Microwave processing of carbon fibre polymer composites: a review

Abstract

Microwave processing of carbon fibre-reinforced polymer (CFRP) composites is gaining increasing attention as an alternative to conventional thermal processing techniques. It has the potential to reduce part processing times and alleviate the bottleneck that often exists in composite component production. The present study reviews the current research and the state-of-the-art of microwave processing of CFRP composites. The benefits and limitations of microwave processing compared with traditional thermal processing methods are examined. The mechanical properties of microwave-cured composites are compared against conventionally-cured counterparts with varying levels of success. Current industrial microwave heating technologies are also examined. It is evident that microwave processing of polymer composites is promising but has notable limiting factors which have stunted its uptake in industry. Due to their prevalence in literature, CFRP composites with thermosetting matrices are the focus of this review. However, potential applications to carbon fibre-reinforced thermoplastics are also briefly described.

Introduction

Carbon fibre-reinforced polymers (CFRPs) in recent decades have gained significant attention and are now widely used in the aerospace industry and other high-performance industries. The high cost and long process times involved in curing thermosetting CFRPs in the conventional fashion using an autoclave has led to an interest in the development of alternative curing methods that are commonly referred to as ‘out-of-the-autoclave’ (OOA) methods. Microwave processing is one such OOA technique being investigated. ¹ Microwave heating has been in existence for some 60 years; however, the microwave processing of composite materials is a relatively new concept that has potential applications in a wide variety of industrial fields. ²

A significant amount of recent research has been conducted into the use of microwaves to cure CFRPs and similar material systems. ^{3 -13} However, the use of this technology remains limited and will most likely remain this way for at least the near future until numerous challenges are overcome. ^5,14,15 While traditional thermal approaches such as autoclave curing are currently more established, microwave processing is poised to play an increasingly important role in composite part fabrication. ¹⁶

This article examines the research and development of microwave processing of CFRPs based on thermosetting matrices. This review first examines the background of microwave processing with respect to the curing of CFRPs and then explores its benefits, limitations and effects on mechanical performance. Industrial applications and the applications of microwave processing to CFRPs with thermoplastic matrices are also discussed.

Microwave curing mechanisms

Microwaves form part of the electromagnetic spectrum (Figure 1) and have frequencies ranging from 300 MHz to 300 GHz. In general, microwave furnaces are comprised of three major parts: the microwave source (typically a magnetron), the transmission lines and the applicator. In short, the source produces the microwaves, the transmission lines transmit the microwaves to the applicator and the applicator transfers the microwave energy to the material being heated. Applicators determine how the microwaves will be distributed throughout a material and are the critical component for achieving properly cured composites. Types of applicators include: waveguides, single-mode cavities and multi-mode cavities. Single-mode applicators produce predictable non-uniform fields that will create one hot spot. Multi-mode applicators are used in domestic microwave ovens and produce multiple hot spots.

Figure 1.

Electromagnetic wave spectrum. ¹

Microwaves as a source of heating

Microwaves heat dielectric materials, such as thermosetting polymers, via an electromagnetic field interaction with the material on a molecular level. The microwaves penetrate the material thickness and transmit energy through the process of polarization. There are numerous kinds of polarization. Electronic polarization involves the displacement of electrons from their equilibrium position in relation to the nucleus. Atomic polarization occurs when different kinds of atoms in a molecule move relative to each other. Dipole polarization involves the rotation of molecular dipoles to match the applied electromagnetic field. Resistance to this reorientation creates friction in the material and, in turn, creates thermal heat. Interfacial (Maxwell–Wagner) polarization involves a build-up of charge at a material interface. Dipole polarization and interfacial polarization are the two most significant mechanisms for energy transfer in composite materials. ¹⁷

The dielectric properties of a material ultimately govern the level of polarization. The dielectric constant quantifies the capacitive part of the dielectric response, while the dielectric loss factor quantifies the conductive part of the material response. Materials such as carbon fibres, which have high dielectric loss factors, consequently have a low penetration depth of microwaves. ¹⁴ Therefore, these materials are said to act as reflectors of microwaves. On the other hand, materials with a low dielectric loss factor have a high penetration depth, which in turn means they absorb little energy from microwaves. As a result, materials such as thermosetting polymers, which are in the middle of the conductivity range, are most effectively heated by the transfer of energy through microwaves (Figure 2). This differs from conventional heating where energy is passed most effectively into high conductive (high dielectric loss) materials. ¹⁷

Figure 2.

Relationship between dielectric loss factor and the ability of a material to absorb microwave energy, adapted from Thostenson and Chou. ¹⁷

Thermosetting resins undergo crosslinking during the curing process causing the dielectric properties of the resin to change at a molecular level. The changes in dielectric properties are proportional to the changing viscosity of resin. At the beginning of the curing process, resin viscosity is relatively low, and the dielectric properties cause the resin to couple well with the microwaves as the dipoles of the resin are relatively free to move and align with the electromagnetic field. As crosslinking takes place, the resin becomes more viscous and this causes the dielectric loss factor to decrease. At this stage, the thermoset will become more transparent to microwaves as the molecular dipoles are less free to orientate themselves to the electromagnetic field, making the material more resistant to microwave heating. The variation in dielectric properties throughout the curing process can be exploited via in situ monitoring of the cure to optimize the use of microwaves during processing. ^10,17 To this end, Zhang et al. ⁸ developed an off-line method of dielectric measurement that can be applied to the microwave curing process.

While there are other bands of electromagnetic radiation that could potentially be used to heat and cure thermosetting polymers, microwaves are the most appropriate. This is because microwaves can penetrate the polymer effectively, while not posing a significant health hazard like some other electromagnetic waves such as gamma radiation. ¹⁸

The microwave effect

There is some debate about the precise mechanisms involved in the microwave curing of thermosetting polymer composites. ¹⁹ Some researchers have suggested microwaves do not change the curing reaction, rather just increase the rate of reaction through volumetric heating. ²⁰ For example, research by Mijović et al. ²¹ and Mijović and Wijaya ²² compared the mechanisms and rate of reactions during thermal and microwave curing of numerous epoxies, polyimides and bismaleimides. The curing reaction was monitored through in situ remote near-infrared spectroscopy. It was proposed that the microwave field did not accelerate the kinetics of the curing process and that the kinetics of both processes remained the same. The difference in curing time noted in the tests was attributed to the different temperature distribution caused by volumetric heating from the electromagnetic field.

Many contradict this notion by suggesting microwave curing contributes some kind of non-thermal phenomena, widely known as the microwave effect. ¹⁷ For example, Mijović and Wijaya ²² found that microwave curing of an epoxy produced a broader glass transition range compared with a thermal cure. They believed this could have been caused by a difference in the curing mechanism. The presence of the microwave effect is often linked to the initial accelerated cure that can occur when the viscosity of the thermoset is still relatively low. As previously noted, during this period of low viscosity the thermoset more readily couples with the electromagnetic field causing the material to heat rapidly which in turn accelerates the curing process. There are even instances noted in the literature where rapid crosslinking from microwave processing has trapped unreacted functional groups inside a rigid molecular network. ¹⁸

In general, many unexpected experimental results are often attributed to the microwave effect. Critics of the microwave effect state that poor temperature measurement and lack of control of the experimental conditions can produce errors which are mistaken for the microwave effect. ¹⁷ Wang and Liu ¹⁸ state that there is a lack of direct evidence to distinguish the thermal and microwave polymerization mechanisms since estimations have been constructed by measuring the glass transition temperature or curing degree. For example, Degamber and Fernando ²³ and Degamber et al. ²⁴ have used low-cost optical fibre sensors to monitor the temperature of an epoxy resin during a microwave cure process. Such sensors may not be sensitive enough to contribute to the understanding of the microwave effect. More research is required to clarify the existence and impact of the microwave effect during the curing of carbon fibre composites.

Benefits

Thostenson and Chou ¹⁷ have reviewed many of the fundamental advantages of using microwaves to cure CFRPs. Polymers typically have low thermal conductivities, and this creates long processing times as heat is conducted slowly throughout the material thickness. This can often produce an uneven temperature distribution in the composite and in turn leads to a non-uniform cure. Because microwaves penetrate the volume of a polymer in depositing energy, it is possible to achieve uniform volumetric curing of the polymer which can increase overall part quality. By interacting with the polymer at a molecular level, microwaves can dramatically increase the rate of the curing reaction, thus reducing the total processing time. The reduction in processing times and the associated energy savings have the potential to create significant cost savings in the production of CFRPs, compared with other traditional processing methods like autoclave curing (Figure 3). Improved processing and shorter process times of carbon nanotube-reinforced polymers are also possible. ^26,27 Moreover, during microwave curing only the product in the chamber is heated, not the chamber itself; hence there is no cool-down time required for the microwave oven.

Figure 3.

Relative costs plotted against production rate for several common composite processing techniques and the estimated space of microwave processing. Plot created with CES Edupack 2018. ²⁵

Microwave curing of CFRPs can also create superior adhesion of the fibre–matrix interface and this has been widely reported. ² Drzal et al. ²⁸ have attributed this to selective heating of the carbon fibres. The fibres selectively couple to the microwaves then heat the surrounding polymer via conduction; thus, producing superior interfacial properties. This effect has also been reported in the microwave curing of glass fibre polymer composites. ²⁹ Another distinct advantage of microwave curing is the ability to switch the curing reaction off and on, which is particularly useful in creating hinged profiles during pultrusion. ³⁰ Microwave processing has been further studied in the context of pultrusion processing of carbon fibre composites to increase production rates and reduce energy consumption. ³¹

Limitations

Despite the potential advantages of using microwaves in the curing of polymer composites, the technology has not yet been widely accepted by the industry. Hay and O’Gara ³⁰ have attributed this to equipment issues; the lack of suitable microwave transparent tools for use in the process. This notion is supported by a study into costs of using alternative curing methods in the production of CFRPs. ³² The study contradicted thinking that microwave curing can provide a cost benefit over conventional thermal curing. It was shown that while microwave curing did produce faster part cycle times, it could not compensate for the high equipment costs to implement the process. The process requires higher cost consumable materials (breather clothes, bagging mats and sealers) when compared to a conventional autoclave. Moreover, the size of the working chamber is significantly smaller than what is available in conventional autoclaves. It was suggested that if microwave curing is going to be adopted by the industry, its associated costs need to be reduced. To this end, Hatori et al. ³³ proposed an open microwave heating system of polymer resin using interdigital electrode array film and dispersed carbon nanotubes. This system was built to address the high costs and the limits on the size of curable products associated with enclosed-type microwave ovens. To address tooling issues Nuhiji et al. developed a CFRP tool that was successfully utilized to cure CFRP panels in the laboratory and industrial microwaves. The conductive carbon fibres in the tool facilitated the fast heat transfer across the part.

Some suggest that the overall energy consumption of the microwave process is relatively high when compared to a conventional thermal oven; however, this was thought to have little impact on the overall cost. ³² The initial investment cost of the microwave oven is high, which is another significant disadvantage to the process. ⁵ The heating behavior of new complex tool geometries typically need to be tested to be fully understood; highlighting the need for process monitoring and simulation tools, which has been the focus of recent research. ^3,34

Early research suggested only relatively thin unidirectional CFRP laminates could be cured using microwaves and this is a significant pitfall. ^35,36 This was attributed to the high dielectric loss of the carbon fibres that cause the microwaves to be reflected and thus leaving the middle plies of a thick laminate uncured. The selective coupling of the microwaves to the carbon fibres can produce hot spots (Figure 4) and electrical arcing. ⁵ While this can increase interfacial properties, it can also cause a non-uniform cure. It has also been shown to burn holes in vacuum bag consumables. ³⁷ More recent work has shown that it is possible to cure thick CFRP cross-ply laminates using single-mode microwave cavities. ³⁸ It should be noted that the problem of lack of penetration through the thickness is not as prevalent in curing glass fibre/epoxy composites. ^{3,6,39 -41} For example, Thostenson and Chou ³⁹ have shown that microwaves can produce a superior cure over a conventional autoclave for a one-inch thick glass fibre/epoxy composite.

Figure 4.

Schematic of hot spot formation within CFRP laminates that can occur with microwave curing, adapted from. ⁵

In general, close monitoring and control of process parameters are required to produce a uniform cure of CFRP laminates with microwaves. ^{3,14,16,42 -44} This is because a uniform cure in the microwave process relies upon the electromagnetic field being uniform, which can be difficult to achieve. ^14,39 The high electromagnetic power heating autoclave inset oven system (HEPHAISTOS) microwave oven ⁴⁵ (further discussed in the Industrial Applications section) overcame the non-uniform electromagnetic field issue using a hexagonal heating chamber and 12 independently temperature-controlled microwave antennas. Other studies have suggested that another solution to the non-uniform cure caused by non-uniform electromagnetic fields within the microwave chamber is to combine microwave heating with conventional heating using heated tooling. ^46,47 However, this method is likely to be less effective for thick components. In many conventional microwave ovens, the adverse effect of the non-uniform electromagnetic field within the chamber is often reduced by rotating the component being processed on a turntable or with rotating reflectors used to change the distribution of the electromagnetic field in the chamber. These methods attempt to average the power distribution, ¹⁷ although not always practical, especially in industrial-size ovens used to cure large components.

The microwave curing process can produce CFRPs that have a significantly high void content which can cause poor mechanical performance in comparison to conventionally-cured CFRPs, especially when there is a lack of pressure in the process ⁴⁸ (see Table 1). There is also some safety concern surrounding the potential leakage of microwaves during the curing process. ⁴⁴

Table 1.

Void contents observed in CFRP composites cured by autoclave and microwave. ⁴⁸

Type of CFRP composite	Void content (%)
System 1 – autoclave	0.1
System 2 – autoclave	0.1
System 1 – microwave	19.3
System 2 – microwave	8.9

CFRP: carbon fibre-reinforced polymer.

Mechanical performance

For microwave curing to be a viable substitute for a conventional cure, it is critical that the process produces components with comparable mechanical properties. Previous studies have examined tensile, compression, flexural and interlaminar shear properties of microwave-cured CFRPs. The mechanical properties of microwave-cured CFRPs depend on both process and material-related factors ⁵ (Figure 5). The different microwave process parameters and material systems used in each of the previous studies have caused both positive and negative effects on the mechanical performance of CFRPs. These are discussed in this section of the review.

Figure 5.

Factors that contribute to the mechanical performance of CFRP cured with microwaves, adapted from Mgbemena et al. ⁵

Lye and Boey ⁴⁴ noted the potential to produce CFRPs with superior tensile strength in relation to the extent of cure using microwave processing (Figure 6). This potential largely arises from a uniform cure via volumetric heating in the application of microwaves. However, this potential is often not realized, and this was attributed to the creation of voids from electrical arcing and hot spots in the laminates. Voids can also be created from the presence of moisture in the thermoset. ⁴⁹

Figure 6.

Plot of tensile strength against extent of cure for a thermally-cured and a microwave-cured composite, adapted from Lye and Boey. ⁴⁴

Balzer and McNabb ³⁷ studied the effect of microwave curing on the tensile strength of woven CFRPs. It was shown that only 30 min was needed to fully cure the CFRP using microwaves, in comparison to 180 min required by the autoclave. The cured specimens were tested in tension. The maximum tensile strength of the autoclave cured specimens was significantly higher than the microwave-cured specimens (Figure 7). This was attributed to the large void content that was present in the microwave-cured specimens.

Figure 7.

Comparison of the tensile strength of CFRP cured in an autoclave and a microwave at different temperatures, adapted from Balzer and McNabb. ³⁷

Hang et al. ⁵⁰ showed that microwave-cured composites can have similar curing kinetics as the conventional thermally-cured ones, and that they can exhibit improved interlaminar shear and flexural strengths, but slightly lower tensile and compressive strengths. Improved interlaminar shear and compression strengths were also reported by others ^{4,51 -54} (see Table 2 and Figure 8). Fang and Scola ⁵³ investigated the mechanical properties of a unidirectional carbon fibre-reinforced phenylethynyl-terminated polyimide cured using microwaves. Specimens cured in the microwave were subjected to a vacuum and pressure via use of a custom mould. This process produced low void (less than 0.5%) content in all specimens. The mechanical properties of the microwave-cured specimens were compared to conventionally-cured specimens. Higher flexural strength, moduli and apparent laminar shear strength were achieved in the microwave-cured composites. Day and Samoladas ⁵⁵ and Day et al. ⁵⁶ used Raman spectroscopy to investigate the micromechanics of model Kevlar–epoxy and carbon–epoxy composites following microwave and conventional curing. The results indicated that interfacial shear strengths of the microwave-cured composites were similar to thermally-cured ones. Similarly, Meyer ⁵⁷ observed similar shear strengths in CFRPs cured with microwaves and an autoclave and Zhou et al. ⁵⁸ observed enhancements in interlaminar fracture toughness of carbon fibre/bismaleimide composites processes with microwave curing. Xu et al. ^59,60 observed comparable flexural strength and interlaminar shear strength of carbon fibre/bismaleimide composites cured with microwaves and conventionally in an oven. These studies indicate that with the right process parameters and material systems, it is possible to produce microwave-cured CFRPs with similar or even superior mechanical properties compared to conventionally-cured counterparts. However, if the microwave process parameters are not optimized for a particular CFRP material system, defects (such as voids) can be introduced and these will have an adverse effect on the resultant mechanical performance of the composite.

Figure 8.

(a) Flexural strength and (b) flexural modulus of CFRP cured using conventional heating and microwave heating with VARTM, adapted from Shimamoto et al. ⁵¹

Table 2.

Compression and interlaminar shear strengths of CFRP composites cured by autoclave and microwaves.⁴

Mechanical property	Process	Value (MPa)
Compression strength	Thermal cure	403 ± 9
	Microwave cure	422 ± 26
Interlaminar shear strength	Thermal cure	47 ± 8
	Microwave cure	50 ± 4

CFRP: Carbon fibre-reinforced polymer.

Industrial applications

The most prevalent technology currently used industrially in the microwave curing of CFRPs is the HEPHAISTOS microwave oven (Figure 9(a)). The system has also been used in several recent research studies. ^14,15 Developed by the Karlsruhe Institute of Technology, the HEPHAISTOS system is based on a hexagonal applicator, which is used to produce a uniform microwave field. The major components of the HEPHAISTOS system are depicted in Figure 9(b). Originally, the system employed an operating frequency of 30 GHz; however, in 2002, computational simulations were used to help scale the frequency of the system down to 2.45 GHz. ^61,62 Application of the 2.45 GHz frequency allows for the energy-efficient processing of relatively large CFRP components. The power ranges from 5% (i.e. 510 W) to 100%, with a minimum resolution of 0.1%. It has an internal hexagonal chamber, with a diameter of 1 m and a depth of 1 m.

Figure 9.

(a) Photograph of the HEPHAISTOS microwave oven supplied by Weiss Technik, Germany, (b) Major components of the HEPHAISTOS system, adapted from Feher, Drechsler and Filsinger. ⁶¹

Initially, two potential directions for the HEPHAISTOS were considered: a standalone system and an upgrade for an existing autoclave system via the addition of magnetron units. However, the standalone system was favored since it does not require pressurization which is a significant contributor to the cost-intensive operation of an autoclave. ^61,62 Once the HEPHAISTOS system had achieved the desirable 2.45 GHz frequency, the technology was patented and is now licensed to Weiss Technik.

The hexagonal working chamber of the HEPHAISTOS produces a relatively uniform microwave field, compared to the field produced by a circular chamber ⁶³ (Figure 10). Conventional metal tools can be used in the working chamber, meaning microwave transparent materials are not required in material processing. ⁶³ Fibre optic sensors in the chamber allow for temperature measurement. No arcing occurs at the carbon fibres during processing. ⁶²

Figure 10.

Relatively uniform microwave field produced by (a) hexagonal HEPHAISTOS applicator and (b) a circular applicator, adapted from Feher et al. ⁶²

GKN Aerospace has performed experiments with the HEPHAISTOS microwave oven to evaluate the microwave curing process of thin (4–5 mm thick) thermosetting CFRP aircraft components. ⁴⁵ Tests were done on three different OOA prepegs: MTM-44-1 (Advanced Composites Group), M56 (Hexcel) and Cycom 5320 (Cytec Engineered Materials). Thermocouples on the part and tool have been used to monitor the temperature during the microwave process. They showed that temperature (and therefore extent of cure) can be manipulated in different sections of the part through shielding. An 80% reduction in energy used compared to autoclave curing and a reduction of 40% of the total cycle time were reported. However, more work was needed to properly benchmark the performance of the microwave-cured components. ⁴⁵ Significant challenges have arisen in using the microwave oven. The choice of microwave compatible consumables and tuning of the process to be compatible with complex part geometries were deemed significant obstacles that are especially pertinent to aerospace structures. ^5,45

The technology employed by the HEPHAISTOS microwave oven overcomes many of the previous setbacks involved in the microwave curing of CFRPs and suggests this type of technology represents the way forward for industry adoption. Moreover, the HEPHAISTOS microwave oven can also be used with traditional metal tools, overcoming the issue of the need for microwave transparent non-metallic tooling.

Applications to thermoplastic composites

A limited amount of research has been performed on the application of microwaves in processing carbon fibre-reinforced thermoplastics. Ku et al. ⁶⁴ demonstrated that carbon fibre-reinforced thermoplastics can be processed using variable frequency microwaves (VFM). By cycling frequencies between 2.5 GHz and 18 GHz into the microwave applicator, it is believed that a uniform field can be produced without arcing occurring at the carbon fibres. VFM experiments were performed on the processing of carbon fibre-reinforced low-density polyethylene, with a 33% weight fraction of fibres. It was found that the optimum frequencies to process the material were 8.5–9 GHz, and 10–12 GHz. These frequencies produced the least amount of reflectance of the microwaves by the carbon fibres, and therefore the most effective heating of the material. It was also shown that stable component dimensions could be attained using the VFM process and that process temperature could be controlled via an online computer. However, the high cost and power consumption of a high power VFM facility mean they are not particularly suitable for large-scale industrial applications. ⁶⁴

Benítez et al. ⁶⁵ have successfully processed carbon nanofibre-reinforced high-density polyethylene composites using microwaves. Specimens with a varying weight fraction of carbon nanotubes were heated using a conventional domestic microwave oven (550 W, 2.45 GHz). The specimens with a weight fraction of 10% or less of carbon nanotubes were successfully processed without a reduction in mechanical or dielectric properties. The specimens with over 10% weight fraction of carbon nanofibres had reduced mechanical properties after microwave processing. In particular, the failure strain was reduced by at least 50%. The study shows that the microwave processing of carbon nanofibre thermoplastics can be effectively achieved. However, there is still a lack of understanding surrounding the effect of microwaves on the carbon nanofibres and the existence or non-existence of arcing between nanofibres.

Concluding remarks

Microwave curing processes have the potential to increase the build rate of thermosetting CFRP components and alleviate the bottleneck that can exist in composite component production lines. It also has the potential to reduce manufacturing costs while maintaining or potentially improving part quality. However, for this potential to be realized, it is critical that current costs around the process be reduced and understanding of the process be improved. It is also imperative that the mechanical performance of components processed with microwaves can match that of conventionally-cured components, which to date has proved challenging. For the mechanical properties of components produced by microwave processing to be comparable to conventionally-cured CFRPs, the curing process needs to be performed with careful monitoring and control of the process parameters.

Many academic institutions are currently using domestic or commercial microwave ovens to test the performance of microwave-cured CFRPs against thermally-cured counterparts. This is producing inconsistent results as the quality of the components produced is highly dependent on the quality of the production process. While an industrial scale microwave curing system is available in the HEPHAISTOS microwave oven, limited institutional research has been performed using this technology. This technology overcomes some of the drawbacks involved in the microwave curing of CFRPs and may represent the way forward for industry adoption of this technology.

The full potential of the microwave curing of CFRP composites has not yet been realized. More research is required, particularly with the microwave ovens capable of producing large scale real-world components, rather than relatively small test coupons. Furthermore, research on process control and optimization to improve the cure quality and repeatability would be advantageous.