Glues may make up a relatively small proportion of individual products. Yet with around 14.7 million tonnes produced globally each year, the environmental impacts of adhesives add up. The solvents in liquid glues contaminate soil and groundwater while many wood adhesives contain the carcinogen formaldehyde. Almost all are made from petroleum-based chemicals, contributing to overall emissions. Manufacturers must look to bio-based options to achieve deep sustainability in their products.
The environmental integrity of key green transition technologies also will depend on the availability of low carbon, non-toxic adhesives. The electric vehicle industry is already a major consumer of adhesives, which are easier to deploy than welding for joining different materials. This EV adhesives market is projected to grow a stunning 48 percent from 2021 to 2030. Minimising the environmental harms of future transport depends on generating a high-volume supply of green adhesives, starting now.
Luckily, bio-based alternatives for both medical and industrial purposes are hitting the market, making it easier than ever for companies to make the green chemicals transition.
Thermoplastic adhesives: the workhorses of the packaging industry
Found inside coffee cups and other food packaging, thermoplastic adhesives are the most common class of adhesives. These practical glues are easy to work with. They are solid at room temperature and capable of being melted before application. The problem is that their stickiness comes from petroleum-based polymers.
Packaging sealants based on renewable bio-polymers are now available from several major adhesives manufacturers. One of them is the UK Sealock Group, which recently unveiled a thermoplastic adhesive polymer created from corn and sugar cane. Although marketed as a way of securing the inner coating of paper cups, it can be used for any consumables packaging and in disposable cutleries. Combined with fully degradable packaging materials, this sealant makes it possible to achieve products with the lowest possible environmental impact.
Dow Packaging is the largest chemicals company to have entered the bio-adhesives market. In 2021, their released their new bio-based AFFINITY GA polyolefin elastomers (POE). This is a hot melt adhesive, a type of thermoplastic adhesive that comes in the form of a solid polymer block. Once heated, melted, and applied to surfaces, it solidifies upon cooling. Usually, hot melts contain Ethylene-vinyl acetate (EVA), the same petroleum-derived polymer used in clingfilm. Dow’s AFFINITY adhesive instead uses bio-polymers that do not compromise on performance. It is also cost-competitive with oil-based polymer glues, making it an attractive alternative for packaging manufacturers.
The EU’s 2019 legislation banning single used plastics have pushed European companies to adopt more bio-based adhesives in their packaging. However, for bio-glues to make a substantial dent in the petroleum-based adhesives market, renewability will not be enough. To incentivise industry, bio-based glues will also need to deliver unique features that synthetics cannot offer. This is what Conagen have achieved with their de-bondable adhesive released in March 2022. It is made from natural biomolecules engineered using fermentation technologies and is capable of bonding metals, plastics, and glass in cars, boats, and planes.
What distinguishes Conagen’s adhesive from other industrial glues is that it is reversible, meaning it can re-heated to its original state and used again. This is rare for industrial adhesives as their strength normally owes to a chemical curing process that causes permanent structural changes to the substance. Conagen’s reversible glue opens the way for waste-saving and circular opportunities. For example, manufactured parts do not have to be thrown out if bonded incorrectly and electronic devices can be easily dismantled at the end of their lives.
Conagen’s success in developing and scaling a high-performance biobased adhesive owes to their experience in creating other fermented bio-materials at industrial volumes. However, specialised companies focused on bio-adhesive innovations are also springing up.
Industrial bio-adhesives inspired by nature
The world’s most versatile and heavy-duty glues are found in the animal kingdom. In the last decade, scientists have been prospecting for sticky chemicals in the bodies of coastal animals. Many research groups have converged on the mussel as a model organism that could inspire the next generation of adhesives.
The shoreline is a volatile environment where adhesive capabilities offer small creatures a huge survival advantage against the wear and tear of rain, wind, and waves. The mussel is a exemplar of this. It emits hundreds of threads loaded with a protein called cathechol, made from a special amnio acid known as DOPA. This substance is sticky enough to attach to Teflon.
After a decade of development supported by the Office of Naval Research, Purdue University researchers recently managed to replicate and enhance this mussel glue. They extracted its active ingredient catechol and inserted it into a biomimetic polymer. This brand-new hybrid material is called poly-cathechol-tyrene or PCS. It is 17 times stronger than the natural glue produced by mussels and promises applications in medicine, aerospace engineering, automotives, cosmetics, and construction. Bio-based glues like this are highly competitive with petro-based versions. Not only are they degradable and renewable, but they also come with unique properties. PCS for example is fully water-resistant, something that synthetics have always performed poorly on.
Just this March, the Purdue lab found investors for PCS. Their technology has been licenced by Mussel Polymers Inc, a specialist in spinoff commercialisation created by Wardenclyffe Chemicals Inc. PCS is now the first new adhesive chemical formula to be commercialised in decades.
Bio-adhesives for medicine
Bio-based glues are attracting strong interest from the biomedical materials sector. They are readily biodegradable, a property essential for treating internal injuries but rare among synthetic materials. The human body is also more likely to accept materials from other living organisms since it shares their fundamental biological building blocks.
Compared to bio-based industrial adhesives, those developed for medical purposes undergo a longer research and trial process. Even the most high-performance organic adhesives require laboratory tinkering or synthetic additives before they can serve specific medical functions. The dynamic environments inside living bodies place very particular demands on bio-based adhesives. They must cling onto tissue firmly whilst also being able to accommodate any structural changes that occur during the tissue healing process. Their chemical composition must be such that it does not disrupt key biological processes, particularly those that aid healing.
Nonetheless, looking to nature provides useful R&D shortcuts. Organic substances offer ready-made, highly complex chemical formulations which are difficult to engineer from scratch. Sandcastle worms, another coastal animal, secretes a highly water-resistant glue they use to bind sand and shell particles into protective shelters. The US startup RevBio have used this compound to create an orthopaedic bio-glue for bone fracture repair. This innovation utilises an amino acid called OPLS found in the worm’s sticky protein. Combined with calcium phosphate, it produces a gloopy material that sets solid when applied to bone. This bio-cement can help heal breaks as well as augment bone density in women suffering from osteoporosis. Currently, this technology is undergoing its first human clinical trials.
Other innovations in medical bio-adhesives are currently seeking investment. Like the team at Purdue, researchers at Northwestern University have been looking at mussel glue. Led by Dr Nathan Gianneschi, they have created a wound-healing medical glue that fixes live cells and surrounding molecules in place. Rather than simply lifting adhesive proteins from the mussel, however, they altered its structure. While the amino acid chains in mussel glue naturally form a linear chain, Gianneschi’s team arranged them into parallel lines. Once implanted onto a synthetic polymer structure, it creates one of the world’s strongest medical glues. Gianneschi is awaiting a patent on his restructured amino acids.
In August 2021, MIT researchers announced another new medial bio-adhesive inspired by barnacles. This potentially life-saving substance attaches to surfaces covered in blood. It seals within 15 seconds of application, improving substantially on existing blood-stemming materials that take minutes to set. Their innovation could make the difference between life and death in an emergency response situation.
Many more natural bio-glues are waiting to be explored. Among them are the residues that bind pollen to flower stigma, salamander slime that attach to fatty substances, and the binding materials of butterfly cocoons. Thanks to their complex bio-molecular formations, these organic sealants will outperform synthetics on therapeutic value, degradability, and biocompatibility.