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The Chemistry of Waterborne Polyurethane Coatings

published on March 5, 2020

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 has stimulated the development of alternative technologies based on waterborne polyurethane systems.

New, eco-friendly waterborne technologies allow 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 coatings develop, they are forecasted to exhibit >5% CAGR. Polyurethane dispersions (PUDs) will have the highest growth rate as the technology of PUDs continues to advance.   Applications for PUDs 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 Coatings

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

Polyols Isocyanates Curatives Specialty Intermediates
Polyether Polyols HDI Ethylene Diamine Dimethylol Propionic Acid (DMP)
PPG based HDI Trimer DETA Polyaspartic Acid
PTMEG based HDI Biuret   Methylethyl Ketoxamine (MEKO)
Polyester Polyols IPDI   Dimethyl Pyrazole (DMP)
Adipate based IPDI Trimer   Caprolactam (Ɛ-CAP)
Acrylic Polyols H12MDI    
Caprolactones MDIs    

 

Of the polyether-based systems, PTMEG-based PUDs exhibit the highest mechanical properties, flexibility, and water resistance.  In the polyester category, polyacrylates, polycaprolactone, and polycarbonate-based systems offer the highest performance. All aliphatic systems exhibit exterior durability and very good gloss retention.

 

Waterborne Polyurethane Dispersions (PUDs)

Modern PUDs are stable, user-friendly, and exhibit properties approaching those of solvent-based systems. These systems may be formulated as air-dried or baked coatings for all types of substrates, both flexible and rigid in nature.  Solids levels can range from 30 to more than 50 weight percent of polyurethane in water.

PUDs consist of polyurethane particles dispersed in water.  There are many techniques for achieving dispersions, including the incorporation of a carboxylic acid moiety in the polyurethane backbone to serve as an internal emulsifier.  Formulators can achieve this internal emulsifier by using a carboxylated diol like dimethylolpropionic acid (DMPA), which is co-reacted with a polyol and isocyanate and then neutralized with a base, tertiary amines, or ammonium hydroxide, facilitating the dispersion of the polyurethane particles in water.  

DMPA levels vary in the range of 4-8 weight percent of the prepolymer weight.  These PUDs 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.

While the majority of PUDs are anionic, you’ll also see cationic PUDs used.  In these cases, a tertiary amine-diol like N-methyldiethanol amine (MDEA) is copolymerized, and a weak volatile acid like acetic acid is used to create the cationic charge.  The PUDs are cationic in the wet stage, but they convert to a non-ionic polymer upon film formation. The cationic charge can provide certain advantages, such as better adhesion to surfaces that are hydrophobic.

 

Two-Component PUDs

In 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 like MDI. Formulators may also use hydroxy-terminated polyurethane dispersions in a 2K formulation where chain extension and crosslinking are affected using isocyanates like 1,6- hexane diisocyanate (HDI) trimers.  Here again, they achieve polyurethane dispersions by incorporating a carboxylated diol like DMPA, 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 PURs 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 dispersions in water.  

Hydroxy-functional PURs are formulated with the blocked isocyanate.  After application and baking, the blocked isocyanate thermally deblocks, and the isocyanate can then react in a traditional manner with the hydroxy-functional prepolymer to afford a crosslinked PUR. You control the deblocking temperatureby the selection of the blocking agent.   When MEKO is used as the blocking agent, a high temperature baking cycle is required ~ 150-170 °C. DMP unblocks at lower temperatures, ~ 110-120 °C. Ɛ-CAP deblocks at higher temperatures, ~160-180 °C. The chemistry associated with blocked isocyanates is depicted below, where H-Block represents the blocking agent like DMP, MEKO, Ɛ-CAP, etc. Other crosslinking agents include carbodiimides (via carboxylic acid moieties) and melamines.       

 

One-Component Systems

Thermally-cured 1K PUR systems have also been formulated with the above chemistry. A protecting group is reacted into an isocyanate prepolymer.  The choice of polyol and isocyanate, and the NCO/OH ratio control the coating properties; the protecting moiety controls the thermal curing profile.  1K 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 with 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 to form a dry film coating.

 

Other Chemistries

Formulators have employed moisture-cure chemistries in 1K systems.  Examples include silane end-capped prepolymers.  Curing is affected by the formation of siloxane moieties on exposure to heat, which chain-extends and crosslinks the prepolymer.

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

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

Waterborne-hybrid polyurethane-acrylic systems (PUAs) have been developed combining polyurethanes with hydroxyl functional polyacrylates in a single dispersed particle. Polyacrylates are known for achieving good weatherability, chemical resistance, 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 the water sensitivity when incorporated into the PUD formulation.

 

Summary and Conclusions

Environmental considerations favoring low-VOC alternatives to solvent-based polyurethane coatings have driven a steady shift to aqueous polyurethane dispersions.  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 urethane intermediates can address your unique 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.