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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

A series of azo functionalized diols were synthesized through diazotization which involves the reaction of amine with phenol and 2,6-dimethyl phenol. Four different amines have been used to prepare five bisphenols. These bisphenols were converted to their corresponding cyanate esters by treatment with cyanogen bromide (BrCN) in the presence of triethylamine (Et3N). The chemical structures of the prepared compounds were characterized with Fourier Transform Infrared, 1H-NMR, 13C-NMR spectroscopy, and elemental analysis. Dynamic curing behavior was investigated using differential scanning calorimetry. The maximum curing temperature of these cyanate esters are in the range of (186–208°C). Tg values of the polycyanurate networks are in the range of 245–276°C. The thermal properties of cured cyanate ester were studied at a heating rate of 10°C min−1 in N2 atmosphere. The polymers showed excellent thermal stability (T10 was found to be in the range 405–438°C) and the percentage of char yield at 800°C were found to be 30–49. The flame retardancy of the cyanate ester resins have been studied using limited oxygen index value which is in the range of 29.5–37.1 at 800°C. POLYM. ENG. SCI., 55:47–53, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Cyanate ester (CE) resins have received considerable attention in the past few years because of their importance as thermosetting resins for use in the aerospace, electronics, and adhesive industries [1-6]. CE monomers usually undergo thermal polycyclotrimerization to give polycyanurates, i.e., sym-triazine rings linked by aryl ether linkages, resulting in a uniformly cross-linked structure. The cured CE resins thus obtained have both better thermal properties and good processability in comparison with epoxy resins, phenolic and bismalimide resins. CE resins have their own unique properties such as good strength, low dielectric constants, radar transparency, low water absorption, and superior metal adhesion [7-10]. These properties make them the resin of choice for high performance applications where such properties are required. Bisphenol A dicyanate (BACY) is a commercial CE monomer, and has been widely used in a variety of fields. However, the search to achieve further improvements in performance and reduction in cost is never ending. Over the past decade, many new cyanate monomers emerged, including phosphorus-containing dicyanates [11, 12], silicon-containing dicyanates [13, 14], aromatic ether and ketone-containing dicyanates [15, 16], 2,7-dihydroxynaphthalene dicyanate [17], etc.

The key physical properties that serve as a basis for identifying “improved” polycyanurate systems include: (i) ease of processing, (ii) glass transition temperatures generally in the range of 200–300°C, (iii) mechanical properties such as elastic modulus and impact strength, (iv) thermo oxidative stability, and (v) moisture absorption. Many approaches have been reported to improve the above mentioned properties of CE by changing the structure of the starting resin which influences the properties of the final cured cyanurate polymer [18-22]. In the prior works attempt insertion of azo group in phenol novolac resin at a molecular level has recently been shown to impart improvements in thermal properties, mechanical properties, flame retardance, and char yields in cured epoxy resins [23, 24]. However, insertion of azo group at the molecular level into the monomer connecting structures of CEs cured network seems a logical starting point for tailoring the molecular structure to achieve improved physical properties (thermal properties, flame retardancy). In this article we present synthesis of new azo-based CE resins, solubility and their structural confirmation are discussed, and thermal properties and flame retardancy were evaluated.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Materials

4,4-diamino diphenyl sulphone and 4,4′-diamino diphenyl methane (DDM) were purchased from Aldrich, India. Aniline was purchased from SRL, India. 2,6-dimethyl phenol was purchased from Alfa Aesar, India. Resorcinol and cyanogen bromide were purchased from Spectrochem, India, and used as received unless it is stated. 4, 4′-diamino biphenyl methane, phenol and triethyl amine were obtained from E-Merck,-India. Triethyl amine was distilled from CaH2 prior to use.

Measurements

FT-IR (Fourier Transform Infrared) spectra were obtained using a ABB spectrophotometer. The samples were prepared in the form of pellet with KBr. 1H-NMR spectra were recorded using Jeol Ex-500 spectrophotometer at an operating frequency of 500 MHz. The samples, dissolved in DMSO-d6 were scanned from 0 to 12 ppm using TMS as internal reference. For 13C-NMR, the samples were scanned from 0 to 180 ppm. Elemental analysis was performed on a Carlo Erba EA 1108 micro analyzer. Melting points were determined on an electrothermal melting point apparatus, IA 6304 using capillary tubes and are uncorrected. Differential scanning calorimetry (DSC) studies were carried out using TA instruments model Q10 series. The DSC was run from room temperature to 300°C at a heating rate of 10°C/min in flowing nitrogen (60 ml/min). Thermogravimetric analysis (TGA) was carried out with TA instruments Q600 series at a heating rate of 20°C/min both in air and N2 atmosphere.

Synthesis of Bisphenols Containing Azo Linkage

The synthetic procedure for the preparation of 5-(phenylazo)−1,3-dihydroxy benzene is given. About 4.9 ml (0.052 mol) of aniline was taken in a 250-ml, three-necked round bottomed flask, Conc. HCl (16 ml) and water (16 ml) were added and stirred continuously for about 10 minutes at 0°C. About 4 g of NaNO2 was weighed and dissolved in water (20 ml) while maintaining the temperature at 0°C. This solution was added in drops to the mixture taken in the three-necked round bottomed flask maintained at 0°C, over a period of 45 minutes. The resulting reddish brown solution is benzene diazonium chloride. To this solution resorcinol (5.94 g, 0.054 mol.) in 10% NaOH (45 ml) maintained at 0°C was added in drops over a period of 45 minutes. After complete addition, the mixture was left as such for another 30 minutes, with occasional stirring. An orange-red precipitate was obtained, which was filtered, recrystallized from 75:25 water/methanol mixture and dried in a vacuum oven at 60°C. All other diphenols were synthesized using the same procedure. The synthetic (Schemes 1 and 2) for the preparation of all the diphenols is given bleow. The FT-IR (Fig. 1) spectrum shows absorptions at about 1594–1600 cm−1 because of the [BOND]N[DOUBLE BOND]N bond. The band at 3250–3500 cm−1 may be assigned to O[BOND]H stretching. The disappearance of sharp bands because of NH2 at 3334 cm−1 confirms the completion of the reaction.

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Scheme 1. Synthesize of bis aryl hydroxy azo compounds.

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Scheme 2. Synthesize of bis aryl hydroxy azo compounds.

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Figure 1. FT-IR spectrum of BHDAM.

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Synthesis of Bis Aryl Cyanato Azo Compounds

To a four-necked, round-bottomed flask equipped with a stirrer and nitrogen inlet, dry acetone was added. The reaction mixture was gradually cooled to −15°C to −5°C, and then CNBr (0.093 mol) was added. 5-(phenylazo)−1,3-dihydroxy benzene (10 g, 0.046 mol) and triethylamine (21.2 g, 0.093 mol) in dry acetone were added gradually over 2 h, and was maintained at the same temperature for two more hours. After the reaction was completed, the white Et3N.HBr salt was filtered. The filtrate was diluted with CH2Cl2 (100 ml) and extracted with water to remove residual Et3N.HBr. The organic phase was dried over Na2CO3 and then distilled to remove the solvent. All other prepared bisphenols were converted into the corresponding dicyanate ester monomers in a similar way (Scheme 3).

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Scheme 3. Synthesize of bis aryl cyanato azo compounds.

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5-(phenylazo)−1,3-dicyanato benzene (BCDA). Yield 85%, m.p.104 (°C) elemental analysis(%) theoretical C; 63.14, H; 2.58, N; 21.01, found C;63.26, H;2.64, N; 21.19. FT-IR (cm−1) – 2272 (OCN stretch), 1600 (N=N stretch). 1H-NMR (DMSO-d6, ppm): 7.96 (2H, d, Ha), 7.47(1H, t, Hb), 7.34 (2H, d, Hc), 7.58 (1H, d, Hd), 6.47(1H, d, He), 7.05 (1H, s, Hf). 13C-NMR (DMSO-d6, ppm), 145.5 (C1), 123.3 (C2), 129.2(C3), 131.1 (C4), 146.9 (C5), 126 (C6), 108.9 (C7), 162.2 (C8), 104(C9), 160.1 (C10), 109 (C11).

5-(4′-cyanato benzeneazo)−2,6-dimethyl-1-cyanato benzene (BCMDA). Yield 84% m.p.109 (°C) elemental analysis(%), theoretical C; 65.16, H; 3.76, N; 18.78, found C; 65.01, H; 3.89, N; 18.82. FT-IR (cm−1) – 2920 (CH3 stretch), 2271(OCN stretch), 1595 (N=N stretch). 1H -NMR (DMSO-d6, ppm): 7.78 (2H, d, Ha), 7.61 (2H, d, Hb), 7.74 (2H, s, Hc), 2.35(6H, s, Hd). 13C-NMR (DMSO-d6, ppm), 160.9 (C1), 116.3 (C2), 124.4 (C3), 145.3 (C4), 146.2 (C5), 115.5 (C6), 126.3 (C7), 14.5 (C8), 156.9(C9), 108.3 (C10).

Bis(4′-cyanato-4-azobenzene)methane (BCDAM). Yield 83% m.p.126 (°C) elemental analysis (%) thereotical, C; 70.21, H; 3.16, N; 17.87, found C;70.38, H; 3.20, N; 17.70. FT-IR (cm−1) - 2269(OCN stretch), 1600 (N=N stretch). 1H NMR (DMSO-d6, ppm): 7.30 (4H, d, Ha), 8.14(4H, d, Hb), 8.06 (4H, d, Hc), 7.81 (4H, d, Hd), 4.12(2H, s, He). 13C-NMR (DMSO-d6, ppm), 109.2 (C1), 158.4 (C2), 116.8(C3), 124.4 (C4), 151.2(C5), 149.5 (C6), 122.1 (C7), 129.8 (C8), 143.2(C9), 44.2 (C10).

Bis(4′-cyanato-2-chloro-4-azobenzene) methane (BCCDAM). Yield 86%, m.p.131 (°C) elemental analysis(%), theoretical C; 60.88, H; 2.49, N; 15.38, found C; 60.90, H; 2.38, N; 15.45. FT-IR (cm−1) - 2259 (OCN stretch), 1599 (N=N stretch). 1H -NMR (DMSO-d6, ppm): 7.33 (2H, d, Ha), 7.78 (2H, d, Hb), 7.72 (4H, d, Hc) 7.58 (4H, d, Hd), 7.26 (2H, s, He), 3.83 (2H, s, Hf). 13C-NMR (DMSO-d6, ppm), 108.9 (C1), 159.8 (C2), 116.5 (C3), 124.6 (C4), 145.7 (C5), 146.6 (C6), 125 (C7), 125.9 (C8), 144.5 (C9), 44.9(C10), 128.6 (C11), 129.4 (C12),

Bis(4′-cyanato-4-azobenzene)sulphone (BCSDA). Yield 88%, m.p.139 (°C) elemental analysis (%) (theoretical) C; 60.76, H; 2.74, N; 15.98, S; 5.85. found C; 60.66, H; 2.87, N; 15.89, S; 5.99. FT-IR (cm−1) – 2240 (OCN stretch), 1605 (N=N stretch). 1H -NMR (DMSO-d6, ppm): 8.33(4H, d, Ha), 8.19 (4H, d, Hb), 7.98 (4H, d, Hc), 7.79 (4H, d, Hd). 13C-NMR (DMSO-d6, ppm), 142.1 (C1), 128.9 (C2), 123.3 (C3), 158 (C4), 151.1 (C5), 124.4 (C6), 117.8 (C7), 159.2 (C8), 108.9(C9).

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Monomer Synthesis

All the dicyanate monomers were synthesized by reacting the diols with cyanogen bromide in the presence of triethyl amine as shown in Scheme 3. FT-IR (Fig. 2) was used to identify the presence of cyanate group. The bands observed at 2235 and 2270 cm−1 confirm the presence of –OCN group. The band at 1575–1600 cm−1 corresponds to N=N. The disappearance of –OH band at 3325 cm−1 shows the completion of the reaction. Figure 3 shows the FT-IR spectrum of polycyanurate. Polymerization of the CE groups can be easily monitored by the disappearance of the band at 2200–2300 cm−1 because of the stretching vibration of –OCN in the FT-IR spectrum and the appearance of new absorptions at 1596 and 1355 cm−1 for N–C=N and N–C–O stretching vibrations, respectively. The high reactivity of the cyanate functional groups require the cyanation reaction be conducted at temperature below −5°C in order to prevent side reactions like formation of iminocarbonate. The conversion of bisphenol to CE monomer was also monitored using 1H-NMR (Figs. 4 and 5). The spectrum clearly shows the singlet signal at 4.1 ppm for CH2 proton. The signals around 6.8–8.4 ppm are because of aromatic protons. The singlet signal at 2.35 ppm is because of –CH3 protons. The absence of signal in the region 9 ppm confirms the conversion of –OH to –OCN. Figure 6 and 7 shows the 13C-NMR spectrum of the dicyanate esters. The azo (C-N=N-C) carbon was found at 145.1 ppm. The carbon of the cyanate group is found at 109.8 ppm. Aromatic carbons appear in the wide range of 115.9–156.1 ppm. The -CH2 group appears at 39.5. These data support the structure of the synthesized CEs containing azo linkages. Elemental analysis data also support the formation of expected CE as they agree well with the calculated data. The solubility of the CE monomers were tested in various solvents and the results are summarized in Table 1. All the cyanate ester monomer were readily soluble both in strong dipolar solvents and in common organic solvents such as acetone, CH3OH, CHCl3, THF, ethyl acetate DMAc, DMF, NMP, and DMSO. The good solubility of these polymers in low boiling point solvents is a benefit to prepare the polymer films or fiber reinforcement composites at low processing temperature.

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Figure 2. FT-IR spectrum of BCDAM.

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Figure 3. FT-IR spectrum of cured BCDAM.

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Figure 4. 1H-NMR spectrum of BCSDA.

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Figure 5. 1H-NMR spectrum of BCDAM.

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Figure 6. 13C-NMR spectrum of BCSDA.

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Figure 7. 13C-NMR spectrum of BCDAM.

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Table 1. Solubility of cyanate ester monomers.
MonomerAcetoneCH3OHCHCl3THFEthyl acetateDMAcDMFNMPDMSO
  1. ++ fully soluble, + partially soluble insoluble.

BCDA++++++++++++++++
BCMDA++++++++++++++++
BCDAM++++++++++++++++
BCCDAM++++++++++++++++
BCSDA+++++++++++++++++

DSC Analysis

CEs react simply by heating via a highly selective cyclotrimerization reaction to produce (aryloxy)- 1,2,3- triazines (ether ketone). No secondary reactions take place or only to a very small extent, and thus, in the case of dicyanate esters, highly cross-linked polymers (so-called polycyanurates) with a very regular chemical structure are obtained [18]. Thermal behavior of CEs such as melting points, polymerization temperatures (the thermally induced polymerization of the synthesized compound could be observed from the exothermic peaks of DSC thermogram), processing windows of the obtained compounds were measured by DSC. The thermal characteristics data are presented in Table 2. Figure 8 shows the DSC curves of the CEs. The onset temperature of the endothermic peak was taken as the melting point of the CE. All the dicyanates show an onset of cure at around 150°C and maximum curing temperature of these CEs are in the range of 186–208°C. The reactivity of the CE depends on the backbone of the monomer. The peak maximum temperature of dicyanate with single azo group is lower than that of a dicyanate with two azo groups present in the monomer. The glass transition temperature was obtained by cooling the sample and re-running of the same sample. The glass transition temperature (Tg, Fig. 9) is in the range of 245–276°C. The Tg is lowest for the CE BCMDA. This may be because of the unsymmetrical structure of the cured BCMDA (pendent like structure).

Table 2. Thermal transitions of cyanate esters by DSC.
MonomerTi (°C)Tp (°C)Tf (°C)TgΔH (J/g)
  1. Ti—Initial curing temperature.

  2. Tp—Peak curing temperature.

  3. Tf—End curing temperature.

  4. Tg—Glass transition temperature.

BCDA(A)156186220253212
BCMDA(B)170191215245207
BCCDAM(C)172202234258211
BCDAM(D)178208231276197
BCSDA(E)178204218271191
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Figure 8. DSC curve of dicyanate ester.

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Figure 9. DSC curve of polycyanurates (Tg). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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TGA Analysis

A number of studies of the thermal and thermo-oxidative stability of aryl dicyanate esters have been presented but it is generally accepted that the decomposition mechanism of polycyanurates proceeds in several stages, involving reaction of unconverted cyanate functionality with air born moisture to form an iminocarbonate intermediate before the rapid rearrangement to a carbamate. At temperatures greater than 200°C this will undergo decarboxylation very easily to yield amine and gaseous carbon-di-oxide. The onset weight loss is also observed at higher temperature in an inert atmosphere than in the presence of air and large residue of char are usually characteristic of the degradation of polycyanurates in an inert atmosphere upto 800°C. The degradation products include carbon monoxide, hydrogen, cyanuric acid and its derivatives, phenol and bisphenols. The thermal properties of the CEs were determined using TGA at a heating rate of 20°C min−1. The temperature at which 10% loss occurs was determined from the original thermogram (Fig. 10) and the results are given in Table 3. The T10 value of CEs are in the range 405–438°C in N2. Sample BCSDA shows a higher T10 value than the other four because of the presence of sulfone group in the polymer network. The char yield at 800°C shows some variations according to the different substituents. The char yield is in the range of 30–49%. The char yield determines the flame retardency of a polymer. The flame retardency of the cured CEs were evaluated from their LOI value. The LOI value was found to be in the range of 29.5–37.1. If it has a higher retardency, then it has greater tendency to self extinguish a flame. The polycyanurates form a carbonaceous char during burning that protects the underlying material and further enhance fire resistance. Thus, the developed CE can be considered as better flame retardant materials.

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Figure 10. Thermograms of polycyanurates.

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Table 3. Thermal stability of polycyanurates.
Polymer codeTGA data
T10 °CChar yield %LOI
  1. T10—10% weight loss temperature.

  2. LOI—Limiting oxygen index.

BCDA (A)4053330.7
BCMDA (B)4123029.5
BCCDAM(C)4254535.5
BCDAM (D)4294937.1
BCSDA (E)4384133.9

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

A series of azo based dicyanate resins were synthesized from biphenol precursors by treatment of cyanogen bromide (CNBr) in the presence of triethylamine. The structure of the resins and its precursors were confirmed by FT-IR, NMR, and elemental analysis. Tg values of the cured CE are in the range of 245–276°C. The thermograms of cured CEs show single stage decomposion in the range of 405–438°C. The T10 for all the CEs are in the range of 456–762°C. The BCSDA shows the higher thermal stability because of the presence of sulfone group present in the back bone of the polymer. The char yield is in the range of 30–49%. The LOI values of the CEs were found to be 29.5–37.1, which shows their good flame-retarding property. All the CE monomers are good soluble in polar aprotic solvents such as NMP, DMAc, and DMSO. They are even soluble in low boiling solvents such as chloroform, acetone, and methanol. Thus, this series of azo containing CE polymer network demonstrated a good combination of properties and may be of interest for electronic industry and structural composite applications.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES