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Silk could be used to replace many microplastics commonly incorporated into products used for agricultural chemicals, paints, cosmetics, and detergents, MIT scientists have said.
Some microplastics are intentionally added to a variety of products, amounting to an estimated 50,000 tons a year in the EU alone. The EU has already declared that these added, nonbiodegradable microplastics must be eliminated by 2025, so the search is on for suitable replacements, which do not currently exist.
The microplastics widely used in industrial products generally protect some specific active ingredient (or ingredients) from being degraded by exposure to air or moisture, until the time they are needed.
They provide a slow release of the active ingredient for a targeted period of time and minimise adverse effects to its surroundings. The materials used today for such microencapsulation are plastics that persist in the environment for a long time and with no currently available substitute that is both practical and economical.
Unlike the high-quality silk threads used for fine fabrics, the silk protein used in the new alternative material is widely available and less expensive.
While silkworm cocoons must be painstakingly unwound to produce the fine threads needed for fabric, for this new use non textile-quality cocoons could be used and the silk fibres would simply be dissolved using a scalable water-based process.
The processing is so simple and tunable that the resulting material can be adapted to work on existing manufacturing equipment, potentially providing a simple “drop-in” solution using existing factories.
In lab tests, the researchers demonstrated that the silk-based coating material could be used in existing, standard spray-based manufacturing equipment to make a standard water-soluble microencapsulated herbicide product, which was then tested in a greenhouse on a corn crop. The test showed it worked even better than an existing commercial product, inflicting less damage to the plants.
“There is a strong need to achieve encapsulation of high-content actives to open the door to commercial use,” said MIT professor Benedetto Marelli
“The only way to have an impact is where we can not only replace a synthetic polymer with a biodegradable counterpart, but also achieve performance that is the same, if not better.”
By precisely adjusting the polymer chain arrangements of silk materials and addition of a surfactant, it is possible to fine-tune the properties of the resulting coatings once they dry out and harden. The material can be hydrophobic (water-repelling) even though it is made and processed in a water solution, or it can be hydrophilic (water-attracting), or anywhere in between depending on the application.
“To encapsulate different materials, we have to study how the polymer chains interact and whether they are compatible with different active materials in suspension,” said MIT postdoc Muchun Liu.
The payload material and the coating material are mixed together in a solution and then sprayed. As droplets form, the payload tends to be embedded in a shell of the coating material, whether that’s the original synthetic plastic or the new silk material.
The method can make use of low-grade silk, unusable for fabrics, of which large quantities of which are currently discarded because they have no significant uses. It can also employ used, discarded silk fabric, diverting that material from being disposed of in landfills.
Currently, 90 per cent of the world’s silk production takes place in China, largely because China has perfected the production of the high-quality silk threads needed for fabrics. As the new process being proposed uses bulk silk which has no need for that level of quality, production could easily be ramped up in other parts of the world to meet local demand, should the process become widely used.
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