Q&A About Polyurethanes
July 1, 2008
Editor’s note: This new feature from Bayer MaterialScience, an industry leader in polyurethane chemistry, will provide formulating help to readers. Their team of experts can help you improve green strength and heat resistance, or adjust the reactivity of an adhesive, as well as provide solutions to improve the tensile strength, elongation, and UV stability of a sealant. Questions regarding contact bonding and laminating with polychloroprene can be addressed as well. Please see below to find out how to submit a question.
Question: Our company currently makes waterborne sealants based on acrylic polymers. What advantages could be obtained from using a polyurethane dispersion?Depending on the needs of your application, you may see significant performance improvements in using a polyurethane polymer. It is important that you evaluate the proper polyurethane, as products are available that span a range of hardness and flexibility levels.
There are polyurethanes dispersions that have been specifically designed for use in sealants. These dispersions can be formulated with typically used fillers (e.g., calcium carbonate) and plasticizers (e.g., butyl benzyl phthalate) to produce sealants that can outperform commercial acrylic formulations. The property improvements occur in the areas of tensile strength, elongation and tear resistance. Polyurethane dispersions for this type of application are based on aliphatic polyisocyanates to prevent discoloration. Optimal results are obtained with a system based solely on a polyurethane polymer, but it has been found in many applications that you may also use a more cost-effective blend of a polyurethane and a lower-cost polymer to achieve the desired performance improvement.
Question: I am rather new to polyurethane chemistry. Could you explain the difference between a polymeric MDI and an MDI prepolymer?You have raised an interesting question regarding the nomenclature used within the polyurethane industry. MDI (4,4’-diphenylmethane diisocyanate) is one of the basic isocyanates used for adhesive, sealant and elastomer applications. MDI monomer is commercially produced by way of a distillation process. It is a solid material at room temperature, making it difficult to handle. MDI monomer can be reacted with polyether or polyester polyols in a variety of ratios to produce “prepolymers.” These prepolymers are often liquid at room temperature, thus making them easier to handle. They can be produced at a variety of isocyanate contents depending on the application (e.g., a low-NCO content ~3% with a polyether for a moisture-curing sealant or at a higher NCO content [16%] for a moisture-curing wood adhesive). High-performance elastomers are often made with polyester-based prepolymers.
In contrast, polymeric MDI is composed solely of MDI as a mixture of various oligomers composed of 2, 3 and 4+ aromatic rings that are linked together. Polymeric MDI is available in a number of grades that differ mainly in viscosity and functionality. Prepolymers can also be made that are based on polymeric MDI. This is an interesting topic that we could perhaps elaborate on in a subsequent column.
For additional information on the topics addressed, or to ask another question, please e-mail firstname.lastname@example.org with the subject line “Polyurethane Q&A."