E L S E V I E R Materials Chemistry and Physics 52 (1998) 272-276

MATERIALS CHEMISTRYAND

PHYSICS

Materials Science Communication

Effects of CTBN on the cure characteristics of DGEBA/MDA/PGE-AcAm system

J.Y. Lee a, H.K. Choi b, M.J. Shim °, S.W. Kim a,, Department of Chemieal Engineering, The Universio' of Seoul, Seoul 130-743, South Korea

b Division of Chemisto', NITQ, Kwacheon 427-010, South Korea • Department of Life Science, The University ofSeoul, Seou1130-743, South Korea

Received 13 June 1997: received in revised form t7 July 1997; accepted 4 September 1997

Abstract

Effects of carboxyl-terminated butadiene acrylonitrile robber (CTBN) on the cure characteristics of diglycidyl ether of bisphenol A (DGEBA)/4,4'-methylene dianiline (MDA)/phenyl gtycidyI ether (PGE)-acetamide (AcAm) system were studied by autocatalytic cure rate expression. Cure rate at initial stage increased with the increment of the CTBN content, however, at some high stage cure rate was inverted due to the diffusion controt of formed CTBN domain. The total conversion was also decreased with the increase of the CTBN content. © 1998 Elsevier Science S.A.

Keywords: Cure "kinetics; Epoxy; Autocatalyst; CTBN; Diffusion control

1. Introduct ion

It has been well known that there are many accelerators such as -OH, -COOH, -SO3H, -CONHz, CONHR, etc., to donate hydrogen bonds to the amine-epoxide reaction and the acceleration mechanism is illustrated by the formation and decomposition of the termolecular transition state [ 1-6 ].

In this system, the acceleration effects o f - O H and - C O O H groups were studied. The - O H group was generated by the cure reaction of diglycidyI ether of the bisphenol A (DGEBA)/4 ,4 ' -methy lene dianiline (MDA) system and was also from the introduced phenyl glycidyl ether ( P G E ) - acetamide (AcAm). The - C O O H group was from the addition of carboxyl-terminated butadiene acrylonitrile copolymer (CTBN). To analyze the accelerated cure rate, the following autocatalytic cure rate expression was used [7-1o].

d ~ &= ~ =(kl +k z od")(1-o~) n ( 1 )

dt

* Corresponding author. Department of Chemical Engineering, The Uni- versity of Seoul, 90 Jeonnong-Dong, Dongdaemun-Gu, SeouI I30-743, South Korea. Tel.: +82-2-210-2447; fax: +82-2-210-2310, 216-0570; e-mail: swkim@scucc.scu.ac.kv

0254-0584/98/$19.00 © I998 Elsevier Science S.A. All rights reserved PII $ 0 2 5 4 - 0 5 8 4 ( 9 7 ) 0 2 0 4 9 - X

dR

m + n = 2 (3)

In( ( \ &P/(1-ceP)2 . . . . kt 2--m )klo~-'~/( m-- 2o~p) )

m--- (4) lnc%

~2 = (2 -m)k t@- '~ (5) m - 2 ~ p

Where, & is the cure rate, c~ is the cure degree of monomers, m and n are reaction orders, % is the degree of cure at exo- thermic peak and &p is the cure rate at exothermic peak. kl represents the cure rate constant associated with the non- catalytic reaction and catalytic reaction o f - O H and - C O O H groups from the initial formulation, and k2 represents the cure rate constant associated with the autocatalytic reaction of the - O H group generated from the reaction between amine- epoxide groups.

2. Exper iment

DGEBA-type epoxy resin (Epon 828, Shell Co.) was cured by 4,4'-methylene dianiline (MDA) with the addition

J,Y. Lee era[./Materials Chemisto' and Physics 52 (1998) 272-275 273

of synthesized phenyl glycidyI ether (PGE)-ace tamide (AcAm) and carboxyl-terminated butadiene acrylonitrite copolymer (CTBN I300 X 8, B.F. Goodrich Co.). The for- mulation of CTBN was as follows. The molecular weight of the CTBN was 3600 and acrylonitrile content was i8%.

HOOC-[(7- CH2--CH:CH--C H2-)~.,C- CI4 ,-- C H--)TJT COOH

CN

DGEBA, P G E - A c A m ( 10 phr) and CTBN (0, 5, 10, 15 and 20 phr) were well mixed at 80°C for 5 min and MDA (30 phr) was added to the mixture at 80°C to prevent the cure reaction during the mixing. To study the cure kinetics, the sample was analyzed using the isothermal DSC method at 80 ~ 120°C and the data were introduced to Eqs. ( 1 ) - ( 5 ) .

3. Results and discussion

All DSC thermograms showed that the rate of heat gener- ation increased at the initial stage, exhibited a maximum value and decreased as increasing cure time. Such curve meant that the chemical reaction of the system was catalyzed by the

product and this was referred to as an autocatalytic reaction. In this epoxy system, the hydroxyl group produced by the cure reaction of epoxy-amine groups acted as a catalyst.

With the assumption that the rate of heat generation was proportional to the cure rate [11,12], isothermal cure rate curves for D G E B A / M D A / P G E - A c A m / C T B N (10 phr) system at 80, 90, 100, 110 and 120°C are shown in Fig. 1. With the increment of cure temperature, the increment in the maximum value of cure rate, @, and the shift to a shorter time, tp at &p were observed and these meant that the more monomers participated in the cure reaction in a shorter time and at a higher temperature. The relationships between @, tp and cure temperature are shown in Fig. 2 and they are expressed as the follows.

I +13.16 lnc% = - 6 " 1 9 X 1 0 3

10

I • 80oC , " , • 9000

% / \ * 10000 'E 6 ' • 110°0

('4

2 × 4 .d

0 10 20 30 40 50 60

T ime (min)

Fig. I. Cure rate versus time for DGEBA/MDA/PGE-AcAm/CTBN ( I0 phr).

-2 4

3

-3

' ~ : 2 5-

..Z

1

"'~ 215 216 217 218 2. 0

1/T x lOa (K-l)

Fig. 2. Maximum cure rate, &p and the time, tp to be % as a function of reciprocal absolute temperature for DGEBA/MDA/PGE-AcAm/CTBN ( i0 phr).

1.0

0.8

0.4

0.2 . ,,, 110oc

0.0 o 120oC

0 10 20 30 40 50 60

T ime (min)

Fig. 3. Conversion versus time for DGEBA/MDA/PGE-AcAm/CTBN ( 10 phr).

1 In tp=7.77X103 ,~ --18.84

Plots of monomer conversion, c~ as a function of time for D G E B A / M D A / P G E - A c A m / C T B N ( I 0 phr) were obtained from the integration of cure rate curves in Fig. 1 and they are displayed in Fig. 3. All the curves showed s-shape and these also meant that the cure reaction of the epoxy system followed the autocatalytic mechanism. At low con- version, the curves were shifted to the left along the time-axis in accordance with the increasing cure temperature. However, the value of maximum conversion for the curves of three high temperatures decreased with increasing cure temperature, and the maximum values for 80 and 90°C could not be compared here because the conversion was not reached to the maximum value. At some higher conversion, epoxy matrix became vit- rified and CTBN domains were produced. So the functional groups of the epoxy system were effectively quenched and the cure reaction became diffusion limited [13-15] . When curing temperature increased, the quenching effect increased

274 J. E Lee et aL / Materials Chemistr3' and Physics 52 (1998) 272-276

10

.e_ E

~5 ;,<

d

8

6

4

2

0 .0

• 80 °C

" 90 oC

100°C

• 110°C

i . i , r ~ . .

0.2 0 . 4 0.6 0.8 1.0

c~ Fig. 4. Cure rate versus conversion for DGEBA/MDA/PGE-AcAm/CTBN ( t0 phr).

due to no arranging time of the monomers, therefore the maximum conversions were inverted at three high tempera- rares. Fig. 4 shows the cure rate versus conversion for DGEBA/MDA/PGE-AcAm/CTBN (10phr) . At low conversion, cure rate increased with the increment of cure temperature, however, the maximum cure rate appeared at lower conversion. At high conversion, when cure temperature increased, the maximum conversion and the cure rate increased at 100°C like the general cure reaction of the epoxy system, however, above 100°C the cure rate inverted at about

oe = 0.50 ~ 0.64 and the maximum conversion decreased due to the quenching effect explained above.

Kinetic parameters k], k2, in and n are easily obtained from Eqs. (2 ) - (5 ) and are listed in Table 1. To obtain the acti- vation energy and pre-exponential factor for k~ and k2, the following Arrhenius equation was used [ 16].

kf=Ai exp(-EaJRT) (6)

where k; represents rate constants for k] and k2, As is the corresponding pre-exponential factor, Eas is the activation energy and R is the gas constant. The relationship between - Ink and 1/Tis plotted in Fig. 5. The activation energy was calculated from the slopes and the pre-exponential factor was obtained from the intersect, and the values are listed in Table 1. Using the same method, the kinetic parameters for the epoxy systems with various CTBN contents were calculated and are listed in Table 1. k2 was higher than k[ for all systems with different CTBN content, and with the increment of CTBN content, k~ increased while k2 decreased. The former could be explained by the role of the generated -OH group and the latter by the role of the added -COOH group. This occurs because k~ was associated with the non-catalytic and catalytic reaction of the -OH group in PGE-AcAm and the -COOH group in CTBN from the initial formulation, and k2 represents the "kinetic rate constant associated with the auto- catalytic reaction of the -OH group generated from the reac- tion between amine-epoxide groups.

Table 1 Kinetic parameters for the DGEBA/MDA/PGE-AcAm/CTBN system

CTBN content Temp. kX I0 ~ (min -~) (phr) (K)

Ea (kcatmol -I ) A X 10 -4 ( ra in- i ) m n

k} k. Eal Ea2 AI A2

0 353 0.40 363 0.66 373 1.12 383 1.48 393 2.24

5 353 0.52 363 1.05 373 1.55 383 Z03 393 3.12

10 353 O.82 363 1,36 373 2.10 383 3,42 393 4.11

15 353 1.11 363 1.54 373 2.25 383 3.72 393 4,41

20 353 1.34 363 1.97 373 2.75 383 3.85 393 5,63

5.40 7,48

10.82 16.55 26,19

3.31 5.24 7.79 9,71

12,02 3.04 4.i4 6.95 7.86

10,48 2,31 3.48 4,09 7.65

1[.21 1.79

3.14 5.t4 7.94

10,17

11.74

11.74

11.42

10.03

9,74

10.87 7.63 26.97 1,08 0.92 t.01 0.99 0.96 1.04 0.95 1.05 0.92 1.08

8.84 7.92 1.08 0.65 1,35 0.72 1.28 0.74 1.26 0,75 1,25 0.44 1.56

8,56 10,27 0.62 0.72 1.28 0.76 1.24 0,69 1.31 0,54 1.46 0.32 1,68

10.87 1.75 tl ,58 0.62 1.38 0,61 1.39 0.37 1.63 0.45 1.55 0.45 1.55

12.20 1.40 66,66 0,61 1,39 0,69 1.31 0,60 1,40 0.43 1.57 0.42 1.58

J.Y. Lee e~ al. / Materials Chemistry and Physics 52 (1998) 272-276 275

i

8

k 1 4

3

2

1

0 215 ' 216 ' 21.7 218 219 l/q" X 103 (K -1)

Fig. 5. Relationship between -In k and I/T.

A'N~al O _ - CTBN Content (phr) - - e w 0 phr

3 • ,10 phr

E" t-

~ ' 2

X

1

°o 1'o 2'0 3'o 4'0 Time (min)

Fig. 6. Comparison of cure rate versus time for the DGEBA/MDA/PGE- AcAm system with various CTBN contents at 100°C.

1.0

0.8

0.6

0.4

//r ?/ * 13 phr ~/? / / • 10 phr o 2 y ,

0.0 I ~ . I , I , I , I , ~ J

10 20 30 40 50 60 Time (rain)

Fig. 7. Comparison of conversion versus time for the DGEBA/MDA/PGE- AcAm system with various CTBN content at I00°C.

4. Conclusions

From these results, the following conclusions were obtained. 1. Isothermal conversion curves for the epoxy systems with

various CTBN contents showed an s-shape and this meant that all systems followed the autocatalytic mechanism.

2. With the increment of CTBN content, the cure rate at the initial stage increased due to the catalytic role of the - COOH group in CTBN, while at some high conversion

stage the cure rate inverted and total conversion decreased due to the diffusion control of formed CTBN domains.

3. With the increase of CTBN content, k I increased and k2

decreased, however, all k2 were higher than kl. This meant that the catalytic effect o f - C O O H increased, while the autocatalytic effect o f - O H decreased.

Fig. 6 shows the effect of CTBN content on the cure rate as a function of time for the D G E B A / M D A / P G E - A c A m

system at 100°C. With increased content of CTBN, the max-

imum values appeared at a shorter time and the initial cure rates were faster, and these results meant that the cure rate of

the epoxy system was accelerated by the -COOH group in

CTBN. By the integration of the curves in Fig. 6, conversion

as a function of time is displayed in Fig. 7. All the curves

showed an s-shape regardless of the CTBN content, and this

meant that all formulations followed an autocatalytic mech-

anism. It was also found that the conversion of the system at

the initial stage was made higher when more CTBN was added. However, the conversion was inverted at c~-- 0.75 for

10 phr of CTBN and at o~ = 0.60 for 20 phr of CTBN, and the

maximum conversion decreased with the increment of CTBN content due to the diffusion control of CTBN domain formed

during the cure reaction.

Acknowledgements

This work was supported by Sun Kyong group.

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