Q&A About Polyurethanes
Q: I am trying to improve the performance of a printing ink. Can polyurethane resins be a useful component in this application?A: Although polyurethane products are normally considered coating, adhesive or sealant resins, solvent-borne polyurethane products have long been used as modifiers in nitrocellulose-based inks. Nitrocellulose is used in a variety of applications because of its low cost; however, it lacks flexibility and adhesion when printed onto films or difficult substrates. Polyurethane resins, which are well-known for both their excellent adhesion and high range of flexibility (tailoring from 10 to > 1,000% elongation is possible), are often the resins of choice to increase nitrocellulose performance in printing inks.
In recent years, waterborne inks and 100%-solid UV-curable inks have grown in popularity. Low- or zero-VOC waterborne inks are preferred because of drives to reduce worker exposure to VOCs and meet related VOC regulations. On the other hand, UV-curable inks, with their fast cure times, offer increased productivity benefits.
Polyurethane resins offer unique benefits to both ink technologies. In waterborne inks, waterborne dispersions of fully reacted polyurethanes remain ideal additions for applications that require high flexibility combined with durability. Aliphatic waterborne polyurethane resins have a long history of use in coatings and adhesives because of their ability to impart excellent weatherability in coatings and to strengthen adhesives. Ink formulators are now taking advantage of these benefits by using polyurethane dispersions alone or by blending them into their ink formulations.
Polyurethane resins excel in UV-curable inks by adding flexibility. Moreover, UV-curable polyurethane resins offer the added benefit of minimizing shrinkage, which can be as high as 10% in inks that do not contain polyurethane products. More recently, new polyurethane products that are both waterborne and UV-curable have caught the attention of ink formulators because they not only offer the combined benefits of both technologies but also have the potential to address odor and migration issues in 100%-solid UV-curable inks.-Dr. Jim Thompson and Dr. Serkan Unal
Q: Our company prepares prepolymers based on MDI isocyanates and polyether polyols, but the final prepolymer occasionally has a very high viscosity or even gels. What might cause this problem?A: The viscosity and ultimate stability of prepolymers can be influenced by a variety of factors, including reaction temperature and the consistency of the raw materials used. One additional important factor is the balance of acidity and basicity present in the isocyanate and the polyol. It is known that side reactions that lead to increased viscosity and gelation are catalyzed by strong bases (e.g., alkoxide ions).
The conventional process for the production of polyether polyols uses potassium hydroxide to initiate the polymerization of propylene oxide, which can lead to polyols with residual amounts of basic impurities. Polyol manufacturers can take steps to neutralize this basicity or it can be accomplished at the point of prepolymer production. One way to overcome this undesired catalytic effect is to add small amounts of strong acids (e.g., phosphoric acid) to the polyol to neutralize the residual basicity.
Polyether polyols are also available that are made using a patented organo-metallic propoxylation catalyst. The polyols resulting from this process do not contain the residual basic impurities that can be found in polyols made by the conventional process. This advanced process also yields the added benefit of producing polyethers with a functionality closer to its nominal value, which leads to urethane polymers with improved mechanical properties. Polyurethane sealants are a typical application that would benefit from using these advanced polyether polyols.
Care must be taken when contemplating a switch of polyol suppliers, however, since a switch could result in a change in the fundamental properties of the polyol, thus introducing changes in the final prepolymer.-Dr. Jeffrey F. Dormish
For additional information on the topics addressed, or to ask another question, e-mail firstname.lastname@example.org with the subject line “Polyurethane Q&A.”
Any views or opinions expressed in this column are those of the authors and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media.