A persistent luminescent (photoluminescent or afterglow) coating disperses a rare-earth activated alkaline-earth aluminate pigment, mainly strontium aluminate doped with europium and dysprosium (SrAl2O4:Eu2+,Dy3+, emitting about 520 nm green), into a clear or light continuous binder so the film stores energy while lit and keeps emitting for hours after the excitation source is removed. It needs no electricity or wiring, which is why it is used for escape-route signage, edge outlines, equipment location and some decorative work. It is unrelated to historic radioactive (radium-based) glow materials and to electroluminescent (EL) devices, both in how they are charged and in how they are regulated.
TL;DR — Afterglow brightness and duration are set jointly by the emitting centre (Eu2+) and the trap levels (Dy3+). Under DIN 67510-1 a typical commercial reading is 185/24 mcd/m2 at 10/60 min, which is Class C; higher D and E classes come from raising pigment areal loading and topcoat transmission. The hard engineering is not whether it glows but three fears: water (alkaline aluminates deliquesce and lose light, so SiO2 encapsulation is needed), settling (particle density about 3.5 g/cm3 needs strong thixotropy), and topcoat yellowing (a yellowing binder blocks the excitation light). To be both bright and long you need a dry film of 150-300 um, a pigment volume fraction of 50-70 percent of solids, and a high-transmission non-yellowing clear coat.
Key data and limit anchors
- Emission and excitation: SrAl2O4:Eu2+,Dy3+ peaks near 520 nm (green); its excitation band spans roughly 250-480 nm, so daylight, LED and fluorescent lamps all charge it. The blue-green system Sr4Al14O25:Eu2+,Dy3+ emits near 480 nm.
- Brightness and life scale: versus classic zinc sulfide (ZnS:Cu) glow powder, an aluminate system is typically about one order of magnitude (roughly 10x) higher in both initial brightness and afterglow duration; a standard charge leaves a readable afterglow for 8 h or more, falling fast in the first 60 min and then decaying on a near-hyperbolic long tail.
- DIN 67510-1:2020 brightness classes (afterglow threshold in mcd/m2 at 2/10/30/60 min after charge): Class C 690/140/45/20, Class D 1100/260/85/35, Class E 2200/520/170/70; a product sold as 185/24 is a 10/60 min reading and is Class C. Field checks follow DIN 67510-2 and product requirements refer to ISO 17398.
- Particle size and areal loading: common grades are 10-90 um. Coarse grains catch more light per unit and are brighter but roughen the film; fine grains give a thin even film but emit less per particle. The glow pigment often needs a 50-70 percent volume fraction of solids to absorb and re-emit enough.
- Hydrolysis red line and encapsulation: alkaline-earth aluminate reacts with water and CO2 to form strontium hydroxide Sr(OH)2.8H2O plus aluminium hydroxide and loses luminescence; pigment density is about 3.5-3.6 g/cm3. Waterborne and high-humidity systems must use SiO2, alumina or phosphate encapsulated grades, and anti-settling relies on a thixotropic package such as fumed silica (see nano silica dispersion, anti-settling and matting).
- Topcoat requirement: you need a high-transmission, yellowing-resistant clear coat (aliphatic polyurethane or fluorocarbon, avoiding aromatic yellowing; see the hardener logic in white coating yellowing resistance). No optical brightener may be added to the clear or the colour coat, or it will steal excitation and emission light and disturb the afterglow reading and the safety-sign colour coordinates.
Mechanism and failure paths
Trap and slow release: why it can store and give back light
Eu2+ is the emitting centre, and its allowed 4f to 5d transition gives a strong, broad yellow-green band. Dy3+ (and other trivalent rare-earth co-dopants) create trap levels inside the lattice: under light, electrons are captured by traps, and after the source is removed they are released back to Eu2+ by thermal probability and re-emit. Trap depth and density set the afterglow duration. The long afterglow of strontium aluminate comes from these relatively deep intrinsic and doped traps, not from a simple forbidden transition of an ordinary phosphor.
Fear of water: hydrolysis loss of alkaline aluminates
SrAl2O4 is sensitive to water and carbon dioxide. The surface hydrates first into non-luminescent hydrated strontium hydroxide and aluminium hydroxide, forming a chalky deactivating shell, and the rising alkalinity can also attack the resin. The fix is an inert coating shell on the grain (SiO2, phosphate or alumina) that keeps water away from the pigment; this is the same interface-decides-everything problem discussed for the TiO2/SiO2 surface chemistry of anti-fogging hydrophilic nanocoatings. Solvent-borne and 100-percent systems can use bare powder for higher brightness, but waterborne lines must use encapsulated powder, and the shell slightly dilutes efficiency: a priced, honest trade.
Topcoat yellowing and settling: two engineering dimming paths
The phosphor is an inorganic crystal with a very long life, so the impression that a mark gets dimmer over use usually comes from outside it: a yellowing binder or clear blocks 250-480 nm excitation light, dense pigment sinks and starves the surface layer, and soiling masks the face. The first two are formulation problems, answered by a non-yellowing aliphatic clear and strong anti-settling thixotropy; the topcoat toughness and clarity trade is explored in balancing hardness and toughness in nanoceramic topcoats.
Formulation and application engineering
Binder choice: waterborne, UV and solvent
Indoor signage, toys and decor go waterborne (encapsulated powder, low-VOC resin); fast set-off and high clarity point to UV cure; outdoor or industrial location points to two-component polyurethane. On any line the pigment must be wetted without breaking the encapsulation shell, so dispersion is low to medium shear: over-long bead or sand milling shatters the shell and lets hydrolysis start. After dispersion, thixotropy locks the grains in place; a well-dispersed, anti-settling package such as nano silica dispersion is what protects loading and film uniformity.
Film thickness, double-layer loading and back reflection
Emission is a volume effect: a single dry film of 150-300 um is a normal starting point. For more light use a pigmented base plus a clear top, or two loading layers. A pale or reflective substrate (foil or a white primer) bounces downward-emitted light back to the front and effectively raises output; a dark or black substrate absorbs it and charging efficiency drops sharply.
Clear coat and weatherability
The clear must be highly transparent, non-yellowing and brightener-free. Outdoor parts use aliphatic polyurethane or fluorocarbon and are controlled for post-ageing yellowing (delta YI) and transmission after accelerated testing such as ISO 4892 or QUV. When clear and colour coat have closely matched refractive index, light escapes more easily; too much interface scattering traps it and dulls the glow.
Signage compliance and the structural-colour contrast
For escape-route use, safety colours and symbols follow ISO 3864 and the persistent-luminescence performance follows ISO 17398 and the DIN 67510 series: measure the 10 and 60 min brightness after charge to assign C/D/E, and confirm legibility within 12 h. Factory control can use integrated brightness or a simple luminance meter to sample pigment loading and film uniformity. If you achieve the daytime look with structural colour instead of pigment (see photonic-crystal structural colour), remember it does not change the remove-the-light afterglow path; the two are different mechanisms.
Three glow systems compared
| System | Emission peak | Relative brightness and afterglow | Water resistance | Typical use | Engineering note |
|---|---|---|---|---|---|
| SrAl2O4:Eu2+,Dy3+ (green) | about 520 nm | High, afterglow 8 h or more | Poor, needs SiO2 or alumina encapsulation | Escape signs, outlines, equipment location | Waterborne needs encapsulated powder; low-medium shear dispersion |
| Sr4Al14O25:Eu2+,Dy3+ (blue-green) | about 480 nm | Medium-high, blue-shifted colour | Poor, also humidity-sensitive | Coloured signs, decorative parts | Blend with the green grade for white or cyan |
| ZnS:Cu (classic yellow-green) | about 530 nm | Low, roughly one tenth of an aluminate | Better, and cheap | Low-demand decor, toys | Do not select when brightness and life matter |
Frequently asked questions
Can an afterglow coating replace an emergency light?
No. It stores and releases light passively, brightness falls monotonically with time, and it is meant for escape guidance, edge outlines and equipment location, not for delivering task illumination. Emergency lighting has its own separate standard.
Can a waterborne formula use non-encapsulated glow powder?
Effectively no. Bare alkaline aluminate hydrolyses on contact with water and loses light while pushing pH up and attacking the resin. A waterborne line must use SiO2 or alumina encapsulated grades and accept the small brightness loss the shell brings.
How long to charge, and how long does it glow in the dark?
It depends on illuminance and time. Sunlight or strong light near-saturates in minutes; a few hundred lux indoors needs longer. After the light is removed the first 60 min stay bright (these are the DIN 67510 10 and 60 min rating points) and then a long tail decays; a good aluminate product keeps a readable afterglow for 8 h or more.
It stopped glowing after years; is that pigment decay?
Mostly not. The inorganic phosphor itself is very long lived. A dimmer look usually comes from a yellowing clear or binder blocking light, from pigment sinking so the surface is starved, or from soiling. Countermeasures: non-yellowing aliphatic clear, strong anti-settling thixotropy, and a cleanable face design.
Can the daytime appearance be fully clear or dark?
Not fully clear. Glow powder is pale yellow to yellow-green grains, and at high areal loading the film looks hazy. A dark or black substrate absorbs excitation light and cuts charge-discharge efficiency sharply. When you must hide the daytime look, you trade loading against clarity and output.
Which standard does an export customer inspect against?
Persistent-luminescence safety signs are generally referred to ISO 17398 and inspected with DIN 67510-1 (grading at production) and DIN 67510-2 (field measurement): charge, then measure the 10 and 60 min brightness to assign C, D or E, and confirm legibility within 12 hours. Colour and symbol layout follow ISO 3864.
Last updated: 2026-10-01 | Standards and literature: DIN 67510-1:2020 / DIN 67510-2, ISO 17398, ISO 3864; Materials 3(5):2536-2566 (doi:10.3390/ma3052536), Coatings 13(4):808 (doi:10.3390/coatings13040808). Author: Kexin New Materials (Guangdong) Co., Ltd. technical team. Trade terms: EXW/FOB only.