Cure induced stress generation and viscoelasticity in polymer coatings.

Abstract

Coatings solidified by free-radical polymerization and crosslinking (curing) reactions initiated with ultraviolet (UV) light do so quickly and at room temperature. Low viscosity monomer or oligiomer makes the use of volatile solvent unnecessary, decreasing energy use and making the process more environmentally friendly but photoinitiators can be toxic, limiting certain applications. Stress may be generated by a changing specific volume during cure, and stress-induced defects are undesirable. The goal of this research is to understand stress generation in UV irradiated coatings and to model stress generation and viscoelasticity seen during curing. Two new mathematical models were created to accomplish viscoelastic stress modeling. The first, a network model, uses a two-dimensional network of one-dimensional elements to replicate deformation in the coating. The second uses continuum momentum conservation and linear viscoelastic equations. Inertial forces can be neglected and a substitution performed, making the solution more rapid and simple with standard finite element methods. Stress generation in uniformly cured coatings depends on how quickly the specific volume and physical properties change. Reaction kinetics, volume, and stress are calculated simultaneously. Rapid initiation from high initiator concentration or UV light intensity delays volume change, generating more stress because the volume changes with a higher modulus. An optimum curing schedule would insure the actual specific volume and its equilibrium value remain the same. Inhomogeneities in the substrate or the presence of defects change the stress field. Knowing forces on the coating boundaries suggests defect locations and types. Probing the types of geometries and surface roughnesses seen in different types of coatings shows that restricted deformation increases stress concentrations and surface forces seen. Also, avenues for reducing stress via relaxation are discussed. The two-dimensional stress profiles used in these analyses are not possible to measure experimentally, making computational modeling essential. The models developed and methodology presented may be extended to other UV cured coatings or to other methods of coating solidification. Process windows of allowable final conversion-stress-energy-time states suggest what tradeoffs must be made to meet constraints.