Fabio Aguirre, M Asjad Shafi, Susan Falcone-Potts, The Dow Chemical Company, discuss the rheokinetic modelling of fusion bonded epoxy powder coatings at standard and  low application temperature.

Currently, there is a strong interest in designing FBE formulations for high tensile strength steel pipelines1 as they require FBE coatings to be applied at 40-60°C lower than the typical 240°C application temperature used to coat conventional steel pipelines for more than 40 years2. 

At commercial production rates the curing time is 2-3min, and at these lower temperatures (as low as 180°C), standard FBE coatings do not develop the properties required to provide effective corrosion protection.

In order to minimise the time to develop an optimum FBE formulation for low application temperature we explored the use of mathematical models as scale up tools. In this approach, data is collected from commonly available lab scale setups and then used to predict the FBE’s behaviour under actual industrial application conditions.

Structure and property developments

All FBE formulations for pipe coatings contain solid epoxy resins. A typical isothermal curing profile for FBE epoxy formulations is shown in figure 1. (see print edition) The figure shows the degree of conversion, reaction rate, and formulation viscosity as a function of time.

The kinetics of the initial curing reaction (figure 1; region 1 – see print edition) depends upon the type and nature of the functional groups and the amount of catalyst in the formulation. In the autocatalytic region (figure 1; region 2 – see print edition) the molecular weight and viscosity show a stronger, nonlinear dependence on the degree of cure. However, the glass transition temperature still increases linearly with the degree of cure.

As the curing reactions continue the degree of crosslinking increases and macroscopic gels start forming. The point at which the macroscopic gels first appear is commonly referred to as gel point (figure 1; region 3 – see print edition). For almost all industrial applications, the glass transition temperature (Tg) of the material in this region is still significantly below the final cured Tg.

As the curing reactions proceed further, the glass transition temperature begins to approach the curing temperature (figure 1; section 4 – see print edition). This limits the mobility of the molecular segments and their ability to diffuse towards each other and, consequently, the rate of the curing reaction is limited.

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