A secondary battery pack comprising: - a battery pack enclosure composed of an enclosure top panel and an enclosure bottom panel which when sealed to each other provide a substantially airtight battery pack enclosure, - at least one array of battery cells within said enclosure bottom panel which are electrically connected to one another and are arranged in an upright manner such that axes of the cells are parallel to each other, - an isocyanate-based material A which fills partially or fully the open space of said battery pack enclosure and/or fills partially or fully the open space within said array of battery cells and/or covers partially or totally said battery cells, and wherein said isocyanate-based material A is prepared by mixing and curing a composition X comprising: (a) at least one isocyanate compound, (b) at least one organic compound having at least two epoxy groups or at least one active hydrogen-containing compound or a mixture of active hydrogen-containing compounds selected from the group consisting of polyols, polyamines, polyamides, polyimines and polyolamines. (c) at least one catalyst, (d) optionally at least one blowing agent, (e) optionally at least one adhesion promoter, and (f) optionally at least one additive, and wherein for 100 parts by weight of composition X it further comprises: from 0.1 part by weight to 30 parts by weight, preferably from 0.1 part by weight to 20 parts by weight, and even more preferably from 4.5 parts by weight to 30 parts by weight of at least one organopolysiloxane polymer Y comprising either as a terminal group or as a pendant group at least one polyoxyalkylene moiety end-capped by a hydroxyl group. - a battery pack enclosure composed of an enclosure top panel and an enclosure bottom panel which when sealed to each other provide a substantially airtight battery pack enclosure, - at least one array of battery cells within said enclosure bottom panel which are electrically connected to one another and are arranged in an upright manner such that axes of the cells are parallel to each other, - an isocyanate-based material A which fills partially or fully the open space of said battery pack enclosure and/or fills partially or fully the open space within said array of battery cells and/or covers partially or totally said battery cells, and wherein said isocyanate-based material A is prepared by mixing and curing a composition X comprising: (a) at least one isocyanate compound, (b) at least one organic compound having at least two epoxy groups or at least one active hydrogen-containing compound or a mixture of active hydrogen-containing compounds selected from the group consisting of polyols, polyamines, polyamides, polyimines and polyolamines. (c) at least one catalyst, (d) optionally at least one blowing agent, (e) optionally at least one adhesion promoter, and (f) optionally at least one additive, (a) at least one isocyanate compound, (b) at least one organic compound having at least two epoxy groups or at least one active hydrogen-containing compound or a mixture of active hydrogen-containing compounds selected from the group consisting of polyols, polyamines, polyamides, polyimines and polyolamines. (c) at least one catalyst, (d) optionally at least one blowing agent, (e) optionally at least one adhesion promoter, and (f) optionally at least one additive,