Chloe Zhang
Plastic is a versatile material used in a wide range of applications, from engineering to food packaging. It is well known that most materials expand when heated and contract when cooled. However, the behaviour of plastic is not always so straightforward. Some plastics, such as Nylon and Acetal, have been observed to expand when cooled, a phenomenon known as negative thermal expansion (NTE). This unusual behaviour is not limited to a specific type of plastic and is also observed in other materials such as ceramics and graphite. The cause of NTE varies across materials but is often related to the geometry of molecules and the rotational energy of bridging atoms. While NTE plastics can expand when cooled, they do not always return to their original size, with some plastics used for shrink-wrapping exhibiting irreversible contraction upon heating. The choice of plastic for specific applications, such as cold environments, must consider the unique properties of different plastics, including their response to temperature changes.
| Characteristics | Values |
|---|---|
| Expansion or Contraction in Cold | Some plastics expand when cooled, a phenomenon known as Negative Thermal Expansion (NTE). However, plastics used in shrink-wrapping do not expand back to their original size when cooled. |
| Expansion or Contraction in Heat | Plastics tend to contract when heated. |
| Impact of Temperature on Properties | Cold temperatures can deteriorate the properties of plastics, decreasing impact resistance and causing a temporary loss of elasticity, making the material more brittle. |
| Plastics with Cold Resistance | ABS plastic performs well in temperatures as low as -20 °C. Polytetrafluoroethylene (PTFE) can be used at temperatures as low as -240 °C. Ultra-high-molecular-weight polyethylene (UHMW) is a high-density material that tolerates cold temperatures. |
| Expansion Rate of Specific Plastics | Nylon and Acetal have an expansion rate that increases slightly at temperatures over 60°C. For every 10°C increase or decrease, a 100mm nylon6 rod will expand or contract by 0.12mm. |
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What You'll Learn
Some plastics expand when cooled
While plastic generally expands as temperature increases, some plastics exhibit negative thermal expansion (NTE), meaning they expand when cooled. This phenomenon is not exclusive to plastics and is observed in other materials such as ceramics, oxides, phosphates, cyanides, and graphite. The cause of NTE varies depending on the material but is often related to the geometry of molecules, including crystal structure and polymer arrangement.
In one observation, a dry solid block of nonporous plastic material was placed in a freezer, and upon retrieval, it was found to have expanded globally by a few inches. This behaviour is attributed to the material's NTE coefficient, which causes it to increase in size as its temperature decreases. It is important to note that not all plastics exhibit NTE, and the property is not unique to a specific type of plastic.
The thermal behaviour of plastics is an essential consideration in product development. For example, when selecting a plastic material for a specific application, manufacturers must consider the environmental temperature range the part will be exposed to. The choice of plastic becomes particularly critical when mating plastic with other materials, such as metal, that have different thermal expansion rates. If the dimensional change is obstructed, excessive stress can be induced in the plastic, leading to potential failure.
Additionally, the glass transition temperature (Tg) plays a crucial role in the ductility of plastics. Below Tg, materials like polymers, rubber, and cotton candy transition from being ductile to brittle. This transition occurs because the molecules lose their ability to stretch and slip past each other, concentrating stress in a small area, which can lead to cracking and fracture.
While some plastics expand when cooled, it is important to note that certain plastics, such as those used in shrink-wrapping, do not exhibit reversible behaviour. These plastics undergo partial polymerization during heating, resulting in shrinkage. However, they do not expand back to their original size when cooled, making them useful for applications like heat-shrink tubing.
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Plastics with NTE expand when cooled
Typically, when plastic is exposed to cold temperatures, it contracts and becomes more brittle. However, there is a class of plastics that exhibit a phenomenon known as Negative Thermal Expansion (NTE), where they expand when cooled within a certain temperature range. This behaviour is observed in plastics that shrink upon heating. The cause of NTE varies across materials but is generally related to the geometry of molecules, including crystal structure and polymer arrangement. For example, in oxides and cyanides, the contraction upon heating is attributed to the rotational energy of "bridging" atoms or groups.
Nylon is one such plastic that exhibits notable expansion and contraction rates. It has been observed to expand or contract by 0.12 mm per 10°C change in temperature, which is approximately ten times the expansion rate of steel. Acetal is another plastic that behaves similarly to Nylon, with its expansion rate increasing slightly at temperatures over 60°C.
It is important to note that plastics used in shrink-wrapping processes do not exhibit NTE. These plastics are only partially polymerised during manufacturing and become fully polymerised when heat is applied, resulting in irreversible shrinkage.
While plastics with NTE expand when cooled within a specific temperature range, they may not necessarily maintain this behaviour at extremely low temperatures. Very low temperatures can deteriorate the properties of plastics, reducing their impact resistance and causing a temporary loss of elasticity. Therefore, when considering the application of plastics, it is crucial to select the appropriate material based on the expected temperature range and desired mechanical properties.
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Plastics become brittle when cold
Plastic is a versatile material with numerous applications. However, it is susceptible to damage from cold temperatures, which can cause it to become brittle and break. This phenomenon is a result of the restricted movement of plastic's long, chain-like molecules, which impedes their ability to collectively dissipate stress, leading to cracks and fractures.
The ductility of plastics, or the ability of their molecules to stretch and slip past each other, is crucial for their resilience. When exposed to cold temperatures, the molecules in plastics slow down and arrange themselves into a more ordered, crystalline structure. This change diminishes the flexibility of the plastic, making it more prone to cracking.
The chemical structure of plastic significantly influences its cold resistance. Plastics with flexible polymer chains are less likely to become brittle in cold conditions. Additionally, additives such as plasticizers, stabilizers, and impact modifiers can enhance the cold resistance of plastics. Processing conditions, such as temperature and pressure, also play a role in the final product's cold resistance.
While many plastics become brittle in the cold, there are exceptions. For instance, ABS plastic can withstand temperatures as low as -20 °C (-4 °F), and polytetrafluoroethylene can be used at temperatures as low as -240 °C (-400 °F). These cold-resistant plastics are invaluable in industries such as aerospace and automotive, where certain components must endure extreme cold temperatures.
The impact of cold temperatures on plastics highlights the importance of selecting the appropriate plastic for specific applications. By understanding the chemical composition, additives, and processing methods that contribute to a plastic's cold resistance, engineers and designers can ensure the durability and functionality of products and infrastructure in cold environments.
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Plastics can deform when heated
While the effects of temperature on plastics are typically observed at high heat levels, excessively low temperatures can also have an impact. Prolonged exposure to heat while under a load or force can cause plastic to deform or "creep" over time. Most thermoplastic materials have a heat distortion temperature (HDT) of less than 500 degrees Fahrenheit. As the temperature increases, the material's stiffness decreases, and it begins to soften and lose its stiffness. If the plastic is heated for long enough or exceeds its operational temperature range, it will start to distort.
The impact of heat on plastics is particularly evident in everyday scenarios, such as when microwaving food in plastic containers. The plastic may soften and lose its stiffness, and if heated excessively, it can become unrecognizable from its original form. This highlights the importance of selecting the appropriate plastic thermoforming material for specific applications, ensuring it can withstand the intended temperature range.
The deformation of plastics due to heat is also observed in processes like shrink wrapping. Certain plastics used in shrink wrapping do not expand back to their original size when cooled. These plastics undergo partial polymerization, becoming fully polymerized when heated, resulting in irreversible shrinkage.
Additionally, the mechanical properties, chemical resistance, electrical conductivity, and material fatigue of plastics can be influenced by increased temperatures. Exceeding the approximate heat deflection temperature of a material can lead to distortion. The specific temperature thresholds and performance vary across different plastic materials, and factors like part geometry and material thickness further affect their behavior under extreme temperatures.
It is worth noting that the phenomenon of negative thermal expansion (NTE) exists, where some plastics expand when cooled. This behavior is related to the geometry of molecules and is not limited to a specific type of plastic. However, not all plastics exhibit NTE, and the presence of this property does not serve as a means of identification for a particular plastic.
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Plastics with ductility can resist breaking
Plastics can either expand or contract when exposed to cold temperatures, depending on the type of plastic. Some plastics, such as those used in shrink-wrapping, do not expand back to their original size when cooled. This is because they become fully polymerised when heat is applied and this process is not reversible.
While plastics can be designed to withstand cold temperatures, they are still susceptible to deterioration when exposed to extreme cold. The cold can cause a significant decrease in impact resistance and a temporary loss of elasticity in polymers, making them harder and more brittle, and therefore more likely to break.
To prevent plastics from breaking in cold temperatures, certain plastics with ductile properties can be used. Ductility is a measure of the degree of plastic deformation that has been sustained at the point of fracture. Ductile materials can be stretched and exhibit a lot of plastic deformation before breaking. This is because ductile materials can absorb more energy prior to failure than brittle materials.
Polymers are generally considered ductile materials as they allow for plastic deformation. The plastic deformation of ductile materials is important as it can indicate the potential failure of the material. The point at which a material exhibits ductile behaviour versus brittle behaviour depends on the material and the temperature. The minimum temperature at which a material transitions from brittle to ductile behaviour is known as the ductile-brittle transition temperature (DBTT).
Some plastics formulated for cold temperatures include ABS, which performs well at temperatures as low as -20°C, and polyetheretherketone (PEEK), a thermoplastic used for applications requiring superior mechanical properties. Ultra-high-molecular-weight polyethylene (UHMW) is another plastic that can tolerate cold temperatures and is often used in abrasion-resistant liners.
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Frequently asked questions
Most plastics expand when cooled. This phenomenon is known as negative thermal expansion (NTE). However, plastics used in shrink-wrapping do not expand back to their original size when cooled.
The cause of NTE differs for different materials but is almost always related to the geometry of molecules (crystal structure or polymer arrangement). In oxides and cyanides, the contraction upon heating is attributed to the rotational energy of "bridging" atoms/groups.
The cold can quickly deteriorate the properties of plastics, causing a significant decrease in impact resistance. Polymers become harder and more brittle, making them more likely to crack or break.
