Solutions to Blistering: Modification of a Poly(urea-co-urethane) Coating Applied to Concrete Surfaces

Author: Eleni Papadopoulos

Papadopoulos, Eleni, 2009 Solutions to Blistering: Modification of a Poly(urea-co-urethane) Coating Applied to Concrete Surfaces, Flinders University, School of Chemical and Physical Sciences

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Blistering, during cure, resulting in subsequent coating delamination has been found to occur in a poly (urea-co-urethane) cement composite coating used for the protection of new and existing industrial concrete surfaces. Refurbishment of the blistered material, leads to time delays and increased costs. The reaction kinetics within this product is complex due to one of the reactants being water dispersed. The presence of water causes carbon dioxide evolution during cure through the decomposition of carbamic acid formed from the reaction between water and isocyanate functional groups of the resin. Water also reacts with cement and lime in the reactive aggregate, thus a multitude of reactions occurs, during cure of these complex materials. Due to the complexity of the composite coating, the cure chemistry for the polyurea and polyurethane reactions were investigated individually, with reference to the reactant ratios used by the industry. Although the competing reactions between isocyanate and hydroxyl groups and isocyanate and water molecules has been studied extensively for monomeric systems, carbon dioxide reduction in these systems has resided in the use of catalysts to increase the rate of the preferred reaction. This solution was not economically viable nor was it practical for the industry partners. Solutions were therefore aimed at the reduction or elimination of carbon dioxide in the coating formulation through the addition of a slower reacting polyol. Glycerol was chosen as the slower reacting polyol, because of its ability to reduce the viscosity of the polyol emulsion as well as increase gel times, improving the coating's pot-life and application ability. The relative rates of cure of the polyurethane/polyurea reactions have not previously been attempted for oligomeric isocyanates with tri-functional polyols, such as castor oil and glycerol dispersed in water. There was a lack of literature describing cure of oligomeric isocyanates and polyol emulsions and the rate of carbon dioxide evolution from these systems. A novel Pressure Differential Scanning Calorimetry (PDSC) technique was used to understand the cure of the polyurea reaction as well as the carbon dioxide evolved from this system and was compared to volumetric tests for this system. The effect of changing the ratios of isocyanate and the reactive components of the polyol emulsion on carbon dioxide evolution was therefore investigated. Understanding how these changes affect the application of the coating system was also taken into consideration through rheological assessment of the polyol formulation. Hydrated lime or quicklime, added to polyurethane coatings was found to aid in the absorption of carbon dioxide, therefore improvements to the cement aggregate by the addition of hydrated lime or quicklime was also considered. The curing mechanism for the composite coating containing the cement and inert fillers was out of the scope of this thesis.

Keywords: polyurethane,bulk polymerisation,PMDI,reaction kinetics,composite coating

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2009
School: School of Chemical and Physical Sciences
Supervisor: Dr. Stephen Clarke