The three most common materials from which drink containers are constructed are glass, foam and plastic. When choosing which type to use, one consideration is which keeps hot drinks hot and cold drinks cold. Glass, foam and plastic have very different insulating properties.
Properties of Glass, Foam and Plastic
Foam drinking containers are made of expanded polystyrene (EPS), a form of polystyrene plastic filled with tiny air bubbles. Plastic drink containers, such as soda bottles, are commonly made of Polyethylene Terephthalate Polyester (PET). The glass used for drink containers is made of recycled glass, high-purity silica sand, sodium carbonate and various chemical salts to purify and strengthen the material.
Conductive Heat Transfer
Liquids warm or cool because energy migrates from hot to cold. If the drink is warmer than the room, the heat energy will travel from the drink to the cooler air. Conversely, if the drink is cooler than the room, the heat energy from the warmer air will travel into the drink. The rate of the heat transfer depends on the insulating properties of the container.
Thermal Conductivity Coefficient
All materials conduct heat; in Physics, the property is defined as the quantity of heat energy transmitted through a material in a given amount of time, provided a temperature difference exists between two sides of the material. Designated as “k,” the coefficient is W/mK, where W equals watts of energy, m equals the thickness in meters and K is the temperature in kelvins. Generally, the higher the k-value of a material, the more heat it conducts. In practical terms, the higher the k-value of a container, the faster a drink will warm or cool to match room temperature. Glass has a k-value of 1.05, EPS foam has a k-value of .03 and PET plastic has a k-value of .4. Based on this analysis, liquid will retain its temperature best in an EPS foam container.
Another way to express how well a container maintains temperature is to examine the insulation properties. Commonly called the R-value, the formula is L/k, where L is the thickness of the material and k is the thermal conductivity coefficient. The higher the R-value, the better the material insulates or maintains temperature. For containers of the same thickness, the R-value of glass is .95L, for EPS foam is 33L and for PET plastic is 2.5L. This means that a PET plastic cup would have to be more than ten times as thick as the EPS foam cup to have the same insulating properties. The glass cup would have to be more than 30 times as thick.
References and ResourcesThe Engineering Toolbox: Thermal Conductivity of Some Common Materials
The Engineering Toolbox: Heat Loss Through Buildings Due to Transmission
AZO Materials: Polyethylene Terephthalate Polyester (PET, PETP) – Properties and Applications
The Engineering Toolbox: Conductive Heat Transfer
MIT: Beverage Containers: Manufacturing, Recycling, and Public Policy