This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 4891−4898
Mussel-Inspired General Interface Modification Method and Its Application in Polymer Reinforcement and as a Flame Retardant Hao Wang,† Xuan Zhou,† Masroor Abro,‡ Ming Gao,† Meigui Deng,† Zhi Qin,† Yingjuan Sun,† Lina Yue,† and Xiaoqian Zhang*,† †
College of Environmental Engineering, North China Institute of Science and Technology, Beijing 101601, P. R. China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
‡
S Supporting Information *
ABSTRACT: Inspired by the remarkable adhesion of mussels, the mimicking of natural adhesive molecules has been widely used for surface modification. In the present study, an economical and easily available biomimic material named as tannic acid−Fe3+ (TA−Fe3+) was first directly used as a surface modifier, carbonization agent, smoke inhibitor, and flameretardant synergist. Compared with the flame-retardant magnesium hydroxide (Mg(OH)2), TA−Fe3+-modified Mg(OH)2 endowed polyamide 6 (PA 6) with improved mechanical performance and flame-retardant properties. The flame-retardant and smoke-suppressant properties were tested by the limiting oxygen index and cone calorimeter tests. The flame-retardation mechanism was investigated by thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy. The tensile strength could increase up to 90%, and the modified flame retardant was found to have higher UL-94 grade with the same dosage of flame-retardant additives. The peak heat release rate, total heat release, peak of smoke production rate, and total smoke production were significantly reduced. The synergistic effect between TA−Fe3+ and Mg(OH)2 was also discussed. This study provides new insights into the direct utilization of a biomimicking adhesive molecule, TA−Fe3+, to realize simultaneous composite reinforcement and flame-retardant property enhancement. Meanwhile, because of the extensive synergies of flame-retardant metal oxide with iron element and the universal growth characteristics of TA−Fe3+, it has potential applications in the preparation of various flame-retardant polymers.
1. INTRODUCTION Polyamide 6 (PA 6) has received considerable attention in recent years because of its excellent mechanical properties, selflubrication, abrasion resistance, electrical properties, and chemical corrosion resistance. Thus, PA 6 is extensively used in automotive, marine, electronic appliances, industrial machinery, and consumer goods markets.1 However, the applications of PA 6 have been restricted because of its poor flame retardancy.2 Considering green and environmental awareness, the inorganic series of flame retardants, such as metal hydroxides, which are nontoxic, efficient, and smokeless, have gained popularity across the world.3 However, metal hydroxide particles are extremely easy to aggregate because of the strong polarity of the molecules, which results in poor adhesion between the interface of metal hydroxide and the polymer matrix.4 The poor homogeneous dispersion of metal hydroxide particles in the polymer matrix results in a decrease in the mechanical properties.5 Further, a large amount of metal hydroxide is added to the polymer matrix, more than 50 wt %, which can also have a significant influence on the processing performance and mechanical properties of the resulting materials.6 Therefore, to improve the mechanical performance and flame-retardant properties simultaneously, developing a high-efficient synergist via reasonable compounding and surface © 2018 American Chemical Society
modification becomes one of the most important problems in polymer and materials science. Recently, mussels have aroused intensive interest because of their strong adhesive abilities resulting from their foot protein.7,8 Researchers show that catechol and amino groups in the foot protein play a key role in the adhesion process.9,10 Lee and Messersmith selected dopamine as a bioinspired adhesive molecule and employed polydopamine (PDA) coatings as a general method for surface modification.11 Up to now, PDA has been widely applied in the preparation of adhesives,12 inorganic mineralization,13 membrane separation,14 wettability,15 bioactivity,16 and antibacterial surfaces.17 Recently, Li et al.18 used PDA as a surface-reacted layer to prepare ultrafine Fe(OH)3 on PDA coatings and a high-effective flameretardant synergy was obtained. However, the high cost of PDA limits its application in the industry. Now, scientists and researchers are attempting to explore lower-cost materials with similar properties, such as the tannic acid−iron complexes (TA−Fe3+)19 and polyamine−catechol compounds.20 Received: January 29, 2018 Accepted: April 20, 2018 Published: May 4, 2018 4891
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega TA−Fe3+ complexes have potential applications in the industry because of their low cost, their convenience in preparation, and their ability to form the modification coating on the material surface via chelation quickly. The use of TA− Fe3+ for surface modification can result in the iron ion to be dispersed at the atomic level.21 It is noteworthy that the iron salt is often used as a fire-retardant and smoke-suppression agent individually or as part of a broader system.22 By virtue of these advantages, rational utilization of biomussel-inspired TA− Fe3+ and a metal hydroxide flame retardant to obtain a highperformance flame-retardant material was inspiring. TA−Fe3+ and the metal hydroxide in their combined state were applied in a combustible PA 6 material for the first time in this work. The flame-retardant properties were investigated by using the limiting oxygen index (LOI), UL-94 vertical burning tests, thermogravimetric analysis (TGA), and cone calorimeter testing (CCT). Scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopic analysis were employed to observe the morphology and structure of char residues by PA 6 composites after combustion. Finally, the role of TA−Fe3+ in the system was also discussed. In the current study, Mg(OH)2 has been chosen as the metal hydroxide flame retardant and PA 6 is used as the matrix. Surprisingly, TA−Fe3+-modified coating not only improved the mechanical property but also increased the flame-retardant effect, resulting in multiple effects working together to obtain better comprehensive performance. Further analysis was carried out for studying the mechanical and film-retardant properties. Besides, SEM and FT-IR spectroscopy were employed to study the morphology and structure of char residues by PA 6 composites after combustion.
Figure 1. Schematic of the PA 6/TA−Fe3+/Mg(OH)2 compound.
2. RESULTS AND DISCUSSIONS 2.1. Characterization of the PA 6/TA−Fe3+/Mg(OH)2 Compound. Imitation mussel materials have been widely used for surface modification because of their universal strong adhesion characteristics. In this study, TA−iron, a cheap and broadly available imitation mussel material, grew on the surface of Mg(OH)2, and through mixing, extrusion, granulation, and injection with PA 6 matrix, a series of composite materials were prepared. The schematic of the prepared structure is shown in Figure 1. TA−Fe3+ could be evenly distributed on the surface of Mg(OH)2 because of the strong hydrogen-bonding interaction between the hydroxyl of TA and PA 6. The 200 mesh-sized Mg(OH)2 showed micron-sized particles and there was almost no change seen after TA−Fe3+ modification (Figure S1). The coordination constant of TA−Fe3+ was lg K = 24.6 in 0.1 M NaCl solution at 20 °C,24 which demonstrated the reliable stability of the chelate. The crystalline structure of Mg(OH)2 had no obvious change after the TA−Fe3+ modification (Figure S2). The FT-IR spectra of pristine Mg(OH)2, TA−Fe3+, and TA−Fe3+/Mg(OH)2 are illustrated in Figure 2a. The strong absorption peak at 3700 cm−1 was due to the stretching vibration caused by O−H bond in the crystals.25 The peaks in 1307 and 1177 cm−1 were assigned to the aromatic C−O asymmetrical stretch,25 which demonstrated the presence of TA. Mg(OH)2 is an ionic compound and has strong molecular polarity which is not favorable for its dispersion in PA 6. The contact angle of 0° was found for Mg(OH)2 indicating hydrophilicity. However, the contact angle was found to be 20° after it was modified with TA−Fe3+, which proved that the modification process overshadowed the molecular polarity of
Figure 2. (a) FT-IR spectra of pristine Mg(OH)2, TA−Fe3+, and TA− Fe3+/Mg(OH)2 and photographs of a water droplet on (b) initial and (c) modified Mg(OH)2.
Mg(OH)2. The modified material also exhibited hydrophilicity, which was beneficial to dispersion. The TGA curve of TA−Fe3+ is depicted in Figure 3. It could be seen that TA−Fe3+ had only about 12 wt % loss at 280 °C, which was due to the dehydration of TA and elimination of TA.26 The major degradation of TA−Fe3+ occurred at around 400 °C, whereas the main decomposition of TA occurred at 350 °C, which was consistent with the literature.26 In comparison, the decomposition temperature of the complex
Figure 3. TGA curve of TA−Fe3+ under air atmosphere. 4892
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega
an increasing amount of unmodified Mg(OH)2. At the same time, both the plasticity and the elongation of the material were decreased. The rupture elongation of PA 6 composites with 45 wt % Mg(OH)2 was decreased by 80% in comparison with that of the PA composite containing 15 wt % Mg(OH)2. The composite materials with modified Mg(OH)2 showed better mechanical properties such that the elongation and tensile strength of the composites at the low dosage (15 and 30 wt %) were improved. The elongation of 45 wt % modified Mg(OH)2 composites was slightly decreased but the tensile strength was increased by 90%. This might due to the multiple hydroxyl groups of TA which could produce stronger hydrogen-bonding forces with the nylon polymer, as described in the Figure 1. Thus, the resulting materials had improved mechanical properties because of the surface modification, which was also observed in the SEM photos of the tensile fracture surface shown in Figure 6. As shown in Figure 6, the sample section of PA 6 with unmodified 45 wt % Mg(OH)2 had obvious Mg(OH)2 lumps, and there was a distinct phase separation at the interface between Mg(OH)2 and PA 6. This indicated that Mg(OH)2 was more prone to self-aggregation due to the strong polarity interaction of Mg(OH)2 with itself because of the incompatibility between Mg(OH)2 and PA 6. The defects of these phases were more likely to produce stress concentrations when the sample was stretched, resulting in reduced mechanical properties of the sample. However, there was no obvious phase separation in the cross section of TA−Fe 3+ -modified composites, and large agglomeration of Mg(OH)2 was diminished. This indicated that the surface modification of the bioadhesive mussel coating could greatly improve the compatibility between the interfaces of the composites and metal hydroxide, resulting in the improvement of their mechanical properties. At the same time, more drawing phenomena occurred at the cross section of the composite with modified Mg(OH)2. This revealed a stronger interaction between the modified composites, which was related to the formation of hydrogen bonds between the TA−Fe3+-modified layer and nylon. 2.3. Combustion Behavior. As depicted in Figure 7, after adding 0, 15, 30, and 45 wt % Mg(OH)2 flame retardant into PA 6, the LOI values of PA 6 were increased from 22.1 vol % to 23.7, 25.1, and 29.1 vol %, respectively. It could be stated that a higher oxygen density was required to initiate and sustain the combustion of the specimen after the addition of Mg(OH)2 into neat PA 6. It was nonobvious in low content, which was due to Mg(OH)2 by blocking the moisture released during thermal decomposition, leading to reducing the temperature in the combustion zone. In contrast with the LOI value of 23.7% for 15 wt % Mg(OH)2 in PA 6, the same content of added TA−Fe3+/Mg(OH)2 showed the LOI value of 25.6%, which was an improvement by 1.9%. This phenomenon not only appeared in low mass fractions but also in the higher mass fractions, for example, 30 wt % (LOI value 27.5%) and 45 wt % (LOI value 31.2%), which indicated that Fe3+ with atomic-level distribution distinctly hindered the ignition behavior. Interestingly, 30 wt % modified Mg(OH) 2 /PA 6 compound (containing less flame-retardant additives) compared with 45 wt % unmodified Mg(OH)2/PA 6 compound showed the same UL-94 classification. It was revealed that there existed a synergistic effect between the TA−Fe3+ and Mg(OH)2 system, and thus, increasing the TA−Fe3+/Mg(OH)2 content was a
slightly increased, which indicated better thermal stability of TA−Fe3+ than that of TA. The processing temperature of the composite material was 220 °C at which the structural stability and chemical properties of TA−Fe3+ could be definite. In the processing via the single-screw extruder, shear forces led to higher temperatures, which might have influenced the stability and chemical properties of TA−Fe3+. Therefore, FT-IR spectroscopy analysis for the TA−Fe3+ complex heated at 220, 250, and 280 °C in the air was conducted. As shown in Figure 4, the curves of TA−Fe3+ after the heat treatment at 220, 250, and 280 °C went through slight changes,
Figure 4. FT-IR spectra of TA−Fe3+ itself after heat treatment at 220, 250, and 280 °C.
such as H bonding at 3325 cm−1, CO stretching vibration at 1680 cm−1, aromatic C−O symmetric stretching vibration at 1590 cm−1, aromatic C−O asymmetric stretching vibration at 1338 and 1187 cm−1, C−O−C stretching vibration at 1020 cm−1, and C−H out-of-plane bending at 750 cm−1. The above results did not reveal any obvious changes in the chemical structure of the TA−Fe3+ complex after heat treatment at 280 °C, and this showed that TA−Fe3+ could attach to the surface of Mg(OH)2 in the extrusion process. 2.2. Mechanical Properties of the Composites. Generally, a major shortcoming with respect to traditional inorganic flame retardants is the deterioration of mechanical properties resulting from their poor homogeneous dispersion. On the basis of the interfacial modification method of musselinspired materials, the initial interest was to improve inorganic filler dispersion and mechanical properties. The mechanical properties of the composites were measured by tensile testing, and the results are shown in Figure 5. The tensile strength of the material was found to increase first and later decrease with
Figure 5. (a) Stress−strain curves of PA with different composites and (b) the partially enlarged image of (a). 4893
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega
Figure 6. SEM results of the microstructure of the tensile failure section of the composites with 45 wt % (a) Mg(OH)2 and (b) modified Mg(OH)2.
Figure 7. (a) LOI and UL-94 results of PA 6/Mg(OH)2 and PA 6/Mg(OH)2/TA−Fe3+ and (b) the experimental (exp.) and calculated (cal.) TGA curves of TA−Fe3+/Mg(OH)2.
Figure 8. Measured (a) HRR and (b) THR curves of PA 6, PA 6/Mg(OH)2, and PA 6/TA−Fe3+/Mg(OH)2 with the fire-retardant mass fraction of 45%.
more effective approach to enhance the flame retardancy of neat PA 6. To investigate the synergistic effect of TA−Fe3+/Mg(OH)2, their thermal decomposition behavior was investigated by using TGA under air, as shown in Figure 7b. The experimental (exp.) curve was obtained by TGA and the calculated (cal.) curve was calculated according to the weight percentage of every component (as seen in Figure S3) in the compound.27 The results revealed that TA−Fe3+/Mg(OH)2 had a three-step degradation in the TGA curve. There was little degradation before 380 °C, which might due to the dehydration of TA, elimination of acetic acid, and decarboxylation.26 At this stage, the exp. value was smaller than the cal. value, which meant that Mg(OH)2 catalyzed the degradation of TA−Fe3+. From 380 to 420 °C, the degradation was mainly due to the decomposition of Mg(OH)2. After 420 °C, the exp. value was much larger than the cal. value, which meant that there was a remarkable synergistic effect between MgO [decomposition by Mg(OH)2]
and TA−Fe3+. Mg(OH)2 and its decomposition product MgO combined with the Fe element to facilitate the TA degradation to form a refractory aromatic compound and char, which was effective in reducing mass/heat transfer. After 680 °C, the compound was burned because of the presence of oxygen. Cone calorimeter testing (CCT) is regarded as the most useful bench-scale tests that can simulate real-world fire conditions.28 To identify the role of TA−Fe3+ and Mg(OH)2 during the combustion process of the PA samples, CCT was used. The detected samples included neat PA 6, PA 6 with 45 wt % Mg(OH)2, and PA 6 with 45 wt % TA−Fe3+/Mg(OH)2. Heat release rate (HRR) is a measure to test the heat release per unit surface area of burning materials, which is believed to have the greatest influence on burning behavior.29 The HRR curves for each sample are presented in Figure 8a. It was shown that the neat PA burned fast after ignition with the peak of heat release rate (PHRR) of 512 kW/m2. The PHRR value was significantly reduced in the presence of Mg(OH)2 (263 kW/ 4894
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega m2), and it reached 195 kW/m2, a 26% reduction, with the incorporation of TA−Fe3+. This indicated that TA−Fe3+ chelate-modified Mg(OH)2 improved the flame-retardant properties of the composite. There was no residue found for neat PA 6 after CCT. In the case of PA 6 with Mg(OH)2/TA− Fe3+, the char was more compact and stronger (as shown in Figures S4 and S5), whereas for PA 6 with Mg(OH)2, it was more vulnerable and fragile, which indicated that unmodified Mg(OH)2 was less effective in reducing mass/heat transfer of the composite. The total heat release (THR) curves presented in Figure 8b exhibit results similar to those of HRR. The THR curves showed that neat PA 6 released a total heat of 138.2 MW/m2 at the end of burning, whereas it was 99.1 and 83.9 MW/m2 in the case of PA 6/Mg(OH)2 and PA 6/TA−Fe3+/Mg(OH)2, respectively, with a 16% reduction relative to PA 6/Mg(OH)2. The THR values indicate that the TA−Fe3+ chelate showed prominent synergy with Mg(OH)2 and yielded a higher flameretardant efficiency. Figure 9 shows the mass loss (ML) curves of different samples. After the cone calorimeter test, there were 0, 26.2, and
synergistic effect between TA−Fe3+ and Mg(OH)2 and the catalytic ability of Fe2O3 facilitated the generation of compact char which improved the flame-retardant properties of the composite material simultaneously. The smoke production rate (SPR) and total smoke production (TSP) are the important factors in death by fire.29 The SPR and TSP curves in Figure 10 revealed that the addition of 45 wt % Mg(OH)2 in the PA 6 matrix could notably suppress combustion, while the peak SPR value (0.245 m2 s−1) of the compound was the highest among all samples, as shown in Figure 10a. Additionally, the TSP value of PA 6 was higher than that of PA 6/Mg(OH)2 (2.1 m2) after 1377 s which indicated Mg(OH)2 as a smoke suppressor for PA. TA−Fe3+ chelate-modified Mg(OH)2 illustrated the prominent smoke suppression effect. PA 6/TA−Fe3+/Mg(OH)2 (45 wt %) gave rise to a remarkable reduction of peak SPR and the TSP values to 0.177 m2 s−1 and 1.307 m2, revealing the reduction percentage of 28 and 38% relative to PA 6/Mg(OH)2, respectively. Simultaneously, the PA 6/TA−Fe3+/Mg(OH)2 specimen emitted smoke after 200 s and took longer time to reach the maximum smoke density, which signified that the iron salt made the spline a flameless combustion model by effectively reducing the burning rate and the amount of smoke. This was due to the inherent smoke inhibition of iron31 and the prominent synergistic effects with Mg(OH)2. Apart from the abovementioned LOI, HRR, THR, ML, SPR, and TSP, another important parameter for evaluating flame retardation of polymeric materials is the emission of toxic gases which are a secondary fire risk. The toxic gases generated by combustion principally are CO accompanied with CO2.32 Therefore, the determination of CO from various materials is conducive to illustrate the relative risks. Figure 11 illustrated that the CO production (COP) was slightly increased over the period of 440 s when Mg(OH)2 was
Figure 9. ML as a function of time.
32.4 wt % of residue left, respectively, for neat PA 6, PA 6/ Mg(OH)2, and PA 6/TA−Fe3+/Mg(OH)2. PA 6/TA−Fe3+/ Mg(OH)2 showed the lowest ML value, which indicated that PA 6/TA−Fe3+/Mg(OH)2 had the highest residual carbon. This was due to the following two reasons: first, the Mg(OH)2 and Fe element facilitated the TA degradation and then formed the refractory aromatic compound and char (as seen Figure 7b) and second, the Fe3+ ion in TA−Fe3+ was transformed into Fe2O330 after the combustion process (seen the X-ray photoelectron spectroscopy (XPS) results in Figure S6), which also promoted the catalytic charring behavior. The
Figure 11. CO production as a function of time.
Figure 10. (a) SPR and (b) total SPR as a function of time. 4895
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega added into PA 6, whereas 45 wt % PA 6/TA−Fe3+/Mg(OH)2 showed an increase in COP after 500 s. This was attributed to the better flame-retardant properties of TA−Fe3+/Mg(OH)2 and the resulting incomplete flammability of the composites. 2.4. Proposed Mechanism. The mussel-inspired biomimic TA−Fe3+ chelate coating grew evenly on the surface of flameretardant Mg(OH)2, which reduced the polarity of the Mg(OH)2 surface and resulted in the reduction of agglomeration, and additionally, it favored the distribution of Mg(OH)2 in the PA 6 matrix. The formation of holes became hard when broken, thus improving the mechanical properties of the material. It was noteworthy that because of the structure of TA and nylon, there were a large number of hydrogen bonds, which also improved the mechanical properties of the compound. While considering the fire resistance, the uniform distribution of Mg(OH)2 could improve the utilization of flame-retardant materials and enhance the flame-retardant properties. During combustion, Fe2O3 formed by uniformly dispersed iron ions on the material surface catalyzed charring more effectively, which in turn improved fire retardancy. This phenomenon could be attributed to the strong protective effect from the formation of a carbonaceous layer. Meanwhile, the phenyl ring compound formed by the Mg(OH)2 and Fe element synergistic and thermal decomposition of TA was surmised to cross-link the matrix material for the formation of char. The above reasons resulted in compact and tough carbon, which was beneficial to reduce the overall loss of mass, heat transfer, and slow the degradation of the polymer, thus enhancing the flame retardancy in the condensed phase. The iron element and Mg(OH)2 as smoke inhibitors served to reduce the smoke content during combustion by their synergistic effects, thus depressing possible smoke poisoning in realistic fires.
Suzhou Enhanry Advanced Materials Co., Ltd. All of the chemicals were used as received. 4.2. Preparation of TA−Fe3+-Modified Mg(OH)2. TA (2g) and 2 g of ferric trichloride were dissolved in 200 mL of deionized water separately to form solutions. Mg(OH)2 (400 g) was dispersed in 600 mL of deionized water. Then, solutions of both TA and ferric chloride were added into the Mg(OH)2 solution simultaneously, followed by fast and vigorous stirring for 2 min. Meanwhile, NaOH was used to adjust the pH to 8. The mixed solution was filtered and washed. The overall procedure was repeated three times. Later, the product was dried in a vacuum oven at 90 °C. The dry compound was pounded in a mortar and then sieved through a 200 mesh screen to produce TA−Fe3+-modified Mg(OH)2. 4.3. Preparation of the PA Composite Material. The PA 6/Mg(OH)2 composite and PA 6/TA−Fe3+/Mg(OH)2 composite were prepared with a single-screw extruder. To mix granulation, it was partly injected into a standard spline and then related mechanical performance test was conducted. The other composite was heated in the mold and cured, and then, the sample was cut precisely, which was subjected to the fire testing. The composites added contained 15% Mg(OH)2 and were denoted as 15 wt % PA 6/Mg(OH)2. 4.4. Characterization. 4.4.1. LOI Test. The LOI test was conducted with the JF-3 oxygen index test instrument (Jiangning, China) in terms of the standard LOI test, ASTM D 2863-97. The sample size for the LOI measurement was 100 × 10 × 10 mm3. Each measurement was repeated at least five times, and the results were reported as the average LOI value in this work. 4.4.2. Vertical Burning Tests. The vertical burning test was performed with a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Company, China). The samples used were 130 × 12.7 × 3 mm3 according to the UL94 test (ASTM D3801 standard). The procedures of conducting this test as well as the definitions and units of vertical burning rate were referred based on the literature.23 Each measurement was repeated at least five times, and the results were reported as the average value in this work. 4.4.3. Mechanical Properties. The mechanical properties were analyzed using a WSM-20KB universal testing machine (Changchun, China) according to GB/T 1040-2006. Three samples were used and the results were averaged for this analysis. 4.4.4. Thermogravimetric Analysis. TGA was carried out on a HCT-2 thermal analyzer (Beijing Hengjiu Scientific Instrument Factory) under air atmosphere at a heating rate of 10 °C min−1. Samples (4 mg), at room temperature initially, were heated from room temperature to 800 °C. 4.4.5. Scanning Electron Microscopy. The surface morphology of the specimens was investigated by SEM using JEOL 7500. The samples were sputter-coated in gold before scanning to provide an electrically conductive surface. The acceleration voltage used for imaging was 5 kV. 4.4.6. Cone Calorimeter Testing. A Cone calorimeter ASTM M 1354 was used for predicting the HRRs of materials and products. Approximately, 13.1 MJ of heat was released per kilogram of oxygen consumed. The external heat flux was 55 kW m−2 because it corresponds to a common heat flux in a mild fire scenario. The samples with dimensions of 100 × 100 × 3 mm3 were exposed to a Stanton Redcroft cone calorimeter with a heat flux of 55 kW m−2. The averaged cone data
3. SUMMARY The low-cost bio-inspired mussel adhesion material TA−Fe3+ has been successfully grown on the surface of Mg(OH)2, a flame retardant, and the modified flame retardant material could be evenly dispersed in the PA matrix. Meanwhile, the functional group such as the hydroxyl from TA formed strong interactions with PA 6 by hydrogen bonding, which significantly improved the mechanical properties of the resulting composites. Iron was evenly dispersed on the surface of Mg(OH)2; the Mg(OH)2 and Fe element synergistically facilitated the TA degradation to form the refractory compound and char during the combustion process and ferric oxide was yielded simultaneously. This encouraged the formation of a dense carbon layer by catalytic charring at the interface, which increased the amount of carbon residue resulting in reducing quality loss and heat transfer. At the same time, modified Mg(OH)2, a smoke inhibitor, significantly reduced the smoke density by synergistic effect with Mg(OH)2. On the account of the general growth and adhesion characteristics of TA−Fe3+, this can be regarded as a low-cost, convenient, and general method to prepare high-strength and efficient flame-retardant materials. 4. EXPERIMENTAL SECTION 4.1. Materials. TA, ferric trichloride, and sodium hydroxide were purchased from Beijing Chemicals Co. (Beijing, China). PA 6 chips (relative viscosity of 2.5, density of 1.127 g cm−3) and Mg(OH)2 (5000 mesh, purity ≥ 95%) were provided by 4896
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega
retardant polypropylene composites. J. Membr. Sci. 2009, 44, 1324− 1332. (5) Wang, Z.; Chen, Z.; Fan, W.; Nie, W. Effects of Surface Modifiers on Mechanical and Rheological Properties of Halogen-Free Flame Retarded Polyethylene Composites. Polym.-Plast. Technol. Eng. 2006, 45, 191−196. (6) Song, L.; Hu, Y.; Lin, Z.; Xuan, S.; Wang, S.; Chen, Z.; Fan, W. Preparation and properties of halogen-free flame-retarded polyamide 6/organoclay nanocomposite. Polym. Degrad. Stab. 2004, 86, 535−540. (7) Waite, J. H.; Tanzer, M. L. Polyphenolic Substance of Mytilus edulis: Novel Adhesive Containing L-Dopa and Hydroxyproline. Science 1981, 212, 1038−1040. (8) Lin, Q.; Gourdon, D.; Sun, C.; Holten-Andersen, N.; Anderson, T. H.; Waite, J. H.; Israelachvili, J. N. Adhesion mechanisms of the mussel foot proteins mfp-1 and mfp-3. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 3782−3786. (9) Kang, S. M.; You, I.; Cho, W. K.; Shon, H. K.; Lee, T. G.; Choi, I. S.; Karp, J. M.; Lee, H. One-Step Modification of Superhydrophobic Surfaces by a Mussel-Inspired Polymer Coating. Angew. Chem. 2010, 49, 9401−9404. (10) Wu, J.; Zhang, L.; Wang, Y.; Long, Y.; Gao, H.; Zhang, X.; Zhao, N.; Cai, Y.; Xu, J. Mussel-Inspired Chemistry for Robust and SurfaceModifiable Multilayer Films. Langmuir 2011, 27, 13684−13691. (11) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426−430. (12) Lee, H.; Lee, B. P.; Messersmith, P. B. A reversible wet/dry adhesive inspired by mussels and geckos. Nature 2007, 448, 338−341. (13) Long, Y.; Wu, J.; Wang, H.; Zhang, X.; Zhao, N.; Xu, J. Rapid sintering of silver nanoparticles in an electrolyte solution at room temperature and its application to fabricate conductive silver films using polydopamine as adhesive layers. J. Membr. Sci. 2011, 21, 4875− 4881. (14) Zhang, C.; Lv, Y.; Qiu, W.-Z.; He, A.; Xu, Z.-K. Polydopamine Coatings with Nanopores for Versatile Molecular Separation. ACS Appl. Mater. Interfaces 2017, 9, 14437−14444. (15) Zhang, L.; Wu, J.; Wang, Y.; Long, Y.; Zhao, N.; Xu, J. Combination of bioinspiration: a general route to superhydrophobic particles. J. Am. Chem. Soc. 2012, 134, 9879−9881. (16) Yang, Y.; Li, X.; Qiu, H.; Li, P.; Qi, P.; Maitz, M. F.; You, T.; Shen, R.; Yang, Z.; Tian, W. Polydopamine Modified TiO2 Nanotube Arrays for Long-Term Controlled Elution of Bivalirudin and Improved Hemocompatibility. ACS Appl. Mater. Interfaces 2018, 10, 7649−7660. (17) Mallinson, D.; Mullen, A. B.; Lamprou, D. A. Probing polydopamine adhesion to protein and polymer films: microscopic and spectroscopic evaluation. J. Membr. Sci. 2017, 53, 3198−3129. (18) Li, Z.; Liu, L.; González, A. J.; Wang, D.-Y. Bioinspired polydopamine-induced assembly of ultrafine Fe(OH)3 nanoparticles on halloysite toward highly efficient fire retardancy of epoxy resin via an action of interfacial catalysis. Polym. Chem. 2017, 8, 3926−3936. (19) Ejima, H.; Richardson, J. J.; Liang, K.; Best, J. P.; van Koeverden, M. P.; Such, G. K.; Cui, J.; Caruso, F. One-step assembly of coordination complexes for versatile film and particle engineering. Science 2013, 341, 154−157. (20) Wang, H.; Wu, J.; Cai, C.; Guo, J.; Fan, H.; Zhu, C.; Dong, H.; Zhao, N.; Xu, J. Mussel Inspired Modification of Polypropylene Separators by Catechol/Polyamine for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 5602−5608. (21) Wang, Z.; Kang, H.; Zhang, W.; Zhang, S.; Li, J. Improvement of interfacial interactions using natural polyphenol-inspired tannic acidcoated nanoclay enhancement of soy protein isolate biofilms. Appl. Surf. Sci. 2017, 401, 271−282. (22) Idumah, C. I.; Hassan, A.; Affam, A. C. A review of recent developments in flammability of polymer nanocomposites. Rev. Chem. Eng. 2015, 31, 149−177. (23) Bian, X.-C.; Tang, J.-H.; Li, Z.-M.; Lu, Z.-Y.; Lu, A. Dependence of flame-retardant properties on density of expandable graphite filled rigid polyurethane foam. J. Appl. Polym. Sci. 2007, 104, 3347−3355.
obtained from three replicated experiments are reported in this work. 4.4.7. FT-IR Spectra. The FT-IR spectra were measured on an IS 5 (Nicolet) spectrophotometer in the range of 4000−600 cm−1 at a resolution of 0.5 cm−1. The samples were first ground and then mixed with KBr to form pellets.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00182. SEM photos, XRD patterns, TG curves, photos of the composites; and XPS spectra (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Xiaoqian Zhang). ORCID
Xiaoqian Zhang: 0000-0003-4963-6379 Author Contributions
The manuscript was written through contributions of all authors. Funding
The authors are grateful for financial support from the Hebei Province Science Foundation for Youths (E2017508045, E2018508091), the Foundation of Hebei Educational Committee (QN2017409), and the fundamental research funds for the Central Universities (3142016001, 3142017071, 3142017065). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Prof. Jian Xu and Ning Zhao at the Institute of Chemistry, Chinese Academy of Sciences (China) for providing technical support in SEM experiments and water contact angle testing. We thank Dr. Qilong Tai at Suzhou Enhanry Advanced Materials Co., Ltd for providing sample and test help.
■
ABBREVIATIONS TA, tannic acid; Mg(OH)2, magnesium hydroxide; PA 6, polyamide 6; PDA, polydopamine; LOI, limiting oxygen index; SEM, scanning electron microscopy; FT-IR, Fourier transform infrared spectroscopy; TG, thermogravimetry; CCT, cone calorimeter testing; HRR, heat release rate; PHRR, peak of heat release rate; THR, total heat release; ML, mass loss; SPR, smoke production rate; TSP, total smoke production; COP, CO production
■
REFERENCES
(1) Kashiwagi, T.; Harris, R. H., Jr.; Zhang, X.; Briber, R. M.; Cipriano, B. H.; Raghavan, S. R.; Awad, W. H.; Shields, J. R. Flame retardant mechanism of polyamide 6-clay nanocomposites. Polymer 2004, 45, 881−891. (2) Wu, Z.-Y.; Xu, W.; Liu, Y.-C.; Xia, J.-K.; Wu, Q.-X.; Xu, W.-J. Preparation and characterization of flame-retardant melamine cyanurate/polyamide 6 nanocomposites by in situ polymerization. J. Appl. Polym. Sci. 2009, 113, 2109−2116. (3) Sertsova, A. A.; Marakulin, S. I.; Yurtov, E. V. Metal compound nanoparticles: Flame retardants for polymer composites. Russ. J. Gen. Chem. 2017, 87, 1395−1402. (4) Chen, X.; Yu, J.; Guo, S.; Lu, S.; Luo, Z.; He, M. Surface modification of magnesium hydroxide and its application in flame 4897
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
Article
ACS Omega (24) Su, X. B.; Dai, L. J.; Chen, S. P.; Lu, Y. D.; Wu, Z. H. Study on Complexation Properties of Black Wattle Tannin with Metals Ions. Chemistry & Industry of Forest Products 2007, 27, 37−41. (25) Shu-Yu, W.; Wen-Zhi, H.; Chang, L.; Guang-Ming, L.; Fei-Er, Z. Characterizations and preparation of Mg(OH)2 nanocrystals through ultrasonic-hydrothermal route. Res. Chem. Intermed. 2016, 42, 4135− 4145. (26) Xia, Z.; Singh, A.; Kiratitanavit, W.; Mosurkal, R.; Kumar, J.; Nagarajan, R. Unraveling the mechanism of thermal and thermooxidative degradation of tannic acid. Thermochim. Acta 2015, 605, 77− 85. (27) Ma, D.; Zhao, P.; Li, J. Effects of Zinc Phytate on Flame Retardancy and Thermal Degradation Behaviors of Intumescent Flame Retardant Polypropylene. Polym.-Plast. Technol. Eng. 2016, 56, 1167− 1176. (28) Schartel, B.; Hull, T. R. Development of fire-retarded materialsinterpretation of cone calorimeter data. Fire Mater. 2007, 31, 327−354. (29) Liu, Y.; Wang, J.-S.; Deng, C.-L.; Wang, D.-Y.; Song, Y.-P.; Wang, Y.-Z. The synergistic flame-retardant effect of O-MMT on the intumescent flame-retardant PP/CA/APP systems. Polym. Adv. Technol. 2010, 21, 789−796. (30) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441−2449. (31) Liu, L.; Chen, X.; Jiao, C. Influence of ferric phosphate on smoke suppression properties and combustion behavior of intumescent flame retardant epoxy composites. Iran. Polym. J. 2015, 24, 337− 347. (32) Wesolek, D.; Kozlowski, R. Toxic gaseous products of thermal decomposition and combustion of natural and synthetic fabrics with and without flame retardant. Fire Mater. 2002, 26, 215−224.
4898
DOI: 10.1021/acsomega.8b00182 ACS Omega 2018, 3, 4891−4898
