Material Choices and Life Cycle Assessment
Choosing the Right Material
Chemists routinely select materials by measuring and comparing their physical properties against a product specification. A product specification is a list of the requirements a material must meet for a particular use — for example, a bicycle frame needs to be strong in tension, lightweight, and resistant to corrosion. The key properties used to compare materials include: melting point, softening temperature (for polymers), electrical and thermal conductivity, strength (in tension and compression), stiffness, flexibility, brittleness, hardness, density and ease of reshaping.
The four main categories of engineering materials are: ceramics (glass, clay), polymers (plastics), composites (concrete, carbon fibre reinforced polymers), and metals. Composites allow the properties of several materials to be combined — for example, steel-reinforced concrete is strong in both tension (from the steel) and compression (from the concrete).
Materials are selected by comparing measurable physical properties against a product specification. Key properties include strength (tension and compression), hardness, density, electrical conductivity, melting point and ease of reshaping. The right choice depends on which properties are most critical for the application.
What Is a Life Cycle Assessment?
Choosing a material by properties alone is not enough — we must also consider the environmental cost of the whole product’s life. A life cycle assessment (LCA) evaluates the environmental impact of a product at every stage of its life. The four main stages are:
1. Extracting and processing raw materials — energy, land disruption, water use, and emissions from mining or harvesting. 2. Manufacturing the product — energy and water used in factories, waste generated, and transport of components. 3. Using the product — energy consumed during use, maintenance materials, and transport of the product throughout its working life. 4. Disposal at end of life — whether the product goes to landfill, is incinerated (releasing gases), or is recycled or reused.
A common LCA comparison is a single-use plastic bag against a paper bag. At first glance, a paper bag seems better because it is biodegradable. But an LCA reveals a more complex picture: making one paper bag uses more water and more energy than making one plastic bag, and produces more greenhouse gases during manufacture. The plastic bag, however, is non-biodegradable and accumulates in ecosystems if not collected. If the plastic bag is reused several times or recycled, its total environmental impact may be lower than multiple paper bags used once each. This shows why LCA judgements are not straightforward — the answer depends on usage pattern, disposal infrastructure and which impacts are given greatest weight. Different analysts can reach different conclusions, which means LCAs have an element of subjectivity and must be evaluated critically.
Recycling and Reuse
At the end of a product’s life, the preferred outcome (from lowest to highest environmental impact) is: reuse → recycling → incineration with energy recovery → landfill. Reusing a product requires no energy for reprocessing — it simply extends the life of existing materials. Recycling reduces the need to extract new raw materials and saves energy compared with making products from scratch. For example, recycling aluminium uses only a small fraction of the energy needed to extract it from its ore by electrolysis.
The PET (polyethylene terephthalate) drinks bottle provides a well-studied example. Collected PET bottles can be shredded, melted and remoulded into new bottles or extruded into fibres for clothing and insulation. This recycling is economically viable because PET is abundant in the waste stream, has a reliable market, and the energy saving compared with making new PET from crude oil is substantial. However, viability depends on factors including: the finite nature of crude oil (making virgin PET increasingly expensive), collection infrastructure, sorting complexity, and consumer behaviour.
LCAs are limited by incomplete data, difficulties in quantifying some impacts (such as noise, visual impact or biodiversity loss), and the choices analysts make about which factors to weight most heavily. Companies may selectively publish LCA data that supports favourable marketing claims — so when reading an LCA, consider who funded it and what data was included or excluded. Decisions about sustainability require weighing scientific evidence alongside values about which environmental impacts matter most. This is a core example of how science and society interact in making policy decisions.
An LCA covers four stages: raw material extraction, manufacturing, product use, and disposal. It assesses water use, energy use, emissions and waste at every stage. LCAs can be partly subjective — different analysts may reach different conclusions depending on what they measure and how they weight different impacts.