Sweeter polymers

Plant based sugars are increasingly of interest as feedstocks to make greener, better performing plastics – including a polymer filler derived from sugar beet. Lou Reade reports

Biomass has become a rich source of monomers for making commercial bioplastics.

Polylactic acid (PLA) – the most widely used bioplastic – is typically derived from corn, for example. Other polymers, such as polyhydroxyalkanoate (PHA), are produced naturally by microorganisms. Even ‘conventional’ polymers can be made from non-fossil resources: petchems company Braskem, for example, makes some of its polyethylene (PE) from sugarcane instead of oil. The sugar is fermented into ethanol, which is then dehydrated into ethylene – and polymerised to form PE. The company is currently raising production by 30% – to 260,000t/year – at its plant in Triunfo, Brazil.

Braskem’s process is the most celebrated example of turning sugar into plastics. However, the final polymer retains no trace of any of the molecular structure of sugar. In other cases, the chemical structure of sugars – such as ring structures and specific linkages – are retained, in order to create polymers with unique features. One example is an elastomer with improved mechanical properties; another is a copolymer of PLA, which is more degradable than the ‘original’. Researchers have also looked to use previously discarded waste streams to make materials such as foamed plastics.

Strong elastomer

Researchers at Birmingham University, UK, have used a sugar called isosorbide to create a novel elastomer or synthetic rubber with superior elastic recovery and toughness, compared with conventional elastomers. It is also degradable and mechanically recyclable, according to research leader Andrew Dove (Angew. Chemie; doi: 10.1002/anie.202115904).

The aim of the research was to understand the stereochemistry, which plays a critical role in the material’s properties. Because the isosorbide has relatively ‘disordered’ chains, it promotes beneficial hydrogen bonding – increasing the polymer’s strength, elastic recovery and optical transparency.

‘Hydrogen bonding also allows the material to retain its properties after recycling,’ says Dove. An elastomer – or rubber – typically gets its properties from covalent crosslinking. These bonds will be degraded when a material is mechanically recycled. In Dove’s material, the hydrogen bonds naturally re-form after recycling – meaning that physical properties are maintained.

‘These covalent bonds are usually broken during recycling,’ he says. ‘Hydrogen bonding is like “dynamic” crosslinking.’

The team also used a different sugar –isomannide – to create an elastomer. The isomannide-derived material did not form hydrogen bonds, so had similar – though inferior – properties. As part of the research, the team created transparent thin films from both isosorbide and isomannide. These were thermally stable to 120°C. They were subjected to various tests, including uniaxial stretching, to determine their physical properties.

Typically, isosorbide-based polymers have high glass transition temperatures – making them stiff and brittle at room temperature. This is due to the presence of a fused ring structure that takes up lots of free volume. However, the new elastomer does not have the usual ‘hard-soft’ combination of ‘domains’ seen in typical elastomers. Instead, it uses a more precise chemistry to create an alternating structure – which helps to determine the physical properties.

‘The precise repeating unit is the key difference with conventional synthetic polyurethane elastomers,’ says Dove. ‘It means you don’t get the same type of phase separation.’

The research has been patented in collaboration with US-based Duke University. They are now looking for partners to help commercialise the material. Because the new elastomers are bio-derived – and both recyclable and degradable – Dove says they could be attractive for applications such as packaging. This is because packaging companies are increasingly interested in using sustainable materials. However, he says cost would be an issue due to the way they are produced.

The material was originally envisaged for biomedical applications. ‘Tissues in the body – such as tendons and skin – are elastic,’ he says. ‘In addition, the tensile curves for these materials are unusual.’

He says the elastomers have a ‘J-shaped’ stress-strain curve that more closely resembles the behaviour of natural tissues rather than synthetic elastomers. Their degradability also makes them an attractive proposition for biomedical applications, he says.

People say that PLA [polylactic acid, the most widely used bioplastic] is “biodegradable”, but that’s a tricky word. To degrade completely, PLA needs industrial composting conditions.
Antoine Buchard Centre for Sustainable and Circular Technologies, Bath University, UK

Sulfur links

At Bath University, UK, Antoine Buchard is using sugar chemistry to create new monomers and polymers. ‘Our starting point was to take carbohydrates and – with a minimum amount of chemistry – transform them into viable monomers,’ he says. ‘Once you have the monomer, you can use modern polymerisation techniques to make bio-derived materials that conserve the sugar core in the polymer backbone.’

A key part of the research involves replacing oxygen links with sulfur links, which helps to transform properties such as degradability, he says. The wide variety of available sugars – with their many functional groups – means that chemical properties can be ‘tuned’.

‘We’re trying to re-construct carbohydrate polymers using the tools of modern chemistry,’ he says.

In 2021, Buchard and his co-researchers reported how a sugar derivative called oxetane – containing a four-membered cyclic ether ring – undergoes ring-opening copolymerisation (ROCOP) with carbon disulfide. This creates a polythiocarbonate polymer (Polymer Chem.; doi: 10.1039/d1py00753j). The team employed a commonly used catalyst called CrSalen to lower the reaction temperature, allowing polymerisation to take place with lower energy input.

‘In this case, we think we can run at a lower temperature than oxetane alone, or with cyclic anhydride,’ he says. ‘That’s because of the presence of sulfur.’

Buchard says his team has since developed a cheaper, aluminium-based catalyst that works more efficiently than CrSalen – and is the subject of a future paper. Although CS2 is a notoriously toxic chemical, Buchard says that using it in a reaction like this may be a way of ‘remediating’ it – and ‘trapping’ it within a polymer structure.

While there are noticeable effects of replacing oxygen with sulfur, he says these are difficult to predict with certainty. In ongoing research, Buchard has replaced oxygen with sulfur in various positions and tried to figure out the consequences. ‘Sometimes properties are improved, sometimes they are reduced. The reality is not clear,’ he says.

However, one thing is certain: the presence of sulfur in a polymer linkage makes the material less thermally stable, more degradable and more sensitive to UV light than its oxygen counterpart. This makes it potentially important as the basis for degradable, bio-derived polymers.

‘On the back of oxetane/CS2 work, we realised that the linkages were degradable using UV light, but you don’t want a material that completely collapses,’ he says.

Breaking down

Buchard has also studied how sulfur linkages could help PLA to degrade more readily. ‘People say that PLA is “biodegradable”, but that’s a tricky word,’ he says. ‘To degrade completely, PLA needs industrial composting conditions.’

The most difficult step in degradation is to break the ‘big chains’, he says; once this has happened, shorter chains are more amenable to hydrolysis in the open environment.

Earlier in 2022, Buchard and colleagues reported how a copolymer of PLA will degrade under UV light. Here, L-lactide – the monomer used to make PLA – is copolymerised with a cyclic xanthate monomer (Chem. Comm.; doi: 10.1039/d2cc01322c). This introduces sulfur-containing thionocarbonate and thioester linkages. This polymer, with around 3% of sulfur-containing linkages, was found to lose 40% of its mass within six hours of UV exposure.

Buchard says the mechanism behind the degradation needs to be studied in more detail. Testing also needs to be performed on ‘real life’ objects. So far, it has only been done on powders. ‘This has the potential to increase the degradability of PLA without significantly affecting its material properties,’ he says.

Cooking wood

In June 2022, Swiss scientists produced a PET-like material from sugars derived from biomass. They transformed what they called ‘lignocellulosic biomass’ into a di-ester-containing monomer in 83% yield. This monomer can be polymerised using a variety of aliphatic diols to produce a polymer that is tough, heat-resistant and has a good oxygen barrier – potentially useful in packaging.

‘We essentially just “cook” wood or other non-edible plant material, such as agricultural waste, in inexpensive chemicals – to produce the plastic precursor in one step,’ says Jeremy Luterbacher, Associate Professor at EPFL in Switzerland. ‘By keeping the sugar structure intact within the molecular structure of the plastic, the chemistry is much simpler than current alternatives.’

EPFL researchers have created a PET-like material from natural sugars, and used it to create this 3D-printed structure
Alain Herzog (EPFL)

The polymer can be processed by standard injection moulding, extrusion and even 3D printing. The researchers have made packaging films, fibres that could be spun into clothing or textiles, and filaments for 3D printing. ‘What makes the plastic unique is the presence of the intact sugar structure,’ says Luterbacher. ‘It’s easy to make, because you don’t have to modify what nature gives you, and simple to degrade because it can go back to a molecule that is already abundant in nature.’

Although biodegradation studies still need to be performed, the polymer can be chemically recycled – via methanolysis at 64°C – and eventually depolymerised in room-temperature water (Nature Chem.; doi: 10.1038/s41557-022-00974-5).

Adding value

Whether used in food packaging or an automotive component, bioplastics can help to reduce a product’s carbon footprint. But this is largely negated if the polymer formulation incorporates conventional additives, themselves often derived from crude oil.

Plastics additives perform many functions, from preventing degradation to improving processability, so the focus is on developing additives, fillers and reinforcements with greener credentials.

One recent example comes from the Fraunhofer Institute for Environmental, Safety and Energy Technology (Fraunhofer Umsicht) in Oberhausen, Germany. Rodion Kopitzky, of the department of circular and bio-based plastics, has investigated the potential of sugar beet pulp (SBP) – a residue from the sugar industry – as a replacement for conventional fillers in biopolymer formulations. The main potential benefit is that SBP could act as a cheap, bio-based and biodegradable filler for bio-based compounds, says Kopitzky (Polymers; doi: 10.3390/polym13152531).

SBP has been considered for several applications, including as an animal feed and a solid fuel. For use as a plastics filler, Kopitzky compounded the SBP with several bioplastics, including PLA and polybutylene succinate (PBS). However, several factors limit the effectiveness of SBP as a filler. Firstly, its high pectin content gives it a high water-absorption capacity, compared with other fillers or typical agricultural fibres. In addition, compatibility between the SBP and polymer matrix is typically poor – though could be overcome using a compatibiliser such as glycerol, he says.

Scandinavian researchers are building a pilot plant to make compostable PLA bioplastic from soya molasses
Nordic Soya

Soya source

A consortium of four Scandinavian companies has developed a way of making compostable PLA bioplastic from waste products from the production of soya. The partners say the method offers a more sustainable alternative to using sugarcane – a food crop – as a raw material. Finnfoam, Brightplus, VTT and Nordic Soya have spent four years exploring how to convert soya molasses – which is unsuitable for consumption and typically incinerated – into a bioplastic foam.

‘The process will produce PLA from the side streams of soy production,’ said Finnfoam CEO Henri Nieminen. ‘This is a sustainable alternative to sugar- and corn-based PLA.’

Nordic Soya processes soya at a plant in Finland. Soya molasses left over from its processing was used as the raw material in the research. Globally, residues from soya production could produce around 22m t of bioplastic/year, according to the partners. They are building a pilot plant in Finland, which they plan to convert to a full-scale plant by the end of 2023.

Finnfoam intends to use the material to make thermal insulation for buildings. Its ecological quality is enhanced by the fact that thermal insulation also serves as a carbon sink, helping to reduce the carbon footprint of buildings.

‘Side streams that are unusable in food production can now be used to make high value bio-based products,’ said Jarkko Leivo, Technology Director at project co-ordinator Brightplus. ‘We can modify the material’s properties, such as its transparency and thermoformability, or improve its chemical resistance and reusability.’

Sugars and other types of biomass are unlikely to replace oil as the main source of polymers any time soon. Nova Institute, a German research organisation, says bioplastics production will reach almost 3m t by 2025. This is less than 1% of all global plastics production.

However, with pressure on organisations to improve environmental credentials – and consumers demanding greener products – production of plastics derived from sugar molecules, rather than oil fractions, is likely to keep accelerating.