Introduction to polymers in sustainable engineering

In 2011, the human population surpassed 7 billion (http://www.prb.org/Publications/Datasheets/2011/world-population-data-sheet/population-bulletin.aspx) raising fresh debate as to the ability of the earth to support our ever increasing numbers and levels of consumerism. However, the majority of this growth in population is occurring in the poorest countries, with the highest mortality rates and lowest levels of consumption. In fact, the greatest pressure on resources will come from increased consumerism in the developed and developing countries. Birth rates generally decline as populations become wealthier and healthier. Hans Rosling argues that by eliminating poverty, we can stabilise world population size (http://www.gapminder.org/videos/population-growth-explained-with-ikea-boxes/), but we will require new sustainable technologies to enable and maintain these improvements. Therefore, the main challenge for manufacturing is to meet the growing needs of these new consumers, whilst minimising the impact on the earth's resources through sustainable technologies. It was originally intended to publish a special issue that would focus entirely on bio-based polymers and oils. However, the scope has been expanded to include a wider range of approaches for improving sustainable polymer use to include methods and technologies in remanufacturing and recycling.

The use of renewable materials in manufacturing will become increasingly necessary to replace the use of current finite resources, such as fossil fuels, as they are increasingly depleted. However, renewable materials are only sustainable if their use does not exceed the limits of the system's ability to regenerate them. Thus, we begin this issue with an examination of the fundamental question, global carrying capacity, for renewable polymer production. The paper by Colwill et al. examines how the global capacity for biopolymer production by 2050 might be constrained by feedstock availability when other competing demands on agricultural land use are accounted for. This is followed by a paper that remains at a system level, considering the green supply chain for plastic films. Golghate and Pawar suggest the five steps that are required to develop a framework for the co-existence of ecosystems and plastic industry for a better environment. The third paper, by Doll and Sharma, investigates the physical properties of bio-based lubricant oils and shows how the kinematic viscosity and pour point of one such oil (soya) can be controlled through its blending in specific ratios with a synthetic ester. The fourth paper, by Chevali and Ulven, examines the effect of extrusion screw speed on biocomposite thermomechanical properties and suggests that optimising the residence time as a means towards achieving a balance of properties. This is followed with a review, by Medina-Gonzalez et al., of the use of supercritical CO₂ in several cellulose applications to improve subsequent processing or to create alternative material structures with unique characteristics.

Renewable materials, however, are only part of the solution. Managing our material resources throughout their lifecycle, in the most efficient and sustainable manner, will be vital in ensuring that we have sufficient materials to meet our future manufacturing needs. One such approach to utilising waste polymers is considered in the next paper, by Henry et al., which investigates the effects of combining rubber crumbs with recycled aggregates and fly ash on the mechanical properties and environmental impact of concrete. We continue the theme of finding alternative applications for recycled waste with our final paper of this issue, by Wang et al., which reports on a carbon footprint analysis of sign substrate material made from recycled e-waste in comparison to aluminium.