Polyurethane dispersions (PUDs): Binders for high-performance water-based coatings

What are polyurethane dispersions?

Polyurethanes (PU) are well-established polymeric binders used in coating applications. They require a good balance of mechanical, chemical, and long-term durability. With stricter environmental regulations and sustainability requirements, PU technology has moved towards waterborne polyurethane dispersions (PUDs).

In solvent-borne polyurethane systems, the polymer is dissolved in organic solvents. In contrast, PUDs are essentially stabilized colloidal dispersions of polyurethane particles in water. Their particle sizes range from 20-300 nm. The size depends on the chemistry and the manufacturing process. Following the application, water evaporates. The dispersed particles then coalesce to form a continuous film, as in coatings based on latex or other polymer dispersions.

One major advantage of PUDs, beyond their low VOC content, is that they can form mechanically strong, durable films without crosslinking. The performance of these thermoplastic films is often comparable or better than that of crosslinked (thermoset) films based on some other binder chemistry. This is attributed to the unique structure and morphology of polyurethanes.

Unlike polymer solutions (used in solvent-borne or high-solid coatings), the viscosities of polymer dispersions are independent of their molecular weight. This enables the formulators:

• To prepare PUDs with very high molecular weight without encountering viscosity issues.
• To use thermoplastic PUDs to achieve high performance (something that is not practical with solvent-borne PU systems).
• To functionalize PUDs with reactive functional groups and use an appropriate crosslinking system to further enhance the performance of their coatings.

How are PUDs prepared?

PU resins for coatings are derived from the reaction of polymeric polyols and polyisocyanate compounds. Depending on the final form of PU desired, the reaction uses catalysts, solvents, modifiers, and chain extenders. These resins are generally hydrophobic and soluble in polar solvents based on their composition and molecular weights. They are not soluble or dispersible in an aqueous medium.

To offer PU resin as a stable dispersion in water, the polymer backbone must carry sufficient hydrophilicity. This is achieved by incorporating hydrophilic domains (typically anionic) during synthesis. Such hydrophilically modified PU resins, when dispersed in water under shear conditions, form stable dispersions, called PUDs.

Steps involved in PUD formation

The following are the three steps that lead to the formation of polyurethane dispersions.

1. STEP 1: Involves the reaction between polyols and diisocyanates, using an excess of NCO groups over OH groups. This leads to the formation of isocyanate-functional intermediates, called prepolymers. The type of polyols, catalyst, and NCO/OH ratio are selected based on the molecular weight, particle size, and performance requirements of the final coatings. A compound with a hydrophilic group [dimethylol propionic acid (DMPA)] is also incorporated in this reaction. This imparts sufficient hydrophilicity for eventual dispersion and stabilization.

    

   

2. Step 2: The acid-functional groups on the prepolymer are neutralized by the addition of just a sufficient quantity of a base. The base is typically a volatile tertiary amine. This produces anionic hydrophilic domains on the prepolymer backbone.

    

   

    

3. Step 3: The neutralized prepolymer is added to water under high shear conditions. This is followed by the quick addition of the chain extender (typically a low molecular weight, water-soluble diamine). In this step, particle formation takes place by selective orientation of the prepolymer chains. This causes the hydrophobic chains, along with NCO groups, to be pushed away from water. This forms the core of the particles, while the hydrophilic domains are oriented at the particle-water interface. 

    

   The chain extender molecules added in this step diffuse into the particles formed and react with -NCO groups, resulting in a rapid buildup of molecular weight (chain extension). This also increases in particle size. Due to the very high reactivity of amine towards -NCO groups, this reaction takes precedence over that of the NCO–water reaction. However, a small amount of urea linkages (due to the reaction between NCO and water) is almost always formed when using a prepolymer process.

    

   

PUD architecture and structure-property relationships

PU resins for coating applications are derived from a reaction between polyols and isocyanates in the presence of a catalyst.

➤ Both polymeric and monomeric polyols, having -di or multiple functionality, are used. This depends on the PU technology and its end-use application.

• Polymeric polyols are the most important components in PUDs. A wide variety of polymeric polyols are commercially available, varying in their chemistry, functionality, and molecular weights. The chemistry of polymeric polyols and their molecular weight are important considerations for PUDs. This is because these factors significantly influence the final film properties.
• Monomeric polyols (low molecular weight) are frequently used as modifiers or chain extenders.
• In PUDs, di-functional polyols are the most commonly used ones with polyester, polyether, polyacrylic, and polycarbonate backbones.
• Recently, many suppliers have developed bio-based polyols that impart special performance properties, in addition to improving the sustainability profile of the product.

➤ Aliphatic isocyanate compounds are invariably used over aromatic compounds. This is due to the outstanding weatherability and color retention properties of aliphatic films over their aromatic counterparts. The selection of the type of isocyanate compounds and their functionality affects many properties and the cost of the final product.

In addition to isocyanate compounds and polyols, PUD requires a special raw material that ties into the PU chains while conferring hydrophilic moieties. These are monomeric diols with sulfonic acid or carboxylic acid functionality. One of the most commonly used compounds is DMPA.

A special characteristic of DMPA is that its acid functional group does not exhibit appreciable reactivity toward isocyanate compounds. Hence, they are retained during the prepolymer formation step. These acid functional groups are then deprotonated using a base to generate hydrophilic sites for subsequent dispersion and stabilization in water.

The amount of the hydrophilizing agent used in PUDs is carefully calculated. It can control the formation and stabilization of dispersion and particle size, among other factors. Excess hydrophilization is undesirable as it can make final films more water sensitive.

Soft segments and hard segments

One of the defining characteristics of polyurethane chemistry is the existence of segmented morphology. Many outstanding properties of polyurethane coatings are attributed to this segmented morphology.

• Soft segments: Originate primarily from polymeric polyols. They contribute to flexibility, elasticity, toughness, adhesion, and low-temperature performance.
• Hard segments: Originate primarily from isocyanates and chain extenders. They contribute to hardness, abrasion resistance, tensile strength, and chemical resistance.

The ratio of soft and hard segments strongly influences coating performance. Besides, many polyurethane systems exhibit microphase-separated morphology. Here, soft and hard segments tend to organize into nanoscale domains due to differing polarity and intermolecular interactions. 

This morphology explains why polyurethanes can achieve combinations of high strength, good flexibility, and excellent abrasion resistance that are difficult to achieve with many competing polymers.

Soft and hard segments in PUDs and their contribution to coating performance

When designing formulations, the right selection of polyol type and proportion, isocyanate, and chain extender is essential. This helps achieve the desired film morphology and performance for your target application.