Catalysis of Urethane Systems Catalyst Concentration

06.02.2019

Catalyst Concentration

Metal catalyst concentration in a urethane system is normally discussed in terms of the catalyst metal content and resin solids of the system. For example, a typical DBTDL level for a two-component urethane system might be 0.02-0.03% of the catalyst as supplied based on total resin solids of the paint. However, with the Sn content of DBTDL being approximately 18%, it is more appropriate to note the typical Sn level to be 0.0036¬0.0054% of total resin solids. This is a particularly important consideration when comparing DBTDL with one of a list of catalyst alternatives. Catalysts are formulated at different metal concentrations, and, therefore, replacement of DBTDL with another catalyst based on equal weight of the catalyst as supplied is usually not appropriate. As a first approximation, the catalysts can be used at an equal metal level to DBTDL, with the exception of aluminum catalysts, which should be approximately 10 times the level of tin. Certain zirconium complexes have demonstrated an ability to provide very fast dry times of two-component isocyanate cross-linked coatings at room temperatures and under colder conditions. DBTDL is usually slower than the zirconium complexes when compared at equal metal concentrations. In most studies, similar or faster dry times were obtained using zirconium concentrations that were at least one-third the concentration of tin.

Table 2. Acylic/hexamethylene diisocyanate clearcoat

Figure 6. Low-temperature cure response

In another examination of reaction rate, dry times of a solvent-borne acrylic/isocyanate clearcoat (Table 2) catalyzed with zirconium and with DBTDL was evaluated. Catalyst levels were based on achieving a similar dry time at room temperature. The zirconium metal level was one-fifth the level of tin. Dry times were compared at 5°C and 24°C. Figure 6 illustrates similar dry times at 24°C and greater increase in dry time of the DBTDL system at 5°C compared to the zirconium catalysts.

Catalyst Selectivity

Acceleration of an isocyanate reaction is a selective process. Urethane formation is usually preferred over urea, allophanate, or biuret formation. Therefore, a catalyst that would selectively enhance urethane formation would be of considerable interest. Generally, most catalysts that promote urethane formation will also accelerate side reactions. However, side- reaction rates can vary greatly depending on the catalyst. The selectivity of a metal catalyst can be related to the metal type, steric hindrance of alkyl groups, and attached ligands. The activity of tertiary amines with respect to specific reactions is related to amine basicity and accessibility of the nitrogen electron. Together, these properties determine amine catalyst strength. Amine catalyst strength increases as basicity increases and steric hindrance decreases. A particular concern is the reaction of isocyanate with water. The eventual product of an isocyanate/water reaction is polyurea with an intermediate reaction that produces a primary amine and releases carbon dioxide (CO2). This side reaction can be detrimental to the quality of a formulated coating. For example, because the resulting product would contain less urethane and more polyurea, certain resistance properties of the coating are affected.

Also, sufficient generation of CO2 causes gassing, which will reduce the time that the paint is usable, and cause gloss reduction in applied films. Gloss reduction can be related to gassing and formation of insoluble polyurea.

In solvent-based systems, water can be carried into a formula by pigments, resins, solvents, and other additives. It can also be present as atmospheric humidity. Naturally, in water-reducible systems, catalyst selectivity is a major concern. Results of recent studies suggest that certain zirconium complexes are more selective catalysts than DBTDL catalysts have been developed that can provide very fast dry times by selective catalysis of two-component acrylic and polyester urethane coatings cross-linked with HDI trimers and biurets. A qualitative comparison of the effect that zirconium and DBTDL have on the reaction of water and isocyanate visually suggests that the reaction rate is faster with DBTDL than with zirconium (Fig. 7). Samples, in the image, were prepared by blending HDI trimer, catalyst, and water. The DBTDL samples developed air bubbles (CO2 generation) quickly, particularly the sample with 2.0% water, which had a significant amount of gassing within the first hour. Zirconium samples developed minimal gassing even after 1 week. Figure 7 shows the difference between the two catalysts after aging the formulation overnight in the presence of 2.0% water. The lack of gassing in the zirconium vial does not necessarily establish it as a more selective catalyst than DBTDL. This result could also indicate hydrolysis of the zirconium catalyst causing deactivation. Therefore, additional tests were conducted to examine catalyst selectivity. HDI Trimer with 2.0% Water

Figure 7. Water reaction with isocyanate

Selectivity of different metal compounds were analyzed by FTIR spectroscopy. Catalysts were added to a solution of butyl isocyanate, 2-ethyl hexanol, and water (molar ratio 1:1:2) and the final product was analyzed. Examination of urethane and urea peak heights provided a quantita¬tive analysis of the selectivity characteristics of the catalysts. Distinct differences were observed in the urethane and urea absorption ratios with the different catalysts. Additional FTIR studies have supported this observation.

Viscosity Stability

Viscosity stability, or pot life, of the blended polyol/polyisocyanate is limited; hence, mixing of the two components is done just prior to application. As the reaction proceeds in the pot, products of higher molecular weight are generated, resulting in a more viscous solution that eventually becomes a gel. The maximum application viscosity of a paint depends on the application method. Higher viscosity results in poor flow and leveling or large droplet formation during spray application. Application equipment becomes less efficient as viscosity increases beyond double the initial viscosity. The objective of the formulator is to develop a coating system that gives a long pot life but reacts quickly when applied as a film. Fast reaction after application is important. Before curing, the film is very sensitive to dirt and to mechanical damage. An effective method of increasing the pot life of a tin-catalyzed polyhydroxy/polyisocyanate without affecting the rate of cure is by adding a diketone to the tin compound and presumably forming a labile complex with the catalyst. The preferred diketone compound used for the purpose of extending the pot life of metal-catalyzed two- component isocyanate coatings is 2.4-pentanedione. A chelating agent is a ligand that has two or more points of attachment to a metal atom. As a volatile chelating agent, 2,4-pentanedione can essentially “block” the metal compound from catalyzing the reaction until the coating is applied. In a thin film, the chelating agent evaporates, leaving the free catalyst to accelerate the reaction. In these coatings, the addition of a β-diketone improves pot life but usually does not influence cure. The stabilizing effect of 2,4-pentanedione depends on the metal catalyst used to accelerate the reaction. A particular metal complex that has provided unusually good dry time and pot life characteristics with 2,4-pentanedione is based on aluminum. Although this catalyst has demonstrated excellent catalytic activity in two-component urethane coatings, the unique attribute of the product is realized when it is used with 2,4-pentanedione, primarily in its stabilizing capabilities. The polyester/isocyanate formulation in Table 3 an aluminum complex and DBTDL with and without 2,4-pentanedione. Each of the catalyzed systems were formulated to 70% solids with an NCO/OH ratio of 1.1:1.0. The level of aluminum required was much higher than the level of tin (0.08% aluminum compared to 0.004% tin on resin solids); however, the higher aluminum level did not adversely affect resistance properties of cured films.

Table 3. Polyester/hexamethylene diisocyanate clearcoat

Note: 2,4 PD = 2,4-pentanedione

Table 4 contains data comparing dry time and pot life of the catalyzed systems. Both catalysts yielded films that had a surface dry time of less than 1 h. The viscosity of each system was also very similar. The addition of 1.5% 2,4-pentanedione had minimal affect on dry times, but it had a significant affect on pot life (Fig. 8)

Table 4. Enamel properties

Figure 8. Pot-life extension of a two-component polyester/hexamethylene diisocyanate trimer with 2,4-pentanedione

Modification with 2,4-pentanedione increased the time to reach double the initial viscosity from 18 to 63 min for the DBTDL system, and 12 min to 5.5 h for the system catalyzed with aluminum. Figure 8 illustrates a double viscosity increase with up to 1.8% 2,4-pentanedione added on resin solids. All films developed equal hardness with 200+ methyl ethyl ketone (MEK) rubs, 160+ direct/reverse impact, and 100% adhesion after 1 week of ambient cure. Single-component polyol/isocyanate coatings that are stable under ambient conditions are possible when the free isocyanate is reacted with a volatile blocking agent. At elevated temperatures, the blocking agent will release from the isocyanate and, preferably, escape the film. Catalyzed blocked isocyanate systems can fully cure under coil (i.e., 1 min at 205°C) and general industrial (i.e., 25 min at 120°C) baking conditions. The rate of cross-linking depends on cure time and temperature, the blocking agent, reactivity of the isocyanate and polyol, film thickness, and catalysis. In some cases, catalyst addition is not needed because the temperature required to free the isocyanate sufficiently increases the polyol/isocyanate reaction rate. Metal catalysts accelerate the reaction of the free isocyanate with the coreactant resin, but there is no definitive evidence that they lower the deblocking temperature of the blocked isocyanate. Commercial isocyanates are blocked by forming relatively weak bonds with oximes, phenols, alcohols, e-caprolactam, 3,5-dimethylpyrazole, triazole, and diethyl malonate. Catalysis of blocked isocyanate coatings can be achieved with tin, zinc, or bismuth compounds. Cobalt carboxylates are also catalysts for these systems; however, color issues limit their use to primer applications. Cure-response studies were conducted that compared several metal catalysts in different blocked isocyanate systems to determine potential minimum cure bake conditions. The formulations were based on cross-linking an acrylic polyol with HDI blocked with methyl ethyl ketoxime (MEKO), 3,5-dimethylpyrazole, and e-caprolactam. Catalysts included were DBTDL, zinc octoate, and bismuth carboxylate. The metal concentration was 0.1% on total resin solids and the NCO/OH ratio was 1.0:1.0 for each system. The studies indicate the minimum temperature required to achieve adequate cure (100 double MEK rubs) after a 20-min dwell time with the given catalysts. The MEKO systems sufficiently cured as low as 140°C with each catalyst. Tin and bismuth catalysts performed equally in the 3,5-dimethylpyrazole system, generating 100 double MEK rubs at 130°C. Catalysis with zinc produced undercured films that wrinkled with 3,5-dimethylpyrazole. Catalyst addition to a e- caprolactam blocked HDI system may not be necessary. Studies have shown that catalyzed and uncatalyzed paints can achieve 100 double MEK rubs at about 170°C. The cross-linking reaction rate of the polyol and isocyanate without catalyst is sufficient at the temperature required to deblock the isocyanate.