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Photopolymerization behaviors of hyperbranched polyphosphonate acrylate and properties of the UV cured film
Progress in Organic Coatings 65 (2009) 417–424
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Photopolymerization behaviors of hyperbranched polyphosphonate acrylate and properties of the UV cured film Hailong Wang 1 , Songpan Xu 1 , Wenfang Shi ∗ State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
a r t i c l e
i n f o
Article history: Received 19 December 2008 Received in revised form 16 March 2009 Accepted 18 March 2009 Keywords: UV curing Hyperbranched Flame retardant Phosphorus Thermal degradation
a b s t r a c t A novel hyperbranched polyphosphonate acrylate (HBPPA), used as a reactive-type flame retardant in UV curable systems, was successfully synthesized by the reaction of di(acryloyloxyethyl) benzenephosphonate (DABP) with N-(2-aminoethyl)-piperazine, and characterized by FTIR, 1 H NMR and GPC measurements. HBPPA was blended with DABP as a monomer in different ratios to obtain a series of flame retardant resins. Their maximum photopolymerization rates (Rpmax ) and final unsaturation conversion (Pf ) in the cured films in the presence of a photofragmenting initiator were investigated. The results showed that the Pf increased along with HBPPA content and the pure HBPPA has the maximum value of 81.0% in the photo-DSC analysis. Their flame retardancy was monitored by the limiting oxygen index (LOI), and showed that the UV cured films greatly expanded when burning, and the degree of expansion increased along with HBPPA content. However, the LOI values varied from 36.0 to 39.0, which can be ascribed to the condensed phase mechanism. Their thermal degradation behaviors were investigated by thermogravimetric analysis and in situ FTIR spectroscopy, and showed that the phosphonate group of HBPPA first degraded to form poly(phosphoric acid)s at around 300 ◦ C, which had a major contribution to form the compact char to protect the sample from further degradation. The dynamic mechanical thermal properties were studied by dynamic mechanical thermal analysis and showed a good miscibility between HBPPA and DABP. The crosslinking density and Tg of the cured films decreased along with the content of HBPPA in the blend. © 2009 Elsevier B.V. All rights reserved.
1. Introduction UV curing systems have been widely used in coatings, inks, and adhesives, due to a lot of benefits over conventional thermalcuring systems such as rapid cure, low energy consumption, high efficiency, and low VOCs [1–3]. This technology is satisfying with new requirements for traditional or advanced applications, since it can offer a broad range of the changes in formulation and curing conditions; besides, the UV cured films exhibit desired hardness, excellent chemical resistance and low shrinkage. However, because the traditional UV cured products consisted of acrylates are generally flammable, it demands to develop flame retardant systems to reduce the fire hazards in the applications [4]. Compared with traditional halogen-type flame retardants, halogen-free flame retardants, especially that phosphorus/ nitrogen-containing compounds, have received much attention for the absence of toxic gases and smokes during combustion. Their flame retardant mechanism has been widely investigated as the
∗ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi). 1 These authors contributed equally to this paper. 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.03.005
condensed phase as well as synergy effects of phosphorous and nitrogen. A compact expanding char was formed at the beginning of burning, which decreases the amount of flammable volatile gases reaching the flame zone, accordingly to protect the polymeric material from further degradation and combustion [5]. As the lower viscosity and higher curing rate compared to linear counterparts with similar molecular weight, hyperbranched polymers have been reported to be possible to apply for UV curable coatings. Traditionally, hyperbranched polymers are prepared mainly by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups. Kakimoto and co-workers [6] developed a novel approach concerning the polycondensation of a diamine (A2 ) with trimesic acid (B3 ). However, if the functional groups in a monomer possess equal reactivity, nonlinear polymers and infinite networks as manifested by gelation are formed with gel point being predictable. Moreover, the approach for preparing hyperbranched polymers from a B B2 type monomer and an A2 type monomer has been developed [7–9]. In the B B2 monomer there are one B functional group and two B functional groups that have different reactivity with A group due to different chemical environment. In Flory’s gelation theory, if the B groups have different reactivity, soluble hyperbranched polymers with high molar mass can be obtained.
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Therefore, in this work we report the synthesis of a novel phosphorus–nitrogen containing hyperbranched polyphosphonate acrylate (HBPPA) used as a flame retardant oligomer in UV-coatings by employing an A2 +B B2 type polycondensation. DABP was synthesized as an A2 component. N-(2-aminoethyl)-piperazine was used as a B B2 component. The synthesized HBPPA was characterized with FTIR and 1 H NMR spectroscopy. The molecular weight and its distribution were determined by gel permeation chromatography (GPC). The photopolymerization kinetics, the dynamic mechanical thermal properties and mechanical behaviors of the cured films were also investigated. The flame retardancy of the UV cured films was characterized by the limiting oxygen index (LOI) and the thermostability was characterized by TGA and in situ FTIR analysis. 2. Experimental 2.1. Materials Na2 SO4 , NaHCO3 , Na2 S2 O3 , toluene, methylene dichloride and N-(2-aminoethyl)-piperazine were used as received. Phenylphosphonic dichloride (BPOD) and triethylamine (TEA) were distilled prior to use. All the above chemicals were purchased from Shanghai First Reagent Co., China. Hydroxylethyl acrylate (HEA), supplied from Dong-fang chemical Co. Beijing, China, was distilled under vacuum and dried over a 4 Å molecular sieve before use. ␣-Hydroxyketone (Iragcure 184), by Runtec Chemical Co., Ltd., China, was used as a photoinitiator. 2.2. Synthesis of di(acryloyloxyethyl) benzenephosphonate (DABP) Into a 500 mL round-bottom flask equipped with a mechanical stirrer, 150 mL of toluene solution containing 55.73 g HEA (0.48 mol) and 48.57 g TEA (0.48 mol) was added. Then a solution containing 39.00 g BPOD (0.2 mol) and 80 mL of toluene was added into the above reactant dropwise through an addition funnel under stirring at 0 ◦ C using an ice bath, and kept at ambient temperature for 10 h after finishing the addition. The formed triethylamine hydrochloride salt was removed by filtration. The filtrate was extracted by HCl (1 M), NaHCO3 (saturated) aqueous solution and distilled water twice, and then dried over sodium sulfate. After solvent being removed under vacuum, a colorless liquid product was obtained and named DABP. The schematic outline of synthesis route for DABP is given in Scheme 1.
2.3. Synthesis of HBPPA 5.16 g (0.05 mol) N-(2-aminoethyl)-piperazine was dissolved in 200 mL of chloroform using a 500 mL round-bottomed flask. 80 mL of chloroform solution containing 32.30 g DABP was added into the above reactant dropwise through an addition funnel, and left stirring for another 30 h at room temperature, followed by removing chloroform under vacuum, and finally obtained a lightly yellow liquid product, hyperbranched polyphosphonate acrylate, named HBPPA. The schematic outline of synthesis route for HBPPA is given in Scheme 2. 2.4. Sample preparation The mixtures of DABP and HBPPA in different ratios together with 1.5 wt% Irgacure 184 were stirred until the homogenous blends formed and then exposed to a medium pressure mercury lamp (1 kW, Fusion UV systems, USA) to obtain the cured films, denoted as DABP00, DABP20, DABP40, DABP60, DABP80, and DABP100, according to the ratio of DABP to HBPPA. The compositions of the resins are listed in Table 1. 2.5. Measurements The FTIR spectra were recorded with an MAGNA-IR 750 spectrometer (Nicolet Instrument Co., USA). The in situ FTIR spectra at different temperatures were recorded to monitor the thermodegradation process of the cured films. The temperature was raised from 200 to 450 ◦ C with a heating rate of 2 ◦ C min−1 , and kept at setting temperature for 10 min. The 1 H NMR spectra and 13 C NMR spectrum were recorded with a DMX-300 MHz instrument (Bruker, Switzerland). The thermogravimetric analysis (TGA) was carried out on a Shimadzu TG-50 instrument using a heating rate of 10 ◦ C min−1 in air. The molecular weight distribution was determined by gel permeation chromatography (GPC) with two linear Styragel columns HT3, HT4 and a column temperature of 60 ◦ C. A Waters 1515 pump and a Waters 2414 differential refractive index detector (set at 30 ◦ C) were used. The eluent was DMF with 1 g L−1 BrLi at a flow rate of 1.0 mL min−1 . The tensile storage modulus (E ) and tensile loss factors (Tan ı) of the cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 5 Hz and a heating rate of 10 ◦ C min−1 in the range of −50 to 160 ◦ C on the sheets of 25 mm × 5 mm × 1 mm. The crosslink density (e ) as the molar number of elastically effective network chain per cube centimeter of the coating was calculated from the storage modulus in the rubbery plateau region according to [10]: e = E /3RT , where E is the elastic storage modulus, R is the ideal gas constant, and T is the temperature in Kelvin. The mechanical properties were measured with an Instron Universal tester (model 1185, Japan) at 25 ◦ C with a crosshead speed of 25 mm min−1 . The photopolymerization rate was monitored in air by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system BHG-250 (Mejiro Precision Co., Japan). The incident light intensity Table 1 Compositions of the resins.
Scheme 1. Schematic illustration for the synthesis of DABP.
Sample
Initial mixture
Weight ratio
Phosphorus content (%)
Nitrogen content (%)
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
HBPPA DABP/HBPPA DABP/HBPPA DABP/HBPPA DABP/HBPPA DABP
– 0.20/0.80 0.40/0.60 0.60/0.40 0.80/0.20 –
7.39 7.66 7.93 8.20 8.47 8.74
5.02 4.01 3.01 2.01 1.00 0
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Scheme 2. Schematic illustration for the synthesis of HBPPA.
at the sample pan was measured to be 2.4 mW cm−2 with a UV power meter. The unsaturation conversion (Pt ) was calculated by the formula, Pt = Ht /H∞ , where Ht is the heat effect within t seconds, H∞ is the heat effect of 100% unsaturation conversion. The DSC curves were unified by the weight (g) of samples. The polymerization rate is defined by mmolc c g−1 s−1 , namely, the variation of unsaturation concentration (mmolc c g−1 ) per second. For calculating the polymerization rate and H∞ , the value, H0 = 86 J mmol−1 , for the heat of polymerization per acrylic unsaturation was taken. The limiting oxygen index (LOI) values of UV cured films were measured using a ZRY-type instrument (made in Jiangning, China) with the sheets of 120 mm × 6.5 mm × 3 mm according to the standard ASTM D635-77.
3. Results and discussion 3.1. Characterization The synthesis of DABP was performed by the reaction of BPOD and HEA, as illustrated in Scheme 1. The chemical structure of DABP was characterized with FTIR, 1 H NMR and 13 C NMR. As shown in Fig. 1, the FTIR spectrum of DABP exhibits characteristic absorption at 1251 cm−1 corresponding to vibration with P O bond. The absorption bands observed at 1732, 1636, 1410, and 810 cm−1 demonstrate the existence of acrylate group. The 1 H NMR and 13 C NMR spectra of DABP show the resonances corresponding to all protons and carbons of the given structure, as shown in Figs. 2 and 3, respectively. Both IR and NMR results are consistent with the expected molecular structure. The synthesis of HBPPA was performed by the Michael addition reaction between DABP and N-(2-aminoethyl)-piperazine, as shown in Scheme 2. The FTIR spectrum of HBPPA, as given in Fig. 4, shows the strong absorption at 1255 cm−1 (for P O) and 1040, 980 cm−1 (for P O C), revealing the formation of phosphonate
Fig. 1. FTIR spectrum of DABP.
Fig. 2.
1
H NMR spectrum of DABP.
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Fig. 3.
13
C NMR spectrum of DABP.
structure. Moreover, the absorption bands observed at 1732, 1635, 1410, and 810 cm−1 indicate the existence of acrylate group. The 1 H NMR spectrum of HBPPA with the assignments as shown in Fig. 5, shows three groups of characteristic peaks at 5.80–6.60 ppm, which prove the existence of acrylate group in HBPPA. The peaks observed at 2.20–3.00 ppm are assigned to the H atoms (Ha , Hb , Hc , Hd , He and Hf ), the substituent groups attached to the same carbon are nitrogen atoms; the peaks, assigned with Hg and Hh that the substituent group attached to the same carbon is carbonyl group, can be observed at 4.00–4.40 ppm. The peaks observed at 7.20–8.00 ppm are obviously signs of H atoms attached with phenyl group. As shown in Fig. 6, the number average molecular weight (Mn ) of HBPPA was measured experimentally to be 4200 g mol−1 by GPC using DMF as an eluent, and its polydispersity is 1.29.
Fig. 5.
1
H NMR spectrum of HBPPA.
3.2. Photopolymerization kinetics The photopolymerization kinetics, which can be characterized by the peak maximum (Rpmax ) and the final degree of double bond conversion (Pf ), is one of the important parameters for characterizing the UV curing process [11]. Figs. 7 and 8 show the photopolymerization rates and unsaturation conversion versus irradiation time obtained from Photo-DSC measurements at room temperature, by which the changes in Rpmax and Pf were obtained, as shown in Fig. 9.
Fig. 4. FTIR spectrum of HBPPA.
Fig. 6. GPC trace of HBPPA.
Fig. 7. Photopolymerization rates of the resins with different contents of HBPPA versus irradiation time.
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Table 2 DMTA results and crosslinking density of the cured samples. Sample
Ts (◦ C)
Tg (◦ C)
Ts /Tg
e (mmol/cm3 )
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
16 20 40 64 68 101
25 30 51 82 90 120
0.97 0.97 0.97 0.95 0.94 0.95
2.3 3.0 3.8 5.0 6.8 10.2
early-formed gels could hardly take part in further polymerization [13]; on the contrary, the lower concentration of double bonds can reduce the curing rate of the sample with low content of DABP, and thus prevent the gel-point of system appearing prematurely. As a result, there are fewer double bonds trapped in the crosslink network that could take part in further polymerization. This is the key point that the sample contained less DABP exhibits higher double bond conversion (Pf ). 3.3. Dynamic mechanical thermal properties Fig. 8. Unsaturation conversion of the resins with different contents of HBPPA versus irradiation time.
From Fig. 7, it is obvious that the photopolymerization rate, similar to that of other acrylate systems reported in the literature [12], shows a steep increase at the beginning of irradiation, reaching to the maximum rate, Rpmax , and then drops rapidly; besides, as shown in Fig. 9, the Rpmax values increase with increasing content of DABP according to the higher concentration of double bond of DABP (the double bond concentration of DABP is 5.65 mmol/g compared with 1.2 mmol/g for HBPPA). However, from Fig. 8 and the curve of Pf in Fig. 9, an opposite trend can be observed for Pf compared with Rpmax . The Pf increases from 54.6% to 81.0% with increasing HBPPA content in DABP/HBPPA blend. As described above, the concentration of double bond of DABP is much higher than that of HBPPA. Accordingly, during the curing process the gelation of the DABPrich blends occurs earlier than that with less content of DABP to form the three-dimensional gel structure, which can restrict the diffusion and mobility of macroradicals and pendant double bonds in the microgel. As a result, the radical termination rate decreases and the polymerization rate increases rapidly. As the reaction proceeds, the higher crosslinking level eventually limits the mobility of monomers and oligomers as well as the radicals, which means the propagation reaction is controlled by the diffusion of fragments in the system. The reactive species are trapped in the crosslink network and the polymerization rate begins to decrease till the reaction finally stops. This implies the double bonds trapped in the
Fig. 9. Rpmax and Pf changes with different contents of HBPPA.
The dynamic mechanical thermal analysis (DMTA) was utilized to investigate the dynamic mechanical behavior of the UV cured films. The crosslinking density of a polymer can be estimated from the plateau of elastic modulus in its rubbery state by DMTA. As shown in Table 2, the crosslinking density increases from 2.3 to 10.2 mmol/cm3 along with the increase of DABP content. Considering with the chemical structure of the network, the behavior is expected because of the lower concentration of double bond and a long flexible molecular chain of HBPPA, which usually lead to looser network. The incorporated hyperbranched polyphosphonate can reduce the rigidity of the polymer chains to make chain motion possible at lower temperature. Fig. 10 also shows the plots of loss factor (Tan ı) versus temperature. The glass transition temperature (Tg ) of crosslinked material can be detected as the relaxation peak of the loss factor. As can be seen from Fig. 10, there is an obvious decrease in Tg with increasing HBPPA content (from 120 to 25 ◦ C, as listed in Table 2), which can be ascribed to the lower crosslinking density and a higher level of flexibility and mobility of HBPPA chains. Moreover, the analysis for the height and width of relaxation peak shows the trends in the crosslinking density and network homogeneity. The value of Tan ı is the ratio of viscous component to elastic component. And the height of the peak is usually associated with the segmental mobility and relaxing species; that means, the higher peak often indicates the higher segmental mobil-
Fig. 10. DMTA curves of the cured samples with different DABP contents.
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Fig. 11. TGA curves of the cured samples with different DABP contents.
ity and more relaxing species. As seen from Fig. 10, the peak of Tan ı increases with increasing HBPPA content, which indicates that the networks for the HBPPA-rich samples are looser. Ts is the softening point defined as the extrapolated onset of the drop of storage modulus, as shown in Table 2. The Ts /Tg ratio expresses the width of Tan ı peak by the rule that a higher Ts /Tg ratio often leads to a narrower Tan ı peak [13,14]. The Ts /Tg values of all samples are listed in Table 2, as seen that the values remain with 0.95 approximately. The uniformity of the Tan ı peak indicates the absence of any network heterogeneity, which also implies the good miscibility between DABP and HBPPA. 3.4. Thermal degradation behavior The TGA curves of HBPPA/DABP blends in air are shown in Fig. 11, and the data are listed in Table 3. There are three characteristic temperature regions observed. The first region can be assigned to the decomposition of phosphonate, whereas the second is due to the thermal pyrolysis of side chains and the formation of char. The third region is attributed to the decomposition of unstable structures in the formed char [15,16]. The chemical structural changes during thermal degradation are demonstrated by the following in situ FTIR analysis. As seen from Fig. 11, it is obvious that the UV cured film with more DABP content exhibits relatively higher thermostability at lower temperature compared with the film with less DABP. This can be explained by that all the cured films possess similar crosslinked networks, whereas the cured film with more DABP content possesses higher crosslinking density due to the higher double bonds concentration, as described above. That means the increased DABP content results in the lower segmental mobility and less relaxing species; consequently, the networks for the HBPPA-rich samples are looser and have lower initial decomposition temperatures as listed in Table 3. Whereas, the DABP-rich samples exhibit
Fig. 12. FTIR spectra of cured DABP00 film during the thermal degradation in the range of RT to 500 ◦ C.
higher efficiency in char formation at elevated temperature for the higher content of phosphorus. 3.5. Thermal degradation mechanism The chemical structure changes during the thermal degradation of cured DABP00 and DABP40 were monitored by in situ FTIR. The spectra are shown in Figs. 12 and 13, respectively. As seen from the graphs, the curves show little difference because of the similar molecular structure that HBPPA and DABP possess. Moreover, the crosslinking network from DABP00 and DABP40 were combined by the same chemical bond. The main peaks and bands of cured films are [16]: 1. 2. 3. 4.
2958 cm−1 the stretching vibration of C H bond; 1733 cm−1 , the stretching vibration of C O bond; 1450 cm−1 , the deformation vibration of C H bond; 1260 cm−1 , the stretching vibration of P O bond;
Table 3 Thermogravimetric analysis data and flame retardancy of the cured samples. Sample
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
Temperature recorded at specific weight loss (◦ C) 10%
80%
219 237 239 242 245 289
657 665 628 607 529 529
Residue (%) (800 ◦ C)
LOI
2.5 3.9 4.4 6.2 7.5 10.1
36 36 37 39 37 36
Fig. 13. FTIR spectra of cured DABP40 film during the thermal degradation in the rang of RT to 500 ◦ C.
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Fig. 14. Photographs of the sample after and before combustion: (a) and (A) DABP; (b) and (B) BPPA20 DABP80 ; (c) and (C) HBPPA40 DABP60 ; (d) and (D) HBPPA60 DABP40 ; (e) and (E) HBPPA80 DABP20 ; (f) and (F) HBPPA.
5. 1170 cm−1 , the stretching vibration of C O C bond; 6. 1040 and 980 cm−1 , the stretching vibration of P O C bond; 7. 1080 and 880 cm−1 , the symmetric and asymmetric vibration of P O P bond. As described above, the decomposition of cured HBPPA film is considered to be divided into three stages: firstly the degradation of phosphonate group, then ester group and alkyl chain, finally unstable structures in the formed char. The result can be further demonstrated by their in situ FTIR spectra. As seen from the graphs, the quick decrease of the absorption peak at 1040 and 980 cm−1 and its disappearance completely over 300 ◦ C indicate the degradation of P O C group, which means the phosphonate group degraded at lower temperature. In addition, the decrease of the P O absorption peak at 1260 cm−1 and the new peak appeared
at 1290 cm−1 when raising temperature over 330 ◦ C indicate the existence of poly(phosphoric acid) at high temperature. The new peak at 1147 cm−1 is assigned to the stretching vibration of P O C and PO2 /PO3 in phosphate–carbon complexes; moreover, the peaks appeared at 1080 and 881 cm−1 are assigned to the symmetric and asymmetric stretching vibration of P O P bond. This indicates that some phosphate groups link to each other by sharing one oxygen atom, resulting in the formation of poly(phosphoric acid), such as P2 O5 and P4 O10 [17–19]. Besides, it can be seen that the absorbance at 1170 cm−1 for C O C bond in DABP40 decreases more slowly than that in DABP00 when raising the temperature, which means the ester group in the cured film becomes more stable with DABP addition because of the increased crosslinking density. This result can also be demonstrated by the same degradation tendency for C O bond at 1733 cm−1 in two samples.
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3.6. Flammability The LOI value, the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support the combustion, can be used as an indicator to evaluate the flame retardancy of a polymer. As listed in Table 3, the LOI values of the cured samples remain over 36.0 with a slight increase form 36.0 to 39.0 and then decrease to 36.0. The expanding charred crusts formed after the DABP/HBPPA blends burned are shown in Fig. 14. It can be seen the degree of expansion increases with increasing HBPPA content. As reported in the previous work in our laboratory [5,20–22], the flame retardants containing phosphorus and nitrogen generally possess a synergistic effect between phosphorus and nitrogen as they are burning; the combustion of phosphorus content form a compact char while the nitrogen-containing compounds decompose to yield unflammable gases, causing the char to expand and hence provide insulating crusts. In the condensed phase mechanism of a flame retardant, the char formation can reduce the combustible volatiles; moreover, the larger the degree of expansion, the thicker the insulating layer will be formed to protect the underlying material from burning. Therefore, the LOI value increases primarily along with the increase of HBPPA content. However, with the continuous increasing of HBPPA content, the char yield decreases and the exorbitant nitrogen content would destroy the char structure, for which the LOI value of the resin decreases. That is, when the sample contains 40% HBPPA, the synergistic effect of phosphorus and nitrogen has the most distinct influence to the cured film, resulting in a peak value of LOI. 4. Conclusions A novel hyperbranched polyphosphonate acrylate (HBPPA) has been synthesized and characterized successfully. HBPPA was blended with DABP in different ratios to obtain a series of UV curable resins. The photopolymerization kinetics study shows that the Rpmax decreases with increasing HBPPA content in the HBPPA/DABP blend due to the lower concentration of double bond. The Pf increases linearly from 54.6% to 81.0%, resulting from that HBPPA has much longer spacer chain than DABP. The data from the dynamic mechanical thermal analysis shows that the crosslinking density and Tg of the cured film decrease along with the content of HBPPA; moreover, it is found that HBPPA has good miscibility with DABP.
The result of LOI shows that the cured samples greatly expanded when burning, and the LOI values increased from 36.0 to 39.0 and then decreased to 36.0 along with the increase of DABP, which was controlled by the two factors: the degree of expansion and the final char yield. The TGA and RT-FTIR results show that the cured films exhibit relatively lower thermostability at lower temperature, which is related with the content of phosphorus and the crosslinking density in the cured film. Acknowledgement The authors 50633010).
gratefully
acknowledge
NSFC
Project
(No.
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Photopolymerization behaviors of hyperbranched polyphosphonate acrylate and properties of the UV cured film Hailong Wang 1 , Songpan Xu 1 , Wenfang Shi ∗ State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
a r t i c l e
i n f o
Article history: Received 19 December 2008 Received in revised form 16 March 2009 Accepted 18 March 2009 Keywords: UV curing Hyperbranched Flame retardant Phosphorus Thermal degradation
a b s t r a c t A novel hyperbranched polyphosphonate acrylate (HBPPA), used as a reactive-type flame retardant in UV curable systems, was successfully synthesized by the reaction of di(acryloyloxyethyl) benzenephosphonate (DABP) with N-(2-aminoethyl)-piperazine, and characterized by FTIR, 1 H NMR and GPC measurements. HBPPA was blended with DABP as a monomer in different ratios to obtain a series of flame retardant resins. Their maximum photopolymerization rates (Rpmax ) and final unsaturation conversion (Pf ) in the cured films in the presence of a photofragmenting initiator were investigated. The results showed that the Pf increased along with HBPPA content and the pure HBPPA has the maximum value of 81.0% in the photo-DSC analysis. Their flame retardancy was monitored by the limiting oxygen index (LOI), and showed that the UV cured films greatly expanded when burning, and the degree of expansion increased along with HBPPA content. However, the LOI values varied from 36.0 to 39.0, which can be ascribed to the condensed phase mechanism. Their thermal degradation behaviors were investigated by thermogravimetric analysis and in situ FTIR spectroscopy, and showed that the phosphonate group of HBPPA first degraded to form poly(phosphoric acid)s at around 300 ◦ C, which had a major contribution to form the compact char to protect the sample from further degradation. The dynamic mechanical thermal properties were studied by dynamic mechanical thermal analysis and showed a good miscibility between HBPPA and DABP. The crosslinking density and Tg of the cured films decreased along with the content of HBPPA in the blend. © 2009 Elsevier B.V. All rights reserved.
1. Introduction UV curing systems have been widely used in coatings, inks, and adhesives, due to a lot of benefits over conventional thermalcuring systems such as rapid cure, low energy consumption, high efficiency, and low VOCs [1–3]. This technology is satisfying with new requirements for traditional or advanced applications, since it can offer a broad range of the changes in formulation and curing conditions; besides, the UV cured films exhibit desired hardness, excellent chemical resistance and low shrinkage. However, because the traditional UV cured products consisted of acrylates are generally flammable, it demands to develop flame retardant systems to reduce the fire hazards in the applications [4]. Compared with traditional halogen-type flame retardants, halogen-free flame retardants, especially that phosphorus/ nitrogen-containing compounds, have received much attention for the absence of toxic gases and smokes during combustion. Their flame retardant mechanism has been widely investigated as the
∗ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi). 1 These authors contributed equally to this paper. 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.03.005
condensed phase as well as synergy effects of phosphorous and nitrogen. A compact expanding char was formed at the beginning of burning, which decreases the amount of flammable volatile gases reaching the flame zone, accordingly to protect the polymeric material from further degradation and combustion [5]. As the lower viscosity and higher curing rate compared to linear counterparts with similar molecular weight, hyperbranched polymers have been reported to be possible to apply for UV curable coatings. Traditionally, hyperbranched polymers are prepared mainly by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups. Kakimoto and co-workers [6] developed a novel approach concerning the polycondensation of a diamine (A2 ) with trimesic acid (B3 ). However, if the functional groups in a monomer possess equal reactivity, nonlinear polymers and infinite networks as manifested by gelation are formed with gel point being predictable. Moreover, the approach for preparing hyperbranched polymers from a B B2 type monomer and an A2 type monomer has been developed [7–9]. In the B B2 monomer there are one B functional group and two B functional groups that have different reactivity with A group due to different chemical environment. In Flory’s gelation theory, if the B groups have different reactivity, soluble hyperbranched polymers with high molar mass can be obtained.
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Therefore, in this work we report the synthesis of a novel phosphorus–nitrogen containing hyperbranched polyphosphonate acrylate (HBPPA) used as a flame retardant oligomer in UV-coatings by employing an A2 +B B2 type polycondensation. DABP was synthesized as an A2 component. N-(2-aminoethyl)-piperazine was used as a B B2 component. The synthesized HBPPA was characterized with FTIR and 1 H NMR spectroscopy. The molecular weight and its distribution were determined by gel permeation chromatography (GPC). The photopolymerization kinetics, the dynamic mechanical thermal properties and mechanical behaviors of the cured films were also investigated. The flame retardancy of the UV cured films was characterized by the limiting oxygen index (LOI) and the thermostability was characterized by TGA and in situ FTIR analysis. 2. Experimental 2.1. Materials Na2 SO4 , NaHCO3 , Na2 S2 O3 , toluene, methylene dichloride and N-(2-aminoethyl)-piperazine were used as received. Phenylphosphonic dichloride (BPOD) and triethylamine (TEA) were distilled prior to use. All the above chemicals were purchased from Shanghai First Reagent Co., China. Hydroxylethyl acrylate (HEA), supplied from Dong-fang chemical Co. Beijing, China, was distilled under vacuum and dried over a 4 Å molecular sieve before use. ␣-Hydroxyketone (Iragcure 184), by Runtec Chemical Co., Ltd., China, was used as a photoinitiator. 2.2. Synthesis of di(acryloyloxyethyl) benzenephosphonate (DABP) Into a 500 mL round-bottom flask equipped with a mechanical stirrer, 150 mL of toluene solution containing 55.73 g HEA (0.48 mol) and 48.57 g TEA (0.48 mol) was added. Then a solution containing 39.00 g BPOD (0.2 mol) and 80 mL of toluene was added into the above reactant dropwise through an addition funnel under stirring at 0 ◦ C using an ice bath, and kept at ambient temperature for 10 h after finishing the addition. The formed triethylamine hydrochloride salt was removed by filtration. The filtrate was extracted by HCl (1 M), NaHCO3 (saturated) aqueous solution and distilled water twice, and then dried over sodium sulfate. After solvent being removed under vacuum, a colorless liquid product was obtained and named DABP. The schematic outline of synthesis route for DABP is given in Scheme 1.
2.3. Synthesis of HBPPA 5.16 g (0.05 mol) N-(2-aminoethyl)-piperazine was dissolved in 200 mL of chloroform using a 500 mL round-bottomed flask. 80 mL of chloroform solution containing 32.30 g DABP was added into the above reactant dropwise through an addition funnel, and left stirring for another 30 h at room temperature, followed by removing chloroform under vacuum, and finally obtained a lightly yellow liquid product, hyperbranched polyphosphonate acrylate, named HBPPA. The schematic outline of synthesis route for HBPPA is given in Scheme 2. 2.4. Sample preparation The mixtures of DABP and HBPPA in different ratios together with 1.5 wt% Irgacure 184 were stirred until the homogenous blends formed and then exposed to a medium pressure mercury lamp (1 kW, Fusion UV systems, USA) to obtain the cured films, denoted as DABP00, DABP20, DABP40, DABP60, DABP80, and DABP100, according to the ratio of DABP to HBPPA. The compositions of the resins are listed in Table 1. 2.5. Measurements The FTIR spectra were recorded with an MAGNA-IR 750 spectrometer (Nicolet Instrument Co., USA). The in situ FTIR spectra at different temperatures were recorded to monitor the thermodegradation process of the cured films. The temperature was raised from 200 to 450 ◦ C with a heating rate of 2 ◦ C min−1 , and kept at setting temperature for 10 min. The 1 H NMR spectra and 13 C NMR spectrum were recorded with a DMX-300 MHz instrument (Bruker, Switzerland). The thermogravimetric analysis (TGA) was carried out on a Shimadzu TG-50 instrument using a heating rate of 10 ◦ C min−1 in air. The molecular weight distribution was determined by gel permeation chromatography (GPC) with two linear Styragel columns HT3, HT4 and a column temperature of 60 ◦ C. A Waters 1515 pump and a Waters 2414 differential refractive index detector (set at 30 ◦ C) were used. The eluent was DMF with 1 g L−1 BrLi at a flow rate of 1.0 mL min−1 . The tensile storage modulus (E ) and tensile loss factors (Tan ı) of the cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 5 Hz and a heating rate of 10 ◦ C min−1 in the range of −50 to 160 ◦ C on the sheets of 25 mm × 5 mm × 1 mm. The crosslink density (e ) as the molar number of elastically effective network chain per cube centimeter of the coating was calculated from the storage modulus in the rubbery plateau region according to [10]: e = E /3RT , where E is the elastic storage modulus, R is the ideal gas constant, and T is the temperature in Kelvin. The mechanical properties were measured with an Instron Universal tester (model 1185, Japan) at 25 ◦ C with a crosshead speed of 25 mm min−1 . The photopolymerization rate was monitored in air by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system BHG-250 (Mejiro Precision Co., Japan). The incident light intensity Table 1 Compositions of the resins.
Scheme 1. Schematic illustration for the synthesis of DABP.
Sample
Initial mixture
Weight ratio
Phosphorus content (%)
Nitrogen content (%)
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
HBPPA DABP/HBPPA DABP/HBPPA DABP/HBPPA DABP/HBPPA DABP
– 0.20/0.80 0.40/0.60 0.60/0.40 0.80/0.20 –
7.39 7.66 7.93 8.20 8.47 8.74
5.02 4.01 3.01 2.01 1.00 0
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Scheme 2. Schematic illustration for the synthesis of HBPPA.
at the sample pan was measured to be 2.4 mW cm−2 with a UV power meter. The unsaturation conversion (Pt ) was calculated by the formula, Pt = Ht /H∞ , where Ht is the heat effect within t seconds, H∞ is the heat effect of 100% unsaturation conversion. The DSC curves were unified by the weight (g) of samples. The polymerization rate is defined by mmolc c g−1 s−1 , namely, the variation of unsaturation concentration (mmolc c g−1 ) per second. For calculating the polymerization rate and H∞ , the value, H0 = 86 J mmol−1 , for the heat of polymerization per acrylic unsaturation was taken. The limiting oxygen index (LOI) values of UV cured films were measured using a ZRY-type instrument (made in Jiangning, China) with the sheets of 120 mm × 6.5 mm × 3 mm according to the standard ASTM D635-77.
3. Results and discussion 3.1. Characterization The synthesis of DABP was performed by the reaction of BPOD and HEA, as illustrated in Scheme 1. The chemical structure of DABP was characterized with FTIR, 1 H NMR and 13 C NMR. As shown in Fig. 1, the FTIR spectrum of DABP exhibits characteristic absorption at 1251 cm−1 corresponding to vibration with P O bond. The absorption bands observed at 1732, 1636, 1410, and 810 cm−1 demonstrate the existence of acrylate group. The 1 H NMR and 13 C NMR spectra of DABP show the resonances corresponding to all protons and carbons of the given structure, as shown in Figs. 2 and 3, respectively. Both IR and NMR results are consistent with the expected molecular structure. The synthesis of HBPPA was performed by the Michael addition reaction between DABP and N-(2-aminoethyl)-piperazine, as shown in Scheme 2. The FTIR spectrum of HBPPA, as given in Fig. 4, shows the strong absorption at 1255 cm−1 (for P O) and 1040, 980 cm−1 (for P O C), revealing the formation of phosphonate
Fig. 1. FTIR spectrum of DABP.
Fig. 2.
1
H NMR spectrum of DABP.
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Fig. 3.
13
C NMR spectrum of DABP.
structure. Moreover, the absorption bands observed at 1732, 1635, 1410, and 810 cm−1 indicate the existence of acrylate group. The 1 H NMR spectrum of HBPPA with the assignments as shown in Fig. 5, shows three groups of characteristic peaks at 5.80–6.60 ppm, which prove the existence of acrylate group in HBPPA. The peaks observed at 2.20–3.00 ppm are assigned to the H atoms (Ha , Hb , Hc , Hd , He and Hf ), the substituent groups attached to the same carbon are nitrogen atoms; the peaks, assigned with Hg and Hh that the substituent group attached to the same carbon is carbonyl group, can be observed at 4.00–4.40 ppm. The peaks observed at 7.20–8.00 ppm are obviously signs of H atoms attached with phenyl group. As shown in Fig. 6, the number average molecular weight (Mn ) of HBPPA was measured experimentally to be 4200 g mol−1 by GPC using DMF as an eluent, and its polydispersity is 1.29.
Fig. 5.
1
H NMR spectrum of HBPPA.
3.2. Photopolymerization kinetics The photopolymerization kinetics, which can be characterized by the peak maximum (Rpmax ) and the final degree of double bond conversion (Pf ), is one of the important parameters for characterizing the UV curing process [11]. Figs. 7 and 8 show the photopolymerization rates and unsaturation conversion versus irradiation time obtained from Photo-DSC measurements at room temperature, by which the changes in Rpmax and Pf were obtained, as shown in Fig. 9.
Fig. 4. FTIR spectrum of HBPPA.
Fig. 6. GPC trace of HBPPA.
Fig. 7. Photopolymerization rates of the resins with different contents of HBPPA versus irradiation time.
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Table 2 DMTA results and crosslinking density of the cured samples. Sample
Ts (◦ C)
Tg (◦ C)
Ts /Tg
e (mmol/cm3 )
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
16 20 40 64 68 101
25 30 51 82 90 120
0.97 0.97 0.97 0.95 0.94 0.95
2.3 3.0 3.8 5.0 6.8 10.2
early-formed gels could hardly take part in further polymerization [13]; on the contrary, the lower concentration of double bonds can reduce the curing rate of the sample with low content of DABP, and thus prevent the gel-point of system appearing prematurely. As a result, there are fewer double bonds trapped in the crosslink network that could take part in further polymerization. This is the key point that the sample contained less DABP exhibits higher double bond conversion (Pf ). 3.3. Dynamic mechanical thermal properties Fig. 8. Unsaturation conversion of the resins with different contents of HBPPA versus irradiation time.
From Fig. 7, it is obvious that the photopolymerization rate, similar to that of other acrylate systems reported in the literature [12], shows a steep increase at the beginning of irradiation, reaching to the maximum rate, Rpmax , and then drops rapidly; besides, as shown in Fig. 9, the Rpmax values increase with increasing content of DABP according to the higher concentration of double bond of DABP (the double bond concentration of DABP is 5.65 mmol/g compared with 1.2 mmol/g for HBPPA). However, from Fig. 8 and the curve of Pf in Fig. 9, an opposite trend can be observed for Pf compared with Rpmax . The Pf increases from 54.6% to 81.0% with increasing HBPPA content in DABP/HBPPA blend. As described above, the concentration of double bond of DABP is much higher than that of HBPPA. Accordingly, during the curing process the gelation of the DABPrich blends occurs earlier than that with less content of DABP to form the three-dimensional gel structure, which can restrict the diffusion and mobility of macroradicals and pendant double bonds in the microgel. As a result, the radical termination rate decreases and the polymerization rate increases rapidly. As the reaction proceeds, the higher crosslinking level eventually limits the mobility of monomers and oligomers as well as the radicals, which means the propagation reaction is controlled by the diffusion of fragments in the system. The reactive species are trapped in the crosslink network and the polymerization rate begins to decrease till the reaction finally stops. This implies the double bonds trapped in the
Fig. 9. Rpmax and Pf changes with different contents of HBPPA.
The dynamic mechanical thermal analysis (DMTA) was utilized to investigate the dynamic mechanical behavior of the UV cured films. The crosslinking density of a polymer can be estimated from the plateau of elastic modulus in its rubbery state by DMTA. As shown in Table 2, the crosslinking density increases from 2.3 to 10.2 mmol/cm3 along with the increase of DABP content. Considering with the chemical structure of the network, the behavior is expected because of the lower concentration of double bond and a long flexible molecular chain of HBPPA, which usually lead to looser network. The incorporated hyperbranched polyphosphonate can reduce the rigidity of the polymer chains to make chain motion possible at lower temperature. Fig. 10 also shows the plots of loss factor (Tan ı) versus temperature. The glass transition temperature (Tg ) of crosslinked material can be detected as the relaxation peak of the loss factor. As can be seen from Fig. 10, there is an obvious decrease in Tg with increasing HBPPA content (from 120 to 25 ◦ C, as listed in Table 2), which can be ascribed to the lower crosslinking density and a higher level of flexibility and mobility of HBPPA chains. Moreover, the analysis for the height and width of relaxation peak shows the trends in the crosslinking density and network homogeneity. The value of Tan ı is the ratio of viscous component to elastic component. And the height of the peak is usually associated with the segmental mobility and relaxing species; that means, the higher peak often indicates the higher segmental mobil-
Fig. 10. DMTA curves of the cured samples with different DABP contents.
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Fig. 11. TGA curves of the cured samples with different DABP contents.
ity and more relaxing species. As seen from Fig. 10, the peak of Tan ı increases with increasing HBPPA content, which indicates that the networks for the HBPPA-rich samples are looser. Ts is the softening point defined as the extrapolated onset of the drop of storage modulus, as shown in Table 2. The Ts /Tg ratio expresses the width of Tan ı peak by the rule that a higher Ts /Tg ratio often leads to a narrower Tan ı peak [13,14]. The Ts /Tg values of all samples are listed in Table 2, as seen that the values remain with 0.95 approximately. The uniformity of the Tan ı peak indicates the absence of any network heterogeneity, which also implies the good miscibility between DABP and HBPPA. 3.4. Thermal degradation behavior The TGA curves of HBPPA/DABP blends in air are shown in Fig. 11, and the data are listed in Table 3. There are three characteristic temperature regions observed. The first region can be assigned to the decomposition of phosphonate, whereas the second is due to the thermal pyrolysis of side chains and the formation of char. The third region is attributed to the decomposition of unstable structures in the formed char [15,16]. The chemical structural changes during thermal degradation are demonstrated by the following in situ FTIR analysis. As seen from Fig. 11, it is obvious that the UV cured film with more DABP content exhibits relatively higher thermostability at lower temperature compared with the film with less DABP. This can be explained by that all the cured films possess similar crosslinked networks, whereas the cured film with more DABP content possesses higher crosslinking density due to the higher double bonds concentration, as described above. That means the increased DABP content results in the lower segmental mobility and less relaxing species; consequently, the networks for the HBPPA-rich samples are looser and have lower initial decomposition temperatures as listed in Table 3. Whereas, the DABP-rich samples exhibit
Fig. 12. FTIR spectra of cured DABP00 film during the thermal degradation in the range of RT to 500 ◦ C.
higher efficiency in char formation at elevated temperature for the higher content of phosphorus. 3.5. Thermal degradation mechanism The chemical structure changes during the thermal degradation of cured DABP00 and DABP40 were monitored by in situ FTIR. The spectra are shown in Figs. 12 and 13, respectively. As seen from the graphs, the curves show little difference because of the similar molecular structure that HBPPA and DABP possess. Moreover, the crosslinking network from DABP00 and DABP40 were combined by the same chemical bond. The main peaks and bands of cured films are [16]: 1. 2. 3. 4.
2958 cm−1 the stretching vibration of C H bond; 1733 cm−1 , the stretching vibration of C O bond; 1450 cm−1 , the deformation vibration of C H bond; 1260 cm−1 , the stretching vibration of P O bond;
Table 3 Thermogravimetric analysis data and flame retardancy of the cured samples. Sample
DABP00 DABP20 DABP40 DABP60 DABP80 DABP100
Temperature recorded at specific weight loss (◦ C) 10%
80%
219 237 239 242 245 289
657 665 628 607 529 529
Residue (%) (800 ◦ C)
LOI
2.5 3.9 4.4 6.2 7.5 10.1
36 36 37 39 37 36
Fig. 13. FTIR spectra of cured DABP40 film during the thermal degradation in the rang of RT to 500 ◦ C.
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Fig. 14. Photographs of the sample after and before combustion: (a) and (A) DABP; (b) and (B) BPPA20 DABP80 ; (c) and (C) HBPPA40 DABP60 ; (d) and (D) HBPPA60 DABP40 ; (e) and (E) HBPPA80 DABP20 ; (f) and (F) HBPPA.
5. 1170 cm−1 , the stretching vibration of C O C bond; 6. 1040 and 980 cm−1 , the stretching vibration of P O C bond; 7. 1080 and 880 cm−1 , the symmetric and asymmetric vibration of P O P bond. As described above, the decomposition of cured HBPPA film is considered to be divided into three stages: firstly the degradation of phosphonate group, then ester group and alkyl chain, finally unstable structures in the formed char. The result can be further demonstrated by their in situ FTIR spectra. As seen from the graphs, the quick decrease of the absorption peak at 1040 and 980 cm−1 and its disappearance completely over 300 ◦ C indicate the degradation of P O C group, which means the phosphonate group degraded at lower temperature. In addition, the decrease of the P O absorption peak at 1260 cm−1 and the new peak appeared
at 1290 cm−1 when raising temperature over 330 ◦ C indicate the existence of poly(phosphoric acid) at high temperature. The new peak at 1147 cm−1 is assigned to the stretching vibration of P O C and PO2 /PO3 in phosphate–carbon complexes; moreover, the peaks appeared at 1080 and 881 cm−1 are assigned to the symmetric and asymmetric stretching vibration of P O P bond. This indicates that some phosphate groups link to each other by sharing one oxygen atom, resulting in the formation of poly(phosphoric acid), such as P2 O5 and P4 O10 [17–19]. Besides, it can be seen that the absorbance at 1170 cm−1 for C O C bond in DABP40 decreases more slowly than that in DABP00 when raising the temperature, which means the ester group in the cured film becomes more stable with DABP addition because of the increased crosslinking density. This result can also be demonstrated by the same degradation tendency for C O bond at 1733 cm−1 in two samples.
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3.6. Flammability The LOI value, the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support the combustion, can be used as an indicator to evaluate the flame retardancy of a polymer. As listed in Table 3, the LOI values of the cured samples remain over 36.0 with a slight increase form 36.0 to 39.0 and then decrease to 36.0. The expanding charred crusts formed after the DABP/HBPPA blends burned are shown in Fig. 14. It can be seen the degree of expansion increases with increasing HBPPA content. As reported in the previous work in our laboratory [5,20–22], the flame retardants containing phosphorus and nitrogen generally possess a synergistic effect between phosphorus and nitrogen as they are burning; the combustion of phosphorus content form a compact char while the nitrogen-containing compounds decompose to yield unflammable gases, causing the char to expand and hence provide insulating crusts. In the condensed phase mechanism of a flame retardant, the char formation can reduce the combustible volatiles; moreover, the larger the degree of expansion, the thicker the insulating layer will be formed to protect the underlying material from burning. Therefore, the LOI value increases primarily along with the increase of HBPPA content. However, with the continuous increasing of HBPPA content, the char yield decreases and the exorbitant nitrogen content would destroy the char structure, for which the LOI value of the resin decreases. That is, when the sample contains 40% HBPPA, the synergistic effect of phosphorus and nitrogen has the most distinct influence to the cured film, resulting in a peak value of LOI. 4. Conclusions A novel hyperbranched polyphosphonate acrylate (HBPPA) has been synthesized and characterized successfully. HBPPA was blended with DABP in different ratios to obtain a series of UV curable resins. The photopolymerization kinetics study shows that the Rpmax decreases with increasing HBPPA content in the HBPPA/DABP blend due to the lower concentration of double bond. The Pf increases linearly from 54.6% to 81.0%, resulting from that HBPPA has much longer spacer chain than DABP. The data from the dynamic mechanical thermal analysis shows that the crosslinking density and Tg of the cured film decrease along with the content of HBPPA; moreover, it is found that HBPPA has good miscibility with DABP.
The result of LOI shows that the cured samples greatly expanded when burning, and the LOI values increased from 36.0 to 39.0 and then decreased to 36.0 along with the increase of DABP, which was controlled by the two factors: the degree of expansion and the final char yield. The TGA and RT-FTIR results show that the cured films exhibit relatively lower thermostability at lower temperature, which is related with the content of phosphorus and the crosslinking density in the cured film. Acknowledgement The authors 50633010).
gratefully
acknowledge
NSFC
Project
(No.
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