Q&A About Polyurethanes
Q: The use of waterborne polychloroprenes for foam-bonding applications was discussed in a previous column (ASI October 2009). How do I formulate an adhesive that will develop bond strength very rapidly?A: Basing your adhesive on a fast-crystallizing polychloroprene latex with the optional use of a nanoparticulate silica sol is a good starting formula. In addition, adhesives with fast strength development can be developed using techniques that destabilize the polychloroprene latex and cause it to “break” or rapidly coagulate when sprayed on the substrate. The adhesive instantly develops a high degree of tack, and bond strength develops quickly. Fast-breaking adhesives can be made by way of two basic formulation types: 1.) A two-component approach that involves co-spraying a formulated polychloroprene adhesive with a salt solution such as calcium chloride at the point of use. The salt causes the polychloroprene to coagulate and the substrates can be quickly bonded. 2.) A high-performance, one-component adhesive can be generated by reducing the system’s pH from ~13 to ~9. The pH adjustment is typically accomplished by adding glycine to the recipe. This reduced-pH formulation will “break” during the spraying process.
Recently, a new approach of injecting carbon dioxide into the adhesive to lower its pH was introduced. The injection is made shortly before the adhesive is applied. These formulations demonstrate improved performance compared to the salt-spray system. In addition, they allow immediate bonding and offer a pot life ranging from two hours to two days. It should be noted that medium- and slow-crystallizing polychloroprenes are often used with each of these formulating techniques to adjust adhesive performance. - Dr. Jeffrey F. Dormish
Q: Over the last several years, VOC content restriction has made my solventborne coating obsolete. I need the properties a solventborne coating offers, but without the solvent. What should I consider?A: Waterborne polyurethane coatings. Over the last several years, advancements in water-based polyurethane coatings have made them more robust compared to solvent-based coatings. Since the carrier is water (rather than solvent), the coatings tend to have very low VOC levels, which meet most environmental regulations.
There are several different types of coatings to consider: two-component waterborne polyurethane coatings, single-component coatings based on water-based polyurethane dispersions (PUDs), or a single-component system tweaked with small amounts of water-dispersible polyisocyanates or other functional additives.
A two-component water-based polyurethane consists of an “OH” functional dispersion that will react with a water-dispersible polyisocyanate. Modifications in the production of water-dispersible polyisocyanates have led to raw materials with extremely low viscosity, allowing for easy mixing (including, in many cases, hand mixing) of the two components. The reaction between the polyol and polyisocyanate yields superior polyurethane coating performance, with extremely low VOC levels.
Your second option is a single-component waterborne coating based on polyurethane dispersions (PUDs). Waterborne PUDs are dispersions of polyurethane particles in water. There are several new types of polyurethane dispersions that offer improved properties over traditional one-component systems, including self-crosslinking PUDs and UV-light-curable PUDs. Self-crosslinking PUDs are produced by adding a material that oxidatively cures when exposed to air. UV-curable PUDs have acrylate functionality. Coating formulations based on a UV-curable PUD will crosslink when irradiated with UV light. In both cases, the additional crosslinking of the coating will offer improved properties while maintaining the advantages of a one-component application.
The final option is adding a small amount of water-dispersible polyisocyanates, polyfunctional aziridines or polycarbodiimides to a single-component system. This will enhance coating properties by increasing crosslinking in the system, but will introduce a shortened pot life, which should be considered during the coating application. - Peg Kendi
For additional information on the topics addressed, or to ask another question, e-mail email@example.com with the subject line “Polyurethane Q&A.”