What is a solvent? Types, production and uses

Solvent is a liquid that dissolves or dilutes another substance without chemically altering it. In the chemical industry, the solvent usually does not remain in the final product — it does its job and evaporates. Nevertheless, solvent selection largely determines a formulation's applicability, drying speed, film quality and occupational safety profile.

What is a solvent?

A liquid's behavior as a solvent is defined by polarity, hydrogen-bonding capacity and evaporation rate. Formulators quantify these three axes using Hansen solubility parameters: the closer the parameters of the solvent and the polymer to be dissolved are to each other, the better the dissolution. This is the measurable version of the "like dissolves like" rule.

In practice, a single solvent is rarely sufficient. Solvent blends are used in paints and inks: a fast-evaporating component handles initial drying, a medium-speed component manages flow and leveling, and a slow component takes care of surface smoothness.

How is it produced?

• Hydrocarbon solvents (naphtha, mineral spirits, xylene, toluene): obtained through distillation and reforming of crude oil; hydrogenation may then be applied to reduce aromatic content.
• Oxygenated solvents (acetone, MEK, ethyl acetate, butyl glycol): produced through chemical synthesis — oxidation of alcohols, esterification, or addition of ethylene oxide derivatives to alcohol.
• Bio-based solvents (ethyl lactate, 2-methyltetrahydrofuran, d-limonene): produced through fermentation or processing of agricultural raw materials.

Recovery is an integral part of production. A significant portion of the solvent used in industry is collected via distillation and reused; this is a decisive factor both in terms of cost and emissions.

Types and varieties

Which sectors use it?

• Paints and coatings: the largest field of use; viscosity adjustment and film formation.
• Adhesives and inks: application consistency and fast drying.
• Pharmaceuticals: synthesis medium, crystallization and purification; residual solvent limits are strictly monitored here.
• Industrial cleaning: degreasing, metal surface preparation.
• Cosmetics: perfume and nail polish formulations.
• Electronics: circuit board cleaning and photoresist processes.

Turkish market and production

In Türkiye, solvent supply relies largely on refinery and petrochemical output as well as imports, while blending, thinner production and recovery services are widespread domestically. The size of the paint industry is the main determinant of solvent demand.

Current topics in the sector's agenda include: VOC regulations and the resulting shift toward water-based, high-solids and solvent-free systems; updates to hazardous substance classifications; the spread of solvent recovery (relevant to both cost and waste management); and the performance-cost balance of bio-based substitutes. Current news on these topics is listed below.

Technical properties

The most frequently checked figures on a solvent data sheet are, in order, dissolving power, evaporation behavior and safety profile.

In practice, relying on a single figure can be misleading. For example, a solvent selected based solely on boiling point may evaporate quickly under high ambient humidity, cooling the surface and causing blushing on the film due to condensation. This is why fast, medium and slow components are balanced together in a formulation.

Points to consider in selection and procurement

• Design a blend, not a single solvent. The balance between fast, medium and slow components determines film quality more than any single substance.
• Read the safety data sheet before formulating. Flash point and health classification directly affect storage and ventilation investment.
• Set up the recovery scenario from the outset. If usage volume is predictable, investment in distillation often pays for itself in a short period.
• Clarify the purity specification. Water and acid content in particular can disrupt curing in moisture-sensitive systems.
• Monitor regulatory changes. Substituting a solvent that becomes subject to restriction is a reformulation process that can take months; it is necessary to start early.

Frequently asked questions

What is VOC, and how does it relate to solvents?

VOC (volatile organic compound) refers to organic substances that evaporate easily at room temperature. Most conventional solvents are VOCs. Since regulations impose VOC limits per product category, formulators either reduce the solvent ratio, switch to water-based systems, or prefer solvents that do not fall under the VOC definition.

Do water-based systems eliminate solvents entirely?

No. Water-based paints typically still contain a small proportion of co-solvent; this is necessary for film formation and freeze resistance. Still, the total VOC load is significantly lower than that of the solvent-based equivalent.

Are thinner and solvent the same thing?

Thinner is not a single substance but a solvent blend prepared to dilute a specific type of paint. Cellulosic thinner, synthetic thinner and polyurethane thinner have different compositions; using the wrong thinner disrupts the paint's curing and surface quality.

In which cases does solvent recovery make sense?

When usage volume is consistent and the contamination profile is predictable. The purity of solvent collected through distillation is often directly sufficient for non-critical applications (such as equipment cleaning). Return on investment is generally calculated based on the combination of solvent price and disposal cost.

Which parameter is most critical when selecting a solvent?

There is no single parameter; but in practice a three-way balance is established: dissolving power (it must actually be able to dissolve the polymer), evaporation rate (it must suit the application method), and health, safety and regulatory profile. Optimizing one while ignoring the others tends to come back as blistering, orange-peel effect and adhesion problems in the field.