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Preparation, characterization of microencapsulated ammonium polyphosphate and its flame retardancy in polyurethane composites
Materials Chemistry and Physics 173 (2016) 205e212
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation, characterization of microencapsulated ammonium polyphosphate and its flame retardancy in polyurethane composites Ming-Yuan Shen a, Wei-Jen Chen a, Chen-Feng Kuan b, Hsu-Chiang Kuan b, Jia-Ming Yang c, Chin-Lung Chiang c, * a
Department of Aviation Mechanical Engineering, China University of Science and Technology, Hsinchu County, 303, Taiwan Department of Computer Application Engineering, Far East University, Tainan, 744, Taiwan Green Flame Retardant Material Research Laboratory, Department of Safety, Health and Environmental Engineering, Hung-Kuang University, Taichung, 433, Taiwan b c
h i g h l i g h t s A novel microencapsulated flame retardant was synthesized using in situ polymerization technology. The microencapsulation of ammonium polyphosphate with the polymer resin resulted in improved hydrophobicity. Polyurethane composites have excellent thermal stability and flame retardance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 28 November 2015 Accepted 2 February 2016 Available online 12 February 2016
In this study, a novel microencapsulated flame retardant containing ammonium polyphosphate (APP) and an 4,40 -oxydianiline-formaldehyde (OF) resin as the core and shell material was synthesized using in situ polymerization technology. The structure and performance of OF microencapsulated APP (OFAPP) were characterized using Fourier transform infrared spectroscopy and scanning electron microscopy. The thermal properties of OFAPP were systematically analyzed through thermogravimetric analysis. Flame retardancy tests, such as limiting oxygen index (LOI) and UL-94, were conducted to evaluate the effect of varying the composition of APP and OFAPP in silanol-terminated polyurethane (Si-PU) composites. The results indicated that the microencapsulation of APP with the OF resin resulted in improved hydrophobicity. The results also revealed that the flame retardancy of the Si-PU/OFAPP composite (LOI ¼ 37%) was higher than that of the Si-PU/APP composite (LOI ¼ 23%) at the same additive loading. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Fourier transform infrared spectroscopy (FTIR) Thermogravimetric analysis (TGA)
1. Introduction Polyurethane (PU) has received considerable attention in recent years because of its unique polymeric characteristics and favorable physicochemical properties [1e4]. Furthermore, PU can be specifically designed to satisfy the diverse requirements of conventional polymers used in coating, adhesive, foam, and thermoplastic elastomer applications [1,5,6]. However, the thermal instability and flammability of PU are major limitations of the material [7]. Therefore, numerous researchers have studied this specific problem to improve the thermal stability and flame retardancy of PU [8,9].
* Corresponding author. E-mail address: [email protected] (C.-L. Chiang). http://dx.doi.org/10.1016/j.matchemphys.2016.02.006 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Flame retardants composed of ammonium polyphosphate (APP) have been developed for many years and have shown favorable performance in retarding the production of flames. Ammonium polyphosphate can play the role of flame retardant due to a strong dehydrating agent generated by polyphosphoric acid is, which can promote to form carbon residue on the surface of polymer matrix. The carbon residue containing phosphorus covered on the surface, which can block oxygen to prevent the spread of flammable gases and carbon gasification [10,11]. However, a flame retardant comprising solely APP alone cannot effectively yield favorable outcomes; therefore, researchers have adopted synergistic effects by combining APP with other flame retardants to effectively improve the flame retardancy of APP [12,13]. Hygroscopicity is a flaw associated with APP-containing flame retardants because it can reduce the effects of flame retardancy when APP is added to
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polymeric composites [14,15]. Therefore, microencapsulation technology has been applied to modify APP to enhance the flame retardancy of APP surfaces. In addition, this approach facilitates converting the hydrophilicity of APP to hydrophobicity, which increases the compatibility of composite materials and decreases the hygroscopicity of APP [14,15]. Wu et al. [16] used polyvinyl alcohol (PVA) to modify melamineformaldehyde resin to coat ammonium polyphosphate (VMFAPP), and to flame retardant polypropylene. The results showed that the microcapsules can greatly reduce the heat release rate of materials, and PVA content of melamine-formaldehyde prepolymer materials played an important role on water-resistant and flame retardant of the microcapsules. The silicon microcapsule has been widely used to encapsulate material by solegel process. Ni et al. used silica gel to microencapsulate APP and it used to flame retardant thermoplastic polyurethane [17]. Through microencapsulation, the water solubility, hydrophilicity, thermal properties of APP are greatly improved. In the current study, PU was first modified to incorporate silicon to form siloxane linkage in the PU structure. Polysiloxanes offer high thermal stability up to 300 C. That resistance results from the presence of SieO bonds, for which the dissociation energy is higher (ca. 460.5 kJ/mol) in relation to CeO (358 kJ/mol), CeC (304 kJ/mol) and CeNH (98 kJ/mol) bonds [18,19]. A solegel reaction was subsequently applied to form a PU network structure that facilitates increasing the thermal stability of PU. An APP-containing flame retardant was then microencapsulated to produce carbonization agents on the surface, which convert APPs that are initially unexpandable to APPs exhibiting expandability. Moreover, microencapsulation converts the property of APPs from being hydrophilic to being hydrophobic, which enhances the compatibility of substrates to flame retardants.
2.3. Preparation of 4,40 -oxydianiline-formaldehyde microencapsulated APP To prepare 4,40 -oxydianiline-formaldehyde microencapsulated APP (OFAPP), ODA (10 g) and formaldehyde (5.99 g) were added to a reaction tank, to which 40 mL of THF was added. Subsequently, ammonia solution was used to adjust the pH of the solution in the reaction tank to 8e9. The solution was then heated at 60 C for 10 min, yielding a transparent solution following reaction completion. APP (40 g) and ethanol (500 mL) were placed in another reaction tank, preheated, and stirred at 80 C for 0.5 h. Next, the transparent prepolymer was transferred into the APPcontaining reaction tank, to which HCl was added to adjust the pH to 3e4. The reaction was allowed to proceed at 80 C for 3 h, yielding powdered products. Following filtration, the powders were extracted and rinsed several times in ethanol and then placed in a convection oven at 80 C for 12 h. The reaction is shown in Scheme 2. 2.4. Preparation of Si-PU/OFAPP composites To synthesize the Si-PU/OFAPP composites, the OFAPP flame retardant was added to the Si prepolymer prepared according to Scheme 1. Next, 0.5 mL of H2O was added and the temperature was adjusted to 70 C, and the solution was observed for any reduction in solvent volatility or increase in viscosity. When the viscosity increased, the Si-PU/OFAPP product was placed in a mold at room temperature for 6 h before drying at 80 C in a vacuum oven for 12 h. The final product (Si-PU/OFAPP composites) was then removed from the oven and cooled at room temperature. 2.5. Measurements
2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI, purity 98%), ethylenediamine (purity 99%), 3-aminopropyltriethoxysilane (APTS, purity 99%), 4,4'-oxydianiline (ODA, purity 98%), and formaldehyde (37 wt% sol., stab. 10%e15% methanol) were purchased from Acros Chemical Co, New Jersey, United States. Anhydrous stabilized tetrahydrofuran (THF) was supplied by Lancaster Co., Morecambe, Lancashire, United Kingdom. Arcol polyol 1007 (polyether polyols 700) were purchased from Bayer Material Science Ltd, Kaohsiung, Taiwan. Ammonium polyphosphate (APP, phase II, n > 1000) was purchased from San Jin Chemicals Corporation, Kaohsiung, Taiwan.
2.2. Preparation of Si-PU IPDI (12.6 g) and arcol polyol 1007 (20 g) were placed in a fourneck flask inserted in an 80 C oil bath and mechanically stirred in N2 atmosphere. Subsequently, 1 g of the metal catalyst dibutyltin dilaurate was added into the four-neck flask. After 1.5 h of reaction, the temperature was reduced to 50 C when viscosity of the prepolymer increased to a level similar to that of maltose. Next, THF (50 mL) and APTS (12.6 g) were added into the four-neck flask and stirred for 0.5 h, after which 0.5 mL of H2O was added. The temperature was then raised to 70 C, to reduce the volatility of the solvent and increase the solution viscosity. When the viscosity increased, the product (Si-PU) was placed in a container at room temperature for 6 h before vacuum drying at 80 C for 12 h in a vacuum oven. The product was removed after 12 h and cooled at room temperature. The reaction is specified in Scheme 1.
The FTIR spectra of the materials were obtained between 4000 and 400 cm1 using a Nicolet Avatar 320 FT-IR spectrometer from the U.S.A. Thin films were prepared by solution-casting. A minimum of 32 scans were signal-averaged with a resolution of 2 cm1. The mean particle size and distribution of samples were measured by optical particle sizing instrument (Zeta-Sizer3000HS, Malvern Corporation, English). Before the measurement, the samples were dispersed in ethanol, and sonicated for 15 min. The morphology of the fractured surface of the composites was examined using a scanning electron microscope (SEM) (JEOL JSM 840A, Japan). The thermal degradation of the composites was examined using a thermogravimetric analyzer (TGA) (Perkin Elmer TGA 7) from room temperature to 800 C at a heating rate of 10 C/min in an atmosphere of nitrogen. The measurements were made on 6e10 mg samples. The LOI test was carried out following the ASTM D 2836 Oxygen Index Method, using a test specimen bar that was 7e15 cm long, 6.5 ± 0.5 mm wide, and 3.0 ± 0.5 mm thick. The sample bars were suspended vertically and ignited using a Bunsen burner. The flame was removed and the timer was started. The concentration of oxygen was increased when the flame on the specimen was extinguished before it had burned for 3 min or burning 5 cm away from the bar. The oxygen content was adjusted until the limiting concentration was determined. The vertical burning test was done inside a fume hood. Samples were held vertically with tongs at one end and burned from the free end. Samples were exposed to ignition source for 10 s then they were allowed to burn above a cotton wool until both sample and cotton wool extinguished. Observable parameters were recorded to assess fire retardancy. The UL 94 test classifies the materials as V-0, V-1 and V-2 according to the time period needed before self-extinction and the occurrence of flaming dripping after removing the ignition source. V-0 is the most ambitious and desired classification.
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3. Results and discussion 3.1. Structural identification of Si-PU In this study, PU containing silanol was fabricated, the structures of which were observed using Fourier transform infrared spectroscopy (FTIR). Fig. 1 shows the FTIR spectra of IPDI, polyol, and prepolymer, which reveal that the NCO peak of IPDI is located at 2256 cm1 and that polyol contains considerable eOH groups (at 3480 cm1). The reaction between the two components yielded the prepolymer. The spectra show that the -NCO peak diminished in intensity and the prepolymer contained -NCO reactive functional groups on its two sides for subsequent reaction processes [20]. In addition to the variations in the eNCO functional group, new eNH
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(3320 cm1), C]O (1717 cm1), and CeN (3320 cm1) peaks formed after the reactions between eNCO and eOH to produce urethane linkage. According to the spectra, the linkages of urethane appeared and the eOH peaks diminished [20e22], indicating the formation of the prepolymer. Fig. 2 illustrates the FTIR spectra of the prepolymer and Si-PU; these spectra were analyzed to determine whether the prepolymer reacted with APTS, as an indication of endcapping process. Fig. 2 indicates the complete disappearance of the eNCO peak in the terminated Si prepolymer, suggesting that the APTS reacted with the prepolymer, thereby forming a structuring comprising silanol at two ends of the prepolymer. After the hydrolysis of the terminated Si prepolymer, eOH functional groups were observed on the silanol at both ends as reflected by the increase in the
Scheme 1. Reaction of Si-PU composites.
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Scheme 2. Reaction of OFAPP.
number of eOH peaks in the Si-PU spectra shown in Fig. 2. Furthermore, the SieOCH2CH3 is located at 780 cm1, where the peaks also diminished in intensity after the hydrolysis reaction [23]. Thus, hydrolysis facilitated the subsequent condensation reactions.
According to the results, Si-PU composite materials were successfully fabricated through a series of reactions.
Fig. 1. FTIR spectra of IPDI, PEG, and prepolymer.
Fig. 2. FTIR spectra of the prepolymer, Si prepolymer, and Si-PU.
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Fig. 3. FTIR spectra of APP and OFAPP.
3.2. Structure characterization of OFAPP Fig. 3 depicts the FTIR spectra of APP before and after the microencapsulation. The spectra were analyzed to preliminarily determine the structures of APP and peak variations. These spectra show typical APP peaks at 3200 (NeH), 1253 (P]O), 1070 (PeO symmetric stretching), 1017 (symmetric vibration of PO2 and PO3), 882 (PeO asymmetric stretching) and 800 cm1(PeOeP) [24]. Several new peaks were also observed, suggesting the formation of microencapsulated OFAPP, such as at 3375 (eOH), 2823 (CeH), 1625 (C]C), 1495 (benzene), 1308 (CeN), 1120 (CeOeC), and 823 cm1 (para-position of benzene) [25e27]. In the OFAPP spectra, the peaks representing the OF shell and typical APP peaks were identified. This result suggests that APP was successfully microencapsulated, forming the microencapsulated APP flame retardant.
Fig. 5. SEM photographs of (a) APP and (b) OFAPP particles. The insets are pictures of the water contact angle.
3.3. Size distribution of OFAPP Fig. 4 shows the particle size distributions of APP and OFAPP. As shown in the diagram, the particle size of APP is around 15 mm, and that of OFAPP is around 26 mm. Obviously, the thickness of OFAPP is
larger than that of APP, meaning OF resin was wrapped onto the surface of APP particles. The variations in particle size distribution also confirmed the successful fabrication of the microencapsulated flame retardant. 3.4. Surface morphology and hydrophobicity of OFAPP
Fig. 4. Particle size distributions of APP and OFAPP.
Fig. 5 shows the changes in the surface morphology of APP before and after the microencapsulation. As show in Fig. 5(a), the APP particles, predominantly presented in the form of long rods, exhibit smooth surface morphologies, with partial cracks on the surface. Fig. 5(b) shows the OFAPP particles, the surface morphology of which differed from that of APP in that a coarsesurfaced shell was observed on the surface of the OFAPP particle. In addition, no cracks as those observed on the surface of APP particles were identified. In summary, an additional outer shell on the APP particles was observed after the microencapsulation, revealing a surface morphology considerably different to that of OFAPP. The insets show a water contact angle of 22.49B for APP; this is primarily attributed to the hydrophilicity of NHþ 4 in APP, thus resulting in low water contact angles [27]. OFAPP exhibited a water contact angle of 74.92 , indicating increased hydrophobicity, which is attributable to the benzene structures on the microencapsulated shell. APP featured superior water compatibility and hence a lower
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water contact angle than OFAPP did; this indicates that APP is hydrophilic and OFAPP is hydrophic. Therefore, the microencapsulation of shell materials can reduce the attack of water vapor and increase the compatibility of APP with polymers due to its hydrophobicity when mixed with it, thus improving the dispersion of additives in the polymer matrix. 3.5. Thermal properties of Si-PU/OFAPP composites According to the data shown in Fig. 6(a) and Table 1, pristine PU has a Td5 of 306 C, which increased to 328 C after PU was modified into Si-PU. This result can be attributed to the Si element migrating on the surface of PU, subsequently forming the SieOeSi char layers, which improved thermal resistance and enhanced antioxidative effects. Td5 represented the initial degradation temperature. There are two components to affect the property including silicon and phosphorus. The materials containing silicon will possess better thermal stability due to that SieO bonding have higher bond energy. On the other end, the materials containing PeO bond due to lower bond energy. These two factors affected the final property of the materials. We can find out the trend for Si-PU/ OFAPP 10e40% in Table 1 is not consistent for Td5. Consequently, thermal decomposition was delayed to a point that it commences at
Fig. 6. (a) TG and (b) DTG curves of pure PU, Si-PU, and Si-PU/OFAPP 10e40% composites.
a higher temperature [28,29]. In the APP and OFAPP composites, APP released NH3 and H2O at 220e400 C, reacting with the outer shell; therefore, the Td5 temperature ranged between those of pure PU and Si-PU. Furthermore, the weight of Si-PU (Fig. 6(a)) continued decreasing after 500 C because of the composite material's inability to withstand high thermal energy. Nevertheless, the weight of Si-PU stabilized at 600 C. In the OFAPP composite, complex reactions constantly occurred at 220e500 C; therefore, OFAPP composites decomposed earlier than Si-PU did, suggesting that the temperature at which OFAPP decomposed was lower than that for Si-PU. However, the thermal stability of OFAPP composites after 500 C was superior to that of the Si-PU composite. This is because the series of precedent reactions resulted in dense barrier layers that prevented the thermal destruction of polymers in the inner layers, thus explaining the high amount of char residue at the high temperature stage. Fig. 6(b) depicts the variations of the thermal decomposition rates. The decomposition rates of pure PU and Si-PU are 30.4 and 16.1 wt%/min, respectively. The rates for both the APP- and OFAPPadded composites were lower than that of Si-PU (16.1 wt%/min), and only the sample Si-PU/OFAPP 40% exhibited a rate (17.8 wt %/min) closest to that of Si-PU. This can be explained by the excellent thermal stability of the char layer produced by Si-PU at 500 C. When temperatures exceeded 500 C, the Si-char layer could no longer resist high temperatures and the structure gradually disintegrated. Furthermore, in the OFAPP-added composites, a series of reactions occurred at 220e500 C including the volatilization of inflammable gases and the formation of char layers. Therefore, the thermal decomposition rate of this composite was lower than that of Si-PU because the APP- and OFAPP-added composites both exhibited thermal stability at high temperature (>500 C). In addition, compared with pure PU, the thermal decomposition rates of Si-PU/ OFAPP 10%e40% were 24.8, 22.7, 19.7, and 17.8 wt%/min, respectively, showing a decreasing trend. When the composition of OFAPP was increased by 40% (i.e., Si-PU/OFAPP 40%), the thermal decomposition rate slowed by 12.6 wt%/min (approximately 58%), suggesting improved thermal stability. The curves shown in Fig. 7 present comparisons of the differences in thermal stabilities between the APP- and OFAPP-added composites. Fig. 7(a) indicates that the thermal stability of Si-PU/ OFAPP is superior to that of Si-PU/APP after approximately 400 C. The char produced on the Si-PU/APP and Si-PU/OFAPP samples at 800 C were 20.8 wt% and 23.7 wt%, respectively, implying that the microencapsulated OFAPP demonstrated enhanced thermal stability at a high temperature. Finally, the integral procedural decomposition temperature (IPDT) data shown in Table 1 were obtained according to the thermal stability assessment method for polymeric composites developed by Doyle [30]. In these data, higher IPDT represents better thermal stability [31]. The results for pristine PU, Si-PU, SiPU/OFAPP 10%, Si-PU/OFAPP 20%, Si-PU/OFAPP 30%, and Si-PU/ OFAPP 40% were 336, 540, 611, 687, 771, and 852 C, respectively, whereas that for Si-PU/APP 30% was 717 C. The IPDTs for all the samples demonstrated an increasing trend, meaning that the initial material modification and the subsequent addition of OFAPP flame retardants both contributed to improving the thermal stability of the composites; compared with the thermal stability of pristine PU, those of Si-PU and Si-PU/OFAPP 40% increased by 160% and 253%, respectively. Furthermore, the IPDT of Si-PU/APP 30% and Si-PU/ OFAPP 30% differed by 54 C, which further represented that the thermal stability of the microencapsulated OFAPP was superior to that of APP.
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Table 1 Thermal properties of pure PU, Si-PU, and Si-PU/OFAPP 10e40% composites. Sample
Td5a ( C)
Rmaxb (wt%/min)
IPDTc ( C)
Char residue (wt%) at 800 C
Pure PU Si-PU Si-PU/OFAPP 10% Si-PU/OFAPP 20% Si-PU/OFAPP 30% Si-PU/OFAPP 40% Si-PU/APP 30%
306 328 319 331 324 317 319
30.4 16.1 24.8 22.7 19.7 17.8 23.5
336 540 611 687 771 852 717
0.9 12.2 17.1 20.5 23.7 26.9 20.8
a b c
Td5 indicates the temperature at which the weight loss of the sample reaches 5%. Rmax corresponds to the maximum thermal degradation rate. The entire degradation process is considered in the IPDT.
3.6. Flame retardancy of Si-PU/OFAPP composites Fig. 8 present the limiting oxygen index (LOI) of the composite materials and the values obtained from Underwriters Laboratories' Standard for Safety and Flammability of Plastic Materials for Parts in Devices and Appliances testing (UL-94). Pristine PU exhibited an LOI of 17%, failed the UL-94 test, and demonstrated dripping effects, meaning that pristine PU is flammable. After PU was modified, SiPU exhibited only a slight improved LOI (18%), but demonstrated antidripping effects from UL-94 results. Next, the LOI values of the 10%, 20%, 30%, and 40 wt% OFAPP-added composites were 20%, 24%,
Fig. 7. (a) TG and (b) DTG curves of Si-PU/APP 30% and Si-PU/OFAPP 30% composites.
37%, and 41%, respectively. Among these samples, the Si-PU/OFAPP 20% and Si-PU/OFAPP 30% measured LOI values that differed by 13%. These samples demonstrated a marked advancement from V-1 to V-0 (the top level) in the UL-94 ratings, suggesting that excellent flame retardancy can be obtained when a sufficient concentration of OFAPP is added. In the Si-PU/APP 30% and Si-PU/OFAPP 30% samples, the LOI values were 23% and 37% and their UL-94 rankings were Fail and V0, respectively. A comparison indicated that Si-PU/OFAPP 30% exhibited satisfactory flame retardancy because of the excellent synergistic effect between the outer shell of OFAPP and internal core of APP. In addition, the subsequent reaction with the substrate Si char layer formed dense and expandable char layers, which provided excellent flame retardancy. The flame retardancy data of the composite materials after hydrothermal treatments (75 C for 24 h) showed that the LOI value of the Si-PU/APP 30% composite dropped from 23% to 21%, whereas that for the Si-PU/OFAPP 40% composite decreased from 41% to 40%. Si-PU/APP 30% was not ranked in the UL-94 test before the hydrothermal treatment; therefore, variations cannot be observed through the treatment process. The UL-94 results for Si-PU/OFAPP 30% before and after hot water treatment showed a drop in ranking from V-0 to V-1. Regarding the flame retardancy before and after the treatment, the LOI values of the APP-added composites varied considerably, whereas the LOI values of the OFAPP-added composites remained unchanged. In summary, the LOI and UL-94 results showed that dripping effects were no longer present in the modified Si-PU composite. The OFAPP- and APP-added composites demonstrated considerable differences in their flame retardancy. Moreover, the OFAPP-added composite exhibited no apparent variations in flame retardancy after the hydrothermal treatments, suggesting that
Fig. 8. LOI values of the pristine PU, Si-PU, Si-PU/APP 30%, and Si-PU/OFAPP 10e40% composites.
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microencapsulation effectively enhanced the flame retardancy of the OFAPP flame retardant. 4. Conclusion A novel microencapsulated flame retardant was developed through microencapsulation, in which the surface of APP was encapsulated with an OF resin, thereby forming an OFAPPcontaining flame retardant. The TGA results showed that adding 40% flame retardant in the polymer matrix resulted in approximately 26.9 wt% of char and high flame retardancy (LOI ¼ 41%). The TGA curves verified the presence of the protective mechanism of OFAPP at various temperatures; this mechanism increased the thermal stability of OFAPP and promoted char formation. According to the comparison of the APP before and after microencapsulation, the microencapsulated OFAPP demonstrated excellent flame retardancy in terms of thermal stability. Overall, the findings of this study revealed that the novel microencapsulated flame retardant can enhance the flammability, flame retardance, and the applicability of PU. Acknowledgments The authors would like to express their appreciation to the National Science Council of the Republic of China for financial support of this study under grant NSC-102-2221-E-241-003-MY3. References [1] H.T. Jeon, M.K. Jang, B.K. Kim, K.H. Kim, Synthesis and characterizations of waterborne polyurethane-silica hybrids using sol-gel process, Colloid Surf. A Physicochem. Eng. Asp. 302 (2007) 559e567. [2] K.M. Zia, H.N. Bhatti, I.A. Bhatti, Methods for polyurethane and polyurethane composites, recycling and recovery: a review, React. Funct. Polym. 67 (2007) 675e692. [3] K. Song, Y. Zhang, J. Meng, E.C. Green, N. Tajaddod, H. Li, M.L. Minus, Structural polymer-based carbon nanotube composite fibers: understanding the processing-structure-performance relationship, Materials 6 (2013) 2543e2577. [4] G.T. Howard, Biodegradation of polyurethane: a review, Int. Biodeterior. Biodegrad. 49 (2002) 245e252. [5] X. Chen, L. Huo, C. Jiao, S. Li, TG-FTIR characterization of volatile compounds from flame retardant polyurethane foams materials, J. Anal. Appl. Pyrolysis 100 (2013) 186e191. [6] A.M. Borreguero, P. Sharma, C. Spiteri, M.M. Velencoso, M.S. Carmona, J.E. Moses, J.F. Rodriguez, A novel click-chemistry approach to flame retardant polyurethanes, React. Funct. Polym. 73 (2013) 1207e1212. [7] S.S. Pathak, A. Sharma, A.S. Khanna, Value addition to waterborne polyurethane resin by silicone modification for developing high performance coating on aluminum alloy, Prog. Org. Coat. 65 (2009) 206e216. [8] H.A. El-Wahaba, M.A. El-Fattah, N.A. El-Khalik, C.M. Sharaby, Synthesis and performance of flame retardant additives based on cyclodiphosph (V) azane of sulfaguanidine, 1, 3-di-[N/-2-pyrimidinylsulfanilamide]-2, 2, 2.4,4,4hexachlorocyclodiphosph(V)azane and 1, 3-di-[N/-2pyrimidinylsulfanilamide]-2, 4-di[aminoacetic acid]-2, 4-dichlorocyclodiphosph (V) azane incorporated into polyurethane varnish, Prog. Org.
Coat. 74 (2012) 615e621. [9] L. Gao, G. Zheng, Y. Zhou, L. Hu, G. Feng, Y. Xie, Synergistic effect of expandable graphite, melamine polyphosphate and layered double hydroxide on improving the fire behavior of rosin-based rigid polyurethane foam, Ind. Crop. Prod. 50 (2013) 638e647. [10] A. Riva, G. Camino, L. Fomperie, P. Amigouet, Fire retardant mechanism in intumescent ethylene vinyl acetate compositions, Polym. Degrad. Stab. 82 (2003) 341e346. [11] J.W. Gu, G.C. Zhang, S.L. Dong, Q.Y. Zhang, J. Kong, Study on preparation and fire retardant mechanism analysis of intumescent flame-retardant coatings, Surf. Coat. Tech. 21 (2007) 7835e7841. [12] C.X. Zhao, Y. Liu, D.Y. Wang, D.L. Wang, Y.Z. Wang, Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol), Polym. Degrad. Stabil. 93 (2008) 1323e1331. [13] Z.Z. Xu, J.Q. Huang, M.J. Chen, Y. Tan, Y.Z. Wang, Flame retardant mechanism of an efficient flame-retardant polymeric synergist with ammonium polyphosphate for polypropylene, Polym. Degrad. Stabil. 98 (2013) 2011e2020. [14] K. Wu, Z. Wang, H. Liang, Microencapsulation of ammonium polyphosphate: preparation, characterization, and its flame retardance in polypropylene, Polym. Compos. 29 (2008) 854e860. [15] Z. Wang, K. Wu, Y. Hu, Study on flame retardance of Co-microencapsulated ammonium polyphosphate and Dip Zheng pentaerythritol in polypropylene, Polym. Eng. Sci. 48 (2008) 2426e2431. [16] K. Wu, L. Song, Z.Z. Wang, Y. Hu, Microencapsulation of ammonium polyphosphate with PVA- melamine-formaldehyde resin and its flame retardance in polypropylene, Polym. Adv. Tech. 19 (2008) 1914e1921. [17] J.X. Ni, L.J. Chen, K.M. Zhao, Y. Hu, L. Song, Preparation of gel-silica/ammonium polyphosphate core-shell flame retardant and properties of polyurethane composites, Polym. Adv. Technol. 22 (2011) 1824e1831. [18] G. Camino, S.M. Lomakin, M. Lazzari, Polydimethylsiloxane thermal degradation. Part 1. Kinetic aspects, Polymer 42 (2001) 2395e2402. [19] G. Camino, S.M. Lomakin, M. Lazzari, Polydimethylsiloxane thermal degradation. Part 2. The degradation mechanisms, Polymer 43 (2002) 2011e2015. [20] J. Luo, X. Wang, J. Li, X. Zhao, F. Wang, Conductive hybrid film from polyaniline and polyurethanesilica, Polymer 48 (2007) 4368e4374. bi, P.M. Wladyka, G. Marosi, Flax fibre [21] K. Bocz, B. Szolnoki, A. Marosi, T. Ta reinforced PLA/TPS biocomposites flame retarded with multifunctional additive system, Polym. Degrad. Stabil. 106 (2014) 63e73. [22] X. Chen, L. Huo, C. Jiao, S. Li, TG-FTIR characterization of volatile compounds from flame retardant polyurethane foams materials, J. Anal. Appl. Pyrolysis 100 (2013) 186e191. [23] Y.H. Han, A. Taylor, M. Mantle, K. Knowles, Sol-gel-derived organic-inorganic hybrid materials, J. Non Cryst. Solids 353 (2007) 313e320. [24] X. Wang, H. Pang, W. Chen, Y. Lin, G. Ning, Nanoengineering core/shell structured brucite@polyphosphate@amine hybrid system for enhanced flame retardant properties, Polym. Degrad. Stabil. 98 (2013) 2609e2616. [25] I. Poljansek, M. Krajnc, Characterization of phenol-formaldehyde prepolymer resins by in line FTIR spectroscopy, Acta Chim. Slov. 52 (2005) 238e244. [26] F. Salaün, I. Vroman, Influence of core materials on thermal properties of melamine-formaldehyde microcapsules, Eur. Polym. J. 44 (2008) 849e860. [27] Z. Zheng, J. Yan, H. Sun, Z. Cheng, W. Li, H. Wang, X. Cui, Microencapsulated ammonium polyphosphate and its synergistic flame-retarded polyurethane rigid foams with expandable graphite, Polym. Int. 63 (2014) 84e92. [28] Y.L. Liu, C.Y. Hsu, W.L. Wei, R.J. Jeng, Preparation and thermal properties of epoxy-silica nanocomposites from nanoscale colloidal silica, Polymer 44 (2003) 5159e5167. [29] C.S. Wu, Y.L. Liu, Y.C. Chiu, Y.S. Chiu, Thermal stability of epoxy resins containing flame retardant components: an evaluation with thermogravimetric analysis, Polym. Degrad. Stabil. 78 (2002) 41e48. [30] C.D. Doyle, Estimating thermal stability of experimental polymers by empirical thermogravimetric analysis, Anal. Chem. 33 (1961) 77e79. [31] Y. Qian, P. Wei, P. Jiang, Z. Li, Y. Yan, J. Liu, Preparation of a novel PEG composite with halogen-free flame retardant supporting matrix for thermal energy storage application, Appl. Energy 106 (2013) 321e327.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation, characterization of microencapsulated ammonium polyphosphate and its flame retardancy in polyurethane composites Ming-Yuan Shen a, Wei-Jen Chen a, Chen-Feng Kuan b, Hsu-Chiang Kuan b, Jia-Ming Yang c, Chin-Lung Chiang c, * a
Department of Aviation Mechanical Engineering, China University of Science and Technology, Hsinchu County, 303, Taiwan Department of Computer Application Engineering, Far East University, Tainan, 744, Taiwan Green Flame Retardant Material Research Laboratory, Department of Safety, Health and Environmental Engineering, Hung-Kuang University, Taichung, 433, Taiwan b c
h i g h l i g h t s A novel microencapsulated flame retardant was synthesized using in situ polymerization technology. The microencapsulation of ammonium polyphosphate with the polymer resin resulted in improved hydrophobicity. Polyurethane composites have excellent thermal stability and flame retardance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 28 November 2015 Accepted 2 February 2016 Available online 12 February 2016
In this study, a novel microencapsulated flame retardant containing ammonium polyphosphate (APP) and an 4,40 -oxydianiline-formaldehyde (OF) resin as the core and shell material was synthesized using in situ polymerization technology. The structure and performance of OF microencapsulated APP (OFAPP) were characterized using Fourier transform infrared spectroscopy and scanning electron microscopy. The thermal properties of OFAPP were systematically analyzed through thermogravimetric analysis. Flame retardancy tests, such as limiting oxygen index (LOI) and UL-94, were conducted to evaluate the effect of varying the composition of APP and OFAPP in silanol-terminated polyurethane (Si-PU) composites. The results indicated that the microencapsulation of APP with the OF resin resulted in improved hydrophobicity. The results also revealed that the flame retardancy of the Si-PU/OFAPP composite (LOI ¼ 37%) was higher than that of the Si-PU/APP composite (LOI ¼ 23%) at the same additive loading. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Fourier transform infrared spectroscopy (FTIR) Thermogravimetric analysis (TGA)
1. Introduction Polyurethane (PU) has received considerable attention in recent years because of its unique polymeric characteristics and favorable physicochemical properties [1e4]. Furthermore, PU can be specifically designed to satisfy the diverse requirements of conventional polymers used in coating, adhesive, foam, and thermoplastic elastomer applications [1,5,6]. However, the thermal instability and flammability of PU are major limitations of the material [7]. Therefore, numerous researchers have studied this specific problem to improve the thermal stability and flame retardancy of PU [8,9].
* Corresponding author. E-mail address: [email protected] (C.-L. Chiang). http://dx.doi.org/10.1016/j.matchemphys.2016.02.006 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Flame retardants composed of ammonium polyphosphate (APP) have been developed for many years and have shown favorable performance in retarding the production of flames. Ammonium polyphosphate can play the role of flame retardant due to a strong dehydrating agent generated by polyphosphoric acid is, which can promote to form carbon residue on the surface of polymer matrix. The carbon residue containing phosphorus covered on the surface, which can block oxygen to prevent the spread of flammable gases and carbon gasification [10,11]. However, a flame retardant comprising solely APP alone cannot effectively yield favorable outcomes; therefore, researchers have adopted synergistic effects by combining APP with other flame retardants to effectively improve the flame retardancy of APP [12,13]. Hygroscopicity is a flaw associated with APP-containing flame retardants because it can reduce the effects of flame retardancy when APP is added to
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polymeric composites [14,15]. Therefore, microencapsulation technology has been applied to modify APP to enhance the flame retardancy of APP surfaces. In addition, this approach facilitates converting the hydrophilicity of APP to hydrophobicity, which increases the compatibility of composite materials and decreases the hygroscopicity of APP [14,15]. Wu et al. [16] used polyvinyl alcohol (PVA) to modify melamineformaldehyde resin to coat ammonium polyphosphate (VMFAPP), and to flame retardant polypropylene. The results showed that the microcapsules can greatly reduce the heat release rate of materials, and PVA content of melamine-formaldehyde prepolymer materials played an important role on water-resistant and flame retardant of the microcapsules. The silicon microcapsule has been widely used to encapsulate material by solegel process. Ni et al. used silica gel to microencapsulate APP and it used to flame retardant thermoplastic polyurethane [17]. Through microencapsulation, the water solubility, hydrophilicity, thermal properties of APP are greatly improved. In the current study, PU was first modified to incorporate silicon to form siloxane linkage in the PU structure. Polysiloxanes offer high thermal stability up to 300 C. That resistance results from the presence of SieO bonds, for which the dissociation energy is higher (ca. 460.5 kJ/mol) in relation to CeO (358 kJ/mol), CeC (304 kJ/mol) and CeNH (98 kJ/mol) bonds [18,19]. A solegel reaction was subsequently applied to form a PU network structure that facilitates increasing the thermal stability of PU. An APP-containing flame retardant was then microencapsulated to produce carbonization agents on the surface, which convert APPs that are initially unexpandable to APPs exhibiting expandability. Moreover, microencapsulation converts the property of APPs from being hydrophilic to being hydrophobic, which enhances the compatibility of substrates to flame retardants.
2.3. Preparation of 4,40 -oxydianiline-formaldehyde microencapsulated APP To prepare 4,40 -oxydianiline-formaldehyde microencapsulated APP (OFAPP), ODA (10 g) and formaldehyde (5.99 g) were added to a reaction tank, to which 40 mL of THF was added. Subsequently, ammonia solution was used to adjust the pH of the solution in the reaction tank to 8e9. The solution was then heated at 60 C for 10 min, yielding a transparent solution following reaction completion. APP (40 g) and ethanol (500 mL) were placed in another reaction tank, preheated, and stirred at 80 C for 0.5 h. Next, the transparent prepolymer was transferred into the APPcontaining reaction tank, to which HCl was added to adjust the pH to 3e4. The reaction was allowed to proceed at 80 C for 3 h, yielding powdered products. Following filtration, the powders were extracted and rinsed several times in ethanol and then placed in a convection oven at 80 C for 12 h. The reaction is shown in Scheme 2. 2.4. Preparation of Si-PU/OFAPP composites To synthesize the Si-PU/OFAPP composites, the OFAPP flame retardant was added to the Si prepolymer prepared according to Scheme 1. Next, 0.5 mL of H2O was added and the temperature was adjusted to 70 C, and the solution was observed for any reduction in solvent volatility or increase in viscosity. When the viscosity increased, the Si-PU/OFAPP product was placed in a mold at room temperature for 6 h before drying at 80 C in a vacuum oven for 12 h. The final product (Si-PU/OFAPP composites) was then removed from the oven and cooled at room temperature. 2.5. Measurements
2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI, purity 98%), ethylenediamine (purity 99%), 3-aminopropyltriethoxysilane (APTS, purity 99%), 4,4'-oxydianiline (ODA, purity 98%), and formaldehyde (37 wt% sol., stab. 10%e15% methanol) were purchased from Acros Chemical Co, New Jersey, United States. Anhydrous stabilized tetrahydrofuran (THF) was supplied by Lancaster Co., Morecambe, Lancashire, United Kingdom. Arcol polyol 1007 (polyether polyols 700) were purchased from Bayer Material Science Ltd, Kaohsiung, Taiwan. Ammonium polyphosphate (APP, phase II, n > 1000) was purchased from San Jin Chemicals Corporation, Kaohsiung, Taiwan.
2.2. Preparation of Si-PU IPDI (12.6 g) and arcol polyol 1007 (20 g) were placed in a fourneck flask inserted in an 80 C oil bath and mechanically stirred in N2 atmosphere. Subsequently, 1 g of the metal catalyst dibutyltin dilaurate was added into the four-neck flask. After 1.5 h of reaction, the temperature was reduced to 50 C when viscosity of the prepolymer increased to a level similar to that of maltose. Next, THF (50 mL) and APTS (12.6 g) were added into the four-neck flask and stirred for 0.5 h, after which 0.5 mL of H2O was added. The temperature was then raised to 70 C, to reduce the volatility of the solvent and increase the solution viscosity. When the viscosity increased, the product (Si-PU) was placed in a container at room temperature for 6 h before vacuum drying at 80 C for 12 h in a vacuum oven. The product was removed after 12 h and cooled at room temperature. The reaction is specified in Scheme 1.
The FTIR spectra of the materials were obtained between 4000 and 400 cm1 using a Nicolet Avatar 320 FT-IR spectrometer from the U.S.A. Thin films were prepared by solution-casting. A minimum of 32 scans were signal-averaged with a resolution of 2 cm1. The mean particle size and distribution of samples were measured by optical particle sizing instrument (Zeta-Sizer3000HS, Malvern Corporation, English). Before the measurement, the samples were dispersed in ethanol, and sonicated for 15 min. The morphology of the fractured surface of the composites was examined using a scanning electron microscope (SEM) (JEOL JSM 840A, Japan). The thermal degradation of the composites was examined using a thermogravimetric analyzer (TGA) (Perkin Elmer TGA 7) from room temperature to 800 C at a heating rate of 10 C/min in an atmosphere of nitrogen. The measurements were made on 6e10 mg samples. The LOI test was carried out following the ASTM D 2836 Oxygen Index Method, using a test specimen bar that was 7e15 cm long, 6.5 ± 0.5 mm wide, and 3.0 ± 0.5 mm thick. The sample bars were suspended vertically and ignited using a Bunsen burner. The flame was removed and the timer was started. The concentration of oxygen was increased when the flame on the specimen was extinguished before it had burned for 3 min or burning 5 cm away from the bar. The oxygen content was adjusted until the limiting concentration was determined. The vertical burning test was done inside a fume hood. Samples were held vertically with tongs at one end and burned from the free end. Samples were exposed to ignition source for 10 s then they were allowed to burn above a cotton wool until both sample and cotton wool extinguished. Observable parameters were recorded to assess fire retardancy. The UL 94 test classifies the materials as V-0, V-1 and V-2 according to the time period needed before self-extinction and the occurrence of flaming dripping after removing the ignition source. V-0 is the most ambitious and desired classification.
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3. Results and discussion 3.1. Structural identification of Si-PU In this study, PU containing silanol was fabricated, the structures of which were observed using Fourier transform infrared spectroscopy (FTIR). Fig. 1 shows the FTIR spectra of IPDI, polyol, and prepolymer, which reveal that the NCO peak of IPDI is located at 2256 cm1 and that polyol contains considerable eOH groups (at 3480 cm1). The reaction between the two components yielded the prepolymer. The spectra show that the -NCO peak diminished in intensity and the prepolymer contained -NCO reactive functional groups on its two sides for subsequent reaction processes [20]. In addition to the variations in the eNCO functional group, new eNH
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(3320 cm1), C]O (1717 cm1), and CeN (3320 cm1) peaks formed after the reactions between eNCO and eOH to produce urethane linkage. According to the spectra, the linkages of urethane appeared and the eOH peaks diminished [20e22], indicating the formation of the prepolymer. Fig. 2 illustrates the FTIR spectra of the prepolymer and Si-PU; these spectra were analyzed to determine whether the prepolymer reacted with APTS, as an indication of endcapping process. Fig. 2 indicates the complete disappearance of the eNCO peak in the terminated Si prepolymer, suggesting that the APTS reacted with the prepolymer, thereby forming a structuring comprising silanol at two ends of the prepolymer. After the hydrolysis of the terminated Si prepolymer, eOH functional groups were observed on the silanol at both ends as reflected by the increase in the
Scheme 1. Reaction of Si-PU composites.
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Scheme 2. Reaction of OFAPP.
number of eOH peaks in the Si-PU spectra shown in Fig. 2. Furthermore, the SieOCH2CH3 is located at 780 cm1, where the peaks also diminished in intensity after the hydrolysis reaction [23]. Thus, hydrolysis facilitated the subsequent condensation reactions.
According to the results, Si-PU composite materials were successfully fabricated through a series of reactions.
Fig. 1. FTIR spectra of IPDI, PEG, and prepolymer.
Fig. 2. FTIR spectra of the prepolymer, Si prepolymer, and Si-PU.
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Fig. 3. FTIR spectra of APP and OFAPP.
3.2. Structure characterization of OFAPP Fig. 3 depicts the FTIR spectra of APP before and after the microencapsulation. The spectra were analyzed to preliminarily determine the structures of APP and peak variations. These spectra show typical APP peaks at 3200 (NeH), 1253 (P]O), 1070 (PeO symmetric stretching), 1017 (symmetric vibration of PO2 and PO3), 882 (PeO asymmetric stretching) and 800 cm1(PeOeP) [24]. Several new peaks were also observed, suggesting the formation of microencapsulated OFAPP, such as at 3375 (eOH), 2823 (CeH), 1625 (C]C), 1495 (benzene), 1308 (CeN), 1120 (CeOeC), and 823 cm1 (para-position of benzene) [25e27]. In the OFAPP spectra, the peaks representing the OF shell and typical APP peaks were identified. This result suggests that APP was successfully microencapsulated, forming the microencapsulated APP flame retardant.
Fig. 5. SEM photographs of (a) APP and (b) OFAPP particles. The insets are pictures of the water contact angle.
3.3. Size distribution of OFAPP Fig. 4 shows the particle size distributions of APP and OFAPP. As shown in the diagram, the particle size of APP is around 15 mm, and that of OFAPP is around 26 mm. Obviously, the thickness of OFAPP is
larger than that of APP, meaning OF resin was wrapped onto the surface of APP particles. The variations in particle size distribution also confirmed the successful fabrication of the microencapsulated flame retardant. 3.4. Surface morphology and hydrophobicity of OFAPP
Fig. 4. Particle size distributions of APP and OFAPP.
Fig. 5 shows the changes in the surface morphology of APP before and after the microencapsulation. As show in Fig. 5(a), the APP particles, predominantly presented in the form of long rods, exhibit smooth surface morphologies, with partial cracks on the surface. Fig. 5(b) shows the OFAPP particles, the surface morphology of which differed from that of APP in that a coarsesurfaced shell was observed on the surface of the OFAPP particle. In addition, no cracks as those observed on the surface of APP particles were identified. In summary, an additional outer shell on the APP particles was observed after the microencapsulation, revealing a surface morphology considerably different to that of OFAPP. The insets show a water contact angle of 22.49B for APP; this is primarily attributed to the hydrophilicity of NHþ 4 in APP, thus resulting in low water contact angles [27]. OFAPP exhibited a water contact angle of 74.92 , indicating increased hydrophobicity, which is attributable to the benzene structures on the microencapsulated shell. APP featured superior water compatibility and hence a lower
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water contact angle than OFAPP did; this indicates that APP is hydrophilic and OFAPP is hydrophic. Therefore, the microencapsulation of shell materials can reduce the attack of water vapor and increase the compatibility of APP with polymers due to its hydrophobicity when mixed with it, thus improving the dispersion of additives in the polymer matrix. 3.5. Thermal properties of Si-PU/OFAPP composites According to the data shown in Fig. 6(a) and Table 1, pristine PU has a Td5 of 306 C, which increased to 328 C after PU was modified into Si-PU. This result can be attributed to the Si element migrating on the surface of PU, subsequently forming the SieOeSi char layers, which improved thermal resistance and enhanced antioxidative effects. Td5 represented the initial degradation temperature. There are two components to affect the property including silicon and phosphorus. The materials containing silicon will possess better thermal stability due to that SieO bonding have higher bond energy. On the other end, the materials containing PeO bond due to lower bond energy. These two factors affected the final property of the materials. We can find out the trend for Si-PU/ OFAPP 10e40% in Table 1 is not consistent for Td5. Consequently, thermal decomposition was delayed to a point that it commences at
Fig. 6. (a) TG and (b) DTG curves of pure PU, Si-PU, and Si-PU/OFAPP 10e40% composites.
a higher temperature [28,29]. In the APP and OFAPP composites, APP released NH3 and H2O at 220e400 C, reacting with the outer shell; therefore, the Td5 temperature ranged between those of pure PU and Si-PU. Furthermore, the weight of Si-PU (Fig. 6(a)) continued decreasing after 500 C because of the composite material's inability to withstand high thermal energy. Nevertheless, the weight of Si-PU stabilized at 600 C. In the OFAPP composite, complex reactions constantly occurred at 220e500 C; therefore, OFAPP composites decomposed earlier than Si-PU did, suggesting that the temperature at which OFAPP decomposed was lower than that for Si-PU. However, the thermal stability of OFAPP composites after 500 C was superior to that of the Si-PU composite. This is because the series of precedent reactions resulted in dense barrier layers that prevented the thermal destruction of polymers in the inner layers, thus explaining the high amount of char residue at the high temperature stage. Fig. 6(b) depicts the variations of the thermal decomposition rates. The decomposition rates of pure PU and Si-PU are 30.4 and 16.1 wt%/min, respectively. The rates for both the APP- and OFAPPadded composites were lower than that of Si-PU (16.1 wt%/min), and only the sample Si-PU/OFAPP 40% exhibited a rate (17.8 wt %/min) closest to that of Si-PU. This can be explained by the excellent thermal stability of the char layer produced by Si-PU at 500 C. When temperatures exceeded 500 C, the Si-char layer could no longer resist high temperatures and the structure gradually disintegrated. Furthermore, in the OFAPP-added composites, a series of reactions occurred at 220e500 C including the volatilization of inflammable gases and the formation of char layers. Therefore, the thermal decomposition rate of this composite was lower than that of Si-PU because the APP- and OFAPP-added composites both exhibited thermal stability at high temperature (>500 C). In addition, compared with pure PU, the thermal decomposition rates of Si-PU/ OFAPP 10%e40% were 24.8, 22.7, 19.7, and 17.8 wt%/min, respectively, showing a decreasing trend. When the composition of OFAPP was increased by 40% (i.e., Si-PU/OFAPP 40%), the thermal decomposition rate slowed by 12.6 wt%/min (approximately 58%), suggesting improved thermal stability. The curves shown in Fig. 7 present comparisons of the differences in thermal stabilities between the APP- and OFAPP-added composites. Fig. 7(a) indicates that the thermal stability of Si-PU/ OFAPP is superior to that of Si-PU/APP after approximately 400 C. The char produced on the Si-PU/APP and Si-PU/OFAPP samples at 800 C were 20.8 wt% and 23.7 wt%, respectively, implying that the microencapsulated OFAPP demonstrated enhanced thermal stability at a high temperature. Finally, the integral procedural decomposition temperature (IPDT) data shown in Table 1 were obtained according to the thermal stability assessment method for polymeric composites developed by Doyle [30]. In these data, higher IPDT represents better thermal stability [31]. The results for pristine PU, Si-PU, SiPU/OFAPP 10%, Si-PU/OFAPP 20%, Si-PU/OFAPP 30%, and Si-PU/ OFAPP 40% were 336, 540, 611, 687, 771, and 852 C, respectively, whereas that for Si-PU/APP 30% was 717 C. The IPDTs for all the samples demonstrated an increasing trend, meaning that the initial material modification and the subsequent addition of OFAPP flame retardants both contributed to improving the thermal stability of the composites; compared with the thermal stability of pristine PU, those of Si-PU and Si-PU/OFAPP 40% increased by 160% and 253%, respectively. Furthermore, the IPDT of Si-PU/APP 30% and Si-PU/ OFAPP 30% differed by 54 C, which further represented that the thermal stability of the microencapsulated OFAPP was superior to that of APP.
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Table 1 Thermal properties of pure PU, Si-PU, and Si-PU/OFAPP 10e40% composites. Sample
Td5a ( C)
Rmaxb (wt%/min)
IPDTc ( C)
Char residue (wt%) at 800 C
Pure PU Si-PU Si-PU/OFAPP 10% Si-PU/OFAPP 20% Si-PU/OFAPP 30% Si-PU/OFAPP 40% Si-PU/APP 30%
306 328 319 331 324 317 319
30.4 16.1 24.8 22.7 19.7 17.8 23.5
336 540 611 687 771 852 717
0.9 12.2 17.1 20.5 23.7 26.9 20.8
a b c
Td5 indicates the temperature at which the weight loss of the sample reaches 5%. Rmax corresponds to the maximum thermal degradation rate. The entire degradation process is considered in the IPDT.
3.6. Flame retardancy of Si-PU/OFAPP composites Fig. 8 present the limiting oxygen index (LOI) of the composite materials and the values obtained from Underwriters Laboratories' Standard for Safety and Flammability of Plastic Materials for Parts in Devices and Appliances testing (UL-94). Pristine PU exhibited an LOI of 17%, failed the UL-94 test, and demonstrated dripping effects, meaning that pristine PU is flammable. After PU was modified, SiPU exhibited only a slight improved LOI (18%), but demonstrated antidripping effects from UL-94 results. Next, the LOI values of the 10%, 20%, 30%, and 40 wt% OFAPP-added composites were 20%, 24%,
Fig. 7. (a) TG and (b) DTG curves of Si-PU/APP 30% and Si-PU/OFAPP 30% composites.
37%, and 41%, respectively. Among these samples, the Si-PU/OFAPP 20% and Si-PU/OFAPP 30% measured LOI values that differed by 13%. These samples demonstrated a marked advancement from V-1 to V-0 (the top level) in the UL-94 ratings, suggesting that excellent flame retardancy can be obtained when a sufficient concentration of OFAPP is added. In the Si-PU/APP 30% and Si-PU/OFAPP 30% samples, the LOI values were 23% and 37% and their UL-94 rankings were Fail and V0, respectively. A comparison indicated that Si-PU/OFAPP 30% exhibited satisfactory flame retardancy because of the excellent synergistic effect between the outer shell of OFAPP and internal core of APP. In addition, the subsequent reaction with the substrate Si char layer formed dense and expandable char layers, which provided excellent flame retardancy. The flame retardancy data of the composite materials after hydrothermal treatments (75 C for 24 h) showed that the LOI value of the Si-PU/APP 30% composite dropped from 23% to 21%, whereas that for the Si-PU/OFAPP 40% composite decreased from 41% to 40%. Si-PU/APP 30% was not ranked in the UL-94 test before the hydrothermal treatment; therefore, variations cannot be observed through the treatment process. The UL-94 results for Si-PU/OFAPP 30% before and after hot water treatment showed a drop in ranking from V-0 to V-1. Regarding the flame retardancy before and after the treatment, the LOI values of the APP-added composites varied considerably, whereas the LOI values of the OFAPP-added composites remained unchanged. In summary, the LOI and UL-94 results showed that dripping effects were no longer present in the modified Si-PU composite. The OFAPP- and APP-added composites demonstrated considerable differences in their flame retardancy. Moreover, the OFAPP-added composite exhibited no apparent variations in flame retardancy after the hydrothermal treatments, suggesting that
Fig. 8. LOI values of the pristine PU, Si-PU, Si-PU/APP 30%, and Si-PU/OFAPP 10e40% composites.
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microencapsulation effectively enhanced the flame retardancy of the OFAPP flame retardant. 4. Conclusion A novel microencapsulated flame retardant was developed through microencapsulation, in which the surface of APP was encapsulated with an OF resin, thereby forming an OFAPPcontaining flame retardant. The TGA results showed that adding 40% flame retardant in the polymer matrix resulted in approximately 26.9 wt% of char and high flame retardancy (LOI ¼ 41%). The TGA curves verified the presence of the protective mechanism of OFAPP at various temperatures; this mechanism increased the thermal stability of OFAPP and promoted char formation. According to the comparison of the APP before and after microencapsulation, the microencapsulated OFAPP demonstrated excellent flame retardancy in terms of thermal stability. Overall, the findings of this study revealed that the novel microencapsulated flame retardant can enhance the flammability, flame retardance, and the applicability of PU. Acknowledgments The authors would like to express their appreciation to the National Science Council of the Republic of China for financial support of this study under grant NSC-102-2221-E-241-003-MY3. References [1] H.T. Jeon, M.K. Jang, B.K. Kim, K.H. Kim, Synthesis and characterizations of waterborne polyurethane-silica hybrids using sol-gel process, Colloid Surf. A Physicochem. Eng. Asp. 302 (2007) 559e567. [2] K.M. Zia, H.N. Bhatti, I.A. Bhatti, Methods for polyurethane and polyurethane composites, recycling and recovery: a review, React. Funct. Polym. 67 (2007) 675e692. [3] K. Song, Y. Zhang, J. Meng, E.C. Green, N. Tajaddod, H. Li, M.L. Minus, Structural polymer-based carbon nanotube composite fibers: understanding the processing-structure-performance relationship, Materials 6 (2013) 2543e2577. [4] G.T. Howard, Biodegradation of polyurethane: a review, Int. Biodeterior. Biodegrad. 49 (2002) 245e252. [5] X. Chen, L. Huo, C. Jiao, S. Li, TG-FTIR characterization of volatile compounds from flame retardant polyurethane foams materials, J. Anal. Appl. Pyrolysis 100 (2013) 186e191. [6] A.M. Borreguero, P. Sharma, C. Spiteri, M.M. Velencoso, M.S. Carmona, J.E. Moses, J.F. Rodriguez, A novel click-chemistry approach to flame retardant polyurethanes, React. Funct. Polym. 73 (2013) 1207e1212. [7] S.S. Pathak, A. Sharma, A.S. Khanna, Value addition to waterborne polyurethane resin by silicone modification for developing high performance coating on aluminum alloy, Prog. Org. Coat. 65 (2009) 206e216. [8] H.A. El-Wahaba, M.A. El-Fattah, N.A. El-Khalik, C.M. Sharaby, Synthesis and performance of flame retardant additives based on cyclodiphosph (V) azane of sulfaguanidine, 1, 3-di-[N/-2-pyrimidinylsulfanilamide]-2, 2, 2.4,4,4hexachlorocyclodiphosph(V)azane and 1, 3-di-[N/-2pyrimidinylsulfanilamide]-2, 4-di[aminoacetic acid]-2, 4-dichlorocyclodiphosph (V) azane incorporated into polyurethane varnish, Prog. Org.
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