Chemistry of Paints

Paints are complex chemical mixtures designed to form a durable, decorative, and protective film when applied to a surface. Modern paints are sophisticated formulations containing 4–5 main classes of ingredients.

Main Components of Paint

Component	% by mass	Main function	Chemical examples
Binder (Resin)	20–50%	Film formation • adhesion • cohesion • durability	Alkyds, acrylics, polyurethanes, epoxies, vinyls
Pigment	5–30% (decorative) 40–70% (anticorrosive)	Color • opacity • UV protection	TiO2, iron oxides, carbon black, phthalocyanines
Extender / Filler	10–50%	Cost reduction • mechanical properties • rheology control	CaCO3, talc, kaolin, silica, barytes
Solvent / Carrier	10–60% (varies greatly)	Application viscosity • film formation aid	Water, mineral spirits, esters, ketones, glycol ethers
Additives	0.1–10% total	Specific performance improvements	Driers, dispersants, thickeners, biocides, defoamers, UV absorbers

Important Resins

Resin Type	Chemical Characteristic	Engineering Application
Alkyds	Polyester modified with fatty acids.	General-purpose industrial machinery.
Epoxies	Contain epoxide groups; high cross-link density.	Heavy-duty floor coatings, marine environments.
Polyurethanes	Formed by Isocyanate + Polyol reaction.	Automotive finishes, UV-resistant topcoats.
Acrylics	Esters of acrylic or methacrylic acid.	Exterior architectural coatings due to color stability.

Binders – Film-Forming Heart of Paint

Most important classification criterion: chemistry of the binder.

Type	Chemistry	Drying / Curing Mechanism	Typical Uses
Alkyd	Polyester + fatty acids	Oxidative crosslinking (autoxidation)	Architectural gloss, wood coatings
Acrylic (waterborne)	Acrylic / methacrylic copolymers	Coalescence + minor crosslinking	Wall paints, exterior emulsions
2K Polyurethane	Polyol + polyisocyanate	Chemical reaction (–OH + –NCO)	Automotive, industrial floors
Epoxy	Epoxy resin + amine / polyamide	Ring-opening addition	Primers, chemical-resistant coatings
Alkyd-emulsion (hybrid)	Alkyd dispersed in water	Oxidative + coalescence	Low-odor "oil-based" paints

Examples: Various Pigment Used for Painting

• Different Color Pigments
• White: TiO₂, ZnO
• Red: Pb₃O₄, Fe₂O₃
• Blue: Ultramarine blue, cobalt blue
• Green: Cr₂O₃, phthalocyanine green
• Yellow: ZnCrO₄, PbCrO₄, litharge
• Black: Carbon black, lamp back
• Orange: basic lead cromate, cadmium orange
• Brown: burnt umber
• Metallic: Cu powder, Zn dust, Al
• Metal protective pigments: Pb₃O₄, blue lead.
• Organic pigments – azo, phthalocyanine, quinacridone, diketopyrrolopyrrole (DPP), perylene
• Functional pigments:
• Anticorrosive: zinc phosphate, zinc chromate (phasing out), calcium borosilicate
• UV barrier: nano-TiO₂, ZnO
• IR reflective ("cool roof"): complex inorganic color pigments

Pigment Volume Concentration (PVC)

In coating engineering, the ratio of pigment to binder determines the film's physical properties. This is known as the PVC.

$$PVC = \frac{V_p}{V_p + V_b} \times 100$$

Where V_p is the volume of pigment/fillers and V_b is the volume of the non-volatile binder.

• Low PVC (< 35%): High gloss, high flexibility, and excellent barrier properties. Typical of topcoats.
• Critical PVC (CPVC): The point where there is just enough binder to coat all pigment particles. Exceeding this leads to porosity.
• High PVC (> 60%): Matte finish, breathable, but lower mechanical strength. Typical of primers and interior wall paints.

Mechanism of Film Formation

The transition from a liquid to a solid protective film occurs through two primary pathways:

Physical Drying

Common in lacquers, this involves the simple evaporation of the solvent. The polymer chains, already present in the liquid, entangle as the solvent leaves, forming a solid layer. This process is reversible; the film can be re-dissolved in the original solvent.

Chemical Curing (Cross-linking)

In engineering-grade paints like epoxies or polyurethanes, a chemical reaction occurs. Small molecules (monomers or oligomers) react to form a 3D polymer network. This is often triggered by:

• Oxidation: Reaction with atmospheric oxygen (e.g., Alkyd resins).
• Polyaddition: Mixing two components, like a resin and a hardener (e.g., Epoxy).
• UV Radiation: Rapid polymerization triggered by ultraviolet light.

Solvents Today (2026 perspective)

Strong regulatory pressure continues worldwide:

• VOC limits are very low in EU, California, China Class I zones
• Waterborne systems now dominate architectural coatings (>80% in most developed markets)
• High-solid 2K systems and powder coatings growing in industrial segment
• Bio-based solvents (e.g. lactate esters, terpenes) gaining niche acceptance

Important Additives (small %, large impact)

Additive class	Purpose	Common examples
Driers	Speed up autoxidation	Cobalt, manganese, zirconium octoates / neodecanoates
Thickeners / Rheology modifiers	Anti-sag, brushability	HEUR, HASE, hydrophobically modified cellulose, fumed silica
Dispersants	Pigment stabilization	Polyacrylic salts, polyurethane dispersants, fatty acid derivatives
Coalescents	Help film formation in waterborne paints	Texanol, butyl cellosolve, Optifilm
Biocides / Fungicides	In-can & dry-film preservation	MIT/CMIT (phasing out), BIT, zinc pyrithione, IPBC
UV absorbers & HALS	Photostability	Benzotriazoles, hindered amine light stabilizers

Environmental Considerations

The "Chemistry of Paints" is currently shifting toward Green Chemistry. Traditionally, paints relied on Volatile Organic Compounds (VOCs) which contribute to smog and health issues. Engineers are now prioritizing:

• Water-borne systems: Replacing organic solvents with water.
• Powder coatings: Eliminating solvents entirely by using electrostatic spray and heat curing.

Summary – Modern Paint Families (2026)

1. Waterborne acrylic emulsions – interior & exterior walls
2. Water-reducible alkyds – decorative enamels
3. High-solid 2K polyurethane & polyaspartic – industrial & automotive
4. Waterborne 2K epoxy & polyurethane – low-VOC industrial primers/topcoats
5. Powder coatings – appliances, architecture, automotive parts
6. UV-curable coatings – wood, graphic arts, 3D printing finishes