From ultrafiltration to nanofiltration hollow fiber membranes: a continuous UV-photografting process

A new way to prepare nanofiltration membranes, consisting of in-line external modification of the skin of a polysulfone ultrafiltration hollow fibre skin, is described. The paper presents a continuous process (dip-coating followed by photografting) and the influence of the operating conditions on the membrane characteristics. This study is focused on a simplified model based on the evaluation of the two sources of monomer available for grafting: one is monomer contained in the thin film drawn from the dip bath, second is contained in the pores of the membrane. The results show that two extreme types of modification can be produced, depending on the experimental conditions: a high rate coupled with a short dip contact time leads essentially to an external grafting whereas a low rate coupled with a long dip residence time leads essentially to an “in pore” grafting.

Fig. 6. Light energy received influence on water permeability (CAA = 5%, Cr = 0.5%, dipping contact length = 97 cm, Lp0 = 100 l.h!1.m!2 bar!1). amount of monomer available in the pores increases with contact time. From these data, we can conclude that a residence time of 5 s is sufficient to provide enough monomer from the inside of the pores to reach the maximum degree of grafting achievable in the given operating conditions. 4.5. Conjugated effect of light intensity and line rate Fig. 6 shows that Lp decreases as received energy increases. In these experimental conditions, grafting was not limited by the amount of In the dip photografting method, monomer is present in the superficial liquid film and inside the membrane pores. For a given solution concentration, the quantity of monomer available for grafting depends on the liquid film thickness and on the monomer concentration profile inside the pores. It therefore depends on the operating rate and on the contact time in the monomer solution during the dipping. An increase in rate gives an increase in superficial film thickness and a decrease in the amount of monomer in the pores. These phenomena, coupled with the light power of the radiation received and the UV residence time, influence the final permeability. For a fixed rate, permeability is a decreasing function of the energy received: a higher energy gives a higher radical concentration leading to a larger number of grafted polymer chains and thus to a lower permeability. Two extremely different types of modification can be produced: a high rate coupled with a short contact time leads essentially to an external graft layer whereas a low rate coupled with a long contact time leads essentially to an “in-pore” graft layer. So by adjusting the operating conditions, membrane permeability and retention can probably be modulated to achieve the desired properties.