Acrylic Resin for Paint & Coatings: Types, Properties, Application

What are Acrylic Resins?

An acrylic resin is a polymeric material containing acrylic monomers. These resins can be available in solution, dispersion, or solid form. The monomers are usually:

Esters of acrylic, methacrylic acids or their derivatives, and
Can be functionalized by introducing different chemical groups (R groups).

Other monomers can also be incorporated into the polymer chains. This results in achieving different properties or lower costs.

Typical Acrylic Monomers: Acrylic acid based (Left), Methacrylic acid based (Right)

Typical performance profile

In general, Acrylic resins show good chemical and photochemical resistance. Key parameters of an acrylic resin are:

Tg (Glass transition temperature)
Average Molecular weight of the polymers, and
Polymer molecular weight distribution

These parameters impact the resin properties (viscosity, dispersion…) and the final film/coating obtained (flexibility/hardness...). Its applications range from solvent-based and water-based industrial coatings to architectural coatings.

Main Categories of Acrylic Resins

Depending on their composition, we can divide acrylic resins into 2 different categories:

Pure Acrylic Resins

These contain only acrylic monomers. On each monomer different functionalizations (R groups) are possible. The most common ones include:

Simple hydrogen atoms, lead to the presence of carboxyl groups in the polymer.

Non-reactive groups, for instance, alkyl chains contain only Carbon and Hydrogen. These may prevent reactions with other compounds. Thus, improving the resin's chemical resistance.

Reactive groups contain Hydroxy functions which could react with isocyanates or melamines. Some contain glycidyl functions (epoxy group) that will react with amines and carboxylic acids. These groups allow bonding between polymer chains to form stronger polymeric material.

They influence resin properties, applications, and final properties of the film/coating obtained. H functionalizations and the presence of carboxyl groups can improve the adhesion on a substrate. A large number of carboxyl groups will also help to solubilize the resin in water. To get a resin with specific properties or to reduce its cost, different monomers can be incorporated into the acrylic polymer.

Complex Acrylic Resins

Styrene is the most used and the resulting resins are known as Styrene-Acrylic. Styrene monomers are significantly less expensive than acrylic ones. They are known to increase water resistance and alkali resistance and improve hardness. However, Styrene-Acrylic resins are often subject to yellowing and chalking. They have severe issues that reduce their potential applications.

Browse the complete range of acrylic resins available in our database:

Forms of Acrylic Resins: Features, Properties, and Uses

Acrylic resins are available under different forms, like:

Thermoplastic Acrylic Resins
Crosslinking resins
Acrylic Latexes

In solvent-based acrylic resins, the resin is solubilized in a solvent or a solvent blend. While water-based resins are formulated in water. A very specific class of water-based resins is latexes. Latexes are emulsions of acrylic resins that become water-resistant once the water evaporates.

Thermoplastic Acrylic Resins

The polymers composing the resin do not contain any reactive group. Thus, the polymer chains are not cross-linked. To improve the interaction between the different polymer chains, high molecular weight polymers are used. Thermoplastic resins normally soften and can be reshaped with an increase in temperature. This property makes these resins the ideal candidates for some industrial processes such as:

injection molding,
compression molding, or
extrusion

The main uses of these resins include inks and adhesives.

Thermoplastic Acrylic Resins
Physical form	Available as small beads, flakes, high solid content, viscous solution, or dispersion
Functionalization	No reactive groups on acrylic monomers
Properties	Water resistance, yellowing resistance, gloss retention, fast drying, alkali resistance, good adhesion
Applications	Inks, exterior applications that require metal protection, and adhesives

Crosslinking Resins

Crosslinking resins can be cured to promote chemical interactions between different polymer chains. Curing can lead to more complex polymeric structures and stronger materials. It can occur in different conditions that depend mainly on the active group present in the polymers.

In the presence of reactive groups, acrylic resins can be crosslinked. These groups allow the interaction between two different polymer chains. This can occur at a certain temperature or under UV light. A catalyst may also be added to promote and accelerate the chemical reaction.

We can distinguish between two types of crosslinking systems:

Externally crosslinked resins

They require a curing agent, a chemical that will react with the polymers. In this case, the R group is commonly a hydroxyl-functionalized chain. It allows the reaction with melamine or isocyanate curing agents. This kind of formulation (resin + curing agent) can be provided where they are already mixed together as:

The 2K are used in particular when heating in an oven is not possible. In 1K, isocyanate curing agents can be “blocked”, or made unreactive at room temperature. The curing of the resin will occur only at higher temperatures in an oven (stoving coatings).

Carboxyl groups are also present on the polymer chains of crosslinking resins or as free acrylic acids. They can act as catalysts for the curing reaction and improve coating adhesion. Moreover, other curing agents like epoxies can react with the carboxyl groups. These acrylic resins can be provided in the solvent phase but if the number of carboxyl groups in polymers is high enough, they can be soluble in water. In this case, they are commonly identified as water-thinnable. In water-based systems, a co-solvent can also be present to improve resin compatibility.

Finally, emulsions of thermoset acrylic resins are also available. Emulsions usually allow:

higher solid content at the same viscosity compared to water-thinnable ones and
alkali resistance is usually better as they require less carboxyl groups

Self-crosslinking resins

Acrylic resins can be available as self-crosslinking versions (rather solvent-based or water-based). Some R groups in the copolymer structure are blocked amide (alkoxymethyl acrylamides) groups. For example, N,N-bis-butoxy-methylamide. During the curing process, they react with the hydroxyl groups available on the copolymers leading to a cross-linked network. The curing takes place usually in an oven at elevated temperatures.

Compared to resins crosslinked with curing agents, self-crosslinked resins have:

increased hardness
gloss, and
chemical resistance

Crosslinking Resins
Physical form	Available as solution in solvent, water thinnable, water dispersion
Functionalization	Reactive groups on acrylic monomers (hydroxyl, carboxyl) Presence of curing agents (epoxy, melamine, isocyanate) Self-crosslinked with functionalized specific groups (alkoxymethyl acrylamides)
Properties	Excellent mechanical resistance, good durability with time, weatherability, chemical resistance, flexibility, gloss retention, increased hardness
Applications	Industrial (automotive, OEM, can coatings), stoving coatings, appliances and white goods

Acrylic Latexes

Acrylic latexes are emulsions of acrylic polymeric particles in water. There exist acrylic emulsions that can be cross-linked with curing agents. Coalescence is the main mechanism used to obtain a paint film or coating from latex.

After application, the latex is left to dry and the water evaporates. The polymeric particles get in contact with each other, interact, and coalesce to form a continuous film. To obtain coalescence and a good film, the Tg of the polymer needs to be below the film-forming temperature. This allows the deformation of the particles and diffusion of polymer molecules. The Minimum Film-forming Temperature (MFFT) is thus an important parameter to consider when selecting an acrylic emulsion.

Acrylic Latexes
Physical form	Available as water emulsions
Functionalization	Coalescence is the main mechanism to obtain a film. However, reactive groups (hydroxyl, glycidyl, carboxyl…) can be incorporated in the resin (R groups) to achieve further crosslinking.
Properties	Yellowing resistance, gloss retention, alkali resistance, flexibility, improved emulsion stability using pH buffers
Applications	Architectural wall paints, masonry, industrial and automotive coatings

Which factors to consider while selecting acrylic resins?

Glass Transition Temperature (Tg)

The Glass Transition Temperature (Tg) is the temperature at which a polymeric material will go from a glassy solid state to a liquid state. The Tg of an acrylic resin is defined by the resin formulation. This parameter has a key role in the hardness/flexibility of the final paint film. The following rules can help in selecting a resin formulation with suitable Tg:

Tg will strongly depend on the resin monomers (methacrylate monomers have a higher Tg than acrylate ones)
Tg increases with the degree of cross-linking (number of cross-links between 2 polymer chains)
Higher the Tg , harder (less flexible) the film obtained

The factors that influence the final Tg include:

presence of other monomers (e.g., styrene)
nature of R groups - Reactive or non-reactive
Tg of crosslinking agents used - (e.g., melamine or isocyanate)

Viscosity

The viscosity of an acrylic resin depends on the solid content. However, the average molecular weight and molecular weight distribution of the polymers will also have an impact. Usually, the following rules apply:

For the same solid content the higher the average molecular weight of the polymer the higher the viscosity.
When the average molecular weight is the same, the narrower the molecular weight distribution, the lower the viscosity.

It is important to notice that the average molecular weight has no influence on the viscosity of latex emulsions. In this specific case, viscosity depends on the particle size and size distribution.

Hydroxyl Value Number

The hydroxyl value is an indicator of the reactivity of the acrylic resins functionalized with hydroxyl functions (i.e., the number of OH groups available). It is usually expressed as the KOH mass in mg equivalent to the amount of acetic acid reacting during the acetylation of 1g of resin. The higher the hydroxyl value, the higher the reactivity (and thus the cross-linking possibilities).

Acid Value

The acid is an indicator of the number of carboxyl groups present in the copolymer. It is usually expressed as the amount of KOH needed to neutralize 1g of resin (See DIN 53402 or ISO 2114). The number of carboxyl groups has an impact on the adhesion properties of the resin and the solubility in water. The higher the acid value, the higher the number of carboxyl groups.

Minimum Film-forming Temperature for Acrylic Dispersion

The Minimum Film-forming Temperature (MFT) is the minimum temperature under which the acrylic latex will lead to a cracked material rather than a continuous film.

For acrylic latexes designed for architectural applications (wall paints), the MFT is usually below 5°C.
For latexes designed for industrial applications, where oven curing is used, the MFT can be higher.

pH (For Water-based or Dispersion)

Water-based acrylic resins are neutralized with acid or basic buffers to improve resin stability. During the formulation of the coating the pH may evolve and the dispersion can become unstable and coagulate:

If the initial pH is acidic a risk of coagulation of the particles is possible if the pH increases during the paint formulation.
If the pH is basic, the dispersion can usually tolerate higher pH but not a lower one.

Acrylic Resins – Commercial Grades for Paints, Coatings and Inks

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