Proceedings of the 9th Annual ISC Graduate Research Symposium ISC-GRS 2015

Proceedings of the 9th Annual ISC Graduate Research Symposium
ISC-GRS 2015
April 27, 2015, Rolla, Missouri
CURE KINETICS OF A CARBON FIBER/BISMALEIMIDE PREPREG SYSTEM
Sudharshan Anandan
[email protected]
Department of Mechanical and Aerospace Engineering
Advisors: Drs. K. Chandrashekhara and Thomas Schuman
ABSTRACT
Bismaleimide (BMI) resins are generally used in applications
involving high ambient temperatures. The mechanical
properties of cured composites are determined by the degree of
cure, which depends on the kinetic model. A cure kinetic model
is an integral part of process simulation as the rate constants are
crucial for accurately predicting the degree of cure and the
amount of heat generated. In the current work an isothermal
based cure kinetic model for a new generation of BMI prepreg
is developed. The resin used is AR4550 from Aldila reinforced
with IM7G carbon fibers. Curing of the prepreg will be studied
using Differential Scanning Calorimetry (DSC) and the data
will be analyzed using an nth order reaction model.
1. INTRODUCTION
Bismaleimides (BMI) are high temperature polymers which
have higher service temperature compared to conventional
epoxy resins. BMIs also exhibit good tack and drape, and an
epoxy-like cure behavior. They also possess good thermal
stability and fatigue resistance even at high temperatures [1].
BMI based composites are generally manufactured in an
autoclave. However, acquiring and operating large autoclaves,
required for traditional manufacturing of high performance
composites can be expensive. Moreover, the part size is limited
by the size of the autoclave. Use of out-of-autoclave (OOA)
processing can also result in reduced core crush and core
stabilization (sandwich structures), use of low cost tooling and
production flexibility [2]. Composites for aerospace
applications are manufactured from prepregs. Prepregs or preimpregnated materials are reinforcement materials (carbon
fiber, glass fiber etc…) which are infused with the resin. They
can be laid on a mold and cured without any further addition of
resin. A new generation of BMI based prepregs have been
developed, which are suitable for OOA curing [3]. Carbon/BMI
composites are used in high temperature tooling and the
aerospace industry. Composites made with BMI OOA prepregs
were evaluated in the NASA CoEx (Composite for Exploration)
project. Due to a limitation in tooling, the cure temperature was
limited to 177 °C (350 °F) [4, 5]. Samples were found to have
poor consolidation at specific regions
and the green strength (strength before post cure) was evaluated
[6]. It was found that the green strength increased with higher
base cure temperatures. An improvement in mechanical
properties was noticed when samples were post-cured.
He, et al. [7] investigated the effect of post cure cycles on
IM7/5250-4 composites. It was found that glass transition
temperature increased with post-cure temperature. Flexure
properties and mode II fracture toughness were also enhanced
for composites post cured at 218°C, as compared with those
post cured at 190 °C or at 246 °C.
Mechanical properties of fiber reinforced polymers are
dependant on the degree of cure. A cure kinetics model is
required for process simulation which can be used to study
processing-structure-property relationships for thick and
complex shaped composite components.
Several experimental techniques such as DSC [8-11], FTIR
[12], near infra-red spectroscopy and UV reflection
spectroscopy can be used to characterize thermosetting
polymers. DSC is one of the most widely used techniques to
obtain cure kinetic parameters of exothermic reactions.
Figure 1. DSC exotherm for BMI prepreg cured at 200 °C
Figure 1 shows a typical DSC curve for the BMI prepreg.
The ultimate heat of reaction is obtained by integrating the
curve to find the area under the exotherm. The degree of cure,
α, at any time, t, is calculated using equation 1,
In a previous study, carbon/BMI composite laminates were
manufactured using the same material as in the current study
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α=
∆𝐻𝑡
∆𝐻𝑈
(1)
where, ∆𝐻𝑈 is the total heat of reaction and ∆𝐻𝑡 is the heat of
reaction at time, t.
Two kinetic models are commonly used to characterize the
curing of thermosetting resins: nth order type and autocatalytic
type mechanisms [13]. The nth order kinetic model is given by
equation 2,
𝑑α
= f(1 − 𝛼)
𝑑𝑡
(2)
where, f(1 − 𝛼) can take the form,
f(1 − 𝛼) = 𝑘(1 − 𝛼)𝑛
(3)
where, 𝛼 is the degree of cure, t is the time, 𝑘 is the reaction
rate constant and 𝑛 is the reaction order.
According to nth order kinetics, the rate of reaction is
highest when α is low. The rate of reaction is affected by the
amount of unreacted resin only.
If the rate of reaction is also affected by the reaction
products, the autocatalytic reaction model (equation4) can be
used. Here f(1 − 𝛼) takes the form,
f(1 − 𝛼) = 𝑘𝛼 𝑚 (1 − 𝛼)𝑛
(4)
where, 𝛼 is the degree of cure, t is the time, 𝑘 is the reaction
rate constant and 𝑚, 𝑛 are reaction orders. In this case the
maximum rate of reaction occurs at around 20-40% conversion.
Loustalot and Grenier [14] investigated the curing
mechanism of epoxy resin systems in the presence of glass and
carbon fibers. They concluded that the presence of fibers did
not change the reaction mechanism or network structure, but
did lead to notable differences in the reaction rate, especially at
low temperatures.
Guo, et al. [10] studied the cure kinetics of a commercial
T700/BMI prepreg system. An nth order reaction model was
used and a first order reaction mechanism was reported. A
higher cure temperature resulted in an increase in reaction rate.
The effect of the presence of fibers on the curing parameters
was significant. The presence of carbon fibers restricted the
mobility of the reacting species. The rate constant was lower
for prepreg compared to that of neat resin.
Boey, et al. [15] studied the cure kinetics of a modified
BMI system. The nth order reaction model was found to be
suitable to describe the reaction mechanism over the selected
temperature range. Higher isothermal cure temperature lead to
reduced reaction order and increased rate of reaction. The
reaction mechanism was dependent on the isothermal cure
temperature.
In the current study, the cure kinetics of an OOA BMI
prepreg system is studied using isothermal DSC techniques. An
appropriate cure kinetic model is chosen and cure parameters
are extracted using curve fitting based on nonlinear least
squares. The variation of cure kinetic parameters with
isothermal cure temperatures is investigated.
2. MATERIALS
In this study IM7G/AR4550 BMI unidirectional prepreg system
(Aldila Composite Materials) was used. AR4550 is a toughened
BMI resin system, ideal for OOA curing. The unidirectional
prepreg contains 35% resin by weight with a fiber areal weight
of 200 g/m2. It has low tack compared to epoxy prepregs and
has a shelf life of two weeks.
Figure 2. Dynamic DSC scan of BMI prepreg
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A dynamic DSC scan was conducted at the rate of 5 °C per
minute (Fig. 2). The onset temperature was found to be 145.5
°C. The exotherm contains two peaks as shown in figure x.
Isothermal cure temperatures for BMI based systems range
between 177 °C to 192 °C. The samples are postcured at
temperatures higher than 200 °C.
3. METHODOLOGY
Differential Scanning Calorimetry (DSC) can be used to
measure heat flow in and out of a sample under controlled
thermal conditions. Curing of thermoset resins are generally
exothermic in nature and therefore DSC can be used to provide
information about total heat evolved during the reaction and
glass transition of the cured sample.
Samples were encapsulated in aluminum pans and cured in
the DSC. A cure cycle similar to the manufacturer
recommended cure cycle for BMI prepreg was followed. The
sample temperature was raised equilibrated at 25 °C. The
temperature was then increased to 125 °C, the point of
minimum resin viscosity, and held constant for 60 minutes.
This is done in order to maximize the mobility of reacting
groups. The temperature was then quickly increased to the
curing temperature. Various cure temperature options are 170
°C, 180 °C, 190 °C and 200 °C. Each sample was cured for four
hours to ensure that the reaction was complete.
4. RESULTS AND DISCUSSION
The heat of reaction was calculated as the area under the
exotherm and degree of cure was calculated at time, t using
equation (1). The variation of degree of cure, α, and rate of
𝑑α
reaction, , for an isothermal cure temperature of 200 °C is
𝑑𝑡
shown in fig. 3.
The reaction mechanism follows nth order kinetics
behavior. For thermosets that follow nth order kinetics, the rate
of reaction peaks at low degrees of cure and decays over time.
The rate of conversion is proportional to the concentration of
unreacted material. Figure 4 shows the change in reaction rate
with degree of cure. In all cases, the reaction rate peaks at low
values of α. Therefore, nth order reaction models were chosen to
model the cure kinetics of the prepreg system.
Figure 4. Reaction rate vs degree of cure
Figure 5 shows the degree of cure variation with a change
in isothermal cure temperature. With a decrease in cure
temperature, the cure reaction shifts to a longer cure time. In
case of isothermal cure temperatures of 170 °C and 180 °C,
complete cure was not achieved after 2 hours. Composite
panels cured at lower temperatures require higher cure times.
Insufficient cure durations will lead to poor mechanical
properties.
Figure 5. Degree of cure vs time
Figure 3. Degree of cure vs time at 200 °C
The rate constant and order of reaction for an nth order
reaction at each isothermal temperature are obtained using
curve fitting via non-linear least curves method using
Levenberg-Marquardt algorithm.
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Table 1. Cure kinetic parameters
Temperature (°C)
170
180
190
200
k (min-1)
0.03345
0.04227
0.05814
0.04936
n
1.74
1.78
1.50
0.98
Table 1 shows the variation of rate constant of the first
order reaction with cure temperature. An isothermal cure
temperature of 190 °C is also associated with the maximum rate
constant. This is close to the recommended cure temperature for
the BMI prepreg. The rate constant drops when the temperature
is increased to 200 °C. A possible explanation for this
phenomenon is restriction in molecular mobility due to
increased crosslinking which takes place at high temperatures.
The order of reaction also reduces with an increase in cure
temperature. Similar behavior was reported by Boey, et al [15].
This is due to a change in reaction mechanism which is
dependent in isothermal cure temperature.
7. CONCLUSIONS
Curing of OOA BMI prepreg was studied using isothermal
DSC techniques. The reaction mechanism of the studied BMI
system was fit to an nth order cure kinetics model. It was found
that the rate of reaction as well as order of reaction were
dependent on cure temperature. Initially, the rate of reaction
increased with an increase in isothermal cure temperature. At
200 °C, the rate of reaction dropped due to increased rate of
crosslinking which retards the reaction. Maximum rate of cure
was found to occur at a cure temperature of 190 °C. When
lower cure temperatures are used due to tooling considerations,
longer cure times are essential to ensure good quality composite
components.
8. ACKNOWLEDGMENTS
Support from Intelligent Systems Center and Center for
Aerospace Manufacturing Technologies (CAMT) is gratefully
acknowledged. The authors also thank Gurjot Singh Dhaliwal
for his contribution to this work.
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