An Effective Way To Flame-Retard Biocomposite with Ethanolamine

Mar 31, 2015 - In order to improve the flame retardancy of WPC, a single flame retardant of ammonium polyphosphate (APP) modified via ion exchange ...
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An Effective Way To Flame-Retard Biocomposite with Ethanolamine Modified Ammonium Polyphosphate and Its Flame Retardant Mechanisms Ya-Hui Guan, Jian-Qian Huang,* Jun-Chi Yang, Zhu-Bao Shao, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Biocomposite of wood flour (WF)/polypropylene (PP) composite (WPC) is not easily flame-retarded because of the different flame retardant mechanisms of PP and WF. In order to improve the flame retardancy of WPC, a single flame retardant of ammonium polyphosphate (APP) modified via ion exchange reaction with ethanolamine, which is named as ETAAPP, was used to prepare flame-retardant WPC. The flammability was investigated by limiting oxygen index (LOI), UL-94 vertical burning test, and cone calorimeter. The results show the flame retardant properties of the flame-retardant WPC are improved greatly. The limiting oxygen index is 43.0%, which is increased by 71.6% compared with that of WPC with the same content of APP. And the vertical burning test can pass UL-94 V-0 rating. The results of cone calorimeter test show that the heat release rate (HRR) and total heat release (THR) of the WPC with ETA-APP are decreased in comparison with WPC/APP. The flame-retardant mechanism of WPC/ETA-APP system was investigated by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). It is found that the hydroamine groups and phosphate acid in ETA-APP might promote the etherification and dehydration reactions in WPC/ETA-APP, which facilitated the formation of stabile char residue of WPC. Consequently, the flame-retardant efficiency is improved greatly. The flexural properties of WPC/30 wt % ETA-APP increase a lot in comparison with WPC/30 wt % APP, which is because of the better compatibility of ETA-APP with WF.

1. INTRODUCTION Wood flour and plastic composite (WPC) is a kind of biocomposite that combines the favorable performance of both wood flour and plastic.1 Compared to other fillers, the natural wood flour reinforced polymer composites are more environmentally friendly and are widely used in transportation, military applications, building and construction industries, packaging, consumer products, etc.2−6 As organic materials, polymers and wood flour are very sensitive to flame. Thus, the improvement of the flame retardancy of composite materials has become important for complying with the safety requirements of WPC products. But some key problems still exist concerning improving the flame-retardant efficiency of WPC. First, the poor compatibility of flame retardants with WF deteriorates not only the flame-retarding effect but also the mechanical properties of the biocomposite. Furthermore, polymers used in WPC have higher fire hazard properties than wood because plastic has a higher chemical heat content and can melt, which may lead to a risky scenario.7 Lastly, WPC is not easily flameretarded because wood flour and PP have different flame retardant mechanisms. For environmental concerns, halogen-free flame retardation has attracted great attention in recent years because halogencontaining flame-retardant materials can produce a lot of smoke and toxic gases during the combustion process.8,9 Among the halogen-free flame retardants, the intumescent flame retardants (IFRs) have been increasingly used to retard the flammability of WPC. An IFR additive is actually a system that consists of an © XXXX American Chemical Society

acid source, a blowing agent, and a charring agent or char. To flame-retard wood or biocomposites, no charring agent seems to be necessary, as the heteroatoms of cellulose and the aromatic rings of lignin act as charring agents for residue formation.10 However, the flame retarding efficiency of the flame-retardant WPC without additional charring agents is not good enough. Nevertheless, as we all know, the preparation processes of carbonization agents are very complicated,11,12 and the solvent is harmful to the environment. These carbonization agents also have many drawbacks such as moisture absorption, exudation, and incompatible with matrix.13 So the ideal pathway is to find some new IFRs that are composed of a single compound that gathers all the advantages of acid source, carbonization agent, and blowing agent. APP derivatives should be the best candidate, since APP gathered two advantages of acid source and blowing agent. Ammonium polyphosphate (APP, (NH4)n+2PnO3n+1), as a common and popular inorganic phosphorus/nitrogen flame retardant (FR), has been used in biocomposite extensively because of low cost, low toxicity, and higher thermal stability.14−16 One of the most important modifications of APP is microencapsulation with melamine− formaldehyde resin, urea−formaldehyde resin, polyurethane, or epoxy resin shell, etc. They had been studied in detail, and the Received: January 12, 2015 Revised: March 21, 2015 Accepted: March 23, 2015

A

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Results of the LOI Test and UL-94 Testa UL-94 formulation WPC WPC/10 wt WPC/20 wt WPC/25 wt WPC/30 wt WPC/30 wt PP/30 wt %

% ETA-APP % ETA-APP % ETA-APP % ETA-APP % APP ETA-APP

PP (wt %)

WF (wt %)

ETA-APP (wt %)

APP (wt %)

LOI (%)

dripping

ratingb

32.5 29.0 25.5 23.7 22.0 22.0 70.0

60.0 53.5 47.0 43.8 40.5 40.5 0

0 10.0 20.0 25.0 30.0 0 30.0

0 0 0 0 0 30.0 0

19.0 22.0 25.5 28.0 43.0 29.0 30.5

little no no no no no yes

NR NR NR NR V-0 V-0 NR

a The content of PP-g-MA, lubricant (PETB), and anti-oxygen (1010) in WPCs was 4.0, 3.0, and 0.5 wt %, respectively. bNR: no rating in UL-94 test.

waterproofing of APP particles was improved largely.17 Qu et al. found that microencapsulated ammonium polyphosphate with silicon-containing compounds could also lead to a decrease in the particle’s water solubility.18 Another important application of APP is combining APP with carbonization agent and blowing agent to obtain IFR. Li and He added the mixture of APP and pentaerythritol (PER) into biocomposite.19 The results showed that the LOI value was 26.1% when the content of the mixture was 28 wt %, and the mixture of APP and PER obviously decreased the mechanical properties of LLDPE-WF composites. Le Bras et al. dealt with the effect resulting from an intumescent PP/fiber/APP/PER/melamine system.20 The result showed that the flame retardancy of the composite with APP/PER/melamine system was not better than APP alone. Zhang et al. investigated the mechanical properties and flame retardancy of wood-fiber/PP/APP/silica composites.22 The LOI reaches 27.9% when the addition of APP is 40 phr. Whereas when APP and silica are 20 and 10 phr in the composite, respectively, the LOI improves a little. However, the mechanical properties’ obvious decrease can be observed with more than 4 phr silica. Schartel et al. explored the flame retardant properties of PP flax compounds containing APPbased fire retardant which is a mixture of APP comprising around two-thirds and different synergistic materials.23 The results showed that when the contents of APP based fire retardant were 30 wt %, the LOI values were 26%. The literature shows that the flame-retardant efficiency and mechanical properties of WPC are not ideal when APP was combined with other carbonization sources and/or blowing sources. To explore an effective way to flame-retard the biocomposite, a single compound of APP modified via ion exchange reaction with ethanolamine according to our previous work was adopted. Ammonium polyphosphate (APP) was modified from inorganic phosphorus/nitrogen FR into organic phosphorus/nitrogen IFR by ion exchange reaction with ethanolamine. The modified APP (ETA-APP) is a single compound that gathers all the advantages of acid source, carbonization agent, and blowing agent. It also displays better compatibility with wood flour because wood flour and ETA-APP all contain a large number of −OH groups. The flame retardancy of WPC/ ETA-APP was investigated by limiting oxygen index (LOI), UL-94, and cone calorimeter tests. Meanwhile, the thermal stability of WPC/ETA-APP composites was investigated by TGA. A possible FR mechanism of WF/PP/ETA-APP composite was proposed according to the results of TGA, FTIR, and XPS. And the mechanical properties were also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene (PP, T30S) was purchased from Lanzhou Petroleum Chemical Co., China. Maleic anhydride grafted polypropylene (PP-g-MA, CMG5001) was supplied by Shanghai Rizhishen New Technology Co., Ltd. The lubricating agent (PETB) was purchased from Chengdu HuiCheng New Materials Co., Ltd. Antioxygen (1010) was purchased from Chengdu TongLi Additives Co., Ltd. The wood flour (WF, pine, 80 mesh) was supplied by Qingdao Fumulin Wood Plastic Composite Co., Ltd. China. Commercial APP (type I) was supplied by Changfeng Fire Retardants Co., Ltd. (Sichuan, China). Ethanolamine (AR) and ethanol (AR) were purchased from Kelong Chemical Reagent Co., Ltd. (Sichuan, China). ETA-APP was synthesized according to the procedure reported previously.21 2.2. Sample Preparation. All samples shown in Table 1 were prepared in a Banbury mixer (SX300) at 180 °C and 50 rpm for about 7 min and then pressed on a curing machine at 190 °C for 3 min to form sheets. The wood flour was dried at 100 °C in an air-circulating oven for 24 h. ETA-APP and APP were dried in the oven at 80 °C for 8 h. 2.3. Measurements. The LOI value was tested on an HC2C oxygen index meter (Jiangning, China) according to ASTM D2863-97, and the size of the sample is 130 mm × 6.5 mm × 3.2 mm. The UL-94 vertical burning level was tested on a CZF2 instrument (Jiangning, China) according to ASTM D3801, and the dimension of the sample is 130 mm × 13 mm × 3.2 mm. The flammability of the sample was measured with a cone calorimeter (Fire Testing Technology). The samples with a size of 100 mm × 100 mm × 3 mm were exposed to a radiant cone at an external heat flux of 50 kW m −2. Thermogravimetric analysis (TGA) was carried out on a TG 209 F1 (NETZSCH, Germany) thermogravimetric analyzer from 40 to 700 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere with a flowing rate of 50 mL min−1. Fourier transform infrared spectra (FTIR) were obtained using a Nicolet FTIR 170 SX spectrometer. The samples were heated to the corresponding temperature in TG 209 F1 (NETZSCH, Germany) at a heating rate of 40 °C min−1 under a nitrogen atmosphere and maintained for 10 min. X-ray photoelectron spectroscopy (XPS) of the condensed products of the IFR-WPC system at different temperatures was recorded by a XSAM80 (Kratos Co., U.K.), using Al Kα excitation radiation (hν = 1486.6 eV). The samples were heated to the corresponding temperature in TG 209 F1 (NETZSCH, Germany) at a heating rate of 40 °C min−1 under a nitrogen atmosphere and maintained for 10 min. B

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scanning electronic micrograph (SEM) observed on a Philips XL-30S was used to investigate the char residues obtained after combustion by cone calorimeter tests and cryogenic fracture surfaces of the composites. SEM graphs were recorded after gold coating surface treatment. The flexural property was measured by a CMT4104 tester according to GB/T 9341-2008 at the speed of 20 mm min−1 at room temperature. The sizes of the sample are 150 mm × 10 mm × 4 mm. The Notched Izod impact property was measured by Notched Izod impact instrument (ZBC1400-2) according to GB/T 1843-2008 at room temperature. The sizes of the samples are 80 mm × 10 mm × 4 mm.

3. RESULTS AND DISCUSSION 3.1. LOI and UL-94 Tests. In order to investigate the flame retardance of the WPCs, the LOI tests and vertical burning tests (UL-94) were carried out at the room temperature. The presence of 30 wt % ETA-APP in the WPCs with different content of WF enhanced their flame-retardant properties. The LOI value increases with increasing WF; when the weight ratio of WF in WPC is 60 wt %, the highest LOI value is achieved (Supporting Information Table S1). Table 1 shows the results of the LOI and UL-94 tests of WPCs with 60 wt % WF and different content of ETA-APP. When the flame retardant is incorporated in WPC, the weight ratio of WF and PP is decreased together under the ratio of WF to PP in WPC. The following results in this paper are based on WPC/30 wt % ETA-APP system in which the weight loading of WF in WPC is 60 wt %. As a comparison, we also tested the LOI and UL-94 of WPC/30 wt % APP and PP/30 wt % ETA-APP Table 1 shows that WPC is a highly flammable material with a low LOI value (19.0%) and no rating in UL-94 test by burning out with little melt-dripping after ignition. However, the flame retardance could be improved when ETA-APP or APP is incorporated. It could be concluded from Table 1 that with the content of ETA-APP increasing, the LOI values of the WPCs/ETA-APP increase greatly. When ETA-APP is 30 wt %, the best flame retardance could be achieved: LOI value of the composite reaches 43.0%, and a V-0 classification at the thickness of 3.2 mm has been achieved. However, when the filling amount of ETA-APP decreases to 25 wt %, the LOI value decreases greatly, suggesting that 30 wt % of ETA-APP should be ideal. As a comparison, the LOI of WPC/30 wt % APP is 30.0%, which is much lower than WPC/30 wt % ETA-APP. Obviously, ETA-APP is much more effective for promoting the LOI value of WPC than APP. It is notable that PP/30 wt % ETA-APP has a LOI value (30.5%) which is much lower than WPC/30 wt % ETA-APP and no rating in UL-94 test by burning out with heavy melt-dripping after ignition. The reason for this interesting phenomenon is that some interaction may exist between ETA-APP and WF which improved the flame retardance of WPC. 3.2. Cone Calorimetric Analysis. The cone calorimetry is an effective method to evaluate the flammability of materials. In order to investigate the effect of 30 wt % ETA-APP on the flame retardancy of WPC, cone calorimeter test was performed. The heat release rate (HRR), the peak of HRR (PHRR), and the total heat release (THR) are used to evaluate flame retardancy of WPCs. Figure 1 and Table S2 show the results of cone calorimeter of the flame-retardant WPCs at an incident heat flux of 50 kW m−2.

Figure 1. Heat release rate (HRR) curves of WPC, WPC/30 wt % APP, and WPC/30 wt % ETA-APP.

Figure 1 shows the representative HRR versus time for WPCs with and without flame retardants. The HRR of WPC without flame retardant shows the typical shape of a wooden sample, with one peak at the beginning, and sustains above 250 kW m−2 for about 130 s.24 PHHR value of WPC without flame retardant is 296 kW m−2, while WPC/30 wt % APP and WPC/ 30 wt % ETA-APP have much lower PHRR of 202 and 176 kW m−2, respectively. As Figure 1 illustrates, WPC/30 wt % APP shows two peaks. The first peak is at the beginning (202 kW m−2) and then is followed by the second peak (171 kW m−2). This is probably because of the worse quality of char layer developed. The gases break through the char, which makes the HRR increase significantly. WPC/30 wt % ETA-APP shows only one peak at the beginning (176 kW m−2) and then decreases gradually (Figure 1). The data reveal that either APP or ETA-APP could restrain the combustion of WPC. However, when APP was modified via ion exchange reaction with ethanolamine to flame-retard WPC, the flame-retarding effect is even better. This is mainly because the ETA-APP acts with a condensed-phase mechanism and the char formed is stronger. From Table S2, it can be seen that THR of WPC/30 wt % ETA-APP decreases more than that of WPC/30 wt % APP. At the end of burning, neat WPC releases a total heat of 53 MJ m−2, while WPC/30 wt %APP and WPC/30 wt % ETA-APP release 39 and 34 MJ m−2, respectively. Compared with neat WPC, the decreasing in THR for WPC/30 wt % ETA-APP is larger than that of WPC/30 wt % APP at the same burning range, indicating that APP modified via ion exchange reaction with ethanolamine could reduce the THR of the WPC. The lower THR value is because ETA-APP can prohibit the decomposition of WPC and form a continuous char layer on the surface which acts as a barrier of heat and volatilization of decomposition products. Obviously, the char residue of WPC/ 30 wt % ETA-APP (47.0%) is much higher than that of WPC (23.2%) which is proof of the above conclusion. All the results above show that ETA-APP is able to improve the flame retardant properties of WPC greatly. In order to clarify the morphology of the char formed during combustion, the outside surface of char residues was studied by SEM. The SEM micrographs of the char residue after cone calorimeter test for different systems are shown in Figure 2. The char residue of WPC/30 wt % APP (Figure 2(a-1, a-2)) is continuous with some holes and flocculent structures on the surface. This is because the cross-linked polyphosphoric acid formed on heating could provide a stable softened glassy coating on the surface of the polymer, but it is not compact C

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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WPC decomposes in two decomposition steps which are related to the decomposition of WF and PP, respectively. Table 2 summarizes the characteristics for the two decomposition steps. It can be seen that Tonset of WPC is 281 °C and the maximum decomposition temperatures are 367 and 482 °C, respectively. The initial decomposition temperature of WPC/ 30 wt % ETA-APP is 256 °C, which is lower than that of WPC/ 30 wt % APP. Addition of ETA-APP to the WPC accelerates the first step of decomposition greater than APP. It is because ETA-APP can promote the decomposition of wood flour in WPC and promote the char formation which can protect the matrix from heat and decomposition. Char formation is a basic aspect of IFR additives because the char reduces the combustion rate of polymeric materials by not allowing the oxygen to easily reach the combustion zone.25 The presence of ETA-APP or APP decreased T1peak of the wood flour in WPC from 367 °C to 282 °C and 307 °C, respectively. This also indicates that ETA-APP can promote the degradation of wood flour greater than APP. The char residue of WPC is 12.1 wt % at 700 °C, whereas the char residues of the WPC with ETA-APP or APP are 34.7 and 27.5 wt %, respectively. This indicates that ETA-APP might interact with WPC especially WF which can effectively promote char formation of the composites. The intumescent char layer effectively protects the underlying substrate from the attack of heat. To investigate if there is some interaction between ETA-APP and WF, the experimental and the theoretical TG and weight loss rate curves of WF/ETA-APP (7:3 in weight) are studied as shown in Figure 3. The calculated data of WF/ETA-APP are a linear combination of the TG curves of the individual components of the mixture, which are representative of a noninteracting phenomenon. The data of thermal degradation are shown in Table 2. As can be seen from the figure, the experimental and the theoretical TG curves of WF/ETA-APP (7:3) are very different. The amount of residue char of WF/ ETA-APP (7:3) is always lower than the theoretical data until 380 °C, while the degradation rate of the mixture is basically lower than the theoretical rate when the temperature was higher than 350 °C. This phenomenon shows that a more steady structure was formed in the low temperature zone, owing to the reaction between WF and ETA-APP, which could enhance the thermal performance of the WPC/ETA-APP composite in the comparatively high temperature zone. For a further study on the chemical interaction between WPC (especially WF in the WPC) and ETA-APP, FTIR tests at different temperatures were carried out. Figure 4b shows the FTIR spectra of the condensed products of WPC/30 wt % ETA-APP at room temperature, 256 °C (Tonset), 282 °C

Figure 2. SEM micrographs of residual char after cone calorimetry test for (a-1, a-2) WPC/30 wt % APP and (b-1, b-2) WPC/30 wt % ETAAPP.

enough. So the decomposed gases easily permeate through the char layer. However, the intumescent char of WPC/30 wt % ETA-APP shown in Figure 2(b-1, b-2) is continuous and compact with a glassy film covering the char layer, which is helpful for improving the quality of char layer. Then the char layer is expanded by the blowing effect of noncombustible gases during burning. 3.3. Flame-Retardant Mechanism. To investigate the interaction between WPC and ETA-APP during thermal degradation, the TG and DTG curves of WPC/30 wt % ETA-APP are studied as shown in Figure 3.

Figure 3. TGA and DTG curves of WPC,WPC/30 wt % APP, WPC/ 30 wt % ETA-APP, WF/ETA-APP (7:3)exp (WF/ETA-APP as 7:3 in weight), and WF/ETA-APP (7:3)cal in N2.

Table 2. Thermal Degradation Data of WPC, WPC/30 wt % APP, WPC/30 wt % ETA-APP, WF/ETA-APP (7:3)exp (WF/ETAAPP as 7:3 in Weight), and WF/ETA-APP (7:3)cal in N2 sample

Tonset a (°C)

Tmax1 b/Rmax1 c (°C/% min−1)

Tmax2 b/Rmax2 c (°C/% min−1)

char residue (%) at 700 °C

WPC WPC/30 wt % APP WPC/30 wt % ETA-APP WF/ETA-APP(7:3)expd WF/ETA-APP(7:3)cale

281 261 256 253 267

367/−12.4 307/−7.8 282/−8.4 310/−14.4 371/−12.3

482/−17.9 498/−21.5 495/−15.6 516/−3.8 585/−3.4

12.1 27.5 34.7 37.8 22.5

a

Tonset, the temperature at which mass loss is 5 wt %. bTmax, the temperature at which the mass loss rate is maximum. cRmax, the maximum mass loss rate value. dWF/ETA-APP (7:3)exp, the experimental result of WF/ETA-APP (WF/ETA-APP as 7:3 in weight). eWF/ETA-APP (7:3)cal, the computational result of WF/ETA-APP (WF/ETA-APP as 7:3 in weight). D

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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species is the main reason for the increase of residue, which is observed in the TGA test. Furthermore, the residue is stabilized and shows increased stability during thermo-oxidative decomposition. This might be an important reason for the better flame retardancy of ETA-APP than APP. X-ray photoelectron spectroscopy (XPS) is utilized to study the atomic concentration of the condensed products of WPC/ 30 wt % ETA-APP, which were obtained at the typical thermal degradation temperatures of 256 °C (Tonset), 281 °C (Tmax1), 495 °C (Tmax2), and 600 °C, repectively. Figure 5 shows the Figure 4. FTIR spectra of condensed products of WPC/30 wt % APP and WPC/30 wt % ETA-APP at different temperatures in N2.

(Tmax1), and 495 °C (Tmax2) as suggested in Figure 3. As a comparison, we also give the FTIR spectra of the condensed products of WPC (Supporting Information Figure S1) and WPC/30 wt % APP at the same temperatures (Figure 4 a). From Figure 4, the peaks of NH4+ located at 3400−3030 cm−1 gradually decreased and disappeared, indicating the occurrence of the thermal decomposition of NH4+ in APP and ETA-APP. Actually, the O−H stretching peak of cellulose is also located at ∼3300 cm−1, indicating that the H2O vapor is decomposed from the WF with the increasing of temperature (Supporting Informationy Figure S1). In Figure 4 and Figure S1, a very strong band at 2950 cm−1 is the absorption of C−H symmetric and asymmetric stretching of aliphatic chain, which disappears when the temperature reaches near 300 °C, indicating the decomposition of aliphatic chain. Figure 4 a shows that the structure of P−N located at around 714 cm−1 becomes more obvious at 261 °C for APP system, while the P−N for ETAAPP becomes more obvious at around 710 cm−1. It can be observed that the peak located at 1532 cm−1 corresponding to ETA salt (NH3+−CH2−) is observed at 256 °C for WPC/30 wt % ETA-APP and disappears when the temperature reaches 495 °C. At 256 °C, the peaks of P−O−C (aliphatic) located at 766 and 993 cm−1 are also observed. The plentiful P−O−C structure should be formed in the reaction among the hydroxies of wood flour, NH 3 + −CH 2 −CH 2 −OH and unreacted NH4+O−−P in APP. Meanwhile, the peaks of P−N−C are observed at 1104 and 710 cm−1 with the temperature increased, so more of the stable residue is formed when the temperature is increased to 495 °C. In addition, the absorptions at 2927 and 2850 cm−1 disappear gradually from 281 to 495 °C, and the absorption at 1643 cm−1 becomes wider, which means that some structures containing −CC− appear.28 For WPC/30 wt % APP system, the peaks at around 1150−900 cm−1 may correspond to the vibration of P−O. The formation of P−O−P, etc. contributes to the flame retardation of APP. Nevertheless, for WPC/30 wt % ETA-APP, these peaks at around 1150−900 cm−1 are ascribed to the vibration of P−O, C−N, and C−O. During heating, the peaks at 2927 and 2850 cm−1 vanish and the peaks of PO (1250 cm−1), P−O (1088, 883 cm−1) become obvious. PO and P−O are attributed to the formation of polyphosphates during the thermal decomposition of ETA-APP.28 The results indicate that the stable char residues consisted of P−N−C, P−O−C, −CC− and aromatic structures are formed during combustion process of WPC/ ETA-APP. Generally, the P−O−C and P−N−C structures can also improve the stability of the residue, and the P−O−C and P−N−C cross-linked structures can further facilitate the formation of char residue by aromatization under high temperature (around 500 °C).26,27 Cross-linkage of the residue

Figure 5. C 1s, N 1s, O 1s, and P 2p X-ray photoelectron spectroscopy spectra of the condensed products of WPC/30 wt % ETA-APP at different temperatures.

XPS spectra of C 1s, N 1s, O 1s, and P 2p of WPC/30 wt % ETA-APP at different temperatures. The C1s spectra of WPC/ 30 wt % ETA-APP are shown in Figure 5a. At 256 °C, the peaks at 284.5 eV are assigned to C−H and C−C of aliphatic and aromatic species in the residue. The bands around 285.0 eV are the contributions of C−O−C, C−OH, C−N, and C−O− P.29 Interestingly, a new peak around 286.3 eV appears at 495 °C, which could be ascribed to the bonds of CO, CC, and CN,29,30 indicating that more steady cross-links are further formed by aromatization at relatively high temperature. These results are in accordance with the FTIR spectra of the condensed products. Figure 5b shows the N1s spectra of WPC/ 30 wt % ETA-APP. There are two peaks in this figure; one peak around 399.5 eV is the contribution of CN, and the other peak around 401.5 eV is assigned to the N−H bond of NH4+.31,32 The O1s spectra are presented in Figure 5c. The peak at 531.6 eV can be assigned to double binding O in phosphate and carbonyl groups (CO, PO). The other peak around 533.0 eV can be assigned to −O− in C−O−C, P− O−C, P−O−P, and/or C−OH groups.29,31,32 The relative intensity of 533.0 eV rises at high temperature, demonstrating the reaction between wood flour/PP composite and the phosphoric acid(s). The P2p spectra of WPC/30 wt % ETAAPP are shown in Figure 5d. The peaks between 134 and 135 eV can be assigned to P−O−C and/or PO3− groups in the phosphorus-rich cross-links.32,33 The XPS data for the condensed products of WPC/30 wt % ETA-APP at different temperatures are presented in Table S3. In detail, the C content decreases significantly from 88.26 wt % at 256 °C to 35.15 wt % at 600 °C, which is due to the incomplete degradation of PP, wood flour, and ETA-APP. The content of O increases from E

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Possible pyrolysis mechanism of biocomposite of WPC/30 wt % ETA-APP.

retardancy properties of WPC with the same content of WF and ETA-APP are better, so we chose this system as the object of our study. Figure 7a shows the flexural properties of WPC,

9.27 to 43.92 wt % when the temperature is increased from 256 to 600 °C, which might be due to the formation of the aldehydes, ether, alcohol, ketone, etc. after the degradation of wood fiber and the relative decrease of the content of C. Compared with the change of O and C contents, the change of N content is not obvious. At first it increases from 0.57 to 1.66 wt % when the temperature is increased from 256 to 495 °C, because of the formation of P−O−N, P−C−N and P−N bonds, and then decreases slightly with the degradation of part of the char residue. The content of P keeps increasing from 1.90 to 19.70 wt % when the temperature is increased from 256 to 600 °C, which should be due to the formation of P−O, P− O−C, P−C−N, and P−N bonds during the thermal decomposing process. The results of XPS are consistent with that of FTIR analysis of condensed products. According to the results of TG curves, FTIR spectra, and XPS, a potential char-forming mechanism of WPC/30 wt % ETA-APP is shown in Figure 6. The adsorbed water in wood flour is probably eliminated first when the biocomposite was heated. Taking cellulose in wood flour for example, the phosphoric acid produced by ETA-APP catalyzes the cellulose to degrade earlier in the initial stage. The chain scissions of the cellulose take place with the formation of several smaller molecules. At the same time, accompanied by the release of NH3 and H2O in the decomposition of ETA-APP, some stable compounds containing P−N−C, P−O−C, etc., which can react with the hydroxyls of the cellulose to form a stable carbon layer, are formed. With increasing the temperature, part of P−N−C and P−O−C decomposes under the catalysis of P−OH, accompanied by the release of H2O and NH3. The compact and stable char layer consisting of P−N−C, P−O−C, CN is foamed by the gases to form an intumescent layer which is heat insulation and oxygen barrier. Finally, the excellent flame retardant performance of ETA-APP was achieved because of the presence of the barrier formed. Cross-linking also occurs in lignin, hemicelluloses, and polyose, but for better clarity, Figure 6 only shows cellulose. 3.4. Mechanical Properties. The flexural modulus of WPC increases sharply with increasing content of WF, and the flexural strength decreases with increasing content of WF (Supporting Information Figure S2). The changes of flexural modulus and flexural strength of WPC are irregular with increasing content of fire retardant when the content of WF is the same (Supporting Information Figure S2). But the flexural properties of WPC with 60 wt % WF and 30 wt % ETA-APP are comprehensively better. At the same time, the flame

Figure 7. (a) Flexural strength, flexural modulus, and (b) impact strength of WPC, WPC/30 wt % APP, and WPC/30 wt % ETA-APP.

WPC/30 wt % APP, and WPC/30 wt % ETA-APP. Despite the presence of the compatibilizer, the addition of APP shows a negative impact on the flexural properties of the composites. The flexural strength of WPC/30 wt % APP system decreases about 20% compared to that of WPC. This could be attributed to the poor compatibility of APP with polymer. The other reason for the decrease of flexural property is the existence of the cavities within the samples. Deterioration of the mechanical properties of the filled and unfilled plastics with the addition of flame-retardants has been reported by some researchers.34,35 However, the flexural modulus and the flexural strength of WPC/30 wt % ETA-APP system decrease about 8.6% compared to that of WPC. Figure 7b shows the notched impact strength of WPC, WPC/30 wt % APP, and WPC/30 wt % ETA-APP. Adding APP shows a little negative effect on the notched impact strength of WPC. However, adding ETA-APP shows a positive effect on the impact strength of WPC. This may be because of the good dispersion of ETA-APP in the composite, which leads to the increasing of the mechanical properties of WPC. In order to differentiate the particle dispersion in WPC, we investigated the cryogenic fracture surface of WPC, WPC/30 wt % APP, and WPC/30 wt % ETA-APP by SEM. The surfaces of APP (Figure 8a) showed APP particles with clean surface. After being modified, the surfaces of ETA-APP particles (Figure 8b) are rough, and it seems that some particles clung to each other. From Figure 8c it is easy to observe the close contact between WF and matrix. WF is embedded in the matrix with no gap between them. This is proof of the strong adhesion that appeared at the contact zone between the polymer and WF due F

DOI: 10.1021/acs.iecr.5b00123 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

promote the formation of a stable carbon layer. The good compatibility of WF with ETA-APP improves the mechanical property of flame-retardant WPC.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of condensed products of WPC at different temperatures in N2, displayed in Figure S1; flexural modulus and flexural strength of WPC with different content of wood flour and different content of ETA-APP, displayed in Figure S2; results of the LOI test and UL-94 test of WPC/30 wt % ETAAPP with different content of wood flour, listed in Table S1; data obtained from cone calorimeter test of neat WPC, WPC/ 30 wt % APP, and WPC/30 wt % ETA-APP, listed in Table S2; XPS data of the condensed products of WPC/30 wt % ETAAPP at different temperatures, listed in Table S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. SEM micrographs of (a) APP particle, (b) ETA-APP particle, and the cryogenic fracture surfaces of (c) WPC, (d) WPC/30 wt % APP, and (e) WPC/30 wt % ETA-APP, respectively. WF is marked by arrow, and flame retardants are marked by circle.

AUTHOR INFORMATION

Corresponding Authors

*J.-Q.H.: e-mail, [email protected]; tel, +86-28-85410755; fax, +86-28-85410259. *Y.-Z.W.: e-mail, [email protected]; tel, +86-28-85410755; fax, +86-28-85410259.

to the hydrogen interactions shown previously by FTIR. Some studies in the literature have shown that when adhesion was not good, there were voids around the WF, and WF pull-out could be observed.36,37 The surface of WPC/30 wt % APP (Figure 8d) shows randomly scattered APP particles with clean surface, and some of the particles even agglomerated in the fracture surface of WPC (marked by circle in Figure 8d) and some voids around the WF (marked by arrow in Figure 8d). This indicates that APP particles can be pulled out easily from the WPC matrix by breaking the interface because of the poor adhesion between APP and the WPC matrix, thus decreasing the mechanical performance of WPC (Figure 7). From Figure 8e it can be seen that there is no obvious aggregation of ETA-APP particles. WF (marked by arrow in Figure 8e) is embedded in the matrix with no gap between them, and the ETA-APP (marked by circle in Figure 8e) disperses uniformly in the matrix without clear surface. Furthermore, WF is coated with the matrix and the failure occurs in the matrix and not at the filler’s surface. Hence, we confirm the good interaction between WF and matrix containing ETA-APP for our composites. This is why the mechanical properties of WPC with ETA-APP are improved compared with WPC/30 wt % APP.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (Grants 50933005, 51121001), the Excellent Youth Foundation of Sichuan (Grant 2011JQ0007), and Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT1026).

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4. CONCLUSIONS Ammonium polyphosphate (APP) modified via ion exchange reaction with ethanolamine was incorporated into the biocomposite (WPC) to prepare the flame retardant biocomposite. The effect of the new flame retardant (ETAAPP) on flame retardant properties of WPCs was investigated. The fire performance of flame-retardant WPC with ETA-APP is improved significantly. WPC containing 30 wt % ETA-APP could not only give a high LOI value of 43.0% and UL-94 V-0 rating without dripping but also decrease its heat release rate obviously. ETA-APP can change the thermal degradation behavior of WPC, especially WF. It induces the degradation of WPC and reduces the Tmax1 of the decomposition peaks of WPC and then promotes WPC to form char. The char residue of WPC/30 wt % ETA-APP reaches 34.7 wt % at 700 °C based upon TGA test. There are some interactions between the decomposed products of WF and ETA-APP, which could G

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