Let’s Talk About Measuring Color Shift and Gloss Reduction of Finish Coats

KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 12 – December 2024

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Introduction

As certified coating inspectors, you may be asked to measure the color and gloss of finish coats applied to industrial structures to verify that the correct color and sheen have been applied, or to serve as baseline values for future comparative measurements over the life of the coating. Since the AMPP CIP training/certification courses do not specifically address color & gloss measurement, I felt it was appropriate to discuss the topic in this forum. Bridges, tanks, power generating plants, and other facilities use color for branding, safety, or basic aesthetics to appease the public, as tanks and bridges are frequently visible from public neighborhoods. For commercial “brick & mortar” facilities, color and gloss is frequently part of a brand and retention is paramount to the image they intend to portray. Amusement parks extensively use bright colors to attract visitors and to provide them with specific “experiences.” They also use it for wayfinding. Color can be so critical that drawdowns of the formulated topcoat colors (on special coated paper known as Leneta Charts) selected by a facility owner may be a required submittal prior to the start of a project.

This issue of the Certified Coating Inspector Forum describes the application of color and gloss measurements to industrial and commercial structures, briefly describes color and gloss values, and highlights the instrumentation that can be used in the shop or field to measure color difference and gloss retention. Some of the information in this Forum was excerpted from an article titled, “Problem Solved: My Paint Looks Bad – Can I Prove it has Faded,” published in Durability and Design in the Fall of 2017.

Application of Color and Gloss Measurements to Industrial and Commercial Structures

Solar radiation from the sun can cause deterioration of the resin system (and pigments on occasion) in the coating over time, resulting in color shift and gloss reduction. Depending on the coating type, formulation, color, and location of the structure, color and gloss shift can occur in as little as 6 months. Conversely, some finish coats under similar conditions, such as fluorourethane, may retain their color and gloss for 25+ years.

Bright colors and darker colors tend to shift more quickly, while lighter, pastel colors tend to retain color longer. A case study in the importance of coating formulation for color stability is the Hell Gate Bridge[1], a steel through-arch railroad bridge between Astoria in
Queens, Randall and Ward’s Islands, and the Bronx, NY. The bridge spans a stretch of the East River known as Hell Gate. In the early 1990’s, the bridge was completely recoated for the first time in over 80 years. The color selected for the topcoat was known as “Hell Gate Red” a deep natural red. The 4-coat system was composed of two coats of epoxy, a pigmented polyurethane coat, and a clear topcoat designed to preserve the red color of the bridge. Unfortunately, the bridge began to fade to a pink color even before the repainting was completed. The problems with color retention on the Hell Gate Bridge were reportedly caused by using a pigment that was not able to resist solar radiation, and a clear urethane that was not formulated properly to protect the pigmented urethane undercoat. Subsequent recoating with a fluorourethane finish coat (FEVE fluorinated) in the same Hell Gate Red has resulted in only minor shifts in color and gloss after 25 years.

The gloss or “sheen” level of a coating is selected primarily for aesthetics, although scrub-ability and scuff resistance are improved with higher gloss coatings. Choices can range from flat (matte) to high hide matte, eggshell, satin, semi-gloss, gloss, and high gloss, however industrial protective coatings are not always available in each of these levels. The gloss of a protective coating changes over time; these changes may be caused by aging of the coating or exposure to solar radiation from the sun. Finish coats designed for exterior use are typically formulated with light stabilizers so that the gloss retention is maximized. Polyurethane, polysiloxane, fluorourethane, and even acrylic latex resin systems tend to retain their gloss longer than vinyl, alkyd, epoxy, and other resin types, but in some cases are significantly more costly. Changes in gloss more than about 5 G.U. (gloss units) can be detected by the human eye, but minor reductions in gloss or the need to quantify a change in gloss can only be determined using gloss meters.

Color and gloss measurements can be obtained at multiple stages. They can be acquired from sample drawdowns to verify the formulated (or tinted) finish coat matches the specified color and gloss. Measurements can also be obtained on the structure post-application to verify that the correct color and gloss have been applied, as well as to serve as baseline values, then compared to measurements taken throughout the life of finish coat to assess color shift and gloss retention.

Industry Standards Governing Color Measurement

There are four ASTM standard methods for measurement of color. These include:

• ASTM E1349 Standard Test Method for Reflectance Factor and Color by Spectrophotometry Using Bidirectional (45°:0° or 0°:45°) Geometry
• ASTM E1331 Standard Test Method for Reflectance Factor and Color by Spectrophotometry Using Hemispherical Geometry
• ASTM D2244 Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
• ASTM E1164 Standard Practice for Obtaining Spectrometric Data for Object-Color Evaluation

Measuring Color

Color measurements can be determined using portable hand-held colorimeters similar to the one shown in the photo below.

A good description of color measurements (colorimetry) was provided in an article by Rick A. Huntley and Richard A. Burgess (Senior Coating Consultants with KTA-Tator, Inc.), published in the December 2015 Journal of Protective Coatings and Linings, “What You See Isn’t Always What You Get… The Problem with Aesthetics.”  In the article, the authors state, “The color of a coating can be quantified by using a colorimeter. The colorimeter measures the spectral energy of light reflected from the coating over the visible spectrum. The instrument then assigns the resultant measurement standard color coordinates on what is known as a color space.  There are various color spaces, each having different coordinates for a color, but one of the most common is the L*a*b* color space. In that space, L* represents the darkness of the color on a scale of 0 to 100, with 0 representing the darkest black and 100 representing the brightest white. The a* and b* coordinates represent the color and have numbers from -100 to 100, with positive a* representing red shades, negative a* representing green shades, positive b* representing yellow shades and negative b* representing blue shades.” (Figure 1).

Figure 1 – L*a*b* Color System Coordinates

To determine if there are differences in color, measurements at two different locations are compared, or the measurements at the same location, but at two different points in time, are compared. Differences or changes are designated using the symbol ∆ (delta).  For example, if differences between light to dark or black to white were of interest, the investigator would look to the ∆L* between the measurements.  Most often, though, the concern is the change in total color, termed ∆E (delta E). ∆E is the sum of the squared values for ∆L*, ∆a*, and ∆b*.  

Color change that is perceptible to the human eye is generally accepted as 3∆E or more for light colors, and 2∆E or more for dark colors, provided the original color is available for direct comparison.     

The best way to determine if a color shift has occurred is to measure the color at the time of installation to establish a baseline.  Measurements should be taken at specific locations and the locations documented, so that future measurements can be made in the same spots.  Drawdowns of the color samples can also be obtained during the material selection process (Photo 2), or small test panels can be painted at the time of the initial application. The samples should be stored in a folder or envelope (out of sunlight) and can be held to the surface in the future for both a visual assessment of change, as well as side-by-side instrument readings. 

Photo 2

Industry Standards Governing Gloss Measurement

There is one ASTM standard method for measurement of gloss: ASTM D523 Standard Test Method for Specular Gloss, which describes the procedures for measuring gloss of nonmetallic specimens at 20°, 60°, and 85° incidence angles.

The reason gloss is measured using 3 different geometries, 20°, 60°, and 85°, is to differentiate low gloss to high gloss levels.  The angle recommended for measurement depends on the value of the measurement obtained at a designated starting point (60° angle), as shown in the Table 1.

Table 1: Recommended Measurement Angles

Gloss meters are available to measure at one, two, or all three angles. Generally, meters that measure at multiple angles are more costly than single angle meters. Gloss measurements taken at a specific angle cannot be converted to different angle.

Calculating Gloss Retention

When determining gloss retention, the gloss level (in Gloss Units, or G.U.) of a newly applied coating is compared with the gloss level (in G.U.) after a certain number of months/years of exposure. 

Gloss retention is usually expressed as a percentage:

Percent Gloss Retention = 100 x (Gloss_initial – Gloss_final)/Gloss_initial

For example, if the measurement of the newly applied coating is 70.6 G.U. and the measurement after 5 years exposure is 33.5 G.U., the percent gloss retention is calculated as:

Percent Gloss Retention = 100 x (70.6 – 33.5)/70.6 = 52.5%

It is important that the initial and interim/final gloss measurements are obtained using the same angle and the same meter when percent gloss retention is determined.

Summary

Color and gloss (sheen) is a science that influences our “psyche.” It affects how we feel in an environment. It is used for branding, safety indicators, or basic aesthetics. Bright colors with high gloss can energize us, while soft colors with a lower gloss may have a calming effect. Determining whether manufactured batches of coatings match the selected color and sheen level may be required on industrial or commercial coating projects where color and gloss are critical aspects of the final product. I predict you will see greater emphasis placed on color selection and color and gloss retention as our expectations grow for longer lasting finish coats.

[1] https://lumiflonusa.com/case-studies/bridges/the-story-of-hell-gate-bridge-and-its-faded-coating/