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1 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices Synthesis and Characterization of Bismaleimide (1,3- bismaleimido benzene) Vinyl Ester Oligomer Modified Unsaturated Polyester Interpenetrating Matrices 1 R. Josephine Sharmila, 2 M. Alagar* and 2 R. Suresh Kumar 1 Department of Chemistry, Easwari Engg. College, Chennai , India 2 Department of Chemical Engineering, Anna University, Chennai , India Received: 5 April 2005 Accepted: 8 August 2005 ABSTRACT Bismaleimide - vinyl ester oligomer modified unsaturated polyester interpenetrating network systems were developed. Unsaturated polyester was modified with 10%, 20%, 30% (by wt) of vinyl ester oligomer with benzoyl peroxide as curing agents. The vinyl ester oligomer toughened unsaturated polyester matrix systems were further modifi ed with 5%, 10% and 15% (by wt.) of 1,3- bismaleimido benzene. Thermal behaviour of the matrices were characterised by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and heat defl ection temperature analysis. Mechanical properties were studied by ASTM methods. Data obtained from mechanical and thermal studies indicated that the introduction of vinyl ester oligomer into unsaturated polyester improves the fracture toughness with marginal reduction in the values of heat defl ection temperature, and glass transition temperature. However, the introduction of BMI into VEO incorporated unsaturated polyester resin enhanced both thermal and mechanical behaviour of unsaturated polyester resin. INTRODUCTION Conventional structural material for high performance engineering and aerospace applications is replaced recently by reliable high performance composite materials. Composite materials are lightweight, has better impact strength, good thermal stability and improved mechanical properties. The unique properties of high strength to weight ratio of composites led to active research on two main aspects of composites namely reinforcements *Corresponding author. Fax: address: mkalagar@yahoo.com (Dr. M. Alagar) Rapra Technology, 2006 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

2 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar and matrix resins. Advanced composites exhibit desirable physical and chemical properties that include lightweight coupled with high stiffness and strength along with direction of the reinforcing fibre, dimensional stability, temperature and chemical resistance. Advanced composites are replacing metal components in a large number of applications, particularly, in the field of aerospace industry. Composite materials are widely used in engineering and aerospace applications not only due to their physico-mechanical and chemical characteristics but also to the capability of changing their properties according to the required applications. Polymeric composites continue to demonstrate a marked impact on the materials world due to their lightweight, greater stiffness, higher operating temperature, higher reliability and affordability. One of the major advantages of using polymers in structural composites is the ease of fabrication of complex structural shapes. The polymeric resin systems used in manufacturing advanced composites are thermoplastic and thermosetting. Some of the more common thermosets include epoxies, polyester, vinyl ester, phenolics, amino resins, bismaleimides, polyimides and polyamides. Unsaturated polyester resins have been used in the development of high performance light weight fibre reinforced composites, for manufacturing marine crafts, automotive parts (1) etc., because of their excellent mechanical (2) and light weight properties. Unsaturated polyester resins exhibits many desirable properties, such as high strength and modulus, excellent chemical and solvent resistance, good thermal, and electrical properties, outstanding adhesion to various substrates, and easy processability under various conditions. Unsaturated polyester resins are also widely used as moulding compounds or encapsulation materials for electronic components. However, unsaturated polyester has high cross-linking density and high shrinkage during cure, which affects its impact behaviour. In order to improve impact behaviour and strain to failure, the liquid rubber (3-6), low profile additives like poly (methylmethacrylate), poly(vinyl acetate) and poly propylene glycol (7-9), and rubbers like amine-terminated acrylonitrile butadience (ATBN), carboxylterminated acrylonitrile butadiene (CTBN), hydroxyl-terminated acrylonitrile butadiene (HTBN) (10) and polyurethanes (11) have been used for unsaturated polyester. The modified products have good impact behaviour but failed to increase the thermo mechanical properties of cured unsaturated polyester resin in the high temperature region. Hence, a suitable polymeric material is needed to retain the desirable properties of unsaturated polyester resin like stiffness and glass transition temperature (T g ). To achieve this, development of an intercrosslinked polymer network having flexible and rigid molecular segments can be considered. Intercrosslinking network mechanism provides matrix materials with single glass transition temperature 212 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

3 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices when compared with polymer blends, which normally exhibit unusual glass transition temperature. Vinyl ester oligomer and bismaleimides are chosen as flexible and rigid segments for the modification of unsaturated polyester resin. VEO is the most suitable polymer because of its versatile behaviour like flexibility, high thermal and thermo-oxidative stability, high moisture resistance, and good dielectric properties. Though the incorporation of VEO into unsaturated polyester systems improves its impact strength, stress-strain properties, but reduces the glass transition temperature. In order to enhance the thermal properties, modification of VEO toughened unsaturated polyester system is carried out with rigid materials like bismaleimides is essential. The introduction of BMI into epoxy (12,13), siliconised epoxy (14), unsaturated polyester modified epoxy (15) resin enhanced thermomechanical properties. In the present work it is proposed to use bismaleimides, a reactive component to modify unsaturated polyester (16) and is expected to increase thermomechanical properties. Bismaleimides are a leading class of thermosetting polyimides. Bismaleimides have superior thermo mechanical properties viz., high cross linking density, high Tg, high char yield, high thermal stability, excellent fire resistance, specific strength and specific modulus and low water absorption. BMI resins are produced from the reaction of a diamine and maleic anhydride. Polymerization occurs through the maleimide double bond, without the evolution of volatiles, in the range C. BMI resins are important for high temperature applications and they can be used temperature up to 230 C. The major applications for BMI structural composites are in aero-engines and military aircraft. Hence, an attempt is made in the present work to modify the commercially available polyester resin using vinyl ester and bismaleimide to improve thermo mechanical properties without an appreciable loss in strength by forming an intercrosslinked network of BMI-vinyl ester modified unsaturated polyester using BPO as curing agent. EXPERIMENTAL Materials General purpose unsaturated polyester resin, epoxy resin (diglycidyl ether of bisphenol A, DGEBA) LY556, having an epoxy equivalent of about ; Ciba-Geigy (India) and acrylic acid (Merck, Darmstadt, Germany) were used as received. VEO and BMI were prepared in the laboratory. Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

4 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar Preparation of vinyl ester oligomer Vinyl ester oligomer was prepared using epoxy resin (LY 556) and acrylic acid. 1 mol bisphenol A-type epoxy resin and 2 mol acrylic acid is mixed in RB flask at 70 C for 4 h with constant stirring to obtain vinyl ester oligomer The reaction for the preparation of vinyl ester oligomer is given in Scheme 1 and characterization of VEO was reported in elsewhere in detail (17,18). Preparation of unsaturated polyester - VEO blends A known amount of unsaturated polyester resin (100 g), and varying amounts of VEO (10, 20 and 30 g) was taken in a round bottomed flask and stirred at room temperature 30 C for 1 h. After obtaining homogenous mixture, benzoyl peroxide (2% by wt) were mixed at 50 C for 10 min with constant stirring. It was then subjected to vacuum to remove the entrapped air and then cast and cured at 60 C for 4 hours and 100 C for 2 h. The castings were post cured at 120 C for 2 h and finally removed from the mold and characterised. Preparation of 1,3 - bismaleimido benzene Bismaleimide was prepared according to the reported procedure (19). In a three-necked flask fitted with paddle stirrer, reflux condenser and nitrogen inlet, 600 ml of acetone, 1.0 mol (98.1 g) of maleic anhydride and 0.5 mol (54.1 g) of m-phenylenediamine were added. Rapid formation of precipitate of bismaleiamic acid occurred on mixing the reactants together, and the mixture was allowed to stand for 30 min to complete the reaction. To the reaction vessel were now added 1 g of nickel acetate and 25 ml of triethylamine and Scheme 1. Synthesis of vinyl ester oligomer 214 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

5 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices the mixture was refluxed. By means of pressure equalizing funnel ml of acetic anhydride was added to the refluxing mixture and heating was continued for an additional 3 h. The mixture was diluted with 500 ml of water and chilled to crystallize the bismaleimide. The resulting bismaleimide was filtered and recrystallized from acetone. The reaction for the preparation of bismeleimide is given in Scheme 2 (synthesis of 1,3-bis(maleimido)benzene) Preparation of BMI-VEO-UP blend A typical BMI-VEO-UP matrix was prepared as follows. A known amount of VEO modified UP as mentioned in previous paragraph is taken in round bottomed flask and varying amounts of 1,3 bismaleimide benzene was dissolved at 100 C under vigorous stirring. The temperature was brought down to 50 C after complete dissolution. Benzoyl peroxide (2% by wt) was added as curing agent. The product was subjected to vacuum to remove entrapped air and then cast and cured at 60 C for 4 h and 100 C for 2 h. The castings were post cured at 120 C for 2 h and finally removed from the mold and characterized. Scheme 2. Synthesis of 1,3-bismaleimido benzene Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

6 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar TEST METHODS Thermal studies Differential Scanning Calorimeter DSC 2910 (TA instruments USA) was used to determine glass transition temperature (Tg) in the temperature range between 50 C and 250 C at a heating rate of 10 C /min in nitrogen atmosphere. Thermogravimetric analysis (TGA) of the polymer matrices was carried out using a Thermal analyst 2000 (TA instruments USA) at a heating rate of 10 C /min in an inert atmosphere to determine thermal stability of the matrices. Heat deflection temperature It is also called (DTUL) Deflection Temperature of polymeric material under flexural load. Heat distortion temperature apparatus was used to determine HDT as per ASTM- D648 using specimens of size 120 mm in length, and 13 mm in thickness are kept in oil bath under a load of 1.82 MPa. The temperature was raised at a rate of 2 C /min and the temperature was noted when the specimen deflected by 0.25 mm. Tensile and flexural properties Tensile properties are studied as per ASTM - D3039 using Instron testing machine (Model 6025 UK) at 2 mm/min cross head speed, using specimen with a width of 25 mm, length of 150 mm, and thickness of 3 mm. The flexural strength are studied as per ASTM D790 using specimen with 10 mm in width and 90 mm in length using Instron testing machine (Model 6025 UK). Unnotched Izod impact test The unnotched Izod impact strength is studied as per ASTM D using specimens width 10 mm and length 63 mm. Water absorption Samples were tested according to ASTM D570 using specimens of 60 mm square and 3 mm thickness. Test specimen was immersed in water for 24 h at 30 C and the percentage of water absorbed by the specimen is calculated using the equation given below. % increase in weight = (W 2 - W 1 ) x 100 /W 1 Where W 1 is the initial weight of the sample. W 2 is the final weight of the sample after immersed in water for 24 h at 30 C. 216 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

7 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices RESULTS AND DISCUSSION Thermal properties The DSC thermogram of unsaturated polyester and vinyl ester modified unsaturated polyester showed a decrease in glass transition temperature (Tg) (20). It is observed that Tg of unsaturated polyester decreases with increasing vinyl ester oligomer content. This may be explained as the increase in chain length creates more free volume, which makes the chain flexible. In contrast, the introduction of BMI into unsaturated polyester and vinyl ester modified unsaturated polyester increases the Tg values. This may be attributed to the formation of intercross linking network, which in turn decreases molecular relaxation and increases rigidity thereby increases glass transition temperature. The glass transition temperature of bismaleimide modified VEO-unsaturated system, VEO modified UP system and unmodified UP systems are presented in Figure 1. Thermo gravimetric analysis (TGA) is used to assess the thermal stability and utility of polymer matrices for thermal insulation applications. The TGA curves of unmodified unsaturated polyester, vinyl ester modified unsaturated polyester matrix systems showed an increase in degradation temperature Figure 1. DSC traces of a) unmodified UP resin b) VEO (10% by wt.) modified UP c) BMI (10% by wt.) modified UP d) BMI (10% by wt.) modified VEO (10% by wt.) UP system Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

8 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar which implies that the presence of VEO molecular segments in unsaturated polyester matrix enhances the thermal stability and decrease in percentage weight loss (20). The inclusion of BMI into VEO-unsaturated polyester showed an increase in degradation temperature. This may also due to the formation of intercrosslinking network between unsaturated polyester and bismaleimide and the thermally stable heterocyclic aromatic rigid structure of BMI. The degradation temperature of BMI modified VEO unsaturated polyester is shown in Figure 2. Cure reaction behaviour The DSC thermograms of VEO (10% by wt.), BMI (10% by wt.), VEO BMI (10% by wt.) modified UP systems are presented in the Figure 3. The unsaturated polyester-bpo reaction starts at 99 C and reaches peak maximum at C. From the figure it is observed that the DSC thermogram of systems show unimodel reaction exotherms. The unimodel reaction exotherm of vinylester modified unsaturated polyester is due to the lower concentration of vinylester. Further the vinylester incorporated system, the reaction commences at 90.5 C C. The peak maximum temperatures obtained for VEO (10% by wt.), BMI (10% by wt.), VEO BMI (10% by wt.) modified UP systems are 102 C, 126 C, 118 C respectively. The curing temperature decreases with increase in the concentration of vinyl ester. This confirms the reaction between unsaturated polyester and vinyl ester, which accelerates the reaction rate and reduces the curing temperature. The incorporation of BMI (10% by wt.) into UP show a unimodel reaction exotherm. This is also due to the lower concentration of bismaleimide. For the bismaleimide incorporated system the reaction commences at 104 C and reaches the peak maximum at 126 C. The presence of BMI monomer within the network structure increases the crosslinking density since BMI is tetrafunctional while styrene in the polyester chain is bifunctional. The slight decrease in the peak maximum confirms the higher crosslinking density of BMI incorporated system (16). Heat deflection temperature The thermomechanical behaviour of unmodified unsaturated polyester, vinyl ester modified unsaturated polyester and BMI modified vinyl ester - unsaturated polyester systems are assessed using HDT measurements and the values are presented in Table 2. The introduction of vinyl ester into unsaturated polyester decreases the values of HDT due to lowering of crosslinking density. The 218 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

9 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices Figure 2. TGA curves of VEO-UP (10% by wt.) with a) 0% bismaleimide b) 5% bismaleimide c) 10% bismaleimide d) 15% bismaleimide Figure 3. DSC thermograms of a) unmodified UP b) VEO (10% by wt.) modified UP c) BMI (10% by wt.) VEO modified UP d) BMI (10% by wt.) modified UP system during cure Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

10 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar incorporation of BMI into vinyl ester modified unsaturated polyester enhances the values of HDT by 15.5%. This may be attributed to high network and heterocyclic aromatic structure in association with rigid molecular structure of BMI. MECHANICAL PROPERTIES Tensile properties The values of tensile strength and tensile modulus of unmodified unsaturated polyester, unsaturated polyester modified with varying percentages of VEO and BMI are given in Table 1. The values of tensile strength of modified unsaturated polyester with 10%, 20%, 30% (by wt) of vinyl ester increases the value by 3.9%, 14.6% and 26.4% respectively than that of unmodified unsaturated polyester. This may be explained due to the higher degree of interpenetrating network formed between the unsaturated polyester chains and VEO molecules. Similarly the introduction of bismaleimide into VEO modified unsaturated polyester, further enhanced the values of tensile strength according to their weight percentage. This may be due to the increased cross-linking density and rigidity. It was observed that the improvement of tensile strength by 51%, 63%, 70% was noticed in case of introduction of 5%, 10%, 15% bismaleimide. A similar trend is observed in the case of tensile modulus. Flexural properties The values of flexural strength and flexural modulus of unmodified unsaturated polyester and unsaturated polyester modified with varying percentages of VEO and BMI are presented in Table 1. The incorporation of 10%, 20% 30% (by wt) of VEO into unsaturated polyester resin increases the flexural strength by 19.4%, 37.7% and 44.3% respectively than unmodified unsaturated polyester (system A). This may be explained due to the formation of chain entanglement between unsaturated polyester and VEO matrix. A similar trend is observed in the case of flexural modulus. The introduction of 5%, 10%, and 15% BMI (by wt) into VEO modified unsaturated polyester resin further enhances the values of flexural strength and modulus as observed in the case of tensile strength. The increase in the value of flexural strength is influenced by the presence of aromatic rigid skeleton, which in turn increases the flexural behaviour. Unnotched Izod Impact strength The values of impact strength of unsaturated polyester and modified unsaturated polyester with varying percentages (by wt) of VEO and BMI are listed in Table 1. The incorporation of VEO into unmodified unsaturated polyester 220 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

11 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices Table 1. Mechanical properties of unmodified unsaturated polyester, vinyl ester modified unsaturated polyester, BMI modified unsaturated polyester systems Matrix System UP/VEO/ BMI Composition Tensile Strength (MPa) Tensile Modulus (MPa) Flexural Strength (MPa) Flexural Modulus (MPa) Unnotched Izod Impact Strength A 100/00/ B 100/10/ C 100/20/ D 100/30/ E 100/10/ F 100/10/ G 100/10/ H 100/00/ I 100/00/ J 100/00/ resin increases the toughness behaviour of unsaturated polyester resin by 10%, 17.5% and 25.5%. The increase may be explained due to the excess free volume created by long flexible ether chain. Further, the introduction of 5%, 10%, and 15% BMI (by wt) into VEO modified unsaturated polyester decreases the value of impact strength. The decrease may be attributed to the restricted chain mobility due to the formation of increased intercrosslinked network structure. However, the values are found to be higher than that of the unmodified system. Water absorption The water absorption behaviour of unmodified unsaturated polyester and modified unsaturated polyester systems are presented in Table 2. The incorporation of vinyl ester into unsaturated polyester enhances the water absorption behaviour. This is due to hydrophilic nature of vinyl ester linkages, whereas the inclusion of BMI into vinyl ester modified unsaturated polyester improves water repellant behaviour. This may be due to the fact that more effective network is formed on increasing bismaleimide concentration, which in turn increases the cross-linking density and rigidity. Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

12 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar Table 2. Data on heat deflection temperature and water absorption (%) of BMI-VEO-UP matrices Matrix System UP/VEO/BMI Composition Heat Deflection Temperature ( C) Water Absorption (%) A 100/00/ B 100/10/ C 100/20/ D 100/30/ E 100/10/ F 100/10/ G 100/10/ H 100/00/ I 100/00/ J 100/00/ CONCLUSION It is concluded from the data that forming an interpenetrating network with vinyl ester oligomer enhanced the impact behaviour of unsaturated polyester matrix. However, the impact strength was decreased by the incorporation of BMI into VEO modified unsaturated polyester (Systems E to G) by the formation of rigid intercrosslink network structure. Further mechanical studies and thermal properties viz., glass transition temperature, heat distortion temperature and thermal stability of bismaleimide modified VEO incorporated unsaturated polyester matrix showed an increased trend, which confirmed the network formation. In addition, mechanical studies and thermal characterisation indicate the introduction of VEO into unmodified system improved impact behaviour and thermal stability. Further it was also observed that the water intake behaviour increases with increasing VEO concentration and decreases with increasing BMI content. Among the different matrix systems developed and studied, the system G exhibits better performance. The morphological studies and electrical properties of BMI-VEO-UP matrix systems are under progress. 222 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006

13 Synthesis and Characterization of Bismaleimide (1,3-bismaleimido benzene) Vinyl Ester Oligomer Modifi ed Unsaturated Polyester Interpenetrating Matrices ACKNOWLEDGEMENT The authors thank Dr. A. Ashok Kumar, Dr. K. Dinakaran and S. Premkumar, Chemical Engineering Department, Anna University for their support. REFERENCES 1. Margolis, J.M., Advanced Thermoset Composites. Industrial and Commercial Applications, Van Nostrand Reinhold, New York (1986). 2. Burns, R.B., Polyester Moulding Compounds, Marcel Dekker Inc., New York (1992). 3. Crosbie, G.A., Phillips, M.G., J. Mater. Sci., 20, 182, McGarry, F.J., Riew, C.K. and Rowe E.H., Polym. Eng. Sci., 18, 78, Martuscelli, E., Musto, P., Rogosta, G., Scarinzi, G. and Totti, E. J., Polym. Sci. Phys. Edn., 31, 629, Abbate, M., Martuscelli, E., Musto, P., Ragosta, G. and Leonard, M.J., J. Appl. Polym. Sci., 62, 2107, Suspene, L., Fourquier, D. and Yang, Y.S., Polymer, 32, 1593, Atkins, K.E., Polymer Blends, Eds., D.R. Paul, and S. Newman, Academic Press, New York, (1978), Chap. 2, p Subramaniam, R. and McGarry, F.J., Proc. 48 th Ann. Conf. SPI/CI. 14-C (1993). 10. Tong, S.N. and Wu, P.T.K., J. Reinf. Plast. Comp., 9, 299, Frish, H.L., Frish, K.C. and Kempler, D., Polym. Eng. Sci., 4, 646, Woo, M., Chen, L. B. and Seferis, J.C., J. Mater. Sci., 22, 3665, Kim, D.S., Han, M.J. and Lee, J.R., Polym. Eng. Sci., 35, 1353, Ashok Kumarm A., Alagar M. and Rao R.M.V.G.K., J. Appl. Polym. Sci., 81, 38, Dinakaran, K. and Alagar, M., J. Appl. Polym. Sci., 85, 2853, Abbate, M., Martuscelli, E., Musto, P., Ragosta, G. and Scarinzi, G., Polymers for Advanced Technologies, 7, 519, Fan, L.H., Hu, C.P. and Ying, S.K., Polymer, 37, 975, Fan, L.H., Hu, C.P., Zang, Z.P. and Ying, S.K., J. Appl. Polym. Sci., 59, 1417, Ashok Kumar, A., Alagar, M. and Rao, R.M.V.G.K., Polymer, 43, , Dinakaran, K. and Alagar, M., J. Appl. Polym. Sci., 86, 2502, Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3,

14 R. Josephine Sharmila, M. Alagar and R. Suresh Kumar 224 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 3, 2006