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Thermal and UV-curing behavior of phosphate diacrylate used for flame retardant coatings
Progress in Organic Coatings 59 (2007) 318–323
Thermal and UV-curing behavior of phosphate diacrylate used for flame retardant coatings Xilei Chen, Yuan Hu ∗ , Chuanmei Jiao, Lei Song State Key Lab of Fire Science, University of Science and Technology of China, Anhui 230026, PR China Received 15 October 2006; received in revised form 12 January 2007; accepted 1 May 2007
Abstract A novel phosphorus-on-skeleton compound has been synthesized by allowing phosphorus oxychloride to react with 1-oxo-4-hydroxymethyl2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (PEPA) and 2-hydroxyethyl acrylate (HEA). Its structure was characterized by Fourier transformed infrared spectroscopy (FTIR) and 1 H nuclear magnetic resonance spectroscopy (1 H NMR). The UV-curing behavior was investigated using FTIR. Results show that the conversion of the unsaturated bond of the monomer exposed to UV light for 100 s is approximately 84%. Flame-retardant effectiveness was estimated from the limiting oxygen index (LOI) and thermal stability was characterized by thermogravimetric analysis (TGA). The LOI value was 39 and the char yield was 53% at 600 ◦ C. TG data indicate that the material undergoes degradation in three characteristic temperature stages, which may be attributed to the decomposition of the phosphate, thermal pyrolysis of aliphatic chains, and degradation of an unstable structure in char, respectively. These were further characterized by real time Fourier-transform infrared measurement. It is proposed that the flame retardant action results from decomposition of phosphate to form poly(phosphoric acid), which catalyses the breakage of bonds adjacent to carbonyl groups to form an intumescent char, preventing the sample from burning further. © 2007 Published by Elsevier B.V. Keywords: UV curing; Phosphate; Flame retardant; Thermal degradation
1. Introduction Photo-polymerisable resins are now being increasingly used in various applications, mainly in the coating industry, graphic arts, and microelectronics, replacing conventional thermally cured solvent-based coatings and adhesives [1,2]. However, these resins are combustible in the presence of an ignition source, thus this limits their use in application which require flameretardant performance. Traditionally, flame retardant materials are achieved by physically blending flame retardants as additives with polymers. However, the high concentration of additives usually leads to difficulty in curing and opacity, especially for UV curable systems [3]. Another efficient way by which to reduce the flammability of the cured films is to chemically bond flame retardant units to the polymer mainchain, i.e. by using curable flame retardants. A series of multifunctional oligomers/monomers have been used as flame retardants for UV curable systems [4,5]. Poly(bisphenyl acryloxyethyl phos-
∗
Corresponding author. Tel.: +86 551 3601664; fax: +86 551 3601664. E-mail address: [email protected] (Y. Hu).
0300-9440/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2007.05.002
phate) has been blended in different ratios with urethane acrylate EB220 to obtain a series of UV curable flame-retardant resins [6]. Compounds containing phosphorus represent a growing group of additives which are viewed as environmentally friendly flame retardants. They have been widely used as flame retardant materials, which function differently from other flame retardants such as inorganic materials and halogenated compounds. It has been reported that a phosphorus-containing flame-retardant functions in two ways: (1) interrupting the exothermic process and thus suppressing combustion by capturing free radicals and (2) increasing the char yield by redirecting the chemical reactions yielding carbon rather than CO or CO2 , and through the formation of a surface layer of protective char [7]. It has been suggested that an investigation of the mechanism of thermal decomposition would provide insight into the flame retarding effect in a phosphorus-containing polymeric system [8]. Therefore, it would be interest to study the effects of phosphorus on the formation of charred crust during polymer combustion, and determine the composition of the char residue. However, few reports address the use of flame-retardant oligomers and monomers in UV curable systems or the thermal degradation
X. Chen et al. / Progress in Organic Coatings 59 (2007) 318–323
of these systems despite the fact that UV curing technology is developing rapidly and is being widely used in many fields. In this instance, a novel flame retardant multifunctional monomer (MDMPE) based on phosphorus oxychloride, PEPA and HEA was designed and synthesized. The flame retardancy and thermal behavior of the UV cured film were characterized by LOI and thermogravimetric analysis, respectively. The chemical structure changes during the thermal degradation of the film were monitored by real time FTIR. 2. Experimental 2.1. Materials Phosphorus oxychloride (POCl3 ) and triethylamine (TEA) were distilled before use. 2-Hydroxylethyl acrylate (HEA), supplied from Dong-fang Chemical Co., Beijing, China, was ˚ molecular distilled at reduced pressure and dried over a 4 A sieves before use. PEPA was purchased from Yueyang Huihong Chemical Co., Hunan Province, China. 2-Hydroxy-2-methyl-1phenyl-1-propanone (Darocur 1173), kindly supplied as a gratis sample by Ciba Specialty Chemicals (Switzerland), was used as a photoinitiator. Other reagents were used as received without further purification. 2.2. Synthesis [9] Phosphorus oxychloride (0.33 mol) was placed in a 500 ml round-bottomed flask filted with a calcium chloride drying tube and stirred. PEPA (0.3 mol) in 200 ml of acetonitrile was then added dropwise at 5 ◦ C over a period of 1 h. After addition of the PEPA solution, the mixture was stirred for 24 h. Unchanged phosphorus oxychloride and solvent were removed by distillation at reduced pressure (8 Torr) and elevated temperature (120 ◦ C). The residue contained as a major product PEPA dichlorophosphate (PDCP) and a minor product of diPEPA chlorophosphate (DPCP). A 23 g sample of the residue was placed in another reaction kettle and 0.2 g of CuCl2 was added. After raising the temperature to 60 ◦ C, 30 g (0.23 mol) of HEA in 30 ml of acetonitrile was added dropwise to the reactor over a period of 1 h. After 12 h, the solution was placed in a rotary evaporator to remove any unchanged reactants. The solution was then neutralized with sodium hydrogen carbonate and dried with anhydrous magnesium sulfate. The final product was a mixture of DMPE (major product) and MDPE (minor product). This mixture was designated as MDMPE. The distribution of the chemical structures of MDMPE was shown by HPLC to be 89% DMPE and 11% MDPE, respectively. IR (NaCl) (cm−1 ): 1290 and 1268 ( P O); 1458 and 856 (C H); 1196 (C O C); 1030 and 980 ( P O C); 1725 ( C O); 1636; 1411; 810 ( C CH). 1 H NMR (CDCl ): δ (ppm) = 3.60–3.74 (exocyclic 3 C CH2 O), 3.91–4.05 (P O CH2 CH2 O ), 4.28–4.32 (P O CH2 CH2 O ), 4.60–4.75 (intra-annular C CH2 O), 5.85–5.89 (CH CH2 , trans), 6.12–6.17 (CH CH2 , cis), 6.43–6.47 (CH CH2 ).
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Anal. Calcd for MDMPE: C 37.85, H 4.99, O 43.22; Found: C 37.83, H 4.98, O 43.24. 2.3. UV irradiation process of MDMPE The ultraviolet light source used for irradiation is a lamp (80 W/cm2 , Lantian Co., Beijing), which emits light in the near UV (characteristic wavelength, 340–360 nm). The UV irradiation was carried out in an air atmosphere. The polymerization mixture was prepared by adding 3 wt% photoinitiator (Dorocure 1173) into the original MDMPE. A thin uniform film of the mixture was laid on two glass plates. The samples on the glass plates were put in the UV irradiation apparatus and were irradiated for different times. The distance between the MDMPE samples and the UV lamp was about 10 cm. 2.4. Measurements 2.4.1. FTIR analysis FTIR spectroscopy (EQUINOX55, Bruker Co. Germen) was employed to characterize the MDMPE synthesized using thin KBr discs. The transition mode was used and the wavenumber range was set from 4000 to 400 cm−1 . Real time Fourier transform infrared (FTIR) spectra was recorded using a Nicolet MAGNA-IR 750 spectrophotometer equipped with a ventilated oven having a heating device. Powders of the cured sample were mixed with KBr powders, and the mixture was pressed into a disc, which was then placed into the oven. The temperature of the oven was raised at a heating rate of about 2 ◦ C/min. Dynamic FTIR spectra were obtained in situ during the thermo-oxidative degradation of the cured sample. 2.4.2. 1 H NMR spectra analysis 1 H NMR measurement was conducted on an Avance 300 spectrometer (Bruker Biospin, Switzerland, frequency: 1 H 300 MHz) at room temperature using CDCl3 as the solvent. Teramethylsilane (TMS) in CDCl3 was used as internal standard. 2.4.3. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the cured sample was performed using a DT-50 (Sahimadzu, Japan) instrument. About 10.0 mg of the UV cured sample was put in an alumina crucible and heated from 25 to 800 ◦ C. The heating rate was set as 10 K/min (air atmosphere, flow rate of 150 ml/min). 2.5. Determination of conversion of unsaturated bond by FTIR FTIR spectroscopy (EQUINOX55, Bruker Co. Germen) was employed to determine MDMPE relative conversion percentage (CP) of unsaturated bond during the UV curing process. The original MDMPE and photoinitiator mixture (liquid) was spread on the KBr plates and cured under UV irradiation directly. The relative conversion percentage of unsaturated bond in the UV-cured films was determined by monitoring the stretching vibration of the acrylate carbon–carbon double bond at 1636 cm−1 [10]. The spectra were normalized with respect to
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the intensity of the carbonyl peak at 1733 cm−1 as an internal standard to account for variations in sample thickness and instrument recording [11]. The unsaturated bond content (A0 ) of the uncured sample containing photoinitiator was defined as 100%. If the unsaturated bond content of the sample irradiated with t time was defined as At , the CP of unsaturated bond could be calculated as follows: A0 − At CP (%) = × 100% A0
3. Results and discussion 3.1. Synthesis and characterization of monomer DMPE is a phosphorus-containing acrylate. It can be UV cured to form a phosphorus-containing acrylate polymer. In view of the location of phosphorus, this type of polymer can be regarded as phosphorus-on-pendant polymers. On the other hand, DMPE consists of two acrylate monomers and can be copolymerized into a type of phosphorus-on-skeleton polymer.
Scheme 1. Synthesis route of MDMPE.
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Fig. 1. The infrared spectrum of MDMPE.
In the synthesis of phosphorus-on-skeleton polymers, the PEPA dichlorophosphate (PDCP) intermediate was first prepared. However, the by-product diPEPA chlorophosphate (DPCP) was also produced. Hence, after the UV curing polymerization, the unwanted product MDPE was also present (Scheme 1). The final acrylate monomer DMPE was characterized by FTIR and 1 H NMR. Evidence for the condensation of PEPA with phosphorus oxychloride was clearly confirmed with absorptions present at 1299 cm−1 attributed to P O bond of phosphate and at 1266 cm−1 assigned to P O bond of PEPA. As the FTIR spectrum given in Fig. 1 shows, the C H stretch of the aliphatic CH2 occurs at 2957 cm−1 . Confirmation of the structure of the DMPE monomer was ascertained by the carbonyl absorption peak at 1725 cm−1 and the P O C peak (aliphatic carbon) at 1026 cm−1 [12]. 3.2. UV-curing behaviors of UV-curable MDMPE The UV-curing behavior of MDMPE can be monitored by infrared spectroscopic techniques. That is, during the curing progress, by monitoring the change of peak area of reactive groups, such as vinyl initiated by UV irradiation, the relative conversion percentage (CP) of double bond and the cross-linking extent for the reaction can be established [10,11]. However, the shortcoming of this method is that its accuracy is slightly affected by the sample themselves and the presence of additives. Fortunately, there was no addition of additives except for the little photoinitiator which has no absorption at 1636 cm−1 . In this instance, the FTIR spectroscopy was employed to determine MDMPE’s relative CP of unsaturated bond during UV curing progress. The CP of unsaturated bond for MDMPE mixture samples versus irradiation time in an air atmosphere is presented in Fig. 2. As can be seen from Fig. 2, the CP of the unsaturated bond for MDMPE increases with the increasing of irradiation time. When the sample is exposed to UV light in air atmosphere for 100 s, the CP of unsaturated bond is nearly 84%. This indicates that MDMPE is very sensitive to UV light just as expected.
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Fig. 2. Conversion percent of unsaturated bond for UV cured MDMPE.
3.3. Thermal stability of the UV-cured MDMPE A thermogravimetric analyzer (TGA) was employed to investigate the thermal stability of UV-cured MDMPE. The TG and DTG curves during heating are presented in Fig. 3. It can be seen that the temperature corresponding to 5 wt% loss and the temperature of maximum weight loss rate for the cured film is about 307 and 332 ◦ C, respectively. It is very interesting that when the temperature is raised to 772 ◦ C, there is a second weight loss maximum which may be attributed to the rapid decomposition of the unstable char. In air, the weight loss was brought about by the oxidation of the polymer at high temperature. As a result of the oxidation process, the char yield decreased as the temperature increased. The residual weight of the sample is about 36% at 800 ◦ C. Compared with other phosphate containing acrylates, such as TAEP, DAEEP, both the decomposition temperature and residual weight (ceramic yield) of the UV-cured MDMPE is high [13]. The main reason is that PEPA pendant groups were introduced into the poly(phosphate acrylate). It has been reported that the decomposition pathway of pure PEPA is release of phosphoric acid followed by formation of a foamed carbonized residue [14]. At the higher temperature, the formation of char is more
Fig. 3. Thermagravimetry curves for UV cured MDMPE.
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Fig. 4. Real time FTIR spectra for the degradation of UV cured MDMPE at different temperature.
obvious for the material than that for a phosphate containing acrylate system without pendant PEPA groups. 3.4. Real time FTIR of the UV-cured MDMPE The chemical structure changes of the cured film during the thermal degradation was monitored by real time FTIR. The spectra from 3200 to 500 cm−1 are shown in Fig. 4. The main peaks and bands of cured film are: 1. 2952 cm−1 , stretching of CH2 ; 1452 and 1391 cm−1 , the deformation vibration of C H; 846 and 749 cm−1 , the rocking vibration of C H; 2. 1730 cm−1 , the stretching vibration of C O bond; 3. 1165 cm−1 , the stretching vibration of C O C bond; 4. 1064, 1026, 980, 846 cm−1 , the stretching vibration of P O C bond [12]; 5. 1262, 1257 cm−1 , the stretching vibration of P O bond [15]. The decomposition of the cured MDMPE film can be divided into three parts: the degradation of phosphate groups, the degradation of ester groups and the degradation of alkyl chain. The quick decrease in relative intensities at 1026 and 980 cm−1 above 150 ◦ C and then the complete disappearance above 260 ◦ C clearly indicate the complete degradation of the P O C functionality (Fig. 5). Furthermore, the peaks at 1083 and 1016 cm−1 assigned to the stretching vibration of P O P and PO2 /PO3 in phosphate carbon complexes, respectively [16], and the peaks at 1083 and 882 cm−1 assigned to the symmetric and asymmetric stretching vibration of P O P band [17,18] appear above 260 ◦ C. This indicates that some phosphate groups link to each other by sharing one oxygen atom, leading to the formation of poly(phosphosphoric acid), which can further decompose to form phosphorus oxides, such as P4 O10 . The disappearance of strong peaks at 1257 and 1262 cm−1 and the appearance of two new peaks at 1283 and 1149 cm−1 in the FTIR spectra [19] implies that the phosphate group deviates from the aliphatic structure and forms poly(phosphoric acid) or re-links to the aromatic structures at the temperatures over
Fig. 5. Relative peak intensity at 1730 cm−1 for C O bond, 1450 cm−1 for C H bond and 1026 cm−1 for P O C bond in the FTIR spectra for the UV cured MDMPE film as a function of temperature.
270 ◦ C. The formation of aromatic structures is suggested by the appearance of new peaks at 749, 667 and 590 cm−1 [20]. Because there are interfering peaks attributed to phosphate at around 753, 840 and 1380 cm−1 , the peaks around 2800, 3000 and 1452 cm−1 were selected to study the decomposition of C H bonds. The peak at 2984 cm−1 for the C H of cured film disappears completely at above 350 ◦ C, as shown in Fig. 4. This is due to the fast decomposition of the P O C linkage. Fig. 5 contains FTIR spectra to demonstrate the changes of relative intensities of the peaks at 1730 cm−1 for the C O bond, 1450 cm−1 for the C H bond and 1026 cm−1 for the P O C bond of the cured MDMPE film with increasing temperature. It is very clear that the absorbance at 1026 cm−1 for the P O C bond decreases rapidly with increasing temperature from 150 to 290 ◦ C. The absorbance at 1450 cm−1 attributed to the C H bond decreases at a lower rate with increasing temperature from 150 to 380 ◦ C. The absorbance at 1730 cm−1 assigned to the C O bond decreases slowly with increasing temperature from 150 to 320 ◦ C. All these absorbance decreases slowly above these temperature ranges. The sharp decrease in intensity of the C H and C O absorption is due to the rapid decomposition of the unstable P O C unit at low temperature. 4. Conclusions A novel phosphorus-on-skeleton compound (DMPE) has been successfully synthesized using phosphorus oxychloride to react with PEPA and HEA. The monomer showed high photopolymerization response, and the cured film displayed high flame retardance. The results indicate that the degradation of the UV cured MDMPE film can be divided into three steps. From 160 to 270 ◦ C, the degradation is mainly attributed to the rapid degradation of phosphate groups. From 270 to 350 ◦ C, poly(phosphoric acid) is formed, which catalyses the breakage of bonds to carbonyl groups to form polynuclear aromatic structures. When the temperature is raised above 550 ◦ C, some unstable structures in
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the char are decomposed, resulting in the formation of phosphorus oxides and some volatile aromatic molecules. The char yield at 800 ◦ C exceeds 36%. Acknowledgements The National Natural Science Foundation of China (No. 50476026) Specialized Research Fund for the Doctoral Program of Higher Education (20040358056), Program for New Century Excellent Talents in University, and National 11th Five-Year Program are gratefully acknowledged. References [1] A. Valet, Prog. Org. Coat. 35 (1999) 223. [2] K. Maag, W. Lenhard, H. Loffles, Prog. Org. Coat. 40 (2000) 93. [3] T. Randoux, J.C. Vanovervelt, H. Van den Bergen, G. Camino, Prog. Org. Coat. 45 (2002) 281. [4] H.B. Liang, A. Asif, W.F. Shi, J. Appl. Polym. Sci. 97 (2005) 185.
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[5] H.B. Liang, A. Asif, W.F. Shi, Polym. Degrad. Stab. 87 (2005) 495. [6] Z.G. Huang, W.F. Shi, Polym. Degrad. Stab. 91 (2006) 1674. [7] Y.L. Chang, Y.Z. Wang, D.M. Ban, G.M. Zhao, Macromol. Mater. Eng. 289 (2004) 703. [8] Y.L. Liu, G.H. Hsiue, C.W. Lan, Y.S. Chiu, Polym. Degrad. Stab. 56 (1997) 291. [9] T.L. Wang, Y.L. Cho, P.L. Kuo, J. Appl. Polym. Sci. 82 (2001) 343. [10] K.A. Berchtold, B.H. Lu, L. Lovell, J. Nie, C.N. Bowman, Macromolecules 34 (2001) 5103. [11] H. Wei, Y. Lu, W. Shi, H. Yuan, Y. Chen, J. Appl. Polym. Sci. 80 (2001) 51. [12] R. Kwiatkowski, A. Wlochowicz, J. Mol. Struct. 516 (2000) 57. [13] H.B. Liang, W.F. Shi, Polym. Degrad. Stab. 84 (2004) 525. [14] A.I. Balabanovich, Thermochim. Acta 435 (2005) 188. [15] C.S. Wang, J.Y. Shieh, Eur. Polym. J. 36 (2000) 443. [16] S. Bourbigot, M.L. Bras, R. Delobel, J.M. Tremillon, J. Chem. Soc. Faraday Trans. 92 (1996) 3435. [17] M. Bugajny, S. Bourbigot, Polym. Int. 48 (1999) 264. [18] M. Le Bras, S. Bourigot, B. Revel, J. Mater. Sci. 34 (1999) 5777. [19] S.W. Zhu, W.F. Shi, Polym. Degrad. Stab. 2 (217) (2003) 233. [20] R. Setnescu, S. Jipa, T. Setnescu, W. Kappel, S. Kobayashi, Z. Osawa, Carbon 37 (1999) 1.
Thermal and UV-curing behavior of phosphate diacrylate used for flame retardant coatings Xilei Chen, Yuan Hu ∗ , Chuanmei Jiao, Lei Song State Key Lab of Fire Science, University of Science and Technology of China, Anhui 230026, PR China Received 15 October 2006; received in revised form 12 January 2007; accepted 1 May 2007
Abstract A novel phosphorus-on-skeleton compound has been synthesized by allowing phosphorus oxychloride to react with 1-oxo-4-hydroxymethyl2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (PEPA) and 2-hydroxyethyl acrylate (HEA). Its structure was characterized by Fourier transformed infrared spectroscopy (FTIR) and 1 H nuclear magnetic resonance spectroscopy (1 H NMR). The UV-curing behavior was investigated using FTIR. Results show that the conversion of the unsaturated bond of the monomer exposed to UV light for 100 s is approximately 84%. Flame-retardant effectiveness was estimated from the limiting oxygen index (LOI) and thermal stability was characterized by thermogravimetric analysis (TGA). The LOI value was 39 and the char yield was 53% at 600 ◦ C. TG data indicate that the material undergoes degradation in three characteristic temperature stages, which may be attributed to the decomposition of the phosphate, thermal pyrolysis of aliphatic chains, and degradation of an unstable structure in char, respectively. These were further characterized by real time Fourier-transform infrared measurement. It is proposed that the flame retardant action results from decomposition of phosphate to form poly(phosphoric acid), which catalyses the breakage of bonds adjacent to carbonyl groups to form an intumescent char, preventing the sample from burning further. © 2007 Published by Elsevier B.V. Keywords: UV curing; Phosphate; Flame retardant; Thermal degradation
1. Introduction Photo-polymerisable resins are now being increasingly used in various applications, mainly in the coating industry, graphic arts, and microelectronics, replacing conventional thermally cured solvent-based coatings and adhesives [1,2]. However, these resins are combustible in the presence of an ignition source, thus this limits their use in application which require flameretardant performance. Traditionally, flame retardant materials are achieved by physically blending flame retardants as additives with polymers. However, the high concentration of additives usually leads to difficulty in curing and opacity, especially for UV curable systems [3]. Another efficient way by which to reduce the flammability of the cured films is to chemically bond flame retardant units to the polymer mainchain, i.e. by using curable flame retardants. A series of multifunctional oligomers/monomers have been used as flame retardants for UV curable systems [4,5]. Poly(bisphenyl acryloxyethyl phos-
∗
Corresponding author. Tel.: +86 551 3601664; fax: +86 551 3601664. E-mail address: [email protected] (Y. Hu).
0300-9440/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2007.05.002
phate) has been blended in different ratios with urethane acrylate EB220 to obtain a series of UV curable flame-retardant resins [6]. Compounds containing phosphorus represent a growing group of additives which are viewed as environmentally friendly flame retardants. They have been widely used as flame retardant materials, which function differently from other flame retardants such as inorganic materials and halogenated compounds. It has been reported that a phosphorus-containing flame-retardant functions in two ways: (1) interrupting the exothermic process and thus suppressing combustion by capturing free radicals and (2) increasing the char yield by redirecting the chemical reactions yielding carbon rather than CO or CO2 , and through the formation of a surface layer of protective char [7]. It has been suggested that an investigation of the mechanism of thermal decomposition would provide insight into the flame retarding effect in a phosphorus-containing polymeric system [8]. Therefore, it would be interest to study the effects of phosphorus on the formation of charred crust during polymer combustion, and determine the composition of the char residue. However, few reports address the use of flame-retardant oligomers and monomers in UV curable systems or the thermal degradation
X. Chen et al. / Progress in Organic Coatings 59 (2007) 318–323
of these systems despite the fact that UV curing technology is developing rapidly and is being widely used in many fields. In this instance, a novel flame retardant multifunctional monomer (MDMPE) based on phosphorus oxychloride, PEPA and HEA was designed and synthesized. The flame retardancy and thermal behavior of the UV cured film were characterized by LOI and thermogravimetric analysis, respectively. The chemical structure changes during the thermal degradation of the film were monitored by real time FTIR. 2. Experimental 2.1. Materials Phosphorus oxychloride (POCl3 ) and triethylamine (TEA) were distilled before use. 2-Hydroxylethyl acrylate (HEA), supplied from Dong-fang Chemical Co., Beijing, China, was ˚ molecular distilled at reduced pressure and dried over a 4 A sieves before use. PEPA was purchased from Yueyang Huihong Chemical Co., Hunan Province, China. 2-Hydroxy-2-methyl-1phenyl-1-propanone (Darocur 1173), kindly supplied as a gratis sample by Ciba Specialty Chemicals (Switzerland), was used as a photoinitiator. Other reagents were used as received without further purification. 2.2. Synthesis [9] Phosphorus oxychloride (0.33 mol) was placed in a 500 ml round-bottomed flask filted with a calcium chloride drying tube and stirred. PEPA (0.3 mol) in 200 ml of acetonitrile was then added dropwise at 5 ◦ C over a period of 1 h. After addition of the PEPA solution, the mixture was stirred for 24 h. Unchanged phosphorus oxychloride and solvent were removed by distillation at reduced pressure (8 Torr) and elevated temperature (120 ◦ C). The residue contained as a major product PEPA dichlorophosphate (PDCP) and a minor product of diPEPA chlorophosphate (DPCP). A 23 g sample of the residue was placed in another reaction kettle and 0.2 g of CuCl2 was added. After raising the temperature to 60 ◦ C, 30 g (0.23 mol) of HEA in 30 ml of acetonitrile was added dropwise to the reactor over a period of 1 h. After 12 h, the solution was placed in a rotary evaporator to remove any unchanged reactants. The solution was then neutralized with sodium hydrogen carbonate and dried with anhydrous magnesium sulfate. The final product was a mixture of DMPE (major product) and MDPE (minor product). This mixture was designated as MDMPE. The distribution of the chemical structures of MDMPE was shown by HPLC to be 89% DMPE and 11% MDPE, respectively. IR (NaCl) (cm−1 ): 1290 and 1268 ( P O); 1458 and 856 (C H); 1196 (C O C); 1030 and 980 ( P O C); 1725 ( C O); 1636; 1411; 810 ( C CH). 1 H NMR (CDCl ): δ (ppm) = 3.60–3.74 (exocyclic 3 C CH2 O), 3.91–4.05 (P O CH2 CH2 O ), 4.28–4.32 (P O CH2 CH2 O ), 4.60–4.75 (intra-annular C CH2 O), 5.85–5.89 (CH CH2 , trans), 6.12–6.17 (CH CH2 , cis), 6.43–6.47 (CH CH2 ).
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Anal. Calcd for MDMPE: C 37.85, H 4.99, O 43.22; Found: C 37.83, H 4.98, O 43.24. 2.3. UV irradiation process of MDMPE The ultraviolet light source used for irradiation is a lamp (80 W/cm2 , Lantian Co., Beijing), which emits light in the near UV (characteristic wavelength, 340–360 nm). The UV irradiation was carried out in an air atmosphere. The polymerization mixture was prepared by adding 3 wt% photoinitiator (Dorocure 1173) into the original MDMPE. A thin uniform film of the mixture was laid on two glass plates. The samples on the glass plates were put in the UV irradiation apparatus and were irradiated for different times. The distance between the MDMPE samples and the UV lamp was about 10 cm. 2.4. Measurements 2.4.1. FTIR analysis FTIR spectroscopy (EQUINOX55, Bruker Co. Germen) was employed to characterize the MDMPE synthesized using thin KBr discs. The transition mode was used and the wavenumber range was set from 4000 to 400 cm−1 . Real time Fourier transform infrared (FTIR) spectra was recorded using a Nicolet MAGNA-IR 750 spectrophotometer equipped with a ventilated oven having a heating device. Powders of the cured sample were mixed with KBr powders, and the mixture was pressed into a disc, which was then placed into the oven. The temperature of the oven was raised at a heating rate of about 2 ◦ C/min. Dynamic FTIR spectra were obtained in situ during the thermo-oxidative degradation of the cured sample. 2.4.2. 1 H NMR spectra analysis 1 H NMR measurement was conducted on an Avance 300 spectrometer (Bruker Biospin, Switzerland, frequency: 1 H 300 MHz) at room temperature using CDCl3 as the solvent. Teramethylsilane (TMS) in CDCl3 was used as internal standard. 2.4.3. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the cured sample was performed using a DT-50 (Sahimadzu, Japan) instrument. About 10.0 mg of the UV cured sample was put in an alumina crucible and heated from 25 to 800 ◦ C. The heating rate was set as 10 K/min (air atmosphere, flow rate of 150 ml/min). 2.5. Determination of conversion of unsaturated bond by FTIR FTIR spectroscopy (EQUINOX55, Bruker Co. Germen) was employed to determine MDMPE relative conversion percentage (CP) of unsaturated bond during the UV curing process. The original MDMPE and photoinitiator mixture (liquid) was spread on the KBr plates and cured under UV irradiation directly. The relative conversion percentage of unsaturated bond in the UV-cured films was determined by monitoring the stretching vibration of the acrylate carbon–carbon double bond at 1636 cm−1 [10]. The spectra were normalized with respect to
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the intensity of the carbonyl peak at 1733 cm−1 as an internal standard to account for variations in sample thickness and instrument recording [11]. The unsaturated bond content (A0 ) of the uncured sample containing photoinitiator was defined as 100%. If the unsaturated bond content of the sample irradiated with t time was defined as At , the CP of unsaturated bond could be calculated as follows: A0 − At CP (%) = × 100% A0
3. Results and discussion 3.1. Synthesis and characterization of monomer DMPE is a phosphorus-containing acrylate. It can be UV cured to form a phosphorus-containing acrylate polymer. In view of the location of phosphorus, this type of polymer can be regarded as phosphorus-on-pendant polymers. On the other hand, DMPE consists of two acrylate monomers and can be copolymerized into a type of phosphorus-on-skeleton polymer.
Scheme 1. Synthesis route of MDMPE.
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Fig. 1. The infrared spectrum of MDMPE.
In the synthesis of phosphorus-on-skeleton polymers, the PEPA dichlorophosphate (PDCP) intermediate was first prepared. However, the by-product diPEPA chlorophosphate (DPCP) was also produced. Hence, after the UV curing polymerization, the unwanted product MDPE was also present (Scheme 1). The final acrylate monomer DMPE was characterized by FTIR and 1 H NMR. Evidence for the condensation of PEPA with phosphorus oxychloride was clearly confirmed with absorptions present at 1299 cm−1 attributed to P O bond of phosphate and at 1266 cm−1 assigned to P O bond of PEPA. As the FTIR spectrum given in Fig. 1 shows, the C H stretch of the aliphatic CH2 occurs at 2957 cm−1 . Confirmation of the structure of the DMPE monomer was ascertained by the carbonyl absorption peak at 1725 cm−1 and the P O C peak (aliphatic carbon) at 1026 cm−1 [12]. 3.2. UV-curing behaviors of UV-curable MDMPE The UV-curing behavior of MDMPE can be monitored by infrared spectroscopic techniques. That is, during the curing progress, by monitoring the change of peak area of reactive groups, such as vinyl initiated by UV irradiation, the relative conversion percentage (CP) of double bond and the cross-linking extent for the reaction can be established [10,11]. However, the shortcoming of this method is that its accuracy is slightly affected by the sample themselves and the presence of additives. Fortunately, there was no addition of additives except for the little photoinitiator which has no absorption at 1636 cm−1 . In this instance, the FTIR spectroscopy was employed to determine MDMPE’s relative CP of unsaturated bond during UV curing progress. The CP of unsaturated bond for MDMPE mixture samples versus irradiation time in an air atmosphere is presented in Fig. 2. As can be seen from Fig. 2, the CP of the unsaturated bond for MDMPE increases with the increasing of irradiation time. When the sample is exposed to UV light in air atmosphere for 100 s, the CP of unsaturated bond is nearly 84%. This indicates that MDMPE is very sensitive to UV light just as expected.
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Fig. 2. Conversion percent of unsaturated bond for UV cured MDMPE.
3.3. Thermal stability of the UV-cured MDMPE A thermogravimetric analyzer (TGA) was employed to investigate the thermal stability of UV-cured MDMPE. The TG and DTG curves during heating are presented in Fig. 3. It can be seen that the temperature corresponding to 5 wt% loss and the temperature of maximum weight loss rate for the cured film is about 307 and 332 ◦ C, respectively. It is very interesting that when the temperature is raised to 772 ◦ C, there is a second weight loss maximum which may be attributed to the rapid decomposition of the unstable char. In air, the weight loss was brought about by the oxidation of the polymer at high temperature. As a result of the oxidation process, the char yield decreased as the temperature increased. The residual weight of the sample is about 36% at 800 ◦ C. Compared with other phosphate containing acrylates, such as TAEP, DAEEP, both the decomposition temperature and residual weight (ceramic yield) of the UV-cured MDMPE is high [13]. The main reason is that PEPA pendant groups were introduced into the poly(phosphate acrylate). It has been reported that the decomposition pathway of pure PEPA is release of phosphoric acid followed by formation of a foamed carbonized residue [14]. At the higher temperature, the formation of char is more
Fig. 3. Thermagravimetry curves for UV cured MDMPE.
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Fig. 4. Real time FTIR spectra for the degradation of UV cured MDMPE at different temperature.
obvious for the material than that for a phosphate containing acrylate system without pendant PEPA groups. 3.4. Real time FTIR of the UV-cured MDMPE The chemical structure changes of the cured film during the thermal degradation was monitored by real time FTIR. The spectra from 3200 to 500 cm−1 are shown in Fig. 4. The main peaks and bands of cured film are: 1. 2952 cm−1 , stretching of CH2 ; 1452 and 1391 cm−1 , the deformation vibration of C H; 846 and 749 cm−1 , the rocking vibration of C H; 2. 1730 cm−1 , the stretching vibration of C O bond; 3. 1165 cm−1 , the stretching vibration of C O C bond; 4. 1064, 1026, 980, 846 cm−1 , the stretching vibration of P O C bond [12]; 5. 1262, 1257 cm−1 , the stretching vibration of P O bond [15]. The decomposition of the cured MDMPE film can be divided into three parts: the degradation of phosphate groups, the degradation of ester groups and the degradation of alkyl chain. The quick decrease in relative intensities at 1026 and 980 cm−1 above 150 ◦ C and then the complete disappearance above 260 ◦ C clearly indicate the complete degradation of the P O C functionality (Fig. 5). Furthermore, the peaks at 1083 and 1016 cm−1 assigned to the stretching vibration of P O P and PO2 /PO3 in phosphate carbon complexes, respectively [16], and the peaks at 1083 and 882 cm−1 assigned to the symmetric and asymmetric stretching vibration of P O P band [17,18] appear above 260 ◦ C. This indicates that some phosphate groups link to each other by sharing one oxygen atom, leading to the formation of poly(phosphosphoric acid), which can further decompose to form phosphorus oxides, such as P4 O10 . The disappearance of strong peaks at 1257 and 1262 cm−1 and the appearance of two new peaks at 1283 and 1149 cm−1 in the FTIR spectra [19] implies that the phosphate group deviates from the aliphatic structure and forms poly(phosphoric acid) or re-links to the aromatic structures at the temperatures over
Fig. 5. Relative peak intensity at 1730 cm−1 for C O bond, 1450 cm−1 for C H bond and 1026 cm−1 for P O C bond in the FTIR spectra for the UV cured MDMPE film as a function of temperature.
270 ◦ C. The formation of aromatic structures is suggested by the appearance of new peaks at 749, 667 and 590 cm−1 [20]. Because there are interfering peaks attributed to phosphate at around 753, 840 and 1380 cm−1 , the peaks around 2800, 3000 and 1452 cm−1 were selected to study the decomposition of C H bonds. The peak at 2984 cm−1 for the C H of cured film disappears completely at above 350 ◦ C, as shown in Fig. 4. This is due to the fast decomposition of the P O C linkage. Fig. 5 contains FTIR spectra to demonstrate the changes of relative intensities of the peaks at 1730 cm−1 for the C O bond, 1450 cm−1 for the C H bond and 1026 cm−1 for the P O C bond of the cured MDMPE film with increasing temperature. It is very clear that the absorbance at 1026 cm−1 for the P O C bond decreases rapidly with increasing temperature from 150 to 290 ◦ C. The absorbance at 1450 cm−1 attributed to the C H bond decreases at a lower rate with increasing temperature from 150 to 380 ◦ C. The absorbance at 1730 cm−1 assigned to the C O bond decreases slowly with increasing temperature from 150 to 320 ◦ C. All these absorbance decreases slowly above these temperature ranges. The sharp decrease in intensity of the C H and C O absorption is due to the rapid decomposition of the unstable P O C unit at low temperature. 4. Conclusions A novel phosphorus-on-skeleton compound (DMPE) has been successfully synthesized using phosphorus oxychloride to react with PEPA and HEA. The monomer showed high photopolymerization response, and the cured film displayed high flame retardance. The results indicate that the degradation of the UV cured MDMPE film can be divided into three steps. From 160 to 270 ◦ C, the degradation is mainly attributed to the rapid degradation of phosphate groups. From 270 to 350 ◦ C, poly(phosphoric acid) is formed, which catalyses the breakage of bonds to carbonyl groups to form polynuclear aromatic structures. When the temperature is raised above 550 ◦ C, some unstable structures in
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the char are decomposed, resulting in the formation of phosphorus oxides and some volatile aromatic molecules. The char yield at 800 ◦ C exceeds 36%. Acknowledgements The National Natural Science Foundation of China (No. 50476026) Specialized Research Fund for the Doctoral Program of Higher Education (20040358056), Program for New Century Excellent Talents in University, and National 11th Five-Year Program are gratefully acknowledged. References [1] A. Valet, Prog. Org. Coat. 35 (1999) 223. [2] K. Maag, W. Lenhard, H. Loffles, Prog. Org. Coat. 40 (2000) 93. [3] T. Randoux, J.C. Vanovervelt, H. Van den Bergen, G. Camino, Prog. Org. Coat. 45 (2002) 281. [4] H.B. Liang, A. Asif, W.F. Shi, J. Appl. Polym. Sci. 97 (2005) 185.
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