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Preparation of novel c-6 position carboxyl corn starch by green method and its application in flame retardance of epoxy resin Shui-dong Zhang, Fang Liu, Hua-Qiao Peng, Xiang-Fang Peng, Saihua Jiang, and JunSheng Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03266 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Industrial & Engineering Chemistry Research

Preparation of novel c-6 position carboxyl corn starch by green method and its application in flame retardance of epoxy resin

Shuidong Zhang1, Fang Liu1, Huaqiao Peng2, Xiangfang Peng1, Saihua Jiang1∗, Junsheng Wang3*

1

Lab for Micro Molding and Polymer Rheology, School of Mechanical and

Automotive Engineering, South China University of Technology, Guangzhou 510640, China 2

The Second Research Institute of CAAC, Chengdu 610041, China

3

Tianjin Fire Research Institute of the Ministry of Public Security, Tianjin 300381,

China

∗

Corresponding author: Junsheng Wang & Saihua Jiang. Fax: +86-2087110029, E-mail address:

[email protected] & [email protected]. 1

ACS Paragon Plus Environment


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Abstract Novel c-6 position oxidized corn starch (OST) with high carboxyl contents (26.3 -54.5%) was prepared by a green method, using hydrogen peroxide as the oxidant. The as-obtained OSTs were then used as flame-retardant carbon sources with microencapsulated ammonium polyphosphate (MFAPP) in epoxy resin (EP). Compared to EP, EP/MFAPP/OST composites obtained exhibit significantly enhanced flame retardancy. The introduction of only 6.25 wt% OSTs and 6.25 wt% MFAPP results in remarkably increased limiting oxygen index and decreased heat release rate, and all composites can reach UL94 V-0 rating. Thermogravimetric analyses and Cone calorimeter results suggest both OSTs and MFAPP have good catalytic charring effects, and the increased carboxyl content benefits to the char formation of the composites. Owning to the formation of compact char on the sample surface during combustion, the transfer of oxygen, heat and flammable gas products is inhibited; the flame retardancy of EP/MFAPP/OST composites is thus remarkably enhanced.

2


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1. Introduction Due to its renewability, biodegradability, abundance and low cost, starch has many diverse applications in food production, biodegradable plastic, adhesives industry and so on. The hydroxyl groups of starch are easily subjected to various types of reactions including oxidation, esterfication, etherification, etc, obtaining various kinds of modified starch. Among them, the oxidized starch (OST) often exhibits excellent physicochemical properties including low viscosity, high stability, and excellent film forming and binding properties which are markedly different from those of starch.1 To date, OST has been widely applied in the fields such as food,2 paper coatings,3, 4 biomaterial,5 thermoplastic starch and so on.1, 6, 7 However, few people has reported its new application in flame retardace of polymers. Intumescent flame-retardant (IFR) system is a good halogen-free flame-retardant technique for polymers.8, 9-10 Its flame-retardant mechanism is based on the charred layer acting as a physical barrier which slows down heat and mass transfer between gas phase and condensed phase.11 The conventional IFR system comprises three parts: acid source, carbonization agents and blowing agents. Carbonization agents are mainly hydroxyl-containing compounds, such as polyols and polysaccharides.8-10 Recently, many high-efficient carbonization agents have been synthesized,12-14 among which starch has been demonstrated as a promising carbonization agent due to its virtue of natural sources.9,

15

Furthermore, from our previous work, carbonized

efficiency of OSTs was increased with increasing carbonyl contents, which is also beneficial to the flame retardancy of polymers.4, 16 These results above indicate that 3



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OST could be a highly efficient flame-retardant carbonization agent. According to the references, OSTs can be synthesized by the oxidation of starch. The

useful

oxidants

include

alkaline

hypochlorite,

2,

2,

6,

6-tetramethylpiperidine-1-oxyl (TEMPO),16, 17 periodate and peroxide.18-21 Under the oxidation condition, the hydroxyl groups of OST are converted into carbonyl groups and carboxyl groups.8,

22, 23

Owning to its environment-friendly and non-toxicity,

H2O2 has attracted greater interest than other oxidants.24 H2O2 can decompose into hydrogen ions and water during oxidation, which makes it environmental friendly. Although researchers have successfully prepared OSTs by H2O2,25-27 most OSTs obtained still has a low oxidation degree, and the carbonyl contents in OSTs are relatively low. In our previous study, we proposed a new method to prepare OST with high carbonyl content (higher than 55%).7 H2O2 was used to oxidize gelatinized starch, and carbonyl and carboxyl groups could be controlled by varying the H2O2/starch molar ratios. EP is one of the most widely exploited reactive polymeric resins. To reduce its flammability and broaden its application, some IFRs have been used.16 Wang group successfully improved the flame retardancy of EP by using DODPP, metal and ammonium polyphosphate.9, 16 They demonstrated that the addition of IFR system could reduce the flammability of EP significantly. The objective of this study was to investigate the selectivity of primary alcohol group of starch oxidized by H2O2, and the flame-retardant effect of OST as a carbonization agent in EP resin. Structure of OSTs with different carbonyl contents was characterized by Fourier Transform 4


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Infrared Spectroscopy (FTIR) and

13

C-NMR. Physicochemical properties of OSTs

were examined by intrinsic viscosity, thermogravimetry analysis (TGA) and Thermogravimetric analysis-infrared spectrometry (TG-IR) tests. In addition, OST as well as MFAPP was introduced to EP to prepare EP/MFAPP/OST composites. Limiting Oxygen Index (LOI), Vertical burning (UL-94) and Cone calorimeter (CCT) tests were used to evaluate the flame retardancy of EP/MFAPP/OST composites. SEM was employed to investigate the char morphology of composites after combustion. The detailed flame retardant mechanism was proposed. 2 Materials and Methods 2.1 Materials Corn starch containing 11-13% of moisture was obtained from Langfang Starch Company (Technical Grade, Langfang, Hebei, China). Epoxy resin (EP, CYD-128) was purchased from China Petrochemical Corporation (Yueyang, China). Polyether amine curing agent (D230, Baxxodur®EC301) was purchased from BASF SE; Ammonium polyphosphate (APP) was supplied by Sichuan Haiwang flame retardant materials Co. (Dujiangyan, China). Microencapsulated ammonium polyphosphate (MFAPP) was prepared by our lab according to the method of Lin et al.28 All other chemicals and solvents were of analytical grade (99.5%) and used as received. 2.2 Preparation of OST The OST was prepared by the oxidation of starch, with hydrogen peroxide as the oxidant. The detailed oxidation procedure was illustrated in Scheme 1, according to our previous work.7 Starch slurry was prepared by adding 50 g starch and 200 mL 5



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distilled water into a 1000 mL three-necked round bottom flask equipped with a mechanical stirring and heating device. The mixture was heated to 80 oC with modest stirring to gelatinize the starch about 0.5 h. After that, 250 mL distilled water was added and the temperature decreased to 35 oC. 12.5 mg FeSO4·7H2O was dissolved in 50.0 mL distilled water before being added to the gelatinized starch. The FeSO4 solution was first added to the starch slurry, followed by the addition of H2O2 in 0.5 h. The pH value of H2O2 was maintained 6.2 ± 0.2 controlled by 0.1 M NaHCO3 solution. Molar contents of H2O2 were based on the glucose unit of starch molecules (161 g/mol). The mixture was kept at 35 oC with modest stirring for 2 h. When the reaction was completed, the oxidized starch was precipitated in 1000 mL ethanol and then separated by centrifuging. The product was obtained after drying in the vacuum oven at 50 oC for 24 h, and then at 80 oC for 24 h. Finally, the dried products were ground by a high-speed knapper. 2.3 Preparation of EP/MFAPP/OST composites A typical preparation of epoxy based composites (EP/MFAPP/OST) containing 6.25 wt% OST and 6.25 wt % MFAPP was illustrated as below: 27.5 g curing agents (D230) and 82.5 g epoxy resin (EP) were added into a plastic beaker and mixed for 10 min. The mixture was placed in a vacuum chamber at 40 °C for 1 h. Then, 7.86 g OST and 7.86 g MFAPP were mixed with the pre-mixture (110 g) for 5 min. Subsequently, the mixture was mixed under magnetic stirring with a speed of 2000 rpm for 3 min until a homogeneous mixture was formed. The mixture was placed in a vacuum chamber at 40 °C for 12 h. Finally, the EP/MFAPP/OST composites were cured at 6


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60 °C for 2 h and at 80 °C for 2 h, respectively. After curing, the composites were permitted to cool to room temperature. The EP/MFAPP/PER and EP/MFAPP/ST composites were prepared by the analogous procedure, in which OST was replaced by the same amout of PER or ST. 2.4 Carbonyl and carboxyl content determination of OST Carbonyl content determination 1.0 g dry sample was firstly dispersed in distilled water (300 mL) to form homogeneous solution by heating. pH of the above solution was then adjusted to 3.2 by adding 0.15 M aqueous HCl and 60.0 mL of a hydroxylamine hydrochloride solution (hydroxylamine hydrochloride, 25 g; 500 mL 0.1 M NaOH). The gelatinized starches were used as a control. Samples were run in triplicate, and the coefficient of variation was 1%. The carbonyl content, which reflects the extent of oxidation, was expressed as the amount of carbonyl groups per 100 glucose units. Carboxyl content determination 1.0 g dry sample was slurried in water (100 mL) and 0.5 M NaOH was added to keep the pH above 10. After stirring for 1 h, the mixture was back-titrated with 0.15 M HCl to the phenolphthalein end-point. Conversion factors were determined using oxalic and citric acid as standards. The gelatinized starch was used as a control. Samples were run in triplicate and the coefficient of variation was 1%. 2.5 Intrinsic viscosity measurement of OST About 25.0 mg OST was weighted and dissolved in 25.0 ml dimethylsulfoxide (DMSO). An Ubbelohde viscometer at 25.0±0.1°C was used to measure the intrinsic 7



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viscosity [η] of oxidized starch and control gelatinized starch samples at about 1.0 mg/ml dissolved in dimethylsulfoxide (DMSO). Intrinsic viscosity could be obtained by one point method according to the Equation 1.

[η ] =

2(η sp − ln η r )

(1)

C

The viscosity average molecular weight (Mv) was calculated by Mark- Houwink Law:

[η ] = KM v a , K = 8.5 × 10−3 , a = 0.76

(2)

2.6 Characterization Fourier transform infrared (FTIR) spectra were obtained from samples in KBr pellets using a 170SX FTIR spectrophotometer (Nicolet, Madison, WI, USA). Solid state

13

C-NMR spectra were recorded using a Bruker AVANCE III with

proton decoupling at room temperature. TGA of (5-10 mg) samples previously kept at 0 % relative humidity were carried out using a Perkin-Elmer Pyris thermal analyzer under nitrogen atmosphere at a heating rate of 10 oC/min. The range of scanning temperature was from 40 oC to 500 o

C. TG-IR was performed using the TGA Q5000 IR thermogravimetric analyzer that

was linked to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from 40 to 600 °C. The heating rate was 10 °C/min (nitrogen atmosphere, flow rate of 50 mL/min). LOI values of all composite samples were obtained at room temperature on an oxygen index instrument (JF-3) produced by Jiangning Analysis Instrument Factory, 8


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China, according to GB/T2406-93 standard. The dimensions of all samples were 120×6.5×3.0 mm3. Vertical burning ratings (UL-94) of all samples were measured on a CZF-2 instrument (Jiangning Analysis Instrument Factory, China）with sample dimensions of 130×12.5×3 mm3. The burning time after first ignition (t1), the burning time after second ignition (t2) and total combustion time (t1+t2) were recorded from the UL-94 tests based on average burning time of five specimens. The combustion properties of EP and EP composites were carried out on a cone calorimeter based on ASTM E1354/ISO5660. Every specimen with the sizes of 100×100×1.5 mm3 (Due to the high expansion degree, 3.0 mm standard thickness could not be measured) wrapped in an aluminum foil was exposed horizontally to a heat flux of 35 kW/m2. Three parallel runs were performed for each sample to obtain averages. Scanning electron microscopy (SEM) was used to examine the morphology of the char residue obtained on CONE test by using a FEI QuanTa200 SEM, whose accelerating voltage was 1.5 kV. The surface of char residues was sputter-coated with gold layer before examination. 3. Results and Discussions 3.1 Effect of hydrogen peroxide content on oxidation Table 1 shows the carbonyl and carboxylic contents of all OST samples with different molar ratios between H2O2 and ST (MR). As MR values increased from 0.75 to 3.0, the carbonyl and carboxyl contents were raised from 30.7% to 56.2%, and 9



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26.3% to 54.5%, respectively. It is evident that the differences between carboxyl contents and carboxyl contents is reduced as MR rises. According to the available literature, hydroxyl groups in starch were mainly oxidized to aldehyde or ketone groups at lower MR, and tended to oxidized to carboxyl groups at higher MR.7 With the catalysis of FeSO4, H2O2 rapidly decomposed into hydroxyl radicals (HO•), the active free radicals oxidized the hydroxyl groups to aldehyde group, then to carboxyl groups. 29, 30 Moreover, oxidation occurred under ambient conditions at the pH=6.2 with high peroxide levels.31,32 Therefore, it was expected that these conditions would be favorable for the transformation of H2O2 to HO•, which was respondent to preparation of OST with higher carboxyl content. However, the oxidation efficiency decreased when the MR increased from 1.5 to 3. It was attributed to that only part of HO• anticipated the oxidation, the other part of H2O2 would decompose at high concentration. When MR increased from 0.7 to 1.5, the carboxyl content of OST markedly increased from 26.3% to 47.6%, which indicates that H2O2 promoted the oxidation of hydroxyl groups to carboxyl groups under these conditions.7 The above results revealed that carboxyl content of OST was controlled by MR values. 3.2 FTIR spectroscopy Due to the oxidation, the hydroxyl groups in ST are oxidized to carbonyl and carboxyl groups, which can be characterized by FTIR spectra. The FTIR spectra of ST and OST are shown in Figure 1. In the case of ST spectra, the absorption peaks at 2927 cm-1 and 1465 cm-1 were attributed to -CH2-. The absorption at 1640 cm-1 is assigned to H2O bending vibration, which is due to the water absorption by hydroxyl 10


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groups in ST molecules.7, 33 Compared with ST spectrum, the spectra of OST (Figure 1b-e) displayed a new absorption peak at 1735cm-1, corresponding to the vibration of C=O, suggesting the formation of carbonyl groups after the oxidation of hydroxyl groups.7,

34, 35

The intensity of C=O peak at 1735 cm−1 increases as MR rises,

indicating the increase of oxidation degree. From the titration result of carbonyl and carboxyl content, C=O from carboxyl groups was respondent to the appearance of this new absorption peak. Furthermore, the absorption intensity at 1640 cm-1 assigned to -OH groups was decreased as MR values increased. This result demonstrated that the -OH groups in ST has been oxidized. In summary, the FTIR results confirmed that native ST was successfully oxidized to carboxyl starch by H2O2. 3.3 13C-NMR spectroscopy When 2, 2, 6, 6-tetramethyl-1-piperidinyl oxoammonium ion (TEMPO), NaBr, and NaOCl are corporated to oxidize starch, it achieves a selective oxidation of the primary alcohol groups.3 Due to the hydroxyl groups of C2, C3 and C6 on starch, D glucose ring can be oxidized to carboxyl groups by H2O2, so it need to be further determined by 13carbon solid-state nuclear magnetic resonance (13C-NMR).7 Figure 2 presents the difference of 13C-NMR of native ST and OST. The chemical shifts at 61, 74, 73, 71, 79 and 101 ppm were attributed to C6, C3, C2, C5 and C4 of starch anhydroglucose, respectively.36 Compared with ST spectrum, all OST spectra (Figure 2b-e) appeared a new chemical shift at 173.6 ppm, indicating the introduction of a carboxyl group at C6 by the selective oxidation. The peak intensity at 61 ppm decreased with the increase of carbonyl content. Moreover, there were no chemical 11



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shifts found in the range from 200 to 230 ppm, indicating the absence of carboxyl groups from the oxidation of secondary alcohol groups.16, 37 Furthermore, as shown in the local amplification, it could be observed the peak intensity at 173.6 ppm increased with the increasing carboxyl content. The

13

C-NMR spectra further confirmed that a

selective oxidation of the primary alcohol groups has been achieved by H2O2. Furthermore, the selectivity for the primary alcohol group oxidation was generally almost 100%. The oxidation mechanism of gelatinization starch oxidized by H2O2 is a radical chain reaction, as described in Scheme 2. H2O2 will be catalyzed to form hydroxyl radical (HO•) by FeSO4 (Stage 1). This highly reactive free radical oxidizes the glucose unit by subtracting hydrogen from a C-H group on the sugar ring, forming a radical (R•CHOH) that further reacts with H2O2 yielding aldehyde groups (Stage 2 and 3).16, 24 Then aldehyde groups are oxidized to carboxyl group by hydroxyl radical (HO•) (Stage 4). Maybe it is hard to cleave the C2-C3 bonds of the glucose units of corn starch by H2O2, oxidation rates in primary alcohols is faster than in secondary hydrogen group. As a result, it increased the selectivity for primary alcohol oxidation.25 3.4 Viscosity average molecular weight (Mv) of OST ST is easy to hydrolyze when pH value is higher (such as in the alkaline solution) or lower than 7.0, which results in the decrease of Mv.16 To discern the influence of carbonyl content on the degradation of OST prepared by H2O2 at pH=6.2, intrinsic viscosity of OST was measured and Mv was calculated, and the corresponding results are presented in Figure 3. Mv of OST had a marked decrease and the degree of 12


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degradation depended on the carboxyl content. Compared to Mv of ST (70.45×105), Mv of OST26.3 was 2.42×105. When the carboxyl content increased from 37.8 to 55.4%, Mv of OST gently decreased from 1.79×105 to 1.43×105. The decrease in Mv results from the partial cleavage of the glucosidic linkages.20,

26

These results

confirmed an increase in carbonyl content of OST, and a corresponding reduction in Mv when MR increased to 3.0. H2O2 oxidized mainly branched chain of molecular chain in the initial stage of oxidation reaction,32 it resulted in remarkable decrease of Mv because of the bond rupture. During the further reaction, the decrease in glycosides bond rupture became mild, which was attributed to the insignificant decrement in Mv. 3.5 Thermal properties TGA, DTG curves of ST and OSTs with different carboxyl contents are presented in Figure 4, and the values of T-5% (the temperature corresponding to 5% weight loss), Tmax and residues are listed in Table 2. Figure 4 reveals the relationship between the carboxyl content of OST and the temperatures for thermal decomposition. ST showed the highest thermal stability with a T-5% of about 275.9 °C. There was a reduction in thermal stability in parallel with the increase in carbonyl content of OST.7, 16 The thermal stability for OST26.3 and OST37.8 (the difference in carboxyl content was 11.5%) is similar. However, the thermal stability of OST47.6 and OST54.5 (the difference in carboxyl content was 6.9%) was markedly decreased with increasing carbonyl contents. There are probably two factors to influence the thermal stability of OST: (a) the decrease of molecular weight caused by oxidation. The 13



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oxidation of ST by H2O2 leads to the depolymerization of molecular chains, and the lower molecular weight of OST have a negative influence on thermal stability. (b) The catalytic effect of carboxyl groups. Carboxyl groups in OST could catalyze the dehydration of glucose units, which would results in the earlier degradation of OST.38 Although thermal stability of OST was decreased, the char residue of OST was increased with the increase of carboxyl content. Compared with that of ST, the char residue of OST26.3 was increased from 11.5 wt% to 21.1 wt%. Starch undergoes intermediate physical and chemical changes during the thermal degradation.39 It starts first with a scission of glycosidic bonds, then pyrolysis of native starch occurs at 300 o

C and results in releasing volatiles, such as CO2, CO, H2O, acetaldehyde, furan and

2-methyl furan.39 During thermal decomposition of OST, additional reactions could be evolved due to the scission of carboxyl groups. As carboxyl group could catalyze the dehydration of glucose, it may increase the conversion of glucose to carbon and reduce the volatiles. The char residue was thus increased. TGA results showed that the thermal stability of starch was dependent on the carboxyl content. The introduction of carboxyl group decreased the thermal stability, but increased the char residues of starch. Moreover, carboxyl groups would catalyze glucose to form char. The novel properties would endow OST to be currently of great interest as a highly efficient carbonization agent of IFR. 3.6 TG-IR Analysis of ST and OST FTIR spectra for pyrolysis products of ST and OST at maximum decomposition rates are shown in Figure 5. The decomposition gases include CO, CO2, H2O, CH4, 14


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C2H4, and CH3OH.40, 41 In Figure 5a, the absorption peaks between 3600 and 3800 cm-1 were attributed to the stretching vibration and vibration absorption of -OH (moisture gas), respectively. The absorption peaks at 2970 cm-1 and 2910 cm-1 were attributed to the special absorption peaks of -CH3 group. The absorption peaks at 2380 and 2262 cm-1 were attributed to the formation of carbon dioxide and carbon monoxide. The fine vibrational structures centered at 1730 and 1680 cm-1 indicate the formation of C=O groups by carboxyl group and acetaldehyde, respectively.44 The absorption peaks at 1060 and 660 cm-1 were attributed to the bending vibration of C-O group and O-H deformation, and it could be attributed to methanol combining with the absorption peaks at 2970 and 2910 cm-1.29, 32, 41 The presence of CO2, CO, CH3OH and carbonyl compounds in the spectra suggests the scission of glycosidic linkages and strong bonds in the backbone of starch.41,

42

The Figure 5b-e show that the

intensity of absorption peaks at 2970, 2910, 1680, 1060 and 660 cm-1 were decreased significantly. That is because carboxyl group could catalyze the degradation of glucose and promote the char formation at higher temperature, which is consistent with TGA results. The results revealed that when the carboxyl content increased, the acetaldehyde and methanol contents in decomposition gases decreased. The merit of the oxidation by H2O2 would decrease content of combustible acetaldehyde and methanol gas, significantly. The higher char residue amount and lower combustible volatile amount evinced by TGA and TG-IR results above would make OST a promising carbonization agent candidate for IFR system. 15



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3.7 Flame retardancy of EP composites To evaluate the flame-retardant efficiency of OST in polymers, OST with different carboxyl contents as well as MFAPP (a common acid source agent in IFR system) was incorporated into EP matrix to prepare EP/MFAPP/OST composites. For comparison, PER, a common carbonization agent, was also introduced into EP/MFAPP system to fabricate EP/MFAPP/PER composite. The as-obtained composites were then characterized by LOI and UL-94 tests, and the corresponding results were given in Table 3. Compared to pure EP, all composites exhibit increased LOI, which suggested the enhanced flame retardancy of all composites. Compared with EP/MFAPP/PER with LOI value of 24.9, the LOI value of EP/MFAPP/OST was much larger (range from 26.2 to 29.5), indicating the higher flame-retardant efficiency of MFAPP/OST than MFAPP/PER. That may be attributed to the better catalytic-charring efficiency of MFAPP/OST than MFAPP/PER. The protective char layer formed on the sample surface could barrier the transfer of heat, oxygen and flammable gas products, which would improve the flame retardancy of EP. In the system of MFAPP/OST, both MFAPP and OST could catalyze the degradation of the matrix. MFAPP can degrade firstly and form phosphoric acid during combustion, and the obtained phosphoric acid could acts as an acid catalyst, and accelerates the chain scission of EP matrix to form char residue. In addition, the TGA result of OST above (Figure 4) indicated that OST with carboxyl groups could also promote the char formation, and the carboxyl group content was a beneficial factor for the char formation. That was why the LOI values of EP/MFAPP/OST raise as carbonyl groups 16


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increases. However, when carboxyl group content was beyond 47.6%, the LOI value of EP/MFAPP/OST is decreased. For example, the EP/MFAPP/OST54.5 exhibited a lower LOI value than EP/MFAPP/OST47.6. The similar phenomena could be found in UL-94 data. From Table 3, the introduction of MFAPP/PER could not make EP achieve UL94 V-0 rating, whereas all EP/MFAPP/OST composites could reach UL94 V-0 rating. The t1, t2 and t1+t2 during UL94 test are decreased significantly as carboxyl group content rises (≤47.6%). However, when more carboxyl group was formed in OST, the flame retardancy of EP/MFAPP/OST composites is reduced. Both UL94 and LOI results demonstrate that the MFAPP/OST is a good IFR, and exhibits even higher flame-retardant efficency than MFAPP/PER and MFAPP/ST. The carboxyl group content of OST at 47.6% is a turn point for the flame retardancy of EP/MFAPP/OST composites. To further investigate the good flame retardancy of EP/MFAPP/OST composites, EP/MFAPP/PER, EP/MFAPP/ST, EP/MFAPP/OST47.6 were characterized by Cone calorimeter (CCT). Heat release rate (HRR), peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) obtained from CCT tests are the key parameters for fire hazard evaluation of materials, especially PHRR is regarded as the most important parameter. Figure 6A shows the HRR curves of EP, EP/MFAPP/PER, EP/MFAPP/ST and EP/MFAPP/OST47.6 vs. temperatures. The corresponding THR and TSP values are plotted in Figure 6B and C, respectively. From Figure 6 and Table 4, the PHRR (1340 kW/m2) and THR (36.3 MJ/m2) of EP were very high. With introduction of three different kinds of IFRs (MFAPP/PER, MFAPP/ST and 17



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MFAPP/OST47.6), both PHRR and THR of the composites shifted to lower values. Compared to EP/MFAPP/PER and EP/MFAPP/ST systems, the EP/MFAPP/OST47.6 exhibited lower PHRR (400 kW/m2) and THR (13.4 MJ/m2), further confirming the better flame-retardant effect of MFAPP/OST than MFAPP/PER and MFAPP/ST. The lower TSP of EP/MFAPP/OST47.6 than those of EP/MFAPP/PER and EP/MFAPP/ST indicated the improved smoke suppression of the composites. As shown in Table 4, the char amount of EP/MFAPP/OST47.6 is much higher than those of EP/MFAPP/PER and EP/MFAPP/ST, which further confirms the better catalytic charring effect of MFAPP/OST47.6 than EP/MFAPP/PER and EP/MFAPP/ST, consistent with the TGA results. Higher char amounts contributes to the flame retardancy enhancements of composites. Based on the LOI, UL-94, CCT, and TGA results above, it is concluded that flame retardancy of EP/MFAPP/OST composites was mainly influenced by the catalytic charring effect of MFAPP/OST. During the composite combustion, the char was formed on the surface of samples, which acted as a stable protective layer to retard the transfer of heat, flammable gas products and oxygen. As a result, the flammability of the composites reduced. To explain the detailed flame-retardant mechanism of EP/MFAPP/OST composites, Figure 7 presents the residual char of all composites at the end of LOI test. It was evident that pure EP could only form little char after combustion, whereas all flame-retardant

composites

including

EP/MFAPP/PER,

EP/MFAPP/ST

and

EP/MFAPP/OST produce char residue, further demonstrating that the char formation was the main reason for the flame retardancy enhancement. Additionally, as can be 18


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seen in Figure 7, the residual char amount of EP/MFAPP/OST was much higher than those of EP/MFAPP/PER and EP/MFAPP/ST. As demonstrated above, for the EP/MFAPP/OST system, besides MFAPP, OST also possessed catalytic charring effect which was different from PER and OST. The char formed on the surface can reduce the flammability of composites by retarding the diffusion of heat, oxygen and flammable gases. It was respondent to the reason that EP/MFAPP/OST show the better flame retardancy than the other two kinds of composites. Figure 8 presents the representative SEM images of char residues for EP/MFAPP/PER, EP/MFAPP/ST, EP/MFAPP/OST47.6 and EP/MFAPP/OST54.5. Compared to EP/MFAPP/PER, EP/MFAPP/ST and EP/MFAPP/OST54.5, the char of EP/MFAPP/OST47.6 was more compact and dense, which means that the moderate OST is required for good char structure, and excessive carboxyl groups may deteriorate the char compactness and quality. Furthermore, different from the flat surface of the char for EP/MFAPP/ST, some raised bubble could be found in the char surface of EP/MFAPP/OST47.6, implying its better intumescent effect than the char of EP/MFAPP/PER and EP/MFAPP/ST. The compactness and good intumescent effects were beneficial to the flame retardancy of char. These two factors combined with the higher char amounts make EP/MFAPP/OST47.6 present the best flame retardancy. On the other hand, according to the TG-IR analysis, due to the char formation caused by MFAPP/OST, the flammable gases such as methanol in gas phase reduced, which also benefits to the flame retardancy improvements. 4. Conclusions 19



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In this work, we have successfully prepared novel c-6 position carboxyl corn starch (OST) with high carboxyl content (from 26.3% and 55.5%) by a green oxidation method. It has been confirmed that the OST is obtained from the oxidation of alcoholic hydroxyl group of starch by H2O2. The as-fabricated OST as well as MFAPP was then introduced into EP matrix to form EP/MFAPP/OST composites, in which OST was used as a carbon source, and MFAPP was used as acid source. The introduction of MFAPP/OST remarkably improves the flame retardancy. With the addition of 6.25 wt% OST and 6.25 wt % MFAPP, the LOI values of composites are increased, maximally by 7.0, and all composites can reach the UL94 V-0 rating. The PHRR, THR and TSP of composite are much lower than those pure EP. Additional, the flame retardancy of the composites is influenced by the carboxyl group content in OST. As carboxyl content increases from 0 to 47.6%, the LOI values of EP/MFAPP/OST rises from 22.5% to 29.5%, and the total combustion time (t1+t2) is decreased gradually, which is attributed the improved char amount resulted from the catalytic charring effect of MFAPP/OST. The compact char formed on the sample surface can reduce the flammability by retarding the diffusion of oxygen, heat and flammable gas products. When the carboxyl group content is beyond 47.6%, the flame retardancy of composites is slightly reduced. That is because the excessive carboxyl groups may deteriorate the char compactness and quality. This work demonstrates OST is not only a good flame retardant, but also an excellent carbon source, much better than PER and ST. Acknowledgments 20


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The authors wish to acknowledge the financial support of the National Science Fund for 50903032 and U1333126, Fund Research Grant for Science and Technology in Guangzhou, China (2014J4100038), the Fundamental Research Funds for the Central Universities (2015ZZ020), Natural Science Foundation of China (No. 51503067),

Natural

Science

Foundation

of

Guangdong

Province

(No.

2014A030310122) and China postdoctoral Science Foundation (No. 2015M572309). Supporting Information Solubility index of PER, starch and all OST samples (Table S1). This supporting information is available free of charge via the Internet at http://pubs.acs.org.

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of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6. Polym. Degrad. Stabil. 2007, 92, 1528-1545. (33) Luo, F. X.; Huang, Q.; Fu, X.; Zhang, L. X.; Yu, S. J., Preparation and characterisation of crosslinked waxy potato starch. Food Chem. 2009, 115, 563-568. (34) Kweon, D. K.; Choi, J. K.; Kim, E. K.; Lim, S. T., Adsorption of divalent metal ions by succinylated and oxidized corn starches. Carbohyd. Polym. 2001, 46, 171-177. (35) Para, A., Complexation of metal ions with dioxime of dialdehyde starch. Carbohyd. Polym. 2004, 57, 277-283. (36) Lian, X. J.; Wang, C. J.; Zhang, K. S.; Li, L., The retrogradation properties of glutinous rice and buckwheat starches as observed with FT-IR, C-13 NMR and DSC. Int. J. Biol. Macromol. 2014, 64, 288-293. (37) Liu, J. H.; Wang, B.; Lin, L.; Zhang, J. Y.; Liu, W. L.; Xie, J. H.; Ding, Y. T., Functional, physicochemical properties and structure of cross-linked oxidized maize starch. Food Hydrocolloid. 2014, 36, 45-52. (38) Aggarwal, P.; Dollimore, D., A thermal analysis investigation of partially hydrolyzed starch. Thermochim. Acta 1998, 319, 17-25. (39) Yang, Z.; Liu, X.; Yang, Z.; Zhuang, G.; Bai, Z.; Zhang, H.; Guo, Y., Preparation and formation mechanism of levoglucosan from starch using a tubular furnace pyrolysis reactor. J. Anal. Appl. Pyrol. 2013, 102, 83-88. (40) Baker, R. R.; Bishop, L. J., The pyrolysis of non-volatile tobacco ingredients 26


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Figure captions Scheme 1. Schematic illustration for the preparation of oxidized starch. Scheme 2. Synthetic route for oxidized starch. Figure 1. FTIR curves of (a) ST, (b) OST26.3, (c) OST 37.8, (d) OST47.6 and (e) OST54.5. Figure 2. 13C-NMR spectra of (a) ST, (b) OST26.3, (c) OST37.8, (d) OST47.6 and (e) OST54.5. Figure 3. Viscosity-average molecular weights of ST and all OST samples. Figure 4. TGA and DTG curves of (a) PER, (b) ST, (c) OST26.3, (d) OST37.8, (e) OST47.6 and (f) OST54.5. Figure 5. FTIR spectra for the pyrolysis products of ST and OSTs (at maximum decomposition temperatures): (a) ST, (b) OST26.3, (c) OST37.8, (d) OST47.6 and (e) OST54.5. Figure 6. (A) HRR, (B) THR and (C) TSP curves obtained from Cone calorimeter test: (a) EP, (b) EP/MFAPP/PER, (c) EP/MFAPP/ST and (d) EP/MFAPP/OST47.6. Figure 7. Residual char at the end of LOI test: (a) EP, (b) EP/MFAPP/PER, (c) EP/MFAPP/ST,

(d)

EP/MFAPP/OST26.3,

(e)

EP/MFAPP/OST37.8,

(f)

EP/MFAPP/OST47.6 and (g) EP/MFAPP/OST54.5. Figure 8. SEM images of char residue for EP based composites: (a) EP/MFAPP/PER, (b) EP/MFAPP/ST, (c) EP/MFAPP/OST47.6 and (d) EP/MFAPP/OST54.5.

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Samples ST OST26.3 OST37.8 OST47.6 OST54.5

Table 1 The parameters of ST and all OST samples. Molar ratio of Carboxyl Carbonyl H2O2/ST (MR) content (%) content (%) 0 0 0 0.75:1 26.3 30.7 1:1 37.8 38.1 1.5:1 47.6 48.8 3:1 54.5 56.2

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Mv (×104 g/mol) 70.45 2.42 1.79 1.70 1.43


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Samples PER ST OST26.3 OST37.8 OST47.6 OST54.5

Table 2 TGA data of PER, ST and all OST samples. T-5%(°C) Residue (%) Tmax（°C） 225.0 290.7 0.9 244.1 306.7 11.5 203.1 303.5 21.1 201.0 303.0 22.0 184.6 297.2 22.6 175.0 296.3 23.4

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Table 3 Flame retardancy of pure EP and EP based composites. Samples LOI (%) UL-94 test t1 t2 t1 + t2 EP EP/MFAPP/PER EP/MFAPP/ST EP/MFAPP/OST26.3 EP/MFAPP/OST37.8 EP/MFAPP/OST47.6 EP/MFAPP/OST54.5

22.5 24.9 30.1 26.2 27.3 29.5 25.6

— 5.6±0.3 1.7±0.2 1.2±0.1 0.9±0.1 0.7±0.1 1.6±0.1

— >50.0 7.2±0.3 3.6±0.2 2.8±0.1 2.1±0.1 6.5±0.3

31


— >50.0 8.9±0.3 4.8±0.2 3.7±0.1 2.8±0.1 8.1±0.3

UL-94 rating No rating No rating V-0 V-0 V-0 V-0 V-0


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Table 4 Cone calorimeter parameters of EP and EP based composites. Char yield (%) Sample TTI pHRR THR TSP 2 2 2 (s) (kW/m ) (MJ/m ) (m /kg) EP 29 1340 36.3 20.36 5 EP/MFAPP/PER 24 422 20.6 7.46 30 EP/MFAPP/ST 24 457 15.2 6.49 32 EP/MFAPP/OST47.6 22 400 13.4 5.85 48

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Scheme 1. Schematic illustration for the preparation of oxidized starch.

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Scheme 2. Synthetic route for oxidized starch.

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Figure 1. FTIR curves of (a) ST, (b) OST26.3, (c) OST 37.8, (d) OST47.6 and (e) OST54.5.

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Figure 2. 13C-NMR spectra of (a) ST, (b) OST26.3, (c) OST37.8, (d) OST47.6 and (e) OST54.5.

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Figure 3. Viscosity-average molecular weights (Mv) of ST and all OST samples.

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Figure 4. TGA and DTG curves of (a) PER, (b) ST, (c) OST26.3, (d) OST37.8, (e) OST47.6 and (f) OST54.5.

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Figure 5. FTIR spectra for the pyrolysis products of ST and OSTs (at maximum decomposition temperatures): (a) ST, (b) OST26.3, (c) OST37.8, (d) OST47.6 and (e) OST54.5.

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Figure 6. (A) HRR, (B) THR and (C) TSP curves obtained from Cone calorimeter test: (a) EP, (b) EP/MFAPP/PER, (c) EP/MFAPP/ST and (d) EP/MFAPP/OST47.6.

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Figure 7. Residual char at the end of LOI test: (a) EP, (b) EP/MFAPP/PER, (c) EP/MFAPP/ST, (d) EP/MFAPP/OST26.3, (e) EP/MFAPP/OST37.8, (f) EP/MFAPP/OST47.6 and (g) EP/MFAPP/OST54.5.

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Figure 8. SEM images of char residue for EP based composites: (a) EP/MFAPP/PER, (b) EP/MFAPP/ST, (c) EP/MFAPP/OST47.6 and (d) EP/MFAPP/OST54.5.

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