Alexei Volkov
Plastic is made of long molecules that look like chains (polymers). When plastic items are manufactured, these chains are stretched and moulded into shape. When exposed to heat, the chains loosen up and pull each other back together, causing the plastic to contract and shrink. This is why a plastic bag or bottle shrivels up when heated. Some plastics are also sensitive to moisture and can warp or swell when exposed to water or high humidity.
| Characteristics | Values |
|---|---|
| Reason for swelling | Exposure to heat |
| Effect of heat on plastics | Increase in molecular movement and expansion |
| Volume change | Increase |
| Plastic state change | From thin sheet to thick blob |
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What You'll Learn
Plastic shrinks when heated due to manufacturing stresses
Plastic is known to swell up when heated due to several factors. Firstly, plastics have a low melting point and soften easily when heated, becoming more liquid-like than most other materials. This liquid state, coupled with surface tension, causes the plastic to pull back into a minimum surface shape, often a ball. As most plastics are in thin sheets, they tend to "roll up" when heated, giving the appearance of shrinkage.
However, the phenomenon of plastic shrinkage is more complex than a simple size reduction. Plastic sheets are manufactured by melting the plastic and either rapidly cooling it or slowly cooling it through a process called annealing. This cooling process locks the polymer chains into a high-strain orientation, which is an energetically unfavourable state. Therefore, when plastic is reheated, the polymer chains are free to move and relax into a low-energy, curled orientation with less bulk. This re-entanglement of polymer chains as they cool causes them to occupy less space, resulting in the plastic shrinking.
The manufacturing process plays a significant role in the shrinkage behaviour of plastic. During manufacturing, plastic is often stretched over a mould or rapidly cooled, creating internal or residual stress within the material. When reheated, the plastic releases this built-up stress, returning to its natural, unstretched state. This release of stress can occur during transportation or storage if the plastic is exposed to high temperatures.
Additionally, the type of plastic and its processing conditions influence its response to heat. For example, thermoplastic semi-crystalline materials, such as polyethylene, have a sharp melting point and low flexural modulus. When heated, they soften and may bend or contract due to the difference in thermal expansion between the two sides. On the other hand, thermoset materials like phenolic are not melted during the forming process and tend to burn rather than contract when heated.
To summarise, plastic shrinkage when heated is a result of the release of manufacturing stresses, the re-entanglement of polymer chains, and the material's natural tendency to return to a lower-energy state. Environmental factors, such as temperature and humidity, can further influence the shrinkage behaviour of plastic.
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Plastic is made of long molecules that can be stretched and moulded
Plastic is a general term for materials that can be formed and moulded under heat and pressure. The process of moulding plastic involves heating it until it becomes liquid, at which point it can be poured into a mould and allowed to cool and harden. This process is known as polymerisation, where monomers (smaller molecules) are joined together to form polymers (larger molecules).
Polymers are the chemical class of materials that make up all modern plastics. They are large molecules, consisting of a chain of repeating smaller molecules (monomers). These chains can be thousands of units long, and the backbone of the chain links a large number of repeat units. The chains can be formed in different patterns, which influences the shape of the polymer and its molecular weight and other chemical/physical properties.
The two most common types of polymers are condensation polymers and polymers that melt with heat (thermoplastics). Condensation polymers are formed by joining two or more different monomers, and examples include polyester and nylon. Thermoplastics, as the name suggests, melt with heat and are easy to recycle. The most common kind of plastic in plastic bags is polyethylene, which is a thermoplastic.
The process of moulding plastic involves heating it until it becomes liquid, at which point it can be poured into a mould and allowed to cool and harden. This is how plastic products are given their shape. The moulding process can also involve stretching the plastic over a mould, which is how most vacuum-formed plastic products are made. When plastic is heated, its molecules move around more and bump into each other, causing expansion. However, because most plastic products are thin sheets, they tend to "roll up" when heated, giving the appearance of shrinking.
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Degraded plastics are more susceptible to water damage
Plastic is a common material used in a wide range of applications, from toys and kitchen utensils to electrical items and construction. While plastic is designed to be durable, it is not indestructible and can degrade over time. This degradation is often accelerated by exposure to environmental factors such as heat, sunlight, and moisture, which can cause the polymers in plastic to break down and release chemicals called plasticizers, resulting in a sticky residue.
One of the main factors contributing to plastic degradation is heat. High temperatures can accelerate the breakdown of plastic polymers, increasing the rate at which plasticizers migrate to the surface. This is why it is important to avoid exposing plastic items to extreme temperatures, direct sunlight, or heat sources like stoves and ovens. Prolonged exposure to heat can also cause plastics to become soft and malleable, affecting their strength and appearance.
Another factor that contributes to plastic degradation is water. Water can have a detrimental effect on plastics, especially when combined with other factors such as heat and chlorine. Chlorinated water used to kill microbes, for example, can slowly attack certain types of plastics, particularly when the water is heated. Additionally, the combination of water and heat can lead to a process called "thermal oxidation," which can further degrade plastics.
The degradation of plastics can also occur through a process called "polymer weathering." This is caused by the combined effects of UV-light, oxygen, water, and pollutants. Over time, exposure to these elements can cause the plastic to embrittle and break apart, eventually forming microplastics. The additives and chemicals used in plastics can also leach out into the environment, leading to potential ecological and environmental concerns.
Biodegradable plastics, such as poly-caprolactone (PCL) and poly lactic acid (PLA), are designed to be more susceptible to degradation than traditional plastics. This is due to their chemical structure, which includes ester linkages that are more easily broken down by microbial enzyme systems. While biodegradable plastics offer an environmentally friendly alternative to traditional plastics, they may also be more prone to pretreatment processes that can accelerate their degradation.
In summary, degraded plastics are more susceptible to water damage due to the breakdown of their polymer structures. This degradation is often accelerated by environmental factors such as heat, sunlight, and moisture, which can cause the plastic to become brittle, sticky, or tacky. Additionally, the combination of water with other factors such as heat and chlorine can further exacerbate the degradation process. Therefore, it is important to protect plastic items from these factors to prolong their lifespan and maintain their appearance.
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Hydrostatic pressure affects the permeability of plastics
Plastic is a polymer that is created through the process of rapid cooling, which orients long polymer chains. When exposed to heat, this orientation is disrupted, causing the plastic to become more liquid and to pull back into a minimum surface shape, often a ball. This process can be harnessed to fuse plastic together, creating a thicker and stronger material. However, this also means that plastic exposed to heat may become sticky as the polymers break down and plasticizers migrate to the surface.
Hydrostatic pressure can have various effects on the permeability of plastics to water and other substances, depending on the exposure conditions and the mechanism of permeation. While some materials, such as butyl rubber, exhibit decreased permeability with increasing hydrostatic pressure, others like polyethylene, polyurethane, and nylon show little to no change in permeability. The specific volume and degree of crystallinity of the plastic also play a role in permeability under hydrostatic conditions.
The decrease in permeability with increasing hydrostatic pressure can be attributed to the compression of free volume, which in turn lowers diffusion and permeability. This relationship between hydrostatic pressure and permeability is important to consider when selecting materials for applications where imperviousness is crucial, as simple swelling tests may not accurately predict water penetration.
Additionally, the exposure of plastics to high hydrostatic pressures can affect their swelling behaviour. While it is known that water flow through porous and non-porous materials increases with increasing hydrostatic pressure, the correlation between swelling and permeability is more complex. In the case of rubbers exposed in the ocean, hydrostatic pressure does not increase swelling, indicating that other factors also influence the swelling behaviour of plastics under pressure.
In summary, hydrostatic pressure can indeed affect the permeability of plastics, with the specific effects depending on the type of plastic, exposure conditions, and mechanism of permeation. The complex relationship between hydrostatic pressure, permeability, and swelling behaviour highlights the importance of considering multiple factors when selecting materials for applications where impermeability is a critical requirement.
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Low humidity makes plastics more brittle
While plastic can become sticky due to a process called polymer degradation, which is often accelerated by exposure to heat, sunlight, and moisture, low humidity can also negatively impact plastics. Low humidity can cause materials like polyamides (e.g. nylon) to become unnaturally stiff and brittle. This is because such materials absorb water over time, and low humidity causes them to lose water, resulting in brittleness. This phenomenon is reversible, as the mechanical properties of the material will return to normal when exposed to nominal humidity levels. However, it is important to note that irreversible consequences, such as brittle failures, can occur when these materials are exposed to extremely dry conditions for prolonged periods.
To prevent plastics from becoming brittle in low-humidity environments, it is recommended to store plastic items in sealed glass or vapour-proof containers with dry indicating silica gel. The silica gel will absorb any moisture in the air and turn pink when it needs to be replaced. Additionally, it is important to monitor the humidity levels at least once a year to ensure that they do not drop too low.
It is worth noting that not all plastics are equally affected by humidity. For example, hydrophobic materials like polyethylene and silicone are less susceptible to changes in humidity levels compared to hygroscopic materials like polyamide-based plastics (nylon, PEBAX, etc.).
Furthermore, while low humidity can cause plastics to become brittle, high humidity can also negatively impact plastics by accelerating their degradation. This is especially true for materials like cellulose esters and ebonite, which are highly susceptible to damage from high humidity conditions. Therefore, it is important to maintain moderate humidity levels to prevent unnatural physical changes and degradation of plastics.
Overall, understanding the impact of humidity on plastics is crucial for predicting and managing the aging behavior of plastic products and ensuring their longevity. By controlling storage conditions, such as humidity and temperature, and selecting appropriate plastic materials, we can minimize the risk of degradation and maintain the performance and integrity of plastic items.
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Frequently asked questions
Plastic is made of long molecules that look like chains (polymers). When plastic is manufactured, these chains are stretched and pushed into the desired shape. When exposed to heat, these chains loosen up and pull each other back together, causing the plastic to contract and take up less space.
High humidity can cause mould on plastics. Low humidity, on the other hand, can make plastics brittle, fragile, and prone to fracture. Swelling due to moisture absorption is quantified by the Coefficient of Moisture Expansion (CME).
No, only plastics that are formed and shaped from a molten state react by shrinking when reheated. Cross-linked plastics, where the polymer chains are welded together, often behave differently.
Yes, some plastics are water-sensitive and can rapidly absorb water and swell when immersed. Plastics with organic fillers or water-absorbent fillers like wood flour, commonly used in historic plastics, are more prone to swelling.
