Analysis of Optical Properties of Luminescent Pigments for Ink

This type of pigment is completely different from the general pigment, and it has a completely new concept in optical properties.
The difference between luminescent pigments and general pigments lies in the fact that the former can absorb certain forms of energy and convert it into photon with emission energy in the heat emission, thus causing luminescence. For general pigments, they can only convert the absorbed energy into heat in the visible region or near visible region, so there is no luminescence phenomenon.

The color of a typical pigment has only selective reflection in the visible spectrum. This selective absorption and reflection gives the general pigment a color. Some absorb the spectrum of this kind of spectral reflectance, and some absorb that kind of spectral reflectance of this kind of spectrum, thus forming countless colors. The colors of the luminescent pigments can all emit rays or be combined with other emitted rays. It does not have much absorption effect in the visible spectral region.

When the blue and yellow colors of the general pigment are equally mixed, green color is obtained (red and blue are subtracted). When the blue and yellow luminescent pigments are properly mixed, white or highly saturated color light is obtained.

Some materials emit visible light after being irradiated with ultraviolet light and have a brilliant sense of color. This phenomenon of absorbing invisible light and emitting visible long-wavelength light is called a cold glow phenomenon. When (ultraviolet) light ceases to illuminate, the substance that stops emitting light is called a fluorescent (body) substance, and if it continues to emit light, it is called a phosphorescent (body) substance.

Luminescent Pigments are sometimes transliterated into Luminescent Pigments. Generally divided into organic and inorganic two categories, the former mainly refers to daylight fluorescent pigments, and the latter refers to zinc sulfide, cadmium sulfide pigments.

Daylight fluorescent pigments are now widely used in the ink and coating industry.

First, organic light-emitting pigments

Organic light-emitting pigments currently mainly refer to daylight fluorescent pigments.

Fluorescent pigments have the property of absorbing light at specific frequencies and regenerating these absorption energies at low rates (long wavelengths). Many natural products such as quinine and various mineral phosphors are only sensitive to ultraviolet light, and many synthetic organic compounds and inorganic pigments can only generate bright fluorescence in the visible spectrum under the excitation of strong ultraviolet light. color.

What is to be discussed here is a pigment that can generate fluorescence after being excited in the ultraviolet region and the visible spectrum region. In the case of daylight fluorescent pigments, because its emission light also includes (additional) ordinary reflection colors, thereby enhancing its emission range (energy), it looks stronger than daylight.

(I) Composition characteristics

Luminescent pigments can increase brightness and visibility due to fluorescence. After they absorb energy, almost all molecules are excited in the low energy state (ground state). Due to the absorption of the quantum emitted in the ultraviolet region and the visible region, the time is very short (about 10-5 seconds), and as a result, electrons are transited to orbits of higher energy levels. When this transition occurs, it can be said that the molecule is in an excited state. A molecule can have many excited states, and each excited state has a certain form of vibration. Absorption of emission energy by a molecule is a quantum process. The energy of photons (photons) can be defined as E=hv, E is energy, h is Planck's constant, and v is the frequency of light absorption. The absorbed energy corresponds to the change in the state of one molecule, and it must be strictly equal to the energy of the photon. For a given molecule, it can only absorb quantum of a certain frequency, and the molecular structure of matter determines these frequencies. For many molecules, their absorption bands are wide, including daylight fluorescent pigments.

In the excited state, the vibration relaxation is faster than the light emission, so the vibration energy disappears immediately due to the collision of adjacent molecules until the molecule enters the low vibration level of the first excited state. Most molecules lose their residual electrons and vibrational energy (due to internal conversion and other deactivation processes) after reaching the lower vibrational levels of the first excitation (single-plex) state. When this happens, the molecules fall into the fundamental energy state without emission. Therefore, in this process, if a molecule selectively absorbs part of the incident light and reflects a part of the remaining light, a color is formed. For example, when a white light falls on a non-fluorescent (ordinary) orange pigment, only the orange wavelength is reflected, and the remaining light is absorbed and converted into heat.

It may also be that some of the excited molecules lose their vibrational energy and then encounter the emission transition to the ground state, which results in fluorescence or phosphorescence, depending on whether the molecule is in an excited singlet or triplet state. The term triplet is used to describe an electronic state in which the rotation of all electrons in a molecule is paired. Since most molecules are singlet in the ground state energy level, if

The rotation of the electrons does not change during absorption, and the excited state is still singlet. When a molecule enters the ground state directly from the excited singlet state with the emission of a photon, fluorescence occurs. The emission lifetime from the excited singlet state to the singly singlet state is 10-9 to 10-7 seconds. These fluorescence spectra are often a specular image of long wavelength absorption bands.

The fluorescence emission is longer than the excitation emission, and the wavelength is longer because the molecule is subjected to vibration deactivation (process) prior to fluorescence emission. According to the Planck relationship, emitted light has very little energy and is of low frequency (long wavelength).

If the excited molecule encounters an electron reversal of the excited triplet state, it will tend to the ground state due to the emission process (phosphorescence). Since the rotation needs to be changed, the emission lifetime of phosphorescence is much longer than that of fluorescence, and the emission lifetime of organic phosphorescent compounds can be from 10-4 to several seconds. Inorganic polycrystalline pigments have a longer life. These pigments include zinc, cadmium, tellurium, antimony sulfides or silicates. Most crystalline phosphors are doped with small amounts of activators such as copper, aluminum, silver, and the like. These inorganic phosphors emit light longer than organic phosphors, sometimes up to 12 hours.

Fluorescent dyes: Although people have done a lot of research on the relationship between molecular structure and fluorescence activity, the results obtained are still just a lot of concepts. Absorption of light is mainly due to the presence of non-bonding electrons, such as in oxygen atoms, in aldehydes. Or non-localized mobile electron-π electrons have emerged. When there are a large number of π electrons in the molecule, the difference between the energy levels becomes smaller, so that the emission of long wavelengths of lower energy (visible light) may excite the molecules. Some aliphatic compounds, such as carotene, have a large number of conjugated bond systems that absorb light, and these compounds have a strong color. Therefore, aromatics with a symmetric conjugated bond system can produce more intense colors and often have fluorescence. Substituents on aromatic fluorescent substances have an effect on fluorescence, and both ortho and para-direct groups tend to increase fluorescence, while meta--direct

The group tends to reduce fluorescence. After extensive research on fluorescent dyes, it was found that most fluorescent dyes contain an anthracycline system in which the ninth and tenth carbons are replaced by groups such as CH, O, N, NH, and S. π electronic system. Electron acceptors, such as halogens, tend to attract electrons, reducing the chance of fluorescence.

Most daylight fluorescent pigments are made from a number of dyes that can change ultraviolet and short wavelength visible light to visible fluorescence. Daylight fluorescence is even worse in the afternoon or day, because the short-wavelength light in daylight occupies a higher proportion.

It must be noted that general orange pigments and daylight fluorescent orange pigments look similar under white light, except that the fluorescent pigments are slightly brighter. However, under the blue or green light, the general orange pigment becomes black (it does not reflect these wavelengths). The fluorescent orange is bright orange.

Daylight Fluorescent Pigments: The attractiveness of daylight fluorescent pigments is incomparable with ordinary pigments. It has exceptional brightness, which makes people look particularly eye-catching and dazzling. For example, the spectral reflection curves of two orange pigments (a common pigment, a fluorescent pigment). It was measured with a modified spectrophotometer. The sample was first irradiated with white light and then measured with a photocell. It can be seen that the absorption of both can reach up to 600 nm, and thereafter, the two are rapidly rising. The curve of the fluorescent pigment reaches the highest peak at 625 nm. Here, it is “obvious reflection” and it is about incident light. More than twice. The obviousness of the peak indicates that the fluorescent pigment has a relatively high purity or saturation. The amount of ultraviolet light, blue light, and green light in the light source significantly affects the amount of fluorescence.

The spectrophotometric data can also be converted to tristimulus values ​​(tristimulus values), and then in C. I. E marked on the color map. It can be seen that almost all daylight fluorescent pigments have a relatively high excitation purity and are close to the spectral trajectory at the edges of the graph. Lemon yellow, yellow orange and red orange have more than 90% purity and are close to the spectral trace. Although some common pigments have similar purity, they are relatively dark (low brightness).

This figure can be thought of as a color cone, the bottom of the brightness is low, 90 ~ 100% of the brightness in the upper part. A bright, general orange pigment with a brightness of about 15%, but with a similar hue and purity to a daylight fluorescent pigment, its brightness can be as high as 55%. On the other hand, most daylight fluorescent pigments are in C. I. E color is three-dimensional, so they are called a new area in color.

The characteristic data of some representative fluorescent pigments are shown in the table below.

Item data
Density (g/cm3)
1.36

Oil absorption (g/100 g)
47-54

Decomposition point (°C)
180-200

Softening point (°C)
145-155

Refractive rate (shot) 1.64

Impermeability (water)
excellent
Impermeability (Ethanol)
Good can
Impermeability (methyl ethyl ketone)
difference
Impermeability (mineral oil)
excellent
Impermeability (linseed oil)
excellent
Impermeability (Toluene)
difference
Impermeability (dioctyl phthalate)
Good can
Alkalinity can be - poor
Acid resistance (oxidant, reducing agent)
Good can
Lightfastness Can-Poor
Average particle size (microns) standard 3-4 (maximum 40-50)

Average particle size (microns) medium 2.5 (max 10-15)

Average particle size (micron) ultrafine 1.2-1.4 (maximum 4-5)

Note: Within the average particle size, the data in parentheses is the largest particle data.

Fluorescent dyes only fluoresce in relatively dilute solutions, and after exceeding a suitable concentration, fluorescence extinguishes due to molecular collisions, re-absorption of emitted light, or other processes. If the solution is frozen into a hard glass, no emission deactivation effect is greatly prevented. Therefore, when the solution is changed from a solution state to a plexiglass or a solid state of a plastic, fluorescence is enhanced. Some resins have been found to have no flow effect on the dye molecules. They not only enhance fluorescence but also improve light fastness.

Some of the major daylight fluorescent pigments are prepared by adding a dilute solution of a dye to a triazine-modified sulfonamide resin. This type of resin is a very brittle organic glass, which is formed by the condensation of tosylamine-formaldehyde with a triazine such as melamine or benzoguanamine.

Fluorescent pigments can also be prepared by adding fluorescent dyes to modified glycerine, phthalic acid or vinyl resins.

A few organic compounds also exhibit fluorescent phenomena in the undissolved state, such as some aromatic aldehydes such as aldehyde-linked nitrogen (2-hydroxy-1-naphthaldehyde).

In the characteristic data of fluorescent pigments, the particle size is a very important indicator. Especially for the ink industry, the fluorescence of the pigment is very strong, but when the particles are too large to print, isn't it too?

Since the resin used as a fluorescent pigment at room temperature has a low hardness and toughness, it is not easy to finely pulverize the pigment particles. To improve its hardness and brittleness, it is easy to crush, so low-temperature (cold) smashing is one of the better processes.

The standard particle size is generally 3.5 μm, which can be used in the coating industry. The medium size particle size can be used for gravure printing inks and fabric inks. The ultrafine particle size is mainly used for offset printing, lead printing and flexographic inks. Among them, the smallest particle size can reach 0.25-0.5 microns.