Maya Ellison
Plastic is a versatile material with a wide range of applications, from consumer goods to healthcare. However, its properties are greatly affected by temperature. When heated, plastic typically softens and becomes more pliable, which aids in shaping and cutting. Yet, extreme heat causes plastics to lose their strength and toughness, leading to deformation and melting. Interestingly, despite expanding when heated, heated plastic sheets often appear to contract, curling in on themselves due to the release of internal stresses. Conversely, freezing temperatures also negatively impact plastic, making it more brittle and prone to damage. Thus, understanding the effects of temperature on plastics is crucial for their effective use and preservation.
| Characteristics | Values |
|---|---|
| Effect of heat on plastic | Plastics tend to expand when heated but contract when cooled |
| Heat disrupts the orientation of polymer chains in plastic, causing them to relax into a low-energy state, resulting in shrinkage | |
| Heat treatment of plastics can prolong their lifespan and relieve internal stresses | |
| Impact of temperature on plastic | Constant exposure to high temperatures causes plastic to lose strength and toughness |
| Extreme cold temperatures make plastic vulnerable to damage or breakdown | |
| Different types of plastics, like thermoplastics and thermoset plastics, behave differently when heated |
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What You'll Learn
Plastic sheets contract when heated
Plastic is a versatile material with varying characteristics depending on its type. Some plastics are durable and can withstand heavy impact, while others are more flexible and less prone to breaking. However, one factor that can significantly impact plastic is temperature, especially heat.
When plastic sheets are heated, they tend to contract, which might seem contradictory to the behaviour of most other materials. This contraction occurs due to the unique way plastic sheets are produced. During manufacturing, plastic sheets are rapidly cooled, which freezes the polymer chains in an oriented, stretched state. This rapid cooling process enhances the toughness of the plastic sheet.
However, when these plastic sheets are heated above their glass transition temperature, the polymer chains are no longer locked in that high-strain orientation. They begin to relax and shift into a low-energy orientation, resulting in the plastic sheet shrinking. This shrinkage is not due to a decrease in volume but rather a reconfiguration of the plastic's shape. The plastic goes from a thin sheet to a thicker, more compact form.
Additionally, the difference in thermal expansion between the two sides of the plastic sheet creates internal stress. As the temperature increases, the plastic softens, and when it can no longer withstand the internal stress, it bends. This bending contributes to the overall contraction of the plastic sheet.
It's important to note that different types of plastics may respond differently to heat. For example, thermoplastic sheets, like phenolic, often burn when heated instead of contracting. Understanding the specific characteristics of the plastic type is crucial to predicting its behaviour when exposed to heat.
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Heat treating plastic
Heat treatment is a series of controlled heating and cooling processes used to alter the physical or chemical properties of various materials. Heat treating plastic is necessary for most industries that work with polymers and similar synthetic materials.
Normalizing
Normalizing is a type of heat treatment that gives plastic a higher resistance to internal stresses and makes a uniform structure. Industries use normalizing to get a calculable microstructure. Once this stage ends, it enhances the plastic’s mechanical features, meaning the polymer can resist stronger impacts. However, the plastic can shrink during the normalizing process by up to 4%.
Annealing
Annealing is the second type of heat treatment procedure. It is mostly used to change the properties of metals, but glass and plastic can also undergo the annealing process to improve their physical and chemical properties. Once the annealing process is done, the polymer and other synthetic materials are more malleable and have improved ductility and wear resistance. Annealing is required in all types of plastic moulding.
Heat Treatment Applications
Heat treatment of plastics is a cheap way to produce resistant plastics. Heat treatment can prolong the lifespan of plastics and relieve internal stresses caused by manufacturing methods. The heat softens the ingredients so that the plastic can be shaped or cut more easily. It also improves the material’s strength and prepares it for further heat treatments.
Limitations
Plastics that are constantly exposed to high temperatures will eventually lose their strength and toughness. Materials exposed to high heat for extended periods will wear down much quicker than those exposed to moderate temperatures. This is because plastics are good insulators, so they trap heat, making it more likely to melt or deform a plastic object.
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Plastic toughness and temperature
The toughness of a plastic material determines its ability to resist impact damage and crack propagation in real-world applications. Toughness is distinct from hardness and strength. A tough material can absorb substantial energy through deformation before failing, while a brittle material will shatter upon impact.
Plastic materials are affected by their surroundings, and temperature plays a significant role in their durability. Prolonged exposure to high temperatures will cause plastics to lose their strength and toughness, becoming more prone to cracking, chipping, and breaking. This is known as thermal degradation. The rate of degradation is directly proportional to the temperature and duration of exposure.
Plastics subjected to higher temperatures for longer durations will wear down much faster than those exposed to moderate temperatures. This is because plastics are good insulators, trapping heat and making it more likely to melt or deform. However, it is important to note that the exact temperature thresholds and performance will vary for each type of plastic. Factors such as part geometry and material thickness will also influence the material's properties under extreme temperatures.
On the other hand, exposure to freezing temperatures can also negatively impact plastics. At 0 degrees Celsius or 32 degrees Fahrenheit, plastics become more vulnerable to damage or complete breakdown. Their molecular structure changes, making them more brittle. To prevent this, plasticizers can be used to maintain flexibility and elasticity in cold conditions.
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Plastic's response to extreme temperatures
The effect of extreme temperatures on plastics depends on the type of plastic and the manufacturing process. Generally, plastic tends to expand when heated and contract when cooled. However, plastic has a low melting point and softens easily under heating, so it becomes more liquid than most other materials. This causes the plastic to pull back due to surface tension.
Plastic sheets, for example, are produced through rapid cooling, which keeps the polymer chains oriented in a way that makes the sheets flat. When heated above its glass transition temperature, the polymer chains are no longer locked in this high-strain orientation, and they relax into a low-energy orientation, resulting in a curled and bending form that shrinks the bulk material. This phenomenon is observed in thin plastic sheets, which appear to roll up when heated, although their total volume increases.
Prolonged exposure to high temperatures will eventually cause plastics to break down, wear down, melt, or deform. This is because plastic is a good insulator, trapping heat with no escape, which then affects the plastic itself.
On the other hand, exposure to freezing temperatures (0°C or 32°F) can also negatively impact plastics, making them more vulnerable to damage or complete breakdown. At these low temperatures, the molecular levels of plastic change, causing the material to become brittle. Plasticizers can be used to prevent this damage by reducing molecular changes and increasing flexibility and elasticity.
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How plastic's behaviour changes with heat
The behaviour of plastics changes with heat, and this can be leveraged in various applications. When heated, plastics tend to expand, soften, and become more pliable. This is because the heat disrupts the oriented polymer chains, allowing them to relax into a lower-energy state. However, due to their low melting point, they can quickly go from a solid to a more liquid state, and their total volume may decrease as they reconfigure from a thin sheet to a thick blob. This is why plastics tend to roll up when heated, giving the appearance of shrinking.
The effect of heat on plastics can be intentionally used in processes like heat treating, where ovens are employed to heat polymers to specific temperatures, softening them for easier shaping or cutting. Heat treating can also improve the material's strength and prepare it for subsequent heat treatments. This method is commonly used in industries such as technology, healthcare, and industrial manufacturing.
However, prolonged exposure to high temperatures will cause plastics to break down over time, resulting in a loss of strength and toughness. This is because plastics are good insulators, trapping heat and making them more susceptible to melting or deformation. Therefore, while heat can be beneficial in treating plastics, excessive heat will negatively impact their quality and integrity.
Additionally, the behaviour of plastics when heated can vary depending on their type and manufacturing process. For example, thermoplastic sheets may burn or bend when heated, while thermoset materials like phenolic are not melted during the forming process. The presence of internal stresses, manufacturing methods, and thickness can also influence how plastics behave when exposed to heat.
Understanding the behaviour of plastics under different temperature conditions is crucial for maintaining the integrity of plastic products and ensuring their longevity. While heat can be advantageous in certain applications, it can also accelerate the breakdown of plastics, underscoring the importance of considering the specific characteristics and limitations of different plastic types.
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Frequently asked questions
When plastic is heated, it tends to expand and soften. However, due to its low melting point, it can quickly turn into a more liquid state.
Plastic sheets are produced through rapid cooling, which keeps the polymer chains oriented in a way that makes them flat. When heated, these polymer chains are no longer locked in this high-strain orientation and relax into a low-energy, curled state, resulting in the sheet shrinking.
Extreme heat will cause plastic to wear down and break down over time. The quality of plastic diminishes when exposed to high temperatures, and it will eventually lose its strength and toughness.
Heat treatment can be used to improve the strength of plastic. Heating polymers to a specific temperature softens them, making it easier to shape or cut. This process also relieves internal stresses and prepares the plastic for further heat treatment.
Freezing temperatures cause plastic to become more vulnerable to damage or complete breakdown. The plastic becomes more brittle at the molecular level when exposed to freezing temperatures, increasing the likelihood of breakage.
