Converting Industrial Alkali Lignin to Biobased Functional Additives for

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Converting Industrial Alkali Lignin to Biobased Functional Additives for Improving Fire Behavior and Smoke Suppression of Polybutylene Succinate Lina Liu, Guobo Huang, Ping'an Song, Youming Yu, and Shenyuan Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00955 • Publication Date (Web): 24 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Sustainable Chemistry & Engineering

Converting Industrial Alkali Lignin to Biobased Functional Additives for Improving Fire Behavior and Smoke Suppression of Polybutylene Succinate Lina Liu,†,‡ Guobo Huang,±,§ Ping’an Song,*,†,‡ Youming Yu,*,†,‡ Shenyuan Fu*,†,‡ †Department

of Materials, College of Engineering, Zhejiang A&F University, 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

* Corresponding Author E-mail: [email protected] (Dr. P. Song) ABSTRACT: The inherent flammability of biodegradable polybutylene succinate (PBS) restricts extremely the growing applications as packaging and construction materials, meanwhile only a minority of industrial alkali lignin has been effectively utilized until now. To address these two challenges, herein we have converted alkali lignin into one biobased additive for PBS by chemically modified lignin with phosphorous, nitrogen and the zinc (II) ions. Cone calorimetry results show that addition of 10 wt% of modified lignin (PNZn-lignin) reduces the peak heat release rate and total heat release of PBS strikingly by 50% and 67%, respectively. Moreover, the total smoke production is decreased noticeably by 50 %. Observations of char residues indicates that adding PNZn-lignin leads to a compact, intact and thick char layer responsible for such enhanced properties. This work offers a new strategy for 1

ACS Paragon Plus Environment


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reducing the flammability and smoke release of PBS, promoting high value-added utilization of industrial lignin, and designing biobased advanced polymeric materials.

KEYWORDS: Polybutylene succinate (PBS), Alkali lignin, Flammability, Smoke Suppression

2


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INTRODUCTION The past several decades have seen serious impacts of petroleum-based polymers on the environment, such as “white pollution” because of their extensive consumption. This has driven the development of biopolymers made from renewable resources as alternatives.1-4 Among these biopolymers, polybutylene succinate (PBS), a succinic acid-based biodegradable aliphatic polyester, has recently emerged as a promising alternative to traditional fossil plastics due to its excellent biodegradability, ease of processing, good flexibility, and comparable mechanical properties to traditional plastics like polypropylene.1-5 More important is that PBS is expected to be “all green” in the future since the monomers can be obtained facilely by bacterial fermentation.6 As compared with another biopolymer, polylactic acid (PLA), besides comparable mechanical strength and modulus, PBS has much higher strain at break or toughness and higher softening point. Thus, PBS has found a variety of applications as agricultural films, one-off daily necessities, and packing materials until now. However, the inherent flammability and melt dripping upon burning restrict extremely the potential application of PBS, especially in construction such as foaming products, packing and electric and electronic fields where low flammability is normally required.5,7-11 Until now, the most primary approach to reduce the flammability is to add flame retardant additives into PBS. For instance, Chen et al. reported that incorporation of nanosized carbon black could improve the flame retardancy of PBS to some extent, probably attributed to the formation of high-quality carbon layer during combustion.7 Yang et al. found that the flammability of PBS can 3



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be reduced by adding nitrogen-phosphorous flame retardants, such as melamine phosphate, melamine phosphite and melamine hypophosphite.8 Recently, Hu et al. investigated the effect of fumed silica or graphene on ammonia polyphosphate (APP)/melamine flame retarding PBS, and better flame retardancy and anti-dripping properties are observed when three components are combined, exhibiting strong synergistic effects between them.9,10 To improve the compatibility of APP with the PBS matrix, they prepared the microencapsulated APP (MAPP) by coating with ethyl cellulose. A V-0 rating for PBS could be achieved when MAPP and the char-forming agent (CFA) were combined when the ratio was tuned properly, whereas the tensile strength of the composites was not decreased dramatically.11 Kuan et al. has found that water crosslinking can enhance the flame retardancy and nondripping performances of APP/PBS composites treated with tetraethoxysilane.12 Despite above great efforts, these flame retardant additives are expected to be biobased materials to make PBS composites completely “green”.13 Evidently, previous research has been devoted mainly to the traditional phosphorus-nitrogen based additives, whereas the use of biobased flame retardant additives have been involved hardly so far. Industrial alkali lignin is mainly produced in abundance as a by-product in both paper-making industry and lignocellulosic ethanol industry.14 Unfortunately, only a minority of industrial lignin has found potential applications until now in spite of extensive discovery studies on its comprehensive utilization.15-19 Therefore, extending the high value-added utilization of lignin remains a huge challenge. In fact, currently 4


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the biggest issue limiting the effective utilization of lignin lies in its vastly variable and complex nature, and another bottleneck for the application of lignin in polymer materials is the incompatibility with the polymer matrices.4 Chemical modification has been widely reported to be one promising approach for promoting the effective utilization of industrial lignin in polymers. Recently, lignin has shown great potential as biobased flame retardant additive because of its high char yield after decomposition from its aromatic framework (about 40 wt% at 900 oC).20,21 It is reported that the presence of lignin can reduce effectively flammability of polymers including polypropylene (PP) and ABS.21-23 In addition, APP, aluminum hydroxide, and melamine phosphate have been widely used to create the synergy with lignin, and the thermal degradation temperature is further delayed and the heat release rate of PP is decreased effectively.22 Very recently, Ferry et al.13 modified lignin by grafting molecular or macromolecular phosphorous compounds and the heat release rate of PBS was found to be remarkably reduced due to the formation of the char layer arising from the degradation of lignin. Likewise, our previous work has also clearly demonstrated that functionalization alkali lignin by grafting phosphorus and nitrogen elements can significantly reduce the flammability of PP and its composites, and adding a very low loading level of metal compounds strengthens the effect.24-26 The results show that the metal ion holds a capability of promoting the dehydrogenation of the polymer and catalyzing the char formation in the condensed phase. To improve the fire behavior and smoke suppression of PBS and promote the high 5



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value-added utilization of industrial lignin, in this paper we functionalize alkali lignin by grafting nitrogen and phosphorous as well as zinc ions via coordination. As expected, the results show that incorporating 10 wt% of modified lignin is able to considerably reduce the heat release rate and smoke production of PBS. This work provides a new approach for extending the durable application of advanced PBS and promoting the highly effective utilization of alkali lignin, and also contributes to the design of the fully biobased high-performance polymer materials. EXPERIMENTAL SECTION Materials. Polybutylene succinate (PBS) (Mw: 190,000) was purchased from Anqing hexing Chemical Co., Ltd (Anqing, China). Lignin used in the work was wheat straw alkali lignin (OH: 6.65 wt %, Mw: 1388, polydispersity index: 1.26) with a density of about 1.30 g/cm3, and purchased from Shandong Quanlin Paper Co., Ltd. (Gaotang Town, China). Other chemical agents including diethyl phosphite (DEP), polyethyleneimine (PEI) (Mw:10,000), zinc acetate (Zn(Ac)2), 37% formaldehyde (HCHO), sodium hydroxide (NaOH), hydrochloric acid (HCl) and ethanol are analytical grade and used as received without further purification. Preparation of PNZn-lignin. The alkali lignin was purified and functionalized with PEI prior to the synthesis of PNZn-lignin. The purification of lignin (O-lignin) and the fabrication of amino-modified lignin (A-lignin) were carried out according to the same procedure reported in our previous work,26 as shown in Sheme.1. With regard to the preparation of PNZn-lignin, typically 5.0 g of A-lignin, 0.01 mol (0.081 mL) of formaldehyde and 0.015 mol (2.07 g) of DEP were dissolved in 100 mL 6


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deionized water under stirring. Then, a certain amount of NaOH solution (2 wt %) was added to make the pH value of the mixture reach ~10.0, and the reaction was allowed to continue at 70 oC for 5h. Subsequently, 5.5 g of Zn(Ac)2 was introduced and the reaction continued at 70 oC for another 5 h. Finally, the resulting product was obtained by washing with distilled water for at least 3 times followed by drying at 60 o

C under reduced pressure until the weight did not change. The dark brown solid was

designated as PNZn-lignin, as shown in Scheme.1. Composites Fabrication. PBS and its blends based on O-lignin and PNZn-lignin were fabricated via melt compounding using a ThermoHaake Torque Rheometer at 120 oC for 10 min with a rotor speed of 60 rpm for each sample. As for the designation of the samples, PBS/x-PNZn-lignin, x refers to the mass fraction of PNZn-lignin in the composite, for instance, PBS/5.0-PNZn-lignin contains 95 wt% of PBS and 5.0 wt% of PNZn-lignin. To evaluate the effect of modification, the PBS composite containing 5.0 wt% of O-lignin (PBS/5.0-O-lignin) was also prepared according to the same protocol. Characterization. FT-IR spectra were obtained on a Bruker Vector 22 FT-IR spectrometer with the KBr pellet pressing method. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo ESCALAB 250 spectrometer with the power of 150 W, beam spot at 500 um and energy analyzer fixed at 30 eV. Elemental analysis (EA) measurements were carried out on a Vario EL elemental analyzer (Elementar Analysensysteme GmbH, German). Each sample was measured three times and the average value were reported. Atomic Absorption Spectroscopy (AAS) 7



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test was carried out on a SCT-127 AAS (Thermo iCE3000) atomic absorption spectrometer (USA). 0.1g of PNZn-lignin was adding into a solution with 5 mL nitromurlatic acid and boiled for 30 min followed by diluting the mixture into a solution concentration of 10 mg/L. The solution was subsequently measured and the content of Zn element was obtained by comparing the resultant absorbance with the standard curve. Hydrogen spectra nuclear magnetic resonance (1H NMR) of lignin and PNZn-lignin (~15 mg sample dissolved in 0.5 ml 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 microscope (SEM) images were recorded on a S4800 (FEI, Japan) SEM at an accelerating voltage of 5 kV and the element content in the char residue was determined by energy dispersive X-ray analysis (EDAX). Thermogrametric analysis (TGA) tests were performed on a TA SDTQ600 (TA Instruments) thermogravimetric analyzer. About 8.0 mg sample was heated from room temperature to 700 °C at a heating rate of 20 °C/min under N2 or air 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= 1.54Å) at room temperature. The flammability and smoke production of samples with a size of 100×100×3.0 mm3 were 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 5 % and the data reported 8


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here were the means of triplicate experiments. RESULTS AND DISCUSSION Characterization of PNZn-lignin. The synthetic route to prepare modified lignin (PNZn-lignin) is shown in Scheme 1. IR spectra were employed to characterize the structure change of alkali lignin after chemical modification. For the IR spectrum of O-lignin (Fig. 1a), several absorption bands located at 3400-3500 cm-1 (υO-H, aromatic and aliphatic hydroxyl groups), 2970-2820 cm-1 (υC-H, in CH3, CH2 and CH groups), 1650 cm-1 (υC=C, characteristic peaks of aromatic rings), and 1200-1020 cm-1(υC-O, associated with primary alcohol groups) are clearly found.25,26 Besides absorption peaks observed in O-lignin, the in-plane-deformation vibration of N-H bonds centered at 1510 cm-1 and the stretching vibration of C-N bonds located at 1124 cm-1 are determined in the IR spectrum of A-lignin (see Fig. 1b), strongly indicating the successful grafting of PEI chains onto the lignin molecule.26 With regard to PNZn-lignin, as shown in Fig.1c, the band at 1610 cm-1 arises from the combination of stretching and bending vibration of P=O bonds, whereas the absorption peak at 1210 cm-1 is assigned to be the stretching vibration of P=O bonds. Moreover, the absorption bands at 1124 and 1050 cm-1 are respectively due to the stretching vibration of C-N bond in P-C-N segments and P-O bond.27,28 Moreover, the absorption peaks at 637 and 500 cm-1 (Zn←O) and at 425 cm-1 (Zn←N) appear in the IR spectrum of PNZn-lignin, obviously suggesting that the zinc ions are successfully introduced into the PNZn-lignin structure via the coordination reaction.29,30 The IR results indicate that alkali lignin has been successfully functionalized by phosphorus, 9



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nitrogen and zinc ions. 1

H NMR is used to further characterize the structure change of lignin before and

after modification. 1H NMR spectrum of O-lignin (Figure 2a) clearly exhibits the chemical shifts at 9.2 ppm (H-e), 7.6-6.3 ppm (H-c, H-c′, H-f), 5.2 ppm (H-a) and 3.7-3.4 ppm (H-b, H-d), agreeing well with the previous reports.31-33 As for PNZn-lignin, the signals of aromatic protons move up to high chemical shifts at around 8.1-7.8 ppm (H-c, H-c′) (see Figure 2b) because the H-f was substituted by carbon atoms via the Mannich addition reaction with formaldehyde and DEP. In addition, multiple signals are clearly observed between 3.4 and 2.4 ppm (3.4 ppm, 2.9 ppm, 2.7 ppm, 2.6 ppm, 2.4 ppm), which are attributed to methyl/methylene protons lined to N and O atoms (H-b, H-d, H-f, H-g, H-h and H-j marked in the structure of PNZn-lignin of Fig. 2b). Specifically, because of the presence of H-h, H-g, and H-f, the relative intensity of chemical shift at around 3.4 ppm becomes much stronger as compared with that of d-DMSO. Besides,

13

C solid-state NMR is also employed to

confirm IR and 1H NMR analysis. Similar to previous reports,26,34-36 it is clearly observed in NMR of O-lignin that the aromatic ring carbons signal peaks appear at 152-142 ppm (C-1,C-2), 137-124 ppm (C-4, C-5), 115ppm (C-6) and 105 ppm (C-3) (Figure 3a). In addition, the chemical shifts of hydroxyethyl carbons (C-7) and methoxyl carbons (C-8) are also detected at 74 ppm and 56 ppm, respectively.34-36 With respect to PNZn-lignin, the strong signals between 23 (multiple peaks) and 33 ppm are clearly detected, and they are attributed to the aliphatic carbons (C-12, C-13, C-14, C-9). Simultaneously, these strong signals leads to the significant reduction in 10


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the relative intensity of aromatic carbon (C-1~C-6). In addition, the hydroxyethyl carbons signals (C-7) at 75 ppm increases remarkably because of the overlapping effect of and C-11 signal from DEP. Thus, both 1H NMR and 13C SNMR results show that lignin were chemically modified by PEI and DEP. XPS measurement can provide more detailed evidence for identifying the elemental composition and type of chemical bonds of PNZn-lignin. As shown in Figure 4A, in O-lignin 78 wt% of C and 22 wt% of O are determined by XPS. With regard to A-lignin, it shows 8 wt % of N element in addition to 73 wt% of C and 9 wt% of O, very close to the element analysis (EA) results (75 wt% of C, 13 wt% of O, 6.1 wt% of H and 5.9 wt% of N), suggesting the introduction of PEI chains into the lignin structure. As compared with A-lignin, besides C, O and N, 8.0 wt % of P and 6.0 wt % of Zn elements are detected, and 74 wt% of C, 15 wt% of O, 6.4 wt% of H and 3.6 wt% of N are found by EA in PNZn-lignin. Meanwhile, the AAS test shows that PNZn-lignin contains about 7.0 wt% of Zn element. XPS and EA results further verify that the alkali lignin was chemically modified with P, N and Zn elements. To ascertain the relative ratio of Zn←N and Zn←O coordination bonds, the XPS spectrum of Zn2p is split into two peaks by origin software. As presented in Figure 4B, two peaks centered at 1021.7 eV and 1023.1 eV can be ascribed to Zn-O and Zn-N bonds, respectively, and a ratio of 39/61 is obtained by comparing their individual area. Thermal Stability. TGA tests allow us to determine the thermal stability and char-forming capability 11



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of modified lignin. In nitrogen atmosphere pristine lignin (O-lignin) starts to degrade from 248 oC (Ti, the initial degradation temperature, where 5wt% mass loss takes place) and decomposes most rapidly at about 340 oC (Tmax), leaving a high char residue of 59 wt% at 620 oC (see Figure 5A).26 In comparison, PNZn-lignin exhibits a similar Ti of 249 oC to O-lignin but a much higher Tmax of 358 oC (an increase 18 oC). Meanwhile, the char residue shows a slight decrease, around 56wt%, indicating that PNZn-lignin display a marginally enhanced thermal stability that the pristine lignin. In comparison, the TGA in air atmosphere can help us evaluate the thermal oxidation stability and char-forming capability a material. As given in Figure 5B, for O-lignin besides the same Ti to that in nitrogen, it exhibits two maximum mass loss peaks (Tmax) occurring at 331 oC and 501 oC, respectively, with a char residue of 25 wt% at 620 oC. As for PNZn-lignin, in spite of the same Ti of 248 oC, the Tmax is shifted up to 349 oC and a much higher char residue of 55 wt% is left, more than double of that of O-lignin. Such high char fully indicates that PNZn-lignin holds a strong char-forming capability, which is able to contribute to protecting the polymer phase when their composites are exposed to the fire. Figure 6 shows thermal degradation behaviors of PBS and its composites in the nitrogen condition. The PBS matrix starts to degrade at 354 °C (Ti) and shows a Tmax of 410 °C, similar to the previous results.10 Incorporating 2.5 wt% of O-lignin increases Ti by 11 oC (365 oC) but hardly changes Tmax of PBS. As indicated in Figure 5A, O-lignin shows a much lower Ti of only 248 oC while Ti of PBS takes place at 354 o

C, and thus theoretically the presence of O-lignin should make PBS become 12


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thermally unstable, namely resulting in a lower Ti. The significant increase in Ti is most likely due to the protection action of the char residue (2.4 wt%) created by O-lignin at relatively low temperature. This can be explained by the fact that most of O-lignin (Tmax: 340 °C) has decomposed and formed a protective char before PBS starts to degrade from 354 °C. Likewise, adding an equal loading level of PNZn-lignin leads to a similar thermal degradation behavior to O-lignin, but a marginally higher Ti (366 oC) and char residue. However, the Ti of the PBS composites decreases whereas Tmax hardly changed with further increase in the loading level of PNZn-lignin. For instance, 10 wt% of PNZn-lignin makes Ti of PBS reduce by 51 oC (down to 303 oC) because of the much lower thermal stability of modified lignin than PBS, as also evidenced by a small thermal decomposition peak at around 300 oC. Similar phenomenon was also observed by Hu’s work, where the lower thermal stability of APP or MAPP reduced the Ti of PBS.9-11 Nevertheless, such loading level of PNZn-lignin generates a high char residue of 10 wt% as compared with only 0.3 wt% for the pure PBS at 500 oC. The char residue has a positive thermal protection effect on the polymer in the composite, thus retaining the Tmax of PBS, which in turn reflects the huge influence in protecting the polymer upon exposed to fire. Fire Behavior. Cone calorimetry is widely used to evaluate the fire hazards of materials because it is able to simulating the real combustion behavior of a material in fire. As shown in Figure 7A&C and Table 1, the PBS matrix is extremely flammable, showing a time to ignition (tign) of 70 s, a peak heat release rate (PHRR) of around 13



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500 kW/m2 very close to that reported in the literature and a THR of 18.8 kJ/g.13 As shown in Figure 7C, after ignition the mass of PBS rapidly reduces because of combustion and leaves some char residue. Moreover, the combustion is companied by the generation of droplets floating out along the tank during the combustion, with the picture inserted in Figure 7A. Although the droplets can take away some heat from the combustion zone, they are very harmful since it can ignite other goods around them and trigger a huge fire. Compared with PBS, all PBS composites display shorter time to ignition regardless of the kind of lignin to different degree. However, the presence of lignin or PNZn-lignin effectively restricts the generation of droplets or dripping, which is very conductive to reducing the flammability of PBS (see Table 1). In addition to a slightly shorter tign, adding 2.5 wt% of O-lignin slightly decreases the PHRR of PBS to 420 kW/m2, but THR is increased from 18.8 to 21.6 kJ/g, similar to the trend of effective heat of combustion (see Figure 7D), which is most likely due to the fact that the presence of O-lignin effectively restricts the dripping of PBS during burning, thus increasing the relatively effective heat of combustion of PBS. Meanwhile, adding O-lignin seems to hardly affect the mass loss of PBS during combustion and the only difference lies in the increase of char residue relative to the PBS matrix (see Figure 7C). By contrast, incorporating an equal loading level of PNZn-lignin also leads to a shorter tign, slightly reduction in the PHRR, about 447 kW/m2 as compared with 500 kW/m2 for pure PBS. More importantly, the THR is remarkably decreased by 33% (about 7.2 kJ/g), suggesting that adding a very low loading level of PNZn-lignin can 14


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effectively reduce the heat release of PBS during a fire. Interestingly, Figure 7C shows that the presence of PNZn-lignin makes the mass of PBS composites lose much quickly than O-lignin, which is probably because Zn ions can catalyze the degradation of lignin and PBS to some extent. With increasing loading level of PNZn-lignin, both PHRR and THR steadily decrease considerably (see Figure 7B). For instance, PBS/10-PNZn-lignin exhibits very low PHRR and THR, about 244 kW/m2 and 6.1 kJ/g, respectively, reduced by 51% and 68% relative to that of the PBS bulk (see Figure 7A &B, Table 1). In addition, the effective heat of combustion shows similar trend to PHRR (see Figure 7D), and the mass loss curve also shows that PBS/10-PNZn-lignin burns very slowly and the majority of mass is left as char residue (see Figure 7C). As compared with Ferry’s work,13 they shown that adding 20 wt% of alkali lignin reduces the PHRR and THR of PBS by 48% and 14%, respectively, and the THR hardly reduced even though adding 2.0 wt% of phosphorus-containing copolymers decreased the PHRR of PBS/lignin by 52%. Meanwhile, the addition of 20 wt% of APP/CFA mixture made the PHRR reduce by 47 %, but decreased THR only by 4 %.11 Evidently, for their systems with a loading level of additives double of our system (10 wt% PNZn-lignin), the magnitude of reduction in PHRR and THR are still lower than our value. Therefore, PNZn-lignin can be a highly effective flame retardant additive for PBS and other polymers in terms of reducing the heat release and effective heat of combustion. Similar to the change trend of PHRR, the average mass loss rate (AMLR) of PBS displays a monotonous reduction with increasing PNZn-lignin content. However, 15



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adding pristine lignin leads to a much higher AMLR of 0.075 g/s as compared with 0.060 g/s for the neat PBS (see Table 1). When the loading level of PNZn-lignin is 10 wt%, the AMLR is strikingly decreased by 53 % (only 0.028 g/s), indicating that PNZn-lignin can effectively slow down the combustion of the PBS composite. Moreover, with the addition and increase of lignin and PNZn-lignin, the char residue increases growingly after cone tests. PBS/2.5-PNZn-lignin shows a residue of 17.5 wt%, relative to 15.3 wt% for the PBS/2.5-O-lignin and 9.3 wt% for the pure PBS. The same loading level of PNZn-lignin leads to higher char residue than O-lignin because of the higher char-forming capability in N2 condition as evidenced by TGA results (see Fig. 5A) since the degradation of PNZn-lignin in the condensed phase is anaerobic and all the oxygen is basically consumed in the flame. Unexpectedly, upon adding 10 wt% of PNZn-lignin, the char residue is enhanced up to 54.6 wt%, far higher than that of 9.3 wt% of pure PBS (see Figure 7C, Table 1). If taking 55 wt% (determined by TGA) as the char residue of PNZn-lignin after the cone test, we can calculate the theoretical char residue, about 22.9 wt% (=0.1×55 wt%+0.9×19.3 wt%), which is still much far smaller than 54.6 wt%. Such high residue created after the cone test is probably because of the thermal protection action of the char created at earlier stage during combustion. In other words, the char layer can isolate the heat flow, oxygen and small combustible molecules, and prevent the heat from transferring from the upper flammable zone to the underlying polymer. This in turn, results in the formation of the more char residue. Additionally, the high char residue is also the most key reason for the largely reduced PHRR, THR and AMLR. 16


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Smoke Suppression. Besides heat release rate and mass loss, the smoke produced during the combustion of a material is more critical because the smoke generated during the fire can dramatically reduce visibility of the fire hazard site and thus make it more difficult for the trapped people to escape if they inhaled the toxic smoke gases.23 Average specific extinction area (ASEA) and total smoke production rate (TSR) are two key parameters to evaluate how much smoke produced during the fire. Generally, smoke mainly derives from small unstable carbon particles and cyclic compound formed during combustion. As shown in Figure 7E and Table 1, interestingly, addition 2.5 wt% of O-lignin or PNZn-lignin leads to higher ASEA and TSR as compared with the pure PBS. The difference between O-lignin and PNZn-lignin is that the latter causes less smoke than the former at the same loading level because the presence of P, N and Zn in the latter structure. As for the PBS matrix, it shows an ASEA of 88 m2/kg and a TSR of 275 m2/m2, relative to 184 m2/kg and 538 m2/m2 for PBS/2.5-PNZn-lignin. However, with further increasing loading level of PNZn-lignin, both parameters reduce extraordinarily, strongly indicating that PNZn-lignin can effectively enhance smoke suppression performance of PBS. For instance, 10 wt% of PNZn-lignin makes ASEA and TSR reduce noticeably by 54% and 55%, respectively, as compared with that of pure PBS. Such reduction in smoke production is partially because the presence of Zn ions in PNZn-lignin can reduce the formation of cyclic aromatic compounds via its capability of bonding unstable carbon particles to residue, therefore preventing these solid particles from escaping to the flame zone and the air.37,38 It is reported that Zn-containing compounds, such as ZnO 17



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and ZnB4O7, can effectively reduce the smoke release of a material during fire.37,38 In fact, both ZnO and Zn2P2O7 are detected in the char residue, as discussed in the following section. Char Residue Analysis. Investigating the char residue helps us understand the action mechanism of PNZn-lignin in PBS. Digital photographs offer the macroscopic visual observations on the char of composites created after combustion. Obviously, the char residue from PBS (Figure 8A) shows a very thin morphology with some ruptured blocks, and this discontinuous layer leads to a high heat release, whereas that of PBS/2.5-O-lignin or 2.5-PNZn-lignin (Figure 8B&C) displays much thicker char layer despite some cracks. Compared with O-lignin, the equal loading level of PNZn-lignin results in a more compact char layer with less cracks due to the higher char residue. As expected, incorporating 10 wt% of PNZn-lignin leads to a whole, intact and compact char layer (see Figure 8D), as evidenced by the high char residue (54.6 wt%). Therefore, it is reasonable to conclude that it is the thick, compact char layer that is primarily responsible for largely reduced heat release and slow mass loss during combustion. In comparison, SEM images allow us observe carefully the morphology and structure of the char residue on a microscopic scale. As shown in Figure 9A1&A2, the char residue of PBS displays a loose surface and the char is not continuous, and such char is harmful for isolating heat and combustible gas, and thus total heat release (THR). In addition, 81.7 wt% of carbon and 18.3 wt% of oxygen are determined by EDAX. As for PBS/2.5-O-lignin, shown in Figure 9B1&B2, incorporation of pristine lignin fails to create an intact char layer which seems to be 18


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looser and more fragile relative to that of PBS. The char is found to contain slightly higher carbon of 85.5 wt% and less oxygen with respect to that of PBS. By contrast, unlike the loose structure of PBS/2.5-O-lignin, addition of 2.5wt% PNZn-lignin into PBS changes apparently the structure of char layer, that is, the char becomes much firmer and more intact, despite some holes and cracks. Meanwhile, at a higher magnification there are some large blocks observed, which can help enhance the strength of char structure (Figure 9C2). Once the content of PNZn-lignin increases up to 10 wt%, a much more continuous and compact char layer and a whole char block (Figure 9D1&D2) is apparently observed. Such compact and intact char is able to not only insulate the combustion heat between the upper char layer and the underlying material, thus protecting the underlying materials; but also can effectively prevent the combustible gases productions due to the thermal decomposition of the underlying polymer from diffusing to the flame zone. As a result, all fire parameters including HRR, THR and AMLR are dramatically decreased, which means that the fire hazard of PBS is remarkably reduced. Meanwhile, 1.1 wt% P and 1.5 wt% Zn are detected in the char residue, besides C and O elements. The N element is not found mainly because the PEI chain segments tend to degrade and release as NH3 gas upon heated at high temperature. The chemical structure of the char residue of PBS and its composites after cone tests is analyzed by IR, which can help us to further understand the action mechanism of lignin or PNZn-lignin in PBS. It is readily to observe that char residues of PBS and PBS/2.5-O-lignin exhibit basically same IR spectra (see Figure 10a&b), while that of 19



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PBS/2.5-PNZn-lignin shows a much different spectrum (Figure 10c). For the char of PBS and PBS/2.5-O-lignin, three strong absorption bands, respectively, located at 3710~3000 (centered at 3445 cm–1) belonging to the stretching vibration of hydroxyl groups (υO-H), at 1640 cm-1 (υC=C), indicating the formation of unsaturated carbon structures or polyaromatic carbonaceous species, and at 1100 cm-1 (υC-O). In addition, a weak absorption band between 2950 and 2850 cm-1 is also found due to the stretching of CH2 groups.26,27,39 These absorption peaks demonstrate that the presence of pristine lignin basically has no effect on the char structure which is mainly composed of both graphitic and amorphous carbon skeleton containing around 15 wt% of oxygen (see Figure 9A1 and B1). By contrast, with regard to the char of PBS/2.5-PNZn-lignin, besides absorption peaks observed above, absorption bands in at 1725 (υC=O) cm-1, 1157(υP=O) cm-1, 1050 (υP-O-C) cm-1 and 750 (υP-C) cm-1 are also determined.27,40 Moreover, double peaks appearing at 972 and 995 cm-1 indicates the presence of P2O74- groups in the char, which means that the phosphorus element mainly transforms into phosphoric acid and pyrophosphoric acid (P2O74-) species. As well-known, these phosphoric acid species have strong dehydration effect on oxygen-containing compounds, including PBS and lignin, in our system. Besides, the peak at 505 cm-1 belongs to the vibration peaks of Zn-O. To reveal the bond type of Zn (II), XRD analysis is adopted, which can also provide useful information for understand the effect on the char-forming of PBS and lignin. As shown in Figure 11, characteristic peaks of Zn2P2O7 (14 o, 17 o, 22 o, 25 o, 29 o, 31 o, 42 o

) and ZnO (31 o, 35 o, 45 o, 48 o, 61 o and 68o) are clearly found. This means that the 20


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majority of Zn (II) ions chemically bonds with P2O74- and the rest with O, which can form sticky but stable materials and strengthen carbonaceous layer, thus reducing the heat release dramatically. Meanwhile, the generation of zinc compounds (ZnO and Zn2P2O7) is primarily responsible for markedly reduced smoke production observed above because of their strong smoke suppression effect on polymers.37,38 Besides the diffraction peaks of Zn compounds, a broad diffraction peak at around 26o and peaks at 44 o and 58 o are also found, which are respectively related to the amorphous and graphitic carbon in the char, as evidenced by IR results (C=C bonds). To verify the presence of graphitic structure and determine the hybridization state of carbon atoms in the residue char, Raman spectroscopy is carried out, as shown in Figure 12. Apparently, the char residue of PBS only displays a visible weak band at 1587 cm-1 (G-band), which seems to indicate the presence of only graphitic structure in the char (Figure 12a), as evidenced by IR results. Actually, the carbon framework consists of many defects of long-range disordered amorphous carbon because of the presence of oxygen-containing groups (EADX and IR results). As for the char from PBS/2.5-O-lignin composite (Figure 12b), both G-band and D-band are detected at 1593 and 1351 cm-1, respectively. Generally, G-band is related to the vibration of sp2-hybrided carbon atoms in graphite layers, whereas 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.41,42 This suggests that there are graphitic and disordered carbon structures coexisting in the char residue. Meanwhile, the relative 21



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intensity ratio (R), namely IG/ID 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. The R value of from PBS/2.5-O-lignin’s char is 1.12, indicating a relatively low graphitization degree. With regard to the char of PBS/2.5-PNZn-lignin, as shown in Figure 12c, both G-band and D-band appear but shift to high wave numbers centering at 1600 and 1353 cm-1, respectively, relative to the char of PBS/2.5-O-lignin. Such blue shift is probably due to the participation of P and Zn (II) ions in the formation chemical bonds with C or O atoms, thus activating the sp3-hybridized carbons. In addition, its R value is 1.23 larger than that of the char from PBS/O-lignin char residue, suggesting a relatively higher graphitization degree, agreeing well with XRD results (see Figure 11). In fact, no XRD peaks can be determined for the char of PBS/O-lignin. Based on above analysis of morphology, structure and chemical composition, it is basically clear that it is the combination of the strong dehydration action of phosphoric acid species, the excellent smoke suppression effect of Zn (II) ions and high char-formation capability of lignin that enables PBS/PNZn-lignin to generate a compact, thick and strong char layer, thus leading to largely reduced flammability and smoke release. CONCLUSIONS Modified lignin (PNZn-lignin) was fabricated by chemically functionalizing alkali lignin with phosphorous, nitrogen and Zn2+ ions. Incorporation of PNZn-lignin can effectively increase to some extent the thermal stability of PBS. Adding 10 wt% of 22


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PNZn-lignin, PHRR and THR of PBS are remarkably reduced by 51% and 68%, respectively. Meanwhile, the ASEA and TSR are decreased respectively by 54 % and 55% as compared with that of PBS. Impressively, the char residue is enhanced from 9.3 wt% for pure PBS to 54.6 wt% for PBS filled with 10 wt% of PNZn-lignin. It is the combination of the strong dehydration action of phosphoric acid species, the strong smoke suppression effect of Zn (II) ions and high char-formation capability of lignin that leads to a compact, thick and strong char layer and makes PBS show largely reduced flammability and smoke release. This strategy not only extends the high value-added utilization of industrial lignin wastes, but also provides a novel approach for creating biobased PBS materials with low flammability and smoke production in fire, making such modified PBS composites find potential application in construction, packaging and electric and electronic fields. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], * E-mail: [email protected], * E-mail: [email protected] ACKNOWLEDGMENTS Lina Liu as the co-first author and Ping’an Song 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 23



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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). Notes The authors declare no competing financial interest. REFERENCES (1) Liu, G. M.; Zheng, L. C.; Zhang, X. Q.; Li, C. C.; Jiang, S. C.; Wang, D. J. Reversible lamellar thickening induced by crystal transition in poly(butylene succinate). Macromolecules 2012, 45, 5487-5493. (2) Wu, D.F.; Yuan, L. J.; Laredo, E.; Zhang, M.; Zhou, W. D. Interfacial properties, viscoelasticity, and thermal behaviors of poly(butylene succinate)/polylactide blend. Ind. Eng. Chem. Res. 2012, 51, 2290-2298. (3) Fujimaki, T. Processability and properties of aliphatic polyesters, ‘BIONOLLE’, synthesized by polycondensation reaction. Polym. Degrad. Stab. 1998, 59, 209-214. (4) Thakur, V. K.; Thakur, M. K.; Raghavan, P.; Kessler, M. R. Progress in green polymer composites from lignin for multifunctional applications: A review. ACS Sustainable Chem. Eng. 2014, 2, 1072-1092. (5) Gallo, E.; Schartel, B.; Acierno, D.; Russo, P. Flame retardant biocomposites: synergism between phosphinate and nanometric metal oxides. Eur Polym J. 2011, 24


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30


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Captions of Figures Scheme 1 Schematic representation for the synthetic route to the functionalized lignin (PNZn-lignin). Fig. 1

Infrared spectra of a) O-lignin, b) A-lignin and c) PNZn-lignin in the wavenumber range of A) 4000-500 cm-1 and B) 750-400 cm-1.

Fig. 2

1

H NMR spectra of a) O-lignin and b) PNZn-lignin using d6-DMSO as the

solvent. Fig. 3

13

Fig. 4

A) XPS spectra of a) O-lignin, b) A-lignin and c) PNZn-lignin with their

C solid-state NMR spectra of a) O-lignin and b) PNZn-lignin.

elemental compositions inserted, and B) Zn2p spectrum in PNZn-lignin showing relative ratio of Zn2+←O and Zn2+←N coordination bonds. Fig. 5

TGA and DTG curves for O-lignin and M-lignin (A) under nitrogen condition and (B) under air condition. a Ti 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 oC.

Fig. 6

A) TGA and (B) DTG curves of the neat PBS and its composites based on O-lignin and PNZn-lignin under N2 condition at a heating rate of 20 °C /min.

Fig. 7

A) Heat release rate, B) total heat release, C) normalized mass loss, D) effective heat of combustion and E) total smoke production curves of PBS and its composites based on O-lignin and PNZn-lignin at a heat flux of 35 kW/m2

31



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Fig. 8

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Digital photos of composites for A) PBS, B) PBS/2.5-O-lignin, C) PBS/2.5-PNZn-lignin and D) PBS/10-PNZn-lignin after cone calorimeter measurements.

Fig. 9

SEM images of the residue char for A1, A2) for the PBS matrix, B1, B2) for PBS/2.5-O-lignin, C1, C2) for PBS/2.5-M-lignin composite and D1, D2) for PBS/10-PNZn-lignin composite after cone calorimeter tests.

Fig. 10

FT-IR spectra of the char residue for a) PBS, b) PBS/2.5-O-lignin and c) PBS/2.5-PNZn-lignin after cone tests.

Fig. 11

XRD diffraction pattern of the char residue for the PBS/2.5-PNZn-lignin after the cone test.

Fig. 12

Raman spectra of the char residue for a) PBS, b) PBS/2.5-O-lignin and c) PBS/2.5-PNZn-lignin after cone tests.

32


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Scheme 1

Figure 1

33



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Figure 2

Figure 3

34


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Figure 4

Figure 5

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Figure 6

Figure 7

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Figure 8

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Figure 9

Fig. 10

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Fig. 11

Fig. 12

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Table 1. Detailed combustion data of PBS, PBS/O-lignin and PBS/PNZn-lignin composites collected from cone calorimeter measurements. Residue a (wt%)

AMLR a (g/s)

ASEAa (m2/kg)

TSR a (m2/m2)

Y

9.3±0.2

0.060±0.006

89±3

274±7

21.6±1.6

N

15.1±0.3

0.075±0.005

255±6

884±11

447±11

14.4±0.8

N

17.8±0.1

0.053±0.003

184±4

538±13

69±1

349±8

9.8±0.6

N

21.4±0.3

0.043±0.002

73±2

121±7

68±1

244±7

6.1±0.5

N

54.6±0.8

0.028±0.002

41±1

125±5

Run

tign a (s)

PHRR a (kW/m2)

THR a (kJ/ g)

1

70±1

500±19

18.8±1.9

2

64±1

416±10

3

66±1

4 5

Droplets (Y/N)

Run 1-5 represents the PBS matrix, PBS/2.5-O-lignin, PBS/2.5-PNZn-lignin, PBS/5.0-PNZn-lignin, and PBS/10- PNZn-lignin, respectively. a

tign, PHRR, THR, Droplets, Residue, AMLR, ASEA and TSR refer to the time to ignition, peak heat release rate, total heat release, dripping from the sample, residue after cone tests, average mass loss rate, average specific extinction area and total heat release, respectively.

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Converting Industrial Alkali Lignin to Biobased Functional Additives for Improving Fire Behavior and Smoke Suppression of Polybutylene Succinate Lina Liu, Guobo Huang, Ping’an Song, Youming Yu, Shenyuan Fu

For Table of Contents Use Only

A green strategy has been developed to convert industrial lignin waste into high value-added biobased additive for creating biobased high-performance polymer materials. *E-mail: [email protected] (Prof. Dr. Y. Yu), [email protected] (Prof. Dr. S. Fu)

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Converting Industrial Alkali Lignin to Biobased Functional Additives for

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