What are Fluorescent Pigments and how do they work?

Blog Archive | 8 minutes  | Author: Erin White , BSc.

What are Fluorescent Pigments?

Pigments that can be stimulated by light to give brighter and more brilliant colours than conventional pigments are known as fluorescent pigments. Often, it is UV light that is responsible for this brilliance, so these pigments can also be known as UV fluorescent pigments. When an abundance of UV light is present, for example in a blacklight, they will be seen as bright and eye-catching colours. If the same eye-catching advantages are wanted without the use of a blacklight, pigments that are stimulated by daylight are needed.

 

In this technical article 

What are Daylight Fluorescent Pigments?

Daylight Fluorescent Pigments (DFPs), such as the Aurora SRA and Aurora AQA ranges, are pigments that are stimulated by daylight to fluoresce, giving bright colours that can be used in a variety of different applications. They consist of fluorescent dyes encapsulated in resins or polymers to create fluorescent pigment powders.

Fluorescent coloured objects are seen three times earlier than objects coloured with conventional pigments due to the fluorescent light given off, meaning they are utilised in applications where catching the eye is beneficial. These include cosmetics, sportswear, stationary, warning signs and safety gear (Figure 1 and 4). Packaging and advertising can also take advantage of drawing the eye more easily. As such, fluorescent pigments are available for several different materials, including paint, ink, oil and water. Many of the applications listed above make use of fluorescent pigments for plastics.

 

Examples of products and applications using daylight fluorescent pigments. These include a high-viz jacket on a building site and brightly coloured stationary including highlighters. It also shows a vial of a fluorescent liquid.

Figure 1: Examples of fluorescence and applications for fluorescent pigments.

 

Features of Daylight Fluorescent Pigments needed for application

To be efficient as a fluorescent pigment, there are some important features DFPs should have. Pigment luminosity, or brilliance, is of obvious importance as is heat and light stability. Solvent resistance and water resistance describe how well the pigment can be introduced to a substance without dissolving or forming a gel. The particles should remain suspended in the substance in order to be effective. For some applications, other features such as opacity or transparency may need to be considered as well.

 

What is Fluorescence?

Fluorescence and luminescence in everyday vernacular are often used interchangeably, but fluorescence is a type of luminescence, specifically a type of photoluminescence. Other types of luminescence include phosphorescence, also a type of photoluminescence, and chemiluminescence. As the prefix photo- suggests, light is a key factor for photoluminescence, in this case, fluorescence, to occur. Simply put, a molecule can absorb a photon of light and therefore its energy, and then reemit a photon of lower energy and higher wavelength. The emitted light is fluorescence or fluorescent light.

Why does Fluorescence occur?

To understand why fluorescence occurs, a brief understanding of the quantum mechanical make-up of a molecule is needed. An atom is made up of a net positive nucleus, which holds in place negative electrons. The electrons are held in discrete energy levels. A system will always favour the lowest energy state and so when atoms combine to make molecules, they do so because this lowers their energy and makes them more stable. When molecules form, new energy levels, known as orbitals, are formed that the electrons can exist in. For each atom or molecule, the energy levels are certain and discrete, and the system is said to be quantised.

The lowest possible energy system, known as the ground state, is always favoured and so the system exists primarily in this state. When energy is put into the system, a molecule is said to be in an excited state and will quickly undergo processes that emit energy from the system to return it to the ground state. One such process is fluorescence.

 

How does Fluorescence work?

When light waves hit the molecule, a photon is absorbed and gives the system energy, which promotes an electron from the ground state (S0) to an excited state (Sn). There are a few processes the molecule can then undergo (Figure 2), but the ones relevant here are vibrational relaxation, internal conversion and, of course, fluorescence.

 

A simplified Jablonski diagram showing the energy processes involved for fluorescence to occur. An arrow upwards represents excitation from the ground state to the second excited state. A squiggly arrow downwards represents vibrational relaxation between different vibrational levels. A squiggly arrow horizontal represents internal conversion between two vibrational levels in two different energy states. An arrow downwards between the first energy state and the ground state represents the emission of a photon in fluorescence.

Figure 2: A simplified Jablonski diagram showing the energy levels in a molecule and the different energy processes involved for fluorescence to occur.

 

Vibrational relaxation

Within each energy state, there are further, smaller energy levels called vibrational levels. The electron will drop between these from wherever it was promoted until it is at the lowest vibrational energy level within that excited state (v = 0). This drop is called vibrational relaxation and it emits a small amount of energy as heat energy.

Internal conversion

When at the lowest vibrational energy level of that state, the electron will need to drop to the next energy state. If in any excited state except the first one (ie Sn+1), it will do this by moving from a low vibrational energy level of the higher excited state to a high vibrational energy level of the excited state below it which sits at the same energy value. This is an isoenergetic process, meaning no energy is lost or gained.

Fluorescence

When, by vibrational relaxation and internal conversion, the electron reaches the lowest vibrational level of the first excited state (S1, v = 0), fluorescence will occur as the electron drops down to the ground state. The remaining energy will be emitted in the form of a photon of light. Due to the previous vibrational relaxation, this energy is not the same amount of energy that was originally absorbed, but lower. This lower energy means that the photon emitted will have a lower frequency and higher wavelength (Figure 3).

 

An electromagnetic spectrum showing UV light at higher frequency and therefore lower wavelength than visible light. Visible light is then enlarged to show the rainbow of light included. The equations E = h x nu (where v is the greek that represents frequency) and E = h x c divided by lambda (where lambda is the greek letter that represents wavelength) are included to show the inverse relationship between the two.

Figure 3: An electromagnetic spectrum with wavelength and frequency ranges for each type of light. As the equation shows, energy is indirectly proportional to wavelength so when the emitted light is at a lower energy it is at a higher wavelength. This higher wavelength means the emitted light is in the visible spectrum and so we can see it.

 

How are Fluorescent Colours Made?

Due to the consistency of the certain energy levels of a molecule, most of the fluorescence emitted light will be of the same wavelength each time. Different wavelengths correspond to different colours and so a set colour will be seen for a set molecule. Due to the vibrational relaxation and internal conversion first, this colour is also independent of the wavelength of light absorbed.

As can be seen in the electromagnetic spectrum (Figure 3), UV light is at lower wavelengths than visible light and so for DFPs, the light absorbed is the UV light within normal daylight and then the light emitted is in the higher wavelength of the visible range so that the human eye can see it.

 

Part of a hand shown holding a bottle of fluorescent yellow nail vanish. The nails are painted with fluorescent vanish in yellow and pink.

Figure 4: An example of an application of fluorescent pigments in the cosmetic industry in nail varnish.

 

Types of Daylight Fluorescent Pigments

Conventional DFPs

Melamine formaldehyde encapsulated pigments are among the most common types of DFPs. These offer high fluorescence as well as excellent solvent resistance and heat and light stability. Our Aurora SRA range is used in a wide spectrum of applications from safety signage through to novelty applications is available in a range of different colours. 

New hybrid polymer DFPs

Formaldehyde is now known to have carcinogenic effects so in some industries formulators are looking to new formaldehyde-free technologies.  Historically, low solvent resistance was the main reason that formaldehyde-free DFPs were not widely used as where incorporation into solvents was required, the encapsulated pigments would dissolve creating a gel. Optimisation of these resins went through several iterations but these were at the compromise of solvent resistance, pigment luminosity or stability. A hybrid polymer, as found in the Aurora AQA range, has been found to not only match but in some cases improve upon the formaldehyde-containing versions for solvent resistance without compromising on other features (Figure 5).

 

A radar chart with two lines on - one for Aurora SRA and one for Aurora AQA. It shows the Aurora AQA either matching or exceeding the Aurora SRA on the following categories. Heat stability, formaldehyde free, light stability, solvent resistance, colour luminosity and water resistance.

Figure 5: A radar chart showing some important features that DFPs need and how the Aurora SRA and Aurora AQA series match up against them.

 

Luminosity intensity tests show similar results between equivalent colours of the two ranges with reflections 2-3 times higher than traditional non-fluorescent pigments. Many colours are available including fluorescent pink, fluorescent orange and fluorescent yellow pigments.

The Blue Wool Scale (BWS) measures how much colour degradation occurs on a sample when compared to an otherwise identical sample left in a completely dark room. This is the light fastness, or light stability, of the pigment. Here, Aurora AQA exceeded their conventional counterparts, making them suitable for use in paints and spray cans.

Other applications require the pigment to be able to withstand high temperatures and so the heat stability becomes important. Here again, the new hybrid polymer DFPs, show resistance to comparable temperatures to the same colour in the Aurora SRA range. Temperatures up to 240 °C could be reached maintaining colour strength and 280 °C with the pigment remaining stable.

Summary

Daylight Fluorescent Pigments can create vivid and eye-catching effects.  Conventional daylight fluorescents such as our Aurora SRA range were based on melamine formaldehyde technology however formaldehyde free alternatives can not only provide good replacements of traditional DFPs but can, in some cases, improve upon them. With the colours easily replicated, good stability in both aqueous and solvent based products and high heat and light stability, the Aurora AQA can be widely used in a range of luminous applications.  For further insight and for recommendations for your formulation, contact your account manager or call us to discuss your requirements.

Author: Erin White , BSc.

Erin studied at the University of York where she earned a BSc in Chemistry and has just completed a MSc in Atmospheric Chemistry. She has recently joined us and will be covering areas across all markets.