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Test method for lightfastness of lithographic printing ink

2022-12-29

Sunlight consists of shortwave ultraviolet, visible light and infrared rays. While ultraviolet rays are the primary factor leading to ink degradation, temperature and humidity as secondary factors will also accelerate the rate of degradation.

The lithographic printing ink is exposed to strong sunlight near the window glass, and the environment where the ink on the car dashboard is in summer is rougher. These environments have high ultraviolet rays, high temperatures, and high relative humidity. humidity. The exposure sites for this study were chosen in Florida and Arizona due to the harsh exposure conditions available at these two sites

Lab
Environment.

Choose from 8 colors commonly used in lithographic printing inks: Yellow #1, Yellow #2, Yellow #3, Magenta, Violet, Orange, Red and Purple. Trials used een Little Joe print proofer to provide typical offset film build for prints. The inks are printed on standard 70lb coated paper. Samples of each color are subjected to all types of exposure testing.

 

Ink samples were tested at Weathering Research Services in Florida and Arizona. All samples are installed on the glass-framed lighting fixture, and the lighting angle is 45° to the south, in order to best receive the sunlight through the glass. The sunlight spectrum through glass was chosen because it simulates the harsh indoor lighting conditions.

The color change of the ink sample was measured before, during and after exposure. The color was measured using a spectrophotometer according to ASTM D2244. The color change of each sample is represented by the value of AE.

Florida exposure tests begin each season: Fall Equinox (2002-09-21), Winter Solstice (12-21-2002), Lentenequinox (3/21/03) and Summer Solstice (6/21). /03). Arizona exposure testing begins in the fall (7/10/02). Table 1 shows the total light energy (i.e. irradiance) in MJ/m2. This value is the conclusion reached by various outdoor exposure tests after 90 days of exposure in Florida and Arizona.

 

Figure 1 is a graph of the lightfastness of 8 color inks in the Florida fall test.

 

As shown in the picture, the durability of inks varies greatly. Some pigments are very durable, others are not. After 90 days of exposure, most of the ink samples were too severely faded for analysis. However, after 35 days of exposure, the lightfastness performance of the ink samples began to widen the gap, both good and bad. Therefore, 35 days is used to evaluate the lightfastness of the ink in various outdoor exposure tests.

 

This graph shows very clearlythe durability range of 3 different inks of the same color. Although the color is the same, their lightfastness performance is completely different. Yellow A significantly outperformed Yellow B and Yellow C. In fact, Yellow A is a lightfast yellow ink suitable for artwork or outdoor applications, while Yellow B and Yellow C are mostly used for general commercial printing.

Effects of seasonal variations

Due to the short duration of natural exposure (35 days), the test was repeated in each season to determine the effect of the time of year on ageing. The results showed that testing at different times of the year had no effect on the ranking of lightfast properties. The sequence was the same between seasons of exposure, but there were some differences in the rate of aging.

To quantify the difference in aging rate from season to season, comparedwe averaged values ​​of ΔE over 35 days of exposure. For example, the average AE for all inks in the Florida winter exposure test was 21, while the average AE for all inks in the Florida fall exposure test was 44. In this example, the fall trials were about twice as tough as the winter trials. Sample Absolute Values ​​They cannot be compared at different times of the year due to differences in aging rates due to the seasons. This inconsistency may not show up in accelerated lightfastness testing.

Table 2 AE color changes of inks in various outdoor exposure tests

 

Table 2 shows the AE of all ink samples after 35 days of exposure in Florida and Arizona Value:

Figure 3 Correlation of Ink Lightfastness

 

Figure 3 is a comparison table of Florida's 35-day fall and 45-d exposure testsagent in winter. The product performance ratings of the two exposure test results agree very well.

 

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