Dielectric method for monitoring the curing of polyester-melamine coatings

A method has been developed for monitoring the curing of coatings by means of dielectric measurements. The features of the method have been demonstrated by monitoring the curing of polyester-melamine coatings.

A sensor has also been developed, consisting of two comb-electrodes for the dielectric measurements and a temperature-dependent resistance for temperature measurements applied on a glass substrate. This has allowed the use of a thin layer of clear or pigmented coating and therefore approaches coating application practice. During curing, both dielectric properties (the dielectric permittivity and loss) and temperature have been measured.

From the dielectric data, the electrical conductivity has been calculated. The conductivity has been shown to depend on the degree of curing as well as on the temperature and the amount of solvent in the coating. Most of the solvent was evaporated before curing started. The increase in the glass transition temperature of the coating during curing has been determined using the WLF equation and the data for uncured and fully cured coatings. The glass transition temperature has been used as a measure for the extent of the reaction.

Curing of two different polyester-melamine coating systems has been monitored. The first system was cured both quasi-isothermally at 130 °C and at four different heating rates. The second system, a coil-coating system, was developed for fast curing at high temperatures. This system was cured at four different heating rates, and also flash cured by placing for a few minutes in an oven at a temperature above 250 °C.

For both isothermal curing and curing at a constant heating rate, the extent of the reaction has been determined with time. Assuming simple kinetics, the reaction constant and the order of the reaction have been determined.

It is concluded that the dielectric method is fast and convenient for monitoring curing, under conditions approaching coating application practice.