Preparation of Cellulose Acetate - Ceramic Composite Membranes
6.3 Results and discussion
6.3.1 Surface morphology
2. Using the values of for same CA concentration and varying dipping time a graph was plotted. The data points in the graph were fitted using suitable linear/nonlinear functions by regression analysis.
Rcoat
3. From the curve fit equation obtained from regression analysis, value was determined using Eq. (6.5) by extrapolating data points to
Rint
=0 t . 4. The values of Rtopwere calculated using Eq. (6.6).
During UF of BSA solutions,RBSA was evaluated using the following equation
coat UF
BSA R R R
R = − sup − (6.8)
where RUF refers to the hydraulic resistance during UF experiments.
Figures 6.2a and 6.2b present the SEM image of top surface and cross section of ceramic support, respectively. It can be observed from Fig. 6.2a that the top surface of the support contains both macropores and mesopores. Figure 6.3 present SEM images of the top surface of composite membranes prepared by varying CA concentration (2 - 8 wt %) and dipping time (30 - 150 s). From the figure, it was observed that with an increase in both CA concentration and dipping time, the pore size of the membrane decreased. The SEM image of 2 wt % CA membrane (M-2-30) indicates that almost no change occurred to the porous structure in the membrane top surface. It could be further observed that the coverage of porous structure by CA increased with increasing dipping time from 30 s to 150 s. When the concentration of CA was increased to 4 %, the top surface consisting of both macropores and mesopores of support was totally covered by the CA layer and lead to the formation of a porous polymeric structure (M-4-30, M-4-90). The porous structure of the polymeric film was contributed by the evaporation of the solvent (acetone) from the membrane surface.
(a) (b)
Figure 6.2: SEM images of the ceramic support (a) top surface (Magnification: 5 KX) and (b) cross section (Magnification: 150 X).
With an increase in dipping time from 30 to 150 s, the pore sizes of the membranes were observed to decrease very significantly due to deposition of more CA on the support.
Formation of CA porous structure accounts for an increase in the effective membrane porosity of the membranes (M-4-30, M-4-90 and M-4-150) compared to the membranes prepared with 2 wt % CA concentration (M-2-30, M-2-90, M-2-150). This was attributed to the lack of formation of porous CA structure with 2 wt % CA solution concentrations. Finally, the membrane prepared with dipping time of 150 s (M-4-150) was observed to have the least pore sizes of all the membranes prepared at 4 wt % CA. Membranes prepared with 6 wt % CA concentration (M-6-30, M-6-90, M-6-150) indicated that a stable porous structure was formed. The pore sizes of the membranes were observed to be smaller than those obtained with 4 wt % CA concentration. With an increase in dipping time, the pore sizes were reduced further due to additional deposition of CA on the support. The membranes prepared with 8 wt
% CA (M-8-30, M-8-90, M-8-150) were observed to possess lowest pore size among all the prepared membranes.
Figure 6.4 presents the cross sectional SEM images of the membrane prepared using varying CA solution concentrations (2 - 8 wt %) and dipping time (30 s - 150 s). Comparing the SEM image of ceramic support in Fig. 6.2b with SEM images of 2 wt % CA membranes (M-2-30, M-2-90, M-2-150), it could be observed that CA solution penetrates into the pores of the support due to very low solution viscosity and density (almost the same as the solvent). As a result, the porosity of the membranes was reduced. Further, SEM images indicate that the concentration was not enough to form a film on the support. With an increase in dipping time, more CA penetration occurs in the support as observed for membrane M-2-150. Therefore, CA solution contributes more towards the blocking of ceramic porous structure rather than
8
2
CA concentration (wt %)
6
4
30 90 150 Time (s)
Figure 6.3: Variation of top surface morphology with dipping time and CA concentration. (Magnification: 5 KX).
8
2
CA concentration (wt %)
6
4
30 90 150 Time (s)
Figure 6.4: Variation of cross-section morphology with dipping time and CA concentration. (Magnification: 150 X).
creating a thin film on the top surface. SEM images for 4 wt % CA membrane (M-4-30, M-4- 90 and M-4-150) shows similar trend like 2 wt % CA membranes. With an increase in dipping time, a tendency of creating film is observed. For dipping time of 150 s, a film like structure was formed on the top surface indicating the dominance of CA surface coverage over ceramic matrix blockage. The measured film thickness was about 0.6 µm. Due to less ceramic matrix blockage the effective porosity of the M-4-30, M-4-90 and M-4-150 membranes were more compared to M-2-30, M-2-90 and M-2-150 membranes. When the dip coating solution concentration was increased to 6 wt % CA, initially no film growth was observed for membrane M-6-30. With an increase in dipping times, a stable film was formed for M-6-90 and M-6-150. The measured film thickness for these membranes was 8 and 12 µm, respectively. With an increase in CA concentration in acetone, viscosity and density of the solution increased. Due to this, penetration tendency of the 8 wt % CA solution was less. As a result, less ceramic matrix blockage took place to obtain high effective porosity of the membranes compared to those prepared with 2 and 4 wt % CA concentrations. For 8 wt % CA membranes (M-8-30, M-8-90, M-8-150), a stable film growth was observed for all deposition times which increased with increasing deposition times. The measured film thickness of these membranes was 12, 22 and 32 µm for M-8-30, M-8-90 and M-8-150, respectively. Almost no penetration of the CA into the ceramic matrix was observed due to very high viscosity and density of the CA solution.
Based on these observations from SEM, it could be concluded that with an increase in CA concentration and dip coating time, the average pore size of the membranes decreases. This conclusion was in good agreement with results presented in the literature [98]. For the chosen ceramic support, CA concentrations varying from 6 to 8 wt % tend to be the optimal
combinations to yield UF membranes (pore diameter ranging between 10 nm to 100 nm) [3]
with tailored pore size distributions. On the other hand, MF membranes (pore diameter >100 nm) [3] can be prepared using low to moderate concentrations of CA solution concentration (2 to 4 wt %). An increase in CA concentration was found to have profound effect in altering morphological properties than the dipping time, except for M-4-150 and M-6-30 where an increase in dipping time was found to yield similar effect as CA concentration.