The molecules of waterborne polyurethane are not simply dissolved in water. Instead, a large number of molecules are aggregated into spherical colloidal particles floating in water, which prevents colloidal particles from precipitating and forms stable polyurethane dispersions through affinity between colloidal particles and water.

从以上图我们可以看到，聚氨酯粒子能够在水中稳定存在是依靠粒子自身重力和粒子与水的亲和力的平衡。若亲和力大于或等于重力，则粒子可以稳定存在，树脂亦可稳定；若亲和力小于重力，则粒子沉降，树脂则发生沉淀或分层。
From the above picture, we can see that the stability of the polyurethane particles in the water depends on the equilibrium between the particle's own gravity and the affinity of the particles to the water. If the affinity is greater than or equal to the gravity, the particles will be stable and the resin will be stable. If the affinity is less than the gravity, then the particles will sink and the resin will precipitate or stratified.

二，如何保证聚氨酯树脂有足够的亲和力。
Two, how to ensure that the polyurethane resin has sufficient affinity.
现在通常的作法是在水性聚氨酯的分子链中接入和水有亲和力的基团或链段，一般是带有阳离子、阴离子或非离子链的材料。若聚氨酯分子中接入的是阳离子，则该法制得的水性聚氨酯为阳离子型，阴离子和非离子亦然。这些亲水基团或链段处于聚氨酯粒子表面，形成带电层或胶体层对粒子产生包覆效果，且相邻粒子的保护层相互排斥，防止粒子团聚而失稳。
Now, the usual way is to connect the molecular chains of waterborne polyurethane and water with affinity groups or segments, usually with cationic, anionic or non-ionic chains. If the polyurethane molecules are connected with cations, the aqueous polyurethane obtained by this method is cationic, anionic and nonionic. These hydrophilic groups or segments are on the surface of polyurethane particles, forming charged or colloidal layers to produce particles, and the protective layers of neighboring particles repel each other to prevent particle aggregation from destabilizing.

三，高固含和低固含水性聚氨酯稳定体系的差别。
Three, the difference between high solid and low solid water bearing polyurethane stabilizer system.
高固含水性聚氨酯粒子往往比低固含的大很多，一般认为30%固含树脂的粒径在50nm左右，而50%以上固含树脂的粒径往往大于200nm，这样导致粒子自身重力往往大很多，根据重力/亲和力平衡原理，高固含树脂的粒子与水应具有更强的亲和性。为了维持较强的稳定性，聚氨酯粒子必须具有更强的保护层，所以必须选择更强电解性粒子或链段接入聚氨酯分子中。
High solid aqueous polyurethane particles are much larger than the low solid content, generally 30% solid resin particle size of about 50nm, while more than 50% solid resin particle size is greater than 200nm, so that the particle gravity often much larger, according to the gravity / affinity balance principle, particle and water resin with high solid content should have a stronger affinity. In order to maintain strong stability, polyurethane particles must have stronger protective layer, so we must choose stronger electrolytic particles or segments to access polyurethane molecules.

高固含和低固含水性聚氨酯稳定体系的差别。

水性聚氨酯已经发展了数十年，上世纪60年代，美国、德国等发达国家就着手水性聚氨酯的研究，中国在这方面研究较晚，真正开始系统性研究是在改革开发以后。经过数十年的发展，我国的水性聚氨酯已取得长足的进步，但与发达国家相比，差距还是很明显的，特别是高性能特种化产品相对偏少，基础研究没有形成系统性。
Waterborne polyurethane has been developing for decades. In the 60s of last century, the developed countries such as the United States and Germany began to study waterborne polyurethane. China's research in this field is rather late. The real systematic research is after the reform and development. After decades of development, China's waterborne polyurethane has made great progress, but compared with developed countries, the gap is still very obvious, especially the high performance special products are relatively few. Basic research is not systematic.