01 Polyurethane

The structure of polyurethane is formed by the reaction of polyisocyanates (e.g., diisocyanate OCN-R-NCO) and polyols (e.g., diol HO-R-OH), where urethane segments are repeating structural units. Polyurethane structures have structures similar to amide and ester groups; therefore, the chemical and physical properties of polyurethane are between those of polyamides and polyesters.

Synthesis of Waterborne Polyurethane

Waterborne polyurethane (WPU) is a binary colloidal system using water as a medium, including aqueous solutions, aqueous dispersions, and aqueous emulsions. Polyurethane particles are dispersed in a continuous aqueous phase, also known as waterborne PU or water-based PU. It has advantages such as being non-toxic, non-flammable, environmentally friendly, energy-saving, safe and reliable, not easily damaging the coated surface, and easy to operate and modify.

Classification of Waterborne Polyurethanes

Based on appearance, waterborne polyurethanes can be classified into polyurethane aqueous solutions (particle size <1nm), polyurethane dispersions (particle size 1nm～100nm), and polyurethane emulsions (particle size >100nm).

Based on the charge properties of the hydrophilic groups, waterborne polyurethanes can be classified into anionic, cationic, and nonionic waterborne polyurethanes.

Based on the synthetic monomers, waterborne polyurethanes can be classified into polyether-type, polyester-type, and polyether/polyester hybrid types.

Based on product packaging, waterborne polyurethanes can be classified into single-component and two-component waterborne polyurethanes.

02 Raw Materials for the Synthesis of Waterborne Polyurethanes

Polyisocyanates

Polyisocyanates used in the synthesis of waterborne polyurethanes include two main categories: aromatic and aliphatic. Aromatic polyols mainly include TDI (toluene diisocyanate) and MDI (diphenylmethane diisocyanate); aliphatic polyols mainly include HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate).

Oligomer polyols used in the synthesis of waterborne polyurethanes mainly include two categories: polyether type and polyester type. They constitute the soft segment of polyurethane.

Chain extenders are commonly used in the synthesis of waterborne polyurethanes to adjust the molecular weight and the ratio of soft to hard segments. Chain extenders are mainly polyfunctional alcohols or amine compounds.

Hydrophilic agents (hydrophilic chain extenders) are chain extenders that can introduce hydrophilic groups onto the main chain of waterborne polyurethane macromolecules. They are functional monomers used in the preparation of waterborne polyurethanes. These chain extenders contain carboxyl groups, sulfonic acid groups, or tertiary amine groups. Polyurethanes with these groups become water-soluble after neutralization and ionization. Neutralizing Agent (Salt-forming Agent)

A neutralizing agent is a reagent that can form salts with carboxyl, sulfonic acid, or tertiary amine groups. Neutralizing agents used in waterborne polyurethanes include triethylamine, ammonia, hydrochloric acid, and acetic acid.

Solvents

Waterborne polyurethane prepolymers have high viscosity during preparation. To facilitate mass and heat transfer, especially in the later emulsification stage, appropriate solvents must be added.

Water

Water is the dispersion medium in waterborne polyurethanes. To improve stability, deionized water or distilled water should be used.

03 Synthesis Mechanism

The synthesis of waterborne polyurethanes can be divided into two stages:

The first stage is pre-step polymerization, where oligomeric diols, chain extenders, waterborne monomers, and diisocyanates are polymerized stepwise from solution to generate waterborne polyurethane prepolymers with a molecular weight on the order of 10³. The second stage is the dispersion of the neutralized prepolymer in water.

Depending on the chain extension reaction, self-emulsification methods mainly include the acetone method and the prepolymer dispersion method.

04 Emulsification Principle

Using neutralizing agents or salt-forming agents, the side groups (COOH) or tertiary amines (NR3) of waterborne polyurethane are dispersed in water under high-speed stirring. The selection principle for neutralizing agents or salt-forming agents is to ensure good resin stability, light color, good appearance, and economic availability.

During emulsification, the ideal state is that the hydrophobic portions of the polyurethane macromolecular chains are coiled and aggregated at the center of the latex. Hydrophilic groups are distributed on the surface of the latex particles and point outwards towards the surrounding aqueous phase. The splitting of ion bonds at the particle interface forms an electrical double layer. Anions (cations) chemically bonded to the polyurethane backbone remain fixed on the particle surface, while ions migrate to the aqueous phase surrounding the particles, forming a charge layer with an N potential on the microsphere surface, thereby enhancing the stability of the aqueous dispersion.

05 Preparation Methods

(1) Acetone Method (Phase Inversion Method)

This method involves synthesizing a PU prepolymer with hydrophilic groups in acetone, slowly adding water, and when a certain amount is reached, a phase inversion occurs. Water becomes the continuous phase, while the PU particles swollen by the solvent become the discontinuous phase, forming a PU aqueous dispersion. Before adding water, the -NCO groups can be sealed, and crosslinking is achieved through heating during use.

(2) Prepolymer Mixing Method (Solid Self-Dispersion Method)

This method involves first preparing a prepolymer with hydrophilic terminal -NCO groups, then dispersing it in water to form an aqueous dispersion. This method requires the addition of a small amount of solvent to reduce its viscosity.

(3) Melt Dispersion Method

A prepolymer with hydrophilic terminal -NCO groups is first prepared, reacted with urea to generate a hydrophilic biuret prepolymer, dispersed in water, and reacted with formaldehyde for chain extension or crosslinking.

(4) Ketoimine & Ketoazines Method

This method uses a blocked diamine (ketoimine or ketoazines) as a latent chain extender added to a hydrophilic -NCO-terminated prepolymer. The two do not react. When the mixture is dispersed in water, the hydrolysis rate of the ketoimine is faster than the reaction rate of -NCO with water, releasing the diamine which reacts with the prepolymer to generate chain-extended polyurethane 2-urea.

06 Performance Improvement of Waterborne PU

This method introduces a three-dimensional structure during the synthesis of waterborne PU to improve water resistance. There are two methods for introducing internal crosslinking:

① Introducing a three-dimensional structure during prepolymer synthesis, using a very small amount;

② Using a triamine for chain extension in water.

A. Self-crosslinking

This method introduces crosslinkable groups during the synthesis of waterborne PU, such as blocked isocyanates, crosslinkable double chains, and self-polymerizable hydroxymethyl groups.

B. External crosslinking

This method adds a crosslinking agent during the application of waterborne PU to achieve crosslinking.

C. Blending Modification

Research on blending waterborne PU with waterborne PA has improved water resistance while reducing costs.

D. Copolymerization Modification

PU and PA are waterborne polymers with a PNS structure.