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Question: Why is it always recommended that isocyanates be protected from water?
Answer: Isocyanates have a fundamental reaction with materials that contain a reactive hydrogen group, such as the hydroxyl group in a polyester polyol, a polyether polyol, an amine group or water. The reaction of isocyanates with hydroxyl groups in polyether or polyester polyols is desired to allow urethane groups to form and a high-molecular-weight polyurethane polymer to be produced. The reaction with water produces carbon dioxide gas and an amine, which further reacts with isocyanate groups to form a polyurea linkage. This can be a desired reaction in the case of a moisture-curing adhesive or coating, or it can be an undesired side reaction in an improperly stored container of polyisocyanate. In the latter case, the moisture in the air reacts with isocyanate groups and can build pressure in the container due to the generation of carbon dioxide, or it can form an undesired hard film on the surface of a liquid polyisocyanate. For this reason, once a container of polyisocyanate is opened, it should always be purged with dry nitrogen to expel any moisture in the head space. This process will extend the polyisocyanate’s shelf life.
The next question elaborates on the positive aspects of the reaction with water.
Question: What factors should I consider when developing a moisture-curing adhesive?
Answer: Moisture-curing adhesives are very useful materials for bonding substrates that naturally contain a residual amount of moisture, like wood. Adhesives of this type are most often based on aromatic isocyanates, such as MDI or polymeric MDI, which cure faster than aliphatic polyisocyanates. Aliphatic systems are most often used to produce coatings that are clear and non-yellowing. The aromatic products can be based on a polyether or polyester backbone, depending on the application. Prepolymers that are liquid at room temperature are most often based on a polyether backbone and have a viscosity of a few thousand centipoises to allow easy application. By varying the isocyanate content from low to high, the hardness of the final adhesive layer can be adjusted from soft to hard. A prepolymer with a very low isocyanate content (<3 %NCO) could be used for a moisture-curing sealant. Amine-based catalysts are often used to adjust reactivity. A prepolymer that is solid at room temperature based on crystalline polyester polyols can be used to prepare a reactive hot-melt polyurethane adhesive. This material needs to be heated to temperatures of about 120°C/250°F to be applied. As the bead of adhesive cools to room temperature, it solidifies and the adhesive begins to strengthen. Within a few days, the remaining isocyanate content reacts with moisture to develop a high-molecular-weight polymer with its final polymer properties.
Advances in polyurethane chemistry have led to the development of specialty isocyanate prepolymers. They include products with a low residual monomer content (to comply with European product-labeling regulations) and prepolymers that cure at a faster rate without the use of external catalysts.
For additional information on the topics addressed, or to ask a question, e-mail firstname.lastname@example.org with the subject line “Polyurethane Q&A.”