Solvents Selection for Industrial Coatings

Why solvents are required in paints and coatings?

• Pigments
• Additives, and
• Binders

• Solvents control the viscosity for the application
• Solvents have an important effect on film quality, which is strongly dependent on the solvent's evaporation rate during drying

Method to Estimate Solvating Power of Solvents

Basic Principles of Hansen Solubility Parameters

• Hansen Solubility Parameters are a set of 3 numbers that describe the way solvents (but also polymers) will behave with other molecules (Do they want to be near each other or not).


• Each one of the 3 parameters, δD, δP and δH, represents a type of interactions: London dispersion forces, polar forces and hydrogen bonding forces respectively.


• As dispersion forces are similar for most of the common solvents/organic molecules δD does not vary much.


• δP and δH are more important here and can differ a lot from solvent to solvent. These numbers are good indicators of the polar properties and ability of a solvent to form hydrogen bonds respectively.


• The smaller the δP the better the ability to dissolve the non-polar resin, the higher the δH the better the ability to form hydrogen bonds.

Tips to Find Suitable Hydrocarbon Solvent(s)

• Aliphatic solvents are linear, branched or cyclic hydrocarbon chains such as pure solvent like hexane.
• Aromatic solvents feature a benzene group (cyclic structure of 6 carbons) like Toluene and Xylene.
• Aliphatic and cyclic hydrocarbons blends are usually well known as Mineral or White Spirit and Special Boiling Point Spirit. Blends of aromatic solvents are also available.

1. Special Boiling Point Spirit (Flash Point < 21°C) includes different grades with different flash points and fixed boiling ranges. They are very fast evaporating solvents and thus are used for fast drying coatings.


2. Mineral or White Spirit (Commonly with flash point > 21°C) are also available in different grades with different flash points and fixed boiling ranges. Their names can usually refer to the flash point (30°C, 40°C, 60°C …). They are commonly used for oil-based and alkyd resins.


3. Aromatic hydrocarbons blends (sometimes called Naphtha solvents) are usually aromatic petroleum fractions (C9 to C13) with different grades having fixed boiling ranges. They are commonly used in many industrial coatings as part of the solvent systems even if they try to be avoided when possible. In general, aromatic solvents have higher dissolving power than aliphatics.


4. Toluene and xylene are commonly used with phenolic and amino formaldehyde in heat-curing systems as well as with alkyd resins.


5. Spirits of Turpentine are specific solvents made from the distillation of tree resins and composed of different terpenes. There are commonly used for oil-based systems.

Ketones as Solvents in Paints and Coatings

• Acetone – A fast evaporation solvent used in cellulosic coatings
• Methyl isobutyl Ketone – A medium evaporation all around solvent used in many systems
• Methyl Amyl Ketones – A low evaporation solvent with good solving power properties
• Isophorone – A very low evaporation solvent used in heat-curing systems. It is known to improve the wetting of surfaces and pigments.

Types of Esters Used as Solvents

• Ethyl Acetate – A fast evaporation solvent widely used in many fast drying systems
• Butyl Acetate – It is widely used, its moderate evaporation rate makes it perfect, during drying, to avoid surface defects of the film (blushing, cratering...)
• Propylene Glycol Mono Methyl Ether Acetate – A moderate evaporation, solvent is also used in lots of systems. It has a greater (but limited) miscibility with water compared to other esters.
• Butyl Glycol Acetate – It is a slow evaporation solvent with very good solvating power making it suitable to improve the flow and gloss of coatings cured at high temperature

When you should select alcohol-based solvent?

• Ethanol – A high evaporation solvent, able to dissolve very polar resins but unable to dissolve very non-polar film formers like
• Butanol – A moderate evaporation solvent widely used in many systems. Among other properties, this solvent is known to be able to decrease the viscosity (even in small quantities) in non-polar resin systems like alkyds paints and in some waterborne coatings

Using Glycol Ether as Solvents

• Ethylene Glycol MonoButyl Ether – It is usually known as Butyl Glycol, is a very versatile solvent. It has a higher (even if very slow) evaporation rate than most of the other glycol ethers and is widely used in both solvent-borne and waterborne coatings.


• Propylene Glycol Methyl Ether – Its moderate evaporation rate and full miscibility with water make it a strong candidate for lots of coating systems.


• Dipropylene Glycol n-Butyl Ether – It is a very slow evaporating solvent making it a very good coalescing agent.

Find Suitable Solvents for Coatings

Replacing Solvents in Industrial Coatings with Hansen Solubility Parameters