Accepted Manuscript
Review
Accelerated Microwave Curing of Fibre-reinforced Thermoset Polymer Com-
posites for Structural Applications: A Review of Scientific Challenges
Chinedum Ogonna Mgbemena, Danning Li, Meng-Fang Lin, Paul Daniel
Liddel, Kali Babu Katnam, Vijay Thakur Kumar, Hamed Yazdani Nezhad
PII: S1359-835X(18)30362-2
DOI: https://doi.org/10.1016/j.compositesa.2018.09.012
Reference: JCOMA 5181
To appear in:
Composites: Part A
Received Date: 8 June 2018
Revised Date: 31 July 2018
Accepted Date: 10 September 2018
Please cite this article as: Ogonna Mgbemena, C., Li, D., Lin, M-F., Daniel Liddel, P., Babu Katnam, K., Thakur
Kumar, V., Yazdani Nezhad, H., Accelerated Microwave Curing of Fibre-reinforced Thermoset Polymer
Composites for Structural Applications: A Review of Scientific Challenges, Composites: Part A (2018), doi: https://
doi.org/10.1016/j.compositesa.2018.09.012
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1
Accelerated Microwave Curing of Fibre-reinforced Thermoset
Polymer Composites for Structural Applications: A Review of
Scientific Challenges
Chinedum Ogonna Mgbemena
1,2
, Danning Li
1
, Meng-Fang Lin
1
, Paul Daniel Liddel
1
, Kali
Babu Katnam
3
, Vijay Thakur Kumar
1
and Hamed Yazdani Nezhad
1,
*
1
Enhanced Composites and Structures Centre, School of Aerospace, Transport and Manufacturing,
Cranfield University, MK43 0AL, UK
2
Department of Mechanical Engineering, Federal University of Petroleum Resources, Effurun, Nigeria
3
School of Mechanical, Aerospace and Civil Engineering, University of Manchester, M13 9PL, UK
*Corresponding author: h.yazdani-nezhad@cranfield.ac.uk, Tel.: +44 (0)1234750111
Abstract
Accelerated curing of high performance fibre-reinforced polymer (FRP) composites via
microwave heating or radiation, which can significantly reduce cure time and increase
energy efficiency, has several major challenges (e.g. uneven depth of radiation penetration,
reinforcing fibre shielding, uneven curing, introduction of hot spots etc). This article reviews
the current scientific challenges with microwave curing of FRP composites considering the
underlying physics of microwave radiation absorption in thermoset-matrix composites. The
fundamental principles behind efficient accelerated curing of composites using microwave
radiation heating are reviewed and presented, especially focusing on the relation between
penetration depth, microwave frequency, dielectric properties and cure degree. Based on
this review, major factors influencing microwave curing of thermoset-matrix composites are
identified, and recommendations for efficient cure cycle design are provided.
Keywords: microwave curing, thermosetting polymers, depth of penetration, dielectric
constant, fibre-reinforced polymer composite
Nomenclature
Parameter
Definition
Microwave radiation frequency
Cure kinetics model function
Curing rate constant in Arrhenius expression
Microwave speed in polymer
Time (duration)
Material constant (frequency factor) in Arrhenius expression
Material constant matrix
Depth of microwave penetration
Activation energy in Arrhenius expression
Electric field intensity
Conjugate of electric field strength
Concentration level
Average power
Microwave energy
Gas constant (=1.987 cal K
-1
mol
-1
or 8.314 J K
-1
mol
-1
)
Absolute temperature in Kelvin (K)
Cure temperature
Glass transition temperature
Degree of cure
Strain tensor
Permittivity of free space (8.8514×10
-12
F/m)
Composites Part A: Applied Science and Manufacturing, Volume 115, December 2018, pp. 88-103
DOI:10.1016/j.compositesa.2018.09.012
Published by Elsevier. This is the Author Accepted Manuscript issued with: Creative Commons Attribution Non-Commercial No Derivatives License (CC:BY:NC:ND 4.0).
The final published version (version of record) is available online at DOI:10.1016/j.compositesa.2018.09.012. Please refer to any applicable publisher terms of use.
2
Relative dielectric constant
Effective loss factor or relative loss factor
Effective conductivity
Stress tensor
Loss tangent coefficient
Contents
1 Introduction .................................................................................................................... 2
2 Fibre-reinforced thermoset composites cure .................................................................. 4
2.1 Thermoset polymer cure ......................................................................................... 4
2.2 Cure of high performance structures ....................................................................... 5
2.3 Kinetics models for efficient cure cycle design ........................................................ 6
2.4 Quantification of cure state in a microwave curing process ..................................... 7
3 Accelerated curing of thermoset polymers and composites ............................................ 9
3.1 Existing methods..................................................................................................... 9
3.2 Evidence on challenges with microwave thermoset polymer and composite cure . 11
3.3 Dielectric properties of composites........................................................................ 14
3.4 Microwave heating based on dielectric properties ................................................. 15
4 Challenges with microwave curing for structural applications ....................................... 16
4.1 Non-uniform microwave curing evidences ............................................................. 16
4.1.1 Non-uniform microwave curing in dissimilar material systems ........................ 16
4.1.2 Non-uniform microwave curing at constant microwave power ........................ 17
4.1.3 Non-uniform microwave curing of thick structures .......................................... 18
4.2 Scientific challenges: ............................................................................................ 19
4.2.1 Challenge 1 – Thermal conductivity of resins: ................................................ 19
4.2.2 Challenge 2 – Microwave shielding: ............................................................... 19
4.2.3 Challenge 3 – Microwave arcing: ................................................................... 19
4.2.4 Challenge 4 – Microwave cavity: .................................................................... 20
4.2.5 Challenge 5 – Microwave radiation characteristics: ........................................ 22
5 Design considerations for efficient microwave curing ................................................... 23
6 Concluding remarks ..................................................................................................... 25
Acknowledgements
References
1 Introduction
Advanced Composites provide opportunities to tailor material properties and manufacturing
processes in order to achieve ultra-light, high-performance, environment-friendly engineering
structures—reducing fuel consumption and emissions and helping combat climate change
[1]. Polymer composites are playing an indispensable role in different industries. Fibre-
reinforced polymer (FRP) composites are light-weight materials with high strength-to-weight
ratio and their properties can be tailored for modern high performance structural applications
3
allowing for efficient engineering solutions to severe and varying operating conditions e.g.
dynamic and impact events on composite aircrafts [2, 3]. Different types of fibres and
polymers ranging from glass to carbon fibres are currently being used in composites
materials. The aerospace sector currently holds the largest share (nearly 40%) of the global
composite market with its distinctive certification requirements for advanced high
performance composites [4, 5] which makes it one of the top markets for carbon fibre use.
Superior rigidity and functional properties distinguish high performance polymer composites,
usually reinforced by carbon fibres, from other composites with their intense use for
transport, energy efficiency, property tailoring. The global polymer composites market is
expected to generate an income in excess of £30 billion in 2021 at the compound annual
growth rate of 5.1% from 2016 to 2021 [6], with current global end-product market size of
£70 billion as estimated in [7] and market share of Europe, North America and Asia at 24%,
37% and 32%, respectively [8]. The composites industry has experienced high penetration
into key markets, including automotive and aerospace sectors, and has been identified as
one of the key pervasive technologies for future manufacturing according to the UK
Department for Business, Innovation & Skills [9, 10]. These factors are the main driving force
behind the ongoing global substitution for metals in order to mitigate the attendant problem
of climate change [11, 12]. This is more evident in aerospace sector than other sectors,
which is the major source of stratospheric pollution [13, 14] e.g. via ten tons of CO
2
emission
per 1000km (approximately per one hour flight) [15]. The aerospace industry provided ample
opportunities for the expanding maintenance, repair and overhaul (MRO) market with the
introduction of new aircrafts such as the Boeing Dreamliner 787, Airbus A350 XWB, and
Bombardier CS-100, with more than 50% composites by weight being used in their primary
structure [16].
Moreover, growing demand for polymer composites in critical structures gave rise to an
urgent need for a reliable and rapid composite repair and fastener-less joining, e.g. adhesive
bonding especially in the maintenance, repair and overhaul (MRO) sector in aerospace [5]. It
is currently accepted that efficient polymer processing in composites and bonded joints are
paramount to critical structural applications as they offer flexible geometry, property tailoring
and improved strength-to-weight ratios which lead to improvements considering
environmental impact [11, 12]. In adhesively bonded structures, the quality of the bonded
composites has a strong relationship with the variabilities caused by process parameters
such as temperature, curing duration and rate [17]. Therefore any variation in curing of
thermoset polymers can potentially behave as defect precursor to the polymer integrity in the
form of either weak (low-strength) or kissing bonds [18-20]. The latter one can dramatically
lower down the adhesion properties [18]. Structures containing zero-thickness bond
deficiencies which could be a result of improper cure are difficult to assess. This becomes
important in relation to large bond areas where non-uniform temperatures and curing may
exist.
Rapid curing as a tool for energy saving and mass production is not new: various types have
been explored for industrialisation such as radiation cure at ambient temperature (electron
beam, UV) and at high temperature (infrared, laser, microwave) [21, 22], and conventional
conduction/convection thermal curing (induction, ultrasonic, resistance heating). However,
curing process-induced degradation seems inevitable no matter what technique is used
implying that a fully controlled processing cycle should be designed that also depends on the
geometric features and composite material (e.g. carbon and epoxy). For instance, the
presence of weak bonds introduced by rapid conventional curing, at twice the specified rate,
can lead to >40% reduction in the ultimate failure load of adhesively bonded single-lap joint
with 25mm x 25mm bond area [18, 23]. This drawback can become critical in structural
applications and a barrier to the growth of rapid polymer processing e.g. in aerospace MROs
where the average duration for in-service composite repair to perform successfully at the
flight line takes 15 hours according to Commercial Aircraft Composite Repair Committee
(CACRC) [24]. The presence of fibre reinforcements such as carbon can also influence
temperature gradient through the thickness and thus the depth of penetration, which can
become widespread in a rapid conventional heating. If a controllable accelerated curing
4
process with high depth of penetration (e.g. microwave) at a structural scale is developed via
controlling this gradient, it could have a significant economic impact on composites
manufacturing and operations (e.g. aircraft manufacturers and airlines), particularly those
running short duration domestic flights [25].
This review will focus on the identification of major challenges with energy-saving high-
volume microwave processing technique for thermoset composites via addressing the state-
of-the-arts in academic researches and industrial applications. A framework to address the
underlying knowledge behind accelerated microwave curing of polymer composites in critical
structures is lacking in many review articles which has been central to shaping the current
review. Microwave processing has frequently been presented as a means of rapidly
heating/curing resins or FRP composites in a highly homogenous volumetric manner when it
is compared to conventional heating [26-28]. It therefore appears well-recommended to use
for the curing of resins either in isolation or as part of reinforced composites during
production, or as a means of curing sections of polymer bonds, and therefore for bonded
joints. However, the mechanical performance of resins can deteriorate due to microwave
exposure [29-33] and the presence of carbon fibres that prevent penetration, suggesting that
microwave is not currently a reliable means for the manufacturing of high performance
composite structures. These issues with the quality of microwave processed composites, in
association with variabilities caused by curing parameters (e.g. radiation parameters, curing
time and temperature) dictate a requirement for fully controllable microwave curing.
Therefore, the information collated by this review will be analysed to determine the
interactions between microwave energy, temperature gradient, penetration depth and the
curing state so as to gather principle scientific basics for design of a microwave curing
procedure. The analysis will be used for the development of concluding remarks for
proposing an in-situ controlled rapid microwave processing.
2 Fibre-reinforced thermoset composites cure
2.1 Thermoset polymer cure
The term ‘curing` in thermosetting polymers refers to the transition of liquid resin and
hardener components to a solid vitrified material [34]. Curing is initiated when the
components are stoichiometric and physically mixed together. Polyester, phenolic and epoxy
resins are among the mostly used thermoset polymers with epoxy popularly used in high
performance composite structures.
Epoxy is mainly referred to as a chemical group comprised of two carbon atoms bonded to
an oxygen atom. Epoxy curing is an exothermic process where heat is released during the
chemical reaction [34-36]. The heat level applied to the epoxy mixed with hardener cure
accelerator is related to the strengthening of the material by cross-linking of polymer chains
which can be achieved by conventional heating, electron beams, chemical additives or
accelerated curing (e.g. microwave, radiofrequency, ultra-violet radiation [22]). Simple epoxy
is a three-member ring structure which is referred to as alpha-epoxy or 1,2-epoxy with R as
C-H groups as is shown in Figure 1(a). Epoxy molecules are normally neutral electrically
however possess a dipole moment (uneven electric charge) with partial negative charge
() at oxygen side and consequently positive () at the other sides.
![Figure 5: Schematic illustration of dipolar polarisation versus ionic polarisation as two different dielectric heating mechanisms (e.g. in an epoxy polymer) occurring due to low and high electromagnetic radiation, respectively [88]](/figures/figure-5-schematic-illustration-of-dipolar-polarisation-2uckwb5k.webp)
![Figure 10: Temperature profiles for carbon fibre-reinforced epoxy composite laminates at different power levels during microwave heating [90] .](/figures/figure-10-temperature-profiles-for-carbon-fibre-reinforced-5wuox13o.webp)