Indi an Journal of Engineering & Materials Sciences Vol. 8, December 200 I, pp. 381-385

Spectroscopic studies on curing of novolac epoxy resin-Polyamide and polyamine hardener systems in presence of a polysulphide

P S Makashir, NT Agawane, R R Mahajan & J P Agrawal* High Energy Materials Research Laboratory, Sutarwadi, Pune 411021, India

Received 17 November 2000; accepted 26 July 2001

Novolac epoxy resins have recently been reported for inhibition of hydroxy terminated polybutadiene (HTPB) based composite propellants and their elongation has been improved by the incorporation of some Oexibilizers. In the present investigat ion . curing behaviour of a novolac epoxy resin (Dobeckot E4) by hardeners (polyamide EH 411 and polyamine H 758) in conjunction with a polysulphide Oexibilizer has been studied using infrared (lR) spectroscopy. It is observed that the rate of curing of Dobeckot E4 by hardeners such as hardener EH411 and hardener H 758, is higher as compared to polysulphide Oexibilizer LP-33. Further, rate of curing of epoxy resin by hardeners such as EH411 and H758 enhances in the presence of Oexibili zer LP-33. The curing studies by infrared (IR) spectroscopy suggest that hardeners EH41l1H758 react with epoxy ring of epoxy resin and as a result, epoxy ring opens while curing. Similarly, - SH group of polysulphide Oexibilizer LP-33, reacts with the epoxy ring of resin and opens it as curing progresses and thus enhances the rate of curing of epoxy resin in conjunction with hardeners EH411lH758. This is also supported by the gel time and exotherm peak temperature data.

Epoxy resins possess a number of valuable properties due to which they find innumerable applications1.2 One of the important applications of novolac epoxy. resin (Dobeckot E 4), next generation of epoxy resins, is their application for 'inhibition of composite propellants' used for various rockets/missiles3

.4 and it

is mainly because of a unique combination of properties such as chemical inertness, toughness, better bonding with a variety of substrates, negligible shrinkage on curing etc.s A commercially available novol ac epoxy resin, (Dobeckot E4), however, does not possess high elongation which is essential in order to ensure successful performance of inhibited rocket propellants over a wide range of temperatures6 ,i.e., -40°C to +60°C. It is reported in the literature that the elongation of these resins is improved by the incorporation of either long chain molecules (dibutyl phthalate, tricresy l phosphate etc.-referred as plasticizers) which remain unreacted after curing in the resin networks7 or long chain molecules (epoxy diluents, liquid rubbers etc.-referred as flexilibizers) which react during curing and become integral part of the 3-dimensional networks3

. The use of a reactive epoxy diluent/liquid rubber is preferred because they become integral part of the 3-dimensional networks and therefore, do not migrate during subsequent ageing/storage. The most important member of

* For correspondence

flexibilizers is liquid polysulphides, which impart flexibility of high order7

.12.

The mechanism of curing of epoxy resins using polyamide/polyamine hardeners is reported in the literature1.2. However, the curing mechanism of epoxy resin by polyamine/polyamide in conjunction with a polysulphide flexibilizer has not yet been reported .

The object of this investigation is to study curing of Dobeckot E4 by hardeners such as EH411 (polyamide) and H 758 (polyamine), in the presence of a polysulphide flexibilizer (Thiokol LP-33) with the help of lR spectroscopy. We have studied the rate of curing of Dobeckot E4 by polyamide and polyamine hardeners individually as well as in the presence of a polysulphide flexibilizer, Llsing IR spectroscopy and correlated the same with gel time data.

Experimental Procedure

Materials Novolac epoxy resin Dobeckot £4, polyamide

hardener EH411, polyamine hardener H758 of Dr. Beck and Co. (India) Ltd., Pune and liquid polysulphide LP-33 of Thiokol Chemical Corporation, New Jersey, USA, were used for these studies. The specifications of these materials are:

Novolac Epoxy Resin, Dobeckot E4-An epoxy resin based on cashew nut shell liquid (CSNL) and is an amber coloured clear liquid with density (at 25°C),

382 INDIAN J. ENG. MATER. SCI., DECEMBER 2001

g/mL: 1.064±0.002; viscosity (at 25DC), cPs: 17, 120±1000 and epoxide equivalent: 330±35.

Polyamine Hardener 758-lt is a low viscosity polyamine and is a fast curing agent for Dobeckot E4 at ambient temperature.

Polyamide Hardener, EH 411-A room temperature curing hardener and is a yellowish brown, clear and viscous liquid with density (at 25 DC), g/mL: 0.98±0.0 I, and viscosity (at 25 DC), cPs: 17,750±100.

Liquid Polysulphide LP-33-A mobile liquid with a viscosity (at 25 DC), cPs: 1400-1650 and (Mn): 1000±150. This is a thiol terminated , saturated, linear liquid polymer and consists of 0.5% cross-linking agent.

Methods

Gel time and exotherm peak temperature (G I and EPT )

Studies on gel time and exotherm peak temperature of di fferent formulations of E4, hardeners EH4111H758 and LP-33 were carried out, as reported earlier l3

•

The time corresponding to a sudden build-up of viscosity (known as gelling) of known weight of resin at a definite temperature after adding hardener is defined as gel time (Gt) and the highest temperature which is recorded during gelling is defined as exotherm peak temperature (EIYr) 13. The gel time was measured by a Gelation Timer, Model GT-4 (M/S. Techne Cambridge Ltd ., Cambridge, UK) and EJYf was measured by inserting a laboratory thermometer (with a least count of 1 DC) in the resin and recording the rise in temperature periodically while determining gel time.

The gelation timer consists of a disc type plunger, hanged from a linkage which is crank driven by a sy nchronous motor and a digi ta l counter (in minutes), also connected to the synchronous motor. The plunger falls in a liquid polymer under gravity and is pulled­up by the synchronous motor. At the gel point, rigidity of polymer supports weight of the disc resulting in stoppage of the synchronous motor as well as digital timer by a relay mechanism and gel time is displayed on the digital counter.

A weighed quantity of resin (l00 g), was taken in an aluminium cup (45 mm diameter and 77 mm length) and immersed in a constant temperature water bath by holding the cup in a clamp. It was immersed in such a way that the resin level was below that of the water level of the water bath. The bath temperature was maintained at 40±1 DC. When the

resin attains the temperature of the bath , required quantity of hardener was added separately and mixed thoroughly. The plunger was operated simultaneously followed by an insertion of a thermometer in the resin. The digital timer also starts displaying time when the plunger goes up and down. The temperature of the resin was also recorded by the thermometer every minute. The plunger and timer stop at gel point and gel time is displayed on the digital timer/counter. The temperature of resin was recorded till it became maximum and then declined by a few degrees. A mean of three readings was taken and reported as Gt

and EJYf •

IR studies Perkin-Elmer FfIR Spectrophotometer, Model

1605, was used to record IR spectra of Dobeckot-E4 by incorporation of hardeners and polysilphide flexibilizer in different proportions . The IR spectra were recorded by the smear method, in the frequency range 4400-440 cm·1, using medium speed of sC<!:lI1ing.

A number of formulations based on Dobeckot E4, polyamide hardener (EH411) and polyamine hardener (H 758) were made and studied in terms of tensile strength (TS) and elongation (E)14. Based on a combination of TS and E, formulati ons [Dobeckot E4: 100+Hardener EH 411 :50 and Dobeckot E4: 100+Hardener H758 : 10] were selected for studying the effect of liquid polysulphide (LP-33). The data generated on gel time, tensile strength, elongation, dioctyl adipate absorption, heat resistance and flame retardance suggested that 20 parts of LP-33 were optimal in order to impart characteristics suitable for inhibition of composite propellants 15 Therefore, these formulation s were selected for this study.

Calculation of rate of curing To study the rate of curing of Dobeckot E4 by

incorporation of hardeners EH 411, H 758 and polysulphide flexibilizer LP-33, in selected proportions, IR spectra were recorded at regular time intervals I6

.17

.

The variation in the intensity of the band due to terminal epoxy ring at 912 cm-1 was used to monitor the curing of epoxy resin during IR studies l6

. For I I . 18-20 h f . f . d h ca cu atll1g t e ractlon 0 epoxy resll1 cure , t e

peak intensity obtained immediately after mixing Dobeckot E4+LP-33, Dobeckot E4+EH 411, Dobeckot E4+H 758, Dobeckot E4+EH411+LP-33 and Dobeckot E4+H7S8+LP-33 was taken as

MAKASHIR el 01.: CURING OF NOVOLAC EPOXY RESIN 383

respective initial concentration and curing rate of epoxy resin was calculated using formula,

C . Change in concentration at time 't' 100

unng rate = x Initial concentration

Results and Discussion Epoxy resins are converted into 3-dimensional

networks held together by covalent bonds 1.2 as a result of crosslinking reaction. This conversion of liquid or friable brittle solids into a tough crosslinked polymer is called curing and is achieved by the addition of curing agents which serve as initiators for resin homo­polymerization. The polyfunctional curing agents function as reactants or co-monomers in polymerization, leading to mainly formation of 3-dimensional networks composed of resin molecules crosslinked via curing agents. In most of the curing reactions, actual mechanism is ionic in nature, but it may be anionic or cationic as well, depending upon the curing agent. Tertiary amine curing agents21

convert the hydroxyl group of an epoxy resin to an alkoxide ion, which reacts with epoxide to create a new alkoxide ion and combines with second epoxide molecule forming an ether and regenerating alkoxide ions to combine with the third epoxide molecu le. Thus this chain reaction continues till the entire mass is converted to a solid mass. The first alkoxide ion of

100.----------------------------------,

80 ,..... ~ 0 '-I

w u

60 z <t: t--t: ~ (j)

z 40 <t: ct t-

20

4000 3000 2000 1600 1200 800 600

WAVE NUMBER/cm-1

Fig. I-FfIR spectra of Dobeckot E4

a chain may be a part of the resin or it may come from a phenolic hydroxyl group in a catalyst. In nutshell, epoxy groups are readily opened up by the available ions/active hydrogens and polymerization of epoxy

. 22 resins occurs . IR spectra of Dobeckot E4, and polysulphide

flexibilizer LP-33 are given in Figs I and 2 respectively . IR spectrum of Dobeckot E4 (Fig. 1) shows bands due to epoxy group23 at 1246, 1042 and 912 cm· l

, whereas IR spectra of hardeners EH411 and H758 have absorption bands due to -NH stretching mode (amide and amino groups respectively) in the frequency range of 3250 - 3450 em-I. The IR spectra of Dobeckot E4 by incorporation of hardeners EH411lH758 in the proportion '4, ' 5 of 100:501100:10 have been recorded immediately after mixing and then, at a regular time interval.

Similarly, the IR spectra of Dobeckot E4 by incorporation of polysulphide flexibilizer (LP-33) in the proportion of 100:20 have been recorded immediately after mixing and then at a regular time interval. It is seen from these IR spectra that there is a loss in the intensity of band at 2560 cm·1 due to -SH stretching,

IR spectra of Dobeckot E4 by incorporation of hardener EH4111H758 and a flexibili zer LP-33 have also been recorded immediately after mixing and then, at a regular time interval. Fig. 3 represents the variation in band intensities ' 9.

2o for different groups

10 0 ,..... ~ 0

'-' ~

" w 80 () 2560 z <t: t--t: ~ 60 (j) Z « (k:' t-

40

20

4000 3000 2000 1600 1200 800 600

WAVE NUMBER!cm-1

Fig. 2-FfIR spectra of polysulphide flexibi lizer LP-33

384 INDIAN J. ENG. MATER. SCI., DECEMBER 2001

with time, i.e., progressive curing. The intensity of the bands is not found to vary uniformly with time, as is normally expected. Instead, intensities of bands con'esponding to epoxy group and -SH group show preferential loss (2560 cm·I). The variation in the intensity of band due to terminal epoxy ring at 912 cm·1 has been used to monitor the curing of epoxy resin during IR studies l6

. It is evident that the loss in intens ity of epoxy group is faster by incorporation of hardener EH4111H-758 than only polysulphide.

During curing studies by IR, there is a preferential drop in intensi ty l s.2o of epoxy ring band l 6 at 912 em-I

and -NH stretching band of primary amide group of the hardener EH 411, with time when incorporated in Dobeckot E4 (Fig. 3). This suggests that there is an interaction between primary amide group of EH 411 and epoxy rings leading to opening of epoxy ring while curing. Similar results are obtained for

MIN .

340

300

230

160

110

55

0

4000 3000 2000 1600 1200 800 470

WAVE NUMBER / c m- 1

w <..:J z <l: r-t: ~ cJ)

z <t: ~ r-

Fig. 3-FTIR spectra of Dobeckot E4+EH411 Lp·33 ( 100:50:20) as a function of time.

polyamine hardener H 758 also when incorporated in Dobeckot E4. The curing reactions l

1.2 1 of Dobeckot

E4 in presence of hardener EH 411 and hardener H 758 may be shown as

(i) Polyamide hardener

(ii) Polyamine hardener

Similarly, on incorporation of polysulphide LP-33 in Dobeckot E4, there is a preferential loss in intensi ty l s.2o of -SH band and also loss in the intensity of bands corresponding to epoxy group, from the beginning of the reaction. Thi s suggests that mercaptan, i.e., -SH group of polysulphide reacts with the epoxy ring of Dobeckot E4 in the following manner, as cunng progresses.

(iii) Polysulphide flexibilizer

CH2-CH-+R' SH- R· S· CH2-CH-

~O/ I OH

However, reaction of epoxy group with mercaptan (-SH) group of polysulphide flexibilizer is not so vigorous as with an amine/amide group of polyamine/polyamide curing agents2

.11

.

Fig. 4, represents the rate of curing 1S.20 of Dobeckot

E4 by incorporation of polysulphide flex ibilizer LP-33, polyamide hardener EHAII and polyam ine hardener H 758 in selected proporti ons. Thi s figure also suggests that the rate of curing of Dobeckot E4 by incorporation of hardeners EH411 or H 758 is higher than that of polysulphide flexibilizer, LP-33 alone. Further, when flexibilizer LP-33 is incorporated in Dobeckot E4-hardeners EH 411 or H 758 systems, rate of curing of Dobeckot E4 enhances.

The liquid polysulphide LP-33 (20 parts) has been added to Dobeckot E4-hardener (EH4111H758) system and effect of its incorporation on gel time and exotherm peak temperature is given in Tab le I. It is seen that the gel time of Dobeckot E4 - EH 411/ H758 is reduced on incorporation of LP-33 which infers that

MAKASHIR et al. : CURING OF NOVOLAC EPOXY RESIN 385

80 E

" ~ 60 w'

~ Q:

(!) 40 Z C

cr ::> u

20

A

100 200 300 400 500 TIME/ MIN

Fig. 4-Rate of curing of Dobeckot E4 by incorporation of hardeners and fl exibilizer as a function of time [ (A) E4+LP-33 ( 100:20), (B) E4+EH411 (100:50) , (C) E4+H758 (100: 10), (D) E4+EH411+LP-33 (100:50:20) and (E) E4+H758+LP-33 ( 100: 10:20)]

Table I- Effect of liquid polysu lphide LP-3 3 on gel time and exotherm peak temperature of Dobeckot E4 - hardener sys tems

Formulation Gcl time (G/) Exotherm peak min. temperature (En)

°C E4 + H 758 35 46.2 100:10

E4 + H758 + LP 33 23 47.8 100 : 10:20

E4+EH411 38 43.4 100:50

E4 + EH 411 + LP 33 25 46 .0 100:50:20

the rate of curing is enhanced by liquid polysulphide flexibi lizer.

Thus the gel time data of various formulations (Table 1) also supports the observations of IR curing studies, i.e. , liquid polysulphide accelerates rate of curing of Dobeckot E4-polyamide/polyamine systems. This may be explained on the basis of the fact that the liquid polysulphides have terminal -SH group which participate during curing and enhances

f . I I 1 ?4 I . d' h I' 25 h rate 0 cunng ' '- . t IS reporte 111 t e Iterature t at G, and Ep/ are inversely related to each other and as G, decreases in the presence of a polysulph ide (LP-33), Ep/ is expected to increase which is observed ex peri men ta1\ y .

Conclusions The present investigations show that the rate of

curing of Dobeckot E4 novolac epoxy resin by hardeners EH4111H758 is higher as compared to polysulphide flexibilizer LP-33. Also, the hardness EH411/H758 react with epoxy ring of the epoxy resin, resulting in the enhancement of the rate of curing of the epoxy resin .

Acknowledgements Authors are thankful to the Director, HEMRL for

his interest in this work .

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