Effect of Epoq and@ismaIk'mide Structure on the C p r o p d s of Epoq'lienolic-

Bimahimide M a t e S y s t e m

a 1 . t of the results discusedin this chapter ispu6listied

1. "Epoa-fl&lpfienoG Bismabimide mat*system - Property dependency on epo~r structun."

IntenrcltMlaCconferi mm 2006, fim, Dec.26-29,2006

2. 'Epov-JnyCphenoC- Bismabimde mat*system - Property &pendency on nature of bismaI2mde"

Communicrrted to J. o f ~ p p G c d & , ~ S c z i m z

Ihe properties of the tematy mat* system (Ern) o6tained 6y the reaaive bhnding of E m aayC phenol and cBirmaGimidi +nd on the structure and prqpnries of its components, Ihe i n a n c e of the stnuturd variations of epoxy resin and 6imahimidi on the tr5mnaC physicaf a d mechanical properties of the tmnag 6hnd was examined: E(FB compositions were preparedusing 6im&h&, diil$Cphenolandtline d i f J n t epoxy resin systam vie. WmGu epoxy, LY-556 anda tri epoxy, fiaving &fierent p/jvicat chemiaf and mohcuhr characteristics. Ihe &ct of smcturaf variations of @%I resin on tlk pe$onnunce of the E(FB system was s t d i d w'ng ER0 comparitwns containing three difleant typar of 6ismaIk 'des-BMI~ @MIM a d @ M E - in i-om6ination with the mGu epoxy and &l$C 6rrphenoCA. 272s chapter summarises the s t d m canied out fbr the cue cfiaracterizatwn andp@nnance evaluation of tliese matr&systems a n d t h i r g h s composites.

7.1 Introduction In the previous chapter we have studied the effect of different types of

reinforcements on the performance of the fiber reinforced EPB composites. The

influence of reinforcement type, architecture, orientation and stacking sequence

on the performance of the composite was also evaluated. The present chapter

deals with the effect of variations in the chemical and molecular characteristics of

the matrix components on the performance of their ternary blends. The influence

of these matrix modifications on their composite strength characteristics was also

studied. The molecular characteristics of the nowlac epoxy diallaylbisphenol - bismaleimide (EPB) system was modified in two ways.1) by changing the type of

epoxy and 2) by changing the nature of the bismaleimide.

The versatility in form, modification, properties, hardening methods, cure

conditions and application is probably the most outstanding characteristic of epoxy

resin. Its utility can be further widened by the modification of the same using

suitable means to tailor its property to suit the different system requirements. The

cure of epoxy resin is complicated and it is useful to visualize the process in

several stages1. Multifunctional (tetra- and tri functional) epoxy resins are used for

most of the existing aerospace applications. N, N' tetraglycidyl- 4, 4'-

diaminodiphenyl methane (TGDDM) epoxy resin cured with DDS is the commonly

used system. Two most frequently encountered trifunctional epoxies are triglycidyl

- p aminophenol (TGPAP) and tris epoxy -novolac (TEN). The TEN resin is a

semi solid at room temperature. This type of structure results in a very tightly

crosslinked network upon curing and the resin has a very high heat distortion

temperature and outstanding thermo -oxidative stability compared to other types

of epoxy resins2. The glass transition temperature of over 300% can be attained

by curing it with DDS'. This resin is therefore intended for applications in

advanced composites which demand property retention up to 1 7 7 ' ~ dry and 2 3 2 ' ~

wet. Novolac epoxy resins, heing multifunctional can produce denser crosslinked

networks compared to common epoxy resins4. The bisphenol A novolac epoxy

resin contains aromatic rings in its molecular back bone . as a result it may

achieve excellent mechanical and thermal performance. The synthesis of

bisphenol A novolac epoxy resin is reported by several author^.^'

Chapter 7 21 1

Another important parameter concerns the thenal resistance of these

materials and, as would be expected, the choice of wring agent dictates eventual

glass transition temperature of the resin. When the difference between the cure

temperature T. and glass transition temperature T, is small, the cure reaction

become diffusion controlled because the molecular mobility is reduced close to the

glass transition temperature. If the cure temperature is considerably less than T,,

vitrification may oc:cur before gelation and then further reaction may be inhibited.

Elevated temperature resins are those that cure to yield somewhat

inflexible molecular structures. Rigidity can be built in to the cured matrix in

several ways: through the incorporation of aromatic groups, an increase in the

number of reaction sites (epoxy groups) per molecule or a reduction in the

distance between the reaction sites. The three primary classes of epoxy resins

used in composite application are phenolic glycidyl ethers, aromatic glycidyl

amines and cycloaliphatics'. In order to improve the thermal and mechanical

properties of epoxy resin, modifying the molecular backbone and lor increasing

the number of epoxide group functionality are employed by many

Changes in the basic structure of the backbone linking the two maleimide

end groups causes changes in its toughness and other physical properties.

Generally the aromatic linkages have higher rigidity than the aliphatic ones. This

rigidity normally contributes to high melting point, glass transition temperature,

thermo oxidative resistance, decomposition temperature and modulus of a

polymeric material. However the processability and fracture resistance of aromatic

BMI resins are drastiially reduced by the increased molecular rigidity. The studies

relating to the influence of BMI nature on the adhesive and thermo- mechanical

characteristics of o, 0'- diallylbisphenol A (DABA) - BMI system showed that the

presence of polar groups such as sulphone and ether resulted in enhanced

cohesive strength of the matrix".

The earlier studiestz it was observed that in diallyl bisphenol novolac

(ABPF) cured using structurally different bismaleimides (BMls), the adhesive

properties are nol: significantly affected by the stmctural variations of the

bismaleimides. The reported general trend in adhesive properties of the different

bismaleimides in this study was in the order BMIS> BMlE zBMlP >BMIM.

BMI resins are reactive towards many reactive species and curing reactions such

as thermal polymerization, addition reactions, Diels Alder reactions, have been

developed to increase their application performance. The type and extent of each

reaction depend on the chemical and molecular characteristics of the

bismaleimide. Maleimides react with ally1 phenols through Alder-ene reacti~n'~."

to give rise to cyclic network structures with improved high temperature

performan~e.'~ The applicability of BMI-allyl phenol resins as matrices for

advanced composites have been studied extensive~y.'~ The properties of BMI

resins strongly depend on their synthesis conditions. However, A DSC kinetic

study has shown that the structure of BMI does not have much influence on the

reaction kinetics".

The structural changes and the resulting property variations of the epoxy

and the BMI systems can influence the performance of the ternary EPB blend

formed by the reaction between epoxy - phenol and bismaleimide. Hence, two

series of studies were carried out , one to study the effect of structural variations

of epoxy and second to study the influence of bismaleimide type on the

performance and properties of Me EPB neat systems and their glass composites.

This chapter details the results of these studies.

7.2 Materials and Methods

7.2.1 Materials

The materials used in the study constitute three different types of epoxies

viz. novolac epoxy, Ly-556 and triphenylomethane triglycidyl ether (tri epoxy) and

three different bismaleimides viz. 2. 2 -bis 4-(4 maleimidophenoxy) phenyl

propane-BMIP, Bis (4-maleimidophenyl) methane- BMlM and Bis (4-maleimido

phenyl) ether- BMIE. Sources of epoxies, bismaleimides, DABA and TPP are

given in chapter 2. The characteristics of different epoxies and bismaleimides are

given in Tables 7.la and 7.1 b.

Chapter 7 213

7.2.2 Preparation of Ternary Blend.

Epoxy resin and diallyl bisphenol A are weighed in a 100 ml reaction

bottles so that they are in their stochiometric equivalent ratios. Known weights (0.5

weight %) of TPF' was added to these flasks. Calculated amounts of bisphenol a

bismaleimide was then added to this so that the quantities of epoxy, allyl phenol

and bismaleimide are in their stoichiometric equivalent ratios. The resin blends

required for analysis were prepared by dissolving them in AR acetone, heating to

7 0 ' ~ for completo dissolution and removal of acetone by evaporation in a water

bath at 70%. Complete removal of solvent was achieved by heating the same in a

vacuum oven at 70 '~ . The same procedure was repeated for these systems with

different epoxies (E2 & E3) to get the EPEE2 and EPEE3 respectively. A second

set of samples was prepared using the same epoxy (El) and diallyl bisphenol A

and different types of bismaleimides (BMI-I. BMI-2 and BMI-3) to get EPB-B1.

EPEB2 and EPB-B3 respectively. The cure conditions of the ternary blends were

fixed based on their cure characteristics obtained from DSC themnograms and

Table7. l a Characteristics of different epoxies

Type of epoxies St~cture

Novolac /"\

-

Ly-556

(E2)

Tri epoxy

(E3)

Equivalent weight

185.2

181.2

153.3

Chapter 7 214

isothermal and non- isothermal rheograms. The curing was effected by heating

the ternary blends in vacuum oven at the prefixed heating schedule. The final

curing was done by heating the same at 2 5 0 . ~ for five hours. The different EPB

systems are identified in Table 7.2.

7.2.3 Characterisation of the raw materials

7.2.3.1 Characterisation of epoxy resins

The molecular characteristics of the epoxy resins were evaluated by

determining their molecular weight distribution using the gel permeation

chromatographic technique. The IR spectra were recorded following the analysis

conditions mentioned in chapter.2.

7.2.3.2 Characterisation of bisrnaleirnidee

The three bismaleimides used for the study were characterized for their

thermal behavior by recording their DSC thermograms at a heating rate of 10

' ~ lm in in nitrogen atmosphere. Their IR spectra were also recorded to ObSeNe the

difference in their structural characteristiis.

Table7.l b Characteristics of di i rent Bismaleirnides

Equivalent.

Bismaleimide Structure Weight

Chapter 7 215

Table7.2 Identification of di i rent EPB systems

r----I I I Identification of EPB Components of the temary blend

I 1 1 EPB-El Epoxy-El. DABA. BMI-I I

PI EPB-E2

Epoxy-€2, DABA, BMI-I

I Epoxy-E3. DABA. BMI-1

Epoxy-El. DABA. BMI-I

I 5 1 EPB-B2 Epoxy-El , DABA. BMI-2

7.2.4 Characterization of the ternary blend

7.2.4.1 Spectroscopic characterization

The IR spectra of all the EPB systems were recorded before and afler

cure reaction to check the cure completion under the optimized cure conditions.

1 4 EPB-63

7.2.4.2 Cure characterization using DSC

The cure reactions of the resin systems were studied using a Meltler

DSC-20 analyzer at a heating rate of 5'~lmin in nitrogen atmosphere. Separate

DSC thermograms were recorded to study the Epoxy-Allyl phenol reactions for

the three epoxy systems and the Epoxy-allyl phenol- bismaleimide reactions for

the systems with i) three different epoxies (El, E2 & €3) and same bismaleimide

(BMIP) and ii) different bismaleimides (BMI-I, BMI-2 8 BMI-3) and same epoxy

(Novolac epoxy).

Epoxy-El. DABA. BMI-3

7.2.4.3 Rheological Characterisatlon

Rheological analysis was carried out for all the EPB systems. The

analysis conditions and the procedure are given in chapter 2. The advancement of

cure reaction with respect to temperature was studied first, followed by the

Chapter 7 216

isothermal rheological studies at temperature corresponding to the large scale

crosslink formation observed in the non-isothermal rheological studies.

7.2.4.4 Thermo gravimetric Analysis:

Thermo gravimebic analysis was performed on all the cured ternary

blends with epoxy and bismaleimide structural variations for checking their thermal

stability.

7.2.4.5 Debtmination of glass transition temperature

The non-isothermal DSC analysis was performed on the cured polymer for

the determination of its glass transition temperature. A preliminary DSC analysis

was done in nitrogen atmosphere using a sample mass of 15 rng at a heating rate

of 2O0Chnin. The sample was cooled to 50% The heating and cooling was

repeated twice and the final analysis was done at a heating rate of 2"Clmin. The

temperature corresponding to the midpoint of the shifled base line in the DSC

curve is taken as the glass transkion temperature.

7.2.4.6 Evaluation of adhesive properties

The adhosive properties of the EPB matrix systems with bismaleimide

structural variations (EPB-B1, EPEB2 and EPB-83) were evaluated by

determining their lap shear strength as per the standard pmcedure ASTM D-1002.

The specimen preparation and lapshear testing was carried out as per the

conditions given in chapter 5, following the time temperature cure schedule

optimised for the system using the above mentioned analysis techniques. The

adhesive properties were evaluated at elevated temperatures to compare the high

temperature performance of the different EPB systems.

7.2.5 Preparation of Laminates:

EPB-glass laminates were prepared in the manner described in chapter 5.

using the different EPB matrix systems containing diiW%nt epoxies ( EPB-El.

EPEE2 and EPEE3) and same bismaleimide (BMIP) and also those containing

the same epoxy (EPN) and three different bismaleimides (EPEBI. EPEB2 and

Chapter 7 218

The characteristic epoxy absorption at 915 an-' was observed in the IR spectrum

of the three epoxy resins. The IR spectrum shows that EPN contains some OH

groups. The SEC traces of these three epoxy resins are given in Fig 7.2

Fig.7.2 SEC traces of ~ b x i e s (El. EZ and E3)

The molecular weight distribution pattern obtained for the three resins were found

to be different. From GPC traces, LY-556 (E2) mainly contained the expected

molecular species with smaller concentration of chain extended ones. The EPN is

constituted by a mixture of oligomers (in significant proportion). The proportion of

higher oligomers in the hiepoxy is approximately 20% as deduced from the

relative areas under the GPC peaks.

7.3.1.2 Characterisation of Bismaleimides

The FT-IR and DSC techniques were used for the characterization of

three bismaleimides- BMI-I, 2 and 3. The IR spectra of the three bismaleimides

are given in Fig.'7.3. All bismaleimides showed the characteristic C=O absorption

at 1710 cm.'. The aromatic C=C absorption appeared at 1500 an" The peaks at

830 and 690 cm' are characteristic of the C-H bending vibration of the maleimide

groups. BMI-3 showed the characteristic C-0-C stretch vibration at 1250 cm.'

unlike the other two.

Chapter 7 217

EPBB3).The same catalyst concentration (0.5 % of the weight of epoxy &

diallylbisphenol) was used in all cases.

7.2.6 Characterisation af composites The composites were characterised for their mechanical properties such

as flexural strength (FS), compressive strength (CS), Inter laminar shear strength

(ILSS) and Interfacial shear strength and thermal & physical properties such as

linear expansion, density, resin-content, void-content and water absorption

following the respective ASTM procedures given in Table 2.2 of chapter 2.

7.3 Results and discussion

7.3.1 Characterisation of Raw materials 7.3.1.1 Characterisation of Epoxy Resins

The epoxy resins were characterized by FT-IR and size exclusion

chromatography (SEC) analyses. The FT- IR spectra of the three epoxy resins are

given in Figure 7.1.

Fig.7.1 IR spectra of Epoxies (El, E2 and E3)

Chapter 7 219

Fig.7.3 IR spectra of Bismaleimides (BMI-I. BMI-2 8 BMI-3)

The cure characteristics of the three bismaleimides were compared using their

DSC curves given in Fig. 7.4.

Fig 7.4 IISC thermograms of bismaleimides (EM-I, BMI-2 8 BMI-3)

The melting endotherm were found to be 72'C. 159.4'C and 172.9"C for BMI-I,

BMI-2 & BMI-3 respectively. The cure reaction initiated more or less at the same

temperature for BMI-2 and BMI-3, while that for BMI-2 it was comparatively at a

lower temperature. The corresponding exotherm peak temperatures were in the

order BMI-I>BMI-3 =-BMI-2. The diirence in temperature between the melting

and decomposition is largest in the case of BMI-f, which allows a wider

processing window for the system, while it was minimum for BMI-2.

7.3.2 Cure characterisation of the ternary polymer blend

7.3.2.1 IR spectroscopy:

The IR spectra of the different EPB systems (EPB-E2 & EPB-E3) with the

same bismaleimide-BMI-lare given in Fig. 7.5.a Bb. The corresponding IR

spectra of the EPB-B2 and EPB-B3 with different bisrnaleimides in combination

with novolac epoxy are given in Fig.7.6a & b

Fig 7.5a IR spectrum of the EPEE2 system before and after curing

As in the case of EPB-El I EPB-B1 described in chapter.5 and Fig. 5.5, the

characteristic absorption due to ally1 at 917 crn-' and maleimide at 690cm.' (=C-H

Chapter 7 221

bond) disappeared in the cured resin in the case of EPB-€2 and E3. The C=O

absorption intensity considerably diminished as expected. Similar pattern was

seen in all cases. Similarly Me intensity of the absorption at 830cm.' diminished.

Infact, this absorption is a combination of =C-H (maleimide) and C-H (aromatic)

bending vibrations. The former only disappears on curing.

Fig 7.5b IR spectrum of the EPB-E3 system before and after curing

Fig 7.6a IR spectrum of the EPEE2 system before and after curing

Chapter 7 222

Fig 7.6b IR spectrurr~ of the EPEB3 system before and after curing

7.3.2.2 Differential Scanning Calorimetry

The DSC cure thermograms of the 1:lblends of epoxy- diallyl bisphenol

(EP) systems with different epoxies (EP-El, EP-E2 & EP-E3) are given in Fig. 7.7.

The peak temperature corresponding to the epoxy-ally1 phenol reaction was not

found to alter with the change in the epoxy type. The allyl polymerization started at

about 230% in all cases. The allyl polymerization being a low enthalpy reaction

was difficult to be distinctly detected in DSC.

The DSC thermograms of EPB systems with different epoxies (EPB-El.

EPEE2 and EPB-E3) are given in Fig. 7.8. While keeping the BMI same (BMIP),

variation of epoxy is not found to cause significant difference in cure pattern. The

phenol-epoxy, Ene reaction, Wagner-Jauregg and Diels-Alder reactions all occur

more or less in the same temperature regions in all the three cases. The

incorporation of tri epoxy in the EPB system resulted in comparatively insignificant

enthalpy change for the phenol - epoxy reaction for EPEE3 system. The Ene

reaction occurred in the temperature range of 150- 175 'C and the Diels-Alder

reaction occurred in the temperature range of 190- 2 2 0 ' ~ in all the three cases. In

the case of EPB system containing LY-556, the ene reaction occurred at a

comparatively lower temperature and the enthalpy change due to the DiCAlder

Chapter 7 223

reaction was negligible. The curing of the unsaturated groups in the Alder-ene

adduct occured beyond 250'C.

ti- * " C d. A Temperature OC

Fig.7.7 DSC thermograms of Epoxy-Phenol (EP) Systems wfih different Epoxies (TPP. 0.5%)

The influence of the bismaleimide structural variations on the cure reaction of the

EPB system was studied using three bismaleimides B1, 82 and 83. The DSC

cutves of the in the EPB systems with same epoxy (novolac epoxy), diallyl

bisphenol and different bismaleimides are shown in Fig.7.9. The pattern of DSC

cure curves were found to be different. The phenol-epoxy. Ene, Wagner-Jauregg

and Diels-Alder reactions occured almost at the same temperature range in all the

three cases. However, there is a difference in the relative enthalpy change in

these reactions particularly for EPB-62. In this case relative exothermicity of

phenol -epoxy reaction was maximum and occurred at a slightly higher

temperature and that for Ene- reaction was minimum among the three systems

studied. This is only apparent as the absolute concentration of DABA and EPN is

Chapter 7 224

less in ~ ~ l p when compared to the other two, because of the higher molecular

weight of me particular BMI in the stoichiometrically equivalent blend.

"r-

172.8

-__---' . , EP-E2

\_--__ .----- --

300 Temperature OC

Fig.7.8 DSC cure thermograms of EPB systems with different epoxies (TPP 0.5%, H.R- 10"CImin))

& EPB-

3 :\& O 1 EPB-82 EPBSI

0 0

-0 1 50 100 150 200 250 300

Tenperature('C)

Fig, 7.9 DSC cure thermograms of EPB SyStemS with different Bimaleimides ( TPP- 0.5%. H.R-1O"CI min)

Chapter 7 225

A weak exothem at 250'C in EPB-B3 can be attributed to the self polymerization

of BMI-3 that might not have been incorporated in the matrix by the Alder-ene

reaction.

7.3.2.3 Rheological cure characterization of the blends

The rheological cure characterisation of the different blends was canied

out to get a better insight in to their cure profile. The complex viscosity (q3. storage shear modulus (G') and loss shear modulus (G") of the ternary blends

EPB-El. E2 8 E3 with different epoxies and EPB 81. 82 and 83 with different

bismaleimides were monitored as a function of temperature (T) using the

rheometer. The variation of G' with respect to temperature for these two sets of

EPB systems are given in Figures 7.10 and 7.1 1. The non-isothermal rheograms

of these EPB systems gave a better insight in to the temperature dependence of

their cure reaction with epoxy and bismaleimide structural changes. The cure

reaction was monitored as a function of time under isothermal condition to

optimize the processing conditions of their composites.

Fig 7.1 0 Dependence of storage shear modulus of EPB systems with nature of epoxies.

Chapter 7 226

The isothenal rheograms of these matrix systems, recorded at 250'~

revealed that the storage shear modulus levels off after about five hours at this

temperature, indicating the cure completion of the system. The rheOgramS

supplemented the DSC observation of the cure reaction.

The non-isothermal rheograms obtained for the EPB system with different

epoxies showed that Me temperatures corresponding to the gelation is not

diiering much with change in epoxy type, while those corresponding to the

cmsslinking was found to be slightly different.

Fig 7.11 Dependence of storage shear modulus of EPB systems with nature of bismaleimides

The relative change in modulus boilt up by epoxy-phenol reaction (100-

150'~) is insignificant. The Ene reaction (150-175'~) caused marginal increase

while W - J reaction (175-200'~) increased the modulus significantly. A major

change in E' is seen for Diels-Alder reaction (175-220'~). The exotherm observed

in DSC beyond 2 5 0 ' ~ (associated with a curing phenomena) was evident in the

rheogram too. This is attributed to the residual unsaturation originated from the

Chapter 7 227

Alder-ene reaction. EPEE3, the triepoxy incorporated system, has already built

up a crosslinked network. The relative increase in modulus thmugh the curing of

residual unsaturation is naturally not very significant in this case. The overall

cmsslinking is affected also by the epoxy functionally and the resultant cmsslinks.

The ene reaction is found to be the major contributor of the " rheo cure".

In systems with varying BMI nature, the onset of gelation is facilitated by

the molecular mobility. In BMI-I with flexible spacers, the ene reaction is facilitated

more than the other two. Between EPB-B2 and 83. BMI-3 having a flexible ether

group, facilitated this reaction. However the temperature scale difference is subtle.

Interestingly, the modulus build up is maximum for BMI-I. This is due to the fact

that the absolute concentration of BMI is more in this ternary blend. It appears that

all the unsaturation groups in EPB-B2 and 83 are not consumed in cmsslinking

due probably to their structural rigidity in comparison to BMI-I. As a result, this

curing takes place at a higher temperature (-280'C). Further, the increase in

modulus due to residual unsaturation in polymerization is insignificant in EPB-B2

and 83 in comparison to B1.

7.3.3 Characterisation of the cured EPB system

7.3.3.1 Thermogravimetric Analysis

The TGA thermograms recorded for the cured EPB blends with different

types of epoxies are shown in Fig 7.12 and the temperatures corresponding to the

initiation, peak,and completion of decomposition reaction are given in Table 7.3.

Table 7.3 Thermal decomposition characteristics of EPB systems with epoxy sbuctural variation

Chapter 7 228

The temperature of initiation of decomposition for the EPB systems with epoxies

El , E2 & E3 were found to be in the order €3 > E l >E2. The nature of epoxy

network has got a decisive role on the onset of thermal decomposlion. The

novolac epoxy and tri epoxy showed comparable thermal stability as far as the T,

and T, are concerned. The trend in rate of decompostion was in line with the

epoxy functionality and resulting crosslink density of different systems. The rate of

decomposition was lowest for EPN and highest for LY-556 which was reflected in

the residue at a given temperature (in the TGA thermogram). Thus the rate of

decomposition was lowest for EPN and highest for LY-556 which was reflected

also in the residue at a given temperature (in the TGA thermogram).

Fig 7.12 TGA. thermograms of cured EPB blends with epoxy structural variations (H.R 10°Clmin)

The variation in TG profiles among the EPB systems with different BMls

was very benign. Though the initial decomposition temperature is a few degrees

lower for EPBB1 system vis- a- vis the rest, this has got a reduced rate of

Chapter 7 229

decomposition at higher temperature regime. The thermal stability of EPBB2 was

wmparatively higher and that for EPB-B1 (containing BMI-I) was the minimum.

The thermal stabilities of the EPB systems with bismaleimide shuctural variation

indicated that their thermal stabilities are in the order EPB-B2> EPB-63 > EPB-B1.

The corresponding thermograms are given in Fig. 7.13 and the relevant

temperature data are given in Table 7.4. The maximum thermal stability obtained

for the EPB-B2 system may be due to its higher crosslink density resulting from

the shorter distance between the maleimide groups in BMI-2. EPB-63 has more or

less the same environment as EPBB2 and the difference between the two is

within experimental scatter. Even though the presence of more aliphatic groups in

BMI-1 was expected to contribute to its lower thermal stability in comparison to Me

systems containing BMI-2 and BMI-3, this k offiet by the high concentration by

weight of BMI in the ternary blend. This is also reflected in the comparatively good

char residue of the system.

EPB-B3

EPB-B2

E

m > EPB-BI

Fig 7.13 The TGA thermograms of cured EPB blends with Bismaleimide structural variation ( H.R 1O0C/min, N2 )

Chapter 7 230

Table 7.4 Thermal decomposit~on characteristics of EPB systems with bismaleimide structural variation

Residue (%) 33.8 34.1 33.7

7.3.3.2 Evaluation of Adhesive strength

The adhesive properties of EPB systems with different epoxies (EPB

E l ,EPB-E2 & EPBE3) and those with different bismaleimides (EPB-61. EPEE2

8, EPBB3) were evaluated by determining their lap shear strength. The material

performance evaluated at different climatic conditions is given in Tables. 7.5 and

7.6 respectively. For the system with epoxy variation. the adhesive strength was

found to be very good for those containing nowlac epoxy and tnepoxy. The high

temperature retention of LSS was found to be proportional to their expected

crosslink density ie. EPN > triepoxy > diepoxy Table.7.6 Adhesive properties of

EPB systems with bismaleimide variations

EPB-B3 Reference1

Temperature -r EPB-61

Table.7.5 Adhesive properties of EPB systems with epoxy structural variations

EP&B2

Lapshear strength (kglcm2)

Chaoter 7 231

Table.7.6 Adhesive properties of EPB systems with bismaleimide

structural variations

I Reference * I Lapshear strength (kg/cm2) I

For the system with bismaleimide variation, the result indicated an

improvement in the adhesive strength of the material (for EPBB2) up to 120°C

and there after it showed a reducing tendency. At 150"C, the strength retention is

comparatively poor for EPB system with BMI-I, as in this case, the T, is lower

than the other two cases. The marginally enhanced polarity contributed by the

ether group in BMI-3 is reflected in its better LSS in comparison to BMI-2. The

high temperature (150°C) performance of EPB-B2 was found to be the best

among the three. This might have been contributed by the reduced distance

between the maleimide groups in BMI-2. In the case of EPB-B3 even though the

distance between the maleimide groups in BMI-3 is practically the same as for 82.

the presence of flexible ether linkage in its structure might have contributed to its

inferior strength retention in comparison to EPB-B2.

7.3.4 Characterisation of EPBglass composites

The glass laminates prepared using the three component EPB resin

systems containing different epoxies and bismaleimides were characterized for

their mechanical, thermo-mechanical and physical properties.

7.3.4.1 Mechanical Properties

The mechanical properties -compressive, flexural and interlaminar shear

strength -of the EPB composites were found to vary with the nature and properties

of the components of the EPB matrix system. The strength of these composites

Chapter 7 232

measured under dierent loading environments, such as tension, compression

and flexure, are summarized in Tables 7.7 and 7.8. The general trend in these

properties was EPB-El > EPB-E3 > EPB-€2 for the epoxy variation and EPB-B1>

EPB-62 > EPEE3 for the systems with different types of bismaleimides. The

improved strength of the EPB-El system containing novolac epoxy may be due to

its higher functionality compared to the other two systems. In the case of the

system containing tri-functional epoxy EPB-E3, its properties are better than the

di-functional epoxy, LY-556. By and large, the strength of the EPB systems with

epoxy structural variation was in proportion to their epoxy functionality. The better

flowability and flexibility of the di-functional epoxy (LY-556) based system was

reflected in good flexural strength for its composites. EPN appear to have the best

combination with BMI-1 and DABA to give a stronger composite.

Table 7.7 Mechanical characteristics of EPB composites with different epoxies

Reference-

Property 1

Table 7.8 Mechanical characteristics of EPB composites with different bismaleimides.

Reference-

IFSS (kglcm') j - 7 1 Compressive strength

Flexural strfngth

Chapter 7 233

When the structural dependence of BMI on the mechanical performance of the

EPB system was examined, BMI-I and BMI-3 with flexible spacers were found to

have distinct advantage over BMI-2. The interlaminar shear strength, compressive

strength and interfacial shear strength of EPB-BI and EPB-B2 were comparable,

while the increase in flexural strength of EPB-Bi was'much higher compared to

the other two systems. In the case of BMI-I, its moderate crosslink density (large

spacing between imide groups) and better wmpatability with diallyl bisphenol

(similarity in structure) could furnish a good matrix system with better flexural

characteristics.

7.3.4.2 Thenno Mechanical Analysis

The thermo mechanical analysis of the different EPB composite systems

were carried out to determine their linear expansion coefficient (a). The linear

expansions of the samples were in the range of 3 . 2 ~ 1 0 ~ to 3.4~10.~ for the

compositions with different epoxies and between 3 .1~10 '~ and 3.4 ~ 1 0 . ~ for the

systems with different bismaleimides. The variation in To of the neat systems with

different bismaleimides obtained from their DSC analysis was found to range from

178 to 193°C

7.3.4.3 Physical properties of EPB Composites

The thermo physical properties of the composites viz. density, water

absorption, coefficient of linear expansion, resin I reinforcement wntent etc. give

information regarding its quality and suitability for specific application. The

properties evaluated for different EPB systems are given in Tables 7.9 and 7.10.

The structural variations of the epoxy do not cause significant change the physical

and thermo-physical properties of the EPB matrix system. Based on physical

parameters, the wmposites were found to be of good quality and we can infer that

the change in mechanical properties described previously is not a consequence of

the defects in composites. The resin wntent in these wmposites determined by

matrix digestion showed a variation from 20.4 to 21.6 for EPB systems with

different epoxies and from 20.4 to 22.3 for those with different bismaleimides.The

water absorption values were found to be more or less the same.

Chapter 7 234

Table 7.9 Therrno physical properties of EPB composites with different epoxies

Reference-.

Property 1 Density(g1-x)

I I I

Table 7.10 Thermo physical properties of EPB composites whh different bismaleimides

EPB-E2

1.83

Water absorption(%) I 2.9 I I I

Density (glcc) 11.82 1.84 I

1.83

EPB-E3

1.83

i Water absorption(%) 1 ~-TcT-+- 1.9 1 .O

2.0

Resin content (%)

Resin content (%) 1 20.4 21.8 I

22.3 I

2.3

20.4 21.4

7.4 Conclusions The structural variation of epoxy was found to influence the properties of the EPB

matrix system as well as its composites. The epoxy variation in the EPB system

has not caused any significant variation in its cure pattern as observed in the DSC

thermogram and non-isothermal rheogram. The variation in the nature of BMI has

resulted in significant variation in the relative enthalpies of the different cure

reactions observed in the DSC curve. The ftieological behavior of the BMI

modified systems showed marginal shift in the different stages of reaction.

21.6

Chapter 7 235

A comparative evaluation of the thermal stability of the systems revealed

that the incorporation of tri epoxy improved the thermal properties of the EPB

matrix system. The trend in the thermal stabilities of the EPB systems with epoxy

variation based on their TI values was found to be E3> El> E2. Among the

bismaleimide modified systems, thermal stability was found to be maximum for

EPEBI. The higher crosslink density resulting from the shorter distance between

the maleimide groups in BMI-2 has contributed to its superior thermal stability.

The adhesive strength and the strength retention at elevated temperature

were found to be very good for EPB systems containing novolac epoxy and tri

epoxy, while LY-556 was inferior in this respect. These properties were found to

depend on their crosslink density (EPN > Tri-epoxy, di-epoxy). In the BMI

modified systems. EPB-B2 showed the best thermal characteristics. EPB-B2 and

83 were good in respect of adhesive strength and its high temperature retention.

The marginally enhanced polarity of BMI-3 has reflected in its better LSS in

comparison to BMI-2. But it was reversed for the high temperature retention,

where the flexibility dictated the property. The high temperature performance of

these systems followed the trend in their T, values. The glass transition

temperature was maximum for BMI-2, due to the higher cross link density resulting

fmm the lower distance between the maleimide groups.

The trend in the strength of the composites was in the order EPB-€1,

EPEE3 > EPEE2 for EPB systems with epoxy variation and EPEE1 > EPB-B2 >

EPB-B3 for bismaleimide modified systems. In general the strength of EPB

systems with epoxy struchrral variation was in proportion to their epoxy

functionally. The better flowability and flexibility of the difunctional epoxy (LY-556)

based system was reflected in good flexural strengthfor its composites. When the

structural dependence of BMI on the mechanical performance of the EPB system

was examined. BMI-I and BMI-3 with flexible and polar ether spacers exhibited

distinct advantage over BMI-2.

The EPB system formed by the reactive blending of novolac epoxy with

BMI-I was found to yield a ternary blend with improved mechanical performance

at ambient conditions, while that with BMI-2 was found to be the best with respect

to high temperature performance.

Chapter 7 236

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