July 2003 Q&A

The Importance of pH in Waterbornes

What effects does the pH have in a waterborne coating?

The pH will have a major effect on several properties of a coating. Most waterborne coatings are formulated at a pH between 7.5 and 9, though some systems can be on the acidic side, such as 4. The specific target needs to be maintained for the additives, pigments and resins to work together.

The first, and probably the most obvious effect, is on the viscosity. Many thickeners are based upon carboxylic acid-containing polymers or polymers that interact with the hydronium ion or hydroxide. The ionic interactions along the polymer chain, either by having the ionic groups as a part of the chain or associated with the chain, cause the chain to become elongated, which increases the viscosity. Any change in the pH can result in an increase or decrease in viscosity in these systems.

A second area is pigment dispersion stability. The point of zero charge on pigments is the pH at which the surface of the particle has no charge. When the pH is increased above this value it is usually negatively charged, and below this point it can become positively charged. Generally we try to keep the pH above this value so that all the pigments are negatively charged. If all particles are negative, they will repel each other. However, if there are positive and negatively charged particles, flocculation may occur. A shift in the pH due to amine loss or carbon dioxide absorption can therefore result in flocculation as the pH decreases.

A third effect is on the stability of the paint. Some resins will flocculate if the pH shifts. This effect is very similar to that of pigments, since it is usually with carboxylic acid groups on the surface of the polymer particle that are as the salt at high pH but under lower pH are existing as the acid, which can aggregate. The pH can also affect the wetting of the paint on the surface, due to the pH sensitivity of the wetting aids. Corrosion can also be accelerated if the pH is lower or higher, depending upon the type of metal being painted.

The final major issue is on the hydrolytic stability of the resin. The pH can accelerate the rate of hydrolysis of the resin. The effect can be seen in waterborne urethanes, alkyds, acrylics, epoxy and polyesters. In general, amides, esters, epoxy and similar groups that can be hydrolyzed may show property changes with time in the can during storage. If the groups are in the backbone, the molecular weight will be lowered, and thus the tensile strength of the coating will be lowered, as may the hardness. If the groups are in the side chain, the effect can be in increased water sensitivity and other properties.

In general, the pH of a waterborne coating will have a profound effect on many of the paint properties. Therefore, proper pH at the time of shipping and during use is critical. It is important to note that in a dip tank or any open tank system, the carbon dioxide from the air may be absorbed into the paint and shift the pH during the day as can the loss of amine, which was added to the paint to make the pH higher. If the pH shifts, the paint’s properties will be altered. Work with the supplier in an effort to control the pH.

Exposing Alkyds to Moisture

Will acid etching allow alkyds to be used on galvanized steel or concrete?

The problem with both galvanized steel and concrete or plaster-type products is that the surface is alkaline. The alkaline surface can react with the esters of the alkyd to form metallic soaps, which are well known as mold release agents. In effect, they will not allow the coating to stick to the surface, and the paint will peel off. If the surface is washed with a dilute acid, the alkaline residues are removed and, if rinsed well and dried, will allow an alkyd to stick to these substrates.

However, if the substrate becomes wet, as in the concrete or plaster-type materials, the alkaline material within the substrate will migrate to the paint interface and create the metallic soap, and the paint will peel. On galvanized steel, the moisture that can permeate through the paint can in exterior exposure cause failure through the formation of the metallic soaps again. Therefore, in general, alkyds should be avoided for both these types of substrates. Only in areas where it will not be exposed to any moisture, including from the backside, can it be used with good results.