Lightfastness is a property of a colourant such as

dye Juan de Guillebon, better known by his stage name DyE, is a French musician. He is known for the music video of the single "Fantasy Fantasy is a genre of speculative fiction that involves supernatural or Magic (supernatural), magical ele ...

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

that describes its resistance to fading when exposed to light. Dyes and pigments are used for example for

dyeing Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular ...

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, and different types of fabric. At first, the word "textiles" only referred to woven fabrics. However, weaving is no ...

plastic Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...

s or other materials and manufacturing

paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...

s or printing inks. The bleaching of the color is caused by the impact of

ultraviolet radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

in the chemical structure of the

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s giving the color of the subject. The part of a molecule responsible for its color is called the

chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...

. Light encountering a painted surface can either alter or break the

chemical bond A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...

s of the pigment, causing the colors to

bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color from (i.e. to whiten) fabric or fiber (in a process called bleaching) or to disinfect after cleaning. It often refers specifically t ...

or change in a process known as photodegradation. Materials that resist this effect are said to be lightfast. The

electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...

of the sun contains

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s from gamma waves to radio waves. The high energy of

in particular accelerates the fading of the dye. The

photon energy Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequenc ...

of UVA-radiation which is not absorbed by atmospheric

ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...

exceeds the dissociation energy of the carbon-carbon single bond, resulting in the

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

of the bond and fading of the color.

Inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...

colourants are considered to be more lightfast than organic colourants. Black colourants are usually considered the most lightfast. Lightfastness is measured by exposing a sample to a lightsource for a predefined period of time and then comparing it to an unexposed sample.

Chemical processes

During the fading, colourant molecules undergo various chemical processes which result in fading. When a UV-photon reacts with a molecule acting as colourant, the molecule is excited from the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...

to an excited state. The excited molecule is highly reactive and unstable. During the

quenching In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, suc ...

of the molecule from excited state to ground state, atmospheric

triplet oxygen Triplet oxygen, 3O2, refers to the ''S'' = 1 electronic ground state of molecular oxygen (dioxygen). Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradi ...

reacts with the colourant molecule to form

singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemistry, inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are Radical (chemistry), spin p ...

and

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

oxygen radical. The oxygen atom and the superoxide radical resulting from the reaction are both highly reactive and capable of destroying the colourants.

Photolysis

Photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

, ''i.e.,'' photochemical decomposition is a chemical reaction where the compound is broken down by the photons. This decomposition occurs when a photon of sufficient energy encounters a colorant molecule bond with a suitable dissociation energy. The reaction causes homolytic

in the chromophoric system resulting in the fading of the colourant.

Photo-oxidation

Photo-oxidation, ''i.e.,'' photochemical

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

. A colorant molecule, when excited by a photon of sufficient energy, undergoes an oxidation process. In the process the chromophoric system of the colorant molecule reacts with the atmospheric oxygen to form a non-chromophoric system, resulting in fading. Colorants which contain a

carbonyl group In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...

as the chromophore are particularly vulnerable to oxidation.

Photoreduction

Photo-reduction, ''i.e.,'' photochemical reduction. A colorant molecule with an unsaturated

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

(typical to

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

s) or

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

(typical to

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s) acting as a chromophore undergoes reduction in the presence of hydrogen and photons of sufficient energy, forming a saturated chromophoric system. Saturation reduces the length of the chromophoric system, resulting in the fading of the colorant.

Photosensitization

Photosensitization, ''i.e.,'' photochemical sensitization. Exposing dyed cellulosic material, such as plant-based fibers, to sunlight allows dyes to remove hydrogen from the cellulose, resulting in photoreduction on the cellulosic substrate. Simultaneously, the colorant will undergo oxidation in the presence of the atmospheric oxygen, resulting in photo-oxidation of the colourant. These processes result in both fading of the colorant and strength loss of the substrate.

Phototendering

Phototendering, ''i.e.,'' photochemical tendering. As a result of UV light, the substrate material supplies hydrogen to the colourant molecules, reducing the colorant molecule. As the hydrogen is removed, the material undergoes oxidation.

Standards and measure scales

Some organizations publish standards for rating the lightfastness of pigments and materials. Testing is typically done by controlled exposure to

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

, or to artificial light generated by a

xenon arc lamp A xenon arc lamp is a highly specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light to simulate sunlight, with applications ...

. Watercolors, inks,

pastel A pastel () is an art medium that consists of powdered pigment and a binder (material), binder. It can exist in a variety of forms, including a stick, a square, a pebble, and a pan of color, among other forms. The pigments used in pastels are ...

s, and

colored pencil A colored pencil (American English), coloured pencil (Commonwealth English), colour pencil (Indian English), map pencil, pencil crayon, or coloured/colouring lead (Canadian English, Newfoundland English) is a type of pencil constructed of a na ...

s are particularly susceptible to fading over time, so choosing lightfast pigments is especially important in these media. The most well known scales measuring the lightfastness are the Blue Wool Scale, Grey scale and the scale defined by ASTM (American Standard Test Measure). On the Blue Wool Scale the lightfastness is rated between 1–8. 1 being very poor and 8 being excellent lightfastness. In grey scale the lightfastness is rated between 1–5. 1 being very poor and 5 being excellent lightfastness. On ASTM scale the lightfastness is rated between I-V. I is excellent lightfastness and it corresponds to ratings 7–8 on Blue Wool Scale. V is very poor lightfastness and it corresponds to Blue Wool scale rating 1. The actual lightfastness is dependent on the strength of the radiation of the sun, so lightfastness is relative to geographic location, season, and exposure direction. The following table is listing suggestive relations of the lightfastness ratings on different measure scales and the relation relative to time in direct sunlight and normal conditions of display: away from a window, under indirect sunlight and properly framed behind a UV protective glass.

Test procedure

The relative amount of fading can be measured and studied by using standard test strips. In the workflow of the Blue Wool test, one reference strip set shall be stored protected from any exposure to light. Simultaneously, another equivalent test strip set is exposed under a light source defined in the standard. For example, if the lightfastness of the colourant is indicated to be 5 on the Blue Wool scale, it can be expected to fade by a similar amount as the strip number 5 in the Blue Wool test strip set. The success of the test can be confirmed by comparing the test strip set with the reference set that was stored protected from the light.

In graphical industry

In printing, organic pigments are mainly used in the inks, so the shifting or bleaching of the color of a printing product due to the presence of UV light is usually just a matter of time. The use of organic pigments is justified primarily by their inexpensive cost compared to inorganic pigments. The particle size of the inorganic pigments is often larger than that of organic pigments, thus inorganic pigments are often not suitable to be used in

offset printing Offset printing is a common printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithography, lithographic process, which ...

. In

screen printing Screen printing is a printing technique where a mesh is used to transfer ink (or dye) onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen in a "flood stroke ...

, the particle size of the pigment is not the limiting factor. Thus it is the preferred printing method for printing jobs requiring extreme lightfastness. The thickness of the ink layer affects the lightfastness by the amount of pigment laid on the substrate. The ink layer printed by screen printing is thicker than that printed by offset printing. In other words, it contains more pigment per area. This leads to better lightfastness even though the printing ink used in both methods would be based on the same pigment. When mixing printing inks, the ink with the weaker lightfastness defines the lightfastness of the whole mixed color. The fading of one of the pigments leads to a tone shift towards the component with better lightfastness. If it is required that there will be something visible from the printing, even though its dominant pigment would fade, then a small amount of pigment with excellent lightfastness can be mixed with it.

References

External links

Doing your own lightfastness testsPupulandia: Onko taide ikuista – tai kuuluuko sen olla?
*{{cite journal , doi= 10.1177/004051756303300707 , title = The Lightfastness of Dyes: A Review, journal = Textile Research Journal, volume = 33, issue = 7, pages = 528, year = 1963, last1 = Giles, first1 = Charles H, last2 = McKay, first2 = Robert B, s2cid = 101986249 Pigments Color Photochemistry Properties of textiles Dyeing