Waterborne polyurethanes
Introduction
In the past, market demands for high performance and appearance were the pacemaker in the development of resins for surface coatings. Mainly solventborne systems were in use. A change has been introduced by the trend to be more environmentally conscious. As a result, coatings regulations have been implemented to restrict solvent emissions (volatile organic compounds, VOC), especially when coatings are being applied. This trend was led by government regulations like the Clean Air Act in the United States and the German TA-Luft. Low VOC technologies have since gained a larger share in the paint market. Alongside high-solids, powder and radiation coatings it is predicted, for example in Europe, that waterborne systems will come to dominate with good growth in segments other than the already established waterborne applications in the building and automotive electrodeposition coatings market (Table 1) 1, 2, 3, 4, 5.
Requirements for industrial coatings with higher performance, formerly set by solventborne coatings, could not be matched by standard waterborne systems. Solventborne polyurethanes are known to perform on a high level. Therefore it was a logical step, in the trend of reducing solvents, to convert this technology to the aqueous phase.
Even though one component waterborne polyurethanes have been known for a long time, they were considered a specialty. Patents date back about 50 years. Their preparation, compared to solventborne systems, is more challenging because of working in the aqueous phase. The problems have been overcome and various preparation methods are available today. In the beginning, mostly high molecular, non-reactive types were developed. But because of their property limitations, developments were initiated resulting in new classes of chemically crosslinked (postcuring) and reactive waterborne two-component polyurethanes.
A wide variety of performance levels allows a broad range of applications. The properties vary from tough, elastic, abrasion-resistant coatings for floors to highly elastic films for plastics and textiles. Today, their success can be measured by the high standards they have reached in many applications.
This paper presents some basic principles in chemistry and developments of one- and two-component waterborne polyurethanes. It describes examples for their use in coatings applications based on Bayhydrole® and Bayhydur®/Desmodur® raw materials of Bayer AG.
Section snippets
Waterborne polyurethanes
The basic chemical components of waterborne polyurethanes are all building blocks which are known from solventborne polyurethanes. Mainly they are diisocyanates, polyols, amines, catalysts and additives.
A number of reactions in the preparation process are important. The polyaddition reaction of isocyanates (excess) with polyols leads to polyurethane polymers. In a second reaction, the remaining isocyanate groups react with amines to form ureas. As the reaction takes place in water (dependent on
Preparation processes
Developments over the last 30 years have led to many processes for converting polymers containing urethane and urea groups into waterborne two-phase systems. The basic principle for the processes involves two steps. The first is the preparation of a low to medium molecular weight isocyanate prepolymer from di- or polyols with di- or polyisocyanates. In a second step, the prepolymer chain is extended and dispersed in water in different ways by introducing hydrophilic solubilizing groups.
Non-reactive waterborne polyurethanes
The building of higher molecular structures in solventborne monophase polyurethanes leads to high viscosity. One major reason is the interaction of the polymer molecules by hydrogen bonding and Van-der-Waals forces. The application of these solventborne polyurethanes in coatings applications is therefore limited.
The high viscosity increase of high molecular solventborne polyurethanes can only be avoided by the use of two-phase systems like aqueous dispersions. In these dispersions, the
Postcuring
Non-reactive higher molecular polyurethane dispersions are well suited for the applications already mentioned. In some aspects they do not satisfy high quality requirements. The reason is the insufficient crosslinking of the film-forming macromolecules, which leads to a lack of solvent and chemical resistance. To improve those properties, different chemical, postcuring mechanisms are available. These fall into two major groups:
- 1.
one-component systems allow the formulation of storage-stable
Conclusions and outlook
Environmental requirements to reduce VOCs and performance properties favor the development of polyurethanes for waterborne applications. In recent years, many production processes for polyurethane dispersions have been developed. Polyurethane chemistry offers wide scope for formulating products thanks to the wide range of raw materials available. With the introduction of chemical crosslinking in one-component and, recently, in waterborne two-component polyurethanes, the superior high
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