Indian Journal of Chemical Technology Vol. 10, May 2003 , pp. 257-264

Articles

Modification of epoxy system for industrial applications: Preparation and characterization

M Suguna Lakshmi & B S R Reddy •

Industrial Chemi stry Laboratory , Central Leather Research Institute, Chennai 600 020, India

Received II Janua ry 2002; revised received 5 December 2002: accepted 6 January 2003

Modified DGEBA epoxy resin system was prepared using epoxy phenolic novolac (EPN), bisphenoi A cyanate ester prepolymer and polyether sulphone (PES). DGEBA epoxy resin modified with cyanate ester prepolymer offers process advantage in making prepregs for the preparation of structural composite materials. Further, the DGEBA epoxy resin modified with EPN and PES along with cyanate ester prepolymer was fully polymerised by curing at 200°C for 4 h. The cured resin was characterised by FT-IR, thermogravimetric analysis and differential scanning calorimetric methods. The moisture resistance properties were studied for the cured resins. The morphology studies were done for cyanate ester monomer, cyanate ester-epoxy polymer and PES-modified cyanate ester-epoxy polymers. Carbon fibre laminates were made based on PES-modified cyanate ester and epoxy polymer. Mechanical properties of the laminates were evaluated.

Epoxy resin is one of the most important thermosetting resins employed for high performance industrial applications 1

-6

. However, epoxy networks are typically rather brittle and display low fracture toughness in contrast to high toughness, high modulus characteristics required for high performance applications. Moreover, the nature and amount of the crosslinking agent used for making epoxy networks also induces relatively high moisture absorption characteristics. ln addition to this , lack of toughness has limited the application characteristics of these systems. In order to improve these above-mentioned drawbacks in properties, the DGEBA was chosen as model epoxy system for modification (scheme 1).

Scheme !- Structure of diglycidyl ether of bisphenol A (DGEBA)

Though DGEBA is known to produce highly crosslinked thermosets upon curing, the degree of actual crosslinking is limited. More tightly crosslinked resins are required in high performance composites for use at elevated temperatures. To

*For con-espondence (E- mail: incluL·hcm:'OOO @yahou.com; Fax: +91-44-491 1589).

increase the degree of crosslinking, it is intended to introduce epoxy phenolic novolac (EPN) component (scheme 2) into the epoxy system, which may result in a more crosslinked network structure in the region.

n

Scheme 2-Structure of epoxy phenolic novolac (EPN)

To reduce the water absorption of the cured resin7-9

,

bi sphenol A cyanate ester (scheme 3) was selected as a curing agent to the epoxy resin.

Scheme 3-Structure of bi sphenol A cyanate ester

Several workers 10-15 have investigated the

modification of epoxy resins with polyether sulphone (PES) (scheme 4) . PES is introduced into the system to act as a thermopl asticizer to lower the brittleness and thereby improve the toughness of the system.

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Scheme 4--Structure of polyether su i phone (PES)

The objective of the present work involved the preparation of modified epoxy resin system by incorporating multi-components and evaluation of its spectroscopic, thermal and morphological studies. Further, preparation of carbon fiber laminates from modified epoxy resin system and evaluation of their mechanical properties have also been reported.

Experimental Procedure Materials

The epoxy resins DGEBA, EPN (L Y556 and EPN-1138 of Ciba-Geigy Ltd., India) , PES (Victrex 3600 p, ICI Ltd. USA) and cobalt naphthenate and nonyl phenol (Aldrich) were used as received . Bisphenol A, cyanogen bromide, nonyl phenol, triethylamine, acetone and dichloromethane were procured from S.D. Fine Chemicals, India. Bisphenol A was purified by recrystallisation. Cyanogen bromide was used as received . Triethylamine and acetone were dried over anhydrous calcium chloride and distilled.

Synthesis of cyanate ester-monome/6

A batch scale of 100 g cyanate ester was synthesized by reacting 74 .6 g (0.7 mole) cyanogen bromide with 81 g (0.35 mole) of bisphenol A in the presence of 90 g (0.89 moles) triethylamine as a catalyst at ooc (scheme 5).

CH1 l-<0)--oo 2 ( Cz H.I I.1N t H + 2 CNBr I Q oC CH3

Scheme 5- Synthesis of bisphenol A cyanate ester

Indian J. Chern. Techno!. , May 2003

The reaction mixture was filtered under vacuum and the filtered liquid was poured into cold distilled water to isolate the product. The obtained product was fu rther purified by recrystallization in methanol :water (l: 1 VIV) at 5°C to get white crystalline product. The

yield was 80% (78 g) and the melting point was 75-780C. The sy nthesized cyanate ester monomer was characterised by Ff-IR, 1H/ 13C-NM R spectroscopic methods.

Preparation of cyanate ester prepolymer To I g of cyanate ester 18 mg of 2% cobalt

naphthenate solution in ethylmethyl ketone and 10 mg nonyl phenol were added. This mixture was heated at 95°C for 3 h and quenched to obtain a liquid cyanate ester.

Curing and co-curing of cyanate ester resin The curing and the co-curing of cyanate ester with

epoxy resins were made by employing the compositions as given in Table 1. The formulations were heated to 90°C and stirred fo r 0.5 h to obtain homogenous liquid. In formulation C, the PES was dissolved in dichloromethane and added to the epoxy and cyanate ester blend . All the formulations were heated to 90°C and kept under vacuum for 0.5 h and further subjected to the high temperature by slowly ratsmg the temperature to obtain void free homogeneous samples. Thus, the samples were cured at 180°C for 2 h and cooled. The appearance of the cured samples of formulations A, B and D were transparent, hard and brown in colour, while the formulations C and E were opaque. The formulations A, B and C were characterised by Ff-IR, DSC and TGA methods, while the formulations D and E was analysed exclusively by DSC studies .

Table !-Formulations (% WIW) , Tg and % moi sture absorption data of epoxy-cyanate ester

System DGEBA EPN TGDDM CYANATE DDS PES Tg (OC) %Moisture absorption

A ]()() 250 1.1

B 60 40 225 1.6

c 30 30 40 25 210 1.6

D 60 40 260 2.7

E 65 35 262 4.5

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Suguna Lakshmi & Reddy: Modification of epoxy system for industrial application Articles

Moisture absorption studies The cured specimens were placed in water which

was maintained at 80°C. The moisture absorption of the specimens immersed in water was checked by weighing them periodically for every 4 h. The specimens were removed from water, neatly wiped with a tissue paper, weighed immediately and placed back into hot water. This procedure was repeated till the samples attained the saturation of the moisture absorption (no change in weight) .

Fabrication of carbon fiber laminate For the formu lation C, an 8ply ( 12"x6") carbon

cloth (T-300 plain weave, Toray Company) laminate was fabricated. Each ply was impregnated with the resin, allowed to stand for 25 days at RT and stacked one on the other. Also a 1" width Teflon strip was placed in the mid layer, along one side to act as a crack initiator. The plies were placed in a laminating press and heated under pressure. It was cured at 180°C for 2 h and post -cured at 200°C for 4 h. The cured laminate was tested for flexural strength, flexural modulus, tensile strength, tensile modulus and Mode 1/Mode II fracture toughness properties.

Instruments Nicolet Ff-IR spectrophotometer model 20 DXB

was used for studying the IR spectroscopy (400-4000 cm- 1

) of monomers and cured samples . Analyses were done using solid KBr pellets. 1H and 13C-NMR of the compounds synthesized were run on a Bruker 320 MHz spectrometer at room temperature using CDCb as a solvent and tetramethylsilane (TMS) as an internal standard. Differential Scanning Calorimetric (DSC) analysis was performed using a Du Pont 200 instrument. The heating rate for DSC was I 0°C per min in the nitrogen atmosphere. Thermal stability of the formulations A, B and C (Table 1) were investigated on Mettler 3000 TG equipment. A S440 model Scanning Electron Microscope was employed to examine the morphology of cured samples. The surfaces of the samples were vacuum coated with gold . The universal testing machine model H T E-S­Series, Hounsfield equipments Ltd , UK was used for the following tests, employing ASTM specifications (Tensile test: ASTM-0 3479-76 and ASTM-D 790-84a). Mode I Fracture toughness (G 1c) was measured using Double Cantilever Beam (DCB) method . Mode II Fracture toughness (G 1;c ) was measured using the End Notched Flexure (ENF) 17 method .

4000

(b)

2000 Wavenumber(cm-1)

1000 ~0

8 7 (c)

160

6

II

~ 4 3 2 PPM

l 120 80

PPM

0

1 40 0

Fig . I (a)- FT-IR spectra of cyanate ester monomer; (b) - 1H­NMR spectra of cyanate ester monomer and (c)- 13C-NMR spectra of cyanate ester monomer.

Results and Discussion Characterization of cyanate ester monomer FT-IR Spectra

The cyanate ester monomer shows strong absorption peak around 2236-2273 cm- 1 for the presence of - OCN and a doublet peak around 1351-1383 cm- 1 for the presence of isopropyl group of bisphenol A cyanate ester. The peaks at 1169 and 1198 em_, are due to C-0 stretching of phenol. The

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broad peak around 1593-1631 em·' implies the C=C stretch in the benzene ring. The stretches at 811 and 831 cm- 1 show the 1,4-substitution in the aromatic compound (Fig. 1 a).

1H and 13C-NMR Spectra

In the 1H NMR spectrum, aromatic protons of the benzene ring appear around 7.0-7.2 ppm. The protons of the isopropyl group were observed at 1.5-1.6 ppm (Fig lb). In the 13C-NMR spectrum, the OCN group was observed at 151 ppm, which confirms the formation of OCN group. The carbons present in the phenyl rings were seen between 108-128 ppm while the quaternary carbon present in the isopropyl group was observed at 42.6 ppm. The methylene group present in the isopropyl group was observed at 31.3 ppm (Fig lc). The 1H and 13C-NMR spectra confirm the absence of phenolic proton and the formation of OCN group.

Characterization of cyanate ester prepolymer Though epoxy-cyanate system is known for good

adhesion, high temperature resistance, and low moisture absorption properties, the physical state (solid) of the cyanate ester possess difficulty in the preparation of its prepregs (the impregnated fibers which are in the ready form for making composite materials). Prepregs made by impregnating in the solution of epoxy-cyanate mixture becomes dry once the solvent get evaporated . On the other hand, prepregs made by impregnating in the melt liquid of epoxy-cyanate mixture also becomes brittle after the prepregs gets cooled. Since prepregs made by solution and/or melt impregnation methods are dry, it is difficult to stick them together. In the present studies, cyanate ester prepolymer is in the liquid form for making better prepregs. Both the epoxy-cyanate systems prepared were found to be in the semi-solid or liquid form which forecasts that the prepergs made with them will have required tack properties. Therefore, attempts were made to gel the cyanate ester monomer so as to prepare the homogenous epoxy-cyanate liquid system for application in making tacky prepregs. The gelation was carried out by the partial polymerisation of the cyanate ester monomer. The polymerisation reaction proceeds via polycyclotrimerisation by non-catalytic (thermally) and catalytic methods . Non-catalysed thermal polymerisation reaction is possible and the reaction is supposed to be catalysed by impurities including moisture and residual phenol from synthesis. Thermal

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Indian J . Chern . Techno!. , May 2003

polymerisation requires high temperatures around 150°C and above. Here the reaction proceeds faster and results in highly crosslinked solid resin.

The cobalt naphthenate, manganous octoate and copper acetyl acetonate catalysts were employed 18

·19

for polycyclotrimerisation. Nonyl phenol was used as a co-catalyst. This catalytic system was employed because of its miscibility in neat resins , low temperature curing and good latency at room temperature. These catalysts perhaps help bringing cyanate groups together into the ring forming proximity while the hydroxyl group donates protons for imido carbonate formation and step transfer20

·2 1

.

To obtain low crosslinked cyanate ester, it was necessary to optimize the quantity of the catalyst and the temperature at which it was to be trimerised. After conducting a number of trial experiments varying the catalyst/co-catalyst concentration, the curing conditions were optimized. The co-catalyst (nonyl phenol) also acts as a solvent for the metal catalysts, which is sparingly soluble in neat resins. When the nonyl phenol concentration was increased, it I . . h 22 p astiCizes t e cyanate ester . Hence, the liquid

cyanate obtained in the present investigation may be due to the plasticizing effect of nonyl phenol present in the mixtures . The cyanate ester prepolymer prepared under present experimental conditions was found to be in the liquid form at room temperature which is advantageous for making tacky prepregs . The pot life of the liquid resin was also observed and was found to be 25 days.

Characterization of cyanate ester prepolymer-epoxy prepregs

The prepreg plies made wi th cyanate ester prepolymer were placed one above the other and stacked well. On the physical observation, the plies were in stack with each other for a minimum period of 25 clays. Also, the liquid cyanate ester prepolymer can be easily formulated with other liquid epoxy resins to impregnate the fiber for making various composite materials. The prepregs made out of liquid cyanate prepolymer-epoxy system need not have to be heated to melt the cyanate so as to stick the prepregs together for making composites .

FT-·IR Spectra The cyanate ester prepolymer (Fig. 2a) shows

ab orption peak at 1598 cm- 1 which represents the tri azine ring formation. The strong absorption at 2236-2273 cm- 1 shows the presence of - OCN. The

Suguna Lakshmi & Reddy: Modification of epoxy system for industrial application Articles

(a)

,...., fl. ........ (1) u c 0 .....

.. ~ E Ill (b) c e ..-

4000 2000 1000 500

Wavenumbers (cm-1)

Fig. 2 (a)-Ff-IR spectra of cyanate ester prepolymer and (b)­Ff-IR spectra of cyanate ester homopolymer,

Table 2-Ff-IR spectral .ssignmen: . · l ~ puxy-cyanate ester polymers

Chemical class Absorbing Reference Wave number group compound (cm- 1)

Epcxy resin Epoxy ether DGEBA 1245, 1242 EPN

Arylcyanurate Tri azine Trimer 1599, 1598

Novolac resin -CH2- EPN 2971 , 281 3

Sui phone so2 PES 1150

Oxazo1ine - NCO Cyanate 2280 ol mer

peaks for the presence of OCN as well as the triazine ring lead to assumption that the cyanate ester is partially polymeri sed to yield a cyanate ester prepolymer.

Table 3-Thermal data of epoxy-cyanate ester formul ations (A, 8 , and C)

Formu- lOT Decom-lation (0 C) position

range (OC)

A 410 410-670

8 421 421-592

c 376 376-592

Temp. (0C) vs weight loss % of polymer

40 60 80 90

425 514 564 593

410 428 518 550

400 526 583 609

Characterisation of epoxy-cyanate ester polymer FT-IR Spectra

Homo- and blends of different compositions were prepared (Table I) and characterised by Ff-IR (Fig. 2b). The characteristic infrared spectral absorptions are listed in Table 2. The spectra confirm the presence of the original constituents like methylene in the novolac resin , sulphone in the PES resin and epoxy ether in the epoxy resins. The triazine and isocyanurate rings formed during the curing reaction have also been confirmed.

Ther mogravimetric analysis

Thermal properties of cyanate ester homopolymer (A), epoxy-cyanate polymer blend (B) and the PES modified epoxy-cyanate polymer blends (C) (Table 3) were investigated by recording the thermograms of these systems in air (Fig. 3). The cyanate ester homo polymer undergoes two-step degradation. The maximum decomposition of crosslinked cyanurate rings shows at 410°C. The co-cured cyanate epoxy polymer undergoes multiple degradations since the initial decomposition temperature (IDT) of the systems B and C coincide with the IDT of the cyanate polymer. The cyanurate crosslinked structure that

. •)J decomposed around 400°C was conftrmect-· . It was noted that IDT of the system C was less than the IDT of the systems B and A. This may be due to the flexible functional groups present in PES, which did influence the cyanate-epoxy system to lower the thermal stability . It can be concluded that the system (A) and (B) possess similar thermal stability . The system (C) though shows slightly lower IDT, its decomposition range was similar to the system B.

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--------

c ·a (\

CTI

100 200

- - - - 0 ---- b ----'C

I"· I \ I \ ' . I \

~ \ ' r1 . J I \ ! \ \ I I I

II I 0

I ~ I ,I

f1rv,. ~ l·

Fig . 3 (a) - Thermogram of cyanate ester homopolymer; (b)- Thermogram of epoxy-cyanate ester polymer blend and (c)-Thermogram of PES modifi ed epoxy-cya nate es ter po lymer blend .

Differential scanning calorimetric and moisture absorption studies

The Tg and the % of moi sture absorption properties of epoxy-cyanate systems were compared with the properties of conventional epoxy-am ine cured sys te m.

Cyanate ester homopolymer (A) gave high Tg (250°C) and the lowest moisture absorption ( 1.1 %). The epoxy-cyanate systems (8 , C and D) were compared with epoxy-amine (TGDDM-DDS) sys tem (E). The Tg·s observed were in the same range for the epoxy res in c ured with cyanate and amine. But, the moisture abso rpt ion is much less in epoxy-cyanate systems when compared with epoxy-amine system (£). Further, it was observed that the presence of EPN m

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Indian J. Chern. Tec h110I. , May 2003

M~- - . -- ... 1-4 ...... 411 ......... . .

Fig. 4--Scanning electron micrographs: I-S EM of cyanate ester homopolymer; 11-SEM of epoxy-cyanate ester polymer blend; and III - SEM of PES modified epoxy-cyanate ester polymer blend.

the DGEBA epoxy-cyanate system, Tg val ue was fo und to be slightly higher with respect to the Tg of DGEB A epoxy-cyanate system. Though the PES mod ified epoxy-cyanate system shows less Tg. the

Suguna Lakshmi & Reddy : Modification of epoxy system for industrial application Articles

moi sture resis tance property has not been compromised. Thus, the PES modified epoxy-cyanate system shows good Tg (210°C) and low moisture absorption (1 .6%). The low moisture absorption in the case of cyanate-cured epoxies may be due to the formation of strong networks and the absence of hydrophilic amino groups to some extent.

Scanning electron microscopy (SEM)

There are reports on toughening of the highly crosslinked thermosetting polymers with high modulus and high Tg thermoplastics as studied by SEM24-26. In a thermoset/thermoplastic blend the components are usually mixed together and the crosslinking of the epoxy occurs in situ. The increase of the molecular weight of the epoxy during curing may result in phase separation even if the temperature is kept constant27 . One of the most studied thermoplastics for the modification of epoxy resin is the polyether sulphone (PES). PES is added to increase viscosity during hot lay up of the prepregs for the manufacture of composites28.

Morphology SEM photographs of the cold snap surfaces for the

cured systems are given in photomicrographs I to Ill (Fig. 4). The cyanate ester homopolymer (I) morphology appears to be transparent and homogeneous under the magnification at 800 x. The blend of DGEBA and EPN with cyanate ester monomer (II) appears nearly transparent. Nevertheless, some amount of phase separation was observed which may be the immiscible regions formed during the reaction of epoxies with cyanate ester monomer. The phase separation of PES in epoxy-cyanate ester (III) was observed at 800 x magnification . During the curing reaction, the epoxy rings opens up and produce polar hydroxyl groups. The related enthalpy of mixing seems to cause the PES to separate from the solution. On the other hand, with more than 20% of polysulphone content, the cyanate ester/polysulphone blends were phase separated by spinodal decomposition27 . The phase separation during the curing process seems to be an important parameter because it provides a two-phase

system to allow the formation of micro-voids for the shrinkage compensation. Phase separation may be caused by a change in the miscibility of the monomer, prepolymer and catalyst during curing. Therefore, the thermoplastic/thermosetting system undergoes several steps before it reaches a maximum reaction rate point. Initially before the reaction, at room temperature, the two systems remain separately. Phase one remairs as a continuous phase and the phase two may perhaps be dispersed in the continuous phase. During heating, the systems become miscible and transparent. Further, the reaction takes place in the miscible systems to form highly crosslinked having high molecular weight micro-gels. This decreases the miscibility of the overall reacting system and at this point phase separation occurs. The separated micro-gel gets crosslinks themselves to produce a macrostructure and di splays as co-continuous phases in the two phase network structure.

Evaluation of carbon fiber laminate properties The mechanical properties of the laminates are

given in Table 4. It can be seen that the flexural and tensile properties are good while the fracture toughness properties are high in the order of 0.32 (KJ/M2)29, compared to standard epoxy system (0.080 KJ/M2)3o_

Conclusion DGEBA epoxy resm was modified with EPN,

cyanate ester and PES to obtain an epoxy resi n system, which offers improved properties. The EPN was included to enhance the degree of cross-linking while the cyanate ester was introduced to improve the moisture resistance and to establish the less densed crosslinked structure upon curing. Further, the cyanate ester monomer was prepolymerised by varying the catalyst and co-catalyst concentration. The liquid cyanate ester prepolymer had a pot life of 25 days. The cyanate ester prepolymer-epoxy blend offers process advantage for making epoxy prepregs . The DGEBA epoxy resin modified system shows high glass transition temperature and low moisture absorption. The morphology study of PES modified epoxy resin system by SEM shows phase separation.

Table 4-Mechanical properties of the modified epoxy composite

Properties Resin content (Volume%)

Values 55

Flexural strength (Mpa)

750

Flexural modulus (Gpa)

55

Tensile strength (Mpa)

560

Tensile modulus (Gpa)

61

Fracture toughness (KJ/M 2)

G1c Gnc

0.32 1.16

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The related enthalpy of mixing of epoxy with PES, which seems to cause the PES to separate. The flexural, tensile and toughness properties of the composite, are seems to be very good.

Acknowledgement One of the authors (MS) is grateful to Mr M K

Sridhar, Ms P Kanakalatha, and Ms Chandra Ajay, National Aerospace Laboratories, Bangalore for their help in the present work.

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