The diversity of polyurethane dispersion (PUD) polymers – polyurethane polymers dispersed in water – is remarkable in its scope. Over the past 40+ years, the number of offerings on the market have increased dramatically as producers have found more and more raw materials options to create products that exhibit a broad range of performance characteristics. As a result, the number of products on the market have proliferated. The intent of this document is to illustrate some of the major compositional varieties and their associated properties. It is not meant to be comprehensive by any means, but to indicate the range of performance possibilities for the formulator/applicator.
Polyurethane polymers as a class of materials is defined as a reaction product of a polyol and an isocyanate. The simple equation is shown below.
In the example above R represents the type of polyol and R’ represents the type of isocyanate. The major categories of polyols and isocyanates are discussed below.
Polyols are hydroxyl-functional (-OH) intermediates that are available in a variety of structures and molecular weights (or lengths). The most commonly used types in commercially available polyurethane dispersions are polyesters, polyethers and polycarbonates. The characteristics generally associated with polyurethane films based on these are described below.
- Polyesters – Good chemical-resistance as well as UV-stability and oxidation resistance. The polyol of choice for applications such as floor coatings (wood and concrete), wood furniture finishes and DIY brushing varnishes and other end-uses where mar- and abrasion-resistance is required. Depending on the molecular weight (length), they can be designed with a range of flexibilities to suit applications from rigid to very flexible substrates. Aliphatic, polyester-based PUD’s are the combination of choice in the industrial and architectural coatings market, and for some automotive applications.
- Polyethers – Better hydrolysis resistance properties than polyesters and lower cost relative to either polyesters or polycarbonates. They typically produce soft, flexible polyurethane films that are excellent for textile and leather base coat applications. Their shortcomings (yellowing, softening) become evident when films are exposed to heat and UV. Consequently, PUD’s based on polyethers are generally best suited to interior applications.
- Polycarbonates – Historically, polycarbonate-based PUD’s have been considered the Cadillac of the PUD market. Their cost was typically at the high end of the polyol range, but they have become competitive with high-end polyesters in recent years. The benefits exhibited in PUD films include excellent UV-stability and hydrolysis resistance as well as good chemical resistance. They often find application in automotive interior (on leather and plastic substrates) and exterior coating end-uses where long-term durability is critical. They offer the UV- and oxidation-resistance of the best polyester polyols and the hydrolysis resistance of polyether polyols.
It is possible to use different types of polyols in combination to achieve properties with characteristics of each. For instance, polyester/polyether blends can provide a balance of performance and economics for applications where cost is important.
The choice of polyols can also influence the level of co-solvent (typically a volatile organic compound or VOC) required to coalesce the PUD film. Softer polyester and polyether polyols allow film formation to occur at ambient room temperatures with no co-solvent addition, while harder polyols will need the assistance of a co-solvent or heat (or both) to attain optimal film properties.
Isocyanates (intermediates with -NCO functional groups) come in two general classes: aliphatic and aromatic. This is related to the presence of unsaturation, or double bonds, in the main structure — aliphatic types have no unsaturation. The commonly used aliphatic types are isophorone diisocyanate (IPDI) and 4,4’-diisocyanato dicyclohexlmethane (H12MDI), often referred to as hydrogenated MDI. The aromatic types are toluene diisocyanate (TDI) and to a much lesser extent methylene diphenyl diisocyanate (MDI).
- Aliphatic – H12MDI and IPDI are the isocyanates of choice for PUD’s used in most industrial and architectural coating applications due to the UV-stability or non-yellowing nature of their structures. Consequently, PUD’s with good color stability are achievable when an aliphatic isocyanate is reacted with a polyester or polycarbonate polyol. Typically, H12MDI based PUD’s are harder and offer better chemical-resistance performance than IPDI-based counterparts all other things being equal. Conversely, IPDI-based PUD’s are generally softer and selected for applications where flexibility is required. The difference in cost between the two aliphatic isocyanates fluctuate over time, but both are usually higher than aromatic types.
- Aromatic – TDI, and MDI to a far lesser extent, are the aromatic isocyanate options for synthesizing PUD’s. As noted previously, the UV-stability of these materials restricts their use to applications where film “yellowing” is not important. End-uses would include some leather finishes, laminating adhesives and other binders. Typically, aromatic isocyanates are reacted with polyether or lower cost polyester polyols for economic reasons. From a processing perspective, these intermediates require a high degree of control and workers must wear personal protective equipment (PPE) when handling due to low LD50 ratings.
Urethane – Acrylic Hybrids
Acrylate monomers are used in the production of urethane acrylic hybrids to impart properties such as improved weathering resistance, adhesion and lower cost. They can be created through an emulsification process where the polyurethane micelle is used as a seed from which the larger particle is grown. Particle morphology can be varied by the choice of monomer(s) and the amount incorporated to attain specific performance attributes. Typical end-uses include wood floor and sports court finishes, automotive interior parts, wood and plastic furniture finishes and DIY varnishes.
Urethane-acrylic hybrids are also made by simple blending of acrylic emulsions and PUD’s. While technically not true hybrids, they can provide similar performance characteristics by altering blend ratios of a variety of different polymers including self-crosslinking acrylics and PUD’s. This approach also gives the formulator a wider choice of components from which to design adhesives, coatings and inks.
With an understanding of the basic attributes of the diisocyanates and polyols typically used to produce PUD’s, we can begin to consider how we might combine them to achieve desired properties in a particular PUD.
Polyols are the building blocks of PUD’s that offer the greatest variety of structures and molecular weights because they come in three main chemical types and may, or may not, contain some branching in addition to their primarily linear forms. They constitute the “soft” segment in PUD’s with molecular weights generally between ~ 500 and up to ~ 8000. The low molecular weight types are used for the harder PUD’s while the higher molecular weight types are used to make soft, highly extensible PUD’s.
The three most common polyols used in PUD synthesis are: Polyether diols, Polyester Diols, and Polycarbonate Diols. Other types can be, and have been, used but these three constitute the vast majority of those used commercially. The choice of which type to choose depends directly on their relative strengths and weaknesses with respect to desired performance properties.
Polyether polyols fall mainly into two types: Polypropylene Glycols (PPG’s) and Polytetramethylene Ether Glycols (PTMEG’s).
The PPG types are the more widely used due mostly to lower prices and they offer good flexibility and hydrolysis resistance. The PTMEG types offer those properties too, as well as low hysteresis. Any article expected to have a long service life with repeated deformation and rebound will be most appropriately made with a PTMEG polyol. The chief weakness of polyether polyols is their susceptibility to oxidation and degradation by exposure to UV radiation.
Polyester polyols come in many types. Some are appropriate for use in PUD’s and some, due to their poor hydrolysis resistance, are not used commercially in water-borne polymers. Those that are used offer good to very good hydrolysis resistance and both good oxidation resistance and good UV resistance. These last two properties generally determine the applications where Polyester-based PUD’s are used. They are mostly employed for use in high-performance PUD’s that must be able to perform over reasonably long lifetimes while exposed to ambient conditions involving sunlight and oxidative attack (outdoor exposure). Polyester polyols are generally considerably more expensive than polyether types.
Polycarbonate Polyols are the highest-performance type of polyols generally used to make PUD’s. They offer the hydrolysis resistance of Polyether Polyols as well as the oxidation and UV resistance of the best Polyester Polyols. They are quite expensive (though less so than in the past) and are not normally used in short life-cycle products. Their superior combination of properties makes them ideally, or even uniquely, well-suited to use in products demanding long term performance in challenging environments. In general, they are used in those applications where Polyether and Polyester PUD’s simply cannot measure up.
Diisocyanates come in a number of both Aromatic and Aliphatic structures but, for this discussion, we will restrict the information solely to the Aliphatic types as PUD’s used for coatings must nearly always offer color stability (Aromatic types darken under exposure to light). By far, the most commonly used diisocyanates for coatings PUD’s are H12MDI and IPDI. There are others but, again, their usage volumes are far lower than those of these two.
As opposed to the Polyols discussed above, the diisocyanates don’t differ much in molecular weight. The choice of which one to use is based, rather, on their molecular structures and the property profiles those structures best support.
H12MDI consists of two aliphatic ring structures and is completely symmetrical about a central methyl group. Both halves are mirror-images of one another, and this type of structure facilitates design of harder, more chemical-resistant types of PUD; those designed for topcoat use where hardness, chemical resistance, and abrasion resistance are required. Protective coatings designed for use on rigid substrates like wood, metal, ceramics, and many plastics are primarily made with H12MDI.
IPDI, on the other hand, consists of only one aliphatic ring structure and is asymmetrical. One Isocyanate group is attached directly to a ring carbon while the other is attached to an alkyl group substituent which is, in turn, attached to a ring carbon. This distinctly non-linear structure confers properties on the PUD’s made from it which favor their use on mostly flexible substrates like fibers, fabrics, leather, and flexible plastics.
One of the reasons PUD properties span the gamut from chewing gum to glass is because of the ability to combine their various constituents in an almost limitless variety of polyol molecular weights and chemistries, coupled with either of the two diisocyanates described above.
In this regard, PUD’s vary far more in structure, properties, and uses than many other polymer types. This is why it is possible to make extremely soft coatings for fabrics, for example, which barely affect the hand and drape of the goods, but which can still provide many property improvements like non-fray, non-wicking, non-rustling (quiet), properties and yet still improve strength and durability. These types will typically offer elongations of 400-1000%.
At the other end of the property spectrum, the very hard coatings offer superb non-marring abrasion resistance as well as chemical and stain (graffiti) resistance, etc. to rigid surfaces and substrates which lack those properties natively. And, this protection is provided at very low film thicknesses and add-on weights. These types will typically offer elongations of 10-250%.
Between the extremes of very soft and very hard are a world of intermediate elongation PUD’s suitable for use in protecting a huge array of semi-flexible and semi-hard substrates.
Authors: W. Otterbein, J. Zermani