Polymers for Advanced Technologies Volume 5, pp. 287-291

Cure Behavior and Thermal Stability of an Epoxy: New Bismaleirnide Blends for Composite Matrices Dae Su Kim’*, Mi Jung Han’, Jae Rock Lee’ and Ji Young Chang2 ’ Engineering Material Laborato y, Korea Research Institute of Chemical Technology, PO Box 9, Daedeog-Danji, Taejeon 305-606, Korea

Department of Advanced Chemistry, Ajou University, Suwon, Kyungkido 441-308, Korea

ABSTRACT

A new bismaleimide (BMI) resin was synthesized to formulate epoxy(tetraglycidy1 diaminodiphenyl meth- ane; TGDDM) - bismaleimide thermoset blends for composite matrix applications. 4,4’-diaminodiphenyl methane ( D D M ) was used as an amine curing agent for the TGDDM. A Fourier transform infrared (FTIR) spec- troscopy was employed to characterize the new BMl resin. Cure behavior of the epoxy-BMl blends was studied using a differential scanning calorimeter (DSC). DSC thermograms of the thermoset blends indicated two exothermic peaks. The glass transition temperature of the thermoset blends decreased with BMl content. Thermogravimetric analysis (TGA) was carried out to investigate thermal degradation behavior of the cured epoxy-BMl thermoset blends. The new BMl resin reacted partially with the DDM and weak intercrosslink- ing polymer networks were formed during cure of the thermoset blends.

KEYWORDS: Epoxy, Bismaleimide, Blend, Cure, Thermal analysis

INTRODUCTION Modification of epoxy resins has been carried out to improve the impact strength of cured epoxy parts by incorporating thermoplastic polymers [l, 21, especially those such as the low molecular weight liquid rubbers (ATBN: amine terminated butadiene acrylonitrile copolymer, CTBN: carboxyl terminated butadiene acrylonitrile copolymer) [3-51 and poly(ethy1ene oxide) [6 ] . The impact strength of an

* To whom all correspondence should be addressed.

epoxy system composed of diglycidylether of bisphe- no1 A (DGEBA) and triethylenetetramine (TETA) was considerably increased by adding the liquid rubbers (ATBN and CTBN) [5, 71.

In order to modify properties of thermosetting polymers, thermoset-thermoset blend systems have also been studied. For example, bismaleimide- triazine(biscyanate) (BT) type resins have been de- veloped for laminating applications. Gotro et al. [8] carried out a characterization study of a bismaleimide/biscyanate/epoxy thermoset blend for printed circuit board applications. Instead of blend- ing bisimide resins with the epoxy resin, Scola and Pater [9] used an amine curing agent containing bisimide linkage in order to improve the thermal stability and mechanical properties of the neat epoxy resin. They made a superior graphite fiber- reinforced composite using the bisimide amine cured epoxy resin as a composite matrix.

The thermal stability of epoxy resin would be improved by introducing higher temperature perfor- mance resins such as polyimide and cyanate resin. Woo et al. [lo] reported that the co-cured structure of epoxy(TGDDM + DDS)-bismaleimide (diphenyl- methane BMI (60 wt%) + tolylene BMI (40 wt%)) thermoset blend was a homogeneous inter- crosslinked network. According to their results, the thermal stability (glass transition temperature, T,) of epoxy resin increased linearly with BMI compo- sition. Cure kinetics and chemorheological study for the epoxy (DGEBA) - cyanate(4,4-diphenyl methane dicyanate) blend matrices to enhance thermal stabi- lity and electrical properties of epoxy resin were studied by Chen [ll].

Polyimide networks can be obtained by conden- sation or addition polymerization. Condensation type polyimides are difficult to process because of

CCC 1042-7147/94/050287-05 0 1994 by John Wiley & Sons, Ltd.

Received 5 October 1993 Revised 8 December 1993

288 I Kim et al.

Maleic Anhydride p-Phenylene Diamine Butane-Tetracarboxylic Dianhydride

I f

New Bismaleimide

FIGURE 1. A synthetic scheme of the new BMI resin.

some disadvantages such as insolubility and infusi- bility [12]. Addition type polyimides such as bisma- leimides were developed mainly to overcome pro- cessing disadvantages [13, 141. Thermoplastic [15] and elastomeric [16] modifiers have been incorpor- ated to improve the toughness of neat polyimides because the cured neat polyimides have low impact and fracture toughness. McGreil and Jenkins [17] incorporated a reactively terminated thermoplastic particles in the laminated polyimide composites to improve the interlaminar toughness.

A new bismaleimide resin was synthesized in this study to modify thermal stability and toughness of the tetrafunctional epoxy system. The newly syn- thesized BMI resin was characterized using FTIR spectroscopy. Cure behavior of the epoxy-BMI blends was investigated using a DSC for several blend compositions in this study. Thermogravimetric analysis was carried out to investigate thermal degradation behavior of the epoxy-BMI blends.

EXPERIMENTAL New BMI Synthesis

In order to modify a tetrafunctional epoxy resin, a new bismaleimide (BMI) resin was synthesized from maleic anhydride, p-phenylenediamine and butane tetracarboxylic dianhydride. A simple schematic rep- resentation of synthesizing the new BMI resin is shown in Fig. 1. The butane tetracarboxylic dianhyd- ride shown was synthesized from butane tetracar- boxylic acid by dissolving it in liquid acetic anhyd- ride, and then refluxing the solution for 3 hr, followed by cooling, filtering and drying sequen- tially. In order to synthesize the new BMI, p-phenylene diamine and maleic anhydride were dissolved separately into each DMF (dimethyl- formamide) beaker, and then the two solutions were mixed into one DMF solution, in which the butane tetracarboxylic dianhydride synthesized in this

study was dissolved. The DMF solution was refluxed for 12 hr, and then precipitated by methanol. After drying the precipitates, the new BMI was obtained.

Materials

The epoxy resin and amine curing agent used in this study were TGDDM (MY 720 from Ciba-Geigy) and DDM (4,4-diaminodiphenyl methane from Tokyo Chem.). Epoxy equivalent weight of the TGDDM was 120-135 g/eq. and density of the resin was 1.21 g/cm3 at 25°C. The amine hydrogen equivalent weight of the DDM was 50 g/eq. H. Considering an autocatalytic reaction mechanism of epoxy-amine reaction and the functional groups contained in the new BMI resin, 25 phr of DDM was used at first. The epoxy-BMI blend system containing the stoichio- metric amount (40 parts per hundred of resin (phr)) of DDM was also investigated.

The amount of BMI resin added to the epoxy resin ranged from 0 to 20 phr with respect to the TGDDM used.

FTIR Spectroscopy

A qualitative analysis for the newly synthesized BMI resin was carried out using FTIR spectroscopy (Div- ision FTS-80, Digilab). FTIR spectra of the new BMI resin were obtained in the range between 4000 and 400 cm-'. A powder specimen was made by pressing a mixture (the new BMI and KBr powders) into a parallel disc shape.

Thermal Analysis Using DSC and TGA

In order to study cure behavior of the epoxy-new BMI blends and to measure the glass transition temperatures of cured thermosetting blends, a dyna- mic DSC method was employed. A DuPont DSC 910 and DuPont 9900 thermal analyzer were used. The scanning rate employed for the dynamic DSC meth- ods was lO"C/min. The scanning range was from 25 to 300°C.

Cure Behavior and Thermal Stability of Epoxy-BMI Blend / 289

A DuPont 951 thermogravimetric analyzer (TGA) was employed to investigate thermal degradation behavior of the cured epoxy-BMI thermoset blends. The scanning rate used for the TGA study was also lO"C/min.

RESULTS AND DISCUSSION Characterization of the New BMI by FTIR Spectroscopy In order to improve thermal stability and toughness of the tetrafunctional epoxy resin, a new BMI pre- cursor for polyimide was synthesized.

The FTIR spectrum of the new BMI is shown in Fig. 2. The strong absorbance peak at 1740-1700 cm-' was caused by the carbonyl groups (-CO-) composing a part of the imide group. The peak indicated at 1650-1600 cm-' seemed to be due to the carbon-carbon double bonds (-C=C-) from the reactant maleic anhydride. The absorbance peaks at 3500-3200 cm-' and 1500 cm-' were considered to be due to the unreacted carboxyl group (-CO-OH) and -NH- group of amic acid respectively. The absorbance peak at 1200 cm-' was due to the C-N single bonds of amic acid. The weak absorbance peak at about 800 cm-' was due to the aromatic linkage (-C6H4-).

Rao [18] reported that other characteristic absor- bance peaks for imide groups were observed in the range of 1390 cm-' (imide 11), 1110 cm-' (imide 111) and 700 cm-' (imide IV). The FTIR spectra of the new BMI resin synthesized in this work also showed the imide characteristic peaks at 1360 cm-', 1100 cm-' and 670 cm-'. From the result of FTIR absorbance peaks, it was found that the imidization reaction was not performed completely, and the new BMI resin consisted of bisimides and amic acids. The unreacted functional groups (-COOH and -NH-) of the amic acids may react with the epoxy group when the new BMI resin is blended with the epoxy resin. So it was expected that the epoxy-new BMI thermoset blend system might have an intercrosslinked struc- ture after cure.

"1

0 I 4 4 ) 0 3 C a Q 2 M o 1500 XKK) 500

W e NumEor (an-')

FIGURE 2. FTIR spectrum of the new BMI resin.

A 25 phr DOM 100Clmin

1 I I 1 0 100 200 300

Temp. ( "C )

FIGURE 3. DSC thermograms for the epoxy-BMI thermoset blends.

Cure Behavior of the Epoxy-New BMI Blends Figure 3 shows dynamic DSC thermograms for the epoxy-new BMI blends containing 0-20 phr of BMI. The curing agent (DDM) added to the blends was 25 parts per hundred of epoxy resin by weight (0.65 stoichiometry to the oxirane rings contained in the neat epoxy resin) since the epoxy curing reaction occurred usually through the autocatalytic reaction mechanism and the new BMI resin also contained some functional groups (-COOH and -NH-) reac- tive to the oxirane ring of the epoxy resin.

Several chemical reactions can occur during the curing process of the epoxy-new BMI thermoset blend system: (a) ring opening reaction of epoxy resin due to the active amne hydrogen of DDM; (b) autocatalytic reaction of epoxy resin due to -OH group produced through the ring opening reaction (a); (c) additive reaction of amine (DDM) to the double bond of BMI, Michael addition; (d) homo- polymerization of BMI through the double bond of BMI; (e) ring opening reaction of epoxy resin due to the carboxyl group (-COOH) remaining in the new BMI resin; ( f ) ring opening reaction of epoxy resin due to the amine hydrogen (-NH-) remaining in the BMI; (g) further imide ring formation by conden- sation reaction of the amic acid.

The first DSC thermogram from the top in Fig. 3 shows dynamic curing behavior of the neat epoxy systems (TGDDM + DDM) containing different amounts of DDM. An exothermic reaction peak due to the reaction between the oxirane ring of epoxy group and the active amine hydrogen from DDM was observed at about 190°C when the heating rate was 10"C/min. All the DSC thermograms for the epoxy- BMI blends indicated bimodal peaks indicating two distinct reaction mechanisms. The exothermic reac- tion peaks due to incorporation of the new BMI were observed at about 140°C. The exothermic peaks at 140°C seemed to be caused by reaction (c) (Michael addition). The first peak became large as the content of the new BMI was increased because the reaction heat contributed by the new BMI was increased.

The activation energy for the ring opening reac- tion of epoxy resin by the active amine hydrogen of aromatic amine usually ranges from 42 to 50 kJ/mol

290 I Kim et al.

0 40 phr DDM

[19]. The activation energy for the Michael addition ranges from 38 to 46 kJ/mol [20], while the activation energy for the BMI homopolymerization through the double bond is about 58 kJ/mol [21]. Major reactions occurring during cure of this epoxy-BMI blend were considered to be reactions (a) and (c). Reaction (d) is also important but it needs a high temperature (at least 180°C [20]) for activation. The reaction heat due to reactions (e) and ( f ) seemed to be very small and it would contribute to the second peak which is mainly due to reaction (a). Reaction (g) could also occur, but it seems to be negligible because it needs very high temperature (250-300°C) [22].

Figure 4 shows the heats of reaction per unit mass as a function of the new BMI composition in the epoxy-BMI blends containing 25 and 40 phr of DDM. The composition of BMI is expressed as the weight percentage of BMI in the epoxy-BMI blends except for the DDM weight. The heats of reaction for the pure epoxy resin (TGDDM + DDM) were 370 J /g for 25 phr of DDM and 393 J /g for 40 phr of DDM respectively. Compared with the neat epoxy reac- tion, the heat of reaction for the neat bismaleimide is usually low because the BMI homopolymerization involves only the energy difference between a dou- ble bond and a single bond. Woo et al. [lo] reported that the heat of reaction for the BMI homopolymer- ization of diphenylmethane BMI (60 wto/o) and tolyl- ene BMI (40 wt%) mixture was 120 J/g. The heat of reaction for the blends decreased with BMI compo- sition because the number of functional groups per unit mass might decrease as the BMI composition was increased.

If the new BMI resin contains only pure bis- imides and there is no correlation between the epoxy-amine reaction and the new BMI-amine or BMI homopolymerization, the heat of reaction curve in Fig. 4 should have a linear relationship with BMI composition. The nonlinear relationship between the heat of reaction for the epoxy-BMI blends and BMI composition indicated indirectly that a copoly- merization reaction between three components of epoxy-BMI-DDM would occur. Consequently, the resultant structure of the cured epoxy-BMI blend would be an intercrosslinked network.

The glass transition temperatures of the cured epoxy - BMI thermoset blends, containing 25 phr of

183.1

177.0

0 I p h r l B M I

5

7- 10 146.6 t

20 I 25phr DDM

100 200 300

Temp.( OC )

FIGURE 5. Glass transition temperatures of the cured epoxy-BMI blends.

DDM, are shown in Fig. 5 as a function of BMI composition. The glass transition temperature of the cured neat epoxy resin was 183.1"C. Tg decreased as the new BMI content in the blend was increased. The crosslinking density of the epoxy-BMI blend de- creased with BMI composition because some portion of amine hydrogen from DDM reacted with the new BMI resin through the Michael addition reaction. This seemed to be the main reason for the Tg decrease with BMI composition. Another important reason for the Tg decrease is that the bisimide contents in the new BMI resin might be not high enough. From a viewpoint of molecular structure, there is the possi- bility that the cured epoxy-BMI blends have worse structure (low crosslinking density) than the neat epoxy system (TGDDM + DDM) because amic acid contained in the new BMI reacts partly with the epoxy group to form an intercrosslinked structure. Even though the intercrosslinking network would be formed during cure of the blends, the average mol- ecular weight between crosslinking points was increased with BMI composition. The single Tg observed from the dynamic DSC scan for each epoxy-BMI blend indicated that the morphology of the cured thermoset blend has one homogeneous phase.

Thermal Degradation of the Epoxy-New BMI Blends Figure 6 shows the TGA data for the cured epoxy- BMI blends with BMI composition of 0-20 phr and 25 phr of DDM. The neat epoxy resin started to degrade at about 280°C. The initiation temperature of the thermal degradation process decreased slightly with BMI composition. This is due to the fact that the crosslinking density of the epoxy-BMI blend system decreased as the composition of BMI was increased. It was considered that the portion of flexible chains in the cured epoxy-BMI blend was increased with BMI composition. But final residual char yield increased with BMI composition because the BMI resin had higher thermal stability than epoxy resin. The degradation rate of the epoxy-BMI blend system decreased with BMI composition even though the

Cure Behavior and Thermal Stability of Epoxy-BMI Blend / 291

20 .'\ ------

I I I 406

1 0 200 600 800

Temp.[ O c ]

FIGURE 6. TGA curves for the cured epoxy-BMI blends.

blend containing the higher BMI composition degraded earlier than the neat epoxy resin.

Thermal stability of the epoxy resin was not improved, and this result proved indirectly the fact that crosslinked polyimide was not produced suffi- ciently during cure of the epoxy-BMI blends since the newly synthesized BMI resin did not contain enough bisimides to form polyimide networks.

From the results that the glass transition temper- ature (measured by DSC at the heating rate of 10°C/ min) of the neat epoxy resin decreased with BMI composition, it was considered that the new BMI resin synthesized in this study behaved like a thermoplastic modifier. When a thermoplastic modi- fier is incorporated into a brittle thermoset polymer in order to improve the impact and fracture tough- ness, the sacrifice of thermal (T,) and some mech- anical (modulus) properties is not avoidable. But compared with the epoxy resin containing the usual thermoplastic modifiers such as ATBN and CTBN, thermal stability of the epoxy-BMI thermoset blend studied in this work was decreased only negligibly. Even though the new BMI resin behaved as a thermoplastic modifier, it would be better than the usual thermoplastic modifiers at improving the impact strength of the neat epoxy resin.

CONCLUSIONS The characterization study of the new BMI resin using FTIR showed that the BMI resin contained imide rings as well as amic acids. The DSC thermo- grams of the epoxy (TGDDM + DDM)-BMI thermo- set blends indicated two reaction exothermic peaks. The first peak of the thermograms was due to the reaction (Michael addition reaction) caused by the BMI resins and the second peak due to the epoxy curing reaction with DDM. The glass transition temperature of the thermoset blends decreased with BMI composition because the crosslinking density

decreased with BMI composition. Even though the thermal degradation process started earlier in the case of high BMI composition, the degradation rate decreased with BMI composition. The new BMI resin reacted partly with amine curing agent, and hence a weak intercrosslinking polymer networks were formed during cure of the thermoset blends. According to the TGA results (initiation temperature of thermal degradation), the thermal stability of the epoxy resin was not improved by blending the new BMI resin since the bisimide contents in the new BMI resin were not enough. The new BMI resin can be a better modifier than the usual thermoplastic modifiers for the neat epoxy resin.

ACKNOWLEDGMENT This work was sponsored by MOST (Ministry of Sci. & Tech.) of Korea in the national project category.

REFERENCES 1.

2.

3.

4.

5.

6. 7. 8.

9.

10.

11. 12. 13.

14.

15.

16.

17.

18. 19. 20,

21. 22.

C. B. Bucknall and J. K. Partridge, Polymer, 24, 639 (1983). A. M. Abrahim, T. J. Quinlivan and J. C. Seferis, ACS Polym. Prepr., 26, 277 (1985). J. N. Sultan and F. J. MaGarry, Polym. Eng. Sci., 23, 29 (1973). J. B. Enns and J. K. Gillham, Polym. Eng. Sci., 28, 2567 (1983). D. S. Kim and S. C. Kim, Polym. A d v . Tech., 2, 211 (1990). Q. Guo, X. Peng and Z. Wang, Polymer, 32,53 (1991). F. J. McGarry, Proc. Royal SOC. London, A329,59 (1970). 1. T. Gotro, B. K. Appelt and K. I. Papathomas, Polym. Comp., 8(2), 39, 1987. D. A. Scola and R. H. Pater, 'The properties of novel bisimide amine cured Epoxy/Celion 6000 graphite fiber composites', 13th National SAMPE Tech. Conf., Oct. 13-15 (1981). E. M. Woo, L. B. Chen and J. C. Seferis, J. Mat. Sci., 22, 3665 (1987). Y. T. Chen, PhD Thesis, Univ. of Minn (1993). P. M. Hergenrother, Polym. Prepr., 25(2), 97 (1984). A. Bargain, A. Combet and P. Grosjean, US patent 3562223 (1971). H. D. Stenzenberger, J . Appl . Polym. Sci., Appl . Pol. Symp. , 32, 91 (1977). A. D. Gupta and D. Kumar, J. Appl . Polym. Sci., 30, 3879 (1985). I. K. Varma, G. M. Fohlen, J. A. Parker and D. S. Varma, in Polyimides: Synthesis, Characterization and Applications, K. L. Mittal (Ed.), Plenum, New York (1984). P. T. McGrail and S. D. Jenkins, Polymer, 34(4), 677 (1993). B. S. Rao, J. Polym. Sci., 26, 3 (1988). T. Kamon, Netsukoukasei Jushi, 6, 94 (1985). A. Nagai, A. Takahashi, M. Wajima and K. Tsukanish, Poly. I., 20(2), 125 (1988). L. T. Pappalardo, J. Appl . Polym. Sci., 22, 809 (1977). P. M. Hergenrother, SAMPE Quarterly, 3, 1 (1971).