Alexei Volkov
Plastic is well known for its remarkable ability to resist breaking under stress. However, certain plastics can become brittle and prone to shattering when frozen. This is due to the plastic's transition at everyday temperatures, causing it to lose molecular mobility. While it is commonly understood that heat makes objects expand, and cooling makes them contract, some plastics exhibit negative thermal expansion (NTE). This means they will expand when cooled and shrink when heated. This phenomenon is related to the geometry of the molecules and is observed in various materials beyond plastics.
| Characteristics | Values |
|---|---|
| Do plastics expand when frozen? | Yes, but not all plastics. |
| Why do plastics expand when frozen? | Due to the phenomenon of negative thermal expansion (NTE). |
| What is NTE? | Materials with a negative thermal expansion (NTE) coefficient will increase in size as they are cooled down. |
| Do plastics contract when cooled? | Yes, in some cases, plastics contract when cooled. For example, an empty plastic bottle in a refrigerator or freezer will shrink and crumple in on itself. |
| Do plastics become brittle when frozen? | Yes, many plastics become brittle when frozen. For example, polypropylene has a Tg of between -20 and 0 degrees Celsius, so it can become shatter-prone. |
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What You'll Learn
Plastic with a negative thermal expansion (NTE) coefficient expands when frozen
Plastic, like most materials, expands when heated and contracts when cooled. However, some plastics exhibit a phenomenon known as negative thermal expansion (NTE), where they expand when cooled and contract when heated. This unusual behaviour is not limited to plastics and is observed in various materials, including ceramics, oxides, phosphates, cyanides, and graphite, and even water.
NTE is a physicochemical process where materials exhibit atypical behaviour in response to temperature changes. While most substances expand upon heating, materials with NTE do the opposite; they contract when heated and expand when cooled. This property is quantified by the material's NTE coefficient, which indicates its tendency to increase in size as its temperature decreases.
Not all plastics possess NTE, but this property is also not exclusive to a specific type of plastic. The behaviour of plastics with NTE can be explained by the geometry of their molecules, including crystal structure and polymer arrangement. When heated, the polymer chains in these plastics relax and orient themselves in a way that reduces the overall size of the material.
The presence of NTE in plastics has practical implications. For example, it can be a consideration when mating plastic with another material, such as metal, that may have different thermal expansion rates. Additionally, understanding NTE in plastics can lead to potential applications in engineering, photonics, electronics, and structural design. By combining NTE materials with regular materials, engineers can create composites with tailored or zero thermal expansion properties.
In summary, while most plastics follow the typical behaviour of expanding when heated and contracting when cooled, a notable subset of plastics exhibits negative thermal expansion. This NTE behaviour has sparked interest in various scientific and engineering fields due to its potential applications and unique physicochemical properties.
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Water expands when frozen, causing plastic bottles to burst
It is a well-known fact that heat makes things expand, and cooling makes them contract or shrink. However, water is an exception to this rule—it expands when frozen. This happens because the water molecules can no longer pack together as closely due to the distance imposed by the hydrogen bonds formed when liquid water turns to ice. As a result, more space is required to contain the same initial amount of water. This is why a plastic bottle will crack if you completely fill it with water and leave it in the freezer for too long. As the water freezes, it expands and needs more space than the bottle can sustain, causing it to burst.
Although the strength of a single hydrogen bond is insufficient to burst through a plastic bottle, the combined strength of millions of hydrogen bonds is enough to exert significant pressure. This pressure can cause the plastic bottle to crack or burst, as the water expands and pushes against the sides of the container.
It is important to note that not all plastics behave in the same way when exposed to cold temperatures. Some plastics exhibit a phenomenon known as negative thermal expansion (NTE), where they increase in size as their temperature decreases. This behaviour is observed in various materials, including certain ceramics, phosphates, cyanides, and graphite, and it is influenced by the geometry of the molecules and their crystal structure or polymer arrangement.
On the other hand, many plastics become brittle when exposed to cold temperatures. For example, polypropylene, commonly used in containers, toys, and outdoor furniture, can lose its molecular mobility and become shatter-prone at temperatures below 0°C. This transition occurs within everyday temperature ranges, demonstrating the versatility and complexity of plastic behaviour.
Additionally, the ductility of plastics, or their ability to stretch, plays a crucial role in their resilience. The long, chain-like molecules in plastics can stretch to several times their original length, absorbing energy individually and collectively dissipating stress to prevent breakage. This property is why dropped plastic bottles often bounce instead of shattering and why car bumpers flex instead of denting. However, when plastic bottles are filled with water and frozen, the expansion of water can exceed the ductility limit of the plastic, leading to bursting.
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Plastic becomes brittle when frozen
The phenomenon of plastics becoming brittle when frozen is well-known. This occurs due to the restriction of motion in the plastic's long, chain-like molecules, which are usually free to slip past, around, or through one another. When the temperature drops, this movement is hindered, and the molecules cannot stretch to absorb energy. As a result, the stress from impact becomes concentrated in a small area, leading to the creation of cracks that can propagate into fractures. This transition temperature, where ductility gives way to brittleness, is known as the ductile-to-brittle transition temperature (DBTT), and it varies for different plastics.
Polycarbonate (PC), for instance, is an incredibly durable plastic that is sometimes employed as "bullet-proof glass." However, at extremely low temperatures (-40°F), PC transforms into a brittle state and will shatter like ordinary glass if subjected to high-impact force. Similarly, polypropylene, a low-cost material commonly found in containers, toys, outdoor furniture, and recycling bins, has a transition temperature between -20°C and 0°C. As a result, it can easily lose its molecular mobility and become prone to shattering during cold weather.
The ductility of plastics, or the ability of their molecules to stretch and collectively dissipate stress, is usually responsible for their resilience. This ductility allows plastics to withstand stress and avoid breakage. However, when the temperature drops and the molecules lose their mobility, the risk of fracture increases. This is why plastics become more brittle and prone to shattering at colder temperatures.
Not all plastics exhibit this behaviour uniformly. Some plastics have a negative thermal expansion (NTE) coefficient, which means they increase in size as their temperature decreases. This phenomenon is observed in various materials, including certain ceramics, oxides, and graphite. While not unique to a specific type of plastic, NTE is not a property of all plastics. The occurrence of NTE depends on the geometry of molecules and their crystal structure or polymer arrangement.
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Not all plastics expand when frozen
It is a well-known fact that heat makes things expand, and cooling them down generally leads to contraction. However, not all plastics expand when frozen. While many plastics exhibit this transition at everyday temperatures, becoming brittle when frozen, this is not true of all plastics.
For example, plastics used in shrink-wrapping do not expand back to their original size when cooled. These plastics are only partially polymerised, becoming fully polymerised when heat is applied, and this process is not reversible. The molecules in these plastics do not expand again due to bond formation.
Another example is polypropylene, which is often used in containers, toys, outdoor furniture, and recycling bins. Polypropylene has a transition temperature (Tg) of between -20 and 0 degrees Celsius, meaning it can easily lose its molecular mobility and become shatter-prone at low temperatures.
Furthermore, while some plastics with a negative thermal expansion (NTE) coefficient will increase in size when cooled, not all plastics possess this property. The tendency to expand when frozen is related to the geometry of the molecules, including crystal structure and polymer arrangement.
It is worth noting that the remarkable resilience of modern plastics is due to ductility, the ability of their long, chain-like molecules to stretch. This allows plastics to resist breaking when placed under stress, such as when a plastic bottle bounces instead of shattering when dropped.
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Plastic shrinks when heated
While it may seem counterintuitive, some plastics do indeed shrink when heated. This phenomenon is observed in various materials, including ceramics, oxides, and polymers, and is known as negative thermal expansion (NTE). NTE occurs when materials increase in size as they are cooled down. Although not all plastics exhibit NTE, it is not unique to a specific type of plastic.
The process by which plastics shrink when heated involves the behaviour of polymer chains within the plastic. When plastic is heated above its glass transition temperature, the polymer chains are no longer locked in a high-strain orientation. Instead, they relax into a low-energy orientation, curling and bending in a way that reduces the overall size of the plastic. This change in conformation is entropically favourable, as it allows for more arbitrary bends, reducing the Gibbs free energy and increasing stability.
Additionally, the application of heat energy to plastic imparts additional energy to the molecules within the polymer chains. This causes the molecules to become excited and move apart from each other, creating more space within the plastic. However, if no external energy is applied to lock these molecules in their new orientation during the cooling process, the chains will re-entangle and nestle closely together, resulting in a net shrinkage of the plastic.
It is important to note that not all plastics are designed to shrink when heated. Some plastics, such as heat-shrink tubing, are specifically engineered to exhibit this behaviour. By applying heat to this type of tubing, it releases the mechanical stress, allowing it to shrink and cover electrical connections securely.
Furthermore, the behaviour of plastic when heated can vary depending on its type. For example, thermoplastic and thermoset plastics may respond differently to heat. Thermoplastic, including materials like ABS, acrylic, and PVC, can be influenced by the method of heating, such as uneven heat distribution when heated from a single side. On the other hand, thermoset materials like phenolic are not melted during the forming process, potentially resulting in distinct responses to heat.
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Frequently asked questions
Not all plastics have a Negative Thermal Expansion (NTE) coefficient, but some do. This means that some plastics will expand when cooled and contract when heated.
The cause of NTE varies depending on the material, but it is almost always related to the geometry of the molecules and their crystal structure or polymer arrangement.
Water expands when chilled within a certain temperature range. This is due to the ordered molecular structure of ice, which requires more space to contain the same initial amount of water. As a result, a plastic bottle will crack if it cannot sustain the expansion of the frozen water inside it.
