If you asked someone a decade ago, “Which is better...water-based polyurethanes or solvent-borne urethanes", the answer probably would have been, “solvent-borne.” As coating technology has evolved significantly (and continues to evolve), depending on the application, that answer today would no longer be as cut and dry.
Water-based polyurethanes are typically found in paint and coatings applications, and are most commonly used in wood and textile coatings. They are also making an impact in digital inks as binders, first in textiles and now spreading into packaging. For the market at large, glass sizing is a major application, along with plastic, such as flexible PVC coatings and rigid plastics like ABS for aesthetic and haptic reasons.
The majority of today’s coatings use either water-based polyurethanes or solvent-borne urethanes, where moisture-cured pre-polymers have significant use as protective coatings. Despite water-based advances, solvent-borne urethanes and high solids 2K urethanes still dominate applications in metal coatings, particularly for steel and direct-to-metal because they offer robust barrier properties. The high solids systems also offer reduced shrinkage and thicker film build.
Water-Based Performance Characteristics
One of the key performance characteristics of water-based polyurethanes is reduced VOC in a high molecular weight urethane versus a solvent-borne counterpart. Odor can also be greatly reduced, but that is dependent on the solvent content and type. Water-borne polymers for coating applications typically contain a tertiary amine that can have an odor considered offensive to some people. Newer polymers overall have significantly fewer tertiary amines, which helps minimize odor. Color and longevity characteristics of the final product would be determined by the composition of the urethane itself.
Other performance characteristics include:
- The ability to use water-based polymers on substrates that are sensitive to solvents, such as plastics.
- Being more user friendly and typically lower in toxins like live isocyanate and aromatic solvents, particularly NMP-free polyurethane dispersions (PUDs).
- Very effective crosslinking chemistry can be used in PUDs that is not possible in solution polymers.
- Auto-oxidative crosslinking is easier. It is possible to use auto-oxidative crosslinking in solvent-borne chemistry, but it requires a lower starting molecular weight and/or lower solids compared to a water-based polymer. This leads to faster return-to-service by reaching hardness and resistance properties more rapidly than solvent-borne or high solids materials, especially if there is no accelerated energy cure (like applied heat). For instance, this can be important for a floor coating that will see heavy use.
Achieving Performance With Low VOC or No VOC Coatings
Water-based PUDs can deliver very low VOCs in the final coating properties when properly designed. This desirable property can be combined with very good surface hardness and high performance at low or room temperature (RT). Acrylic emulsions can also provide low VOC coatings and low/RT application, however, these applications tend to have lower performance targets. For a water-based solution, an acrylic emulsion can be designed to impart some urethane like properties, but, acrylic emulsions are more of a complementary polymer rather than an alternative. Urethanes have properties that acrylics don’t and vice versa, particularly when it comes to solvent resistance. Therefore, formulators will blend water-borne urethanes and acrylic emulsions to obtain the best balance of cost and performance for a particular application.
In oven-cured applications, many polymers can deliver high performance and high hardness at low or no VOCs. A high solids or 2K urethane can provide low VOC, high hardness and very high performance at RT, but may require long cure times to attain sufficient properties. A high solids/2K system would also require a professional applicator to properly handle the materials properly as they are typically toxic and require precise mixing and subsequent application in a timely manner due to their limited pot life. A water-based system is much more user friendly in this regard and can be applied by someone with a limited amount of experience, which is ideally suited for wood flooring.
The properties that reduce VOCs and odor also lead to reduced flammability. Water-borne polymers in general, including PUDs, have a lower degree of flammability. That said, some legacy PUD polymers still contain NMP, which is used as a processing solvent and has a high degree of flammability.
Lubrizol Water-Based Polyurethanes
Lubrizol offers a number of water-borne, low VOC and self-crosslinking technologies that meet and exceed environmental requirements and opportunities, including Aptalon™ Polyamide Polyurethanes and Turboset™ Self-Crosslinking Polyurethanes. Contact us to learn more about our water-based polyurethanes and how value can be maximized for specific applications.