TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The amount of the sterically hindered amine compound is 200 to 5000 (preferably 400-3000) ppm, calculated on total weight of the polymer. At least one of the sterically hindered amine compounds is of formula (I) or (II). The gas mixture comprises fluorine in an amount of 0.5 to 10 vol.%, and optionally oxygen in an amount of up to 10 vol.%.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The ethylene homo- or copolymer composition comprises the sterically hindered amine compounds or their N-hydroxy- or N-oxyl derivatives. The ethylene homo- or copolymer composition comprises a combination of at least two sterically hindered amine compounds or their N-hydroxy- or N-oxyl derivatives. The polymer comprises in addition other stabilizers such as sterically hindered amines including all secondary amines whose substitution on the carbons adjacent to the amine nitrogen is such that any single hydrogen does not remain at these positions, so that preference is given to derivatives of 2,2,6,6-tetramethyl-piperidine, substituted either at the 4-position or on the amine nitrogen, and to derivatives of quinoline and of diphenylamine. Preferred Method: The inner surface of a hollow body made by the polymer is subjected to fluorination by a gas mixture comprising fluorine and nitrogen or a noble gas or oxygen under a pressure of 5 to 20 bars at a temperature of -20 to -190 degrees C over a time period of 10 seconds to 10 minutes. SPECIFIC COMPOUNDS - 21 Compounds are disclosed as the sterically hindered amine compounds e.g. 2,2,6,6-tetramethylpiperidine of formula (I'), 2,2,6,6-tetramethyl-piperidine-4-ol, 2,2,6,6-tetramethylpiperidine-4-one, 2,2,6,6-tetramethylpiperidine-4-yl acetate and 2,2,6,6-tetramethyl-piperidine-4-yl 2-ethylhexanoate.    EXAMPLE - High molecular mass polyethylene produced in a gas phase process in the presence of a chromium based Phillips catalyst having a HLMFR (190 degrees C/21.6 kg) of 6 g/10 minutes and a density of 0.946 g/cm3 was treated with Chimassorb 944 (RTM: sterically hindered amine compound as stabilizer) (1800 ppm). The polymer was additionally stabilized with 400 ppm Irganox 1010 (RTM: sterically hindered phenols as stabilizer) and 1100 ppm Irgafos 168 (RTM: stabilizer). The powder was pelletized and stabilized to form pellets (test). Control pellets were formed in the same manner as the test except that no stabilizer was used. From the test/control pellets prepared above, bottles having a planar bottom were prepared by blow molding in a blow molding tool. The bottles had a volume of 500 ml. The bottles were prepared at an inner pressure of 10 bars at a melt temperature of 195 degrees C and had a wall thickness of 1 mm, each. At a wall temperature of 90-100 degrees C the pressure was lowered to 2 bars and the fluorinating gas was introduced under a pressure of 10 bars. As fluorinating gas, a mix of 1 vol.% F2, 2.6 vol.% O2 and 96.4 vol.% N2 was employed having a temperature of -100 degrees C. After a contact time of 60 seconds the fluorinating gas was removed by nitrogen. All bottles were filled with 450 ml of a commercial super fuel having a methanol content of 3 vol.%. The bottles were closed gas tight and their exact weight was noted. They were stored over a time period of 10 days, 40 days and 60 days at a temperature of 50 degrees C in a heating chamber. After each time period their exact weight was taken again and was compared with the noted weight of each bottle. The mass loss (g) after different time period of storage for test/control was: after 10 days = 3.5/5; after 40 days = 8/16; and after 60 days = 11.5/22. The results showed that the presence of sterically hindered amine compounds in the ethylene polymer improved the barrier properties in combination with the fluorinating treatment significantly.