Indian Journal of Engineering & Materials SciencesVol. 3, 1une 1996,pp.1I4-118

Cure kinetics of epoxy formulations with diaminodiphenyl methane

Siddararnaiah", K S 1agadeesh & G PrabhakaraDepartment of Polymer Science and Technology, Sri 1ayachamarajendra College of Engineering,

Mysore 570006, India

Received 4 September 1995; revised, 111anuary 1996

The effects of the addition of triglycidyl-p-amino phenol (TGPAP), diglycidyl aniline (DGA) anddiglycidyl ether of bisphenol-A (DGEBA) epoxy diluents on the curing behaviour of highly viscousN, N, N', N'-tetraglycidyl-4, 4'-diaminodiphenyl methane (TGDDM) epoxy resin with 4,4'·-uiaminodiphenyl methane (DDM) hardener have been investigated by dynami DSC. Dynamic scans havebeen carried out over a temperature range of 30-250°C for different resin formulations. The activa-tion energies (£a) for various epoxy systems have been determined by the Kissenger and Ozawa equ-ations and are in good agreement even for other kinetic parameters. The heat of cure is constantover a wide range of amine concentration. This indicates that any parallel or competing processeswhich might occur must have comparable heat of reaction.

The use of DSC to follow the cure of variousthermosetting polymers is well documented in theliterature I. The study of curing characteristics ofN, N, N', N'-tetraglycidyl-4, 4'-diaminodiphenylmethane (TGDDM) with different types of hard-eners and catalysts by various workers?", formul-ations based on TGDDM continue to offer a lotof scope for making better composites in terms ofproperties.

The effect of the addition of triglycidyl-p-aminophenol (TGPAP) and diglycidyl aniline (DGA) asthe epoxy diluents or modifiers on the TODDM/4, 4'-diaminodiphenyl methane (DDS) andTGDDM/4, 4'-diaminodiphenyl ether (DDE) sys-tems has been reported=". In the present investi-gation, the DSC technique is applied to study thecuring kinetics of TGDDM and TGDDM/TGPAP,TGDDM/DGA and TGDDM/diglycidyl ether ofbisphenol A (DGEBA) (80120) resin formulationsusing 4, 4'-diaminodiphenyl methane (DDM) as acuring agent to understand the behaviour of thereaction.

Experimental Procedure ,Materials-The epoxy resins TGDDM, TGP"AP

and DGA were prepared by the procedure deve-loped by Jagadeesh and Siddaramaiah" and theirpurities were established by various techniquessuch as physical, chemical, HPLC, IR and NMR.The epoxide equivalents of TGDDM, TGPAP andDGA were found to be 117.5, 104.4 and 116,8g!mol respectively. The DGEBA resin (LY 556)was obtained from Ciba-Giegy Ltd. The epoxide

* Author to whom correspondence should be addressed.

equivalent of the DGEBA resin was 175.8g/rnol. DDM from Fluka was used after recrystal-lization.

Methods-To about 2 g of the resin or resinformulation contained in a small capsule the re-quired amount of DDM was dispersed and mixedthoroughly at 115°C. The mixture was maintainedat 115°C and at certain time intervals, a smallportion (5-10 mg) of the mixture was withdrawnand stored at - lOoC in a sealed container to besubsequently subjected to dynamic DSC.

The cure temperatures were determined by Du-Pont 9900 thermal analyzer with 910 DSC mo-dule. The calorimetric measurements were madewith the empty cell as the reference. The instru-ment was calibrated with high purity indium metalunder nitrogen atmosphere. The dynamic scanswere taken in air atmosphere at different heatingrates of 2-20°C/min in a range of 30-300°C. Theruns were made in duplicate and reproducibilityof kinetic data was found to be 98-99%.

Results and DiscussionDynamic DSC studies-The DSC scans were

obtained for various epoxy formulations ofTGDDM with DDM as curing agent at a heatingrate varying between 2 and 20°C/min in air. Atypical thermogram for TGDDM/TGPAP/DDMsystem (80120/30 by weight ratio) taken with aheating rate of lO'Czmin is shown in Fig. 1 as aplot of exothermic heat flow against the tempera-ture and in the integrated form, as a plot of frac-tional conversion versus temperature. Betweenthe room temperature and start of cure exothermat 88°C, a constant baseline was observed at the

SIDDARAMAIAH et al.: CURE KINETICS OF EPOXY FORMULATIONS 115

2-0.-----------------------,

1-0

0·8

0·6

••ItJ

0-4

0·2

0-0100 150 200 250

Ternperctu reo,oc

1-5

01

~! 1-0

a••:r

0-5

O-O'-- __ ----'- --'------__ ~L_ __ _____'_--'

50

Fig. I~DSC scan at 10°C/min heating rate for the stoichiom-etric mixture of TGDDM/TGPAP/DDM (RO/20/30)

Table 1+Dependence of heat of cure on time of mixing atII vc for TGDDM/TGPAP/DDM HW/20!3()) system

Sample Heat of cure Cure time Activationwithdrawing at Q. Jig mill energy

time. min La ± 2, kJ/molo 4RO_R 15_0 97_05 474.6 13.4 !i6.4

10 460.'} 12.7 75.020 445.0 12.1 6~.!i3() 437_6 11.9 67,4

output signal of the thermal analyzer returned tothe haseline at about 215°C. As shown in Fig. I,the rate of fractional conversion increased withthe curing temperature and attained maximumconversion-of ahout98%.

effect of variation of withdrawal time-rurther,the epoxy system was subjected to DSC at differ-ent intervals of time with a constant heating rateof lOoC/min to investigate the relationship be-tween cure time and activation energy. The datafrom dynamic DSC were analyzed for kinetic par-ameters by Arrhenius relation (Table 1i. The va-lues shown in Table 1 conform with the decreasein activation energy with respect to increase in thesample withdrawal time. Also, decrease in curetime is expected because of the partial curing tak-ing place as the sample withdrawing time is in-creased. As the withdrawal is delayed, the systembas more time for isothermal curing at 115°C. Asimilar trend was observed by Barton" and Patelet al".

Effect of variation of hardener (DDM), concen-tration- The values of the peak temperature andactivation energy for the various resin-DDM mix-tures are given in Table 2. The activation energydoes not vary significantly with variation in DDM

Table 2~Curing parameters of TGDDM/TGPAP with differentconcentration of DDM by dynamic DSC ( 10°C) scansDDM Peak Heat of E; ± 2

concentration temperature cure kJ/mol%w/w °C Q.J/g

0 275 53R.R 170.:;23 166 504.6 75.335 160 460.9 75.045 159 451).5 74.855 156 453.0 71.4

100 ISO 427.4 6!i.R

concentration between 23-45%. It shows that theheat of reaction is approximately constant for awide range of amine concentration which suggeststhat all the epoxides react even when there is in-sufficient amounts of primary and secondaryarnine-J?-!'. A likely reason for this is the base-catalyzed addition to hydroxyl (proposed by Ho-rie et al.llI

) groups which result from amine addi-tion, all these reactions having approximatelyequal exothermicity. This is shown in Scheme 1.

/0"RNH + CH2-CH--+-RNH-CH-CH- --·(1)

2 2 II OH

,.....,0"I + c~-cH--·-TH-CH2-~-CH2-TH---(2)

OH R OH

- - -(3)

III(1,lIorlll)

Scheme 1=Curing reactions of epoxy

The peak temperatures of the scans arc givenin Table 2 and they all arc lower for a higher con-centration of DDM. This is because all the cpox-ides react with the primary amine (the hardener).

Effect of variation of heating rate= The kineticparameters were then evaluated from the scans ofthe samples run at different heating rates. Thecurves shown in Fig. 2 were taken at a heatingrate of 2, 5, 10 and 20°C/min for TGDDM/TGPAP/DDM (80/20/30). Similar runs were tak-en for the formulation of TGDDM with DGAand DGEBA. The temperature at which the cur-ing reaction started T, peaked T and ended T'-' P 1

were obtained and they arc presented in Table J,along with the time taken for cure. The reactivitiesof the diluents with curing agent depend upon thelocation of epoxy group in the molecule. It wasobserved that the gelation time for the TGDDM/TGPAP/DDM formulated systems is slightly low-er than that for the TGDDM/DDM system.

116 INDIAN J. ENG. MATER. SCI..JUNE 19W)

Fig. 2- Typical DSC scans at different heating rates forTGDDM/TGPAP/DDM (80/20/30)

Table ~-Curin!! characteristics of different resins formulationscontaining 30";', DDM

Formu- t, t,lation °C T

Resinsystem

C.uretimemin

TGDDM 100 88 163 215 12.7TGPAP 100 83 154 195 11.2TGDDM/TGPAP 80/20 85 158 207 12.2TGDDM/DGA 80/20 !U 162 204 12.1TGDDM/DGEBA 80/20 83 159 207 12.4

A simple and accurate relationship between ac-tivation energy (l::,), heating rate (¢) and peak ex-othermic temperature (I;,) was given byOzawa 12.JJ in the form of:

... (1)

The data from DSC scans at different heatingrates were also analysed in terms of a simple kin-etic model as described by Barton+', The kineticequation is assumed to be of the Arrhenius form.For a given value of fractional conversion, (a), itfollows that

In r=A- EIRT ... (2)

where r is Arrhenius rate constant, A is constant,R is gas constant, T is the absolute temperature.

The plots of In rand log., ¢ versus 1/ T weredrawn and indicated a high degree of linearity as

12·0r------=-~-----_Fmclionol conversion0:0·1 b:0-:2 c:O·,)

d: 0'4 e: 0-5 f :0·69 = 0·7 h = 0·8 i= 0·9

•..c

o

-'),0 cb

a

-6'02~'0~2~.TI---2~1~~2~1--~2'~4--~--2~'6---2~'7--2~'8

10')K-1--,T

Fig. ~-Plots of natural log of cure rate (r) versus reciprocaltemperature for TGODM/TGPAP/DDM (XO/20/30)

0·8

Fractionl Conversion0=0'1b=0'2c=0·3d=0'4e=O'S·=0·69:0·7h=O-8;=0·9

20

1·6

o ·0~--;;:,:---;;~---:~---:,.L-~.L-~..L-~-1-~2·0 2·1 2·2 2-'3 2'4 2·6

.1Q.3 ,K -1T

Fig. 4-Plots of natural log of cure rate (r) versus reciprocaltemperature for TGDDM/DGNDDM (80/20/30)

28

shown in Figs 3-6. From the slopes, activation en-ergy was calculated and plotted against fractionalconversion as shown in Figs 7 and 8. In spite ofexcellent linearity observed in the plots, the in-crease in the apparent activation energy is consist-ent with the reaction which becomes increasinglydiffusion controlled as the viscosity rises", Similarobservations were made by the authors while at-tempting to explain the mechanism of curing ofTGDDM based formulations with DDS and DDS-BFE systems''".

The values of activation energy E, and frequen-

SIDDARAMAIAH et al: CURE KINETICS OF EPOXY FORMULATIONS 117

110

100

90 1·20 ,E " "&-. ...-. /.x.

80 " go 0·9u /e"w "r

""70 "r 0·6.,",../

60 •... 0·3tv"

'50 0·00 1'0 2·1 2·2

Fractional Conversion

Fig. :;;- Plots of log heating rate (¢) versus reciprocal temper-ature for TGDDM/TGPAP/DDM (Hf)!20/30) system

110r-------------------------,

ow

100

90

"0~ 80-..x.

70

60

SO~--~~~~--~~--~--~o 0·2 0·4 0·6 0-8 1·0Fractional Conv~rsion

Fig. 6-Plots of log heating rate (¢) versus reciprocal tem-perature for TGDDM/DGNDDM (RO/20!30) system

cy factor A, were also determined by the Kissing-er" equation.

-In (¢/Tj,;I)= E/R~ -In (ARlEa) ••• (3)

where ¢ is the heating rate, I;, is the peak exoth-ermic temperature, R is the gas constant.

The values calculated from the Kissinger equa-tion are in full agreement with the values obtainedusing Ozawa's method and are given in Table 4.The addition of epoxy diluents like TGPAP, DGAand DGEBA has decreased the activation energyand frequency factor value as expected because ofthe decrease in the overall volume of functionalgroup.

,.( 0,.a = 10b=20c="]Od= 40e ='50f = 609 =70h =80i =90

-Fig. 7-Apparent activation energy I f.,,) as a function of frac-tional conversion (u), for TGDDM/TGPAP/DDM (HO/20/3())

[(0) from Arrhenius equation; (e) from Ozawa equation]

rl 'I.a= 10b= 20c = 30d= 40e= 50f = 609 = 70h= 80i = 90

0·6

0'3

Fig. H-Apparent activation energy (E,,) as a function of frac-tional conversion (a), for TGDDM/DGNDDM (H0/20/30)

[(0) from Arrhenius equation (e) from Ozawa equation]

Table 4-Kinetic parameters evaluated using Ozawa and Kis-singer relatiorrs (epoxy formulations with 30% DDM)

Resin Formu- Ozawa equation Kissinger equationsystem lation

E~±2kJ/mol

InAmin-I

E~+2kJ/mol

InAmin I

TGDDM 100 74.1 19.6 73.6 19.5TGPAP 100 68.0 18.4 66.5 18.0TGDDM/TGPAP 80:20 70.4 18.9 71.2 19.1TGDDM/DGA 80:20 67.9 17.7 68.4 17.8TGDDM/DGEBA 80:20 68.6 17.9 65.3 17.0

llR INDIAN J. ENG. MATER. SCI., JUNE 1996

The reaction between epoxies and amines in-volves several steps and hence the kinetics of theepoxy-amine reaction may be rather complex. Inthe present case the data are treated assumingsimple nth order Arrhenius-type kinetics. Thekinetic. parameters obtained also support energyin the range of 65-74 kJ/mol which is well inagreement with the reported values for variouscpoxyamine systems2•4,6, 7,16.17.

The relative trend in the characteristic tempera-tures and the kinetic parameters can be regardedas a measure of the relative reactivities of the var-ious epoxy systems and this information is usefulin the choice of reactive diluent or hardener tomodify the properties.

ConclusionsThe dynamic DSC cure kinetic study of the

TGDDMIDDM system using the epoxy diluentslike TGPAP, DGA and DGEBA are investigated.It reveals that the use of a diluent lowers the curetemperatures. The systems are. found to followsimple nth order Arrhenius-type kinetic. The par-ameters are in good agreement with the values re-ported for various epoxy-amine systems2.4.6.7.16.17.

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(1991) 1459.15 Kissinger H E, Anal Chern, 29 (1957) 1702.16 Gough L J & Smith I T, J App/ Polym Sci, 3 (1960) 362.17 Jenkins R & Karre L, J Appl Polym Sci, 10 (1966) 303.