Polyurethane dispersions (PUDs) are the polymers of choice for demanding applications. These waterborne polymers are low in VOCs and can be formulated to be in compliance with most federal, state and local emission control laws. One source of the low VOC content is a small amount of cosolvent that is introduced as a processing aid and has a side benefit of coalescing the particles in the dispersion into a smooth, continuous elastomeric film. The requirements of a cosolvent are many (Table 1). M-Pyrol" [N-Methyl-2-Pyrrolidone (NMP)] has been the cosolvent of choice because it meets all the requirements. In addition, it is a good coalescent and has the ability to behave as both catalyst and surfactant1 (Figure 1). However, NMP is not a low-cost solvent. It has a strong tendency to oxidize when in contact with oxidizing reagents and/or strong acids. When subjected to a high-temperature environment (>100 deg C) for more than two hours, discoloration was often experienced. NMP is capable of dissolving or swelling a wide spectrum of protective gears in an industrial environment.

California Proposition 65 requires special labeling of products containing M-Pyrol. Other states and countries may follow with similar enactment. In addition, the superfund amendment and reauthorization act (SARA) require the listing of quantities of M-Pyrol on the MSDS for each product. Other states' right-to-know acts require identification of the M-Pyrol in the product and on the labeling of the product. Worldwide need to replace or eliminate NMP from polyurethane dispersion products (PUD) is thus desirable.

In this article we review the pros and cons of eliminating cosolvents or switching to other organic solvents. Synthetic routes to minimize NMP usage are cited from the literature. Commercially available NMP-free and cosolvent-free aqueous Witcobond® PUD grades are discussed. Examples of a wide range of solid content and average-particle-size PUDs are given. The physical properties of these grades can be hard and mar resistant, strong and tough or soft and pliable. The applications of these solvent-free PUDs in textiles, coatings and adhesives are briefly reviewed. Examples of starting formulations are also given.

Discussion

One important side benefit of removing M-Pyrol is the improvement in color stability. As mentioned earlier, M-Pyrol residue in the dried film tends to cause yellowing, particularly at high temperatures. A good illustration of color improvement is the recent development of Crompton's Witcobond W-290HSC in our laboratories. A film from this M-Pyrol-free product was cast on white Carrara glass and subjected to 140°C for 20 hours in a convection oven. Very slight discoloration was observed. By comparison, a film from a product containing only 2.5% M-Pyrol showed significant yellowing (Figure 2).

The replacement candidate for M-Pyrol in the conventional processes for the prepolymer stage must have the properties shown in Table 1. Very few commercial-grade solvents fit all of the criteria in Table 1. DBE (available from DuPont) fits most of the criteria except it is not water-soluble and it slowly hydrolyzes in water at high or low pH. Similarly, propylene and ethylene carbonates have the best stability at low pH, which limit their use. Other cosolvents such as dimethyl acetamide or ethylene glycol dimethyl ether are either more expensive and do not have the solvating power of M-Pyrol or tend to have a pungent odor.

The other option is to make the PUDs without any cosolvent. This option is the most preferred but is also the most challenging.

The classical way to produce solvent-free PUDs is to use a volatile solvent such as acetone, which is removed from the finished latex by distillation. This technique is commonly referred to as the "acetone process"2. The drawback of this process is the economics. Not only does the equipment have to be explosion-proof, but the cost to distill acetone and dry it before recycling for the next batch or disposing of it by incineration is significant. Additional cost comes from the reactors output since a significant part of the reactor's volume will be occupied by the acetone.

Another limited option for making solvent-free PUDs is via selection of special polyols/isocyanate or using a high NCO/OH ratio to achieve a workable prepolymer viscosity. An example of a special isocyanate that allows for the preparation of a solvent-free PUD is TMXDI (tetramethylenexylene diisocyanate). It is well known that prepolymers from this isocyanate are of lower viscosity than comparable prepolymers made from other isocyanates3. The drawback of this method is the relatively high cost of this isocyanate and the relative softness of the resulting PUD. Such PUDs may be good adhesives, but they will lack the hardness and mar resistance for topcoats. Hardness can be increased for products derived from TMXDI by increasing the level of hard segment and/or crosslink density of the polymer but these measures usually incur further additional costs.

Crompton has developed a patented process4 where acrylates and similar monomers are used as diluents to the prepolymer. After the prepolymer is converted to latex, the vinylic monomers are trapped inside the urethane particle and subsequently polymerized with free radicals at the latex stage to yield a solvent free core-shell type urethane/acrylate alloy (Figure 3). When the particles coalesce into a continuous film, there is a minimum of phase boundaries between the urethane and acrylic polymers resulting in films of superior properties as compared to a urethane acrylic blend, and they are as good as pure urethane (Table 2). If a hard vinylic polymer is used, it acts as a reinforcer (filler) to the urethane and increases overall hardness and mar resistance. By the same mechanism, when a soft, vinylic monomer is used, the resulting polymer acts as a plasticizer to a hard urethane, thereby reducing coalescent demand and lower film-forming temperature. This technology allows us to produce products with a wide range of properties. An example of this technology is Witcobond A-100. This product has excellent properties as a metal primer and coating or as a wood varnish for kitchen cabinets and concrete coating (roofing tiles, garage floors and the like). The properties of this PUD are shown in Table 3. Another advantage of this type of product is superior weathering. Figure 4 shows comparative QUV aging of W-A100 with other urethanes and urethane acrylic blends. Other solvent-free PUDs that are prepared by different proprietary processes are shown in Table 4.

With the exception of Witcobond W-170, the rest of the PUDs in Table 4 are aliphatic. Of these aliphatic grades, one is non-ionic. The solid content ranges from 30-67%, and the Tg is from -47 to +77 °C. Tensile strength is from 1200 - 8200 psi, and Sward hardness is from 10-64.

Applications

In addition to the general coating and adhesives for various substrates mentioned above, these grades are particularly suited for applications listed in Table 5. In Table 6 are some suggested grades for the various textile applications. It is noteworthy that most of these grades have excellent color stability at relatively high temperatures. This color fastness allows for curing at relatively high temperatures without having to worry about yellowing.

Another application area where these grades are used to an advantage is in printing inks. The inks derived from blends of Witcobond A-100 and Witcobond W-290H not only have good printability, excellent resistance to rubbing, and easy clean-up but also provide excellent viscosity versus shear as shown in Figure 5. The overprint varnish provides for higher-gloss, mar rub and better chemical resistance.

In certain printing papers, the solvent-free non-ionic or the cationic dispersions offer wide formulating latitude at a wide pH range. The cationic, in particular, provides for good ink receptivity in inkjet printing and good adhesion to polyethylene-coated papers.

In leather coating, the advantages are not only the low-VOC and high-solid content but also good coalescence at room temperature. This forms a film of high tensile with high elongation that provides a tough coating with a good feel, low-temperature flexibility, and is crosslinkable with good interlayer adhesion.

Conclusions

California Proposition 65 makes the presence of M-Pyrol in the finished product undesirable due to the requirement of special labeling for potential toxicity. The elimination of M-Pyrol changes the manufacturing, formulation and application of waterborne polyurethane dispersions. Crompton's patented technology utilizes reactive acrylate diluents in manufacturing of M-Pyrol free, low-VOC Witcobond PUDs. The finished urethane/acrylate alloy has excellent properties for many applications.

A wide array of M-Pyrol-free and 100% polyurethane/ polyureas are also commercially available from Crompton's Witcobond polyurethane dispersions. Superior yellowing resistance and other properties make them suitable for demanding applications in coatings, inks and adhesives.

References

1 MSDS for 1- methyl -2- pyrrolidone, BASF Corporation, 1995.
2 Dieterich, D. and Reiff, H. Adv. Urethane Science & Technology vol 4,112 (1976).
3 Kobylanska, I.; Konkus, D. M.; and Ley, D. A. Adhesives Age, April 2001.
4 Loewrigkeit, P.; Van Dyke, K. A. U.S. Patent 4,644,030; 1987.

(Witcobond is a registered trademark of Crompton Corporation and/or its subsidiaries. M-Pyrol is a registered trademark of ISP Management, Inc.)

This paper was presented at the PIC Conference, October 1, 2003, Los Angeles, CA.

For samples and literature, contact 877/873.0273 (U.S.) or 732/826.5901(outside U.S.). For technical inquiries, contact 914/784.4954, or bechaib@cromptoncorp.com.