Paint & Coatings Industry - April 2008 - (Page 67)

PU Crosslinkers Applications Resin Design – Co-Reactants and Crosslinkers Extensive resin design has taken place to allow 2K-waterborne polyurethane systems to reach their current state of performance. Work had to be done on both the resin or Bayhydrol® polyol portion, as well as the Bayhydur® polyisocyanate crosslinkers. Either research thrust would make for interesting reading. However, the focus of this report is to discuss the chemistry and development of the polyisocyanate crosslinkers that have been introduced from the early 1990s until today. A number of properties are important to coatings formulators. One of the important properties of coatings is their clarity and overall appearance. The importance of effectively dispersing an isocyanate crosslinker can be easily demonstrated simply by observing the appearance of two similar systems. Water-Dispersible Development The development of water-dispersible polyisocyanates can take a couple of different conceptual approaches. One of the first is whether an internal or an external emulsifying agent should be used. A wide range of surfactants are available commercially. However, at Bayer MaterialScience the decision was made very early on to focus on internal emulsifying agents. It was felt that using external surfactants could lead to problems such as blistering, decreased water resistance and blushing. This is mainly due to the ability of an external surfactant to migrate through the coating to the surface. Generation 1 Products Since an internal surfactant is reacted into the backbone of the polyisocyanate resin it does not have the ability to migrate through the coating. Conceptually, either an ionic or non-ionic emulsifying agent could be used. The initial development work was focused on modifying HDI polyisocyanates with monofunctional hydrophilic polyethers as the emulsifying agent. These were incorporated into the standard hydrophobic polyisocyanate crosslinker through a urethane linkage. This was the first generation of hydrophilic products (Figure 3). The generic structure shown is based on an HDI isocyanurate trimer. Generation 1 products based on HDI have a good overall blend of properties. They are relatively easy to disperse and they form stable emulsions. They have good reactivity and can be used in a wide range of formulations. By varying the MW and amount of polyether it is possible to achieve higher chemical resistance at the expense of ease of dispersibility. It is also possible to use lower viscosity starting polyisocyanates to get a lower-viscosity, water-dispersible crosslinker. This improves dispersibility, but can also lead to a somewhat lower functionality. Another feature of this product line is the ability to tailor the products for adhesive applications, having higher functionality and less water PA I N T & C O A T I N G S I N D U S T R Y 67 Dispersibility Importance In this first example it can be seen that a standard hydrophobic polyisocyanate is not properly dispersed in the polyol dispersion when using a hand mix application. In the electron microscope picture on the right in Figure 1 the undispersed domains of polyisocyanate in the polyol matrix are plainly visible. On the left in Figure 1 an actual film is laid over the top half of the system label. The poor dispersion of the polyisocyanate is demonstrated by the opacity of the film. In the second system a hydrophilic polyisocyanate is hand mixed with the same polyol dispersion as the first example. In this system, the electron microscope picture shows a uniform surface and even dispersion in the film (Figure 2). Again, on the left an actual film is laid over the system label. In contrast to the first example, this film has high clarity and good appearance due to the excellent dispersibility of the hydrophilic polyisocyanate chosen. Gloss readings as high as 95 are now routinely available. This clearly demonstrates the importance of a good dispersion on coating properties.

Table of Contents Feed for the Digital Edition of Paint & Coatings Industry - April 2008

Paint & Coatings Industry - April 2008 Contents Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News Snapshots of Success – Automotive Applications Designing UV-Curable Materials for High-Temperature Optical Fiber Applications The Future of UV Coatings is Now! Milling Media Review Part 1 – Bead Density Effect Preparation and Properties of UV-Curable Polyurethane Acrylate Resins for Metal Surfaces Conquering Color Harmony High-Performance PU Crosslinkers for Two-Component Waterborne Applications Decorative Film Laminates How Long Will the Gilding Glitter? Gaining a Competitive Advantage Using Benchmarking Supplier Showcase Materials Watch Products and Literature Classifieds Advertiser Index Automotive Color Trends New England Museum Solves Century-Old Vanderbilt Mansion Mystery

Paint & Coatings Industry - April 2008

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