The Chemistry of Waterborne Polyurethane Coatings

What is a polyurethane coating technology? Polyurethanes have long been known to produce coatings that exhibit high toughness, abrasion resistance, enhanced aesthetics, and durability. 

Solvent-based polyurethanes have traditionally set the performance standard for high-durability coatings. However, environmental considerations for low-VOC alternatives and reduced exposure to solvents have stimulated the development of alternative technologies based on waterborne polyurethane systems, also known as polyurethane dispersion.

But what is polyurethane dispersion and how does it impact your final result? PUD polyurethane dispersion is a newer, eco-friendly waterborne polyurethane coating technology that allows one-component (1K) and two-component (2K) coatings to be formulated with high durability, good substrate adhesion, stain, water, and abrasion resistance, toughness, and corrosion protection. The 2K waterborne polyurethanes achieve the best overall properties, but 1K systems can be formulated with minimal trade-offs while offering ease of use and clean-up.

As waterborne polyurethane coating technologies develop, they are forecasted to exhibit >5% CAGR. Polyurethane dispersion (PUD) will have the highest growth rate as the technology of PUDs continues to advance. Applications for polyurethane dispersion coating technology include wood and concrete architectural coatings, automotive finishes, undercoat primers and sealers, textile and leather coatings, metal and plastics coatings, industrial and maintenance coatings, and inks.

Intermediates Used in Waterborne Polyurethane Coating Technology

Waterborne polyurethane coatings are generally based on polyether, polyester, and polyacrylate soft segments, with aliphatic isocyanates to provide good UV resistance. The intermediates used in formulating waterborne polyurethane dispersion coatings include the categories of polyols, isocyanates, curatives, and specialty intermediates found in the table below:

Of the polyether-based systems, PTMEG-based PUDs exhibit the highest mechanical properties, flexibility, and water resistance, making them a great selection for the protection of moving, vibrating, or stretching parts. 

In the polyester category, polyacrylates, polycaprolactone, and polycarbonate-based systems offer the highest overall performance of the available options. All aliphatic systems exhibit exterior durability and excellent gloss retention, giving them a superior appearance in many situations.

Contact us to learn more!

Waterborne Polyurethane Dispersion (PUD)

Modern PUD polyurethane dispersion technology is stable, user-friendly, and exhibits properties approaching those of solvent-based systems without the environmental and health costs. These systems may be formulated as air-dried or baked waterborne polyurethane coatings for all types of substrates, both flexible and rigid in nature. Solids levels can range from 30% to over 50% polyurethane in water by weight.

PUDs consist of polyurethane particles that have been dispersed in water. There are many techniques that are used for achieving dispersions, which can include the incorporation of a carboxylic acid moiety into the polyurethane backbone to serve as an internal emulsifier in the waterborne polyurethane coating. 

Formulators can achieve this internal emulsifier by using a carboxylated diol such as dimethylolpropionic acid (DMPA), which is co-reacted with a polyol and isocyanate and then neutralized with a base, tertiary amines, or ammonium hydroxide. This process facilitates the dispersion of the polyurethane particles in water.

DMPA levels vary in the range of 4-8% by weight of the prepolymer weight. These forms of polyurethane dispersion possess an anionic charge in the wet stage, but they convert to a non-ionic polymer upon dry film formation and volatilization of the amine. The higher the level of DMPA, the smaller the particle size of the PUD, which improves film-forming properties in the waterborne polyurethane coating.

While the majority of PUD polyurethane dispersion formulations are anionic, you’ll also see cationic PUDs used in some situations. In these cases, a tertiary amine-diol such as N-methyldiethanol amine (MDEA) is copolymerized, and a weak volatile acid such as acetic acid is used to create the cationic charge in the compound. 

Though the PUD polyurethane dispersions are cationic in the wet stage, they convert to a non-ionic polymer upon film formation. The cationic charge can provide certain advantages in particular situations, such as providing better adhesion to surfaces that are hydrophobic.

Two-Component PUDs

In a two-component polyurethane dispersion, also known as a 2K system, formulators can cure isocyanate-terminated prepolymers with polyamines, such as ethylene diamine (ED) and diethylenetriamine (DETA). Cure occurs because the amine curatives are more reactive with the isocyanate termination versus water. 

Also, aliphatic isocyanates exhibit lower reaction profiles vs. aromatic isocyanates such as MDI. Formulators may also use hydroxy-terminated polyurethane dispersions in a 2K formulation where chain extension and crosslinking are affected using isocyanates such as 1,6- hexane diisocyanate (HDI) trimers. Here again, they achieve polyurethane dispersions by incorporating a carboxylated diol such as DMPA, which is neutralized with ammonia hydroxide, tertiary amines, or metal bases. The moderate reaction speeds between a hydroxyl end-functionality and an isocyanate can be increased by using urethane catalysts.

An attractive 2K chemistry for waterborne polyurethane dispersion is based on polyols in combination with water-dispersible, emulsifiable (using surfactants), blocked polyisocyanates. The blocking group protects the isocyanate moiety under normal conditions and permits dispersion in water.

Hydroxy-functional polyurethane dispersion is formulated with the blocked isocyanate. After application and baking, the blocked isocyanate thermally deblocks, allowing the isocyanate to react in a traditional manner with the hydroxy-functional prepolymer to develop a crosslinked waterborne polyurethane dispersion. You can control the deblocking temperature through proper selection of the blocking agent. When MEKO is used as the blocking agent, a high-temperature baking cycle is required at ~ 150-170°C. 

By comparison, DMP unblocks at lower temperatures of ~ 110-120°C. Ɛ-CAP deblocks at higher temperatures of ~160-180°C. The chemistry associated with these blocked isocyanates is depicted below, where H-Block represents the blocking agents such as DMP, MEKO, Ɛ-CAP, etc. Other crosslinking agents include carbodiimides (via carboxylic acid moieties) and melamine compounds.

One-Component Systems

Thermally-cured one-component polyurethane dispersion, also known as 1K PUR systems, have also been formulated using the above chemistry, with a protecting group being reacted into an isocyanate prepolymer. The choice of polyol and isocyanate, and the NCO/OH ratio will control the coating properties; the protecting moiety controls the thermal curing profile. For this reason, 1K waterborne polyurethane coating systems based on this technology are attractive coating products.

High molecular weight, anionic-modified polyurethanes with DMPA in the backbone can be dispersed under shear in water by using a neutralizing agent such as TEA or ammonia, forming a water-dispersible PUR. The dispersed polyurethanes can be coated with air drying and the concomitant liberation of the ammonia or TEA neutralization agent, forming a dry film coating.

Other Chemistries

Some formulators have employed moisture-cure chemistries in 1K systems, with examples including silane end-capped prepolymers. In this situation, curing is impacted by the formation of siloxane moieties upon exposure to heat, which chain-extends and crosslinks the prepolymer.

Much like 1K chemistries, high molecular weight, anionic-modified polyurethanes incorporating DMPA in the backbone can be dispersed under shear in water using TEA or ammonia as a neutralizing agent to form a water-dispersible polymer backbone. The dispersed polyurethanes may be coated with air drying and the concomitant liberation of the ammonia or TEA neutralization agent.

Starting with a water-dispersible prepolymer featuring pendant carboxylic acid moieties in the polymer backbone, 1K coatings have also been formulated using fatty acid components that can be oxidatively cured through air-induced crosslinking of the fatty acid when facilitated by metal salts.

Waterborne-hybrid polyurethane-acrylic systems (PUAs) have been developed by combining polyurethanes with hydroxyl functional polyacrylates in a single dispersed particle. Polyacrylates are known for achieving good weathering, resistance to industrial chemicals, good appearance characteristics, and doing so at lower costs. 

The hydroxy-functional acrylates are crosslinked with blocked isocyanates or aliphatic isocyanates to produce high-quality coatings. An alternative to this chemistry is to end-cap a polyol/DMPA/isocyanate prepolymer with HEMA (hydroxyethyl methacrylate) followed by dispersion in water using TEA and subsequent copolymerization with MMA, BA, and other acrylic monomers.

Organofunctional silanes such as aminopropyl functional silanes (e.g. A-1100), are effective in reducing water sensitivity when incorporated into the polyurethane dispersion formulation.

Summary and Conclusions

Environmental considerations favoring low-VOC alternatives to solvent-based polyurethane coatings have driven a steady shift to waterborne polyurethane dispersion options. 

New eco-friendly PUD technologies have gained significant attention and market traction, as they allow 1K and 2K coatings to be formulated with high durability, good mechanical properties, high adhesion characteristics, water and abrasion resistance, and good aesthetic properties. 

As with all polyurethanes, PURs offer broad options in polymer design that allows them to meet requirements in a wide variety of diverse applications.

To explore how our broad portfolio of waterborne polyurethane intermediates can address your unique waterborne polyurethane coating requirements, partner with the expert teams at Gantrade Corporation. 

Our teams, armed with a wealth of technical knowledge and expertise, can guide you to the best solutions for your applications. Contact Gantrade today to get started with the right chemical products for your company's needs.