Alexei Volkov
Plastic is a broad term for polymers with long chain-like molecules that can be flexible or brittle. Over time, exposure to heat, light, and oxygen can cause chemical reactions within these polymers, breaking down their chains and making the plastic more rigid and brittle. This process can be slowed by adding antioxidant chemicals to the plastic, but eventually, even non-biodegradable plastics will degrade. The rate of degradation depends on the specific type of plastic and the environmental conditions it is exposed to.
| Characteristics | Values |
|---|---|
| Cause of brittleness | Exposure to heat, UV radiation, sunlight, and oxygen |
| Chemical reaction | Breakage of polymer chains |
| Effect | More rigid or crumbly |
| Solution | Remelt the plastic and add new plasticizers |
| Prevention | Add antioxidant chemicals |
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What You'll Learn
The role of plasticisers
Plasticisers are added to plastics to make them softer and more flexible. The flexibility of plastics is due to the movement of the long chain molecules within them. Plasticisers create space between these molecules, allowing them to move more easily when force is applied.
One commonly used plasticiser is dinonylphthalate, which was used throughout the last century. However, this plasticiser is volatile and tends to evaporate over a couple of years, causing the plastic to become brittle again. To restore the flexibility of the plastic, it must be remelted and new dinonylphthalate added. More recently, less volatile plasticisers have been discovered, which are more effective at maintaining the flexibility of plastics over time.
The use of plasticisers is particularly relevant for products that need to remain flexible, such as Barbie dolls, which are made of PVC and require plasticisers to preserve their softness. However, the evaporation of plasticisers from such products can pose a challenge for preservation.
The susceptibility of plastics to becoming brittle over time is influenced by various environmental factors, including heat, light, and oxygen, which can induce chemical reactions within the polymer chains. These reactions can lead to the breakdown of the chains, making them more rigid and crumbly, or the formation of cross-links between chains, resulting in increased rigidity. Additionally, exposure to heat during the melting process can affect the organisation of polymer strands at the molecular level, impacting the strength and properties of the plastic.
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The effect of temperature changes
However, when plastic is subjected to high temperatures, its properties can change. For example, when plastic is melted and then cooled, it can become brittle. This is because the heating and cooling process erases the original structure of the polymer strands, affecting the way they organise at the molecular level. The strength of a polymer is often dependent on the length of the polymer chains, and when these chains are broken down, the shorter chains are more prone to falling apart, resulting in a more brittle material.
Additionally, exposure to heat and UV radiation over time can cause chemical reactions within the polymer, leading to increased brittleness. The heat can break down the polymer chains, making them more rigid and crumbly. This is particularly true for plastics exposed to sunlight, which degrades the plastic through oxidation. Antioxidant chemicals are added to plastics to stabilise them, but over time these antioxidants deplete, leaving the plastic vulnerable to rapid weakening and embrittlement.
On the other hand, lower temperatures can also impact the brittleness of plastic. When the motion of the molecules is restricted due to cold temperatures, their ability to stretch and slip past one another is hindered. This restriction can lead to a concentration of stress in a small area, resulting in cracks and fractures, causing the plastic to become brittle.
The specific effects of temperature changes on the brittleness of plastic depend on various factors, including the type of plastic, the presence of additives, and the rate of temperature change. Overall, while plastic does not last forever, understanding the impact of temperature can help in preserving plastic items and predicting their longevity.
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The impact of sunlight and UV radiation
Plastics are polymers, which means they are made up of long chains of molecules. The flexibility and strength of a plastic depend on the ability of these chains to move and slip past each other. UV radiation can cause these chains to break down, making the plastic more rigid and brittle. This is particularly true for plastics that are exposed to sunlight and UV radiation for prolonged periods, such as outdoor furniture or car interiors.
The impact of UV radiation on plastics can be mitigated through the use of antioxidant chemicals, which are added to most plastics during the manufacturing process. These antioxidants stabilize the plastic against oxidative degradation by preventing the breakdown of polymer chains. However, over time, these antioxidants can become depleted, leaving the plastic vulnerable to the effects of UV radiation.
In addition to the direct impact of UV radiation on polymer chains, sunlight can also contribute to the degradation of plastics through heat absorption. When plastics are heated, they can undergo a process called the glass transition, where the polymer chains reorganize, affecting the strength and flexibility of the plastic. Prolonged exposure to sunlight can cause repeated glass transitions, leading to a greater likelihood of chain breakdown and, consequently, embrittlement.
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Polymer chain length
Polymers are plastics and they have molecules that are long chains. The flexibility of polymers is determined by the ability of these chains to move over each other when force is applied. The length of these chains is critical to the strength of the polymer material. Longer chains are stronger because they can intertwine, while shorter chains, when broken down, are more prone to falling apart.
The polymer chains can be manipulated to be flexible in a few ways. One way is to make the chain molecules branched and twiggy, which means they cannot pack together tightly and have space between them to move when force is applied. Another way is to have bonds between the chains, like silicones, which can change how rigid they are.
Environmental factors such as heat, light, and oxygen can cause chemical reactions within the polymer, breaking down the chains and making them more rigid or crumbly. This can also result in more cross-links between the chains, making them even more rigid.
The strength and material properties of a polymer depend on the organisation of the polymer strands at the molecular level and the relative proportions of amorphous (disorganised) and crystalline (organised) portions. When plastic is melted, it erases the structure produced by the original manufacturing method. As the liquid cools, the polymer transitions from a semi-solid state, where the strands can reorganise, to a truly solid material.
Over time, polymers can become sticky, as seen in the case of the musical instrument, the clavinet, where the hammers that struck the strings became sticky, altering the sound produced.
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$51.43
Ductility
The ductility of a material refers to its ability to deform under tensile stress—this is often described as the material's ability to stretch without breaking. The ductility of plastics is dependent on several factors, including their chemical composition, molecular structure, and environmental conditions.
Plastics are polymers, which means they are made up of long chains of molecules. The length of these polymer chains is crucial to the ductility of plastics. Longer chains result in stronger plastics because they can absorb more energy by stretching. When a force is applied to a plastic object, its ductility allows it to deform and stretch instead of breaking. This is because the long polymer chains can slip past, around, or through one another, distributing the impact energy and preventing a concentrated stress point that could lead to cracking or fracturing.
However, over time, plastics can become brittle due to various factors. One significant factor is the exposure to environmental conditions such as heat, light (especially UV radiation), and oxygen. These elements can cause chemical reactions within the polymer chains, breaking them down and making the plastic more rigid and brittle. Additionally, some plastics contain plasticizers, which are added to make the material softer. However, these plasticizers can evaporate over time, causing the plastic to lose its flexibility and become brittle.
The rate at which plastics become brittle can vary depending on their composition and exposure to environmental factors. For example, antioxidant chemicals are often added to plastics to stabilize them against oxidative degradation caused by sunlight and heat. However, over time, these antioxidants deplete, leaving the plastic vulnerable to rapid weakening and embrittlement.
Furthermore, the molecular structure of plastics also plays a role in their ductility. Amorphous polymers, which have a more random molecular arrangement, tend to be more transparent but weaker than crystalline polymers with a more organized structure. The way these polymer strands organize at the molecular level during manufacturing can significantly impact the strength and ductility of the final plastic product.
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Frequently asked questions
Yes, exposure to heat, light, and oxygen can cause chemical reactions within the polymer, breaking down the chains and making the plastic more rigid or crumbly.
The strength of a polymer material depends on the length of the polymer chains. When you melt and cool plastic, you erase the structure produced by the original manufacture method, and the shorter chains are more prone to falling apart, making the plastic brittle.
Ductility is the ability of plastic's long, chain-like molecules to stretch and absorb energy. When molecules are restricted in motion, they can't stretch, and the stress remains concentrated in a small area, leading to cracks and fractures.
Yes, by remelting the plastic and adding a plasticizer such as dinonylphthalate, the polymer can become softer again.
