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The kinetics of reaction of epoxy powder coatings

Prosser, J. L. (1992). The kinetics of reaction of epoxy powder coatings. (Unpublished Doctoral thesis, City, University of London)

Abstract

The kinetics of reaction of a number of reactive epoxy powders (epoxy resins containing hardeners and, variously, pigments, a flow control agent, or no additives), using IR spectroscopy and thermal analysis, have been studied.

Spectroscopic methods involving absorbance and reflectance techniques were initially promising, but were then found to be unacceptable because of poor repeatability.

Dynamic thermal analysis showed that sample size was more important than particle size; an epoxy powder divided into two portions of different particle size gave the same heats of reaction, glass transition temperature and peak temperature according to sample mass; particle size had a negligible effect.

Pigmentation affected kinetics. Ti02 and Si02 had a marked effect on exotherms, in that Ti02 slightly retarded reaction and Si02 gave multiple exotherms; an adsorption/release mechanism of low molecular weight resin components on surface active pigments is postulated.

Variation in heating rate produced a significant break in the course of the reaction, the break occurring at a heating rate in the 10-15 degree per minute heating range; this was detected using a Kissinger analysis; the powder contained a dicyandiamide hardener and was tested in a Differential Scanning Calorimeter. Another powder, containing o-tolylbiguanide as hardener and tested in a Differential Thermal Analyser had two distinct exotherms, the more pronounced the faster the heating rate; the phenomenon is attributed to sequential reaction of primary and secondary amines with a contribution from the change in viscosity dependence on the heating rate.

Comparison of oven and DSC dynamic heating studies showed that a DSC experiment was not sufficient to predict the results of an oven heating cycle with any accuracy; this was true for both convection and IR heating cycles.

A reactive epoxy powder at four constant temperatures gave a stepped conversion/time plot, the two steps being at the same degree of conversion for three temperatures; at the highest temperature, only one step was detected. A reactant exhaustion/renewal cycle similar to a Lotka-Volterra cycle is considered.

The importance of the dependence of viscosity changes (and hence molecular mobility) on heating rate is emphasised.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
Departments: School of Science & Technology
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
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