Specialty pigment –Fluorescent pigments

Excerpt:

Chemtip Laboratories

9, Shams Industrial Estate,

Near Parmar Ind. Estate,

Vasai Phata, Vasai (E).

Abstract:

Specialty pigments are those which offer certain effects which are special such as glow, interference, brightness etc.. They are costly and consumed in less quantity. Fluorescent pigment is one type of specialty pigment. An emission of light by a luminescent pigment under excitation is fluorescent. Fluorescent pigments are bright and intense in colour and glow in day light pigment (also called daylight fluorescent).Unlike fluorescence, phosphorescent material does not immediately re-emit the radiation it absorbs.Absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation. (e.g., glow-in-the-dark). Fluorescent pigments are classified into three categories viz. Day light fluorescent pigment, UV fluorescent pigments and IR Fluorescent Pigments. Fluorescent pigments are solid dye solutions in polar resin carriers. Fluorescent pigment manufacturing process involves dissolution of dyes in polymer matrix and pulverizing the resultant matrix into fine powder. Polymer matrix can be thermoplastic resin such as P- toluene sulphonamide (PTS amide), melamine and formaldehyde, polyamide, polyester, acrylate and can be thermoset plastics such as Ptoluene sulphonamide (PTS amide), melamine and formaldehyde and benzoqunamine formaldehyde . Fluorescent pigments are bright and attractive coloured pigments but poor light and thermal stability. Fluorescent pigments are used in plastics, paints, inks, textiles, cosmetics, and security papers.

Introduction

SPECIALTY pigments are those which offer certain effects which are special such as glow, interference, brightness etc.. They are costly and consumed in less quantity. Despite their limited use, these pigments bring unique effects to plastics/paints/inks/textiles and personal care products, thus they are an important class of color additives for the decorative cosmetic industry. Specialty pigments includes effect pigments as discussed in earlier articles, fluorescent pigments, phosphorescent pigments, thermochromics, photochromic etc..

Each specialty Pigment has a specific attribute that makes them stand out when compared to regular pigments. Luminescent Pigments: A material that emits light (visible, IR or UV) upon suitable excitation, without becoming incandescent. Fluorescent Pigment: The emission of light by a luminescent pigment under excitation (e.g., daylight fluorescent). It absorbs light at low wavelength (high energy) such as UV, Visible and emits it at a higher wavelength ( Lower energy) of bright colours. Phosphorescence Pigments :Unlike fluorescence, phosphorescent material does not immediately re-emit the radiation it absorbs.Absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation. (e.g., glow-in-the-dark). Thermochromics pigments: Reversibly altering color upon exposure to a heat source. Photochromic Pigments: Reversibly altering color upon exposure to a UV rich light source. There are many forms of energy that selected luminescent pigments can absorb and convert to luminescence.

Fluorescent pigments

When material is exposed to light it absorbs light of shorter wavelength and immediately emits lights of higher wavelength, fluorescent effect is observed. These pigments absorb various light frequencies (visible and invisible to the human eye) and reemit them, producing intense visible colors that appear to glow, even in daylight. This fluorescent phenomenon can be explained by following figure1a. When a luminescent pigment molecule is excited by light energy, they are excited to higher singlet energy states, which then revert to their original ground state by emitting a portion of their absorbed energy. When reflection is measured as function of wavelength, in case of fluorescent pigment the reflection increases sharply as compared to conventional pigment ( Fig.1b).

Fluorescent pigments are thus classified into three categories

  • Day light fluorescent pigment and

  • UV fluorescent pigments.

  • IR Fluorescent Pigments

Daylight fluorescent pigments

Fluorescent Pigments are solid dye solution in polymer matrix. They are bright with intense colour and glow in day light. Hence called Day light Fluorescent pigments. Daylight fluorescent pigments are an American invention. In the 1930s, Bob and Joe Switzer experimented with ways of combining certain dyes and resins that produced colors far brighter than normal and that had the unique effect of "glowing" under UV or black light. Initially, these new colors and effects found their way into magic shows, stage shows and movie promotion posters. During World War II, these pigments were diverted to the manufacture of bright signal panels as used by the army. After the war, this technology was used in outdoor billboards where the message "jumped off the boards.”

Composition of day light fluorescent pigments

Fluorescent pigments is made up of polymer matrix and dyes. So fluorescent pigments are solid dye solutions in polar resin carriers. Fluorescent dyes in their undissolved state do not fluoresce. Once these dyes are dissolved in a thermoplastic or thermoset carrier resin and are ground into a fine powder, they become fluorescent pigments. Only a few fluorescent dyes that are commercially available can be made into pigments.

  • Dyes used for fluorescent pigments

The dyes used to prepare day light fluorescent pigments are either basic dyes, solvent soluble dyes. Basic dyes such as basic Rhodamine B, Basic violet, Basic yellow etc. solvents dyes are such solvent yellow, methylene blue, methyl violet etc.. Depending upon the colour to be manufactured combination of different dyes are used. To obtain the desired shade a colour additive rule is followed. Different composition of dyes are used to prepare fluorescent pigments colours like yellow, chrome, orange, pink, red, Magenta, Blue, green etc.

Polymer system used for fluorescent pigments

To prepare day light fluorescent pigments different kinds of resins are used they can thermoplastics or thermoset. There are different types of carrier resins for daylight fluorescent pigments.

Thermoplastic Pigment (Melamine Formaldehyde Type)

One of the oldest types of carrier resins is based on para toluene sulfonamide (PTSA) --melamine -- formaldehyde, which is fully condensed resins and melt at a range of about 115-135 deg C, are clear, brittle and easily pulverized into a pigmentary form. (Used in printing inks, textile printing, paper printing etc..)

Thermoset Pigment (Melamine Formaldehyde Type)

These types were the next to be developed. Through post curing and some chemical modification of the PTSA-MF resins, solvent resistance was increased and softening temperature was raised to 175 deg C. (Used in printing inks. Solvent based paints, PVC inks)

Thermoset Pigment (Benzoguanamine Formaldehyde Type)

These are specifically developed for plastics to minimize the plating tendency of all other types. They contain only benzoguanamine formaldehyde type matrix. (May be used in printing inks.)

Thermoplastic Pigment (Polyamide Type)

These types were developed for use in plastics. They are based on polyester and polyamide or combination of both with melting points from 110-150 deg C, depending on the intended application.

Thermoplastic Pigment (Polyester Type)

These also were developed for use in plastics. They are based on polyfunctional glycols and phthalic anhydride. They have lower softening points (around 90 deg C) and are considered less heat stable than PA types.

Thermoplastic Pigment (Vinyl or Acrylic Type)

These are mainly used for calendered vinyl and the matrix is based on PVC or an acrylic resin. They do not contain any formaldehyde and display very high color strength.

Aqueous Dispersions (Acrylic Type)

These are a newer development in fluorescent pigment manufactured and are specifically designed for printing and writing inks. They are manufactured by adding fluorescent dyes into acrylic monomer and then polymerizing, thus creating colored latex particles (globules). They are formaldehyde free and have a very fine particle size (sub micron), which is very suitable for ink manufacture. (Used in printing inks)

Manufacturing process

To manufacture day light fluorescent pigment different process are followed depending upon the grade of polymer system. Fluorescent pigment manufacturing is formulation based process. There are number of ingredients such as dyes, dispersing agents, catalyst, solvents etc. Sequence of addition and composition is the heart of fluorescent pigment manufacturing process. Fluorescent pigments are available in major eight colours viz Yellow, Chrome, Orange, Pink, Red, Magenta, Green, Blue. However, there are number of shades possible by variation of dyes composition. The basic process involves following steps.

Preparation of Solid Dye solution in polymer matrix

In case of PTS amide melamine formaldehyde resin system, a polymer is prepared by reacting these raw materials in appropriate ratio and a combination of dyes are added to polymer matrix. To obtain best fluorescent effect, dyes should be dissolved molecular level in polymer matrix. This can be obtained by controlling manufacturing parameters viz; pH, Temperature, solution of dyes and stage of addition.

Grinding pulverising polymer matrix

The resultant solid dye solution of polymer matrix which is obtained by above process is further undergoes heat treatment to make brittle enough to pulverize either in reactor or dryer. So obtained polymer matrix is grounded to fine particles with pulverize them with micro-pulverizer or jet mill to obtain desire particle size and its distribution. The strength of fluorescent pigments depends upon the particle size and its distribution.

In case of aqueous dispersion of acrylic type fluorescent pigment dispersion an aqueous dispersion of acrylic polymer is prepared by selecting appropriate acrylic monomers such as methyl methacrylate, ethyl methacrylate, butyl methacrylate and water dispersible monomer such sulphonated acrylate. In aqueous dispersion of acrylic polymer, a dyes solution is dissolved to obtain day light fluorescent pigment dispersion.

Properties of day light fluorescent pigments3

  • Fluorescent pigments emit light of higher wavelength so there are bright and intense coloured pigments.

  • Some solvents are easily affected by solvents such acetone, isopropyl, toluene etc., so they cannot be used in solvent based applications. Grades based on benzoguanamine can be used for solvent based applications due to their high solvent resistant properties.

  • Thermal stability of fluorescent pigments is quite poor. They can be used upto 200°C. Certain fluorescent pigment based on polyamide resin can be used upto 300°C .

  • Fluorescent pigment has very poor light stability so their uses are limited.

Uses

  • Day light fluorescent pigments are widely used in plastic industry typical examples in the plastic field are toys, sport articles, packaging materials, safety helmets, traffic cones etc..

  • Certain grade of fluorescent pigments is used in challenging formulations that are solvent based and /or require extreme processing temperatures. These products are used in nail enamels and hair spray formulations.

  • Day light fluorescent pigments are used in paints industry especially indoor applications such as painting of kids room, in paints for ambulances, fire engines, advertising signs, aerosols and clay pigeons.

  • Fluorescent pigments are mainly used for mass coloration of paper, photocopy, and decorative applications. It is also used in textile industry in the design of colors for printed fibers, pure or halftone fluorescent pigments are recommended in order to freshen up the conventional shades and to bring in more depth in color variances.

  • Fluorescent pigments are used in ink manufacturing, typical examples in the graphic arts field are printing packaging and posters, large screen-printed panels or small labels, safety labeling and stationary.

  • Speciality applications of fluorescent pigment are in temporary markings. These are also used in Non-Destructive Testing and pencils.