Catalysis of Polyurethane Systems: Catalyst Concentration

Catalyst Concentration

Metal catalyst concentration in a urethane system is typically discussed in terms of the metal content of the catalyst and the resin solids of the system. For example, a typical DBTDL level in a two-component urethane system may be 0.02-0.03% of the catalyst based on the total resin solids given in the paint. However, it is more appropriate to note that while the Sn content of DBTDL is approximately 18%, a typical Sn level would be 0.0036-0.0054% of the total resin solids. This is an important consideration, particularly when comparing DBTDL with one of the alternative catalyst lists. Catalysts are formulated at different metal concentrations, and therefore, replacing DBTDL with an equal weight of another metal catalyst is generally not appropriate. As an initial approach, catalysts to be used other than aluminum catalysts can only be used at a metal level equal to the required DBTDL when the tin level is approximately 10 times higher. Certain zirconium complexes have demonstrated the ability to provide very fast drying times for two-component isocyanate cross-linked paints at room temperature and under cooler conditions. DBTDL is generally slower than zirconium complexes when compared at equal metal concentrations. In most studies, similar or faster drying times were obtained by using zirconium concentration of at least one-third the tin concentration.

Table 2. Acrylic/hexamethylene diisocyanate varnish

Figure 6. Effect of low-temperature curing

In another examination of reaction rate, drying times of a solvent-based acrylic/isocyanate varnish catalyzed with zirconium and DBTDL (Table 2) were evaluated. Catalyst levels were based on similar drying time at room temperature. The zirconium metal level is one-fifth of the tin level. Drying times were compared at 5°C and 24°C. Figure 6 shows similar drying times at 24°C and a higher increase in drying time of the DBTDL system at 5°C compared to zirconium catalysts.

Catalyst Selection

Acceleration of an isocyanate reaction is a selective process. Urethane formation is generally desired to occur rather than urea, allophanate or biuret formation. Therefore, the importance of a catalyst that will selectively increase urethane formation will increase considerably. In general, most catalysts that accelerate urethane formation also accelerate side reactions. However, side reaction rates can vary considerably depending on the catalyst. Selection of a metal catalyst is related to the metal type of the catalyst, steric hindrance of alkyl groups and attached ligands. The activity of tertiary amines according to specific reactions is related to the basicity and accessibility of the nitrogen electron. Along with these properties, it determines the strength of the amine catalyst. As basicity increases, amine catalyst strength increases and steric hindrance decreases. Of particular interest is the reaction of isocyanate with water. The likely end product of an isocyanate/water reaction is polyurea formed through 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 paint. For example, since the resulting product will contain less urethane and more polyurea, certain resistance properties of the paint are adversely affected. Furthermore, insufficient CO² release will reduce the usable life of the paint and lead to gassing, which causes a loss of gloss in applied films. Loss of gloss can be attributed to gassing and formation of insoluble polyurea.

In solvent-based systems, water may be introduced into the system by pigments, resins, solvents and other additives. It may also enter the system as atmospheric moisture. In water-reducible systems, catalyst selection is naturally a very important matter.

Results from recent studies show that some zirconium complexes are more selective catalysts than DBTDL. Based on these studies, zirconium catalysts have been developed that provide very fast drying times with selected catalysis for two-component acrylic and polyester urethane paints cross-linked with HDI trimers and biurets. Qualitative comparison of the effect of zirconium and DBTDL on the reaction of water and isocyanate visually suggests that the reaction rate with DBTDL is faster than with zirconium (Figure 7). The examples in the image were prepared by mixing HDI trimer, catalyst and water. DBTDL samples form air bubbles (CO2 release) rapidly, particularly the sample containing 2.0% water, producing significant gas release within the first hour. Zirconium samples, on the other hand, produced minimal gas release even after 1 week. Figure 7 shows the difference between the two catalysts after the formulation with 2.0% water was left overnight. The absence of gassing in the zirconium container does not make it significant as a more selective catalyst than DBTDL. This result may also demonstrate hydrolysis of the zirconium catalyst causing deactivation. For this reason, additional tests should be conducted to examine catalyst selection. 2.0% Aqueous HDI Trimer

Figure 7. Water reaction with isocyanate

Selection of different metal compounds was analyzed by FTIR spectroscopy. Catalysts are added to a solution of a butyl isocyanate, 2-ethyl hexanol and water (molar ratio 1:1:2) and the final product is analyzed. Examination of urethane and urea peak heights provided quantitative analysis of the selectivity properties of the catalysts. Differences were observed in urethane and urea absorption ratios with different catalysts. Additional FTIR studies supported this observation.

Viscosity Stability

The viscosity stability or pot life of a mixed polyol/polyisocyanate is limited; therefore, mixing of the two components is done immediately before application. As the reaction proceeds in the pot, products of higher molecular weight are formed, resulting in a more viscous solution that eventually reaches a gel state. The maximum application viscosity of a paint depends on the application method. Higher viscosity results in poor flow and surface smoothness during spray application or formation of large droplets. As viscosity increases more than twice the initial viscosity, application equipment becomes less efficient. The formulator's objective is to develop a paint system that provides long pot life but reacts quickly when applied as a film. Rapid reaction after application is important. Before hardening, the film is highly susceptible to contamination and mechanical damage. An effective method to increase pot life without affecting the cure life of a tin-catalyzed polyhydroxy/polyisocyanate is the addition of a diketone to the tin compound and probably formation of a variable complex with the catalyst.

The diketone compound used to extend pot life of metal-catalyzed two-component isocyanate paints is 2,4-pentanedione.

A chelating agent is a ligand having two or more attachment points to a metal atom. As a volatile chelating agent, 2,4-pentanedione can essentially "block" the metal compound from catalyzing the reaction until the paint is applied. In a thin film, the chelating agent volatilizes and releases the catalyst to accelerate the reaction. In these paints, addition of a β-diketone extends pot life but generally does not affect curing. The stabilizing effect of 2,4-pentanedione is dependent on the metal catalyst used to accelerate the reaction. A special metal complex catalyst providing unusually good drying time and pot life properties with 2,4-pentanedione is aluminum-based. Although this catalyst exhibits excellent catalytic activity in two-component urethane coatings, the product's specific property when used with 2,4-pentanedione is essentially achieved through stabilization effect. The polyester/isocyanate formulation in Table 3 shows an aluminum complex with and without 2,4-pentanedione and DBTDL. Each catalyzed system is at 70% solids and each is formulated at a 1.1:1.0 NCO/OH ratio. The required aluminum level is much higher than the tin level (0.08% aluminum on solid resin compared to a tin level of 0.004%); however, the higher aluminum level did not adversely affect the resistance properties of the cured films.

Table 3. Polyester/hexamethylene diisocyanate varnish

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

Table 4 contains data comparing drying time and pot life of catalyzed systems. Both catalysts provide films with surface drying time of less than 1 hour. The viscosity of each system is also quite similar. Addition of 2,4-pentanedione at 1.5% had minimal effect on drying time but had significant effect on pot life (Figure 8).

Table 4. Enamel properties

Note: 2,4 PD = 2,4-pentanedione; TRS = total resin solids.

Figure 8. Extension of pot life with 2,4-pentanedione in a two-component polyester/hexamethylene diisocyanate trimer

Modification with 2,4-pentanedione extends the time to double viscosity for the DBTDL system from 18 to 63 minutes and for the initially aluminum-catalyzed system from 12 minutes to 5.5 hours. Figure 8 shows doubling of viscosity with the addition of 2,4-pentanedione up to 1.8% over resin solids. All films showed equal hardness with 200+ methyl ethyl ketone (MEK) rub, 160+ direct/reverse impact test and 100% adhesion after curing for 1 week at room temperature. Preparation of one-component polyol/isocyanate paints that are stable under ambient conditions is possible when free isocyanate is reacted with a volatile blocking agent. At high temperatures, the volatile blocking agent separates from the isocyanate and exits the paint film. Catalyzed blocked isocyanate systems can be fully cured at 205°C for 1 minute and under general industrial baking conditions at 120°C for 25 minutes. Cross-linking rate may vary depending on cure time and temperature, blocking agent, reactivity of isocyanate and polyol, film thickness and catalyst. In some cases, catalyst addition is not necessary because the temperature required to release the isocyanate increases the polyol/isocyanate reaction rate sufficiently. Metal catalysts accelerate the reaction of free isocyanate with co-reactive resin, but there is no definitive evidence that they lower the deblocking temperature of blocked isocyanate.

Commercial isocyanates can be blocked with oximes, phenols, alcohols, e-caprolactam, 3,5-dimethylpyrazole, triazole and diethyl malonate, forming relatively weak bonds. Catalysis of blocked isocyanate paint can be accomplished with tin, zinc or bismuth compounds.

Cobalt carboxylates are also effective catalysts for these systems; however, they can only be used in primers due to color problems. Cure studies were conducted comparing several metal catalysts in different blocked isocyanate systems to determine the lowest baking conditions required for curing. The formulations here were prepared by cross-linking acrylic polyol with HDI using methyl ethyl ketoxime (MEKO), 3,5-dimethylpyrazole and e-caprolactam as blocking additives. DBTDL, zinc octoate and bismuth carboxylate were used as catalysts. Metal concentration was given as 0.1% over total resin solids and the NCO/OH ratio for each system was 1.0:1.0. The studies show the minimum temperature required with the given catalysts to obtain adequate curing (100 MEK rubs) after a 20-minute waiting period. MEKO as a blocking additive provided systems with adequate hardening up to 140°C with all catalysts. 3,5-dimethylpyrazole systems equally with tin and bismuth catalysts achieved 100 MEK rubs at 130°C. When 3,5-dimethylpyrazole was used as blocking additive and zinc as catalyst, wrinkled insufficiently cured films were formed. An HDI system blocked with e-caprolactam may not require catalyst. Studies showed that both catalyzed and uncatalyzed paints could achieve 100 MEK rubs at approximately 170°C. The cross-linking reaction rate of polyol and isocyanate without catalyst is sufficient to reach the temperature required for deblocking the isocyanate. M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.    

References 1. Dombrow B.A. Polyurethanes-, Reinhold Plastics Applications Series; Reinhold: New York, 1957; 6. 2. Calbo, L.J. Ed. Handbook of Coatings Additives, Marcel Dekker: New York, 1992; Vol. 2, 263. 3. Thiele, L.; Becker, R. Catalytic Mechanisms of Polyurethane Formation. Adv. Urethane Sci. Technol. 1993, 4. Blank, W.J. New Developments in Catalysis; FATIPEC Macromolecular Symposia No. 187; Adler H.-J.P., 5. Florio, J.J. Troubleshooting metal catalyzed urethane systems. Paint&Coatings Industry 2000, 16 (10), 80. 6. Bayer Corp. Aliphatic isocyanate, hexamethylene diisocyanate trimer, 100% active, 181 g/eq., Desmodur N-3300. 7. Florio, J.J. Non-tin metal catalysts for urethane coatings. Paint&Coatings Industry 1997, 13 (10), 110. 8. Bayer Corp. Water-dispersible polyisocyanate, 100% solids, 220 g/eq., Bayhydur XP-7007. 9. Wicks, Z.W.; Jones, F.N.; Pappas, S.P. Organic Coatings, Science and Technology, 2nd Ed.; Wiley Interscience: New York, 1999; 196.