Kinetic Analysis of Curing Reaction

Transcription

1 Chapter 4 Kinetic Analysis of Curing Reaction This chapter deals with the kinetic analysis of epoxy resinltert-butyl PEEK blends cured with 4,4'diaminodiphenylsulfone (DDS). The curing reaction was followed by using isothermal differential scanning calorimetry (DSC). Phenomenological model developed by Kamal was used to explain the curing kinetics. The curing reaction followed autocatalytic mechanism even afler the addition of thermoplastic. The extent of reaction was found to depend on the cure temperature and composition of the blends~ Parts of the results of this chapter i. have been published in Polymer, 44, 3687, 2003 ii. have been submitted for publication in Colloid and Polymer Science

2 136 Chapter 4 mong the thermosetting polymers used, epoxy resins find a wide range of A applications as adhesives, coatings, sealants, as matrices for high performance composites in aerospace industry etc. The wide range of applications arise from the properties such as easy processability, good chemical and corrosion resistance, good adhesion to various substrates and easy cure But due to the inherent briffle nature of the epoxy resin owing to the high crosslink density, it is necessaly to improve the toughness for its use in many advanced applications. Considerable efforts have been made in the past to improve the toughness by blending with reactive liquid rubbers.%' Improvement in toughness was achieved with the lowering of other properties like glass transition temperature (T,), thermal and oxidative stability. Studies had revealed that blending with thermoplastics such as poly(ether su~fone),~'~ po~~(etherimide).".'~ po~y(phenyleneoxide),'~ phenoxy resint4 and could enhance the fracture toughness without sacrificing strength, stiffness, T, or any other desirable properties. The ultimate properties of thermoplastic toughened epoxy resin strongly depend on the morphology generated on curing. The morphology of the blends can be controlled by proper selection of curing agent and curing conditions. For example, phenolphthalein poly(ether ether ketone) (PEK-C)lepoxy blends cured with maleic anhydride and hexahydrophthalic anhydride gave homogeneous morphology while phthalic anhydride cured blends were heterogeneous." Zhong et a1.18 found that curing temperature is important in determining the morphology of diaminodiphenylmethane (DDM) cured DGEBNPEK-C blends. Blends cured at 80 C were homogeneous and those cured at 150 C were heterogeneous. Recently Martinez et all9 observed similar behaviour in DGEBNpolysulfone blends cured with DDM. So it is clear that cure reaction and generated morphology are closely related. Hence it is necessary to study the curing reaction of blends in detail. An understanding of the mechanism and rate of reaction is essential to establish processing-morphology-property relationships in thermosets. The reaction of epoxide with amine involves several reactions, like addition of amine to epoxide, homopolymerisation of epoxy by etherification or ionic polymerisation, cyclisation and various side transformations and degradation

3 Klnehc analysis of 137 reactions. In most cases the addition of amine is the strongly predominating reaction. The addition of arnine to epoxide is given in Scheme 4.1 /O\ R-CH-CH, OH + NYR, & R-CH-CH-NHR, I Scheme 4.1: Curing reaction of epoxy resin with arnine curing agent The reaction of epoxide with a primary amine affords a secondaty amine and a hydroxyl group. The reaction of secondary amine with another epoxide gives tertiary arnine and hydroxyl gro~p.20'2' The curing reaction of epoxy-amine systems was monitored by several researchers using different techniques like fourier transform infra red spectroscopy (FTIR).~~~~~ Raman spectros~opy.~~~~~ dielectric spectroscopy.z7 thermal scanning rheornetryz8 and DSC.'~~ DSC is a widely accepted tool for monitoring cure reaction since excellent results were obtained with a small amount of sample in a relatively short time span. Both isothermal and dynamic measurements were used to follow the cure reaction. DSC has two advantages; (i) it is the react~on rate method that permits to measure with great accuracy both the rate of reaction and degree of conversion and (ii) the DSC cell may be considered as a mini reactor without temperature gradient. DSC kinetics provides variables required for solution of the heathass transfer equation namely, heat flow and heat generation. The basic assumption for the application of DSC technique to the cure of the thermoset polymers is that the rate of reaction is proportional to the measured heat flow (4). 4 AH Rute of retrction, -- = - dl where, AH being the enthalpy of curing reaction The amine curing of epoxy resin was found to be autocatalytic. The hydroxyl group formed by amine epoxide addition is an active catalyst. Hence the curing

4 138 Chapter 4 reaction generally shows an accelerating rate in its earlier stages.35~3%elatively few systematic studies were done on the cure kinetics of blends of epoxy resins with thermoplastics. It has been found that the reaction mechanism remained autocatalytic irrespective of the addition of thermopla~tic.~'~~~ One among the exceptions was polycarbonate modified epoxy resin cured with amine where the cure reaction followed nth order kinetic^.^' In this chapter the cure kinetics of epoxy resin toughened with tert-butyl PEEK polymers is reported. DSC was used to follow the curing reaction. The effect of curing conditions and composition of the blends on the curing kinetics was investigated. Two blend systems were used for kinetic studies. Epoxy resin modified with random and hydroxyl terminated PEEK (PEEKT and PEEKTOH12) with pendent tert-butyl group were investigated in detail. 4.1 Differential scanning calorimetry In isothermal mode the overall curing process can be analysed. The polymerisation involves several reactions including primary and secondary amine attack on epoxy group, homopolymerisation, etherification and degradation." This makes the analysis of polymerisation kinetics from dynamic DSC measurements too difficult. In contrast isothermal DSC measurements of epoxy curing and their interpretation was rather more inf~rmative.~~ Many authors have previously noted that the heat of reaction obtained from dynamic DSC measurements is lower than that obtained from isothermal DSC measurement^.^^.^^ It is also accepted that isothermal experiments generate more reliable kinetic parameters. Therefore, the curing reaction was followed using isothermal DSC at different temperatures. Dynamic DSC measurements were done for the diglycidyl ether of bisphenol- A (DGEBA)IDDS mixture at 10, 7.5, 5 and 2.5"CImin. to determine the total heat of reaction. The dynamic heating curves at various heating rates are shown in Fig. 4.1

5 K;netic analysis of ' Temperature ("C) Figure 4.1: Dynamic DSC scans of neat resin at different heating rates It was observed that peak maximum shifted towards the lower temperature side as the heating rate is lowered. The total heat of reaction (AH.,) for the neat epoxy system was found to be 392Jlg. This value was taken as total heat of reaction for epoxy-amine reaction. The extent of conversion for epoxylpeekt and epoxyipeektoh12 blends cured at 180 C. 165 C and 150 C for 3hrs, 5hrs and 6hrs respectively are shown in Fig. 42(a-c) and Fig. 43(a-c) respectively.

6 140 Chapter , - lophr PEEKT phr PEEKT a - 30phr PEEKT phr PEEKT SOphr PEEKT Time (min.) Figure 4.2a: Conversion against time plot for neat resin and epoxy/peekt blends cured isothermally at 180 C Figure4.2b: Conversion against time plot for neat resin and epoxy1peekt blends cured isothermally at 165 C

7 Kinetic analysis of A- 2Ophr PEEKT a - 3Ophr PEEKT -0-4Ophr PEEKT -0-50phr PEEKT Time (min.) Figure 4.2~: Conversion against time plot for neat resin and epoxy1peekt blends cured isothermally at 150 C Sphr PEEKTOHlZ phr PEEKTOHI2 -*- 1 Sphr PEEKTOH Tlme (mln ) Figure 4.3a: Conversion against time plot for neat resin and epoxyipeektoh12 blends cured isothermally at 180 C

8 142 Chapter 4 Time (rnin.) Figure 4.3b: Conversion against time plot for neat resin and epoxyipeektoh12 blends cured isothermally at 165 C -- 5phr PEEKTOHI2 IOphr PEEKTOHIZ - 15phr PEEKTOHIZ Time (min.1 Fiaure 4.3~: Conversion against time plot for neat resin and epoxylpeektoh12 " blends cured isothermally at 150 C

9 Kinetic analysis of All the blends showed an increase in the extent of conversion in the beginning followed by a levelling off indicating that the curing reaction came to a stop. Also the time required to reach the plateau region increased with lowering of curing temperature. The extent of reaction decreased with the addition of PEEKT and PEEKTOH12 to epoxy resin. This is due to phase separation of the blends. As the curing reaction proceeds, phase separation will occur at certain conversion and time. During phase separation some of the lightly crosslinked epoxy resin or DDS could be trapped in the separated out thermoplastic phase. The lightly crosslinked epoxy resin is not as reactive as the epoxy rich phase or there may not be enough DDS to cure the resin leading to decrease in the over all curing reaction. This effect was more pronounced with increase in thermoplastic concentration. It could be better said that the probability for the reaction between epoxy resin and hardener decreased as the thermo~lastic content increa~ed.~~ The rate of reaction daldt versus time plot for epoxylpeekt and epoxy1 PEEKTOHI2 blends are shown in Fig. 4.4(a-c) and Fig. 4.5(a-c) respectively. The rate of reaction increased after starting the curing; reached a maximum and decreased. The rate of reaction decreased with increase in PEEKT or PEEKTOH12 in the blends. Similar behaviour was observed for all the blends -- -@- 20phr PEEKT 30phr PEEKT 40phr PEEKT 1 -,- 50phr PEEKT Figure 4.4a: Reaction rate against time plot for neat resin and epoxylpeekt blends cured at 180 C

10 144 Chapter lophr PEEKT -A- 2Ophr PEEKT -a- 3Ophr PEEKT a D W Tune (mln ) Figure 4.4b: Reaction rate against time plot for neat resin and epoxylpeekt blends cured at 165 C Time (rnin.) Figure Reaction rate against time plot for neat resin and epoxylpeekt blends cured at 150 C

11 Klnetlc analysis of 145 Time (min.) Figure 4.5a: Reaction rate against time plot for neat resin and epoxylpeektoh12 blends cured at 180 C Figure 4.5b: Reaction rate against time plot for neat resin and epoxyipeektoh12 blends cured at 165 C

12 146 Chapter phr PEEKTOHtZ --- IOphr PEEKTOHI? * - 15phr PEEKTOHI? Time (min.) Figure 4.5~: Reaction rate against time plot for neat resin and epoxyipeektoh12 blends cured at 150 C Reaction rate of 10phr PEEKT and PEEKTOH12 modw epoxy resin cured at different temperatures are shown in Fig. 4.6 and 4.7 respecbvely. The reaction rate decreased with the decrease in wring temperature. The rate of readn was not much affected by the molecular weight of thermoplastic. The time to reach peak maximum increased with lowering of cure temperature and with increase in modifier content in the blends d r E 0 O W Time (rnin.) Figure 4.6: Reaction rate vs time plot for epoxyil0phr PEEKT blends cured at 180,165 and 150 C

13 K;nef;c analysis of Time (min.) Figure 4.7: Reaction rate vs time plot for epoxyil0phr PEEKTOH12 blends cured at 180, 165 and 150 C From Figs. 4.4 and 4.5, it was observed that the time required to aftain maximum rate varied with the composition of the blends and curing temperature. It increased with increase in PEEKT or PEEKTOHI2 in the blend and with lowering of cure temperature. The maximum in reaction rate against time plot is typical of autocatalytic mechanism. The phenomenological model developed by ~amal~' was used for isothermal kinetic analysis. The general equation assumed for the curing reaction of epoxy-amine system is given in equation 4.2. where, a is the fractional conversion at time t, k, and k, are the rate constants with two different activation energies, m and n are the kinetic exponents of the reaction and m + n gives the overall reaction order. The kinetic constants k, and k2 depend on temperature according to Arrhenius law. where, Ai is the pre exponential constant. Eai is the activation energy, R is the gas constant and T is the absolute temperature.

14 Since there are two kinetic constants k, and k2, two activation energies E, and Ea2 were obtained by plotting Inkl and Ink2 versus IT. The slopes of these plots were then used to calculate the activation energies E, and Ea2 respectively. A typical plot of Ink, and Ink2 against 1/T for epoxyipeekt blend is shown in Fig. 4.8a and 4.8b. The other systems also showed similar behaviour x - C TT x 1000 (min-') Fig..4.8a: Ink1 against 1/T plot for neat epoxy and epoxyipeek7 - blends /l x 1000 (mln ') Fig. 4.8b: Ink,aga~nst 1TT plot for neat epoxy and epoxy/peekt blends In order to determine kinetic parameters, the experimental value of conversion (a) and the reaction rate for the complete course of reaction were determined and

15 Kinetic analysis of adjusted with the kinetic equation. Several methods were available for the calculation of parameters in equation 4.2 from isothermal DSC curves." parameters k,, k, m and n were determined without any constraints on them. In our study the The kinetic parameters obtained afler a large number of iteration are given in Tables 4.1 and 4.2 respectively for epoxy/peekt and epoxylpeektoh12 blends. T m+n k~xlo-3 kzxl03 Ear Ea2 (" c) (min~') (mi"-') lna' InA2 (kj molkl) (k~ rno1~1) Neat epoxy EpoxyMOphr PEEKT Epoxyl20phr PEEKT Epoxyl40phr PEEKT EpoxylSOphr PEEKT Table 4.1: Autocatalytic model constants for PEEKT modified DGEBA epoxy

16 150 Chapter 4 Neat epoxy Table 4.2: Autocatalytic model constants for PEEKTOH12 modified DGEBA epoxy blends The overall reaction order m + n was in the range 2.5 to 3.5 up to 20phr PEEKT. The value m was in the range 0.6 to 1.2 for the neat resin as well as the blends. The value of n was between 1 7 to 2.7 for the neat resin and all the blends and higher values were obtained above 20phr blends. PEEKTOH12 modified epoxy resin also showed similar values for rn and n at all compositions except for 20phr blend. Activation energies for the curing reaction of the blends exhibited higher values compared to the neat resin. This means that the polymers hinders with the reaction between epoxide and amine.

17 The plot of dddt vs. a for the experimental data and the data obtained by autocatalytic model given in equation 4.3 for epoxy/peekt and epoxyipeetctoh12 blends are given in Fig. 4.9(ac) and 4.10(ac) respectively. The open symbols represent the experimental data and the solid line represents the theoretical value. Figure 4.9a: <' Reaction rate against conversion plot for experimental and autocatalytic ft for neat resin and epoxy1peekt blends cured at 180 C 0 1 /- ' Neat epoxy I lophr Figure4.9b:,L Reaction rate against conversion plot for experimental and autocatalytic fit for neat resin and epoxy/peekt blends cured at 165 C

18 152 Chapter 4 Figure4.9~: Reaction rate against conversion plot for experimental and autocatalytic fit for neat resin and epoxyipeekt blends cured at 150 C li Figure 4.10a: Reaction rate against conversion plot for experimental and autocatalytic it for neat resin and epoxyipeektoh12 blends cured at 180 C

19 Figure 4.10b: Reaction rate against conversion plot for experimental and autocatalytic fit for neat resin and epoxyipeektoh12 blends cured at 165 C Figure 4.10~: Reaction rate against conversion plot for experimental and autocatalytic fit for neat resin and epoxyipeektoh12 blends cured at 150 C r' The experimental data agreed well with the model predictions at lower conversion ie during the initial stages of cure. But at higher conversions, the values

20 154 Chapter 4 predicted by the model are high compared to the experimental data. This is due to vitrification of the system. This meant that the cure reaction was controlled by diffusion during the final stages of cure. The difference from experimental data was high at lower curing temperatures. A semiempirical relationship based on free volume concept was used to explain diffusion control in cure reaction^.^^,^^ When the conversion reaches a critical value a, d~ffusion becomes the controlling factor and the rate constant kd is given by where, kc is the rate constant for chemical kinetics and C is the diffusion coefficient. Equation 4.4 corresponds to an abrupt change from chemical control to diffusion control of the curing reaction when conversion reaches a,. But the onset of diffusion control is gradual and there is a region where both diffusion and chemical factors are controlling. The overall rate constant can be expressed in terms of kd and k as follows This equation combined with equation 4.5 gives the diffusion factor f(a) When a is much smaller than a,, u <<,a,, f(a) is approximately unity and the reaction is kinetically controlled and the diffusion effect is negligible. As a increases, f(a) decreases and will approach zero where the reaction effectively ceases. The effective reaction rate at any conversion is equal to the chemical reaction rate multiplied by f(~t).~'

21 Kinetic analysis of The value of f(a) was taken as the ratio of the experimental reaction rate to the reaction rate predicted by the autocatalytic model. The value of f(a) was around one during the early stages of cure. As the curing reaction proceeds further f(a) decreases markedly due to the onset of diffusion control. The plot of f(a) versus a for 20phr PEEKT and 15phr PEEKTOHI2 blends are shown in Fg and 4.12 respectively. Figure 4.11: Plot of diffusion factor against conversion at different curing temperatures for epoxyi20phr PEEKT blends Figure 4.12: Plot of diffusion factor against conversion at different curing temperatures for epoxyll5phr PEEKTOHI2 blends

22 156 Chapter 4 The decrease in f(u) and hence in effective reaction rate due to diffusion control was evident from the figures. The other blends also showed the same trend. a, gives the gradual change of the system from chemical control to diision control. The values of a, and C are obtained by applying non linear regression to f(a) versus a data to equation 4.6. The values of a, and C for PEEKT and PEEKTOH12 modified epoxy resin are given in Table 4.3 and 4.4 respectively. Neat epoxy EpoxyllOphr PEEKT Epoxyl20phr PEEKT Epoxyl30phr PEEKT Epoxyl40phr PEEKT Epoxyl50phr PEEKT Table 4.3: Values of critical conversion a,and C parameters for epoxy1peekt blends cured at different temperatures

23 Kinetic analysis of Tcure ("C) -.- a,: C Neat epoxy - Table4.4: Values of critical conversion a, and C parameters for epoxy1 PEEKTOHI2 blends cured at different temperatures The value of a, generated by non-linear regression increased slightly as the cure temperature increased. i e the change from chemical control to diffusion control occurred at higher conversions. The critical conversion was also affected by the

24 158 Chapter 4 composition of the blends. Compared to neat resin the change to diffusion control occurred at lower conversion for the blends. This is due to the increased viscosity of the system due to the addition of modifier. a, gives the state of the system at which the curing reaction became more diffusion controlled. But it cannot be said exactly that at what particular conversion diffusion control begins, since the change from chemical control to diffusion control is a gradual process. 4.2 Conclusion The cure kinetics of DGEBNPEEKT and DGEBNPEEKTOH blends using DDS as curing agent was investigated. The curing reaction was followed by DSC. Isothermal measurements were performed at and 150 C. The extent of conversion decreased with increasing modifier content and with lowering of curing temperature. The decrease in curing reaction in the blends was due to phase separation. The neat epoxy resin as well as the blends exhibited autocatalytic mechanism. The rate of reaction decreased with the addition of PEEKT and PEEKTOH and the time to reach maximum rate shifted towards longer times. Lowering the cure temperature further reduced the rate of reaction. Kinetic analysis was performed using the phenomenological model developed by Kamal. The autocatalytic model agreed well with the experimental data up to the vitrification point and afterwards the reaction became diffusion controlled. A diffusion factor was introduced to explain the reaction during the later stages of cure. The kinetic data fit well with the experimental data for the full course of reaction with the inclusion of diffusion factor. 4.3 References 1. B. Ellis, Ed., Chemistry and Technology of Epoxy Resins. Blackie Academic 8 Professional, Glasgow, C. A. May. Y. Tanaka. Epoxy Resin Chemistry and Technology, Marcel Dekker. New York, B. U. Kang, J. Y. Jho, J. Kim, S. S. Lee, M. Park. S. Lim, C. R. Choe, J. Appl. Polym. Sci., 79, 38, 2001

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