Omar Khalid
Glow-in-the-dark plastic is made using phosphors, a group of substances that radiate visible light after being energised. The phosphor is mixed into plastic and moulded to make glow-in-the-dark objects. The most common phosphors used in glow-in-the-dark toys are Zinc Sulfide and Strontium Aluminate, the latter being more modern and having a longer persistence. The light from the sun or a lamp energises the phosphors in the plastic, and in the dark, you can see their atoms slowly losing this energy in the form of a dim glow.
| Characteristics | Values |
|---|---|
| Chemicals used | Phosphors, Strontium Aluminate, Zinc Sulfide, Europium |
| Chemical process | Photoluminescence, Thermoluminescence, Triboluminescence, Radioluminescence, Crystalloluminescence |
| Energy source | Light, heat, pressure |
| Persistence | Modern glow-in-the-dark products can last several hours |
| Colour | Green, as it is the brightest and longest-lasting colour |
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What You'll Learn
- Phosphors: substances that radiate visible light when energised
- Strontium Aluminate: a modern phosphor with a long persistence
- Chemiluminescence: chemical reactions that produce light, like in glow sticks
- Bioluminescence: biochemical reactions in living cells that cause a glow
- Thermoluminescence: the release of light from heat, e.g. chlorophone
Phosphors: substances that radiate visible light when energised
Phosphors are chemical substances that radiate visible light when energised by certain types of light energy. They are used in a wide range of applications, from TV screens and fluorescent lights to cosmetic creams and postage stamps. Some phosphors emit light only while being energised, and these are called fluorescent. Others, called phosphorescent, continue to emit light after the source of energising energy is removed. Phosphorescent phosphors are used in glow-in-the-dark products, which are typically energised by normal light and have a long persistence (glow time).
Phosphors can be natural or synthetic. Natural phosphors are found in the exoskeleton of scorpions, in teeth, and in fingernails. Chemists have also created hundreds of synthetic phosphors, such as copper doped zinc sulfide (ZnS:Cu), which is the most common phosphor used and yields a blue-green light. ZnS:Cu is often used in cosmetic creams for Halloween makeup.
The persistence of a phosphor generally increases as the wavelength increases. Phosphors can be classified into two categories based on their persistence time: fluorescent substances, which emit energy immediately and stop glowing when the exciting radiation is turned off, and phosphorescent substances, which emit energy after a delay and continue to glow after the radiation is turned off, decaying in brightness over time.
Phosphors can be energised by various sources, including visible light, ultraviolet rays, and electron beams. When a phosphor is exposed to radiation, its orbital electrons are excited to a higher energy level. As these electrons return to their former level, they emit the extra energy as light of a certain colour. This process is known as luminescence.
Moisture can impair the lifetime of phosphors, and they may also lose efficiency due to changes in the valence of activators, degradation of the crystal lattice, diffusion of atoms, or chemical reactions with the environment.
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Strontium Aluminate: a modern phosphor with a long persistence
Glow-in-the-dark objects contain phosphors, substances that radiate visible light after being energised. Strontium aluminate is a modern phosphor with a long persistence and is frequently used in glow-in-the-dark objects. It was discovered by Yasumitsu Aoki (Nemoto & Co.) and patented in 1994. Strontium aluminate is now the longest-lasting and brightest phosphorescent material commercially available. It is superior to its predecessor, copper-activated zinc sulfide, being about 10 times brighter and longer-lasting.
Strontium aluminate is often used in glow-in-the-dark toys, where it replaces the cheaper but less efficient copper-activated zinc sulfide. It can also be used as a phosphor in fluorescent lamps in photocopiers and other devices. The glow intensity depends on the particle size, with larger particles generally producing a better glow.
Strontium aluminate can be formulated to phosphoresce at various wavelengths, resulting in different colours. The most common formulation emits a green light with a wavelength of 520 nm. It can also be formulated to emit aqua or blue-green light with a wavelength of 505 nm, or blue light with a wavelength of 490 nm. Strontium aluminate can also be formulated to phosphoresce at longer wavelengths, such as yellow to red, but these emissions are often dimmer.
Strontium aluminate phosphors can be prepared by the sol-gel process, and the wavelengths produced depend on the internal crystal structure of the material. The phosphor is usually fired at about 1250 °C, and subsequent exposure to temperatures above 1090 °C is likely to cause a loss of its phosphorescent properties. Strontium aluminate is insoluble in water and has a pH of approximately 8. It can be coated onto metal surfaces to create a long-persistence phosphor. This coating process can also be applied to paper sheets via spray-coating followed by thermal fixation, resulting in a phosphorescent paper surface.
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Chemiluminescence: chemical reactions that produce light, like in glow sticks
Glow sticks, also known as light sticks, chem lights, light wands, light rods, and rave lights, are self-contained, short-term light sources. They are used in a variety of settings, including recreation, military, and emergency services. They are also used in marine, transportation, and mining industries.
Glow sticks contain two separate compartments, each with a different chemical solution. One solution contains a diphenyl oxalate compound and a dye, and the other solution contains hydrogen peroxide. When the glow stick is bent, the glass ampoule containing one of the solutions is broken, and the solutions mix, producing light through chemiluminescence. This process does not involve the release or absorption of heat, making it a "cold-light".
Chemiluminescence is a chemical reaction that produces light. In the case of glow sticks, the reaction is initiated by the oxidation of the oxalate ester in the presence of hydrogen peroxide and is catalysed by a base such as sodium acetate. The initial oxidation product is 1,2-dioxetanedione, which decomposes to electronically excited carbon dioxide. This process alone does not produce light, but when a fluorescent dye is added, it captures the energy from the carbon dioxide and releases it in the form of visible light. The energy of the light produced depends on the structure of the molecule, allowing different colours to be achieved.
The light produced by chemiluminescence in glow sticks cannot be turned off and can only be used once. The reaction continues until one of the reactants is used up, at which point the glow stick will stop emitting light.
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Bioluminescence: biochemical reactions in living cells that cause a glow
Bioluminescence is a fascinating phenomenon where certain living organisms emit a visible glow through biochemical reactions in their cells. This occurs when specific chemicals, known as phosphors, absorb and store energy from light exposure. These phosphors then slowly release the stored energy in the form of dim visible light, creating a distinctive glow.
Phosphors are essential to understanding bioluminescence. They are substances that have the unique ability to radiate visible light after being energised. This process of energy absorption and release is known as phosphorescence, and it is a critical mechanism behind bioluminescence. Phosphors can be natural or synthetic, and they play a vital role in both biological and artificial glow-in-the-dark phenomena.
In the context of bioluminescence, the phosphors involved are typically natural and inherent to the organisms exhibiting this trait. These phosphors are often found in specific organs or body parts, such as the tails of fireflies or the tentacles of certain marine animals. The biochemical reactions within the cells of these organisms activate the phosphors, causing them to emit a distinct glow.
The colour of the light emitted in bioluminescence is often green. This is because the human eye is particularly sensitive to green light, making it appear brighter to us. Additionally, the most common, affordable, and non-toxic phosphor also happens to glow green, further contributing to its prevalence in bioluminescent organisms.
Beyond bioluminescence, phosphors are also utilised in various artificial glow-in-the-dark applications. For instance, they are mixed with plastic to create toys, stickers, and other novelty items. Strontium aluminate is a commonly used phosphor in these products due to its long persistence and enhanced brightness compared to alternatives like zinc sulfide.
In summary, bioluminescence is a captivating phenomenon where specific chemicals, namely phosphors, undergo biochemical reactions within living cells, resulting in the emission of visible light. This process, known as phosphorescence, involves the absorption and release of energy, creating a characteristic glow that is often green in colour due to both biological sensitivities and the nature of commonly used phosphors. Understanding bioluminescence not only provides insight into the wonders of nature but also contributes to our knowledge of artificial glow-in-the-dark technologies.
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Thermoluminescence: the release of light from heat, e.g. chlorophone
Thermoluminescence is a process in which light is emitted from heat. The phenomenon is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon heating of the material. This process is distinct from black-body radiation.
High-energy radiation creates electronic excited states in crystalline materials. In some materials, these states are trapped for extended periods due to localized defects or imperfections in the lattice, interrupting normal intermolecular or inter-atomic interactions in the crystal lattice. These states are not stable, as they are constantly affected by vacuum fluctuations.
When the material is heated, the trapped states are able to interact with phonons, allowing the release of light. The amount of light emitted is directly proportional to the original dose of radiation received. This property of thermoluminescence is used in thermoluminescence dating to determine the age of buried objects that have been heated in the past.
While the sources do not explicitly mention chlorophone, they do discuss the thermoluminescent properties of other plastics. For example, the thermoluminescent properties of LiF (TLD-100) single crystals have been studied in relation to plastic deformation. It was found that after exposure to ionizing radiation, the intensity of the thermoluminescent glow decreases with the amount of plastic deformation.
Additionally, the effect of plastic deformation on the thermoluminescence of highly pure KBr crystals has been investigated, showing that plastic deformation increases the area under the thermoluminescence peaks without introducing any additional glow peaks.
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Frequently asked questions
Plastic glows in the dark due to the presence of phosphors, which are substances that radiate visible light after being energized.
Phosphors are a group of substances that radiate visible light after being energized. Some phosphors are natural, like those found in your teeth and fingernails, while chemists have also created many others.
Phosphors store energy when exposed to light and then slowly release it in the form of visible light. This process is called phosphorescence, a type of photoluminescence.
Two examples of phosphors used in glow-in-the-dark products are Zinc Sulfide and Strontium Aluminate. Strontium Aluminate is newer and has a longer persistence than Zinc Sulfide.
The human eye is particularly sensitive to green light, so green appears brightest to us. Green phosphors are also affordable, non-toxic, and have the longest glow time.
