I have prepared PVDF membrane by NIPS method. I obtained good hydrophobicity. (When the water is dropped on the PVDF membrane it is rolling). But the membrane is soft and it is easy to tear. The thickness of membrane is about 500 micron. When cold saline water is circulated on the the Feed side, no water drops are coming out from the permeate side which indicates that the hydrophobicity remains. But the feed is hot saline water (60-80 C), it passes through the membrane, made the membrane hydrophilic and everything gets collected from the permeate side. The contact angle greatly reduced after hot water circulation.
At high temperatures, the hydrophobocity greatly reduced, the membrane pores gets wetting nature.
The mechanical strength of the membrane is very poor.
How to avoid these problems so that it is usable for DCMD applications?
If you have permeation of liquid water, it could be due to defects or large pore size and porosity. Macrovoids could compromise the barrier properties and the mechanical properties of the membrane(in general PVDF has good mechanical properties and your membrane is quite thick.). Have you performed SEM analyses?
What about the salt in the feed? Fouling could affect the contact angle.
Dear Sergio Santoro
I am using PVDF membrane for desalination of saline water ( 1% NaCl). The membrane withstands even 2 bar pressure when Room Temperature water circulation on the feed side. but goes bad only hot water (even at 50 C at free flow ) circulation, suggesting that pore were opened.
I have to record SEM later!
Fouling occurs only after several hours of running. in my case, feed hot saline water is coming out suddenly through permeate side
What is the NIPS method? Does it generate PVDF fibers membrane or PVDF membrane with pores?
Does this happen at lower temperature (e.g. 40oC)? When the hot saline water contact with the cool PVDF membrane, NaCl might recrystallize at the surface of PVDF membrane. In this case, the membrane may be fouled much faster.
Besides, I think there may be another possibility. The surface tension of water decreases with the increase of the temperature of the water. At higher temperature, the surface tension of the water is lower than the critical surface tension that your PVDF membrane can hold. I suggest that you can test the contact angles of liquids with different surface tension (e.g. 72.8 mN/m to ~30 mN/m) on you PVDF membranes.
Dear Fang Li
NIPS is Non-solvent Induced Phase Separation method used to separate the PVDF from its solution. During Phase separation, PVDF is formed as membrane with pores. It may be flat sheet or fibers. My case is Flat sheet.
The crystal formation of hot saline water at cold PVDF surface is possible at nearly saturated concentration. I am using only 1 % NaCl which is very dilute. So fouling at dilution is not possible.
I agree that the surface tension decreases with increase in temperature. I checked the contact angle of PVDF membrane before and after heating to 80 C. The contact angle greatly reduced.
Another possible reason is pore size.
The pore size of the membrane may be high so that water enters easily. So i am attacking these problems.
Also i want to know, is there any additive to increase the mechanical strength of polymer.
Thanks
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