Article pubs.acs.org/IECR
Synthesis and Properties of a Photopolymerizable CarbeneMediated Poly Phosphinate Flame Retardant by Carbene Polymerization Yong Yu,†,‡ Shengling Jiang,§ and Fang Sun*,†,‡ †
State Key Laboratory of Chemical Resource Engineering, ‡College of Science, and §College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: A novel photopolymerizable poly phosphinate (poly ethyl (4-acrylamidebenzyl)phosphinate, P-NH-AC) flame retardant was synthesized by a carbene polymerization and characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and gel permeation chromatography (GPC). The effect of P-NH-AC on the kinetics of photopolymerization, thermal stability, combustion behaviors, and physical and mechanical properties of the UVcured materials were investigated by real-time infrared spectroscopy (RT-IR), thermogravimetric analysis (TGA), thermogravimetric analysis/infrared spectrometry (TGA-IR), the limiting oxygen index (LOI), and the cone calorimetric test (CCT). For the systems with P-NH-AC, the thermal stability was improved with the increase of the P-NH-AC; however, the final residue of all systems was low. The addition of 5% P-NH-AC increased the LOI from 29.0 to 32.0. The addition of P-NHAC significantly decreased the heat release rate (HRR), total heat release (THR), and total smoke production (TSP) of the resin. Moreover, P-NH-AC can also improve physical and mechanical properties of the materials.
1. INTRODUCTION The photopolymerization technique is one of the fastest developed environmentally friendly polymerization technologies because of its distinct advantages such as solvent-free formulations, low energy consumption, room temperature treatment, high efficiency, and spatial and temporal control.1,2 UV-curable materials have been widely used in many areas such as coatings,3,4 encapsulations,5 inks,6 and adhesives.7 It is universally known, however, that UV-curable materials, like other polymer materials, are highly combustible, which restricts their applications in some fields, for example, optical fibers, wooden furniture, and electronic devices. Therefore, UVcurable flame retardants have attracted much attention in recent years. Especially flame retardants containing phosphorus have received considerable research interest because of the virtue of the high efficiency and environmental protection. There are no halogen acids during the combustion process of flame retardants containing phosphorus.8−10 Moreover, they have exhibited an efficient fire retardancy either due to a condensed phase mechanism involving polymer charring or due to a gas phase flame inhibition mechanism by PO• radicals.11 Besides, they are of relatively low toxicity.12 Xing13 synthesized a phosphorus-containing monomer used as a coating flame retardant, which showed excellent flame retardance. Chen14 studied the flammability and thermal behavior of the cured film of a phosphate monomer (DGTH), and the results indicated that the cured film had a good thermal stability and a high limiting oxygen index (LOI) value (it reached 48). Shi15 synthesized two kinds of phosphorus-containing monomer and investigated their thermostability and combustion behaviors, demonstrating that phosphate groups make a large contribution to the formation of a compact char layer as a barrier to protect the material from further burning. © 2014 American Chemical Society
Compared with traditional polymerization methods, carbene polymerization has high stereoselectivity, which endows the polymer with an unique structure and performance that cannot be reached by other polymerization methods.16−18 For example, Hetterscheid16 obtained poly(ethyl 2-ylidene-acetate) by carbene polymerization, and the polymer had syndiotactic structure leading to a high degree of crystallinity. Doulut19 studied the influence of the tacticity on the retardation process in PMMA, and the results indicated that the presence of isotriad sequences enhanced molecular mobility in the studied temperature range. However, photopolymerizable phosphoruscontaining prepolymers with flame resistance synthesized by carbene polymerization have been rarely reported. The photopolymerizable phosphorus-containing prepolymers would have unique structures, leading to a good flame resistance and the improvement of performances of the UVcurable materials. In this paper, a novel photopolymerizable poly phosphinate (poly ethyl (4-acrylamidebenzyl)phosphinate, P-NH-AC) flame retardant was synthesized by carbene polymerization and characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and gel permeation chromatography (GPC). The effect of the content of P-NH-AC on the kinetics of photopolymerization, the thermal stability, the flame resistance, and the physical and mechanical properties of the UV-cured materials were investigated by real-time infrared spectroscopy (RT-IR), thermogravimetric analysis (TGA), thermogravimetric analReceived: Revised: Accepted: Published: 16135
May 17, 2014 September 22, 2014 October 6, 2014 October 6, 2014 dx.doi.org/10.1021/ie502023v | Ind. Eng. Chem. Res. 2014, 53, 16135−16142
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was 83%. 1H and 31P NMR spectra of compound b are shown in the Supporting Information, Figures S3 and S4. 1 H NMR (δ, CDCl3, ppm): 11.46 (−PO(OH)−); 7.28 (−C6H5); 3.83−3.90 (−OCH2CH3); 3.00−3.05 (Ph−CH2−); 1.17−1.21 (−OCH2−CH3). 31P NMR (δ, CDCl3, ppm): 26.53. 2.2.3. Synthesis of Compound c. Compound b (10.00 g) was added into a three-necked flask equipped with a mechanical stirrer and ice bath, and then mixed acid (HNO3 3 mL + H2SO4 4 mL) was dropwise added into the flask. The reaction mixture was stirred below 10 °C to precipitate compound c. Compound c was filtrated and recrystallized with ethanol to get pure compound c, which was a white solid. The yield of compound c was 63%. 1H and 31P NMR spectra of compound c are shown in the Supporting Information, Figures S5 and S6. 1 H NMR (δ, DMSO, ppm): 11.71 (−PO(OH)−); 7.54− 7.55, 8.17−8.19 (−C6H4−); 3.91−3.94 (−OCH2CH3); 3.26, 3.32 (−Ph−CH2−); 1.15−1.19 (−OCH2−CH3). 31P NMR (δ, DMSO, ppm): 21.70. 2.2.4. Synthesis of Compound d. Compound c (2.45 g) was refluxed in SOCl2 (1.62 g) under nitrogen atmosphere for 12 h. Excess thionyl chloride was removed under reduced pressure to obtain compound d. The yield of compound d was 99%. 1H and 31P NMR spectra of compound d are shown in the Supporting Information, Figures S7 and S8. 1 H NMR (δ, CDCl 3 , ppm): 7.50−7.53, 8.21−8.23 (−C6H4−); 4.21−4.34 (−OCH2CH3); 3.61−3.67 (−Ph− CH2−); 1.35−1.39 (−OCH2−CH3). 31P NMR (δ, CDCl3, ppm): 21.50. 2.2.5. Synthesis of Compound e (P-NO2). Compound d was added into a diethyl ether solution of CH2N2 at 0 °C, and then the solution was stirred at room temperature for 48 h to obtain compound e (P-NO2), which precipitated as a yellow solid. Then the precipitate was filtrated and washed with diethyl ether and ethyl acetate, respectively. The 1H NMR spectrum of compound e is shown in the Supporting Information, Figure S9. 1 H NMR (δ, CDCl3, ppm): 7.47−7.52, 8.18−8.2 (−C6H4−); 3.4−4.2 (−OCH2CH3, −Ph−CH2−, −P−CH2−); 1.25−1.27 (−OCH2−CH3). GPC: Mn = 547, Mw = 1842, polydispersity index (PDI) = 3.36. 2.2.6. Synthesis of Compound f (P-NH2). Compound e (0.50 g) and 20 mL of methanol were added into a threenecked flask equipped with a mechanical stirrer and cooler, and then Zn dust (0.54 g) was added into the mixture followed by NH4Cl (0.66 g). The reaction mixture was warmed to 45 °C, and then returned to room temperature within 15 min. Subsequently, ammonia−water (NH3·H2O) (4 mL) and ethyl acetate (6 mL) were added. After filtration, the solvent was removed under reduced pressure to obtain compound f (PNH2), which was purified by column chromatography. The 1H NMR spectrum of compound f is shown in the Supporting Information, Figure S10. 1 H NMR (δ, D2O, ppm): 6.77−6.79, 7.07−7.09 (−C6H4−); 3,76−3.81 (−OCH2CH3); 3.44−3.47 (−P−CH2−); 2.79−2.95 (−Ph−CH2−); 1.13−1.14 (−OCH2−CH3). GPC: Mn = 638, Mw = 1792, PDI = 2.8. 2.2.7. Synthesis of Compound g (P-NH-AC). A solution of compound f (0.50 g) in 20 mL of THF and K2CO3 (0.38 g) were added into a flask with a magnetic stirrer, and then acryloyl chloride (1.14 g) was dropwise added into the flask. After being stirred at room temperature for 48 h, the reaction mixture was filtered, and the resulting filtrate was concentrated
ysis/infrared spectrometry (TGA-IR), the limiting oxygen index (LOI), the cone calorimetric test (CCT), the contact angle of water, and pencil hardness.
2. EXPERIMENTAL SECTION 2.1. Materials. Triethyl phosphite (P(OEt)3) was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Benzyl chloride, thionyl chloride (SOCl2), hydrochloric acid (HCl), nitric acid (HNO3), concentrated sulfuric acid (H2SO4), potassium carbonate (K2CO3), sodium hydroxide (NaOH), ammonium chloride (NH4Cl), and zinc powder (Zn) were all provided by Beijing Chemical Works. Acryloyl chloride was supplied by Beijing Ouhechem Co. 2-Hydroxyethyl acrylate (HEA), trimethylolpropane triacrylate (TMPTA), and isobornyl acrylate (IBOA) were obtained from Eternal Specialty Chemical (Zhuhai) Co., Ltd. The photoinitiator 2-hydroxyl-2methyl-1-phenylpropane-1-one (Darocur 1173) was obtained from Ciba Geigy Co. Diazomethane (CH2N2) was synthesized by following previously reported procedures.20 Tetrahydrofuran (THF) and trichloromethane (CHCl3) were dried by Na and P2O5, respectively, before use. 2.2. Synthesis of Poly Ethyl (4-Acrylamidebenzyl)phosphinate (P-NH-AC) (Scheme 1). 2.2.1. Synthesis of Scheme 1. Synthetic Route of P-NH-AC
Compound a. Triethyl phosphite (23.2 g) and benzyl chloride (20.0 g) were added into a single-necked flask, and then the mixture was stirred at 140 °C for 12 h. Subsequently, the unreacted triethyl phosphite in the mixture was removed under reduced pressure to obtain compound a.21 The yield of compound a was 89%. The 1H and 31P NMR spectra of compound a are shown in the Supporting Information, Figures S1 and S2. 1 H NMR (δ, CDCl3, ppm): 7.30 (−C6H5); 3.98−4.03 (−OCH2CH3); 3.12−3.17 (Ph−CH2−); 1.21−1.25 (−OCH2− CH3). 31P NMR (δ, CDCl3, ppm): 26.50. 2.2.2. Synthesis of Compound b. Compound a (2.28 g) was refluxed in 50 mL of ethanolic sodium hydroxide (1.05 g) for 12 h, and most of the ethanol was removed by distillation. The reaction mixture was diluted with 30 mL of H2O, acidified, and extracted with CHCl3, and then the oil layer was separated and dried over Na2SO4. The CHCl3 was distilled under reduced pressure. The residual materials slowly crystallized in a refrigerator to obtain compound b. The yield of compound b 16136
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where W0 is the weight of liquid film before curing and W is the weight of cured film which was obtained by the following procedure: the liquid film was first irradiated under a highpressure mercury lamp (365 nm, 10 mW/cm2) for 90 s, and then extracted with ethanol for 30 min and dried to constant weight.
by removal of the solvent to obtain compound g (P-NH-AC), which was purified by column chromatography. 1 H NMR (δ, CD3OD, ppm): 7.15−7.17, 7.34−7.36 (−C6H4−); 5.62−5.65, 6.20−6.36 (CH2CH−), 3.63−3.77 (−OCH2CH3); 3.60−3.63 (P−CH2−); 3.26 (−OCH3); 2.85− 2.90 (Ph−CH2−); 1.07−1.10 (−OCH2CH3). 13C NMR (δ, CD3OD, ppm): 162.95 (CO); 125.88−136.42 (C of Ph, CC); 59.09 (−CH2−); 50.85 (O−CH3−); 33.48−35.44 (Ph−CH2); 16.71 (−CH3). 31P NMR (δ, CD3OD, ppm): 21.33, 21.45. GPC: Mn = 1012, Mw = 2149, PDI = 2.12. 2.3. Characterizations. The Fourier transform infrared (FTIR) spectra were recorded according to a Nicolet 50XC spectrometer (Nicolet, USA) and scanned between 400 and 4000 cm−1. The 1H NMR, 13C NMR, and 31P NMR spectra were recorded by an AV 400 MHz spectrometer (Bruker, Germany). The molecular weight of the prepolymer was determined by a Waters 515-2410 gel permeation chromatograph (GPC, Waters, USA). Tetrahydrofuran, 1.0 mL/min, was used as the mobile phase. The GPC instrument was calibrated using multiple linear polystyrene (PS) standards. The limiting oxygen index (LOI) and cone calorimetric analysis were used to characterize the flame retardancy of all samples. The LOI is the minimum volume percentage of the oxygen concentration maintaining the combustion of a polymer sheet. The LOI values were measured using an oxygen index meter (JF-3, China) with sheet dimensions of 100 mm × 6.5 mm × 3 mm according to ASTM D2863-97 at room temperature. The cone calorimeter tests were conducted to research the fire performance on an FTT cone calorimeter according to ISO 5660-1. Testing samples (100 mm × 100 mm × 3 mm) were irradiated at a heat flux of 40 kW/m2, corresponding to a mild fire scenario. Thermogravimetric analysis (TGA) was performed on a TGA/DSC1 1100 SF thermogravimetric analyzer (NETZSCH, Germany) at a heating rate of 10 °C/min. A sample of 4−6 mg was examined under nitrogen and air at a flowing rate of 30 mL/min at temperatures ranging from room temperature to 800 °C. We have performed three replicas of TGA measurement, and the experimental errors for the weight and temperature are ±0.5 wt % and ±1 °C, respectively. The contact angles of water on the UV-cured film surface were measured on a contact angle microscope (OCA20, Data Physics Co., Germany). Pencil hardness apparatus AR015 (Tianjing Instrument Co.) was employed to measure the hardness of cured films. 2.4. Preparation Procedure of UV-Cured Film. The monomers and P-NH-AC were mixed with a certain weight ratio, and then photoinitiator 1173 (0.1 wt %, relative to resin) was added to the mixture to form a stock photosensitive liquid. The photosensitive liquid was coated onto a mold made from glass slides, and subsequently exposed under a high-pressure mercury lamp with 365 nm wavelength (10 mW/cm2) for 90 s to obtain the UV-cured film. 2.5. Photopolymerization. To study the kinetics of the photopolymerization of P-NH-AC, all samples with P-NH-AC were investigated by real-time infrared spectroscopy (RT-IR; Nicolet 5700, Thermo Electron, USA, equipped with an extended range KBr beam splitter and an MCT/A detector) according to previously reported procedures.22 2.6. Calculation of Gel Yield. The gel yield is expressed as gel yield = (W / W0) ·100%
3. RESULTS AND DISCUSSION 3.1. Characterization of the P-NH-AC Molecule. The 1 H, 13C, and 31P NMR spectra of P-NH-AC are shown in the Supporting Information, Figures S11, S12, and S13, respectively. Figure S11 in the Supporting Information shows that the signals of aromatic hydrogen of P-NH-AC at 7.15−7.17 and 7.34−7.36 ppm, and the signals of CH2CH− at 5.62−5.65 and 6.20−6.36 ppm, were observed in the spectrum of P-NHAC. The peaks at 3.63−3.77, 3.60−3.63, and 2.85−2.90 ppm were attributed to the protons in −OCH2−, −P−CH2, and Ph−CH2−, respectively. The peaks at 3.26 and 1.07−1.10 ppm were assigned to the protons of methyl in −OCH3 and −OCH2CH3. Figure S12 in the Supporting Information shows that the characteristic absorption peaks of CO, aromatic carbon, −CH2−, O−CH3−, Ph−CH2, and −CH3 appear at 162.95, 125.88−136.42, 59.09, 50.85, 33.48−35.44, and 16.71 ppm, respectively. The spectra of 1H NMR and 13C NMR indicate that P-NH-AC was successfully synthesized. As shown in Scheme 2, the phosphoryl carbene inserted at αC because of the steric hindrance; thus, P-NH-AC presents a Scheme 2. Reaction Sites of Phosphoryl Carbene Insertion
linear structure.23 The repeated unit of P-NH-AC synthesized by phosphoryl carbene insertion polymerization was CP( O)C, and this structure would enhance the solvent and water resistances of the polymer. The stereoregularity of P-NH-AC was investigated by 31P NMR spectra. The phosphorus spectrum demonstrated that the chemical shift of phosphorus was single and the peak at 21.4 ppm was splitting, which indicated that the chemical environment of phosphorus was single and P-NH-AC possessed a syndiotactic structure.23 3.2. Photopolymerization Kinetics. The final double bond conversion (DC) and the rate of polymerization (Rp) are the most important parameters characterizing the photopolymerization of a resin. The effect of the ratio of P-NH-AC to the monomers (HEA/TMPTA, 5/1 wt %) on the photopolymerization kinetics is shown in Figure 1. The rates of polymerization of the systems decreased with the increase of the content of P-NH-AC, and the final double bond
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poor, leading to the breaking of the structural homogeneity of the materials. 3.4. Analysis of Real-Time Infrared Spectra (RT-IR). Real-time infrared spectroscopy (RT-IR) was used to evaluate the thermal degradation process of materials. Figure 4 depicts the RT-IR spectra of UV-cured films with P-NH-AC at different pyrolysis temperatures in N2 atmosphere, and the main absorption peaks in the spectra are listed in Table 3. As the temperature rose, the relative intensities of the characteristic peaks at 2972, 2870 cm−1 (C−H) and 1044, 880 cm−1 (P−O−C) decreased, and these peaks almost disappeared at 400 °C. The result was ascribed to the instability of the P− O−C bond, which can induce the decomposition of the C−H bond. As is shown in Figure 4, the intensity of the peaks at 1044 cm−1 (P−O−C absorption peaks) decreased, and disappeared above 500 °C, which indicated the decomposition of the phosphinate. The appearance of the new peaks at 1092 and 891 cm−1 (P−O−P absorption peaks) above 450 °C indicated the formation of complex phosphorus oxides.24,25 The new peaks at 1268 and 1154 cm−1 demonstrated the rearrangement from an aliphatic ester structure to an aromatic ester structure. The increase of intensities of peaks at 1621 cm−1 proved that there may be precursors of the aromatic structure.26,27 The intensity of the peak at 1730 cm−1, which was assigned to the vibration of the CO group, began to decrease at about 300 °C and almost disappeared around 600 °C, indicating that the structure of CP(O)C and benzene catalyzed the decomposition of materials to form a compact carbon layer. This carbon layer can delay the further decomposition of the materials. 3.5. Thermal Degradation Mechanism of UV-Cured Films. TGA-IR analysis is aimed at understanding the effect of P-NH-AC on the thermal degradation of materials. Figures 5 and 6 show three-dimensional TGA-IR spectra of the gas phase in the thermal degradation of UV-cured films containing 0 and 5 wt % P-NH-AC, respectively. The peaks in the regions around 3400−4000, 2250−2400, and 1600−1900 cm−1 were noted. The characteristic absorption peaks represented corresponding products, such as H2O (3400−4000 cm−1), CO2 (2250−2400 cm−1), carbonyl (1763 cm−1), alkane (3000−2800 cm−1), and phosphate-containing compounds (1063, 939 cm−1).28,29 It was easily found from Figures 5 and 6 that the time to reach the maximum intensity of the absorption peak of the UVcured films with P-NH-AC was delayed, which indicated that the thermal stability of UV-cured films was enhanced by the introduction of P-NH-AC.
Figure 1. Effect of the content of P-NH-AC on photopolymerization kinetics of the system. System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
conversions of all systems reached above 95% after the irradiation for 2 min. The results were attributed to the reduction of the double bond content of the photopolymerization system with the increase of the content of P-NH-AC. 3.3. Thermogravimetric Analysis. The thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) curves of UV-cured films of the system with P-NH-AC in nitrogen and air are presented in Figures 2 and 3. The temperatures of 10% weight loss and the peak temperatures at the maximum weight loss rate for the examined UV-cured films are listed in Tables 1 and 2. It was discovered that the test atmosphere significantly affected the thermal decomposition of the system. TG curves show that oxygen accelerated the degradation of the early stage. T10% in air was 30−40 °C lower than that in nitrogen, while Tmax1 and Tmax2 in air were increased and the system without P-NH-AC presented a twostep thermal degradation. These results demonstrated that oxygen was involved in the degradation reactions and the reaction mechanism is very complex. Whether in air or nitrogen, for the systems with P-NH-AC, the thermal stability was improved with the increase of the P-NH-AC, but the final residue was low though it slightly increased with the increase of the P-NH-AC. Compared with the system without P-NH-AC, some system with a small amount of P-NH-AC presented the decrease of the thermal stability instead. This may be because the compatibility of P-NH-AC with acrylate monomers was
Figure 2. (a) TG and (b) DTG curves of UV-cured films with P-NH-AC in N2. System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %. 16138
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Figure 3. (a) TG and (b) DTG curves of UV-cured films with P-NH-AC in air. System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
Table 1. Thermal Decomposition Data of UV-Cured Films with P-NH-AC in N2a P-NH-AC content (wt %) 0 0.5 1 1.5 2 5
T10% (°C) 331 313 315 318 323 341
± ± ± ± ± ±
1 0 1 1 0 1
Tmax1 (°C) − 302 310 317 324 330
± ± ± ± ±
Tmax2 (°C) 0 1 1 1 1
421 416 427 432 432 446
± ± ± ± ± ±
Table 3. Assignments of the Peaks in IR Spectra
char (wt %, 800 °C) 1 1 1 0 1 1
3.7 5.7 6.5 6.9 7.6 8.0
± ± ± ± ± ±
0.1 0.1 0.2 0.1 0.1 0.1
absorption peak (cm−1)
assignment
2972, 2870 1731 1453 1273 1180 1044, 880
−CH2−, −CH3 CO C−H PO C−O−C P−O−C
a
System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
Table 2. Thermal Decomposition Data of UV-Cured Films with P-NH-AC in Aira P-NH-AC content (wt %) 0 0.5 1 1.5 2 5
T10% (°C) 225 275 285 290 295 296
± ± ± ± ± ±
0 1 1 1 0 1
Tmax1 (°C) 389 375 386 387 390 391
± ± ± ± ± ±
Tmax2 (°C) 2 1 1 1 1 1
436 505 507 508 511 523
± ± ± ± ± ±
Figure 5. TG-IR spectra of UV-cured film without P-NH-AC. System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
char (wt %, 800 °C) 1 1 0 1 1 1
0 0.6 1.8 2.5 2.7 3.3
± ± ± ± ±
0.1 0.1 0.1 0.2 0.1
System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
Figure 6. TG-IR spectra of UV-cured film with 5% P-NH-AC. System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
Figure 7 shows the plots of the absorption intensities of alkane, CO2, and CO as a function of the temperature change. Compared with the materials without P-NH-AC, the temperature to reach the maximum intensities of the absorption peaks of alkane, CO2, and CO during the thermal decomposition of the materials with P-NH-AC was enhanced. This also proved
that the introduction of P-NH-AC improved the thermal stabilities of the materials. 3.6. LOI Tests. LOI values of UV-cured materials with different contents of P-NH-AC are listed in Table 4. The LOI value of the UV-cured materials containing 5 wt % P-NH-AC reached 32%, and the LOI value rose with the increase of the
a
Figure 4. IR spectra of cured films at different thermal degradation temperatures: (a) room temperature to 400 °C; (b) 450−700 °C. System composition: HEA/TMPTA = 5/1 (wt %); P-NH-AC, 5 wt %; photoinitiator 1173, 1 wt %. 16139
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Figure 7. Relationship between intensity of characteristic peak and temperature for evolved products. (a) 0 and (b) 5 wt %.
Table 4. LOI Values of Systems with P-NH-ACa P-NH-AC content (wt %)
LOI (%)
0 0.5 1 1.5 2 5
29.0 29.1 29.3 30.1 31.1 32.0
selected isobornyl acrylate (IBOA) to be the photopolymerizable monomer in order to easily prepare samples for the cone calorimetric test. 3.7.1. Heat Release Rate (HRR). The heat release rate (HRR), particularly the peak heat release rate (PHRR), has been recognized as one of the most important parameters to evaluate fire safety.33 The measured heat release rate curves of each sample are shown in Figure 9 for comparison. It can be
a
System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
content of P-NH-AC, demonstrating that P-NH-AC had a good flame resistance. Digital photographs of the residual chars obtained after LOI tests are presented in Figure 8. The residual chars of all systems
Figure 9. Heat release rate (HRR) as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1 (wt %).
observed that the acrylate resin without P-NH-AC burnt at 20 s after ignition and the HRR reached a sharp peak with a peak heat release rate (PHRR) of 1745 kW/m2. When 1 and 5 wt % P-NH-AC were added into the acylate resin, the resin burnt rapidly (15 s) and the peak HRR decreased to 1465 and 1245 kW/m2, respectively. Moreover, the average of the HRR (avHRR) of the resin with P-NH-AC was much lower than that without P-NH-AC. The results suggest that the addition of PNH-AC can remarkably improve the flame retardancy of the resin. 3.7.2. Total Heat Release (THR). Figure 10 shows curves of total heat release (THR) for each sample. It can be seen that the THR value of the resin without P-NH-AC is always higher than that with P-NH-AC, which suggests that P-NH-AC could reduce the yield of fuel during combustion. At the end of burning, the total heat release of the resin without P-NH-AC was 157 MJ/m2, while those with 1 and 5 wt % P-NH-AC were reduced to 131 and 118 MJ/m2, respectively. The significant decrease in the THR of the flame retardant indicated that a part of the polymer was not completely burnt. This may be because during the burning process a char was formed on the surface of the matrix, which served as a thermal insulation layer to inhibit
Figure 8. Appearances of residual chars of cured films with P-NH-AC. (a) 0, (b) 0.5, (c) 1.0, (d) 1.5, (e) 2.0, and (f) 5.0 wt %.
exhibited two layers, and the outer layer of the residual char was intumescent, while the inside layer of the residual char was compact and dense. The increase of the content of P-NH-AC resulted in the increase of the intumescent char. Furthermore, there was no molten drop during the combustion process. The results indicated a good flame resistance of the UV-cured materials with P-NH-AC. 3.7. Cone Calorimeter. The cone calorimeter has been used for quantitative material flammability analysis.30−32 The cone calorimeter can provide a wealth of information from its simulation of real-world fire conditions, such as the heat release rate (HRR), the time to ignition (TTI), the peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP). Therefore, we have used the cone calorimeter to identify the role of P-NH-AC during the combustion process of UV-curable acrylate resin samples and 16140
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Table 5. Properties of Cured Films with P-NH-ACa P-NH-AC content (wt %)
gel yield (%)
0 0.5 1 1.5 2 5
99.6 97.8 97.4 96.9 96.2 91.3
hardness
water absorption (%)
θ(H2O) (deg)
γSd(H2O) (mN/m)
2B HB H 2H 2H H
24.8 24.2 23.9 20.6 16.1 14.2
50 52 53 55 66 70
149.2 144.3 141.8 136.8 109.4 99.5
a
System composition: HEA/TMPTA = 5/1 (wt %); photoinitiator 1173, 1 wt %.
compatibility of P-NH-AC with the monomers and the decrease of the gel yield of UV-cured films. The cured films all exhibited high water absorption and small contact angles, resulting from the hydrophilic hydroxyl group of HEA. However, the water absorption and surface dispersion energy of the UV-cured films with P-NH-AC decreased with the increase of the content of P-NH-AC, and the contact angle showed a contrary trend. The result may be due to the C P(O)C structure and the highly regular architecture of PNH-AC. The introduction of P-NH-AC into the UV-curable materials enhanced not only the flame resistance, but also the mechanical properties of the materials due to its unique molecular structure.
Figure 10. Total heat release (THR) as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1 (wt %).
polymer pyrolysis and prevent the evolution of combustible gases to feed the flame.34 3.7.3. Smoke Release. The smoke production during the fire test is another important parameter to characterize the fire behavior, as the smoke and its toxicity are main causes of death during fire disasters. Figure 11 shows the total smoke
4. CONCLUSIONS A novel photopolymerizable polyphosphinate (P-NH-AC) used for a UV-curable flame retardant was synthesized by carbene polymerization and characterized by IR, NMR, and GPC. The effect of the content of P-NH-AC on the photopolymerization kinetics and the flame resistance were investigated. The results showed that the rate of polymerization of the systems decreased with the increase of the content of P-NH-AC, and the final double bond conversion of all systems reached above 95% after irradiation for 2 min. TG curves show that oxygen accelerated the degradation of the early stage. T10% in air was 30−40 °C lower than that in nitrogen, while Tmax1 and Tmax2 in air were increased. Whether in air or nitrogen, for the systems with P-NH-AC, the thermal stability was improved with the increase of the P-NH-AC; however, the final residue was low though it slightly increased with the increase of the P-NH-AC. In comparison with the system without P-NH-AC, some system with a small amount of P-NH-AC presented a decrease of the thermal stability instead due to the breaking of the structural homogeneity of the materials. The combustion behaviors of the UV-curable acrylate resin are improved by introducing P-NH-AC. The LOI value of the resin with 5 wt % P-NH-AC was increased to 32.0; that is, that the LOI value was increased by 3.0 units as compared to that without P-NH-AC. The results of the cone calorimeter demonstrated that the addition of P-NH-AC significantly decreased the HRR, THR, and TSP of the resin. All results indicate that P-NH-AC has a significant effect on the flame retardancy of the UV-curable acrylate resin. Furthermore, with the increase of the content of P-NH-AC, the hardness and the contact angle of UV-cured films increased, while the water absorption and dispersion surface energy decreased. It is proved that the introduction of P-NH-AC into UV-curable materials can enhance not only the flame resistance, but also the physical and mechanical properties of the materials.
Figure 11. Total smoke production (TSP) as a function of time. System composition: IBOA/photoinitiator 1173 = 100:1 (wt %).
production (TSP) of the composites during the cone calorimeter test. The TSP of the resin without P-NH-AC was 33 m2, while those with 1 and 5 wt % P-NH-AC decreased to 30 and 27 m2. These results prove that P-NH-AC provides superior smoke suppression and has an obvious flame retardant effect. 3.8. Performances of UV-Cured Films with P-NH-AC. The hardness, water absorption, contact angle, and surface dispersion energy of the UV-cured films with P-NH-AC are listed in Table 5. It was observed that the hardness of the UV-cured films with P-NH-AC was improved by increasing the content of P-NHAC, which was ascribed to the rigid benzene ring structure and syndiotactic structure of the P-NH-AC molecule. However, when the content of P-NH-AC in the UV-cured film increased to 5%, the hardness began to reduce because of the poor 16141
dx.doi.org/10.1021/ie502023v | Ind. Eng. Chem. Res. 2014, 53, 16135−16142
Industrial & Engineering Chemistry Research
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Article
(16) Hetterscheid, D. G. H.; Hendriksen, C.; Dzik, W. I.; Smits, J. M. M.; Eck, E. R. H. V.; Rowan, A. E.; Busico, V.; Vacatello, M.; Castelli, V. V. A.; Segre, A.; Jellema, E.; Bloemberg, T. G.; Bruin, B. D. Rhodium-mediated stereoselective polymerization of “carbenes”. J. Am. Chem. Soc. 2006, 128, 9746−9752. (17) Noels, A. F. Carbene chemistry: stereoregular polymers from diazo compounds. Angew. Chem., Int. Ed. 2007, 46, 1208−1210. (18) Blencowe, A.; Caiulo, N.; Cosstick, K.; Fagour, W.; Heath, P.; Hayes, W. Synthesis of hyperbranched poly(aryl ether)s via carbene insertion processes. Macromolecules 2007, 40, 939−949. (19) Doulut, S.; Demont, P.; Lacabanne, C. Influence of tacticity on the α retardation mode in amorphous poly(methyl methacrylate). Macromolecules 2000, 33, 3425−3430. (20) Boer, T. J.; Backer, H. J. Diazomethane. Org. Synth. 1963, 4, 250. (21) Griitzmacher, H.; Pritzkow, H. Reaction of Stannyl-Substituted Phosphorus Ylides with Boron Trifiuoride Diethyl Etherate: Evidence for Formation of Transient Triphenylphosphonio-Substituted Stannaethenes. Organometallics 1991, 10, 938−946. (22) Li, S. J.; He, Y.; Nie, J. Photopolymerization of hybrid monomer 3-(1-propenyl)oxypropyl acrylate. J. Photochem. Photobiol., A: Chem. 2007, 191, 25−31. (23) Xu, J. X.; Dong, W. X. Polyphosphinate monomer, polyphosphinate and preparation method of polyphosphinate. China Patent CN 102250140 A, Nov 13, 2011. (24) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy: A Guide for Students of Organic Chemistry; Saunders: Orlando, FL,1979. (25) Bugajny, M.; Bourbigot, S.; Le Bras, M.; Delobel, R. The origin and nature of flame retardance in ethylene-vinyl acetate copolymers containing Hostaflam AP 750. Polym. Int. 1999, 48, 264−270. (26) Yuan, H.; Xing, W.; Zhang, P.; Song, L.; Hu, Y. Functionalization of cotton with UV-cured flame retardant coatings. Ind. Eng. Chem. Res. 2012, 51, 5394−5401. (27) Le Bras, M.; Bourbigot, S.; Revel, B. Comprehensive study of the degradation of an intumescent EVA-based material during combustion. J. Mater. Sci. 1999, 34, 5777−5782. (28) Liu, Y.; Zhao, J.; Deng, C. L.; Chen, L.; Wang, D. Y.; Wang, Y. Z. Flame-retardant effect of sepiolite on an intumescent flameretardant polypropylene system. Ind. Eng. Chem. Res. 2011, 50, 2047− 2054. (29) Suppakarn, N.; Jarukumjorn, K. Mechanical properties and flammability of sisal/PP composites: Effect of flame retardant type and content. Composites, Part B 2009, 40, 613−618. (30) Siat, C.; Bras, M. L.; Bourbigot, S. Combustion behaviour of ethylene vinyl acetate copolymer-based intumescent formulations using oxygen consumption calorimetry. Fire Mater. 1998, 22, 119. (31) Schartel, B.; Hull, T. R. Development of fire-retarded materials interpretation of cone calorimeter data. Fire Mater. 2007, 31, 327. (32) Wang, D. Y.; Liu, Y.; Wang, Y. Z.; Artiles, C. P.; Hull, T. R.; Price, D. Fire retardancy of a reactively extruded intumescent flame retardant polyethylene system enhanced by metal chelates. Polym. Degrad. Stab. 2007, 92, 1592. (33) Gilman, J. W.; Jackson, C. L.; Morgan, A. B. Flammability properties of polymer-layered silicate nanocomposites: Polypropylene and polystyrene nanocomposites. Chem. Mater. 2000, 12, 1866. (34) Lin, J. S.; Liu, Y.; Wang, D. Y.; Qin, Q.; Wang, Y. Z. Poly(vinyl alcohol)/Ammonium polyphosphate systems improved simultaneously both fire retardancy and mechanical properties by montmorillonite. Ind. Eng. Chem. Res. 2011, 50, 9998.
ASSOCIATED CONTENT
S Supporting Information *
1
H, 13C, and 31P NMR spectra of compounds a, b, c, d, e, f, and g. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-64449336. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (Grant 51273014) is gratefully acknowledged.
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REFERENCES
(1) Tarle, Z.; Meniga, A.; Ristic, M.; Sutalo, J.; Pichler, G.; Davidson, C. L. The effect of the photopolymerization method on the quality of composite resin samples. J. Oral Rehabil. 1998, 25, 436−442. (2) Sebra, R. P.; Anseth, K. S.; Bowman, C. N. Integrated surface modification of fully polymeric microfluidic devices using living radical photopolymerization chemistry. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1404−1413. (3) Maag, K.; Lenhard, W.; Löffles, H. New UV curing system for automotive applications. Prog. Org. Coat. 2000, 1, 93−97. (4) Sepeur, S.; Kunze, N.; Werner, B.; Schmidt, H. UV curable hard coatings on plastics. Thin Solid Films 1999, 351, 216−219. (5) Deker, C. Kinetic study and new applications of UV radiation curing. Macromol. Rapid Commun. 2002, 23, 1067−1093. (6) Cathal, D.; O’Connell; Michael, J. H.; David, M.; Simon, E. M.; Gordon, G. W. Liquid ink deposition from an atomic force microscope tip: deposition monitoring and control of feature size. Langmuir 2014, 30, 2712−2721. (7) Christian, H.; Souleymane, C.; Anita, R.; Gina, L. F.; Christoph, W. Light-induced bonding and debonding with supramolecular adhesives. Appl. Mater. Interfaces 2014, 6, 4713−4719. (8) Chen, X. L.; Jiao, C. M. Thermal degradation characteristics of a novel flame retardant coating using TG-IR technique. Polym. Degrad. Stab. 2008, 93, 2222−2225. (9) Ç akmakçı, E.; Mülazim, Y.; Kahraman, M. V.; Apohan, N. K. Flame retardant thiol−ene photocured coatings. React. Funct. Polym. 2011, 71, 36−41. (10) Price, D.; Bullett, K. J.; Cunliffe, L. K.; Hull, T. R.; Milnes, G. J.; Ebdon, J. R.; Joseph, P. Cone calorimetry studies of polymer systems flame retarded by chemically bonded phosphorus. Polym. Degrad. Stab. 2005, 88, 74−79. (11) Liao, F. H.; Zhou, L.; Ju, Y. Q.; Yang, Y. Y.; Wang, X. L. Synthesis of a novel phosphorus-nitrogen-silicon polymeric flame retardant and its application in poly(lactic acid). Ind. Eng. Chem. Res. 2014, 53, 10015−10023. (12) Paul, C. H.; Daniel, B.; Walter, G. Organophosphate flame retardants and plasticizers in indoor air. Chemosphere 2004, 57, 781− 787. (13) Xing, W. Y.; Song, L.; Hu, Y.; Zhou, S.; Wu, K.; Chen, L. J. Thermal properties and combustion behaviors of a novel UV-curable flame retarded coating containing silicon and phosphorus. Polym. Degrad. Stab. 2009, 94, 1503−1508. (14) Chen, X.; Hu, Y.; Jiao, C. M.; Song, L. Preparation and thermal properties of a novel flame-retardant coating. Polym. Degrad. Stab. 2007, 92, 1141−1150. (15) Zhu, S.; Shi, W. Synthesis, characterization and flame retardancy of methacrylated phosphate/diphosphate. Polym. Int. 2004, 53, 266− 271. 16142
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