Construction of Hierarchical Natural Fabric Surface Structure Based

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40168−40179

Construction of Hierarchical Natural Fabric Surface Structure Based on Two-Dimensional Boron Nitride Nanosheets and Its Application for Preparing Biobased Toughened Unsaturated Polyester Resin Composites Fukai Chu,† Dichang Zhang,‡ Yanbei Hou,† Shuilai Qiu,† Junling Wang,† Weizhao Hu,*,† and Lei Song*,† Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 25, 2019 at 05:59:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China ‡ Department of Physical Science, University of California, Irvine, California 92697, United States S Supporting Information *

ABSTRACT: It has been a big challenge to prepare the unsaturated polyester resin (UPR) composites with good fire safety, interfacial quality, and impact strength in an environmentally friendly way. In this study, to improve interfacial performance of fabric-reinforced UPR composites, nontoxic two-dimensional hexagonal boron nitride (h-BN) nanosheets were assembled on the surface of ramie fabrics, where sodium alginate acts as a green dispersant to disperse h-BN sheets during the process. Then, the biobased phosphorus-containing toughening agent (PCTA) was synthesized to simultaneously improve the impact strength and fire safety of the composite. With application of h-BN nanosheets-assembled fabric (AF) and 20 wt % of PCTA, the AF/ UPR@PCTA20 composite presented the maximum 41.2% decrease in the value of peak heat release rate and a maximum 17.8% decrease in the value of total heat release, which also reached V-0 rating in the vertical burning test. Meanwhile, the AF/UPR@ PCTA20 composite showed an obvious increase in limiting oxygen index, from 24.0 to 29.5% compared with RF/UPR. The flame retardant mechanism was investigated from gas phase and condensed phase. Furthermore, compared to neat RF/UPR composite, the AF/UPR@PCTA20 composite showed a significant 68.8% improvement in impact strength, implying an extreme toughening effect of PCTA on UPR composites. The research provides a viable green method for the development of environmentally friendly UPR composites in the future. KEYWORDS: unsaturated polyester resin, natural fabric, composite interphase, phosphorus-containing biobased toughening agent, fire safety, impact strength energy consumption, detrimental fiber hairy, and the difficulty for decomposition limited the use of glass fiber/fabric. In recent years, biobased composites prepared with natural fiber/ fabric have attracted great attention because of their low price and environmental friendliness. Moreover, natural fabricreinforced UPR composites perform equivalent mechanical properties to glass fiber/fabric-reinforced composites.9−11

1. INTRODUCTION As a thermoset material, unsaturated polyester resin (UPR) is particularly attractive in the automobile industry and high-rise buildings because of its low cost and high anticorrosion property.1,2 Considerable research efforts have been devoted to the fabrication of fiber/fabric-reinforced UPR composites.3−5 For example, glass fiber/fabric is commonly used in fiber/ fabric-reinforced polymer composites owing to its exceptional advantages such as high strength and chemical stability.6−8 However, as renewable resources and environment-friendly society have become main topics of the modern times, the high © 2018 American Chemical Society

Received: September 4, 2018 Accepted: October 26, 2018 Published: October 26, 2018 40168

DOI: 10.1021/acsami.8b15355 ACS Appl. Mater. Interfaces 2018, 10, 40168−40179

Research Article

ACS Applied Materials & Interfaces

with enhanced thermal stability and flame retardant effect, owing to the advantages of high thermal stability and barrier effect of 2D h-BN sheets. To further solve the problem of intrinsic poor toughness and fire risk of UPR composites, the effective phosphoruscontaining toughening agent (PCTA) can be synthesized with toughening and flame retardant effect simultaneously. Some researchers have synthesized PCTAs applied for different thermoset materials, such as epoxy 34,35 and phenolic foams,36−38 and the detailed information was summarized in Table S2. However, very few literature studies have focused on the toughness and flame retardant performance improvement of UPR at the same time, with PCTAs. In accordance with the green and sustainable ideality, some renewable materials can be considered as biobased toughening agents for UPR, such as cardanol, which is derived from cashew nut shell and has been proved to be of significant plasticizing effect.39−41 Furthermore, with the reactive hydroxyl groups and nonconjugated CC bonds, phosphorus elements can be easily introduced to prepare a biobased PCTA. In this work, to improve interfacial adhesion between RFs and UPR, 2D h-BN nanosheets were applied to modify the surface of RF through a versatile layer-by-layer (LbL) method, where sodium alginate (SA) acts as a green dispersant to disperse h-BN sheets during the process. Phosphoruscontaining cardanol-based toughing agent was then synthesized to simultaneously improve the toughness and fire safety property of the UPR composites, through the oven vacuum bag manufacturing method. The thermal stability, fire safety, flame retardant mechanism, and impact strength were investigated. With the use of nontoxic h-BN sheets and biobased materials such as SA, RF, and cardanol, it is expected the research herein provides a viable green method for the development of environmentally friendly UPR composites in the future.

Ramie fabric (RF) has great physical properties and is supposed to play the key role in reinforcing and toughening environment friendly UPR composites.12−14 As shown in Table S1, ramie fibers show better tensile strength and young’s modulus compared to other typical natural fibers.13,15,16 However, it should be pointed out that there are two vital issues for RF-reinforced UPR composites: (1) how to secure the mechanical safety properties of the RF-reinforced UPR composites. Three factors influence the mechanical performance of the composites. (a) Interface adhesion quality. It is well established that good compatibility between natural fiber/ fabric and UPR matrix is a prerequisite for excellent mechanical performance of the fiber/fabric-reinforced UPR composites. As a result of the rough surface and abundant possible reaction sites, the fibers can effectively interlock with UPR matrix and transfer stress from UPR matrix to fibers by good interface adhesion.16−18 However, there are a great deal of hydroxyl groups in the surface of fiber/fabric. The hydrophilic properties from hydroxyl groups lead to a very poor interface performance with hydrophobic matrixes. The hydrophilic groups weaken the waterproof property of fibers, resulting the swell of fibers in moisture conditions and thus interface debonding.17,19 To prepare a good reinforcing agent, surface modification must be done for RF. (b) Manufacturing method of natural fiber/fabric-reinforced UPR composites. It has proved that the vacuum bagging method showed better mechanical performance than that of the hand lay-up method in preparing UPR composites.20 In addition, void reduction mechanisms, including “through-thickness air diffusion” and “in-plane flow to the laminate edges”, have been investigated during oven vacuum bag processing.21 (c) Poor toughness of UPR matrix. In general, impact strengths of natural fabric/UPR composites are better than pure UPR with fabric acting as a reinforcement.22,23 However, some strategies should be adopted to further obtain toughness for the fabric-reinforced UPR composites;20,24 (2) how to ensure the fire safety of RFreinforced UPR composites. Both RF and UPR are composed of the elements of carbon, hydrogen, and oxygen, which determine their poor fire resistance.25−27 Meanwhile, crosslinked polystyrene chain segment in UPR can release a large amount of heat and poisonous gases when burning, which aggravates the fire risk of RF/UPR composites.28,29 One of the most efficient method to improve the interfacial performance is introducing nanomaterials on fabric surface.30 Recently, many novel two-dimensional (2D) nanosheets, such as hexagonal boron nitride (h-BN), have exhibited great application prospect for fabrication of functional fabrics. It has been reported that porous BN nanosheets were coated on the surface of cotton fabric and melamine sponge, and the modified materials showed high oil/water separation efficiency.31 Meier et al. used h-BN sheets to consolidate the carbon fabric through-the-thickness impregnation and then prepared modified carbon fabric/epoxy composites.32 In another example, BN/pyrocarbon coatings were successfully prepared on SiC fibers via the dip-coating technique. The SiC fiber-reinforced ceramic composites containing the BN/PyC coatings showed relatively superior mechanical behavior and unchanged dielectric property because of low Young’s modulus, good interfacial properties and low electrical conductivity of h-BN.33 On the basis of that, h-BN sheets can be a key role in improving the interfacial property for RFreinforced UPR composites. Furthermore, it would have more helpful prospects in fabrication of fabric-reinforced composites

2. EXPERIMENTAL SECTION 2.1. Materials. Hexagonal boron nitride (h-BN), polyethyleneimine (PEI, Mw = ca. 70 000), and phenyl dichlorophosphate (PDCP) were obtained from Sinopharm Chemical Reagent Co., Ltd. SA, sodium hydroxide (NaOH), hydrochloric acid (HCl), trimethylamine (TEA), dichloromethane, sodium sulfite, and anhydrous magnesium sulfate were purchased from Sinopharm Chemical Reagent Co., Ltd. 3-Chloroperbenzoic acid (m-CPBA) was bought from Anhui Wotu Chemicals Co., Ltd. Plain RFs were supplied by Hunan Huasheng Dongting Co., Ltd. UPR (named 196) was bought from Hefei Chaoyu Chemical Co., Ltd. Cardanol was purchased from Cardolite Corporation. 2.2. SA-Assisted Exfoliation and Dispersion of h-BN. SA (2.5 g) and deionized water (250 mL) were added into a three-necked bottle to prepare SA aqueous solution after stirring at 50 °C for 2 h. Then, bulk h-BN (0.5 g) was dispersed and exfoliated in SA solution through stirring and bath ultrasonication for 10 h. Subsequently, the mixture was centrifuged at 9000 rpm for 10 min, and the sediments were washed five times with deionized water to remove the excess unabsorbed SA. The sediments were dispersed in 250 mL of deionized water again with the sonicating process lasted for 2 h. To remove the un-exfoliated h-BN from the stable SA-BN nanosheets dispersion, the mixture was centrifuged at 1500 rpm for 10 min and the supernatant was reserved. 2.3. LbL Assembly. First, RFs were tailored into the same size (150 mm × 150 mm) and treated with NaOH solution to remove the wax and oils in the surface. Then, 0.5 wt % PEI cationic solution was prepared and the pH was adjusted to 9 with HCl. Anionic solution (0.1 wt %) was obtained through the SA-BN supernatant above. Afterward, RFs were immersed in these two solutions alternately for 40169


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Figure 1. (a) Schematic illustration of the h-BN sheets dispersed with SA. (b) Digital photos of the water−oil emulsion without and with 1.0 wt % of SA. (c) Digital images of h-BN nanosheets dispersions without and with SA. (d) FTIR curves of bulk h-BN and SA modified h-BN sheets. (e) TGA results of h-BN and SA-BN nanosheets. (f,g) TEM images for bulk h-BN and SA-assisted exfoliated BN sheets. (h) XRD curves of bulk h-BN and SA-modified h-BN sheets. 10 times. The fabrics were washed with deionized water to clean the unabsorbed components after each immersion. 2.4. Synthesis of PCTA. Cardanol (0.15 mol), TEA (0.15 mol), and dichloromethane (150 mL) were mixed in a three-necked bottle with a reflux condenser and a magnetic stirrer. PDCP (0.075 mol) was dissolved into 30 mL of dichloromethane and the solution was added dropwise into the mixture above in 2 h at room temperature. Then, the system was heated at 70 °C for 7 h. After white precipitate was filtered, the organic layer was washed with deionized water and dried with anhydrous magnesium sulfate. The solvent was removed through rotary evaporation, and the brown product bicardanylphosphate (BCP) was obtained. Subsequently, the epoxidization of BCP was as follows:39 m-CPBA (0.2 mol) was slowly added into the mixture of BCP (0.05 mol) and dichloromethane (200 mL), and the system was stirred for 6 h at 0 °C. After that, the product was filtered and the organic layer was washed with sodium sulfite solution and deionized water and then dried with anhydrous magnesium sulfate. A yellow liquid epoxide (E-BCP) can be obtained after filtration and rotary evaporation. Finally, the PCTA was prepared by reacting E-BCP (0.05 mol) and 9,10-dihydro-9-oxa-10-phosphahenanthrene-10-oxide (DOPO; 0.2 mol) in the three-necked bottle at 160 °C for 6 h. 2.5. Fabrication of Ramie Fabric-Reinforced UPR Composites. The preparation of the composite was through the oven vacuum bag manufacturing method.21,42−44 First, UPR and different contents of PCTA were blended homogeneously with 2 wt % benzoyl peroxide acting as the initiator. Then, the mixture was put on the surface of RFs by the hand lay-up method. Subsequently, the sample above was transferred to the prepared vacuum bag. The system was arranged in the oven at 70 °C for 4 h and 120 °C for 3 h, with a pump worked continuously to ensure the vacuum condition during the

curing process. The formulation of the composites is shown in Table S3.

3. RESULTS AND DISCUSSION 3.1. SA-Assisted Dispersion h-BN Sheets. SA is a renewable and eco-friendly material, mainly derived from sea algae. The polar −COO− and −OH group and nonpolar −CH moieties in the polymer chains mean the potential water/oil amphiphilic properties of SA, which allow SA to serve as a green dispersant for h-BN sheets. To better illustrate the amphiphilic property, 1.0 wt % of SA was blended with 30% cyclohexane and 70% water. As shown in Figure 1b, for the system without SA, there is an obvious stratification between cyclohexane and water. However, the water−oil emulsion system is formed with the addition of SA after ultrasonic agitation. The fact demonstrates that SA has an affinity to both polar and nonpolar solvents, even though the hydrophobic −CH species is less representative.45−48 Figure 1a shows this hydrophobic interaction between SA and h-BN sheets. When bulk h-BN was stirred and ultrasonicated with SA solution (10 mg/mL), the solid content of the supernatant obtained after washing off the unabsorbed SA can reach 0.86 mg/mL, compared with 0.52 mg/mL for the supernatant without addition of SA. After standing for 10 h at room temperature, some aggregates appear in the h-BN dispersion without SA, while SA-BN solution is more stable (Figure 1c). Meanwhile, the SA-assisted dispersed h-BN nanosheets solution exhibits a 40170


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Figure 2. (a,b) Photographs of ramie and RFs. (c) Schematic illustration of the LbL-assembled process with PEI cationic solution and SA-BN anionic dispersion. (d) XPS results of raw RFs and AFs. (e,g,i) SEM images of raw RFs. (f,h,j) SEM images of assembled RFs. Insets in (i,j) are the EDX images of raw fabrics and SA-BN AFs, respectively.

make it possible to disperse less-stacked BN sheets in water stably. Thus, SA-BN nanosheet dispersion has great potential as the anionic deposition solution in the LbL assemble technique. Figure 2a,b shows the ramie known as “China grass” and ramie woven fabric, respectively. Also, the LbL assemble process is depicted in Figure 2c. With PEI solution and SA-BN dispersion acting as cationic and anionic solution, respectively, LbL assembly technology was operated on the surface of RFs. The mass increment is 14.2% based on the mass change after and before the LbL process. The scanning electron microscopy (SEM) test was carried out to identify the change of surface morphology. After the RFs are assembled with 10 bilayers, a thin layer and uniform distributed sediments can be observed on the surface of assembled RFs (Figure 2f). Meanwhile, the SEM images show that the compact coatings can also connect the neighboring fibers (Figure 2h,j). Compared with the AF, pure RFs present typical smooth surfaces with little surface adhesive materials appearing (Figure 2e,g,i). The energydispersive X-ray (EDX) test was operated to verify the deposits originated from SA-BN sheets and PEI. Compared with the pure RF, the B element is detected for LbL-assembled RF, indicating that SA-BN sheets are surely absorbed on the surface of RF. X-ray photoelectron spectroscopy (XPS) was further applied to confirm the successful assembly of SA-BN sheets and PEI in the surface of RF (Figure 2d). The coexistence of B 1s and N 1s on the surface of modified RF is evidenced by the shoulders observed on the peak at 190.56 and 398.19 eV, respectively. The elemental scale is summarized in XPS figure, with 5.26% B and 5.33% N appearing in the surface of LbL-modified RF. In addition, the X-ray maps of lowmagnification SEM images for SA-BN AFs show that h-BN sheets are evenly distributed on the surface of RFs, indicating the construction of hierarchical surface structure (Figure S1). To confirm the product structure in the synthetic route of PCTA (Figure 3), FTIR analysis was first used. As depicted in Figure 4d, the peaks at 2854 and 2926 cm−1 can be ascribed to the stretching vibration of −CH2 group from aliphatic chain. Upon treatment with epoxidation, the curve of E-BCP shows the new peak at 910 cm−1, which can be attributed to epoxide group.39 In addition, the characteristic peak at 3009 cm−1 in BCP disappears in the spectrum of E-BCP, indicating that the CC in the side chain of cardanol has been changed into the

deeper color, indicating the use of SA facilitated the dispersion of h-BN nanosheets. To better interpret the dispersion mechanism of SA-BN nanosheets, some characterizations were applied. The Fouriertransform infrared spectroscopy (FTIR) spectra were carried out for both bulk h-BN and SA-BN sheets (from the dried supernatant). As depicted in Figure 1d, the absorption bands at 1384 and 805 cm−1 can be assigned to the B−N in-plane stretching vibration and the B−N−B out of plane bending vibration, respectively.49 Compared with the bulk h-BN, the curve of SA-BN shows the new stretch at 2927 cm−1 which originates from −CH2 moieties of SA. The thermogravimetric analysis (TGA) results also show some difference between hBN and SA-BN sheets (Figure 1e). A 5.50 wt % weight loss is recorded for SA-BN, while the weight of bulk h-BN stays unchanged even at 800 °C. The weight loss stage between 200 and 600 °C in SA-BN is attributed to thermal degradation of absorbed SA. The results of FTIR and TGA show that some SA polymers certainly interact with h-BN sheets because of the amphiphilic properties of SA. In the meanwhile, the steric hindrance50 and the electrostatic repulsive forces generated by the charged carboxyl groups in SA promote the flakes to disperse stably in water. The crystalline structures of bulk hBN and SA-BN sheets were confirmed by X-ray diffraction (XRD). As shown in Figure 1h, peaks of bulk h-BN and SA-BN sheets appearing near 26.8°, 41.7°, 44.0°, 50.2°, and 55.3° can be well-indexed to the (002), (100), (101), (102), and (004) planes of h-BN, respectively.51 Obviously, the intensity of the SA-BN sheets is lower than that of bulk h-BN, which manifests less-ordered stacking SA-BN sheets.52 In addition, the full width at half-maximum (fwhm) from the (002) peak can also assess the stacked layers. The fwhm value for SA-BN (0.271°) is larger than that of bulk h-BN (0.260°), indicating the lessstacked layers for SA-BN sheets.53 The result was further confirmed by transmission electron microscopy (TEM) images (Figure 1f,g). Bulk h-BN has a bulkier and more stacked structure, while SA-BN sheets present a smaller size and a less layer-stacked morphology. In brief, the results of XRD and TEM prove that bulk h-BN is exfoliated into thin layers, which facilitate the dispersion of the sheets in water in return. 3.2. Characterization of Assembled Ramie Fabric and PCTA. SA-assisted dispersion and exfoliation of bulk h-BN 40171


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in the spectra of PCTA, illustrating the reaction between EBCP and DOPO. NMR measurements were also employed to confirm the structure of cardanol-based toughing agent. Figure 4a−c shows the 1H NMR spectra of BCP, E-BCP, and PCTA, respectively. The signal from 5.2 to 5.5 ppm in the spectra of BCP (Figure 4a) is caused by proton of the unsaturated −CHCH− in the side chain of cardanol.39 However, these peaks almost disappear in the curve of E-BCP. Instead, there appear new peaks at 2.8−3.2 ppm, which can be ascribed to the proton of −CH−O−CH− species (Figure 4b), indicating that the CC double bonds from cardanol have been transformed to epoxide species.41 In addition, the changed chemical shifts from 2.0 ppm (label 4 in Figure 4a) to 1.5 ppm (label 4′ in Figure 4b) also validate the epoxidation reaction of BCP. For the 1H NMR spectrum of PCTA (Figure 4c), the signals of epoxy group disappear, indicating that DOPO has reacted with E-BCP. Also, new peaks at 7.3−8.0 ppm can be attributed to the proton of DOPO’s phosphaphenanthrene group. In order to further verify the structure of the cardanolbased toughing agent, 31P NMR was carried out. There is only one intensive peak at −17.6 ppm for BCP (Figure 4e), resulting from the phosphate moiety of PDCP. For PCTA, two signals appear in its spectrum of 31P NMR. The peak with label a in Figure 4f has the same origin with that of BCP, and the other peak at −39.3 ppm is associated with the P atom of DOPO. The results above prove the phosphorus-containing cardanol-based toughing agents are synthesized successfully. 3.3. Thermal Behavior Analysis. The TGA curves of RF and its UPR composites under a nitrogen atmosphere are plotted in Figure 5a,b, accompanied by their derivative thermogravimetric analysis (DTG) curves. The representative information such as the temperature at mass loss of 10% (T10%) and two maximum thermal degradation temperatures (Tmax1 and Tmax2) is listed in Table S4. For RFs, the mass decrements stem from the thermal rupture and degradation of cellulose polymer chains. However, there are some differences in degradation temperatures between raw fabrics and modified fabrics. Compared with raw RF, the T10% of SA-BN/PEI AF rises from 319 to 338 °C and Tmax also has an obvious increment, indicating the enhanced thermal stabilities of modified fabric. The reason why thermal properties are

Figure 3. Synthetic route of PCTA.

epoxide group.41 The new stretch at 1485 cm−1 in the spectrum of PCTA can be designated to P−Ph, originating from the DOPO structure. Significantly, there is no P−H signal

Figure 4. (a,b,c) 1H NMR results and (d) FTIR curves for BCP, E-BCP, and PCTA, respectively. (e,f) 31P NMR images of BCP and PCTA. 40172


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Figure 5. TGA and DTG curves of (a) RFs and (b) RF/UPR composites.

Figure 6. HRR vs time curves obtained from (a) MCC and (b) cone calorimeter for the RF/UPR composites.

flammable and its PHRR value is as high as 198 W/g. However, with the use of h-BN sheets-AF (AF/UPR), the PHRR value is decreased by 10.0%. Furthermore, the values of PHRR become lower when introducing PCTA. For example, the PHRR value is changed to 158 W/g with 20 wt % PCTA being incorporated (AF/UPR@PCTA20), decreased by 20.2% compared with RF/UPR. The results imply that both h-BN nanosheet and PCTA can inhibit the HRR and improve the fire safety of RF-reinforced UPR composites. To further explore the fire safety of the RF/UPR composites in a real environment, a broadly employed bench-scale test instrument cone calorimeter60 was operated to obtain the HRR curves (Figure 6b). Some important parameters including PHRR, time to ignition (TTI), total heat release (THR), and the ratio of PHRR and time to PHRR (FGI)61 are listed in Table S5. As depicted, the PHRR value of neat RF/UPR composite can reach up to 754 kW/m2. When h-BN sheetmodified RFs are used, the PHRR value of AF/UPR composite is reduced to 692 kW/m2, demonstrating the improved flame retardancy. On incorporating PCTA into the UPR matrix, the PHRR data of all composites are moved to lower values and gradually decrease as PCTA content rises. With the addition of 20 wt % PCTA, the PHRR of the composite is significantly decreased to 443 kW/m2, corresponding to 41.2% decrement compared with raw fabric/UPR composite. When it comes to THR values, the value of all modified composites is lower than that of the neat one. When introducing an increasing content of PCTA into UPR matrix, the THR values tend to decrease. In particular, the incorporation of 20 wt % PCTA brings about a minimum THR value, which is reduced by 17.8% compared to the neat one. This can be ascribed to the barrier effect of absorbed h-BN sheets and the carbonization catalysis of phosphorus-containing flame-retardant PCTA, forming more stable char layers and then preventing the release of heat and inflammable gas. From Table S5, it can be found that the use of AF (AF/UPR) results in an enlarged TTI, while incorporation of PCTA brings about a lowered TTI. This

improved can be attributed to the adherent h-BN plates that possess significant thermal stability.54 Additionally, the amount of char residue formed after the thermal decomposition process also increases, implying that the SA-BN/PEI LbL assembly is beneficial to char formation. For RF/UPR composites, there are mainly two thermal decomposition stages as observed in their DTG curves. The first thermal degradation stage is connected with the scission of cellulose macromolecular in natural fabric during the temperature-rise period, and the second one is related to pyrolysis of the UPR matrix.55 Compared with neat RF/UPR composite, the application of SA-BN/PEI-coated fabric in AF/UPR composite results in the higher T10% and Tmax1, with the slightly higher char residues being formed. For example, the T10%, Tmax1, and char residue for AF/UPR composite are higher than those of neat RF/UPR, exhibiting that the AFs benefit the thermal stabilities of the UPR composites. However, with the addition of PCTA, all RF/UPR composites pyrolyze in advance. The T10% and Tmax1 of the composites decrease as PCTA contents increase, which can be assigned to the worse stability of PO and P−O than common C−C bonds.25 The amount of char residue of RF/UPR composites increases slightly with content of introduced PCTA rising. For example, the char yield for AF/UPR@PCTA20 is 9.8 wt %, while that of AF/UPR is 8.1 wt %. The reason is that P element in PCTA allows it to catalyze the pyrolysis of natural fabric and UPR matrix and thus inspires char formation. The formed char can act as the protective layer to restrain the delivery of oxygen, mass loss, and release of gas pyrolysis.56−58 3.4. Combustion Properties Analysis. As an easy screening measurement, microscale combustion calorimetry (MCC) was carried out to analyze the combustion properties of the RF/UPR composites with just a little bit samples.59 Heat release rate (HRR) curves derived from MCC are exhibited in Figure 6a, where peak HRR (PHRR), one of the most important information to evaluate the fire risk of the UPR composites, is summarized. Raw RF/UPR composite is 40173


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values, such as hydrocarbons (2946 cm−1), CO2 (2357 cm−1), CO (2175 cm−1), carbonyl compounds (1760 cm−1), and aromatic compounds (908 cm−1), indicating decreased pyrolysis hazards and improved fire safety. The reduced pyrolysis products can be ascribed to the interruption of free radical reaction which plays a key role in the improvement of flame retardant efficiency. On the other hand, more compact and complete char layer can also facilitate the reduction of toxic pyrolysis products, which acts as a reinforced blocked layer, hindering the release of gaseous pyrolysis products. Digital photos and SEM pictures of the char residues for RF/UPR, AF/UPR, and AF/UPR@PCTA composites are obtained after the cone calorimeter test. As observed in Figure 9, there are only tiny amounts of fragmentary fibers reserved in the char residues after combustion for RF/UPR, and the joints of fibers were seriously fractured (Figure 9a). Nevertheless, the AF/UPR composite retains the skeleton of the fabrics, indicating that the existence of h-BN sheets on the surface of RF would benefit the char formation (Figure 9b). When incorporating increasing content of PCTA, the AF/UPR@ PCTA composites exhibit a fully compact and well-integrated char layer, which would effectively contribute to the inhibition of heat and pyrolysis transfer in the combustion zone (Figure 9c,d). The much more compact protective char layer for AF/ UPR@PCTA20 composite can be ascribed to the char-forming catalytic action of the PCTA during combustion. To verify this favorable effect, the RT-FTIR technique was operated to investigate the RF/UPR composites at different temperatures. As can be seen from Figure 10, the characteristic peaks for RF/ UPR are as follows: −OH of physical absorbed water (3428 cm−1), −CH2 (2927 cm−1), CO (1732 cm−1), C−O−C (1263 and 1118 cm−1), and the aromatic compounds (1448, 755, and 705 cm−1). However, AF/UPR@PCTA20 composite shows slightly different RT-FTIR spectra to the neat RF/UPR composite, with some new absorption peaks appearing at 1132 and 1016 cm−1. These new peaks originate from the structure of PO and P−O−P and indicate the formation of pyrophosphoric acid, which plays a key role in catalyzing the formation of char barriers. To further explore the elemental composition of the char layer, the XPS test was also applied for char residue of AF/ UPR@PCTA20 composite after the cone calorimeter test (Figure 11a−d). The O 1s peaks at 531.8, 532.9, and 533.8 eV should be ascribed to CO/PO, P−OH, and P−O−P/C− O−C, respectively. The C 1s peak centered at 284.8 eV should be assigned to the C−C bonds, the peaks centered at 286.2 and 288.5 eV corresponds to C−O and CO bonds. P 2p peak at 133.8 eV is assigned to OP−O, originating from the thermal decomposition of DOPO structure in PCTA. The signals at 134.6 and 135.3 eV should be ascribed to P−O−P and PO3, which can correspond to the structures of pyrophosphoric acid and metaphosphoric acid, respectively.64 In addition, 15.56 at. % of B 1s element and 8.12 at. % of N 1s element in the XPS results confirm that h-BN sheets facilitate the formation of char layer. The FTIR spectra can also be used to analyze the composition of char residues (Figure 11e). Compared to the FTIR spectrum of RF/UPR, AF/UPR@ PCTA20 presents the peak of PO and P−O−P derived from pyrophosphoric acid and metaphosphoric acid, indicating that PCTA plays a vital role in the formation of char layer. On the other hand, the formation of char residue can be partly ascribed to the gas phase effect that has been discussed in gas phase analysis. Because of incomplete combustion, the release

trend is in correspondence with the TGA results as mentioned above. The utilization of h-BN sheets-AF enhanced the thermal stabilities of the composites and the prior pyrolysis of PCTA promotes the UPR matrix to form the barrier char. FGI refers to the ratio between PHRR and time to PHRR, and a lower FGI value represents better flame retardancy. The FGI of RF/UPR composite is 4.9 kW/(m2·s), but that of the AF/ UPR@PCTA20 composite is reduced to 3.4 kW/(m2·s), a remarkable 30.6% reduction compared to the neat composite, indicating the dramatically improved fire safety. The fire safety of the composites was also measured by UL-94 tests and limiting oxygen index (LOI). Not only the AF/UPR@PCTA20 composite can reach V0 rating, but also the LOI value is 29.5%, compared with 24.0% of neat RF/UPR composite. 3.5. Mechanism for Improved Fire Safety Properties. To illuminate the significantly improved flame-retardant effect in gas phase, the TG−FTIR test was first conducted.62 Figure 7

Figure 7. (a) 3D images and (b) FTIR spectra of released pyrolysis products for RF/UPR, AF/UPR, and AF/UPR@PCTA20 at the maximum evolution rate, derived from TG−FTIR technique.

shows 3D images of TG−FTIR and FTIR curves obtained from the pyrolysis procedure of RF/UPR composites at the maximum evolution rate. As shown in Figure 7b, the FTIR curve of AF/UPR is similar to that of neat RF/UPR and some typical thermal decomposition products can be obviously identified. For example, as can be observed, the peaks at 2870, 2946, 3034, and 3074 cm−1 can be ascribed to hydrocarbons;63 the characteristic peaks at 2357 and 2175 cm−1 are assigned to CO2 and CO, respectively; the signals of pyrolysis products anhydrides, carbonyl compounds, and aromatic compounds appear in 1806, 1760, and 908 cm−1, respectively. However, compared to RF/UPR and AF/UPR composites, some new adsorption bands appear in the FTIR curve of the AF/UPR@ PCTA20 composite. The new adsorption signals of 1174 cm−1, 982 cm−1, which were referred to PO and P−O in H3PO2, demonstrate phosphate and its derivatives formed in the gas pyrolysis phase. It is known that the free radicals P• and PO• can be generated from some phosphorus-containing substance, such as PH3 and H3PO2, which then react with OH• and H• free radicals and interrupt the radical reaction.25 The intensity curves for total pyrolysis products and typical pyrolysis gas of RF/UPR, AF/UPR, and AF/UPR@PCTA20 composites are extracted from TG−FTIR results (Figure 8). The intensity of total pyrolysis volatiles of AF/UPR@PCTA20 composite is significantly lower than that of RF/UPR and AF/UPR composites. With the addition of phosphorus-containing biobased toughening agents, the maximum absorbance intensity of representative gaseous products presents lower 40174


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Figure 8. TG−FTIR results of pyrolysis products from RF/UPR and AF/UPR@PCTA20: (a) Gram−Schmidt curves, (b) hydrocarbons, (c) CO2, (d) CO, (e) carbonyl compounds, and (f) aromatic compounds, respectively.

Figure 9. Digital photos and SEM images of carbon residues obtained after the cone calorimeter test for (a) RF/UPR, (b) AF/UPR, (c) AF/ UPR@PCTA10, (d) AF/UPR@PCTA15, and (e) AF/UPR@PCTA20 composites.

of small molecular pyrolysis products such as CO2, hydrocarbons and carbonyl compounds is inhibited. Thus, the carbonization reaction for fabric-reinforced UPR composites is further catalyzed because anhydrides are formed in condensed phase, which dehydrate the UPR matrix under high temperature. Therefore, the observation of char structure corresponds to the mechanism of improved fire safety deducted above. The presence of PCTA could not only inhibit the living radical reaction in gas phase but also promote the formation of

complete char layer structure in condensed phase together with h-BN sheets. The flame retardant mechanism is depicted in Figure 12. 3.6. Tensile and Impact Strengths. The RF/UPR composite presents 73.7 MPa and 11.2 kJ/m2 in tensile strength and impact strength, respectively, significantly 26.5 MPa and 8.6 kJ/m2 larger than that of neat UPR, indicating the enhancement effect of RFs on UPR composites. Compared to the RF/UPR composite, the AF/UPR composite with AFs 40175


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plasticizing effect derived from the long alkyl molecular chains in PCTA. However, the long alkyl chains of cardanol group make it possible to obtain a better impact strength for UPR composites. As observed in Figure 13, the AF/UPR composite presents a slightly larger impact strength than the RF/UPR composite, increasing from 11.2 to 11.9 kJ/m2 . The introduction of PCTA into UPR results in the further improvement in impact strength. As the PCTA content increases to 20 wt %, the impact strength of AF/UPR@ PCTA20 composite reaches up to 18.9 kJ/m2, the significant 68.8% improvement compared with RF/UPR, implying an extreme toughening effect of PCTA on UPR composites. The toughening effect of PCTA can be derived from the long alkyl structure of biobased cardanol, which act as a kind of soft phases, bringing the stiff UPR a better impact resistance performance.65,66 Thus, the future looks bright for the application of biobased materials as modifying agents.

4. CONCLUSIONS In this work, to improve the fire safety and impact strength of natural fabric-reinforced UPR composites, a phosphoruscontaining cardinal-based toughening agent was synthesized. The 2D h-BN sheets were assembled on the surface of RFs to improve the interfacial quality and inhibit heat release, with SA acting as a green dispersant during LbL assembly. After assembly, the T10% and Tmax show an increment compared to raw fabric in the TGA test, indicating the improved thermal stability of RF. The AF/UPR@PCTA20 composite presents a 41.2% maximum decrease in the PHRR value and a 17.8% maximum decrease in the THR value, implying enhanced flame retardant performance. In addition, the composite reaches V-0 rating in UL-94 test, and the LOI value increases from 24.0 to 29.5% compared with RF/UPR. The flame retardant mechanism was investigated from gas phase and condensed phase. The presence of PCTA could not only inhibit the living radical reaction in gas phase but also promote formation of complete char layer structure in condensed phase. The 2D h-BN sheets also facilitate the formation of char residues. Furthermore, the AF/UPR@PCTA20 composite shows a significant 68.8% improvement in impact strength

Figure 10. RT-FTIR spectra of (a) neat RF/UPR and (b) AF/UPR@ PCTA20 composites at different pyrolysis temperatures.

shows a slightly enhanced tensile strength, indicating the improved interfacial properties, which can be confirmed by the SEM images of fractured surfaces for RF/UPR and AF/UPR composites in Figure S2. The fractured surface of RF/UPR is extremely rough and disorganized, and many ramie fibers are pulled out of the UPR matrixes, leaving holes and cracks in the fractured surface morphology (Figure S2a−c). The results indicate the terrible interfacial performance between raw RFs and UPR matrixes. Compared with neat RF/UPR composite, AF/UPR shows little exposed fibers and it has a good interface between assembled ramie fibers and UPR, without holes and cracks in the surfaces (Figure S2d−f). When PCTA was introduced into the matrix, the tensile strength of the composites decreased because of the

Figure 11. (a) XPS spectrum; (b−d) high-resolution spectra of O 1s, C 1s, and P 2p regions; and (e) FTIR curves of the char residues for AF/ UPR@PCTA20 composite after the cone calorimeter test. 40176


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Figure 12. Schematic illustration of the mechanism for improved fire safety of modified RF/UPR.

*E-mail: [email protected]. Phone/Fax: +86-551-63600081 (L.S.). ORCID

Weizhao Hu: 0000-0002-3896-4775 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financially supported by the National Natural Science Foundation of China (no. 51603200).

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(1) Hou, Y.; Hu, W.; Gui, Z.; Hu, Y. Effect of Cuprous Oxide with Different Sizes on Thermal and Combustion Behaviors of Unsaturated Polyester Resin. J. Hazard. Mater. 2017, 334, 39−48. (2) Wang, D.; Kan, Y.; Yu, X.; Liu, J.; Song, L.; Hu, Y. In Situ Loading Ultra-small Cu2O Nanoparticles on 2D Hierarchical TiO2graphene Oxide Dual-nanosheets: Towards Reducing Fire Hazards of Unsaturated Polyester Resin. J. Hazard. Mater. 2016, 320, 504−512. (3) Rorrer, N. A.; Dorgan, J. R.; Vardon, D. R.; Martinez, C. R.; Yang, Y.; Beckham, G. T. Renewable Unsaturated Polyesters from Muconic Acid. ACS Sustainable Chem. Eng. 2016, 4, 6867−6876. (4) Thakur, V. K.; Thakur, M. K. Processing and Characterization of Natural Cellulose Fibers/Thermoset Polymer Composites. Carbohydr. Polym. 2014, 109, 102−117. (5) Lavoratti, A.; Scienza, L. C.; Zattera, A. J. Dynamic-Mechanical and Thermomechanical Properties of Cellulose Nanofiber/Polyester Resin Composites. Carbohydr. Polym. 2016, 136, 955−963. (6) Naresh, K.; Shankar, K.; Velmurugan, R. Reliability Analysis of Tensile Strengths Using Weibull Distribution in Glass/Epoxy and Carbon/Epoxy Composites. Composites, Part B 2018, 133, 129−144. (7) Gao, Y.; Liang, X.; Liu, Y.; Bao, W.; Li, S.; Wu, C. Effect of Electrical Stress on Glass Fiber Reinforced Polymer Used in High Voltage Composite Insulator Under Wet Environment. Compos. Sci. Technol. 2018, 155, 151−159. (8) De Luca, F.; Sernicola, G.; Shaffer, M. S. P.; Bismarck, A. “Brickand-Mortar” Nanostructured Interphase for Glass-Fiber-Reinforced Polymer Composites. ACS Appl. Mater. Interfaces 2018, 10, 7352− 7361. (9) Fidanovski, B. Z.; Spasojevic, P. M.; Panic, V. V.; Seslija, S. I.; Spasojevic, J. P.; Popovic, I. G. Synthesis and Characterization of Fully Bio-Based Unsaturated Polyester Resins. J. Mater. Sci. 2018, 53, 4635−4644.

Figure 13. Tensile strength and impact strength of RF/UPR composites.

compared to the neat RF/UPR composite, indicating a good toughening effect of PCTA on UPR composites, because of the long alkyl structure of biobased cardanol. With the use of nontoxic h-BN sheets and biobased materials such as SA, RF, and cardanol, it is expected the research provides a viable green method for the development of environmentally friendly UPR composites in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15355. Characterization techniques; mechanical properties of several typical natural fibers and E-glass; summarization of different PCTAs in refs;34,38 formulations and detailed data of TGA and cone calorimeter test for RF/UPR composites; X-ray maps of SA-BN AFs; and SEM images of fractured surface for RF/UPR and AF/UPR composites (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone/Fax: +86-55163602353 (W.H.). 40177


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Construction of Hierarchical Natural Fabric Surface Structure Based

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