One of the key purposes of your travel mug is keeping your coffee hot. Have you ever wondered how is that miracle achieved? It all comes down to efficient insulation. Your coffee is not that different than your home – keep it well insulated and it’ll stay warm in the cold winter nights. There are several common insulation methods for travel mugs. We’ll try to explain a little about the science behind them.
Thermal conductivity is key
Although there’s a complicated scientific formulation for thermal conductivity (watts per meter kelvin) it basically means how well does heat propagate through a material. It’s high for metals – that’s why we cook in metal cookware. It’s low for silicon which is why it is often used for cooking gloves and placemats. The lower the thermal conductivity, the less heat will ‘escape’ from your coffee. Why not make our travel mug from silicon you ask? Because we’d also like it to be stable, and robust. Some travel mug lids are made of silicon though.
Ceramics is one option
One class of materials that have low thermal conductivity are ceramic materials. Ceramics (stemming from the greek word ‘for pottery’) are materials that are prepared by heating followed by cooling and are very suitable for insulation. The advantage of a ceramic tumbler – pretty good insulation and they are often very decorative. The downside – they are often heavy and also tend to break.
Double walls can do the trick
Another material that has a pretty low thermal conductivity and is much lighter is air. Since you can’t pour coffee into thin air – you trap it between two walls. The same principle that applies to double paned windows applies to travel mugs. Double walled mugs are very common, provide good insulation and are relatively light. Common materials for the doubled wall insulated mugs can be glass – especially beautiful when drinking colorful tea or stainless steel.
Foam insulation is basically another way of trapping air, foam is any substance that is formed by trapping gas in it. Polystyrene (or ‘styrofoam’) is a cheap popular polymer that is made mostly of air that is often used for insulation in lower-end mugs.
Vacuum is the ultimate insulation
One thing that has even lower thermal conductivity than air is vacuum. To understand why we have to dive just a tiny bit more into thermal conductivity. In a nutshell, heat is conducted through a material by the atoms in the material bumping into each other and transferring energy to one another (gross simplification, any physicists in the audience please sigh and nod reproachfully). Vacuum is the absence of any material and has no atoms. In theory, the perfect vacuum should transfer no heat at all. In reality though, creating an almost perfect vacuum is challenging making good vacuum based travel mugs pricey. The closer you get to a good vacuum the better your insulation gets allowing you to keep your coffee hot while keeping your tumbler lightweight and stylish.