Time to unlearn what you thought you knew about phase changes and packaging materials, like PCMs and gel packs
Did you know there is a difference between heat and temperature? They are connected, but not the same thing. In general, misconceptions like these result from people creating their explanations for how the world works.
These ideas are often formed long before a person arrives at their first science class, which serves their purpose well. Numerous studies and anecdotal evidence demonstrate that people tend to cling to these ideas even with discrepant events and explicit instruction.
When our labs begin work on testing a packaging solution, we start with a thorough understanding of the principles of thermodynamics and how the packaging elements (i.e., cooler, refrigerant, packing procedures) work together to influence both the temperature stability of packaging and how long the temperature can remain stable.
To this end, we thought we would throw a bit of fun knowledge out there to help change how phase change materials work and why thermodynamic testing is essential to your cold chain packaging.
Let’s start with a quick, less than scientific definition of phase changes. A phase change is a physical process of transition from one state (solid, liquid, gas, plasma) to another. These changes occur when adequate energy is supplied to the medium (or enough is removed) and can also happen when pressure on the system is changed. During this process of transition, the energy is gained or lost results in the maintenance of a constant temperature that is used to provide a stable temperature source for your package. Gel packs and PCMs follow this same process when moving from a solid, frozen gel to a liquid.
Here are five myths that may change your understanding of phase change materials used in packaging:
Myth #1 – During a phase change in matter, the temperature of matter also must change
Heat energy causes the kinetic energy in the ice molecules to increase and results in a temperature increase.
The ice molecules vibrate with more kinetic energy, so when you increase the molecules’ temperature, you increase their kinetic energy. When the temperature reaches 0°C, the solid ice changes to liquid water. While in this phase change, the added heat energy does not change into the kinetic energy of the molecules because the temperature stays the same and instead changes into potential energy. The potential energy is the energy stored in the bonds.
The temperature remains constant, changing from a gas to a liquid and from a liquid to a solid when the molecules come together to form bonds and release energy. When solid ice converts to liquid water, the added heat energy breaks the bonds between the ice molecules, which change into liquid water molecules.
The same happens when liquid water converts to steam where the temperature remains constant, and the added heat energy is converted to potential energy to break the bonds between the liquid water molecules, which change into steam molecules.
Myth #2 – States of matter and phases of matter are the same thing
Matter is anything that has mass and occupies space. States of matter are the physical forms taken by the phases of matter. Although the state and phase don’t mean the same thing, you’ll often hear the two terms used interchangeably.
States of matter are solids, liquids, gases, and plasma. Phase can be classified on structure, state, and composition. Therefore, the state is a subset of the phase. State changes can be between a solid and a liquid, a liquid and a gas, and a solid and a gas. Phase change can be a structure change, phase change, or composition change.
When Ferrite (alpha Iron) is heated, it converts to austenite (gamma Iron). Both ferrite and austenite are solids; therefore, they have the same state but different structures. The structure of ferrite is BCC (body-centered cubic), whereas the structure of austenite is face-centered cubic (FCC). Since the structure is different, there are different phases even though they are in the same state.
Another example is when delta iron is subjected to heat; it converts to a liquid. Since it changes from a solid to a liquid, there is a state change and a phase change.
Myth #3 – Phase change materials are most effective when they actually change phases
Phase-change materials (PCMs) absorb and release heat through basic characteristics of matter. As the surrounding temperature rises, the heat absorbed by the material melts it from solid to liquid. This works in the opposite direction, releasing heat as the material solidifies again. As long as the PCM remains in-between states of matter it will provide a constant temperature while it phases. The goal is to have the PCM always phasing and never completing the process. This ensures the material, much like Goldilocks’ porridge, always feels just right.
Myth #4 – Water always begins freezing at 0 °C
When liquid water loses thermal energy, it changes to solid ice; however, there is one more thing needed in addition to the loss of thermal energy, a crystal seed. This seed is a small impurity from which an ice crystal can grow. It may be another particle in the water or come from the container holding the water. This is the reason not all water freezes at 0°C. Under some conditions, water can get as cold as -40°C in a process called supercooling. This occurs in pure water and usually in a totally smooth container.
Learn anything today?
Little did you know that thermodynamics could be interesting and relevant to your cold chain packaging. As an ISTA Certified packaging laboratory, we must understand these theories and apply them. Email us if you have any questions or additional myths you’d like us to debunk.