Solvents are small but mighty. These industrial chemicals are everywhere in industry, with total annual use estimated at 20 million tonnes annually.
Biobased solvents can lower the environmental footprints of industries as wide-ranging as mining and nutraceuticals and as novel as printed electronics. Here are the areas where these green chemicals are making a mark.
Phasing out toxic petrochemicals
A solvent is any substance that dissolves a material without changing anything chemically about it. Water is the classic example, capable of dissolving more substances than any other liquid.
Pharmaceuticals, printing, engineering and construction all need various solvents to manufacture their wares. Yet most are still made from crude oil, resulting in chemicals with damaging health effects.
Today, many industrial solvents are volatile, flammable, and explosive, as well as toxic to human health and the environment. Shockingly, they have even found their way into the food and ingredients industries – petroleum-derived chloroform or dichloromethane are commonly used to extract amino acids, flavonoids, proteins, and phenolic compounds from plants.
There is increasing demand for ‘greener’ biobased solvents that are lower impact, lower cost, biodegradable, and easily obtained from renewable sources. Many of the major solvent-using industries are starting to adopt them.
Paints and inks
The paint and coatings industry makes up the biggest slice of solvent demand at around 46% with the most common chemicals being ethyl acetate, butyl acetate and methyl ethyl ketone.
Paints rely on liquid solvents to suspend pigments and render them mixable and spreadable. They can also influence the paint drying process, the paint’s thickness and flow. After they are applied, solvents are meant to evaporate into the air.
Some paints can go fully solvent-free and many of those on the market today are water-based. However, some kind of solvent is still essential in certain applications, for example, protective paints that guard the layer underneath.
Biobased solvents that can be used in coatings are n-butanol, acetone and propylene glycol. While plant-based solvents can be just as toxic when released into nature – they are after all, powerful chemicals – they are considered to release fewer air pollutants than their petrochemical versions.
Chicago-based Vertec is a major biobased solvent supplier that offers a range of solvents for coatings and paints made from corn, soybeans, citrus and other plant feedstocks.
Larger players in the chemicals and biobased spaces usually have their own offerings. European industrial giant Umicore supplies biobased solvents such as its Valires RD derived from soybean. Agribusiness corporation Cargills also produces a biosolvent named ‘Agri-Pure’, a vegetable derived substance with multiple uses including fro dissolving inks and paints.
Valorising winery waste
Thanks to their ability to selectively draw out target compounds from different kinds of waste streams, biobased solvents can bridge between different segments of the bioeconomy, creating circular pockets within it.
One example of this is how biobased solvents could forge stronger supply chains between the viticulture and organic healthcare industries. Researchers found a way of drawing out bioactive compounds like phenolic compounds from winery waste using biobased 2-MeTHF – molecules with antioxidant and antimicrobial properties that are commonly used as a functional ingredient.
2-MeTHF has gained traction as a substitute for hexane, a solvent made from crude oil which is used to extract edible oils from plants and in cleaning agents. The substance is also a renewable alternative for dichloromethane, used in paint removal, adhesives, cleaning fluids, chemical processing and foam manufacturing.
Conventional oil-based hexane has been classified as neurotoxic or environmentally unsafe with inhalation leading to dizziness, nausea and headaches. 2-MeTHF still holds chemical hazards – it is highly flammable – yet exhibits far fewer health concerns and can be made from corncobs or even waste ingredients such as bagasse. This green chemical is generally quite abundant – suppliers are concentrated in China, with the US in second place followed by Germany in the United Kingdom.
A solvent for printed electronics
Green solvents are also finding uses in electronics. While the sector may seem distant from the chemical industries we normally associate with solvent use, emerging tech for printable devices is poised to fuel demand for sustainable inks.
Cyrene is one of the best known recent examples of a truly innovative biobased solvent for electronics. Originally developed in the Green Chemistry labs at the University of York, it was awarded a ‘Bio-based Innovation of the Year’ prize in 2017 for its wide-ranging potential uses and its potential to underpin more sustainable low-cost printed electronics.
Made from plant cellulose, the solvent was able to replace traditional solvents in graphene ink – the kind used in inkjet printers, where graphene flakes are dispersed in solvents.
The development of Cyrene was important from a commercial perspective because the product could lower the health impacts of printed electronics, a potentially high-growth industry.
A 2018 Nature article showed that biobased Cyrene solvent even performed when used to screen-print a graphene antenna – a high frequency antenna used in enhancing radio communications.
The paper confirmed that inks made using Cyrene can be used to print other kinds of devices too. Cyrene’s performance in printed electronics owes to how the biobased solvent does not interfere with the conductive properties of the graphene.
Extracting from trees
Biobased solvents can support sustainable scaling in the bioeconomy at large. As well as dissolving and suspending materials evenly, some can selectively extract chemical compounds from another material – something critical for processing raw materials.
The extractive power of certain solvents can be used to transform formerly value-less waste byproducts into feedstock, all without the need for environmentally harmful chemicals. Wood is a key example.
The lignin inside woody material is in high demand as a biomaterial input as it can form the basis of a huge variety of materials, chemicals, and fuels. Yet despite its abundance, lignin is relatively costly to extract and process. Since all wood contains lignin, a sustainable and cheap biobased solvent could unlock vast volumes of as-yet uneconomic waste wood from the forestry industry.
Today, lignin is extracted using solvents that are harmful for the body and the environment. These include sulfuric acid and sodium hydroxide. Many researchers have recently converged on biobased chemicals known as deep eutectic solvents (DES) as a replacement for these. DESs are milder than the chemicals generally used in industry and require far lower temperatures to be effective, further saving on environmental impacts.
Among those investigating DESs for lignin extraction are scientists at the Wageningen University in the Netherlands. A group at its Food and Biobased research centre have pioneered an early investigation into a partly biobased lignin solvent based on lactic acid and choline chloride.
This biobased solvent is designed to boost lignin’s industrial applications by making the extraction process still efficient while avoiding some of the environmental impacts of conventional chemicals. The lignin which results from the DES developed by the team is of higher quality than that of conventional solvents.
Extracting with trees
One of the many applications of lignin is as a solvent. Some types can even be used to extract metals. The organisation Swedish Mining Innovation is researching whether current fossil-based ‘flotation reagents’ could be partially or totally replaced with lignin particles. Flotation reagents are a key metals processing technology, used to separate target metals from other materials but many are toxic to ecosystems.
In a pre-study, researchers at Swedish Mining Innovation found that lignin particles could selectively pick up on copper and other minerals, a process now protected under a Swedish patent.
In semi-pilot plant trials, the LIGNOFLOT researchers found that just very small concentrations of the lignin particles were necessary to improve flotation performance. Thanks to the biobased material, the use of petrochemical xanthate in the flotation process could be reduced by 75%, cutting the hazards associated with the process.
Biobased solvents are also being tried in lithium ion batteries for EVs. A team of Italian and German chemists reported a ‘novel bio-solvent’ lactic acid could be used as the electrolyte within the batteries.
The electrolyte component constitutes the ‘in-between’ of a lithium ion battery, a space where the ions move between the cathode and anode on either side. Once combined with conducting salts, the biobased material showed hgh electrochemical performance in a lab-scale battery demonstration.
This experimentation with biobased solvents in advanced industrial applications shows how mature the field has become. Attention to green solvents appears to only be increasing because the use of crude oil chemicals have become toxic in a PR sense.
Growing awareness of the problem has already led large players in the pharma industry such as Pfizer and GSK to throw resources at the problem. In the 2000s, Both companies have produced guides and classifications intended to inform industry best practices on sustainable solvent selection, highlighting the most toxic chemicals in usen today alongside their potential replacements, often biobased.
Biobased solvents must hit a large number of additional criteria that oil-based solvents do not have to: as well as performing well, it must be obtained from renewable feedstock such as plants, available widely and potentially cost-competitive, as well as being biodegradable without producing toxins. Although biobased solvents must do more than petrochemicals in order to compete, it is an area set to grow thanks to industry demand and considerable research attention.
