Low Coefficient of Thermal Expansion

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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: [email protected]

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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




4/13

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




5/13

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)





6/13

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





7/13

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





8/13

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





10/13

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





11/13

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





12/13

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





13/13

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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