Preparation of polyethylene film containing porous structure

Abstract

A new method of preparing porous polyethylene film was investigated. Low density polyethylene (LDPE) and tapioca starch were mixed together to produce 2, 4, 6, 8, 10, and 12 wt% starch blends. Each blend was melt extruded to obtain LDPE/starch films having thicknesses of 50, 80, and 100 µm. The porous structure of the films was formed by removing starch particles from the films using acidic hydrolysis and enzymatic hydrolysis. For acidic hydrolysis, the films were immersed in solutions of HC1, H2S04, and HN03 under various conditions while a solution of a-amylase was employed for enzymatic hydrolysis. For acidic hydrolysis, starch particles were best removed using 5 N HNO3 at 65℃ which gave a reduction in starch level of approximately 85%, whereas for enzymatic hydrolysis the reduction was much lower at about 35%. The amounts of starch removal correlated well with scanning electron micrographs where more pores were observed in HNO3 hydrolyzed film. The concentration of HNO3 solution and hydrolysis temperature both played important roles in starch removal. The reduction in starch level increased with increasing acid concentration. At temperatures below the gelatinization temperature range of tapioca starch, starch removal was much lower than that within the gelatinization temperature range. It was found that as the starch content increased from 2 wt% to 12 wt% the microstructure of the films changed from dense to porous structure. An increase in the film thickness resulted in a decrease in starch removal. Mechanical properties of porous films obtained from both nitric acid hydrolysis and enzymatic hydrolysis were lower than those of untreated LDPE/starch film. The gas permeabilities (P) of the film containing 12 wt% starch before hydrolysis were 45.75, 51.61, 65.13, 19.97, and 43.84 barrers for nitrogen, carbon dioxide, ethylene, propane, and propylene gases, respectively, and 484.40, 506.84, 601.50, 162.88, and 176.52 barrers after nitric acid hydrolysis. The selectivity or separation parameters of porous, HN03 hydrolyzed film were P(N2)/P(C3H8) = 2.97, P(N2)/P(C3H6) = 2.74, P(C02)/P(C3H8) = 3.11, P(C02)/P(C3H6) = 2.87, P(C2H4)/P(C3H8) = 3.69, and P(C2H4)/P(C3H6) = 3.41. The dramatic increase in gas permeabilities coupled with good selectivity indicates that the porous films have good potential for use in industrial gas separation.