Color, color comparison and color fastness of geomembranes

The Geosynthetic Institute is developing a new geomembrane specification for colored polyethylene geomembranes. It is called GRI Standard GM-39 “Standard Test Method for Determining the Color, Color Comparison and Colorfastness of Geomembranes.” This method is needed to access color difference as a Manufacturing Quality Control (MQC) tool to control production or to evaluate colorfastness as a durability evaluation.

The surface color and subsequent colorfastness of a geomembrane is one of the basic appearance properties project stakeholders use to evaluate quality. For an exposed geomembrane, one does not want to have extreme color differences between rolls, as this situation might be visually unappealing. This method can be used in conjunction with laboratory or field procedures for evaluating the relative resistance of geomembranes to change in color when exposed to UV radiation or weathering. Geomembranes may change in color because of exposure over time. The test method is applicable to all commonly available colored geomembranes such as black, white, green and blue, regardless of texturing or scrim reinforcement.

This test method uses colorimeters to capture, communicate and evaluate color as shown in Figures 1 and 2. For geosynthetics, color measurement devices help ensure the color being produced matches the color that originally was specified. The industry uses colorimeters for QC because they use wavelengths that are only in the visible range, while spectrophotometry can use wavelengths in a wider range, which is the preferred choice for research and development. Colorimeters are affordable, portable and easier to use, making them suitable choices for basic color measurement applications.

The ability of a polymer to resist fading after being subjected to the elements is a basic durability challenge of any colored geomembrane. Fading can be evaluated in the following five ways:

• Color fastness due to abrasion
• Color fastness due to washing/dry cycling
• Color fastness due to chemical solutions (e.g., acid/base, salt, chlorine)
• Color fastness due to sunlight
• Color fastness due to heat and/or pressure

The ability of a plastic material to resist deterioration caused by exposure to the above is an indication of the quality of the geomembrane and stability of the polymeric formulation. This practice is intended to induce property changes associated with end-use conditions.

There is color everywhere, but the perception of that color changes based on changing parameters. Each person has a different visual perception of an object, depending on the object’s material composition, surface texture and the type of light directed at it. Color is described as what someone sees when light is reflected from an object at a different wavelength; the brain then interprets these signals.

Colorimeters help measure the color of the geosynthetic by using “L,” “a” and “b” values. It is important to know what these values mean. The colorimeter can be set to measure single points on a surface or compare two parts against each other. The values are as follows:

• The “L” refers to the lightness of the part ranging on a scale of 0 to 100. Zero is pure black (dark) and 100 is pure white (light).
• The “a” refers to the red-green range. A negative “a” value is green and a positive “a” value is red. Zero is neutral and unlike the “L” value, “a” has no defined range limit.
• The “b” refers to the blue-yellow range. A negative “b” value is blue and a positive “b” value is yellow. The higher the “b,” the more yellow it is. Like “a,” “b” also has no defined range limit.

After using a colorimeter, one can use the calculation of Delta E (ΔE) to combine all three values to compare to another sample or a standard coupon. When comparing two samples, ΔE can give an overall difference to help determine if the sample is close in color to the standard. Limits can be set depending on the application. The closer to 0, the closer in color. Generally, the human eye can differentiate anything over a ΔE of 2.0–3.0. Less than 2.0 is considered to be a very close color match, but again, it is dependent on a company’s chosen standard and the application. In general, geomembranes should be able to maintain a Delta E (ΔE) of 4 or below.

Each individual sees color differently, so not every person should make decisions on color matching, because some people differentiate color better than others. When a color is matched, the geomembrane should be in its final form. Color difference can be an MQC tool to control production and colorfastness can be used as a durability evaluation. This method can be used in conjunction with laboratory or field procedures for evaluating the relative resistance of geomembranes to change in color when exposed to UV radiation or weathering.

The GRI GM-39 test method will help ensure color conformity among geosynthetic rolls. We admit that this new standard has been missing from our repertoire of test methods for years. However, the final draft is being reviewed and GRI GM39 will soon be available as an MQC and durability check.