Maya Ellison
Plastic is generally believed to be impermeable, which is why many liquids are stored in plastic containers. However, this is not entirely accurate, as plastics are permeable to both water and gases. The permeability is low, but not zero, and plastics can absorb moisture. Over time, water can enter plastics by osmosis, causing them to swell. This is a problem for old boats with plastic hulls, which can develop large blisters. The permeability of plastic depends on its type and thickness, and the length of time it is exposed to water.
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What You'll Learn
Plastic polymers are long chains of molecules
Polymers can be thought of as long chains of beads. When a sharp object like a pencil is pushed through a plastic bag, the polymers move apart, but they also seal the gap around the pencil due to their flexibility. This prevents water from leaking out. The surface tension of the water also plays a role, as a certain outward pressure is required to overcome the inward pressure due to surface tension.
While plastics are generally considered impermeable, they are slightly permeable to water and gases. The permeability is low, but over very long periods, a perceptible change can occur. This is why boat hulls, which are in constant contact with water, need to be carefully constructed to prevent water from entering through small imperfections by osmosis.
The belief that plastics are completely impermeable may stem from their widespread use in storing liquids. However, the permeability of plastics depends on the specific type of plastic and the conditions it is exposed to.
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Plastic is flexible and lightweight
While plastic is generally believed to be impermeable, it is in fact permeable to water and gases. However, the permeability is low, and perceptible changes occur only after very long periods of time. The permeability of plastic depends on its type and the presence of imperfections such as small air bubbles. For example, the plastic in boat hulls can act as a partially permeable membrane, allowing water to enter through osmosis and causing swelling. Similarly, plastic balls filled with water can become indented over time as water escapes through microscopic gaps in the surface.
The flexibility and lightweight nature of plastic contribute to its widespread use and popularity. Polypropylene (PP), for instance, is one of the most flexible thermoplastics globally and is known for its durability, heat resistance, and ability to withstand repeated stress without cracking. It is commonly used in laboratory equipment, automotive parts, medical devices, and food containers. Another flexible thermoplastic is polyethylene terephthalate (PET), which boasts excellent chemical resistance, high strength-to-weight ratio, and shatterproof qualities. It finds applications in optical devices, packaging, and bottles.
Polycarbonate plastic, known for its strength and impact resistance, also exhibits inherent design flexibility. Unlike glass or acrylic, polycarbonate sheets can be easily cut and cold-formed on-site, making them versatile in applications such as greenhouses, DVDs, and police riot gear. Additionally, some plastics can be formulated to possess specific characteristics. For instance, PVC can be manufactured to be rigid or flexible, with the latter finding use in plumbing products, clothing, and medical tubing.
The softness and hardness of plastics, typically measured using a durometer, play a crucial role in their flexibility. Materials with a hardness of Shore A 35 and below are considered very flexible but may struggle to bear their weight and be challenging to assemble. On the other hand, materials with a hardness above Shore D 60 might not be classified as rubbery or elastic. When selecting a plastic material, it is essential to consider not only softness or hardness but also other properties such as chemical resistance, UV-resistance, and service temperature.
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Plastic is partially permeable to water
Plastic is often used to store liquids and is generally believed to be impermeable. However, plastic is partially permeable to water and gases. The permeability is low, but not zero, and perceptible changes can occur over long periods. The rate of permeability depends on the type of plastic.
For example, a boat hull can be in the water for years. If there are imperfections in the hull construction, such as small air bubbles, the plastic can act as a partially permeable membrane. Water enters these imperfections by osmosis, causing them to swell into large blisters. This process only becomes a problem for older boats as it takes years for the damage to occur.
The permeability of plastic to water is also demonstrated in an experiment where a plastic bag containing water is stabbed with a pencil. The water does not leak out because the polymers in the plastic move apart and seal the gap around the pencil. However, if the plastic bag is punctured with a sharp object, water will leak out. This is because a certain amount of outward pressure is required for the water to escape through the puncture hole, and the puncture hole created by the sharp object is not large enough to generate the required pressure.
The flexibility of the polymer molecules in plastic items contributes to their ability to seal gaps and prevent water leakage. This property of polymers allows them to be used in various applications, such as plastic bags, bottles, and packaging, where leak-proof characteristics are essential.
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Plastic thickness impacts permeability
Plastic is a polymer formed by long chains of molecules that create a "molecular ball" structure. These balls have small voids that allow smaller molecules like oxygen and water vapour to pass through. However, the permeability of plastics is low and not zero. Over time, a perceptible change can occur, and water can penetrate plastic.
The thickness of plastic impacts its permeability. Thicker plastics, like those used in boat hulls, can remain submerged in water for years. On the other hand, thinner plastics, like plastic bags, are more susceptible to water penetration. The thickness of the plastic relates to the size of the voids in the molecular structure, with thicker plastics having smaller voids that restrict the passage of molecules.
The type of polymer used in plastic production also affects its permeability. Different polymers have varying sizes of voids, which allow molecules to pass through more or less easily. Additionally, processing parameters during production and ambient temperatures during use can alter the barrier properties of plastics, impacting their permeability.
To measure the permeability of plastics, companies employ specific sensors that detect the transmission rate of molecules through the material. This information is vital for product development, ensuring that permeability values remain acceptable during design modifications, such as changes in thickness.
In summary, while plastic is not completely impermeable, its thickness plays a significant role in determining its permeability. Thicker plastics exhibit lower permeability due to their smaller voids, while processing, temperature, and polymer type also influence the barrier properties of plastic materials.
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Plastic is not entirely impermeable
The permeability of plastics to water is demonstrated in an experiment where a plastic bag containing water is stabbed with a pencil. Surprisingly, the water does not leak out immediately. This is because the polymers, or long chains of molecules, in the plastic move apart and seal the gap around the pencil, preventing water from escaping. However, this experiment does not prove that plastic is entirely impermeable to water.
In another example, plastic toy balls filled with water were observed to change from a firm to an indented state over a year. This indicated that water had escaped through the ball's surface via diffusion, resulting in a loss of volume and change in shape. Similarly, boat hulls made of plastic can be affected by water permeation. If there are imperfections in the hull construction, such as small air bubbles, water can enter these imperfections by osmosis, causing them to swell into large blisters over time.
The level of permeability depends on various factors, including the type of plastic, the presence of imperfections, and the duration of exposure to water. While plastic may provide a temporary barrier to water, it is not completely impermeable, and alternative materials like metals should be used if absolute impermeability is required.
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Frequently asked questions
Plastic items like bags and bottles are made of polymers, which are long chains of molecules. When you stab a pencil through a plastic bag, the polymers move apart and seal the gap around the pencil, preventing water from leaking out.
Boat hulls are much thicker than plastic bags. If there are imperfections in the hull construction, water can enter by osmosis and cause swelling over time.
No, all plastics are at least slightly permeable to water and gases. The permeability is low, but when very long periods of time are involved, a perceptible change can occur. If absolute impermeability is required, metals should be used.
While plastics are not completely impermeable, they are generally considered impermeable because many liquids are stored in plastic containers without issues.
