Medicamentos para la prevención de complicaciones

Wei et al used microwave and thermal energy to cure diglycidyl ether of bisphenol A (DGEBA) / diaminodiphenyl sulfone (DDS) and a DGEBA / meta phenylene diamine (mPDA) [26]. They used Fourier Transform Infrared (FTIR) to measure the extent of cure,and thermal mechanical analysis (TMA) to determine the glass transition temperature (Tg). Their findings show that the reaction rate constants of the primary

etherification reaction is negligible for both microwave and thermal cure for the DGEBA / mPDA system [26]. For the DGEBA / DDS system, the reaction rate constants of the primary amine-epoxy reaction are greater than those of the secondary amine- epoxy reaction; and etherification reaction is only negligible at low cure temperatures for both thermal and microwave cure [26]. They also found that particularly at higher isothermal cure temperatures, the vitrification time is shorter in microwave cure than in thermal cure for both the DGEBA / mPDA and DGEBA / DDS systems [26].

Navabpour et al used dynamic and isothermal curing methods to study the cure kinetics of a commercial epoxy resin system RTM6 using a microwave heated calorimeter and a conventional differential scanning calorimeter [54]. The resins cured isothermally using microwave heating were found to have larger values of preexponential factor and higher values of activation energy then resins cured using thermal heating [15]. It was observed that the reaction orders were similar for both microwave and thermal heating. This suggested that the mechanisms of curing were similar. For the dynamic curing, the data revealed that microwave cured resin had higher preexponential factor and activation energy than thermal curing. The two heating methods gave different reaction orders [15]. This result implied that the curing mechanisms using microwave and thermal heating are different [15].

Navabpour et al also carried out near – infrared spectra during dynamic curing of the resin, and they found out that there was more rapid reaction of the amine groups in microwave curing than in conventional curing of the epoxy resin [15].

Nesbitt et al also used dynamic and isothermal curing methods to compare the curing kinetics of diglycidyl ether of bisphenol-A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine-phenol complex acting as an accelerator) using a conventional differential scanning calorimetry and a microwave heated power compensated calorimeter [55].They found that dynamic microwave curing of the resin produced higher reaction rates and shorter cure times than conventional heating. Also, microwave cured samples had higher values of preexponential factor and activation energy than conventional curing [16]. The orders of the reaction for microwave and conventional heating methods were similar. The glass transition temperatures of the

resin cured using conventional heating was higher than resins cured using microwave heating for each heating rate used in curing the resin [16]. The activation energy obtained during isothermal curing of the resin using microwaves was lower than the activation energy obtained during conventional curing [16]. They also found that the glass transition temperature of the conventionally cured resin was higher than that of the microwave-cured resin [16].

Hill et al used fiber-optic FT-NIR spectroscopy to study the thermal and microwave cure process for the epoxy resin diglycidyl ether of bisphenol A DGEBA with 4 4’- diaminodiphenyl sulfone (DDS) and 4,4’-diaminodiphenyl methane (DDM) [4]. They found the rates of reaction of primary amine and secondary amine to be similar for microwave and thermal cure processes. They also concluded that there was no special effects of microwave radiation on the kinetic parameters of either the primary amine or the secondary amine reactions [17]. They also found both systems to be characterized by a negative substitution effect [17].

Wei et al used electromagnetic radiation and conventional heating using thin film sample configurations to cure stoichiometric mixtures of DGEBA / DDS and DGEBA / mPDA isothermally. The extent of cure was measured by Fourier transform infrared spectroscopy (FTIR), while the glass transition temperatures were measured directly from the cured thin film samples using Thermal Mechanical Analysis(TMA) [56]. Microwave radiation was observed to have stronger effects on the DGEBA / DDS system than the DGEBA / mPDA system. Compared to conventional heating, it was observed that there were significant increases in microwave cured DGEBA / DDS samples, while there were only slight increases in the microwave cured DGEBA / mPDA samples. After gelation, the microwave cured samples had higher glass transition temperatures than the thermally cured samples [18]. The magnitude of increase of glass transition temperature between microwave and thermally cured samples was much more significant in DGEBA / DDS system than in DGEBA / mPDA system [18].

dynamic scanning calorimetry to measure and compare both the degree of cure, and the glass transition of samples cured in the thermal and microwave fields at the same temperature [28]. They found out that cure proceeded slightly faster in the thermal field than in the microwave field. They also observed that the glass transition temperature range was broader in the microwave field, initiating a probability that there is a difference in the cure mechanism of epoxy systems in the microwave and thermal fields [9].

Marand et al used in-situ measurements of microwave dielectric properties and infrared spectroscopy to compare reaction mechanisms of a DGEBA / DDS epoxy system undergoing isothermal cure at different temperatures using thermal and microwave heating [57]. Their findings revealed that the rate of cross-linking was much higher in samples cured by microwave radiation than samples cured thermally. At higher temperatures especially, this higher cross-linking rate appeared to lead to an entrapment of the unreacted epoxy and amine groups within the resin matrix, and in the microwave cured samples, this led to an overall lower degree of cure [19]. Marand stressed that his conclusions were limited to the epoxy systems he examined, and that in other molecular systems, acceleration of reactions by microwave energy may lead to overall faster reaction rates, void of the possibility of cross-linking reactions [19].

Wallace et al cured PR500 epoxy resin using a conventional oven and a commercial microwave oven. Modulated Differential Scanning Calorimetry MDSC, Dynamic Thermal Analysis, Infrared Spectroscopy, and solid-state NMR spectroscopy were used to compare the cured resins [58]. Their investigations showed that in microwave-cured samples, the epoxy-amine reaction is more dominant than the other possible curing reactions, including the epoxy-hydroxyl reaction. At the same degree of cure, Infrared spectroscopy revealed that the intensity of the hydroxyl and the amine bands was more in the thermally cured sample [58]. This indicated that during microwave cure, the amine-epoxy reaction was more dominant under these conditions. –CH2OH group is

formed in the epoxy-hydroxyl reaction. Solid State NMR spectroscopy showed that there were a larger number of –CH2OH groups in the thermally cured sample, hence, the

Wallace et al inferred that from the results of the IR spectroscopy, solid-state NMR and DMA, microwave curing of epoxy samples under these conditions lead to the increase of the amine-epoxy reaction compared to the epoxy-hydroxyl reaction, leading to a different network structure revealed by DMA. Wallace suggested that this could be responsible for the widening of the glass transition temperature which is commonly observed in microwave cured epoxy resins [20].

CHAPTER THREE

3.0 EXPERIMENTAL