The Forest Products Laboratory of the US Forest Service has opened a $1.66 million (US$1.7 million) pilot plant for the production of cellulose nanocrystals (CNC) from wood by-products materials such as wood chips and sawdust. Prepared properly CNCs are stronger and stiffer than Kevlar or carbon fibers. Source: Gizmag
Putting CNC into composite materials results in high strength, low weight products. In addition, the cost of CNCs is less than 10% of the cost of Kevlar fiber or carbon fiber. These qualities have attracted the interest of the military for use in lightweight armor and ballistic glass (CNCs are transparent), as well as companies in the automotive, aerospace, electronics, consumer products, and medical industries.
Cellulose is the most abundant biological polymer on the planet and it is found in the cell walls of plant and bacterial cells. Composed of long chains of glucose molecules, cellulose fibers are arranged in an intricate web that provides both structure and support for plant cells. The primary commercial source for cellulose is wood, which is essentially a network of cellulose fibers held together by a matrix of lignin, another natural polymer, which is easily degraded and removed.
Wood pulp is produced in a variety of processes, all of which break down and wash away the lignin, leaving behind a suspension of cellulose fibers in water.
At present the yield for separating CNCs from wood pulp is about 30%. There are prospects for minor improvements, but the limiting factor is the ratio of crystalline to amorphous cellulose in the source material.
Short-term goals for the cost of CNCs is around $10 per kilogram, but large-scale production should reduce that figure to one or two dollars a kilo.
CNC’s greatest nemesis is water.
There are several approaches to make CNC composite materials viable choices for real world applications. The simplest, but most limited, is to choose applications in which the composite will not be exposed to water.
Another is to alter the surface chemistry of the cellulose so that it becomes hydrophobic, or water-repelling. This is easy enough to do, but will likely substantially degrade the mechanical properties of the altered CNCs.
A third approach is to choose a matrix material that is hydrophobic, and preferably that forms a hydrophobic interface with CNCs. While not particularly difficult from a purely chemical viewpoint, there is the practical difficulty that interfaces between hydrophobic and hydrophilic materials are usually severely lacking in strength.
Perhaps the most practical approach will simply be to paint or coat CNC composite materials in some material that keeps water away.
