Flame Retarding Cyanate Ester Resin with Low Curing Temperature

Article pubs.acs.org/IECR

Flame Retarding Cyanate Ester Resin with Low Curing Temperature, High Thermal Resistance, Outstanding Dielectric Property, and Low Water Absorption for High Frequency and High Speed Printed Circuit Broads Xiangxiu Chen, Guozheng Liang,* Aijuan Gu,* and Li Yuan Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application Department of Materials Science & Engineering College of Chemistry, Chemical Engineering and Materials Science Soochow University, Suzhou, Jiangsu 215123, P. R. China S Supporting Information *

ABSTRACT: High-end electric products require high frequency and high speed printed circuit broads (HFS-PCBs), while high performance resin is the key for fabricating HFS-PCBs. A new resin system (DPDP/CE) was developed by copolymerizing 10(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DPDP) and 2,2′-bis(4-cyanatophenyl) propane (CE). Compared with CE resin postcured at 250 °C for 4 h, the DPDP/CE system that was only postcured at 220 °C for 2 h has outstanding flame retardancy, greatly reduced water absorption, low dielectric loss, and high thermal resistance. For DPDP1.4/ CE resin with 1.4 wt % phosphorus content, its dielectric constant and loss at 1 GHz are 2.71 and 0.005, respectively, and hardly change after staying in boiling water for 100 h. Different from UL-94 V-2 rating of CE resin, the flame retardancy of DPDP1.4/ CE resin is desirable UL-94 V-0 rating, resulting from both gas-phase and condensed-phase mechanisms. These attractive features suggest that the DPDP/CE system is suitable to fabricate HFS-PCBs for high-performance electric products.

1. INTRODUCTION Printed circuit broads with higher frequency, higher speed, and better reliability (HFS-PCBs) have been urgently required by electric industry,1−3 so the corresponding resins for HFS-PCBs should have more and higher performances. This means that the resins not only should have better dielectric properties (lower dielectric constant and loss), better moisture resistance, high thermal stability, and good flame retardancy,4,5 but also should have a better curing feature (low curing temperature and short curing time) to guarantee the better properties that the given resin is expected to exhibit. Cyanate ester (CE) resin is famous for its extremely low dielectric constant and loss over wide temperature ranges as well as high thermal stability, so CE has been regarded as the right candidate with the highest competition for fabricating functional materials, especially HFS-PCBs.6,7 However, like almost all polymers, CE resin does not have good flame retardancy, which only belongs to V-2 level from the vertical burning UL-94 tests (UL-94 V). Besides, CE resin has low water absorption at room temperature, but this feature does not exhibit at high temperature, thus resulting in the corrosion of integrated circuits.8 In addition, CE resin should be cured at high temperature (230 °C or even higher) for a long time, so the final products tend to have big residual stress and poor reliability.9,10 More and more attention has been directed at developing high performance flame-retarding resins based on CE resins; however, very little literature has been reported, especially, no literature reported a resin for HFS-PCBs. Four kinds of flame retardant CE resins were reported by using phosphoruscontaining, bromine-containing, and silicone-containing flame retardants or inorganic fillers. Due to the harmfulness to the © 2015 American Chemical Society

environment, the use of halogenated compounds has been forbidden worldwide, and then phosphorus-containing flame retardants have been widely used to improve the flame retardancy of polymers including CE resin owing to their environmentally friendly feature and high flame retarding efficiency.11,12 Typically, Mathew’s13 group synthesized a kind of phosphazene-containing CE resin, of which the flame retardancy was tested to be UL-94 V-0 rating; however, the glass transition temperature (Tg) was only 160 °C, about 90 °C lower than the value of commercial CE resin. Lin and associates14 incorporated 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) into CE resin, when the phosphorus content is as large as 2.0 wt %, the modified CE resin showed a flame retardancy of UL-94 V-0 rating without changing the dielectric properties, but the Tg decreased about 79 °C. Moreover, the gel time at 170 °C remarkably decreased from 927 to 197 s; hence the reactivity of the modified CE resin is very high. This is not beneficial to be used in the large-scale product. Zhang et al.15 synthesized a kind of phosphoruscontaining CE resin, which was proved to be effective in improving the flame retardancy and gaining outstanding thermal resistance; however, it proved to be difficult in overcoming the harsh curing conditions. In recent years, our group has carried out lots of work on developing high performance flame retarding CE resin. One way is synthesizing phosphorus-containing hyperbranched polysiloxane (P-HSi), which proved to be effective in improving Received: Revised: Accepted: Published: 1806

November 2, 2014 January 17, 2015 January 20, 2015 January 20, 2015 DOI: 10.1021/ie504333f Ind. Eng. Chem. Res. 2015, 54, 1806−1815

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Industrial & Engineering Chemistry Research the flame retardancy, thermal resistance, and dielectric properties;16 however, the synthesis of P-HSi should be carefully controlled. Another approach for improving the flame retardancy of CE resin is the incorporation of hybridized expandable graphites or carbon nanotubes;17,18 however, these fillers are electric conductors, and will bring a remarkable increase of the dielectric constant and loss, and thus the modified CE resins are not suitable for fabricating PCBs. Therefore, developing a high performance CE resin that simultaneously exhibits low curing feature, high flame retardancy, good thermal and water resistance, low dielectric constant, and loss for HFS-PCBs is still a big challenge. Our research reported herein is to make progress on the above challenge. Specifically, a series of modified CE resins with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DPDP) were prepared, and the curing behavior and integrated performances of DPDP/CE resins were studied. Some attractive results were found, and the origin behind especially the flame retarding mechanism was intensively discussed.

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P nuclear magnetic resonance (31P NMR) spectra were obtained on a Bruker Avance 400 spectrometer (Germany) at 400 MHz, and DMSO-d6 was used as the solvent. A scanning electron microscope (Hitachi S-4700, Japan) coupled with energy disperse X-ray spectrometer (EDS) was employed to observe the morphologies of the fractured surfaces of chars. The resolution of the secondary electron image was 1.5 nm under 15 kV. All samples were dried at 105 °C for 6 h before tests. Dielectric constant and loss were performed on an Agilent E4991A measurement system (USA) at 30 °C with a twoparallel-plate mode over a wide frequency range from 1 MHz to 1 GHz. The dimensions of each sample were (50 ± 0.02) × (50 ± 0.02) × (1.0 ± 0.02) mm3, and all samples were dried under vacuum at 105 °C for 1 h before tests. Thermogravimetric (TG) analyses were conducted using a PerkinElmer TGA-7 (USA) at a heating rate of 10 °C/min with a flow rate of 100 mL/min in a nitrogen atmosphere. The initial decompose temperature (Tdi) was defined as the point of intersection of the tangent of the onset temperature, and the maximum degradation temperature (Tmax) was the tangent of the maximum degradation rate temperature. Thermogravimetric analysis-infrared spectrometry (TG-IR) was performed using TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer and FTIR spectrometer (Nicolet 6700, USA) to determine the vapor products of the sample during the decomposition. The sample was heated from 40 to 800 °C with a heating rate of 10 °C/min in a nitrogen atmosphere. DMA Q800 apparatus from TA Instruments (USA) was used to conduct the dynamic mechanical analysis (DMA) with a single cantilever clamp geometry and a heating rate of 3 °C/ min at 1 Hz. The dimensions of each sample were (35 ± 0.02) × (13 ± 0.02) × (3.0 ± 0.02) mm3. X-ray diffraction (XRD) patterns were obtained using a MERCURY charge-coupled device X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation at a scanning rate of 2 °/min. The water resistance was measured according to GB/10342008. In detail, a sample with dimensions of (50 ± 0.02) × (50 ± 0.020) × (4 ± 0.1) mm3 was put into boiling water (100 ± 1 °C) and maintained for a given length of time. After that, the sample was removed from the water, dried with filter paper, weighted, and then immersed in the water again; all these subsequent steps should be done quickly. The water absorption for each sample was taken as the average value of three specimens. UL-94 V tests were performed with ASTM D635-77, and the dimensions of each sample were (125 ± 0.02) × (13 ± 0.02) × (3.0 ± 0.02) mm3. For each system, five sample bars were tested. Limiting oxygen index (LOI) values were measured on a Stanton Redcraft Flame Meter (U.K.) according to ASTM D2863/77. The dimensions of each sample were (100 ± 0.02) × (6.5 ± 0.02) × (3 ± 0.02) mm3. Flammability of resins was characterized using a cone calorimeter performed in an FTT device (U.K.) according to ISO 5660 with an incident flux of 56 kW/m2. Typical results from cone calorimetry were reproducible to within about ±10%, and the data reported here were the averages of triplicate. The dimensions of each sample were (100 ± 0.02) × (100 ± 0.02) × (3 ± 0.02) mm3.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. CE used herein was 2, 2′-bis(4cyanatophenyl) propane made in Jiangdu Maiya Chemical Co. Ltd. of Jiangsu Province in China. DPDP (the content of phosphorus atom is 9.6 wt %) was purchased from Eutec Chemical Co., Ltd. (China). 2.2. Preparation of CE and DPDP/CE Prepolymers. CE was heated to 150 °C with stirring and maintained at that temperature for 2 h, which was CE prepolymer. Appropriate quantities of CE and DPDP were mixed with stirring at 150 °C until a clear and brown liquid was obtained, which was then kept at 150 °C for 2 h to obtain a homogeneous liquid, and coded as DPDPn/CE prepolymer, where n represents the content of phosphorus in the prepolymer, taking values of 0.5, 0.8, 1.1, and 1.4 wt %. 2.3. Preparation of Cured CE and DPDP/CE Resins. CE prepolymer was thoroughly degassed at 150 °C and poured into a preheated (150 °C) metal mold, followed by curing and postcuring using the procedure of 150 °C/2 h+180 °C/2 h +220 °C/2 h and 250 °C/4 h in an air oven, successively. After that, the cured sample was demolded and slowly cooled to room temperature. DPDP/CE resin was also prepared using the above process except that the curing and postcuring procedures were 150 °C/ 2 h+180 °C/2 h+220 °C/2 h and 220 °C/2 h, respectively. 2.4. Measurements. Differential scanning calorimeter (DSC) analyses were done using DSC 200 F3 (Netzsch, Germany) over the temperature range between 50 and 320 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The sample was prepared by thoroughly mixing DPDP and CE at room temperature for 30 min, and then the mixture was put into a polyethylene bag and ground into fine powders for DSC tests. The content of phosphorus in the DPDP/CE mixture was 0.5, 0.8, 1.1, or 1.4 wt %. Gel time at 170 °C was measured on a temperaturecontrolled hot plate by the standard knife method.19 The time required for the resin to stop legging and became elastic was recorded as the gel time. Fourier transform infrared (FTIR) (KBr pellets) spectra of resins were recorded between 400 and 4000 cm−1 with a resolution of 2 cm−1 on a Nicolet 5700 infrared spectrometer (USA) at 25 °C. 1807

DOI: 10.1021/ie504333f Ind. Eng. Chem. Res. 2015, 54, 1806−1815

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Industrial & Engineering Chemistry Research

compared with that in DPDP as shown in the 31P NMR spectra of DPDP and DPDP0.8/CE prepolymer (Figure 4), while the reaction between DPDP and CE is related to the loading of DPDP. Specifically, when the amount of DPDP is trace, the main role of DPDP is catalyzing CE to form triazinering structure (Figure 3b (1) and (2)); however, with a large content of DPDP, DPDP can also copolymerize with triazinering structure and form the phosphorus-containing product (Figure 3b (3)).10,21−23 The copolymers of phosphoruscontaining product may lead to reduced cross-linking density (Xdensity) compared with the value of CE resin, which can be further confirmed by the calculation based on DMA analyses below. On the other hand, as the concentration of CE is much larger than that of DPDP in the DPDP/CE system, the triazine structure is still the main product of the cross-linked structure. For a cured thermosetting resin, besides the chemical structure, the aggregation state structure that represents the arrangement and stack of molecular chains is an important aspect, which can be reflected by the Xdensity. Figure 5 gives the Xdensity values of CE and DPDP/CE resins calculated with a semiempirical equation (eq 1) based on DMA analyses.24

3. RESULTS AND DISCUSSION 3.1. Effect of DPDP Content on the Curing Behavior of CE and the Cross-Linked Structure. Figure 1 shows the gel

Figure 1. Gel times of CE and DPDP/CE prepolymers at 170 °C.

time at 170 °C for DPDP/CE prepolymers with various contents of phosphorus. The gel time decreases with the increases of the phosphorus content, indicating that DPDP/CE prepolymers have faster curing reaction than CE. This statement is further and fully confirmed by the DSC curves of CE, DPDP, and DPDP/CE mixtures as shown in Figure 2.

ρ = G′/3RT

(1)

where G′ is the storage modulus of the cured resin in the rubbery plateau region above Tg (Figure S1). Herein, G′ is chosen as the modulus at the absolute temperature T, which is 25 °C higher than Tg; R is the gas constant. All DPDP/CE resins have lower Xdensity values than the CE resin, and the more the DPDP content is, the lower the Xdensity value. This is expected as the coreaction product between DPDP and CE has lower compactness than the cyclotrimerization of CE. 3.2. Tg. Tg reflects the movement capacity of molecular chains with the increase of temperature, which is the maximum application temperature of a thermosetting resin.25 Tg is often defined as the peak (maximum) temperature in the tan δtemperature plot from DMA tests.26 As shown in Figure 6, the Tg values of DPDP1.1/CE and DPDP1.4/CE resins are 269 and 259 °C, about 95% and 91% of that of CE resin, respectively; however, DPDP0.5/CE and DPDP0.8/CE resins have similar Tg values as the CE resin. These results are interesting because the modified CE resins with phosphaphenanthrene have obviously decreased Tg values.13,14 In other words, the DPDP/CE system with suitable loading of DPDP still has outstanding Tg as CE resin. 3.3. Water Absorption. The water absorptions of the original and modified CE resins are shown in Figure 7. All DPDP/CE resins have much lower water absorptions than the CE resin. Especially, when the phosphorus content is as small as 1.4 wt %, its water absorption in boiling water for 100 h is only 0.23%, about 51% of the value of CE resin. This is an attractive feature for fabricating reliable electric products. The reason behind Figure 7 can be found from the FTIR spectra of cured original and modified CE resins (Figure 8). For the cured CE resin, the characterization peaks of the −OCN group (2235 and 2274 cm−1) are still observed, indicating that −OCN groups are not completely consumed; that is, CE is not completely cured even the postcuring temperature is as high as 250 °C. On the other hand, no absorption peaks of the −OCN group are found in the spectra of DPDP/CE resins, although the postcuring temperature is only 220 °C. As −OCN groups are easy to absorb water, it is reasonable to find that DPDP/CE resins have better water/

Figure 2. DSC traces of CE, DPDP, and DPDP/CE mixtures.

Each sample has an exothermic peak, while the whole peak shifts toward the direction of low temperature and becomes wider with the addition and the increase of the DPDP content. The peak temperature for the DPDP/CE mixture is about 21− 76 °C lower than that of CE. Furthermore, the DPDP/CE mixture has much smaller exothermic enthalpy (670.0−705.0 J/ g) than CE (721.0 J/g), and the exothermic enthalpy decreases as the content of DPDP increases. All these results demonstrate that, different from CE resin, the DPDP/CE system has a moderate curing reaction with reduced curing temperature; this is attractive for large-scale production. It is known that the curing mechanism of CE is the cyclotrimerization of −OCN groups (Figure 3a).20 This also exists in the DPDP/CE system; besides, there are additional chain extensions through forming iminocarbonate from the reaction between hydroxyl groups in DPDP and triazine rings (Figure 3b).10 This is also proved by the different chemical shift of the phosphorus atom in the DPDP/CE prepolymer 1808


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Figure 3. Cyclotrimerization of −OCN groups (a) and the coreactions between DPDP and CE (b).

moisture resistance than the CE resin. The good water resistance further plays a positive influence on the remaining dielectric properties for samples after staying in the boiling water for 100 h as discussed below. 3.4. Dielectric Properties. Figure 9 shows dependence of the dielectric constant and loss on frequency for various resins. All DPDP/CE resins have lower dielectric constant and loss than CE resin, for example, the dielectric constant and loss at 1 GHz of DPDP1.4/CE resin are 2.71 and 0.005, about 94% and 89% of that of the CE resin, respectively. The dielectric properties of a resin are known to be dependent on the orientation and relaxation of dipoles, so the presence and the number of polar groups play a key role on the dielectric properties. DPDP contains hydroxyl groups, which will increase

Figure 4. 31P NMR spectra of DPDP and DPDP0.8/CE prepolymer.

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Figure 5. Cross-linking densities of CE and DPDP/CE resins.

Figure 8. FTIR spectra of DPDP, cured CE, and DPDPn/CE resins.

Figure 6. Overlay curves of tan δ vs temperature of cured CE and DPDP/CE resins.

Figure 9. Dependence of dielectric constant (a) and loss (b) on frequency for original and treated resins in boiling water for 100 h. Figure 7. Water absorptions of cured CE and DPDP/CE resins in boiling water for 100 h.

properties over the whole frequency range tested as original ones; however, both the dielectric constant and loss of treated samples are bigger than those of the original resins because of the existence of absorbed water. Note that compared with treated samples, the increased degree in dielectric properties of DPDP/CE resins is much lower than that of the CE resin. For example, the increased degrees in dielectric constant and loss of DPDP/CE resins at 1 GHz are 2.6−3.5% and 0.4−3.2%, respectively, while those of the CE resin are 3.8% and 10.5%, clearly suggesting that the DPDP/CE resins have much better dielectric properties and stability after water absorption. 3.5. Flame Retardancy and Mechanism. Three classic methods were used to evaluate the flame retardancy of DDPDP/CE resins, and the typical data are summarized in

the dielectric constant and loss; however, on the other hand, as discussed above, the hydroxyl group can catalyze −OCN to form triazine-ring structure, and triazine-ring structure has low dielectric constant and loss due to its symmetric structure.27 In addition, the cured CE resin still has −OCN groups, while DPDP/CE resins do not have. Therefore, the addition of DPDP has two competitive effects on the dielectric properties of the resins, and the data shown in Figure 9 suggest that the positive effect plays the domain role. For the samples after treating in the boiling water for 100 h, all treated resins also exhibit very good stability of dielectric 1810


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Industrial & Engineering Chemistry Research Table 1. Typical Flame Retarding Data of Cured Resins from Three Tests UL-94 V resin

level

Max (t1, t2)

t1+t2

LOI (%)

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

EHC (MJ/kg)

MLR (g/s)

CE DPDP0.5/CE DPDP0.8/CE DPDP1.1/CE DPDP1.4/CE

V-2 V-2 V-1 V-1 V-0

100 69 30 14 8

246 152 89 61 43

25 29 38 40 42

59 64 66 68 87

384 277 245 227 208

35 31 29 28 26

18.0 14.5 13.0 12.0 11.5

0.053 0.046 0.034 0.028 0.025

toward the right, suggesting that DPDP/CE resins are more difficult to be ignited than the cured CE resin, and the results are consistent with the TTI values shown in Table 1. The result is contrary to those previously reported,29,30 in which DOPO modified resins have shorter TTI than the original resins. The difference between DOPO and DPDP is that DPDP has an additional phenyl-ring with nonflammable feature, and this is beneficial to increase the TTI.31 To reveal the above data, the integrated changes of effective combustion heat (EHC) and mass loss rate (MLR) are often used.32 As shown in Table 1 and the curves of mass loss versus time for DPDP/CE and CE resins (Figure 11), the EHC and

Table 1. All DPDP/CE resins have higher LOI values than the CE resin, and as the phosphorus content increases, the LOI value of the DPDP/CE resin increases. For example, the LOI of DPDP1.4/CE resin is about 1.7 times that of CE resin, indicating that the DPDP/CE resins have better flame retardancy. A similar statement is also concluded from the UL-94 V tests. In detail, CE and DPDP0.5/CE resins are UL-94 V-2 grades, while with the increase of the phosphorus content, the flame retardancy gradually improves, and a desirable UL-94 V-0 rating is achieved for the DPDP1.4/CE resin. This result is better than that of the DOPO/CE system,14 in which the content of phosphorus should be equal to or larger than 2.0 wt % to get a UL-94 V-0 level, so DPDP has better flame retarding effect. The Max (t1, t2) value represents the maximum time that a material needed to self-extinguish after the first or second ignition and the removal of the flame; the (t1+t2) value is the time plus that a polymer took to self-extinguish after two ignitions.28 Based on Table 1, it is known that, although both DPDP0.5/CE and CE resins show the UL-94 V-2 level, the DPDP0.5/CE resin has significantly shortened Max (t1, t2) and (t1+t2) values compared with the CE resin. Moreover, the Max (t1, t2) and (t1+t2) values decrease as the content of DPDP in the DPDP/CE resins increases, meaning that the presence of DPDP can improve the self-extinguishing ability of the resins. To obtain more fire characteristic parameters, cone calorimeter tests were conducted. Figure 10 presents the

Figure 11. Overlay curves of mass loss versus time for DPDP/CE and CE resins.

MLR values of the DPDP/CE resin are about 19−36% and 13−53% lower than that of the CE resin, respectively, meaning that the condensed phase plays a domain role in the flame retardancy. These data are in good agreement with the thermal degradation behaviors of DPDP/CE resins as shown in Figure S2 in the Supporting Information. Specifically, compared with the CE resin, DPDP/CE resins not only have lower Tdi values due to the difficulty of the breaking down of P−C bond33 but also have higher char yield (Yc) values at 800 °C. All these results reflect that a condensed mechanism plays an important role. This statement will be further supported by the different morphologies and carbon structures between CE and DPDP/ CE resins. As the flame mechanism is not dependent on the amount of flame retardants, but is dependent on the species of flame retardants, so the SEM photos, XRD patterns, and TG-IR spectra of DPDP0.8/CE and CE resins were provided for discussing the flame mechanism. Figure 12 shows the SEM images and digital photos of CE and DPDP0.8/CE resins after cone tests. Due to the large loss during the combustion, the char of the CE resin does not have regular shape and is made up of many holes. While DPDP0.8/CE char is compact and almost remains the shape of the original resin, suggesting that

Figure 10. Overlay curves of HRR as a function of time for CE and DPDP/CE resins.

overlay curves of heat release rate (HRR) as a function of time for DPDP/CE and CE resins. Compared with the CE resin, each DPDP/CE resin shows a wider and lower HRR peak, and the peak heat release rates (PHRRs) of DPDP/CE resins are about 54−72% that of CE resin; similar results are also found in the total heat release (THR; Table 1), indicating that DPDP/ CE resins have much better flame retardancy than the CE resin. Furthermore, the curves of DPDP/CE resins slightly shift 1811


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of DPDP0.8/CE is obviously stronger, indicating that the addition of DPDP increases the degree of graphitization during combustion, however, the reason behind this is not clear. Previous studies show that the residual char with high degree graphitic structure is obviously beneficial to improve the flame retardancy through forming protective layers in the condensed phase and thus preventing the polymer from external radiation and heat feedback.36,39 To further confirm whether there is a gas-phase flame retarding role or not, TG-IR spectra of CE and DPDP0.8/CE resins were recorded to analyze the gases evolved during the thermal decomposition. Figure 14 presents three-dimensional TG-IR spectra. The most intensive absorbance band in the two spectra (2400−2300 cm−1) is assigned to carbon dioxide (CO2).40,41 Compared with the spectrum of CE resin (Figure 14a), that of the DPDP0.8/CE resin (Figure 14b) shows stronger peaks originating from water (4000−3400 cm−1), aromatic compounds, and carbonyls (1500−1800 cm−1)42 when the degradation time was 3000−4700 s. To obtain more detail information on the gases evolved, Figure 15 gives FTIR spectra at typical temperatures of CE and DPDP0.8/CE resins. For the CE resin, its initial decompose temperature (Tdi) and the maximum degradation rate temperature (Tmax) of the CE resin are 430 and 443 °C, respectively (Figure 16). The main absorption peaks of gaseous decomposition products at Tdi resulting from the stretching vibrations of water (4000−3400 cm−1), CO2 (2362, 2313 cm−1),43 CO of carbonate (1742 cm−1),42 C−N of isocyanurate ring (1688 cm−1), C−H of aromatic rings (1600 and 1510 cm−1),44 C−H of CAr-O−CH2 (1256 cm−1), and C− O (1177 cm−1). These peaks also appear in the FTIR spectra at Tmax, besides, there are new peaks assigned to the stretching and deformation vibrations of C−H in methane group (3010, 1344 cm−1),44 the stretching vibration of −CH3 and−CH2− (2924 cm−1) in hydrocarbons,44 the deformation vibration of ammonia (966, 931 cm−1) and C−H in the aromatic rings (816, 750 cm−1).45 The Tdi and Tmax of the DPDP0.8/CE resin are 428 and 446 °C, respectively (Figure 16). The FTIR spectrum of the DPDP0.8/CE resin is similar to that of the CE resin, except that the former shows an absorption peak reflecting P−CAr (1425 cm−1)35,44 at 446 and 500 °C, demonstrating that DPDP plays a flame retarding role in the gas phase. As previously reported, the phosphorus-containing compounds are easy to decompose and form free radicals, which will catch the combustible free radicals of H* and OH*, so the phosphorus can act as the flame inhibition in the gas phase.46 Note that different from the presence of the absorption peaks at 816 and 750 cm−1 until 800 °C in the TG-IR spectra of CE resin, these peaks do not exist in the spectrum of the DPDP/ CE resin at 600 °C, reminding us that DPDP/CE resins have more residue at high temperature than the CE resin, so the disappearance of absorption peaks at 816 and 750 cm−1 in the spectrum of the DPDP/CE resin indicates that the DPDP/CE resin has a much better thermal stability. In addition, the absorption peaks of water in the spectrum of the DPDP/CE resin are stronger than those of the CE resin, implying that water may play a dilute effect on the flame retardancy of the DPDP/CE resin. From above discussion, it is concluded that the flame retardancy mechanism of DPDP/CE resins contains both gas phase and condensed phase mechanisms.

Figure 12. SEM images and digital photos of residual chars for cured CE and DPDP/CE resins after combustions.

the DPDP0.8/CE char formed during combustion is so stable that it can hinder the heat transfer. For phosphorus-containing flame retardants, phosphoric acid formed during thermal decomposition is easy to produce pyrophosphoric acid, which will catalyze the dehydration reaction, and generate carbocations. At high temperature, pyrophosphoric acids turn into metaphosphoric acid and their corresponding polymers, which then take part in char formation. The resultant carbonized layer (char) isolates and protects the polymer from the flames and limits the volatilization of fuel and oxygen diffusion and, consequently, reduces combustion, and prevents the formation of new free-radicals.34,35 In other words, the condensed-phase mechanism is responsible for the outstanding flame retardancy of the DPDP/CE system. Figure 13 gives the XRD patterns of CE and DPDP0.8/CE chars after cone calorimeter tests. Both patterns show diffraction peaks at 2θ = 24° and 44°, assigned to the reflections of graphite planes (002) and (101), respectively,36,37 while no other peaks are visible, suggesting that the CE and DPDP0.8/CE chars are glassy carbon.38 However, compared with the diffraction peaks of graphite for the CE resin char, that

Figure 13. XRD patterns of the CE char (a) and DPDP0.8/CE char (b) after cone calorimeter tests. 1812


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Figure 14. Three-dimensional TG-IR spectra of the evolved gases produced by CE (a) and DPDP0.8/CE (b) resins.

4. CONCLUSIONS The whole curing peak of the DPDP/CE system is about 21− 76 °C lower than that of CE. Compared with the CE resin postcured at 250 °C, DPDP/CE resins postcured at 220 °C have remarkably higher and better performance without sacrificing high thermal resistance, including remarkably reduced water resistance, better dielectric properties, and outstanding flame retardancy. Both the gas phase and condensed phase play effects on the flame retardancy of DPDP/CE resins.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the overlay curves of the dependence of storage modulus on temperature of cured CE and DPDP/CE resins. Figure S2 shows the overlay TG and DTG curves of resins in a nitrogen atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected]. *Tel: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank National Natural Science Foundation of China (21274104) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financially supporting this project.

Figure 15. FTIR spectra of pyrolysis products at typical temperatures for CE (a) and DPDP0.8/CE (b) resins.

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Figure 16. Overlay TG and DTG curves of CE and DPDP0.8/CE resins in a nitrogen atmosphere from TG-IR tests.

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Flame Retarding Cyanate Ester Resin with Low Curing Temperature

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