effect of epoq and@ismaik'mide structure on the cpropds of...
TRANSCRIPT
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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