Polymer discovery gives 3D-printed sand super strength

Oak Ridge National Laboratory has developed a novel polymer that strengthens sand in additive manufacturing applications. This 6.5-cm 3D-printed sand bridge held 300 times its weight. Credit: Dustin Gilmer/University of Tennessee in Knoxville
Oak Ridge National Laboratory, Department of Energy, developed a new polymer that can bind and reinforce silica sand in binder jet additive manufacturing. This is a 3D printing method used by industries to prototype and produce part.

The printable polymer allows for sand structures with complex geometries, exceptional strength and is also water-soluble.

Nature Communications published the study. It shows that a 3D-printed, sand bridge can support 300 times its weight at 6.5 cm. This feat is comparable to the 12 Empire State Buildings located on Brooklyn Bridge.

Binder jet printing is faster and cheaper than traditional 3D-printing methods. It allows you to create 3D structures out of a variety powdered materials. This gives you advantages in both cost and scaleability. This concept is similar to inkjet printing except that the printer head uses a liquid polymer instead of ink to bind powdered materials like sand. It builds up a 3D design layer-by-layer. The binding polymer gives printed sand its strength.

This team utilized their polymer expertise to create a PEI binder, which doubled the strength and durability of sand parts, compared to conventional binders.

Binder jetting parts are porous at first when they are removed from the printer bed. You can strengthen them by adding cyanoacrylate to the design. This fills in any gaps. The second step gave a eight-fold increase in strength over the first. This made the polymer sand composite more durable than any other building material, even masonry.

"This application requires a specific type of polymer. We sought out properties such as solubility that would yield the best results. "Our key finding was in our PEI binder's unique molecular structure that makes it reactive to cyanoacrylate, which gives us exceptional strength," stated Tomonori Saito from ORNL, who is the lead researcher.

While conventional binders can make parts denser by adding infiltrate materials like super glue, none of these have been able to match the performance of the PEI binder. The PEI binder is strong because of the way it reacts with cyanoacrylate during curing.

Oak Ridge National Laboratory scientist Tomonori Saito displays a 3D-printed sandcastle at ORNL's DOE Manufacturing Demonstration Facility. Credit: Oak Ridge National Laboratory scientist Tomonori Saito shows an 3D-printed sandcastle in the DOE Manufacturing Demonstration Facility (ORNL).

The super-strength of sand could be used to develop tooling for composites manufacturing.

Silica sand, a readily available and cheap material, is being used to create composite parts in the aerospace and automotive sectors. Lightweight materials like carbon fiber and fiberglass are wrapped around 3D printed sand cores or "tools" and heat cured. Silica sand has two advantages: it is washable and does not shrink when heated. A water-soluble binder is important for composite applications. It allows for a simple washout with tap water to remove the silica sand and leaves a hollow composite form.

"To guarantee accuracy when tooling parts, you need material that doesn't change shape throughout the process. This is why silica has been so promising. Dustin Gilmer, a University of Tennessee Bredesen Center student who was the study's lead writer, said that it has been difficult to overcome structural weaknesses in sand parts.

The current sand casting cores and molds are not suitable for industrial use. Commercial methods such as washout tooling apply heat and pressure to the sand, which can cause it to fail or break on the first attempt. For large-scale manufacturing and rapid part production, stronger sand parts will be needed.

Gilmer stated, "Our high-strength, polymer sand composite increases the complexity of parts that are possible to be made using binder jetting methods. This allows for more complex geometries and broadens applications for manufacturing and tooling.

The industry partner ExOne licensed the novel binder for research and won a 2019 R&D 100 Award.

The journal article is entitled "Additive manufacturing strong silica structures enabled by polyethyleneimine binder."

More information: Dustin B. Gilmer and colleagues, Additive Manufacturing of Strong Silica Sand Structures Enabled by Polyethyleneimine Binder, Nature Communications (2021). Journal information: Nature Communications Dustin B. Gilmer and colleagues, Additive Manufacturing of Strong Silica Sand Structures Enabled by Polyethyleneimine Binder, (2021). DOI: 10.1038/s41467-021-25463-0