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Creating renewable plastics that don’t cost the Earth

Bioplastics of the future come in all shapes, colours and sizes. Achim Raschka

Imagine a future where packaging is made entirely from waste material and biodegrades to harmless by-products. Or where your home’s cavity wall insulation foam is made from captured CO2 emissions. Or where construction materials, vehicle components and engineering plastics are sophisticated biological composites comprised of tough cellulose fibres embedded in naturally derived polymers.

Such inventions are or are poised to enter the mainstream, driven by considerable consumer and economic pressure to replace conventional plastics made from petrochemicals with new materials, derived from natural sources such as plants or gases like CO2. Sustainable polymers like these offer some intriguing advantages compared to conventional petrochemical polymers, most of which were discovered more than 50 years ago.

Sustainable polymers are made from natural raw materials. While that need not in itself mean they are any greener than conventional materials, over the whole lifecycle of manufacture, use and disposal they can provide substantial gains. This is particularly obvious when they’re made from waste materials. For example if CO2 emissions from power stations are used to make insulation foam, this represents a means to lock-up carbon emissions and also put them to long-term use insulating homes, thus reducing emissions further.

Another key aspect of sustainable polymers is that they naturally contain oxygen in the form of oxides of carbon, carbon dioxide or carbohydrate for example. Petrochemicals are hydrocarbons (reduced forms of carbon) which means oxygen must be added to them, a process that often requires the use of toxic reagents or catalysts.

Some bio-derived polymers (although not all) are biodegradable. This can be an advantage in situations where recycling is not an option such as in some packaging or agricultural applications. In most other cases they are recyclable - although it’s important to ensure new bio-polymers don’t contaminate conventional plastics recycling streams.

The sophisticated structures of natural materials could bring improvements in the properties of new polymers. Using the natural chemistry of renewable resources more cleverly has to be a future goal, for example with built-in degradation, improved barrier properties for airtight packaging, and enhanced biodegradability, strength, or heat resistance.

Plastic from plants

Polylactic acid, or PLA, is a sustainable polymer derived from corn starch that has been on the market for a decade, mainly as disposable packaging. An important aspect of PLA chemistry is its chain tacticity - the arrangement of its polymer chains. By changing the stereochemistry of the molecules - the patterns in which they’re arranged - different properties can be emphasised.

Our team at Imperial College London have developed a new catalyst to prepare a new, more heat resistant form of PLA which will widen the range of uses PLA can be put to. Producing the new material cost-effectively will be the next challenge, but this class of material could replace common tough polyesters currently used for such things as housings for household appliances.

Adding cellulose, nature’s reinforcing agent, to polymers to improve strength is a method that aims to mimic the way plants and trees generate the strength to support their structures. Composite materials like this, with cellulose fibres reinforcing a matrix or resin composed of a naturally derived polymer, could deliver materials tough enough even for the vehicle industry, where bioplastics have struggled to match the properties of petrochemical plastics and resins. Recent research from KTH Sweden and from Imperial College London has highlighted different approaches to improving the compatibility between the cellulose fibres and polymers which will be necessary in order to make better composites in future.

Making solid CO2 gains

Other research has focused on polymers created from feedstocks other than corn starch. For example, Hillmyer and Tolman in Minneapolis have reported an interesting class of thermoplastic elastics prepared from the ester lactide and an extract of menthol from spearmint. In Konstanz, Germany, Mecking and co-workers have developed efficient chemical processes to transform natural fatty acids (which are the well known polyunsaturates found in oil crops such as rapeseed) into polymers with properties similar to polyethylene.

A route that has attracted considerable attention is the use of CO2 emissions to make polymers. Using suitable catalysts, it’s possible to co-polymerise CO2 with an epoxy to form polycarbonates which are between 30-50% CO2 by mass. This substantial sequestration of CO2 provides not only interesting materials but a means to put carbon emissions to use. These polycarbonates are also suitable to replace conventional petrochemicals in the manufacture of polyurethanes, widely used as insulation materials, stuffing foams, filler and in clothes (such as Spandex).

Many companies and academic research groups worldwide are working intensely on how to create processes that will sequester as much CO2 as possible. At Imperial College London, we have developed an intriguing class of catalysts, based on inexpensive zinc and magnesium, which use CO2 very productively at pressures as low as one atmosphere and using carbon dioxide which is heavily contaminated with water.

Such naturally derived polymers clearly have a bright future, with some materials already commercially available and others arriving in the next three to five years. The pace of research in this area is rapid and accelerating. Today, the major use is in packaging, but longer term these materials will expand to into most if not all markets that plastics currently rule.

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