Uncatalyzed systems

As far as the uncatalyzed system is concerned, it doesn t appear that rhodium(I) in all its various forms, including the rhodium(III)-hydride or even the rhodium(II) species, is capable of generating hydrogen by itself. Rather, there must be subsequent reactions involving rhodium(I) and other species that ultimately produce the hydrogen. 

The model satisfactorily described the cure behavior for the entire range as experimentally monitored by FTIR, DSC, and torsional braid analysis (TBA). This model satisfactorily explained the cure behavior of both catalyzed and uncatalyzed systems over a wide range of temperature and throughout the curing process. The authors proposing the kinetic model considered the reaction to be triggered by the adventitious water and phenol impurities (whose reactions with the cyanate ester is considered as an equilibrium reaction). Catalysis by the added metal ions, which stabilizes the imino carbonate intermediates by complex-ing, is also considered. The model has considered all possible reaction paths and intermediates as detailed in Sect. 4 and depicted in Scheme 14. Considering the various reactions, expressions could be obtained for the individual apparent empirical rate constants of the second order auto catalytic model in terms of the actual rate constants and equilibrium constants. 

Table I summarizes the kinetic parameters extracted from the DSC data for these formulations. The reaction in the uncatalyzed systems gave a poor fit to an nth order reaction model especially above 50% reacted. This may be due to autocatalysis by carbamate groups (J, 2 as they form or complexities due to minor impurities in these systems. Second order kinetics provided a reasonably good fit to the early portion (< 50%) of the reaction and were used to obtain the apparent pre-exponential factors (A) and activation energies (E) shown in Table I. Relatively small values of E and A were obtained in both the extended and unextended formulations without catalyst.

These workers showed that if water is removed during the reaction. Reactions 7-9 occurred in approximately the ratio 2 1 1 in an uncatalyzed system. A higher degree of selectivity for the hydroxy ester (Reaction 7) was observed to occur in the base-catalyzed reaction that uao m-r.nr.oaH to nroceed as follows ... 

Catalysis plays an important role in the deblocking or thermal dissociation of the blocked isocyanates. Notably organometallie compounds and tertiary amines are capable of lowering both the deblocking temperature and time as compared to the uncatalyzed systems. Wicks (61) has pointed out that since most of the deblocking reactions are carried out in the presence of hydroxyl... 

Already, many years before titanium-doped NaAlH4 was recognized as a possible hydrogen storage medium [152], Dymova et al. [153,154] showed that the two-step dissociation of NaAlH4 into NaH, Al and hydrogen (Eq. (6.14)) is in fact reversible. However, due to the poor kinetics of the uncatalyzed system, the reverse reaction requires unpractically severe reaction conditions (200 00 °C, that is, above the 183 °C melting point of the tetrahydride, and 100-400 bar). 

The PCI measurements carried out on the uncatalyzed system in desorption mode in the temperature range of liquid NaAlH4 (Eq. (6.14)) (Fig. 6.26) resulted in values of 6.4 and 32.1 kj mol for the first and the second dissociation step respectively [154]. These values are significantly higher than those based on measurements of PCIs on the Ti(OBu")4 catalyzed solid NaAlH4 (Eq. (6.15), Fig. 6.27) [133], 4.5 and 15.7kJ molfi. For this reason, PCIs of metal-doped and undoped NaAlH4 deserve to be re-examined (see also Section 6.6.2.7). 

DSC thermograms for the uncured epoxies are shown in Figure 3. The uncatalyzed system exhibits two reaction peaks, as observed in our earlier work (12). The lower temperature shoulder is attributable to amine-epoxy reaction and the higher temperature peak is due to hydroxyl and homopolymerization reactions. The... 

Kinetic analyses were performed on the initial reaction peaks for the uncured epoxy systems according to the method of Borchardt and Daniels (13). The calculated kinetic parameters are shown in Table 1. When BF MEA is added to the uncatalyzed system, the reaction order increases and the activation energy and... 

The DSC results for the unreacted systems exhibited two reaction peaks for the uncatalyzed system and two additional lower temperature peaks for the catalyzed systems. DSC experiments indicate that BF MEA increases the rate of polymerization and decreases the reaction temperatures for the TGDDM-Novalac-DDS epoxy system. As the catalyst concentration is increased, the importance of the low temperature catalyzed reactions increases. 

Although attempts were made to evaluate cure kinetics in both catalyzed and uncatalyzed resins, only thermograms of the uncatalyzed system yielded results that were readily interpretable in... 

Combining the experimentally determined conversion curves for DSC and FT-IR along with the degree of cure curve from DMA.for the uncatalyzed system (Figure 9) shows clear agreement between DSC and FT-IR results. The onset of the curing reaction measured by DMA, however, starts about 10 C higher than the onset of the chemical reaction as measured by either DSC or FT-IR. 

Epoxy/anhydride reactions vary significantly depending on the presence of a catalyst. In uncatalyzed systems, the anhydride reacts withhydroxyl... 

The amount of a catalyst in polymerization not only determines the transition times of morphological changes but also significantly affects the morphology of final products. Figure 2.13 shows various final morphologies of LCPs synthesized from different levels of catalyst concentration. For the catalyzed systems with low catalyst concentrations of 0.002 wt% to 0.007 wt%, their morphologies are quite similar to that of an uncatalyzed system at the same reaction... 

Previous works confirm that the bulk copolymerization reaction of P(OBA/ ONA) is a bimolecular second-order reaction for both catalyzed and uncatalyzed systems [37]. The reaction rate equation can be expressed as follows ... 

Figure 2.14 The dependence of 1/(1 — P) on the reaction time for an uncatalyzed system and a catalyzed system with 0.005 wt% CH3COONa. Monomer composition is 73/27 ABA/ANA. Reaction temp ature is 250 C. 

FIGURE 42.10 Intensity ratio of peaks at 135 and 104 D as a function of melt mixing time at 270°C. Squares refer to the uncatalyzed system, circles to the catalyzed one. Solid symbols 25 keV Ga primary ions, open circles 10 keV Ar ions. The dashed line following the points pertaining to the catalyzed system has been drawn as a guide for eye. Figure from Reference [154]. 

Several considerations regarding uncatalyzed system apply to the reactions of acids with epoxy groups ... 

Reactions such as these occur at a slow rate at temperatures of less than 120°C to 140°C in uncatalyzed systems. 

---