Preliminary Analysis of the Existing Morphology of Waterborne Polyurethane Emulsion Particles!

Waterborne polyurethane resin is a kind of polymer material, the full name is polyurethane resin, English Polyurethane, referred to as PU. Polyurethane resins are used for coatings, coatings or adhesives. For the convenience of construction, a large amount of organic solvents are usually required for dissolution. These solvents mainly include toluene, xylene, dimethylformamide (DMF), etc. It poses a great threat to human health.

If the organic solvent is replaced by non-toxic water, such polyurethane products will become environmentally friendly and harmless. How to make the water-incompatible polyurethane resin stably dissolve in water is the subject of many waterborne polyurethane researchers.
 
At present, the most effective method is to insert groups having affinity with water, such as carboxyl group, amino group, sulfonic acid group or polyethoxy group, on the molecular chain of polyurethane resin. So how do these hydrophilic groups enable water-insoluble polyurethane molecules to dissolve in water?
 
First of all, we need to have an understanding of the existence of water-based polyurethane.
The molecules of water-based polyurethane are not simply dissolved in water, but a large number of molecules agglomerate into spherical colloidal particles "floating" in the water. The affinity of the colloidal particles and water prevents the precipitation of the colloidal particles and forms a stable polyurethane dispersion.

Second, how to ensure that the polyurethane resin has sufficient affinity.
The common practice now is to insert groups or segments that have affinity with water in the molecular chain of water-based polyurethane, generally materials with cationic, anionic or non-ionic chains. If cations are inserted into the polyurethane molecules, the water-based polyurethane prepared by this method is cationic, as are anions and nonions. These hydrophilic groups or segments are located on the surface of the polyurethane particles, forming a charged layer or a colloid layer to coat the particles, and the protective layers of adjacent particles repel each other to prevent the particles from agglomerating and destabilizing.

Third, the difference between high-solids and low-solids water-based polyurethane stabilization systems.
High-solid water-containing polyurethane particles are often much larger than those with low-solid content. It is generally believed that the particle size of 30% solid-containing resin is about 50 nm, while the particle size of more than 50% solid-containing resin is often larger than 200 nm, which leads to the particle’s own gravity. Many, according to the principle of gravity/affinity balance, high solid resin particles should have stronger affinity with water. In order to maintain strong stability, the polyurethane particles must have a stronger protective layer, so stronger electrolytic particles or segments must be selected to be incorporated into the polyurethane molecules.

Water-based polyurethane has been developed for decades. In the 1960s, developed countries such as the United States and Germany started research on water-based polyurethane. China is relatively late in research in this area, and the real systematic research is after reform and development. After decades of development, my country's water-based polyurethane has made great progress, but compared with developed countries, the gap is still very obvious, especially the relatively small number of high-performance special products, and the basic research has not been systematic.