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Applications of Polymer, Composite, and Coating Materials

Construction of Hierarchical Natural Fabric Surface Structure Based on Two-dimensional Boron Nitride Nanosheets and Its Application for Preparing Bio-based Toughened Unsaturated Polyester Resin Composites Fukai Chu, Dichang Zhang, Yanbei Hou, Shuilai Qiu, Junling Wang, Weizhao Hu, and Lei Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15355 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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ACS Applied Materials & Interfaces

Construction of Hierarchical Natural Fabric Surface Structure Based on Two-dimensional Boron Nitride Nanosheets and Its Application for Preparing Bio-based Toughened Unsaturated Polyester Resin Composites

Fukai Chu a, Dichang Zhangb，Yanbei Hou a, Shuilai Qiu a, Junling Wang a, Weizhao Hua, *, Lei Song a, *

a State

Key Laboratory of Fire Science, University of Science and Technology of China, 96

Jinzhai Road, Hefei, Anhui 230026, PR China. bDepartment

of Physical science, University of California, Irvine, CA 92697, USA,

Corresponding authors *Tel./Fax: +86-551-63600081. E-mail: [email protected]. (Lei Song) *Tel./Fax: +86-551-63602353. E-mail: [email protected]. (Weizhao Hu) ORCID: Weizhao Hu: 0000-0002-3896-4775

KEYWORDS: unsaturated polyester resin, natural fabric, composite interphase, phosphorus-containing bio-based toughening agent, fire safety, impact strength 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

ACS Paragon Plus Environment

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environmentally friendly way. In this study, to improve interfacial performance of fabricreinforced UPR composites, the nontoxic two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheets were assembled on the surface of ramie fabrics, where sodium alginate (SA) 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

and

20

wt%

of

PCTA,

the

AF

(assembled

fabrics)/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 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 development of environmentally friendly UPR composites in the future.

1. INTRODUCTION As a thermoset material, URP is particularly attractive in automobile industry and highrise buildings due to its low cost and high anti-corrosion property.1-2 Considerable research


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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

friendly society have become main

renewable

resources

topics of the

modern

and times,

environmentthe

high energy

consumption, detrimental fiber hairy and the difficulty for decomposition limited the use of glass fiber/fabric. In recent years, bio-based composites prepared with natural fiber/fabric have attracted great attention because of their low price and environmental friendliness. Moreover, natural fabric-reinforced UPR composites perform equivalent mechanical properties to glass fiber/fabric reinforced composites.9-11 Ramie fabric has great physical properties and is supposed to be 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 ramie fabricreinforced UPR composites: (1) How to secure the mechanical safety properties of the ramie fabric-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 water-proof 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 ramie fabric. (b) Manufacturing method of natural fiber/fabric-reinforced UPR composites. It has proved that vacuum bagging method showed better mechanical performance than that of 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) The 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 ramie fabric-reinforced UPR composites. Both ramie fabric 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 ramie fabric/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 2D nanosheets, such as hexagonal boron nitride (h-BN), have exhibited great application prospect for fabrication of functional


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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-thethickness impregnation, and then prepared modified carbon fabric/epoxy composites.32 In another example, BN/Pyrocarbon coatings were successfully prepared on SiC fibers via dip-coating technique. The SiC fiber-reinforced ceramic composites containing the BN/PyC coatings showed relatively superior mechanical behavior and unchanged dielectric property, due to low Young’s modulus, good interfacial properties and low electrical conductivity of h-BN.33 Based on that, h-BN sheets can be a key role in improving the interfacial property for ramie fabric-reinforced UPR composites. Furthermore, it would have more helpful prospects in fabrication of fabric-reinforced composites with enhanced thermal stability and flame retardant effect, owning 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 phosphorus-containing toughening agent can be synthesized with toughening and flame retardant effect simultaneously. Some research have synthesized phosphoruscontaining toughening agents applied for different thermoset materials, such as epoxy34-35 and phenolic foams36-38, and the detailed information was summarized in Table S2. However, very few literature has focused on the toughness and flame retardant performance improvement of UPR at the same time, with phosphorus-containing toughening agents. In accordance with the green and sustainable ideality, some renewable



materials can be considered as bio-based 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 non-conjugated C=C bonds, phosphorus elements can be easily introduced to prepare a bio-based phosphoruscontaining toughening agent. In this work, to improve interfacial adhesion between ramie fabrics and UPR, 2D h-BN nanosheets were applied to modify the surface of ramie fabric through a versatile Layerby-Layer (LbL) method, where sodium alginate (SA) acts as a green dispersant to disperse h-BN sheets during the process. Phosphorus-containing 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 bio-based materials such as SA, ramie fabric and cardanol, it is expected the research herein provides a viable green ethod for development of environmentally friendly UPR composites in the future. 2. EXPERIMENTAL SECTION 2.1. Materials. Hexagonal boron nitride (h-BN), polyethyleneimine (PEI, Mw=ca. 70000) and phenyl dichlorophosphate (PDCP) were obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium alginate (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


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acid (m-CPBA) was bought from Anhui Wotu Chemicals Co., Ltd. Plain ramie fabrics 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 three-necked bottle to prepare SA aqueous solution after stirring at 50 ℃ for 2 h. Then, bulk h-BN (0.5 g) was dispersed and exfoliated in SA solution through stirring and bath ultra-sonication 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 250ml deionized water again with the sonicating process lasted for 2 h. To remove the unexfoliated h-BN from the stable SA-BN nanosheets dispersion, the mixture was centrifuged at 1500rpm for 10 min and the supernatant was reserved. 2.3. Layer-by-Layer assembly. First, ramie fabrics were tailored into the same size (150 mm x 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. 0.1 wt% anionic solution was obtained through the SA-BN supernatant above. Afterwards, the ramie fabrics were immersed in these two solutions alternately for 10 times. The fabrics were washed with deionized water to clean the unabsorbed components after each immersion. 2.4. Synthesis of phosphorus-containing cardanol toughing agent (PCTA). Cardanol (0.15 mol), TEA (0.15 mol), 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 dichloromethane and the solution was added dropwise into the mixture above in 2 h at room temperature. Then the system was heated at 70 ℃ 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 follows39: 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 ℃. After that, the product was filtered and the organic layer was washed with sodium sulfite solution, 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 phosphoruscontaining cardanol toughing agent (PCTA) was prepared by reacting E-BCP (0.05 mol) and DOPO (0.2 mol) in the three-necked bottle at 160 ℃ 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 PCPA were blended homogeneously with 2 wt% BPO acting as initiator. Then, put the mixture on the surface of ramie fabrics by hand lay-up method. Subsequently, the sample above was transferred to the prepared vacuum bag. The system was arranged in the oven at 70 ℃ for 4 h and 120 ℃ 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.


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RESULTS AND DISCUSSIOIN 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 non-polar -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 were 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 non-polar 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 ultra-sonicated 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 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 FTIR spectra was 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 cm-1 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 TGA results also show some difference between h-BN and SA-BN sheets (Figure 1e). A 5.50 wt% weight loss is recorded for SA-BN, while the weight of bulk hBN stays unchanged even at 800 °C. The weight loss stage between 200 ℃ and 600 ℃ 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, due to 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 h-BN and SA-BN sheets were confirmed by XRD. As shown in Figure 1h, peaks of bulk h-BN and SA-BN sheets appearing near 26.8°, 41.7o, 44.0o, 50.2o and 55.3o 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 a less ordered stacking SA-BN sheets.52 In addition, the full width half max (FWHM) from (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 less stacked layers for SA-BN sheets.53 The result was further confirmed by TEM images (Figure 1f and 1g). Bulk h-BN has a bulkier and more stacked structure, while SA-BN sheets present a smaller size and a less layers-stacked morphology. In brief, the results of XRD and TEM prove that bulk h-BN is exfoliated into thin layers, which


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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 make it possible to disperse less-stacked BN sheets in water stably. Thus, SA-BN nanosheets dispersion has great potential as the anionic deposition solution in LbL assemble technique. Figure 2a and 2b show the ramie known as “China grass” and ramie woven fabric, respectively. And 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 ramie fabrics. The mass increment is 14.2% based on the mass change after and before LbL process. SEM test was carried out to identify the change of surface morphology. After the ramie fabrics are assembled with 10 bilayers, a thin layer and uniform distributed sediments can be observed on the surface of assembled ramie fabrics (Figure 2f). Meanwhile, the SEM images show that the compact coatings can also connect the neighboring fibers (Figure 2h and 2j). Compared with the assembled fabric, pure ramie fabrics present typical smooth surfaces with little surface adhesive materials appearing (Figure 2e, 2g and 2i). EDX test was operated to verify the deposits originated from SA-BN sheets and PEI. Compared with the pure ramie fabric, B element is detected for LbL assembled ramie fabric, indicating SA-BN sheets are surely absorbed on the surface of ramie fabric. XPS was further applied to confirm the successful assembly of SA-BN sheets and PEI in the surface of ramie fabric (Figure 2d). The coexistence of B 1s and N 1s on the surface of modified ramie fabric is evidenced by the shoulders observed on the peak at 190.56 eV and 398.19 eV, respectively.



The element scale is summarized in XPS figure, with 5.26% B and 5.33% N appearing in the surface of LbL-modified ramie fabric. In addition, the X-ray maps of low-magnification SEM images for SA-BN assembled fabrics show that h-BN sheets are evenly distributed on the surface of ramie fabrics, indicating the construction of hierarchical surface structure (Figure S1). To confirm the product structure in the synthetic route of PCTA (Figure 3), FT-IR analysis was firstly 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 the C=C in side chain of cardanol has been changed into epoxide group.41 The new stretch at 1485 cm-1 in spectrum of PCTA can be designated to P-Ph, originating from DOPO structure. Significantly, there is no P-H signal in the spectra of PCTA, illustrating the reaction between E-BCP and DOPO. NMR measurements were also employed to confirm the structure of cardanol-based toughing agent. Figure 4a-4c show the 1H NMR 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 the C=C double bonds from cardanol have been transformed to epoxide species.41 In addition, the


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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 1H NMR spectrum of PCTA (Figure 4c), the signals of epoxy group disappear, indicating DOPO has reacted with E-BCP. And 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 cardanol-based 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 behaviors analysis. The TGA curves of ramie fabric and its UPR composites under nitrogen atmosphere are plotted in Figure 5a and 5b, accompanied with their derivative thermogravimetric analysis (DTG) curves. The representative information such as the temperature at mass loss of 10% (T10%), two maximum thermal degradation temperatures (Tmax1 and Tmax2) are listed in Table S4. For ramie fabrics, 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 ramie fabric, the T10% of SA-BN/PEI assembled fabric rise from 319 ℃ to 338 ℃ and Tmax also has an obvious increment, indicating the enhanced thermal stabilities of modified fabric. The reason why thermal



properties are 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 increase, implying the SA-BN/PEI LbL assembly is beneficial to char formation. For ramie fabric/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 result 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 that of neat RF/UPR, exhibiting that the assembled fabrics benefit the thermal stabilities of the UPR composites. However, with the addition of PCTA, all ramie fabric/UPR composites pyrolyze in advance. The T10% and Tmax1 of the composites decreases 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 ramie fabric/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 inspire 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


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3.4. Combustion properties analysis. As an easy screening measurement, MCC was carried out to analyze the combustion properties of the ramie fabric/UPR composites with just a little bit samples.59 Heat release rate (HRR) curves derived from MCC are exhibited in Figure 6a, where peak heat release rate (PHRR), one of the most important information to evaluate the fire risk of the UPR composites, is summarized. Raw ramie fabric /UPR composite is flammable and its PHRR value is as high as 198 W/g. However, with the use of h-BN sheets-assembled fabric (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 both h-BN nanosheet and PCTA can inhabit the heat release rate and improve the fire safety of ramie fabric-reinforced UPR composites. To further explore the fire safety of the ramie fabric/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 sheets-modified ramie fabrics are used, the PHRR value of AF/UPR composite is reduced to 692 kW/m2, demonstrating the improved flame retardancy. On incorporating PCTA into 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, down 17.8% compare to the neat one. This can be ascribed to the barrier effect of absorbed h-BN sheets and the carbonization catalysis of phosphoruscontaining 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 assembled fabric (AF/UPR) result in an enlarged TTI, while incorporation of PCTA bring about a lowered TTI. This trend is in correspondence with the TGA results as mentioned above. The utilization of h-BN sheets-assembled fabric 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


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improved flame-retardant effect in gas phase, TG-FTIR test was firstly conducted.62 Figure 7 shows 3D images of TG-FTIR and FTIR curves obtained from pyrolysis procedure of ramie fabric/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 cm-1, 2946 cm-1, 3034 cm-1 and 3074 cm-1 can be ascribed to hydrocarbons63; The characteristic peaks at 2357 cm-1 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 cm-1, 1760 cm-1 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 cm1,

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, 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 bio-based toughening agents, the maximum absorbance intensity of representitive gasous products present lower 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 play 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 pyropysis products, which acts as a reinforced blocked layer, hindering the release of gasous pyrolysis products. Digital photos and SEM pictures of the char residues for RF/UPR, AF/UPR and AF/UPR@PCTA composites are obtained after cone calorimeter. 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, AF/UPR composite retains the skeleton of the fabrics, indicating the existence of h-BN sheets on the surface of ramie fabric 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 phosphorus-containing toughening agent during combustion. To verify this favorable effect, RT-FTIR technique was operated to investigate the ramie fabric/UPR composites at different temperatures. As can be seen from Figure 10, the characterictic 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 cm-1 and


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1118 cm-1), and the aromatic compounds (1448 cm-1, 755 cm-1 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 appraring at 1132 cm-1 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 fomation of char barriers. To further explore the elemental composition of the char layer, XPS test was also applied for char residue of AF/UPR@PCTA20 composite after cone calorimeterc (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 PO-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 comfirm that h-BN sheets faciliate the formation of char layer. The FTIR spectra can also be used to analyse the compsotion 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 PCTA plays a vital role in the formaiton 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. Due to incomplete combustion, the release of small



molecular pyrolysis products such as CO2, hydrocarbons and carbonyl compounds are inhabited. Thus, 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 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 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 ramie fabrics on UPR composites. Compared to RF/UPR composite, AF/UPR composite with assembled fabrics shows a slightly enhanced tensile strength, indicating the improved interfacial properties, which can be confirmed by the SEM images of fractured surface 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 ramie fabrics 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 matrix, the tensile strength of the composites


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decreased due to the 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 RF/UPR composite, increasing from 11.2 kJ/m2 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 bio-based 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 bio-based materials as modifying agents. 4. CONCLUSION In this work, to improve the fire safety and impact strength of narural fabric-reinforced UPR composites, a phosphorus-containing cardinal-based toughening agent was synthesized. 2D h-BN sheets were assembled on the surface of ramie fabrics to improve the interfacial quality and inhibit heat release, with sodium alginate acting as a green dispersant during LbL assembly. After assembly, the T10% and Tmax show an increment compared to raw fabric in TGA test, indicating the improved thermal stability of ramie fabric. The AF/UPR@PCTA20 composite presents the 41.2% maximum decrease in PHRR value and a 17.8% maximum decrease in 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 facilitates the formation of char residues. Furthermore, the AF/UPR@PCTA20 composite shows a significant 68.8% improvement in impact strength compared to the neat RF/UPR composite, indicating a good toughening effect of PCTA on UPR composites, due to the long alkyl structure of bio-based cardanol. With the use of nontoxic h-BN sheets and biobased materials such as SA, ramie fabric and cardanol, it is expected the research provides a viable green method for development of environmentally friendly UPR composites in the future. SUPPORTING INFORMATION Characterization techniques: Mechanical properties of several typical natural fibers and E-glass (Table S1); Summarization of different phosphorus-containing toughening agents in Ref. 34-38; Formulations and detailed data of TGA and cone calorimeter for ramie fabric/UPR composites (Table S3-S5); X-ray maps of SA-BN assembled fabrics (Figure S1); SEM images of fractured surface for RF/UPR and AF/UPR composites (Figure S2). CONFLICT OF INTEREST The authors declare no conflict of interest. ACKNOWLEDGMENTS The research was financially supported by the National Natural Science Foundation of China (No. 51603200).


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Figure Captions 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. Figure 2 (a, b) Photographs of ramie and ramie fabrics. (c) Schematic illustration of



LbL assembled process with PEI cationic solution and SA-BN anionic dispersion. (d) XPS results of raw ramie fabrics and assembled fabrics. (e, g, i) SEM images of raw ramie fabrics. (f, h, j) SEM images of assembled ramie fabrics. Inset in (i, j) is the EDX images of raw fabrics and SA-BN assembled fabrics, respectively. Figure 3 Synthetic route of phosphorus-containing toughening agent (PCTA). Figure 4 (a, b, c) 1H-NMR results, (d) FTIR curves for for BCP, E-BCP and PCTA, respectively. (e, f) 31P-NMR images of BCP and PCTA. Figure 5 TGA and DTG curves of (a) ramie fabrics and (b) ramie fabric/UPR composites. Figure 6 HRR vs time curves obtained from (a) MCC and (b) cone calorimeter for the ramie fabric/UPR composites. Figure 7 3D images (a) and FTIR spectra (b) of released pyrolysis products for RF/UPR, AF/UPR and AF/UPR@PCTA20 at the maximum evolution rate, derived from TG-FTIR technique. 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 cone calorimeter for (a) RF/UPR, (b) AF/UPR, (c) AF/UPR@PCTA10, (d) AF/UPR@PCTA15 and (e) AF/UPR@PCTA20 composites. Figure 10 RT-FTIR spectra of (a) neat RF/UPR and (b) AF/UPR@PCTA20 composites


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at different pyrolysis temperatures. Figure 11 (a) XPS spectrum, (b, c, 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 cone calorimeter test. Figure 12 Schematic illustration of Mechanism for improved fire safety of modified ramie fabric/UPR. Figure 13 Tensile strength and impact strength of ramie fabric/UPR composites.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Figure 8



Figure 9


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Figure 10



Figure 11


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Figure 12



Figure 13


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Abstract Graphic


Construction of Hierarchical Natural Fabric Surface ... - ACS Publications

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