Surfactants encompass a variety of compounds that are used in various ways. The ‘surface-active agents’ as the name implies, are the compounds used to alter surface phenomena. These chemicals will reduce tension & improve wetting & spreading (dispersants). When surfactants are added to a coating, it concentrates at the interface between the liquid & the substrate because surfactant molecules are made up of two groups: a soluble group & an insoluble group. The insoluble end tends to be pulled in. These will be treated individually, but in each case keep in mind that we are really talking about compounds that affect surface activity.
In an overall sense, surfactants are classified according to the electronic charge associated with the molecules. They fall into four categories: non-ionic, anionic, cationic & amphoteric.
- Nonionic Surfactants: Nonionic surfactants usually are prepared by the addition reaction of ethylene oxide to hydrophobic compounds that contain one or more active hydrogen atoms. These surfactants do not carry a charge nor do they dissociate. Their surface-active character comes from the oxyethylene portion of the molecule. A particular advantage of the nonionic surfactants is that they are compatible with ionic surfactants. In such combinations, they import good freeze-thaw stability to aqueous systems & are less harmful to mechanical properties than the ionic compounds. Nonionic surfactants generally perform well over a range of ph values, & they will usually foam less than anionic & cationic surfactants in complex coating formulations. At the same time they might have environmental concerns.
- Anionic Surfactants: These surfactants carry a negative charge on the hydrophilic portion of the molecule. They are usually phosphates, sulfates & sulfonates. Anionic surfactants are good emulsifying & wetting agents, but they do not impart freeze-thaw stability. Generally speaking, anionic surfactants perform best in alkaline coating systems. Typically, the surfactants are added into the formulation during paint manufacture either in the grinding or let-down stage depending upon the formulation. These additives have also been tested as post-paint formulation additives & have exhibited comparable properties. It has been speculated that use of the surfactant before the paint formulation stage like use of phosphate esters in emulsion polymerization as well as polymerization stabilizer, or additives in pigment dispersion, should not only benefit the property of the paint but reduce the detrimental effects of any additional surfactants into the paint system.
- Cationic Surfactants: These surfactants carry a positive charge. As a rule, cationic surfactants do not play a significant role in emulsion polymerization. Compounds such as alkyl trimethyl ammonium chloride typify these surfactants. Generally speaking, cationic surfactants perform best in acidic coating system.
- Amphoteric Surfactants: Amphoteric surfactants exhibit both positive & negative character. Amphoteric compounds have been developed for the prevention of flocculation of carbon black & other pigments in high solids systems. They have also been used to prevent viscosity increases where polymer solubility is causing rheology problems.
Contribution to Properties
Surfactants comprise a very minor portion of a system as the quantity used is based on the total weight of the system. However, even though they are used in small quantities, surfactants make a major contribution to the following properties.
- Pigment wetting & dispersing:Anionic surfactants are the major type used to improve wetting & dispersion of pigments. They are essential for good dispersion, which allows one to optimize hiding power as well as flow & leveling. Both anionic dispersant & nonionic wetting agents contribute to overcome pigment floating & settling. Each pigment particle requires a particular amount of wetting agent to disperse & stabilize it optimally. Excessive amounts of surfactant should not be used because this can lead to shade & other appearance changes, moisture & water sensitivity, along with an accompanying loss of adhesion & poor resistance. Pigment wetting involves the replacement of the pigment/air interface with a pigment/liquid interface. This replacement is accomplished by dispersing the pigment in the liquid by mechanical means. The dispersant is responsible for keeping the pigment particles separated thus preventing agglomeration & settling during storage. This is accomplished by one or both of two mechanisms
(a) Electrostatic repulsion
When the hydrophilic portion of the surfactant is absorbed on the pigment particles, the ionic portion of the molecule or hydrophile will be found in the liquid phase. This charged portion of the molecule will repel particles that contain like electrostatic charges on other particles and result in a stabilized system. Anionic surfactants function by this mechanism.
(b) Steric hindrance
Steric hindrance takes place when the hydrophobic portion of a nonionic surfactant is absorbed onto the surface of the pigment particles & the long hydrophilic chain in the liquid medium. And this acts as a mechanical bumper that prevents the stabilized pigment particles from too closely approaching each other. This is the mechanism by which nonionic surfactants function.
- Color Development:This property is achieved or optimized by the proper or correct dispersion of the pigment particles in a coating formulation. The more finely divided the insoluble pigment is in the paint, the more uniform & better (true) will be color development. Color development is a direct result of the efficiency or effectiveness of the pigment dispersion. This is dependent on the type of surfactant used for dispersion and also on mechanical grinding of the pigment.
- Washability:As mentioned earlier, nonionic surfactants & other surfactants that contain an oxyethylene segment will have an effect on moisture sensitivity. Nonionic surfactants usually are less severe in causing moisture sensitivity as compared to ionic surfactants. Proper selection of the nonionic surfactant used is important to obtaining the best adhesion & scrub-ability.
- Foam: Use of the surfactant or surfactant blend can modify the foaming characteristic of the paint being prepared. Low-molecular-weight nonionic surfactants will foam less than high-molecular-weight nonionic surfactants. Nonionic surfactants will cause less foam than ionic surfactants.
- Freeze-Thaw Stability:Ethylene &/or propylene glycol are usually added to paints to obtain freeze-thaw stability. A mixture of nonionic & anionic surfactants properly blended will yield a synergistic balance between the respective properties of the individual surfactants. Properly blended combinations of these surfactants will often provide excellent free-thaw stability & allow decreased levels of ethylene glycol to be used.
- Package or Storage Stability:Surfactants play an important role in obtaining good coating stability & good in-can or storage stability in particular. They are useful in reducing settling.
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