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["title_eng"]=>
string(95) "Pigmentation principles: Pigments and paints with effects – when a color is not only a color "
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string(80) "Principi pigmentacije: Efektni pigmenti i efektne boje - kad boja nije samo boja"
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So far in this series, we've talked about pigments that provide color by absorbing and reflecting light. There is, however, a whole group of pigments that don't work on this principle — or complement it in subtle ways. These pigments are collectively called effect pigments, and they are responsible for metallic sheens, pearlescent effects, color changes with viewing angle, and many other visual effects that classic pigments can't reproduce.
1. What are effect pigments and how do they differ from classic ones?
Classical pigments are solid particles that absorb certain wavelengths of visible light and reflect others. The color we see is relatively independent of the angle from which we view it and the intensity of the light source — red remains red whether we view it straight on or at an angle. Effect pigments impart color or visual effect by mechanisms that are dependent on geometry — the angle of incidence of light and the angle of view. As a result, their appearance changes with changes in the angle, intensity, or direction of light. It is this dynamism that makes them visually appealing and functionally different from classical pigments. Effect pigments are mostly lamellar — thin, plate-like particles that are oriented parallel to the substrate in the paint film. This orientation is crucial to their visual effect.
2. Metallic pigments
Metallic pigments are tiny metal flakes — most often made of aluminum, copper, or bronze — that reflect light like small mirrors. Thanks to their lamellar shape and metallic surface, they achieve high reflectivity and a characteristic metallic sheen that cannot be achieved with classic pigments. Aluminum flakes give a silver metallic effect. They can be uncolored (the natural silver color of aluminum) or colored with interference coatings — then they are called colored metallics and change color with the viewing angle. Copper and bronze flakes give a warm golden or reddish-golden glow, depending on the alloy composition. They are often used in decorative paints, tampons, and miniature paints. Characteristics of metallic pigments:
• High reflectance — specular reflection that gives a metallic sheen
• Dependence on particle orientation — poor dispersion or application gives an uneven effect
• Sensitivity to oxidation — copper and bronze can darken over time without a protective coating
• Electrical conductivity — aluminum flakes are conductive, which is relevant in some applications

3. Mica and pearlescent pigments
Mica is a natural mineral that can be cleaved into extremely thin, transparent flakes. The paint industry uses synthetic and natural mica flakes, usually coated with thin layers of metal oxides — most commonly titanium dioxide (TiO₂) or iron oxide. The effect is created by the interference of light: light is reflected on the upper and lower surfaces of a thin oxide layer, and these two reflections mutually enhance or cancel each other depending on the thickness of the layer and the wavelength of the light. By controlling the thickness of the oxide layer, almost any color of the spectrum can be achieved. The thickness of the layer of just a few hundred nanometers determines whether the particle will be gold, blue, green or red. The pearlescent (pearlescent) effect is created when multiple layers of mica of different thicknesses interfere with each other — the result is a deep, multi-layered shine that resembles pearl or shell surfaces. Unlike metallics, mica is not metallic — its particles are transparent and light passes through them. Because of this, mica simultaneously provides a glossy effect and a certain level of transparency, which metallic pigments cannot.
4. Holographic and chrome pigments
Holographic pigments contain micron-sized diffraction gratings — surfaces with an extremely regular microstructure that decompose white light into spectral colors. The result is a characteristic rainbow effect that changes rapidly and dramatically with the viewing angle. Each particle is actually a small prismatic element. Chromic (chameleon) pigments are a more advanced version — multilayer structures that display two or more specific colors depending on the viewing angle, unlike the holographic effect that displays the entire spectrum. A typical example is a pigment that appears gold at a right angle and green at a 45° angle. These pigments are particularly challenging to apply — the irregular orientation of the particles in the paint film destroys the effect, so they require careful application and a compatible binder.
5. Thermochromic and photochromic pigments
Thermochromic pigments change color with temperature. The mechanism can be different: some use liquid crystals that change structure at a certain temperature, others are based on chemical compounds that reversibly change structure (leuco dyes). Most commercial thermochromic pigments change color at a certain “transition” temperature — below it they have one color, above it another (usually becoming colorless or pale). Photochromic pigments change color upon exposure to UV radiation. Indoors (without UV) they are colorless or pale; in sunlight they become colored. The mechanism is also reversible — the color is restored when the UV source is removed. They are used in sun-darkening glasses, safety inks, and decorative applications. Both types of pigments have limited durability — the color change cycles gradually degrade the active component. They are not suitable for permanent applications exposed to strong UV radiation or high temperatures.
6. Fluorescent pigments
Fluorescent pigments absorb light — including UV radiation invisible to the eye — and re-emit it as visible light of a longer wavelength. This process, which we described in the diagram of the interaction of light and matter, is called fluorescence. The result is a characteristic “glowing” color that appears brighter than a normally colored surface — because the pigment not only reflects visible light but also adds emitted light from the UV part of the spectrum. This effect is called daylight fluorescence because it is also visible in ordinary daylight, which contains a UV component. Fluorescent pigments are organic compounds — as a rule, they have poor lightfastness. The UV radiation that activates them also degrades them, so fluorescent colors lose their intensity relatively quickly when exposed to sunlight.
7. Phosphorescent pigments
Phosphorescence is similar to fluorescence in that the pigment absorbs energy and emits it as visible light — but unlike fluorescence, the emission persists after the light source is removed. Phosphorescent materials "store" energy in excited electronic states that are slowly discharged, emitting light for minutes or hours. Modern phosphorescent pigments are based mainly on strontium aluminate (SrAl₂O₄) activated with europium and dysprosium. These pigments are significantly more durable and brighter than the older zinc sulfide pigments that have been used for decades, and do not contain radioactive components like earlier "glow-in-the-dark" materials. Phosphorescent pigments are inorganic, which means they have good light fastness and chemical stability. Their color is typically white or slightly yellowish in daylight, and green or blue-green in the dark.
8. Practical application and disadvantages of effect pigments
Effect pigments require more care when applying than classic pigments. A few key points:
• Particle orientation is crucial.
• The substrate affects the effect.
• The binder must be compatible.
• Coverage is limited.
• Dispersion requires attention. As described in the first article in this series, poor dispersion leads to an uneven effect. With metallic and mica pigments, poor dispersion is visible as shiny dots instead of a uniform shine.
Conclusion
Effect pigments extend the possibilities of color far beyond what classical absorption and reflection can achieve. From the metallic luster of aluminum flakes to the pearlescent reflection of interference mica, from the dramatic color change of holographic pigments to the quiet luminescence of phosphorescents — each of these pigments is based on a different physical principle. Understanding these principles helps in selecting the right pigment for the desired effect and in avoiding common application mistakes.
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Dosad smo u ovoj seriji govorili o pigmentima koji daju boju apsorpcijom i refleksijom svjetlosti. Postoji, međutim, cijela skupina pigmenata koji ne funkcioniraju na tom principu — ili ga nadopunjuju na neuočljive načine. Ove pigmente zajednički nazivamo efektnim pigmentima, i oni su odgovorni za metalne sjajeve, biserni odsjaj, promjenu boje s kutom gledanja i mnoge druge vizualne efekte koje klasični pigmenti ne mogu reproducirati.
1. Što su efektni pigmenti i po čemu se razlikuju od klasičnih
Klasični pigmenti su čvrste čestice koje apsorbiraju određene valne duljine vidljive svjetlosti i reflektiraju ostale. Boja koju vidimo relativno je neovisna o kutu iz kojeg gledamo i o jakosti izvora svjetlosti — crvena boja ostaje crvena bez obzira gledamo li je ravno ili pod kutom. Efektni pigmenti daju boju ili vizualni efekt mehanizmima koji su ovisni o geometriji — o kutu upada svjetlosti i kutu gledanja. Zbog toga se njihov izgled mijenja s promjenom kuta, jakosti ili smjera svjetlosti. Upravo ta dinamičnost čini ih vizualno privlačnima i funkcionalno drukčijima od klasičnih pigmenata. Efektni pigmenti uglavnom su pločasti (lamelarni) oblika — tanke, pločaste čestice koje se u filmu boje oričentiraju paralelno s podlogom. Ta orijentacija ključna je za njihov vizualni učinak.
2. Metalik pigmenti
Metalik pigmenti su sitne metalne ljušице — najčešće od aluminija, bakra ili bronce — koje reflektiraju svjetlost poput malih ogledala. Zahvaljujući lamelarnom obliku i metalnoj površini, postižu visoku refleksiju i karakterističnu metalnu blistavost koja se ne može postići klasičnim pigmentima. Aluminijske ljušice daju srebrni metalik efekt. Mogu biti neobojen (prirodna srebrna boja aluminija) ili obojene interferentnim premazima — tada se nazivaju obojeni metalici i mijenjaju boju s kutom gledanja. Bakarne i brončane ljušice daju topli zlatni ili crvenkasto-zlatni odsjaj, ovisno o sastavu legure. Često se koriste u dekorativnim bojama, tamponima i bojama za minijature. Karakteristike metalik pigmenata:
• Visoka refleksija — specijarna (zrcalna) refleksija koja daje metalnu blistavost
• Ovisnost o orijentaciji čestica — loša disperzija ili nanošenje daje neravnomjeran efekt
• Osjetljivost na oksidaciju — bakar i bronca mogu potamniti s vremenom bez zaštitnog premaza
• Električna vodljivost — aluminijske ljušice su vodljive, što je relevantno u nekim primjenama

3. Mica i perlescentni pigmenti
Mica je prirodni mineral koji se može cijepati u iznimno tanke, providne listiće. U industriji boja koriste se sintetski i prirodni listici mice, obično prevučeni tankim slojevima metalnih oksida — najčešće titan-dioksida (TiO₂) ili oksida željeza. Efekt nastaje interferencijom svjetlosti: svjetlost se reflektira na gornjoj i donjoj površini tankog oksidnog sloja, a te se dvije refleksije međusobno pojačavaju ili poništavaju ovisno o debljini sloja i valnoj duljini svjetlosti. Kontrolom debljine oksidnog sloja može se postići gotovo svaka boja spektra. Debljina sloja od samo nekoliko stotina nanometara određuje hoće li čestica biti zlatna, plava, zelena ili crvena. Perlescentni (biserasti) efekt nastaje kada više slojeva mice različitih debljina međusobno interferiraju — rezultat je duboki, višeslojni sjaj koji podsjeća na biserne ili školjkaste površine. Za razliku od metalika, mica nije metalna — njezine čestice su prozirne i svjetlost prolazi kroz njih. Zbog toga mica istovremeno daje i efekt sjaja i određenu razinu prozirnosti, što metalik pigmenti ne mogu.
4. Holografski i kromski pigmenti
Holografski pigmenti sadrže mikronske difrakcijske rešetke — površine s izuzetno pravilnom mikrostrukturom koja razlaže bijelu svjetlost na spektralne boje. Rezultat je karakteristični dugini efekt koji se brzo i dramatično mijenja s kutom gledanja. Svaka čestica zapravo je mali prizmatski element. Kromski (chameleon) pigmenti su naprednija inačica — višeslojne strukture koje prikazuju dvije ili više određenih boja ovisno o kutu gledanja, za razliku od holografskog efekta koji prikazuje cijeli spektar. Tipičan primjer je pigment koji izgleda zlatno pod pravim kutom i zeleno pod kutom od 45°. Ovi pigmenti posebno su zahtjevni za primjenu — neredovita orijentacija čestica u filmu boje uništava efekt, pa zahtijevaju pažljivo nanošenje i kompatibilno vezivo.
5. Termokromni i fotokromni pigmenti
Termokromni pigmenti mijenjaju boju s promjenom temperature. Mehanizam može biti različit: neki koriste tekuće kristale koji mijenjaju strukturu pri određenoj temperaturi, drugi se temelje na kemijskim spojevima koji reverzibilno mijenjaju strukturu (leukobojila). Većina komercijalnih termokromnih pigmenata mijenja boju pri određenoj „prijelaznoj“ temperaturi — ispod nje imaju jednu boju, iznad nje drugu (obično postaju bezbojna ili blijeda). Fotokromni pigmenti mijenjaju boju izlaganjem UV zračenju. U zatvorenom prostoru (bez UV) su bezbojna ili blijeda; na suncu postaju obojeni. Mehanizam je takoer reverzibilan — boja se vraća uklanjanjem UV izvora. Koriste se u naočalama koje se tamne na suncu, sigurnosnim tintama i dekorativnim primjenama. Oba tipa pigmenata imaju ograničenu trajnost — ciklusi promjene boje postupno degradiraju aktivnu komponentu. Nisu pogodni za trajne primjene izložene jakom UV zračenju ili visokim temperaturama.
6. Fluorescentni pigmenti
Fluorescentni pigmenti apsorbiraju svjetlost — uključujići UV zračenje nevidljivo oku — i ponovno je emitiraju kao vidljivo svjetlo više valne duljine. Ovaj proces, koji smo opisali u dijagramu interakcije svjetlosti i tvari, naziva se fluorescencija. Rezultat je karakteristična „sijajuća“ boja koja izgleda svjetlija od obično obojene površine — jer pigment ne samo reflektira vidljivu svjetlost nego i dodaje emitiranu svjetlost iz UV dijela spektra. Taj efekt se naziva dnevna fluorescencija (daylight fluorescence) jer se vidi i na običnom dnevnom svjetlu koje sadrži UV komponentu. Fluorescentni pigmenti su organski spojevi — u pravilu imaju lošu svjetlostalnost. UV zračenje koje ih aktivira istovremeno ih i degradira, pa fluorescentne boje relativno brzo gube intenzitet izložene suncu.
7. Fosforescenti pigmenti
Fosforescencija je slična fluorescenciji u tome što pigment apsorbira energiju i emitira je kao vidljivo svjetlo — ali za razliku od fluorescencije, emisija traje i nakon uklanjanja izvora svjetlosti. Fosforescenti materijali „pohrane“ energiju u pobuđenim elektroničnim stanjima koja se sporo prazne, emitirajći svjetlo minutama ili satima. Suvremeni fosforescenti pigmenti temelje se uglavnom na stroncijevom aluminatu (SrAl₂O₄) aktiviranom europijem i disprozijumom. Ovi pigmenti znatno su trajniji i svjetliji od starijih cinkovih sulfidnih pigmenata koji su se koristili desetljećima, i ne sadrže radioaktivne komponente kao raniji „svijetleci u mraku“ materijali. Fosforescenti pigmenti su anorganski, što znači da imaju dobru svjetlostalnost i kemijsku stabilnost. Boja im je u pravilu bijela ili blago žutača pod dnevnim svjetlom, a zelena ili plavo-zelena u mraku.
8. Praktična primjena i mane efektnih pigmenata
Efektni pigmenti zahtijevaju više pažnje pri primjeni nego klasični pigmenti. Nekoliko ključnih napomena:
• Orijentacija čestica je ključna.
• Podloga utječe na efekt.
• Vezivo mora biti kompatibilno.
• Pokrivnost je ograničena.
• Disperzija zahtijeva pažnju. Kao što je opisano u prvom tekstu ove serije, loša disperzija dovodi do neravnomjernog efekta. Kod metalik i mica pigmenata loša disperzija vidljiva je kao sjajne točkice umjesto ravnomjernog sjaja.
Zaključak
Efektni pigmenti proširuju mogućnosti boje daleko izvan onoga što klasična apsorpcija i refleksija mogu postići. Od metalnog sjaja aluminijskih ljušica do bisernog odsjaja interference mice, od dramatične promjene boje holografskih pigmenata do tihe luminiscencije fosforescenata — svaki od ovih pigmenata temelji se na drukčijem fizikalnom principu. Razumijevanje tih principa pomaže pri odabiru pravog pigmenta za željeni efekt i pri izbjegavanju uobičajenih grešaka pri primjeni.
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