both neat PP and PP/lignin2%-FD@50 were subjected to isothermal TGA in air at 200 oC, which is close to the common melt processing temperature of PP. As can be seen in Figure 9, the onset of the weight loss occurs at ~2.0 h for neat PP, whereas it is delayed to ~2.5 h for PP/lignin2%-FD@50. This again demonstrates the antioxidant function of the freeze-dried lignin in PP. Thermal properties of PP/lignin composites


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To further understand the underlying mechanisms for the changes in mechanical properties brought by the incorporation of lignin and UV irradiation, the thermal properties of the PP/lignin composites before and after the UV irradiation are also investigated. From DSC analysis results (Table 2), it is clear that the addition of lignin decreases Tg slightly, which is due to the increased chain segmental mobility in PP-lignin interphase. PP/lignin2%-FD@50 exhibits the lowest Tg, which is consistent with its largest interfacial area brought by the thinner sheet morphology of the lignin particles in this sample and their more uniform dispersion in PP at a relatively low lignin content. The reduced Tg provided a strong evidence for the proposed toughening mechanism, i.e., the observed toughening effect can be attributed to enhanced relaxation in PPlignin interphase regions, although the craze near the interface may also contribute. The peak melting temperatures (Tm) during the first heating scans, the heat of fusion (∆Hm), the peak crystallization temperatures (Tc) during the first cooling scans, and the crystallization enthalpy (∆Hc) of the molded samples are also shown Table 2 (the DSC curves are given in Figure S3). On one hand, the ∆Hm and Tm show little change, implying that the crystallinity and crystal size of the PP matrix in the molded samples are not significantly affected by the addition of lignin. Thus, it is confirmed that the modulus improvement given by lignin is mainly due to the reinforcement effect of the rigid lignin. On the other hand, upon cooling in DSC, the incorporation of lignin into PP gives rise to an increase of Tc by ~ 7 °C for all the composite samples. The increase of Tc may be ascribed to the nucleating effect of the lignin in PP.43-44 Furthermore, the addition of freeze-dried lignin also slightly decreases the crystallization enthalpy (∆Hc), indicating that the presence of lignin in the polymer matrix slightly hinders the growth of PP crystals. This effect seems to be slightly more significant for the freeze-dried lignin obtained from the more dilute solution, again confirming their better dispersion in PP.


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Table 2 and Figure S3a show that the addition of lignin lowers the thermal degradation temperature (Td) of the polymer, which is defined as the temperature corresponding to 5% weight loss in the heating process. It is probably due to the oxygen-containing groups of lignin that accelerate the thermal degradation in N2 atmosphere. It is worth noting that the increase of lignin content from 2 wt% to 6 wt% causes a bounceback of the Td. It is believed that the phenolic groups of lignin can also act as a free radical scavenger in the TGA process, retarding the thermal degradation of the polymer, that is, when PP/lignin composites start to degrade, the phenolic groups of lignin may reduce the PP degradation rate to some extent if a large amount of lignin is present in PP. The thermal properties of the PP/lignin composites before and after the UV irradiation are also compared. As shown in Table 3 and Figure S4, the Tg of neat PP drops about 7 °C (from 4.6 to -11.4 °C), whereas the Tg of PP/lignin2%-FD@50 is almost unchanged after the UV irradiation. The reduced Tg for neat PP can be attributed to the increased chain mobility caused by reduction of molecular weight of PP after the UV radiation. Besides, the Td of neat PP (in N2) drops by 62 °C (from 397 to 335 °C) after the UV exposure, whereas the Td of PP/lignin2%FD@50 only exhibits a slight drop of about 13 °C (from 356 to 343°C). This further confirms that the UV irradiation induces the chain scission of the PP chains, making them break down into volatile small molecules faster in the subsequent TGA tests.

CONCLUSIONS In this work, alkali lignin was freeze-dried and then incorporated into PP by melt blending. It is found that the freeze-dried lignin particles from the solution of lignin concentration of 50 mg/mL exhibit a sheet-like morphology with thickness of about 200 nm, and they can be easily


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break down into small pieces by melt compounding with PP. This gives the corresponding PP/lignin composite more uniform filler dispersion and much larger interfacial area than this counterpart with as-received lignin, which contains spherical or irregular shaped particles with sizes of tens of microns. The results of tensile mechanical tests show that the Young’s modulus and elongation at break of the PP/lignin composites are improved by using the freeze-dried lignin as the additive in comparison with that of neat PP or PP with the same amount of as-received lignin. In particular, the elongation at break is drastically increased with only 2 wt% of freezedried lignin obtained from the solution of lignin concentration of 50 mg/mL, revealing the significant toughness effect of lignin in PP matrix. The toughening effect is confirmed by the rough fractured surfaces of the composite with fiber pull-out near the interface, and its reduced Tg. The significant toughening effect of lignin can be attributed to the crazing near the interface and enhanced relaxation in PP-lignin interphase. The sheet-like lignin is also an excellent free radical scavenger that greatly slows down UV-induced and thermo-oxidative degradation of PP owing to its large contacting area with PP. In a sharp contrast to neat PP, the tensile mechanical properties of the PP/lignin composites show much less drop after UV irradiation for 14 days. In conclusion, it is demonstrated that freeze-drying is a simple and efficient route to renewable multifunctional fillers for non-polar polymers.

ASSOCIATED CONTENT Supporting Information. SEM image and analysis showing size distribution of the as-received lignin, BET specific surface area (SSA) and BJH pore volume analysis results of lignin particles, typical TEM images of PP/lignin composites containing 2 wt% freeze-dried lignin from the solution with conc. of 50


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mg/mL, thermal and mechanical properties of PP/lignin composites before and after UV irradiation are included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel: +65 6790 4585

ACKNOWLEDGMENT This work was supported by Science and Engineering Research Council of the Agency for Science, Technology and Research (A*Star) and Ministry of National Development, Singapore under Grant 132 176 0011. Seyed Ismail Seyed Shahabadi thanks Nanyang Technological University, Singapore, for providing Ph.D. scholarship in the course of this work.

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FIGURES

Figure 1. SEM images of (a) the as-received alkali lignin, (b) freeze-dried lignin at lignin concentration of 100 mg/mL, (c) freeze-dried lignin at lignin concentration of 50 mg/mL (inset scale bar: 1 µm) .


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Figure 2. N2-adsorption isotherms of the as-received lignin and freeze-dried lignin prepared by freeze-drying of an aqueous solution with lignin concentrations of 50 mg/ml, and their specific surface areas (SSA). Inset: a picture showing that lignin aerogel obtained from the solution of lignin concentration of 50 mg/ml; after free-drying its apparent volume remains almost the same as the initial aqueous solution.


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Figure 3. POM (×100) of the composites of PP with (a) 2 wt% as-received lignin and (b) 2 wt% freeze-dried lignin from the solution with lignin conc. of 50 mg/mL.


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Figure 4. Tensile stress-strain curves of neat PP (i), and the PP/lignin composites containing 2 wt% as-received lignin (ii), 2 wt% freeze-dried lignin from the solution with lignin conc. of 100 mg/mL (iii), 2wt% freeze-dried lignin from the solution with conc. of 50 mg/mL (iv), and 6% freeze-dried lignin from the solution with lignin conc. of 50 mg/mL (v).


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Figure 5. SEM images showing (a) the fractured surfaces of neat PP and (b) PP/lignin2%FD@50; the insets show fiber pulling-out in the regions near lignin particles.


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Page 28 of 34

Figure 6. Typical tensile curves of neat PP and PP/2%lignin-FD@50 before and after UV irradiation.


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Figure 7. Tensile mechanical properties of the PP/lignin composites before and after UV irradiation. (a) Young’s modulus, (b) yield stress, (c) yield strain and (d) elongation at break of neat PP, PP/lignin composites with 2 wt% as-received lignin (no FD-2%), 2 wt % freeze-dried lignin at lignin concentration of 100 mg/mL (FD@100-2%), 2 wt% freeze-dried lignin at lignin concentration of 50 mg/mL (FD@50-2%), and 6% freeze-dried lignin at lignin concentration of 50 mg/mL (FD@50-6%).


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Page 30 of 34

Figure 8. FTIR spectra of neat PP before and after UV irradiation, and that of PP/lignin2%FD@50 before and after UV irradiation.


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Figure 9. Weight loss of neat PP and PP/lignin2%-FD@50 as a function of time at 200 oC in air atmosphere.


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Page 32 of 34

TABLES. Table 1 Mechanical property of neat PP and lignin containing PP composites with tensile speed of 50 mm/min. Sample neat PP PP/lignin2%noFD PP/lignin2%FD@100 PP/lignin2%FD@50 PP/lignin6%FD@50

Lignin conc. in FD (mg/mL)

Lignin content (wt%)

Young’s modulus (MPa)

Yield stress (MPa)

--

0

616±16

39.1±0.8

0.177±0.008 54.0±5.0

no FD

2%

628± 9

38.8±0.6

0.145±0.006 35.3±7.2

100

2%

637±13

38.3±0.8

0.137±0.003 57.8±15.5

50

2%

641±6

38.6±1.0

0.136±0.004 95.5±34.9

50

6%

655±17

37.4±1.0

0.123±0.003 55.0±7.3

Yield strain (mm/mm)

Elongation at break (%)

Table 2 Thermal properties of the PP/lignin composites and neat PP before the UV irradiation Sample

Tg (°C)

Tm (°C)

(J/g)

Tc (°C)

-∆Hc (J/g)

Td (°C)

neat PP

-4.6

161.8

60.8

110.9

89.6

397.3

PP/lignin2%-noFD

-6.3

158.9

59.2

116.7

89.4

354.0

PP/lignin2%-FD@100

-7.1

159.4

59.1

117.4

88.1

359.0

PP/lignin2%-FD@50

-7.8

161.0

58.4

117.4

87.3

355.7

PP/lignin6%-FD@50

-5.2

161.1

57.0

117.6

85.9

377.7

∆Hm


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Table 3 Effects of UV exposure on thermal properties of the PP/lignin composites and neat PP Sample

Neat PP PP/lignin2%FD@50

UV Exposure

Tg (°C)

Tm (°C)

∆Hm (J/g)

Tc (°C)

∆Hc (J/g)

Td (°C)

Before

-4.6

161.8

60.8

110.9

89.6

397

After

-11.4

160.2

57.0

111.0

92.6

335

Before

-7.8

161.0

58.4

117.4

87.3

356

After

-6.7

160.9

59.1

116.8

89.2

343


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Page 34 of 34

Sheet-Like Lignin Particles as Multifunctional Fillers in Polypropylene Fenggui Chen, Wanshuang Liu, Seyed Ismail Seyed Shahabadi, Jianwei Xu, Xuehong Lu*

For Table of Contents Use Only (TOC) Freeze-dried sheet-like lignin particles are incorporated into polypropylen by melt compounding, resulting in significantly increased interfacial area. The tensile mechanical properties and UV stability of the composites are hence drastically enchanced.


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Sheet-Like Lignin Particles as Multifunctional ... - ACS Publications

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