Journal of Membrane Science, 43 (1989) 143-148 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PERVAPORATION SEPARATION OF ACETIC ACID-WATER MIXTURES USING MODIFIED MEMBRANES. PART II. GAMMARAY-INDUCED GRAFTED POLYACRYLIC ACID (PAA)-NYLON 6 MEMBRANES*
R.Y.M. HUANG and Y.F. XU** Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario NZL 3Gl (Canada) (Received August 12,1987; accepted in revised form August 10,1988)
Summary Graft copolymers of polyacrylic acid (PAA) onto Nylon 6 and blended PAA-Nylon 6 films containing varying amounts of PAA were prepared by the direct radiation technique; aqueous solutions of acrylic acid (AA) containing cupric sulphate were exposed to various radiation doses. The percent grafting could be adjusted by varying the concentration of acrylic acid and the radiation dose, and up to 35% grafting was obtained. The PAA-Nylon 6 blend graft copolymers were tested for the pervaporation separation of acetic acid-water mixtures and gave high separation factors a! (water/acetic acid) of up to 300 and good flux rates of 40-60 g/m*-hr.
In Part I of this series of investigations of the pervaporation separation of acetic acid-water mixtures with blended membranes, the results of polyacrylic acid (PAA)-Nylon 6 blends which were ionically crosslinked by aluminum nitrate were reported [ 11.The present paper deals with the pervaporation separation of the acetic acid-water system using polyacrylic acid-Nylon 6 graft copolymer membranes prepared by the gamma-ray-induced graft copolymerization of acrylic acid onto Nylon 6 and PAA-Nylon 6 blended substrates. Experimental
Details of the experimental procedures, irradiation cell and the pervaporation apparatus used for the pervaporation test runs are similar to those presented in Part I of this series [ 11. *Based in part on a paper presented at the 36th Canadian Chemical Engineering Conference, October 6-8,1986, Sarnia, Ontario, Canada. **Permanent Address: Department of Chemical Engineering, Qinghua University, Beijing, China.
0 1989 Elsevier Science Publishers B.V.
Materials Nylon 6 with molecular weight 42,000 and polyacrylic acid (PAA) with molecular weight 150,000 (in 25% aqueous solution) were obtained from Polysciences Inc. Acrylic acid (AA) monomer was Baker grade from J.T. Baker Chemical Co. The acetic acid, aluminum nitrate (Al ( NO3 ) ,.9H,O ), dimethyl formamide (DMF) and cupric sulphate (CuS04.5H,0) used were all Baker analyzed reagent grade obtained from the J.T. Baker Chemical Co. Formic acid (88% and 90% ) was obtained from Fisher Scientific. Membrane
Preparation of the polyacrylic acid (PAA)-Nylon 6 blended substrate The method for the preparation of the PAA-Nylon 6 blended substrate film has been described in the previous paper of the series [ 11. Preparation of graft copolymer membranes The substrate film was soaked in the grafting solution consisting of different concentrations of acrylic acid monomer in water and 0.04 M cupric sulphate for 12 hours prior to irradiation in stoppered 6-in. test tubes. After deaerating by vacuum, the test tubes were irradiated in a gamma cell 20 cobalt-60 irradiation unit (dose rate 0.09 Mrad/hr) to the required doses at room temperature. The resulting membrane was washed with deionized water and then immersed in it for at least 24 hours to remove to homopolymer before further use. Results and discussion Grafted membranes As reported in the previous paper [ 11, compared with ionically crosslinked PAA membranes, the blended PAA-Nylon 6 membranes have much higher selectivities, which is attributed to the fact that blending PAA with relatively inert Nylon 6 adjusted the hydrophilic-hydrophobic balance for the system to be separated. If this is true, grafting PAA onto a Nylon 6 backbone should have at least the same effect and give even better results because the grafting produces a more homogeneous state between PAA and Nylon 6 than blending them together. Chemical modifications of nylon by graft copolymerization with PAA have been reported by several investigators [2-41. A considerable amount of PAA homopolymer is formed during the grafting reaction; however, in order to avoid this, several metal salts dissolved in the monomer solution can be used to reduce homopolymer formation [ 51. Of the various metal ions, Cu*+ has been shown to be the most effective in suppressing homopolymerization. Trivedi and Mehta  studied the effect of CuSO, concentration on the degree of
grafting of acrylic acid onto Nylon 6. They found that maximum grafting yields were obtained at 0.02 M CuSO, concentration. In order to prevent excessive PAA homopolymerization, a 0.04 M CuSO, concentration was selected for the present study. Figure 1 shows the percent grafting for the different irradiation doses and different concentrations of AA in the grafting solution. As can be seen, the percent grafting increases considerably with irradiation dose while only slight increases with increase of AA concentration in the grafting solution were observed. The same trend was also previously reported by Trivedi and Mehta [41* Table 1 shows the effect of the proportion of PAA in the PAA-Nylon 6 blended substrate films on the pervaporation results. 25% PAA in blended substrate films yielded the best results in terms of separation factors and permeation rates. As shown in Fig. 2, the percent grafting obtained for a 25% blended substrate is lower than that for a Nylon 6 substrate, particularly in the case of low monomer concentrations of grafting solution. The results of pervaporation runs which were carried out at 15°C are shown in Tables 1 and 2. 40
Fig. 1. Percent grafting vs. irradiation dose for Nylon 6 substrates (CuSO, in grafting solution 0.0417 M). TABLE 1 Effects of PAA content in grafted PAA-Nylon 6 blended substrates on pervaporation performance (operating temperature 15°C) Content of PAA in substrate
Gamma-ray irradiation conditions
Feed concentration (wt.% HAc)
Permeation rate (g/m*-hr)
1M A.4 05MA
Fig. 2. Percent grafting vs. irradiation dose for 25% PAA blended polyacrylic acid (PAA)-Nylon 6 substrate ( CuSO, in grafting solution 0.0417 M). TABLE 2 Pervaporation performance for membranes with PAA radiation grafted onto blended PAA-Nylon 6 substrates Operating temperature: 15’ C; cupric sulphate in grafting solution: 0.04 M Gamma-ray irradiation conditions
Membrane thickness (mil)
Feed concentration (wt.% HAc)
Permeation rate (g/m’-hr)
0.5 1.0 2.0
1.8 2.6 9.7
1.2 1.2 1.2
48.4 48.9 48.6
37.6 38.2 42.8
288.8 277.3 205.1
As can be seen, the radiation dose and the concentration of the grafting solution influence the percent grafting, the separation factor and permeation flux of the grafted membranes. The exact location of the polyacrylic acid grafted onto the blended membrane is presently under investigation using transmission and scanning electron microscopy and will be described in a separate paper. A mathematical model is also being developed to describe the hydrophilichydrophobic balance criteria using solubility parameters, diffusion, solubility and absorption data of the two liquid components in the modified membrane as reported in our previous publication [ 11.
The selectivity of binary pervaporation can be generally expressed in terms of the separation factor: a(B/A)
YEJY* acid) =---x13/xA
here X is the feed composition, Y is the permeate composition and component B is the preferentially permeating component. Figure 3 shows the effect of total PAA content in the blended membranes on the pervaporation separation factor. Experiments were conducted at 15 oC and for 50 wt.% acetic acid concentration in the feed mixtures. One can see that the selectivity of the membrane shows a peak value at about 26 wt.% total PAA content. The peak width of the separation factor curve is so narrow that the total PAA content in the membrane had to be carefully controlled in order for the membrane to have a high pervaporation selectivity. Figure 4 shows a plot of permeation rate vs. total PAA content. Because the permeation rate always increases at the expense of separation factor for the same type of membrane, the pervaporation flux has a minimum value at the peak value of the selectivity as shown in Fig. 3. As reported in the previous paper [ 11, the optimum PAA content in the
Fig. 3. Separation factor vs. total polyacrylic acid (PAA) content of membranes (feed concentration = 50 wt.% acetic acid in water, pervaporation temperature = 15 ’ C ) .
Fig. 4. Permeation rate vs. total polyacrylic acid (PAA) content of membranes (feed concentration = 50 wt.% acetic acid in water, pervaporation temperature = 15 ’ C ) .
blended PAA-Nylon 6 membranes is 40 wt.% for the separation of 50 wt.% acetic acid-water mixtures, which is quite different from the grafted membranes prepared in this study. On the other hand, the separation factors obtained from grafted membranes are much higher than those for the blended membranes reported previously [ 11. Conclusions The separation of 50 wt.% acetic acid-water mixtures was studied by the pervaporation process using membranes prepared by grafting PAA onto Nylon 6 or blended PAA-Nylon 6 backbones. Higher selectivities and permeation fluxes were obtained for the grafted membranes studied in this paper than for the blended ones previously reported [ 11. The membranes were found to be stable for the period of the experiments. Acknowledgements The authors wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC ) for their financial support of this study.
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