Amine-activated epoxy systems

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. 

DSC/FT-IR allows the chemist to monitor the chemical changes of the material itself as it is heated. The coupling of an FT-IR to a TGA or DSC provides the chemical information necessary for interpretation of the thermal event. The generation of DSC traces during the cure of an amine activated epoxy system as a fimction of activator-resin ratio indicates that the mix ratio will influence the rate of... 

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. 

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. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

Added low molecular weight hydroxyl groups accelerate the reaction between amines and epoxy as noted in Figure 17. This representation shows the fractional loss of epoxy for Epon 828 and Resin systems I, II and III, each of which has increasing hydroxyl concentrations. Table I. There is an obvious disappearance of linearity when one compares Epon 828 to systems I, II and III this change is probably due to the more active added hydroxyls as opposed to the formed hydroxyls. 

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. 

Triforine cure accelerator Dipentaerythrityl acrylate cure activator, oxidation butyl rubber Benzothiazyl disulfide cure inhibitor, RTV systems Dimethicone, vinyidimethyl-terminated cure moderator, RTV systems Dimethicone, vinyidimethyl-terminated cure promoter, polyesters Benzene phosphinic acid cure promoter amine-cured epoxies Furfuryl alcohol curing agent... 

Epoxy-amine systems follow an addition step-growth polymerisation mechanism. The two principal reactions of primary and secondary amines with epoxy oligomers are shown in Reaction scheme 1 [30]. These reactions are catalysed by acids, phenols and alcohols (e.g. impurities in commercial epoxy resins). The presence of water causes a tremendous acceleration, but does not alter the network structure. The hydroxyl groups formed by the amine-epoxy addition steps are also active catalysts, so that the curing reaction usually shows an accelerating effect in its early stage (autocatalysis). 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

Dyakonov and co-workers [72] determined overall activation energies for the thermal degradation of aromatic amine cured epoxy resin systems based on the diglycidyl ether of bisphenol. 

The limiting case of R21 —> oo may also be considered. Although this situation does not occur for an epoxy-amine system, it is of interest to determine the effect of an instantaneous activation of the second available... 

Other authors observed the same inconsistent results for other epoxy-anhydride-tertiary amine systems. For example, Peyser and Bascom (1977) observed first-order kinetics under isothermal and dynamic conditions however, the activation energy for dynamic runs was E = 104.2 kJ mol-1, much larger than the value for isothermal runs, E = 58.6 kJ mol-1. 

For Lewis base and Lewis base salt catalyzed anhydride cured epoxy reactions, Fischer (7) proposed that the initial step of an anhydride, epoxide, and tertiary amine system was the activation of the anhydride (reaction 1) ... 

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