Hydrophilic modification of polypropylene membranes and films by catalytic ozonation
Authors: Gu Hong Bin 
Published: May 2021

Commercial polypropylene microfiltration membranes (PPMM) and biaxial oriented polypropylene (BOPP) films were ozonated in aqueous and gaseous media, respectively, followed by graft polymerization of acryl amide (AAm), hydroxyethyl methacrylate (HEMA), and polyethylene glycol (PEG) to improve their surface hydrophilicity. The efficiency of ozonation conducted in the gaseous and aqueous phases was compared, the gaseous phase ozonation was found slightly more effective in generating peroxide groups, while the aqueous phase ozonation was found more effective in grafting polymerization. Scavengers were added to the aqueous phase ozonation, results indicated that both the radical groups and the molecule ozone contributed to the peroxide generation. The free radical groups contributed maximum 25% and 32% for ozonation of PPMM and BOPP, respectively, and the molecule ozone contributed the percentage remaining. Results also showed that the concentration of peroxides generated on the surfaces of PPMM or BOPP increased with the applied ozone dose and ozonation time in both phases.

Copper sulfate hydrate (CuSO4·5H2O) and ferric chloride hydrate (FeCl3·6H2O) were added in the aqueous phase ozonation as homogenous catalysts, results showed that the peroxide generation rate of PPMM and BOPP was improved comparing to that of ozonation without catalyst. The peroxide generation of PPMM showed 17% increase by adding copper catalyst, and 16% increase in peroxide generation was observed in ferric catalyzed ozonation of BOPP. The mechanism of the aqueous phase ozonation was investigated, along with that of catalytic ozonation. An enhanced radical process was found for catalytic ozonation in this study.

The hydrophilicity of PPMM and BOPP was improved by graft polymerization of AAm, HEMA and PEG
initiated by the peroxides. The aqueous phase ozonation was found more effective in grafting. A washing test was conducted using distilled water blending with 10% isopropyl alcohol. When the ozonated membranes and films were washed and compared to the non-washed ones, it was found that the gaseous phase ozonated PPMM or BOPP lost more peroxides than their aqueous phase counterpart after washing. The washing tests showed that the aqueous phase ozonation could induce a better graft polymerization, because part of the tested peroxides from the gaseous ozonation was washed away in the cleaning and grafting process.

The improved hydrophilicity of PPMM was indicated by the contact angle reduction from 129° to 91° for AAm grafting; from 126° to 74° for HEMA grafting; and from 126° to 88° for PEG grafting; Fourier Transform Infrared (FTIR) measurements showed additional peaks of functional groups, such as amine (N-H) and amide (–N-C=O) functional groups from the grafted AAm (CH2=CH-CO-NH2); and the Scanning Electron Microscope (SEM) images confirmed amorphicity changes of the graft polymerization. X-ray Diffraction (XRD) diffractogram revealed the crystallinity changes of ozonated and grafted PPMM. Bovine serum albumin (BSA) was used to test the filtration performance of virgin and grafted membranes, the filtration tests demonstrated the improvement in anti-fouling effect of the modified PPMM; and the SEM images of the fouled and washed membranes revealed the pore blockage and recovering on the surface.

The hydrophilicity of the grafted BOPP was also improved, indicated by the contact angle reduction of AAm grafted film from 80° to 56°. The FTIR showed additional peaks of N-H and –N-C=O functional groups of grafted AAm. SEM images indicated amorphicity changes of the graft polymerization. The film modified by the aqueous phase ozonation showed its advantages of better graft polymerization, hydrophilicity, and protein adsorption.
The results of this study positively impacted the industrial using of PPMM to elongate its duration time of filtration, and improved the applications of BOPP in biomedical areas