CHAPTER 4 TOUGHENING/MODIFICATION OF BISMALEIMIDES WITH ALLYL COMPOUNDS PART 1: BISMALEIMIDE-ALLYL PHENOL RESIN SYSTEM (BMIP/DABA)

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1 CHAPTER 4 TUGHENING/MDIFICATIN F BISMALEIMIDES WITH ALLYL CMPUNDS This chapter deals with the synthesis, characterization, blending and processing of carbon fibre and glass fibre reinforced composites based on two component Bismaleimide resin systems. Development of the allyl and Bismaleimide components with different backbones, processing, and evaluation of their carbon fabric reinforced composites are described in this chapter. The allyl components chosen for the study were (i) Diallyl bisphenol A (DABA) and (ii) Allylated Novolac (AN). The BMIs chosen were (i) 2, 2 - bis 4-[(4 -maleimido phenoxy) phenyl] propane (BMIP) and 4, 4 bismaleimidodiphenylmethane (BMPM). The chapter has been divided in to two parts based on the resin systems. PART 1: BISMALEIMIDE-ALLYL PHENL RESIN SYSTEM (BMIP/DABA) 4.1 Introduction Bismaleimides (BMI) are one of the important matrix resin systems for high temperature advanced composites. The copolymerised products of BMIs have improved performance characteristics. o,o -diallylbisphenol A (DABA) is a reactive co-monomer which improves toughness of the cured system and enhances the processability. In the present part of the study, the co-reaction and network formation of 2, 2 -bis 4-[(4 -maleimido phenoxy) phenyl] propane (BMIP) and DABA in different compositions are investigated. The effect of resin composition on thermal and mechanical properties of glass fibre reinforced composites is studied. Carbon and glass composites were also prepared for a selected composition of the blend and the mechanical properties were studied. 4.2 Experimental Preparation and Characterization of neat BMIP/DABA resin blends Preparations of blend are described in Chapter-3, section 3.2. University of Mysore 58

2 4.2.2 Curing Curing of Bismaleimide resin (BMIP) Curing of BMIP occurs through the thermal polymerization of double bonds at around 250 C. Figure 4.1 shows the general cure reaction. N N 2,2' bis-4-[4-maleimido phenoxy) phenyl] propane 250 C N N Fig. 4.1: Curing of Bismaleimide Curing of the blend Curing of BMIP and DABA involves three major steps as follows: i) BMIP and DABA monomers are known to co-react at 140 o C via the 'ene' reaction to form the 'ene' adduct. The 'ene' adduct is penta functional, allyl, propenyl and maleimide and two hydroxyl groups. As a result, these are capable of chain extension and cross linking. (ii) The ene adduct reacts with BMIP monomer via Wagner-Jauregg reaction at C and forms a di-adduct. (iii) Diels-alder reaction takes place in the o C range resulting in a tri-adduct and cross-linked products at high temperatures. Etherification occurs above 240 C. Figure 4.2 illustrates the general curing reaction of BMIP/ DABA system. University of Mysore 59

3 The cure behavior of BMIP and BMIP/DABA polymer blend system is studied using DSC. Figure 4.3 and 4.4 show the DSC trace of the curing of BMIP and BMIP: DABA blend (2: 1 mol ratio) composition respectively. N BMIP N + H DABA H N N Wagner-Jauregg BMIP/ C N Ene reaction 140 C Di adduct H Ene adduct H Diels-Alder Reaction BMIP/ C N N H N Thermal Rearrangement Tri adduct Cross- linked Product Allyl cross linking N N N Tri adduct Fig. 4.2: General curing reaction of BMIP/ DABA system University of Mysore 60

4 Endo Exo Heat flow (W/g) Endo Exo a Temperature ( C) b Fig. 4.3: DSC thermogram of (a) BMIP and (b) BMIP: DABA (2:1) molar ratio University of Mysore 61

5 In the case of the pure BMIP uncured resin, the material displayed a sharp melting peak at 165 o C and a single cure exotherm from o C. In the case of BMIP DABA blend the endotherm shifted to 120 o C. The shifting of the endotherm can be explained by the reaction and inclusion of the flexible and bulky groups in DABA which interferes with the close packing of the aromatic basic bismaleimide leading to the chain extension and shifting of the melting peak to lower temperature regime. The polymerization initial temperatures of these bismaleimide are in the small temperature regime. This indicates that the reactivity of double bonds of bismaleimide was not obviously affected but instead the chain length and the structure were affected. However, the range between T m and T 1 ( T) is enlarged to 55 o C from 5 o C by insertion of flexible linkages and bulky group in to the structure of bismaleimide resins of DABA. This means that BMIP have a large processing temperature range between their melting points and polymerization initial temperature, at which the matrix has good fluidity and wettability while manufacturing fibre-reinforced composite. The thermal behavior of the modified resin and the neat resin are tabulated in Table 4.1 below: Table 4.1: Thermal data of DABA modified and unmodified BMI resins from DSC analysis Resin T m a T i b T c T exo d H e ( o C) ( o C) ( o C) ( o C) (J/g) BMIP BMIP/DABA a T m, Melting point b T i, polymerization initial temperature c T, T i -T m d Texo, polymerization peak temperature e H, Enthalpy of polymerization from T i to 300 o C University of Mysore 62

6 Study of curing by FT-IR Figure 4.4 shows the FTIR spectra of (a) BMIP, (b) BMIP/DABA uncured and (c) the BMIP/DABA cured blend. From the Figure 4.4 (a), the peak at 1776 cm -1 is due to the symmetric stretch of C=. The =C-H of maleimide is observed at 3100 cm -1. The C-N-C stretch of maleimide is observed at 1380 cm -1 and C=C of maleimide at 692 cm -1 respectively. From Figure 4.4 (b), the peaks at 913 cm -1 & 1637 cm -1, 1170 cm -1, 1715 cm -1 and 3454 cm -1 are assigned to the allyl, ether, carbonyl and hydroxyl groups respectively. Peaks at 2968 and 2876 cm -1 correspond to the -CH 3 groups in BMIP and DABA. From the Figure 4.4 (c), the reaction between allyl and maleimide groups leading to the ene adduct is confirmed by the absence of peaks at 913 and 1637 cm -1 corresponding to the allyl group. It is also observed that the intensity of carbonyl group of maleimide at 1715 cm -1 is decreasing with the advancement of cure reaction. It has been reported previously that the -C= intensity decreases significantly on polymerising the maleimide ring in the presence of allyl groups. This is attributed to the transformation of maleimide to succinimide during polymerisation, resulting in a change in the absorptivity of carbonyl peak without considerable change in the frequency of absorption. A decrease in absorption intensity of the peak corresponding to C N bond (1395 cm -1 ) is also observed. University of Mysore 63

7 % Transmittance %T % Transmittance a b c Wavenumbers (cm-1) Fig. 4.4: FTIR Spectrum of (a) BMIP, b) BMIP/DABA uncured and (c) Cured BMIP/DABA blend respectively University of Mysore 64

8 Thermal analysis of the cured blend The TGA-DTA thermograms give information about the decomposition profile, decomposition temperature and also the residue. Figure 4.5 shows the thermal decomposition pattern of the DABA: BMIP blends. It is observed that the char residue increases with increase in BMIP in the blend. This is expected as BMIP leads to enhanced cross linking and the TGA results are tabulated in Table 4.2. As the allyl content increases, the thermal stability in terms of Tmax decreases due to the presence of large number of unsaturation in the cured matrix. The peak decomposition temperatures are in the same range for different compositions as shown in Table 4.2. It is also evident from Table 4.2 that, as the bismaleimide content increases, char residue increases systematically. Cured samples Decomposition Temp. (Max) C Char yield (%) BMIP MM MM MM Fig. 4.5: TGA-DTA of BMIP/DABA cured blends (in N 2 ) University of Mysore 65

9 Table 4.2: Thermal stability of cured samples Cured samples DABA: BMIP Decomposition Temp. (Max) C Char yield (%) 0: :1 (MM 1) :1.5 (MM -11) :2 (MM -12) Composites fabrication Preparation of laminates As described in chapter 3, section Evaluation of Mechanical properties The mechanical properties of glass fabric reinforced composites with different resin compositions 1:1, 1:15, 1:2 and 1:3(DABA: BMIP) were evaluated and the results are compiled in Table 4.3. It is clear that the composite containing BMIP and DABA in the ratio 2:1 exhibit the optimum mechanical properties. This composition was evaluated further in carbon fabric reinforced composites also. University of Mysore 66

10 Table 4.3: Mechanical Properties of glass fabric reinforced composites Reference (DABA:B MIP) Flexural Property Strength (MPa) Modulus (MPa) x10-3 ILSS (MPa) Impact Energy (J) Compressive Strength (MPa) Tensile Strength (MPa) (0 ) Unmodified BMI : : : : Mechanical property comparison of Carbon and Glass composite Composites were prepared based on the optimum composition of 1:2 (DABA: BMIP) using carbon and glass fabrics. The mechanical properties of the composites are compiled in Table 4.4. University of Mysore 67

11 Table 4.4: Mechanical properties of composites based on different reinforcements Composite Strength (MPa) Flexural Modulus (MPa) X 10-3 ILSS (MPa) Impact Energy (J) Compressive Strength (MPa) Tensile Strengt h (MPa) GFRP (1:2) CFRP (1:2) It is clear that the ILSS which indicates, the interfacial adhesion is comparable for the carbon and glass fabric composites due to the better interaction with the matrix as expected. Tensile strength is dominated by the properties of the reinforcements and hence is superior for carbon-based system. It is also noteworthy that the impact strength of the CFRP system is superior. University of Mysore 68

12 PART II BISMALEIMIDE - ALLYL PHENL FRMALDEHYDE RESIN SYSTEM 4.3 BMPM- AN BLENDS 4.4 Introduction Allyl phenol formaldehyde (Allyl Novolac-AN) co-react together with Bismaleimide to form a variety of polymer systems with useful properties. In such reactive blends, further improvement in properties is possible by way of structural modification of either the BMI or the phenolic ring. The properties of the resultant matrix depend on the molecular structure and relative ratio of the two reactants, and the extent of cure. This portion of the chapter deals with the synthesis, characterization, blending and processing of carbon fibre reinforced composites based on Bismaleimidodiphenylmethane (BMPM) and Allyl Novolac systems. 4.5 Synthesis of Allylated Novolac (AN) The mode of operandi for the synthesis of Allyl Novolac is as follows. The phenol to allyl phenol ratio in the reactants is maintained at 50:50 mole ratio so that the final product (allyl novolac) will be having 50 % allyl groups in the chain. The allyl novolac- prepolymer is synthesized in a 1.0 L glass reactor equipped with a thermometer, reflux condenser, and stirrer. The synthesis was done by co-reacting phenol (14.6g, 0.47mol), 2- allyl phenol (21.02g, 0.47mol) and formaldehyde (34%, 20.3ml). The reaction is catalysed by adding oxalic acid (5%). After adding the reactants at ambient temperature in to the glass reactor, the reaction temperature was raised to 75 C and maintained for 10hrs. After the completion of the reaction, the reaction mixture was washed with distilled water and subjected to azeotropic distillation to remove water. The excess solvent in the reaction was then removed by evaporation at reduced pressure. The product was then dried at 50 C under vacuum for 8hrs. The characterization of allyl University of Mysore 69

13 novolac resin was carried out by NMR, FTIR and GPC. A typical synthesis scheme of allyl novolac is shown in Figure 4.6 H H H + CH 2 H + H H CH 2 [ ] n Fig. 4.6: Typical synthesis of Allyl Novolac 4.6 Results and Discussion Allyl novolac was synthesized and characterized. The characterization included spectral, chromatographic and hydroxyl value Characterization of Allyl novolac Determination of Hydroxyl Value The hydroxyl value of the synthesized polymers varied between mg KH/ g. Hydroxyl value of the sample is determined by acetylated with a solution of acetic anhydride/ pyridine at reflux temperature. Excess acetic anhydride is acetic acid by the addition of water. The total free acid is then titrated with standard KH solution. A control or a blank experiment is also performed simultaneously. Hydroxyl value = {(V1-V2) xnx56.1}/w Where, V1 and V2 are the volume of NaH consumed by blank and sample, N is the normality of NaH solution, W is the weight of sample taken in grams. University of Mysore 70

14 Spectroscopy Spectroscopic characterization of allyl novolac was done by FTIR (Figure 4.7) and NMR (Figure 4.8) Fig. 4.7: Typical FTIR spectrum of Allyl-Novolac FTIR spectra of the allyl- novolac show that there is a strong absorption at 3419 cm -1, which indicates the presence of hydroxyl groups. The presence of allyl functionality is confirmed by the presence of absorption peak at 1633 cm -1 due to (-C=C-) stretch. C-H stretching is observed at 2977 cm -1, 2843 cm -1, 3013 cm -1 etc. Absorption due to aromatic rings are observed at 1599 cm -1 University of Mysore 71

15 H c 4.93 H H H H H H H 4.96 H 6.30 H g H 6.30 H g H b H e H f H a H d Fig. 4.8: NMR spectra of Allyl Novolac The NMR Spectra of allyl novolac is shown in Figure 4.8. The chemical shift values of allyl proton, hydroxyl protons and aromatic protons are shown in the spectrum. Signals due to aromatic protons appear at 6.75 to 7.5 ppm. The signals due to allylic protons occur at 5 ppm. From the integrated spectra, the ratio of allylic protons to aromatic protons was found to be 50:50 which confirms the extent of allylation Gel Permeation Chromatography The GPC analysis showed that the number average molecular weight of the polymer synthesized is in the range of having a Poly dispersity index (PDI) of 1.7. The Gel permeation Chromatogram of the polymer synthesized is shown in Figure 4.9 University of Mysore 72

16 Fig. 4.9: GPC of Allyl Novolac Characterization of BMPM The procured BMPM was characterised by FT-IR and DSC for determination of its structure and melting point. The melting point of BMPM was found to be 184 o C. From the DSC thermogram, immediately after the melting, the cure initiation happens, which is due to the homopolymerization of the maleimide groups. Curing by thermal homopolymarization initiates at around 175 C for BMPM. The FTIR spectrum and DSC thermogram of BMPM are shown in Figure 4.10 and Figure 4.11 respectively. University of Mysore 73

17 Fig. 4.10: FT-IR spectra of BMPM Fig. 4.11: DSC thermogram of BMPM University of Mysore 74

18 In the FTIR spectrum, characteristics peaks of BMPM were observed. These peaks are due to Asym C= stretch at 1706 cm -1, Sym C= stretch at 1774 cm -1, =C-H of BMI at 3100 cm -1, C-N-C at 1383 cm -1 and C=C of BMI at 700 cm Blending of BMPM-AN and Characterization of BMPM-AN Blends BMPM and AN were blended in different mol ratios as follows. 1:0.5, 1:1, 1:1.5 and 1:2 (AN: BMPM). Melt mixing was employed for blending monomers. Initially, required amount of BMPM and AN were melted at o C and the molten resins were mixed thoroughly. The blends were characterized by FT-IR and DSC for curing studies. The blend was dissolved in minimum quantity of solvent (acetone) for the preparation of composites Spectroscopic characterization Figure 4.12 shows the FTIR spectra of BMPM-AN blend before cure. The peaks at cm -1, 1597 cm -1, cm -1 and cm -1 are assigned to the carbonyl group, C-C stretch (in-ring ie, aromatic), C-H bend in alkanes and benzene rings respectively. Reaction between allyl and maleimide groups leading to the ene adduct is confirmed by the absence of peaks at 996 cm -1 and 1637 cm -1 corresponding to the allyl group in the cured blend (Figure 4.13). The -C= intensity decreases significantly on polymerizing the maleimide ring in the presence of allyl groups. This is attributed to the transformation of maleimide to succinimide during polymerization, resulting in a change in the absorptivity of carbonyl peak without considerable change in the frequency of absorption. A decrease in absorption intensity of the peak corresponding to C N bond (1384 cm -1 ) is also observed. These observations confirmed the co-reaction of allyl and maleimide functions in league with the established mechanism. University of Mysore 75

19 Fig. 4.12: FTIR spectrum of uncured BMPM-AN blends University of Mysore 76

20 Wave Number (cm -1 ) Fig. 4.13: FTIR Spectrum of cured BMPM AN blends Cure characterization by DSC The cure behavior of BMPM/Allyl novolac polymer blend is studied using DSC as shown in the Figure Thermograms of different blend composition (0.5:1, 1:1, 1.5:1, 2:1) were recorded at a heating rate of 10 C /min over a temperature range of C. In the maleimide dominated system, dual melting was observed (independent melting of the two components) during heating in the range of 110 C to 140 C. Similar two-melting transition phenomenon was reported by other researchers for their mixed bismaleimides systems. According to those explanations, the first melting point was corresponding to the eutectic temperature (Te) and the second one to the reactant which has a higher melting point. This explanation may also hold good for the present system also. The cure initiation is observed at around C, which is due to ene reaction in the range of University of Mysore 77

21 Endo Exo 140 to 260 o C and the Diels Alder reaction at C. From the DSC thermogram it can be observed that as the maleimide content increased in the blends, the Diels Alder reaction is more predominant and the cure exotherm is shifted to the higher temperature regime. In the allyl dominated system (1:0.5 AN: BMPM), three exothers are seen. This may be explained as follows: (i) in the C ene reaction takes place, (ii) the dominant exothermic peak ( C) is represented mainly by Ene reaction and Diels Alder reaction along with BMPM homopolymerization, and (iii) above 280 C, the residual unreacted allyl groups react further via homopolymerization. Fig.4.14: DSC thermogram of BMPM/AN blends University of Mysore 78

22 Thermo gravimetric analysis (TGA) TGA thermograms of cured BMI modified allyl novolac resin with different compositions (0.5:1, 1:1, 1.5:1, 2:1) are shown in Figure It can be seen that there were two stages in the weight loss process of the resin. The elimination of trapped solvent or moisture accounted for the minor weight loss starting from ~150 o C. The major weight loss occurred at a temperature >400 C, due to the decomposition of the polymer network. It was found that the thermal stability increases with increase in the maleimide content. The anerobic char yield also increased with the maleimide content as expected. This is a consequence of the increased crosslinks through the formation of cyclic structures on increasing the maleimide content in the co-cured system. Details of the thermal decomposition characteristic of all the blends are tabulated in Table 4.5 represent the thermal decomposition behavior of the blends on the maleimide content. Fig. 4.15: Thermograms of the cured blends University of Mysore 79

23 Table 4.5 Thermal decomposition characteristics of cured blends Cured samples BMIP- AN Initial Decomposition Temp.(Ti) ( C) Char C (%) 0.5: : : : Dynamic Mechanical Analysis (DMA) of the composites Carbon fabric reinforced composites were prepared using the developed blends of BMPM/AN systems and it s Mechanical and Dynamic mechanical analysis were carried out and is explained in the following sections. The developed composites were characterized by DMA and the overlay of the DMA plots with different composition of the resin systems is given in Figure The variation in storage moduli and the Tan δ of the composites with temperature is plotted. The composites were analysed by non-isothermal (3 C/ min, temperature range C, amplitude 40 μm, frequency 1 Hz), three point bending mode. DMA of the cured composites showed only single transition indicating the existence of single phase. The Tg of the cured composites showed no significant increase in the case of 0.5:1 and 1:1 molar ratio of BMPM: AN blends. However, with increasing maleimide content (2:1, BMPM: AN) Tg shifted from 278 C to 287 C as expected. From the DMA, it is evident that as the maleimide content increased in the blend, the composite became stiffer and the glassy modulus increased. The shift in Tg with respect to the maleimide percentage is represented in Figure University of Mysore 80

24 Fig. 4.16: Dynamic mechanical analysis of composites Fig. 4.17: Shift in Tg with increase in BMI in content BMPM-AN composite University of Mysore 81

25 Mechanical characterization Mechanical property data are essential in the design of structural materials for the specified applications. Flexural strength of a material is the ability to withstand a bending force and the impact strength is the ability of the material to absorb impact energy. The properties of the composites of various compositions are given in Table 4.6. It was observed that, as the BMPM content increases, the flexural strength and modulus of the composites increases up to a BMPM content of 1.5 mole ratio, and thereafter a decrease was observed. This may be due to the brittle nature of the composite imparted by the higher BMI ratio in the system. Hence it is under stood that increasing the flexural strength by means of increasing the BMI content can adversely affect the impact resistance of the structure beyond a limiting value of BMI content (1.5 in this case). The change in mechanical properties with increasing BMPM content is represented in Figure Table 4.6: Mechanical properties of BMPM-AN composites Samples Flexural Strength(MPa) Impact Strength(KJ/m 2 ) Flexural Modulus (GPa) BMIP-AN (0.5:1) BMIP-AN (1:1) BMIP-AN (1.5:1) BMIP-AN (2:1) University of Mysore 82

26 Fig. 4.18: Change in mechanical properties with BMPM SEM Analysis SEM images of the fracture surface of the composites with varying composition such as 0.5:1, 1:1, 1.5:1 and 2:1 are given in Figure 4.19 below. From the figure, reasonably good adhesion of the resin to the fibre surface is observed in all cases. Fibre pullout and debonding are not seen. Fibre slippage is observed in the case of 1.5:1.fibre breakage in the case of 2:1. However, when the BMI content increases, fibre breakage increases and wetting reduces. Among the composites, 1:1 composite showed better morphology in terms of uniform wetting. University of Mysore 83

27 0.5:1 1:1 1.5:1 2:1 Fig. 4.19: SEM images of the fracture surface of the composites of BMPM-AN Cure kinetics -theoretical approach Kinetic models Cure kinetics of resins systems are generally studied using non-isothermal and isothermal methods, the former method includes a single and multi-heating rate and the later involves studying the reaction mechanism at a specific temperature as a function of time. The non-isothermal single heating rate method measures the curing process at a constant heating rate, while the multiheating rate method is an iso-conversional method and is suitable for systems with multiple reactions. The two kinetic analysis models are extensively used to understand and predict the cure behavior of the resin systems. The basic assumption for the application of DSC technique to the cure of the thermosetting resins is that the rate of kinetic process d is proportional to the measured heat flow dt [148] as given in Equation (4.1). University of Mysore 84

28 d (4.1) dt H Δ being the enthalpy of the reaction. The kinetic rate process can be described using Equation 4.2 d K( T) f ( ) (4.2) dt Where is the chemical conversion or extent of reaction and f is assumed to be independent of temperature [149]. The rate constant is K (T) is dependent on temperature and assumed to follow an Arrhenius equation: Ea K( T) Aexp (4.3) RT Where A is the pre-exponential factor or frequency factor and Ea is the activation energy. R is the gas constant (8.314J/mol/K) and T is the absolute temperature. The kinetic parameters of the curing reaction, with special reference to Ea, can be calculated using different computational methods [ ]. Ea is determined by the isoconversional methods using the logarithemic form of the kinetic equation (4.2). d Ea ln ln Af (4.4) dt RT d The slope of ln versus 1/T for the same values of α gives the value of activation dt energy. This assumption is related to the zawa and Kissinger methods which relate activation energy Ea and heating rate ɸ and the peak exotherm temperature T p with the assumption that the extent of the reaction at the exotherm peak is constant and independent of the heating rate [154]. Researchers have also adopted the nth order kinetics (NK) model and model free kinetics (MFK) model to understand, predict and explain the cure kinetics and activation energy [ ]. The NK model is applicable to simple reactions, where the activation energy is constant thoughtout the entire reaction. However in the case of more complex reactions involving several reaction steps, which proceed in parallel and is not completely chemically controlled, the reaction kinetics is understood using the MFK model. The MFK approach is based on the assumption that University of Mysore 85

29 activation energy does not remain constant during a reaction and that the activation energy at a particular conversion is independent of the heating rate, also termed as isoconversion principle. The MFK model also predicts that at increasing heating rates, chemical reactions take place at higher temperature and that the reaction mechanism does not change with heating rate [157]. In order to determine if a particular reaction follows the NK model or the MFK model, it is required to compute the activation energy ( Ea ) as a function of fractional conversion. The molar ratio of the constituents has an influence on the reaction, value of Ea and the final structure. In the case of hot-curing systems, isothermal kinetics approach is adopted wherein the reaction kinetics changes from being chemically-controlled in the liquid state to diffusion-controlled in the glassy state as a function of time at a given temperature. The cross-linking reduces molecular mobility and results in the process changing from being kinetically-controlled to diffusion- controlled. After the reaction becomes diffusion-controlled and then practically stops at a maximum conversion, a greater conversion can only be reached by raising the reaction temperature. Cure kinetics of the resin systems are generally studied using non-isothermal and isothermal methods. The former method includes a single and multi-heating rate and the latter involves studying the reaction mechanism at a specific temperature as a function of time. The non-isothermal single heating rate method measures the curing process at a constant heating rate, while the multi heating rate method is an iso-conversional method and is suitable for systems with multiple reactions. Both the kinetic models are extensively used to understand and predict the cure behavior of the resin systems [17-18]. Cure kinetics of the developed blends was also evaluated by the DSC method. University of Mysore 86

30 Calculation of activatin enrgy, Ea by Kissinger and zawa methods for BMPM/AN blends The kinetic analysis of the combinations of blends was done using the zawa and Kissinger methods. The activation energy equation according to zawa model is R ln Ea Tp (4.5) Where is the heating rate ( C/min), Ea is the activation energy. Tp is the peak maximum temperature ( C) and R is the gas constant. Activation energy was calculated from the slope of the plot of 1 against ( ) Tp The activation energy, Ea according to Kissinger model is based on peak temperature (Tp) corresponding to the exothermal peak position of the dynamic scan. ln Tp 2 1 Tp Ea R (4.6) Where is the heating rate, Ea is the activation energy. Tp is the peak maximum temperature ( C) and R is the gas constant. The activation energy can be obtained from the slope of the plot of Vs. In both cases A was calculated from the relation, Ee A RTm E / RTm 2 (4.7) The peak maxima (Tm) obtained from DSC for all the reaction steps are compiled in Table 4.7. University of Mysore 87

31 Table 4.7: Peak temperatures at different heating rates in DSC experiment for the different BMPM/AN blends Heating Rate ɸ ( C/ min) 2:1 (AN:BMPM) 1:2 (AN:BMPM) Tp1( C) Tp 2( C) Tp3( C) Tp1( C) Tp 2 ( C) The peak temperatures obtained by nonisothermal measurements for the alder ene polymers at different heating rates were treated according to the equations of Kissinger and zawa. In a representative figure (Figure 4.20), the reciprocal temperature 1/Tp is plotted against the logarithm of the heating rate, ln, based on the zawa method and against following the Kissinger equation, respectively for the 2AN:1BMPM formulation. From the slope of the graph, the activation energy Ea was computed. The values are of same order, although the Ea by the zawa method is slightly higher that by the equation of Kissinger. Such difference was noticed by other researchers also [19]. The computed activation parameters for the different formulations are given in Table 4.8. University of Mysore 88

32 a b Fig. 4:20: Typical Kinetic plots by (a) sawa and (b) Kissinger methods for 2: 1 (AN:BMPM) From the kinetic evaluation of the curing of the two formulations, the variation of Ea in the 2AN:1BMPM formulation clearly demonstrates the different reaction pathways for the three curing processes. The trend in Ea is in good agreement with the fact that the ene reaction in general requires more activation energy to overcome the barrier than the analogous Diels Alder reaction [160]. This phenomenon arises in the case of allylic systems because of the stereo electronic requirement of breaking the allylic C-H σ-bond. The activation energy for the third exotherm in the case of 2AN:1BMPM is the highest, which can be attributed to the polymerization of the residual allyl groups. This is due to the fact that, polymerization of allyl groups initiated at high temperature, proceeds with difficultly due to the stability of the resulting allyl radical. The Ea value determined for the third peak is comparable to that of the Ea of the allyl homopolymerization reported earlier[161]. University of Mysore 89

33 Table.4.8: Kinetic parameters of curing for the different formulations zawa Kissinger Reference 1st step 2nd step 3rd step 1st step 2nd step 3rd step Ea(kJ/ A(s -1 ) Ea(kJ A(s -1 ) Ea(k J/ A(s-1) Ea(k J / A(s -1 ) Ea(k J A(s -1 ) Ea(k J/ A(s- 1 ) mole) mole) mole) mole) /mole) mole 1:0.5 (AN:BMPM) x x x x x x :2 (AN:BMPM) x 122 6x x 119 3x University of Mysore 90

34 In the case of 1AN:2BMPM, dominated by maleimide groups, the activation energy calculated both by Kissinger and sawa methods were comparable to the ene reaction step of the first formulation. The frequency factor for all the steps are also compiled in Table 4.8.It can be observed that the pre-exponential factor decreases from ene to Diels Alder reaction in all the cases Kinetic analysis by isoconversional methods DSC thermograms in Figure 4.21, representing the curing pattern of the 2AN:1BMPM and 1AN:2BMPM show that in both cases, the second cure step is predominant. Although the activation energy of this step is determined by Kissinger and sawa methods in the above section which resulted in a single activation energy (Ea) value. bviously, the obtained Ea value can be considered as realistic for this step only if Ea is constant throughout the process. From literature, irrespective of the stoichiometry, all the reactions including ene reaction, Diels Alder reaction and BMI homo-polymerization occur at significant rate between 180 o C to 300 o C, which is in fact represented by the second exotherm [ ]. Hence, it is relevant to determine the conversion dependent activation energy throughout the curing process. Isoconversional DSC analysis is proficient for the approximation of dependence of the activation energy on the extent of conversion. The equation used in the present study for the isoconversional analysis of this system is the multiple curve method of zawa Flynn-Wall. This is originally a rearrangement of the Doyle s temperature integral solution [164] in to the equation and is expressed as follows. AE log log R a log E RT a g c l (4.8) Where is the heating rate, c and l are couple tabulated coefficients and g (α) is the integrated form of the conversion dependent function and independent from the scanning rate at a fixed value of the variable α. The most frequently used values are: c= and l= if Ea/RT =28-50 or c=2.000 and l= if Ea/RT = For BMPM/AN system under consideration, Ea/RT is in the range of 13-20, which is significantly less. Hence the values for c and l are chosen as, c= and l= which actually differ University of Mysore 91

35 from the universal ones[ ]. It is well known that the uniform mechanism up to a given degree of conversion is the fundamental assumption of the isoconversional methods. It means is that, the integral solution g (α) in Eq 4.8, does not depend up on the scanning rate at a fixed value of the variable α. The plot of log versus 1/T yields the apparent activation energy, even if the analytical expression of the velocity equation is unknown [168]. a b Fig. 4.21: Fractional conversion curves as a function of temperature for (a) 2AN:1BMPM and (b) 1AN:2BMPM at various heating rates For the isoconversional analysis, the temperature range under consideration was 160 o C to 320 o C which corresponds to the second exothermic peak. This peak can be deconvoluted from the adjacent peaks and analysed seperately. The original DSC data of the two formulations were transformed in to fractional conversion (α) versus temperature curves at various heating rates and the corresponding plots are shown in Figure As evident from the fractional conversion plots, the change in α value for 2AN:1BMPM reaches the maximum at a temperature of 280 o C at the highest heating rate (15 C) where as in the case of 1AN:2BMPM, the α value reaches a maximum at a temperature of 320 o C at the highest heating rate. University of Mysore 92

36 The dependence of activation energy against conversion is plotted in Figure 4.22 for both the formulations. The formulations showed different tendency for the change in activation energy parameter with respect to change in conversion pattern as is seen in the figure. The behavior in activation energy of the two systems may be explained as follows: In the allyl dominated system (2AN:1BMPM), the co-curing between maleimides and allyl groups may be explained on the basis of the of allyl polymerization mechanism. The allyl groups are difficult to polymerize and therefore the products obtained will have low degree of polymerization due to the chain transfer mediated by the allylic hydrogen [ ] during polymerization. Therefore, Ea is found to increase systematically with increase in conversion percentage. The mean value of the activation energies in the plateau region (40>α<60) is around 105k J/mol which is matching with the Ea values obtained for the second step reaction by Kissinger and zawa methods. The activation energy pattern of the blend where maleimide content is more, (1AN:2BMPM) showed a decrease in value up to 30% conversion and thereafter it increased. The decrease in activation energy may be explained on the basis of diffusion control by melt viscosity of the system [171]. As the molecular weight increases progressively in steps, during low conversion stages, the viscosity decreases drastically in the reaction medium with increase in temperature due to the melting of the excess BMPM, which might eventually have resulted in an increase in mobility of molecular chains in this temperature regime. Thus, the kinetics of each step is decided by its respective activation parameters, reactant concentration and viscosity of the medium. The prevalence of diffusion induced vitrification in the kinetics of the cross-linking is exemplified by the apparent asymptotic increase in activation energy after 60% conversion for this formulation which is again supported by the homopolymerization of excess maleimides groups. The growing molecular chains looses their mobility at higher degree of curing with increasing temperature, which eventually changes the reaction mechanism from being kinetically controlled to diffusion controlled. University of Mysore 93

37 Fig. 4.22: Dependence of activation energy on the extent of conversion for the two resin formulations rder of the reaction The kinetic modelling is generally used for the cure time-temperature prediction of the polymer system. The equation relating time, temperature and fractional conversion for any step is given as, α = 1-{1-A (1-n) t e (-E/RT) } 1/1-n (4.9) Where α is the fractional conversion, A the pre-exponential factor, n the order of reaction, t the time, E the apparent activation energy and T the temperature. The order of the reaction, n was found from the Coats-Redfern treatment for the second step using the relationship, University of Mysore 94

38 * α +,( ) ( ) - (4.10) Where α was obtained by integration of the 2 nd exotherm in typical cases, where the base line is unambiguous. α = Hα/Ht, where Hα is the fractional enthalpy and Ht the functional enthalpy. The kinetic modelling is generally used for the cure time-temperature prediction of polymer systems [173]. The reaction order, n was found from Coats Redfern treatment for the second step of the exotherm using equation (9) Where, α, α - (4.11) And when, α α (4.12) Slope= -Ea/RT (4.13) Intercept =,( ) ( ) - (4.14) The term, ( ) ~1 (4.15) The plot of ln [g (α)/t 2 ] was plotted against 1/T (Figure 4.23) for various assumed values of n, ranging from 1 to 3. The co-relation coefficient (r) for different values of n is determined. From the plots, the best value of n was found to be 1. University of Mysore 95

39 Fig. 4.23: Coats Redfern plot with linear fit for the determination of order of Diels Alder reaction in the second step of curing 4.7 Conclusion In the present work, we attempted to address the brittleness of two different BMIs by co reaction with Diallyl bisphenol A (DABA) and Allyl novolac which are presented in two parts. It was expected that the addition of DABA could improve the toughness of the matrix system and also improve the processability and castability. The conclusion of the first part is as follows: The bismaleimide, 2,2'-bis [4-(4-maleimide phenoxy) phenyl] propane (BMIP) and Diallyl bisphenol A (DABA), possessing closely related backbone structure were blended together in various compositions. The cure characterization of the blends was done by DSC, TGA-DTA, Rheometry and FT-IR spectroscopy. University of Mysore 96

40 Thermal analysis (TGA-DTA) of the cured blends showed that the initial decomposition temperatures were in same range. Char residues were found to increase with increase in BMIP content as expected. Glass-reinforced composites were prepared using BMIP/DABA resin systems and their mechanical properties were evaluated. GBD 120 which contains DABA- BMIP in the ratio of 1:2 exhibits superior mechanical properties compared to other combinations. With this optimized combination, composites of carbon are also prepared and mechanical properties evaluated. Thermal analysis shows similar decomposition pattern and resin content of glass composites were calculated from the residue. The polymers appeared suitable for fabrication of composites for structural composites with good mechanical properties and thermal stability for possible space applications. The conclusion of the second part is as follows: Allylated novolac (50 % allylation) with hydroxyl value of mg KH/ g, and with Mn of was synthesized and characterized. Allyl novolac and BMIs were blended in different molar ratios and characterized. It was expected that the addition of allyl group could improve the toughness of the matrix system and also improve its processability. The characterization of the blends was done by FT-IR DSC and TGA-DTA. Carbon fibre reinforced composites were prepared using all the BMI/AN resin systems and their mechanical properties were evaluated. Thermal analysis (TGA-DTA) of the cured blends showed that the initial decomposition temperatures were in same range. The anerobic char yield also incresed with the maleimide content as expected. From DMA of the BMPM-AN cured composite, showed only single transition indicating the existence of single phase and the Tg of the cured composite increased with increase in maleimide content. Also, it is evident that, as the maleimide content increased in the blend, the composite became stiffer and its glassy modulus increased. University of Mysore 97

41 Mechanical properties such as, flexural strength and impact strength of the BMPM-AN cured composites were evaluated, which shows that, as the maleimide content increases, flexural strength increased gradually but impact strength increased to a certain level and then decreased with higher BMI content. From the SEM images, reasonably good adhesion of the resin to the fibre surface is observed in all cases. The polymerisation kinetics of typical allyl dominated and maleimide dominated reactions as estimated by the Kissinger and zawa methods showed apparent activation energies in the range of k J/mol and the pre-exponential factor decreased from ene to Diels Alder reaction in all the cases. The 2nd curing process was evaluated using various kinetic models: Kissinger and zawa Flynn Wall, and the apparent activation energies obtained from two methods were consistent with each other. University of Mysore 98