Nanostructured forms of carbon can be incorporated in epoxy resin to confer self-sensing properties, heating ability, auto-repair function, damage monitoring function, etc. The curing process of epoxy composites is currently based almost exclusively on thermal curing cycles, for which an oven or an autoclave is used. Alternative curing methods for composite manufactures are of great industrial interest and can help save energy. Methods as the irradiation hardening process, heating with infrared and laser radiation have drawbacks because they present high costs and manifest poor application flexibility. In this paper, an alternative curing strategy, based on the application of an electric field, is proposed. The resin is obtained through the dispersion of carbon nanotubes (3% by weight), which act as nanometric heater elements in the epoxy matrix. The electro-curing is activated by applying an external electric power, which allows tunable cross-linking within the epoxy matrix entrapped between the nanotubes. The initiation and propagation of the crosslinking reaction result to be power/time-dependent. The electro-curing method allows reaching higher Curing Degrees (C.D.) with respect to the conventional process in oven and consequently higher glass transition temperatures. The values of the curing degree, for the different curing conditions and strategies were obtained from Differential Scanning Calorimetry (DSC). The polymerization in oven allows a C.D. of 91%. A C.D. higher than 95% has been obtained after an oven curing cycle of 3 hours at 180°C. The flow of an electric current through the material produces heat, due to the resistive nature of the epoxy/CNT nanocomposites, and therefore the temperature increase of the nanocomposite.The increase of the temperature must be accurately controlled by adjusting the applied power and choosing electro-curing cycles suitable to obtain a good compromise between the material performance and the energy employed. Two different approaches were followed. In the first, a single electro-curing cycle is used (5min@10W+15min@5W - sample EC) and, although the C.D. is 100%, the sample evidences numerous trapped bubbles. The poor temperature control during the fast curing process also affects the final dimensions, and shape of the sample, which results completely compromised. The second approach (EC*) uses a double electro-curing cycle (2min@5W+58min@2W) + (5min@10W+15min@5W) where, in first stage, lower values in the temperature are reached, followed by a second stage where higher applied powers determine higher value of temperature values. The double curing cycle limits the autocatalytic nature of the cross-linking reactions, avoiding the formation of bubbles and locally degraded regions. The electrocuring process results in a higher curing degree and glass transition temperature of the filled material, it result a less energy-intensive thermal process; in fact, the cycle in oven requires about 5 MJ for the curing of the sample, whereas the electro-curing process requires about 12.4 kJ for the complete curing of the same sample.
Electro-Curing: an efficient strategy for the curing of epoxy-nanocomposites
Liberata Guadagno
2021-01-01
Abstract
Nanostructured forms of carbon can be incorporated in epoxy resin to confer self-sensing properties, heating ability, auto-repair function, damage monitoring function, etc. The curing process of epoxy composites is currently based almost exclusively on thermal curing cycles, for which an oven or an autoclave is used. Alternative curing methods for composite manufactures are of great industrial interest and can help save energy. Methods as the irradiation hardening process, heating with infrared and laser radiation have drawbacks because they present high costs and manifest poor application flexibility. In this paper, an alternative curing strategy, based on the application of an electric field, is proposed. The resin is obtained through the dispersion of carbon nanotubes (3% by weight), which act as nanometric heater elements in the epoxy matrix. The electro-curing is activated by applying an external electric power, which allows tunable cross-linking within the epoxy matrix entrapped between the nanotubes. The initiation and propagation of the crosslinking reaction result to be power/time-dependent. The electro-curing method allows reaching higher Curing Degrees (C.D.) with respect to the conventional process in oven and consequently higher glass transition temperatures. The values of the curing degree, for the different curing conditions and strategies were obtained from Differential Scanning Calorimetry (DSC). The polymerization in oven allows a C.D. of 91%. A C.D. higher than 95% has been obtained after an oven curing cycle of 3 hours at 180°C. The flow of an electric current through the material produces heat, due to the resistive nature of the epoxy/CNT nanocomposites, and therefore the temperature increase of the nanocomposite.The increase of the temperature must be accurately controlled by adjusting the applied power and choosing electro-curing cycles suitable to obtain a good compromise between the material performance and the energy employed. Two different approaches were followed. In the first, a single electro-curing cycle is used (5min@10W+15min@5W - sample EC) and, although the C.D. is 100%, the sample evidences numerous trapped bubbles. The poor temperature control during the fast curing process also affects the final dimensions, and shape of the sample, which results completely compromised. The second approach (EC*) uses a double electro-curing cycle (2min@5W+58min@2W) + (5min@10W+15min@5W) where, in first stage, lower values in the temperature are reached, followed by a second stage where higher applied powers determine higher value of temperature values. The double curing cycle limits the autocatalytic nature of the cross-linking reactions, avoiding the formation of bubbles and locally degraded regions. The electrocuring process results in a higher curing degree and glass transition temperature of the filled material, it result a less energy-intensive thermal process; in fact, the cycle in oven requires about 5 MJ for the curing of the sample, whereas the electro-curing process requires about 12.4 kJ for the complete curing of the same sample.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
