Development of an Edge Retentive, 100% Solids
Rigid Polyurethane Coating

Introduction

During 2002-2003, Madison worked closely with the US Navy to formulate 100% solids polyurethane coatings suitable for lining shipboard ballast tanks. These tanks were especially difficult to line due to many irregularities including pipes, angles, beams and other complex configurations. As a result, serious problems of corrosion related failure and costly repairs were common. The photo below shows the extent of the problem in a typical ballast tank. The new technology addressed these inherent problems, provided the US Navy with a viable solution to issues of premature corrosion and ultimately became a staple of all Madison infrastructural coatings. Prior to describing the solution, some technical discussion is of value.

Technical Background

The film formation of a liquid coating is a complex phenomenon. Boiled down though, a liquid coating’s ability to flow and level is dictated primarily by something called surface tension. Think of a water droplet beading off of a freshly waxed car – the surface tension of water is uniform and very high compared to wax, so the water tends to pull away and bead. Coatings, generally speaking, have much lower surface tensions and produce fairly level films on steel surfaces. This is true for typically large and/or flat surfaces. However, highly curved surfaces such as angles and edges have extra surface area. Following the application of a liquid coating to an angle or edge, because of the extra surface energy associated with these areas, there is a greater tendency for the coating to “pull” towards the bulk – away from the edge towards the adjacent flat areas (think of the freshly waxed car). As a result, the coating will tend to be thinner at these edges and thicker in the bulk. These edge defects are known as “small or poor edge retention” and results in easier mechanical damage, allowing the access of chemicals such as water, oxygen, and salts to the steel substrate, and subsequently allowing more aggressive corrosion development.

The Solution

Madison researchers initially investigated the more ‘common’ approaches to getting around the problem, including adjusting the surface tension of the coating through various additives and formulating techniques. They soon realized that because of the 100% solids nature of these coatings, the old approaches that applied to solvent borne coatings were mostly inadequate. However, through a systematic approach, they identified and addressed three effects to solve the problem. In order of relevance, they were:

1. Rheology – simply put, this refers to the ability of a coating to flow, level and recover. They formulated a product that is relatively fluid when applied by spray, levels on the angle or edge and sets-up quickly to prevent pulling and sagging.

2. Surface tension – they considered not only the more ‘common’ bulk effect (over large and flat areas), but focused closely on the gradient or change approaching the angle or edge to finely balance these competing forces.

3. Application technique – most intriguing was the discovery that changes in equipment components and conditions, for example tip size, module and pressure affected edge retention. They optimized these conditions to make it easy for applicators to select the right set-up with minimal experimentation.

Outcomes

Utilizing complex test procedures and microscopic photography, they provided evidence of outstanding edge retention on the new formulations and techniques. Whereas edge retention in the range of 30 - 50% was common for the ‘unmodified’ polyurethane coatings, the new formulations exhibited edge retentions in the range of 75 to 90%. When the equipment was optimized and combined with the new formulations, the results clustered around 90+%.

% edge retention is defined as:

% edge retention = {thickness (dry mils) of coating on edge (A) / thickness (dry mils) of coating on flat (B)} x 100%

To illustrate these concepts, the photos at right show the test measurement areas on an unmodified 100% polyurethane edge cross section and a modified cross section using the optimized spray conditions.

Employing the new coating and technique, Madison personnel supervised the successful lining of a number of ballast tanks and noted very high edge retention. The photo at right shows a freshly lined ballast tank similar to the one shown above. Note the uniform coverage on the piping, angles and recessed areas.

Conclusions

Sufficient edge retention isn’t only a concern in the tight confines of a ship board ballast tank. Indeed, most common industrial structures have areas susceptible to this problem. Among some of the complex structures that have been successfully coated with ample edge retention include the crown and rafters of large water and chemical storage tanks, the flanges and fittings of pipelines, the interior of water cooling towers and the sharp bends or ‘knuckles’ of multi-sided utility poles. Madison coatings with Edge Retention Technology add coating to areas where it’s most needed to provide many years of outstanding corrosion protection. Please contact Madison Chemical Industries for more specific information on coating selection and optimal equipment configuration.