Why is the waterborne polyurethane resin resistant to dirt

Update:20-09-2017
Summary:

Urethane resin added anti-staining agent, dirty refers […]

Urethane resin added anti-staining agent, dirty refers to the object easily adsorbed with dust particles or liquid, wpu coating fastness, dense and smooth, easy to be contaminated, so with a dirty function
What is the scope of application of waterborne polyurethane
Recommended use:
1. Food package printing ink
2. Fabric printing, resistant stone mill denim, mold paint, water-based glue and so on
3. Leather varnish, leather coating