Acetylene-grafted resins derived from phenolics via azo coupling reaction

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EUROPEAN POLYMER JOURNAL

European Polymer Journal 44 (2008) 842–848

www.elsevier.com/locate/europolj

Acetylene-grafted resins derived from phenolics via azo coupling reaction Mingcun Wang a,*, Ming Yang b, Tong Zhao b,*, Jian Pei a,* a

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China

Received 25 July 2007; received in revised form 19 December 2007; accepted 4 January 2008 Available online 11 January 2008

Abstract Novel phenolic resins with high ethynyl contents were realized via azo coupling reaction between phenol units and diazonium of 3-ethynylaniline. If Novolac and high-ortho Novolac resins were used as the starting materials directly, the ratio of ethynyl groups to phenolic rings was ca 70%; while the ratio was 100% for the resin from Friedel–Craft polycondensation of 4-(3-ethynylphenyl)salicyl alcohol. All the resins were readily soluble in acetone and ethanol, and meltable at temperatures below 100 °C. The resins underwent thermal addition-type cure with a broad exotherm of around 140–280 °C, and the starting curing temperature showed a downward drift with increase in the ethynyl content. The thermal properties of the cured resins, determined from thermogravimetric analysis (TGA), were considerably superior to those of Novolacand Resole-type phenolic resins. The initial decomposition temperatures were at ca 400 °C, and the anaerobic carbon yields were ca 80% for all the resins. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Phenolic resins; Ethynyl; Azo coupling; Addition cure; Thermal properties

1. Introduction Phenolic resins continue to play an undisputed place in many areas, and retain industrial and academic interest despite the emergence of more and more high performance polymers [1]. But their further applications were impeded by some problems of brittleness, release of volatiles upon processing * Corresponding authors. Tel./fax: +86 10 62566305 (M. Wang); tel./fax: +86 10 62562750 (T. Zhao); tel./fax: +86 10 62758145 (J. Pei). E-mail addresses: [email protected] (M. Wang), [email protected] (T. Zhao), [email protected] (J. Pei).

and limited shelf life. New chemistry is needed to design and synthesize new phenolic resins with thermally stable addition curable groups. A few modified phenolic resins of this kind have been reported, such as those functionalized by malaimide [2], phenylethynyl [3] and propargyl groups [4]. Additionally acetylene-terminated resins have some prominent advantages over others in view of improved processability, excellent thermal stability, high char yields and good mechanical properties, etc [5,6]. And this kind of resins found a number of applications in thermo-mechanical and ablative materials [7].

0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.01.002

M. Wang et al. / European Polymer Journal 44 (2008) 842–848

To the best of our knowledge, ethynylphenyl functionalized phenolic resins have been very little reported. Ethynylphenyl azo Novolac-type phenolic resins were realized with low ethynyl contents of 20– 49% and were found to exhibit better thermal properties than Resole systems [8]. In this paper, acetylene-grafted phenolic resins derived from azo coupling reaction of different phenol compounds were reported. The synthesis, characterization, cure and thermal properties of the resins were studied here to evaluate the possibility of being high temperature resin. 2. Experimental 2.1. Materials 3-Ethynylaniline (Beijing Chuangqi Chemicals, China) and salicyl alcohol (Beijing Chemical Factory, China) were used as purchased. Sodium nitride, glacial acetate acid, and formalin (36–40% aq.) were all purchased from Beijing Chemical Reagents Incorporation. 2.2. Instruments Gel permeation chromatography (GPC) analysis was done on waters (515 HPLC pump, 2410 refractive index detector) using THF as eluent and polyethylene glycol as standards for calibration. FT-IR spectra were recorded on Perkin-Elmer IR2000 instrument with a resolution of 4 cm1. 1H NMR and 13C NMR spectra were performed on Bruker AV400 spectrometer. DSC analysis was recorded on Mettler–Toledo DSC8222e at a heating rate of 10 °C/min under N2 purging. TGA was carried out on Netzsch STA409pc in N2 atmosphere at a heating rate of 10 °C/min. Elemental analysis was studied on Flash EA1112. 2.3. Synthesis of Novolac resin [9] Novolac resin was prepared by acid catalyzed polycondensation between phenol and formaldehyde. Phenol (9.4 g, 0.1 mol) and oxalic acid (0.2 g) were charged in a flask and heated to 50 °C under stirring. Formaldehyde (2.55 g, 0.085 mol) in 37% aqueous solution was added, then the reaction was heated to 85 °C slowly and stood for 3 h. The yellow resin was washed with boiling water to remove unreacted reagents. After drying at 100 °C, 10 g yellow Novolac resin was obtained. FT-IR

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(KBr, cm1): 3370, 3019, 2910, 1611, 1510, 820, 757. 1H NMR (CDCl3, ppm): 6.4–7.2 (broad, aromatic protons), 2.9–3.2 (broad, methylene protons). 13C NMR (DMSO, ppm): 155–157(aromatic 1-position carbons), 128–134 (aromatic 2, 4-position carbons), 117–120 (aromatic 3, 5-position carbons), 36–37 (2, 40 -methylene carbons), 30.9–32 (2, 20 -methylene carbons). 2.4. Synthesis of high-ortho Novolac resin [10] Novolac resin with high ortho linkage between methylene and phenolic ring is noted as HON resin, and can be prepared by Zn(OAc)2 catalyzed polycondensation of phenol and formaldehyde. Phenol (9.4 g, 0.1 mol), formaldehyde (2.55 g, 0.085 mol, in 37% aqueous solution) and Zn(OAc)2 (0.2 g) were added in a flask and heated at 90 °C for 5 h. HCl solution (0.5 ml, 0.0005 mol, in 3.7% aqueous solution) was added cautiously and the reaction continued for another 2 h. After purification and drying by the procedure as described in 2.3, 8.9 g brown solid was obtained. FT-IR (KBr, cm1): 3325, 3014, 2906, 1594, 1510, 823, 756. 1H NMR (CDCl3, ppm): 6.4– 7.2 (broad, aromatic protons), 2.9–3.2 (broad, methylene protons). 13C NMR (DMSO, ppm): 153–159 (aromatic 1-position carbons), 129–133 (aromatic 2, 4-position carbons), 117–121 (aromatic 3, 5-position carbons), 36.1–36.8 (2, 40 - methylene carbons), 31–32.7 (2, 20 - methylene carbons). 2.5. Synthesis of ethynylphenyl azo Novolac resin (EPAN) [8,11] EPAN resin was synthesized by coupling reaction between 3-ethynylphenyl diazonium chloride and Novolac resin in basic solution. At 0 °C, 3-ethynyaniline (6.45 g, 0.055 mol) was added dropwise to 30 ml 15% HCl solution to form milky slurry. To this slurry, cold sodium nitrite solution (3.45 g, 0.05 mol in 5 ml water) was added slowly, with good agitation, till the reaction turned pink transparent. The azo salt solution then was added to a Novolac resin solution (5.2 g in 40 ml 50/50 (v/v) water/ethanol solution containing 2 g NaOH) at 0 °C to form dark yellow slurry. After 5 h, the slurry was neutralized to precipitate the polymers. The synthesized resin was filtered, washed by distilled water, and dried at 50 °C over night to give 6 g of red powder. FT-IR (KBr, cm1): 3450, 3286, 2106, 1596, 1500, 1265, 997, 898, 815, 796. 1H NMR (CDCl3, ppm): 6.6–7.3 (phenolic aromatic protons), 7.4–8 (ethynylphenyl

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M. Wang et al. / European Polymer Journal 44 (2008) 842–848

aromatic protons), 3.6–4 (ethynyl protons), 3–3.3 (methylene protons). Elemental analysis for N: 9.88%.

2.7. Synthesis of ethynylphenyl azo salicyl alcohol (EPASA) 4-(3-Ethynylphenyl)azo salicyl alcohol (EPASA) was prepared according to the procedure similar to that of EPAN. The compound was a yellow crystalline powder. FT-IR (KBr, cm1): 3398, 3287, 3097, 2120, 1600, 902, 823, 802. 1H NMR (DMSO, ppm): 8.05(s), 7.96–7.97(m), 7.82–7.84(d), 7.06–7.08(d), 5.28(s), 4.64(s), 4.40(s). Elemental analysis for N: calculated 11.11%; found 11.23%.

2.6. Synthesis of ethynylphenyl azo high-ortho Novolac resin (EPAHON) EPAHON resin was prepared according to the procedure similar to that of EPAN. The resin was a dark red powder at a yield of about 100% based on the starting HON precursor resin. FT-IR (KBr, cm1): 3450, 3287, 2107, 1594, 1265, 997, 899, 825, 756. 1H NMR (CDCl3, ppm): 6.5–7.2 (phenolic aromatic protons), 7.4–8.2 (ethynylphenyl aromatic protons), 3.5–4.3 (ethynyl protons), 3–3.2 (methylene protons). Elemental analysis for N: 9.95%.

2.8. Synthesis of ethynylphenyl azo ortho Novolac resin (EPAON) [12] In the EPASA compound, the para position was occupied and one ortho position was free (see

CH

OH

OH

+

oxalic acid

HCHO

(

CH2 )

OH

OH

N 2+

CH2 )(

(

0oC

CH2

)

N2

EPAN resin

Novolac resin CH

OH

OH

+

HCHO

Zn(OAc)2

(

HC

CH2 )

OH

OH

N 2+ (

0oC

CH2 ) (

CH2

)

N2

OH

EPAHON resin

HON resin

CH

HC

N 2+

OH

OH

OH

OH

0oC

HOAc; HCl

CH2 )

(

85oC

N2

N2

HC

HC

EPASA Scheme 1. Synthetic protocol for the resins.

EPAON resin

M. Wang et al. / European Polymer Journal 44 (2008) 842–848 Table 1 Elemental analysis of the resins Resins (designation)

N-content (wt%)

Extent of azo coupling (%)

EPAN EPAHON

9.88 9.95

68.3 69.1

Note: The extent of azo coupling was calculated based on the Ncontent, and the equation is as follow: x = 106N/(28–128N), where the N is the nitrogen-content, 106 is the formula weight of phenolic unit, 28 is the formula weight of azo, and 128 is the difference of the formula weights of ethynylphenyl azo and hydrogen groups.

the results of FT-IR and 1H NMR), so the self condensation of EPASA by the methylol and the free ortho position will result in an oligomer with 100% substitution of phenolic rings by azo units (see Scheme 1). EPASA (3.78 g, 0.015 mol) was dissolved in 20 ml glacial acetate acid. After adding another 0.015 ml HCl solution, the reaction was performed at 85 °C for 6 h, resulting in a dark brown solution. The solution was mixed well with 150 ml water and extracted with 30 ml toluene; the organic phase was washed several times with water and dried over sodium sulfate. After removal of toluene, a brown viscous liquid was obtained. 1H NMR (CDCl3, ppm): 6.8–7.4 (phenolic aromatic protons), 7.4–8.3 (ethynylphenyl aromatic protons), 4.8–5.2 (methylol protons), 3.9–4.1 (ethynyl protons), 2.9–3.2 (methylene protons). 2.9. Thermal cure of the resins All the resins (EPAN, EPAHON, and EPAON) were thermally cured under N2 as per the following

Fig. 1.

13

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schedule: 120 °C, 1 h—150 °C, 1 h—170 °C, 1 h— 190 °C, 4 h—210 °C, 1 h—250 °C, 4 h, to give the respective cured thermosets. 3. Results and discussion 3.1. The resins synthesized from Novolac resins Phenolic resins with varying percents of ethynyl functions were synthesized as per the protocol in Scheme 1. The reaction can be done smoothly at 0 °C in aqueous solution, and hence is environment-friendly. Based on the results from elemental analysis, the functional degrees were determined (Table 1). Although the free para positions of phenolic rings were quite different in the molecules of Novolac and HON resins, the percents of the functions were similar in the final title resins. The relative amounts of 2, 20 -, 2, 40 - and 4, 40 methylene linkages can be seen by 13C NMR and FT-IR spectra (see Figs. 1 and 2). As reported previously [13,14], acid-catalyzed Novolac resin had nearly equal probability for ortho and para substitution, implying a relatively small amount of free para position available. In contrast to this, the Zn(OAc)2 catalyzed HON resin had approximately threefourths of the methylene linked in the ortho positions, indicating that about 50% of the para positions of phenolic rings were free. Although equivalent amounts of diazonium salt were used, the extent of substitution was approximately 70% in both cases; this might be attributable to the steric hindrance. It may be safe to hypothesize that for phenolic resins with different ortho/para ratios, the azo coupling

C NMR spectra of Novolac (down trace) and HON (up trace) resins (DMSO, TMS standard).

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M. Wang et al. / European Polymer Journal 44 (2008) 842–848

volume resulting from the branched structure and the co-planarity of ethynylphenyl azo unit and phenolic ring. EPAN and EPAHON resins were both oligmers (EPAN: Mn = 486, Mw = 1282, Polydispersity = 2.64; EPAHON: Mn = 565, Mw = 1039, Polydispersity = 1.84).

Transmittance

EPAHON

EPAN

HON

3.2. Polycondensation of EPASA

Novolac

4000

3000

2000

1000

Wave number / cm-1

Fig. 2. FT-IR spectra of Novolac, HON, EPAN and EPAHON resins.

reaction was mainly governed by the steric hindrance and not the reactivity of the free positions. These findings improved and richened the reported results [8]. The FT-IR spectra of these ethynyl phenolic resins showed the characteristic absorption of „C–H at 3290 cm1 and –C„C– at 2110 cm1, while the absorption of –N@N– fell in the range of phenyl rings. 1H NMR confirmed the structures: 3.5– 4.3 ppm for „C–H, 6.6–8 ppm for –U–H and 3– 3.4 ppm for –CH2–. Considering the high extent of azo coupling, the EPAN and EPAHON must have much higher molecular weights than their precursors. But interestingly, GPC analyses showed nearly unchanged apparent molecular weights (see Fig. 3). This is possibly a consequence of the reduced hydrodynamic

The coupling of diazonium salts occurs preferably at phenylic para positions [11,15], hence it was reliable to prepare para subsitituted azo salicyl alcohol (EPASA) in a quantitative yield and a defined structure (see Scheme 1). The results of 1H NMR and FT-IR correlated with the structure quite well. EPASA was water insoluble, but it could undergo acid-catalyzed polycondensation in glacial acetic acid. Due to the low reactivity and steric hindrance, the resulting EPAON resin had a low molecular weight and was polydisperse (see Fig. 4). It was viscous and lost the melting point of EPASA compound at 158 °C (see Fig. 6). 3.3. Processing capability and thermal cure EPAN and EPAHON resins melted into fluids upon heating at temperatures below 100 °C which was far ahead of curing, while EPAON was liquid even at ambient conditions. They were all organosoluble in common solvents such as THF, acetone and ethanol. These properties made them applicable to processing. DSC curves of the resins are given in Figs. 5 and 6. The temperature ranges of thermal curing exotherms were 180–280 °C (peak at 223 °C), 150– 280 °C (peak at 204 °C) and 120–280 °C (peak at 120

Intensity / mV

Intensity / mV

100

EPAN Novolac EPAHON

80 60 40

HON

20 0 0

10

20

30

40

Elution time / min

-20 0

10

20

30

40

Elution time / min

Fig. 3. GPC profiles of EPAN and EPAHON (THF eluent, PEG standard).

Fig. 4. GPC profile of EPAON (THF eluent, PEG standard).

M. Wang et al. / European Polymer Journal 44 (2008) 842–848 Table 2 Thermal cure characteristics of the resins

8 6

-1

4

Heat / Jg

847

EPAHON

2

Resins

Ti (°C)

Tmax (°C)

Tend (°C)

EPAN EPAHON EPAON EPASA

180 150 120 180

225 204 207 207

280 280 280 230

Notes: (1) The data from the DSC analyses the resins (N2, 10 °C/ min); (2) The DSC curve for EPASA showed a sharp melting peak and a weak curing exotherm.

0 EPAN

-2 -4

110 50

100

150

200

250

300

350

o

Temperature / C

105 100

Fig. 5. DSC curves of EPAN and EPAHON (N2, 10 °C/min). Residue / %

207 °C) for EPAN, EPAHON and EPAON resins, respectively (see Table 2). Hence the resins had wide processing windows. It had been reported that the cure kinetics of ethynyl groups was greatly affected by their concentration and distribution in the polymeric backbone [16]. Higher concentration will facilitate the cure, which agrees well with the results of the starting curing temperatures in a sequence of EPAON < EPAHON < EPAN. Otherwise, the distribution of the functional groups also has an unavoidable effect; the curing peak temperature and the starting curing temperature lower evidently with a higher regularity, which could be revealed by the results of thermal cure of EPAN and EPAHON resins (see Figs. 5 and 6). All the ethynyl phenolic resins could be thermally cured at 250 °C in 6 h, and monitored by FT-IR spectra from the disappearance of adsorption of ethynyl at 3290 cm1 and 2110 cm1.

95 90 85

1 2 3

80 75 70 65

1 EPAN 2 EPAON 3 EPAHON 4 Resole 5 Novolac

4

60

5

55 200

400

600

800

1000

Temperature / oC

Fig. 7. TGA curves of the cured EPAN, EPAHON and EPAON resins (N2, 10 °C/min).

The thermal cure mechanism for acetylene polymers has not been sufficiently elucidated [17,18], but many studies have shown that there are many mechanisms involved, such as cyclotrimerization, Glaser coupling, Strauss coupling, and linear polymerization, etc [19,20]. However, it is believed that the major part of ethynyl groups cure by linear addition crosslinking just as that of propargyl Novolac resin in our previous report [21].

40 30

3.4. Thermal resistance

Heat / Jg

-1

20 EPAON

10 0 EPASA

-10 -20 -30 50

100

150

200

250

300

350

400

Temperature / oC

Fig. 6. DSC curves of EPASA and EPAON (N2, 10 °C/min).

Reghunadhan [8] reported EPAN resins with ethynyl per cents between 24 and 49% possessed anaerobic char yields superior to widely used Resole resin; at 700 °C the char residues were 70–75% and the initial decomposition temperatures were 370–390 °C. Here, the thermal stability of the high ethynyl phenolic resins noted as EPAN, EPAHON and EPAON was examined by TG analysis. From the TG profiles of the cured resins given in Fig. 7, the thermal decomposition initiated apparently at even higher temperatures of approximately 400 °C versus

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M. Wang et al. / European Polymer Journal 44 (2008) 842–848

Table 3 Thermal decomposition characteristics of the resins Cured resins

Ti (°C)

Tmax (°C)

Tend (°C)

Char yield at 900 °C (%)

EPAN EPAHON EPAON Novolac Resole

380 380 399 360 365

535 538 535 413; 536 408; 545

700 700 700 600 585

81 79 80 62 60

Notes: (1) Novolac-type phenolic resin was cured by adding 10 wt% hexamethylenetetraamine; (2) Resole-type phenolic resin here was ammonia catalyzed product; (3) The data here were from the TG analyses of the cured resins (N2, 10 °C/min).

ca 300 °C for conventional Novolac or Resole phenolic resins [22]. These ethynyl phenolic resins yielded high amount of carbon residue, approximately 80% at 900 °C (see Fig. 7 and Table 3), while the carbon residues for the conventional phenolic resins were lower by approximately 15–20%. Compared with conventional phenolic resins and other modified phenolic resins, the titled resins had remarkable thermal properties [23]. 4. Conclusions Novel multiple ethynyl containing addition-curetype phenolic resins were realized. The resins had suitable processing capability evidenced by their good organosolubility, low temperature meltability and broad processing windows. The resins could be thermally cured, but their exotherms showed quite different starting temperature and peak temperature for the resins derived from various phenolic precursors. The cured resins exhibited high thermal resistance evidenced by the TG results of ca 400 °C initial decomposition temperature and ca 80% char yield at 900 °C.

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