Microdielectric analysis of the polymerization of an epoxy-amine system

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Journal of Non-Crystalline Solids 172-174 (1994) 1012-1016

Microdielectric analysis of the polymerization of an epoxy-amine system C. M a t h i e u a'b, G . B o i t e u x a'*, G . S e y t r e a, R. V i l l a i n h, P. D u b l i n e a u b aLaboratoire d'Etudes des Mat~riaux Plastiques et des Biomatkriaux, URA C N R S no. 507, Universitk Lyon 1, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne cbdex, France bA~rospatiale 12, Rue Pasteur, BP 76, 92152 Suresnes cbdex, France

Abstract Microdielectrometry is used to detect in real time critical events such as gelation or vitrification during the curing of epoxy resins (DGEBA-DDS). These new results are compared with that given by classical methods of investigation (insoluble fractions, dynamic mechanical analysis, viscosimetry). The efficiency of the electrical technique allows one to detect chemical phenomena with parameters sensitive to the modification of the epoxy system during the network formation.

1. Introduction Nowadays, epoxy resins are widely used in adhesives, paintings, materials for electronics and composites. They are advantageous because they display good mechanical properties at rather high working temperatures for a reasonable price. The use of epoxy parts for cars or planes has become conventional (the lack ofvolatiles and their relatively well-understood chemistry make them easily processable) but the optimization of their process is still a challenge. The aim of this article is to show how microdielectrometry can improve the curing of epoxy resins by detecting in real-time critical events like gelation or vitrification. If we refer to the literature available on the subject, what strikes us is a tendency of working with complex industrial systems [1, 2]. That is why our

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study is based on a well-known system (DGEBADDS in stoichiometric proportions) in order to clearly distinguish the two principal phenomena observed during the curing, which are gelation and vitrification.

2. Chemistry of the reagents Fig. 1 shows the formula of the monomers, the characteristics of the mixture components are given in Table 1. All the mixtures are realized with a stoichiometric ratio, r, equal to 1 (an epoxy function for an amino-hydrogen).

3. Experimental devices Dielectric analysis is based on the study of the electrical response of a curing polymer placed between two electrodes and subjected to an alternative field. Generally, the output current is attenuated and

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C. Mathieu et al. / Journal of Non-Crystalline Solids 172-174 (1994) 1012-1016

Abbreviation

Monomers

DGEBA ( n = 0,03)

diglycidyl (hher du bisph~nol A

DDS

diamino-4,4' diph~.nylsulfone

1013

Formula

,o

__

c.H

o

o

o Fig. 1. F o r m u l a of m o n o m e r s and abbrevations.

Table 1 Characteristics of the m o n o m e r s D G E B A and D D S

i'o. = (~/~oe~) -

( B I o ~" + 1)) (#/eo)2

and Monomer

DGEBA DDS

State at room temperature

Molar mass (g)

f

crystallized Tf = 32°C crystallized If = 175°C

348

2

248

4

Supplier

8~ipo~e = [,or(8, - 801(I + (,ot)2)], Dow Chemical Fluka

out-of-phase is compared with the input voltage. The response of such an imperfect dielectric stems from its capacitive component (characterized by its dielectric constant or relative permittivity, 8') and its conductive component (characterized by its loss factor, d'). Two main phenomena contribute to this electrical behaviour: (i) permanent dipoles orienting and vibrating in the electric field; (ii) ionic displacements inside the dielectric due to impurities. Values of 8' and d' are calculated by equations which quantify these relationships for one dipolar process: 8' = 8'io n + ~dipole; 8 t' :

8'iron + 8dipole;

8'~o. : (A/,o ~"+ ")(a/80)~; 8~iipole

=

~u

+

[-(Sr

- - 8u)/(1

+ ((.oz)2)];

where A and B are constants depending on the interface electrode/polymer and taking into account electrode polarization effects, n is a real number between 0 and I, a is the dc conductivity, 8o is the permittivity of free space = 8.85 x 10-14 F/cm, co is the angular frequency, 8, is the low frequency dielectric permittivity, eu is the high frequency dielectric permittivity and t is the single relaxation time of the dipoles. Limited for a long time by a lack of well-adapted instrumentation, dielectric analysis has developed rapidly with microdielectrometry [3,4] which became available after 1982. This technique is presently based on the microdielectrometer Eumetric System III of Micromet Instruments. This system generates sinusoidal signals between I0 -3 and 105 Hz and transmits them to the electrodes of the sensor. Among all the available sensors, we have used the mid-conductivity IDEX sensor. It measures conductivities between 10-14 and 10 -~ S/cm and is only made of comb electrodes. The signal detected is analyzed to determine phase and amplitude. A calibration table transforms these results into permittivity and loss factor. This technique enables one to measure precisely gel times without the vitrification hindering if the adequate solvent is choosen. Generally, it is accepted as the

C. Mathieu et al. / Journal of Non-Crystalline Solids 172-174 (1994) 1012-1016

1014

10 m

I

!

I

I

I

|

I

I

I

|01

10°

iO -I

I

go

~

I

~

I

~

I

I

I

~oa

,os

too

I

I

I

m

.4

.7

120

Time (min) Fig, 2. Mechanical tan 6 for different angular frequencies (from 0.1 to 10 rad/s) versus isothermal curing time at 140°C.

Table 2 Gel times (in min) of the mixture D G E B A - D D S for the different techniques at the different temperatures Technique

Insolubles (in THF) Dynamic mechanical analysis Viscosimetry Microdielectrometry

Temperature (°C) 100

120

140

160

180

670 + 5 -493 + 9

243+3 -223_ 1 225_1

92+ 1 105_1 100_+ 1 114+2

39_+ 1 49_1 44_2 47_1

19_ 1 23 -4-1 20+1 16+5

reference technique for detecting gelation. Practically, gel time is defined as the time of appearance of the first insolubles determined by the presence of cloudiness in THF. Mechanical analysis was carried out with a Rheometrics Dynamic Analyser, model RDS7700. Measurements were made in the dynamic mode between parallel plates at frequencies between 1 and l0 rad/s. The oscillatory strain was choosen near 5% to ensure that the measurements were being made in the linear viscoelastic region.

The point of the mechanical tan 6 curves which is frequency-independent (crossover point) corresponds to the gel point [5,6]. In order, to measure the variation of the viscosity of the medium during the reaction, we have used a viscosimeter Carri-Med with cone-plane plates. The increase of viscosity to a high value marks the occurrence of gelation and/or vitrification. We have arbitrarily determined this high value of viscosity to be 1000 Pa s whatever the temperature, corresponding to the limit of the analysis system.

C. Mathieu et al. / Journal of Non-Crystalline Solids 172 174 (1994) 1012-1016

1015

-7

1400

140°C

1200

120°C

-7.5'

-8" r.~ -8.5. "~

100°£

~" 1000

°C

-9.

;> 4OO-

-9.5-1@ ¢0 110.5"

2OO-

Ca0 -11" O

bO°C 0

•~

6OO-

-11.5-

C~ '~ 0

100

2i)0

360 4~ Time (min)

5~

6OO

Fig. 3. Evolution of the viscosity versus curing time for the different temperatures.

DGEBA+DDS

(a/e=1)

t

(a)

130

15

90

1~.

70

9

5O

7 5 3 I 0

3O I I I I I I I I-:10 120 180 240 300 360 420 480 540 TIME (min)

DGEBA+DDS

(a/e=l)

140C

SZSTI40.DAT IBEX-MID 4~

.................................

(b) I

150

3

130

~

L

9

o

~ X 7oo

T

0

5O

-:1

30

-2

0

60

120 IBO 240 300 360 420 480 540

10

TIME (rain)

Fig. 4. Variations of log e," and e,' between 0.1 Hz and 100000 Hz versus curing time at 140°C obtained with an I D E X

sensor.

2/)0

1C 300

4 400

120 0 C 500

0°C 600

700

Time (rain)

Fig. 5. Superposition of all the conductivity curves obtained from the values of e".

4. Results

19

I 60

~

16o

140C

SI5T140.DAT IDEX-MID 21

-12

The main characteristics of the system D G E B A - D D S in stoichiometric proportions are as follows: glass transition temperature of the initial mixture, Tgo = - 2 ° C ; glass transition temperature of the fully cured material, Tg~ = 218°C; and temperature for which gelation and vitrification occur at the same time, gelTg = 81°C. The isothermal curings have been carried out between 100 and 180°C at temperatures separated by 20°C so as to gradually isolate gelation from vitrification. The gel times obtained for each temperature by this method which is the reference for the subsequent discussion are put together in Table 2. Fig. 2 shows the variations of tan 6 versus isothermal curing time at 140°C. The times corresponding to the independence of tan 6 are given in Table 2 for all the temperatures. Fig. 3 displays the evolution of the viscosity versus curing time for the different temperatures. Table 2 reports the times corresponding to a high viscosity of 1000 Pa s. Fig. 4 presents the variations of e" and e' obtained with an IDEX-sensor versus curing time at 140°C. At the beginning of the polymerization, we clearly observe the preponderant influence of ionic conductivity on g' and e' (electrode polarization effects). Then, dipolar relaxations depending on the frequency of measurement appear as usually in the solid state and are in fact due to vitrification. The

1016

c. Mathieu et al. / Journal of Non-Crystalline Solids 172-174 (1994) 1012-1016

superposition of all the conductivity curves (obtained from the values of e") is plotted in Fig. 5. The criterion choosen to define the gel point of the system is the inflexion observed (when it is possible) on these curves. Indeed, we believe that gelation leads to a decrease of ionic conduction rather than a complete immobilization of ions the size of which is small compared to the still wide track of the polymer network. Table 2 groups together the results obtained by microdielectrometry.

5. Discussion Table 2 can be examined according to two temperature domains: (i) from 120 to 180°C, all the experimental techniques detect in their own ways a gel point which is in agreement with the reference of insolubles; (ii) at 100°C, the vitrification hindering alters the dielectric and mechanical determination of the gel times. The comparison with viscosimetry clearly shows that the electrical technique is sensible to a significant increase of viscosity.

6. Conclusion The isothermal curing of a well-known epoxyamine system ( D G E B A - D D S is stoichiometric proportions) has been followed using microdielectrometry, insoluble fractions, dynamic mechanical analysis and viscosimetry. The ability of the electri-

cal technique to detect efficiently chemical phenomena with parameters sensitive to the modification of the system during the network formation has been shown. The following essential conclusions can currently be drawn. (i) Vitrification is undoubtedly characterized by the relaxation of the dipole which are present in the system, in relation with the segmental motions inside the growing network. (ii) As far as gelation is concerned, when it appears well before vitrification (according to the concept of the T T T diagram), it is indicated by an inflexion point on the curve representing the evolution of ionic conduction with isothermal curing time. These conclusions are in agreement with previous studies realized on model systems displaying either vitrification or gelation [7].

References [1] J. Chottiner, Z.N. Sanjana, M.R. Kodani, K.W. Lengeland G.B. Rosenblatt, Polym. Comp. 3 (2) (1978) 59. I-2] J.G. Maquart, Ann. Comp. l&2 (1986) 109. [3] S.D. Senturia and N.F. Sheppard Jr., Adv. Polym. Sci 80 (1986) 1. [4] D.E. Kranbuehl, S.E. Delos and P.K. Jue, Polym. 27 (1986) 11. 1-5] H.H. Winter, Polym. Eng. Sci. 27 (1987) 1698. 1-6] M. F6ve, Makromol. Chem. Makromol. Chem. Symp. 30 (1989) 95. [7] G. Boiteux, P. Dublineau, M. F6ve, C. Mathieu, G. Seytre and J. Ulanski, Polym. Bull. 30 (1993)441.