Catalyzing Urethane Systems

History of Urethane Catalysts Polyurethane chemistry traces its origins to the mid-19th century, when monobazic isocyanate reactions were first studied by Charles Adolphe Wurtz (1817-1884) and August Wilhelm von Hoffman (1818-1892). Research into isocyanate reactions continued into the 20th century. Otto Bayer's (1902-1982) investigation of isocyanate reactions in Germany during World War II is significant for the commercial potential of isocyanate reactions. The Bayer team developed the diisocyanate polyaddition process used in polyurethane production. His work initially focused on research aimed at producing fibers equal to or superior to nylon. Industrial production of polyurethane paints began shortly thereafter in the early 1940s. Today, polyurethane chemistry is used in applications requiring good hardness and flexibility, wear resistance, chemical resistance and environmental durability, in accordance with management standards. These applications include plastics, adhesives, foams, elastomers, sealants and paints. Due to tightening legal restrictions on volatile organic emissions in the paint sector, low molecular weight resins are increasingly used in solvent-based isocyanate/polyol systems. An important characteristic of these low molecular weight resins is that they also have low viscosity. Paints produced with these resins do not require large amounts of solvent to achieve the necessary application viscosities.

An inherent characteristic of two-component isocyanate/polyol systems using low molecular weight resins is their dependence on a catalyst to accelerate the isocyanate/hydroxyl (NCO/OH) reaction. Metal compounds and/or amines are typically used as catalysts in these reactions.

Film formation in low-solids systems prepared with high molecular weight resins depends more on the physical evaporation of solvent and less on crosslinking reactions. Until recently, organotin compounds were used for metal catalysis of the primary NCO/OH reactions in the paint industry. The development of new tin-free metal catalysts and new research into existing catalysts has revealed that catalyst selection has become an increasingly important and complex matter. These newly developed catalysts can provide new opportunities to reduce old problems. This article aims to develop guidelines for catalyst selection by better understanding the complexities of urethane catalysis and addressing commonly encountered problems. The reaction of isocyanates with hydroxyl groups is relatively slow in the absence of a catalyst. Catalysts are generally used to achieve adequate reaction rates and improved properties in various urethane systems. In aliphatic isocyanate crosslinked systems, catalysts are especially needed to accelerate the reaction. Aromatic isocyanates are generally more reactive than aliphatic types and only require the use of catalysts under certain conditions. As mentioned previously, catalysis of the NCO/OH reaction is normally carried out with metallic compounds or amines. Among metallic compounds traditionally used in industry are tin, bismuth, zinc and manganese-based catalysts. Although some mercury and lead compounds provide the desired catalytic properties, their use is avoided because they are highly toxic. Recently developed zirconium and aluminum compounds have begun to be used in urethane applications. Dibutyltin dilaurate (DBTDL) can be considered a metal catalyst that works well in urethane paints.

This catalyst is quite efficient, meaning a very low catalyst ratio can greatly increase the NCO/OH reaction rate. However, as with any catalyst, DBTDL can encounter certain problems involving reactivity stability, ester group hydrolysis, water/isocyanate reaction catalysis and environmental concerns.

Diazabicyclo [2.2.2] octane is a commonly used tertiary amine catalyst. Tertiary amines have been found to effectively catalyze aromatic isocyanate reactions more so than aliphatic isocyanate reactions. Amine catalysts may have color and moisture sensitivity problems. Where appropriate, combinations of organotin and tertiary amine catalysts have shown synergistic properties. To ensure rapid drying of films mixed with the hardener in two-component urethane systems, catalyst use is preferred, but it must provide good pot life under environmental conditions. The catalyst used must remain active in such a way that when the catalyzed component is stored for extended periods, it has minimal effect on resistance properties. Additionally, the catalyst must be environmentally acceptable. Hexamethylene diisocyanate (HDI) biuret and trimer (isocyanurate) derivatives are examples of aliphatic isocyanates frequently used in the paint industry. The readily accessible NCO groups of these derivatives make them much more sensitive to active hydrogens in catalyzed systems. Isophorone diisocyanates (IPDI) and m-tetramethyleneXylene diisocyanates (TMXDI) respond less to catalysts due to sterically hindered NCO groups. Hydrogenated diphenylmethane diisocyanate (H12MDI), TMXDI and other aliphatic isocyanates like IPDI are not used extensively as crosslinkers in paint systems.

Catalysis Mechanism

Catalysis of an isocyanate/polyol reaction initially occurs through combination with either the isocyanate (Lewis acid mechanism) or the polyol (insertion mechanism) and catalysis by a metal catalyst. The encounter of the catalyst and isocyanate creates more electrophilic reactive sites on the resin. This increased electrophilic character enhances the reaction of the isocyanate with nucleophilic alcohol oxygen (Figure 1). Commonly used organotin compounds follow this mechanism. This relationship is formed through one of the pathways in Figure 1.

Figure 1. Lewis Acid mechanism

Figure 2. Insertion mechanism

Insertion metal catalysts initially combine with polyol or water. The actual insertion catalyst is an alcoholate formed from the combination of polyol with the metal catalyst. The alcoholate reacts with isocyanate to form an intermediate metal complex isocyanate, which then reacts with polyol to form a urethane. This proposed mechanism type is shown in Figure 2. Studies have shown that some zirconium compounds catalyze according to the insertion mechanism. Recent studies have also shown that some bismuth compounds initially associate with the polyol component. The progress of the isocyanate/polyol reaction can be observed using many methods. The reaction can be monitored by observing viscosity increase parallel to the decrease in free isocyanate. Figure 3 is a comparison of data generated using these methods. The initial rate of an uncatalyzed HDI trimer/polyol reaction can be analyzed by measuring viscosity and unreacted isocyanate percentage as the reaction proceeds. Isocyanate concentration is determined by titration method using measured N-dibutylamine excess and titration with hydrochloric acid. The NCO/OH ratio was 1.5:1.0. The initial concentration of unreacted isocyanate was approximately 6.5%. As shown in Figure 3, when the initial viscosity doubles, approximately 6% of the free isocyanate has reacted. The film layer using this uncatalyzed formulation becomes tack-free after 27 hours at ambient conditions (25°C, <50% relative humidity).

Figure 3. Unreacted isocyanate percentage versus viscosity

Reaction rate can also be monitored by infrared spectroscopy through the disappearance of the isocyanate band at 2272 cm-1 as a simple measure of reaction rate. In a known polymer system, pot life and pre-cure times are a function of isocyanate conversion. For example, doubling the viscosity of a high-solids polymer can be achieved with 6% isocyanate conversion, and the applied film may require conversion for 30% of the isocyanate to complete pre-curing. Figure 4 shows the decrease in isocyanate (NCO) content over time with varying levels of DBTDL catalyst. A low catalyst level provides approximately 120 minutes of pre-cure time and approximately 20 minutes of pot life. A medium catalyst level provides 35 minutes of pre-cure time and only 5 minutes of pot life. For high DBTDL levels, pre-cure time is approximately 6 minutes and pot life is only 1 minute.

Figure 4. Effect of catalyst level on reaction rate

  A ligand is a compound that donates electron pairs to form bonds with metals. A ligand that is a chelating agent has two or more attachment points to a metal atom. Chelated metals are sometimes called complex compounds. The importance of the organic portion of the organometallic compound and the metal or ligand has been demonstrated. Catalysts evaluated in a polyester/hexamethylene diisocyanate system are listed in Table 1 according to their reactivity in cast film and pot life (data shown in Figure 5). Catalysts were compared at equal metal weight. In some cases, modification of the ligand significantly affected the metal's ability to catalyze the reaction. Results obtained with zirconium, manganese and aluminum complexes are of particular interest. Zirconium and manganese complexed with 2,4-pentanedione provided rapid reactivity in the study referenced above, however zirconium octoate and manganese octoate did not yield the same results. With an aluminum complex, a system catalyzed with 2,4-pentanedione dried much faster than the uncatalyzed control sample, but pot life was equal to the uncatalyzed control sample.

Table 1. Catalyst effectiveness in a polyester/hexamethylene diisocyanate (HDI) system

Figure 5. Metal catalyst comparison

M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.