Curing reaction studies

Barton, J. M. The Application of Differential Scanning Calorimetry (DSC) to the Study of Epoxy Resins Curing Reactions. Vol. 72, pp. 111 — 154. 

Differential photocalorimetry (DPC) is included here since the instrument used is essentially an adaptation of DSC instrumentation. The photocalorimeter comprises a DSC instrument with a UV/visible source mounted on top, such that light of appropriate wavelength or wavelength region from the source is focused onto the measuring head (both reference and sample pans). The most frequent use of DPC is in the study of polymer cure reactions, but it may also be used to follow such as UV degradation. 

To obtain accurate values of the sol, thin specimens (1 mm) in one study (13) were kept in the solvent for six weeks in another study (14), thin specimens were extracted for more than 18 days in Soxhlet extractors. When the present experimental data were obtained (6), there was little interest in knowing the sol fraction accurately. However, as discussed subsequently, to compute the extent of the curing reactions and the concentration of elastically active network chains, the sol fraction must be known accurately. 

Solid state NMR has been used to study polymers of various classes over the past several years. In particular, the technique has been used to study curing reactions in epoxies (12). polyimides (1), and acetylenic terminated sulfones (13). The ability to observe the evolution of the carbons of the reacting species has been clearly shown to provide valuable information which has been difficult or impossible to obtain with other techniques. The use of 13C solid state NMR techniques is essential for the understanding of curing reactions in high temperature polymers in order to be able to correlate the reaction chemistry with the structural and resulting physical properties. 

In the 80/20 specimens, the changes in E (rt) follow a different pattern. There is little change in E (rt) up to a dose of 2000 Mrads. Between 2000 and 4000 Mrads, E (rt) decreases 13%. At 10,000 Mrads, E (rt) is almost 9% greater than a control. Previous DSC studies have shown that during the first 2000 Mrads, there is substantial additional cure (14). Competition between chain scission of strained bonds and further curing reactions could "balance out"... 

Capitano (18) has shown that the absorption band for TX around 380 nm corresponds to the lowest lying — x electronic transition. Herkstroeter, Lamola and Hammond (79) calculated the triplet energy for TX to be 65.5 kcal/mol. Amirzadeh and Schnabel (20) estimated the quantum yield for triplet formation of TX to be 0.85 and determined the rate constants for a number of the photochemical reactions that take place during the photocuring reaction. We have outlined a reaction scheme for TX based on our studies (Figure 2) which includes several pathways that occur during the curing reaction. 

The DGEBA based epoxies were reacted with various curing agents. These spectra were than compared to the solution spectra of unreacted epoxy. It was shown that the peaks due to the epoxide carbons were not present in the polymerized DGEBA. In their previous study, Sojka and Moniz 481 showed that DGEBA partially cured with piperidine, exhibited a peak near the methylene peak that increased in intensity and complexity with the curing reaction. This peak was assigned to the carboxyl-methine ether carbon and the methylene carbon near the reaction site. These studies addressed the questions ... 

In this Sect, we describe the starting material impurities and their effect on the processing and cure reactions of TGDDM-DDS epoxies. The cure reactions are characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) studies. The BF3 amine catalysts used to accelerate the cure of TGDDM-DDS epoxies are characterized by nuclear magnetic resonance (NMR) spectroscopy studies. 

In this Sect, we discuss 1H, 19F and nB NMR studies of BF3 NH2C2H5 and BF3 NHC5H10 complexes, with principal emphasis on the former. We present the chemical composition of commercial BF3 amine complexes, their thermal stability in the solid state and in solution, the effect of moisture and heat upon their composition, the nature of their interaction with the epoxide and amine components utilized in TGDDM-DDS commercial prepregs, the composition of BF3 amine complexes in commercial prepregs, their thermal stability in the prepregs, and the chemical structure of the predominant catalytic species of the cure reactions of the prepreg. 

In this Sect, we report systematic DSC studies of (i) the constituents of boron tri-fluoride monoethylamine (BF3 NH2C2H5) catalyzed TGDDM-DDS epoxies and their mixtures (ii) the nature of the catalyzed cure reactions and (iii) the environmental sensitivity of the BF3 NH2CzH5 catalyst. DSC studies are also reported on the cure reaction characteristics and environmental sensitivity of commercial prepregs that contain BF3 amine catalysts. 

From these DSC studies together with our NMR observations from Section 3.3 we conclude (i) the peak is associated with BF3 NH2C2H5 catalyzed DDS-TGDDM impurity reactions (ii) the y peak is associated BF3 NH2C2H5 and monofluroborate catalyses of the cure reactions and (iii) the p peak is associated BF4 NH3 C2H5 cationic catalyses of the cure reactions. 

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