Phytic Acid Polyelectrolyte Complex: A Green and

Article pubs.acs.org/IECR

Chitosan/Phytic Acid Polyelectrolyte Complex: A Green and Renewable Intumescent Flame Retardant System for Ethylene−Vinyl Acetate Copolymer Tao Zhang,*,†,‡ Hongqiang Yan,⊥ Lie Shen,§ Zhengping Fang,§,⊥ Xianming Zhang,† Jiajun Wang,†,‡ and Baoyue Zhang‡ †

National & Local United Engineering Laboratory of Textile Fiber Materials and Processing Technology, College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China ‡ Department of Packaging Engineering, College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China § MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ⊥ Lab of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People’s Republic of China S Supporting Information *

ABSTRACT: We describe the preparation and characterization of a green and renewable polyelectrolyte complex (PEC) containing phosphorus, nitrogen and carbon elements, based on the ionic complexation between chitosan and phytic acid. Introduction of this PEC to ethylene−vinyl acetate copolymer (EVA) leads to an improvement of the flame retardancy. As for the EVA/PEC composites with 20.0 wt % of PEC (EVA/20PEC), the char residue at 600 °C is 12 wt % higher than that of the pristine EVA under nitrogen atmosphere. Compared to the pristine EVA, the peak heat release rate and total heat release of EVA/20PEC show 249 W g−1 and 5.6 kJ g−1 decreases, respectively. The char residue of EVA/20PEC is full and compact, demonstrating excellent intumescent effect. Introduction of this PEC also contributes to a slight increase of the Young’s modulus while maintains the excellent ductility. This work provides a new approach for the development of environmentally friendly intumescent flame retardant system. molecule.10 With the nontoxicity, biodegradability and antibacterial activity, CS has been paired with many anion polyelectrolytes such as sodium alginate,11 sodium carboxymethyl cellulose12 and γ-poly(glutamic acid)13 to prepare functional PECs. In view of the special structure and properties of CS, researchers have also explored its contribution to flame retardant systems in recent years, mainly in the form of a multilayer coating.14−16 During combustion, CS can be easily carbonized and release inert gases, which provides a carbon source and a foaming agent for the intumescent flame retardant system.15 Although CS based multilayer coating has been found application in the field of flame retardancy, performances of CS based PECs are still rarely explored. Furthermore, in the form of PEC, the cooperation behavior between CS and other components during combustion still remains unknown. In this contribution, we report the design and preparation of a novel CS based PEC containing phosphorus, nitrogen and carbon elements. We choose the natural and innoxious phosphorus-rich phytic acid (PA) as negatively charged partner to pair with CS, which has been recently used as an effective

1. INTRODUCTION As a class of multicomponent polymeric materials, a polyelectrolyte complex (PEC) is usually formed in aqueous solution by combining at least two oppositely charged polyelectrolytes based on the ionic interaction.1,2 In general, organic precursors, catalysts or reactive agents are unneeded during the preparation of PECs, which can reduce the toxicity at some degree.3 Furthermore, the resultant PEC can easily combine the functional groups of the components without complicated chemical reactions, resulting in excellent physical and chemical properties.4 Therefore, extensive research work has been carried out on the design and fabrication of PECs, which are widely used as biomaterials and membrane materials.5,6 With growing concern about global environmental problems, PECs made from renewable and environmentally friendly materials have aroused more and more interest. Among many employed materials, chitosan (CS) is one of the most promising candidates. As we all know, CS is a kind of biopolymer, which is derived by the deacetylation of natural chitin.7,8 Structurally, CS is composed of a polysaccharide backbone with abundant functional OH and NH2 groups.9 In dilute acid solution with a pH value below 5, CS shows a cationic nature and high charge density, which is due to the protonation of NH2 group and the full solubility of the © XXXX American Chemical Society

Received: August 29, 2014 Revised: November 17, 2014 Accepted: November 22, 2014

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dx.doi.org/10.1021/ie503421f | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Schematic Illustration of the Preparation of PEC from CS and PA

acid source in the multilayer coating on fabric.15 Then, we evaluate the flame retardant performance of this PEC on ethylene−vinyl acetate copolymer (EVA), which is widely used in many applications such as wire and cable and demands high flame retardancy.17−20 Furthermore, the influences of this PEC on the tensile and viscoelastic properties of EVA are also discussed. The results from this work will provide new insight into the strategies for design and preparation of an intumescent flame retardant system based on renewable resources.

2. EXPERIMENTAL SECTION 2.1. Materials. Ethylene−vinyl acetate copolymer (EVA, Dupont Elvax 150, VA content: 32%, MFR: 43 g per 10 min) was purchased from DuPont Corporation (Wilmington, Delaware, USA). Chitosan (CS, deacetylation degree ≥95%) and phytic acid (PA, 50 wt % solution in H2O) were acquired from Aladdin Chemistry Co., Ltd. (Shanghai, China). Concentrated hydrogen chloride (HCl, 36.5−38%) and sodium hydroxide (NaOH, ≥96.0%) were obtained from Hangzhou Chemical Reagent Co., Ltd. (Hangzhou, China). Deionized water with a resistance of 18 MΩ was used for all the experiments. The starting reagents were used as received and without further purification. 2.2. Preparation of PEC. Following a slightly modified procedure,12 dried CS powders were stirred in deionized water for 1 h to form a suspension. Thereafter, 5.0 mol/L HCl was added to the CS suspension, and the stirring was continued for several hours to form a homogeneous solution. The concentration was set at 2.0 wt %, and the pH value was adjusted to 1.5 with 5.0 mol/L NaOH or HCl solution. The PA solution was prepared as 4.0 wt % concentrations using deionized water, and the pH value was adjusted to 1.5 with 5.0 mol/L NaOH or HCl solution. As shown in Scheme 1 and Figure 1, the preparation process of PEC was as follows: the excessive PA solution was added into the CS solution dropwise, yielding a white precipitate, which was collected by filtration, washed with deionized water until neutral, dried at 60 °C under vacuum, and ground to a fine powder. 2.3. Preparation of EVA/PEC Composites. The EVA/ PEC composites containing 0, 5.0, 10.0 and 20.0 wt % of PEC

Figure 1. Images of PA solution (a, 4.0 wt % concentration, pH 1.5), CS solution (b, 2.0 wt % concentration, pH 1.5), addition of PA solution into the CS solution and formation of white aggregates (c), collected PEC before drying (d) and obtained PEC after drying (e).

(designated as EVA, EVA/5PEC, EVA/10PEC and EVA/ 20PEC, respectively) were prepared via melt blending at 110 °C in a Thermo Haake Rheomixer (Typ557-8310, Karlsruhe, Germany) for 8 min with a speed of 60 rpm. After this, the composites were transferred to a stainless steel mold, preheated at 110 °C for 5 min, pressed at 10.0 MPa for 8 min, and cooled to room temperature at 10.0 MPa for 10 min. 2.4. Measurements and Characterization. Fourier transform infrared spectra (FT-IR, 4000−400 cm−1) were obtained from KBr pellets using a Nicolet 5700 spectrometer (Thermo-Nicolet, Madison, Wisconsin, USA) with a resolution of 4 cm−1. X-ray diffraction (XRD) patterns were recorded using a Rigaku D/MAX 2550/PC X-ray diffractometer (Cu Kα radiation, λ = 0.154 nm, Tokyo, Japan) with 2θ varying between 5 and 75°. Thermogravimetric analysis (TGA) was performed with a NETZSCH TG 209 F1 thermogravimetric analyzer (Selb, Germany) under nitrogen and air atmosphere. B



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After the samples were kept at 100 °C for 5 min, the samples were then heated up to 600 °C at a heating rate of 20 °C min−1. Microscale combustibility experiments were carried out using a Govmark MCC-2 microscale combustion calorimeter (MCC; combustor temperature, 900 °C; oxygen/nitrogen flow rate, 20/80 mL/mL; Farmingdale, New York, USA) according to ASTM D 7309-2007 (method A). The specimens (in triplicate) were heated from room temperature to 600 °C at a heating rate of 1 °C/s. The limiting oxygen index (LOI) values were determined by using an oxygen index instrument (JF-3, accuracy: ± 0.4%, Nanjing Jiangning Analytical Instrument Factory, Nanjing, China) according to ASTM D 2863. The UL94 vertical burning test was carried out using a horizontal and vertical burning tester (CZF-3, Nanjing Jiangning Analytical Instrument Factory, Nanjing, China) according to ASTM D 3801. Tensile properties were measured at 20 °C using an Instron 3367 universal testing machine (accuracy: ± 0.5%, Canton, Massachusetts, USA) according to ASTM D 638-10. The initial gauge length was 50 mm, and the crosshead speed of was100 mm/min. The Young’s modulus was determined from the slope of the linear regression performed on the initial points on the stress−strain curves. Average values were obtained from at least five independent tests, and the relative errors are also reported. Rheological properties were characterized by using an Anton Paar Physica MCR 301 rotational rheometer equipped with a CTD 450 convection oven (Graz, Austria). The dynamic frequency scanning tests were performed in the range of 0.1− 100 rad s−1 at 110 °C. The morphologies of the char residues were visualized by using a field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Tokyo, Japan, operated at 3 kV) with an energy-dispersive X-ray (EDX) analyzer.

Figure 2. FT-IR spectra of CS, PA and PEC.

Figure 3 shows the XRD patterns of CS, PA and PEC. In the case of PA, there is a broad peak over the range of 20−30°,

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of PEC. In this study, we have designed and prepared a green and renewable PEC from CS and PA as illustrated in Scheme 1, which combines phosphorus, nitrogen and carbon elements simultaneously. To prepare PEC, the PA and CS solutions were first prepared, as shown in Figure 1a,b. Then, the excessive PA solution was added into the CS solution dropwise, which forms white insoluble aggregates and precipitates immediately, as shown in Figure 1c,d. After drying under vacuum, a light yellow solid was obtained, as shown in Figure 1e, which shows similar phenomena to that of CS/sodium carboxymethyl cellulose (CMCNa) PEC.12 The weight ratio of PA/CS is about 0.95:1, and the obtained PEC is virtually insoluble in dilute NaOH solution and organic solvents such as N,N-dimethylformamide and chloroform. Figure 2 presents the FT-IR spectra of CS, PA and PEC. As can be seen, CS shows characteristic absorptions at approximately 2920 and 2873 (stretching vibration of C−H for CH and CH2 group), 1637 (amide I), 1603 (amide II) and 1159 and 897 cm−1 (saccharine structure).8,21 Furthermore, PA exhibits characteristic absorptions at approximately 1637 (stretching vibration of OPO) and 1176 cm−1 (stretching vibration of PO).22 In comparison, a new absorption band appears at 1540 cm−1 for PEC, which is ascribed to the NH3+ vibrational mode.9,23 Meanwhile, PEC also shows absorption band near 1067 cm−1, which belongs to the stretching mode of (PO3)2−.24 Hence, after adding HCl to CS, the NH2 group can be partially protonated, then the ionic complexation between positively charged CS and negatively charged PA can be formed.

Figure 3. XRD patterns of CS, PA and PEC.

indicating its amorphous structure. As a kind of semicrystalline polymer, CS shows typical peaks at around 10° and 20°.10,11 However, the peak at around 10° almost disappears for PEC. Moreover, the peak intensity at around 20° for PEC reduces significantly, indicating the decrease of degree of crystallinity.12 Obviously, such results are attributed to the interactions between CS and PA. When the ionic complexation between CS and PA happens, NH2 groups in CS form complexes with O POH group in PA, which breaks the hydrogen bonding between NH2 and OH groups in CS and results in the amorphous structure for PEC.7 Hence, these results provide further support for the ionic complexation between CS and PA. Figure 4 shows the key information about the thermal degradation behaviors of CS, PA and PEC under nitrogen atmosphere, and the detailed data are listed in Table S1 (Supporting Information). As for the CS, the 5 wt % weight loss temperature (T5%) and maximum weight loss temperature (Tmax) are 271 and 298 °C, respectively, leaving a char residue of 43 wt % at 600 °C. In comparison, the T5% of PEC is 3 °C lower than that of CS, indicating an earlier decomposition catalyzed by PA. However, the Tmax and char residues at 600 °C of PEC are 17 °C and 13 wt % higher than those of CS, respectively, revealing enhancement of thermal stability and char formation triggered by PA. Obviously, in this case, PA works as an efficient acid source, which is due to the produced C



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Figure 4. TGA (a) and DTG (b) curves of CS, PA and PEC under nitrogen atmosphere.

Figure 5. TGA (a, b (magnified curve in the range of 220−420 °C)) and DTG (c, d (magnified curve in the range of 200−400 °C)) curves of EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC under nitrogen atmosphere.

pyrophosphate and polyphosphate at the earlier stage.15,25 But, we should also pay attention to the role of CS. Even at 600 °C, the char residues are still higher than 40 wt %, which can provide adequate carbon and nitrogen elements during the action of PA. Thus, integrating the advantages of CS and PA, PEC alone possesses rich phosphorus, nitrogen and carbon elements, which is a promising candidate for an intumescent flame retardant. 3.2. Characterization of EVA/PEC Composites. 3.2.1. Thermal Properties. TGA was carried out to study the performance of PEC on the thermal degradation behaviors for EVA/PEC systems. Figure 5 presents the TGA and DTG curves of EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC under nitrogen atmosphere, and the detailed data are listed in Table S2 (Supporting Information). As for the pristine EVA,

the thermal degradation mainly takes place in two steps, which are ascribed to the deacetylation process (300−400 °C) and the following decomposition of polyene (400−500 °C), respectively.19,26 Compared to the pristine EVA, an earlier new decomposition process is observed clearly over the range of 200−270 °C for the EVA/PEC systems, which is believed to be due to the catalyzed deacetylation of EVA by the phosphorus containing derivatives from PEC.27 Meanwhile, as the content of PEC increases, the peak value of DTG curve increases gradually, signifying the higher mass loss rate in this first stage.28,29 As a result, a gradual reduction in the T5% is observed as the content of PEC increases. However, it is worth noting that the peak values of DTG curves in the latter two stages decrease gradually as the content of PEC increases, demonstrating the lower loss rate of gaseous D



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Figure 6. Heat release rate curves (a) and PHRR and THR values (b) of EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC.

3.2.3. Tensile Properties. Figure 7 presents the representative stress−strain curves of EVA, EVA/5PEC, EVA/10PEC and

organic fragments such as acetic acid and unsaturated chains.19,28 Accordingly, the char residues from 500 to 600 °C increase as the content of PEC increases, indicative of the enhancement of thermal stability and char formation at high temperature provided by PEC.20 Similarly, under air atmosphere (see Figure S1 and Table S2, Supporting Information), as the content of PEC increases, the peak values of DTG curves in the latter two stages decrease and the char residues from 500 to 600 °C increase gradually, which shows similar phenomenon to that of EVA/ammonium polyphosphate/pentaerythritol system.30 Hence, these results suggest the importance of the earlier degradation induced by this phosphorus-, nitrogen- and carbon-containing PEC, which can accelerate the formation of protective char layer and restrain the mass loss. 3.2.2. Flammability Properties. MCC was employed to analyze the performance of PEC on the flammability properties for EVA/PEC systems. By simulating the anaerobic pyrolysis and a subsequent reaction of the volatile pyrolysis products with nitrogen/oxygen (80/20) gas mixture under high temperatures, parameters related to fire such as peak heat release rate (PHRR) and total heat release (THR) were obtained,31,32 as shown in Figure 6 and Table S3 (Supporting Information). As can be seen, the heat release rate curve of the pristine EVA shows a sharp peak between 400 and 500 °C, and the average PHRR and THR values are 801 W g−1 and 34.1 kJ g−1, respectively. In comparison, the PHRR and THR values of EVA/PEC systems are shown to decrease with increasing the content of PEC, demonstrating increasingly obvious improvement in the flame retardancy.33,34 For example, the average PHRR values of EVA/5PEC, EVA/10PEC and EVA/20PEC show 137, 191 and 249 W g−1 decreases as compared with the pristine EVA, respectively. The LOI test was also used to assess the effect of PEC on the flammability properties for EVA/PEC systems, and the results are shown in Table S3 (Supporting Information). As for the pristine EVA, the average LOI value is 20.0. In comparison, the average LOI values of EVA/5PEC, EVA/10PEC and EVA/ 20PEC are 20.8, 21.2 and 22.9, respectively, indicating the effectiveness of the use of PEC as flame retardant. In addition, during UL-94 vertical burning test, the EVA/PEC system can reach V-2 rating when the content of PEC is 20.0 wt % (see Table S3, Supporting Information). Compared to the earlier results in the literature, the decrease ratio of PHRR value during MCC test and the improvement of LOI value in our work are comparable to those of polyphosphate esters with the content of 30.0 wt % in EVA.35

Figure 7. Representative stress−strain curves of EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC. The inset shows the magnified curves in the range of 0−80%.

EVA/20PEC, and the detailed data are listed in Table S4 (Supporting Information). As can be clearly seen, the stress− strain curves are similar in shape before and after adding PEC, indicating the unmodified overall mechanical performance of the EVA matrix.19,36 Furthermore, the Young’s modulus and peak stress values are shown to increase slightly with increasing the content of PEC. For instance, the average values of Young’s modulus for EVA/5PEC, EVA/10PEC and EVA/20PEC show 1.1, 1.6 and 3.6 MPa higher than that of the pristine EVA, respectively. As for the stress and strain at break, slight decreases are observed after introducing PEC. However, the strain at break for EVA/PEC system is still very high. Even adding 20.0 wt % of PEC, the average value of strain at break is still 1265%. Thus, introduction of PEC not only improves the flame retardancy effectively but also maintains the excellent ductile nature of EVA. 3.2.4. Viscoelasticity Properties. Figure 8 presents the frequency (ω) dependence of the storage modulus (G′) and complex viscosity (η*) for EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC. Compared to the pristine EVA, higher G′ and η* are observed for the EVA/PEC systems in the whole frequency region. What is more, the slopes (k) of plotting log G′ versus log ω in low ω region for EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC are 1.55, 1.29, 1.15 and 0.99, respectively, showing clear dependence on the content of PEC. According E



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Figure 8. ω dependence of the G′ (a) and η* (b) for EVA, EVA/5PEC, EVA/10PEC and EVA/20PEC at 110 °C. The inset shows the k of plotting log G′ versus log ω in low ω region.

Figure 9. Top view SEM images (first row) and EDX spectra (second row) of the char residues for EVA (a, c) and EVA/20PEC (b, d).

to literature reports,37 the decrease of k can be attributed to the growth and formation of network, and the increase of G′ and η* in low ω region can reflect the extent of network formation, which have been observed in carbon nanotube,38 clay,39 nanocrystalline cellulose40 and ultrafine kaolinite41 filled EVA composites. In this work, PEC possesses plenty of OH groups, which tends to create a network structure based on the hydrogen bonding interactions as the concentration of PEC increases.40 Actually, this network structure provides guarantee for the formation of homogeneous and compact intumescent char layer during the burning process, so as to limit the volatilization of gaseous organic fragments effectively. 3.2.5. Analysis of the Collected Char Residues. To clarify the flame retardant mechanism, the representative char residues for the pristine EVA and EVA/20PEC were collected and analyzed by FE-SEM and EDX, as shown in Figure 9. As for the pristine EVA, the char residues are full of long cracks and wrinkles (see Figure 9a). In comparison, the char residues of EVA/20PEC are full and compact, the surface of which are

covered with big bubbles, demonstrating an excellent intumescent effect15,42 (see Figure 9b). EDX analysis (see Figure 9c,d) clearly shows the presence of phosphorus (P) element in the char residues for EVA/20PEC. More interestingly, the relative amount of carbon (C) element is much higher than that of the char residues for pristine EVA, indicating significant carbonization process triggered by the PEC.43 To gain further insight into the flame retardant mechanism, the collected char residues were also analyzed by FT-IR spectroscopy. Figure 10 shows the FT-IR spectra of the representative pristine EVA before flame test and the char residues for the pristine EVA and EVA/20PEC for comparison. It can be clearly seen that the typical absorption bands belong to the ethylene (around 2922, 2852, 1464 and 1371 cm−1) and vinyl acetate (around 1737, 1240 and 1022 cm−1) group44,45 almost disappear for the char residues of pristine EVA and EVA/20PEC, while the band around 1628 cm−1 ascribed to the stretching vibration of C−C for benzene ring46 appears F



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AUTHOR INFORMATION

Corresponding Author

*T. Zhang. Tel. (Fax): +86 571 86843255. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51103129, 51103135), Zhejiang Provincial Natural Science Foundation of China (Nos. LQ14E030008, LY14E030006), the Science Foundation of Zhejiang Sci-Tech University (No. 13012144-Y) and the Open Fund of the Top Priority Discipline of Zhejiang Provincial High Schools (No. 2014YXQN06).

Figure 10. FT-IR spectra of EVA and char residues for EVA and EVA/ 20PEC.

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obviously, indicating the decomposition of EVA and the subsequent carbonization process. In addition, the char residues for EVA/20PEC also show absorption band at approximately 997 cm−1 ascribed to the stretching vibration of P−O−P group, indicating the presence of phosphoric or polyphosphric acid in the char layer.47,48 Therefore, these results rationalize the observed improvement of flame retardancy for the EVA/PEC systems. During combustion, the PEC alone is able to play multiple roles as an acid source, a carbon source and a foaming agent, which are the integrated advantages of both CS and PA. Practically, just these multiple roles facilitate the intumescent process and control the flame spreading process.

4. CONCLUSIONS In summary, we have successfully employed the ionic complexation between positively charged CS and negatively charged PA to prepare a green and renewable PEC. This PEC is shown to be an effective intumescent flame retardant that was able to improve the flame retardancy of EVA. For example, the char residues from 500 to 600 °C for the EVA/PEC systems increase with the introduction of PEC, demonstrating the enhancement of char formation at high temperature. Furthermore, introduction of PEC also effectively reduces the PHRR and THR values of EVA/PEC systems during the MCC test, especially promotes the formation of highly expanded and stable foamed char residues, implying obvious barrier effect during combustion. Moreover, with the action of PEC, the Young’s modulus of the EVA/PEC systems increase slightly, and the excellent ductile nature of EVA still maintains. This study not only presents a new strategy for the design and fabrication of intumescent flame retardant system based on renewable resources, but also finds potential application of PEC in flame retardancy field.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Table of thermal properties of CS, PA and PEC under nitrogen condition, table of thermal properties of EVA/PEC systems under nitrogen and air conditions, figure of TGA and DTG curves of EVA/PEC systems under air atmosphere, table of microscale combustion calorimetry, LOI and UL-94 results for EVA/PEC systems, and table of tensile properties of EVA/PEC systems. This material is available free of charge via the Internet at http://pubs.acs.org. G



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