▁usually ▁divided ▁in ▁cells . ▁The ▁cells ▁are ▁shall ow ▁to ▁reduce ▁static ▁head ▁– ▁based ▁on ▁the ▁principle ▁of ▁shall ow ▁container ▁geometry . ▁The ▁pack aging ▁material ▁should ▁conduct ▁heat ▁well ; ▁and ▁it ▁should ▁be ▁dur able ▁enough ▁to ▁with stand ▁frequent ▁changes ▁in ▁the ▁storage ▁material ' s ▁volume ▁as ▁phase ▁changes ▁occur . ▁It ▁should ▁also ▁restrict ▁the ▁passage ▁of ▁water ▁through ▁the ▁walls , ▁so ▁the ▁materials ▁will ▁not ▁dry ▁out ▁( or ▁water - out , ▁if ▁the ▁material ▁is ▁h yg ro scop ic ). ▁Pack aging ▁must ▁also ▁resist ▁leak age ▁and ▁cor ros ion . ▁Common ▁pack aging ▁materials ▁showing ▁chemical ▁compatibility ▁with ▁room ▁temperature ▁PC M s ▁include ▁st ain less ▁steel , ▁poly pro py l ene ▁and ▁poly ole fin . ▁ ▁Th erm al ▁compos ites ▁ ▁Th erm al ▁compos ites ▁is ▁a ▁term ▁given ▁to ▁combinations ▁of ▁phase ▁change ▁materials ▁( PC M s ) ▁and ▁other ▁( us ually ▁solid ) ▁structures . ▁A ▁simple ▁example ▁is ▁a ▁co pper ▁mesh ▁imm ers ed ▁in ▁para ff in ▁w ax . ▁The ▁co pper ▁mesh ▁within ▁para ff in ▁w ax ▁can ▁be ▁considered ▁a ▁composite ▁material , ▁dub bed ▁a ▁thermal ▁composite . ▁Such ▁hy brid ▁materials ▁are ▁created ▁to ▁achieve ▁specific ▁overall ▁or ▁bulk ▁properties . ▁ ▁Th erm al ▁conduct ivity ▁is ▁a ▁common ▁property ▁target ed ▁for ▁maxim ization ▁by ▁creating ▁thermal ▁compos ites . ▁In ▁this ▁case , ▁the ▁basic ▁idea