Fundamental differences of waterborne acrylic and solventborne binders

One of the key features of solventborne resins, such as oil-based alkyds or those used in two-component epoxies and polyurethanes, is that they are supplied at a lower molecular weight than the typical waterborne acrylic latex polymer. This means that the polymer chains are shorter, and is necessary for those polymers to dissolve in their carrier solvent and so that the binder can be supplied at a reasonable viscosity. Waterborne acrylics, on the other hand, are supplied as a latex, which is a dispersion of small particles in water. Each particle contains several acrylic polymer chains. The acrylic polymer is not soluble in the carrier (water), and is at a much higher molecular weight, which is advantageous for long term exterior durability. The solventborne binders rely on crosslinking reactions, which occur after the paint dries, in order to increase their molecular weight and improve properties such as durability and chemical resistance. This crosslinking can lead to problems of embrittlement and cracking as it continues over time, which is especially common with oil-based alkyd coatings.

A depiction of how waterborne acrylic latex and typical solventborne polymers are supplied is shown below. This pictures shows that several acrylic polymer chains form a small, spherical particle, whereas the solventborne binders are polymer chains that are truely dissolved in the solvent. What is more difficult to depict in the picture is that the solventborne polymer chains are much shorter (i.e., lower molecular weight) than the waterborne acrylic. The difference in molecular weight can be as much as a factor of 100 times or more.

This basic difference in how the binders are supplied results in very different mechanisms for the formation of coating films. Click here for a description of the basic film formation process of waterborne versus solventborne binders.

In addition, the difference in molecular weight or binder chain length, require that most solventborne coatings undergo some sort of crosslinking reaction after film formation. The crosslinking is needed to increase the chain length, which impacts on properties such as durability and chemical resistance. Some solventborne resins, such as oil-based alkyds, will undergo crosslinking in the presence of oxygen (oxidative crosslinking), while others such as polyurethanes and epoxies require the presence of a hardener which reacts with chemical groups on the binder molecule. Because the acrylic binder is already at a high molecular weight, the crosslinking reaction is not necessary to obtain good durability and chemical resistance. For this reason, most waterborne acrylics are used as one-component systems and are thus ready to use and apply. Many solventborne systems such as epoxies and polyurethanes are two-component systems requiring mixing prior to use and displaying a pot life which limits their use to within a finite time after mixing.

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