Apparent Volumetric Shrinkage Study of RTM6 ResinDuring the Curing Process and Its Effect on theResidual Stresses in a Composite

K. Magniez, Arun Vijayan, Niall FinnMaterials Science and Engineering Division, Commonwealth Scientific and Research Organisation (CSIRO),Geelong, Victoria, Australia

A comprehensive characterization of the volumetricshrinkage of a commercially important aerospace resin(RTM6) during the various stages of the curing processwas studied. The apparent volumetric shrinkage, eval-uated from density measurements at room tempera-ture, was correlated with the progress of epoxide con-version. During the entire curing process, the apparentvolume shrinkage was found to be less than 3% andoccurred before vitrification. A slight re-expansion ofthe resin, attributed to self-antiplasticization effects,was observed during postcuring at 1808C. It was con-cluded that residual stresses were not generated dueto chemical cross-linking during curing but rather fromthermal contraction occurring during the cooling stageafter cure. A photo-elastic method was used to charac-terize residual stresses during cooling in a deliberatelyengineered resin rich hole of a carbon fiber/RTM6composite. The residual stress was found to reachapproximately 28 MPa, which is in good agreementwith the value calculated from the shrinkage and elas-tic moduli. It is proposed that this simple method canbe provide insights useful to the design and materialsselection processes by measuring and localizing resid-ual stresses from resin during curing and or thermalcycling. POLYM. ENG. SCI., 52:346–351, 2012. ª 2011 Societyof Plastics Engineers

INTRODUCTION

During curing of thermoset resins cross-linking reac-

tions between the functional groups will result in the for-

mation of a complex three-dimensional network. With the

progress of the reaction, a densification of the network and

hence a resin volumetric shrinkage is generally observed.

Resin shrinkage often causes manufacturing issues

related to both dimensional control and poor surface quality

of the finished composite part [1, 2]. More importantly, if

the shrinkage occurs in a confined environment, it is directly

converted to residual stress [3] leading to the formation of

stress-induced voids [4], cracks, and delamination [2],

which may impact on the mechanical performance of the

composite. In order to minimize these effects; it is therefore

crucial to understand the cure-related chemical shrinkage.

The parameters inducing volumetric shrinkage have been

attributed to both chemical cross-linking during isothermal

curing and thermal contraction effects during cooling [5].

An adapted schematic representation from Ochi [5] of the

volumetric changes of epoxy resins during curing (at a tem-

perature below the glass transition) and during cooling is

given in Fig. 1. Determination of residual stresses arising

from volumetric shrinkage of epoxy and unsaturated polyes-

ter resins has been extensively reported using methods such

as dilatometry [6–8], torsional parallel plate rheometry [9],

dynamic mechanical analysis [10], bending beam [11, 12],

and changes in buoyancy of the resin [2, 5, 13, 14].

RTM6 is a mono-component resin system which has

been commercially developed by Hexcel. This resin system

is currently used for Vacuum Infusion and Resin Transfer

Moulding (RTM) processes, allowing the fast production

of more cost-effective high-performance aerospace com-

posite parts. The primary purpose of the work presented in

this article was to determine the apparent shrinkage (Fig.

1) of this commercially important resin during the various

stages of the curing process. The apparent shrinkage mea-

surement in this work is equivalent to the one described in

the standard American Society for Testing and Materials

(ASTM) method [15]. The evolution of the volumetric

shrinkage of the resin, calculated from changes in density,

was correlated to the progress of resin conversion. Finally,

the magnitude of the stress generated during the cooling

stage was measured using photo-elastic response.

MATERIALS AND EXPERIMENTAL PROCEDURES

Materials

The high performance HexFlow1 RTM6 (Hexcel, Aus-

tralia) is a one-part epoxy/amine resin system that has been

K. Magniez is currently at Institute for Technology Research and Inno-

vation, Deakin University, Geelong Victoria Australia.

Correspondence to: K. Magniez; e-mail: kmagn@deakin.edu.au

DOI 10.1002/pen.22088

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

specifically developed to fulfill the requirements of the

aerospace industry in advanced RTM processes. The com-

position of RTM6 resin is based on a tetra functional ep-

oxy system and a blend of aromatic amine hardeners [16]

and its glass transition temperature (Tg) is 1968C [17].

Measurement of the Volumetric Shrinkage During Cure

Nine sets of RTM6 samples (52 mm 3 27 mm 3 13

mm in dimension, � 5 g in weight) were cured in a con-

ventional oven using silicon moulds. A standard precure

cycle [17] is performed at 1508C for 180 min, and is fol-

lowed by postcure at 1808C for 90 min (Fig. 2). In this

method, six samples were precured at 1508C for various

periods of time: 50, 60, 80, 120, 150, or 180 min (sam-

ples 1, 2, 3, 4, 5, and 6, respectively). The remaining

three sets were precured for 180 min at 1508C then post-

cured at 1808C for 40, 60, or 90 min (samples 7, 8, and

9, respectively).

The change in density was calculated from the change

in buoyancy by displacing the RTM6 samples in air and

water at ambient temperature (208C) as previously

reported by Ochi [5, 13, 14] and Eom et al. [4]. The test

procedure consisted of measuring the sample weight in

air and then weighing the sample suspended on a wire

and immersed in a water bath using an under-hook bal-

ance (accuracy within 0.05%) according to the standard

ASTM test method [18]. The density was converted to

shrinkage by using the following Eq. 5:

Volumetric shrinkage ð%Þ ¼ 100� 1� r1r

� �(1)

where r1 is the density of liquid resin at 208C (1.117 g/

cm3 [17]) and r is the density of the cured sample.

Additionally, the residual heat for the pre- and post-

cured samples was evaluated using a Perkin-Elmer DSC 7

differential scanning calorimeter in dynamic mode. The

instrument was operated under a nitrogen stream at a flow

rate of 50 mL/min and was systematically recalibrated

between experiments using an indium standard. Samples

weighing between 2 and 7 mg were sealed in vented alu-

minium pans. Dynamic curing was achieved by heating

the sample from 258C to 3008C at a heating rate of 108Cper minute. The epoxide conversion was determined using

the assumption that the heat output correlates directly

with the epoxy/amine reaction. Hence, at time t, the frac-

tional residual epoxide conversion at was calculated using

the equation:

at ¼ DHt

DHtotal

(2)

where DHt is the specific heat flow (due to residual epox-

ide conversion after curing time t), and DHtotal is the total

specific heat flow for the neat resin to achieve 100% cure.

DHtotal was found to be 511 J/g. This value is similar to

that reported by Varley [16]. The fractional residual epox-

ide conversion at was converted to a percentage conver-

sion at time t using the following equation:

%Conversion ¼ 100� ð1� atÞ (3)

Rheological analysis was performed by Varley [19] on

a TA Instruments, ARES strain-controlled rheometer. Iso-

thermal cure at 1508C was performed using a parallel

plate configuration (25 mm top plate, 50 mm bottom

plate) with a gap of 1 mm, and at a frequency of 5 rad/s.

The strain was initially set to 10% but was continuously

adjusted throughout the cure using a looping sequence

from the Orchestrator1 rheology software to prevent

overload of the transducer.

Stress Measurements Using Birefringence

A sample of carbon fiber/RTM6 composite (similar in

shape to the one described in the ‘‘Measurement of the

Volumetric Shrinkage During Cure’’ section) of approxi-

mate dimensions 50 mm 3 2.4 mm 3 10 mm was pre-

pared using three plies of woven carbon fabric into which

a 5.5 mm hole had been punched, followed by an infusion

FIG. 2. Autocatalytic curing process of RTM6 resin.

FIG. 1. Schematic representation of the volumetric changes of epoxy

resin during heating, isothermal curing and cooling.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 347

with RTM6 resin in a silicone mould (Fig. 3). The size of

the punched hole was chosen to allow optical measure-

ment of the residual stresses generated by the resin that

fills the hole. The sample was cured and postcured using

the aforementioned experimental method. The sample was

mounted inside a specially made heating chamber

mounted on an Olympus BX50 polarized light microscope

equipped with a full-wave retardation plate (c ¼ 530 nm)

and fitted with a digital camera. Monochromatic (green)

linearly polarized light was transmitted through a first po-

larizer then through the sample and analyzed with a sec-

ond polarizer with its plane of polarization perpendicular

to the first. Stress-induced birefringence results in changes

in the polarization state of the light passing through each

region of the sample according to the local stress in that

region integrated through the depth of the sample along

the light path. The sample was designed such that the

stress would be in the plane and so equal through its

depth. The in-phase orthogonal polarization components

of the linearly polarized light pass through the sample at

different speeds according to the directions and magni-

tudes of the stresses present and the resultant phase shift

results in elliptical polarization. These localized changes

in polarization result in changes in the transmission of the

light through the analyzer so that images of the sample

display intensity patterns that depend directly on the stress

patterns in the sample. The polarization direction can be

rotated several times as the stresses change with time so

that the local intensity oscillates between minima and

maxima giving rise to dark and light fringes representing

contours of equal principal stress differences.

The temperature of the sample was measured with a

thermocouple attached to its side. A hot air gun attached

to the chamber was used to heat the fully cured sample

up to 1608C (to reproduce the thermal state of the sample

at the end of the curing process); the heating was stopped,

and the sample was allowed to cool naturally while its

temperature was monitored. At every 108C change in

temperature, a photograph was taken of the sample

through the crossed polarizers. As the sample cooled

down, the changing stress was measured by counting the

isochromatics (i.e., fringes) passing the centre of the aper-

ture using the photographs.

RESULTS AND DISCUSSION

The percentage epoxide conversion for the RTM6 sam-

ples precured at 1508C for 50, 60, 80, 120, 150, and 180

min and postcured at 1808C for 40, 60, and 90 min are

given in Fig. 4. As expected, the percentage epoxide con-

version increased monotonically as a result of cross-link-

ing to reach 85% before the postcure process, and

increased from 85% to 95% during the postcure. The

build-up of residual stress for an epoxy resin can occur

during both the curing and cooling processes, as previ-

ously reported by Ochi [5]. During the cure of epoxy res-

ins at a temperature below the glass transition (Tg), resid-ual stresses will be generated at the vitrification point,

and those stresses will increase as a result of cooling

shrinkage [14]. The amplitude of the residual stresses af-

ter curing will therefore depend on both the evolution of

the elastic modulus of the resin after vitrification and the

difference in coefficient of thermal expansion (CTE)

between the resin and any constraints; reinforcements and

moulds [5, 13, 14].

The evolution of the elastic shear modulus with the

cure of RTM6 at 1508C is given in Fig. 5. Before gela-

tion, the elastic shear modulus was found to be very low

FIG. 3. Schematic of the carbon fiber/RTM6 composite sample con-

taining a resin rich region (note that the figure is not to scale for visual

clarity).

FIG. 4. Evolution of the % rate of conversion with curing, showing the

gel point G, and the vitrification point V.

FIG. 5. Evolution of the elastic shear modulus G0 as a function of the

cure at 150 8C, showing the gel point G, and the vitrification point V.

348 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

(i.e., \1 Pa) as the resin behaved like a Newtonian fluid.

After gelation, the resin behaves like an elastic solid, and

its shear modulus had increased by several orders of mag-

nitude at vitrification (i.e., 10 MPa). Kinetic and rheomet-

ric data on isothermally cured RTM6 resin between

1108C and 1808C has previously been reported by Varley

[16]. Varley reported gelation time varying with tempera-

ture from 358 min at 1108C to 18 min at 1808C. Vitrifica-tion time varied from 400 min at 1108C to 29 min at

1808C. In the particular case of curing at 1508C, the gel

and vitrification times were found to be 60.5 min and

80.6 min, respectively, corresponding to fractional epox-

ide conversions of 0.53 and 0.74. These values are con-

sistent with those found in this study: 0.5 and 0.72 at cur-

ing times of 60 and 80 min, respectively. In the case of

RTM6 during isothermal curing at 1508C, residual

stresses could only be generated at the vitrification time

of 80 min (at 74% epoxide conversion), and before vitrifi-

cation, the stresses generated would be negligible.

The percentage apparent volume shrinkage as a func-

tion of percentage conversion is plotted in Fig. 6. It can

be noticed that during the entire curing process, the per-

centage volume shrinkage does not exceed 3%. It can also

be seen that a slight decrease in the volume shrinkage (a

slight re-expansion) was observed during the postcuring

process at 1808C. This decrease can possibly be attributed

to self-antiplasticization effects as previously reported by

Venditti et al. [20]. These effects arise from changes in

free volume where the unreacted end groups attached to a

network are able to fill the spaces.

During cooling, the resin shrinkage is dependent on the

CTE of the resin and will be converted to stress if the

resin is constrained by a material with a different CTE. In

a carbon fiber reinforced composite, temperature gradients

may therefore generate residual stresses between the vari-

ous components. The linear CTE for fully cured RTM6

samples at 258C and 1808C are, respectively, 54.5 31026 K21 and 62.5 3 1026 K21 [21], which corresponds

to a linear shrinkage of 0.91% in cooling from 1808Cdown to 258C. The modulus reported for fully cured

RTM6 resin is 2890 MPa [17], and therefore, the cooling

shrinkage of the resin can be converted into a linear stress

nearing 26 MPa. Assuming RTM6 to be isotropic, the

volumetric thermal expansion coefficient OV is very

closely approximated as three times the linear CTE, OL,

(see equation below).

OV ¼ 1

L3qL3

qT¼ 1

L3qL3

qLqLqT

� �¼ 1

L33L2

qLqT

� �¼ 3

1

L

qLqT

ffi 3OL

(4)

The percentage volumetric shrinkage of RTM6 on

cooling is then approximately 2.73%. Carbon fibers are

not isotropic, and their CTE is negative in the longitudi-

nal direction (20.6 3 1026 K21). The transverse CTE of

carbon fibers is difficult to find in the literature due to the

difficulty of the measurement, but it has been estimated to

be 15 3 1026 K21[22]. The longitudinal CTE of carbon

fibers is almost negligible compared with that of the resin,

and therefore, cooling of a carbon fiber/RTM6 composite

from the cure temperature can be expected to generate

some residual stresses.

An estimation of the magnitude of stress generated

during cooling in a carbon fiber/RTM6 composite was

performed using its photo-elastic response. The photo-

elastic response (isochromatic fringes) in a resin rich

FIG. 6. Evolution of the apparent volumetric shrinkage with the extent

of epoxide reaction, showing the gel point G, and the vitrification

point V.

FIG. 7. Schematic of the three-point-bending apparatus fitted to the

microscope setting. A micrometer was used to deform the RTM6 cured

sample only in the plane while recording the fringe pattern on the ten-

sion side of the beam.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 349

pocket was recorded to analyze the stress distribution pat-

tern. To calibrate the system, a sample of cured RTM6 of

approximate dimensions 3.5 mm 3 9 mm 3 100 mm

was mounted in a three-point-bending apparatus fitted to

the microscope (Fig. 7). A micrometer was used to

deform the sample only in the plane by known amounts

while recording the fringe pattern on the tension side of

the RTM6 beam. Some examples of fringe patterns as

stress increases are shown in Fig. 8. It can be seen that as

the deflection increases the number of fringes, and their

spatial density increase as the stress at the edge and the

stress gradient toward middle of the sample increase. The

device was then moved from the microscope to an Instron

tensile test device and the deflection was repeated with

the load cell pushing against the centre pin carriage of the

three-point bending device to obtain a load-deflection

curve for the beam. The peak stress r at the edge of the

beam was calculated from simple beam theory [23] and

correlated with the fringe number n obtained from the

microscope to obtain the photo-elastic calibration plot for

a pure RTM6 beam (Fig. 9).

The gradient of the plot in Fig. 9 provides a photo-

elastic constant, S, used to obtain the stress r from the

fringe number n:

s ¼ n� S

t(5)

The measurements for calibration of the photo-elastic

properties of RTM6 also gave a value for the elastic modu-

lus of 3240 MPa (in flexure), which was slightly higher than

the value of 2890 MPa reported elsewhere [17]. As linear

shrinkage was measured to be 0.91%, the stress caused by

the differential in CTE between the carbon fibers and the

resin would be approximately 29.5 MPa (26.3 MPa if we

use the reported value in [17]). Using the photo-elastic

method, when the sample cooled from 1608C to 208C, 2.7

FIG. 8. Birefringence images with increasing stress in three-point-bending of RTM6 sample. The image

shows an increasing number of dark fringes from 1 to 4, clockwise from top left when the peak stresses at

the edge of the sample are 7 MPa, 14 MPa, 21 MPa, and 28 MPa.

FIG. 9. Photo-elastic calibration for a pure RTM6 beam in a three-

point-bending test. Stress multiplied by thickness (t) versus Fringe num-

ber, n.

350 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

isochromatics passed through the centre of the aperture in

the woven carbon fabric. Correcting for the difference in

thickness between the calibration beam of pure RTM6 resin

and the composite sample (3.5 mm compared with 2.4 mm)

the stress equates to 27.9 MPa. For comparison, residual

stress analysis from curvature measurement of unbalanced

laminates of both a cyanate ester and a poly (ether sulfone)

resin system, reinforced with intermediate modulus Hercu-

les carbon fiber (IM8), were found to reach 40 and 50 MPa,

respectively, when the temperature decreases from 2008C to

208C. Similarly, measurements of residual stresses using the

deep-hole method showed a maximum residual stress of

about 40 MPa in the fiber direction in HexPly 913 epoxy

resin system reinforced with Toho HTA carbon fiber [24,

25]. Also, given the possible inaccuracies in measuring the

number of fringes, the photo-elastic uniaxial test data used

to determine experimental stress was in fairly good agree-

ment with the values calculated using the modulus measured

in flexion and tension of 29.5 and 26.3 MPa, respectively.

CONCLUSIONS

The volumetric shrinkage of RTM6 resin during the

various stages of the curing process was determined. Up to

vitrification, the elastic shear modulus of the resin is too

low to generate significant residual stresses and almost all

the curing shrinkage occurs before this point. It is, how-

ever, shown that the build-up of residual stress can arise

from cooling shrinkage coupled with the difference in CTE

of the carbon fibers and the resin. Because this occurs after

vitrification, these stresses can be significant; of the order

of 29 MPa. The apparent volumetric shrinkage was less

than 3% during the entire curing process and a slight

decrease (re-expansion) was observed during the postcure

part of the process (1808C) which was attributed to self-

antiplasticization effects in the resin. A photo-elastic

response method was used to measure the magnitude of

stress generated during the cooling of a carbon fiber/RTM6

composite from 1608C to 208C, which were found to reach

approximately 28 MPa. Given the possible inaccuracies in

determining the number of fringes, the photo-elastic result

is in good agreement with those deduced from shrinkage

and moduli. Shrinkage stresses of this magnitude could

generate distortions in unbalanced composite components

and should be considered during the design process.

In addition, the constant thermal expansion and con-

traction experienced by some composites in use can

render them susceptible to microcrack formation in the

long term. This can be a serious performance issue if the

propagation of microcracks significantly compromises the

integrity of the material. As a result, accelerated thermal

cycling to investigate microcracking is sometimes part of

the qualification program of new aerospace composite

materials. In this work, a photo-elastic method was used

to characterize residual stresses during cooling in a delib-

erately engineered resin rich hole. This method could be

applied to the measurement of thermally generated

stresses that occur during thermal cycling of composite

parts, providing insights useful to the design and materials

selection processes by allowing minimization of residual

stresses from resin curing and stress localization during

thermal cycling.

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