Green and Scalable Fabrication of Core-Shell Bio-Based Flame

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Green and Scalable Fabrication of Core-Shell Bio-Based Flame Retardants for Reducing Flammability of Polylactic Acid Zhengquan Xiong, Yan Zhang, Xiaoyang Du, Pingan Song, and Zhengping Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01016 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Sustainable Chemistry & Engineering

Green and Scalable Fabrication of Core-Shell Bio-Based Flame Retardants for Reducing Flammability of Polylactic Acid Zhengquan Xiong,a Yan Zhang,a,* Xiaoyang Du,b Pingan Song,c* Zhengping Fanga

a Lab

of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, 1

Xuefu Road, Ningbo 315100, China b

Ningbo Geely Automobile Research & Development Co., Ltd, 918 Binhai 4th Road, Ningbo 315336,

China c School

of Engineering, Zhejiang A&F University, 666 Wushu Street, Hangzhou, 311300, China

Correspondence

should

be

addressed

at

[email protected]

(Y.

Zhang)

and

[email protected] (P. Song)

Abstract

The design of flame-retardant biocomposites based on bio-based flame retardants (FRs) represents a promising direction for creating sustainable world. To date, it remains a major challenge to explore a green and scalable strategy for the design highly effective, bio-based FRs for bioplastics, such as polylactic acid (PLA). Herein, we have demonstrated a green, facile fabrication approach for core-shell structured bio-based flame retardant (APP@CS@PA-Na) via layer-by-layer assembly using water as the assembly media. With electrostatic interactions, APP@CS@PA-Na was prepared by sequential assembly of ammonium polyphosphate (APP) with positively charged chitosan (CS) and then negatively charged phytic acid salt (PA-Na). The addition of APP@CS@PA-Na can enhance both the flame retardancy and the toughness of 1

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polylactic acid (PLA). With the addition of 10 wt% APP@CS@PA-Na, the resultant PLA composite can pass a UL-94 V-0 rating and meanwhile shows an increased elongation at break by 28.4 % compared with that of neat PLA (8.1 %). Through the analysis of the volatile gases and the residues, the flame retardant mechanism of APP@CS@PA-Na in PLA plays the key role in the condensed-phase. This work will broaden the practical application field of PLA, such as in electric and electronic and fibers fields. Keywords: Polylactic acid; Bio-based flame retardant; Core-Shell; Self-assembly.

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Introduction Polylactic acid (PLA), a bio-degradable thermoplastic, has aroused enormous interest as a promising substitute for petroleum-derived polymeric materials because of its abundant resources including starches and sugars, and excellent mechanical properties.1-3 However, similar to most organic polymers, PLA is highly flammable extremely limiting the practical applications in industry where demanding fire retardancy is required.4 Therefore, it is necessary to develop flame retardant PLA materials without introducing significant non-biodegradable components. It represents a promising strategy to develop bio-based flame retardant additives for PLA taking account of renewability of nature bioresources and added environment burdens due to the use of petroleum-based compounds.5 Intumescent flame retardant (IFR) based on ammonium polyphosphate (APP) has become a widely used flame retardant additive for PLA in recent years owing to its advantages of low smoke, halogen-free, low toxicity and high efficiency.6-8 The typical IFR consists of APP, melamine and pentaerythritol. In order to construct a green IFR system based on APP, many efforts have been made to take full advantage of biomass resources because of their renewability, abundance and biodegradability. For example,

biomass

resources

such

as

starch9,

lignin10-13,

bamboo

fiber14,

deoxyribonucleic acid15, cyclodextrin16,17, cardanol18, castor oil19, phytic acid20, chitosan21 and cellulose22 have been reported to be employed as one component of IFR. The preparation of bio-based flame retardant systems relies on two main approaches, namely chemical bonding and physical blending. One strategy is to combine biomass compounds and existing phosphrous-nitrogen (P-N) containing flame retardants by

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chemical reaction, which, however, usually involves the use of a large amounts of organic solvents in the reaction or post-processes. The other one is to physically mix biomass compounds and P-N-containing flame retardants of. Although less organic solvents are used relative to the first approach, this strategy may give rise to the inhomogeneity of these components, thus negatively affecting the flame retardancy.

Since Iler used layer-by-layer (LbL) assembly to prepare multilayers of colloidal particles in 1966,23 the LbL assembly has been extensively used for the preparation of microspheres and surface modifications of materials. One of its advantages is that it can be carried out in the organic solvent free system. Zhang et. al.24 has recently constructed multilayer coatings containing APP and polyethylenimine onto ramie fabrics by using the LbL assembly. During vertical flame tests, the ramie fabrics treated with flame retardant coating demonstrates a self-extinguishing behavior, showing a significantly enhanced flame retardancy as compared with the pristine sample. Alongi et. al.25 has deposited hybrid organic-inorganic coatings consisting of APP, chitosan and silica on polyester-cotton blends. The flammability and combustion behavior of polyester-cotton fabric blends are found to improve significantly. Therefore, the LbL assembly has been successful to create flame retardant fabrics by constructing a flame-retardant multilayered film on the surface via the alternate deposition of oppositely charged polyelectrolytes. It should be noted that the surface pretreatment is often required to enhance the interfacial bonding between the flame retardant polyelectrolytes and the

4


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material. In addition, the flame-retardant coatings may also suffer a poor durability issue, such as chapping and peeling in use.

Previously, we had successfully fabricated a core-shell flame retardant with APP particles via LbL assembly by using plant-derived diphenolic acid (DPA) as raw material.26 As-prepared core-shell flame retardant is constructed in water and can be readily incorporated into PLA. The incorporation of 10 wt% of this core-shell flame retardant can endow PLA with good flame retardancy, passing UL-94 V-0 rating. Unfortunately, the synthesis of DPA-based polyelectrolyte involves the use of the organic solvent, acetonitrile, in addition to a complicated post-treatment.

This work aims to explore a greener approach for creating core-shell bio-based flame retardants by using water as the solvent for LbL. The positively charged chitosan (CS) and negatively charged phytic acid (PA) are selected for the preparation of ‘green’ flame retardant, both of which derive from biomass. A new bio-based core-shell flame retardant was fabricated through depositing consecutively CS and PA on APP, which was constructed by LbL assembly method in water. It is noticed that the acidic property of PA may accelerate the decomposition of PLA. The pH value of aqueous solutions of PA were neutralized by sodium hydroxide to 6. In this core-shell system (APP@CS@PA-Na), APP was the core with CS and neutralized PA (PA-Na) as the shells. The thermal stability and flame retardancy of APP@CS@PA-Na in PLA were investigated and the mechanism was also discussed from the gas and condensed phases.

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This work demonstrates a universal green strategy for creating core-shell flame retardants with high efficiency for PLA.

Material and Methods Materials Polylactic acid (PLA 4032D) with an molecular weight (MW) of 206 kDa was obtained from Nature Works (USA). Ammonium polyphosphate (APP, P% = 31.0~32.0 wt%, degree of polymerization (DP) > 1000) was provided by Hangzhou JLS Flame Retardants Chemical Co., Ltd (China). Chitosan (CS, deacetylation degree ≥ 95 %, the viscosity of CS is 100-200 mpa.s) and phytic acid (PA, 70 wt % solution in H2O) were purchased from Aladdin-reagent (China). Sodium hydroxide (NaOH) and acetic acid were received from Aladdin-reagent (China). Deionized water was used for all the experiments. Fabrication of APP@CS@PA-Na APP@CS@PA-Na was fabricated according to our previous study26 via LbL assembly method and the scheme is presented in Fig.1 (A). In a typical procedure, 2.0 g APP powder was dispersed evenly in deionized water by mechanical stirring for the purpose of forming a suspension (5 wt%). Then, 20 mL of the CS aqueous solution (1 wt%) dissolved in 1 wt% acetic acid was added into the APP suspension, and the adsorption process was allowed to proceed for 2 minutes with constant stirring of the 6


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mixture. Then, the particles were isolated by centrifugation. In a washing step, 60 mL of deionized water was added. The particles above underwent two centrifuge/wash cycles before the addition of the next layer. Before the absorption of the next layer, the pH of the PA aqueous solution were adjusted to 6 with NaOH. Then, 20 mL of the PA-Na aqueous solution (1 wt%) was added, followed by the same washing protocol. The particles obtained by centrifugation were dried in a vacuum oven for 12 hours at 80 oC. Then the massive product was reduced to powder and dried to constant weight in a vacuum oven for 6 hours at 80

oC.

1.4 g of the core-shell flame retardant

(APP@CS@PA-Na) was obtained. Preparation of flame retardant PLA composites Flame retardants were added into PLA by melt blending method via a ThermoHaake Rheomixer (Polylab, Germany). The mixing process was carried out at 170 oC for 8 minutes with a rotor speed of 60 rpm. Prior to mixing, PLA and flame retardants were dried under vacuum at 80 °C for 12 h to remove the residual water. For comparison, the specimens of neat PLA were prepared by the same procedure. Measurements Scanning electron microscopy (SEM) was performed to investigate the morphologies of the core-shell flame retardants, via a Hitachi S-3400N Field-emission Scanning Electron Microscopy operated at 3 kV. Prior to analysis, the specimens were gold-sputtered for 1.5 min under a high vacuum to increase the conductivity. X-Ray 7



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photoelectron spectroscopy (XPS) was carried out with a Thermo ESCALAB 250 Spectrometer. Thermal gravimetric analysis (TGA) experiments were conducted at a heating rate of 20 oC/min in N2 from 50 to 800 oC on a TGA 209 F1 instrument (NETZSCH, Germany). Limiting oxygen index (LOI) values were measured using an HC-2 Oxygen Index Instrument (Jiangning Analysis Instrument Company, China) with the rectangular sheets of 130 × 6 ×3 mm3, according to ASTM D2863 standard. The vertical burning tests were performed on a CZF-Ш Vertical Burning Tester (Jiangning Analysis Instrument Company, China) with the dimension of the sample sheets of 130 × 13 × 3 mm3, according to ASTM D3801 standard. Cone calorimetry was carried out using a cone calorimeter (FTT) at the heat flux of 35 kW/m2 with the dimension of the sample sheets of 100 × 100 × 3 mm3, according to ISO 5660-1 standard. Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy (TG-IR) measurements were performed from 30-800 oC in N2 with a TGA 209 F1 instrument (Netzsch, Germany), combined with a Thermo Nicolet iS10 FTIR spectroscopy (Thermosher, Germany). The TG instrument is linked to the heated gas cell of the FTIR instrument with a heated line. And the volatiles evolved from TG instrument can be transferred into the gas cell of FTIR through the transfer line by a proper gas flow. The spectra were obtained with a scan interval of 2.23 s, and the resolution of the spectra was 4 cm-1. SEM (ZEISS EVO18 field-emission scanning electron miscroscopy, ZEISS, Co.,

8


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Germany) with EDS (Oxford X-Max 20 EDS, Oxford Instrument Co., UK) was performed to characterize the char surface photograph as well as the components of char of the flame-retardant PLA composites after the cone calorimetric tests. Raman spectroscopy (Renishaw inVia, UK, excitation-beam wavelength: 514 nm) was measured to investigate the vibrational properties of the char after the cone calorimetric tests. Tensile tests were carried out using an electronic universal mechanical testing machine (Model SANS-ZBC1400-2, Shenzhen New Sans Material Testing Co. Ltd., China). Dumbbell specimens with a 25 × 4 × 2 mm3 neck were used. Tests were measured at 25 oC at a constant crosshead speed of 2 mm/min. Notched izod impact tests were carried out using the standard sized rectangular bars on an impact tester (SANS-ZBC1400-2, China), according to ISO180/179. At least eight samples were tested to obtain the average values of mechanical properties.

Results and discussion Characterization of core-shell flame retardants Surface morphologies of the pristine APP, APP@CS and APP@CS@PA-Na are investigated by SEM, as shown in Fig.1 (B), (C) and (D). The surface of APP particles was quite smooth with irregular shapes and sizes. In comparison, obvious changes after coating with CS and PA-Na through electrostatic absorption were observed in Fig.1 (C) and (D). Both APP@CS and APP@CS@PA-Na displayed relatively rough surfaces.

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This indicates the presence of organic layer coatings on APP particles.

XPS is used to characterize the composition of elements on the surface of particles. As shown in Fig.2, for the XPS spectra of the pristine APP (C:12.19%, O: 57.19%, N: 12.84%, and P: 17.78% in atomic ratio), the peaks centered at 134.7 and 190.9 eV are attributed to P2p and P2s.27 The peak intensities of P2p and P2s in APP@CS (C:20.99%, O: 51.11%, N: 12.84%, and P: 14.81% in atomic ratio), and APP@CS@PA-Na (C: 20.29%, O: 49.48%, N:12.65%, P:15.99% and Na:1.59%) are slight lower than that of the origin APP. In contrast, the relative content of element C located at 284.7 eV of them increases to 20.99 % and 20.29 %, respectively. These changes of elemental compositions are attributed to the organic layer on APP particles, since the C contents in CS and PA are much higher than that in APP. Moreover, APP@CS@PA-Na shows the Na1s peak with a mass fraction of 1.59 %, indicating the presence of the PA-Na layer on APP particles. Thermal analysis Fig. 3 (A) and (B) presents the TG and DTG curves of neat APP and its two derivatives with detailed parameters listed in Table 1. It can be observed that the neat APP starts to degrade (Tonset , the temperature at 5 % mass loss) from 337 oC and to decompose most rapidly (Tmax, the temperature at maximum weight loss rate) at 638 oC. As for APP@CS and APP@CS@PA-Na, both Tonset and Tmax decrease gradually because both shells of CS and PA-Na are less thermally stable than the APP core.

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Despite that, the Tonset for APP@CS (307 oC) and APP@CS@PA-Na (281 oC) are still far higher than the common melt blending temperature of PLA (170 oC). This means they are fully meet the melt processing of PLA. At the same time, APP@CS@PA-Na had the lowest maximum weight loss rate, indicating its higher thermal stability. This may be attributed to the phosphate groups of PA-Na to improve the char formation in the core-shell system, which can help slow down the decomposition of the flame retardant. The thermal analysis of PLA with 10 wt% different flame retardants is shown in Fig. 3 (C), (D) and Table 1. The neat PLA displays a Tonset of 346 oC and the Tonset for PLA composites decrease slightly after the addition of different flame retardants. In spite of that, the Tmax for all flame retardant PLA composites shows no significant changes compared to that of neat PLA, which means the main degradation process of PLA is not affected by the presence of flame retardants. In contrast, the maximum weight loss rates at Tmax for PLA/10%APP@CS@PA-Na decrease obviously and this may indicate the earlier char formation upon heated. Such rapider weight loss is consistent to that of APP@CS@PA-Na (see Table 1). Table 1 shows that the residual weight at 600 oC increases with the increase of the shell layer. This is because that both CS and PA can serve as the carbon source for this IFR system. Moreover, it is interesting to find that the experimental-residual weights at 600 oC are higher than the corresponding calculated values for all flame retardant PLA composites. This enhanced char-forming ability of APP@CS@PA-Na is expected to offer a better char layer and

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better flame retardancy.

Fire behavior Limiting oxygen index (LOI) test and vertical burning rate (UL-94) are performed to investigate the fire behaviour of PLA composites, with detailed data summarized in Table 2. The neat PLA gives a LOI value of only 20.0 % and shows severe dripping with no rating during UL-94 tests, which indicates its inherent inflammability. After the addition of the flame retardants, the LOI values and UL-94 ratings of PLA composites are improved to different extents with the different shell structures of core-shell flame retardants. When only CS is used as the shell, the LOI value increased to 29.0 %, which is higher than that of PLA/10%APP (27.2%). The reason is that CS is a good carbon source with the release of NH3 during the combustion, which can cooperate with APP to form a protective char layer and thus endow PLA with better flame retardancy. However, the UL-94 rating of PLA/10%APP@CS is still V-2 the same as that of PLA/10%APP. When both CS and PA-Na are as the shells, the flame retardancy of PLA is further improved. PLA/10%APP@CS@PA-Na shows the highest LOI value of 30.5 % and achieves a V-0 rating. In particular, it can self-extinguish in less than 5 s after removing the igniter without igniting the underlying cotton. Compared with the structure of CS, the phosphate groups in PA-Na may not only promote the carbonization of the flame retardant but also the PLA matrix. Cheng et. al.28 have reported that PA was an efficient char forming agent which has the capacity to improve the flame

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retardancy of PLA fabric through a condensed-phase mechanism. Therefore, the protective char layer formed during the combustion of PLA/10%APP@CS@PA-Na can isolate the PLA from the flame and prevent the heat transfer, thus leading to the better flame retardancy. To understand whether the neutralization of PA were contribute to the improved flame retardancy, both PLA/10%APP@CS@PA and PLA/10%APP@CS@PA-Na are compared in terms of LOI and UL-94 ratings. As shown in Table 2, the flame retardancy

of

PLA/10%APP@CS@PA-Na

is

superior

to

that

of

PLA/10%APP@CS@PA in both LOI and UL-94 tests. As is known, PA and its salts can interact with positively charged molecules,29 since the shell CS is a positively charged polysaccharide with amino groups. However, the hydroxyl groups in PA in aqueous solution tend to interact with each other via hydrogen bonding, whereas the hydroxyl groups in PA-Na (pH = 6) decrease because some of them are neutralized. Hence, it is much easier for the PA-Na to deposit on the CS shell of the APP@CS than PA, resulting in higher content of phosphates on the surface of APP@CS@PA-Na than APP@CS@PA and thus better flame retardancy efficiency. Cone calorimetry is used to investigate the fire behavior of polymeric materials in condensed phase and can provide important fire performance parameters, such as time to ignite (TTI), peak of heat release rate (pHRR), total heat release (THR), and residual mass. As presented in Table 3 and Fig.4, the neat PLA burns violently after ignition, displaying a pHRR of 376 kW/m2 and a THR of 67 MJ/m2. With the increasing shells of

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core-shell flame retardants, the gradual decreases in both pHRR and THR values are observed for flame retardant PLA composites. For instance, the addition of 10% APP@CS@PA-Na reduces the pHRR and THR to 252 kW/m2 and 54 MJ/m2, respectively, a decrease by 33 % and by 19 % compared with those of the pure PLA. Additionally, the residual mass increases to 14.0 wt%, around 8 times of that of the pure PLA (1.5 wt%). The significant decrease in the THR and the residue suggest that the PLA participates in the carbonization process. The data of total smoke release (TSR) obtained from cone calorimeter tests are used to evaluate the smoke suppression effects of as-developed flame retardants on PLA. the TSR value of pure PLA is nearly zero, since the complete combustion of PLA led to the release of CO2 and H2O without any smoke (see Fig.S1). After the addition of APP, the TSR values increase obviously. This is because APP can promote the carbonization of PLA, leading to the incomplete combustion of PLA and thus yielding more smoke. However, the inclusion of APP@CS and APP@CS@PA-Na in PLA lead to the decrease in TSR. Especially for PLA/10% APP@CS@PA-Na, the TSR value is the minimum as compared with other systems. The char layer becomes more compact and continuous with the combination of APP, CS and PA-Na, thereby protecting the PLA matrix from the attack of the fire and suppressing the release of smoke. This can also be manifested by the following analysis of the char. On the other hand, the average effective heat of combustion (EHC) values (see Table 3) decreases from 17.1 to 15.4 MJ/kg

with

the

different

structures

of

the

flame

retardants.

PLA/10%

14


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APP@CS@PA-Na shows the lowest EHC value indicative of the least emission of the fuel into the gas phase, which is well consistent to the TSR results.

Flame retardant mechanism TG-IR analysis TG-IR technique is performed to further study the effect of APP@CS@PA-Na on the

thermal

degradation

of

PLA.

The

gas

products

of

both

PLA

and

PLA/10%APP@CS@PA-Na are characterized during the thermal decomposition in N2. The

FTIR

spectra

of

evolved

gaseous

products

of

pure

PLA

and

PLA/10%APP@CS@PA-Na are shown in Fig.5 (A) and (B) and the IR spectra of their gas products at Tmax were presented in Fig.5 (C). It was noted that there are few differences between the characteristic peaks of the main degradation products for them30, except for the intensity of each peak. The weaker absorption intensities of PLA/10%APP@CS@PA-Na than those of neat PLA suggest that volatile components are suppressed during the degradation process. It should be attributed to the barrier effect of the char formed during the pyrolysis of PLA/10%APP@CS@PA-Na. Fig.6 also shows the total intensity of the gas products. The intensity of carbonyl compounds (1760 cm-1), CO (2128 cm-1) and hydrocarbons (2970 cm-1) peaks of PLA/10% APP@CS@PA-Na all display decreased absorption intensities compared to that of the neat PLA. It is reasonable to deduce that the condensed phase mechanism is primarily responsible for the decrease of combustible gas evolved, which is beneficial to 15



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improving flame retardancy of PLA. Char residue The digital images of char residues for PLA and its composites after the cone tests are shown in Fig.7 (A) - (D). The neat PLA almost does not leave any residue due to poor char-forming capability. With the increase of the shell layer of flame retardants, the residue of PLA composites increases gradually. As for PLA/10%APP@CS@PA-Na, the continuity of the residue improves significantly, which can contribute to insulating the heat transfer, thus providing better thermal barrier effects. SEM and energy dispersive spectrometry (EDS) are performed to study the char structures and compositions of PLA composites formed during cone calorimeter tests. As shown in Fig.7 (B1) - (D1), the char layer of PLA/10%APP@CS@PA-Na is much more continuous than those of the rest flame retardant composites. This char can better isolate the underlying PLA matrix from the air for heat and oxygen transfer and prevent the emission of the flammable gas mixture from escaping. As summarized in Fig.7 (B2) (D2), the P-containing compounds in the char formed by flame retardant composites of PLA increases with the increase of the number of shells on the APP particles. It is due to the accumulation of P-containing compounds on the surface of the char layer, which help improve the quality of the char31 and thus lead PLA to showing lower pHRR and THR. In order to further study the flame-retardant mechanism in the condensed phase, the char is characterized by Raman spectra. The peak at about 1360 cm-1 and 1590 cm-1

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are attributed to D-band and G-band, respectively.32-34 And the ratios of ID/IG can be used to indicate the degree of graphitization of the char. As shown in Fig.7 (B3) (D3), the ID/IG value of PLA/10%APP@CS@PA-Na is lower than that of both PLA/10%APP and PLA/10%APP@CS, indicative of a higher graphitization degree of the char for PLA/10%APP@CS@PA-Na. Therefore, the quality of char is improved as well and the flame retardancy of PLA/10%APP@CS@PA-Na is much better than other PLA composites in this work.

Mechanical properties In order to investigate effects of the core-shell flame retardants on the mechanical properties, tensile and notch impact tests are conducted. The stress-strain curves for PLA and its flame retardant composites are presented Fig.8 with detailed results summarized in Table 4. The neat PLA shows a typical brittle fracture behavior with a low elongation at break of only about 8.1 %. With the addition of different flame retardants, the elongation at break increases gradually with the increasing number of shells on APP particles. Especially for PLA/10%APP@CS@PA-Na, the elongation at break increases by 28.4 % compared with that of neat PLA, which is probably ascribed to improved interfacial interaction between the core-shell flame retardant and the PLA matrix. In view of the structure of PA, although the pH of the PA aquous solution was adjusted to 6, there are still hydroxyls in the structure of PA, since PA had 12 hydroxyls in its phosphate groups. Therefore, the hydrogen bonds between PA and PLA may

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facilitate the toughening of PLA. As for the notched impact strength, it increases with the addition of each flame retardant,

compared

with

that

of

neat

PLA

(3.5

kJ/m2).

For

PLA/10%APP@CS@PA-Na, the notched impact strength increases by 17.1 % compared with that of the PLA, which indicates APP@CS@PA-Na has a certain toughening effect on PLA. However, the addition of APP alone or APP@CS in PLA increases the notched impact strength more than APP@CS@PA-Na. The reason may be that PA, also called inositol hexakisphosphate acid, has the rigid six-number ring, which affect its ability to absorb energy in the notched impact tests.

Conclusions This work has developed a green approach to fabricate a core-shell bio-based flame retardant, APP@CS@PA-Na, by utilizing CS and PA-Na as the shells and APP as the core with water as the media. The addition of 10 wt% of APP@CS@PA-Na endows PLA with the better flame retardancy achieving a high LOI value of 30.5 % and a V-0 rating. This means that as-prepared flame retardant PLA can fully meet the demanding fire retardancy requirement in industry. Meanwhile, pHRR and THR values respectively decreases by 33 % and 19 % compared with those of pure PLA. The APP@CS@PA-Na mainly plays the flame retardant role in the condensed phase. This work offers a green and facile methodology for creating flame retardant PLA and helps broaden its practical application in industry.

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Supporting Information Available: Total smoke rate (TSR) curves of PLA and its biocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (51873196, 21671146), Australia Research Council Discovery Project (DP190102992), the Natural Science Foundation of Ningbo (2018A610070), and Key Research and Development Projects of Zhejiang Province (2019C01098, 2018C01051).

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Figure captions Fig. 1 A) Schematic illustration for the fabrication of APP@CS@PA-Na via LbL assembly from water. SEM photographs of B) APP, C) APP@CS and D) APP@CS@PA-Na.

Fig. 2 XPS spectra of APP, APP@CS and APP@CS@PA-Na.

Fig.3 A) TG and B) DTG curves of APP, APP@CS and APP@CS@PA-Na, C) TG and D) DTG curves of PLA and its flame-retardant composites in N2.

Fig. 4 A) Heat release rate and B) total heat release versus time curves. Fig. 5 3D images of TG-IR results of A) neat PLA and B) PLA/10%APP@CS@PA-Na; C) IR spectra of gaseous products of PLA and PLA/10% APP@CS@PA-Na at Tmax. Fig. 6 Absorbance of volatile products of neat PLA and PLA/10%APP@CS@PA-Na: A) total absorption, B) C-O containing compounds, C) carbon monoxide and D)

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hydrocarbons. Fig. 7 Residue char analysis. A-D) Digital photos, B1-D1) SEM images, B2-D2) EDS spectra and B3-D3) Raman spectra. A) neat PLA, B, B1, B2, B3) PLA/10%APP, C1, C2, C3) PLA/10%APP@CS, and D, D1, D2, D3) PLA/10%APP@CS@PA-Na after cone tests. Fig. 8 Typical stress-strain curves of neat PLA and its flame-retardant composites.

Fig. 1

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Fig. 2

Fig. 3

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Fig. 4

Fig. 5

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Fig. 6

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Fig. 7

Fig. 8 28


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Table 1. Data of TGA for flame retardants, PLA and its flame-retardant composites Residual weight at 600 oC (wt%)

Sample ID

Tonset (oC)

Tmax (oC)

Maximum weight loss rate(wt%/min)

Experimental

Theoretical

APP APP@CS APP@CS@PA-Na PLA PLA/10%APP PLA/10%APP@CS PLA/10%APP@CS@PA-Na

337 307 281 346 344 343 341

638 609 610 382 380 380 382

16.3 16.4 13.6 68.3 60.2 58.8 55.9

66.9 46.7 48.8 0.7 7.4 7.7 8.0

/ / / / 7.3 5.2 5.5

Table 2. Detailed results for PLA and its flame retardant composites obtained from UL-94 and LOI tests. UL-94 LOI Sample ID Cotton (%) Rating Dripping AFT a(s) Ignition PLA NR Yes Yes Burn out 20.0 PLA/10%APP V2 Yes Yes 1.7/1.8 27.2 PLA/10%APP@CS V2 Yes Yes 1.6/1.6 29.0 PLA/10%APP@CS@PA V2 Yes Yes 1.7/1.8 29.4 PLA/10%APP@CS@PA-Na V0 Yes No 1.2/1.3 30.5 a Average

burning time after the first and the second ignition.

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Table 3. Results of cone calorimeter tests for PLA and its flame-retardant composites. Sample ID

TTI (s)

PLA PLA/10%APP PLA/10%APP@CS PLA/10%APP@CS@PA-N a

71 ± 1 70 ± 1 69 ± 1 71 ± 1

pHRR THR 2 (kW/m ) (MJ/m2) 376 ± 11 339 ± 4 310 ± 11 252 ± 9

67 ± 1 60 ± 1 58 ± 0 54 ± 3

Residual Mass (wt%) 1.5 ± 0.1 8.6 ± 2.0 11.2 ± 0.1 14.0 ± 3.0

EHC (MJ/kg) 17.1 ± 0.2 16.4 ± 0.2 16.1 ± 0.1 15.4 ± 0.4

Table 4. Summarized mechanical properties of PLA and its flame-retardant composites. Sample ID PLA PLA/10%APP PLA/10%APP@CS PLA/10%APP@CS@PA-Na

Youngs modulus (GPa) 0.85 ± 0.02 0.74 ± 0.03 0.72 ± 0.03 0.72 ± 0.04

Tensile strength (MPa) 56.7 ± 2.8 40.4 ± 0.9 41.3 ± 0.7 45.3 ± 1.0

Elongation at break (%) 8.1 ± 1.3 8.6 ± 1.2 9.2 ± 0.9 10.4 ± 1.2

Impact strength (kJ/m2) 3.5 ± 0.1 4.8 ± 0.2 4.7 ± 0.2 4.1 ± 0.1

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For Table of Contents Use Only

A bio-based flame retardant, APP@CS@PA-Na, has been developed via a green strategy for reducing flammability of biodegradable polylactic acid.

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Green and Scalable Fabrication of Core-Shell Bio-Based Flame

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