Coating Crosslinking Reactions Using Cycloaliphatic Epoxides
Epoxide ring opening reactions are widely used in the coatings industry. Ring opening reactions lead to coatings with low shrinkage, which is especially important when the polymerization reaction is the final curing step in the coating process. The need for low VOCs has dictated the move to lower-molecular-weight, lower-viscosity ‘resins’ for coatings applications. Materials that at one time were considered crosslinkers for high-molecular-weight resins have now become the resin.
In Table 1, the physical properties of several common commercial epoxides — cycloaliphatic, glycidyl ether and aliphatic — are compared.
In addition to physical properties, chemical reactivity of the epoxide group is a key parameter in determining the use of the epoxide in coating formulations. The key difference between a cyclo epoxide and DGEBA is amine reactivity, especially at ambient temperature. Cycloaliphatic epoxides are essentially unreactive at room temperature with the conventional amine curatives commonly used with DGEBA. Figure 1 illustrates the relative stability or half-life of cyclohexene oxide and phenyl glycidyl ether as a function of apparent pH in methanol/buffer solutions.1 Solutions were buffered with acetic acid and/or triethylamine. Cyclohexene oxide is less reactive at high pH with a long half-life; phenyl glycidyl ether is less reactive at low pH. Effects of pH, pKa and catalyst on cycloaliphatic epoxide reactivity have recently been quantified. 2-4
Cationic Cure of Cycloaliphatic EpoxidesLow-viscosity, solvent-free coatings can be formulated with cycloaliphatic epoxides. Cationic cure can be carried out with either UV or thermally initiated super acids. 5,6 Typically the epoxide will either be homopolymerized or co-polymerized with various epoxides and polyols. Model thermally cured formulations and performance data are shown in Table 2. UV results would be similar, with nearly instantaneous cure at room temperature.
Overall coating performance will depend on formulation and cure variables such as epoxide/OH mole ratio, catalyst level, cure time/temperature, polyol and epoxide functionality, and glass-transition temperature.7
Thermal Cure of Acid/Anhydrides with Cycloaliphatic EpoxidesThe rapid reaction of cycloaliphatic epoxides with acids and anhydrides had commonly been used to produce mechanical and electrical goods using liquid injection molding techniques.8,9 These all-aliphatic parts have both excellent weatherability and electrical properties. A typical two-pack system has a cycloaliphatic epoxide and an alcohol or polyol on side A, which is then mixed with the anhydride curative and baked in a mold. Systems are often catalyzed with either Lewis acids or tertiary amines.9 Lewis acid catalysts tend to favor epoxide homopolymerization and amines tend to favor esterification. These systems often need to be cured for up to 2 to 3 hours. This cure schedule is not acceptable for coatings applications. However, low-viscosity, two-pack coatings can be formulated with cycloaliphatic epoxides and anhydrides. Table 3 shows a model for 100%-solids, two-pack coating formulations.
The 1.1/1.0/0.25 equivalent ratio (epoxide/acid/alcohol) was previously used to study the reaction rates of epoxide-221 with anhydride in the presence of an alcohol initiator by wet chemical methods.10 That study looked at a variety of catalysts for the overall reaction. Since the epoxide can both react with the acid to form ester and homopolymerize due to acid catalysis, the cured product can contain both ester and ether groups from the epoxide ring opening. Depending on the catalyst and epoxide acid mole ratio, different levels of ester and ether can be formed. Table 4 contains initial rate data fit to pseudo first-order kinetics11 calculated from the original wet chemical study. The data shows that the choice of catalyst has a significant impact on both reaction rate and ester/ether initial kinetic ratio of the curing product.
Since the acid/anhydride and the catalyst can both catalyze epoxide homopolymerization and the epoxide/acid reaction, the ‘ideal’ stoichiometry (ratio of moles of epoxide to moles of carboxyl groups) will depend on, among other factors, the choice of catalyst.
Ambient Cure of Two-Pack Coatings Containing Cycloaliphatic Epoxides and an Acid-Functional ResinSomewhat less recognized is the potential for ambient cure of cycloaliphatic epoxides with acid-containing resins in a two-pack coating formulation. Table 5 summarizes coating cure data for multifunctional cycloaliphatic epoxides with a model acid-functional polyester. The model polyester was prepared by reacting a caprolactone triol of about 540 Mn with phthalic anhydride. This acid-functional caprolactone-based polyester was then used to cure the epoxide at room temperature. Cure rates and performance improved as the functionality of the epoxide was increased. The higher functional epoxides were prepared as described in reference 7. FTIR kinetics studies12 of the rate of ester and ether formation showed the amine catalyst gave the highest initial pseudo first-order ester formation rate and the highest ester/ether kinetic ratio for the reaction of the tri-acid and tri-epoxide (see Table 6).
Cure of Waterborne FormulationsThe relatively slow reaction rate of cycloaliphatic epoxides at high pH (see Figure 1) but good reactivity with carboxylic acids suggests they can be used as crosslinkers for waterborne anionic, amine-stabilized dispersions.13,14 The epoxy can be added either to the dispersion as a reactive coalescent, or it can be dispersed with surfactants and added to the polymer dispersion as a separate dispersion.15 A typical dispersion formula would contain 60% epoxide, 40% water with 1.5% surfactant (silicone or fluorinated type), and 0.1 cellulosic thickener, such as CELLOSIZE® QP-4400. The epoxide is added under high shear to the surfactant/water thickener mixture. Since amines are also catalysts for the epoxide reaction, this application must be considered as an emerging technology.16
ConclusionLow-viscosity cycloaliphatic epoxides can be used to prepare low- to zero-VOC coatings. The aliphatic backbone of the cycloaliphatic epoxide inherently has excellent weatherability. In reactions with carboxylic acids, the ratio of ester to ether is a function of both the epoxide/acid ratio and the catalyst. Higher functional cycloaliphatic epoxides give better coating-cure performance at ambient temperatures.
AcknowledgmentsThe author would like to thank the following for their helpful discussions: A.C. Ashcraft, J.W. Carter, J.K. Braddock, J.V. Koleske, D.M. Back, N. Kumabe, T.A. Upshaw, G.A. Vandezande and K.T. Lamb. The following participated in the lab work discussed above: K.T. Lamb, J.K. Braddock and G. Gaines.
For more information on crosslinking resins, contact Union Carbide Corp., Research and Development Department, Specialty Polymers and Products, PO Box 670, Bound Brook, NJ 08805.