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string(105) "Pigmentation principles: Binders and solvents - what holds the paint in place and what allows it to flow "
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string(98) "Principi pigmentacije: Veziva i otapala - što drži boju na mjestu i što joj omogućuje da teče"
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So far in this series, we've written about the visible and tangible properties of paint — why paint has a certain shade, why it covers or doesn't cover, why it flows more easily or more slowly. This article is about what lies beneath it all: binders and solvents, the components that determine what remains on the surface when everything else evaporates.
1. What is a binder and what is its role
A binder is a component of paint that holds pigment and filler particles together and ensures their adhesion to the substrate. When the paint dries or sets, the binder forms a continuous film that encapsulates the pigment and protects it from mechanical and chemical influences. The binder determines most of the application properties of the finished paint: film flexibility, hardness, gloss, chemical resistance, adhesion and durability. Two coatings with the same pigment but different binders can behave completely differently — one may be flexible and waterproof, the other brittle and sensitive to moisture.
2. How a binder works — film formation
Depending on the type of binder, the film is formed in one of two basic ways:
• Physical drying
• Chemical crosslinking (polymerization) Acrylics technically undergo physical drying — water evaporates, and acrylic polymer particles combine into a film through a process called coalescence.
Once formed, the acrylic film is chemically stable and practically insoluble in water, but soluble in some organic solvents.
3. Types of binders
Acrylic binders
Acrylic binders are synthetic polymers dispersed in water (latex). They are the most widely used in modern use — they are used in acrylic paints for miniatures, wall paints, primers and varnishes. Characteristics:
• Water is the solvent — easy cleanup, low toxicity
• Fast drying — an advantage for working in layers, a disadvantage for wet-on-wet techniques
• Flexible and waterproof film after drying
• The color changes slightly when drying — acrylics are known to darken as they dry (due to a change in the refractive index of the binder)
Oil binders
Oil paints use drying oils as a binder — most often linseed oil, but also walnut, poppy and safflower. Each oil has slightly different properties: linseed oil dries the fastest and gives a strong film, but it can turn yellow over time; poppy and safflower oils are less yellow but dry more slowly. Features:
• Drying by oxidative polymerization — slow (hours to days), but the film is extremely durable
• Long open time — ideal for wet-on-wet techniques and mixing on the palette
• Rich, deep gloss that acrylics have difficulty reproducing
• Solvents are organic — mineral organic solvents (white spirit, xylene, etc.)
• Some pigments in oil-based paints can yellow over time due to the binder
Alkyd binders
Alkyds are synthetic polymers based on oils and acids — a kind of modernization of oil binders. They dry by oxidative polymerization like oils, but faster and with more predictable properties. They are common in industrial coatings, primers, and decorative paints for metal and wood. Characteristics:
• Faster drying than pure oils, but slower than acrylics
• Hard and resistant film — good chemical and mechanical resistance
• Solvents are organic — mineral organic solvents (white spirit, xylene, etc.)
• May turn yellow over time, similar to oil binders
Gum arabic and other water-soluble binders
Gum arabic is a natural resin used as a binder in watercolors and some inks. It is soluble in water and dries by physical drying — the film remains soluble in water even after drying, which is a characteristic of watercolors (possibility of reactivation). In the same category of water-soluble binders are casein (from milk), egg tempera (egg yolk as an emulsion of oil and water) and gouache binders. Each of them gives a different film with different properties.
4. What are solvents and what is their role
A solvent is a substance that dissolves or disperses a binder, thereby controlling the viscosity of the paint, enabling its application, and determining the drying speed. After application, the solvent evaporates — and this evaporation process is actually the first stage of “drying” the paint. It is important to distinguish a solvent from a thinner. A true solvent dissolves the binder — it molecularly breaks it down and homogeneously incorporates it into the solution. A thinner only lowers the viscosity, but does not necessarily dissolve the binder. In practice, these terms are often used interchangeably, but the difference becomes important when talking about compatibility.
5. Types of Solvents
Water
Water is a solvent for acrylic binders, gum arabic, and other water-soluble binders. The advantages are low toxicity, no flammability, and easy cleanup. Disadvantages include relatively slow evaporation compared to organic solvents and sensitivity to temperature and humidity — drying is significantly slowed down at low temperatures or high humidity.
Turpentine and mineral organic solvents
Turpentine is a natural distillate of coniferous resin and a traditional solvent for oil paints and alkyds. It has a characteristic odor and good solvating effect. Due to its strong odor and potential toxicity with prolonged exposure, in modern use it is increasingly being replaced by mineral organic solvents (white spirit, xylene, etc.) or odorless isoparaffin-based thinners. Mineral organic solvents are a fraction of petroleum with a slightly milder odor than turpentine, but similar properties. Odorless thinners are more refined and have minimal odor — more suitable for indoor use.
Isopropanol and ethanol
Alcohols are used as solvents for some types of inks, varnishes, and shellacs. Isopropanol (IPA) is widely available and is used for cleaning brushes, thinning alcohol-based inks, and various technical applications. It is not compatible with acrylic binders in large quantities — it can destabilize the emulsion.
6. Binder-solvent interaction
Each binder has its own compatible solvents — those that dissolve or disperse it properly. Using an incompatible solvent can lead to: • coagulation — the binder precipitates out of solution in the form of lumps • emulsion destabilization — the paint separates into phases • poor film formation — the paint does not bond properly, remains sticky, or peels • dissolution of previously applied layers — especially a problem with varnishes and topcoats A practical example: acrylic paints are water-based. Adding mineral spirits or turpentine to acrylic paint will not thin it — it will cause the binder to coagulate and destroy the paint. Conversely, adding water to oil paint does not thin it properly — water and oil do not mix.
7. Media, additives and retarders
Media are formulated additives that modify the properties of the paint without disturbing the ratio of the components. For acrylic paints, these can be: drying retarders (retarders) that extend the open time and enable wet-on-wet techniques, liquid media that lower viscosity while maintaining gloss, matte media that reduce gloss, or glaze media that increase transparency. For oil paints, the media can be: varnish (damar, mastic) which accelerates drying and gives shine, siccatives (driers) which chemically accelerate oxidative polymerization, but whose excessive use can cause the film to crack over time. General rule: media are formulated for specific binders and should not be used interchangeably. Acrylic paint medium will not work in oil paint and vice versa.
Conclusion
The binder is what makes the pigment and solvent into paint — it forms a film, holds the pigment to the substrate, and determines the durability of the coating. The solvent controls the viscosity and drying speed, but it must be compatible with the binder. Understanding this trio — pigment, binder, solvent — provides a solid foundation for understanding almost any paint or coating you will work with.
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Dosad smo u ovoj seriji pisali o vidljivim i opipljivim svojstvima boje — o tome zašto boja ima određenu nijansu, zašto pokriva ili ne pokriva, zašto teče lakše ili teže. Ovaj tekst bavi se onim što je ispod svega toga: vezivima i otapalima, komponentama koje određuju što ostaje na podlozi kad sve ostalo ispari.
1. Što je vezivo i koja mu je uloga
Vezivo je komponenta boje koja drži čestice pigmenta i punila zajedno i osigurava njihovo prianjanje na podlogu. Kada boja osuši ili veže, vezivo formira kontinuirani film koji enkapsulira pigment i štiti ga od mehaničkih i kemijskih utjecaja. Vezivo određuje većinu uporabnih svojstava gotove boje: fleksibilnost filma, tvrdoću, sjaj, kemijsku otpornost, adheziju i trajnost. Dva premaza s istim pigmentom ali različitim vezivima mogu se ponašati potpuno drukčije — jedan može biti fleksibilan i vodootporan, drugi krhak i osjetljiv na vlagu.
2. Kako vezivo funkcionira — formiranje filma
Ovisno o vrsti veziva, film se formira na jedan od dva osnovna načina:
• Fizikalno sušenje
• Kemijsko umrežavanje (polimerizacija) Akrili tehnički prolaze kroz fizikalno sušenje — voda isparava, a čestice akrilnog polimera se spajaju u film procesom koji se zove koalescencija.
Jednom formirani akrilni film je kemijski stabilan i praktično neotopiv u vodi, ali topiv u nekim organskim otapalima.
3. Vrste veziva
Akrilna veziva
Akrilna veziva su sintetički polimeri dispergirani u vodi (lateks). Najrasprostranjenija su u suvremenoj upotrebi — koriste se u akrilnim bojama za minijature, zidnim bojama, temeljnim premazima i lakovima. Karakteristike: • Otapalo je voda — lako čišćenje, niska toksičnost • Brzo sušenje — prednost za rad u slojevima, mana za tehnike mokro-na-mokro • Fleksibilan i vodootporan film nakon sušenja • Boja se lagano mijenja pri sušenju — akrili su poznatih po tome da tamne dok se suše (zbog promjene indeksa refrakcije veziva)
Uljana veziva
Uljane boje koriste sušiva ulja kao vezivo — najčešće laneno ulje, ali i orašasto, makovo i saflorovo. Svako ulje ima nešto drugačija svojstva: laneno ulje suši najbrže i daje čvrst film, ali može s vremenom požutjeti; makovo i saflorovo ulje manje žute ali sporije suše. Karakteristike:
• Sušenje oksidativnom polimerizacijom — sporo (sati do dani), ali film je iznimno trajan
• Dugo otvoreno vrijeme — idealno za mokro-na-mokro tehnike i miješanje na paleti
• Bogat, duboki sjaj koji akrili teško reproduciraju • Otapala su organska — mineralna organska otapala (white spirit, ksilen i sl.)
• Neki pigmenti u uljanoj bazi mogu požutjeti s vremenom zbog veziva
Alkidna veziva
Alkidi su sintetički polimeri na bazi ulja i kiselina — svojevrsna modernizacija uljanih veziva. Suše oksidativnom polimerizacijom kao i ulja, ali brže i s predvidljivijim svojstvima. Česti su u industrijskim premazima, temeljnim bojama i dekorativnim bojama za metal i drvo. Karakteristike:
• Brže sušenje od čistih ulja, ali sporije od akrila
• Tvrd i otporan film — dobra kemijska i mehanička otpornost
• Otapala su organska — mineralna organska otapala (white spirit, ksilen i sl.)
• Mogu žutjeti s vremenom, slično uljanim vezivima
Guma arabika i druga vodotopiva veziva
Guma arabika je prirodna smola koja se koristi kao vezivo u akvarelima i nekim tintama. Topiva je u vodi i suši fizikalnim sušenjem — film ostaje topiv u vodi i nakon sušenja, što je karakteristika akvarela (mogućnost reaktiviranja). U istoj kategoriji vodotopivih veziva nalaze se i kazein (iz mlijeka), jajčana tempera (žumanjak jajeta kao emulzija ulja i vode) i gvaš veziva. Svako od njih daje drukčiji film s drukčijim svojstvima.
4. Što su otapala i koja je njihova uloga
Otapalo je tvar koja otapa ili dispergira vezivo i time kontrolira viskoznost boje, omogućuje njezino nanošenje i određuje brzinu sušenja. Nakon nanošenja, otapalo isparava — i taj proces isparavanja je zapravo “sušenje” boje u prvoj fazi. Važno je razlikovati otapalo od razrjeđivača. Pravo otapalo otapa vezivo — molekularno ga razgrađuje i homogeno uključuje u otopinu. Razrjeđivač samo snižava viskoznost, ali ne mora nužno otapati vezivo. U praksi se ovi pojmovi često koriste naizmjenično, ali razlika postaje važna kad se govori o kompatibilnosti.
5. Vrste otapala
Voda
Voda je otapalo za akrilna veziva, gumu arabiku i ostala vodotopiva veziva. Prednosti su niska toksičnost, nema zapaljivosti i lako čišćenje. Mane su relativno sporo isparavanje u usporedbi s organskim otapalima i osjetljivost na temperaturu i vlagu — pri niskim temperaturama ili visokoj vlazi sušenje se znatno usporava.
Terpentin i mineralna organska otapala
Terpentin je prirodni destilat smole četinjača i tradicionalno otapalo za uljane boje i alkide. Ima karakterističan miris i dobar solvatacijski učinak. Zbog jakog mirisa i potencijalne toksičnosti pri dugotrajnoj izložnosti, u modernoj upotrebi ga sve više zamjenjuju mineralna organska otapala (white spirit, ksilen i sl.) ili bezmirisni razrjeđivači na bazi izoparafina. Mineralna organska otapala su frakcija nafte s nešto blažim mirisom od terpentina, ali sličnim svojstvima. Bezorisni razrjeđivači su rafiniraniji i imaju minimalan miris — prikladniji za zatvorene prostore.
Izopropanol i etanol
Alkoholi se koriste kao otapala za neke vrste tinta, lakova i shellaca. Izopropanol (IPA) je široko dostupan i koristi se za čišćenje kistova, razrjeđivanje alkoholnih tinta i raznih tehničkih primjena. Nije kompatibilan s akrilnim vezivima u većim količinama — može destabilizirati emulziju.
6. Interakcija veziva i otapala
Svako vezivo ima svoja kompatibilna otapala — ona koja ga pravilno otapaju ili dispergiraju. Upotreba nekompatibilnog otapala može dovesti do: • koagulacije — vezivo se izlučuje iz otopine u obliku grudica • destabilizacije emulzije — boja se razdvaja na faze • lošeg formiranja filma — boja ne veže pravilno, ostaje ljepljiva ili se lupa • otapanja već nanesenih slojeva — posebno problem kod lakova i finalnih premaza Praktični primjer: akrilne boje su na bazi vode. Dodavanje mineralnog terpentina ili terpentina u akrilnu boju neće je razrijediti — izazvat će koagulaciju veziva i uništiti boju. Obrnuto, dodavanje vode u uljanu boju ne razrjeđuje je pravilno — voda i ulje se ne miješaju.
7. Mediji, aditivi i retarderi
Mediji su formulirani dodaci koji modificiraju svojstva boje bez narušavanja omjera komponenti. Za akrilne boje to mogu biti: usporivači sušenja (retarteri) koji produljuju otvoreno vrijeme i omogućuju mokro-na-mokro tehnike, tekući mediji koji snižavaju viskoznost uz zadržavanje sjaja, mat mediji koji smanjuju sjaj, ili glazurni mediji koji povećavaju transparentnost. Za uljane boje mediji mogu biti: lak (damar, mastic) koji ubrzava sušenje i daje sjaj, sikativi (sušila) koji kemijski ubrzavaju oksidativnu polimerizaciju, ali čija prekomjerna upotreba može izazvati pucanje filma s vremenom. Opće pravilo: mediji su formulirani za konkretna veziva i ne smiju se koristiti naizmjenično. Medij za akrilne boje neće funkcionirati u uljanoj boji i obrnuto.
Zaključak
Vezivo je ono što od pigmenta i otapala pravi boju — ono formira film, drži pigment na podlozi i određuje trajnost premaza. Otapalo kontrolira viskoznost i brzinu sušenja, ali mora biti kompatibilno s vezivom. Razumijevanje ove trojke — pigment, vezivo, otapalo — daje solidan temelj za razumijevanje gotovo svake boje ili premaza s kojim ćete raditi.
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