Novel bismaleimide containing cyclic phosphine oxide and an epoxy unit: Synthesis, characterization, thermal and flame properties

European Polymer Journal 42 (2006) 1899–1907 www.elsevier.com/locate/europolj

Novel bismaleimide containing cyclic phosphine oxide and an epoxy unit: Synthesis, characterization, thermal and Xame properties Minda Shau a, Paifeng Tsai b

b,¤

, Wenyu Teng a, Wenhsiag Hsu

c

a Department of Applied Chemistry, Chia Nan University of Pharmacy and Science, Tainan, Taiwan, ROC Department of Occupational Safety and Hygiene, Chia Nan University of Pharmacy and Science, Tainan, Taiwan, ROC c Department of Chemical Engineering, Tung-Fang Institute of Technology, Kaohsiung, Taiwan, ROC

Received 31 July 2005; received in revised form 16 February 2006; accepted 26 February 2006 Available online 18 April 2006

Abstract A new type of bismaleimide resin (EPBMI), containing epoxy unit and phosphorus in the main chain, was synthesized. The structure of the new resin was conWrmed by infrared spectroscopy (IR), 1H NMR and 13C NMR spectroscopies. In addition, the compositions of the new synthesized bismaleimide with two reactants, 4,4⬘-diaminodiphenylsulfone (DDS) and 4,4⬘-diaminodiphenylether (DDE), was used to compare its reactivity and thermal properties with conventional bismaleimide (EBMI). Reactivity was measured by diVerential scanning calorimetry. Thermogravimetric analysis revealed that the polymers, obtained through the reactions between bismaleimides and diamine agents, also demonstrated excellent thermal properties and high char yield. © 2006 Elsevier Ltd. All rights reserved. Keywords: Bismaleimide; Char yield; Flame retardancy; Phosphorus

1. Introduction Due to increased application of polymeric materials in various industrial Welds in recent years [1–3], it is crucial to improve the thermal and Xame resistances of polymeric materials. In general, polymers containing aromatic and/or heterocyclic ring struc-

* Corresponding author. Tel.: +886 6 266 4911x260; fax: +886 6 266 7320. E-mail address: [email protected] (P. Tsai).

0014-3057/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.02.016

tures, such as polybenzothiazoles, polyoxadiazoles, polyquinoxalines and polyimides, are known to have excellent thermal stability [4]. Condensationtype polyimides have certain disadvantages, such as insolubility and infusibility, and, hence, are diYcult to process [5,6]. Addition-type polyimides (bismaleimides) were developed mainly to overcome these processing disadvantages [7–9]. These maleimides and capped prepolymers are cured into a highly cross-linked network by addition reactions without the evolution of volatile by-products. However, due to their high cross-link density, they are often brittle,

1900

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

non-phosphorylated polymers give about a zero char yield on pyrolysis at higher temperature in air. On the other hand, the incorporation of phosphorus into polymer structures can produce adequate char yield on pyrolysis. In this study, a novel bismaleimide, 10-phenylphenoxaphosphine-3,8-di(2⬘-hydroxy-3⬘-(p-maleimidobenzoyloxy)propyl)ester-10-oxide (EPBMI), was prepared by the reaction of 10-phenylphenoxaphosphine-3,8-diglycidylester-10-oxide (EPCAO) with N-(carboxyphenyl)maleimide (p-CPM), accor ding to Scheme 1. Conventional bismaleimide resin, 2,2-diphenyl-propane-p-di(2⬘-hydroxy-3⬘-(p-maleimidobenzoyloxy)propyl)ether (EBMI), containing an epoxy unit, was also prepared according to Scheme 2. The properties of the newly synthesized bismaleimide are compared with those of conventional bismale-

resulting in low impact and fracture toughness. Introduction of a long, Xexible epoxy chain into the backbone of bismaleimides is expected to reduce cross-link density and also to improve fracture toughness by dissipating the impact energy along the entire molecular chain [10]. The incorporation of halogen and/or phosphorus into the polymer backbone can improve Xame resistance [11]. Currently, the incorporation of halogen into polymers is not preferred for environmental reasons. To overcome this problem, polymers can be modiWed by introducing phosphorus into the molecular structure to improve Xame-retardant properties [12,13]. Van Krevelen [14] proposed that the char residue on pyrolysis is linearly proportional to the oxygen index for halogen-free polymer. According to our previous investigations, it was found that

O

O

O

+

NH2

COOH

N

O

COOH

O p -C PM O O O

O C

C

C

O

C

C

O

C

P

O

O

C

+

C

N

COOH

O

Ph

O p -C PM

EPC A O

O

O

N

C

O

N

C

O O

O H2C

H C

C

O

O

O

O C

C

O

C

C

P OH

O

Ph

EPB M I

Scheme 1. Synthesis of EPBMI.

OH

C

O

O

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

1901

O C C

C

C

O

C

O

O

C

C

C

+

N

COOH

O

C

O p -C PM

E PO N 82 8

O

O

N

C

O

C

O

O C

N

C C

C

O

O

O O

C

OH

O

C

C

C

C

OH

EB M I

Scheme 2. Synthesis of EBMI.

imide. It was found that the newly synthesized resin demonstrated adequate thermal property combined with high char yield. 2. Experimental 2.1. Materials N,N-Dimethyl formamide and acetone were dried over P2O5 and then vacuum-distilled. Sodium acetate, acetic anhydride, ethanol, p-aminobenzoic acid, 4,4⬘-diaminodiphenylether (DDE) and 4,4⬘-diaminodiphenylsulfone (DDS) were of reagent grade (Merck) and used without further puriWcation. EBMI and EPCAO were prepared by methods described previously [10,13]. 2.2. Synthesis of N-(carboxyphenyl)maleimide (p-CPM) [10] To a well-stirred solution of p-aminobenzoic acid (41.10 g) in freshly distilled acetone, 29.40 g of maleic anhydride was gradually added at 10 °C over a period of 30 min. The solution was stirred for an additional 30 min at 10 °C and the yellowish precipitate (4-carboxyphenyl maleamic acid) was puriWed

by recrystallization from DMF/ethanol (50:50, w/w) solution. Cyclodehydration of the amic acid intermediate to maleimide was carried out by treating it with the optimum fused sodium acetate and acetic anhydride for 15 min at a temperature of 85 °C. The reaction solution was cooled to room temperature and poured into ice water to obtain a yellowish orange precipitate. The precipitates were recrystallized from ethyl acetate/n-hexane (50:50, w/w) solution, Wltrated and vacuum dried. Yield was 85%, with a m.p. at »225 °C. 2.3. Synthesis of EPBMI 10-Phenylphenoxaphosphine-3,8-diglycidylester-10oxide (EPCAO) (7.41 g) was placed in a three-neck round-bottom Xask equipped with a mechanical stirrer, a thermometer and a nitrogen purge tube, and heated up to 180 °C. A stoichiometric amount of p-CPM was then added and the mixture was vigorously stirred for 90 min during which nitrogen was constantly purged. The reaction product was poured into an aluminum disk, cooled and crushed to a Wne powder. After washed with toluene, the obtained Wne yellow powder was taken as EPBMI. Yield was »45%, with a m.p. at »151 °C.

1902

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

2.4. Characterization and measurements

3. Results and discussion

Melting points were determined with a Gallenkamp MPD350. Infrared spectra were examined using a Perkin–Elmer Model 2000. 1H NMR and 13 C NMR spectra were obtained using a Bruker AMX-400 spectrometer, where the samples were dissolved in d6-DMSO, with TMS employed as an internal standard. Curing cycles and reactivity were measured by diVerential scanning calorimetry (DSC) on a TA 2010. A heating rate of 10 °C/min and a sample size of 5 § 1 mg was used in each experiment. Thermal resistances were evaluated by thermal gravimetric analysis (TGA) on a TA 2050 with a heating rate of 10 °C/min and a sample size of 10 § 1 mg was used in each experiment.

The newly synthesized bismaleimide resin (EPBMI), which contained phosphorous and epoxy groups, was obtained through the reaction between 10-phenylphenoxaphosphine-3,8-diglycidylester-10oxide (EPCAO) and N-(carboxyphenyl)maleimide (p-CPM) according to Scheme 1. The chemical structure of the new bismaleimide resin (EPBMI) was characterized by infrared, 1H NMR and 13C NMR spectroscopies.

2.5. Preparation of tested samples Thermal and Xame properties of the new bismaleimide (EPBMI), containing phosphorus and ethylene oxide units, were evaluated by preparing six polymers: EBMI homopolymer, poly(EBMI/DDE), poly(EBMI/DDS) copolymers, EPBMI homopolymer, poly(EPBMI/DDE) and poly(EPBMI/DDS) copolymers. To obtain polymers with good thermal stability, EPBMI (or EBMI) was polymerized with diamines at a stoichiometric ratio. Curing cycles were determined by DSC thermograms.

3.1. IdentiWcation of the new bismaleimide The reaction between functional monomaleimide (p-CPM) and epoxy group was monitored by the disappearance of the characteristic peak of epoxy group in IR and NMR spectra. It was observed that carboxylic acid in p-CPM reacted completely with the epoxy group in EPCAO. The IR spectrum of p-CPM (Fig. 1) showed a broad absorption peak at 3210 cm¡1 due to the hydrogenbonded carboxylic acid. Strong absorptions around 1710 cm¡1, due to the imide group, were present. The epoxy resin (EPCAO) showed four characteristic absorptions at 1725, 1375, 1180 and 907 cm¡1. The peak at 1725 cm¡1 represents the CBO stretch in the ester group. Absorptions due to PBO at 1180 cm¡1 and ArAP at 1375 cm¡1 were also

Fig. 1. IR spectrum of p-CPM, EPCAO and EPBMI.

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

observed. The characteristic band of the oxirane ring was observed at 907 cm¡1. However, the absorption band at 907 cm¡1 was very prominent, but was absent from the corresponding spectrum of bismaleimide, indicating that the reaction has taken place. The novel bismaleimide also showed a broad band in the region of »3500 cm¡1 due to the associated hydroxyl group formed as a result of the esteriWcation reaction. 1H NMR and 13C NMR spectra of the epoxy resin (EPCAO), p-CPM and the novel bismaleimide are given in Figs. 2 and 3, but the characteristic peaks of the epoxy group at 2.68 and 2.81 ppm were not observed. In the 13C NMR spec-

1903

tra, there was a complete disappearance of epoxy carbon signals in the case of bismaleimide resin, indicating that the addition reaction of N-(carboxyphenyl)maleimide with epoxy group was completed. As a result, the two epoxy carbons shifted to the low Weld (due to the electron-withdrawing eVect of oxygen). Theoretically, bismaleimide should show three peaks in the aliphatic region at 60–75 ppm. However, both bismaleimides gave three more peaks than expected, corresponding to the - and -isomers of secondary and primary alcohols, resulting from the addition of carboxyimide on carbons of the epoxy group [10].

Fig. 2. 1H NMR spectrum of p-CPM, EPCAO and EPBMI.

1904

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

Fig. 3. 13C NMR spectrum of p-CPM, EPCAO and EPBMI.

3.2. Reactivities of EBMI and EPBMI The polymerization reactions were studied by DSC. SigniWcant caution was taken during the DSC study of the polymerization reaction to obtain homogeneous mixtures. Typical DSC thermograms (Fig. 4) demonstrated the reactivity of EPBMI and EBMI in homopolymerization. Polymerization of EPBMI, which exhibits a lower exothermic starting temperature under the same set of reaction conditions, is faster than that of EBMI. It is, therefore,

reasonable to propose that EPBMI is more reactive than EBMI in homopolymerization. The DSC thermograms (Figs. 5 and 6) demonstrate the reactivity of bismaleimides (EBMI and EPBMI) toward diamines (DDE and DDS). The lower reactivity of DDS toward bismaleimide, compared to DDE, could also be attributed to electronic eVects. In the case of DDS, the electron-withdrawing group, –SO2–, reduced the electron density of the amine nitrogen and, consequently, reduced their nucleophilic attack on the double bond of the bismaleimides.

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

Fig. 4. DSC thermograms of two compositions: EBMI and EPBMI (heating rate: 10 °C/min).

Fig. 5. DSC thermograms of two compositions: EBMI/DDE and EBMI/DDS (heating rate: 10 °C/min).

3.3. Thermal and Xame properties To compare the thermal and Xame-retarding properties of the cured polymers, they were divided into two groups: (1) non-phosphorylated polymers – poly(EBMI), poly(EBMI/DDE) and poly(EBMI/ DDS), and (2) phosphorylated polymers – poly(EPBMI), poly(EPBMI/DDE) and poly(EPBMI/ DDS). From PDTs and temperatures of some characteristic weight losses, which are shown in Figs. 7 and 8, the thermal properties of these cured polymers can be compared. From TGA cures, it was found that the new bismaleimide polymers, contain-

1905

Fig. 6. DSC thermograms of two compositions: EPBMI/DDE and EPBMI/DDS (heating rate: 10 °C/min).

Fig. 7. TGA thermograms of poly(EBMI), poly(EBMI/DDE), poly(EBMI/DDS), poly(EPBMI), poly(EPBMI/DDE) and poly(EPBMI/DDS), in a nitrogen environment (heating rate: 10 °C/min).

ing an epoxy unit and phosphorus – poly(EPBMI), poly(EPBMI/DDE), and poly(EPBMI/DDS) – exhibited excellent thermal stability at high temperature, as observed in Figs. 7 and 8. The curves for nonphosphorylated and phosphorylated polymers crossed near the temperature range of 370–430 °C. Moreover, from Figs. 7 and 8 it was also found that the polymers containing phosphine oxide have a lower thermal degradation rate than conventional bismaleimide polymers. Besides, the relatively lower temperature for the 1% weight loss of phosphorylated polymers – poly(EPBMI), poly(EPBMI/DDE) and poly(EPBMI/DDS) – as shown in Figs. 7 and 8,

1906

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

diVusion to increase both thermal stability and Xame retardancy of polymers. The char yield of these cured polymers in nitrogen and air environments are shown in Tables 1 and 2. It was found that the phosphorous-containing polymers – poly(EPBMI), poly(EPBMI/DDE) and poly(EPBMI/DDS) – have a higher char yield than the non-phosphorylated polymers – poly(EPBMI), poly(EPBMI/DDE), and poly(EPBMI/DDS). The char yield increased by up to 33%. Therefore, phosphorous-containing polymers were conWrmed to be more Xame-resistant than non-phosphorylated polymers. 4. Conclusions Fig. 8. TGA thermograms of poly(EBMI), poly(EBMI/DDE), poly(EBMI/DDS), poly(EPBMI), poly(EPBMI/DDE) and poly(EPBMI/DDS), in an air environment (heating rate: 10 °C/ min).

A new bismaleimide (EPBMI) was prepared from EPCAO, which contained epoxy units and cyclic phosphine oxide. The structure of new bismaleimide resin (EPBMI) was conWrmed through IR, 1H NMR and 13C NMR characterization. The synthesized EPBMI, cured by self-polymerization or diamine reagents (DDE and DDS), showed a slower thermal degradation rate than conventional EBMI resin cure with the same diamine agents. In this study, the Xame retardancy of the new bismaleimide resin was signiWcantly improved through the introduction of phosphorus into the polymer structure. The EPBMI-containing polymers also had a higher char yield on pyrolysis.

possibly resulted from the fact that the phosphorus in these phosphorylated polymers became phosphoric acid on pyrolysis, which catalyzed the dehydration of these polymers at lower temperature. The phenomenon of degradation of the phosphine oxide group at a relatively low temperature played an important role in enhancing Xame retardation. The oxidizing degradation of the phosphine oxide group would form a protective layer on the polymer surface and served as a barrier against heat and oxygen Table 1 TGA data of bismaleimide polymers in a nitrogen environment Materials

PDT (°C)

Temperature at characteristic weight loss (°C)

Residue (%) at 700 °C

10%

20%

30%

40%

50%

Poly(EBMI) Poly(EBMI/DDE) Poly(EBMI/DDS) Poly(EPBMI) Poly(EPBMI/DDE) Poly(EPBMI/DDS)

381 323 355 350 311 313

401 357 378 370 351 349

418 383 397 406 417 417

428 397 408 432 511 481

436 410 417 500 595 544

446 423 426 606 – 690

23 22 21 45 55 50

Table 2 TGA data of bismaleimide polymers in an air environment Materials

PDT (°C)

Temperature at characteristic weight loss (°C)

Residue (%) at 700 °C

10%

20%

30%

40%

50%

Poly(EBMI) Poly(EBMI/DDE) Poly(EBMI/DDS) Poly(EPBMI) Poly(EPBMI/DDE) Poly(EPBMI/DDS)

358 314 354 340 301 316

388 350 380 367 350 359

415 372 401 403 431 446

425 396 412 423 527 518

429 401 424 532 558 556

441 401 447 567 570 570

0 0 2 16 16 12

M. Shau et al. / European Polymer Journal 42 (2006) 1899–1907

Acknowledgement This work was supported by the National Science Council, Republic of China, under Grant NSC892216-E-041-007. References [1] Lee H, Neville K. Handbook of epoxy resins. New York: McGraw-Hill; 1967. [2] Sampson RN. Electrical design properties and testing of plastics and elastomers. In: Harper CA, editor. Handbook of plastics, elastomers, and composites. New York: McGrawHill; 1992. p. 1–57. [3] Penn LS, Chiao TT. Epoxy resins. In: Lubin G, editor. Handbook of composites. New York: Van Nostrand Reinhold; 1982. p. 57–88. [4] Park JO, Tang SH. Synthesis and characterization of bismaleimides from epoxy resins. J Polym Sci Part A: Polym Chem 1992;30(5):723–9. [5] Hergenrother PM. Thermally curable oligomers and high temperature polymers thereform. Polym Prepr 1984;25(1): 97–9.

1907

[6] Stenzenberger HD. Recent advances in thermosetting polyimides. Br Polym J 1988;20(5):383–96. [7] Stenzenberger HD, Herzog M, Romer W, Scheiblish R, Reeves NJ. Development of thermosetting polyimide resin. Br Polym J 1983;15(1):1–12. [8] Varma IK, Gupta AK, Varma DS. Addition polyimides I. J Appl Polym Sci 1983;28(1):191–9. [9] Evans JR, Owell RA, Tang SS. Polyimides and polyamides with semiXexible spacers. J Polym Sci, Polym Chem Ed 1985;23(4):971–80. [10] Rao BS. Novel bismaleimides via epoxy–carboxy addition reaction: synthesis characterization and thermal stability. J Polym Sci Part C: Polym Lett 1988;26(1):3–10. [11] Mikroyannidis JA, Kourties DA. Curing of epoxy resins with 1-[di(2-chloroethoxyphosphinyl)methyl]-2,4- and -2,6diaminobenzene. J Appl Polym Sci 1984;29(1):197–209. [12] Ichino T, Hasuda Y. New epoxy-imide resins cured with bis(hydroxyphthalimide)s. J Appl Polym Sci 1987;34(4): 1667–75. [13] Shau MD, Lin CW, Yang WH, Lin HR. Properties of cyclic phosphine oxide epoxy cured by diacids and anhydride. J Appl Polym Sci 2002;84(5):950–61. [14] Krevelen DW. Some basic aspects of Xame resistance of polymeric materials. Polym 1975;16(8):615–20.