Curing Epoxy Resins by Photopolymerization and Their Use in Composites

Today's search for competitive and cost-effective products has increased interest in composite materials, which combine lightness and high mechanical strength and offer a suitable alternative to traditional materials such as metals and ceramics. The applications of composites have expanded across many sectors, from aviation to maritime, automotive to sports equipment. However, the major factor limiting the transition from steel to composite materials is the currently high production costs of composites. In recent years, significant work has been undertaken to make composite materials more competitive. Most composite materials are obtained as a result of thermal curing (hardening) of the matrix material. This reaction occurs either at high temperature with high energy consumption or at ambient temperature over fairly long periods [1].

Demand from sectors using composite materials for mass production has paved the way for the development of new curing mechanisms.

Photopolymerization is an effective, economical and environmentally friendly method that enables rapid conversion of liquid resin systems into cross-linked solid polymers through light energy. Light-cured formulations generally consist of multifunctional monomers and oligomers containing small amounts of photoinitiators that produce reactive species when exposed to ultraviolet (UV) light. In terms of polymerization mechanisms and reactive species, light-cured resins can be divided into two main categories: free radicals that initiate polymerization of monomers such as acrylates or unsaturated polyesters, and cations that initiate polymerization of multifunctional epoxies and vinyl ethers [2]. Advantages of light curing compared with thermal curing: • Reactions that take hours at ambient temperature can be completed within minutes, • Low total energy consumption because heating is not required, • Use of inexpensive equipment and low thermal stresses. Furthermore, because photopolymerization is a reliable and low-hazard method, it is the preferred option for many applications. Due to these advantages, light curing is used not only for applications such as rapid drying of thin polymeric films, coatings, paints, printing inks and adhesives, but also in the production of printing plates, microelectronics and optical discs. Other applications include dental prosthetics and rapid prototyping using stereolithography [2].

Figure 2. Industrial-scale UV curing machine [4]

The use of light in composite materials is an interesting and novel technology that has gained popularity in recent years. While first commercial applications were introduced in the 1970s using resin for dental applications, UV light-cured composites were first reported in the early 2000s, and since then very little progress has been seen in developing innovative applications [1]. The greatest difficulty in using light for composite production is the requirement for photons to penetrate the material to generate reactive species. The presence of fibers as the reinforcement phase can reduce light transmission, further exacerbating this problem. For this reason, while the light-curing mechanism is an excellent technology for coating or thin-film applications, more work is required for the hardening of thicker materials. Literature reports applications where unsaturated polyester, vinylester and acrylates are reinforced with glass fibers and light-cured [1,5,6], but studies on light curing using epoxy resins and fibers together are quite limited.

Photopolymerization of Epoxy Resins

Epoxy resins are high-performance thermoset-based polymeric materials used in many industrial sectors. The epoxy resin market had a value exceeding USD 7.5 billion in 2015, and is typically formulated as two-component systems that are cured thermally in the presence of chemicals such as amines and anhydrides. These formulations are known to have short shelf life and long curing times [7]. Among the broad chemical family of epoxy resins, the most commonly used epoxy type is diglycidyl ether bisphenol-A (DGEBA) based [8].

Figure 3. Chemical representation of DGEBA-based epoxy resin

DGEBA-based epoxy resins can be cured by cationic photopolymerization as an alternative to thermal curing. To initiate cationic photopolymerization, cationic photoinitiators, referred to as photoacid generators (PAGs) (such as triarylsulfonium or diaryl iodonium salts), must be used [9].

Figure 4. Chemical structures of the most frequently used photoinitiators in cationic polymerization. R groups can be H, CH3, CH3O, Cl and NO2 [10]

When these photoinitiators are activated by UV light, they split and produce strong Bronsted acids (HX). These generated acids initiate cationic ring-opening polymerization at the chain ends of the epoxy resin [11]. At the end of the reaction, the structure cross-links and curing (hardening) occurs (Figure 5).

Figure 5. Photopolymerization of epoxy resin in the presence of onium salts.

X is the counteranion and is typically SbF6-, PF6-, or BF4-.

However, besides the high energy of UV light, the short wavelength prevents deep penetration, limiting the photopolymerization reaction to a thin epoxy layer (100-200 μm). Additionally, the presence of fillers or fibers prevents light from penetrating deeper layers [11]. For this reason, using only photopolymerization for thick epoxy resin applications containing reinforcement/filler components is not possible. Recent research has shown that the radical-initiated cationic frontal polymerization (RICFP) method is an effective way for energy-efficient and rapid curing of DGEBA-based epoxy resins throughout their thickness [9,11]. This technique has been shown to overcome typical problems such as short shelf life, limited layer thickness and energy-intensive thermal curing [8].

Radical-Initiated Cationic Frontal Polymerization (RICFP)

The decomposition of photoinitiators can occur either with ultraviolet light or through a redox reaction of a suitable radical; this reaction is referred to as radical-initiated cationic polymerization (RICP). Several basic research articles have been published on this topic [12,13,14]. These reactive radicals can come from classical radical/thermal initiators such as dibenzoyl peroxide (BPO), azobis isobutyronitrile (AIBN), or from unstable C-C compounds called benzopinacol (TPED) (Figure 7). The disadvantage of this method is the requirement to heat the entire formulation for polymerization to occur [9]. To overcome this disadvantage, an auxiliary curing method called frontal polymerization (FP) can be used. Frontal polymerization is a propagating reaction that, following activation of the initiator (photo or thermal), creates a local reaction zone in neighboring areas to sustain the reaction. The reaction progresses through the decomposition of suitable thermal initiators. The heat required for decomposition is provided by the exothermic heat released during frontal polymerization [9]. This method has been studied in detail by Pojman and colleagues [15,16]. The RICFP method combines radical-initiated cationic polymerization (RICP) with frontal polymerization (FP), eliminating the disadvantages of thermal and classical photopolymerization methods for epoxy resins [17]. The RICFP system enables curing of DGEBA-based epoxy resin throughout its thickness in the presence of a thermal initiator and a cationic photoinitiator. This reaction is also applicable to aliphatic and cycloaliphatic epoxy resins as well as DGEBA-based epoxy resins. Onium salts activated by UV light initiate cationic ring-opening polymerization of epoxy resin at the surface. The polymerization heat generated (exotherm) activates the thermal initiator. The radicals produced by the thermal initiator decompose in regions unreachable by UV light and reactivate the onium salts [18]. This provides curing throughout the part thickness in a chain-reaction manner.

Figure 6. Schematic representation of the RICFP method

Figure 7. Activation of thermal/radical initiator (TPED) following cationic photopolymerization and mechanism of operation with diaryl iodonium salt [11]

The RICFP method does not negatively affect the thermomechanical and electrical properties of the cured resin and has many advantages. These include: • Rapid curing, • Long shelf life, • Energy-efficient production, • Readily available formulations, • Moderate cost, • Ability to work with low-viscosity and low-reactivity epoxy resins such as DGEBA, • Curing with both ultraviolet light and localized heat application, • Applicability to complex-shaped parts. Applications of the RICFP method include: • Fastening bolts, • Repair applications, • Automotive and aerospace parts, • Production of epoxy-based composites. In this system, as long as the reaction heat is conserved, curing can occur in the presence of inorganic fillers or fibers.

Use of the RICFP Method in Composites

Few studies in the literature have examined the production of fiber-reinforced epoxy-matrix composites using the RICFP method. In pioneering work by Sangermano and colleagues [11], UV light-initiated reaction to produce epoxy-glass fiber composites was examined for the first time. The prepared epoxy formulation used DGEBA-based industrial-type epoxy resin, iodonium-derived onium salt as photoinitiator, and benzopinacol (TPED) as thermal initiator. Epoxy-based composites prepared by hand layup containing 2 layers of unidirectional glass fiber fabric were cured using the RICFP method in just 1 minute. The properties of the obtained composite were compared with an amine-cured composite cured thermally. The test results of the composites characterized by dynamic mechanical analysis (DMA) and tensile testing are shown in Table 1.

Table 1. Properties of glass fiber-reinforced epoxy-based composites [11]

The cross-linked composite obtained using the RICFP method showed better performance characteristics when compared with the thermally cured composite of the same composition. This method demonstrated that glass fiber-reinforced epoxy composites can be produced at high speeds at ambient temperature while maintaining their thermomechanical properties [11]. In another study by Sangermano and colleagues, the RICFP method was applied to carbon fiber-reinforced composites, and results were obtained that confirmed the previous study [19].

Conclusion

RICFP is a promising technique for rapid and energy-efficient curing of epoxies for numerous applications, including the production of fiber-reinforced composite materials. Further research is needed to push the limits of this approach and develop new methods for producing photo-cured composites. Cenk Kurtuluş Research Assistant Polymer Engineer (PhD Student) Bursa Teknik Üniversitesi Faculty of Engineering and Natural Sciences Department of Fiber and Polymer Engineering    

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