When it comes to plant-based materials, many people may think of something "green", but not very sturdy. However, this is not the case with a new composite material designed by the Massachusetts Institute of Technology (MIT), which is said to be as hard as aluminum and as tough as bone. The main component of the material is cellulose nanocrystals (CNCs). These are obtained from the cellulose fibers that make up the cell walls of trees through an acid hydrolysis process.
At the nanoscale, CNC is stiffer and stronger than Kevlar fibers. Therefore, it was previously hoped that if they were added to existing polymers, they could make these polymers stronger. Unfortunately, when added, CNCs tend to form lumps, and they only form weak bindings to polymer molecules.
To solve these problems, Professor A. John Hart and colleagues first mixed commercially available CNC powder with a synthetic liquid polymer in a proportion optimized to produce a gel. An ultrasonic probe is then used to break any CNC clumps formed in the gel. Doing so allows individual nanocrystals to be dispersed throughout the gel, so they can form a firm bond with the polymer molecules.
Next, some of the gel is extruded through the nozzle of the 3D printer, and some is poured into the mold. In both cases, once the gel is subsequently dried and contracted, the final result is a hard solid consisting of 60% to 90% CNC.
In addition, it was found that the composite material has a microstructure similar to brick and mortar, similar to the strength of the pearl lining of some mollusk shells. This structure has been shown to prevent cracks from spreading in the material.
Tests on a penny-sized film made from this composite material showed that it was stronger, more resilient than some types of bone, and harder than typical aluminum alloys. The researchers also used the material to create a life-size model of human teeth to demonstrate that it could one day be used to produce dental implants.
At the same time, though, scientists are trying to minimize the shrinkage of composites when they dry. In this way, it will be suitable for making larger objects, rather than smaller samples produced so far.
"By manufacturing composites with CNC under high loads, we can give polymer-based materials a mechanical property they never had before," Hart said. If we could replace some petroleum-based plastics with naturally derived cellulose, it would be arguably better for the planet as well. ”