Ethan Clarke
Plastic is a versatile material used in a wide range of applications. Its properties can be significantly affected by temperature. When discussing whether plastic expands or contracts when exposed to cold temperatures, it is essential to distinguish between different types of plastics and their specific behaviours. Some plastics, such as Nylon and Acetal, exhibit notable expansion and contraction rates with temperature changes, while others may demonstrate negative thermal expansion (NTE), a phenomenon where they expand when cooled and shrink when heated. This behaviour is attributed to the unique molecular structure of these materials. Additionally, the cold can deteriorate the properties of some plastics, reducing their impact resistance and causing a temporary loss of elasticity, making them more brittle and prone to cracking or breaking. Understanding the thermal behaviour of plastics is crucial for selecting the appropriate material for specific applications, especially in extreme environments.
| Characteristics | Values |
|---|---|
| Expansion or Contraction in Cold | Some plastics exhibit negative thermal expansion (NTE) and expand when cooled. However, plastics used in shrink-wrapping do not expand back to their original size when cooled. |
| Factors Affecting Expansion or Contraction | The phenomenon of NTE is related to the geometry of molecules (crystal structure or polymer arrangement). |
| Types of Plastics Exhibiting NTE | Nylon, Acetal, and some unnamed plastics. |
| Expansion Rate | Nylon expands and contracts at ten times the rate of steel. Nylon 6 expands or contracts by 0.12 mm per 10°C of temperature increase or decrease. |
| Impact of Cold on Plastics | The cold can deteriorate the properties of plastics, reducing impact resistance and causing a temporary loss of elasticity, making them more brittle and prone to cracking or breaking. |
| Plastics Formulated for Cold Temperatures | ABS performs well in temperatures as low as -20°C. Polytetrafluoroethylene (PTFE) can be used at temperatures as low as -240°C. Polyetheretherketone (PEEK) is a thermoplastic with superior mechanical properties, high tensile strength, and resistance to low temperatures. Ultra-high-molecular-weight polyethylene (UHMW) is a high-density material resistant to cold temperatures and friction. TIVAR 88, a type of UHMW, is a plastic that can tolerate cold temperatures and is used in abrasion-resistant liners. |
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What You'll Learn
Some plastics expand when cooled
While most plastics contract when heated, there are some plastics that exhibit the opposite behaviour, expanding when cooled. This phenomenon is known as negative thermal expansion (NTE), and it is not limited to plastics but is also observed in other materials such as ceramics, oxides, cyanides, and graphite. The cause of NTE varies across different materials but is generally associated with the molecular structure, particularly the geometry of the molecules and their crystal or polymer arrangement.
Nylon, for example, has been observed to expand and contract at a rate roughly ten times that of steel. Its expansion rate can be calculated as 0.12 mm per 10°C change in temperature. This means that a 100 mm nylon rod will expand or contract by 0.12 mm for every 10°C increase or decrease in temperature. Similarly, acetal exhibits a slight increase in its expansion rate at temperatures exceeding 60°C.
The occurrence of NTE in plastics can be influenced by factors such as porosity, moisture content, and the presence of other materials. For instance, a dry, nonporous block of plastic may exhibit different expansion behaviour compared to a porous or wet plastic specimen. Additionally, plastics with glass-filled grades, such as the glass-filled (GF) grades, have been found to exhibit a significant reduction in contraction and expansion tendencies, with a decrease of up to 50% compared to Tufnol.
It is important to note that the expansion behaviour of plastics is not limited to a specific type of plastic. Various plastics have been specifically engineered to withstand extreme environments, including low temperatures. For example, ABS plastic can perform well in temperatures as low as -20°C, while polytetrafluoroethylene (PTFE) can be used at temperatures as low as -240°C, making it suitable for medical, pharmaceutical, and food industry applications. Ultra-high-molecular-weight polyethylene (UHMW), another plastic designed for cold temperatures, is valued for its resistance to friction and low dynamic coefficient of friction.
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Plastics with a negative thermal expansion (NTE) coefficient expand when cooled
Most plastics contract when exposed to cold temperatures, causing a temporary loss of elasticity in polymers. They become harder and more brittle, making them more susceptible to cracking or breaking under pressure. However, some plastics exhibit a unique characteristic known as Negative Thermal Expansion (NTE), where they expand when cooled within a specific temperature range. This phenomenon is observed in various materials, including certain polymers and ceramics.
NTE is an unusual physicochemical process where materials contract upon heating instead of expanding, as is typical. The cause of NTE varies among materials but is often related to their molecular geometry and crystal structure. For example, in oxides and cyanides, the contraction upon heating is attributed to the rotational energy of "bridging" atoms or groups. Water is a well-known example of a substance with NTE, exhibiting this behaviour at temperatures between 0 and 3.98 degrees Celsius.
In the context of plastics, certain polymeric materials used in 3D printing have demonstrated NTE. These polymers include acrylonitrile butadiene styrene, polyethylene terephthalate, thermoplastic polyurethane, and polylactic acid. During testing, these polymers were subjected to heating and cooling cycles, and their thermal expansion and contraction were measured. The results indicated that these polymers exhibited NTE, with their thermal expansion coefficients varying with temperature.
While NTE plastics expand when cooled within a specific temperature range, it is important to note that they may not return to their original size after being heated. This behaviour is observed in plastics used for shrink-wrapping processes, which become fully polymerised when heated and do not revert to their original form upon cooling.
Overall, NTE plastics offer unique advantages and find applications in various fields, including engineering, photonics, electronics, and structural design. By combining NTE materials with traditional materials that exhibit positive thermal expansion, it is possible to create composites with tailored thermal expansion properties, making them valuable in demanding thermal stability applications.
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Plastics deteriorate in cold temperatures
Plastics are increasingly becoming part of modern manufacturing and construction. Therefore, it is important to understand the effects of low temperatures on plastics.
The glass transition happens over various temperatures, so chemists cannot rate this change with one particular temperature. However, the glass transition premature threshold, the lowest temperature in that range, is the defined low temperature for that particular plastic. After that point, the impact resistance of the plastic lowers while the failure rate due to cracking and breaking increases.
To choose the best plastic for a particular application, manufacturers must know how it performs at extreme temperatures, including low temperatures. With the wide range of plastic products available, each one behaves differently under these stressful situations, and that’s why testing is vital. Manufacturers often use ultra-low deep freezers to test plastics at low temperatures.
Some plastics have been specifically developed for use in cold temperatures, where thermal stresses are significant. For example, TIVAR® 88 is used in numerous industries and is prized for its excellent mechanical properties and resistance to various stresses. It can be used at temperatures as low as -200 °C (-328 °F) without losing its properties.
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Plastics used in extreme environments
When exposed to cold temperatures, plastics tend to lose elasticity and become harder and more brittle, making them more susceptible to cracking or breaking. This poses challenges in various applications, especially in extreme environments. To address these challenges, several plastics have been specifically designed to withstand harsh conditions, including extreme temperatures and corrosive substances.
One such plastic is ABS, which maintains its performance in temperatures as low as -20 °C (-4 °F). Polyetheretherketone (PEEK) is another advanced thermoplastic that excels in low-temperature (cryogenic) applications. It boasts superior mechanical properties, including high tensile strength, and offers resistance to high temperatures, chemicals, and abrasion. PEEK is commonly used in bearings, seals, and thermal insulation in cryogenic settings.
For even more demanding applications, polytetrafluoroethylene (PTFE), a fluoropolymer, offers exceptional chemical and thermal resistance. PTFE can withstand temperatures as low as -240 °C (-400 °F), making it suitable for use in the medical, pharmaceutical, and food industries. It is often used in its virgin form but can also be enhanced with fibreglass additives to improve its mechanical properties.
In addition to these materials, high-performance thermoplastics like Ultem®, Radel® R, and DuPont™ Vespel® are well-suited for extreme temperature conditions. When selecting plastics for such environments, it is crucial to consider factors such as strength, thermal expansion, creep strain, and stress relaxation. The operating temperature range and the specific environmental demands play a significant role in choosing the most suitable plastic for the application.
Designers and engineers can employ several strategies to enhance the resilience of plastic products in harsh environments. Increasing the wall thickness of plastic components, for example, improves rigidity and strength, making the product more impact-resistant, especially at lower temperatures. Additionally, incorporating ribs into the design can enhance rigidity and reduce flexing under compression and impact. Other considerations include avoiding adhesives, which can fail over time due to flexing and thermal fluctuations, and opting for alternative fastening methods like screws, bolts, or cable ties.
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Nylon and Acetal plastics expand at temperatures over 60°C
Most plastics expand and contract when heated or cooled. However, 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.
Nylon and Acetal are two plastics that exhibit a unique behaviour when it comes to temperature changes. While most materials expand uniformly in all directions when heated, Nylon and Acetal have been observed to expand slightly more at temperatures over 60°C. This phenomenon is known as negative thermal expansion (NTE) and is also seen in certain other materials, including ceramics, oxides, and graphite.
Nylon, a type of polyamide, has a coefficient of thermal expansion ranging from 90 to 95. This means that for every 10°C change in temperature, its size can change by up to 0.12mm per 100mm of length. For example, a 1000mm-long piece of nylon could increase in length by 1.2mm for every 10°C increase in temperature.
Acetal, on the other hand, is a polymer known for its strength, stiffness, and low coefficient of friction. While its exact coefficient of thermal expansion is not readily available, it is known that Acetal can withstand temperatures up to 82°C before reaching its maximum working temperature.
It is important to note that the expansion rates of Nylon and Acetal are not typically a cause for concern for most applications. However, understanding the unique thermal properties of different plastics is crucial for optimising their functionality and longevity in various applications, from engineering to packaging.
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Frequently asked questions
Most plastics tend to expand when heated and contract when cooled. However, some plastics exhibit negative thermal expansion (NTE), meaning they expand when cooled and contract when heated.
Negative thermal expansion (NTE) is a phenomenon where materials increase in size as they are cooled down. This is caused by the geometry of molecules and is seen in materials such as ceramics, oxides, cyanides, graphite, and some plastics.
Extremely low temperatures can quickly deteriorate the properties of plastics, causing a loss of elasticity and an increase in brittleness, making them more susceptible to cracking or breaking.
Yes, some plastics are specifically designed for low-temperature environments. For example, ABS plastic can withstand temperatures as low as -20 °C, while Polytetrafluoroethylene (PTFE) can be used at temperatures down to -240 °C.
