Heat retaining gel

[piersgrove]

Hi there. I am a newbie at chemistry but I'm trying to learn as quickly as possible. Any help appreciated! I'm looking to develop a new type of hot water bottle that's sealed like a heat pack. I'm looking for suggestions about how to identify the best liquid / gel that I can use in the hot water bottles that once heated will hold its heat for as long as possible (I can top up the heat over an 8 hour period but battery size is limiting on this). Can anyone help me understand how to go about this or simply suggest some liquids or gels to use?