Article pubs.acs.org/IECR
Flame Retardancy and Thermal Properties of Novel UV-Curable Epoxy Acrylate Coatings Modified by a Silicon-Bearing Hyperbranched Polyphosphonate Acrylate Xiaofeng Wang,†,‡ Jing Zhan,*,† Weiyi Xing,† Xin Wang,† Lei Song,† Xiaodong Qian,† Bin Yu,† and Yuan Hu*,†,‡ †
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *
ABSTRACT: A novel silicon-containing hyperbranched polyphosphonate acrylate (HPA), successfully synthesized via the Michael addition polymerization of tri(acryloyloxyethyl) phosphate (TAEP) with (3-aminopropyl)trimethoxysilane (KH550), was incorporated into the epoxy acrylate resin (EA) to prepare UV-curing flame retardant coatings. The study of thermal degradation of these coatings revealed that HPA can catalyze the degradation of EA, contributing to the formation of thermally stable char layer. The residues of EA/HPA-3 (which contains 30 wt % HPA) at various temperatures were analyzed by X-ray photoelectron spectroscopy (XPS) and the results displayed that phosphorus and silicon elements can well protect the carbonaceous char from thermal-oxidative degradation at 600 °C, but do not work at 800 °C. Besides, the investigation of the flammability illustrated that the addition of HPA increased the limiting oxygen index (LOI) value and reduced the peak heat rate release (HRR) and total heat release (THR).
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burning.12 Another reactive-type flame retardant that has attracted researchers’ attention is silicon-containing monomers,13−15 the degradation products of which can improve the thermal oxidative stability of the char layer by their accumulation on the surface of the burning polymer to insulate the heat transformation and the dispersion of oxygen into the underlying polymeric substrate. Hyperbranched polymers, showing the lower viscosity and higher curing rate compared to linear counterparts with similar molecular weight,16 have been reported to be possible to apply for UV curable coatings.17−19 Traditionally, hyperbranched polymers are prepared mainly by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups. However, very few ABx monomers are commercially available, and access to them sometimes involves multistep organic synthesis. An alternative approach is the polymerization of AA′ + B3, presenting many advantages such as commercially available monomers, facile preparation, and easy-to-tailor structures without gelation even at high conversions.20 It has been confirmed that the Michael addition polymerizations of triacryl-containing molecules (B3) and difunctional amine (AA′)20 are of this kind. In the current work, a novel silicon-containing hyperbranched polyphosphonate acrylate was synthesized via the Michael addition polymerization of TAEP (B3) with KH550
INTRODUCTION UV-curable coating technology, which converts reactive monomers into a network through cross-linking reactions initiated by UV radiation,1 has attracted wide attention due to the advantages such as lower energy consumption, less environmental pollution, lower process costs, high-chemical stability, and very rapid curing even at ambient temperature.2−4 Epoxy acrylate resins (EA) are one class of the most important and extensively used reactive monomers in UV-curing systems due to their networks possessing high stiffness and strength, excellent chemical and solvent resistance, good adhesion properties, and so on.5 It is universally known, however, that epoxy acrylate resins are highly combustible, which restricts its application in some fields like optical fiber, wooden furniture, and electronic devices. Therefore, there is an urgent necessity to develop flame-retardant epoxy acrylate coatings to reduce the fire hazards. Among various approaches to endow UV-curing coatings with flame retardancy, the reactive-type flame retardants (RFRs),6 which can chemically bond flame retardant segments to the polymer backbone, are of great interest due to the fact that they overcome the shortcomings of additive-type ones. In this system, the mechanical properties and transparency of cured films are not greatly affected, and they can work longer as they will not exude from polymer materials during use and aging. One of the RFRs that has recieved much attention7−11 is the phosphorus-containing acrylate monomers, which usually form a thermally stable char layer inhibiting heat and oxygen transfer into the polymer bulk and reducing the diffusion of combustible gases into the zone of the pyrolysis during © 2013 American Chemical Society
Received: Revised: Accepted: Published: 5548
December 8, 2012 February 22, 2013 March 26, 2013 March 26, 2013 dx.doi.org/10.1021/ie3033813 | Ind. Eng. Chem. Res. 2013, 52, 5548−5555
Industrial & Engineering Chemistry Research
Article
(AA′). Then a series of UV-curable flame-retardant coatings were prepared by the incorporation of HPA into EA. The flammability property was assessed by limiting oxygen index (LOI), vertical combustion, and microscale combustion calorimeter (MCC). The thermal degradation behavior and mechanism were investigated by thermogravimetic analysis (TGA), real-time Fourier transform infrared (RT-FTIR), and thermogravimetric analysis-infrared spectrometry (TG-IR). The residues of EA/HPA-3 at various temperatures were analyzed by XPS. Besides, the transparency of the coatings to visible light was evaluated by UV/vis spectrometer.
distribution [weight-average molecular weight/number-average molecular weight (Mw/Mn)] were determined at 30 °C on a Waters 515 gel permeation chromatograph equipped with microstyragel columns (103, 104, and 105 Å) and a Waters refractive-index detector. Tetrahydrofuran (THF) was used as the solvent at a flow rate of 1.0 mL/min. Narrow polystyrenes standards were used for the calibration of the molecular weight and molecular weight distribution. Transparency of cured films with a width of 300 ± 50 μm was measured on a UV−vis spectrometer (Perkin-Elmer Lambda 18) between 400 and 800 nm. The X-ray photoelectron spectroscopy (XPS) measurement was carried out using an ESCALAB MK II (VG Co., Ltd., England) spectrometer, with Al Kα excitation radiation (hν = 1253.6 eV) in ultrahigh vacuum conditions. The microscale combustion calorimeter (MCC) tests were carried out using a Govmak MCC-2 microscale combustion calorimeter; 4−6 mg samples were heated to 650 °C at a heating rate of 1 °C/s in a stream of nitrogen flowing at 80 cm3/min. The volatile anaerobic thermal degradation products in the nitrogen gas stream were mixed with a 20 cm3/min stream of pure oxygen prior to entering a 900 °C combustion furnace. Every test was repeated three times, and the deviation of the data obtained was about ±6%. Limiting oxygen index (LOI) was measured according to ASTMD 2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China). The specimens used for the test were of dimensions 100 mm × 6.5 mm × 3 mm. The vertical test was performed on a CFZ-2 type instrument (Jiangning Analysis Instrument Co., China) according to the UL 94 test standard. The specimens used were of dimensions 130 mm × 13 mm × 3 mm. The thermogravimetic analysis (TGA) was carried out on TGA Q5000 IR thermalgravimetric analyzer (TA Instruments). About 4−10 mg of the cured resins were heated from room temperature to 800 °C at a heating rate of 20 °C/min under air atmosphere. Real-time Fourier transform infrared (RT-FTIR) spectra were recorded using the Nicolet 6700 FT-IR spectrophotometer equipment. The sample was placed in the oven and heated at raising temperature at the heating rate of 10 °C/min. Thermogravimetric analysis-infrared spectrometry (TG-IR) of the cured sample was performed using the TGA Q5000 IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FT-IR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from 30 to 800 °C. The heating rate was set as 20 °C/min (nitrogen atmosphere, flow rate of 45 mL/min).
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EXPERIMENTAL SECTION Materials. Epoxy acrylate resin (EB600) was brought from Cytec Industries Inc. KH550 was purchased from Nanjing Shuguang Chemical Group. Co., Ltd. Hydroxylethyl acrylate (HEA) was supplied by Beijing Orient Chemical Co. 2Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur1173), kindly donated by Ciba Specialty Chemicals, was used as a photoinitiator. Other chemicals were obtained from China National Pharmaceutical Group (Shanghai, China). Phosphorus oxychloride (POCl3), KH550, and HEA were distilled prior to use. Other reagents were used as received. TAEP was prepared according to the previous literature.21 Synthesis of HPA. A typical procedure for the preparation of HPA with equal molar ratio of TAEP to KH550 was as follows. TAEP (10.0 g, 25.5 mmol) was added to a solution of KH550 (5.64 g, 25.5 mmol) in 100 mL of tetrahydrofuran. The polymerization was carried out at 50 °C for 48 h with stirring. Then solvent was removed under vacuum, obtaining a colorless sticky liquid with a 98% yield. Preparation of Flame Retardant EA Coatings. The general fabrication of flame retardant EA was as follows. To a vial were added EA, HPA, and 2 wt % Darocur1173 photoinitiator. The contents of the vial were thoroughly dissolved in appropriate amount of dichloromethane under sonication. After the full evaporation of dichloromethane under vacuum drying, the mixture was exposed to the UV irradiation apparatus (80 W/cm, made by Lantian Co., China) for 80 s with 365 nm light at room temperature. Then moisture curing was carried out at 80% relative humidity for 24 h. The formulations of flame retardant EA resins with different contents of HPA were given in Table 1. Table 1. Formulations of the Cured Films and Their Corresponding Flame Retardancy composition (wt %)
a
flame retardancy
sample
EB600
HP
LOI/%
UL-94 rating
EA/HPA-0 EA/HPA-1 EA/HPA-2 EA/HPA-3
100 90 80 70
0 10 20 30
22.0 24.0 25.0 26.5
NRa NR NR NR
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RESULTS AND DISCUSSION Characterization of HPA. HPA was prepared by the Michael addition polymerization of KH550 with equimolar TAEP in the feed according to Scheme 1. The dendritc, linear, and terminal moieties of HPA are also circled in dotted line. The FTIR spectrum of HPA is given in Supporting Information Figure S1. The absorbance peaks at 1270 and 985 cm−1 are assigned to stretching vibrations of PO and POC, respectively.21 The absorbance peaks at 1730, 1630, 1180, and 800 cm−1 are characteristic of acrylate groups.21 Besides, the strong absorbance peak at 1080 cm−1 should be the overlapping result of the SiOC and POC.22 Supporting Information Figure S2 displays the GPC trace of HPA using THF as an eluent. The number average molecular
No rating.
Measurements. 1H NMR measurements were conducted on AVANCE 400 Bruker spectrometer at room temperature using CDCl3 as a solvent. The FTIR spectra of samples were recorded using a Nicolet MAGNA-IR 750 spectrophotometer. The number average molecular weight determined by gel permeation chromatography [Mn(GPC)] and molecular weight 5549
dx.doi.org/10.1021/ie3033813 | Ind. Eng. Chem. Res. 2013, 52, 5548−5555
Industrial & Engineering Chemistry Research
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Scheme 1. Synthesis Route for HPA via the Michael Addition Polymerization of TAEP with KH550
weight (Mn) of HPA is experimentally to be 3308 g/mol, with a polydispersity of 1.59. The 1H NMR spectrum of HPA with the assignment is exhibited in Supporting Information Figure S3. The three groups of characteristic peaks at 5.80−6.56 ppm are obviously observed, confirming the existence of acrylate group in HPA molecular structure. Furthermore, the integral ratio of peak 1 (δ = 5.76−6.80, CH2CHCOO−) to that of peak (2 + 3 + 4 + 5) [δ = 4.07−4.47, OP(OCH2CH2)3] is calculated to be about 1.19/ 4, relative to their initial ratio value of 3/4 before polymerization, indicating that there is 39.7% acryl moieties left in the HPA. In addition, it can be observed that there is the splitting of peak 9 (δ = 1.43−1.75, NCH2CH2CH2Si) and peak 10 (δ = 0.46−0.71, CH2CH2Si), which should be ascribed to the partial hydrolysis of Si−O−C2H5. Transparency of the Cured Films. The transparency of the cured EA films to visible light is shown in Supporting Information Figure S4. One can see that the transmittance of the pure EA films to visible light between 400 and 800 nm is about between 78% and 86%. The addition of 10 wt % HPA results in a slight improvement in transparency, whereas no further enhancement is observed with further increasing the HPA content in EA resin. This implies the good miscibility between HPA and epoxy acrylate resin. Thermal Degradation Behavior of the Cured Films. The thermal stability of the UV-cured films containing different contents of HPA was investigated by TGA. Figure 1 shows their TGA thermograms in air, and the corresponding data was collected in Table 2. From the TGA curves, one can observe that the neat cured epoxy acrylate resin just loses 5 wt % before 375 °C (T5%). In contrast, the modified epoxy acrylate resins are significantly less thermally stable when evaluated by the temperature at 5% weight loss, and the more HPA added, the lower the T5%. The same trends for the thermal stabilities are observed when using the temperature at the first maximum
Figure 1. TGA curves of the cured films.
Table 2. TGA Data of the Cured Films in Air sample
T5% (°C)
T1max (°C)
T2max (°C)
residues (%, 800 °C)
EA/HPA-0 EA/HPA-1 EA/HPA-2 EA/HPA-3
375 326 305 286
431 405 388 386
601 633 633 646
1.0 3.5 6.6 8.7
mass loss rate (T1max) to assess the resistance to heat shock. This may be due to the early degradation of phosphoruscontaining moieties which could promote the formation of a thermally stable phosphorus−carbon structure by reacting with the polynuclear aromatic carbon during degradation, finally improving the thermal stability of residues at high temperature.10,11,23 This point can be confirmed by the better thermal stability of flame retarded EA films than that of neat EA at high temperature. For instance, the temperature at the second maximum mass loss rate (T2max) are in order of EA/HPA-0 (601 °C) < EA/HPA-1 (633 °C) < EA/HPA-3 (646 °C). Moreover, for flame-retardant EA films, the rates of mass loss from segmental decomposition were decreased and the amount of residues shown in Table 2 was increased. All these indicate the enhanced endurance of char residues against thermal 5550
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P−O−C or PO3 groups28 and P2O5,29 respectively, were taken over by the only one peak at 134.9 eV, revealing that phosphorus element in the residue is finally in the state of P2O5. The two peaks of Si2p at 103.4 and 104 eV, which can be ascribed to Si−P and Si−O groups, respectively, were replaced by the only one peak at 104 eV, exhibiting that some phosphorus−silicon compound benefiting the flame retardance finally transformed into the only silicon-containing compound.23 The N1s peaks in the residue at 600 °C can be split into three peaks at 398.8, 400.3 and 402.1 eV, which can be assigned to CN, CN,23 and oxidized nitrogen compounds,30 respectively. The minor peak at 402.1 eV for the N1s peak in the residue at 600 °C becomes its major part with elevating the temperature to 800 °C. In addition, it is noteworthy about the components and evolution of the chemical states for C1s and O1s peaks. As shown in Figure 4, there is a common feature that the peak area at 284.6 eV, which is the contribution of C−C in aliphatic and aromatic species revealing the formation of carbon due to the dehydration during the combustion,23 occupies the overwhelming majority of their respective C1s peaks no matter what the temperature is at 600 or at 800 °C, indicating that the carbon in the residues is mainly in the nonoxidation state. This is possibly due to the fact that carbon, once being oxidized, tends to become the products which are easy to escape from the residues, like CO2, CO, and carbonyl compounds. For the O1s XPS spectra, the area of peak around 533.2 eV, assigned to COC, COH, COOR,31 COP, and POP32 groups, takes up most of their respective O1s peak area, whether the temperature is at 600 or 800 °C. This may be related to the oxidization progress with oxygen as a participant at high temperature that oxygen first forms single bond with other elements, which then transform into double bonds during the further oxidization reaction. Thermal Degradation Mechanism. In order to investigate the degradation mechanism, real time FTIR was chosen to monitor the chemical structure changes of the cured films during the thermal degradation. Figures 5−6 show the changes in the dynamic FTIR spectra obtained from EA/HPA-0 and EA/HPA-3 at different pyrolysis temperatures, respectively. As shown in Figure 5, the characteristic absorbance peaks of the cured EA film at around 3450 (hydroxyl), 2960 (stretching vibration of CH bond), 1730 (stretching vibration of ester CO bond), 1460 (bending vibration of C−H bond), 1240 (antisymmetric stretching vibration of COOC), 1180 (stretching vibration of COC bond), and 830 cm−1 (outof-plane deformation vibration of neighboring H atoms in benzene) are clearly visible. There are little changes in its FTIR spectra below 300 °C, consistent with the fact that no notable weight loss appeared in the TGA before 300 °C.22,33 When the temperature increased from 300 to 400 °C, the relative intensity of the characteristic absorbance peaks of EA decreased sharply, indicating the occurrence of a highly quick decomposition in this temperature range. Then at 500 °C, all peaks nearly disappeared, illustrating that EA decomposes completely. The FTIR spectra of residual products for EA/HPA-3 are shown as Figure 6. It is obvious that there is an overlapping of absorption bands of PO bonds with that of EA around 950− 1090 cm−1, but the degradation during this absorption range still shows the decomposition of POC bonds to some extent. The quick decrease in their relative intensities and then nearly complete disappearance at 300 °C implies the thorough
oxidation. And in addition to the phosphorus-containing moieties, this improvement should also be closely related to the silicon-containing parts, which can form an intercalated Si− O−Si network inside the char structure and a ceramic-like protective material on the surface of char to protect the substrate at middle and later stages of fire.24,25 Chemical Components of the Residual Char. There is no doubt that it is of great importance to know the element composition of the residues and the evolution of the chemical states of the elements vs temperature for an insight into the thermal degradation mechanism. XPS measurement, which can reflect chemical interactions on the atomic scale and can also provide reliable structural characteristics for amorphous surface layers of complex compositions,26 is selected to make such an analysis. XPS spectra of the residues of EA/HPA-3 at various temperatures for 10 min are shown in Figure 2 and the
Figure 2. XPS spectra of the residues of EA/HPA-3 at 600 °C (a) and 800 °C (b) in a muffle furnace for 10 min.
Table 3. XPS Data of the Residues of EA/HPA-3 at 600 °C (a) and 800 °C (b) in a Muffle Furnace for 10 min sample
C1s (%)
O1s (%)
Si2p (%)
P2p (%)
N1s (%)
residue at 600 °C residue at 800 °C
66.57 7.21
21.12 53.2
4.31 22.79
5.57 13.05
2.42 3.76
corresponding data are listed in Table 3. It is quite clear that the residue at 600 °C contains a 66.57% carbon element, while there is just 7.21% carbon element left for the residue at 800 °C. On the other hand, the oxygen content in the residue at 800 °C increases to 53.2% from 21.12% in the residue at 600 °C. It is believed that the carbon content in the char residue indicates the degree of char accumulation, while the oxygen content in the char residue indicates the oxidation degree of char at high temperature.27 This illustrates that the residue at 800 °C has been highly oxidized, relative to that at 600 °C, which seems to be contradictory to the higher content of flame retardant elements in the residue at 800 °C like silicon (22.79%) and phosphorus (13.05%) compared to 4.31% silicon and 5.57% phosphorus in the residue at 600 °C. The reasonable explanation should be that flame retardant elements have also been so thoroughly oxidized that they cannot provide a good shield of char against thermal oxidation, which can be confirmed by the evolution of chemical states of elements with flame retardancy. As shown in Figure 3, there is quite a noticeable change of the chemical states of P2p, Si2p, and N1s when increasing the temperature from 600 to 800 °C. The two peaks of P2p at 133.9 and 134.9 eV, which can be assigned to 5551
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Figure 3. P2p, Si2p, and N1s XPS spectra of the char residues of EA/HPA-3 at 600 and 800 °C in a muffle furnace for 10 min.
Figure 6. Dynamic FTIR spectra of EA/HPA-3 at different pyrolysis temperatures.
breaking of ester groups in EB600.34 This should be why many characteristic absorption bands of flame-retarded EA resin degrade much earlier than that of pure EA. Moreover, a new peak at 1290 cm−1 (the PO vibration for PO−benzene structure)35,36 does not disappear until 500 °C, indicating the higher thermal stability of the formed phosphorus−carbon complexes in the condensed phase. Besides, the still presence of peak at 1100 and 880 cm−1 (characteristic absorption peaks of Si−O−Si bond)37 at 500 °C exhibits that the forming siloxane leaves in the condensed phase. As a result, an effective barrier layer containing both phosphorus element and silicon element was formed, which could protect matrix beneath from decomposing at high temperature. This can be confirmed by the above results from the XPS analysis. Evolved Products of the UV-Cured Films. In order to get a further insight into the thermal degradation mechanism, the TGA-IR technique, which can give the direct identification of the evolved products,38 was chosen to monitor the evolution of volatilized products formed during the thermal degradation. Figure 7 shows 3D TGA-IR spectra of gas phase in thermal degradation of EA/HPA-0 (a) and EA/HPA-3 (b) at heating rate 20 °C/min in nitrogen atmosphere. Peaks in the regions of around 3050−2700, 2250−2350, and 1600−1900 cm−1 are noted. Some of the gaseous decomposition products of the EA are unambiguously identified by characteristic FTIR signals,
Figure 4. C1s and O1s XPS spectra of the char residues of EA/HPA-3 at at 600 and 800 °C in a muffle furnace for 10 min.
Figure 5. Dynamic FTIR spectra of EA/HPA-0 at different pyrolysis temperatures.
decomposition of POC aliphatic structure. It has been reported that its degraded products could act as an acid catalyst, which would accelerate the cleavage of side groups and the 5552
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Figure 7. 3D surface graph for the FTIR spectra of the evolved gases produced by EA/HPA-0 (a) and EA/HPA-3 (b) pyrolysis.
such as CH4 (3016 cm−1), hydrocarbons (2820−2980 cm−1), CO2 (2358 cm−1), carbonyl-containing compounds (1810− 1650 cm−1), and aromatic compounds (669 cm−1).39,40 It can be seen that there is a similarity for the release of gaseous products during the decomposition process. However, there exist obvious differences in the escape process of volatilized products as well, which can clearly be observed from the absorbance of pyrolysis products for EA/HPA-0 and EA/HPA3 vs time. As shown in Figure 8, the escape of the volatilized products for EA/HPA-3 is earlier than that of the neat EA. And the
lagging behind relative to evolved methane of neat EA. Besides, it needs to be noted that the amount of evolved methane for EA/HPA-3 surpasses that for pure EA, which is possibly related to the increase in the aliphatic parts because the incorporation of flame retardant containing many aliphatic moieties reduces the content of aromatic groups. Flammability Performance of the Cured EA Films. The HRR curves for the cured EA films are shown in Figure 9 and
Figure 9. MCC curves of the cured films.
Table 4. MCC Data of the Cured Films sample EA/HPA-0 EA/HPA-1 EA/HPA-2 EA/HPA-3
Figure 8. Absorbance of pyrolysis products for EA/HPA-0 and EA/ HPA-3 vs time: (a) CH4; (b) CO2; (c) carbonyl compounds; and (d) aromatic compounds.
HRC (J/g·k) 294 221 194 166
± ± ± ±
2 6 1 4
peak HRR (w/g) 295 223 196 167
± ± ± ±
2 7 1 3
total HR (kJ/g) 18.1 15.3 14.1 13.7
± ± ± ±
0.4 0.8 0.4 0.6
the corresponding data are listed in Table 4. It is clear that, for EA/HPA-1, the heat both began to release and reached the peak HRR at relatively low temperature compared with that of virgin EA coating, which can be explained that the relatively weak phosphorus-containing moieties show an earlier degradation followed by the catalysis of forming acidic species on EA. Furthermore, the earlier degradation of phosphorus-containing moieties could promote the formation of a thermally stable phosphorus−carbon structure by reacting with the polynuclear aromatic carbon during degradation,12 finally improving the thermal stability of residues at higher temperature. And the silicon-containing parts can promote the formation of a ceramic-like protective layer on the surface of char.13 These factors help reduce the total amount of evolved products and the release rate of volatilized products during the pyrolysis
carbonyl-containing compounds evolved during the shorter time interval compared to that of neat EA, whereas other gas products release during the longer time span. This can be interpreted that the presence of flame retardant leads to the earlier thermal decomposition of EA and especially accelerates the escape of the carbonyl-containing compounds. It has been found that the acidic species degrading from phosphoruscontaining compounds can promote the formation of a protective char layer, which could prevent the combustible gases from transferring to the surface of the materials and feed the flame.41 This point can be confirmed by the lower evolution of carbonyl-containing compounds and aromatic compounds for EA/HPA-3. Moreover, it can further be proved by the fact that methane volatilizes completely until 45 min, which is 5553
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process, which determine the value of peak HRR and THR. When increasing HPA to 30 wt % in EA, the temperature with the peak HRR are further brought forward, yet the peak HRR and THR present a further decrease, indicating that the addition of more HPA contributes to the escape of the less combustible products and the decrease in release rate of flammable products, finally presenting both lower peak HRR and THR. In addition, the LOI and vertical test of the coatings were performed and the results are shown in Table 1. Pure EA coating exhibits an LOI value of 22.0%, and there is an increase for the LOI values of coatings flame retarded by HPA, for instance, the coating containing 30 wt % HPA (EA/HPA-3) is 26.5, increasing by 4.5% relative to that of virgin EA coating. However, all the coatings are clearly not classified in the test of UL-94 flame classification.
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CONCLUSION
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1 shows the FTIR spectrum of HPA. Figure S2 displays the GPC trace of HPA using THF as an eluent. Figure S3 exhibits the 1H NMR spectrum of HPA in CDCl3. Figure S4 illustrates UV−vis analysis of the flame retardant coatings. This material is available free of charge via the Internet at http:// pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax/Tel.: +86-551-63601664. E-mail address: yuanhu@ustc. edu.cn (Y.H.);
[email protected] (J.Z.). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Decker, C. Kinetic study and new applications of UV radiation curing. Macromol. Rapid Commun. 2002, 23, 1067−1093. (2) Maag, K.; Lenhard, W.; Loffles, H. New UV curing systems for automotive applications. Prog. Org. Coat. 2000, 40, 93−97. (3) Randoux, T.; Vanovervelt, J. C.; Van den Bergen, H.; Camino, G. Halogen-free flame retardant radiation curable coatings. Prog. Org. Coat. 2002, 45, 281−289. (4) Valet, A. Outdoor applications of UV curable clearcoats - a real alternative to thermally cured clearcoats. Prog. Org. Coat. 1999, 35, 223−233. (5) Kim, J. W.; Suh, K. D. Amphiphilic urethane acrylate hydrogels having heterophasic gel structure: swelling behaviors and mechanical properties. Colloid Polym. Sci. 1998, 276, 342−348. (6) Ho, T. H.; Hwang, H. J.; Shieh, J. Y.; Chung, M. C. Thermal, physical and flame-retardant properties of phosphorus-containing epoxy cured with cyanate ester. React. Funct. Polym. 2009, 69, 176− 182. (7) Zhu, S. W.; Shi, W. F. Combustion behaviour and thermal properties of UV cured methacrylated phosphate/epoxy acrylate blends. Polym. Degrad. Stab. 2003, 81, 233−237. (8) Liang, H. B.; Huang, Z. G.; Shi, W. F. Different effects of tri(acryloyloxyethyl) phosphate on the thermal degradation of photopolymerized epoxy acrylate and polyurethane acrylate films. J. Appl. Polym. Sci. 2006, 99, 3130−3137. (9) Chen, X. L.; Song, L.; Hu, Y. Flammability and Thermal Degradation of Epoxy Acrylate Modified with Phosphorus-Containing Compounds. J. Appl. Polym. Sci. 2010, 115, 3332−3338. (10) Liang, H. B.; Asif, A.; Shi, W. F. Photopolymerization and thermal behavior of phosphate diacrylate and triacrylate used as reactive-type flame-retardant monomers in ultraviolet-curable resins. J. Appl. Polym. Sci. 2005, 97, 185−194. (11) Wang, Q. F.; Shi, W. F. Photopolymerization and thermal behaviors of acrylated benzenephosphonates/epoxy acrylate as flame retardant resins. Eur. Polym. J. 2006, 42, 2261−2269. (12) Horrocks, A. R.; Davies, P. J. Char formation in flame-retarded wool fibres. Part 1. Effect of intumescent on thermogravimetric behaviour. Fire Mater. 2000, 24, 151−157. (13) Cheng, X. E.; Shi, W. F. UV-curing behavior and properties of tri/di(acryloyloxyethyloxy) phenyl silane used for flame-retardant coatings. Prog. Org. Coat. 2010, 69, 252−259. (14) Lu, S. Y.; Hamerton, I. Recent developments in the chemistry of halogen-free flame retardant polymers. Prog. Polym. Sci. 2002, 27, 1661−1712. (15) Zhang, K.; Shen, M. M.; Wu, K.; Liu, H. F.; Zhang, Y. K. Comparative study on flame retardancy and thermal degradation of phosphorus- and silicon-containing epoxy resin composites. J. Polym. Res. 2011, 18, 2061−2070. (16) Johansson, M.; Hult, A. Synthesis, Characterization, and Uv Curing of Acrylate Functional Hyperbranched Polyester Resins. J. Coating. Technol. 1995, 67, 35−39. (17) Huang, Z. G.; Shi, W. F. Synthesis and properties of a novel hyperbranched polyphosphate acrylate applied to UV curable flame retardant coatings. Eur. Polym. J. 2007, 43, 1302−1312. (18) Huang, Z. G.; Shi, W. F. UV curing behavior of hyperbranched polyphosphate acrylate/di(hydroxylpropyl methacrylate) piperazine and properties of the cured film. Prog. Org. Coat. 2007, 59, 312−317. (19) Wang, H. L.; Xu, S. P.; Shi, W. F. Photopolymerization behaviors of hyperbranched polyphosphonate acrylate and properties of the UV cured film. Prog. Org. Coat. 2009, 65, 417−424. (20) Wang, D.; Zheng, Z. J.; Hong, C. Y.; Liu, Y.; Pan, C. Y. Michael addition polymerizations of difunctional amines (AA′) and triacrylamides (B-3). J. Polym. Sci., Polym. Chem. 2006, 44, 6226−6242. (21) Liang, H.; Shi, W. F. Thermal behaviour and degradation mechanism of phosphate di/triacrylate used for UV curable flameretardant coatings. Polym. Degrad. Stab. 2004, 84, 525−532. (22) Qian, X. D.; Song, L.; Hu, Y.; Yuen, R. K. K. Preparation and thermal properties of novel organic/inorganic network hybrid materials containing silicon and phosphate. J. Polym. Res. 2012, 19, 1.
A novel silicon-containing hyperbranched polyphosphonate acrylate was successfully synthesized via the Michael addition polymerization, and then, a series of UV-curable flame retardant coatings were prepared by the incorporation of HPA into EA. The thermal degradation of the flame-retardant resins was studied using TGA, RT-FTIR, and TGA-IR. It was found that the addition of HPA can effectively catalyze the degradation of EA, which contributes to the formation of thermally stable char layer. The residues at various temperatures were analyzed by XPS, and the results displayed that phosphorus and silicon elements can protect the carbonation char from thermaloxidative degradation at 600 °C well but do not work at 800 °C. The study of the flammability of these coatings revealed that HPA could increase the LOI value and effectively reduce peak HRR and THR, but no samples pass the V0 rating for UL94 classification. Besides, the transparency of all the flame retardant EA resins to visible light can match that of pure epoxy acrylate.
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ACKNOWLEDGMENTS
The work was financially supported by National Basic Research Program of China (973 Program) (2012CB719701), National Natural Science Foundation of China (No.51036007), China Postdoctoral Science Foundation (2012M511418), and National Natural Science Foundation of China (51203146). 5554
dx.doi.org/10.1021/ie3033813 | Ind. Eng. Chem. Res. 2013, 52, 5548−5555
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(23) Chen, X. L.; Hu, Y.; Jiao, C. M.; Song, L. Preparation and thermal properties of a novel flame-retardant coating. Polym. Degrad. Stab. 2007, 92, 1141−1150. (24) Rong, M. Z.; Zhang, M. Q.; Zheng, Y. X.; Zeng, H. M.; Walter, R.; Friedrich, K. Structure-property relationships of irradiation grafted nano-inorganic particle filled polypropylene composites. Polymer 2001, 42, 167−183. (25) Chen, X. L.; Hu, Y.; Song, L. Thermal behaviors of a novel UV cured flame retardant coatings containing phosphorus, nitrogen and silicon. Polym. Eng. Sci. 2008, 48, 116−123. (26) Bertoti, I. Characterization of nitride coatings by XPS. Surf. Coat. Technol. 2002, 151, 194−203. (27) Wang, Z. Y.; Han, E. H.; Ke, W. Effect of acrylic polymer and nanocomposite with nano-SiO2 on thermal degradation and fire resistance of APP-DPER-MEL coating. Polym. Degrad. Stab. 2006, 91, 1937−1947. (28) Jansen, R. J. J.; Vanbekkum, H. Xps of Nitrogen-Containing Functional-Groups on Activated Carbon. Carbon 1995, 33, 1021− 1027. (29) Clark, D. T.; Fok, T.; Roberts, G. G.; Sykes, R. W. An Investigation by Electron-Spectroscopy for Chemical-Analysis of Chemical Treatments of the (100) Surface of N-Type Inp Epitaxial Layers for Langmuir Film Deposition. Thin. Solid. Films 1980, 70, 261−283. (30) Beamson, G.; Briggs, D. High-Resolution Monochromated XRay Photoelectron-Spectroscopy of Organic Polymers - a Comparison between Solid-State Data for Organic Polymers and Gas-Phase Data for Small Molecules. Mol. Phys. 1992, 76, 919−936. (31) Brown, N. M. D.; Hewitt, J. A.; Meenan, B. J. X-Ray-Induced Beam Damage Observed during X-Ray Photoelectron-Spectroscopy (Xps) Studies of Palladium Electrode Ink Materials. Surf. Interface Anal. 1992, 18, 187−198. (32) Shih, P. Y.; Yung, S. W.; Chin, T. S. Thermal and corrosion behavior of P2O5-Na2O-CuO glasses. J. Non-Cryst. Solids 1998, 224, 143−152. (33) Xing, W. Y.; Hu, Y.; Song, L.; Chen, X. L.; Zhang, P.; Ni, J. X. Thermal degradation and combustion of a novel UV curable coating containing phosphorus. Polym. Degrad. Stab. 2009, 94, 1176−1182. (34) Huang, Z. G.; Shi, W. F. Thermal behavior and degradation mechanism of poly(bisphenyl acryloxyethyl phosphate) as a UV curable flame-retardant oligomer. Polym. Degrad. Stab. 2006, 91, 1674−1684. (35) Hsiue, G. H.; Shiao, S. J.; Wei, H. F.; Kuo, W. J.; Sha, Y. A. Novel phosphorus-containing dicyclopentadiene-modified phenolic resins for flame-retardancy applications. J. Appl. Polym. Sci. 2001, 79, 342−349. (36) 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. (37) Wang, Y. Z.; Chen, W. Y.; Yang, C. C.; Lin, C. L.; Chang, F. C. Novel epoxy nanocomposite of low D-k introduced fluorinecontaining POSS structure. J. Polym. Sci., Polym. Phys. 2007, 45, 502−510. (38) Wang, G. A.; Cheng, W. M.; Tu, Y. L.; Wang, C. C.; Chen, C. Y. Characterizations of a new flame-retardant polymer. Polym. Degrad. Stab. 2006, 91, 3344−3353. (39) Xing, W. Y.; Jie, G. X.; Song, L.; Wang, X.; Lv, X. Q.; Hu, Y. A. Flame retardancy and thermal properties of epoxy acrylate resin/alphazirconium phosphate nanocomposites used for UV-curing flame retardant films. Mater. Chem. Phys. 2011, 125, 196−201. (40) Braun, U.; Knoll, U.; Schartel, B.; Hoffmann, T.; Pospiech, D.; Artner, J.; Ciesielski, M.; Doring, M.; Perez-Gratero, R.; Sandler, J. K. W.; Altstadt, V. Novel phosphorus-containing poly(ether sulfone)s and their blends with an epoxy resin: Thermal decomposition and fire retardancy. Macromol. Chem. Phys. 2006, 207, 1501−1514. (41) Wang, X.; Song, L.; Yang, H. Y.; Lu, H. D.; Hu, Y. Synergistic Effect of Graphene on Antidripping and Fire Resistance of Intumescent Flame Retardant Poly(butylene succinate) Composites. Ind. Eng. Chem. Res. 2011, 50, 5376−5383. 5555
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