Cardura Monomer: The Route to Low Viscosity, Low VOC Acrylic Resins
Cardura glycidyl ester is a widely used building block for acrylic polyols (APOs). The epoxy group is typically used to incorporate Cardura monomer into resins via a reaction with acrylic acid or with (meth)acrylic acid. This results in the formation of hydroxyls that can then be used for the curing of the coating.
The highly branched Versatic™ acid part of the molecule differentiates Cardura glycidyl ester from other mono-functional glycidyl esters. The high boiling point of the monomer makes an attractive process for low solvent-containing acrylic polyols possible. The bulky structure also results in a substantial viscosity reduction of the APOs compared to Cardura monomer-free APOs of comparable molecular weight. When the APOs are formulated into coatings, the Versatic group will greatly enhance flow and leveling which leads to clear-coats or topcoats with exceptional appearance. Last but not least, the Versatic group also protects the ester bonds formed during curing, greatly improving the chemical resistance of the coating.
Production of high solid APOs using Cardura glycidyl ester as the initial reactor charge
The conventional way to incorporate glycidyl versatate in an acrylic polyol is via the reaction of the epoxy group with either acrylic or methacrylic acid. The most convenient process is to use Cardura monomer as the initial reactor charge (IRC) where it will function as a reactive solvent (step 1 in Figure 1).
Typically, the Cardura monomer is then heated (step 2) to the desired polymerization temperature (usually between 130° and 190°C). In a 3rd step, (meth)acrylic esters, (meth)acrylic acid, and styrene are dosed together with some peroxide initiator. The acrylic monomers undergo radical polymerization. Simultaneously, the Cardura monomer reacts with the acid, and the hydroxyl functional adduct which is formed is built into the acrylic polyol. The acid and Cardura monomer should be dosed in an approximately equimolar ratio to ensure that all of the Cardura monomer reacts into the APO, and that the target acid value for the APO is achieved.
Figure 1. Using the Cardura monomer as Reactive solvent in the Initial Reactor Charge
This process is illustrated in the following animation.
The epoxy-acid reaction and the radical polymerization reaction occur simultaneously in a convenient one-step process. The presence of the Cardura monomer as a reactive solvent in the IRC makes it possible to limit or completely avoid using traditional, non-reactive solvents for that purpose. At the end of this process, a limited amount of solvent can be added to reduce the viscosity to the desired level.
Figure 2. Incorporation of CE 10 in acrylic resins
As was mentioned above, the incorporation of bulky aliphatic groups in acrylic polyols results in lower viscosity.
The reason for this viscosity reduction is related to hydrogen bridge formation in the APOs. The beneficial effect of Cardura monomer is illustrated in the following animation.
Figure 3 (below) shows the effect of the Cardura 10 content on the viscosity of acrylic polyols with similar molecular weight, hydroxyl value and Tg. This important viscosity reduction allows coating formulators to achieve the required VOC level with resins having a significantly higher molecular weight than conventional resins.
Figure 3. Viscosity of acrylic resins containing various levels of CE 10. The resins have similar solids content, Mw, calculated Tg and hydroxyl value.