Lightfastness in Package Printing

When you take into account the time and effort spent on supplying packaging with precise colors, it follows the expectation that these colors should stay the same during the product use cycle. Noticeable fading or hue shift of a corporate color projects a negative quality and freshness image to the consumer.

Some of the largest product claims result from neglecting to match the required permanence of the print color to the package application. There is no way to bring back a faded package. Fortunately, there are well-established protocols to avoid fading problems based on end-use exposure. It is important this starts at the design stage since the need for more permanence in color usually incurs a cost premium along with some limitations in color gamut.

Basically, color permanence to light and weather exposure is a matter of correct pigment selection. This can be broken into three exposure categories.

  • Standard Low Lightfast

    Most store packaging falls into this category. Exposure to incandescent and fluorescent lighting is generally for less than 30 days. In these inks, it is generally acceptable to use the most fugitive pigments such as fluorescents, rhodamine, and methyl violet.

  • Limited Lightfast

    Some packaging can see short duration outdoor exposure of less than 30 days. An example would be beverage carriers that often are stacked for display outside gas stations. Many low lightfast pigments fade within days under these conditions. More stable yellow, orange, red, and violet pigments are needed here.

  • Outdoor Exposure

    Packaging for garden and landscaping products such as fertilizer and mulch can see outdoor weathering of more than 30 days. The pigments required often are described as “outdoor paint grade” and have proven weather exposure stability. In this category, the stability of the ink vehicle and substrate must be considered.

How To Test

Ink suppliers will have suitable ink sets for all these categories and will be familiar with the following tests.

ASTM D3424-01 Standard Test Methods for Evaluating the Relative Lightfastness and Weatherability of Printed Matter covers the following conditions. Two involve exposure to natural daylight and five involve accelerated procedures in the laboratory.

1.1.1 Test Method 1

Daylight behind window glass;

1.1.2 Test Method 2

Outdoor weathering;

1.1.3 Test Method 3

Xenon-arc apparatus with window glass filters to simulate daylight behind window glass;

1.1.4 Test Method 4

Xenon-arc apparatus with water spray and daylight filters to simulate outdoor weathering;

1.1.5 Test Method 5

Enclosed carbon-arc apparatus without water spray;

1.1.6 Test Method 6

Enclosed carbon-arc apparatus with water spray;

1.1.7 Test Method 7

Fluorescent lamp apparatus to simulate indoor fluorescent lighting in combination with window-filtered daylight.

While there are huge numbers of variables in all of these methods, careful correlation tests can help select suitable ink formulations and avoid field failures.

Setting a Standard

There is always some degree of fade during exposure. Brand owners should have final say in what is acceptable in terms of color changes during product use.

This would start with a visual exposure display generated by one or more of the methods above and captured by instrumental readings using a spectrophotometer and gloss meter to quantify the changes. It is necessary to check and assign individual standards for every brand color since fading shifts are unique to each pigment combination.

What Can Go Wrong?

Generally there are few failures on indoor packaging. As soon as packaging is placed outdoors, many more variables are at work. Here we have differences in sunlight, exposure angle, rain, humidity, temperature, and atmospheric pollutants. As a result, it is possible for the ink vehicle to fail and become powdery or flake off the substrate.

So it is not safe to assume lightfast pigments alone will have enough protection. The ink vehicle and substrate matter equally in outdoor weathering.

Pastel and tint shades also are more vulnerable to fading. For the printer, it is essential that ink rework streams for lightfast inks are not contaminated with non-lightfast colors.

Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and percormance. Contact him at 636-391-8180; djvargent@sbeglobal.net.


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