Omar Khalid
Plastic is a fascinating material that exhibits unique behaviours when subjected to temperature changes. Typically, materials expand when heated and contract when cooled. However, plastic can sometimes defy this expectation, contracting upon heating and expanding when cooled. This intriguing phenomenon, known as negative thermal expansion (NTE), is not limited to plastics but is also observed in certain other materials. So, what causes this unusual behaviour? Let's delve into the world of polymers, molecular bonds, and thermal dynamics to uncover the secrets behind plastic's unpredictable nature.
| Characteristics | Values |
|---|---|
| Expansion and contraction | Plastics tend to expand when heated and contract when cooled. However, some plastics exhibit negative thermal expansion (NTE), contracting upon heating and expanding when cooled. |
| Cause | The expansion and contraction of plastics are due to the disruption of the orientation of polymer chains by heat. Plastics are produced by rapid cooling, which orients the polymer chains in a stretched state. When heated, the chains are no longer locked in this high-strain orientation and relax into a low-energy, curled state, resulting in shrinkage. |
| Exceptions | Some plastics, such as those used in shrink-wrapping, do not expand back to their original size when cooled. This is because they are only partially polymerized during the heating process and do not revert to their original state. |
| Expansion rate | The expansion rate of plastics varies, with Nylon expanding and contracting at ten times the rate of steel. The expansion rate of Nylon 6, for example, is 0.12 mm per 10°C of temperature increase or decrease. |
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What You'll Learn
Plastic production involves rapid cooling
Plastic is derived either from crude oil, natural gas, or coal (synthetic plastic) or from renewable products such as carbohydrates, starch, vegetable fats and oils, bacteria, and other biological substances (biobased plastic). The majority of plastic in use today is synthetic. The production of synthetic plastic involves a process called polymerisation, which links small molecules together into long molecular chains called polymers. These polymers are then kneaded, heated, melted, and cooled into objects of various shapes, sizes, and colours.
The rapid cooling of plastic during production is essential to achieving the desired configuration of long polymer chains. When plastic is heated, the heat disrupts the orientation of these chains, causing them to become more liquid-like and expanding or contracting. However, when cooled, the polymer chains return to their solid-state, and the plastic hardens into its final form.
The rate of expansion and contraction of plastic varies depending on the type of plastic. For example, Nylon and Acetal exhibit an increase in their expansion rate at temperatures over 60 degrees Celsius. At this temperature, a 100mm nylon6 rod will expand or contract by 0.12mm per 10°C of temperature change. This expansion and contraction can become more pronounced as the temperature changes become larger.
Additionally, some plastics, such as those used in shrink-wrapping processes, do not expand back to their original size when cooled. These plastics undergo partial polymerisation when heated, becoming fully polymerised and shrinking, which is not reversible. The formation of new bonds during the heating process prevents the molecules from expanding again when cooled.
Overall, the rapid cooling step in plastic production is critical to achieving the desired structure and properties of the final plastic product. The rate of cooling and the specific type of plastic used will determine the extent of expansion or contraction observed.
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Plastic's polymer chains are disrupted by heat
Plastics are made up of long polymer chains that are oriented in a particular way through rapid cooling during the manufacturing process. This process keeps the chains locked in a high-strain orientation, which makes the plastic tough and strong. However, when plastics are heated, this orientation is disrupted.
The application of heat during the processing of polymers can break polymer chains and decrease molecular weight, causing plastic degradation. This degradation occurs when the covalent connections along a molecular chain's backbone are disrupted, reducing the molecular weight of the polymer due to the shortening of the molecular chains. As a result, the material's performance properties are negatively affected.
When plastic is heated, the polymer chains are no longer locked in their high-strain orientation. They transition to a low-energy orientation, curling and bending in a way that reduces the overall size of the plastic. This change in conformation is entropically favourable as it decreases the Gibbs free energy, making it a more stable shape.
The behaviour of plastic when heated depends on its type. For example, thermoplastic semi-crystalline materials, like polyethylene, are soft and have a low flexural modulus. When heated, they may not contract but instead bend due to the internal stress caused by the temperature differential. On the other hand, thermoset materials like phenolic are not melted during the forming process and may exhibit different responses to heat.
Additionally, some plastics, like those used in shrink-wrapping, do not expand back to their original size when cooled after heating. These plastics become fully polymerised when heat is applied, and the process is not reversible.
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Plastic's low melting point
Most materials expand when heated and contract when cooled. This is because higher temperatures increase the energy of molecules, causing them to move around more and bump into each other, which results in expansion. However, plastics are an exception to this rule. While plastics initially expand when heated, they soon begin to soften and contract. This is because plastics have a low melting point and are formed of long polymer chains that are oriented by rapid cooling during the manufacturing process. When reheated, the polymer chains are disrupted and the plastic returns to its natural, unstretched state.
Some plastics, such as nylon and acetal, have an increased expansion rate at temperatures over 60°C. For example, a 100mm nylon6 rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease. This is ten times the expansion rate of steel.
Plastics used in shrink-wrapping and heat-shrink tubing do not expand back to their original size when cooled. This is because these plastics are only partially polymerized when manufactured and become fully polymerized when heated, causing them to shrink. This process is irreversible due to the formation of new bonds between the molecules.
The phenomenon of materials shrinking when heated is known as negative thermal expansion (NTE) and is observed in various materials, including ceramics, oxides, cyanides, and graphite. The cause of NTE differs for each material but is generally related to the geometry of molecules, such as their crystal structure or polymer arrangement.
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Plastic's surface tension
The surface energy of plastics is a critical factor in determining their behaviour and interactions with other substances. Surface energy quantifies the disruption of intermolecular bonds in the material, and a higher value indicates a stronger affinity between molecules. This property is particularly important when considering coatings, inks, adhesives, and printing on plastics.
Plastics with low surface energy, such as polyolefin plastics like polypropylene and polyethylene, tend to repel liquids and adhesives, making them challenging to bond with other substances. This behaviour is because the molecules on the surface of low-energy plastics have weak attraction forces, especially towards adhesive molecules. Consequently, coatings may not flow well, resulting in issues like fisheyes, pinholes, gaps, or air bubbles.
Conversely, plastics with higher surface energy exhibit stronger intermolecular forces and are more receptive to bonding. For example, engineered plastics, commonly used in manufacturing due to their strength, lightweight nature, and mouldability, fall into the medium surface energy category, making them more amenable to adhesion.
To enhance the surface energy of plastics and improve adhesion, various surface treatment methods can be employed. These include applying heat, oxidation, chemicals, corona treatments, plasma treatments, and abrasion. By altering the chemical structure of the plastic surface, these treatments increase the surface energy, making it more conducive to bonding with inks, coatings, or adhesives.
Additionally, surfactants can be used to reduce the surface tension of liquids, improving their wetting ability and facilitating better adhesion. Instruments like tensiometers and contact angle measuring tools help optimise these processes by quantifying the dynamic surface tension and the efficiency of surfactants.
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Plastic's internal stress
The behaviour of plastics when exposed to heat depends on various factors, including the manufacturing process, material used, and thickness. When heated, plastic materials generally soften and lose their stiffness, and if exposed to enough heat, they will distort. Different types of plastics behave differently when heated. For example, thermoplastics soften and bend when heated, while thermoset plastics like phenolic tend to burn.
Internal stress in plastics can be induced by the manufacturing process, specifically when molten polymer is cooled and shaped, which results in residual stress. This residual stress can lead to distortion and cracking in the plastic part. Stress-relaxation induced distortion can occur when parts with excessive residual stress undergo thermal cycles such as sterilisation or heat sealing, or when exposed to varying ambient temperatures during transportation and warehousing.
The deformation-induced internal stress can be investigated through stress relaxation procedures, and it has been found to be influenced by the ageing or annealing process. However, there is currently no quantitative interpretation of this phenomenon.
Additionally, when thermoplastic sheets are heated unevenly, differences in thermal expansion between the hotter and cooler sides create internal stress. As the temperature increases, the plastic softens, and if it becomes too soft to hold the internal stress, it bends. This is more likely to occur in thermoplastic semi-crystalline materials like polyethylene, which have a low flexural modulus and, therefore, less internal stress.
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Frequently asked questions
Most plastics contract when heated due to the disruption of the polymer chain orientation caused by heat. However, there are certain plastics, such as Nylon and Acetal, that exhibit a slight increase in their expansion rate when the temperature exceeds 60°C.
Plastics are produced through rapid cooling, which keeps the polymer chains oriented in a way that makes them flat. When heated, the polymer chains are no longer locked in this high-strain orientation and relax into a low-energy, curled state, resulting in the contraction of the plastic.
Plastics that have contracted upon heating will usually expand when cooled within the same temperature range. This phenomenon is known as negative thermal expansion (NTE) and is observed in various materials, including certain plastics, ceramics, and graphite.
Some plastics, such as Nylon and Acetal, exhibit a slight expansion when heated above 60°C. This expansion is attributed to the material's response to increasing temperatures, where the higher energy results in more molecules moving around and bumping into each other, causing expansion.
