Ethan Clarke
Plastic is known to contract when heated, which is contradictory to the behavior of most other materials. This phenomenon is called negative thermal expansion (NTE) and is observed in other materials like ceramics, oxides, and cyanides. The contraction occurs because the polymer chains in the plastic, which are oriented in a high-strain manner during the manufacturing process, relax into a low-energy state when heated above their glass transition temperature. This causes the plastic to roll up and appear shrunk, even though its total volume is increasing. Different types of plastics, such as thermoplastics and thermosets, may exhibit varying behaviors when subjected to heat due to differences in their melting points and methods of formation. Understanding the thermal behavior of plastics is crucial when selecting materials for specific applications, especially when mated with other materials like metals, to ensure compatibility and avoid potential issues caused by conflicting thermal expansion rates.
| Characteristics | Values |
|---|---|
| Plastic contracts when heated due to | The unpolymerized material being in a local energy valley. Heating allows it to jump to the polymerized state, where there is more bonding between atoms, resulting in their wanting to be closer to each other. |
| The disruption of the rapid cooling process during production, which keeps polymer chains oriented in a way that makes the sheets flat. | |
| The melting of nanocystallites, which crosslink the sheet, stabilising it. | |
| The difference in thermal expansion creating internal stress. | |
| The material's transition to a liquid state, where surface tension makes it tend towards a spherical shape. | |
| The material's low flexural modulus, making it soft and unable to hold internal stress. | |
| The material's low thermal conductivity, which makes it prone to thermal degradation, causing it to lose strength and toughness and become more susceptible to cracking, chipping, and breaking. |
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What You'll Learn
Plastic contracts due to surface tension
When plastic sheets are produced, they are rapidly cooled to keep the polymer chains oriented in a way that makes the sheets nice and flat. This is a relatively high-strain orientation as it is associated with the energy level of the molecules at the casting temperature. Once the plastic is heated above its glass transition temperature, the polymer chains are no longer locked in that high-strain orientation. They relax to a low-energy orientation, curling and bending in a way that shrinks the bulk material.
The phenomenon of plastic contraction when heated can be explained by the concept of surface tension. Surface tension is the tendency of a liquid's surface to act like a flexible membrane, which is caused by the attraction of molecules at the surface to each other due to intermolecular forces. When plastic is heated, its surface tension increases, and it tends to contract and form a sphere. This is because a sphere has the smallest surface area to volume ratio, which minimizes the surface energy of the liquid. The surface tension of plastics can be modified to optimize adhesion, printing, or other applications. For example, in the automobile industry and packaging, paints and adhesives are formulated with surfactants that reduce surface tension and improve wetting.
Different types of plastics may behave differently when heated. For example, thermoplastic and thermoset plastics have different responses to heat. Thermoplastic sheets are made by melting the plastic and can be further classified into amorphous and semi-crystalline types. Amorphous materials do not have a sharp melting point, while semi-crystalline materials do. Thermoset materials, on the other hand, are not melted to form and may burn when heated.
The shape and thickness of plastic objects also play a role in how they contract when heated. Most plastics are in thin sheets, which makes them more likely to "roll up" when heated. However, their total volume does not decrease; instead, they reconfigure from a thin sheet to a thick blob. This is similar to what happens when aluminium foil is heated until it melts.
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Polymer chains relax when heated
Plastic is a polymer, and polymers are formed of long chains of molecules. When plastic sheets are produced, they are rapidly cooled to keep the polymer chains oriented in a way that makes the sheets nice and flat. This is a relatively high-strain orientation since it is associated with the energy level of the molecules at the casting temperature.
Once the plastic is heated above its glass transition temperature, the polymer chains are no longer locked in that high-strain orientation. They relax to a low-energy orientation, curling and bending in a way that shrinks the bulk material. The plastic gets more liquid-like and surface tension tries to pull it back into a minimum surface area. The precise mechanism is not known, but it is thought that the shrunk conformation is entropically favourable because there are more arbitrary bends. This would decrease the Gibbs free energy, making it a more stable shape.
Different types of plastics may behave differently when heated. For example, thermoplastic and thermoset plastic. Thermoplastic sheet is made by melting the plastic. It further classifies into amorphous and semi-crystalline types. Amorphous material doesn't have a sharp melting point. Semi-crystalline material has a sharp melting point. Thermoset material like phenolic is not melted to form.
The phenomenon of plastic shrinking when heated is known as negative thermal expansion (NTE). This is observed in other materials besides plastics/polymers, such as many ceramic oxides, phosphates, and cyanides. The cause of the phenomenon is different for different materials, but it is almost always related to the geometry of the molecules (crystal structure or polymer arrangement). In oxides and cyanides, the contraction upon heating is often attributed to the rotational energy of "bridging" atoms/groups.
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Different plastics behave differently
The behaviour of thermoplastic amorphous materials, such as ABS, acrylic, PVC, and polycarbonate, depends on how they are heated. Uneven heating, with one side being hotter than the other, results in varying thermal expansion rates, creating internal stress within the material. This stress can lead to bending or deformation. Additionally, the thickness of the material and the manufacturing process also influence how these plastics respond to heat.
Thermoplastic semi-crystalline materials, like Polyethylene, have distinct characteristics. Polyethylene is a soft material with a low flexural modulus, indicating minimal internal stress. When heated close to its melting point, it turns into a liquid state. At this point, surface tension comes into play, causing the material to contract and form a sphere. This behaviour is attributed to the material's tendency to minimise its surface area and adopt a shape that reduces surface energy.
Furthermore, the method of production can also influence the behaviour of plastics when heated. Many plastic sheets are produced through rapid cooling, which locks the polymer chains in a high-strain orientation, resulting in flat sheets. When heated above their glass transition temperature, these polymer chains are released from their constrained state and rearrange into a low-energy configuration, often resulting in curling or bending. This transformation leads to a reduction in the overall volume of the plastic, giving the appearance of contraction.
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Plastic's internal stress causes bending
When plastic sheets are produced, they are rapidly cooled to keep the polymer chains oriented in a way that makes the sheets nice and flat. This is a relatively high-strain orientation since it is associated with the energy level of the molecules at the casting temperature. Once the plastic is heated above its glass transition temperature, the polymer chains are no longer locked in that high-strain orientation. They relax to a low-energy orientation, curling and bending in a way that shrinks the bulk material.
The difference in thermal expansion creates an internal stress. When the temperature gets higher, the plastic gets softer. Until the plastic structure is weakened to a point that it cannot hold the stress, the plastic sheet bends. Another scenario is that the internal stress is already built up during manufacture. It depends on the manufacturing process, the material used, and the thickness. When the plastic is heated evenly, the material softens. When the plastic sheet is so soft that it cannot hold the internal stress, it bends.
Plastic bending is a nonlinear behaviour particular to members made of ductile materials that frequently achieve much greater ultimate bending strength than indicated by a linear elastic bending analysis. In both plastic and elastic bending analyses of a straight beam, it is assumed that the strain distribution is linear about the neutral axis (plane sections remain plane). In an elastic analysis, this assumption leads to a linear stress distribution, but in a plastic analysis, the resulting stress distribution is nonlinear and is dependent on the beam's material.
For a given material and working stress, it can be seen that the load factor is greater than the normal factor of safety used in elastic design. It considers that failure occurs at first yield. It can also be seen that different load factors will be obtained from, say, rectangular and I-sections, even under the same system of loading. Alternatively, by basing the design on a constant load factor, the working stress may be varied to suit the particular section.
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Plastic's geometry changes with heat
The geometry of plastic changes with heat due to the material's unique molecular structure. Plastic sheets are produced by rapidly cooling molten polymer chains, locking them into a high-strain orientation that makes the sheets flat. When heated, the polymer chains are no longer locked in this state and relax into a low-energy, curled configuration, resulting in a decrease in volume. This phenomenon is known as negative thermal expansion (NTE).
While most materials expand when heated, plastics have a low melting point, softening and becoming more liquid-like. This causes the plastic to pull back due to surface tension, resulting in a spherical shape. The total volume of the plastic may increase, but its geometry changes as it transitions from a thin sheet to a thick blob. This effect is commonly observed in items like yoghurt containers, which flatten out into sheets when heated.
The behaviour of plastic when heated depends on its type. Thermoplastic and thermoset plastics, for example, respond differently to heat. Thermoplastic sheets are made by melting the plastic, while thermoset materials like phenolic are not. Amorphous thermoplastics like ABS, acrylic, PVC, and polycarbonate may respond differently depending on how heat is applied. If heated unevenly, one side becomes hotter and expands more, creating internal stress that causes the sheet to bend.
Additionally, the geometry of plastic can be affected by the manufacturing process, material used, and thickness. Plastics used in vacuum forming or shrink-wrapping may not return to their original size when cooled due to partial polymerization. Prolonged exposure to high temperatures can also cause plastic to deform or "creep," losing strength and toughness and becoming more prone to cracking and breaking.
Understanding the geometry changes of plastics with heat is essential for selecting suitable materials in various applications, such as safety, structural, or functional components in medical, transportation, or industrial contexts.
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Frequently asked questions
Plastic contracts when heated due to the material's internal stress and the way it is manufactured. When heated, the plastic becomes softer and loses its stiffness, and if heated enough, it will distort.
At the molecular level, the heat disrupts the polymer chains that are locked in a high-strain orientation. The chains relax and curl, shrinking the bulk material.
When plastic is heated, its material temperature increases, causing it to lose stiffness and soften. This is because the heat disrupts the polymer chains' tough, oriented, and stretched state.
The phenomenon is called negative thermal expansion (NTE). It is observed in plastics, polymers, and other materials.
Prolonged exposure to high temperatures causes plastic to lose strength and toughness, becoming more prone to cracking, chipping, and breaking.
