Fabrication of Green Lignin-based Flame Retardants for Enhancing

Research Article pubs.acs.org/journal/ascecg

Fabrication of Green Lignin-based Flame Retardants for Enhancing the Thermal and Fire Retardancy Properties of Polypropylene/Wood Composites Lina Liu,†,‡ Mengbo Qian,† Ping’an Song,*,†,‡ Guobo Huang,§ Youming Yu,*,†,‡ and Shenyuan Fu†,‡ †

Department of Materials, College of Engineering, Zhejiang A&F University, No. 88 Huancheng North Road, Hangzhou 311300, China ‡ National Engineering and Technology Research Center of Wood-based Resources Comprehensive Utilization, No. 88 Huancheng North Road, Hangzhou 311300, China § School of Pharmaceutical and Chemical Engineering, Taizhou University, No. 1139 City Road, Linhai 317000, China ABSTRACT: Despite growing extensive applications, until now the poor thermal stability and flame retardancy properties of wood−plastic composites (WPC) remain effectively unsolved. Meanwhile, industrial lignin has emerged as a potential component for polymer composites due to many advantages including abundance, rich reactive functional groups, high carbon content and tailored capability for chemical transformations. Herein, we have fabricated one biobased flame retardant based on industrial lignin chemically grafted with phosphorus, nitrogen and copper elements, as the functional additive for the WPC. Compared with unmodified lignin (O-lignin), functionalized lignin (F-lignin) is more effective to improve the thermal stability and flame retardancy of WPC because of presence of the flame retardancy elements (P and N) and the catalytic effect of Cu2+ on the char-formation. The presence of F-lignin not only reduces the heat release rate, total heat release and slows down the combustion process but also decreases total smoke production rate during combustion. The char residue shows that it is the increased char residues and its continuous compact char formed during burning that are responsible for enhanced flame retardancy properties. This work suggests a novel green strategy for improving flame retardancy performance of WPC and promoting the utilization of industrial lignin. KEYWORDS: Lignin, Wood−plastic composites (WPC), Thermal properties, Flame retardancy

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improve mechanical properties, flame retardancy and thermal stability of WPC.7 Guan et al. added ammonium polyphosphate modified via ion exchange reaction with ethanolamine to WPC to increase the limiting oxygen index by 71.6%, and such improvement is mainly attributed to the hydroamine groups and phosphate acid that can promote the etherification and dehydration reactions, thus facilitating the formation of stabile char residue of WPC.8 Lignin, one natural renewable macromolecular material, represents the third largest source of organic matter in the plant kingdom. Previously, scientists primarily focused on the use of natural plant fibers-reinforced polymers,9,10 whereas lignin has recently attracted researchers’ growing interests for its potential applications in polymers because of many advantages including abundance, rich reactive functional groups, high carbon content and tailored capability for chemical

INTRODUCTION Wood−plastic composites (WPC) represent one sort of novel green material and have been finding extensive applications spanning from construction, automobile materials, logistics packing, to decorating materials due to low cost, excellent mechanical performances and partial biodegradability.1,2 Polypropylene (PP), as one of the cost-advantaged commercially available polyolefins, has been widely used in WPC industry as the matrix resin for its excellent mechanical properties and simple processing methods.3 However, because of the chemical structure features of wood and PP, PP-based WPC exhibits poor thermal stability and high flammability, extremely limiting their wide applications in certain fields where high levels of fire safety are required. Therefore, to address the intractable problem of WPC becomes increasingly necessary with its rapid development and wide application. Until now, many efforts have been devoted to improve the flame retardancy properties of WPC.4−6 For instance, Bai et al. reported incorporation of expandable graphite and intumescent flame retardant into WPC could © XXXX American Chemical Society

Received: January 21, 2016 Revised: March 6, 2016

A

DOI: 10.1021/acssuschemeng.6b00112 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

°C under reduced pressure until the weight did not change, and marked as O-lignin or lignin. Preparation of A-Lignin. 25.0 g of purified lignin, 6.0 g of formaldehyde and 6.0 g of polyethylenimine (PEI) was dissolved in 200 mL of distilled water, and the pH of the solution was adjusted to be 10.0 with a 20 wt % aqueous solution of NaOH by magnetic stirring. Then, the reaction was kept at 50 °C for 5 h. After that, the solution pH was adjusted to be 3.0−4.0 with 10 vol % HCl, and the resultant product was subsequently precipitated. The precipitation was washed with distilled water until the washing water turned neutral, and was dried at 80 °C under reduced pressure until the weight did not change. The resultant brown gray solid was designated as A-lignin, as shown in Sheme 1.

transformations. It is reported that about 50 million tons of lignin is produced worldwide annually, mainly in the form of alkali lignin from paper-making industry.11 Unfortunately, except for only a small proportion using as the energy source, about 95% of industrial lignin is disposed as industrial waste materials. Thus, the comprehensive utilization of industrial lignin is highly critical and remains a huge challenge nowadays.12 Recently, modification of lignin for different applications has attracted growing attention.13−16 For example, Movil et al. reported the generation of hydrogen through electrochemical oxidation of waste lignin from pulping mills for energy storage.17 Qin et al. grafted hydrophilic side chains onto the sulfonated alkali lignin with different molecular weights, and the resultant product could be used as dispersants for coal− water slurry.18 Ferry et al. modified lignin with phosphorus molecules to improve the fire behavior of polybutylene succinate.19 Additionally, lignin can also be incorporated in polymeric materials to improve the thermal properties because its crosslinked structure with phenolic groups is capable of generating the high char yield after decomposition. Moreover, improvements in thermal properties and flame retardant properties of lignin-based composites can be strengthened by chemically modifying lignin with phosphorus and nitrogen elements.19,20 Meanwhile, it is reported that adding a very low loading level of metal compounds can create synergistic effects with phosphorus and nitrogen elements in terms of enhancing the flame retardant properties of polymer composites.21,22 The possible mechanism is its capability to promote the dehydrogenation of the polymer and to catalyze char formation in the condensed phase. In this work, we aim to functionalize chemically alkali lignin by grafting nitrogen and phosphorus first and then coordinating with metal ions, and then investigate effects of modified lignin on the thermal and flame retardant properties of WPC. The flame retardant mechanism of this newly functionalized lignin in the WPC system is also discussed in this work.

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Scheme 1. Schematic Representation for the Synthetic Route to the Functionalized Lignin (F-Lignin)

Preparation of F-Lignin. 5.0 g of A-lignin, 0.01 mol (0.081 mL) of formaldehyde and 0.01 mol (1.38 g) of DEP were dissolved in 50 mL of DMF. Then, a certain amount of HCl solution was added to make the reaction mixture turn weakly acidic (pH ∼ 5.0), and the reaction was allowed to continue at 70 °C for 5 h. Subsequently, 5.0 g of Cu(Ac)2 was introduced and the solution was allowed to react for 5 h at 70 °C. Finally, the resulting product was obtained by removing DMF via rotary evaporation, and was washed with distilled water for at least 3 times and dried at 80 °C under reduced pressure until the weight did not change. The dark brown solid was designated as Flignin, as shown in Scheme. 1. Composites Fabrication. Wood−PP (PP/WP) and wood−PP− lignin (PP/WP/lignin) composites were fabricated via melt compounding using a ThermoHaake Torque Rheometer at 180 °C for 10 min with a rotor speed of 60 rpm for each sample. The experimental formulation is listed in Table 1. Characterization. FT-IR spectra were obtained on a Bruker Vector 22 FT-IR spectrometer with the KBr pellet pressing method. Xray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo ESCALAB 250 spectrometer with the power of 150 W, beam spot at 500 μm and energy analyzer fixed at 30 eV. Elemental analysis (EA) measurements were carried out on a Vario EL elemental analyzer (Elementar Analysensysteme GmbH, Germany). Each sample was measured three times and the average value reported. Atomic absorption spectroscopy (AAS) tests were carried out on a SCT-127 AAS (Thermo iCE3000) atomic absorption spectrometer (USA). Hydrogen spectra nuclear magnetic resonance (1H NMR) of lignin and F-lignin (∼15 mg of sample dissolved in 0.5 mL of DMSO-d6) were recorded on a Bruker-600 NMR 600 MHz spectrometer (Advance III, Bruker, Switzerland). 13C solid-state NMR spectra of lignin and F-lignin were measured on a Bruker-400 NMR 400 MHz spectrometer (Advance III, Bruker, Switzerland). Scanning electron

EXPERIMENTAL SECTION

Materials. Polypropylene (PP) is a copolymer of propylene (90% by mole ratio) and ethylene (10% by mole ratio) with Mw and Mn of 300 000 and 63 000, respectively, and was purchased from China Petrochemical Corporation. Anhydride-grafted polypropylene (PP-gMAH, MAH content: 1.0 wt %) was obtained from Shanghai Rizhisheng Co., Ltd. Wood powder (WP) used here was 250 mesh from poplar wood supplied by Taike wood technology Corporation. Lignin used in the work was wheat straw alkali lignin (OH 6.65 wt %, Mw 36 000 g/mol, polydispersity index: 5.3) with a density of about 1.30 g/cm3, and purchased from Shandong Quanlin Paper Co., Ltd. (Gaotang Town, China). Other chemical agents like stearic acid, diethyl phosphite (DEP), polyethylenimine(PEI), copper acetate (Cu(Ac) 2 ), dimethylformamide (DMF), 37% formaldehyde (HCHO), sodium hydroxide (NaOH), hydrochloric acid (HCl) and ethanol were analytical grade and used as received without further purification. Purification of Lignin. The industrial alkali lignin was purified as follows: 25.0 g of lignin was dissolved in 250 mL of distilled water through adjusting the pH to 10.0 using a 20 wt % aqueous solution of NaOH to enable all lignin to dissolve completely with the aid of stirring. The solution was allowed to stir continuously for another 1 h and then 10 vol % HCl was dropped into the solution by adjusting a pH of 3.0−4.0. After the lignin precipitated completely, the precipitation was washed with distilled water at least 3 times until the washing water turned neutral. The purified lignin was dried at 80 B


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Table 1. Formulations of Wood−Polypropylene Composites Based on O-Lignin and F-Lignin by Mass Fraction sample no.

PP (wt %)

PP-g-MAH (wt %)

O-lignin (wt %)

F-lignin (wt %)

wood (wt %)

UL-94 rating

PP/WF PP/WP/5-O-lignin PP/WP/15-O-lignin PP/WP/2.5-F-lignin PP/WP/5-F-lignin PP/WP/10-F-lignin PP/WP/15-O-lignin

76.5 76.5 76.5 76.5 76.5 76.5 76.5

3.5 3.5 3.5 3.5 3.5 3.5 3.5

0 5 15 0 0 0 0

0 0 0 2.5 5 10 15

20 15 5 12.5 15 10 5

no rating no rating V-2 no rating no rating V-2 V-1

Figure 1. Infrared spectra of (a) O-lignin, (b) A-lignin, (c) PN-lignin and (d) F-lignin.

Figure 2. 1H NMR spectra of (A) O-lignin and (B) F-lignin using DMSO-d6 as the solvent. vertical burning test instrument (CZF-2) (Jiangning, China) according to ASTMD 3801 UL-94 standard.

microscopy (SEM) images were recorded on a S4800 (FEI, Japan) SEM at an accelerating voltage of 5 kV. Thermogrametric analysis (TGA) tests were performed on a TA SDTQ600 (TA Instruments) thermogravimetric analyzer. About 8.0 mg of sample was heated from room temperature to 700 °C at a heating rate of 20 °C under N2 atmosphere. Raman spectroscopy was performed on Algega Dispersive Raman-Thermo Nicolet at 514 nm. X-ray diffraction (XRD) was carried out using a Rigaku X-ray generator (Cu Kα radiation with λ = 1.54 Å) at room temperature. Flame retardancy properties of sample (10 × 10 × 3 mm) was evaluated using a cone calorimeter performed in an FTT UK device according to ISO 5660 with an incident flux of 35 kW/m2. Typical results from cone calorimeter are reproducible to within 10% and the data reported here were the means of triplicate experiments. The UL-94 vertical burning tests were used to test the flammability of WPC composites with a size of 125 × 13 × 3 mm on a

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RESULTS AND DISCUSSION Characterization of Functionalized Lignin (F-Lignin). The typical synthetic route to prepare functionalized lignin (Flignin) is shown in Scheme 1. IR spectra of O-lignin, A-lignin and F-lignin were performed to characterize the chemical structure, as shown in Figure 1A,B. For pristine lignin, a broad absorption band at 3400−3500 cm−1 is attributed to the stretching vibration of aromatic and aliphatic OH groups, and the peaks around 2900 cm−1 are assigned to CH stretching vibration of CH3, −CH2− and −CH− groups. The absorption band at 1650 cm−1 belongs to the stretching C


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hydroxyethyl carbons (C-7) and methoxyl carbons (C-8). These observations are well consistent with previous reports.26,27 As compared with lignin, the most noticeable change is that the C-6 signal (115 ppm) shifts to lower the chemical shift and overlaps with the C-3 peak (105 ppm), as evidenced by the considerably increased intensity of the C-3 signal, because of the Mannich addition reaction. Meanwhile, the intensity of the C-7 signal increases remarkably, suggesting the introduction of C−O bonds, which should arise from the hydroxyethyl carbons (C-11) of DEP segment in F-lignin. Moreover, chemical shifts between 50 and 35 ppm belonging to carbon linked with N atoms (C−N), namely C-9, C-13 and C14, and a should signal peak at 24 ppm associated with C-12 are readily observed. All these changes in 1H NMR and 13C NMR results sufficiently indicate the successfully functionalization of lignin by PEI and DEP. XPS measurements were carried out to determine elemental composition of F-lignin, which helps provide another evidence for the fabrication of F-lignin, especially the instruction of cupric ions. As shown in Figure 4A, O-lignin only shows two elements, namely C and O, containing 78 wt % of carbon and 22 wt % of oxygen, very close to elemental analysis (EA) results, showing 74 wt % C, 20 wt % O, 5.7 wt % H and 0.30 wt % N. In comparison, A-lignin displays 73 wt % of C, 19 wt % of O and 8.0 wt % of N (75 wt % C, 13 wt % O, 6.1 wt % H and 5.9 wt % N determined by EA), clearly indicating that lignin was chemically grafted with PEI chains, which is well consistent to IR results. As for F-lignin, besides C, O and N, 3.0 wt % of P and 6.0 wt % of Cu elements are detected (74 wt % C, 15 wt % O, 6.4 wt % H and 3.6 wt % N determined by EA), which further verifies that pure lignin has been successfully chemically functionalized with flame retardant elements. Moreover, AAS results show that the content of copper is about 6.5 wt % in the F-lignin, very close to that (6.0 wt %) determined by XPS tests. To determine the ratio of Cu−N and Cu−O, we split the XPS spectra peak of Cu2P as shown in Figure 4B. The result shows clearly two peaks at 933.6 eV (Cu−O) and 934.5 eV (Cu−N) respectively, and the ratio of Cu−O and Cu−N is about 42/58 by calculating their relative area.28,29 TGA tests allow us to determine the thermal stability and char-forming capability of modified lignin. As shown in Figure 5, pristine lignin (O-lignin) starts to degrade from 250 °C (Ti) and decomposes most rapidly at about 340 °C (Tmax), leaving a high char residue of 59 wt % at 650 °C. In comparison, although F-lignin exhibits a lower initial decomposition temperature (Ti) of only 194 °C,30 probably because of the catalytical decomposition effect of Cu2+ ions on lignin macromolecules, the Ti is still much higher than the common processing temperature, such as 180 °C. Actually, earlier degradation enables F-lignin to produce char layers that can effectively protect the underlying materials from heat and flame. Meanwhile, the maximum mass loss temperature (Tmax) occurs at 330 °C, with a similar char residue of 56 wt % left. These changes further suggest lignin has been successfully functionalized. Thermal Stability. Figure 6 shows thermal degradation behaviors of PP PP/WP and PP/WP/lignin composites in a nitrogen condition. For the PP/WP composite, it starts to degrade at 255 °C (namely, the initial degradation temperature, Ti) and the maximum weight loss temperature (Tmax) occurs at 473 °C. Incorporating O-lignin into PP/WP composite slightly enhances Ti and Tmax, only by about 2 and 2 °C, respectively. The enhanced Ti is likely due to the higher thermal stability of

vibration of CC in benzene rings of lignin with characteristic absorption peaks of aromatic rings found at 1450, 1580 and 1500 cm−1. Besides, the stretching vibration of CO in primary alcohol is also found at 1020 cm−1.1,20 Compared with O-lignin, besides the absorption peaks of IR spectrum of O H, A-lignin shows the in-plane-deformation vibration of NH bonds at 1580 cm−1 and the stretching vibration of CN bonds at 1170 cm−1, indicating that PEI chains are successfully grafted onto the lignin. As for F-lignin spectrum, the band at 1610 cm−1 arises from the combination of stretching and bending vibration of PO bonds, and the absorption peak at 1222 cm−1 is assigned to be the stretching vibration of PO bonds. Moreover, absorption bands at 1090 and 765 cm−1 are respectively due to the stretching vibration PO and PN C.20 Moreover, two new absorption peaks at ∼434 cm−1 (Cu O) and ∼506 cm−1 (CuN)23,24 appear, strongly proving the presence of cupric in F-lignin by the chelation reaction (see Figure 1B). All these IR changes demonstrated that the Olignin was successfully functionalized with phosphorus and nitrogen. 1 H NMR is a powerful tool to characterize the chemical structure of lignin (O-lignin) and F-lignin. As shown in Figure 2A, the chemical shifts at 9.2 ppm (phenolic hydroxyl groups protons, marked by e), 7.6−6.3 ppm (aromatic protons, marked by c, c′ and f), 5.2 ppm (hydroxyl groups, marked by H-a) and 3.7−3.4 ppm (protons in hydroxyethyl/methoxyl groups, H-b, H-d) are found in the 1H NMR spectrum of pure lignin, which agrees well with the previous reports.25−27 Compared with lignin, the signals of aromatic protons prominently turn weak and specifically the peak of f proton disappears (see Figure 2B), indicating that this proton was substituted by carbon atoms via the Mannich addition reaction with formaldehyde and DEP. Moreover, multiple peaks are clearly observed between 3.7 and 2.7 ppm (3.7, 3.5, 3.3, 3.2, 2.9, 2.7 ppm), which are attributed to methyl/methylene protons lined to N and O atoms (H-b, Hd, H-f, H-g, H-h and H-j marked in the structure of F-lignin of Figure 2B). In addition, 13C solid-state NMR is performed to confirm further IR and 1H NMR results. As shown in Figure 3, for pristine lignin, the aromatic ring carbons signals located at 152−142 ppm (marked by C-1,C-2), 137−124 ppm (C-4, C5), 115 ppm (C-6) and 105 ppm (C-3) are observed, and chemical shifts at 74 and 56 ppm are respectively assigned to

Figure 3. 13C solid-state NMR spectra of O-lignin and F-lignin. D


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Figure 4. XPS spectra of (A) O-lignin, A-lignin and F-lignin with their elemental compositions inset, and (B) Cu2p in F-lignin showing relative ratio of Cu2+ ← O and Cu2+ ← N coordination bonds.

O-lignin than the wood powder in the composites, and the higher Tmax is probably related to the protection action of the much higher char residue created at relatively low temperature, as shown in Figure 6A.20 Adding an equal loading level of Flignin leads to a similar thermal degradation behavior to Olignin. However, incorporation of 10 wt % F-lignin can prominently increase both Ti and Tmax of the PP/WP composites by 7 and 5 °C, respectively. This demonstrates that F-lignin is a little better in enhancing the thermal stability of PP/WP than O-lignin due to the flame retardant fictionalization of lignin. Flame Retardancy Properties. The flammability of samples were tests using UL-94, with results reported in Table 1. Adding pristine lignin hardly leads to any improvement in UL-94 classifications, and a V-2 rating is obtained for PP/WP-15-O-lignin. Whereas for PP/WP-F-lignin, adding 15 wt % F-lignin results in a V-1 rating, showing better flame retardancy than its counterpart containing O-lignin. In addition, cone calorimetry is widely used to evaluate the fire hazards of materials for it is capable of simulating real fire hazards. Figure 7 shows the heat release rate (HRR) and total

Figure 5. TGA and DTG curves for O-lignin and F-lignin under nitrogen condition.

Figure 6. (A) TGA and (B) DTG curves of PP/WP, PP/WP/5-O-lignin, PP/WP/5-F-lignin and PP/WP/10-F-lignin composites under N2 condition at a heating rate of 20 °C/min. aTi and Tmax refer to the temperature where 5 wt % mass loss and the maximum mass loss occurs; the char refers to the char obtained at 600 °C. E


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Figure 7. (A) Heat release rate; (B) total heat release curves; (C) smoke production rate; (D) total smoke production rate of typical PP/WP, PP/ WP/5-O-lignin, PP/WP/5-F-lignin and PP/WP/10-F-lignin composites at a heat flux of 35 kW/m2.

Table 2. Detailed Data of PP/WP, PP/WP/O-Lignin and PP/WP/F-Lignin Composites Collected from Thermal Analysis and Cone Calorimeter Measurements no. PP/WP PP/WP/5-O-lignin PP/WP/15-O-lignin PP/WP/2.5-F-lignin PP/WP/5-F-lignin PP/WP/10-F-lignin PP/WP/15-F-lignin

tigna (°C) 18 20 22 19 22 23 24

± ± ± ± ± ± ±

1 1 1 1 1 1 1

PHRRa (kW/m2) 595 611 580 602 542 575 470

± ± ± ± ± ± ±

15 18 20 16 10 12 15

THRa (MJ/m2) 93.9 91.5 85.3 90.2 60.5 72.3 70.2

± ± ± ± ± ± ±

2.2 1.9 1.7 1.5 1.8 1.4 1.7

residuea (wt %) 8.40 9.61 9.72 9.49 10.9 10.3 10.4

± ± ± ± ± ± ±

0.3 0.4 0.5 0.3 0.2 0.2 0.3

AMLRa (g/s) 0.28 0.039 0.037 0.036 0.035 0.032 0.030

± ± ± ± ± ± ±

0.04 0.006 0.004 0.005 0.004 0.003 0.002

TSRa (m2/m2) 12.5 11.7 10.9 11.8 10.2 9.63 9.17

± ± ± ± ± ± ±

0.4 0.5 0.4 0.4 0.3 0.4 0.2

a

tign, PHRR, THR, residue, AMLR and TSR are the time to ignition, peak heat release rate, total heat release, residue after tests, average mass loss rate and total smoke production, respectively.

15% F-lignin hardly changes the PHRR value anymore. As expected, THR demonstrates a similar trend to PHRR for all composites. The addition of pure lignin only marginally reduces the THR of the PP/WP composite (around 93.2 MJ/m2). In comparison, adding 5% F-lignin decreases sharply THR by 36%, about 60.5 MJ/m2, but increasing the loading level of Flignin brings about the increase in THR, a THR of 72.3 kW/m2 for the PP/WP/10-F-lignin. Similar phenomena have been reported before for many metal-catalyzed flame retarded PP systems, which evidently indicates that there is an optimum content of metal catalyst to achieve the best flame retardancy property.28,29 In our system, PP/WP/5-F-lignin contains about 0.33% of cupric ions shows the lowest PHRR and THR values, and excessive Cu2+ ions can destroy the swelling behavior of the char layer or structure, further leading to worse flame retardancy.29 In fact, such a loading level of Cu2+ ions is quite close to the optimum concentration of Ni(HCOO)2

heat release (THR) curves of PP/WP, PP/WP/O-lignin and PP/WP/F-lignin composites, with detailed data listed in Table 2. As shown in Figure 7A, the PP/WP is extremely ignitable and flammable, exhibiting a time to ignition (tign) of 18 s, a peak heat release rate (PHRR) of 595 kW/m2 and a THR of 93.9 MJ/m2. After 5 wt % WP was replaced with the equal loading level of O-lignin, the tign and the PHRR are respectively increased to 20 s and 611 kW/m2, but the THR is decreased slightly. Similarly, 5 wt % of F-lignin makes the tign of PP/WP prolong 4 s (22 s) and reduces the PHRR of PP/WP by 9%, about 542 kW/m2 as compared with 611 kW/m2 for PP/WP/ 5-O-lignin. The delayed tign and reduced PHRR values indicate the improvement in the flame retardancy performance of PP/ WP. However, increasing further the content of F-lignin leads an adverse effect on the flammability of the composite. For instance, PP/WP/10-F-lignin displays a PHRR value of 575 kW/m2, which is higher than that of PP/WP/5-F-lignin even if the experimental error is taken into account. Moreover, adding F


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ACS Sustainable Chemistry & Engineering (0.2−0.3%) for flame retarding PP based on ammonium polyphosphate (APP) and pentaerythritol (petol).31 In addition, introducing lignin considerably increases the char residue of PP/WP composite, especially for F-lignin. For instance, the char residue increases by 14.3 wt %, from 8.4 to 9.6 wt % after adding 5 wt % O-lignin, mainly caused by lignin during burning. Compared with O-lignin, the equal loading level of F-lignin leads to slightly higher char residues (about 10.6 wt %), an increase by 26 wt % relative to PP/WP probably because of the dehydration action of phosphorus acid species and the catalytical char-formation action of Cu2+ on lignin and wood.22,23,30,33 Besides, the average mass loss rate (AMLR) of WPC displays a similar change trend to the PHRR, for example, a AMLR of 0.047 g/s for PP/WP, 0.051 g/s for PP/WP/5-O-lignin and 0.038 g/s for PP/WP/10-F-lignin. Thus, it is clear that F-lignin can confer better flame retardant properties on PP/WP composite than O-lignin. Besides heat release rate and mass loss, the smoke produced during the combustion of a material is equally critical because the smoke generated during the fire can dramatically reduce visibility of the fire, thus making it a more difficult for the trapped people to escape. Smoke production rate (SPR) and total smoke production rate (TSR) are two significant parameters to evaluate the how much of smoke production during the fire. Generally, smoke mainly derives from small unstable carbon particles and cyclic compound formed during combustion. Figure 7C,D demonstrates that the addition of Olignin and/or F-lignin can to some degree decrease both SPR and TSR of WPCs. Compared with PP/WP and the other two systems, the magnitude of reduction in TSR is much more obvious for PP/WP/10-F-lignin, exhibiting a TSR of 9.63 m2/ m2 relative to 12.5 m2/m2 of PP/WP, as shown in Table 2. Such reduction in smoke is partially due to the capability of cupric ions to trap free radicals created during combustion, thus reducing the formation of cyclic compounds, and partially attributed to their capability of bonding unstable carbon particles to residue, as evidenced by the increased char residue, therefore preventing them from escaping to the air.34 Investigation of Char Residues. Investigating the char residue helps us to understand the flame retardancy mechanism of F-lignin in PP/WP because it, as one kind of intumescent flame retardants, theoretically improves the flame retardancy properties polymers via the condense phase mechanism. Figure 8 gives digital photos of PP/WP, PP/WP/O-lignin and PP/ WP/F-lignin composites after cone calorimeter tests. It is clearly observed that the char residue from PP/WP (Figure 8A) is very thin after combustion, whereas the char residue of PP/ WP/O-lignin or F-lignin (Figure 8B,C,D) displays thicker char layer despite some cracks. Particularly, 10 wt % of F-lignin makes the char residue become much thicker as compared with that of PP/WP, which is also evidenced by much higher char residue (18.6 wt %) than 8.4 wt % of PP/WP. Therefore, the high and thick char residue is primarily responsible for the improved flame retardant properties. SEM images allow us to observe the char residue on a microscopic scale. As shown in Figure 9A, the char residue of PP/WP displays a loose surface and many cracks and 84.2 wt % carbon and 15.8 wt % oxygen are determined. These cracks in the char layer are harmful for isolating heat and combustible gas, and thus total heat release (THR) is high and the time to ignition is very short, as listed in Table 2. As for PP/WP/Olignin in Figure 9B, the incorporation of lignin results in the

Figure 8. Digital photos of composites for (A) PP/WP, (B) PP/WP/ 5-O-lignin, (C) PP/WP/5-F-lignin and (D) PP/WP/10-F-lignin after cone calorimeter measurements.

Figure 9. SEM images of the residue char (A1, A2) for PP/WP composite, (B1, B2) for PP/WP/O-lignin composite, and (C1, C2) for PP/WP/F-lignin composite after cone calorimeter measurements.

formation of a more compact residue char layer, and huge cracks disappear, which contributes to improved flame retardancy. The char layer of the PP/WP/O-lignin composite, shown at a higher magnification in Figure 9B2, is more compact than that of PP/WP in Figure 9A2. In comparison, the char residue of PP/WP/F-lignin shows a much more continuous and compact structure (Figure 9C,C1), which is very effective G


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ACS Sustainable Chemistry & Engineering for isolating heat and flammable gas from the underlying materials, thus leading to reduced HRR, THR and THR. Therefore, it is reasonable to conclude that the continuous and compact char layer is primarily responsible for the improved flame retardant properties of WPC. Raman spectroscopy is employed to characterize graphitic structure and to determine the hybridization state of carbon atoms in the residue char. As shown in Figure 10, the char

Figure 11, the IR spectrum of the char of PP/WP primarily exhibits three strong absorption bands, respectively located at

Figure 11. FT-IR spectra of the char residue for (a) PP/WP, (b) PP/ WP/O-lignin and (c) PP/WP/F-lignin composites.

∼3460 cm−1 belonging to the stretching vibration of OH groups (υO−H), 1639 cm−1 attributed to the stretching vibration of unsaturated bonds (CC), indicating the formation of graphitic structures or polyaromatic species, and 1097 cm−1 corresponding to the COH. In addition, a weak absorption peak at 2916 cm−1 is also found due to the stretching of CH2 groups. After careful comparison, the char of PP/WP/O-lignin basically shows a similar IR spectrum to that of PP/WP, which is mainly because WP itself contains lignin. In comparison, besides above several absorption peaks, absorption bands at 1190, 1153 and 1105 cm−1 are respectively attributed to the stretching vibration of CN, PO, POC (POP or CO) groups.22,37,38 Moreover, two peaks center at 964 and 762 cm−1 probably indicating the presence of CuOP/C and CuO/N in the char. This suggests that Cu2+ ions are capable of catalyzing the carbonization of WP and PP, and thus increase improve the flame retardancy, agreeing well with the Raman result. To evidence further the catalytic effect of Cu2+ on the charforming of PP/WP, its chemical covalence was evaluated using the XRD test. Figure 12 clearly shows that besides diffraction peaks of divalent Cu2+, those of univalent Cu+ and Cu substance are also determined. This strongly indicates that the catalytic dehydrogenation effect of Cu2+ on lignin and WP during combustion, accompanied by the chemical reduction of Cu2+ into Cu+ and Cu substance.23 Flame Retardancy Mechanism. On the basis of the above analysis, a reasonable flame retardancy mechanism can be proposed. For the PP/WP/F-lignin system, upon heat treatment, the phosphorus-containing moiety in the F-lignin will decompose to form phosphoric acid, pyrophosphoric acid or polyphosphoric acid species, as proved by IR.22,38 Such acid species are able to promote lignin and WP (mainly containing cellulose, hemicellose and lignin) to generate carbonaceous substances by the dehydration reaction.38 Meanwhile, the presence of Cu2+ can catalyze the dehydrogenation carbonization reaction of WP and lignin.23 Such a carbonaceous layer is able to not only prevent the heat transfer and the diffusion of small combustible products created by the degradation of

Figure 10. Raman spectra of the char residue for (a) PP/WP, (b) PP/ WP/O-lignin and (c) PP/WP/F-lignin composites.

residue of PP/WP displays two visible bands, respectively located at 1583 cm−1 (G-band) and 1359 cm−1 (D-band). Generally, the G-band is related to the vibration of sp2-hybrided carbon atoms in graphite layers, whereas the D-band is associated with vibration of carbon atoms with dangling bonds in the plane terminations of disordered graphite or glass carbons, representing the sp3-hybridized carbon and the presence of defect-like amorphous domains.33,35,36 This suggests that there are graphic and disordered carbon structures coexisting in the char residue of PP/WP. As for PP/WP/5-Olignin, both the G-band and D-band of its char shift to high wavenumbers, centering at 1591 and 1365 cm−1, respectively, which is probably due to the participation of lignin in the charforming process or bond with C or O atoms and further activate the sp3-hybridized carbons.35 Unexpectedly, as compared with PP/WP, the char residue of PP/WP/5-F-lignin exhibits a lower D-band (1351 cm−1) but a higher G-band (1591 cm−1), mainly because of the catalytic char-forming effect of Cu2+ ions and phosphorus acid species on lignin and wood powders (mainly containing cellulose, hemicelluose and lignin), as evidenced by higher char residues, and their chemical bonding with carbon atoms. In addition, the relative intensity ratio (R), namely ID/IG is inversely proportional to an in-plane microcrystalline size and/ or an in-plane phonon correlation length and can indicate the graphitization degree of carbon materials.36 Therefore, the high ID/IG of 1.26 for PP/WP indicates a low graphitization degree, and the presence of O-lignin hardly affects the R value (1.24). However, the presence of F-lignin sharply reduces the R value down to 1.13, indicating a higher graphitization degree, and that Cu2+ ions can catalyze the formation of graphic carbon and/or reduce the generation of amorphous carbon.37 IR is used to determine further the types of chemical bonds in the char residue and the effect of Cu2+ ions. As presented in H


Research Article

ACS Sustainable Chemistry & Engineering Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Lina Liu and Mengbo Qian as the cofirst author contribute equally to this work. The authors gratefully acknowledge Scientific Research Foundation of Zhejiang A&F University (No. 2055210012), National Science Foundation of China (No. 51303162), Natural Science Foundation of Zhejiang Province (No. Q15C160002), Startup Foundation for youth of Zhejiang A&F University (No. 2013F2064). The Program for key Science and Technology Team of Zhejiang Province (No. 2013TD17), Commonweal Project of Science and Technology Agency of Zhejiang Province of China (No. 2013C32073) and the Science and Technology Project of Taizhou City (14GY01).

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Figure 12. XRD diffraction pattern of the char residue for the PP/ WP/F-lignin composite.

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polymer but also isolate the oxygen gas, thereby reducing the heat release rate and slowing down the combustion process, and protecting the underlying polymer.38 Additionally, the nitrogen-containing moiety in the F-lignin decomposes to generate NH 3 when exposed to the heat flux. The incombustible NH3 gas can dilute the O2 concentration of the combustion zone, thus slowing down of the combustion and decreasing the heat release. Hence, adding F-lignin enables WPC to show enhanced flame retardancy. Compared with Flignin, lignin itself can produce a large amount of carbon residue because of its high carbon content, as evidenced by TGA results, that is why adding lignin can also help reduce slightly the flame retardancy.

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CONCLUSIONS Functionalized lignin (F-lignin) has been successfully fabricated via a two-step reaction by grafting phosphorus−nitrogen and coordinating with metal element (Cu2+) to enhance the thermal stability and flame retardancy of PP/WP composite. Compared with pure lignin, incorporation of a equal loading of F-lignin into WPC can further increase the thermal stability of the PP/ WP composites and improve the flame retardancy properties, with PHRR decreased by 9%, THR by 25%, AMLR by 19% and char residue increased by 30% when the loading level of Flignin is 5 wt %. Moreover, the smoke production is also reduced by 30%. The continuous and much compact char layer is primarily responsible for the improved flame retardancy. The presence of cupric (Cu2+) ions in F-lignin are capable of catalyzing the carbonization of WP and lignin, leading to higher char residue and better flame retardancy. This strategy not only provides a novel method for flame retarding polymeric materials but greatly extends the comprehensive utilization of industrial lignin.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*P. Song. Tel. +86 571 63743976. E-mail: pingansong@gmail. com. *Y. Yu. Tel. +86 571 63743976. E-mail: [email protected]. cn. I


Research Article

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Fabrication of Green Lignin-based Flame Retardants for Enhancing

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