Epoxy-amine system, characterization

These results indicate that studies of the donor-acceptor interactions on the model systems are quite justified. This study is the only possible approach to quantitative characterization of all the numerous complexes appearing in the epoxy-amine system. Today, we are making but initial efforts in the thermodynamic studies of the epoxyamine systems. For the time being, we have only managed to estimate the effective thermodynamic characteristics in such systems. The development of an algorithm for both the experimental and the theoretical approach to the study of similar systems still remains an important task. 

Dielectric cure monitoring generally relies on measin-ement of the ionic conductivity (a in eq. 19). The conductivity during cure of epoxy-amine systems have been characterized to establish relationships between conductivity and viscosity (103,104), conductivity and Tg (104), and relationships to the conversion of epoxide (103). Recently, models were established to relate changes in the dipole component of the complex permittivity to the advancement of cure through the Tg-con version relationship, expanding the capabilities of dielectric sensing to monitor cure (102). 

The microwave and thermal cure processes for the epoxy-amine systems (epoxy resin diglycidyl ether of bisphenol A, DGEBA) with 4,4 -diaminodiphenyl sulfone (DDS) and 4,4 -diaminodiphenyl methane (DDM) were investigated for 1 1 stoichiometries by using fiber-optic FT-NIR spectroscopy. The kinetic rate parameters for the consumption of amines were determined by a least squares curve fit to a model for epoxy/amine cure. The activation energies for the polymerization of the DGEB A/DDS system were determined for both cure processes and found to be 66 and 69 kJ/mol for the microwave and thermal cure processes, respectively. No evidence was found for any specific effect of the microwave radiation on the rate parameters, and the systems were both found to be characterized by a negative substitution effect [99]. 

Experimentally, the glass transition has also manifested itself by a sharp increase in relative rigidity (measured by dynamic-mechanical methods) and a simultaneous drastic decrease in the rate constant of the autocatalytic epoxy-amine reaction The mobility or rigidity of the system is a function of reaction conversion a in the pre-gel region it can be characterized by dynamic viscosity which is proportional to M of the reacting system. Beyond the gel point, still in the rubbery region but not close to the gel point, the dynamic modulus, G, is at low frequencies proportional to " (m a 1)... 

The microstructure of epoxy thermosets can be complex, and both molecular and physical microstructures are presumed. Unfortunately, the intractable nature of these materials makes direct structural characterization extremely difficult. The most accessible technique for direct structural characterization is evaluation of epoxy rubber-like properties above Tg. Sometimes, indirect characterization of epoxy structure is possible due to the fact that the chemistry of several epoxy systems is well behaved (e.g., epoxy-amine chemistry). This permits epoxy network structure to be modeled accurately as a function of the extent of the crosslinking reaction(s). This approach has been developed extensively by Du ek and coworkers for amine-linked epoxies ... 

Dyakonov T, Chen Y, Holland K, Drbohlav J, Burns D, Velde D V, Seib L, Solosky E J, Kuhn J, Mann P J and Stevenson W T K (1996), Thermal analysis of some aromatic amine cured model epoxy resin systems - I Materials synthesis and characterization, cure and post-cure , Polym Degrad Stab, 53, 217-242. doi 10.1016/0141-3910(96)00085-7. 

TGA/FT-IR and DSC/FT-IR were utilized in this research to characterize an amine activated epoxy resin system. The specific system under study was a near-monomeric diglycidyl ether of Bisphenol A (2-di-[4-(2,3-epoxy-l-propoxy)-l-phenyljpropane) with an epoxy equivalent weight of 173. The curing agent was composed of a mixture of 10% tertiary amine catalyst and 90% primary cycloaliphatic diamine. Cured and uncured systems were analyzed using these hyphenated techniques. 

An additional way to produce a secondary amine group utilized the reaction of 8 moles of piperazine with 1 mole of the epoxy terminated siloxane oligomer. This reaction was conducted in di-oxane at 60° for about 24 hours. Excess piperazine was removed by washing the oligomer extensively with distilled water. Since the piperazine is water soluble and the oligomer is not, the final system was easily purified to very low levels of residual piperazine in this manner. The product was characterized by titration of the amine endgroups. 

Theoretical treatment of this polymerization is difficult because of the presence of both primary and secondary amine reactions as well as tertiary amine catalyzed epoxy homopolymerization. To obtain kinetic and viscosity correlations, empirical methods were utilized. Various techniques that fully or partially characterize such a system by experimental means are described in the literature ( - ). These methods Include measuring cure by differential scanning calorimetry, infra-red spectrometry, vlsco-metry, and by monitoring electrical properties. The presence of multiple reaction mechanisms with different activation energies and reaction orders (10) makes accurate characterizations difficult, but such complexities should be quantified. A dual Arrhenius expression was adopted here for that purpose. 

The initial step in the program is the development of a characterization project to assess the effect of a number of primary variables on the performance of laminates under irradiation and at 4 K. Typical variables are reinforcement type, resin system, and cure cycle. The materials will represent a cross section of the existing commercial laminate production. Several laminates will be specially fabricated to provide increased radiation resistance. They will utilize commercial, boron-free glass for reinforcement and epoxy systems cured with aromatic amines. All test laminates will be made by a commercial producer under conditions dupli-... 

The TGA/FT-IR research focused on the characterization of cured amine activated epoxy systems. This hyphenated technique was used to quantitate activator-resin ration for the cured system for two different cure schedules. The "zap and "slow step" cures discussed in the experimental section are attractive processes for different reasons. The production line foreman would naturally prefer the "zap" cure since it would produce a high volume of parts in a short period of time. The polymer engineer favors the "slow step" cure since research has shown that this schedule produces the more thermodynamically stable product. The generated first derivative weight loss profiles and specific gas profiles were utilized to determine the cure schedule of these materials. 

Mirabella and Koberstein have previously shown the benefit of DSC/FT-IR for polymer characterization (3,4). In this work, the same epoxy system described above in the uncured state was analyzed by DSC/FT-IR. Thin films of uncured amine-activated epoxies were placed in the sample pan of the FP84 and heated from 25 to 280 °C at 10 C per minute. Changes in the structure of the epoxy as a fimction of temperature were recorded simultaneously by infrared spectroscopy. The sample was relatively transmissive to infirared radiation. The beam transmitted down through the sample, reflected off the aluminum cup, and passed back up through the material. This type of analysis is called reflection/absorption spectroscopy. A "well behaved" absorbance spectrum was generated directly without any need for correction. To produce a sufficient signal on the DSC, the bulk of the sample had to be placed on the reference side. 

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