Page 68 - Plastics News October 2018

tEChnoLogy

New NIST method measures 3D polymer processing precisely

he National Institute of Standards and Technology printed parts. Instead, the performance of fabricated
T(NIST) have derived methods to measure 3D polymer parts depends on printing conditions. NIST's new method
processing precisely Recipes for three-dimensional (3D) measures how materials evolve with submicrometer
printing, or additive manufacturing, of parts have required spatial resolution and submillisecond time resolution
as much guesswork as science. Until now. Resins and other -- thousands of times smaller-scale and faster than bulk
materials that react under light to form polymers, or measurement techniques. Researchers can use SCRPR to
long chains of molecules, are attractive for 3D printing measure changes throughout a cure, collecting critical
of parts ranging from architectural models to functioning data for optimizing processing of materials ranging from
human organs. But it's been a mystery what happens to biological gels to stiff resins.The new method combines
the materials' mechanical and flow properties during the AFM with stereolithography, the use of light to pattern
curing process at the scale of a single voxel. A voxel is a photo-reactive materials ranging from hydrogels to
3D unit of volume, the equivalent of a pixel in a photo. reinforced acrylics. A printed voxel may turn out uneven
Now, researchers at the National Institute of Standards due to variations in light intensity or the diffusion of
reactive molecules. AFM can sense rapid, minute changes
in surfaces. In the NIST SCRPR method, the AFM probe is
continuously in contact with the sample. The researchers
adapted a commercial AFM to use an ultraviolet laser to
start the formation of the polymer ("polymerization")
at or near the point where the AFM probe contacts
the sample.The method measures two values at one
location in space during a finite timespan. Specifically,
it measures the resonance frequency (the frequency of
maximum vibration) and quality factor (an indicator of
energy dissipation) of the AFM probe, tracking changes
in these values throughout the polymerization process.
These data can then be analyzed with mathematical
models to determine material properties such as stiffness
and damping. The method was demonstrated with two
and Technology (NIST) have demonstrated a novel materials. One was a polymer film transformed by light
light-based atomic force microscopy (AFM) technique -- from a rubber into a glass. Researchers found that the
sample-coupled-resonance photorheology (SCRPR) -- that curing process and properties depended on exposure
measures how and where a material's properties change power and time and were spatially complex, confirming the
in real time at the smallest scales during the curing need for fast, high-resolution measurements. The second
process. "We have had a ton of interest in the method material was a commercial 3D printing resin that changed
from industry, just as a result of a few conference talks," from liquid into solid form in 12 milliseconds. A rise in
NIST materials research engineer Jason Killgore said. He resonance frequency seemed to signal polymerization
and his colleagues have now published the technique and increased elasticity of the curing resin. Therefore,
in the journal Small Methods. 3D printing, or additive researchers used the AFM to make topographic images of
manufacturing, is lauded for flexible, efficient production a single polymerized voxel. Surprising the researchers,
of complex parts but has the disadvantage of introducing interest in the NIST technique has extended well beyond
microscopic variations in a material's properties. Because the initial 3D printing applications. Companies in the
software renders the parts as thin layers and then coatings, optics and additive manufacturing fields have
reconstructs them in 3D before printing, the physical reached out, and some are pursuing formal collaborations,
material's bulk properties no longer match those of the NIST researchers say.

68
Plastics N
Plastics News October 2018ews October 2018 72