Mat Res Innovat (2003) 7:125–132 DOI 10.1007/s10019-002-0222-2 ORIGINAL ARTICLE Witold Brostow · Nathaniel M. Glass Cure progress in epoxy systems: dependence on temperature and time Received: 20 August 2002 / Revised: 7 November 2002 / Accepted: 8 November 2002 / Published online: 13 February 2003  Springer-Verlag 2003 Abstract We have developed an analytical formula for the cure progress of epoxy systems as a function of both time t and temperature T. Complex viscosity h* or the storage modulus G' are used as the measures of the cure progress. The equation is based on the shape of the isothermal viscosity vs. time curves typically found for thermoset systems; temperature dependence of the iso- thermal parameters is established, resulting in a single equation. The equation has been tested for two vastly different thermoset epoxy systems and found to provide reliable predictive capabilities. The equation seems applicable for predicting curing progress of most ther- moset systems, without a limitation to epoxies. Moreover, the equation can be used for discriminating accurate experimental results from less accurate ones. Keywords Cure progress · Thermoset epoxies · Epoxy viscosity · Curing temperature · Curing time Introduction and scope A wide range of use of materials, components and coatings based on thermoset epoxies has been amply documented in the literature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. As pointed out by Atkins and Carey [5], the value of the annual production of these epoxies amounts to several billions of US dollars. Thermoset epoxies have to be distinguished from so-called thermoplastic epoxies which are already available commercially [11, 12]. Curing thermoset epoxies produces very interesting effects, pursued for instance by Suzuki and coworkers [13] in terms of positron annihilation spectroscopy to establish formation and sizes of free volume spaces. The progress of curing is governed by time t, temperature T and composition. Effects of these parameters on the properties of the resulting materials can be dramatic. To give an example, an addition of a fluoropolymer to a commercial epoxy changes static and dynamic friction values: depending on the curing temperature, either increases or decreases in the friction values compared to the pure epoxy take place [14]. Similarly in micro- scratch testing the penetration depth and the recovery depth depend strongly on the curing temperature [15]. Predicting the cure properties of a thermoset resin system is a useful tool in the epoxy resin industry. Models of crosslinking or curing have typically been developed on the basis of reaction kinetics or physicochemical simulations [16]; see the following Section for some details. Various industries use such models for closed mold applications [17, 18] or pultruded composites [19, 20]. Coatings are another application where modeling is desired to predict field performance [21]. However, such models are limited in their capabilities due for example to variations in raw materials. There is a need to develop a general equation that is based on the cure profile of any thermoset system that can predict how soon it will fully cure or what the degree of conversion is achieved at any given time at a specific temperature. Such a predictive model could be used to determine a coating type or chemistry needed for a particular application or temper- ature conditions. To this end, an equation was devised to predict the viscosity of a reacting system as a function of time and temperature based on selected experimental data sets. Viscosity is used here as a measure of the progress of curing, although later on the storage modulus is applied similarly, and one can envisage other measures for the same purpose. Extant kinetic models The following discussion is to some extent based on that in [10]. A chemical reaction – including curing – is typically described by a rate equation which relates the W. Brostow ( ) ) · N. M. Glass Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science & Engineering, University of North Texas, P.O. Box 305310, Denton, TX 76203-5310, USA e-mail: brostow@unt.edu URL: http://www.unt.edu/LAPOM/