Sofia Martinez
Plastic and metal are two very different materials, and their behaviours when exposed to heat vary significantly. While both materials expand when heated, the extent of this expansion differs greatly. Plastic has a much higher coefficient of expansion (LCoE) than metal, with Nylon, for example, expanding and contracting at approximately ten times the rate of steel. This is due to the difference in atomic structure between the two materials. Metals have a crystal lattice structure, while plastics are composed of complex molecules that form chains and more intricate configurations. This difference in structure leads to varying thermal expansion rates, which must be considered when mating plastic with metal to avoid unexpected failures.
| Characteristics | Values |
|---|---|
| Thermal Conductivity | Plastic has a much lower thermal conductivity than metals. |
| Thermal Expansion | Plastic expands as temperature increases. Metals also expand with an increase in temperature. However, plastics like Nylon and Acetal can expand up to ten times more than metals like steel. |
| Thermal Degradation | Plastic materials subjected to prolonged exposure to high temperatures will lose strength and toughness, becoming more prone to cracking, chipping, and breaking. |
| Thermal Expansion Coefficient | All materials have a thermal expansion coefficient. Most materials have a positive coefficient and expand upon heating, while a few materials have a negative coefficient and shrink upon heating. |
| Thermal Expansion in Combination | When plastic is mated with another material, such as metal, with conflicting thermal expansion rates, it can induce stress in the plastic part, leading to unexpected failure. |
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What You'll Learn
- Plastic's coefficient of thermal expansion (CTE) is higher than metal's
- Plastic's thermal conductivity is lower than metal's
- Plastic's molecules may boil off when heated, unlike metal
- Plastic's expansion and contraction rates vary with temperature
- Plastic's expansion and contraction must be considered when mated with metal
Plastic's coefficient of thermal expansion (CTE) is higher than metal's
The coefficient of thermal expansion (CTE) is the degree of expansion divided by the change in temperature. As with most materials, plastic expands as the temperature increases. Plastic materials generally have a much lower thermal conductivity than metals, which means they are good replacement materials when thermal insulation is important.
Different plastics have different CTEs. For example, nylon has a CTE of 90-95, while polyimide has a CTE of 30-60. Acetal, a thermoplastic, has a CTE of 80-120. Acetal is a polyoxymethylene (POM) homopolymer with a high-end temperature of 248°F, while the POM copolymer has a temperature range of -40 to 248°F. Nylon, a polyamide, has a temperature range of -40 to 320°F.
Polypropylene, one of the most popular plastic formulations, has a CTE of 100-180 and a temperature range of -4 to 239°F. UHMW Polyethylene, another thermoplastic, has a CTE of 130-200 and a high-end temperature of 203°F.
Polymethylmethacrylate (acrylic), a highly durable and rigid plastic, has a CTE of 70-77 and a temperature range of -40 to 194°F. This plastic is well-suited for environmental temperature swings.
When selecting a plastic material for a specific application, it is important to consider its CTE and temperature range to ensure it is suitable for the environmental conditions it will be exposed to.
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Plastic's thermal conductivity is lower than metal's
The thermal conductivity of a material is a measure of how well it conducts heat. Metals are well-known for their superior heat transfer capabilities compared to other solid materials. However, plastics generally have a much lower thermal conductivity than metals, making them excellent replacement materials when thermal insulation is important.
A demonstration that illustrates this concept involves placing ice cubes on metal and plastic blocks at room temperature. The ice cube on the metal block melts much more quickly than the one on the plastic block, even though the metal may feel colder to the touch. This occurs because metal is a better conductor of heat energy, so it transfers its heat to the ice cube more quickly, causing it to melt faster. On the other hand, plastics are good insulators, so even though they are at a lower temperature, they do not conduct heat energy as efficiently, and the ice cube melts more slowly.
The thermal conductivity of plastics can be altered by using fillers. For example, very low-k plastics are filled with hollow microspheres, which reduce their thermal conductivity while improving other properties such as compressive strength and dimensional stability. On the other end of the spectrum, ultra-high thermal conductivity polymer composites are being developed using mats of vapor-grown carbon fibers, with reported values of up to 660 W/mK.
While plastics generally have lower thermal conductivity than metals, it's important to note that the thermal expansion rates of these materials can be a critical consideration when they are used together. Plastics expand as temperature increases, and if they are mated with metal, which has a different thermal expansion rate, it can induce stress and lead to unexpected failures.
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Plastic's molecules may boil off when heated, unlike metal
Plastic and metal are two very different materials, each with unique properties that make them suitable for various applications. One key difference between the two is their response to heat. Metals typically have a higher thermal conductivity than plastics, which means they conduct heat more efficiently. This property of metal makes it a preferred choice for applications where heat needs to be transferred or dissipated quickly, such as in cookware or heat sinks.
Plastics, on the other hand, have interesting behaviours when subjected to heat. Like most materials, plastic expands as the temperature increases, a property known as the coefficient of thermal expansion (CTE). This expansion can become an issue when plastic is used in conjunction with other materials, such as metal, that have different CTEs. If the plastic's expansion is constrained by the adjacent material, it can lead to excessive stresses and unexpected failures.
The unique molecular structure of plastics also comes into play when heated. Plastics, particularly thermoplastics, consist of intermolecular bonds that stretch and loosen when heated, increasing the distance between the molecules and making the material softer and more pliable. With sufficient heat, the plastic can even melt into a liquid state without undergoing degradation or compositional changes. This is in contrast to metals, which typically have much higher melting points and do not exhibit the same degree of molecular mobility as plastics when heated.
Additionally, it is important to consider the potential health risks associated with heating plastics. Certain plastics, such as polycarbonate bottles, may contain chemicals like bisphenol A (BPA). When exposed to high temperatures, these plastics can release BPA up to 55 times faster than at normal temperatures. BPA is a known endocrine disruptor and has been linked to various health issues, including developmental problems, cancer, and diabetes. Therefore, it is crucial to be cautious when heating plastics, especially those used for food and beverages.
In summary, plastics and metals exhibit distinct behaviours when heated. Plastics have lower thermal conductivity, expand with increasing temperatures, and their molecules stretch and loosen without breaking. Metals, on the other hand, typically conduct heat more efficiently and have different expansion rates. Understanding these differences is essential for selecting the appropriate material for specific applications, ensuring safety, and avoiding potential failures.
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Plastic's expansion and contraction rates vary with temperature
Like most materials, plastic expands as the temperature increases, a phenomenon known as the coefficient of thermal expansion (CTE). This expansion can cause issues when plastic is paired with another material, such as metal, that has a different thermal expansion rate. If the plastic's expansion is obstructed, it can lead to excessive stress and unexpected failure. For instance, nylon expands and contracts at about ten times the rate of steel. The rate of expansion in nylon is approximately 0.12 mm per 10°C of temperature increase or decrease.
The expansion and contraction rates of plastics are crucial considerations in product development. Manufacturers must account for the environmental temperature range the plastic will be exposed to and select appropriate plastic thermoforming materials. A low temperature for an extended period can cause similar damage to a high temperature for a brief period. Therefore, it is essential to consider the time/temperature relationship and the projected service life of the application.
Additionally, plastic materials subjected to prolonged exposure to high temperatures will lose strength and toughness due to thermal degradation. They become more susceptible to cracking, chipping, and breaking, with the rate of degradation proportional to the temperature and exposure time. Higher temperatures and longer exposure times accelerate the deterioration of plastic properties.
It is worth noting that not all plastics behave the same way. Some plastics, like Nylon and Acetal, exhibit an increase in their expansion rate at temperatures above 60°C. This behaviour is known as negative thermal expansion (NTE) and is observed in other materials beyond plastics and polymers, including ceramics and graphite. The cause of NTE can vary, but it is often related to the geometry of molecules or the formation of bonds during cooling.
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Plastic's expansion and contraction must be considered when mated with metal
Like most materials, plastic expands as the temperature increases. This is due to the plastic's coefficient of thermal expansion (CTE). When plastic is mated with another material, such as metal, which has a different thermal expansion rate, it can cause issues. If the plastic's dimensional change is obstructed, it can induce stress in the plastic part, leading to potential failure.
Plastics generally have a much lower thermal conductivity than metals, which means they are excellent replacement materials when thermal insulation is a priority. However, when plastics are used in conjunction with metals, the difference in expansion rates must be considered. For example, Nylon expands and contracts at approximately ten times the rate of steel. A similar case can be observed with UPVC, which has a linear coefficient of expansion (LCoE) almost five times that of Mild Steel.
When designing products that utilize both plastic and metal components, it is crucial to account for the varying expansion rates. This consideration is especially important in applications where the materials are joined or mated together. By understanding the expected temperature range and the resulting dimensional changes, engineers can ensure that the plastic has room to expand and contract without obstruction.
Additionally, the time of exposure to high temperatures also plays a significant role. Prolonged exposure to elevated temperatures can lead to thermal degradation of plastics, causing them to lose strength and toughness. As a result, they become more susceptible to cracking, chipping, and breaking. Therefore, when mating plastics with metals, it is essential to select compatible materials and design the mating interface in a way that accommodates the differential expansion and prevents excessive stress on the plastic components.
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Frequently asked questions
Yes, plastic has a much higher coefficient of expansion (LCoE) than metal. For example, UPVC expands almost 5 times as much as Mild Steel. Nylon expands and contracts at 10 times the rate of steel.
All materials have a "thermal expansion coefficient". Most materials have a positive coefficient, meaning they expand upon heating. However, some materials have a negative coefficient, meaning they shrink upon heating. Plastic has a lower thermal conductivity than metals, and its molecules can rearrange upon heating, which can sometimes result in a reduction in volume.
Plastic pipes should be clipped or supported differently from metal pipes. Plastic pipes should be gently held or supported so that they can move freely through the pipe clip when they expand or contract.
