Material of the month: bio-based polymers

Materials World magazine
,
31 Mar 2017

Simon Frost examines plastics derived from shellfish, bananas and a range of biomass materials. 

You might wonder why this material of the month – the final installation for the time being – is titled bio-based polymers, rather than the more compact ‘bioplastics’. Many would imagine a bioplastic would be derived from biomass sources such as cellulose, sugarcane or corn, but the term is actually a little broader. The most widely accepted definition is a plastic that is either bio-based, biodegradable, or both – so bioplastics actually include fossil-based polymers, too, if they are biodegradable. 

And that’s another somewhat misunderstood term. Several studies have shown that consumers tend to think biodegradable plastic can be flung into the garden composter and will decay like eggshells and onion peel. Most biodegradable plastics, however, will only decompose in an industrial composting facility. You can start to see why bioplastics are the subject of much debate in the packaging industry and environmental policy discussion.

Polylactic acid

An aliphatic polyester that falls into both bio-based and biodegradable classifications, polylactic acid (PLA) is derived from thermoplastically modified starch, and is one of the more established classes of bio-based polymer, appearing and behaving similarly to polyethylene or polypropylene. Products made from PLA include bottles, packaging films and even medical devices such as screws or plates that biodegrade inside the body within a desired timeframe, which can remove the need for follow-up operations.

In Asia, the most common biomass feedstock for PLA is tapioca – the starch extracted from cassava root. North American producers typically use cornstarch, while sugarcane from Thailand and Brazil, and sugar beet, grown widely in Europe, can also be used as raw materials. 

This is where the ‘food-versus-fuel’ argument, which has also been a topic of debate around biofuels, comes into play. Delphine Lévi Alvarès, Product Policy Officer for Zero Waste Europe, said, ‘What is important for us is that we develop criteria to ensure sustainability of bio-based materials. One is that it shouldn’t be competing with food crops. We really need to look at second- or third-generation bio-based materials – we don’t want to fall into the same tricks as we’ve fallen into with biofuels. We want to optimise land and water use and look at possibilities to use contaminated land that couldn’t be used for food crops.’

Although PLA is biodegradable, to support a circular economy and reduce the land footprint required to produce virgin stock, producers are steadily moving away from encouraging biodegradation as an end-of-life solution in favour of recyclability. In March 2017, Sapienza University of Rome, Italy, set up Resources from Urban Bio-Waste, a consortium of 21 entities from eight European countries aiming to develop feedstocks for bioplastics from urban organic waste. 

PLA process

To create PLA, the biomass feedstock is milled to extract the starch and then hydrolysed to convert the glucose into dextrose, which is then fermented to create lactic acid. Ring-opening polymerisation is the most common process to turn lactides into PLA – the lactide rings are chemically opened and linked together to form long chains of polylactide polymer using a metal catalyst in suspension.

As a thermoplastic material, PLA can be heated to its melting point and cooled to solidify repeatedly without significant degradation, making it relatively easy to recycle, in contrast to thermoset plastics such as polyester resin and melamine formaldehyde, which can only be set once.

The most common processing methods for PLA products are injection moulding and blowmoulding, but its low melting temperature also makes it a useful material for 3D printing filaments. 

Bio-PET

Coca-Cola introduced the first version of its PlantBottle, which was made from 30% bio-based polyethylene terephthalate (PET), in 2009. Standard PET, used widely in drinks bottles, pots, tubs and oven-ready trays is a combination of 32.2% monoethylene glycol (MEG) and 67.8% purified terephthalic acid (PTA), which are esterified and converted to a polymer in a polycondensation reactor (a similar method can also be used to produce PLA).

To make its bottles 30% bio-based, Coca Cola replaced the MEG content, from the hydrocarbon ethylene, with bio-MEG derived from the ethanol in sugarcane from Brazil or molasses from India. A benefit of PET made using bio-MEG is that the end product is the same as the material it replaces. It is known as a ‘drop-in’ solution, as it uses existing technology to replace a part like-for-like. Bio-PET shares the properties of toughness and temperature-resistance with its petrochemical equivalent, and can be recycled in exactly the same way as PET, the most widely recycled plastic. 

The 100% bio-PET bottle

In 2015, it was reported that Coca-Cola had distributed more than 35 billion 30% bio-based PlantBottles, but the main announcement was that the world’s largest soft drinks manufacturer had replaced the other 70% – PTA – with a bio-based paraxylene to create a cost-effective, recyclable and 100% bio-based PET PlantBottle at demonstration scale. 

The drinks giant enlisted the biochemical company Virent Inc, USA, in 2011 to develop its BioFormPX technology, which aimed to replace PTA with paraxylene derived from plant sugars through a patented combination of aqueous phase reforming (APR) and conventional catalytic processing. 

The APR process employs heterogeneous catalysts at moderate temperatures and pressures to reduce the oxygen content of the carbohydrate feedstock. The biomass is reformed to generate hydrogen, the alcohols are dehydrogenated and the carbonyls hydrogenated, followed by deoxygenation reactions, hydrogenolysis and cyclisation. 

The intermediate alcohols, ketones, acids, furans, paraffins and other oxygenated hydrocarbons formed during APR undergo further catalytic processing to generate a mixture of nonoxygenated hydrocarbons that can be used as drop-ins for both petroleum fuel and petrochemical products. 

Coca-Cola aims to scale up production of Virent’s BioFormPX material to commercialise the 100% bio-based PlantBottle by 2020 – and competition, the age-old accelerator of technological advance, could improve its chances of achieving that target. Nestlé Waters and Danone, for example, announced in March 2017 that they had partnered with California-based startup Origin Materials to increase their own 100% bio-based PET bottles, aiming to produce >60% bio-based PET by 2018, >75% by 2020 and >95% in 2022. 

A range of feedstocks

At a smaller scale, companies and research institutions are developing bio-based polymers with a variety of natural feedstocks. French start-up Lactips, a spin-out from the University of Saint-Etienne, is expecting its milk protein-based polymers to enter the laundry, water treatment and agrochemicals market in the third quarter of 2017, with dishwasher tablets in the final stage of development.

Banana peel provides the base for polymers developed at the University of Sonora, Mexico. Strips of banana endocarp are immersed in two antioxidants and then dried, lyophilised and mixed with either citric acid or propolis, a resinous mixture produced by honey bees to create a mouldable paste. When mixed with propolis, the biopolymer is food-safe and prevents the proliferation of bacteria and fungi, while the citric acid mixture a thinner, lighter material suitable for grocery bags is produced. 

The University of Strathclyde, UK, has even developed a polymer using a waste product of the seafood industry, spinning out a company called CuanTec in 2017 to manufacture starch-based biopolymers mixed with chitosan, an inedible biopolymer contained in shellfish that boasts antimicrobial and biodegradable properties. 

From cornstarch to bananas, sugar beet to sugarcane, polymers can be derived from an ever-growing range of biomass sources – and while they won’t fully displace fossil-based plastics any time soon, their uptake by industrial giants solidifies their role in the future.