Abstract: A hermetically closed package enclosing a product comprises a wall formed of a packaging material including a first polymer film (7) and a second polymer film (10) being co-extensive with the packaging material, the first polymer film (7) is laminated to the second polymer film (10) by means of an adhesive layer (16). One of the polymer films is provided in a limited region thereof with a layer of a material generating heat when subjected to microwave radiation, i.e. a heat-generating layer (5) having susceptor effect. The heat-generating effect, i.e. the susceptor effect, of the heat-generating layer (5) is selected so as to melt, decompose, soften or weaken the first polymer film (7) and/or the second polymer film (10) in the limited region as seen transversely of the packaging material to such a degree that an opening is formed in the limited region.

Abstract: A packaging material for forming at least a part of a package for a product releasing gasses comprises a one-way pressure relief valve (18) including a first inner film (10), being provided with at least one inlet opening (13, 14) therethrough, and a second outer film (11) covering the at least one inlet opening (13, 14) and being bonded to the first inner film (10) to form a channel portion (12) between the inner and the outer film. The channel portion (12) communicates with the surroundings of the package through at least one outlet opening (15) spaced apart from the at least one inlet opening (13, 14). The pressure relief valve (18) further comprises a liquid (19) provided in the channel portion (12) and solid spacer means (17) provided in the channel portion (12) proximate to the at least one inlet opening (13, 14).