Sabina Alessi1, Clelia Dispenza1, Giuseppe Pitarresi2 and Giuseppe Spadaro1
1Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo 2Dipartimento di Ingegneria Meccanica, Universita di Palermo
Viale delle Scienze 90128, Palermo
Radiation processing indicates all the processes based on the use of ionizing radiation that cause chemical changes in the matter. Practical application of radiation processing have been evolved since the introduction of this technology nearly fifty years ago. The earliest developments are represented by the sterilization of disposable medical products, preservation of food and crosslinking of plastic materials, while the curing of monomeric coatings was developed somewhat later (Woods, 2000; Cleland et al. 2003). In the last years the use of these and other processes has grown more and more and they are widely practiced today to produce heat-shrinkable plastic films for packaging foods or insulation on electrical wires and jackets on multi-conductor cables in order to increase heat tolerance and to improve the resistance to abrasion and solvents. Other important applications include the reduction of the molecular weight by scissoring of polymers, the grafting of monomers into polymers in order to modify their surface properties and the curing of fibre – reinforced polymer composite materials, whose main applications are in automotive and aeronautic/aerospace industries (Clough, 2001; Singh et al. 1996; Lopata et al. 1999; Goodman & Palmese, 2002; Jhonson, 2006; Berejka, 2010). Due to its behaviour regarding the possibility to involve very small particles, like electrons or heavy ions, radiation processing is the ideal way to produce nano-structured systems. Different examples are reported in literature, such as nano-litography devices (Woods, 2000) and nano-hydrogels especially in the biomedical area (Singh & Kuma, 2008; Chmielewski, 2010).
The radiation sources generally used in radiation processing applications can be divided into two main groups, that one regarding the use of natural or artificial isotopes and the other one in which particle accelerators are employed. In the first group artificial radioisotopes, like Cobalt-60 and Cesium-137, are included and the second group comprises electron accelerators, accelerators for the production of positive ions and x-ray generators (Woods & Pikaev, 1990; Spinks & Woods, 1990).
Currently the most widely used sources of ionizing radiations in the industrial processes are Cobalt-60 and electron accelerators.
Cobalt-60 produces у photons of two discrete energy values, 1.332 and 1.173 MeV. Since у photons tend to lose the greater part of their energy through a single interaction, during irradiation a fraction of them are completely absorbed, but the remaining photons are transmitted with their full initial energy.
Electron beams produced by accelerators are mono-energetic and they may be pulsed or continuous. Unlike у photons, electrons lose their energy gradually, through a number of small energy transfers, so that they are slowed down by thin absorbers.
The use of у or e-beam radiations depends upon different considerations, such as the nature and size of the objects to irradiate, the penetration to be realized and the productivity required. For a fixed material and for a fixed distance from the radiation source, the radiation is characterized by the penetration in the material and the rate by which the energy is absorbed by the material. The first parameter depends on the nature and energy of radiation and can be related to the absorbed dose, i. e. the energy absorbed by the unit of weight of the material, measured in Gray (Gy), or more commonly in kGy (1 J/Kg = 1 Gy). The second parameter is the dose rate that is the energy absorbed by the unit of weight of the material per unit of time, measured in Gy/s or more commonly in kGy/h.
In particular gamma rays are more penetrating than electron beams, but generally they are characterized by a lower dose rate. On the other hand у sources offer the advantage of a very simple process, but they suffer from a low flexibility of the plant and from degradation during the time owing to the decrease of the activity of the source. Due to all these considerations у radiation is normally used when high penetration is required without any productivity problems, while for low penetration and high productivity e-beam process is advisable.
The interaction of electromagnetic and particle radiations with matter occurs by means of different processes, even if it always produces fast charged particles, which generate a mixture of ionized and excited species. The overall effect of both types of ionizing radiation is qualitatively similar, since the same types of ionized and excited species are formed in both cases.
When a moving charged particle interacts with the matter, its energy loss gives rise to a trail of excited and ionized atoms and molecules in the same path of the particles. The same effect is caused by an electromagnetic radiation since the energy absorbed is transferred to electrons and positrons and then dissipated along the paths of these particles. In this way the overall result of the absorption of any type of ionizing radiation by matter is the formation of exited and ionized species, giving rise to similar chemical effects.
As already observed, radiation processing finds several applications in different fields. In particular it has been subjected to a very marked interest for the synthesis and modifications of polymeric materials. In fact, the interaction of ionizing radiation with apt monomers can give rise to radical or ionic (mainly cationic) polymerizations.
Radical polymerization of vinyl monomers is easily performed, while cationic process is more difficult due to the possibility that the cationic species can be neutralised by even very small amount of basic impurities (Chapiro, 1962; Crivello, 1999 ).
Also the modification of polymers by ionizing radiation has been subject of studies and several industrial applications. The interaction of ionizing radiation with polymeric materials causes the formation of free radicals that further evolve towards chain scission, with molecular degradation, chain branching and cross-linking with molecular weight increase (Woods & Pikaev, 1990). All these effects coexist, their extent depending on many factors, such as the molecular structure of the irradiated polymer, the presence of air or other gases during irradiation and the operating conditions (temperature, dose rate, etc.). The molecular modifications induced by irradiation can strongly modify the mechanical, electrical and thermal properties of the polymers. Crosslinking for the production of electrical insulating materials or tubes for both high thermal and mechanical resistance are among the most important applications (Jansen & Brocardo Machado, 2005).
Among the ionising radiation induced polymerization processes, one the most stimulating and studied application is the radiation curing of epoxy resins in order to produce polymeric matrices for carbon fibre structural composites, in both aerospace and advanced automotive industries. Compared to thermal curing, the main advantages lie in the reduced curing time, the ambient curing temperature, the greater design flexibility and the higher materials shelf life (Singh et al., 1996; Lopata et al. 1999; Jhonson, 2006). In particular the possibility to carry out the process at mild temperature derives from the fact that it does not need thermal activation. This behaviour makes the process environmentally friendly, energy saving and induces positive effects on the properties of the synthesised materials. In fact the significant reduction of thermally induced mechanical stresses derived from the use of mild process temperature leads to an improvement of the mechanical properties of the cured materials.
The use of polymeric composite materials for transport applications has considerably increased in the last few decades for their favourable strength/weight ratio, with respect to low weight metallic materials traditionally employed in such structures. Other advantages coming from the use of polymeric composites are the improvement of the resistance to both corrosion and chemicals.
The standard thermal cure process induces high quality and performance in these materials and it has been extensively studied and optimized for each specific application (Ellis, 1994; Di Pasquale et al., 1997; Mimura et al., 2000). On the other hand radiation processing has become more and more promising for advanced structures for the several advantages offered. Lopata, Saunders and Singh widely discussed the benefits of electron beam curing for the manufacture of high performance composites and in particular Lopata suggested the importance of this process also for the repairing of such structures (Lopata et al., 1999; Singh et al., 1996; Lopata & Sidwell, 2003).
The first successful attempts of radiation curing were developed on acrylic derivative epoxies, which undergo to polymerization via radical mechanism, but the obtained materials did not meet the required thermal (high glass transition temperature, Tg) and mechanical properties (high elastic modulus and high fracture energy) for aerospace and advanced automotive applications (Woods & Pikaev, 1990).
Materials with enhanced thermal and mechanical behaviours, similar to that of the materials realized via thermal curing, were obtained when the cationic polymerization of epoxy resins was performed through the use of suitable onium salts, already successfully used in the UV induced epoxy curing (Lopata et al. 1999; Crivello, 1999; Crivello, 2002; Crivello, 2005; Bulut & Crivello, 2005). Due to the presence of strongly electronegative groups, onium salts are very acid and make possible the epoxy ring opening and the further cationic attack to the other epoxy monomers with the increase of the chain length.
Most of the commercially available initiators are dyaryliodonium or triarysulfonium salts of weak bases. Several mechanisms for polymerization of epoxies have been suggested (Decker & Moussa, 1991; Crivello, 1999, 2002 and 2005; Bulut & Crivello, 2005) and probably radiation induced polymerization of epoxies can proceed via different mechanisms whose relative contributions might vary from one formulation to another.
During radiation curing, several parameters can greatly influence the final properties of the cured materials. In particular parameters such as the composition of the epoxy resin system (including the catalyst species, the chemical structure of the epoxy, other modification agents etc.), and those related to the process such as irradiation dose, dose rate and curing temperature have a key role in determining the properties of these materials (Fengmei et al. 2002; Degrand et al., 2003 ; Nho et al., 2004; Raghavan, 2009).
A general problem for epoxy cured systems, either thermally or by irradiation, is that both high glass transition temperature and high elastic modulus are accompanied by a brittle behaviour and by a decrease of the toughness, with a poor resistance of the material to crack initiation and growth and with a low fracture energy value (Ellis, 1994; Broek, 1986; Riew & Kinloch, 1993).
The basic goal in toughening crosslinked epoxy resins is to improve their crack resistance and toughness without a significant decreasing of the other important inherent properties, such as the flexural modulus and the thermo-mechanical properties (Tg) of the original epoxy resins. A way to improve the toughness is the incorporation in the monomer of a second component. This has been successfully done for thermally cured systems, incorporating in the monomer a second component into the continuous matrix of epoxy resins through physical blending or chemical reactions (Kim et al., 1999; Mimura et al., 2000).
Unmodified epoxy resins are usually single-phase materials, while the addition of modifiers can turn the toughened epoxy resins into multiphase systems. When modifier domains are correctly dispersed in discrete forms throughout the epoxy matrix, the fracture energy or toughness can be greatly improved.
Among the toughening agents studied for thermally cured systems, the best improvement of toughness, without loosing thermal and mechanical properties, has been obtained by engineering thermoplastics, like poly(ether sulfone), poly(ether imide), poly(aryl ether ketone), poly(phenylene oxide), polyamide etc. (Unnikrishnan & Thachil, 2006; Mimura et al., 2000; Blanco et al. 2003, Park & Jin, 2007). The thermoplastic toughened epoxies form homogeneous blends in the uncured state and can lead to phase separation on curing. The curing and phase separation processes were studied in many papers (Gan et al., 2003; Giannotti et al., 2003; Montserrat et al., 2003; Swier & Van Mele, 1999 and 2003; Tang et al., 2004; Xu et al., 2004; Li et al., 2004; Wang et al., 2004).
In particular the phase diagrams temperature/compositions for epoxy/toughening agent systems as function of the epoxy curing degree are of fundamental importance. In fact, the use of engineering thermoplastics as a method for toughening high performance, thermally curable epoxies, can cause different morphologies (Inoue, 1995; Mimura et al., 2000; Swier & Van Mele, 1999 and 2003). The "homogeneous" morphology results from a single phase that requires the dissolution of the thermoplastic into the epoxy with a curing process without phase separation. This method effectively reduces the crosslinking density of the epoxies, but it provides only modest improvement of the toughness in thermally cured systems. Another kind of morphology that can be obtained is the "second phase" morphology, resulting from two phases formed. In a first type the thermoplastic is the discontinuous phase (thermoplastic particles) and the epoxy the continuous phase. It generally occurs at thermoplastic loadings of about 15% or less, and it provides only modest improvement of toughness in thermally cured systems. A second type of the particulate consists of the epoxy which forms a discontinuous phase (epoxy particles) distributed in a thermoplastic continuous phase (phase inversion) and it generally occurs with a thermoplastic loadings of more than 15-20%. Also in this case there is only modest improvement of toughness in thermally cured systems.
The preferred morphology which allows to realize high fracture toughness in epoxy based systems is the "co-continuous" morphology, which consists of an epoxy continuous phase and a thermoplastic continuous phase. These phases have nanometer or micrometer dimensions and require thermoplastic loading exceeding 15-20%, while the phase size and the toughness are controlled by the thermoplastic backbone, by its reactive end groups, its molecular weight and obviously by the epoxy nature (Mimura et al., 2000).
Up to 2000 the majority of studies and programs focused on the development of ionising radiation curable resin systems that could match the performance of thermally cured matrices for structural composites, while the understanding of how radiation curing takes place and its dependence on both system and process parameters was limited (Singh et al. 1996; Berejka & Eberl, 2002; Singh, 2001; Decker, 1999). On the contrary in the latest years many research efforts have been done toward other directions, in particular investigating the influencing factors of this process in order to provide a careful foundation of radiation curable epoxy based systems. In particular the influence of the catalyst (type and content), the effect of the nature of epoxy resins and of the processing parameters, such as dose and temperature, on the curing degree of the systems, the EB curing mechanism, the role of the onium salt and the influence of the monomer conversion on the glass transition temperature have been investigated (Fengmei et al. 2002; Gang et al., 2002; Degrand et al., 2003; Nho et al., 2004; Raghavan, 2009; Coqueret et al. 2010).
A very important work has been performed by the "Cooperative Research and Development Agreement" (CRADA) sponsored by the Department of Energy Office of Science, NASA Langley Research Center, U. S. Air Force Research Laboratory, U. S. Army Research Laboratory, and several industrial partners. Several epoxy resins systems in the presence or not of toughening agents and cured by electron beam irradiation have been realised. These resin systems showed mechanical, thermal, and physical properties that are significantly better than earlier electron beam curable resins, and are comparable to many thermally cured, high performance, toughened and untoughened epoxies (Janke et al., 2001). This review presents the results obtained by the authors in the study of e-beam curing of epoxy resin systems in order to produce polymeric matrices for carbon fibre composites (Alessi et al., 2005, 2007, a, b and 2010).
The influence of the processing parameters on both the curing reactions and the properties of the obtained materials is presented. In particular the possibility to really perform the process at mild temperature is critically discussed and the consequences of the low-temperature process on the final structures and properties of the cured materials are evidenced.
Also the influence of engineering thermoplastics in the toughening of the cured matrices is illustrated.