Flow, leveling and viscosity stability can be very challenging and problematic in waterborne ambient cure and baked finishes. These issues can affect not only package stability, but also have a profound effect on appearance during and after application. As the vast majority of waterborne coatings are anionic in nature (amine (EU) neutralized), this article will focus on amine neutralized resin types, although if resin polarity, for example from acid functionality, is high enough, anionic resin types can be dispersible or even water soluble without the use of neutralizing amine.
There are two major categories of waterborne paint technologies: water reducible (EU) and latex (EU). For the purpose of this article, water reducible resins are normally made in solvent and then reduced with water to form a resin dispersion in water. Latex resins are made by emulsion polymerization in water. The paints using emulsion resins most often utilize a small amount of organic cosolvent to improve coalescence of the latex particles, as well as substrate wetting.
Other types of waterborne paints utilize a growing number of resin types that include PUD (polyurethane dispersions (EU)) as well as microemulsions. Accordingly, multiple issues can influence the flow, leveling, and appearance of waterborne coatings. This article will primarily examine the impact of 1) resin type, 2) humidity and temperature variation, 3) wetting and 4) flow control. Appearance of the coating during and after application can be affected greatly by these issues.
Resin Effects, Temperature and Humidity
The proper formulation of waterborne coatings must consider many additional factors due to their complexity when compared to most of their organic solvent born counterparts. Hence, most waterborne coatings, in addition to the essential paint ingredients in solvent born finishes (e.g. resin, pigment, solvent, flow additives etc.), also contain additional amine, antimicrobial (EU) agents, wetting (EU) agents, thickeners (EU), associative thickeners (EU), surfactants (EU), and coupling solvents. Lastly, as the diagram below illustrates, the rheological characteristics of water born coatings can be very different with the addition of water or cosolvent than organic solvent borne coatings with the addition of solvent.
The chart below shows the general effects of change in ambient temperature and/or humidity, and dilution, on viscosity upon application and applied appearance on organic solvent born coatings versus that of a typical water born latex.
An additional complication in waterborne coatings is if they are applied at or above the dew point*, with the same amount of water entering the film as evaporating. This can result in excessive flow and thus severe sagging. If the waterborne paint contains one or more volatile cosolvents, under the critical relative humidity, for a volatile cosolvent the volume fraction of water will decrease. Conversely, at a higher relative humidity above the critical relative humidity, the volume fraction of water will actually increase. Accordingly, in a production environment, it is extremely important to control both temperature and humidity to ensure uniform application and appearance.
Wetting
Rheological properties can be very different than solvent borne coatings due to the high surface tension of water. To compensate, most waterborne coatings contain one or more additives to assist wetting and control flow. These additives enable acceptable wetting by reducing surface tension or controlling rheology to minimize excessive flow during and after application. The coating will not spread or wet properly if the surface tension of the paint is higher than the surface energy of the substrate.
Comparison of Surface Tension (liquids) and Surface Energy (solids) of various Materials to Water
If the surface tension of water is very high, surfactants (EU) or wetting agents are added to improve surface wetting, flow and appearance. Surfactants are classified according to the polarity of their head as being nonionic, anionic, cationic or amphoteric respectively.
The addition of less than 0.005% of some surfactants on total formula weight can depress the surface tension of a latex paint from about 65 dynes/cm to about 30 dynes/cm. In the last few years, reactive surfactants have become available as well. Improved hydrophobicity (EU), shear stability, and smaller latex particles are reported to be advantages.
Flow Control
Latex paints exhibit a higher degree of shear thinning than solvent born coatings. Accordingly, latex paints have too low of a viscosity at high shear rates at application to permit adequate film build and too high a viscosity at low shear rates to permit adequate leveling. Associative thickeners have gained in popularity versus the more traditional cellulosic type, as associative thickeners help to resolve these problems to a greater extent. Paints using associative thickeners exhibit less shear thinning, so the viscosity at high shear rates is higher and thus thicker films can be applied to help minimize sagging. Formulations containing associative thickeners also enable a reduction in viscosity at low shear rates so leveling is also improved. Although there are multiple types of associative thickeners, they are essentially medium-low molecular weight hydrophilic polymers containing at least two long chain nonpolar hydrocarbon groups along the chain.
*The dew point is the temperature at which water vapor in air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates, whereas at temperatures below the dew point, water will evaporate.
Suppliers
Surfactants for Waterborne Coatings:
Acme (EU)
Air Products (EU)
Ashland (EU)
Dow
Ethox
Kowa
Rhodia (Solvay)
Associative Thickener Flow Control Agents:
Air Products (EU)
Ashland (EU)
BASF (EU)
BYK (EU)
Celanese (EU)
Coatex
Elementis (EU)
Nexeo
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Hi Ron,
I’m Nabeel from Pakistan, I am much impressed by you.
I have formulated water based inkjet ink , used in Epson Printer. I am facing just one problem
that is Ink tiny droplets get scattered after jetting.
Can you please tell me reason behing this ?
waiting for your kind reply
[email protected]
Hi,
Mr. Ron, your monumental and impressive explanation has given me and others more insights to to dynamics surfactants/ wetting agents in relation to the problem of surface Tension of water in water borne/ latex paints. Pls what is the right quantity of the wetting agent/ surfactant that should be used without affecting paint?
Thank you very much.
Mr. Emma , Sahara
Hi,
Mr. Ron, your monumental and impressive explanation has given me and others more insights to the dynamics of surfactants/ wetting agents in relation to the problem of surface Tension of water in water borne/ latex paints. Pls what is the right quantity of the wetting agent/ surfactant that should be used without affecting the integrity the paint?
Thank you very much.
Mr. Emma , Sahara
Dear Emma:
Thank you for your comment. Please contact me at [email protected] so I can learn more about the paint systems you are using.
Thank you for reading!
Ron