Mathematical modeling of photoinitiated coating degradation: Effects of coating glass transition temperature and light stabilizers
Introduction
Degradation of coatings exposed to solar ultraviolet (UV) radiation, heat, moisture, and other environmental stresses is termed weathering [1]. For thermodynamic reasons, all organic coatings are prone to weathering eventually, at least in principle, converting the organic components to the stable end products CO2 and water [1]. In practice, the changes observed during exposure can be loss of gloss, adhesion or mechanical properties, discoloration, chalking, and environmental etching [2]. To gain knowledge on the degradation processes, it has become standard in the last 100 years to conduct weathering tests, either outdoors (e.g. in the reference climates in Florida and Arizona) or in laboratory equipment [1]. Shortening the time to market for new products is essential. Practical exposure protocols and equipment are continuously being developed and a great variety of exposure scenarios are now possible. In addition, modern scientific tools, such as a range of spectroscopic techniques, are used for early detection of degradation and for establishing chemical degradation pathways. However, estimating the long term behavior of a coating exposed to non-repeatable weathering conditions, on the basis of accelerated and controlled experiments, remains a challenging task (see [3] for a concise review of previous attempts in this direction). In an earlier article [3], a mathematical model, quantifying photoinitiated coating degradation under artificial exposure conditions for densely crosslinked thermoset coatings having high glass transition temperatures, was developed. For the detailed analysis in that work, three case studies with epoxy-amine coatings (both with and without nano-particles) were selected. Experimental data, including transient mass loss and coating thickness reduction data as well as spectroscopic measurements, was available and allowed a thorough verification of model simulations. In the present work, the model is extended to handle non-stabilized thermoset coatings with very low glass transition temperatures, which oxidize rapidly. Furthermore, the effects of adding a UV absorber and a radical scavenger to the coating are investigated. Experimental data for model verification are available in both cases. Finally, it should be mentioned that the model is relevant for industrial protective coatings and does not include a direct description of how the gloss develops, but simulates the development of an intrafilm oxidation profile, mass loss, and erosion rates.
Section snippets
Experimental details of coatings and exposure conditions
The coating type investigated is epoxy-amine networks both non-stabilized and with light stabilizers added. Recent studies (see references in [3]) have used epoxy-amine networks with or without pigments as reference systems because of their abundant use as protective coatings, insulators in electronics, and as structural composites often exposed to direct sunlight. Additionally, epoxy-amine coatings, exposed to subsequent application of a UV radiation resistant topcoat, may suffer from
Original model
The mathematical model was presented and verified against experimental in the earlier work [3] and here will only be given a concise description. The closed-loop chemical mechanism used in the model is given byR + O2 → ROOCH2N + CHOH + 2ROO + O2 → CON(amide) + CO(carbonyl) + H2O(l) + 2ROOHR + R → E-CCwhere E-CC is short for the “bridge” in the epoxy backbone and CH2N and CHOH are the groups in the network vulnerable to hydrogen abstraction. R· and ROO· are radicals. This reduced
Estimation of model parameters
Estimation of physical and chemical constants needed in the model is described in [3]. However, a few additional model estimations are needed for the extended model. A typical value for the molar absorptivity of a UV absorber can be found from data in Schulz [1], where it is stated that a clear coat of 30 μm gives a value of El/Eo = 0.05 with a UV absorber concentration of “a few percent” (taken here as 3 wt% in a coating with a density of 1000 kg/m3) leading to a molar absorptivity of 400 m2/mol.
Results and discussion
To investigate the effects of relevant parameter changes on the behavior of the thermoset coatings, simulations were performed with the mathematical model. Transient developments in the surface concentrations of carbonyl and epoxy backbone groups and oxidation profiles within the coating were considered as output variables in the parameter study.
Conclusions
The purpose of this work has been to illustrate how a fundamental coating model can be used to simulate the degradation of epoxy-amine coatings with low glass transition temperatures with and without light stabilizers present. This is an extension of the original model, which was developed for epoxy-amine coatings with high crosslink densities and high glass transition temperatures. The important rate phenomena, photoinitiated oxidation reactions, intrafilm oxygen permeability, water absorption
Acknowledgement
Financial support by The Hempel Foundation is gratefully acknowledged.
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