High-density polyethylene (HDPE) was compounded with untreated and surface-treated mica (10, 20, 40 wt %) and composites were injection-molded. The composites were radiation crosslinked (100, 300, 700 kGy) and hydrocarbon permeability, tensile impact strength, and tensile strength at 25 and 80 degrees C of the composites were examined. The permeability of HDPE decreased from 7 to 3.6 g/(d X m(2)) by compounding the polymer with 20 wt % mice, and the permeability was additionally reduced to 1.3 g/(d X m(2)) by irradiation of the compounds (700 kGy). When surface-treated mica was used, the permeability of the composite furthermore decreased to about 1.0 g/(d X m(2)). Upon irradiation, the E modulus measured at 25 degrees C increased 5% when the dose was 300 kGy. At 80 degrees C, the corresponding increase was 40%. The tensile impact strength of an unfilled polymer increased more than three times by an irradiation dose of 700 kGy, and for a polymer with 10 wt % mica, the tensile impact strength was twice the level of an unirradiated composite.