Indian Journal of Chemical Technology Vol. 10, July 2003, pp. 408-413

Articles

Thermal behaviour of bismaleimides, allyl nadic-imides and their blends

Indrajeet Singh 1, Anju Srisvastava2 & I K Varrna3*

1 Department of Chemistry, C.C.R (P.G.) College, Muzaffamagar, India 2 Department of Chemistry, Hindu College, Delhi University, India

3 Centre for Polymer Science and Engineering, l.I.T. Delhi, India

Received 3 November 2001 ; revised received 7 February 2003; accepted 12 March 2003

Synthesis and characterisation of bis(4-maleimidophenyl) sulphone (B) and its Michael adducts E and F, prepared by reacting 1 mol of B with 0.33 mols of 414' -diaminodiph-enyl ether and 919-bis(4-aminophenyl)fluorene is described. Curing characteristics of these resins and bis(4-allyl nadicimidophenyl) methane (S) and 1,6-diallyl nadicimidohexane (V) were studied in static air atmosphere by differential scanning calorimetry. · Curing temperatures depended on the structure of the resins and highest curing temperatures were observed in s 'and v samples. Thermal behaviour of blends of B, E, and F with S and V resins was studied as a function 'Qf blends composition. Thermogravimetric analysis of cured resin blends showed alll improvement in thermal stability.

Addition polyimides, such as bismaleimides and bisnadicimides, exhibit outstanding thermooxidative stability, exceptional dielectric properties and excellent resistance to humidity at elevated temperatures 1

•3

• These low molecular weight monomers and oligomers cure thermally to yield a highly crosslinked infusible polymer network which is insoluble in all organic solvents. These resins are inherently brittle due to their high cross-link density and aromatic structure. Cross-link density of maleimide resins has been successfully reduced by chain-extension reaction with appropriate nucleophiles (Michael addition reaction) such as diamines4

•5

, amjno-acid hydrazide6, etc. Stoichiometric

mixing of biamaleimide with diamine leads to the formation of linear polyaspartimides which could be cast into a flexible film with poor thermal stabilit/. Thus by a careful selection of bismalemide and diamine ratio, it is possible to control the crosslink density with retention of outstanding thermal stability of these resins.

Another major problem of these addition polyimides is high curing temperatures and in some cases a very narrow processing window. The problem becomes excerbated in composite fabrication, particularly in large and/or complex composite structures. Blending of addition polyimides with

Part of thi s paper was presented at Macro 95 at Trivandrum *For correspondence (E-mail: ikvarma@hotmail.com; Fax: 26591421 )

several thermoset resins and thermoplastics has been carried out in the past with an rum to improve the processability and performance. Co-curing of allyl nadic-imides with phosphorus containing nadimide resin8 yielded products with improved char yield and better processability. Blends of chain extended bismaleimides and bismaleirrudes showed a decrease in melting point and curing temperature9

. Sirrularly, co-curing of ethynyl terminated oligoimide and methyl nadimides 10 or bismaleimides and nadirrudes 11

resulted in a decrease in curing temperatures and enhancement of char residue. It is thus possible to adjust the curing temperatures over a range of about 1 00°C by changing the composition of the blend.

Studies on curing of allyl-nadicimides and bismaleimides have not been reported. Effect of blending bi smaleimide monomer and chain extended bismaleimides with allyl nadic-imides on curing characteristics and thermal stability of cured network is now being reported here. Three resin samples B, E and F having the structure shown in scheme 1, were synthesised for this purpose.

Experimental Procedure Materials

Na2C03 (BDH), KOH pellets (E.Merck), glacial acetic acid (BDH), methyl ethyl ketone (MEK, BDH), chloroform (E.Merck) and maleic anhydride were used as such. Acetic anhydride (BDH) was distilled before use. The diamines 4,4' -diarruno diphenyl ether (Fluka), 4,4'-diarruno diphenyl sulphone (Fluka) were

Singh et al.: Thermal behaviour of bismaleimides, allyl nadic-imides and their blends

~ -o-~-o-~0 + H2N ~ ;/ ~ ~ ;/ NH2

0 0

2mol

! 1 mol

1~"-o-f-o-"~1 4, 4'-bismaleimidophenyl sulphone (B)

Resin Designation

When X= 0

Scheme I

dried under vacuum. Acetone (BDH) was dried over KzC03 (BDH) and distilled before use. Anhydrous sodium acetate (BDH) was obtained by fusion. Allyl nadic-imides bis (4-allyl nadicimidophenyl) methane (S) and 1,6-diallyl nadicimidohexane (V) of the structure shown in scheme 2, were procured from Ciba-Geigy.

Synthesis of bismaleimide B

E

F

Scheme 2

Articles

(S)

In a three necked flask equipped with a condenser and nitrogen inlet tube, 0.5 mole of 4,4' diaminodiphenyl sulphone was placed and 350 mL of acetone was added. The temperature was raised to 60°C and solution was stirred by a magnetic stirrer. Maleic anhyride (0.10 mole) was then added in several portions and solution was stirred for 2h at 60°C. Cyclization of the amic-acid intermediate was

done by adding fused sodium acetate and acetic anhydride and heating the solution at 60°C for another 2 h. Bismaleimide was isolated by precipitation, using water as a non-solvent. The precipitates were repea-

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uJ LJ z <( ...... ...... i: Vl z <( 0:: ......

~ 0

4600

Indian J. Chern. Techno!., July 2003

1144

400

WAVENUMBER ( Crn-1)

Fig. 1- FT-IR spectrum of resin B

2.-----------------------------~ tedly washed with water and sodium bicarbonate (NaHC03) and finally with water. Purification of bismaleimide was done by using silica gel (60-120 mesh) column using chloroform as the solvent. The solution was concentrated on a rotary evaporator and crystals of bismaleimide separated out on cooling. Yield= 80-88%. m.p. -247-48°C.

272 -4'C

Temperature ( 'c l

Fig. 2- DSC scan of resin B

410

Preparation of chain extended bismaleimides E&F Chain extension reaction of B with 9,9-bis(4-

aminophenyl)fluorene (BAF) and 4,4' -diamino diphenyl ether (DADPE) was carried out in acetone/ methyl ethyl ketone solution using a molar ratio of I :0.33. The appropriate quantities of bi smaleimide and amine were added to the solvent with continuous stirring and the solution was refluxed for 4-5 h till a homogeneous solution was obtained. The solvent was removed under reduced pressure using a rotary evaporator and shining powder of bismaleimide­amine adducts were obtained.

Preparation of resin blends Three blend compositions were prepared by

dissolving bismaleimide/chain extended bismale-

Singh et al.: Thermal behaviour of bismaleimides, allyl nadic-imides and their blends Articles

imides (B, E or F) in acetone or methyl ethyl ketone and allyl nadic-imides (S, V) in the ratio of 1:3,1:1,3:1. These compositions have been designated by writing the letter designation of bismaleimides/ chain extended maleimides and allyl nadic-imides. A subscript indicating the ratio of the two components was appended after the letter designation of the imide resin. For example, BS, BS3 and B3S indicate 1:1, 1:3 and 3:1 w/w ratio of B and S, respectively . Bismaleimides/chain extended bismaleimides and allyl nadic-imide resins were dissolved in acetone or methyl ethyl ketone and solvent removed under reduced pressure using a rotary evaporator. Curing was done at 280°C for I h in a muffle furnace.

Characterization Structural characterization of various resin samples

was done by recording Ff-IR spectra in KBr pellets

110

100

- 90

* ~ BO .!::?'

3 70

I 6o ~

50

(a)

467.9 't r---+---~

I

I , I ',

J ' ,_

---~---___., \ ------------1 I I

525·J'cl ___ _

20

c

16]_

2: ~ Cl

s -~ "' 0

4,

-4

I I

...!.

401~00;---~3~00;-----,~--;;-;b;---==L9,.,!0l,;-0 -...J. B

Temperature ( 'c )

using Perkin-Elmer 580 B infrared spectrophotometer. 1H NMR spectra were recorded in DMSO-d6 using a Jeol JNM-FX 100 Ff-NMR spectrometer. A DuPont 9900 thermal analyser with 910 DSC module was used for studying the curing behaviour of resin formulations. A DuPont 1090 thermal analyser with a 951 TG module was used for assessing relative thermal behaviour of resin formulations. Thermogravimetric traces of 1 0±2 mg size samples were recorded in nitrogen atmosphere (flow rate 50mL/min) at a heating rate of 10°C/min.

Results and Discussion In the IR spectra of various maleimide resin

samples (Fig. 1) the characteristic carbonyl group was observed at 1780±5 em·' and 1720±5 em·' . The other prominent absorptions arising due to phenyl group (1576±10 cm- 1 & 1490±10 cm-1

), C-N (1370±5 cm.1)

and so2 group (1150±5 em·'& 1330±5cm-1) were also observed. In chain extended bismaleimide, presence of NH group was indicated by absorption band at 3380 -3400 cm-1

•

In 1 H NMR spectra, aromatic proton appeared as a multiplet at 7.22-8.08 ppm and the olefinic protons at 6.49 ppm. These studies confirmed the structure of various samples.

The melting and curing behaviour of these resins was investigated by DSC. A sharp endotherm associated with melting ·was observed in bismaleimide B at 247°C. Exothermic transition associated with curing was observed in the temperature range of 248-3500C. The exothermic peak temperature, T exo• was observed at 272°C (Fig. 2). The Te xo decreased on chain extension of B with amines and the values of 189 and 226°C were observed in samples E and F respectively.

In allyl nadic-imide resins the curing exotherm was observed in the temperature range of 230-350°C. Based on the results of the DSC scan, it was decided to carry out the isothermal curing of various resin samples at 280°C for 1 h.

Thermogravimetric analysis of cured resin samples was done to evaluate relative thermal stability. Typical TG trace is shown in Fig 3. The initial decomposition temperature (Ti) and temperature of maximum rate of weight loss (T max) were noted down from these traces and the results are summarized in the Table 1. Char yield at 800°C was also determined. The highest char yield of 44% was observed in F sample and lowest value was in allyl nadic-imide V

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Table !-Thermal behaviour of cured resin samples

Resin formulation T;, oc Tmv.• oC Yc, %

B 468 488 42 E 394 430 35.5 F 385 430 44 s 48 1 497 29.0 v 470 505 18.5

Table 2-Thermal behaviour of cured bismaleimide Band allyl nadicimide blends

Resin formulation T;, oc Tmax• oc Yc,%

BS3 438 473 38 BS 437 469 43 8 3S 467 490 44 BV3 457 503 26 BY 463 493 31 B3V 468 491 37

F:s 0

---------50

~ 0

>=' 40

------

50

40 -----_--a-------- 30

0.5 0.1

Wt. Fraction of B/EIF

Fig. 4-Plot of char yield (N2 atmosphere, 800°C) versus weight fraction of S in the blends of bismaleimide resins B, E and F

(18.5% ). The presence of fused aromatic rings in sample F may be responsible for this behaviour. However, the initial decomposition temperature was decreased by chain extension reaction of bismaleimide resin B.

412

Indian J. Chern. Techno!., July 2003

Table 3--Thermal behaviour of cured chain extended bismaleimide and allyl nadicimide blends

Resin fommlation T; , oc Tma.P oC y"%

ES3 443 486 36 FS3 444 485 42 ES 416 461 40 FS 429 473 45 E3S 407 447 37 F3S 389 468 54 EV3 447 491 22 FV3 453 494 26 EV 416 465 30 FV 433 480 34.5 E3V 407 448 3 1 F3V 407 463 42

F:Y

40 /

/ /

/ /

/

30 __.,....,..,

/ _,.....,..,..,.., /

20 ,...:;.---

E:v

40

----30 --- 30 ~ -------->=' ----o

20 .:o--- 20

B:V 40 //

4Q ·

...>' ........

------30 ,...P---- l) -o,....... - --, -- 20

0.5 1.0

Wt fraction of BIEIF

Fig. 5-Plot of char yield (N2 atmosphere, 800°C) versus weight fraction of V in the blends of bismaleimide resins B, E and F

Blending of bismaleimide B with allyl nadic-irnides affected the thermal stability and the char yield changed with the composition of the blend. An increase in bismaleimide content resulted in an increase in the char residue in these blends (Table 2). The char yields in the co-cured resin samples were also calculated by rule of mixtures. The observed char

Singh et al. : Thermal behaviour of bi smaleimides, allyl nadic-imides and their blends Articles

11+111-

Scheme 3

yields were higher than the calculated values. A synergistic improvement in char yield was observed in blends of bismaleimide B and allyl nadic-imide resin S (Fig. 4). However, replacement of S by V did not show such synergism (Fig. 5).

Blending of chain extended bismaleimide and allyl nadic-imide also affected the thermal behaviour of the cured resins (Table 3). The highest char yield of 54% was observed in F3S blends. In these resin samples also a synergistic behaviour was observed when allyl nadic-imide S was used (Fig. 4), whereas with resin V no synergism was observed with bismaleimide B (Fig. 5). A slight improvement in char yields was observed when chain extended bismaleimide sample E or F were used along with V in the blends (Fig. 5) .

Co-curing of resin blends may result in a polymer network comprising both constituents. Thermal behaviour therefore would be influenced by the structure of network formed . An improvement in the thermal stability on co-curing of allyl nadic-imide S with different bismaleimide samples was observed. Similar behaviour has been earlier observed in blends of allyl nadic-irnides and nadimide resins. This

synergism may be explained on the basis of co-curing of resins according to the reaction scheme 3, proposed earlier for a two component high performance resin system based on bismaleimide and 2,2' -diallyl bisphenol A 12

-15

. Copolymerisation of the constituent takes place via ene-type linear chain extension reaction followed by Diels-Alder adducts formation.

Conclusion Thus the studies have indicated that curing

temperatures of bismaleimide resins can be reduced by blending with allyl nadicimides. Optimised blends of bismaleimides and allyl nadicimide show a synergistic imrovement in thermal stability of cured resins.

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& Canning M, lnt SAMPE Techn Conf, 17 (1985) 147.

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