In modern history, the linear economy has been prevalent, with the input of raw materials for processing and use, followed by waste at the other end. It hadn’t been much of an issue with abundant resources, minimal waste problems, and minor concern with carbon emissions.
However, while many resources are still plentiful, some are becoming more scarce. As waste has piled up, as well, with more difficulty finding a place for disposal, and as carbon emissions have instigated climate dangers, there is an increasing urgency to shift towards a rather circular economy.
What is a circular economy?
A circular economy, in essence, is designed to minimise waste by circulating products and raw materials rather than throwing them away after use. Reducing and reusing (and sometimes recycling) materials in products to eliminate waste is the main tenet of this, instead of using only new raw materials that create new waste.
Why is a circular economy important for the future?
With the increased emphasis on sustainability and less reliance on dwindling resources, a circular economy, even if implemented only in finite areas, would be a major improvement for the future of the planet in many respects. We would see significantly less waste ending up in the ocean, and we would lower our carbon footprint as a global society by decreasing a large portion of production processes.
Furthermore, we would also see:
- an improvement in waste management as landfills would take in less material
better security when it comes to the supply of raw materials
- improved economic competitiveness and economic growth by creating more jobs
- increased innovation across the board
What types of materials are ideal for a circular economy?
There will be a variety of materials needed for a functioning circular economy to exist. Materials and products that can be reused without excess processing, or at least as little of it as possible, are ideal. Recycling, and materials that are ideal for reuse in this manner, can be considered just as important.
However, in the strictest sense of creating an efficient circle with minimal processing, it is better to reuse before recycling when possible. Although reusing products in their current state is preferred, it’s not always realistic.
A circular economy isn’t about using biodegradable materials. It’s not so much that they are unnecessary as much as that they would require new input, thus introducing more material into the circular system. Biodegradable materials are best used in unavoidable situations where single-use items, such as forks and packages, are needed.
Transition metals for circular economy
Titanium is stronger than iron and only half as dense, with a general density of 4.54 g/cm³ and a tensile strength of 434 MPa at 20 °C. This combination makes it one of the best structural materials for the automotive, aerospace and marine industries – and a pillar of the circular economy.
Known for its high strength-to-weight ratio, corrosion resistance, and heat tolerance, (with a thermal melting point of 1668 °C, a specific heat capacity of 520 J/(kg·K), and thermal conductivity of 22 W/(m·K) at 20 °C), it is also used in cookware, personal electronics, and household appliances. It is a highly biocompatible material that is capable of being incorporated into the human body, making it the material of choice for bone implants.
Titanium in its pure form is a silvery metal known for its strength and low density compared to other similarly hard metals. In most industries, however, titanium alloy is much more commonly used. Continue reading here.
Iron is, without a doubt, the most widely used metal. This is due in large part to its natural abundance, low price, and an immense variety of applications. Pure iron has a general density of 7.87 g/cm³, a tensile strength of 540 MPa, and a specific heat capacity of 449 J/(kg·K) at 20 °C. It has a melting point of 1538 °C, yet rusts easily when it comes in direct contact with air.
This would make it problematic for uses in a circular economy, which requires functional materials in all aspects to remain a part within the circle. Luckily, a plethora of alloys, such as the different grades of steel, impart corrosion resistance (and other properties such as flexibility and hardness) to iron, rendering it one of the most versatile metals for worldwide applications in products perfectly suited to a circular economy.
Stainless steel is a class of iron-based alloys with a minimum chromium content of 10.5 wt.%. It is characterised by its superior resistance to corrosion in comparison to other steels. Continue reading here.
- Density: 7.6 g/cm³ at 20 °C
- Elastic modulus: 220 GPa at 20 °C
- Elongation: 46 % at 20 °C
- Poisson’s ratio: 0.3 [-] at 20 °C
- Tensile strength: 810 MPa at 20 °C
- Yield strength: 460 MPa at 20 °C
Gold has great ductility, malleability, and conductivity. That means it can be beaten into extremely thin sheets, is capable of being shaped or bent, and is a superior electrical conductor. It’s also a relatively soft metal, which is usually hardened by alloying with copper, silver, or other metals.
However, it has a relatively high density of 19.3 g/cm³ at 20 °C, and a high melting point of 1065°C. Its monetary value will always keep mining opportunities open, but that same value keeps it within the circle of use. Once gold is incorporated, it doesn’t leave, which is the aspiration for all materials in a truly circular economy.
- Electrical conductivity: 4.30E+7 S/m at 20 °C
- Electrical resistivity: 2.3E-8 – 2.9E-8 Ω·m at 20 °C
- Density: 19.3 g/cm³ at 20 °C
- Elastic modulus: 78 GPa at 20 °C
- Hardness, Vickers: 5 – 28 [-] at 20 °C
- Reduction of area: 91 % at 20 °C
- Tensile strength: 40 – 125 MPa at 20 °C
- Yield strength: 15 – 30 MPa at 20 °C
- Coefficient of thermal expansion: 1.39E-5 1/K at 100 °C
Alkali metals for circular economy
This metal so intertwined with our daily lives is best known for its use in rechargeable batteries. Lithium-ion chargers power nearly every laptop, cell phone, and almost every electric car on the market. Lithium is the lightest of all solid elements, with a general density of 0.54 g/cm³ and a Brinell hardness factor of 5 [-] at 20 °C.
It’s also the most reactive of all metals – and its importance lies in more than just batteries. When lithium is alloyed with some transition metals, it becomes more stable and can be used in specialized aircraft parts, vehicles, and potentially spacecraft. In essence, lithium can help products last a long time in a circular economy that values product longevity.
Other metals for circular economy
Aluminium can be used in a wide range of applications, from electronics to beverage containers. It is infinitely recyclable, reusable, and is used in everything from aircraft and power lines to construction materials such as ladders and doors.
The pure metal is relatively soft, with a Brinell hardness of 245 [-] at 20 °C, but it becomes stronger and harder when alloyed. It has good ductility and malleability, as well as being a good conductor for both heat and electricity, with an electrical conductivity of 38 million S⋅m-1 and thermal conductivity of 221 W/(m·K) at 20 °C.
Aluminium is arguably the most favoured material used in a circular economy because of its reusability and recyclability. It is also considered the “green metal”, as it is one of the most eco-friendly metals available.
The fundamental difference between Cast and wrought aluminum is easy to understand: Cast aluminium is the aluminium that was melted in a furnace and poured into a mold. Wrought aluminium is when the metal is worked in the solid form with the help of specific tools. Continue reading here.
- Density: 2.7 g/cm³ at 20 °C
- Elastic modulus: 70 GPa at 20 °C
- Hardness, Brinell: 245 [-] at 20 °C
- Poisson’s ratio: 0.35 [-] at 20 °C
- Tensile strength: 90 MPa at 20 °C
Plastics can play a crucial role in a circular economy. Since reusability, rather than biodegradability, is key, any plastic that can be used long-term without breaking down will be useful.
Compared to regular polyvinyl chloride (PVC), plasticized polyvinyl chloride (PPVC) has better tensile strength and flammability and lower temperature impact strength. Its general density of 1.16 – 1.35 g/cm³ and maximum allowed stress of 5 – 28 MPa at 20 °C make it a relatively ideal material for reuse.
Add in a maximum service temperature of up to 65 °C and specific heat capacity of between 900 – 1800 J/(kg·K), and it’s no wonder why PPVC is able to withstand difficult working conditions over a long product lifespan. It is already used prominently in pipes, tubes, handrail profiles, cable- and wire sheathing, and floor coverings, to name a few. We will likely see more PPVC in the future as the world moves away from single-use plastics and similar products.
- Density: 1.16 – 1.35 g/cm³ at 20 °C
- Elongation: 170 – 500 % at 20 °C
- Impact strength, Charpy notched: no break
- Impact strength, Charpy unnotched: no break
- Max allowed stress: 5 – 28 MPa at 20 °C at 170-500% strain
Moving towards a circular economy
Given that many of the key materials needed to sustain a circular economy are already widely used and widely available, transitioning to this type of system should be possible. The main issues will be whether consumer demand can shift the thinking away from single-use items and planned obsolescence towards reusable and long-lasting, quality products.
Perhaps it would work best if lifetime costs were taken into consideration, with more durable products warranting a higher price that could be compared against the lifetime cost of lower-quality products that require constant replacements.
It will be interesting to see if we move in this direction or stay closer to what most of us have known our entire lives.