PerspectiveMaterials

Toward a sustainable materials system

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Science  29 Jun 2018:
Vol. 360, Issue 6396, pp. 1396-1398
DOI: 10.1126/science.aat6821

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The circuit board of modern mobile phones contains more than 60 elements from the periodic table.

PHOTO: BLOOMBERG/CONTRIBUTOR/GETTY IMAGES

Global annual resource use reached nearly 90 billion metric tons in 2017 and may more than double by 2050. This growth is coupled with a shift of materials extraction from Europe and North America to Asia. In 2017, 60% of all materials were extracted in Asia, and extraction is expected to rise substantially in Africa over the next decade. Local extraction and processing helps to improve standards of living in the developing world, but also leads to important environmental concerns. Globally, materials production and consumption is coming up against environmental constraints in almost every domain, including species biodiversity, land-use change, climate impacts, and biogeochemical flows. Mitigating the impact of materials use is urgent and complex, necessitates proactive assessment of unintended consequences, and requires multidisciplinary systems approaches.

Materials consumption trends provide context to inform strategies for impact mitigation. Beginning in the mid-1950s, there has been a shift from biomass or renewable materials to nonrenewable substances, such as metals, fossil fuels, and minerals. Effective strategies for mitigating their impacts are different for high-volume materials with structural applications than for specialty materials with functional uses.

Materials Impacts from Extraction to Disposal

Impacts of materials extraction include landscape degradation, habitat loss, waste generation, decreased water quality, and ecosystem pollution, often in ecologically sensitive areas. Furthermore, energy use during this stage is substantial. For example, primary production of metals accounts for ∼8% of total global energy consumption; this energy consumption is expected to increase because of decreasing ore grade.

The impacts of processes such as refining and manufacturing mainly derive from energy use. However, direct emissions from process chemical reactions can also be significant. For example, in cement production, direct CO2 emissions are caused by calcination of limestone into lime—accounting for 50% of cement production–related emissions, the remainder resulting from electricity and fuel consumption (1). Other direct air, water, and soil emissions can also be substantial, causing damage to ecosystems and human health.

Most impacts during materials use originate from the fuel and electricity needed to power machines. However, metals may also corrode during use, leading to environmental release; similarly, chemical contaminants, such as refrigerants, may leak from products during use.

Impacts from disposal include land use from landfilling, loss of resources, and in the case of metals, potential for toxic leachate with landfilling and atmospheric emissions upon incineration. For polymers, degradation and mismanaged disposal leads to long-lasting microparticles that persist and accumulate in the food supply. For minerals, construction waste is a growing environmental concern particularly around land use in certain regions. Game-changing impact reduction can only be expected as a result of unprecedented changes in technology or consumption patterns for materials throughout their life cycle.

Toward a Sustainable Materials System

Materials alone are not particularly useful: People typically do not set out to own coils of steel or forests of carbon nanotubes. They may not even want products, such as cars or phones. What humans desire is the services delivered by materials and products. As a society, we develop infrastructure and devices (or a physical stock of materials) to provide for human activities such as sustenance, shelter, communication, and education. As much as half of annual global materials extraction is used to build up or renew in-use stocks. A sustainable materials system creates and maintains these stocks with minimized material and energy flows.

However, the material flows to support physical stock vary vastly by region. North America consumes 30 tons of material per capita, Europe 21 tons per capita, and all other regions under 10 tons per capita. This variation underscores the imperative to decouple economic development and enhanced quality-of-life from materials consumption. Such a decoupling requires a profound transition in technology design and in how businesses create value from technology. For example, servicizing business models, where use of a product is sold rather than the product itself, aim to make more intensive use of existing stocks. Examples of service business models already exist, but turning products into services at scale has proven challenging and the net environmental impact is uncertain; there may be a trade-off in use of consumables, for example (2).

There are also material-focused transformative strategies. The immense scale of steel and concrete production means that any desire to improve the environmental sustainability of materials must involve a shift in these industries.

A paradigm shift in steel production could involve cost-effective, direct electrolytic reduction of iron oxide to metal, coupled with renewable-energy-based electricity. Advances in molten electrochemistry could support this direct production of iron from oxide feedstock. These approaches could be applicable to other metals, such as copper (3).

Significant environmental improvement in concrete impact requires substantial carbon sequestration or development of scalable alternatives to cement binders. Here, viable strategies will vary depending on whether the concrete is steel-reinforced and whether the available quantity of replacement raw materials can meet demand (4).

Polymers are not as significant by mass as metals, but nevertheless require a shift because of their large volume fraction in waste streams, continued exponential growth, and persistence in the environment. Here, materials innovation involves development of monomers that enable effective separation, chemical recycling, thermoset reprocessing, and enhanced biodegradability (5).

Five Levers to Minimize Materials Impact

Even as tractable, environmentally beneficial, revolutionary business models or technologies emerge, they take time to implement. Therefore, more evolutionary strategies must also be pursued with urgency. We examine five levers for scientists and engineers to consider to minimize impacts: lifetime extension, dematerialization, manufacturing efficiency, substitution, and recovery (see the figure) (6). These individual strategies must be evaluated in concert, in practice, and from a life-cycle perspective, because efficiencies in one dimension may be correlated with increases elsewhere.

Lifetime extension

Because so much of materials consumption feeds physical infrastructure, reductions in demand can be achieved through extended service lifetimes by improving the durability, maintenance, and utilization of existing stocks. Trace contaminants or defects can lead to materials degradation, reducing lifetime and recovery potential. Researchers should test new materials and devices in nonidealized conditions and contribute to accelerated testing efforts; here, funding agencies are critical. Business model innovation should also play a role to counteract planned obsolescence.

Dematerialization and manufacturing efficiency

Design of more effective stocks can also help to make more efficient use of a given material for a given function. Examples of materials engineering success that has led to dematerialization, include solid-state transistors, higher-transmission energy lines, and alloy design (7). Vehicle light-weighting has been enabled by more effective part design and use of alternate materials, including high-strength steels, aluminum, and composites, leading to lower material intensity per part. This strategy also leads to fuel savings. Further alloy development toward both higher strength and higher ductility—properties that are often at odds with each other—could enable profound savings (8). Manufacturing efficiency is often coupled with cost reduction, but despite this correlation with economic savings, there is still room to improve. For example, 25% and 40% of steel and aluminum, respectively, are lost as scrap during production steps such as casting, forming, and fabrication. Even specialty, functional materials, such as carbon nanotubes, have substantial manufacturing losses, with yields often in the single digits (9). There are several efforts to streamline synthesis processes for nanomaterials through direct synthesis routes.

Substitution

A sustainable system uses materials with minimal per kilogram impact. The impact per unit mass includes material supplied from primary or recovered sources. One strategy, therefore, includes substitution (complete or partial) for materials with lower environmental impact. The scientific community has long had a role in developing material substitutes; however, the aim has typically been to improve technical performance or reduce use of toxic or difficult-to-source materials, rather than more comprehensive environmental impact. The substantial time lag between invention and impact assessment has led to myriad cases of ad hoc, reactionary policy, particularly in the area of toxicity and human health impacts. There are promising initial approaches to predict life-cycle and risk assessment of emerging materials by scaling laboratory-level data on materials and energy inputs, coupled with industry handbooks (10). These can be combined with evolving data science methods to probe and suggest material synthesis routes early in development (11). These methods should integrate more closely with experimental research and be validated with production-level information.

Environmental life cycle of materials

Current materials systems are fraught with inefficiencies. Strategies toward sustainability aim to extract more value from materials in use while minimizing extraction and waste flows throughout the life cycle. Interventions must be evaluated in context because improvement in one phase may lead to increased burden elsewhere. See supplementary materials for suggested further reading.

GRAPHIC: N. CARY/SCIENCE

Recovery

Because materials or components derived from nonprimary sources typically require less energy in manufacturing, another strategy is to increase recovery (including reuse or recycling). The ability to retain value and any impact that results will vary by material. Component reuse and repair have long been in decline due to rising product complexity, shortened life cycles, de-emphasis by manufacturers, and societal norms. Design efforts should focus on current perceived limits in the degree of modularity, what parts can be made accessible for replacement, and consumer uptake.

Materials recovery from products is easier when they are pure and valuable; however, many products in use today are mixed. This increase in entropy makes mixed low-value materials expensive and energy-intensive to recycle (12). Energy reclamation may be the best use of mixed plastics given the substantial materials degradation with each recovery cycle, the energy used in sorting, and transportation required to consolidate volumes. For metals, recycling efficiency is limited by the thermodynamics in the processing stage, and the displacement of primary material with recycled material depends on the alloy, product, and overall market (13).

The scientific community needs to consider potential trade-offs quantitatively so as to avoid unintended consequences, in which we improve one aspect to the detriment of another part of the materials system or life cycle. For example, although there are some key opportunities in dematerialization, increased materials efficiency is typically coupled with increased demand or functionality—such as larger or more accessorized cars negating fuel savings. The need to scale these approaches must also be considered.

Charge to the Technical Community

The strategies presented here are not novel; we can find individual examples of success throughout the life cycle and for different materials. The current sustainability challenge is that scientists and engineers must embrace the complexity, anticipate potential trade-offs, quantify multiple performance objectives, and estimate scaled impact during initial research and development.

Recovery practices and technologies have not kept pace with the acceleration of complexity and scale in materials development. A demonstration of this complexity can be seen in the number of elements found in a circuit board: This number has increased from 11 in the 1980s to more than 60 in the circuit board for modern mobile phones (14). Many of these constituents are lost at end-of-life; this loss includes not only the materials but also the value added by the manufacturing process. To overcome this problem, compatibility upon recycling should be considered in alloy design. The complexity of recovery route options and the fate of materials within those options should also be considered in product design. Predictive simulation-based process modeling can help to design refining infrastructure that would recover the most material and to develop integrated policy that addresses both sources and sinks for materials streams (15).

Materials and production systems are driven by economic incentives, but given current understanding of associated externalities, these incentives only tell part of the story. Governance and engagement have become increasingly important; the vast majority of the strategies mentioned in this perspective require policy to support technology transition. Researchers should evaluate their technologies using multiple performance objectives and then communicate their findings in a way that policy-makers find actionable. As a society, we should educate scientists and engineers on how to perform these assessments, engage with stakeholders, and then provide incentives for systems-based environmental analysis coupled with fundamental research. With over 60% of the urban infrastructure that is expected to exist by 2050 yet to be built and urban population doubling in the coming decades, the opportunity exists now to shape the future of humanity.

Read more articles online at scim.ag/TomorrowsEarth

References and Notes

Acknowledgments: We acknowledge useful discussions with J. S. Krones, H. J. Uvegi, and R. J. Myers.

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