...solvent, is commonly a necessary processing solvent included at levels ranging from about 3 to 15%. Since NMP has recently been added to California's Proposition 65 list and has generally become objectionable for use in Europe, it has become desirable to eliminate NMP from these products. Consequently, solvent-free versions of HPDs have been developed that, despite the lack of NMP used in their preparation, have been found to perform favorably compared to analogous solvent-containing polymers (both hybrid and PUD). Like their solvent-containing counterparts, the outstanding properties of the new solvent-free versions are apparently due to their true hybrid nature, which is analogous to an interpenetrating network (IPN) as indicated by a broad glass transition temperature range. Although the NMP-free versions still require coalescing solvents for adequate film formation, they offer greater flexibility in choosing alternate solvents when formulating high performance coatings.

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BACKGROUND

Thermoplastic polyurethanes are well known for their excellent balance of mechanical toughness and chemical resistance. (1-9) Unfortunately, the solvent-based versions require exceedingly high levels of VOC for application by conventional coating techniques. The waterborne versions (polyurethane dispersions or PUDs) require significantly lower VOC and are, therefore, becoming increasingly popular choices as binders for a variety of one-component coatings for wood (floors and furniture), plastic (business machine housings), leather, metal, and concrete. Their superior physical and chemical properties have been attributed to a combination of their molecular structure and hard/soft domain morphology. (9-10)

[ILLUSTRATION OMITTED]

In general, PUDs are prepared by reacting an excess of diisocyanate with a polyol, dispersing the resulting prepolymer in water, and completing the reaction by adding a water-soluble diamine to consume the residual isocyanate and, thereby, chain-extend the prepolymer to a high molecular weight. The dispersed PUD particles are usually anionically stabilized, which is commonly accomplished by incorporating a carboxylic acid-functional polyol into the backbone of the polyurethane and neutralizing the acid groups with a tertiary amine. Thus, in many cases, no external surfactants are present to contribute adversely to the water sensitivity of PUD-based coatings.

[FIGURE 1 OMITTED]

PUDs are available in both aromatic and aliphatic varieties. Aromatic PUDs are not suitable for applications requiring low yellowing and, therefore, the aliphatic PUDs are required for such cases where exposure to direct or indirect sunlight occurs.

Unfortunately, one of the main disadvantages of the aliphatic PUDs is their relatively high cost. As a result, formulators have sought ways to reduce the costs of their coatings. The most popular strategy is to blend the PUD with an acrylic polymer emulsion that costs less than one-half of a standard aliphatic PUD. Although the acrylics reduce the system cost, they also reduce the overall performance of the binder. The reduction in performance can be lower than what would be predicted from an arithmetic rule of mixtures. (11,12) One possible reason for this behavior is that, on a molecular level, the acrylic polymers are not soluble in the polyurethane polymers. Therefore, the polymers remain phase-separated during film formation. Arguably, the resultant phase morphology is at least partly responsible for the diminished performance behavior.

In order to take advantage of the potential cost reduction afforded by the acrylics and maintain a greater share of the advantageous PUD properties, so-called "hybrid" systems have been developed. The hybrids incorporate both the urethane and the acrylic polymers into the same dispersion. As outlined in the simplified process flow diagram (Figure 1), there are generally two methods for preparing HPDs (Type 1 and Type 2). For Type 1 hybrids, a PUD is first prepared, acrylic monomers are added to the PUD, and the acrylic polymer is formed in the presence of the PUD. (13) To prepare Type 2 hybrids, a polyurethane prepolymer is formed, the acrylic monomers are added to the prepolymer, the mixture is dispersed in water, and the urethane and acrylic polymerizations are completed concurrently. (14,15)

The urethane and acrylic polymers in HPDs exhibit improved molecular compatibility versus simple blending. The improved compatibility is demonstrated by the dynamic mechanical analysis (DMA) data that is shown in Figure 2. The simple blend has two distinct tan delta (tan [delta]) peaks, which correspond to the glass transition temperatures ([T.sub.g]) for the phase-separated urethane and acrylic polymers. The hybrid prepared from the first method previously described also shows two [T.sub.g] peaks, but the peaks have become somewhat broader, which is indicative of some limited molecular mixing. In contrast, a Type 2 hybrid, in which the urethane prepolymer and acrylic monomers are homogeneously mixed prior to dispersion and subsequent polymerization, exhibits only a single, very broad tan [delta] peak. The single peak, which spans the temperature range between the theoretical [T.sub.g]s of the urethane and acrylic polymers, is strong evidence for a significant...

NOTE: All illustrations and photos have been removed from this article.




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