How can Polyurethane act as waterproofing? Which molecular structure helps that ability? Any paper about this?

close cell structure bcz no air and water circulation can occur

Hi

These references could help you

1. Liquid-applied waterproofing membrane for roofs comprising a long-chain aldimine

2. Investigation of the Leaching Behaviour of a Novel Cement-Polyurethane Hybrid Material for Waterproofing Purpose

3. Ionomer‐based polyurethanes: a comparative study of properties and applications

4. Preparation and property of waterborne UV‐curable chain‐extended polyurethane surface sizing agent: Strengthening and waterproofing mechanism for cellulose fiber paper

A waterproof "impermeable" material simply prevents the passage of water through its structure. The structure of a typical polyurethane "PU" can be represented by a single constitutional repeating unit –O-R-O--CO-NH-R'-NH-CO- (where R, R' are aliphatic or aromatic organic parts).

When PU is made by strict absence of moisture or water, the polymeric molecules will be long chains of the presented structure. Structure determines properties so upon looking at what sorts of inter-molecular forces are there, we will find many possibilities for hydrogen bonding, dipole-dipole interaction, and London forces. These many secondary bonds will result in a rigid or tight overall structure enhanced by chain entanglement. Such structure will not allow the passage of water molecules.

PU is not unique in water resistance. You know that water-proof PVC rain coats are widespread. The PVC macro molecules have inter-molecular dipole-dipole & London forces only but these were enough to resist penetration of water through them.

I did not consult any paper or book in answering your question. You may be surprised that I asked a question, similar to yours but about PVC, in a polymer chemistry final exam in the year 1985 !

Broadly speaking, any polymer film will act as "waterproofing". As others have pointed out, defects like pinholes must be avoided. I would not put high emphasis on the molecular structure although this may play a role on the physical nature of the barrier, i.e., flexible or rigid barrier. What polyurethane may give you compared to many other polymers, is the ability to formulate the material to alter the structure, and to form the barrier in-situ by a chemical reaction. PVC, does not have that flexibilitry (although flexibility may be altered via plasticizers). Consider the application and the available application methods.

rigid polyurethane may works.

- Polyurethane-based waterproofing composition for the water-proofing of concrete structures. Patent WO2013182234A1.

- Osmosis and the blistering of polyurethane waterproofing membranes by BRIAN HUBBS.

- DOI: • 10.1590/s1517-707620170003.0211