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Engineers, Part L: Journal of Materials DesignProceedings of the Institution of Mechanical
http://pil.sagepub.com/content/226/1/76The online version of this article can be found at:
DOI: 10.1177/1464420711424003
originally published online 13 October 20112012 226: 76oceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials Design and Applications
I Rhoney and R A PethrickLow coefficient of thermal expansion of thermoset composite materials
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Low coefficient of thermal expansionof thermoset composite materialsI Rhoneyand R A Pethrick*
Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK
The manuscript was received on 14 March 2011 and was accepted after revision for publication on 30 August 2011.
DOI: 10.1177/1464420711424003
Abstract: Selection of the correct resin filler combination is important in achieving materialswhich have a low coefficient of thermal expansion (CTE). A study of nanosilica-modified resin, incombination with silica fillers, allows low levels of CTE to be achieved. In this study, a novelcuring agent, ytterbium triflate, is reported. This curing agent provides a stable catalyst system
which can be used to create the viscous composite mixtures but has the facility of effective cureover a relatively narrow temperature range from 70 C to 100 C. A series of formulations wereexamined based on incorporation of fillers with nanoscale silica particles into either the pureresin or a resin which contained nanoscale functionalized silica particles. The filler incorporationleads to a significant increase in the glass transition temperature as determined by dynamicmechanical thermal analysis. The CTE was observed to be lower below Tg than above it.Changing the nanosilica particle size and distribution produced significant changes in thevalues. However, the CTE scaled according to the total silica content; and the values were ingeneral lower than those calculated theoretically. The use of a highly functional o-cresol epoxynovolac demonstrated how increasing functionality raised the Tgand lowered the CTE, but theuse of toohigh a post-cure temperature reversed this trend. Very good results were achieved using3,4-epoxycyclohexylmethyl, 3,4 epoxycyclohexanecarboxylate, which has a low viscosity andallowed high levels of silica to be readily achieved. This article indicates how the adjustment ofthe epoxy selected to be used as the base material and the type of silica particles used allows thevalues ofTgand CTE to be modified in a composite material.
Keywords: thermal expansion coefficient, ytterbium triflate catalysts, epoxy resin, silica fillers,nanocomposites, glass transition temperature, cure data
1 INTRODUCTION
In a number of applications, it is desirable to have
thermoset materials which have a low coefficient of
thermal expansion coefficients (CTE). A composite,
by its very nature, will have a CTE which reflects the
proportions of the resin and filler in the material [1].
Studies of a range of composites have shown that the
CTE can be predicted to lie between the upper and
lower limits of the Schapery theory, which is based on
the simple rule of mixing
c f m1 1
wherec,f, andmare, respectively, the CTEs of the
composite, filler, and matrix, respectively, and the
volume fraction of the filler [2]. The limits are dictated
by the CTE of the filler, which has a value of 8.5 106
per C for glass, and that of a typical thermoset resin
which has a value of90 106 [3, 4]. In practice, the
values observed depend on a series of complex factors
which relate to the density of the polymer matrix and
the volume fraction of low-CTE fillers which can beincorporated into the material. With the advent of
nanoscale materials, the possibility arises in
*Corresponding author: West Chem, Department of Pure and
Applied Chemistry, University of Strathclyde, Thomas Graham
Building, 295 Cathedral Street, Glasgow G1 1XL, UK.
email: r.a.pethrick@strath.ac.uk
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exploring the way in which small particle fillers may
be used to achieve low values of CTE in composite
structures. Deviations from the simple additivity rela-
tion equation (1) have long been recognized [5] and
this article explores the effects of processing condi-tions on the observed values of CTE obtained with
some novel nanocomposites.
In previous papers, the effects of clay nanoparticles
on the rheological and thermal mechanical properties
of various epoxy resin composites have been reported
[611]. The addition of low levels of clay platelets is
found to have a significant effect both on the rheology
and thermal properties such as the glass transition
temperature (Tg). The platelets are able to interact
and significant increase of the viscosity is observed
with levels as low as a few weight percents.
Similarly, the glass transition increased in certaincases by several tens of degrees. However, in a
number of cases, no enhancement was observed
and this was attributed to the clay platelets inhibiting
the formation ofa dense matrix. Ina recent paper [11],
we report the use of a novel curing system, ytterbium
triflate, which introduces the possibility of a pseudo-
latent type of cure for epoxy resins. The object of this
article is to explore how it is possible to achieve mate-
rials with low CTE values by using various combina-
tions of nanofillers and cure regimes.
2 EXPERIMENTAL
2.1 Materials
The characteristics of the resin and the fillers are
summarized in Table 1. The catalyst, ytterbium tri-
flate, was obtained from SigmaAldrich, UK. The
materials were used as received. The epoxy resins
were thoroughly degassed prior to mixing and after
being mixed with the appropriate quantity of filler.
The mixtures were in general stored overnight in a
desiccator before catalysts were added and cure car-ried out. The cure was undertaken in an aluminium
mould of dimensions; 10 10 2 cm3, which had
been treated with a release agent and carried out in
a vacuum oven which had been preheated to the
required temperature. In order to avoid exotherms,
certain materials were first cured at a lower temper-
ature and then post-cured at a higher temperature.
The cured films were then subjected to a series of
thermal tests.
2.2 Coefficient of thermal expansion
Measurements of CTE were carried out using a
Mettler TMA 40 (Mettler-Toledo Limited, Leicester.
A sample of dimension 1 1 cm2 was heated from
30 C to 250 C at 20 C/min, then cooled from
250 C to 30 C at 20 C/min, and then held at 30 C
for 20min, before being heated from 30 Cto250 C at
4 C/min. The first and second runs were to relax the
sample and erase any thermal history. The final run
involves slowly increasing the temperature and
allows determination of the glass transition temper-
ature Tg and the expansion coefficients above andbelow it. Before the sample was inserted in the instru-
ment, the quartz probe was zeroed. The sample was
placed on the sample stage and the probe lowered to
sit on top of the sample. The thickness of the sample
was recorded for future calculations. The supporting
software allows analysis of the gradient at a given
Table 1 General characteristics of materials used in this study
Name Source Comment
Epoxy resinsBisphenol A Huntsman
Bisphenol F Huntsman3,4-Epoxycyclohexylmethyl, 3,4epoxycyclohexanecarboxylate EEC
Aldrich Chemicals
Nanopox A610 Nanoresins AG Monomer EEC contains 40 wt% SiO2 20 nm particlesNanopox A510 Nanoresins AG Monomer DGEBF contains 40 w t% SiO2 20 nmparticlesNanopox A410 Nanoresins AG Monomer DGEBA contains 40 w t% SiO2 20 nm particlesTris(4-hydroxyphenyl)methanetriglycidyl ether (THP)
Aldrich Chemicals
YDCN-500-IP a novolac resin Kukdo Chemical Co.o-Cresol novolac EEW 190210 g/eq. Soft point 5054 C
SilicaAerosil OX50 Degussa AG Surface area 50 m2/g; size 40nm; density 130 g/LAerosil 90 Degussa AG Surface area 90 m2/g; size 20 nm; density 80 g/LAerosil 380 Degussa AG Surface area 380 m2/g; size 7 nm; density 50 g/L
Glass flakeGF100 nm Glassflake Limited 100 nm thick
Exfoliated clayCloisite 30B Southern clay Montmorillonite modified with quater nary ammonium MT2EtOH
Low coefficient of thermal expansion of thermoset composite materials 77
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temperature. Where the graphs indicated a linear
CTE, an average value was calculated below and
aboveTg. Where the plots showed a marked curva-
ture, the values are quoted at a specific temperature.
The temperature calibration of the equipment waschecked using indium and lead. The unit of CTE is
ppm/ C. The data reported are an average of a min-
imum of three experiments. Values were repeated
until the deviation between individual measurements
were less than 2ppm/ C.
2.3 Dynamic mechanical thermal analysis
The method used has been previously described. The
dynamic mechanical thermal analysis (DMTA) mea-
surements were made using MKIII DMTA instrument
(Polymer Laboratories, Church Stretton, UK). Thesamples were of dimensions 5 6 3 mm3 and cut
from the original plaques using a diamond saw.
Temperature scans were performed over a tempera-
ture range from 100 C to 230 C, at frequency
1 Hz, strain 4, and scanning rate 4 C/min. The sam-
ples were clamped with a torque of 40 Nm.
2.4 Strathclyde curometer
The change in viscosity was measured as a function of
time using the Strathclyde curometer [12, 13]. A smallglass vial containing the sample was held in an oil
bath. The temperatures of the oil bath and the
sample were adjusted and maintained isothermally
during the period of the cure. The paddle probe was
lowered into the sample and operated at a frequency
of 2 Hz and amplitude 500 mm. The sensor output is
the amplitude and phase of the probe motion, which
is directly related to the viscosity changes occurring
in the sample. The amplitude of the motion of the
probe which is coupled to the driver by a spring is
damped due to viscosity. If P1(t) and P2(t) are the
instantaneous displacements of the oscillatorydriver and the response of the system damped by
the curing liquid, then
k P1 t P2 t :A:dP2dt
2
wherekis the spring constant coupling the driver to
the probe, the shear viscosity of the fluid, andAthe
geometric factor related to the probe contact area.
Separating real and imaginary parts of the complex
probe motion, one obtains
_P2 _P1
1 :!:A=k 2 3
and
P2 P1::!:A=k
1 :!:A=k 2 4
Using the above equations, it is possible to calcu-
late the shear viscosity from the in-phase and quad-
rature coefficients. The data were recorded using
Picolog software package. The gelation was taken as
the point at which the viscosity of the material
reaches a value of 104 Pas.
3 RESULTS AND DISCUSSION
In this article, cure of the epoxy resins was achieved
using a cationic catalysed cure system based on ytter-
bium (Yb) triflate (11). This cure system is very effi-cient at high temperatures, while at low
temperatures, the activated mixture can be stable
for many hours/months. A catalysed sample was
stored in a refrigerator for a period of 6 months and
it was found that there was no increase in its viscosity
or loss of its reactivity. A preliminary study of the
effects of temperature and catalyst concentration on
the cure characteristics of one of the epoxy EEC
(Fig. 1). The in-phase displacement reflects the
motion of the probe as this is directly coupled to the
spring. The out-of-phase displacement measures the
phase shift of the driver against the probe and is areflection of the energy dissipation in the spring
which is itself a measure of the energy dissipation in
the fluid.
Increase in the temperature of the cure produces a
shift of thepeak in the out-of-phasedisplacement and
a coincidental drop in the in-phase displacement at
shorter times. The drop in the in-phase displacement
is a reflection of the increase in the viscosity of the
media and the peak in the out-of-phase displacement
a consequence of the growing viscoelastic nature of
the media. Gelation of the material corresponds to a
point close to the peak in the out-of-phase displace-ment and this can be used to assess the effect of tem-
perature on the cure. The point at which the in-phase
displacement approaches the baseline coincides with
vitrification of the material. The breadth of the peak
reflects the time taken to transform from a gel to a
glassy state. A plot of the variation of the gelation
time against reciprocal temperature is shown in Fig. 2.
The plot demonstrates a simple Arrhenius type of
behaviour and the activation energy was calculated
from the slope of the curve and found to be 101 3kJ/
mol. A study was undertaken in which the concentra-
tion of the Yb triflate was varied from 0.08 to 0.15 phrand it was found that the activation energy remained
essentially constant.
78 I Rhoney and R A Pethrick
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3.1 Incorporation of platelet fillers
Equation (1) is purely based on volume fraction and itmay be argued that nanoscale platelet fillers may in
principle be able to reduce the CTE to a greater extent
that would be predicted on the basis of their volume
fractions. Two types of platelet fillers were studied:
organically modified clay Cloisite 30B and glass
flakes with nanoscale thickness. The primary concern
addressed in this study was whether the filler has any
effect on the cure of the resin.
3.2 Organically modified clay-containing
material
For this study, Cloisite 30B was selected as this has
been found in previous studies [611] to be one of
the most easily dispersed organically modified clays.
Cloisite 30B contains methyltallow dihydroxyethyl
ammonium ion and has platelets which are
1 nm thick and 1000 nm long. The epoxy EEC was
selected for study and a mixture containing 3 wt% of
Cloisite 30B was prepared and sonicated for a period
of30 min to achieve optimum exfoliation of the clay
platelets. The sample was then allowed to equilibrateovernight. To this dispersion 0.10-phr Yb triflate dis-
solved in ethanol was added and the mixture stirred at
80 C and degassed. A sample wasthen analysed in the
curometer and it was observed that no significant
reaction had occurred after 22 h at 100 C. Attempts
to achieve cure were carried out at higher tempera-
tures but were unsuccessful and the system showed
evidence of degradation occurring and the emission
of acrid fumes. The clay is modified with a quaternary
ammonium salt. The degradation temperature of
Cloisite 30B is 174 C, which would imply that
under the conditions used for the cure, no loss of thequaternary ammonium slat would be expected.
Thermal degradation of these organic modifiers has
been shown to produce aldehydes, carboxylic acids,
and amine-containing residues, and evolve ammonia
and carbon dioxide. A study of the degradation prod-
ucts was not undertaken but it was clear that the pres-
ence of the Yb triflate was able to catalyse the
degradation at temperatures of the order of 130 C
over a period of time. Increasing the catalyst level to
0.20phr allowed partial cure to be achieved in 23 h.
Cure was also attempted using nanomodified resins
and while some degree was achieved in all cases, sig-nificantly longer gelation times were observed com-
pared with the pure resins. It was concluded that the
0 2000 4000 6000 8000 10000 12000 14000
0.0
5.0x102
1.0x103
1.5x103
2.0x103
2.5x103
Time (secs)
0
50
100
150
200
250
OutofPhaseDisplace
ment
InPhaseDisplacement
Fig. 1 Plot of the in phase {left axis} and out of phase displacement {right axis} as a function of timefor cure reactions carried out at: (#) 100 C, () 110 C, () 120 C, and (n) 130 C. Theunits are arbitrary as they related to the spring used
0.0023 0.0024 0.0025 0.0026 0.0027
2.71828
7.38906
20.08554
54.59815
148.41316
Ln
(Time(mins
))
1/Temperature
Fig. 2 Plot of gelation time against reciprocal temper-ature (K1)
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clay was effectively inhibiting the cure. Since the Yb
salt has to dissociate for the active site to initiate poly-
merization, adsorption at anionic sites on the clay
would effectively inhibit polymerization. To test this
hypothesis, the catalyst level was increased to 0.3 phr
and the gelation time reduced to 4.6 h, but only after
the temperature had been successively raised to
140 Candthen160 C, (Fig. 3).The in-phase displace-
ment is the top curve, which falls with increase in time
and the out-of-phase displacement increases and
then falls with increasing time.
3.3 Vermiculite
A sample of 2.0 wt% of vermiculite (Phyllosilicate) in
EEC was prepared and stirred with a high-speed
mixer (Ultra Turrax) for 1 h. Vermiculite is known to
be easily exfoliated and interestingly satisfactory cure
was achieved. This implies that the inhibition effects
observed are peculiar to the Cloisite clays.
3.4 Nano-glass fake
In principle, glass will nothave thesame ability to bind
cations as clay and hence nano-glass flakes should not
exhibit the problemsencounteredwith Cloisite. A mix-
ture of 6.4 wt% glass flake was stirred at 100 C and
allowedto stand overnight. Thesuspension wasunsta-
ble and it was clear that precipitation of the glass was
slowly occurring. The Yb catalyst was added but the
mixture rapidly became viscous; but cure was only
achieved after 6 h. It is apparent that if the mixture
has a high initial viscosity as a consequence of the
filler addition, this inhibits the subsequent cure. Thiseffect can be attributed to the nature of the polymer-
ization process in which diffusion of the oxirane to the
active site is required for propagation of the polymer-
ization. If the platelets interact with the catalyst, then
the polymerization will be inhibited.
3.5 Samples containing silica 0.5mm
To achieve the loading levels required by equation
(1) to reduce the CTE to a low level, it will be nec-
essary to increase the filler level significantly above
those used above. To maintain a low viscosity forprocessing, the filler must be in a near-spherical
form. A mixture was prepared which contained
20g EEC and 10g of silica 0.5 mm. The large silica
particles in the low-viscosity resin tend to separate
out. To prevent sedimentation, 2 g Aerosil 90 was
added. This significantly increases the viscosity, sup-
presses sedimentation, and increases the total silica
content to 37.8 wt%. This mixture has a gelation time
of 7.3 min. Addition of silica does not lead to the
inhibition effects observed with the platelet mate-
rials. Examination of the curometer curves for the
cure of materials which contained silica 0.5 mm andwith added nanosize silica, Fig. 4, indicates that the
cure is influenced by the viscosity of the media. The
initial rise in the out-of-phase displacement is sim-
ilar to that for the simple resin, Fig. 1; however, after
the peak, it drops rapidly, indicating a rapid rise in
viscosity after gelation and the system is rapidly
transformed to a glass state. The glassy state is indi-
cated as the point at which the in-phase displace-
ment becomes close to zero.
3.6 Addition of nanosilica
Two possible routes are available for the incorpora-
tion of nanosilica, either as in situ created
0 10000 20000 30000 40000 50000 60000 700000.0
5.0x102
1.0x103
1.5x103
2.0x103
Temp raised to 160C
Temp raised to 140C
Time (secs)
0
50
100
150
200
250
300
OutofPhaseDisplacement
InPhaseDisplacem
ent
Fig. 3 Plot of the variation of the in phase (top curve) and out of phase displacements (bottomcurve) for a mixture catalysed with 0.3 phr of Yb and containing 3 wt% Cloisite 30B
80 I Rhoney and R A Pethrick
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nanodispersion and available as a modified resin or
as preformed particles which may then be dispersed
in a normal resin.
3.7 Modified resins
Ranges of modified resins are now available which
contain up to 60 wt% of nanosilica particles which
have been created in situ and have viscosities com-
parable to the unmodified resins. Since these mod-
ified resins contain up to 60 wt% of nanosilica, they
provide a route for the increase of silica fillers and
hence lowering of the CTE. A study of Nanopox
A610, which is the EEC modified with 20 nm silica
particles was undertaken. As with the study with
the pure resin, measurements were made at various
temperatures between 100 C and 160 C and a linear
plot similar to that shown in Fig. 2 was obtained.
The activation energy was also investigated as a
function of the catalysts loading between 0.08 and
0.15 phr and found to be independent of loading. Avalue of the activation energy of 76 4 kJ/mol was
obtained, which is significantly lower than the that
of the pure resin 101 3 kJ/mol. Clearly, the
Nanopox catalyses the reaction. The in situcreation
of the nanoparticles involves growth using a solgel
process. The size of the particles is controlled by the
incorporation of an organically modified silane and
this allows the introduction of functional groups
into the surface of the particles. Aiding the develop-
ment of the interface with the resin incorporation of
oxirane functionalities is desirable and these may in
part account for the higher reactivity of these mod-ified resins compared with their normal
counterparts.
3.8 Addition of nanosilica
A range of nanosilica fillers are available and can be
added to the resin to further increase the silica load-
ing. The silica investigated in this study was Aerosil
OX50. The silica in this form has nominally the same
size as that in the particle-modified resin but will
require to be dispersed. The cure curves obtained
were however markedly different (Fig. 5). Whereasin the case of the in situgenerated nanosilica parti-
cles, the out-of-phase displacement drops rapidly
after passing through the peak, in the case of the
Aerosil OX50, there is a much slower drop, implying
that gelation is followed by a slow transition to a vit-
rified form. The functionality ofin situgrown nano-
particle surface can generate a rigid matrix by further
reaction with the resin; in the case of the Aerosil OX50
particles, no such reactions appear to be possible and
the matrix slowly moves to a completely cured form.
It was however noted that increasing the Aerosil OX50
content significantly decreases the time to gelation,Table 2.
3.9 Combination of nanosilica and modified
nanosilica containing resin
To achieve the maximum silica loading, it would be
desirable to combine the nanomodified resin with the
nanosilica particles. A series of mixtures of A610 and
OX50 were investigated (Table 3).
Increasing the temperature from 100 C to 130 C
leads to the decrease in the cure time from 170 to8 min. Increasing the volume of Aerosil OX50 in the
blend, while reducing the volume of Nanopox A610
0 200 400 600 800 1000
0.0
5.0x102
1.0x103
1.5x103
2.0x103
Time (secs)
0
50
100
150
200
250
300
350
OutofPhaseDisplace
ment
InPhaseDisplacem
ent
Fig. 4 Cure curves for an EEC silica 0.5mm with 2 g Aerosil 90 cured at 100
C with 0.1 phr of Ybcatalyst
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leads to a decrease in the gelation time. While the
Nanopox A610 would be expected to promote the
transition from gelation to vitrification, the Aerosil
OX50 raises the base viscosity, which will enhance
the exothermic effects of the reaction leading to a
shortening of the gelation time. A surprising result
was the observation of a reduction in the gelation
time with a decrease in the Yb triflate level. The cat-
ionic cure of epoxy resins involves a chain-like pro-
cess and as such lowering the catalyst level will
decrease the initiation level, but by analogy with
free radical propagation process can lead to anincrease in the chain length of the species created,
which will raise the rate at which the viscosity
increases. The increase in viscosity will increase the
exotherm and hence shorten the gelation time. These
observations emphasize the different ways in which
the nanosilica particles interact with resin and are
involved in the total cure process. The addition of
the Yb triflate in ethanol required the mixture to be
degassed; however, when the Yb catalyst is added
using methyoxymethanol, the viscosity was lowered
and did not require degassing and the gelation time
changes was approximately halved by changing tomethoxymethanol, which is a better solvent for the
catalyst.
3.10 Influence of the resin matrix
The thermal expansion of the resin makes a signifi-
cant contribution to the total CTE. A series of differ-
ent resin systems were investigated, which included
o-cresol novolac epoxy, bisphenol A, and N,Ndigly-
cidyl-4-glycidylaniline; the latter being used to influ-
ence the cross-link density of the matrix. As with EEC,
the addition of Aerosil OX50 leads to a reduction of
the cure time (Table 4). The o-cresol novolac has a
functionality of 2.7 and in principle is capable of
forming a highly cross-linked matrix. While thissystem showed normal gelation and vitrification
behaviour, it was found that the degree of cure was
low and that post-cure was required in most cases.
This observation is consistent with the idea that the
highly branched epoxy resin can form a gel relatively
easily but that completion of the reaction may be a
relatively slow process and require additional energy.
The Aerosil OX50 produced the expected acceleration
of the gelation as did the use of Nanopox A610. The
latter being a low molecular weight bifunctional
epoxy acts a little like a reactive diluent, lowers the
viscosity and aids the reaction and achieves a higherdegree of cure. The Nanopox A410 which has a
bisphenol A base resin was blended with the novolac
0 1000 2000 3000 4000
0
5.0x102
1.0x103
1.5x103
2.0x103
Time (secs)
0
50
100
OutofPhaseDisplacem
ent
InPhaseDisplaceme
nt
Fig. 5 Cure curves for an EEC plus 40 wt% Aerosil OX50 with 0.1phr Yb and cured at 100 C
Table 2 Variation of the gelation time with increasing Aerosil OX50 content
Concentration EEC (wt%) Concentration OX50 (wt%) Total silica (wt%) Yb triflate (phr) Gelation time (min)
71.38 28.55 28.55 0.10 8659.95 39.98 39.98 0.10 2454.52 45.43 45.43 0.10 17
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as a 50/50 wt% mixture to reduce the viscosity and
reduce the requirement for post-cure. The data
obtained are presented in Table 4.
When the Aerosil OX50 is added to a 50/50 mixture
of the novolac and Nanopox A410 bisphenol A resin,
the cure is inhibited and the gel time increases dra-
matically. However, increase in the level of catalysts
from 0.12 to 0.3 phr reduces the cure time to values,
which are comparable to those found in the blend
without the Aerosil OX50. This inhibition effect is par-
ticularly marked since addition of the Aerosil OX50
significantly reduces the total amount of resinpresent in the mixture.
4 SUMMARY OF CURE DATA
The above study indicates how changes in the initia-
tor level can be used to change the gelation and vit-
rification times in a controlled manner. The other
feature emerging from the study is the impact
which the surface modification incorporated in the
in situgrowth of the silica particles in the Nanopox
resins has on the cure process. The particles are able
to catalyse the reaction, lower the activation energy,and shorten the gelation time. These particles also
significantly shorten the time between gelation and
vitrification and lead to asymmetric out-of-phase dis-
placement curves. The functionalized nanosilica par-
ticles are sufficiently well dispersed to add an
insignificant amount to the viscosity of the base
resin. In contrast, the similarly sized nanosilica par-
ticles interact sufficiently in a strong manner during
the dispersions leading to a very significant increase
in the viscosity. The enhancement in the viscosity
arises from the formation of chains of particles at a
nanolevel. The increased viscosity slows the propaga-
tion reaction and increases the time between gelation
and vitrification. The use of a small amount ofmethyoxymethanol as a dispersion solvent for the
catalyst aids the reaction but also helps dispersion
of the nanosilica and lowers the viscosity. The incor-
poration of platelet-type fillers effectively inhibits the
cure of this system. A mixture of 1005 Nanopox A610
with Aerosil OX50 which had a total silica content
of 44.44 wt% was observed to undergo cure with-
out the addition of Yb triflate, which implies that
there is the possibility of surface-activated catalyst
of the epoxy auto-catalytic process. The initiation
of the auto-catalytic process suggests that the sur-
face contains active hydroxyl groups. The reducedactivation for cure is consistent with this
observation.
Table 4 Comparison of gelation times with the addition of OX50 with variation of resin type
Additive Silica content (wt%) Cure temperature (C) Yb (phr) Cure time (min)
o-Cresol novolac resin 100 0.1 684
120 0.1 218 140 0.1 130Aerosil OX50 23.06 140 0.1 44Nanopox A610 120 0.1 130
140 0.1 37
o-Cresol novolac resin Nanopox A410 50/50 wt% mixtureAerosil OX50 46.59 120 0.3 30Aerosil OX50 19.92 100 0.1 1125Aerosil OX50 19.92 120 0.2 180Aerosil OX50 19.92 120 0.4 40Aerosil OX50 19.92 120 0.5 38Aerosil OX50 35.98 120 0.1 744Aerosil OX50 40.67 120 0.3 57Aerosil OX50 42.70 120 0.3 42
Table 3 Variation of gelation time with composition and temperature for Nanopox A610 OX50 blends
and variation of the catalyst level at different temperatures
ConcentrationA610 (wt%)
ConcentrationOX50 (wt%)
Total silica(wt%)
Yb triflate(phr)
Cure temperature(C)
Gelation time(min)
44.42 11.11 55.53 0.10 100 17042.84 14.28 57.12 0.10 100 15444.42 10.91 55.53 0.080 100 11344.42 11.11 55.53 0.080 130 842.84 14.28 57.12 0.080 130 3242.84 10.91 57.12 0.080 160 5
Low coefficient of thermal expansion of thermoset composite materials 83
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5 DESIGN OF COMPOSITES WITH LOW CTEs
On the basis of equation (1), it is desirable to
achieve the maximum level of silica in the mate-
rial and also to use a resin matrix which itself has
a low value of CTE. The formulations summarized
above were subjected to DMTA and thermome-
chanical analyses to determine the glass transition
temperature Tg and the CTE, respectively. The
values of Tg were obtained from the location of
the peak in tan which is typically 10 above the
Tg, as defined by the onset of modulus reduction
obtained from the E data. The CTEs were deter-mined as the average values below Tg and above
Tg or where the plots were curved; a value at a
specific temperature is presented.
5.1 EEC Nanopox A610
The Nanopox A610 contains 40 wt% 20 nm SiO2 par-
ticles, which have been produced in situusing a sol
gel process. Blends of the EEC and Nanopox A610
were examined and the data obtained are presented
in Table 5. For the 100 wt% resin, the values are
obtained from an average of three separate samples,whereas the 100 wt% Nanopox A610 were obtained
using seven samples.
The observed CTE below the Tg for the NanopoxA610 is lower than that predicted on the basis of equa-
tion (1). The Nanopox A610 consists of solgel parti-
cles which are surface functionalized and hence it
would be expected that during the cure reaction, the
silica particles will be able to act as cross-links in the
matrix. The incorporation of the functionalized silica
increasesTgfrom a value of 162C for the base EEC
resin to a value of200 C for the combined resin with a
total loading of 40 wt% silica. Aerosil OX50 contains
40 nm hydrophilic silica particles, has a low surface
area of 50 m2/g, and does not readily form agglomer-
ates. The Aerosil OX50 was incorporated into both theNanopox A610 and the EEC in an attempt to increase
the total silica loading, and the data are presented in
Table 6. Number of formulations were repeated and
the cure conditions varied slightly. At the 57 wt% total
silica loading, the CTE belowTgis 31 ppm/C, which
is significantly lower than the calculated values.
Increasing the total silica content has decreased the
CTE to a value which is 0.35 of that of the resin. The
Aerosil OX50 is produced by an aqueous solgel pro-
cess and will have OH surface functionality, but this
will be different from the functionalities on the parti-
cles in the Nanopox A610.However, the Aerosil OX50 appears to be achieving
reductions in the CTE of the resin which are
Table 6 Variation of the coefficients of expansion (ppm/C) for EEC Nanopox A610 Aerosil OX50
EEC wt%Nanopox
A610 (wt%)Aerosil OX50(wt%)
Nanosilica(wt%)
Silica(wt%)
CTEbelowTg
CTEaboveTg
CTEcalculated Tg(
C)
0 46.13 23.07 30.76 53.82 39 98 40.9 2170 44.42 29.6 25.9 55.53 31 111 39.4 2180 42.84 28.56 28.56 57.12 31 100 38 22371.38 0 28.55 0 28.55 58 134 6355.56 0 44.44 0 44.44 46 96 4954.52 0 45.43 0 45.43 46 119 4859.96 0 39.98 0 39.98 51 119 530 46.13 23.07 30.76 53.82 35 109 410 42.83 28.55 28.55 57.1 38 111 380 44.42 25.91 29.62 55.53 36 104 390 37.49 37.49 24.99 62.48 31 99 33 1890 49.98 16.66 33.3 49.98 55 129 440 42.84 28.56 28.56 57.12 35 109 38 0 42.84 28.56 28.56 57.12 38 130 38 0 42.84 28.56 28.56 57.12 37 111 38 1510 42.84 28.56 28.56 57.12 35 110 38 0 42.84 28.56 28.56 57.12 29 117 38 144
Table 5 Coefficients of thermal expansion for EEC and Nanopox A610
EEC (wt%) A610 (wt%) Nanosilica (wt%) Silica (wt%) CTE belowTg CTE aboveTg CTE calculated Tg(C)
49.9 29.9 19.97 19.97 54 169 70 169100 0 0 0 86 2 228 20 86 1620 59.9 39.9 39.9 48.5 1.5 136 11 53 2000 0 100 100 0.55 0.55
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comparable to the in situ dispersed Nanopox A610
resin. A number of formulations were prepared in
which the EEC and the Nanopox A610 were combined
in various proportions. A series of samples with a totalsilica content of 57.12 wt% were prepared and slightly
different processing conditions used. While there are
variations in the values observed, an averaged value
of 35ppm/ C is lower than the theoretical prediction;
however, the values ofTgwere low. The sample with
the highest silica content, 62.48wt%, showed the
lowest value of CTE at 31 ppm/ C, but surprisingly
Tgwas not as high as that observed with lower levels
of silica. It is possible that at the highest loadings, the
ability for the matrix to be completely cross-linked
will become limited and this will be reflected in
lower values of Tg. Two alternative hydrophilicAerosil samples were studied; Aerosil 200, which has
a particle size of 12 nm but a surface area of 200 m2/g,
is a grade used for controlling the rheology of fluids
and Aerosil 90 which has a particle size of 20nmand a
surface area of 90 m2/g. The Aerosil 200 has a tight
particle distribution size similar to that of Aerosil
OX50 but with a slightly larger mean diameter,
whereas Aerosil 90 has both a large mean diameter
and a broader particle size distribution. The data
obtained are presented in Table 7. The Aerosil 200
with the Nanopox A610 gives a value which is close
to the theoretically predicted value, while with pureEEC, it gives a significant lowering of the CTE. The
high surface area and the hydrophilic functionality
of the surface may allow favourable packing to be
achieved. While Aerosil 200 showed a significant
enhancement of the Tgat 45 wt% loading, the CTE
was not significantly lower than the theoretically pre-
dicted value. Using the large size and broad particle
distribution, Aerosil 90 gives little enhancement ofTgand relatively high values of the CTE. This implies
that the particles are not forming a significantly
improved network or packing together effectively to
enhance the composite structure.If the cross-link density in the resin matrix is a con-
trolling factor, then the use of a highly functional
epoxy,N,Ndiglycidyl-4-glydylaniline, has the poten-
tial of increasing the cross-link density. An addition of
0.36 wt% of this tri-functional epoxy to the resin
formed from Nanopox A610 and Aerosil OX50 pro-duced CTE values 33 ppm/ C belowTgand 99 ppm/C aboveTgand aTgof 208
C at a total silica loading
of 56.98 wt%. Rather than increasingTg, the value is
slightly lowered and however, the CTE above Tg is
slightly reduced.
5.2 o-Cresol novolac epoxy resin
The use of the trifunctional epoxy clearly does not
achieve a significant reduction in the CTE of the
matrix. o-Cresol novolac epoxy resin is typically a
2.7 functional material and hence has the potentialof producing a highly cross-linked matrix. Values of
CTE andTgare presented in Table 8.
The value of theTgfor the o-cresol epoxy novolac
varied significantly with cure conditions. Post curing
the pure resin at 130 C causes an increase from an
initial low value of 86 C to a value of 146 C and on
further increasing the post-cure temperature, a value
of 185 C was achieved. IncreasingTgcauses a corre-
sponding decrease in the CTE below and above Tg,
except that the final post-cure appears to have
increased the CTE. It has previously been observed
that heating epoxy resins above 150 C can lead toether bond scission, and in this case, a reduction in
the degree of cross-linking with a commensurate
increase in the CTE [14, 15]. A sample with Aerosil
OX50 having a total silica content of 23.06 wt% had
aTgof 184C, and CTE values 53 and 152 ppm/ C,
respectively, below and above Tg. This material was
very viscous and it was difficult to achieve higher
silica loadings. To reduce the viscosity and increase
the silica content, Nanopox A510 was blended as a
10wt% component with the novolac and this
increased the silica level to 33.31 wt%. TheTgof this
blend was 174 C but there was a slight reduction inthe CTE values 45 and 130 ppm/ C, respectively,
below and above Tg. Addition of Aerosil OX50
Table 7 Variation of the coefficients of expansion (ppm/C) for EEC Nanopox A610 Aerosil 200 and
Aerosil 90
EEC
(wt%)
Nanopox
A610 (wt%)
Aerosil 200
(wt%)
Nanosilica
(wt%)
Silica
(wt%)
CTE
belowTg
CTE
aboveTg
CTE
calculated Tg(C)
83.19 0 16.64 0 16.64 50 171 73.45 1640 54.5 9.08 36.33 45.41 48 146 48.29 199
EEC
(wt%)
A610
(wt%)
Aerosil
90 (wt%)
Nanosilica
(wt%)
Silica
(wt%)
CTE
belowTg
CTE
aboveTg
CTE
calculated
Tg(C)
79.94 0 19.98 0 19.98 64 176 70.5 162
41.61 24.97 16.64 16.64 33.29 65 147 58.9 165
0 52.14 13.03 34.76 49.79 63 146 46.2 173
Low coefficient of thermal expansion of thermoset composite materials 85
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increases the silica content to 50.72 wt%, leading to a
reduction of theTgto 117C but did achieve a reduc-
tion of the CTE values to 37 and 121 ppm/ C, respec-
tively, below and above Tg. Addition of Nanopox A610
to the novolac produced a Tg value of 184C at
29.98 wt% loading and CTE values of 54 and
138 ppm/ C below and aboveTg. A 50/50 wt% blend
of novolac and Nanopox A510, which is the bisphenol
matrix with added Aerosil OX50, gave CTE values CTE
50 and 160 ppm/ C below and above Tg at 20wt%
silica loading and 34 and 123 ppm/ C below andaboveTgat 46.5 wt% silica. TheTgof the latter mate-
rial was 175 C. Surprisingly, the novolac resin did not
produce significantly lower values of CTE than those
obtained using the EEC cure system.
6 CONCLUSIONS
The study of a range of different polymer systems
containing a combination ofin situdispersed nano-
particles and added nanofillers indicates that equa-
tion (1) is broadly followed (Fig. 6). While the values
for Aerosil 90, which is the broad distribution mate-
rial, are above the theoretical line, the narrow distri-butions of nanofillers Aerosil OX50 and Aerosil 200
produce values which are below the theoretical line.
Fig. 6 Variation of the CTE vswt% silica. Uncertainty in the values of CTE 2 ppm/C which istypically the size of the symbol
Table 8 Variation of the coefficients of expansion (ppm/ C) for o-cresol epoxy novolac Nanopox
A610 Aerosil OX50
o-Cresolepoxynovolac
NanopoxA610 (wt%)
AerosilOX50 (wt%)
Nanosilica(wt%)
Silica(wt%)
CTEbelowTg
CTEaboveTg
CTEcalculated Tg(
C)
99.9 0 0 0 0 103 246 65 8699.9 0 0 0 0 69 163 65 14699.9 0 0 0 0 65 180 65 18576.86 0 23.06 0 23.06 53 152 50 18449.96 29.98 0 19.98 19.98 55 143 59 18474.93 14.99 0 10 10 56 150 62 16574.93 14.99 0 10 10 58 153 62 18024.98 44.97 0 29.98 29.98 54 138 56 18441.64 24.98 16.66 16.66 33.31 45 130 49 17449.96 29.98 0 19.98 19.98 55 155 59 18445.42 27.25 9.08 18.17 27.25 51 149 54 18645.44 27.26 9.08 18.18 27.25 51 137 54 196
86 I Rhoney and R A Pethrick
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The best results are obtained when the low surface
area Aerosil OX50 is combined with the nanofilled
resin, Nanopox A610. This blend of nanoparticles
and reactive functionality produces the highest
values ofTgand the lowest values of CTE. This mate-rial is relatively easy to mix and cure and has the
potential of being used as a composite material for
applications that require low CTE. The o-cresol epoxy
novolac, although in principle able to achieve a
higher cross-link density and by implication a lower
CTE aboveTg, showed that improvements were only
obtained by careful selection of the filler size and also
post-cure temperature.
FUNDING
Support form Samsung is gratefully acknowledged forpart of this research.
Authors 2011
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