SynopsisThe cure behavior of styrene-based thermosets such as orthophthalic polyester, isophthalic polyester, and vinyl ester resins is characterized using both isothermal and dynamic differential scanning calorimetry ( DSC) techniques. In the conventional autocatalyzed kinetic model, it is assumed that the isothermal cure reaction of a thermoset goes to completion, which usually leads to a poor fit with the DSC kinetic data. It appears from the conversion-time profiles that the cure reaction reached a plateau value (a,) less than unity. This is not unexpected, as the molecular mobility of the resin system decreases as the network cures. In this work, we have refined the model to remove this restrictive acsumption by taking the total ultimate conversion (a,) into account in the computation. This improved model permits us a better prediction of the course of the cure reaction of the styrene-based thermosets under isothermal and dynamic conditions. In addition, the cure kinetics of the three thermoset resins are studied and compared on the basis of several kinetic parameters including conversion, cure rate, reaction rate constant, heat of cure reaction, half-life, and Arrhenius parameters. The reactivity of these resins when arranged in ascending order is found to be: vinyl ester < orthophthalic polyester < isoththalic polyester.

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Structure-property relationships of polyester matrices containing various low profile additives such as poly(vinylacetate), polyurethane, poly(methy1 methacrylate), polystyrene, and polycaprolactone were studied. The morphology and particle size of the dispersed phases in the polyester matrices are very dependent upon the type of low profile additives incorporated in the cured resins. Microvoids which are responsible for the degradation of physical properties of resin matrices are usually observed in the polyester phase and/or the dispersed phase of the low-profile resin matrices. Low profile additives, in general, lower the glass transition temperatures of polyester matrices, slightly increase the loss moduli and the loss factors, and slightly decrease the storage moduli. These polymeric additives also degrade tensile properties and improve the fracture toughness of polyester matrices, but do not have significant effects on the thermal stability.

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. Can. J. Chem. 62, 1068Chem. 62, (1984. 3-Aryl-2,4,1O-trioxaadamantane ortho esters (T) undergo a rapid equilibration with a ring-opened dioxan-2-ylium ion (DH+) prior to hydrolysis to product (a 1,3,5-cyclohexanetrioI rnonobenzoate). The cation is stable in concentrated HZS04 solutions where it has been characterized by nrnr spectroscopy. It is observed using uv spectroscopy in dilute acids, and the ratio [DH']/[T] at equilibrium has been measured as a function of acidity. Reversibility of the ring opening is established by the pattern of plots of cation absorbance versus acid concentration and by the observation that solutions containing cation on neutralization or dilution yield ortho ester, not hydrolysis product. Equilibrium constants for the reaction DH+ e T + H+ have been measured by obtaining the acidity function HT for this system. The effects of the aromatic substituent and the steepness of the acidity function plot versus acid concentration are interpreted in terms of a strong intramolecular interaction in the cation between the cationic center and the hydroxyl oxygen. IAlthough the first reaction stage is in principle reversible, hydrolyses are usually carried out in a large excess of water, and under these conditions the probability is low that the cation once formed will undergo the back reaction with the small amount of alcohol being released. This situation could be different with cyclic systems, where a ring opens in the first stage.Here the reverse is an intramolecular ring closure, and this could in principle compete with water addition, even with water as solvent. Such a reversible ring opening has been suggested with cyclic acetals as the explanation behind their diminished hydrolytic reactivity (3). Evidence for reversibility has been obtained in two instances (4, 5 ) , although in both cases the rates of water addition and the ring closure were quite similar.OOCR ring opening since the departing hydroxyl group is forced to remain in close proximity to the cationic center. Lamaty, Moreau and co-workers (6, 7) were the first to suggest that pronounced reversibility does occur. Their suggestion was based on three kinetic observations: enormously retarded (lo6-lo9) rates of hydrolysis compared with those of acyclic analogs, negative entropies of activation, and a substituent effect, k(3-H) > k(3-CH3), opposite to that expected for ratelimiting cation formation. We have reported (8) in preliminary form verification of reversibility, in the actual observation of the 1,3-dioxan-2-ylium ion DHt in equilibrium with ortho ester. Although oxocarbocations can be isolated as stable salts or solutions (9) or observed as transient intermediates (2,(10)(11)(12), this represents the first example where equilibration with an acetal or ortho ester precursor is involved. We have now carried out a detailed study of the system. In this paper we discuss the quantitative aspects of the ring-opening equilibrium.