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Coatings Ingredients
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Coatings Ingredients

Foaming 101: Basics and Foam Control in Paints and Coatings

Foaming in Paints and Coatings
  1. Foaming Issue in Coatings and Inks
  2. What is Foam and Why Does it Occur?
  3. Foam Formation in Paints and Coatings
  4. Defoaming – Basic Principle and Mechanism
  5. How to Avoid Foam?


Foaming Issue in Coatings and Inks


In the paints, coatings and inks industry, foaming is a common issue primarily associated with water-borne or aqueous systems. Sometimes, foam imparts performance issues in water/solvent-based and solvent-free formulations as well.

Typically, foam occurs due to the presence of various components in the formulation, such as surfactants, dispersants, wetting agents, emulsifiers, etc. It is an undesired side effect that can occur during manufacturing, filling or packaging as well as the application of paint systems.

  • Foam increases the production time and makes packaging difficult e.g., unchecked foam can result in an overflow.

  • Foam often leaves surface defects, such as fisheyes, pinholes, and orange peel during film formation and results in loss of gloss and clarity in the coated film. This not only affects the visual appearance but also reduces the protective function of the coating by weakening the dry film.

  • Foam can also create a more permeable paint surface capable of imbibing dirt and other discoloring pollutants.

While it may not be a question of the type of system or process used, foam is always unwanted in a coating. Thus, to avoid any quality issues, foam control agents have become an integral component in paints and coatings formulations. With a proper defoamer or anti-foaming agent, applied films can have a pleasing appearance and will not contribute to corrosion or water sensitivity.


Learn how to select defoamers with fast screening tests as well as determine the optimal dosage & avoid side-effects »

In order to effectively overcome the issues often seen due to foaming, it is important to understand the basics associated with foam formation. Let’s begin by understanding why foam occurs in any coating system.


What is Foam and Why Does it Occur?


Foam is defined as a two-phase stable dispersion in which gas molecules are enclosed by liquid. To understand it better, let us first start with pure liquids.

Pure liquids do not foam. Upon shaking pure liquid, gas incorporated tends to form spherical bubbles (since this involves the least amount of surface energy). However, if the liquid is pure, the bubbles will rise toward the surface and collapse immediately due to the absence of any stabilization. There are no surface-active agents present in pure liquids.

Bubbles in the Pure Liquid
Bubbles in the Pure Liquid
(Credit: Evonik)

As the air in the bubble is expelled, the liquid quickly rushes into the space vacated by the air. Thus, stable foam is never achieved in pure liquids.


"Good To Know" 

Surface-active agent – A surface-active agent, or surfactant, is a substance which lowers the surface tension of the medium in which it is dissolved, the interfacial tension with other phases, and is positively adsorbed at the liquid-vapor interface and other interfaces. The surfactant lowers the surface tension of a liquid by spreading spontaneously over its surface.


On the other hand, foaming results if the liquid is not pure or if it contains more components. The cause of foam is the introduction of gas into the liquid material and stabilization of the bubbles at the surface of the liquid.

One of the primary reasons for foam formation is the presence of other components that reduce the surface tension. This results in the formation of a stable dispersion of a gas in a liquid medium because a surfactant-stabilized double layer forms around air bubbles and entrains them within it. This type of foam is generally referred to as “liquid foam”.


"Good To Know" 

Surface Tension –
Surface tension is the property of a liquid that makes it behave as if its surface is enclosed in an elastic skin. Surface tension is a special case of interfacial tension in which the phases are liquid & gas.


Solving Surface Tension Issues

When gas bubbles are present in a surfactant solution, surfactant molecules adsorb at the surface of the bubble and reduce the surface tension between the bubble and the volume phase. If a bubble rises, this produces bubbles that are enclosed by a thin film (lamella). The lamellae can bond with others, thus forming a foam.

A lamella consists of a double layer of surfactant, one side of which consists of the monolayer of surfactant molecules at the air-liquid interface of the bubble and the other side consists of the monolayer of surfactant molecules that cover the liquid-air interface.

Unstable Bubbles in Pure liquid vs Stable foam in Surfactant Containing Systems
Unstable Bubbles in Pure liquid (L)
Stable foam in Surfactant Containing Systems (R)
(Credit: MÜNZING Chemie)


Structure of Foam


Foam has two different structures that may occur in spherical bubbles and foams containing polyhedral cells. These structures tend to change under gravity.

  • Macrofoam exists primary in polyhedral form and when it gets dry
  • Microfoam consists of bubbles, which were not able to rise to the surface but remain in the liquid form

Aqueous Foam
Close-up photo of an aqueous foam, consisting of a wet, a transient and a dry region. Scale bar is 4 mm


When does foam occur in paints and coatings?


Paints and coatings formulations are not pure liquids. One of the primary reasons for foaming is the presence of additives that reduce surface tension in the coating formulation.

These additives include:

  • Surfactants and wetting agents – They are used to modify the spreading characteristics of the coating
  • Emulsifying agents – They are used to stabilize binder molecules of aq. dispersions
  • Dispersing agents – They are used for the pigments and fillers to facilitate their uniform dispersion in an aqueous medium
  • Leveling agents and other additives – They are used to achieve special paint properties or to improve the process at the production.

Due to the presence of these additives, air bubbles are stabilized like hydrophobic particles by surfactant molecules. These surfactant molecules, having a hydrophilic-hydrophobic character, form a layer around the bubble, orientating the hydrophobic end towards the air (bubble) and the hydrophilic end towards the water. Thus, the interfacial tension between the bubble and liquid is lowered and stabilized.

In paints, coatings and inks, the air gets incorporated into a system resulting in foaming at various stages, such as:

  • Manufacturing – Due to high shear rates (agitation during the polymer/pigment grinding) or when the letdown is stirred into the paste
  • Packaging – By pumping during package filling
  • Handling & Transport – During stirring and pouring of the paint
  • Application – By shear or spraying, dipping, brushing, etc. resulting in film defects due to the formation of air bubbles
  • Drying – By chemical reaction which releasing gases during paint curing, or even through a porous substrate


Foam Formation in Water-based, Solvent-borne & High Solids Coatings


In water-based coatings and inks, the low evaporation rate and high surface tension of water make formulations prone to bubble entrapment and blistering defects. View Defoamers for Aqueous Systems »

Foam problems in solvent-borne coatings (SB) are in some ways distinctly different from waterborne (WB) coatings. The origin of the foam problem can be different. Since the presence of solvent will assist in substrate and pigment surface wetting, the addition of surfactants is minimized and foam stabilized by surfactant can be less of a problem. However, some surface-active species may be present in SB coatings in the form of flow and leveling agents. View Defoamers for Solvent-borne Systems »

In high-solids coatings grinding, mixing, and application can introduce entrained air that can be stabilized by high viscosity. Since resin levels are high, nearness to a phase transition point may be responsible for surface activity that would not normally be present in lower solids formulations. This surface activity in high-solids coatings may be responsible for foam stabilization. Additionally, oligomers present in high-solids coatings can be surface-active. Solutions to foam problems are important and solutions are at hand. View Defoamers for High-Solids Coatings »


What should you consider to find the best suitable defoamer for your product?
Get Selection Tips Here for Free »


Defoaming – Basic Principle and Mechanism


Foam Stabilization and Decay – Key Theories


The stabilization of bubbles by surfactant or surface-active molecules is assumed to be the primary reason behind foam formation in coatings. The degree of foam stability and decay is related to the stabilizing system of the surfactant, explained by different theories, such as:

  • The Marangoni effect (also called the Gibbs–Marangoni effect)
  • Cohesive Strength surfactant mol.
  • Electrostatic repulsion of equally charged surfactant molecules
  • Bubble size
  • Surface Viscosity
  • Surface Transport

The presence of highly surface-active (foam control agents), insoluble molecules in the surface film interrupts foam stabilization, and thus foaming is prevented.

How Do Foam Control Agents Work?


The two main aspects of foam control agents are surface activity and insolubility. The four basic processes by which antifoams disrupt aqueous foam are:
Defoaming Principle
  • Entering,
  • Bridging,
  • De-wetting, and
  • Rupture.

The mechanism proposes that droplets of the antifoam move to the foam lamellae where they provide a point source for rupture of first one air/water interface and then the other. An oil lens is then formed, which bridges the air-water-air foam film. Drainage in the oil lens and foam film takes place until eventually, the film ruptures.

Higher bulk viscosity systems, such as formulations containing thickeners or high binder content, slow down the displacement of the liquid from the lamella and limit the mobility of the entrained air bubbles.

If you wish to calculate the efficacy of your system, it can be mathematically obtained by following equations:

Entry coefficient, E = γwa + γwo - γoa 

Where,
→ γwa = surface tension of foaming liquid
→ γwo = interfacial tension between the defoamer and the foaming liquid
→ γoa = surface tension of the defoamer


If E is negative, the oil droplet is wetted by the aqueous phase. So, E must be positive for an antifoam. i.e. E > 0 is necessary

Spreading coefficient: S = γwa - γwo - γoa 

If S is positive, then the defoamer can spread and push aside the surfactants.

Bridging coefficient: B = γ2wa + γ2wo + γ2oa

If B is positive, then if E allows the drop to enter the wall of a foam lamella, and if the drop radius is large enough, it will bridge the wall.

Tackling foamLearn a great deal with rather little effort
Go deeper into the science of foam formation and uncover the set of do's & don'ts to avoid foam in coatings in this exclusive 4 part series. Discover digital predictive tools to tackle issues related to foaming!

KNOW MORE »


How to Avoid Foam?


Controlling foam is one of the major challenges for both manufacturers and applicators to ensure a smooth and uniform appearance of the paint film. Foam control agents are added to coating and ink formulations to prevent air entrapment and foaming. Foam control agents can be classified as:


These terms provided here are not rigidly adhered to in the industry and frequently they are used interchangeably. But let us discuss them separately to understand their mode of action.


Antifoams/Anti-foaming Agents


Antifoams added in paints to reduce:

  • Foam problems during manufacturing,
  • Aeration during can filling, and
  • Air bubble formation during application.

They promote the release of air from the coating film during drying. An anti-foam agent should not cause pigment flocculation or emulsion instability, possess poor compatibility resulting in defects in the wet or dry film and affect tint color appearance.

Various products are used, including fatty acid esters, metallic soaps, mineral oils, waxes, silicone oils, and siloxanes, sometimes combined with emulsifiers and hydrophobic silicas are used as anti-foaming agents.

Anti-foam agents for a particular paint or coating system are usually evaluated by a method that involves severe agitation and incorporation of air into the paint system, followed by observing the time required by the paint to return to its original density.


Get Access to Available Foam Control Agents Here »
Analyze technical data of each product, get technical assistance or request samples to test them in your application.



Defoamers


Defoamers destroy foam after it has been formed. Defoamers are low surface tension liquids that can enter the foam lamella, or act as a carrier medium to transport hydrophobic particles into the foam lamella; resulting in collapsing the foam lamella.

In emulsion paints, foaming can lead to the undesirable formation of surface defects (craters and pores) in the coating film. Defoamers reduce the surface tension of the liquid to such an extent that the air bubbles in the foam collapse.

For aqueous coatings, two distinct properties of defoamers are important: defoaming itself and deaerating.

  • To destroy macroscopic foam formed in the coating after application – defoaming
  • To remove air trapped during the coating process – deaerating

Deaerators remove (micro-) air bubbles or microfoam from a liquid and help them to rise to the surface

Defoamer must have partial or targeted incompatibility in the coating formulation. If the deaerator is too compatible it would not be effective, while, if it is too incompatible, defects such as craters, fisheyes or turbidity would occur. The defoamer is normally hydrophobic, spreads easily over interfaces, has a low surface tension, is not soluble in the medium and ideally is dispersed as fine droplets.

Defoamer chemistries typically include mineral oils, paraffin, fatty alcohols and their esters, fatty acids and silicones, or hydrophobic alkoxylates.

Defoamers & Antifoams

Silicones as Foam Control Agents


Simple silicone oils are often used as foam inhibiting agents due to their low surface tension. Here the addition of fumed silica also has a positive influence on the defoaming behavior.

The organo-modified silicones e.g. polyether-modified polysiloxanes are an important subgroup of defoamers used in coatings, particularly overcoming silicone incompatibility issues in varnishes and printing inks. Their organic modifications can contain side chains such as polyethers or polyesters, which allow efficient control over the degree of incompatibility. Organo-modified silicones offer excellent defoaming characteristics with little or no side effects such as loss of gloss.


Get Access to Available Foam Control Agents Here »
Analyze technical data of each product, get technical assistance or request samples to test them in your application.



Using Surfactant as Foam Control Agent


The function of surfactant as a defoamer, anti-foaming agent or de-airing agent is related to interference with foam stabilizing surfactants. Strong intra-molecular forces between surfactant molecules play a dominant role in foam stabilization. Surfactants, demonstrating weak intra-molecular interactions at high surface activity replace foam stabilizing components at the air/ liquid interface and as a consequence reducing foaming. The common surfactant examples which show low foaming capabilities are:

  • EO/PO block polymers
  • Alcohol alkoxylates, and
  • Ethylene diamine EO/PO block polymers.

To deal with foaming of a new coating formulation, it is critical to look at the combination of surfactants, wetting agents, water-soluble polymers and antifoams.

Formulation Tip: It is beneficial to use defoamers in combination with a leveling additive. Once the foam bubbles have burst, the surface quickly smooths preventing the formation of dents and pinholes.

Overall in paints and coatings, defoamer selection must be carefully considered to effectively balance defoaming efficiency and coating compatibility. Discover the factors that are vital to select anti-foaming agents or defoamers for your formulation, as well as methods to evaluate efficacy to meet your end-use requirement.

Defoamers for Coatings: Selection Tips, Performance Evaluation & Troubleshooting

Defoamers or Anti-foaming Agents for Paints, Coatings and Inks


View a wide range of foam control agents available today, analyze technical data of each product, get technical assistance or request samples.



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