Ethan Clarke
Metal and plastic are two materials that are used across almost every industry. Metal is often considered stronger for structural and weight-bearing applications, but certain plastics can now surpass metals in strength. The strength of each material depends on the specific type, intended application, and environmental conditions under which the product will be used. Metals, especially alloys, are known for their high strength, wear resistance, and temperature resistance. However, plastics offer lightweight options, corrosion resistance, chemical resistance, and impact resistance.
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What You'll Learn
Metal alloys are stronger than plastics
Recently, a team led by Jan Schroers, a materials scientist at Yale University, has made significant progress in this area. They have developed bulk metallic glasses (BMGs) - metal alloys with randomly arranged atoms, as opposed to the crystalline structure found in ordinary metals - that can be blow-moulded like plastics. This process involves moulding the alloys at low temperatures and low pressures, causing the bulk metallic glass to soften and flow as easily as plastic without crystallizing.
The resulting products are twice as strong as typical steel and can take on a variety of complex shapes, including seamless metallic bottles, watch cases, miniature resonators, and biomedical implants. These shapes cannot be achieved using regular metal processing methods. Additionally, by blow-moulding the BMGs, the team was able to combine three separate steps in traditional metal processing (shaping, joining, and finishing) into one, reducing the time and energy required for processing.
The superior properties of BMGs, combined with the ease and economy of blow moulding, have the potential to significantly impact society, just as the development of synthetic plastics did in the last century. These novel metals offer the strength and durability of metal while maintaining the mouldability and low cost of plastic, making them a highly desirable material for a variety of applications.
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Metals are more durable and temperature-resistant
Metals, particularly alloys, are known for their high strength, wear resistance, and temperature resistance. They have a higher tensile strength than plastics and a higher melting point. This means that metals can be used in extreme hot and cold temperatures without compromising their structural integrity. Stainless steel, for example, is known for its durability and ease of cleaning.
In contrast, plastics are more susceptible to damage from extreme temperatures. Polypropylene, for instance, can degrade when exposed to extreme temperatures and is not suitable for prolonged outdoor use due to UV exposure, which can cause discolouration and cracking.
Metals are also highly recyclable, with aluminium and steel being good examples of metals with low energy recycling requirements. Mining for raw materials is an environmental concern, but metals can be recycled more reliably than plastics, which depend on the availability of local recycling programs and the specific type of plastic.
The durability of metals is further demonstrated by their resistance to wear and tear. While plastics offer impact resistance, metals have a longer lifespan in most applications and are less prone to damage over time. This makes metals ideal for structural and weight-bearing applications, where their strength and durability are crucial.
However, it is worth noting that with advancements in material science, certain plastics now rival or even surpass metals in strength. Additives such as fiberglass can be used to increase the strength of plastics, making them comparable to metal in terms of durability. Despite this, metals still hold an advantage in their ability to withstand extreme temperatures without degradation, contributing to their overall durability and making them the preferred choice in industries such as transportation, aerospace, construction, and energy.
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Plastics are lightweight, flexible, and corrosion-resistant
While metals are stronger than plastics, plastics have their own unique advantages. One of their most notable qualities is that they are lightweight. This makes them highly versatile and suitable for a wide range of applications, from laboratory equipment and automotive parts to packaging materials and clothing.
The flexibility of plastics is another key advantage. This flexibility allows plastics to be easily formed, molded, or fabricated into a vast array of shapes and products. For example, polyvinyl chloride (PVC) can be either rigid or flexible and is used in construction materials, electrical insulation, clothing, and more. Acrylic, a transparent thermoplastic, can be made into shatter-resistant alternatives to glass, such as mirrors and plexiglass. Other flexible plastics, like polypropylene, can withstand repeated stress without cracking, making them ideal for applications such as automotive parts and medical devices.
Plastics also offer excellent corrosion and chemical resistance. High-density polyethylene (HDPE), for instance, has superior corrosion resistance and outperforms low-density polyethylene in physical and chemical properties. PTFE (polytetrafluoroethylene) is known for its unmatched chemical, electrical, mechanical, and thermal properties, finding use in applications requiring anti-stick characteristics, such as bearings and seals. Additionally, plastics like PVC are chemically nonreactive, making them suitable for use in electrical cable insulation and food containers.
The lightweight, flexible, and corrosion-resistant nature of plastics, combined with their ease of processing and low cost, make them a highly valuable and versatile material for numerous industries.
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Metals are easier to recycle than plastics
Metals are generally stronger than plastics due to their ability to be molded into complex shapes with ease and low expense, without sacrificing durability. While both materials have negative environmental impacts, metals are often easier to recycle than plastics.
Plastics and metals are two of the most common forms of packaging waste, with plastics making up 19% of total waste and metals (largely tinplate and aluminum) making up 5%. Plastic takes over 400 years to break down, while tin and aluminum take 50–200 years. As these materials take so long to degrade, they build up in landfills and ecosystems. Plastic can also pollute the land, rivers, lakes, and the ocean, and can contain additives and chemicals that may negatively impact human health and wildlife.
One of the main challenges in plastic recycling is its complexity. There are multiple types of plastics, and they must be separated and recycled in different ways at different recycling centers. Not all plastic can be recycled because it contains carcinogens and can release volatile organic compounds when melted. In contrast, metals like tin and aluminum are much easier to recycle. They can be easily separated from waste flows and landfills using large magnets, which has led to skyrocketing recycling rates in recent years. For example, the EU had a recycling rate of 72% for tin and aluminum in 2009.
Metals can be recycled an infinite number of times, contributing to a more circular economy. Aluminum cans, for example, can be recycled over and over. In comparison, only 9% of all plastic is currently recycled. Once plastic can no longer be recycled, it must be downcycled, sent to a landfill, or incinerated.
While the production of plastic may use less energy than the extraction of aluminum from its ore, the process of recycling plastic often requires additional energy and chemicals. Aluminum recycling reduces the amount of energy used annually in the production of aluminum and tinplate packaging. Overall, metals tend to have a higher recycling rate than plastics, making them easier to recycle and contributing to a more sustainable future.
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Metal is used in structural and weight-bearing applications
Metals are widely used in structural and weight-bearing applications due to their superior strength, durability, and load-bearing capacity compared to plastics.
One of the key reasons metals are favoured in structural engineering is their ability to withstand high loads and stresses without deforming or failing. Steel, for example, is an alloy of iron and carbon that offers unparalleled strength, making it ideal for constructing buildings, bridges, and other infrastructure. The high strength-to-weight ratio of metals like steel means they can bear significant loads while remaining relatively lightweight, which is essential for applications where weight is a critical factor, such as in aerospace or automotive industries.
Metals also possess excellent fatigue resistance, ensuring they can endure repeated loading and unloading without failure. This makes them suitable for applications requiring long-term durability, such as in machinery, tools, and structural components. Additionally, metals can be heat-treated to further enhance their strength and hardness, improving their load-bearing capabilities. For instance, SAE 52100 steel undergoes heat treatment to achieve high strength and resistance to cracking, making it ideal for high-load applications like ball bearings and roller bearings.
The versatility of metals in terms of processing and shaping also contributes to their popularity in structural applications. Metals can be easily moulded, cast, or welded into a wide variety of complex shapes, allowing for the creation of customized structural elements. This versatility, combined with their strength, makes metals indispensable in construction and manufacturing.
While plastics have their advantages in certain applications, they generally exhibit lower strength and stiffness compared to metals, making them less suitable for weight-bearing roles. However, advancements in metallurgy have led to the development of novel alloys that combine the strength of metals with the mouldability of plastics, offering new possibilities for weight-bearing applications.
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Frequently asked questions
Metals, especially alloys, are known for their high strength, wear resistance, and temperature resistance. However, certain plastics, such as PEEK and PEI, now have similar or even superior strength compared to metals. The durability of each material depends on the specific type, intended application, and environmental conditions.
Stainless steel is a metal that is known for its durability and ease of cleaning. It has a higher tensile strength than plastic.
Yes, novel metals have been developed that can be blow-moulded just as easily and cheaply as plastic while maintaining superior strength. These alloys, made from metals such as zirconium, nickel, titanium, and copper, can be used to create complex shapes like seamless metallic bottles and biomedical implants.
