ADVANCES IN COMPOSITE MATERIALS - ECODESIGN AND ANALYSIS

Radiation curing of epoxies as matrices for advanced composites

The occurrence of curing reactions has been checked by the monitoring of the temperature during samples irradiation. The temperature of the irradiated samples depends on different thermal events, whose rate depends on the processing parameters. The temperature increase is due to both the absorption of radiation energy and the heat developed by the exothermic curing reactions, while the temperature decrease is due to heat releasing from the irradiated samples to the environment. The balance of these phenomena determines the temperature of the epoxy resin during irradiation. The heat production rate is low when both the concentration of iodonium salt and the dose rate are low, while the heat released to the environment is mainly related to the surface/volume ratio.

1.1 Untoughened epoxies

E-beam curing of blends of various epoxies/iodonium salts has been carried out.

One example is reported in Fig 1, where the temperature as function of the absorbed dose for Bis(4diglycidiloxy-phenil) methane (DGEBF)/iodonium salt samples (Alessi et al., 2005) is reported. The iodonium salt concentration is constant (0.1 phr) and the experiments have been performed at different dose rates (84, 420 and 840 kGy/h). At low dose rate (84 kGy/h) the temperature curve has a very slight increase and tends to a plateau value of about 50°C. On the contrary at high dose rates (420 and 840 kGy/h) the temperature increases up to 180°C and after decreases, but toward higher values than that reached during the correspondent irradiation at low dose rate.

"Reprinted from Nuclear Instruments and Methods in Physics Research B, 236, Alessi, S., Calderaro, E., Parlato, A., Fuochi, P., Lavalle, M., Corda, U., Dispenza, C., Spadaro, G. Ionizing radiation induced curing of epoxy resin for advanced composites matrices, 55-60, Copyright (2005), with the permission from Elsevier"

In the same figure the thermal profiles of the system without initiator, i. e. without polymerization reactions, are reported for two dose rates conditions, revealing that the major contribution to the plateau value is essentially due to the heat evolved by the radiation absorption. Since during curing very different temperature profiles can be obtained, the production of materials with marked different properties is realized. In fact, in conditions apt to produce marked temperature increase, the epoxy monomer undergoes to a simultaneous radiation and thermal curing, while when the irradiation is performed at low temperature the systems undergoes only to radiation curing.

Dynamic mechanical thermal analysis (DMTA) tests, carried out on samples cured in different processing conditions, and at different temperature profiles, evidence the different structure and properties of the synthesised materials. In Fig. 2 elastic modulus, E', and loss factor, tanS, as function of the temperature for the materials cured in the conditions of Fig. 1 are reported. The sample cured at low temperature presents two broad relaxation peaks. This is an indication of a not uniform structure with networks of different cross linking degrees. This phenomenon can be related to vitrification phenomena during irradiation. In fact polymerization reactions cause the increase of the glass transition temperature of the irradiated system, which soon reaches the low processing temperature. In these conditions the structure becomes rigid and further curing reactions are controlled by the diffusion of the reactive species in the bulk of the polymerising system. Furthermore the storage modulus/temperature curve reported in the same figure presents an increase between the two relaxation temperatures. This can be attributed to post-irradiation thermal curing due to the heating during the DMT A test itself. We can conclude that the low temperature radiation curing is not complete and that the second relaxation peak can be considered the result of the combined effect of radiation and post-irradiation thermal curing. DMTA tests relative to the sample cured at high temperature presents only one relaxation peak, due to the high temperature reached during irradiation, which allows to overcome the vitrification phenomena, giving rise to the formation of a more "uniform" structure. In these conditions a simultaneous radiation and thermal curing is performed. It is interesting to note that the relaxation temperature is significantly lower than the second relaxation temperature of the material cured at low temperature. This can be explained with the more efficient thermal treatment (due to the test itself) when the structure is less rigid.

"Reprinted from Nuclear Instruments and Methods in Physics Research B, 236, Alessi, S., Calderaro, E., Parlato, A., Fuochi, P., Lavalle, M., Corda, U., Dispenza, C., Spadaro, G. Ionizing radiation induced curing of epoxy resin for advanced composites matrices, 55-60, Copyright (2005) with the permission from Elsevier"

As the results indicate that not always radiation process allows to complete curing reactions, in order to check their completeness, samples radiation cured in various conditions have been subjected to post-irradiation thermal treatments.

In Fig. 3 DMTA curves of the sample cured at 150 kGy - 84 kGy/h and of the sample also subjected to a post-irradiation thermal treatment at 175°C for 2 hours are reported. In the last case a single relaxation peak is observed, thus indicating the formation of an uniform structure. Comparing the two curves reported in Fig.3, it is possible to note that the sample post cured after irradiation at low temperature presents a relaxation temperature very close to that of the second relaxation peak related to the only irradiation process, while not significant increase of Tg after thermal curing has been observed for sample irradiated at high temperature, whose correspondent comparison is not here reported. It is possible to conclude that the way to optimise the curing degree and the thermal properties of the radiation cured materials is a suitable combination of radiation curing at low temperature and a post-irradiation thermal treatment. It is important to consider that the post-irradiation thermal curing is performed on already polymerised solid materials, in an oven and out of the mould. In these conditions thermal treatment does not create environmental problems (no volatile emission) and the obtained materials do not present thermally induced mechanical stresses.

"Reprinted from Nuclear Instruments and Methods in Physics Research B, 236, Alessi, S., Calderaro, E., Parlato, A., Fuochi, P., Lavalle, M., Corda, U., Dispenza, C., Spadaro, G. Ionizing radiation induced curing of epoxy resin for advanced composites matrices, 55-60, Copyright (2005), with the permission from Elsevier"

As already cited in the introduction, the polymer matrices for structural carbon fibre composites for advanced automotive and aerospace applications need to meet severe requirements in terms of thermal and mechanical properties. The optimal combination of a "dual curing" process, radiation curing at moderate temperature, followed by a post­irradiation thermal treatment at high temperature, allows to obtain materials with high thermal performances, indicated by glass transition temperatures higher than 170°C.

An alternative way to increase the Tg of the polymer matrix is to use epoxy resins with higher degrees of functionalities. Blends of difunctional DGEBF and trifunctional Tris(4- glycidiloxyphenil) methane (Tactix) resins have been cured by ionizing radiation in the presence of an iodonium salt (Alessi et al., 2007, a).

In table 1, for difunctional/trifunctional blends at different composition, glass transition temperatures, elastic modulus in the rubbery state (at T=Tg+30°C) and tanS maximum are reported. With respect to the 100% DGEBF system, the presence of a trifunctional monomer allows to perform Tg increase up to about 70°C. This effect is related to a strong increase of cross-linking degree, as confirmed by the values of the elastic moduli in the rubbery state and of the maximum of tanS, also reported in the same table.

"Alessi, S., Dispenza, C., Spadaro, G.. Thermal Properties of E-beam

CuredEpoxy/Thermoplastic Matrices for Advanced Composite Materials. Macromolecular Symposia. 2007. 247. 238-243. Copyright Wiley_VCH Verlag GmbH & Co. KGaA. Reproduced with permission"