Catalysis of Urethane Systems

24.12.2018

Urethane Catalyst Background The roots of polyurethane chemistry date back to the mid-19th century when the reactions of monobasic isocyanates were initially studied by Charles Adolphe Wurtz (1817-1884) and August Wilhelm von Hoffman (1818-1892). Research of the isocyanate reactions continued into the 20th century. Dr. Otto Bayer’s (1902-1982) research of isocyanate reactions in Germany during World War II was significant to the commercial potential of the reaction of isocyanates. Dr. Bayer’s group discovered the diisocyanate polyaddition process that is used to produce polyurethanes. His work initially involved research to produce fibers that would be equal or superior to nylon. Production of polyurethane coatings for industrial applications soon followed in the early 1940s. Today, polyurethane chemistry is utilized in applications that require good hardness and flexibility, abrasion resistance, chemical resistance, and environmental etching while complying with governmental standards. These applications can include plastics, adhesives, foams, elastomers, sealants, and coatings. Due to tightening regulatory restrictions on volatile organic emissions in the coatings area, lower-molecular-weight resins are increasingly utilized in solvent-borne isocyanate/polyol systems. An important characteristic of these lower-molecular-weight resins is that they have low viscosities.

Coat-ings produced with these resins do not require the use of large amounts of solvent to achieve required application viscosities. An inherited trait of two-component isocyanate/polyol systems that utilize low-molecular-weight resins is their dependency on a catalyst to accelerate the isocyanate/hydroxyl (NCO/OH) reaction.

This reaction is usually catalyzed with metal compounds and/or amines. Film formation of low-solids systems that use high-molecular-weight resins rely more on physical evaporation of solvent and less on cross-linking reactions. Until recently, the coatings industry has relied mainly on organotin compounds for metal catalysis of NCO/OH reactions. With the development of new non-tin metal catalysts and with recent research of new and existing catalysts, it has become evident that catalyst selection is an important and complex issue that has become more complicated. These newly developed catalysts can provide new opportunities for managing old problems. This article is intended to provide a better understanding of the complexities of urethane catalysis and to developguidelines for catalyst selection by troubleshooting commonly encountered problems. The reaction of isocyanates with hydroxyl groups is relatively slow in the absence of a catalyst. Catalysts are often used to achieve sufficient reaction rates and cured properties for a variety of urethane systems. Aliphatic isocyanate cross-linked systems are particularly dependent on catalysts to accelerate the reaction. Aromatic isocyanates are generally more reactive than aliphatic types and only require the use of catalysts under certain conditions. As previously mentioned, catalysis of the NCO/OH reaction is normally accomplished with metallic compounds or amines.

Among the metallic compounds traditionally used in the industry are catalysts based on tin, bismuth, zinc, and manganese. Certain mercury and lead compounds pro¬vide desirable catalytic qualities; however, they are also very toxic and are usually avoided.

Recently developed zirconium and aluminum compounds have gained interest in a variety of urethane applications. Dibutyltin dilaurate (DBTDL) can be considered the workhorse metal catalystfor urethane coatings. It is efficient; that is, a very low level of catalyst will greatly increase the NCO/OH reaction rate. However, as with any catalyst, certain problems can be encountered with DBTDL, which can include issues of stability-reactivity, hydrolysis of ester groups, catalysis of the water/isocyanate reaction, and environmental concerns. Diazabicyclo [2.2.2]octane, is a commonly used tertiary amine catalyst. It has been demonstrated that tertiary amines more effectively catalyze reactions of aromatic isocyanates than reactions of aliphatic isocyanates. Issues with amine catalysts can include color and moisture sensitivity. Where applicable, combinations of organotin and tertiary amine catalysts have demonstrated synergistic characteristics. It is preferable to use a catalyst for two-component urethane systems that could provide fast dry times of cast films but allow good pot life under ambient conditions. The catalyst should remain active as the catalyzed component is aged for extended periods and have minimal effect on resistance properties. Also, the catalyst should be environmentally acceptable. Hexamethylene diisocyanate (HDI) biuret and trimer (isocyanurate) derivatives are examples of aliphatic isocyanates that are often used in the coatings industry. The NCO groups of these derivatives are easily accessible, making them very responsive to active hydrogens in catalyzed systems. Isophorone diisocyanates (IPDI) and m-tetramethylenexylene diisocyanates (TMXDI) are less responsive to catalysis because of sterically hindered NCO groups. Another aliphatic isocyanate is hydrogenated diphenyl methane diisocyanate (HI2MDI). TMXDI and HJ2MDI are not often used as crosslinkers in coatings systems.

Mechanism of Catalysis

Catalysis of an isocyanate/polyol reaction can occur with the metal catalyst initially associating with either the isocyanate (lewis acid mechanism) or with the polyol (insertion mechanism). Association of the catalyst and the isocyanate creates a more electrophilic reactive site on the resin. This increased electrophilicity enhances the reaction of the isocyanate with the nucleophilic alcohol oxygen (Figure 1). Commonly used organotin compounds follow this mechanism. This association could occur through one of the pathways in Figure 1.

Figure 1. Lewis Acid mechanism

Figure 2. Insertion mechanism.

Insertion metal catalysts initially associate with the polyol or with water. The actual insertion catalyst is an alcoholate formed from the association of the metal catalyst and the polyol. The alcoholate reacts with the isocyanate to form an intermediate metal- complexed isocyanate, which further reacts with polyol to generate a urethane. A proposed mechanism is illustrated in Figure 2. Studies have indicated that some zirconium compounds catalyze according to the insertion mechanism. Recent work also demonstrated that some bismuth compounds initially associate with the polyol component. Progress of the isocyanate/polyol reaction can be observed using several methods. The reaction can be followed by monitoring the viscosity increase and the corresponding decrease in free isocyanate. Figure 3 is a comparison of data generated using these methods. A baseline rate of an uncatalyzed HDI trimer/polyol reaction was analyzed by measuring viscosity and percent unreacted isocyanate as the reaction progressed. Isocyanate concentration was determined by titration method using a measured excess of N-dibutylamine and titrating with hydrochloric acid. The NCO/OH ratio was 1.5:1.0. The initial concentration of unreacted isocyanate was approximately 6.5%. As Figure 3 illustrates, about 6% of the free isocyanate is reacted when the initial viscosity is doubled. A film cast using this uncatalyzed formula became tack-free after 27 h under ambient conditions (25°C, <50% relative humidity).

Figure 3. Viscosity versus percent unreacted isocyanate

The rate of the reaction can also be followed by infrared spectroscopy, with the disappearance of the isocyanate band at 2272 cm-1 wavenumber as a simple measure of the reaction rate. For a known polymer system, the pot life and tack-free times are a function of isocyanate conversion. For example, a doubling of viscosity of a high-solids polymer system might be attained at 6% isocyanate conversion, and the cast film might require 30% conversion of the isocyanate to become tack-free. Figure 4 illus-trates the decrease in isocyanate (NCO) content over time with varying levels of DBTDL catalyst. A low catalyst level provides a tack-free time of approximately 120 min and a pot life of approximately 20 min. A medium catalyst level provides a tack-free coating of 35 min and a pot life of only 5 min. The tack-free time for the high DBTDL level is about 6 min and pot life is only about 1 min.

Figure 4. Catalyst-level effect on reaction rate

A ligand is a compound that donates electron pairs to form bonds with metals. A chelating agent is a ligand that has two or more points of attachment to a metal atom. Chelated metals are sometimes referred to as complexes. The significance of the metal and the organic portion of the organometallic compound, or the ligand, has been demonstrated. Catalysts evaluated in the above-discussed polyester/hexamethylene diisocyanate system are listed in Table 1 according to reactivity in the cast film and in the pot (data illustrated in Figure 5. The catalysts were compared on equal metal weight. In some cases, changing the ligand dramatically affected the ability of the metal to catalyze the reaction. Results with zirconium, manganese, and aluminum complexes were particularly interesting. Zirconium and manganese complexed with 2,4-pentanedione provided fast reactivity in the above-referenced study and zirconium octoate and manga¬nese octoate did not. The dry time of a system catalyzed with an aluminum complex with 2,4-pentanedione was much faster than the uncatalyzed control when the pot life was equal to the control.