Effect of Surface-Modified Ammonium Polyphosphate with KH550 and

Sep 20, 2015 - Ammonium polyphosphate (APP), which is an important component of intumescent flame retardant (IFR), has been modified with (3-aminoprop...
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Effect of Surface-Modified Ammonium Polyphosphate with KH550 and Silicon Resin on the Flame Retardancy, Water Resistance, Mechanical and Thermal Properties of Intumescent Flame Retardant Polypropylene Jian-Chao Liu, Miao-Jun Xu, Tao Lai, and Bin Li* Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, College of Science, Northeast Forestry University, 26 Hexing Road, Harbin, Heilongjiang 150040, People’s Republic of China S Supporting Information *

ABSTRACT: Ammonium polyphosphate (APP), which is an important component of intumescent flame retardant (IFR), has been modified with (3-aminopropyl) triethoxysilane (KH550) and silicon resin to enhance its hydrophobic properties. The structure and properties of the modified ammonium polyphosphate (MAPP) were well-characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), and water contact angle (CA). The water CA of MAPP was as high as 148°, which demonstrated that MAPP possessed excellent hydrophobic properties. MAPP and APP were mixed with triazine, which is a char-foaming agent (CFA), and polypropylene (PP) to prepare PP/MAPP/CFA and PP/APP/CFA composites, respectively. The fire retardancy and thermal degradation behavior of PP/MAPP/CFA and PP/ APP/CFA composites were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetry, and thermogravimetric analysis. The surface and fracture morphologies were evaluated by SEM. The mechanical properties was analyzed by tensile, flexural, and izod impact tests. The results demonstrated that the MAPP imparted excellent flame retardancy of PP/MAPP/CFA systems, along with a higher LOI value, UL-94 rating, and lower heat release rate and total heat released values. The modification enhanced the interfacial adhesion, mechanical stability, and thermal stability of PP/MAPP/CFA composites. Moreover, PP/MAPP/CFA composites with the thickness of 1.6 mm can still pass the UL-94 V-0 rating after treatment with water for 168 h at 70 °C, meanwhile the weight loss rate was decreased from 2.31% for PP/APP/CFA composites to 0.85%, indicating excellent water resistance. This investigation shows a promising formulation for water-resistant intumescent flame-retardant PP composites with extraordinary properties.

1. INTRODUCTION Polypropylene (PP) is extensively used in industry and daily life, in applications such as building materials, automobiles, electronics, and electric materials, because of its low density, good performance-to-cost ratio, easy processability, and good chemical resistance.1 However, its poor flame resistance restricts the application in many fields, where suitable flameretardant performance is required. Traditionally, brominecontaining flame retardants and antimony trioxide synergistic system are the most effective and show a good ratio of property to price in flame-retardant PP, but some of them have been limited in use because of the evolution of corrosive smokes and toxic gases upon burning.2 Metal hydroxides, such as aluminum hydroxide and magnesium hydroxide, are other flame retardants for PP, but the high loading would be seriously destroy the mechanical properties of PP.3 Intumescent flame retardant (IFR) is a promising halogenfree flame retardant and has attracted more and more attention4−10 in recent years, because of its merits, such as antidripping properties, low smoke emissions, and low toxic gas released during burning. Generally, IFR is composed of three parts, namely, an acid source, a char-forming agent (CFA), and a blowing agent.11 A typical and traditional IFR system involves a mixture of ammonium polyphosphate (as an acid source), pentaerythritol (as an CFA), and melamine (as an blowing © XXXX American Chemical Society

agent). In order to obtain more effective IFRs, two novel types of CFAs have been synthesized, including polyol phosphate compounds2,5,12,13 and triazine derivatives.14−17 Much work has been done by our team regarding the synthesis, application, and mechanism of macromolecular triazine derivatives as CFAs of IFR used in PP.15,17−21 Bin and co-workers15 synthesized a novel hydroxyethylamine triazine macromolecule used as a CFA, which had a good ability of char formation itself and compounded with APP to obtain an IFR showing effective flame retardancy. When the loading of the IFR was only 18 wt %, the intumescent flame retardant−PP (IFR-PP) composite could still pass UL-94 V-0 rating. By now, there are still some problems with IFR systems, including the poor compatibility with PP matrix and the low water resistance of APP and IFR. These drawbacks will restrict their wide industrial applications. To address the above problems, several methods can be employed, such as modifying the surface by using silane coupling agents22 and surfactants21,23 and microencapsulation with water-insoluble materials such as gel−silica, silicone oil, melamine (MEL), melamine formReceived: May 5, 2015 Revised: September 13, 2015 Accepted: September 19, 2015

A

DOI: 10.1021/acs.iecr.5b01670 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Preparation route of MAPP.

aldehyde (MF), or polyurethane (PU), via the in situ polymerization method.24−34 Among these methods, surface modifications have been proved to be good methods for improving the interfacial adhesion in a variety of incompatible blends. Microencapsulation is more effective than surface modification to improve the water resistance of APP or IFR, but the results of water resistance is not satisfactory; for example, the exudation of IFR in the composite is not satisfying (such as 3.2%,24 4%,23 etc.) and the water treatment time of composites to pass UL-94 test is not as long as the standard of UL746C (such as 24 h,31 72 h,34 etc.). The above results indicated that it is flawed to directly encapsulate APP with hydrophobic material. It may be caused by the weak interfacial adhesion, because of the rather different polarities of the coating substances and APP itself. In this work, the modified ammonium polyphosphate (MAPP) has been prepared by two steps combined with the surface modification with KH550 to reduce the polarity of APP and microencapsulated with hydrophobic silicon resin having low surface energy. Its structure and compositions were wellcharacterized and confirmed. The obtained MAPP combined with triazine CFA was incorporated into a PP matrix to prepare flame-retardant PP composites. The water-resistant properties, flame retardancy, thermal degradation behavior, and combustion behavior of the flame-retardant PP composites were characterized and discussed.

excitation radiation under ultrahigh-vacuum conditions. Both the surface elemental compositions of APP and MAPP and the elemental C, N, O, P, and Si content on surfaces I and II of the IFR/PP samples (5 mm × 5 mm × 3.2 mm) were measured. The limiting oxygen index (LOI) was measured on a Model JF-3 oxygen index instrument (Jiangning Analysis Instrument Company, China) with sheet dimensions of 130 mm × 6.5 mm × 3 mm, according to the standard oxygen index test ISO 45891996. The UL-94 vertical burning test (UL-94) was conducted by a CZF-2-type horizontal and vertical burning tester (Jiangning Analysis Instrument Company, China) with specimen dimensions of 125 mm × 12.5 mm × 1.6 mm, according to American National Standard ANSI/UL 94-2010. The combustion test was performed on a cone calorimeter (FTT, UK), using a heat flux of 50 kW m−2, based on ISO standard 5660-1. Each specimen, with dimensions of 100 mm × 100 mm × 4 mm, was wrapped in aluminum foil and laid on a horizontal holder. To determine the water resistance of IFR/PP composites, each specimen with a corresponding initial weight (marked W0) was put into distilled water at 70 °C and kept at this temperature for 168 h, and five specimens were tested for one sample. The water was replaced every 24 h, according to UL746C. The treated specimens were subsequently removed and dried constantly in a vacuum oven at 80 °C for 72 h. The above dry specimens were weighted and recorded as W1, and then the mass loss percentages of the specimens can be calculated using the following equation:

2. EXPERIMENTAL SECTION 2.1. Materials. (3-Aminopropyl) triethoxysilane (KH550) was provided by Aladdin Industrial Corporation. Silicon resin (methylpolysiloxane) with a molecular weight of 100 000− 200 000 was supplied by Shandong Dayi Co. Ltd., China. Ammonium polyphosphate (APP) with an average size of 15 μm was purchased by the Shandong Shian Chemical Co., Ltd., China. Triazine CFA with an average particle diameter of 10 μm was synthesized in our laboratory. Polypropylene (PP) resin (homo polymer, melt flow rate = 3.5 g/10 min) were produced by Daqing Huake Co. Ltd., China. 2.2. Measurement. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded with KBr disks by a Spectrum 400 system (Perkin−Elmer, USA). The contact angles (CA) were measured with a drop-shape analysis system (POWERREACH, Model JC2000A) at three different points for each. The powder of MAPP and CFA were homogeneously mixed with the mass ratio of 4:1, and then spread the powder of the mixture on a slide, whose surface was scraped by sliding to get flat. Then, the sample for determination of the CA of an IFR composed of MAPP and CFA was prepared. X-ray photoelectron spectroscopy (XPS) measurement was carried out using a Model ESCA750 spectrometer, with Al Kα

mass loss (%) =

(W0 − W1) × 100 W0

(1)

Thermogravimetry analysis (TGA) tests were carried out by a thermal analyzer (Perkin−Elmer, Model Pyris 1) at a linear heating rate of 10 °C min−1 under the pure nitrogen atmosphere. The range of temperature was 50−700 °C. The weight of the samples was kept within 4.5 ± 0.5 mg. Scanning electron microscopy (SEM) images were obtained by using a Quanta 200 scanning electron microscopy (SEM) system. The surface morphology of the particles containing APP and MAPP and the microcosmic morphology of the surface and interfaces of the PP/APP/CFA and PP/MAPP/ CFA composites were obtained. The specimens were previously coated with a conductive layer of gold. Tensile and flexural tests were completed with Regeer computer-controlled mechanical instrument (Model RGT-20A, Shenzhen Reger Instrument Co., Ltd.). The tensile test was conducted in accordance with the procedures in GB/T 10402006 at a crosshead speed of 20 mm/min. The flexural test was carried out according to Standard GB/T 9341-2008 at a bending speed of 2 mm/min. The notched impact property was B

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Industrial & Engineering Chemistry Research tested using a notched impact instrument (XJC-5, Hebei Chengde Precision Equipment Company), in accordance with the procedures in GB/T 1843-2008, and the depth of notch is 2 mm. 2.3. Preparation of MAPP. The preparation route of MAPP is shown in Figure 1. In a 1000-mL, four-necked flask equipped with stirrer, thermometer and reflux condenser, 8.43 g of KH550, 350 mL of ethanol, and 328.4 g APP were added together at room temperature. The reaction mixture was stirred for 30 min at ambient temperatures and then heated to reflux temperature until there was no ammonia release. Afterward, the silicon resin ethanol solution (12.96 g/150 mL) was added into the reaction mixture and maintained at the reflux temperature for an additional 4 h. After reaction, the solvent was removed by rotary evaporator under reduced pressure and the white solid was dried in a vacuum drying oven at 80 °C for 12 h. Finally, the white powder was obtained. 2.4. Sample Preparation of PP and IFR/PP. The commercial APP, CFA, and the prepared MAPP were dried in a vacuum oven at 80 °C for 12 h. Next the PP samples containing different ratios of IFR (consisting of MAPP and CFA with a mass ratio of 4:1) were mixed initially by a highspeed mixer and then prepared via a twin-screw extruder (SLJ20 Nanjing Jieya Chemical Engineering Equipment Company, China) with a rotation speed of 25 rpm at the following temperature protocol from the feed zone to the die: 175, 180, 190, 185, 180, and 170 °C. Finally, the final samples with suitable size for analysis were achieved by the injection method (injector: HTF86 × 1, Zhejiang Haitian, China) at a temperature profile of 200, 210, 210, and 210 °C. For comparative investigation, the PP/APP/CFA composites were also prepared by above processing with the same mass ratio (4:1) of APP and CFA.

respectively. Moreover, in contrast with APP, the absorption peak intensity of MAPP at 1085 cm−1 slightly increased, which was caused by joint action of the asymmetric Si−O−Si stretching28,35 and P−O symmetric stretching vibration. As a result, silicon exists in the structure of MAPP. XPS is an effective measurement method to study the surface chemical composition of samples without destruction. XPS spectra of APP and MAPP are presented in Figure 3. As shown

Figure 3. XPS spectra of APP and MAPP.

in Figure 3, it can be observed that the spectra for APP appeared five peaks at 133.7, 191.1, 284.5, 399.0, and 530.8 eV, which attributed to P2p, P2s, C1s, N1s, and O2p, respectively.30 Relative to APP, the peak intensity of P2p, P2s, and N1s for MAPP drastically decreased; meanwhile, the peak intensity of the C1s peak increased. Otherwise, two new peaks appeared at 103.1 and 154.1 eV in the spectra for MAPP assigned to Si2p and Si2s,35,36 which were derived from KH550 and silicon resin. The corresponding content of P and N for MAPP are 5.2 wt % and 2.7 wt % (see Table S1 in the Supporting Information), which are much lower than those of APP (26.0 wt % and 23.8 wt %, respectively), and the C content is 23.0 wt %, which is higher than that of APP (15.3 wt %). The changes of the above peaks and elemental compositions indicated that the surface of APP was well-coated by KH550 and silicon resin. The surface morphologies of APP and MAPP particles are presented in Figure 4. It can be observed that the surface of APP was relative smooth.34 In contrast, the surface of MAPP was rough, and some tiny brush, which may be assigned to the silicon resins chains that had adhered onto the particle surface, were observed. The above fact was caused by the modification with silicon resin on the APP surface. The average diameters of APP and MAPP are almost identical22 and were in the range of

3. RESULTS AND DISCUSSION 3.1. Characterization of MAPP. The FTIR spectra of APP and MAPP are presented in Figure 2. The typical absorption

Figure 2. FTIR spectra of APP and MAPP.

peaks of APP include 3208 cm−1 (N−H), 1253 cm−1 (PO), 1072 cm−1 (P−O symmetric stretching vibration), 1022 cm−1 (PO2 and PO3), 883 cm−1 (P−O asymmetric stretching vibration), and 798 cm−1 (P−O−P).34 After modified with KH550 and silicon resin, the spectrum of MAPP show new absorption band at 2967 and 2926 cm−1, corresponding to the stretching vibration of C−H in methyl and methylene groups,30

Figure 4. SEM images of the surface morphologies for (a) APP particles and (b) MAPP particles. C

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flat surface (θ) and the CA on a heterogeneous surface (θr) composed of a solid and air:39

10−15 μm (as revealed in Figure 4), because of the amount of silicon resin and KH550 encapsulated on the surface of APP being only 5 wt %, based on IFR. The wettability of APP, CFA, MAPP, and IFR composed of MAPP and CFA with the mass fraction of 4:1 has been evaluated by static water contact angle (CA) measurements, and the pictures of the water drop shape on the materials surface and the corresponding water CA are presented in Figure 5. The APP and CFA materials are easily drenched by

cos θr = f1 cos θ − f2

Here, f1 and f 2 are the fractions of solid surface and air in contact with liquid, respectively (that is, f1 + f 2 = 1). The water CA of APP on the flat surface and the surface after modification are 0° and 148°, respectively. The value of f 2 is calculated to be 0.97, which indicates that the hydrophobicity of the MAPP surface is mainly produced by the nanoscaled brushes of silicon resin. The nanoscaled polymer brush structure can increase surface roughness drastically, so the air can be trapped in the grooves of the hemispheres and the apertures of brushes. Compared with the previously reported works,24−34 the prepared MAPP possessed more excellent hydrophobicity. The reason was that the surface polarity of APP first decreased by the chemical modification with KH550 and the molecular of KH550 connected with the ammonium groups on APP molecular surface, and then the silicon resins reacted with KH550 and were encapsulated on the particle surface. The presented method with two steps made the hydrophobic silicon resins firmly and tightly coated on the APP surface, which contributed to the excellent and permanent hydrophobic properties of MAPP. It was worth noting that the water CA of IFR that was composed of MAPP and the superhydrophilic CFA (with a mass ratio of 4:1) was 143°, as shown in Figure 5d. This observation was attributed to the fact that the superhydrophilic CFA was encapsulated by MAPP in the IFR system, which resulted in the IFR having hydrophobic properties, with a water CA of 143°. In addition, the IFR that included MAPP and CFA with low surface energy could enhance the compatibility between the IFR and the PP matrix, which can decrease the deterioration of mechanical properties for PP. 3.2. Flame Retardancy. LOI and vertical burning (UL-94) tests are widely used to evaluate the flammability of flameretardant materials. The results of LOI values and UL-94 rating tests for PP/APP/CFA and PP/MAPP/CFA composites before and after water resistance tests are presented in Table S2 in the Supporting Information. The IFR still exhibited very effective flame retardancy in PP.15,20,21 Moreover, LOI values and UL-94 ratings of the IFR-PP composites increased as the loading amount of IFR increased.17 Pure PP matrix was very inflammable, with a large number of droplets, and its corresponding LOI value was only 17.0%. PP/APP/CFA and PP/MAPP/CFA composites with a thickness of 1.6 mm simultaneously passed UL-94 V-0 rating and their LOI values were 30.4% and 31.7%, respectively, when the loading amount of IFR was 22 wt %. By comparison with PP/APP/CFA, LOI values of PP/MAPP/CFA were higher with the same loading of IFR, which indicated that good dispersibility and compatibility of APP can obviously improve the flame retardancy of PP composite. Meanwhile, in comparison with other works on the modification of APP for PP/IFR composites,22,28,32 the LOI value and UL-94 ratings of PP/MAPP/CFA were higher than those at the same loading, suggesting a more positive effect on flame retardancy. The flame retardancy of the IFR-PP composites after water resistance tests (defined as treated IFR/PP) was also investigated by LOI and UL-94 tests. LOI values of the treated PP/APP/CFA and PP/MAP/CFA composites were reduced (see Table S2), with the former being reduced more than the latter. For example, when the IFR loading was 22 wt %, the LOI

Figure 5. Pictures of the water drop shape on the materials surface for (a) APP, (b) CFA, (c) MAPP, and (d) IFR composed of MAPP and CFA; the corresponding CAs are 0°, 0°, 148°, and 143°, respectively.

the water droplet, because APP and CFA are composed of polar units, which results in the superhydrophilic nature of APP and CFA, and the corresponding CAs were both 0°, as revealed in Figures 5a (for APP) and 5b (for CFA). However, the water CA of MAPP was greatly increased from 0° for APP to 148° and the water drop can remain stable on the MAPP surface for a long time. The wettability is generally related to the surface roughness of a certain material, when a water droplet can dip into the groove on the surface of material, the CA of the rough surfaces (θr) can be expressed by Wenzel’s formulation37 as cos θr = r cos θ

(3)

(2)

Here, r is the roughness factor (the ratio of the total surface area to the projected area on the horizontal plane) and θ is the CA measured on the native flat surface. Based on this equation, with increasing surface roughness (r), the actual CA decreases for hydrophilic materials (θ < 90°) and increases for hydrophobic materials (θ > 90°).38 After the modification of APP, some nanoscaled polymer brush of silicon resin with hydrophobic properties adhered on the particle surface and made the surface of APP drastically rough. Obviously, the surface roughness was greatly increased by the chemical modification of silicon resin, so the air can be trapped in the grooves and apertures on the APP surface due to the existing of nanoscaled hydrophobic brushes of silicon resin. It is known that the air trapped in the surface is very important to hydrophobicity, because the water CA of air is regarded to be 180°. In this case, eq 3, as proposed by Cassie and Baxter, can describe the correlation between the CA of a water droplet on a D

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Industrial & Engineering Chemistry Research value of the treated PP/MAPP/CFA (1.6 mm) was 30.6%, and it still successfully passed UL-94 V-0 rating; its LOI value only decreased 1.1 units. While the flame retardancy of the treated PP/APP/CFA composites obviously decreased, and they could not pass UL-94 V-0 rating until the IFR loading was 26 wt %. Thus, the water resistance of PP/MAPP/CFA in this work is more excellent, because of the hydrophobicity of MAPP and its better interfacial adhesion with PP matrix,30 which will be discussed later. 3.3. Water Resistance Analysis. Based on the flame retardance analysis discussed above, hot water treatment has only minor influence on the flame retardancy of the IFR-PP composites. This result was proved by the following hot water treatment test results. Figure 6 shows the mass loss rate (MLR)

Figure 7. Surface morphologies of IFR-PP composites before and after water resistance tests: (a) PP/APP/CFA, (b) PP/MAPP/CFA, (c) treated PP/APP/CFA, and (d) treated PP/MAPP/CFA.

aggregations of flame-retardant particles and cavities appeared on the surface of PP/APP/CFA composites, because of the poor compatibility between the IFR and the PP matrix.30 The cavities easily lead to penetration of hot water, and the IFR particles on the surface were easily attacked by water and hydrolyzed. After being subjected to water treatment, the surface of PP/APP/CFA composites presented a great deal of grooves and cavities, which were attributed to the extraction of IFR, as shown in Figure 7c. Figure 7b shows that the IFR containing MAPP dispersed homogeneously in PP matrix and the surface of the PP/MAPP/CFA composites appeared relative smooth and showed almost no cavities and aggregations, because of the excellent interfacial adhesion between the IFR and the PP matrix. The flawless surface of PP/ MAPP/CFA, which was like a barrier, could greatly prevent the infiltration of water. Moreover, the silicon resin, because of its hydrophobility leads to the great decrease in extraction of MAPP, and thus there were only some grooves and almost no cavities on the surface of PP/MAPP/CFA after the hot water treatment (Figure 7d). The results indicated that hydrophobicity of MAPP and the flawless surface of PP/MAPP/CFA have a good effect on the enhancement of water resistance. By means of XPS analysis, the elemental composition (C, N, O, P and Si) on the cross section with different depths (named surface I and surface II) was obtained (see Table S3 and Figure S1 in the Supporting Information). For the PP/MAPP/CFA composite, the contents of elemental N, O, P, and Si on surface I were lower than those on surface II, while the content of elemental C was higher. The results indicated that, during the molding process of the PP/MAPP/CFA composites, PP migrated to the surface of the samples, because of its better fluidity and lower density than IFR.17 After water resistance tests, the content of elemental Si on surface I was higher than that on surface II, even higher than that on surface I of the composites before water resistance tests. The content of elemental P on the surface of the PP composites was almost identical before and after water resistance tests. The above results were discussed as follows. During the water resistance test, the water molecules would penetrate into the surface of IFR, resulting in a small amount of IFR being extracted by hot

Figure 6. Relationship between the mass loss rate of PP/APP/CFA and PP/MAPP/CFA and the water treatment time.

of IFR/PP with the water treatment time. The mass loss rate represents the amount of the IFR extracted by water. With the increase in water treatment time, the MLR of the IFR/PP composites increased to a certain extent. It is obvious to think that circulating hot water will permeate through the material, from the surface to the inside, leading to gradual extraction. Therefore, the surface of the IFR/PP composites is crucial to its water resistance, which would be discussed next. The MLR of the PP/MAPP/CFA was much lower than that of the PP/APP/ CFA at the same water treatment time. That is, when the water treatment time was 7 days, the MLR of PP/MAPP/CFA and PP/APP/CFA 1.6 mm thick were 0.85 wt % and 2.31 wt %, respectively. Therefore, the IFR that contained MAPP exhibited much better water resistance in PP matrix than the APPcontaining IFR. In fact, the water absorbability of IFRs seriously restricts their applications in the polymer matrix, and it results in a decrease in flame retardancy, electrical insulation, and processing ability.40 Based on our previous report,17,21 the MLR of PP/MAPP/CFA with 7 days of water treatment was much lower than those (3.16 and 0.92), and the treated PP/MAPP/ CFA composites maintained excellent flame retardancy, which proved that the modification of APP was an efficient way to improve the water resistance of IFR/PP. In comparison with other studies on the modification of APP, the exudation of IFR in composites was 3.2%,24 4%,23 which were much higher than the MLR (0.85%) in this research. The comparison indicates that the water resisitance of PP/MAPP/CFA is excellent. Figure 7 gives the surface morphologies of PP/APP/CFA and PP/MAPP/CFA composites before and after water resistance tests. As shown in Figure 7a, a great deal of big E

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Industrial & Engineering Chemistry Research water.17 IFR would hydrolyze and migrate to the surface of the composites. However, the internal APP in MAPP hydrolyzed into soluble phosphorus products that were exuded by hot water,41 and external silicon resin was retained on the surface of the composites due to good compatibility with PP. Thus, the increase in the P content on the surface is not remarkable, while on the surface, the amount of elemental Si on surface I obviously increased, which was beneficial to the improvement on the water resistance of the composites. Consequently, the flame-retardant PP composites consisting of MAPP retained excellent flame retardancy with a lower mass loss rate after water resistance testing. 3.4. Cone Calorimeter Tests. Based on the cone calorimeter test, the heat release rate (HRR) and total heat released (THR) curves recorded during cone calorimeter tests and the corresponding evaluated experimental data are presented in Figure 8 and Table 1, respectively.

The HRR is recognized to be the most important parameter to quantify the size of fire, and an effective flame-retardant system normally shows a lower HRR value.17 From Figure 8 and Table 1, pure PP is a very flammable polymer; the HRR peak value (peak1-HRR) was 1677.1 kW m−2 and THR reached 184.4 MJ m−2. In contrast, the IFR/PP composites showed obviously shorter ignition times, much lower HRR and THR values, and different HRR curve shapes, containing two HRR peaks. The first peak is assigned to the ignition and to the formation of an expanded protective shield;11,42 the second peak is explained by the destruction of the intumescent structure and the formation of a carbonaceous residue.34 The peak1-HRR, peak2-HRR, and THR values of the PP/APP/CFA composite were 291.8 kW m−2, 397.3 kW m−2, and 161.1 MJ m−2, respectively. MAPP made the peak-HRR values of the PP/ MAPP/CFA decrease to 268.1 kW m−2, 271.1 kW m−2, and 140.8 MJ m−2, respectively, and the latter obviously prolonged the heat release process. However, the ignition time (TTI) decreased as the peak1-HRR values of the composites decreased. By comparison with PP/APP/CFA, most of the flammability parameters are remarkably reduced when the APP is modified, and MAPP can more effectively promote char layer formation of the PP/MAPP/CFA composites and increases the strength and stability of the char layer. This result is in agreement with the flame retardancy and TGA results. 3.5. Thermal Stability. From TGA and differential thermogravimetry (DTG) curves of APP and MAPP under nitrogen atmosphere (Figure S2 in the Supporting Information) and the relative data (Table S4 in the Supporting Information), it can be found that the thermal degradation process of APP contains two stages. The first stage was from 270 °C to 500 °C, at which APP decomposed to form polyphosphoric acid by releasing ammonia and water;22 the second stage was above 500 °C. The parameter T1% was used, which usually represented the initial degradation temperature in many related research works such as ours. Compared with APP, the T1% value of MAPP decreased from 270.4 °C to 232.3 °C. The degradation of APP begins with the breaking of the bond connecting the ammonium group with APP, resulting in the production of ammonia.22,28 From the structures of APP and MAPP (Figure 9), it can be concluded that bond 2 in MAPP

Figure 9. Chemical structure of (a) APP and (b) MAPP. Figure 8. (a) Heat release rate (HRR) and (b) total heat released (THR) curves of PP, PP/APP/CFA, and PP/MAPP/CFA.

Table 1. Related Cone Data of PP and IFR/PP Composites TTI (s) peak1-HRR (kW m−2) time to peak1-HRR (s) peak2-HRR (kW m−2) time to peak2-HRR (s) THR (MJ m−2) a

was weaker than bond 1 in APP, which was attributed to the fact that the charge of ammonium group in MAPP will be dispersed by the introduction of the organic groups (KH550). Therefore, the breaking of bond 2 in MAPP to release KH550 would happen at lower temperature than the breaking of bond 1 in APP to release NH3. Thus, the decrease in the T1% value of MAPP, compared to that of APP, was caused by the weaker combination between KH550 and APP. However, the residues at 700 °C of MAPP increased by 13.9%. It can be seen that APP included two thermal degradation peaks, while MAPP had one more than APP did (see Figure S2(b)), which attributed to the degradation of

a

PP

PP/APP/CFA

PP/MAPP/CFA

37 1677.1 210   184.4

21 291.8 55 397.3 570 161.1

16 268.1 40 271.7 700 140.8

Dash symbol () indicates that corresponding data did not exist. F

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Industrial & Engineering Chemistry Research Table 2. TGA Data of PP and IFR/PP Composites sample

T1% (°C)

Tmaxa (°C)

Rmaxb (% min−1)

R500c (wt %)

R600d (wt %)

R700e (wt %)

ΔWff (wt %)

PP PP/APP/CFA PP/MAPP/CFA

279.7 268.2 254.4

356.7 372.4 388.3

19.2 11.9 11.4

0 14.9 14.6

0 12.6 10.6

0 10.7 7.5

 2.3 4.0

a

Tmax = Tpeak of the maximum decomposition stage in DTG curves. bRmax = the peak value of the maximum decomposition stage in DTG curves. R500 = residual ratio of PP/IFR composites at 500 °C. dR600 = residual ratio of PP/IFR composites at 600 °C. eR700 = residual ratio of PP/IFR composites at 700 °C. fΔWf = weight loss from 500 °C to 600 °C. c

silicon resin in MAPP. The peak values of APP were 1.5% min−1 and 5.8% min−1, respectively, the corresponding peak values of MAPP were 2.0% min−1, 3.6% min−1, and 3.4% min−1. In comparison with APP, the value of peak1 for MAPP increased from 1.5% min−1 for APP to 2.0% min−1 because of the coincidence between the decomposition of side chain in silicon resin and the degradation of APP; the value of peak2 for MAPP decreased from 5.8% min−1 for APP to 3.6% min−1, which contributed to the fact that the modification of APP decreased the degradation of APP and improved the thermal stability in higher temperatures that would be beneficial to flame retardancy. From TGA and DTG curves of pure PP, PP/APP/CFA, and PP/MAPP/CFA composites under nitrogen atmosphere (Figure S3 in the Supporting Information), and the relative data in Table 2, it can be seen that, up to 700 °C, the pure PP was completely converted to volatile combustible products with no residue. Enhanced thermal stability was found with the PP/ IFR composites. The main decomposition temperature of PP/ APP/CFA and PP/MAPP/CFA composites were 15 and 31 °C higher than that of pure PP, as indicated by Tmax. The value of R700 indicated that the PP/APP/CFA composites produced the most residue at 700 °C (R700 = 10.7). Compared with PP/ APP/CFA, the microencapsulated samples have lower initial degradation temperatures, which could be attributed to the fact that more IFR consisting of MAPP and CFA with low surface energy migrated to the surface of the PP/MAPP/CFA composites, resulting in the thermal degradation of IFR at lower temperature. It is noteworthy that, for PP/IFR composites, both PP/MAPP/CFA and PP/APP/CFA has a value of ΔWf, which was derived from the further degradation of phosphorus degradation products above 500 °C (defined as PDPs).11,43 In the APP−CFA system, both unreacted APP and P−C complexes were converted to a variety of PDPs when heated.44 Compared to PP/APP/CFA, the value of ΔWf is larger for PP/MAPP/CFA, which indicated that more PDPs were converted by unreacted APP (see Figure S2 and Table S4) and P−C complexes. Both of them were probably in larger amount in PP/MAPP/CFA, because of surface modification of the APP. Otherwise, the PP/MAPP/CFA composites had smaller Rmax values than that of PP/APP/CFA. This may be due to the fact that the modification of APP with the silicon resin could reinforce the strength and the thermal stability of the char layer to impart the char with good quality, protecting the underlying matrix from thermal degradation. 3.6. Morphologies of PP/APP/CFA and PP/MAPP/CFA Composites. In order to investigate the dispersion of flame retardant in the PP matrix, the SEM images of fracture sections for PP/APP/CFA and PP/MAPP/CFA composites are shown in Figure 10. As displayed in Figure 10a, many cavities could obviously be observed, and there are clear interfaces and even gaps between flame-retardant particles and the polymer matrix, because of the fact that the rather different polarities of IFR and

Figure 10. SEM images of fracture sections for (a) PP/APP/CFA and (b) PP/MAPP/CFA composites.

PP make them thermodynamically immiscible and thus cause a weak interfacial adhesion, which plays an important role in the mechanical and other related properties.35 While in Figure 10b, IFR well disperses in the PP matrix and almost no obvious interfaces are observed between fillers and the matrix. A comparison of Figures 10a and 10b shows that the microencapsulated APP improved the compatibility and dispersion of IFR in PP/MAPP/CFA composites,45 because both the silicon resin shell and the PP have alkyl groups and similar polarity.34 Because the silicon resin has increased the interfacial adhesion between the MAPP and the polymer matrix, it will also result in changed mechanical properties,34 which will be analyzed in the following sections. 3.7. Mechanical Properties of PP, PP/APP/CFA, and PP/MAPP/CFA. The tensile, flexural, and notched impact strengths of PP and IFR-PP composites with different IFR loadings were researched (the related data are provided in Table S5 in the Supporting Information). Generally, the trend of tensile strength is different from flexural strength, which indicated that IFR had different effects on tensile and flexural strength. As the IFR loading increased, the tensile strength of IFR-PP composites decreased, because of the formation of more defects (e.g., holes and cracks) during the process for composites. However, the flexural strength of IFR-PP composites increased as the IFR loading increased. The IFR particles hindered the motion of the PP molecular chain, resulting in increased friction during the flexural test, and the friction increased with the IFR loading. Compared with neat PP, the tensile and impact strength of IFR-PP composites decreased, because of the appearance of more defects at the interfaces between IFR and PP during the process for composites. Moreover, IFR2 consisting of MAPP and CFA shows less effect on the mechanical properties of IFR-PP composites, in comparison with IFR1 containing APP and CFA. When compared with pure PP, two types of IFR clearly enhanced flexural strength, which was increased from 31.8 MPa to ∼35.3 MPa; this may be ascribed the IFR particles of the composites hindered the motion of the PP molecular chain during the flexural test.20 Tensile and impact strength of IFR-PP gradually G

DOI: 10.1021/acs.iecr.5b01670 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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ACKNOWLEDGMENTS This work was financially supported by Heilongjiang Major Research Projects (No. GA12A102) and China Postdoctoral Science Foundation (Nos. 2012 M510907 and 2013 T60339).

decreased as the IFR loading increased, because of the weak interaction between filler particles.46 IFR2 presented less effect on the tensile and impact strengths of IFR-PP than IFR1, for instance, when the IFR loading is 26 wt % in the IFR-PP system, the tensile and impact strengths of PP/APP/CFA are 25.7 MPa and 1.9 kJ m−2, respectively, whereas, for PP/MAPP/ CFA, values of 26.7 MPa and 2.3 kJ m−2 are observed. From the analysis of mechanical properties of IFR-PP, IFR2 shows a more favorable effect on the mechanical properties of IFR-PP than IFR1. This is because they contain different APPs, IFR1 with APP, and IFR2 with MAPP. This observation was probably attributed to the fact that the modification of APP improved the interfacial adhesion47 and the modified siliconcontaining material had a positive effect on the enhancement of mechanical properties.21

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01670. Surface elemental compositions of APP and MAPP (Table S1); flame retardancy of IFR/PP and treated IFR/PP composites (Table S2); elemental compositions of PP/MAPP/CFA and treated PP/MAPP/CFA (Table S3); TGA data of APP and MAPP (Table S4); mechanical properties of PP and IFR/PP composites (Table S5); XPS detecting position of surface I and II of the PP/MAPP/CFA composites (Figure S1); TG and DTG curves of APP and MAPP (Figure S2); and TG and DTG curves of PP, PP/APP/CFA and PP/MAPP/ CFA composites (Figure S3) (PDF)

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REFERENCES

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4. CONCLUSION Modified ammonium polyphosphate (MAPP) was successfully prepared by modifying ammonium polyphosphate (APP) with KH550 and silicon resins, and its water contact angle (CA) reached 148°, because of the formation of nanoscaled polymer brushes of silicon resin on the APP particle surface. MAPP was combined with a char-forming agent (CFA) with a mass fraction of 4:1 to prepare intumescent flame retardant (IFR), and IFR showed hydrophobic properties, with a water CA value of 143°. The PP/MAPP/CFA composites with 22 wt % IFR and a thickness of 1.6 mm successfully passed UL-94 V-0 flammability rating before and after water resistance tests. The modification of APP reduced the surface energy of IFR and enhanced the compatibility between the IFR and the PP matrix. Thus, the PP/MAPP/CFA composites showed enhancement in the water resistance and mechanical properties, compared to PP/APP/CFA composites. The incorporation of MAPP/CFA blends promoted the decomposition of PP and char-forming ahead of time and led to a higher thermal stability at high temperature. The introduction of MAPP/CFA blends benefited the formation of the char layer with high strength and thermal stability on the materials surface during burning, which prevented heat transmission and diffusion, leading to reduction of the heat release rate.

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DOI: 10.1021/acs.iecr.5b01670 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b01670 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX