Sandy-dandy invention shows its strength

Building sandcastles just got a whole new meaning, thanks to a manufacturing invention that has a sand-based polymer holding up to 300 times its own weight.

Researchers at the Oak Ridge National Laboratory, US, designed a novel polymer that binds to silica sand. It can be 3D printed into integrated geometries that massively increase the sand’s strength, but it is also water-soluble for getting rid of in a hurry.

In the study, published in Nature Communications, the team 3D printed a 6.5-centimeter bridge that can hold 300 times its own weight – that’s like 12 Empire State Buildings sitting on the Brooklyn Bridge!

The printer uses a liquid polymer – polyethyleneimine (PEI) – to bind to powdered sand, building the structure layer by layer. This doubled the strength of the sand compared to other polymer binders.

When removed from the printer, the structure was porous and had lots of holes, which were filled with a glue called cyanoacrylate. This second step increased the strength a further eight times, making it stronger than any known building material, including masonry.

“Few polymers are suited to serve as a binder for this application,” says lead researcher Tomonori Saito of Oak Ridge National Laboratory.

“We were looking for specific properties, such as solubility, that would give us the best result. Our key finding was in the unique molecular structure of our PEI binder that makes it reactive with cyanoacrylate to achieve exceptional strength.”

This new material could be used to create composite parts in the likes of the automotive and aerospace sectors – lightweight materials such as carbon fibre and fibreglass could be wrapped around 3D-printed sand cores, often called tools, and cured with heat.

The silica sand is particularly useful for this tooling because it doesn’t change shape with heat and can be later “washed out” when the wrapped material is cured, because the polymer is water-soluble.

“To ensure accuracy in tooling parts, you need a material that does not change shape during the process, which is why silica sand has been promising,” says lead author Dustin Gilmer of the University of Tennessee in the US. “The challenge has been to overcome structural weakness in sand parts.”

Previous sand-based tools easily broke apart under heat pressure, so had limited industrial use.

“Our high-strength polymer-sand composite elevates the complexity of parts that can be made with binder-jetting methods, enabling more intricate geometries, and widens applications for manufacturing, tooling and construction,” says Gilmer.

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