Curing of Epoxy Resins via Photopolymerization and Use in Composites

Introduction

In recent years, the constant quest for competitiveness and cost savings has established composite materials as a suitable alternative to traditional materials such as metals or ceramics by combining lightness with high mechanical performance. Their applications extend from marine and aeronautic to automotive and sports and leisure equipment, but the primary limitation of switching from steel to composites appears to be the high operational and material costs of composites. Over the last few years, many efforts have been dedicated to making them more competitive, although further efforts are required to overcome this problem. Composite materials are mostly subjected to thermal curing. This means that the reaction takes place either at elevated temperature, including high energy consumption, or at ambient temperature and thus represents a very slow process. [1]. The demand for mass production in sectors using composite materials has paved the way for the development of new hardening mechanisms. Photopolymerization is an efficient, economic and environment-friendly technique for rapid generation of solid crosslinked polymer networks from liquid resin systems. Photopolymerization formulations typically comprise multifunctional monomers and oligomers with a small quantity of photoinitiator, classified by the reactive species and polymerization mechanism. UV light exposure either produces free radicals, which initiate polymerization of monomers such as unsaturated polyesters or acrylates, or cations, which further polymerization of multifunctional vinyl ethers or epoxides [2].  

• Advantages of light curing compared to thermal curing; • Reactions that take hours at ambient temperature can be carried out in minutes via photopolymerization, • Low energy consumption due to no heating required, • Low cost equipment usage and low thermal stresses.

Since photopolymerization is a reliable and less hazardous method, it is preferred for many applications. When compared with thermal curing, photopolymerization has many advantages and is therefore widely used for curing of relatively thin polymeric films in applications such as inks, adhesives, paints and fast drying varnishes as well as in the manufacturing of optical disks, microcircuits and printing plates. Another field of application is denture prosthesis and rapid prototyping via stereolithography [2].

Figure 2. Industrial scale UV curing machine [4]

The use of light to manufacture composite parts is an interesting and emerging technology that has gained popularity in recent years. The major limitation to using light to produce composites is that light beams need to pass through the material to create reactive species. The presence of fibers as reinforcing components may also worsen the transmission of light. UV curing is a good technology for thin films or coatings, but it requires adjustments for curing thick materials. Some studies have successfully reported the production of glass-fiber reinforced unsaturated polyester, vinyl ester or acrylate composites using UV curing [1,5,6], however, fiber-reinforced epoxy composites produced via photopolymerization are very limited.

Photopolymerization of Epoxy Resins

Epoxy resins are high-performance thermoset-based polymeric materials used in many industrial fields. These resins' market value was more than USD 7.5 billion in 2015 and are mostly formulated in two parts with anhydrides or amines as hardener. These formulations are known to require long curing times at ambient temperature or short curing times with additional heating [7]. One of the most commonly used epoxy compounds is diglycidylether bisphenol-A (DGEBA) [8].

Figure 3. Chemical representation of DGEBA-based epoxy resin

Photopolymerization can be used to activate epoxy curing as an interesting alternative to thermal curing. To carry out photopolymerization of epoxy resin, cationic photoinitiators (e.g. diaryl iodonium, triaryl sulfonium salts) known as photo acid generators (PAGs) are required [9].

Figure 4. Chemical structures of the most commonly used photoinitiators for cationic polymerization. Typical R groups include H, CH3, CH3O, Cl and NO2 [10].

These photoinitiators cleave when exposed to UV light and form strong Bronsted acids (HX). The strong acids are able to initiate cationic ring-opening polymerization of an epoxy resin [11]. At the end of the reaction, the polymeric structure is crosslinked and cured (Figure 5).

X is the counter anion and is generally SbF6-, PF6-, or BF4-.

Nevertheless, photopolymerization of epoxy resin is limited to thin film (100-200 μm) applications due to the low penetration depth of UV light. Moreover, the presence of fillers or fibers in the epoxy matrix can block light penetration to deeper layers [11]. For these reasons, curing of epoxy solely via photopolymerization mechanism is not a sufficient method for thick and reinforced parts. In recent years, radical-induced cationic frontal polymerization (RICFP) has been used as an energy-efficient process to cure DGEBA in thick applications [9,11]. This method is ideal for overcoming typical application issues such as limited layer thickness, high energy requirements in thermal curing and insufficient pot life [8].

Radical-Induced Cationic Frontal Polymerization (RICFP)

The decomposition of the photoinitiator can be stimulated either by light or by a redox reaction with appropriate radicals, leading to the radical-induced cationic polymerization (RICP) process. Several basic research articles have been published on this subject [12,13,14]. The reactive radicals are formed by thermal cleavage of conventional radical thermal initiators (RTIs) such as azobis(isobutyronitrile) (AIBN), dibenzoyl peroxide (BPO), or C-C labile compounds like benzo pinacol (TPED, see also Fig. 7). The drawback of this technique is that the entire formulation needs to be heated for curing [9]. To overcome this problem, an alternative curing method called frontal polymerization (FP) can be used. FP is a reaction in which, after a suitable exciter, a local reaction occurs and initiates further reaction in adjacent zones, and therefore FP is considered a moving reaction. The progress of this reaction is generally due to thermal decomposition of appropriate initiators. The required heat for the reaction is provided by the exothermic heat of polymerization [9]. It was studied in detail by Pojman et al. [15,16]. Radical-induced cationic frontal polymerization (RICFP), the recommended mechanism, combines radical-induced cationic polymerization (RICP) and frontal polymerization (FP) [17]. The RICFP process enables DGEBA-based epoxy resin to cure through the thickness in the presence of a thermal initiator and a cationic photoinitiator. This process can be applied to aliphatic and cycloaliphatic epoxy resins. Formation of the curing catalyst from the cationic photoinitiators is achieved by irradiation with UV light to initiate polymerization. The radical thermal initiator is triggered by polymerization heat. The formed radicals decompose the cationic initiator in regions which cannot be reached by UV light [18]. This cycle proceeds as a chain reaction and curing is carried out through the thickness of the part.

Figure 6. Schematic representation of the RICFP process

Figure 7. Representation of the activation of the thermal/radical initiator (TPED) followed by the operating mechanism with diaryl iodonium salt [11]

The RICFP method does not adversely affect the thermomechanical and electrical properties of the cured resin and has many advantages; • Fast curing, • Long pot life of formulation, • Energy efficient, • Easy to find and common components, • Low cost chemicals, • Works excellently even with low reactive epoxy resins such as DGEBA, • Polymerization can be initiated with UV light or local application of heat, • Applicable to complex shaped parts.

Applications of the RICFP method; • Repair applications, • Automotive and aeronautic parts, • Joint bolts, • Production of epoxy-based composite materials. In this system, the curing process of epoxy resin can occur even in the presence of inorganic fillers or fiber reinforcements as long as the reaction temperature is maintained.

Use of RICFP Method in Composites

In the literature, there are few studies on the production of epoxy-based fiber-reinforced composites via the RICFP process. In pioneering work by Sangermano et al. [11], UV-induced polymerization of an epoxy-glass fiber composite was reported for the first time. The epoxy formulation included a diaryl iodonium hexafluoroantimonate salt as photoinitiator and benzo pinacol as thermal initiator. Glass-fiber reinforced composites consisting of 2-layer unidirectional glass-fiber fabric were prepared by hand layup technique and cured via the RICFP method in just 1 minute. The properties of the UV-cured composite were compared with the composite thermally cured with amine hardener and having the same fiber content. Test results of the composites characterized by tensile testing and dynamic mechanical analysis (DMA) are shown in Table 1.

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

The crosslinked composite produced by RICFP showed slightly better properties in comparison with the thermally cured composite. Through this method, it has been demonstrated that glass-fiber reinforced epoxy composites can be produced at high speed at room temperature while maintaining thermomechanical properties [11]. In another study by Sangermano et al., the RICFP method was applied to carbon-fiber reinforced composites, with results confirming the previous study [19].

Conclusion

RICFP is a promising method for energy-efficient and fast curing of epoxies for many applications, including the production of composite materials. Further research is needed to push the limits of this technique and develop new methods for manufacturing light-cured composites.

References

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