Toughened epoxies

The increase of crosslinking degree obtained by either the combination of radiation and thermal curing or by the mixing of epoxy monomers at different functionalities, causes improvement of thermal properties and significant increase of the elastic modulus. Unfortunately such molecular behaviour can give rise to very brittle materials. As discussed in the introduction the toughness can be increased by the use of apt toughening agents, like high modulus and high Tg thermoplastics.

This has been done in Alessi et al., 2007b, where blends of difunctional DGEBF, polyethersulphone (PES) and iodonium salts have been e-beam cured by ionizing radiation.

First of all it is shown that the presence of PES does not interfere with radiation curing process and that similar results with respect to systems without thermoplastic are obtained. In particular a similar effect of the processing parameters and more specifically of the irradiation dose rate on the temperature during irradiation is observed, with the obtaining of mild temperature profiles at low dose rates and a dramatic temperature increase at high dose rates. Similar comments can be made also for the molecular structure and thermal properties of the cured materials, as shown in Fig. 4 by DMTA curves, where data relative to a DGEBF/PES blends cured at different dose and dose rates are reported. Also in this case, a post irradiation thermal curing allows to uniform the structure and to obtain a sufficiently high value of glass transition temperature.

In the same table 1 glass transition temperatures, elastic modulus in the rubbery state and tanS maximum are reported, for difunctional/trifunctional blends toughened by a PES based thermoplastic. Also in this case the introduction of the trifunctional epoxy monomer causes a marked increase of Tg value, indicating an increase of the cross linking degree, confirmed by the correspondent values of tanS maximum and of the elastic modulus in the rubbery state.

The presence of the thermoplastic can induce a marked effect in the mechanical properties. The presence of PES does not significantly affect the tensile elastic modulus. Our experiments give for difunctional epoxy formulations containing 10PES, e-beam cured at 80

Toughened epoxies

process conditions: a) 80 kGy, 84 kGy/h; b)150 kGy and 84 kGy/h; c) 150 kGy and 840kGy/h; d) 80 kGy and 84 kGy/h and thermally post-cured for 2 h@ 100°C. Iodonium salt concentration: 1phr (per hundred of resin).

"Reprinted from Radiation Physics and Chemistry, 76, Alessi, S., Dispenza, C., Fuochi, P. G., Corda, U., Lavalle, M., Spadaro, G. E-beam curing of epoxy-based blends in order to produce high-performance composites, 1308-1311, Copyright (2007), with the permission from Elsevier"
similar untoughened epoxy resins (Janke et al., 2001). On the contrary a marked toughness increase is observed. In table 2 the results of the fracture toughness test in terms of the critical intensity factor Kic (Broeck, 1986; ASTM D 695-02a., 2002), for both DGEBF neat resin and DGEBF/PES blends, are reported (Alessi et al. 2010). Passing from neat epoxy resin systems to blends, a general KIC increase is observed. It is worth to note that the KIC values relative to the toughened materials are not very far from the best results obtained by thermally cured systems (Janke et al., 2001).

Toughened epoxiesToughened epoxiesIn Fig. 5a-b the images of the specimens tested on tensile and three point bending testing machines are shown.

Подпись: 0b)

Fig. 5. Mechanical tests. a) Tensile: specimen gripped on a testing machine.; b) Fracture toughness: SENB (Single Edged Notched Bending) specimen in a three point bending

configuration.

System

KIC (MPa*m1/2)

0 PES

0.6 ± 0.1

10 PES

1.5 ± 0.2

20 PES 1.2 ± 0.1

Table 2. KIC values for different epoxy resin based systems irradiated at 80kGy and 70kGy/h.

"Polymer Degradation and Stability, 95, Alessi, S., Conduruta, D., Pitarresi, G., Dispenza, C., Spadaro, G. Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites, 677-683, Copyright (2009), with the permission from Elsevier".

As discussed in the introduction the toughness is strongly related to the blend morphology. In Fig 6 the SEM micrographs of the toughened materials, on fractured surfaces of the tested specimens (table 2), are shown (Alessi et al., 2010). It can be observed that in both cases a co­continuous morphology is obtained, where two phases, one epoxy-rich and the second one PES-rich, are interconnected each other.

Подпись: Fig. 6. SEM micrographs for epoxy resin based systems toughened by PES irradiated at 80 kGy and 70 kGy/h. a) PES content : 10 phr; b) PES content : 20 phr.

I 2 hm I

"Reprinted from Polymer Degradation and Stability, 95, Alessi, S., Conduruta, D., Pitarresi, G., Dispenza, C., Spadaro, G. Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites. 677-683, Copyright (2009), with the permission from Elsevier"

2. Conclusions

In this review the application of radiation processing to the synthesis of epoxy based polymeric matrices for carbon fibre composites is presented.

In the introduction the general fundamentals of radiation processing, with reference to its use in the science and technology of polymer materials and in particular to radiation curing of epoxy resin systems, are discussed.

The most important results obtained by our research group are reviewed. The aim of our research is to improve both thermal and mechanical properties of the e-beam cured materials, relating them to the processing parameters for a full scale production in the aerospace and automotive industries.

It is shown that strong differences can be observed in the temperature of the epoxy systems during irradiation. The temperature depends on several parameters, such as the system formulation, the dose rate and the geometry of the irradiated sample. Varying these parameters, the temperature ranges from low to very high values. In these last conditions simultaneous thermal and radiation curing is performed. DMTA analyses evidence that the different temperature profiles during irradiation cause the formation of very different network structures with consequent different thermal behaviour. It is concluded that the best thermal performances are obtained by a combined "dual cure" process, consisting in e – beam irradiation at moderate dose rates and temperatures, followed by an "out of mould" post-irradiation thermal curing on already solid materials. It is very important to note that the heat released from the irradiated systems toward the environment, which is one of the
parameters which affects the temperature profile during irradiation, depends, among others, on the geometry of the irradiated system and in particular on the surface/volume ratio. This means that the irradiation of samples with the same formulation and at the same dose rate, but with different geometry, can have different temperature profiles and the cured materials can have different properties. This is a very important point in the scale up from laboratory to full scale industrial production. The reference parameter can be the temperature profile during irradiation which must be kept constant in the scaling up. Regarding the mechanical properties, it is shown that blends of epoxy resins with engineering thermoplastics, similar to that used for thermally cured formulations, can be successfully cured by e-beam irradiation and the fracture energy values are not far from the best performances obtained through the more traditional thermal curing.

Finally SEM analysis shows that the toughness improvement is strictly related to the formation of co-continuous morphologies.

Figs. 1, 2, 3 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"

Fig. 4 reprinted from:

“Radiation Physics and Chemistry, 76, Alessi, S., Dispenza, C., Fuochi, P. G., Corda, U., Lavalle, M., Spadaro, G. E-beam curing of epoxy-based blends in order to produce high-performance composites, 1308-1311, Copyright (2007), with the permission from Elsevier"

Fig. 6 reprinted from:

"Polymer Degradation and Stability, 95, Alessi, S., Conduruta, D., Pitarresi, G., Dispenza, C., Spadaro, G. Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites, 677-683, Copyright (2009), with the permission from Elsevier".

Data of Table 1 derived from:

"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"

Data of Table 2 derived from:

"Polymer Degradation and Stability, 95, Alessi, S., Conduruta, D., Pitarresi, G., Dispenza, C., Spadaro, G. Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites, 677-683, Copyright (2009), with the permission from Elsevier".