Microencapsulation and Surface Functionalization of Ammonium

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Materials and Interfaces

Microencapsulation and Surface Functionalization of Ammonium Polyphosphate via In-Situ Polymerization and Thiol-Ene PhotoGrated Reaction for Application in Flame Retardant Natural Rubber Can Wu, Xiaodong Wang, Junying Zhang, Jue Cheng, and Ling Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02464 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Industrial & Engineering Chemistry Research

Microencapsulation and Surface Functionalization of Ammonium Polyphosphate via In-Situ Polymerization and Thiol-Ene Photo-Grated Reaction for Application in Flame-Retardant Natural Rubber

Can Wu,† Xiaodong Wang,*, ‡ Junying Zhang,† Jue Cheng,† and Ling Shi*,†

†

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing

University of Chemical Technology, Beijing 100029, China ‡

State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology,

Beijing 100029, China

*Author for correspondence

Ling Shi Email: [email protected]

Xiaodong Wang Email: [email protected]

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ABSTRACT A novel type of microencapsulated ammonium polyphosphate (MAPP) with a triallyl cyanurate (TAC)/SiO2 double-layered shell was synthesized through in-situ polymerization, followed by the thiol-ene photo-grated reaction. With a double bond in the TAC outer shell, MAPP could be utilized as a highly efficient flame retardant for intumescent flame-retarded natural rubber (NR). The chemical structure, morphology and performance of MAPP were characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, water contact angle (WCA), scanning electron microscopy and thermogravimetric analysis. MAPP exhibited a hydrophobic feature with a WCA of 101º due to its organic outer shell, which made MAPP well dispersed in the NR matrix as well as enhanced the compatibility between the MAPP and NR matrix. The resulting intumescent flame-retardant NR compounds did not only achieve excellent flame-retardant performance but also obtained improved mechanical properties due to the presence of 3D-crosslinking networks between the MAPP and NR matrix. The NR/MAPP compounds also presented good water resistance because of the microencapsulation of APP with an SiO2 inner shell, which provided a waterproof barrier for APP. Based on the cone calorimetric results, the NR/MAPP compounds also presented a decline in total heat release and heat release rate but an improvement in ignition time due to the fact that both the SiO2 inner layer and TAC outer shell could enhance the formation of a high-strength thermally stable char layer during combustion, thus preventing heat transmission and diffusion. The significant enhancement in flame-retardant performance was principally ascribed to the synergistic char-forming effect derived from the APP core and TAC/SiO2 shell. This work provides a new idea for development of APP-based flame-retardant additives and also explores their potential applications in intumescent flame-retardant polymeric systems. KEYWORDS: Microencapsulated ammonium polyphosphate; In-situ polymerization; Thiol-ene photo-grated reaction; Flame retardancy; Natural rubber

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1. INTRODUCTION Nowadays, rubbers have become an important widely used polymer materials in our daily life and industrial production, and they can be used to prepare shock absorbers, tires, conveyor belts, and defense products.1 As one of the most broadly used general purpose rubbers, natural rubber (NR) is a type of important strategical material because of its irreplaceability in many vital application areas by synthetic materials,2 and such a situation is due to its unique properties like high resilience and elasticity, efficient heat dispersion resilience and excellent abrasion.3 Most of the manufacturing processes focus on the usage of NR to enhance the quality of relevant products that can be produced especially for specific applications.4 Compared to metals and other counterparts, these NR-based products are expected to present high thermal and chemical stabilities and outstanding mechanical performance under the dynamic condition.5 However, the structural characteristics of macromolecular chain determine a highly flammable nature for NR, resulting in an extreme low limiting oxygen index (LOI) of 18 vol.%. Owing to such an inherent flammable feature, the applications of NR from in some highly demanding fields such as aircraft tire treads and coal mine conveyor belts were greatly restricted.6 To overcome this vital problem, halogenated and halogen-free flame-retardant additives were generally used to improve the flame retardancy of NR. Nevertheless, halogen-containing flame-retarded polymers will produce thick smoke and toxic gases during combustion. Therefore, for environmental concerns, nonhalogen flame-retardant additives for NR have received a great of interest in recent years.7There were a number of studies on these issues. For instance, Wang et al.6reported the use of mesoporous silica MCM-41 and microencapsulated ammonium polyphosphate (APP) with a double-layered shell for an intumescent flame-retardant system with NR and found that a UL-94 V-0 classification was gained for this system at the microencapsulated APP/MCM-41 mass ratio of 39/1. Khanlari et al.8 reported an investigation on the effect of organically modified

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montmorillonite on the thermal stability and flame-retardant performance of the NR-based nanocomposites. They observed that the ignition time of these nanocomposites was greatly postponed with the addition of 3 wt.% of organically modified montmorillonite, and their peak heat release rates decreased by 54% compared to pristine NR. Guler and co-workers9 studied the flame retarding effect of expandable graphite on thermoplastic polyurethane (TPU) containing huntite/hydromagnesite mineral powders and found that there was a synergistic interaction between the expandable and graphite huntite/hydromagnesite for enhancing the flame-retardant performance of TPU. Intumescent flame-retardant modification method was considered as an important fire-resistant pathway with a series of significant advantages such as an excellent anti-dripping capability and extremely low-density of smoke and toxic gases released during combustion. This methodology evidently conforms to a trend in the development of flame-retardant materials.10 Intumescent flame retardants normally consists of three basic ingredients: carbon source, acid source and gas source.11 The flame-retardant system with APP, pentaerythritol and melamine was considered as one of the most broadly studied intumescent flame-retardant systems.11,12 However, the use of APP was restricted due to its low water resistance and poor compatibility with flame-retarded polymers, and in order to overcome these problems, a variety of methodologies have been adopted to modify and decorate the surface of APP particles13, in which microencapsulation became one of the most important solutions.14-16 Shen and co-workers17 encapsulated APP with various materials and explored its flame retardancy in polyurethane (PU) composites. Their experimental results revealed that the microencapsulation of APP with 4,4’-oxydianiline-formaldehyde could lead to a higher LOI for PU composites compared to pristine of APP at the same loading of flame retardants. Wu et al.18 synthesis a type of microencapsulated APP with an epoxy shell through in-situ polymerization and observed an enhanced flame retarding effect of microencapsulated APP on

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polypropylene (PP). Qiu et al.19 prepared a novel type of multifunctional microencapsulated APP with an organic-inorganic hybrid shell containing rich amino groups on the surface of APP, and they found that the formation of char layer was significantly enhanced. As a result, the flame-retardant efficiency of epoxy resins was effectively improved. Zhang and his co-worker20 reported the preparation of a type of microencapsulated APP with reduced graphene oxide in large size and found the use of this type of APP not only could lead to a significant increase in elongation at break and tensile strength of TPU in comparison with pristine APP, but also effectively restrains the melt-dripping phenomenon of TPU-based compounds. Moreover, the modification of APP with coupling agents was also considered as an effective way for improvement of flame-retardant effect.21,22 Liu et al.23 performed a surface modification for APP with (3-aminopropyl) triethoxysilane and silicon resin and found that the resulting APP exhibited an enhanced flame retarding effect on PP as well as good water resistance. Qin et al.13 reported the inorganic modified APP obtained from chemical deposition on the APP surfaces. This inorganic modified APP was found to exhibit a much better flame retarding effect on PP than pristine APP, resulting in a higher LOI and UL-94 V-0 classification for PP-based compounds. In addition, the modification of APP with various surfactants is also considered as an effective way to enhance the flame-retardant efficiency,24-26 because this pathway can significantly improve the compatibility of APP with the polymer matrix. Shao et al.24 reported a surface modification for APP with ethylenediamine by an ion exchange reaction and found that the flame retarding effect of APP on PP was enhanced significantly after such a surface modification. The PP compounds containing 40 wt % of modified APP exhibited a LOI of 32.5 vol % as well as a UL-94 V-0 classification. Shao and co-workers27 prepared the surface-modified APP with piperazine and then incorporated it into poly(vinyl alcohol)/montmorillonite aerogels to obtain a type of APP-based composite flame-retardant additives. The experimental results indicated that the PP compound containing 22 wt % of

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this APP composite achieved a LOI of 31.2 vol %, which was improved by 58.4% compared to pristine APP. In this work, we designed and fabricated a novel type of microencapsulation APP (MAPP) with a double-layered shell based on SiO2 and triallyl cyanurate (TAC). Such a structural design for MAPP is expected to enhance the compatibility between the polymeric matrix and APP and also improve the water resistance of APP. It is evident that the good water resistance can prevent the migration of APP from polymeric matrix effectively and enhances the flame retardancy of APP-modified polymers. In order to verify the flame-retardant effect of MAPP, the resultant MAPP combined with PER and MEL was introduced into a NR matrix to form a series of flame-retardant NR-based compounds. The flame-retardant properties, combustion behaviors, water-resistant performance and thermal degradation of these flame-retardant NR-based compounds were investigated extensively. The goal of the present study is to exploit a new surface modification methodology for APP to improve the flame-retardant efficiency and comprehensive performance of the flame-retardant polymers.

2. EXPERIMENTAL SECTION 2.1. Materials. NR (commercial grade: SCR-5) was commercially supplied by Hainan Natural Rubber Industry Group Co., Ltd., China. APP (phase II) with an average polymerization degree over 1200 was purchase from Hangzhou Jieersi Flame-Retardant Chemical Co., Ltd., China. Silica sol (pH 8.6~9.3, 30 wt %) were commercially obtained from Sigma-Aldrich (USA). A silane coupling agent, 3-mercaptopropyltriethoxysilane (KH-580), was commercially supplied by Shanghai Maclean Biochemical Technology Co., Ltd., China. Triallyl cyanurate (TAC) was purchased from Chengdu West Asia Chemical Industry Co., Ltd., China., 2-Hydroxy-2-methylpropiophenone (Photoinitiator 1173) and ammonia hydroxide were commercially provided by Shanghai Aladdin Biochemical Technology Co., Ltd., China. 6


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Pentaerythritol (PER) and melamine (MEL) were commercially provided by Shandong Taixing New Material Co., Ltd., China.

2.2. Preparation of MAPP. MAPP was synthesized through a facile in-situ reaction, followed by a thiol-ene click chemistry reaction, and its synthetic mechanism is schematically shown in Figure 1. In a typical synthetic procedure: 100 g of APP and 400 ml of ethanol were added into a three-necked round-bottomed flask. The mixture was stirred at 40 °C for 10 min, and then 70 g of silica sol were added and kept stirring for 4 h. The resultant mixture was cooled to 23 °C, and then filtered and washed with ethanol to obtain some white powders as the microencapsulated APP with an SiO2 shell (SiO2@APP). Then, 100 g of SiO2@APP and 400 ml of anhydrous ethanol were blended in a three-necked round-bottomed flask with agitation for 10 min. In succession, 10 g of KH-580 and 2.14 g of ammonium hydroxide was introduced into the resulting mixture. The resultant mixture was heated to a set reaction temperature of 50 °C and kept at the reflux temperature with stirring for an additional 5 h. The reactant was cooled to room temperature after the reaction was completed. The white solid as the organically modified SiO2@APP was then obtained by filtering, washing and drying. Afterwards, 100 g of organically modified SiO2@APP, 0.5 g of Photoinitiator 1173 and 15 g of TAC were incorporated into 400 mL of acetone in a flask, and then the resulting mixture was stirred at room temperature for 2 h under a 365 nm ultraviolet light emitting diode with the irradiation intensity of 100 mW/cm2. Finally, MAPP was obtained by filtering, washing with ethanol and drying at 100 °C for 5 h. 2.3. Preparation of intumescent flame-retardant NR-based compounds. The NR/MAPP flame-retardant compounds were prepared by mixing in a laboratory two-roll mill, and meanwhile the NR/APP ones were prepared as control samples. MEL and PER were also incorporated into these two types of flame-retardant compounds as assistant flame-retardant additives, and the mass ratios of

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MAPP/MEL/PER and APP/MEL/PER were both set to 3/1/1, which was recognized as an optimum mass ratio for most of intumescent flame-retardant polymeric systems because of the best synergism of nitrogen and phosphorus elements according to the reported references.6,10,28 Moreover, some processing additives such as sulfur, stearic acid, zinc oxide and accelerant N-cyclohexyl-2-benzothiazole sulfonamide were also incorporated during the mixing process. The obtained blending compounds were further vulcanized at 143 °C on a hot press with the optimum cure time determined by a MR-C3 rheometer (Beijing Ruida Yuchen Instrument Co., Ltd. China). All of the samples were tailored from the vulcanized sheets and then kept at 23 ºC for 24 h for further characterizations and measurements. 2.4. Characterization. Fourier-transform infrared (FTIR) spectroscopy was conducted to characterize the chemical structures and compositions of microencapsulation samples on a Bruker Alpha-T FTIR spectrometer. X-ray photoelectron (XPS) spectroscopy was also performed with an ESCALAB 250 spectrometer with Al-Kα excitation radiation. The surface elemental compositions and contents of APP and MAPP specimens were derived from XPS analysis. Scanning electron microscopy (SEM) micrographs were obtained from a JEOL-7800F SEM instrument with an acceleration voltage of 10 kV. The compounding samples were fractured cryogenically after immersion in liquid nitrogen, and the resulting fracture surfaces were coated with a conductive golden layer prior to the SEM observation. The static water contact angle (WCA) was measured on an OCA-20 contact angle analyzer at 23 ºC. The MAPP powders were spread on a slide and then scraped to get a flat surface under slight compression. Thermogravimetric analysis (TGA) was performed with a TA Q50 thermogravimetric analyzer in nitrogen at a heating rate of 10 ºC/min. To analyze the constitution of gas species, the a Nicolet 6700 FTIR spectrometer was coupled with the thermogravimetric analyzer, and the TGA-FTIR combination measurements were also carried out at from 40 ºC to 800 ºC a heating rate of 10 ºC/min under an air

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atmosphere and nitrogen atmospheres. The elongation at break and tensile strength were measured with a Meters CMT-4204 universal testing machine at 25 ± 2 ºC in terms of the Chinese National Standard of GB/T 528–2009. The stretching speed was set to 200 mm/min, and the gauge length was set as 20 mm. The water resistance was measured according to the ANSI standard of UL746C. In a typical test procedure, five specimens were measured for each sample. A sample with an initial weight (w0) was kept in distilled water at 80 ºC for 168 h. After the water-absorption treatment, this sample was dried at 100 ºC in vacuum for 24 h. The dried samples were weighted to obtain a weight (w 1), and the weight loss (Xw) of sample could be calculated using the following equation:13,23

X w (%) 

w0  w1 100% w0

(1)

The LOI was measured with an HC-2 oxygen index meter 207 in terms of the standard of ASTM D2863. The dimension for each specimen was set to 100 × 6.5 × 3 mm3. The vertical burning experiments were carried out on a CZF-II horizontal and vertical burning tester in terms of the standard of ASTM D3801. The dimension of specimen was set to 100×13×3 mm3. A cone calorimetric combustion experiment was conducted on an FTT–0007 low oxygen standard cone calorimeter (Fire Testing Technology Ltd., UK) in terms of the standard of ISO 5660. The test specimen with a dimension of 100×100×3 mm3 was laid on a horizontal sample holder and wrapped with an aluminum foil. All of tests were carried out with a heat flux of 50kW/m2, and reported calorimetric data represented the averaged triplicate data.

3. RESULT AND DISCUSSION 3.1. Structural Characterizations of Microencapsulation Samples. FTIR spectroscopy was carried out confirm the microencapsulation of APP with SiO2 and TAC. Figure 2 shows the obtained infrared spectra. As observed from the infrared spectrum of APP, a set of typical absorption peaks appear at 1245

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cm-1 for P=O, at 884 cm-1 for P−O asymmetric stretching vibration, and at 1095 cm-1 for P−O symmetric stretching vibration. On the other hand, an asymmetry stretching vibration peak of Si−OH can be found at 3408 cm-1 in the infrared spectrum of SiO2@APP, which is ascribed to the Si−OH on the SiO2 layer.29This result demonstrates that the SiO2 has been successfully microencapsulated onto the APP surface. As for the infrared spectrum of organically modified SiO2@APP, a new absorption peak can be observed at the 1095 cm-1 as an indicator of the characteristic Si−O−C stretching absorption, indicating that the surface of SiO2@APP has been well coated with KH-580.16 Moreover, the infrared spectrum of MAPP exhibits an intensive absorption peak at 1621 cm-1 corresponding to the stretching vibration of C=N bond in TAC, which identifies that TAC has successfully wrapped the organically modified SiO2@APP to form the layer-by-layer microencapsulated APP. To confirm the formation of layer-by-layered structural shell upon APP, XPS was performed to detect the surface chemical composition and elemental distribution of pristine APP, SiO2@APP, organically modified SiO2@APP and MAPP, and the obtained XPS spectra are illustrated in Figure 3. As observed in the survey XPS spectra in Figure 3a, the peaks at 533.1 eV, 400.7 eV, 286.2 eV, 191.8 eV, and 134.7 eV were assigned to the O 1s, N 1s, C 1s, P 2s and P 2p signals of APP, respectively.31 Furthermore, there are two O 1s deconvoluted peaks at binding energy of 533.0 and 531.0 eV for the O=P and O−P bonds through the curve fitting as observed in the high resolution P 2p and O 1s spectra of APP (see Figure 3b and 3c). Although the aforementioned peaks are also found in the XPS spectra of SiO2@APP, their relative intensity is somewhat enhanced. Meanwhile, two new peaks at 102.7 eV and 154.9 eV are found in the spectra for the Si 2s and Si 2p signals of SiO2@APP derived from SiO2. It is also noted in Figure 3d and 3e that there are a series of deconvoluted peaks for O=P (533.0 eV), O−Si (532.7 eV), O−P (531.5 eV), Si−O−Si (103.1 eV) and Si−O−P (103.8 eV) bonds in the high-resolution spectra of SiO2@APP. These deconvoluted peaks

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in Si 2p and O 1s zones provide a sound evidence for the formation of dense inorganic SiO2 layer. Furthermore, the peaks corresponding to the Si 2p (102.7 eV) and Si 2s (154.9 eV) signals are also enhanced in the spectrum of organically modified SiO2@APP, and a new peak for S 2p appears at 166.0 eV, indicating the presence of Si and S elements originated from KH-580. Moreover, the peaks for Si−O−P (103.8 eV), Si−C (103.3 eV), Si−O−Si (103.1 eV), C−S−H (164.1 eV) and C−S−S−C (163.4 eV) bonds can be clearly distinguished from their deconvoluted Si 2p and S 2p spectra as observed in Figure 3f and 3g, confirming an effective surface modification of SiO2@APP with KH-580.30 It is interestingly found in the XPS spectrum of MAPP that the aforementioned peaks completely disappear, and however the intensity of N 1s peaks at 400.4 eV is enhanced in the presence of N element due to the microencapsulation of organically modified SiO2@APP with a TAC outer shell. Moreover, the intensity of C 1s peak at 284.7 eV increases remarkably due to the existence of TAC layer on MAPP.31 As observed in the high resolution XPS spectra of MAPP of Figure 3h, there are a series of deconvoluted peaks due to the production of radical polymerization bimolecular termination and the formation of C−S−H bond, while Figure 3i exhibits two deconvoluted peaks for C=N−C (401.2 eV) and N−H (399.1 eV). In addition, the surface elemental compositions and atom fraction of MAPP and APP particles are presented in Table 1. It is found that the atom fractions of O, N and P elements in MAPP are determined as 4.10%, 3.96% and 32.46%, respectively. These data are much lower than the relevant atom fractions in APP. The C-atom fraction of MAPP is also higher than that in APP. Such a change in elemental compositions is ascribed to the encapsulation of APP particles with an organic polymeric layer. These XPS results clearly indicated that the evolution of surface elemental composition of MAPP with the encapsulating process and also gave an evidence for the microencapsulation of APP with an SiO2 inner layer and a TAC outer shell. 3.2. Morphology and Surface Hydrophobicity of Microencapsulation Samples. The surface

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morphologies of APP and its microencapsulation samples were observed by SEM, and the resulting micrographs are illustrated in Figure 4, in which the micrographs with a magnification of 2500 reveal the dispersed state of the particles, while those with a magnification of 5000 reflect the surface morphologies of APP and its microencapsulation samples. It is notable that the surface of APP exhibits an obvious change with the evolution of surface modification and microencapsulation with different shell materials, but the average diameters of APP and its microencapsulation samples are almost identically distributed in the range of 15−20 μm. The surface of pure APP is found to looks fairly smooth and regular as seen in Figure 4a and 4b.23 However, the surface of the SiO2@APP presents a large amount of particulate matter after microencapsulation with an SiO2 shell. The increase of surface roughness suggests the existence of SiO2 layer. Furthermore, the organic surface modification for SiO2@APP and subsequent encapsulation seem not to alter its morphology significantly as observed in Figure 4e−4h, because both KH-580 and TAC layer are fabricated onto the surface of SiO2@APP as small molecular compounds, which only resulted in a very thin film fabricated on the surface of SiO2@APP. The SEM observation verified the formation of MAPP with a layer-by-layer shell when combined with the FTIR and XPS results.13 The water resistance of APP and its microencapsulation samples were evaluated by WCA, and the resulting data and relevant digital photographs are presented in Figure 5. As observed in Figure 5, the digital photograph of pure APP clearly demonstrated the water could be easily absorbed immediately on the surface of APP, resulting in a WCA of 7.8º. Owing to a strong polarity of APP surface, the water droplet can permeate immediately after contacting with APP, thus resulting in a small WCA. Moreover, MAPP shows a much higher WCA of 101º compared to pure APP, indicating a transition from hydrophilicity to hydrophobicity for the surface properties of MAPP. Such a transition is ascribed to the formation of a water-insoluble TAC outer shell that well protects the MAPP.23 It is understandable that a large number of

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hydrophobic structures were formed on the surface of APP with a TAC outer layer after in-situ polymerization and thiol-ene photo-grated reaction, and therefore the water droplets can stably exist on the surface of MAPP, thus leading to good water resistance. 3.3. Thermal stability of Microencapsulation Samples. The thermal stability of APP and its microencapsulation samples was investigated by TGA. Figure 6 gives the obtained TGA and DTG curves. These samples are also observed to present two main degradation steps in their thermogram profile. For pure APP, the first stage degradation occurs in the temperature range of 200 ºC–400 ºC due to the decomposition of APP initiated by thermal degradation of unstable groups such as [–OP(O)(ONH4)2] and [–OP(O)(ONH4)–] and removal process of H2O and NH3 during the thermal decomposition of polyphosphate.9,12 However, the MAPP exhibits the first stage degradation taking place in the range of 200–450 °C. The elimination of H2O and NH3 results in the relevant weight loss during the thermal decomposing process of polyphosphate. It is also clearly observed from Figure 6 that, compared to pure APP, MAPP exhibits a lower initial decomposition temperature because of the dehydration condensation between the silanol groups in the SiO2 shell. The presence of these silanol groups has already justified by the FTIR spectrum of MAPP (see Figure 2). The second stage degradation are found to occur in the range of 500–650 ºC due to the major decompositions of both APP and MAPP,9,11 and such main decompositions are caused by the release of polyphosphoric acid, metaphosphoric acid and phosphoric acid. For pure APP, a rapid weight loss rate of 0.92 %/ ºC is detected at 584 ºC due to the breakage of ultra phosphate structure as well as the formation of volatile P2O5. Moreover, it is found that MAPP exhibits a maximum weight-loss characteristic temperature (the peak of weight loss rate) at 606 ºC for its second stage decomposition, which is higher than that of pure APP due to a heat resistance effect of SiO2 and TAC layers on the surface of MAPP. Nevertheless, it is

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noteworthy that there is another major difference between APP and MAPP, which shows that MAPP has a higher char yield at 800 °C. The protective effect of SiO2 layer as well as the char layer formed during combustion contributes to the improvement of thermal stability for MAPP. 3.4. Mechanical Properties of NR-based Compounds. The mechanical performance of intumescent flame-retardant NR/APP and NR/MAPP compounds was investigated. Table 2 shows the obtained test results. It has been broadly accepted that the tensile strength of a polymer/filler compounding system mainly relies on three factors, which include the structure and shape of fillers, the loading of fillers, and the interfacial adhesion between the matrix and fillers.9,20 Pure NR is a soft rubbery polymer with extremely high elongation at break of around 550%. It can bee seen in Table 2 that the NR/MAPP compounds exhibit a significant improvement in elongation at break and tensile strength compared to the NR/APP ones at the equal amount of flame-retardant additives. The NR/MAPP compound presents the elongation at break of 397% and the tensile strength of 24.8 MPa when 45 parts per hundreds (phr) of flame-retardant additives is incorporated. However, the NR/APP compound only shows the tensile strength of 18.4 MPa and the elongation at break of 337%. There is a dramatically decrease observed in tensile strength with an increase of the amounts of flame-retardant additives in NR matrix, which may be ascribed to the poor compatibility and interfacial adhesion between the NR matrix and pure APP. Such a distinct difference in polarity of NR matrix and APP leads to thermodynamical immiscibility and poor interfacial adhesion accordingly. This immiscible effect is disadvantageous to the tensile strength of NR-based compounds. Nevertheless, the tensile strength presents a significant improvement when the same amount of MAPP is incorporated. As mentioned above, the TAC layer on the surface of MAPP could effective reduce the negative impact of APP particles toward the NR matrix, since its double bond structure can significantly improve the interface compatibility between the NR matrix and MAPP.

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According to our design for the chemical structure of outer layer of MAPP, the TAC layer can play a role of vulcanization accelerator in the NR matrix, and it can directly participate in the vulcanization of NR to form a 3D-crosslinking network in NR matrix. Such a mechanism is illustrated in Figure 7. Therefore, MAPP can be used for the NR-based flame-retardant system, and the chemical bonds can be formed between the NR matrix and MAPP to fix the MAPP particles in the matrix. In this case, the compatibility of APP with NR is enhanced significantly, thus leading to a stronger interfacial adhesion between the matrix and MAPP. The fabrication of a TAC outer layer for APP not only can enhance the dispersion APP particles in the NR matrix but also can improve the mechanical properties of the NR-based compounds effectively.

3.5. Morphologies of NR-based Compounds. To further verify the interfacial interaction between APP particles and NR matrix, the fractography of the NR-based compounds with various microencapsulated APP samples was performed with SEM and the obtained micrographs are shown in Figure 8. It is noted that, although the NR/APP compound presents a fairly flat surface with little matrix deformation, the introduction of MAPP brings about more folds and bulges for the fracture of the compounds, which reflects a better interaction between the matrix and fillers. As observed in Figure 8a and 8b, there are lots of cavities appearing on the fracture surface, and some holes and gaps are found at the interface between the NR matrix and APP, indicating a poor interfacial adhesion of pure APP with NR matrix. However, it is noteworthy in Figure 8c and 8d that MAPP is well dispersed in the NR matrix, and the MAPP particles are well surrounded by the NR matrix without any holes and cracks. Such an SEM observation confirmed the good interfacial compatibility between the matrix and MAPP. Obviously, the organic TAC shell of MAPP not only has a similar polarity with NR but also can act as a vulcanization accelerator to form a chemical bond between the NR matrix and MAPP. In this case, the MAPP particles

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are well dispersed in the NR matrix at a strong interfacial adhesion due to the microencapsulation with TAC shell, which is in good agreement with the mechanical data of the NR/MAPP compounds. 3.6. Thermal Stability of NR-based Compounds. The effect of microencapsulation of APP with a TAC/SiO2 double-layered shell on the thermal stability of NR-based compounds containing 45 phr of flame-retardant additives as representative samples was investigated by TGA in nitrogen, and the obtained TGA and DTG curves were presented in Figure 9. The relevant thermal analysis results were also summarized in Table 3. As observed in Figure 9, pure NR exhibits a typical two-step thermal decomposition behavior. The thermal decomposition of pure NR starts at about 220 ºC and ends at 470 ºC. However, both the NR/APP and NR/MAPP compounds show a significant reduction in characteristic decomposition temperature at a weight loss of 10 wt.% (T-10

wt.%)

compared to pure NR because of the

thermal decomposition of intumescent flame-retardant system. Although the degradation process of NR/MAPP compounds is found to be similar to the NR/APP ones, the NR/MAPP compounds exhibit a slightly higher char yield when the temperature is higher than 520 ºC. Such a phenomenon may be due to the TAC shell acting as sources of carbon and gas, which generates a synergistic effect with the intumescent flame-retardant additives, catalyzes the esterification of the system, and results in a stronger expanded carbon layer.32, 33 Moreover, both the NR/APP and NR/MAPP compounds show a weight loss of 5 wt.% (T-5 wt.%) at 259.26 ºC and 247.54 ºC, respectively. These two data are lower than those of 302.58 ºC for pure NR due to the degradation of flame-retardant additives occurring in the first-step degradation. However, pure NR suffers a weight loss of 80 wt.% (T-80 wt.%) at the temperature of 418.47 ºC, whereas the NR/APP and the NR/MAPP compounds present the same weight loss at 460.50 ºC and 498.26 ºC, respectively. The improvement in this characteristic temperature is attributed to the role of the intumescent flame-retardant system, The NR/MAPP compounds are found to achieve a higher char yield than pure NR

16


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and the NR/APP compounds. This phenomenon may be ascribed to the fact that the TAC layer of MAPP is chemically grafted into the NR matrix and thus provides good barrier properties for the NR matrix. This may reduce the heat transfer speed of the polymeric matrix, thereby delaying the degradation of compounds.17, 28, 34 As a result, the thermal stability of NR/MAPP compounds is much better than pure NR and the NR/APP ones. 3.7. TGA-FTIR Analysis of NR-based Compounds. To understand the thermal decomposition mechanisms of the intumescent flame-retardant NR-based compounds, TGA-FTIR combination measurement was performed to investigate the effect of microencapsulation on the thermal decomposition behavior of intumescent flame-retardant NR-based compounds containing 45 phr of flame-retardant additives as representative samples both in air and in nitrogen. Figure 10 show the FTIR spectra of pure NR and its compounds with APP and MAPP and the associated 3D infrared spectra of gaseous products recorded at different temperatures in both nitrogen and air atmospheres. As observed in Figure 10, the detected major decomposition gases of all the NR-based compounds mainly include ammonia (3800–3900 cm-1, 1650–1860 cm-1 and 962 cm-1), water (3600–3700 cm-1), alkanes (2930 cm-1) and carbon dioxide (2351

and 667 cm-1).12,13,35–37 It is highlighted that the two main abs absorption peaks at 2351 and 667 cm-1 can be observed in Figure 10a, which is attributed to absorption bands of carbon dioxide derived from the breakage of the main chain and subsequent thermal decomposition in nitrogen. The absorption peak of the hydrocarbon fragment appearing at 2930 cm-1 also confirms this deduction. These three absorption peaks are found to weaken for the NR/APP and NR/MAPP compounds as seen in Figure 10c and 10e, indicating an improvement in nonflammability. Comparing Figure 10c with 10e, it can be found that the absorption peak of ammonia at 960cm-1 occurs at 240–440 ºC for the NR/APP compounds but at 240–390 ºC for the NR/MAPP compounds, suggesting that MAPP can generate a flame-retardant effect earlier than APP.

17



Moreover, the reaction temperature range of the NR/MAPP compounds become narrower due to a faster reaction speed of MAPP for flame retardancy, and therefore a large amount of ammonia gas is released in a short period, thus improving the flame-retardant efficiency more effectively. Therefore, it is concluded that the NR/MAPP compounds can achieve a more stable char than the NR/APP ones at high temperature, which results in a lower HRR value for the NR/MAPP compounds. In addition, it is observed in Figures 10 that the FTIR spectra and their 3D profiles of gaseous products in air and nitrogen are similar for these NR-based compounds. Only a difference is observed for the absorption peaks of 2110 cm-1 and 2182 cm-1 in the temperature range of 490–640 ºC. This may be due to the lack of oxygen in the sample cell during the rapid pyrolysis of these compounds. Moreover, the inner SiO2 shell of MAPP does not thermally decompose in this experimental temperature range and may possibly migrate onto the char surface to protect the inside NR matrix more effectively. These results undoubtedly suggest that MAPP can prompt the formation of a tight residual char to enhance the flame-retardant performance of NR-based compounds. 3.8. Flame-retardant Performance and Water Resistance of NR-based Compounds. UL-94 vertical burning experiments and LOI measurements are conducted to investigate the flame-retardant performance of intumescent flame-retardant NR compounds with APP and MAPP, and the obtained flame-retardant test results are also presented in the Table 1. It is noted that the current intumescent flame-retardant systems exhibit a high flame-retarding effect on NR. The LOIs of these systems increased with the loading of flame-retardant additives, followed by an improvement in the nonflammable level in the UL-94 vertical burning experiments. 的 Pure NR is found to be a very inflammable polymer, and its LOI is only 17.7 vol.%, and there is no classification available according to the UL-94 vertical burning experimental results. On the other hand, the NR/MAPP compounds present a more significant increasing trend in LOI than the NR/APP compounds with an increase of the APP or MAPP loading. Nevertheless, the NR/MAPP

18


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compounds achieved a V-0 classification in the UL-94 vertical burning experiments when 45 phr of flame-retardant additives were added, whereas the NR/APP ones only reached a V-1 classification. This suggests that there is a good synergistic effect between the internal core of APP and outer shell of TAC.38 It has been widely accepted that the waterproof performance of APP influences its flame-retardant effect, and the microencapsulation of APP with inorganic materials is expected to enhance its water resistance and improved its flame-retardant effect on NR accordingly. The waterproof performance of NR-based compounds containing 45 phr of flame-retardant additives was investigated by a hot water-treatment experiment at 80 ºC, and the resulting results are presented in Figure 11, in which the weight loss of the compounds with a function of water-treatment time indicates the amount of the intumescent flame-retardant additives extracted with water. As observed in Figure 11, pure NR only lost a small amount of weight at the beginning period of water treatment and then was kept a stable weight. However, the NR/APP compound shows a serious weight loss with an increase of impregnating time. This may be due to the fact that the water may permeate through these compounds from surface to inside, leading to gradual extraction of APP. In fact, the water absorbability of intumescent flame-retardant additives can result in a serious restriction in their applications for polymeric materials, because the high water absorbability may lead to deterioration in electrical insulation, flame-retardant performance and processing ability. On the other hand, the NR/MAPP compound presents a gradual weight loss with increasing the water-treatment time. Meanwhile, the NR/MAPP compound is found to have a much lower weight loss than the NR/APP one at the same water-treatment time. These results indicate that MAPP has better water resistance in the NR matrix than APP due to the microencapsulation with a SiO2 inner shell. In this case, the high better water resistance of MAPP evidently favors its flame retarding effect on NR. 3.9. Cone-calorimetric investigation. Cone calorimetry is an effective tool to investigate the burning

19



properties of polymers and is able to provide several useful burning parameters such as total heat release (THR), heat release rate (HRR), total smoke production (TSP), smoke production rate (SPR), peak of heat release rate (PHRR), time to ignition (TTI), time to PHRR and residual weight for a flame-retardant system. The experimental results of pure NR and its flame-retardant compounds containing 45 phr of flame-retardant additives obtained from cone calorimetry are listed in Table 4. TTI is considered as an important parameter for detecting the flame retardancy of polymers, and it represents the time required for the material surface to continuously burn under the preset incident heat flow intensity. Therefore, TTI can be adopted to compare the flame-retardant performance of materials. In general, the higher the TTI is, the better is the flame-retardant performance of a material. As seen in Table 4, pure NR only has a TTI of 6 s, which means it is very easy to burn and the flame spreads rapidly after burning. However, the NR/APP compound exhibits an improved TTI of 30 s, and the TTI of the NR/APP compound further increase to 37 s, indicating a much better flame retarding effect from MAPP. Figure 12 reveals the HRR and THR plots as a function of combustion time for pure NR and its compounds with APP and MAPP. Pure NR is found to have a PHRR of 2395.67 kW/m2. This means that pure NR performs a rapid combustion behavior in the cone testing process, resulting in the completion of total heat release process in a short period. It is found that the NR/APP compound presents a lower PHRR of 1558.24 kW/m2 compared to pure NR, whereas the NR/MAPP compound shows a much lower PHRR of 1118.80 kW/m2 than the NR/APP one. There are two peaks observed in the HRR curves of the NR/APP and NR/MAPP compounds. Usually, the first peak is attributed to carbon layer formation, which can prevent heat release from the flame-retardant compound material and material contact with oxygen.39-41 It is noteworthy that the amplitude of PHRR of the NR/MAPP compound is much lower than that of the NR/MAPP one at any combustion time, which is due to the protective effect of intumescent carbon layer.

20


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The second peak is assigned to the breakage of intumescent carbon layer.39,40 The second peak of NR/MAPP compound exhibits a delay in comparison with the NR/APP one, indicating that the intumescent carbon layer can generate a better barrier effect during combustion. The flame retardancy with a low smoke density is considered as an importance fire-resistant behavior of intumescent flame-retardant polymers. The data of SPR and TSP represent such performance and can be obtained from cone calorimetric tests. Figure 13 illustrates the SPR and TSP plots as a function of combustion time for pure NR and its compounds with APP and MAPP. It is clearly observed that both APP and MAPP can reduce the SPR to 0.17 m2/s of the NR/APP compound and 0.29 m2/s of the NR/MAPP one from 0.49 m2/s of pure NR. On the other hand, pure NR exhibits a high TSP of 31.8 m3, but the TSP of the NR/APP compound is reduced to 20.3 m3. It is surprisingly noted that he TSP of the NR/MAPP compound is reduced to 15.2 m3. These results indicate that MAPP and APP can depress the production of smoke, and however MAPP exhibits more efficient smoke suppression than APP. The data of SPR and TSP further confirm that the microencapsulation of APP not only can impart NR/MAPP compounds with a higher LOI and higher rank of UL-94 classification but also can enhance their flame retardancy significantly. 3.10. Residual Char Analysis. Figure 14 shows the SEM micrographs and optical photographs of residual char obtained from the cone calorimetry tested samples. As observed in Figure 14a, the residual char of pure NR presents some gray large particles, and almost no carbon is found. For the NR/APP compound, there is a residual carbon layer as observed in Figure 14d, and there are also a number of holes with a larger diameter observed on the sample surface.12 It is observed from the SEM photograph in Figure 14e and 14f that there are a few of holes inside the carbon layer, and the carbon layer structure is relatively integrated. The NR/MAPP compound reveals an intact carbon layer as observed from the sample surface in Figure 14g. It is also noted in Figure 14h and 14i that there are numerous micropores on the surface of

21



carbonized char layer, and a lot of “vesicles” exist inside the microspores. Furthermore, no large pore is found on the surface of NR/MAPP compound. It is well known that the char layer has good carbonization and foaming effects, and therefore it can effective prevent the flammable volatiles from releasing and also shields the heat radiation and thermal feedback from outside flame.6,12,13,42 These observation results for residual char confirmed that the flame-retardant effect of MAPP on NR was derived from its excellent charring capability.

4. CONCLUSION MAPP was successfully prepared via in-situ polymerization and thiol-ene photo-grated reaction and then incorporated into NR to establish an intumescent flame-retardant system. The chemical structure and characteristic performance of MAPP were verified by FTIR, XPS, SEM, WCA and TGA. MAPP exhibited a hydrophobic feature with a WCA of 101º due to its organic outer shell. The resultant MAPP was well dispersed in the NR matrix and also enhanced the compatibility between the NR matrix and MAPP. As a result, the resulting intumescent flame-retardant NR compounds not only achieved good flame-retardant performance but also obtained improved mechanical properties with the elongation at break of 397.28% and tensile strength of 24.80 MPa due to the formation of a 3D-crosslinking network between the matrix and MAPP. The NR/MAPP compounds presented good water resistance because of the microencapsulation of APP with the SiO2 inner shell, which provided a waterproof barrier for APP. According to the cone calorimetric results, MAPP also demonstrated the reduced HRR and THR as well as the improved ignition time due to the fact that both the SiO2 inner layer and TAC outer shell could assist the formation of high-strength and thermally stable char layer during combustion and therefore prevented the heat transfer and diffusion. The significant enhancement in flame-retardant performance was principally attributed to the cooperative char-forming effect of the APP core and SiO2/TAC shell. This study provides a novel strategy 22


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for development of APP-based hybrid additives and also explores their potential applications in intumescent flame-retardant polymeric systems.

AUTHOR INFORMATION Corresponding Author *Fax/Tel: +86-10-64438296. E-mail: [email protected]; *Fax/Tel: +86-10-64421693. E-mail: [email protected];

ACKNOWLEDGMENTS The authors thank the financial support of the Nation Key R&D Program of China (Grant no. 2016YFB0302105).

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Table 1 Surface elemental compositions and atom fractions of APP and its microencapsulation samples. P

N

O

C

S

Si

(atom %)

(atom %)

(atom %)

(atom %)

(atom %)

(atom %)

APP

25.82

23.99

38.57

11.62

0

0

SiO2@APP

4.45

1.88

41.46

23.76

0

28.45

Organically modified

1.63

0.86

38.95

24.08

2.69

31.79

4.10

3.96

32.46

36.76

4.50

18.22

Sample

SiO2@APP MAPP

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Table 2 The formulation, flame-retardant performance and mechanical properties of NR-based composites. Sample Mixing formulation

Flame-retardant

code

performance

(phr) NR

APP

MAPP

PER

MEL

Mechanical properties

LOI

UL-94

Tensile strength

Elongation at break

(vol %)

classification

(MPa)

(%)

1

100

0

0

0

0

17.7±0.3

No rating

35.2±0.3

550±11

2

100

3

0

1

1

19.3±0.3

No rating

34.6±0.2

528±14

3

100

9

0

3

3

21.4±0.3

No rating

29.4±0.4

450±8

4

100

15

0

5

5

23.1±0.3

V-2

23.2±0.3

390±8

5

100

21

0

7

7

25.6±0.3

V-1

21.2±0.3

387±9

6

100

27

0

9

9

27.7±0.3

V-1

18.4±0.4

337±7

7

100

33

0

11

11

29.3±0.3

V-0

14.2±0.2

309±3

8

100

0

3

1

1

20.7±0.3

No rating

36.1±0.4

548±15

9

100

0

9

3

3

23.5±0.3

No rating

34.8±0.3

520±10

10

100

0

15

5

5

25.1±0.3

V-1

28.8±0.3

456±9

11

100

0

21

7

7

27.1±0.3

V-1

25.7±0.2

422±5

12

100

0

27

9

9

28.5±0.3

V-0

24.8±0.2

397±5

13

100

0

33

11

11

31.4±0.3

V-0

21.4±0.2

381±7

31



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Table 3 TGA data of NR-based composites the NR-based composites containing 45 phr of flame-retardant additives. Sample

T-5 wt.%

T-80 wt.%

Tmax,1

Tmax,2

Char yield

(ºC)

(ºC)

(ºC)

(ºC)

(wt.%)

Pure NR

302.58

418.47

—

377.09

4.91

NR/APP composite

259.26

460.50

268.37

379.47

16.13

NR/MAPP composite

247.54

498.26

260.12

382.47

17.94

32


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Table 4 Experimental results obtained by cone calorimetry for pure NR and the NR-based composites containing 45 phr of flame-retardant additives. Sample

TTI

PHRR

Time to PHRR

THR

Peak SPR

Char mass

(s)

(kW/m2)

(s)

(MJ/m2)

(m2/s)

(wt %)

Pure NR

6.0±0.3

2395.7±98.7

89.1±2.9

137.7±4.1

0.49±0.01

2.36±0.08

NR/APP composite

30.5±1.4

1558.3±45.2

110.4±3.2

95.9±2.8

0.29±0.01

13.18±0.37

NR/MAPP composite

37.2±1.5

1118.8±23.5

120.6±3.7

78.4±2.3

0.17±0.01

15.19±0.42

33



Figure Captions Figure 1

Schematic synthetic route and reaction mechanism of microencapsulation of APP.

Figure 2

FTIR spectra of (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP.

Figure 3

(a) XPS survey spectra of (1) APP, (2) SiO2@APP, (3) organically modified SiO2@APP and (4) MAPP. High-resolution XPS spectra of (b, c) APP, (d, e) SiO2@APP, (f, g) organically modified SiO2@APP and (h, i) MAPP.

Figure 4

SEM micrographs of (a, b) APP, (c, d) SiO2@APP, (e, f) organically modified SiO2@APP and (g, h) MAPP.

Figure 5

Charts of static water contact angel for (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP.

Figure 6

TGA and DTG curves of (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP.

Figure 7

Schematic representative of formation of 3D-crosslinking networks in the NR/MAPP compounds.

Figure 8

Cryogenically fractured surface of (a, b) NR/APP and (c, d) NR/MAPP compounds containing 45 phr of flame-retardant additives.

Figure 9

(a) TGA and (b) DTG curves of pure NR and the NR-based compounds containing 45 phr of flame-retardant additives.

Figure 10 FTIR spectra and associated 3D charts of gaseous products of (a) pure NR, (b) NR/APP composite and (c) NR/MAPP composite in nitrogen, and (d) pure NR, (e) NR/APP composite and (f) NR/MAPP composite in air. Figure 11 Plots of weight loss as a function of hot water treatment time for pure NR and the NR-based 34


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compounds containing 45 phr of flame-retardant additives. Figure 12 HRR and THR curves of pure NR and the NR-based compounds containing 45 phr of flame-retardant additives. Figure 13 SPR and TSP curves of pure NR and the NR-based compounds containing 45 phr of flame-retardant additives. Figure 14 Digital photographs and SEM micrographs of cone calorimeter residual chars of (a, b, c) pure NR and (d, e, f) the NR/APP and (g, h, i) NR/MAPP compounds containing 45 phr of flame-retardant additives.

35



TOC Graphic 84x62mm (600 x 600 DPI)


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Figure 1 Schematic synthetic route and reaction mechanism of microencapsulation of APP. 107x55mm (600 x 600 DPI)



Figure 2 FTIR spectra of (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP. 107x87mm (600 x 600 DPI)


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Figure 3 (a) XPS survey spectra of (1) APP, (2) SiO2@APP, (3) organically modified SiO2@APP and (4) MAPP. High-resolution XPS spectra of (b, c) APP, (d, e) SiO2@APP, (f, g) organically modified SiO2@APP and (h, i) MAPP. 304x257mm (300 x 300 DPI)



Figure 4 SEM micrographs of (a, b) APP, (c, d) SiO2@APP, (e, f) organically modified SiO2@APP and (g, h) MAPP. 69x99mm (600 x 600 DPI)


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Figure 5 Charts of static water contact angel for (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP. 104x86mm (600 x 600 DPI)



Figure 6 TGA and DTG curves of (a) APP, (b) SiO2@APP, (c) organically modified SiO2@APP and (d) MAPP. 105x87mm (600 x 600 DPI)


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Figure 7 Schematic representative of formation of 3D-crosslinking networks in the NR/MAPP composites. 102x107mm (600 x 600 DPI)



Figure 8 Cryogenically fractured surface of (a, b) NR/APP and (c, d) NR/MAPP compounds containing 45 phr of flame-retardant additives. 99x75mm (600 x 600 DPI)


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Figure 9 (a) TGA and (b) DTG curves of pure NR and the NR-based compounds containing 45 phr of flameretardant additives 106x166mm (600 x 600 DPI)



Figure 10 FTIR spectra and associated 3D charts of gaseous products of (a) pure NR, (b) NR/APP composite and (c) NR/MAPP composite in nitrogen, and (d) pure NR, (e) NR/APP composite and (f) NR/MAPP composite in air. 160x188mm (600 x 600 DPI)


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Figure 11 Plots of weight loss as a function of hot water treatment time for pure NR and the NR-based compounds containing 45 phr of flame-retardant additives. 104x85mm (600 x 600 DPI)



Figure 12 HRR and THR curves of pure NR and the NR-based compounds containing 45 phr of flameretardant additives. 108x86mm (600 x 600 DPI)


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Figure 13 SPR and TSP curves of pure NR and the NR-based compounds containing 45 phr of flameretardant additives. 105x85mm (600 x 600 DPI)



Figure 14 Digital photographs and SEM micrographs of cone calorimeter residual chars of (a, b, c) pure NR and (d, e, f) the NR/APP and (g, h, i) NR/MAPP compounds containing 45 phr of flame-retardant additives. 137x137mm (600 x 600 DPI)


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Microencapsulation and Surface Functionalization of Ammonium

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