Why do dyes fade? A Look at low lightfast dyes
James P. Bernard is Vice President of Colorants at First Source Worldwide and the author of this expert feature
His skill at problem solving has led him through 48 years in the dye industry across virtually all areas of dye use. Once, he advised a university how to dye a bee population destroying crops. Now that’s strategic colour management
When we consider the question, what causes dyes to fade? We are figuring out what dyes are best to use in what situations. Why is understanding fading important? If you choose a dye with poor light fastness, or one that fades quickly, you risk colour variation.
For example, if you dye fabric for outdoor clothing, the sunlight will damage the colour over time. It will fade even faster if you’re using a dye with poor light fastness. Your end customer will not be a happy camper. When choosing dyes, we need to fit them for their end use. This includes their ability to withstand light if needed. We want happy campers!
Understanding the Structure of Dyes
So, why do dyes fade? To answer this, we need to review the characteristics of dyes that provide colour.
Simply put, dyes are organic compounds that absorb light in the visible spectrum. These compounds have a colour bearing group – called a chromophore – that provides the hue. They’re structured with a conjugated system, which has alternating double and single bonds. They also exhibit resonance of electrons. If any one of these characteristics is not in the molecular structure, we’ll lose the colour (1): 1. GENERAL INTRODUCTION TO THE CHEMISTRY OF DYES 1.1
This point is key to understanding why colour fades. I am going to explain using azo dyes as an example, which comprise the largest group of dyes made commercially in today’s world.
Three factors that affect Colourfastness
It is a well-known fact that sunlight causes colours to fade. It is a less known fact that certain chemicals and pH can have the same effect. Let’s explore what happens in all three instances.
The damaging sunlight
When dyes are exposed to sunlight, the ultraviolet light breaks the azo bond in the dye molecule. This can happen over a short or a long period of time depending on the structure of the dye. It is a natural process that is difficult to avoid. Even the most lightfast of dyes will eventually fade or change colour.
pH level variability
In the same manner a very high or very low pH can destroy the azo bond. In some cases, this can happen fast…even less than a minute. I have seen this happen in the lab many times. One second the blue colour is there, and in another second it is gone.
Chemical usage effects
We know what happens when we spill bleach on our favourite sweater or shirt. The colour fades and vanishes. Now, it’s a chemical causing the rupture of the azo bond in the dye molecule.
This reaction is also used in our environment to clean water. Introducing bleach to a coloured waste water system is an effective method of getting rid of the colour. Over time, with repeat injections of bleach a waste water treatment plant can use this in the initial phase of their treatment programme.
Should I not use fading dyes?
Thus far, I may have given you the impression dyes that fade are bad dyes. But, this is not necessarily true. Here are examples of when it is a good thing.
When driving through construction areas, you may notice a graded green mat on sides of the roads. This is hydro mulch. It contains a proprietary mix of products that contains coloured straw and grass seed. They dye the mix green and it fades over time as the grass begins to germinate and grow. By the time the grass is a couple of inches tall, the green colour fades and you’re left with the natural grass colour.
Another use for fading dye is in marking painted ceiling areas. These products are useful when you want to track where you’re painting a ceiling white. Using a tinted red paint that turns white as it dries will allow for an even coat, since you can track where you’ve already applied the paint.
Similarly, when sealing concrete, it is sometimes hard to see where you applied sealant. Some opt to use a sealant that is tinted green and fades to clear when dry. This is another example of where there’s value in using fading dyes.
The curious case of Red Dyes
Why does it seem that red dyes fade faster than other dyes? There are many theories about the cause, but none are definitive. I believe it is a combination of the size of the red dye molecule, the chromophore, and the structure of the dye. The characteristics of these three things produce the perfect storm for the colour red to absorb a lot of energy in shorter wavelengths of light. This makes it weak over time and causes the colour to fade faster.
Keep in mind the fading properties of dyes you’re interested in using for your processes. Be sure to choose the one that best meets your expectations for how it will fade. Doing the research and testing now can save you from the unexpected colour changes down the line.