Surfactants in paint
Encyclopedia
Paint
has four major components: pigments, binders, solvents, and other additives. Pigments serve to give paint its color, texture, toughness, as well as determining if a paint is opaque or not. Common white pigments include titanium dioxide
TiO2 and zinc oxide
ZnO. Binders are the film that forms as a paint dries and they affect the durability, gloss, and flexibility of the coating. Polyurethanes, polyesters, and acrylics are all examples of common binders. The solvent
helps adjust the curing rate and viscosity of the paint in its liquid state. The solvent evaporates off as the paint dries. There are two types of solvents: solvent-borne and water-borne. The additives that are incorporated into paints are a wide range of things which help to effect the properties of the paint and the final coating. The main additives added to many paints are surfactants.
The word surfactant is short for surface active agent. Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or the interfacial tension between a liquid and a solid. In solutions this behavior is known as wetting
, and it occurs as a result of surfactants adsorbing to the air/water interface. Soluble surfactants are also capable of forming micelles and other aggregate structures in solution, leading to a stabilizing effect in latex paints. Surfactants in paint are used to change many end properties of a dried paint, as well as to emulsify paints in their liquid state.
The thermodynamic explanation for demulsification is the gain in Gibbs Free Energy resulting from lowering the total area of high energy surface interactions.
The energy gained from demulsification is dependent on the total area of interface and the surface tension of that interface. Surfactants lower the surface tension (γ) and thus gibbs energy is gained from demulsification. This slows the process of demulsification and stabilizes the latex paint.
The size of the droplets of dispersed polymer in a latex paint can be modeled with the following equation:
The radius of a droplet in the emulsion is dependent on surfactant length, Ls, volume fraction of dispersed phase, φd, and volume fraction of surfactant, φs.
Anionic head groups are negatively charged, and commonly used in cleaning products. Anionic surfactants can be found in products such as shampoos, laundry detergents, and soaps because of their ability to remove dirt from soft mediums such as fabric. Anionic surfactants are easily suspended in water due to the polarity of the charged head group. However, hard water can deactivate the molecule. Some of the more commonly used anionic head groups are sulfates and ethoxylates.
Cationic head groups have a positive charge and cationic surfactants are used in several different applications. One common use for cationic surfactants is in fabric softeners. Cationic head groups are also added to laundry detergent in conjunction with anionic surfactants because they help to improve the dirt removal properties of the anionic surfactants. Cationic head groups also increase the disinfecting properties of household cleaners. Some common cationic surfactants head groups include amines and quaternary ammonium
ions.
Among the many types of surfactants, cationic surfactants are very useful as corrosion inhibitors due of their protective effectiveness in neutral and acidic media.
Nonionic head groups have no charge and they function very well as grease removers. Nonionic surfactants are commonly used in detergents, soaps, and household cleaners. In solutions of hard water, nonionic surfactants are used to help limit the deactivation of ionic surfactants caused by the calcium and magnesium ions. Some common nonionic surfactant head groups include fatty acids and glycols.
Alkyl ether chains are similar to hydrocarbon chains, except with oxygens incorporated within the backbone as well as carbons. There are two alkyl chains commonly used in surfactants: polyethylene oxide and polypropylene oxide. Polyethylene oxide chains have an oxygen and two carbon (-O-C-C-)n repeating unit and has an increased hydrophilic character compared to hydrocarbons. Polypropylene oxide has the same backbone structure as polyethylene oxide but with a methyl group substituent of one of the carbons, and this structure has hydrophobicity between hydrocarbons and polyethylene oxides.
Fluorocarbon chain tails consist of a carbon backbone that has fluorine substituents instead of hydrogens. Fluorocarbons help to lower the surface tension of water and other solvents because of their lypophobic nature even in harsh conditions such as low pH. When fluorocarbons are incorporated into surfactants they are used as stain repellents and incorporated into coatings in order to decrease surface defects.
Siloxane chains consist of a backbone which contains alternating oxygen and silicon atoms. Surfactants with siloxane tails have been found to resist hydrolysis and prevent breakdown polymer chains which can cause cracking in the paint and are thus used in products such as cosmetics, deodorants, defoamer, and soap.
(Syntheses and Properties of New Double-tail Trisiloxane Surfactants with Both Hydrolysis Resistance and High Spreading Ability)
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
has four major components: pigments, binders, solvents, and other additives. Pigments serve to give paint its color, texture, toughness, as well as determining if a paint is opaque or not. Common white pigments include titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
TiO2 and zinc oxide
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
ZnO. Binders are the film that forms as a paint dries and they affect the durability, gloss, and flexibility of the coating. Polyurethanes, polyesters, and acrylics are all examples of common binders. The solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
helps adjust the curing rate and viscosity of the paint in its liquid state. The solvent evaporates off as the paint dries. There are two types of solvents: solvent-borne and water-borne. The additives that are incorporated into paints are a wide range of things which help to effect the properties of the paint and the final coating. The main additives added to many paints are surfactants.
The word surfactant is short for surface active agent. Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or the interfacial tension between a liquid and a solid. In solutions this behavior is known as wetting
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
, and it occurs as a result of surfactants adsorbing to the air/water interface. Soluble surfactants are also capable of forming micelles and other aggregate structures in solution, leading to a stabilizing effect in latex paints. Surfactants in paint are used to change many end properties of a dried paint, as well as to emulsify paints in their liquid state.
Role of surfactants in paint
| % TiO2 by volume | Modulus of Elasticity (MPa) | Modulus of Elasticity: Surfactant Removed (MPa) |
|---|---|---|
| 0 | 8.9 | 6.0 |
| 13 | 22.9 | 22.4 |
| 25 | 60.2 | 89.1 |
| 38 | 169.8 | 416.8 |
| Elasticity of a latex paint is affected by presence of surfactant. Note the affect changes dependent on TiO2 concentration. |
||
Positive Effects
Surfactants affect a wide array of physical properties in paints. Surfactants affect the behavior of a paint not only during the lifetime of the formed coating but also the initial aggregation and film formation of the paint. Surfactants are also used to stabilize the dispersion of polymer particles during emulsion polymerization in paints and other applications. The mechanical stability, freeze-thaw stability and shelf-life of paints are all improved by the addition of surfactants. The addition of surfactants to paint also allow the paint to coat a surface more easily because surfactants increase the wetting of a solution.Negative Effects
The addition of surfactants does not always have a positive affect on all properties. The water resistance of the coating can be decreased with surfactant addition since surfactants can be very water soluble and will easily wash out of a coating. While the type and amount of surfactant determine what properties will be affected, other chemicals in a paint can alter the overall affect the surfactants may have on the paint. Elasticity has been found to either increase or decrease in latex paints depending on the amount of TiO2 present.Emulsification
Latex paints are an emulsion of polymer particles dispersed in water. Macroemulsions in latex paint are inherently unstable and phase separate, so surfactants are added to lower interfacial tension and stabilize polymer particles to prevent demulsification.The thermodynamic explanation for demulsification is the gain in Gibbs Free Energy resulting from lowering the total area of high energy surface interactions.
The energy gained from demulsification is dependent on the total area of interface and the surface tension of that interface. Surfactants lower the surface tension (γ) and thus gibbs energy is gained from demulsification. This slows the process of demulsification and stabilizes the latex paint.
The size of the droplets of dispersed polymer in a latex paint can be modeled with the following equation:
The radius of a droplet in the emulsion is dependent on surfactant length, Ls, volume fraction of dispersed phase, φd, and volume fraction of surfactant, φs.
Classification of surfactants
There are three main categories of surfactants used in paint —ionic, polymeric and electrosteric.According to head group composition
The head group classification of a surfactant is determined by the head group ion type. Ionic surfactants derive their amphiphilicity from a charged hydrophilic head group and tend to be small, low molecular weight molecules. Ionic surfactants will stabilize particles suspended in a paint by electrostatic repulsion and are easily adsorbed and desorbed from a surface due to their small size.Anionic head groups are negatively charged, and commonly used in cleaning products. Anionic surfactants can be found in products such as shampoos, laundry detergents, and soaps because of their ability to remove dirt from soft mediums such as fabric. Anionic surfactants are easily suspended in water due to the polarity of the charged head group. However, hard water can deactivate the molecule. Some of the more commonly used anionic head groups are sulfates and ethoxylates.
Cationic head groups have a positive charge and cationic surfactants are used in several different applications. One common use for cationic surfactants is in fabric softeners. Cationic head groups are also added to laundry detergent in conjunction with anionic surfactants because they help to improve the dirt removal properties of the anionic surfactants. Cationic head groups also increase the disinfecting properties of household cleaners. Some common cationic surfactants head groups include amines and quaternary ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
ions.
Among the many types of surfactants, cationic surfactants are very useful as corrosion inhibitors due of their protective effectiveness in neutral and acidic media.
Nonionic head groups have no charge and they function very well as grease removers. Nonionic surfactants are commonly used in detergents, soaps, and household cleaners. In solutions of hard water, nonionic surfactants are used to help limit the deactivation of ionic surfactants caused by the calcium and magnesium ions. Some common nonionic surfactant head groups include fatty acids and glycols.
According to tail composition
Hydrocarbon chains are long chains which consist of a carbon backbone hydrogen substituents, making them very hydrophobic. Hydrocarbon chains alone form waxes and oils and retain these characteristics when they are incorporated into surfactant. A good example of surfactants containing a hydrocarbon chain are lipids, which form cell membranes.Alkyl ether chains are similar to hydrocarbon chains, except with oxygens incorporated within the backbone as well as carbons. There are two alkyl chains commonly used in surfactants: polyethylene oxide and polypropylene oxide. Polyethylene oxide chains have an oxygen and two carbon (-O-C-C-)n repeating unit and has an increased hydrophilic character compared to hydrocarbons. Polypropylene oxide has the same backbone structure as polyethylene oxide but with a methyl group substituent of one of the carbons, and this structure has hydrophobicity between hydrocarbons and polyethylene oxides.
Fluorocarbon chain tails consist of a carbon backbone that has fluorine substituents instead of hydrogens. Fluorocarbons help to lower the surface tension of water and other solvents because of their lypophobic nature even in harsh conditions such as low pH. When fluorocarbons are incorporated into surfactants they are used as stain repellents and incorporated into coatings in order to decrease surface defects.
Siloxane chains consist of a backbone which contains alternating oxygen and silicon atoms. Surfactants with siloxane tails have been found to resist hydrolysis and prevent breakdown polymer chains which can cause cracking in the paint and are thus used in products such as cosmetics, deodorants, defoamer, and soap.
(Syntheses and Properties of New Double-tail Trisiloxane Surfactants with Both Hydrolysis Resistance and High Spreading Ability)