by Courtney Fox, Ph.D., Research Scientist, Carbon3D, Inc.
Polymeric materials have become an integral part of our lifestyle over the last century. They’re made into an enormous range of products, from simple and inexpensive injection-molded toys and tchotchkes to super-strong bullet-proof vests and high-performance water-separation membranes.
Recent enhancements in both chemical and computational capabilities are giving scientists the ability to create specially designed polymers aimed at solving a host of important materials challenges.
One development that I am particularly excited about is the possibility that a new family of polymers invented last year by my former colleagues at IBM Research-Almaden may greatly expand the materials that can be used in 3-D printing and other additive manufacturing techniques.
Additive techniques – which are estimated to comprise a $3 billion business today that’s growing faster than 20 percent a year – are quite different from traditional “subtractive” manufacturing methods, which take a block of material and cut, carve, mill or etch away bits and pieces to form the desired part. In 3-D printing, a part is instead built up layer-by-layer with precise computer control. Typically, however, the materials must be able to flow through a small nozzle, yet retain its dimensions until it’s cured, or solidified. This leads to a number of problems, such as a limited choice of materials and their finished-part properties, dimensional creep, weakness at the boundaries between layers, and long time it can take to print larger pieces.
As a result, people usually use 3-D printing today only for making prototypes, not end-use mass-manufactured parts. This leads to another criticism: There is significant waste when the prototypes are discarded.
Many of us in the industry are hopeful that the new IBM polymers have the potential to alleviate many of these issues.
The central theme of the new polymers is the clever combination of a solvent and a monomer – a chemical structure that can be chained together to make a polymer – to make a gel that retains its shape. In this condition, the material can be both self-healing and recyclable.
Applying heat (or, for some combinations, ultraviolet light) will cause the monomer to join together, or cure, to form a permanent three-dimensional network of chemically cross-linked bonds that give the finished material its strength and other mechanical properties. Different combinations of monomers and solvents can result in widely diverse materials and properties.
|Dr. Jeannette Garcia in the lab
Jeannette Garcia’s group at IBM previously demonstrated remarkable self-healing properties in pre-cured networks based on nitrogen-containing monomers used to make high-strength thermoset polymers: hemiaminal and hexahydrotriazine. (A thermoset polymer is cured by heating its monomers.)
Tuning the uncured combination so it flows more freely when experiencing shear stress should allow it to flow through tiny 3-D printer nozzles, yet retain its shape after deposition. It has not yet been demonstrated, but the materials’ self-healing properties combined with a continuous curing process could also eliminate interlayer weakness in 3-D printed parts.
Our CLIP technology at Carbon3D takes advantage of the interactions between UV light and oxygen to rapidly cure photopolymer resins. Our continuous process eliminates the need to repeatedly coat a layer of UV curable material, creating a layerless polymer network. Printing using CLIP also lowers the mechanical forces on a 3-D printed object; traditional 3-D printing techniques like stereolithography or fused deposition modeling could not accommodate soft materials like those developed by IBM. We are very excited about the rapidly expanding 3-D printing material landscape.
Hemiaminal materials can also be recycled, as treatment with acid allows recovery of the original starting monomers with sufficient purity to use to make new products. Combined with their high strength, these materials should enable longer product lifetimes using fewer raw materials.
I believe the future is bright for these materials. I look forward to, for instance, someday using the hemiaminal-UV-curable networks described in the IBM group’s Advanced Materials report last month to print very-high-value membranes for batteries or water-purification directly, without using harsh solvents that are often required today.
Those are just a couple of ways that these new materials could increase the beneficial impacts of 3-D printing.
For three years while Dr. Fox was pursuing her Ph.D. at Stanford, she worked at IBM Research-Almaden with Jim Hedrick and Jeannette Garcia as part of the group that created the new family of recyclable and self-healing polymers. She is the lead author on a recent Nature Communications report that describes how the materials are made and the basis for their remarkable properties. Since late 2014 she has been working at Carbon3D, Inc., a startup company that is commercializing its innovative 3-D printing process that is 25 to 100 times faster than conventional 3-D printing methods.