Maya Ellison
Plastic is sensitive to temperature changes, and its properties are affected by both high and low temperatures. When exposed to high temperatures, plastics can soften and lose stiffness, leading to distortion if exposed for prolonged periods. The rate of deformation depends on the temperature and time of exposure, with higher temperatures causing faster degradation. Additionally, plastics expand as temperatures rise, which can induce stress when mated with materials that have different thermal expansion rates, such as metal. On the other hand, freezing temperatures cause plastics to harden and become more brittle, increasing the risk of fracture. Understanding the effects of temperature on plastics is crucial, especially considering their widespread use in various applications, from manufacturing to food packaging.
| Characteristics | Values |
|---|---|
| Plastic loses its stiffness | As the temperature increases, plastic loses its stiffness and becomes softer. |
| Plastic deforms or "creeps" | Prolonged exposure to heat while under a load or force can cause plastic to deform or "creep" over time. |
| Expansion | Plastic expands as temperature increases. The expansion rate of some plastics, such as Nylon and Acetal, increases slightly at temperatures over 60°C. |
| Distortion | Exceeding the heat deflection temperature of a plastic material can cause it to distort. |
| Thermal degradation | Plastic exposed to high temperatures for prolonged periods will lose strength and toughness, becoming more prone to cracking, chipping, and breaking. |
| Polymer degradation | Polymers degrade over time due to factors like heat, light, moisture, and chemicals. According to the Arrhenius law, a 10-degree temperature rise doubles the degradation rate. |
| Plasticizer effect | At freezing temperatures, plastic becomes more brittle. Plasticizers can be used to reduce molecular changes and increase flexibility and elasticity in cold conditions. |
| Brittleness | Low temperatures cause plastics to harden and become more brittle, similar to glass. |
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What You'll Learn
Plastic softens and loses stiffness
When plastic is subjected to an increase in temperature, it softens and loses stiffness. This is because, as with most materials, plastic expands as its temperature increases. This is known as the coefficient of thermal expansion (CTE). The rate of expansion varies depending on the type of plastic. For example, nylon expands and contracts at ten times the rate of steel. A 100mm nylon 6 rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease.
The softening of plastic with increasing temperature is also referred to as its "softening temperature". As the temperature increases, the plastic loses its load-bearing properties. Between room temperature and the onset of the glass transition, the modulus of PC declines by approximately 20% between room temperature and 135°C.
The heat distortion temperature (HDT) of most thermoplastic materials is less than 500°F. Prolonged exposure to heat while subjected to a load or force can cause plastic to deform or "creep" over time. If the dimensional change of the plastic due to expansion is obstructed, excessive tensile, shear, or compressive stress loads can be induced in the plastic, resulting in unexpected failure.
Plastics exposed to high temperatures will eventually lose their strength and toughness. Materials exposed to higher heat for longer durations will wear down much faster than those exposed to moderate temperatures. This is known as thermal degradation. The Continuous Use Temperature Rating predicts the temperature at which a 50% loss of the original mechanical properties will occur after 100,000 hours of continuous exposure at that temperature.
In summary, as the temperature of plastic increases, it softens and loses stiffness, which can lead to deformation and a loss of strength and toughness.
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Plastic becomes more prone to cracking and chipping
Plastic materials are susceptible to damage from both high and low temperatures. When exposed to high temperatures, plastics lose their strength and toughness, becoming more prone to cracking, chipping, and breaking. This degradation occurs at a rate proportional to the temperature and time of exposure. Plastics subjected to higher temperatures for extended periods will wear down significantly faster than those exposed to moderate temperatures for shorter durations.
The vulnerability of plastics to high temperatures is exemplified by the common experience of heating food in plastic containers. The plastic softens and loses stiffness, and if heated beyond its operational temperature range, it may distort or become unrecognizable. This phenomenon underscores the necessity of selecting plastics with suitable temperature properties for specific applications.
The impact of high temperatures on plastics can be quantified through thermal aging tests, which predict the temperature at which a plastic material will experience a 50% loss of its original mechanical properties after prolonged exposure. Additionally, the Arrhenius law states that the degradation rate of plastics doubles with every 10-degree increase in temperature.
While the effects of high temperatures on plastics are well-known, low temperatures can also significantly impact their structure and function. At freezing temperatures (0°C or 32°F), plastics undergo molecular changes, becoming more brittle and susceptible to fracture or complete breakdown. This increased brittleness can create issues when the plastic is under strain.
To prevent damage at low temperatures, plasticizers can be used to reduce molecular changes and enhance flexibility and elasticity. Manufacturers can employ various testing methods, such as retraction, crystallization, brittleness, and stiffening tests, to understand how their plastic products perform under extremely cold conditions and make informed decisions about material selection.
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Plastic deformation or creep
In the context of plastics, creep becomes a concern when they are exposed to elevated temperatures for extended periods. As the temperature rises, the polymer chains in plastics gain more kinetic energy, allowing them to move more freely and disentangle. This movement results in a reduction in the material's stiffness, as observed in the apparent modulus of plastic materials. The decrease in stiffness can lead to excessive deformation, rendering the plastic part unusable for its intended purpose.
Creep deformation in plastics can occur at relatively low temperatures, even at room temperature, depending on the specific plastic. This is in contrast to metals, which typically undergo creep deformation at higher temperatures. The onset of creep deformation is influenced by the melting point of the material, with plastics and low-melting-temperature metals beginning to creep at lower temperatures.
The behaviour of creep in plastics can be modelled using the Kelvin-Voigt model, which takes into account the viscoelastic nature of polymers. By understanding the mechanisms behind creep deformation, engineers can make informed decisions about material selection and design for applications where plastics are subjected to high stresses and temperatures.
Additionally, the effects of temperature on plastics are not limited to creep deformation. Prolonged exposure to high temperatures can lead to thermal degradation, causing plastics to lose strength and toughness, becoming more prone to cracking, chipping, and breaking. Similarly, exposure to extremely low temperatures can cause plastics to harden and become more brittle, increasing their susceptibility to fracture. Therefore, it is crucial to consider the temperature range that a plastic will encounter during its intended use and select a suitable material accordingly.
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Plastic loses its load-bearing properties
Plastic materials are used in a wide variety of applications where they are expected to demonstrate reliability over a wide range of temperatures. However, plastics are prone to losing their load-bearing properties when exposed to high temperatures.
When the temperature increases, plastic materials begin to soften and lose their stiffness. This is because, as with most materials, plastic expands as its temperature increases. This is known as the plastic's coefficient of thermal expansion (CTE). If the plastic is mated with another material, such as metal, which has a different thermal expansion rate, the dimensional change can be obstructed, inducing stresses in the plastic due to excessive tensile, shear, or compressive stress loads. This can result in deformation or "creep".
The rate of deformation is dependent on the time of exposure to high temperatures, with materials exposed to higher heat for longer durations wearing down substantially faster. This is known as thermal degradation, where the plastic becomes more prone to cracking, chipping, and breaking.
The exact temperature thresholds and performance will vary for each different plastic material. For example, a 100mm nylon 6 rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease. Nylon has a heat distortion temperature (HDT) of less than 500°F, while semi-crystalline nylon 6 has a different behavior to amorphous PC, with its amorphous regions becoming mobile as the temperature increases.
Additionally, while the effects of high temperatures on plastic are well-known, low temperatures can also impact the structural integrity of plastic. At freezing temperatures, plastics become more vulnerable to damage or breaking down completely as they harden and become more brittle. This increases the risk of fracture or breakage, especially if the plastic is under strain.
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Plastic becomes more brittle at low temperatures
Plastic materials are used in a wide range of applications, and their reliability is expected over a wide range of temperatures. While the effects of high temperatures on plastics are well-known, low temperatures can also significantly impact their structure and function.
At room temperature, most plastics are semi-flexible and have a low failure rate under stress. However, when exposed to extremely low temperatures, plastics tend to harden and become more brittle. This change in plasticity is similar to what happens to glass at low temperatures. The increased brittleness of plastics at low temperatures raises the risk of fracture or breakage, especially if the plastic is under strain.
The impact of low temperatures on plastics is essential to understand, especially in modern manufacturing and construction, where plastics are increasingly used. To prevent damage during extreme cold, plasticizers can be added to reduce molecular changes and increase flexibility and elasticity.
Additionally, cold temperatures can cause dimensional changes in plastic components, affecting their wear behaviour, friction, and mechanical properties. Manufacturers can use various testing methods, such as retraction, crystallization, brittleness, and stiffening tests, to understand how their plastic products perform under extremely cold temperatures and choose the right plastic for specific applications.
While the focus here is on the effects of low temperatures, it is worth noting that prolonged exposure to high temperatures can also cause plastics to distort, lose strength and toughness, and become more prone to cracking, chipping, and breaking. The exact temperature thresholds and performance will vary for each plastic material, and factors like part geometry and material thickness will influence their behaviour under extreme temperatures.
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Frequently asked questions
Plastic materials exposed to high temperatures will lose strength and toughness, becoming more prone to cracking, chipping, and breaking. They will also expand, soften, and lose stiffness.
Prolonged exposure to high temperatures will cause plastic to wear down faster. It will also distort and deform or "creep".
Time-dependent changes in mechanical properties, such as creep (slow deformation) and stress relaxation (decrease in stress response under sustained deformation), will accelerate with an increase in temperature. Rising temperatures also negatively affect other important properties, such as gas and water vapour barrier properties in food packaging, which is essential for food preservation.
At low temperatures, plastics undergo significant changes in structure and function. They tend to harden and become more brittle, increasing the risk of fracture or breakage. Cold temperatures can also cause a change in the dimensions of a plastic component, which then changes its wear behaviour, friction, and overall mechanical properties.
At freezing temperatures, plastics become much more vulnerable to damage or breaking down completely. Their molecular levels will change, making them more brittle.
