Benzoxazine Laminates and

Dec 4, 2014 - Zhejiang Textile & Fashion Technology College, Ningbo 315211, China. §. MOE Key Laboratory of Macromolecular Synthesis and Functionaliz...
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Flame-Retarding Modification for Ramie/Benzoxazine Laminates and the Mechanism Study Hongqiang Yan,† Huaqing Wang,‡ and Zhengping Fang*,†,§ †

Lab of Polymer Material and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China Zhejiang Textile & Fashion Technology College, Ningbo 315211, China § MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Institute of Polymer Composites, Zhejiang University, Hangzhou 310027, China ‡

S Supporting Information *

ABSTRACT: Flame retardant ramie/benzoxazine resin laminates were prepared and modified by ammonium polyphosphate (APP) and a nitrogen−phosphorus flame retardant (NEWRAY911). The results showed that the limiting oxygen index (LOI) values of the ramie/benzoxazine laminates could be ameliorated by the addition of APP, but did not achieve V0 level in UL94 test. Their mechanical strengths were also partially damaged. To improve the flame retardancy and reduce the mechanical damage to laminates, ramie fabric was modified by NEWRAY911 before compounding. The LOI value of the laminate made by flame retardant ramie fabric could be increased to 44.8%, and the laminates achieved V0 level in UL94 test. Compared to the unmodified laminates, the flame retarding modification of the fabric helped to improve the mechanical properties of laminates. The mechanism of flame retardancy of these laminates was investigated further by thermogravimetric analysis coupled with infrared spectroscopy, microscale combustion calorimetry measurements and scanning electron microscopy. renewable raw materials.13 Therefore, natural materials including cellulose-rich fibers extracted from natural fibers and their polymer composites have regained interest in the last few decades, because of their low cost, low density, good mechanical properties and ample supply of fibers from renewable resources.14 Ramie fiber, which comes from the phloem tissue of the plant known as “china grass”, is one of the oldest textile fibers.15 It has a significantly smaller diameter (10−60 μm) and consequently a higher aspect ratio than sisal fiber (200−300 μm in diameter) of the same length. In addition, its white color might also fulfill the requirement of denture base for esthetic appearance. The mechanical properties of ramie fiber are also the best in hemp fibers. Compared to glass fiber, ramie fiber has a low specific density (1.5 g/cm3) and a reasonably high tensile modulus (61.4−128 GPa), making it a potential alternative to manmade fibers in fiber reinforced-polymer composites.16 Hence, ramie fiber has been studied and applied in fiber-reinforced composites, such as biodegradable plastics including poly(lactic acid) (PLA) and polycaprolactone (PCL) and thermosetting resin including phenolic resin and epoxy resin, etc.17−20 However, because ramie fiber is flammable, a flame retardant is necessary to be applied before the ramie fiber-reinforced composites can be used in most applications. In recent years, ammonium polyphosphate (APP) has been used to improve the flame retardancy of polymer materials to replace the conventional halogen flame retardants.21−25 When heated, APP releases phosphoric acid and ammonia; therefore,

1. INTRODUCTION Fiber-reinforced composites with superb thermal, dielectric and mechanical properties are highly desirable for automobile manufacture, aerospace industry and high-speed printed circuit board fabrication.1 Initially developed for aerospace structures, these high-performance or “advanced” composites are now found in applications from automotive parts to high-performance sporting goods where their properties per unit mass are critical for their performance. The most recent trend is to use advanced composites in civil structures in applications including bridges, wind turbines, and earthquake-resistant buildings. Clearly, there is a great demand for composite structures at present, and there will be additional demand for these materials in the near future.2 Fiber-reinforced polymer composites are usually constructed from high performance matrix resins, such as epoxies, cyanate esters, polyimides, bismaleimides and phenolic resins, and reinforcing fiber materials, such as glass fiber, carbon fibers and natural fibers.3−5 It is well-known that benzoxazine resin, a type of phenolic resin, has unique properties, such as dimensional stability, nearzero shrinkage, in addition to good heat resistance, water resistance, electrical insulation and flame resistance.5−9 Furthermore, benzoxazine resins possess excellent mechanical properties and molecular design flexibility. Their mechanical properties are much better than those of traditional phenolic resins. Hence, benzoxazine resins are generally used as substitutions of traditional phenolic resins in high performance fiber-reinforced composites. Many fiber-reinforced benzoxazine composites have been studied.1,10−12 Nevertheless, increasing pressure from environmental activists, preservation of natural resources and attended stringency of laws passed by developed countries leads to the invention and development of natural materials with a focus on © XXXX American Chemical Society

Received: October 15, 2014 Revised: November 29, 2014 Accepted: December 4, 2014

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it is often used in intumescent flame retarding systems to act as an acid source and gas source.26 Various forms of phosphorus and nitrogen based flame retardants have been developed and used in flame retardant finishing of cellulosic fabrics for a long time, but there are a only few of them that have been commercially successful.27 However, there is renewed interest due to increasing demand for safety and environmental protection standards on flame retardants.28,29 In our recent study,30 a flame retardant (NEWRAY911), which is a composite material containing phosphorus (13.0−16.0 wt %) and nitrogen (40.0−45.0 wt %), has been applied in flame retarding finishing of ramie fabric and offer good flame retardant efficiency on ramie fabric. Furthermore, benzoxazine resin itself is a good carbon source.31 If APP and NEWRAY911 are applied to ramie fiber-reinforced benzoxazine composites, it can be expected the modification provides good flame protection to composites. But the high curing temperature of benzoxazine resin limits the application of ramie fiber in fiberreinforced benzoxazine composites, so few consider ramie fiberreinforced benzoxazine composites.32 Hence, in this paper, methyl p-toluenesulfonate (p-TMS) was used as the polymerization promoter of benzoxazine resin to decrease the curing temperature below 170 °C, which ensures ramie fiber without degradation during this heating process.33 And APP and NEWRAY911 were chosen to ameliorate the flame retardancy of the ramie/benzoxazine resin laminates. The flame retardancy of the ramie/benzoxazine resin laminates was characterized by limiting oxygen index (LOI) and vertical burning test, and the flame retarding mechanism was studied with the aid of a thermogravimetric analyzer coupled with infrared spectroscope (TG-IR), microscale combustion calorimeter (MCC) and scanning electron microscopy (SEM). Their mechanical properties were also characterized to analyze their performance.

dissolved in ethanol and the solution was stirred thoroughly for 90 min. The solvent was then evaporated from the mixture under vacuum at room temperature. In the following step, the mixture of APP and KH550 was added into the benzoxazine resin at selected concentrations, which was stirred for 30 min at 40 °C. The methyl p-toluenesulfonate (5 wt %) was subsequently added and stirred for 10 min. A homogeneous mixture was obtained and was used as the polymer matrix for preparation of the ramie/benzoxazine resin laminates. The formulations of the ramie/benzoxazine resin laminates are listed in Table S1 (Supporting Information). 2.2.3. Preparation of the Ramie/Benzoxazine Resin Laminates. The ramie fabric was chosen as the reinforcing fabric for the composite material. A typical lamination procedure was employed to prepare the ramie/benzoxazine resin laminates. First, the polymer matrix was impregnated with the ramie fabric using a drum-winding type prepregger. Then, the samples were baked in an oven at 120 °C for 5 min to obtain dried prepregs. The dried prepregs were stacked together and laminated between two stainless steel plates at 5−6 MPa using the following heating process: preheat under 120 °C for 14 min, 120 °C for 16 min, 140 °C for 30 min, 160 °C for 30 min and 170 °C for 30 min. The laminated samples were cooled to room temperature before the pressure was released. Test specimens were prepared by cutting and machining the laminates to the desired shape for the characterization of flexural strength, tensile strength, LOI and UL94 rating, etc. 2.3. Measurement. Limiting oxygen index (LOI) was measured by using an HC-2 oxygen index instrument (Jiangning Analytical Instrument Co. Ltd., Nanjing, China) on testing specimens (100 × 6 × 3 mm, 18 layers of ramie prepreg fabric) according to the ASTM D2863-2008 procedure. Vertical burning tests were performed with a CZF-3 vertical burning tester (Jiangning Analytical Instrument Co. Ltd., Nanjing, China) according to the ASTM D3801 standard, with sample dimensions of 130 × 13 × 3 mm (18 layers of ramie prepreg fabric). Tensile strength and flexural strength of the composite castings (24 layers of ramie prepreg fabric), under plain strain condition, were evaluated as per the ASTME399-74 standard. Five testing samples were used to generate the data points for the mechanical tests. Thermogravimetric analysis/infrared spectrometry (TG-IR) was carried out using a TGA 209 F1 thermal analyzer (Netzsch, Germany) interfaced with a Thermo Nicolet iS10 Fourier transform infrared (FTIR) spectrophotometer (Nicolet). Samples were heated to 800 °C at a heating rate of 20 °C/min both in nitrogen and in air. Microscale combustibility experiments were conducted using an MCC-2 microscale combustion calorimeter (Govmark, USA). A 5 mg sample was heated to 750 °C at a heating rate of 1 °C/s in a mixed stream of oxygen and nitrogen flowing at 20 cm3/min and 80 cm3/min, respectively. The cross section of the ramie/benzoxazine resin laminates was prepared by cutting used 10in bridge tile saw (DV61010V, DTC Technology Co., Ltd., Beijing, China). The morphologies of the cross section of the ramie/benzoxazine resin laminates, along with the char residues after keeping at 350 °C in a muffle for 5 min, were investigated using scanning electron microscopy (Hitachi S3400N, operated at 15.0 kV). A conductive layer of gold was coated over the cross section by plasma vapor deposition technique for laminates.

2. EXPERIMENTAL SECTION 2.1. Materials. Plain ramie fabric (830#, 21*21/52*58) was purchased from Seabird Textile Co., Ltd. (Shaoxing, China). Benzoxazine resin (PX-PN 001) was purchased from Zhaoyu Chemical Materials Co., Ltd. (Shanghai, China). Ammonium polyphosphate (APP, P% = 31.0−32.0 wt %, DP > 1000) and APP enwrapped by melamine resin (Mel-APP) were purchased from JLS Flame Retardants Chemical Co., Ltd. (Hangzhou, China). Methyl p-toluenesulfonate (p-TMS) was used as received from J&K Chemical Co., Ltd. (Shanghai, China). Flame retardant (NEWRAY911, NY-005, N% = 40.0−45.0 wt %, P% = 13.0−16.0 wt %) was purchased from New Ray Flame Retardant Factory. (Changzhou, China). Silane coupling agent (KH550) was purchased from Jiangsu Silane Coupling Agent Factory. (Nanjing, China). All materials were industrial products. 2.2. Preparation of Neat Resin Molding and the Ramie/Benzoxazine Resin Laminates. 2.2.1. Flame Retarding Finishing of Ramie Fabric. The flame retardant (NEWRAY911), silane coupling agent (KH550) and deionized water (30:5:65, wt %) were mixed to prepare the flame retarding finishing solution. Ramie fabric was coated with the flame retarding solution using a double-dip-double-nip process on an impregnating mill with a mangle expression of 75%, followed by 5 min of baking in a 105 °C oven. The content of NEWRAY911 was about 18% after flame retarding finishing of ramie fabric. 2.2.2. Preparation of Polymer Matrix for the Ramie/ Benzoxazine Resin Laminates. First, APP and KH550 were B

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3. RESULTS AND DISCUSSION 3.1. Flame Retardant Properties and Mechanical Properties of the Ramie/Benzoxazine Resin Laminates.

Figure 1. Mechanical properties of the ramie/benzoxazine resin laminates.

Mechanical properties of the ramie/benzoxazine resin laminates are shown in Figure 1, and their flame retardant properties are shown in Table S2 (Supporting Information). Figure 2 presents the SEM images of cross section of the ramie/benzoxazine resin laminates. The LOI of the ramie/benzoxazine resin laminate without flame retarding modification (BL0) is 23.2%, and it shows no rating in UL94 test. The increase of LOI of ramie/benzoxazine resin laminate (BL1) indicates a reduced flammability due to the addition of APP, but the laminate still shows no rating in UL94 test. During combustion, APP releases phosphoric acid, metaphosphoric acid, ammonia and water. The phosphoric acid and metaphosphoric acid dehydrate the hydroxyl groups in the structure of benzoxazine and improve char residue of benzoxazine resin. Besides, phosphoric acid and water could absorb a large amount of heat. At the same time, molten coating is plumped up by ammonia to form uniform intumescent char coating to improve flame retardancy of benzoxazine resin. As shown in Figure 1, Tensile strength and flexural strength of BL0 are about 151 and 197 MPa, respectively. The mechanical properties of BL1 are reduced remarkably after adding 1 wt % APP. It illuminates that the addition of APP produce negative effect on the mechanical properties due to the emergence of many cracks in the fiber−matrix interface, which is observed in Figure 2 (BL1). Thus, the interfacial stress transfer effect of the laminate could be deteriorated after the addition of APP, which produces cracks while the laminate is challenged by mechanical pressure. It suggests that although the addition of APP improves the flame retardancy of ramie/ benzoxazine resin laminates, it exerts adverse effects on their mechanical properties. To reduce mechanical damage to the ramie/benzoxazine resin laminate, APP enwrapped by melamine resin (Mel-APP) was used to replace APP in the manufacture of BL2 laminates. Flame retardancy of BL2 is nearly the same as that of BL1, but the tensile strength and flexural strength of BL2 are about 37% and 50% higher than those of BL1, respectively. These results

Figure 2. SEM images of the cross section of the ramie/benzoxazine resin laminates at two different magnifications (first column, ×200; second column, ×500).

indicate that melamine resin could effectively improve the wetting between APP and matrix to minimize the emergence of cracks in the phase of benzoxazine matrix, which have been approved by the SEM image of cross section of BL2 (Figure 2). As could be seen from Table S2 (Supporting Information), the LOI values of laminates BL2 and BL4 increase with increasing content of Mel-APP, but these laminates still show no rating in UL94 test after adding 3 wt % Mel-APP. The mechanical properties of laminates BL2 and BL4 are also deteriorated to a certain extent with increasing Mel-APP content because increase of Mel-APP content could induce to bring more cracks in the phase of benzoxazine matrix (Figure 2, BL2 and BL4). Because ramie fabric is more flammable than benzoxazine resin, the flame retardancy of ramie/benzoxazine resin laminates is limited by ramie fabric. It is easy to find from Table S2 (Supporting Information) that flame retardancy of the laminates could be improved remarkably by using flame retardant (NEWRAY911) modified ramie fabric. The LOI C

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Figure 3. TG curves (a) and DTG curves (b) for the ramie/ benzoxazine resin laminates under nitrogen atmosphere.

Figure 4. TG curves (a) and DTG curves (b) for the ramie/ benzoxazine resin laminates under air atmosphere.

values of the laminates made by flame retardant ramie fabric (BL3 and BL5) are 39.5% and 44.8%, respectively, and they both achieve V0 level in UL94 test. Furthermore, compared to BL2 and BL4, the tensile strengths of BL3 and BL5 are increased by about 7% and 15%, respectively. Their flexural strengths are also increased by about 3% and 10%, respectively. Compared with the SEM images of cross section of BL4 (Figure 2), those of BL5 (Figure 2) show that there is good wetting between ramie fiber and benzoxazine matrix due to the hydrophilic character of flame retardant (NEWRAY911) coated on fiber surface. Therefore, their mechanical strengths are improved after flame retarding finishing of ramie fabric by using flame retardant (NEWRAY911). Thus, it could be concluded that the flame retardancy of laminates could be ameliorated remarkably by flame retarding finishing of ramie fabric, and their mechanical properties are partially improved. Compared to BL0, the LOIs of laminates made by flame retardant ramie fabric are increased by about 70%−93%, and their mechanical strengths are also higher. It is found that the ramie fabric is only slightly damaged by the flame retardant (NEWRAY911) under the condition of low baking temperature (105 °C) during flame retarding finishing and the interfacial attribute between benzoxazine resin and ramie fiber is improved due to the adhesion of flame retardant on the fiber surface. Therefore, by using NEWRAY911 as a modificator for ramie fabric, it is

possible to improve flame retardancy of the ramie/benzoxazine resin laminates while retaining higher mechanical properties. 3.2. Thermal Degradation of the Ramie/Benzoxazine Resin Laminates. The TG-IR technique was used to investigate the mechanism of flame retardancy of the ramie/ benzoxazine resin laminates, by studying the thermal degradation process and its gaseous products. The TG and DTG curves of ramie/benzoxazine resin laminates in nitrogen or air environment are shown in Figures 3 and 4, with the detailed data listed in Tables S3 and S4 (Supporting Information). The corresponding 3D TG-FTIR spectra of the gases formed in their thermal degradation are shown in Figure 5. FTIR spectra of the evolved gases from thermal degradation in nitrogen or air environment are shown in Figures 6 and 7. As shown in Figure 3 and Table S3 (Supporting Information), Te, Tmax1 and Tmax2 of ramie/benzoxazine resin laminate without flame retarding modification (BL0) are 336, 365.8 and 420.3 °C respectively. Te’s, Tmax1’s and Tmax2’s of the laminates are reduced to 287, 330 and 405 °C after adding 1 wt % APP or Mel-APP, and continue to decrease as the content of Mel-APP increases. It is mostly due to the earlier degradation of flame retardant (APP or Mel-APP).34 However, the Mass1 values of the laminates modified by APP or Mel-APP are lower, and their char residues at 700 °C are much higher than that of BL0, which indicate that the addition of APP or Mel-APP slow D

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Figure 5. 3D FTIR spectra of the gas phases of BL0 ((a) N2, (b) air), BL4((c) N2, (d) air) and BL5((e) N2, (f) air) during thermal degradation process under nitrogen and air atmosphere.

affected by the testing environment. As can be seen, the decomposition reaction of laminates is more drastic in an air environment, especially for the second decomposition peak. The second peak values of derivative thermogravimetry (DTG) curves for ramie/benzoxazine resin laminates are almost constant, which indicated that the flame retardants have no effect on the second stage thermal oxidation decomposition. The char residue at 700 °C of the ramie/benzoxazine resin laminate in air environment is increased by about 3% after adding APP and NEWRAY911. Thus, the flame retardants of APP and NEWRAY911 have remarkable effects on the flame retardancy of ramie/benzoxazine resin laminates in an air environment. As shown in Figures 5, 6 and 7, the gas products during thermal decomposition of ramie/benzoxazine resin laminates mainly contain OH (such as phenol, about 3740 cm−1), NH2 (such as amine, about 3500 cm−1), CO2 (2360 cm−1), CO (2180, 2117 cm−1), carbonyl (1740 cm−1), aromatic ring (1300−1600 cm−1) C−H band (such as aliphatic compound, 670 cm−1), etc.35 As observed from Figures 5 and 6, for BL0 at 335 °C, the bands of CO2 and carbonyl derivatives at 2360 and 1740 cm−1 appear, meaning the degradation of ramie fabric occurs. At 365 °C, the characteristic bands of amine and phenol derivatives can be found at 3500 and 3740 cm−1. These results indicate that the degradation of the benzoxazine resin occur after ramie fabric. For BL4, the bands of amine and phenol derivatives at 3500 and 3740 cm−1 and aromatic ring derivatives at 1300−1600 cm−1 appear at 280 °C, indicating the degradation of APP occurred earlier. But the peak intensity of all gas products during thermal decomposition of BL4 decreases to a certain extent in a nitrogen environment after adding APP, which indicated a reduced flammability. And those of BL5 decrease further after adding APP and NEWRAY911 together. These results illuminated that APP and NEWRAY911 have excellent flame retardant effect for the ramie/benzoxazine

the decomposition of laminates and improve char residue. Furthermore, the peak values of DTG curves also decrease gradually as the content of Mel-APP increases, which signified a reduced mass loss rate during decomposition. Phosphate and ammonia, which generated from the decomposition of APP or Mel-APP, could act as the acid source and gas source. Benzoxazine resin itself is a good carbon source.31 Hence, the intumescent flame retardant is formed and provided good thermal insulation and flame protection for ramie/benzoxazine resin laminates. With increasing Mel-APP content, the effects of thermal insulation and flame protection are improved, and the char residue of laminates increases as well. Compared to BL2 and BL4, Te’s, Tmax1’s and Tmax2’s of BL3 and BL5 are decreased significantly, and their char residues at 700 °C increase by about 5%. Simultaneously, the peak values of DTG curves also decrease remarkably due to flame retardant finishing of ramie fabric by NEWRAY911. These results suggest that the flame retardancy of laminates is improved further by the flame retardant finishing of ramie fabric with NEWRAY911. It is because NEWRAY911 produce phosphoric acid and ammonia during the thermal decomposition, which promoted the carbonization of the laminates to form good flame retardant insulation, and improved the thermal stability.30 The effect of NEWRAY911 on thermal stability of the laminates is more significant because it is directly attached to the surface of flammable ramie fiber in laminates. As shown in Figure 4 and Table S4 (Supporting Information), the Te of BL0 is 324 °C in an air environment, which decreased by 12 °C compared to that in a nitrogen environment. Nevertheless, the Te’s of APP and NEWRAY911 modified laminates are almost the same in different testing environments (nitrogen and air). It is possibly because decomposition of the flame retardants (APP and NEWRAY911), which is the first thing that happened in the deposition of flame retarding modified laminates, is barely E

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Figure 7. FTIR spectra of the evolved gases from BL0 (a), BL4 (b) and BL5 (c) at representative decomposition temperatures under air atmosphere.

Figure 6. FTIR spectra of the evolved gases from BL0 (a), BL4 (b) and BL5 (c) at representative decomposition temperatures under nitrogen atmosphere.

compounds at 2360, 2180, 2117 and 670 cm−1 increase and the peaks of phenol, amine and aromatic ring at 3740, 3500 and 1300−1600 cm−1 decrease during the second stage of decomposition in air environment. In the presence of oxygen, the thermal oxidation is more thorough and creates more CO2, CO and aliphatic compounds during high temperature

resin laminates. In Figure 7, compared to the decomposition of the laminates in nitrogen environment, their first stage decompositions little change under air environment because oxygen is not involved in the decomposition during low temperature. However, the peaks of CO2, CO and aliphatic F

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representative heat release rate curves of the ramie/benzoxazine resin laminates were presented in Figure 8, and the detailed data are listed in Table S5 (Supporting Information). As shown in Figure 8, there are two large exothermic peaks in the MCC curves of ramie/benzoxazine resin laminates, which correlated with the decomposition of ramie fabric (low temperature) and the decomposition of benzoxazine resin (high temperature). The MCC curves of BL1 and BL2 are similar, which indicated that the effects of APP and Mel-APP on flame retardancy of laminates were similar. As shown in Table S5 (Supporting Information), TPHRR1’s and TPHRR2’s of laminates BL2 and BL4 decrease after adding Mel-APP, which was consistent with the TG results of those laminates. The HR capacity, THR and PHRR decrease as the content of Mel-APP increased. These results demonstrate that the addition of MelAPP could reduce the combustion speed, burning degree and combustion heat release of laminates. The HR Capacity and THR of BL3 are 131 J·g−1·K−1 and 6.3 kJ·g−1 lower than those of BL0 and the PHRR 1 of ramie fabric exothermic decomposition decreases from 175.6 to 21.9 W·g−1. Compared to BL2 and BL4, the flame retardancy of BL3 and BL5 is significantly improved. The addition of Mel-APP could also be reduced because flame retardant ramie fabric is used for the manufacture of the laminates, which could maintain good mechanical properties of laminates. These results suggest that the flame retardant finishing of ramie fabric using NEWRAY911 can improve greatly the flame retardancy of ramie/ benzoxazine resin laminates. 3.4. Analysis of the Char Residue of the Ramie/ Benzoxazine Resin Laminates. It is well-known that the char layer can improve the flame retardant performance during combustion. Figure 9 presents the SEM images of the cross section for the char residue of ramie/benzoxazine resin laminates after keeping at 350 °C in a muffle for 5 min. Compared with the SEM image of the cross section of BL0 before combustion (Figure 2), the char residue of BL0 (Figure 9) displays obvious delamination and relatively loose structure. The ramie fabric shrinks remarkably and there are many holes in the phase of benzoxazine resin of BL0, compared with those of BL4 and BL5. It is demonstrated that benzoxazine resin and ramie fabric of ramie/benzoxazine resin laminate without flame retarding modification are decomposed acutely after they were kept at 350 °C in a muffle for 5 min. Compared with the char residue of BL0, the number of holes in the phase of benzoxazine resin decreases and its char residue increases drastically in the case of BL4. Simultaneously, the char residue of ramie fabric is retained more due to the adhesion of more matrix on the fiber surface. It illustrated that the addition of APP could be beneficial for benzoxazine resin to forming char residue and improving flame retardancy. Compared with that of BL0 before the combustion (Figure 2, ×200), the cross section of BL5 (Figure 9, ×200 ) swells to some extent. And its char residue is comparatively intact, which almost completely retain the primary architecture. Furthermore, the interspace between fiber and matrix is filled up with intumescent char residue, meaning that there is an effective wetting between fiber and matrix as a result of good adhesion for BL5. Therefore, laminates made from APP and ramie fabric modified by flame retardant (NEWRAY911) show a superior intumescent characteristic, flame retardation and mechanical property.

Figure 8. MCC curves for the ramie/benzoxazine resin laminates.

Figure 9. SEM images of the cross section for the ramie/benzoxazine resin laminates after keeping at 350 °C in a muffle for 5 min at two different magnifications (first column, ×100; second column, ×200).

treatment. Nevertheless, the second stage of thermal oxidation decomposition is little affected by flame retardants (APP and NEWRAY911). 3.3. Microscale Combustibility Analysis of the Ramie/ Benzoxazine Resin Laminates. Further assessment of the flammability properties for the ramie/benzoxazine resin laminates was conducted by microscale combustibility calorimeter experiments (MCC), which simulated the aerobic pyrolysis and a subsequent reaction between the volatile pyrolysis products and a mixture of nitrogen/oxygen (80/20) gas under high temperatures. One can obtain the key combustion parameters, including the value of heat release capacity (HR capacity), the total heat release (THR), the first peak for total heat release (PHRR), temperature for PHRR (TPHRR), which is a good predictor of flammability. The G

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based GF composite laminated by in situ reaction with carboxyl functionalized CNTs. J. Appl. Polym. Sci. 2013, 129, 2629. (2) Nakamura, R.; Netravali, A. N.; Morgan, A. B.; Nyden, M. R.; Gilman, J. W. Effect of halloysite nanotubes on mechanical properties and flammability of soy protein based green composites. Fire Mater. 2013, 37, 75. (3) Xu, M. Z.; Liu, M. D.; Dong, S. H.; Liu, X. B. Design of low temperature self-cured phthalonitrile-based polymers for advanced glass fiber composite laminates. J. Mater. Sci. 2013, 48, 8108. (4) Lin, S. C.; Pearce, E. M High performance thermoset, chemistry, properties, application; Hanser Gardner: Glassgow, U. K., 1993. (5) Huang, K. W.; Kuo, S. W. High-performance polybenzoxazine nanocomposites containing multifunctional POSS cores presenting vinyl-terminated benzoxazine groups. Macromol. Chem. Phys. 2010, 211, 2301. (6) Takeichi, T.; Kawauchi, T.; Agag, T. High performance polybenzoxazines as a novel type of phenolic resin. Polym. J. 2008, 40, 1121. (7) Ghosh, N. N.; Kiskan, B.; Yagci, Y. PolybenzoxazinesNew high performance thermosetting resins: Synthesis and properties. Prog. Polym. Sci. 2007, 32, 1344. (8) Wang, C. F.; Chiou, S. F.; Ko, F. H.; Chen, J. K.; Chou, C. T.; Huang, C. F.; Kuo, S. W.; Chang, F. C. Polybenzoxazine as a moldrelease agent for nanoimprint lithography. Langmuir 2007, 23, 5868. (9) Zhang, T.; Yan, H. Q.; Fang, Z. P.; Peng, M. Synthesis of aromatic diamine-based benzoxazines and effect of their backbone structure on thermal and flammability properties of polymers. Chin. J. Polym. Sci. 2013, 31, 1359. (10) Jubsilp, C.; Panyawanitchakun, C.; Rimdusit, S. Flammability and thermomechanical properties of dianhydride-modified polybenzoxazine composites reinforced with carbon fiber. Polym. Compos. 2013, 34, 2067. (11) Kim, H. Thermal characteristics of basalt fiber reinforced epoxybenzoxazine composites. Fibers Polym. 2012, 13, 762. (12) Kiskan, B.; Ghosh, N. N.; Yagci, Y. Polybenzoxazine-based composites as high-performance materials. Polym. Int. 2011, 60, 167. (13) Jawaid, M.; Abdul Khalil, H. P. S. Cellulosic/synthetic fibre reinforced polymer hybrid composites: A review. Carbohydr. Polym. 2011, 86, 1. (14) Li, H.; Zhang, Y. F.; Liu, Y. Q.; Zhao, G. Z.; Du, R. K. Study on preparation and properties of ramie fiber reinforced composites. Adv. Mater. Res. 2013, 712∼715, 203. (15) Xu, J.; Li, Y.; Yu, T.; Cong, L. Reinforcement of denture base resin with short vegetable fiber. Dent. Mater. 2013, 29, 1273. (16) Gassan, J.; Bledzki, A. K. Effect of cyclic moisture absorption desorption on the mechanical properties of silanized jute-epoxy composites. Polym. Compos. 1999, 20, 604. (17) Yu, T.; Jiang, N.; Li, Y. Study on short ramie fiber/poly(lactic acid) composites compatibilized by maleic anhydride. Composites, Part A 2014, 64, 139. (18) Leja, K.; Lewandowicz, G. Polymer biodegradation and biodegradable polymersA review. Pol. J. Environ. Stud. 2010, 19, 255. (19) Wang, H. G.; Xian, G. J.; Li, H.; Sui, L. L. Durability study of a ramie-fiber reinforced phenolic composite subjected to water immersion. Fibers Polym. 2014, 15, 1029. (20) Gu, Y. Z.; Tan, X. L.; Yang, Z. J. Hot compaction and mechanical properties of ramie fabric/epoxy composite fabricated using vacuum assisted resin infusion molding. Mater. Des. 2014, 56, 852. (21) Tian, N. N.; Wen, X.; Jiang, Z. W.; Gong, J.; Wang, Y. H.; Xue, J.; Tang, T. Synergistic effect between a novel char forming agent and ammonium polyphosphate on flame retardancy and thermal properties of polypropylene. Ind. Eng. Chem. Res. 2013, 52, 10905. (22) Szustakiewicz, K.; Cichy, B.; Gazinska, M.; Pigiowski, J. Comparative study on flame, thermal, and mechanical properties of HDPE/clay nanocomposites with MPP or APP. J. Reinf. Plast. Compos. 2013, 32, 1005. (23) Tang, Q. B.; Wang, B. B.; Shi, Y. Q.; Song, L.; Hu, Y. Microencapsulated ammonium polyphosphate with glycidyl meth-

4. CONCLUSIONS The ramie/benzoxazine resin laminates were prepared and modified by APP and NEWRAY911. After the addition of 3 wt % APP, the LOI of the laminate increased obviously, but did not achieve V0 level in UL94 test. Tensile strength and flexural strength of the laminate were reduced by about 12% and 13% due to the emergence of many cracks in the fiber−matrix interface after adding 3 wt % APP. However, after adding 3 wt % Mel-APP, the tensile strength and flexural strength of BL2 were improved by about 37% and 50%, respectively. But the mechanical properties of laminates were also deteriorated to a certain extent with increasing Mel-APP content. By employing a flame retardant NEWRAY911, the LOI of the laminates could increase to 44.8%, and the V0 level in UL94 test was also achieved. Furthermore, the tensile strength and flexural strength were increased by about 15% and 10%, respectively, which was mostly due to good wetting between NEWRAY911 coated ramie fiber and benzoxazine matrix. As shown from the TG-IR analysis, the degradation rate was slowed down and the char residue of thermal decomposition was heightened substantially in nitrogen environment for the laminates modified by APP and NEWRAY911. Under air environment, the first stage of decomposition was barely changed, but the peaks of CO2 and CO increased and the peaks of phenol, amine and aromatic ring decreased during second stage of decomposition. Likewise, HR capacity, THR and PHRR were decreased remarkably for laminates modified by APP and NEWRAY911. The SEM images revealed that the cross section of laminates happens to swell to some extent, and its char residue was considerably intact, almost completely retaining the primary architecture after modifying by APP and NEWRAY911.

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ASSOCIATED CONTENT

S Supporting Information *

Table of the formulations of ramie/benzoxazine resin laminates, table of flame retardant properties of the ramie/benzoxazine resin laminates, tables of thermal properties of the ramie/ benzoxazine resin laminates under nitrogen and air atmosphere, and table of microscale combustion calorimetry results of the ramie/benzoxazine resin laminates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel. (Fax): +86 574 88130132. E-mail: [email protected] (Z. Fang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (No. 51103129), Zhejiang Provincial Natural Science Foundation of China (LY14E030006), the Ningbo Natural Science Foundation of China (No. 2012A610084) and the Open Fund of Zhejiang Provincial Top Key Discipline of New Materials and Process Engineering (20110939 and 20121126).

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REFERENCES

(1) Xu, M. Z.; Hu, J. Q.; Zou, X. Q.; Liu, M. D.; Dong, S. H.; Zou, Y. K.; Liu, X. B. Mechanical and thermal enhancements of benzoxazineH

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acrylate shell: Application to flame retardant epoxy resin. Ind. Eng. Chem. Res. 2013, 52, 5640. (24) Yang, C. W.; Liang, G. Z.; Gu, A. J.; Li, Y. Flame retardancy and mechanism of bismaleimide resins based on a unique inorganic-organic hybridized intumescent flame retardant. Ind. Eng. Chem. Res. 2013, 52, 15075. (25) Hu, X. M.; Wang, D. M. Enhanced fire behavior of rigid polyurethane foam by intumescent flame retardants. J. Appl. Polym. Sci. 2013, 129, 238. (26) Wu, K.; Zhang, Y. K.; Hu, W. G.; Lian, J. T.; Hu, Y. Influence of ammonium polyphosphate microencapsulation on flame retardancy, thermal degradation and crystal structure of polypropylene composite. Compos. Sci. Technol. 2013, 81, 17. (27) Gaan, S.; Sun, G. Effect of phosphorus and nitrogen an flame retardant cellulose: A study of phosphorus compounds. J. Anal. Appl. Pyrolysis 2007, 78, 371. (28) Rupper, P.; Gaan, S.; Salimova, V.; Heuberger, M. Characterization of chars obtained from cellulose treated with phosphoramidate flame retardants. J. Anal. Appl. Pyrolysis 2010, 87, 93. (29) Wu, W. D.; Yang, C. Q. Comparison of different reactive organophosphorus flame retardant agents for cotton: Part I. The bonding of the flame retardant agents to cotton. Polym. Degrad. Stab. 2006, 91, 2541. (30) Lu, X. C.; Yan, H. Q.; Wang, H. Q.; Cheng, J.; Fang, Z. P. Preparation and properties of flame retarded ramie/phenolic resin composites. Acta Materiae Compositae Sinica 2011, 28, 1. (31) Zhang, J.; Ji, K. J.; Xia, Y. Z. Polymer combustion and flame retardant technology; Chemical Industry Press: Beijing, 2005. (32) Ran, Q. C.; Zhang, D. X.; Zhu, R. Q.; Gu, Y. The structural transformation during polymerization of benzoxazine/FeCl3 and the effect on the thermal stability. Polymer 2012, 53, 4119. (33) Wang, C. F.; Zhao, C. H.; Sun, J. Q. Synthesis and thermal properties of a bio-based polybenzoxazine with curing promoter. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2016. (34) Davies, P. J.; Horrocks, A. R.; Alderson, A. The sensitisation of thermal decomposition of ammonium polyphosphate by selected metal ions and their potential for improved cotton fabric flame retardancy. Polym. Degrad. Stab. 2005, 88, 114. (35) Zhu, Y. F.; Gu, Y. Effect of interaction between transition metal oxides and nitrogen atoms on thermal stability of polybenzoxazine. J. Macromol. Sci., Phys. 2011, 50, 1130.

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