An Acrylic resin is a polymeric material (in solution, dispersion or solid) containing acrylic monomers. These monomers are usually esters of acrylic, methacrylic acids or their derivatives, and can be functionalized by introducing different chemical groups (R groups). Others monomers can also be incorporated in the polymer chains in order to obtain resins with different properties or lower cost.
In general Acrylic resins show good chemical and photochemical resistance. They are commonly used in many different applications, from solvent-based and water-based industrial coatings to architectural coatings.
Key parameters of an acrylic resin are:
- Tg (Glass transition temperature)
- Average Molecular weight of the polymers, and
- Polymer molecular weight distribution
These parameters have an impact on the resin properties (viscosity, dispersion…) and on the final film / coating obtained (flexibility/hardness...).
Typical Acrylic Monomers
Main Categories of Acrylic Resins
Depending on their composition, we can divide acrylic resins in 2 different categories: pure acrylic resins and more complex ones, containing additional monomers as well.
Pure Acrylic Resins
These contain only acrylic monomers. On each monomer different functionalizations (R groups) are possible. The most common ones includes:
- Simple hydrogen atoms, leading to the presence of carboxyl groups in the polymer
- Non-reactive groups, as for instance alkyl chains that contain only Carbon and Hydrogen. These may prevent reactions with other compounds and thus improve the resin chemical resistance.
- Reactive groups, as for instance containing Hydroxy functions, which could react with isocyanates or melamines, or glycidyl functions (epoxy group) that will react with amines, carboxylic acids… Also, these groups will allow bonding between polymer chains (cross-linking) leading to the formation of a stronger polymeric material.
Different functionalizations will have an influence on the resin properties, on its use in different applications and on the final properties of the film / coating obtained. H functionalizations, and thus 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.
In order to obtain a resin with specific properties, or to reduce its cost different monomers can be incorporated into the acrylic polymer.
Complex Acrylic Resins
Styrene is probably the most used and the resulting resins are known as
Styrene-Acrylic. Styrene monomers are significantly less expensive than the acrylic ones. They are known to increase
water resistance and to lead to both
alkali resistance and
improved hardness. However Styrene-Acrylic resins are often subject to yellowing and chalking, severe issues that reduce their potential applications.
» Increase Water Repellency in Your Coating Formualtion!
Major Forms of Acrylic Resins
Acrylic resinsare available under different forms, like:
- Thermoplastic Acrylic Resins
- Cross-linking resins
- Acrylic Latexes
Another distinction can be made between
solvent-based acrylic resins, where the resin is solubilized in a solvent or a solvent blend, and water-based resins, where the resin is formulated in water. A very specific class of water-based resins is
latexes,
emulsions of acrylic resins that become water resistant once the water is let to evaporate.
Thermoplastic Acrylic Resins
In thermoplastic resins, the polymers composing the resin do not contain any reactive group. Thus in these resins 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 when the temperature is increased. This property makes these resins the ideal candidates for some industrial processes, as injection molding, compression molding or extrusion. Main use of these resin include inks and adhesives.
Cross-linking Resins
Crosslinking resins can be cured to promote chemical interactions between different polymer chains. Curing, which can lead to more complex polymeric structures, and thus to stronger materials, can occur in different conditions, that will depend mainly on the active group present in the polymers.
When reactive groups are present, acrylic resins can be cross-linked by allowing the interaction between two different polymer chains. This can occur in specific conditions, as for instance 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-cross-linked resins, which require a curing agent, i.e. a chemical that will react with the polymers, and
- Self-crosslinking resins
In the first case the R group is commonly a
hydroxyl-functionalized chain allowing 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, and curing of the resin will occur only at higher temperature in an oven (stoving coatings).
In addition to hydroxyl functions, carboxyl groups are also normally present on the polymer chains of cross-linking 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, which can react with the carboxyl groups, could also be used in this case.
These acrylic resins can be provided in solvent phase but if the number of carboxyl group on the 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 same viscosity compared to water-thinnable ones and the alkali resistance is usually better as less carboxyl group are needed.
Finally acrylic resins can be available as
self-crosslinking version (rather solvent-based or water-based). In these type of resins some R groups in the copolymer structure are blocked amide (alkoxymethyl acrylamides) groups like N,N-bis-butoxy-methylamide. During curing process (usually in an oven at elevated temperatures), these groups react with the hydroxyl groups also available on the copolymers leading to a cross-linked network. These type of resins usually have increased hardness, gloss and chemical resistance compared to resins cross-linked with curing agents.
» Check out All the Available Crosslinking / Curing Agents Today!
Acrylic Latexes
Acrylic latexes are emulsions of acrylic polymeric particles in water. Although 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 a latex.
After application, the latex is let to dry and the water evaporates. The polymeric particles get in contact with each other, interact and coalesce to form a continuous film. In order to obtain coalescence, and thus a good film, the Tg of the polymer needs to be below the film forming temperature to allow deformation of the particles and diffusion of polymer molecules. The
Minimum Film-forming Temperature (MFT) is thus an important parameter to consider when selecting an acrylic emulsion.
Even if coalescence is the main mechanism to obtain a film, reactive groups (hydroxyl, glycidyl, carboxyl…) can be incorporated in the resin (R groups) to achieve further cross-linking.
» Understand the Significance of Coalescing Agents in your Coating Formulation!
Select the Right Acrylic Resins
Let's understand the key parameters that should be considered in detail while selecting an acrylic resin.
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 on Hardness/flexibility of the final paint film.
The following rules can help 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 presence of other monomers (styrene for example), the nature of the reactive or non-reactive R group present or the Tg of the crosslinking agent used (melamine or isocyanate for example) will of course influence the final Tg.
Viscosity
The viscosity of an acrylic resin depends on the solid content, but the average molecular weight of the polymers in the resin and the molecular weight distribution 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, narrower the molecular weight distribution, lower is the viscosity.
It is important to notice that the average molecular weight has no influence on the viscosity for 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 (ie the number of OH group 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 group present on 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 group has an impact on the adhesion properties of the resin and on 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 waterbased or dispersion)
Water-based acrylic resins are usually 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.