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Thermo gravimetric analysis (TGA) is a technique widely used because of its ... parameters from thermo gravimetric data for polyurethane material would be .... Cardanol-furfural resins with different mole ratios, i.e., 1:0.9, 1:0.8 and 1:0.7 of ...

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Pelagia Research Library Der Chemica Sinica, 2011, 2(5): 103-117

ISSN: 0976-8505 CODEN (USA) CSHIA5

Comparative thermoanalytical studies of polyurethanes using Coats-Redfern, Broido and Horowitz-Metzger methods S. Gopalakrishnan* and R. Sujatha Department of Pharmaceutical Chemistry, Manonmaniam Sundaranar University, Tirunelveli, Tamilnadu, India ______________________________________________________________________________ ABSTRACT Thermo gravimetric analysis (TGA) is a technique widely used because of its simplicity and the information afforded from a simple thermogram. The accurate determination of kinetic parameters from thermo gravimetric data for polyurethane material would be useful in designing the environmental history of polymer devices and its effect on material properties. Twenty four polyurethane sheets have been prepared using cardanol-furfural resins with two diisocyanates, diphenylmethane diisocyanate (MDI)/toluene diisocyanate (TDI) and their thermal stabilities have been measured by Thermogravimetric analysis (TGA). The kinetic parameters such as energy of activation and regression value in the study of the polyurethane decomposition have been determined by exploiting thermo gravimetric (TG) data. The energy of activation for different stages of polyurethane degradation is determined by Coats-Redfern integration method which involves fourteen different kinetic models. Energy of activation has also been calculated using the well known Broido and Horowitz-Metzger approximation methods and are compared with the results obtained from Coats-Redfern method. Key words: Cardanol-furfural resin, polyurethane, thermogravimetric analysis, energy of Activation, regression co-efficient. ______________________________________________________________________________ INTRODUCTION Thermo gravimetric analysis (TGA) is a thermal analysis technique which measures the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere. TGA measurements are used primarily to determine the composition of materials and to predict their thermal stability up to elevated temperatures. However, with proper experimental procedures, additional information about the kinetics of decomposition and in-use lifetime predictions can be obtained. In TGA, typical weight loss profiles are analyzed for the amount or percent of weight loss at any given temperature, the amount or percent of non 103 Pelagia Research Library

S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ combusted residue at some final temperature, and the temperature of various degradation steps. The derivation of kinetic parameters in the study of the polymer decomposition by exploiting thermo gravimetric (TG) data is useful in the determination of rate constants, activation energies (Ea; kJ mol-1), reaction orders (n), and pre-exponential factors (Z; min-1). The values of kinetic triplet (Ea, n, and Z) depend on several factors such as the flow rate and the nature of gas flowing, heating rate, sample mass, as well as the mathematical methods used to evaluate the data[1]. Many studies of thermo gravimetric data have been used for the determination of kinetic parameters. Sreedhar et al have studied the thermal and surface characterization of plasticized starch polyvinyl alcohol blends crosslinked with epichlorohydrin using different kinetic models such as Coats–Redfern, Broido, Friedman, and Chang to calculate the kinetic parameters[2]. The thermal degradation and energy of activation of the degradation process of styrene-maleic anhydride copolymers have been determined by several thermogravimetric analysis and differential scanning calorimetric models[3]. Ravikumar Reddy et al have studied the thermal stabilities of the polymers functionalized phenol formaldehyde polymer resins by TG and DTA studies and the energy of activation was determined by Horowitz and Metzer and Broido methods[4]. The thermal stability of polyurethane-urea-imide coatings have been analyzed in terms of decomposition kinetic parameters derived from Broido and Coats Redfern methods[5]. The kinetic parameters for the thermal degradation of the block copolymers from natural rubber (NR) and diphenylmethane diisocyanate (MDI) based polyurethanes synthesized were derived from the TGA curves by applying an analytical method proposed by Coats and Redfern [6]. The thermal degradation kinetics of rigid polyurethane foams blown with water has been studied by three single heating rate techniques of Friedman, Chang, and Coats–Redfern methods [7]. The kinetics of the thermal decomposition of moisture-cured and chemically crosslinked polyurethanes, with different chain extenders in nitrogen from ambient temperature to 500°C, have been investigated using the Broido and Coats–Redfern methods [8]. The Flynn–Wall, Kissinger, and Ozawa methods have been used to calculate the activation energies of thermal decomposition of Poly (urethane-isocyanurate)[9]. The Flynn–Wall method gives the activation energy at the beginning of thermal decomposition and is fitted with better linear regression than the methods developed by Yang, Coats–Redfern, and Freeman–Carroll. The isoconversional method proposed by Ozawa is a more reliable method to evaluate the activation energy. The results show that the Ea values calculated by the Kissinger and Ozawa methods are comparatively close to each other. However, this is not generally dependent on the thermal stability of the copolyurethane, suggesting the complexity of the thermal degradation of these polymers. This was concluded also by Chang et al [10] and Ronaldo et al [11] for other polyurethane systems. Coats–Redfern equation have been utilized to calculate the kinetic parameters, viz., order of decomposition reaction (n), activation energy (Ea), pre-exponential factor (Z), and rate decomposition constant (k), for the decomposition of blends of cardanol-based epoxidized novolac resin modified with carboxyl-terminated liquid copolymer samples. It was found that the degradation of the epoxies and their blend samples proceeded in two steps[12]. The activation energy values of decomposition of cardanol based polyurethanes also have been determined by Coats–Redfern equation[13]. The thermogravimetric analysis of semi-interpenetrating polymer networks composed of castor oil polyurethanes and cardanol-furfural resin was followed using a computer analysis method for assigning the kinetic mechanism. Various kinetic equations have been used to evaluate the kinetic parameters[14]. In the present study, thermogravimetric analysis of the synthesized polyurethanes based on cardanol-furfural resins using two 104 Pelagia Research Library

S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ diisocyanates viz., diphenylmethane diisocyanate (MDI)/toluene diisocyanate (TDI) have been carried out. The energy of activation for the decomposition of polyurethanes in various steps and the regression co-efficient have been calculated by employing the integral method of Coats and Redfern [15], Broido method[16] and the approximation method of Horowitz and Metzger [17] and the results have been compared. MATERIALS AND METHODS Cardanol was obtained from M/s Sathya Cashew Pvt.Ltd., Chennai, India. Furfural(AR.grade) was received from M/s S.D.Fine chemicals, Adipic acid, epichlorohydrin, diphenylmethane diisocyanate and toluene diisocyanate were received from E.Merck (Germany), Methanol (BDH) was used to dissolve the catalyst.PPG-1200 was received from Aldrich chemicals(USA) and dibutyltin dilaurate was received from Fluka Chemie (Germany).The chemicals were used as received. Methods The polyurethanes were subjected to differential thermal analysis (DTA)/ thermo gravimetric analysis (TGA) studies at a rate of 20°C/min in air using Universal V4.3A TA instruments. Synthesis of polyurethanes Cardanol-furfural resins with different mole ratios, i.e., 1:0.9, 1:0.8 and 1:0.7 of cardanol to furfural were prepared using adipic acid as catalyst and characterized. Another set of resins were synthesized by converting cardanol-furfural resins into the hydroxyalkylated cardanol-furfural resins by epoxidation followed by hydrolysis. The resins were repeatedly washed with distilled water and dried using rotary evaporator. Table I: Polyurethane cure conditions (Cardanol-furfural resin based polyurethanes) Polyurethane

Cardanol:furfural ratio

PU1 PU2 PU3 PU4 PU5 PU6 PU7 PU8 PU9 PU10 PU11 PU12

1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7

Cardanol-furfural resin OH Functionality Number mgKOH/g 130.5 5 149.0 5 137.0 4 130.5 5 149.0 5 137.0 4 130.5 5 149.0 5 137.0 4 130.5 5 149.0 5 137.0 4

Polypropylene glycol-1200 (Functionality)

Isocyanate

2 2 2 2 2 2

MDI MDI MDI MDI MDI MDI TDI TDI TDI TDI TDI TDI

Isocyan ate index (NCO/OH ratio) 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

Cure temperature (°C) 80 80 80 80 80 80 80 80 80 80 80 80

The cardanol-furfural resins/hydroxyalkylated cardanol-furfural resins were maintained under vacuum for 2 h before polymerization. The hard segment polyurethanes were prepared by mixing the cardanol-furfural resins/ hydroxyalkylated cardanol-furfural resins with two diisocyanates viz.,4,4'-diphenyl methane diisocyanate and toluene diisocyanate, keeping the isocyanate index (NCO/OH mole ratio) constant at 1.4.The reaction was carried out at room temperature in the presence of dibutyltin dilaurate as catalyst. Similarly the soft segment polyurethanes were prepared by treating the cardanol-furfural resins/hydroxyalkylated cardanol-furfural resins and 105 Pelagia Research Library

S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ the commercially available polyol, polypropylene glycol-1200,(PPG-1200) with the two diisocyanates viz.,4,4'-diphenyl methane diisocyanate and toluene diisocyanate, keeping the isocyanate index (NCO/OH mole ratio) constant at 1.4. The polyurethanes formed were then allowed to cure for 48 h in a flat surface without any disturbance. The polyurethanes were again cured in a vacuum oven at 80°C for 48 h and subjected to thermogravimetric analysis (TGA). The polyurethane cure conditions were given in Table I and Table II. Table II: Polyurethane cure conditions (Hydroxyalkylated cardanol-furfural resin based polyurethanes) Polyurethane

Cardanol:furfural ratio

PU13 PU14 PU15 PU16 PU17 PU18 PU19 PU20 PU21 PU22 PU23 PU24

1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7 1:0.9 1:0.8 1:0.7

Cardanol-furfural resin OH Functionality Number mgKOH/g 186 10 198 9 195 8 186 10 198 9 195 8 186 10 198 9 195 8 186 10 198 9 195 8

Polypropylene glycol-1200 (Functionality)

Isocyanate

2 2 2 2 2 2

MDI MDI MDI MDI MDI MDI TDI TDI TDI TDI TDI TDI

Isocyan ate index (NCO/OH ratio) 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

Cure temperature (°C) 80 80 80 80 80 80 80 80 80 80 80 80

RESULTS AND DISCUSSION Evaluation of kinetic parameters from thermal degradation The use of thermo gravimetric data to evaluate kinetic parameters of solid-state reactions involving weight loss (or gain) has been investigated by a number of workers [18-20]. Freeman and Carroll have stated some of the advantages of this method over conventional isothermal studies. To these reasons may be added the advantage of using one single sample for investigation. It is assumed that in the majority of polymers that undergo isothermal decomposition, the rate of decomposition is proportional to the concentration of non degraded materials. Traditionally, isothermal and constant heating rate thermo gravimetric analysis have been used to obtain kinetic information with the constant heating rate method developed by Flynn and Wall[21].This method is preferred because it requires less experimental time. However, the Flynn and Wall method is limited to well-resolved single step decompositions and first order kinetics. A reaction rate is defined as the derivative of conversion with respect to time. In a TGA, conversion is defined[22] as the rate of final mass loss to total mass loss corresponding at a particular stage of degradation process, i.e., W β = Wr Where, Wr = W∞

_

W 106 Pelagia Research Library

S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ W∞ = mass loss at the end of the particular stage of reaction W = mass loss up to time t. The rate of conversion in a dynamic TGA experiment at a constant heating rate can be expressed as:

dβ/dt = Q(dβ/dt) = k(T)f(β) Where, Q is the heating rate, k (T) is the rate constant and f (β) is the conversion functional relationship. Arrhenius expression, which describes the temperature dependence of the rate constant, may be expressed as:

k(T) = A exp(

Ea /RT)

Where, A is the pre-exponential factor, Ea is the activation energy, T is the temperature in °K and R is the Universal gas constant. The integral form of the rate equation in a dynamic heating expression experiment may be expressed as:

g (β) = (AEa /QR)p(x) x p(x) = ∫(exp( x/x2)dx and α

x = Ea /RT

Where, g (β) is the integral form of conversion dependence function Several methods using different approaches have been developed for solving the integral p(x) equation. To solve the function p(x) several methods have been developed. Flynn–Wall– Ozawa[23], Friedman[24-26], Chang[27,28], Kissinger[29] and other researchers have solved this function. The controversy arises due to different assumptions and approaches used for solving the function p(x) by different researchers. Budrugeac[30] has suggested that the evaluation of the kinetic parameters of thermal and thermo oxidative degradation of a polymeric material using a single TG curve recorded at a certain heating rate does not lead to reliable results. Vyazovkin and Wight[31] have critically analyzed different methods for the evaluation of kinetic triplet (Ea, Z, and n). These authors noted that different kinetic models ignore the fact that the correlation coefficient and the other statistical measures are subject to random fluctuations. However, some authors[32-36] claim a physical meaning to these parameters, and have shown that a TG curve may be correctly described by several various kinetic models. The Van Krevelen and Horowitz-Metzger methods present the problem of the arbitrary election of the reference temperature. In this study, to obtain reproducible results, the reference temperature was taken as that corresponding to the inflection point in TG curves [37]. Zsako who suggested that the Coats-Redfern method is superior to other methods, because it shows the best linearity of the data [38]. In the present investigation, three different non isothermal methods are used for the computation of the kinetic parameters. All linear plots drawn by methods of least-square and corresponding correlation coefficients are also calculated. The methods employed are the the integral method of Coats and Redfern, Broido method and the approximation method of Horowitz and Metzger.

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Table III: Kinetic functions (integral differential forms) used for data analysis S.No 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Functions MPL’ MPL0 MPL1/3 MPL1/2 MPL2/3 MPL2 R2 R3 A2 A3 A4 D2 D3 D4

Name of functions Mample power law,n=1 Mample power law,n=0 Mample power law,n=1/3 Mample power law,n=1/2 Mample power law,n=2/3 Mample power law,n=2 Contracting cylinder Contracting sphere Avrami-Erofeev equation (n=2) Avrami-Erofeev equation (n=3) Avrami-Erofeev equation (n=4) Valensi (Barrer) Equation Jander Equation Ginstling Equation

g (α) -ln (1-α) α 3[1-(1-α)1/3] 2[1-(1-α)1/2] 3/2[1-(1-α)2/3] (1-α)-1-1 1-(1-α)1/2 1-(1-α)1/3 [-ln(1-α)]1/2 [-ln(1-α)]1/3 [-ln(1-α)]1/4 2+(1-α)ln(1-α) [1-(1-α)1/3]2 1-2α/3-(1-α)2/3

f (α) (1-α) 1 (1-α)1/3 (1-α)1/2 (1-α)2/3 (1-α)2 2(1-α)1/2 3(1-α)2/3 2(1-α) [-ln(1-α)]1/2 3(1-α) [-ln(1-α)]2/3 4(1-α) [-ln(1-α)]3/4 1-ln(1-α)-1 3/2(1-α)2/3 3/2[(1-α)1/3-1]-1

Rate controlling process Chemical reaction Chemical reaction Chemical reaction Chemical reaction Chemical reaction Chemical reaction Phase boundary reaction symmetry Phase boundary reaction spherical symmetry Assumes random nucleations and its subsequent growth n=2 Assumes random nucleations and its subsequent growth n=3 Assumes random nucleations and its subsequent growth n=4 Two dimensional diffusion and its subsequent growth Three dimensional diffusion Three dimensional diffusion spherical symmetry

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Coats - Redfern method For a first-order reaction process, Coats and Redfern provided an approximation. This is an integral form of the rate equation. The simplified form of the equation is g(α) AR (1 2RT/Ea) Ea Log = Log φEa T2 2.303 RT Where, T = Temperature A = Pre-exponential term R = Gas constant Ea = Energy of activation φ = Heating rate and α is given by _ W0 Wt α = _ W0 W f Where, W0 = Initial weight of the sample Wt = Residual weight of the sample at the temperature Wf = Final weight of the sample Various kinetic functions used for the analysis are presented in Table III. Regression analysis has been carried out for the twenty four polyurethane sheets synthesized in the present investigation. The computer is essential for processing the enormous amount of data involved in this analysis. Many packages based on FORTRAN, BASIC compilers have been developed to determine the kinetic mechanism of decomposition. But these packages have certain limitations such as they do not allow the graphical representation of the analysis. These packages are not user-friendly and also limit themselves by fixing pre exponential factor, activation energy and correlation index. Thermo gravimetric analysis can best be represented graphically by MICROSOFT EXCEL. In this method spread sheets are used with built in capabilities of graphics according to the procedure suggested by Nair et al [40].The spread sheets are used for data entry such as temperature range, initial weight of the sample and the residual weight of the sample at the temperature.

Broido method Broido has developed a model and the activation energy associated with each stage of decomposition was also evaluated by this method. The equation used for the calculation of activation energy (Ea) is: lnln

1 Y

=

-Ea

1

R

T

+ Constant

where Y =

Wt

W∞

W0

W∞

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Where, Y is the fraction of the number of initial molecules not yet decomposed; Wt- the weight at anytime t; W∞- the weight at infinite time (= zero) and W0 - the initial weight. A plot of ln ln (1/Y) vs. 1/T gives an excellent approximation to a straight line. The slope is related to the activation energy. Approximation Method of Horowitz and Metzger Horowitz and Metzger have demonstrated the method of calculation of energy of activation of polymeric substances. The equation used for the calculation of energy of activation (Ea) is: lnln

Wo Wt

=

Ea θ RTs2

Where, θ is the difference between the peak temperature and the temperature at particular weight loss (θ = T – Ts); W0 is the initial weight; Wt is the weight at any time t; Ts is the peak temperature; and T is the temperature at particular weight loss. A plot of ln ln (W0)/ (Wt) vs. θ gives an excellent approximation to a straight line. From the slope, the activation energy (Ea) is calculated. The kinetic parameters of all the twenty four polyurethanes by Coats Redfern, Broido and Horowitz Metzger methods have been determined. The kinetic parameters include temperature range of different possible stages of reaction, activation energy (Ea) and the regression values (R2). Representative computerized plots (log [g(α)/T2] verses 1/T) of polyurethanes based on Coats Redfern equation is presented in Figure 1. The straight line nature of plots is in accordance with the theory. From the slope of the straight line, the energy of activation, Ea has been calculated for all the polyurethane sheets. Energy of activation corresponding to the maximum R2 value for the hard and soft segment polyurethanes synthesized using cardanol-furfural resins and hydroxyalkylated cardanol-furfural resins for all the stages obtained are collectively presented in Table IV and Table V respectively.

Figure 1. Coats-Redfern plot

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Table IV: Energy of activation of hard and soft segment polyurethanes based on cardanol-furfural resins with maximum R2 value by Coats-Redfern method Polyurethanes

Stage 1 R2 Ea Kcal/mol

PU1 PU2 PU3 PU4 PU5 PU6 PU7 PU8 PU9 PU10 PU11 PU12

0.954 0.955 0.985 0.929 0.965 0.983 0.945 0.933 0.961 0.962 0.959 0.964

16.240 14.680 17.956 12.089 12.707 12.501 11.577 12.954 12.030 12.850 12.954 12.656

Function MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2 MPL2

Stage 2 R2 Ea

Function

Kcal/mol

0.993 0.983 0.961 0.948 0.925 0.957 0.941 0.975 0.925 0.999 0.875 0.994

MPL2 D2 D3 MPL2 MPL2 MPL2 MPL2 D3 D2 A4 MPL2 D3

9.682 9.779 10.373 6.882 7.628 8.682 9.007 10.176 10.995 1.445 9.530 11.600

Stage 3 Ea R2

Function

Kcal/mol

0.969 0.949 0.993 0.908 0.860 0.842 0.883 0.905 0.942 0.811 0.968 0.921

11.453 16.037 2.609 8.575 8.914 9.101 11.943 14.377 12.397 7.470 12.024 6.005

D2 D2 MPL0 D4 D4 D2 D2 D2 D2 D3 D3 D2

Table V: Energy of activation of hard and soft segment polyurethanes based on hydroxyalkylated cardanolfurfural resins with maximum R2 value by Coats-Redfern method

Polyurethaness

Stage 1 R2 Ea Kcal/mol

PU13 PU14 PU15 PU16 PU17 PU18 PU19 PU20 PU21 PU22 PU23 PU24

0.993 0.995 0.989 0.985 0.917 0.888 0.996 0.990 0.982 0.989 0.986 0.980

20.789 19.857 19.568 21.690 19.947 20.537 17.282 13.426 13.232 22.711 12.349 12.883

Function MPL2 MPL2 MPL2 D2 D2 D2 MPL2 MPL2 MPL2 D3 MPL2 MPL2

Stage 2 R2 Ea

Function

Kcal/mol

0.995 0.992 0.986 0.866 0.983 0.963 0.994 0.990 0.957 0.996 0.993 0.990

9.586 12.562 12.986 6.916 18.848 12.842 9.020 10.293 17.775 4.328 3.254 3.200

MPL2 MPL2 MPL2 D2 D3 MPL2 MPL2 D2 D3 MPL0 MPL0 MPL0

Stage 3 R2 Ea

Function

Kcal/mol

0.965 0.978 0.983 0.970 0.991 0.938 -

13.592 16.854 16.586 7.477 11.495 1.447 -

D2 D2 D2 D2 D2 MPL0 -

From Table IV and Table V, it is observed that the Mampel equation gives reasonable energy of activation and maximum R2 value for both the hard and soft segment polyurethanes. Thus it may be concluded that in most of the cases the Mampel mechanism correctly represents the first and second degradation stages. In addition to the Mampel mechanism involving chemical reactions, a few cases also indicate two dimensional and three dimensional diffusions. Most of the polyurethanes involve two dimensional or three dimensional diffusion mechanisms in their third stage of degradation where the disintegration of polyurethane moiety takes place. Representative Broido plot is shown in Figure 2. A plot of ln ln (1/Y) against 1/T gives an excellent approximation to a straight line. The slope is related to the activation energy. The kinetic parameters of the cardanol-furfural resins/ hydroxyalkylated cardanol-furfural resins based polyurethanes by Broido method are presented in Table VI and Table VII respectively.

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________

Figure 2. Broido plot Table VI: Kinetic parameters of cardanol-furfural resins based polyurethanes by Broido method Polyurethanes PU1

PU2

PU3

PU4

PU5

PU6

PU7

PU8

PU9

PU10

PU11

PU12

Decomposition Temperature range (°C) 200-330 420-500 540-630 200-330 400-520 530-590 200-330 400-520 530-600 200-390 400-500 500-620 200-410 420-490 500-620 200-340 390-500 510-600 190-360 421-511 511-571 200-360 410-490 500-570 200-360 410-480 500-570 195-355 390-480 480-560 210-360 380-480 490-590 200-360 390-480 490-580

Regression value (R2) 0.913 0.996 0.968 0.961 0.986 0.901 0.985 0.973 0.952 0.933 0.964 0.915 0.971 0.951 0.853 0.985 0.967 0.836 0.954 0.961 0.856 0.911 0.982 0.905 0.938 0.943 0.943 0.960 0.997 0.892 0.961 0.875 0.980 0.963 0.996 0.951

Activation energy in Kcal/mol 18.293 08.496 15.668 15.767 08.308 23.214 19.049 09.807 19.781 12.964 05.870 10.109 12.039 07.066 11.631 14.006 08.428 10.380 12.176 08.668 18.148 12.369 09.688 18.086 11.510 09.692 14.241 13.096 05.153 06.622 13.981 08.616 08.284 13.287 08.049 08.423

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Table VII: Kinetic parameters of hydroxyalkylated cardanol-furfural resins based polyurethanes by Broido method Polyurethanes PU13

PU14

PU15 PU16 PU17 PU18 PU19

PU20

PU21 PU22 PU23 PU24

Decomposition Temperature range (°C) 200-340 360-470 480-570 200-330 350-470 480-580 210-340 360-480 490-590 200-350 360-500 190-350 360-500 190-350 360-500 220-330 400-470 480-570 220-330 400-500 500-580 200-350 360-440 450-600 200-350 360-490 200-350 360-480 200-350 360-500

Regression value (R2) 0.989 0.975 0.964 0.968 0.979 0.958 0.988 0.965 0.946 0.985 0.908 0.951 0.989 0.903 0.971 0.995 0.995 0.962 0.991 0.994 0.973 0.984 0.966 0.890 0.991 0.979 0.986 0.980 0.982 0.947

Activation energy in Kcal/mol 20.235 8.975 11.889 18.568 11.256 15.639 17.585 11.865 15.253 17.291 06.259 12.709 11.752 11.449 10.616 17.569 06.992 11.085 14.423 09.536 17.179 14.126 11.542 05.293 12.682 11.687 12.630 09.930 12.146 11.198

Representative Horowitz and Metzger plot is shown in Fig.3. A plot of ln ln (W0)/ (Wt) vs. θ gives an excellent approximation to a straight line. From the slope, the activation energy is calculated.

Figure 3. Horowitz-Metzger plot

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ The kinetic parameters of the cardanol-furfural resins/ hydroxyalkylated cardanol-furfural resins based polyurethanes by Horowitz and Metzger method are presented in Table VIII and Table IX. Table VIII: Kinetic parameters of cardanol-furfural resins based polyurethanes by Horowitz and Metzger method Polyurethanes PU1

PU2

PU3

PU4

PU5

PU6

PU7

PU8

PU9

PU10

PU11

PU12

Decomposition Temperature range (°C) 200-330 420-500 540-630 200-330 400-520 530-590 200-330 400-520 530-600 200-390 400-500 500-620 210-410 420-490 500-620 200-340 390-500 510-600 190-360 421-511 511-571 200-360 410-490 500-570 200-360 410-480 500-570 195-355 390-480 480-560 210-360 380-480 490-590 200-360 390-480 490-580

Regression value (R2) 0.875 0.9939 0.9762 0.934 0.991 0.912 0.972 0.969 0.961 0.882 0.951 0.936 0.934 0.939 0.879 0.966 0.950 0.857 0.917 0.976 0.926 0.859 0.984 0.917 0.895 0.953 0.953 0.922 0.999 0.907 0.928 0.850 0.980 0.928 0.994 0.962

Activation energy in Kcal/mol 22.383 08.705 14.184 19.210 08.487 24.176 23.605 10.032 19.535 15.501 05.252 10.648 13.648 06.486 12.668 18.867 07.745 10.359 14.567 8.728 18.821 14.378 09.792 18.888 13.305 09.974 14.532 16.192 04.694 07.312 17.966 08.101 08.273 16.458 08.932 08.126

From the results obtained by the above three methods, it is observed that the hard segment polyurethanes have the higher energy of activation when compared to that of soft segment polyurethanes. This is due to higher cross link density of hard segment polyurethanes. In both the cases of diphenylmethane diisocyanate(MDI) and toluene diisocyanate (TDI) treated hard and soft segment polyurethanes based on cardanol-furfural resins, the energy of activation for the first stage degradation is higher when compared to that of second stage of degradation. Moreover the energy of activation in all the three stages is greater in MDI treated polyurethanes than the TDI treated polyurethanes indicating the higher stability of the former ones.

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S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ Table IX: Kinetic parameters of hydroxyalkylated cardanol-furfural resins based polyurethanes by Horowitz and Metzger method Polyurethanes PU13

PU14

PU15 PU16 PU17 PU18 PU19

PU20

PU21 PU22 PU23 PU24

Decomposition Temperature range (°C) 200-340 360-470 480-570 200-330 350-470 480-580 210-340 360-480 490-590 200-350 360-500 210-350 360-500 190-350 360-500 220-330 400-470 480-570 220-330 400-500 500-580 200-350 360-440 450-600 200-350 360-490 200-350 360-480 200-350 360-500

Regression value (R2) 0.985 0.962 0.942 0.985 0.928 0.965 0.986 0.955 0.987 0.996 0.928 0.977 0.984 0.946 0.958 0.989 0.993 0.973 0.983 0.993 0.980 0.972 0.969 0.906 0.986 0.990 0.965 0.990 0.960 0.966

Activation energy in Kcal/mol 23.323 9.597 14.808 19.850 14.125 16.126 19.895 13.986 17.253 19.991 07.015 15.792 13.092 15.582 11.783 22.150 09.834 10.097 17.861 10.032 16.864 17.229 13.591 15.202 14.245 8.526 13.510 09.143 12.802 09.294

The results also indicate that in the case of hard segment polyurethanes, the activation energy is the lowest in the temperature range of 350°-500°C indicating that this is the fastest step. The back bone of polyurethanes is –CO-NH- linkage and the stability of the polyurethanes depend upon the breaking temperature of –CO-NH- bond which lies in the temperature range 350°500°C of the degradation reaction. This is bound to happen since in the first stage (200°-350°C), the decrosslinking of the long alkyl side chain occurs and in the last stage (above 500°C), the decomposition of polyurethane moiety occurs. By judging the activation energy in the three temperature ranges, it could be predicted that the first and last steps are slow compared to the second step where the breaking up of urethane linkages take place. In the case of soft segment polyurethanes also, the second step is found to be the fastest step. It has also been inferred from the activation energy data that the polyurethanes derived from lower mole ratios of cardanol:furfural (1:0.7 and 1:0.8) are comparatively more stable than that of the higher mole ratio of cardanol:furfural (1:0.9) derived polyurethanes. The same trend is observed in the case MDI and TDI treated polyurethanes based on hydroxyalkylated cardanolfurfural resins. 115 Pelagia Research Library

S. Gopalakrishnan et al Der Chemica Sinica, 2011, 2(5):103-117 ______________________________________________________________________________ CONCLUSION From the Coats Redfern method, it is concluded that the Mampel equation gives reasonable energy of activation and maximum R2 value for both the hard and soft segment polyurethanes. In addition to the Mampel mechanism involving chemical reactions, a few cases also indicate two dimensional and three dimensional diffusions. The thermal activation energy (Ea) with maximum R2 value of most of the polyurethanes by Coats Redfern method is comparable with the wellknown Broido method and the approximation method of Horowitz – Metzger. The energy of activation (Ea) is higher for the first stage than the second stage of degradation in all the polyurethanes. The energy of activation (Ea) for the first stage of decomposition of both hard and soft segment polyurethanes is almost the same in many cases. But the energy of activation (Ea) for the second and third stages of decomposition of hard segment polyurethanes is higher than that of the soft segment polyurethanes. The hard segment polyurethanes are having higher crosslink density. So energy needed to break the core structure is high when compared to that of soft segment polyurethanes. It is concluded that the hard segment polyurethanes are thermally more stable than the soft segment polyurethanes and also the lower mole ratios of cardanol: furfural (1:0.7 and 1:0.8) derived polyurethanes are comparatively more stable than that of the higher mole ratio of cardanol: furfural (1:0.9) derived polyurethanes. Acknowledgements One of the authors(RS) wish to thank the University Grant Commission, Southern region, Hyderabad, the director of Collegiate Education, Chennai and the Principal, Sarah Tucker College, Tirunelveli for selecting under FDP Programme. REFERENCES [1]. B.Sreedhar, M.Sairam, D.K.Chattopadhyay, P.A.S.Rathnam, D.V.Mohan Rao, J. Appl Polym. Sci., 2005, 96, 1313. [2]. B.Sreedhar, D.K.Chattopadhyay, M.Sri Hari Karunakar, A.R.K.Sastry, J.Appl. Polym. Sci., 2006, 101, 25–34. [3]. Shashi Baruah, C.Narayan, Laskar, J.Appl. Polym. Sci., 1996, 60, 649-656. [4]. A.Ravikumar Reddy, K.Hussain Reddy, J.Appl. Polym. Sci., 2004, 92, 1501– 1509. [5]. K.Aswini, D.K.Mishra, B.Chattopadhyay, K.V.S.Sreedhar, N.Raju, J. Appl. Polym. Sci., 2006,102, 3158–3167. [6]. Prema Sukumar, V.Jayashree, M.R.Gopinathan Nair, M.N.Radhakrishnan Nair, J. Appl. Polym. Sci., 2009, 111, 19–28. [7]. Xiao-BinLi, Hong-Bin Cao, Yi Zhang, J.Appl.Polym.Sci., 2006, 102, 4149-4156. [8]. D. K. Chattopadhyay, B.Sreedhar, K.V.S.N.Raju, J.Appl.Polym.Sci., 2005, 95, 1509–1518. [9]. M. A.Semsarzadeh, A. H.Navarchian, J.Appl.Polym.Sci., 2003, 90, 963–972. [10]. T. C.Chang, Y.S Chiu, H.B.Chen, S.Y.Ho, Polym. Degrad. Stabil., 1995, 47, 375. [11]. A. C. Ronaldo, C. R. N. Regina, L. L.Vera, Polym. Degrad. Stabil., 1996, 52, 245. [12]. Ranjana Yadav, Deepak Srivastava, J. Appl. Polym. Sci., 2009,114, 1694–1701. [13]. Kattimuttathu, I. Suresh, S.Vadi, Kishanprasad, Ind. Eng. Chem. Res., 2005, 44, 45044512. [14]. D.K.Mishra, B.K.Mishra, S.Lenka, P.L.Nayak., Polym.Eng.Sci., 1996, 36, 8. [15]. A.W.Coats, J.P.Redfern, Nature., 1964, 201, 68. [16]. A.Broido, J Polym. Sci., Part A-2: Polym Phys 1969, 7, 1761. [17]. H. W.Horowitz, G.Metzger, Anal. Chem.,1963, 35, 1464. [18]. Krevelen, D. W.Van, Heerden, C.Van, F.Huntjens, J. Fuel., 1951, 30, 253. 116 Pelagia Research Library

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