Excise exemption for polypropylene staple fibre tow and tops: duty waived under Central Excise Rules for a limited period. Polypropylene staple fibre tow and tops classified under sub heading 5501.90 are exempted from the whole of the duty of excise specified in the Central Excise Tariff Act, 1985, by notification issued under sub rule (1) of rule 8 of the Central Excise Rules, 1944, with the exemption operative for a limited specified period.