Q&A About Polyurethanes
Question: In past columns, you discussed MDI monomer and polymeric MDI polyisocyanates. Can you describe other standard MDI-based polyiso-cyanates and their typical applications?
Answer: This is a broad question, but important in understanding the breadth of polyurethane chemistry. Many standard MDI-based products are available that can be classified as liquid-modified MDIs or MDI prepolymers. For example, carbodiimide-modified MDI is a low-viscosity liquid (approximately 35 cps) with a high isocyanate content (approximately 29%) that can be used as a starting material for prepolymer preparation for customers unequipped to handle molten or fused 4,4’-MDI. It is also suitable as a reactant in two-component systems. In addition, urethane-modified MDI with an isocyanate content of 23% is a typical product most often employed as a ready-to-use isocyanate component for a two-component adhesive or elastomer. It can also be used as a starting material to prepare a prepolymer with a lower isocyanate content. It has a higher viscosity (approximately 600 cps) than the carbodiimide-modified product and will often lead to higher-viscosity prepolymers. Other typical urethane-modified prepolymers contain 3–16% isocyanate content. The low-isocyanate-content prepolymers (3%) are higher in viscosity (8000 cps) and are often used for moisture-curing sealant applications or as a rubber crumb binder. They have a high polyether polyol content, which produces a softer, more elastic urethane polymer. Various prepolymers are available with higher isocyanate contents (10–16%) and intermediate viscosity levels (1000–6000 cps); these can be used as part of a one-component moisture-curing adhesive for wood bonding or they can be used as the isocyanate portion of a two-component adhesive for FDA-compliant packaging applications.
From this brief overview, you can see that a company does not need to have its own reactor capability to supply an adhesive with an isocyanate tailored for a specific end use.
Question: In your March 2009 column, you discussed how isocyanates should generally be protected from water. How can isocyanates be used directly with water-based polymers?
Answer: You have brought up an interesting aspect of isocyanate chemistry. Some polyisocyanates can be dispersed in water and will generate stable emulsions in various waterborne polymers, such as polyurethane, polyvinyl acetate, and acrylics. This is most often accomplished with polyisocyanates that have been modified with hydrophilic groups (nonionic or ionic). The emulsified polyisocyanates will act as crosslinkers for the waterborne polymers, thereby improving their heat and water resistance. The polyisocyanates will also react with the water medium that carries the base polymer, so there is a defined pot life for these systems once the isocyanate has been added. Products based on aromatic polyisocyanates (e.g., polymeric MDI) have a much shorter pot life in water than those based on aliphatic polyisocyanates. Water-dispersible polyisocyanates can be used in applications such as wood bonding, automotive interior trim, furniture and flexible-packaging adhesives.
For additional information on the topics addressed, or to ask another question, please e-mail email@example.com with the subject line “Polyurethane Q&A."